Week 2: Understanding ecosystems

Introduction

Figure 1

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A woodland, illuninated by light from the sun low on the horizon, with the tree trunks silhouetted against the sunlight.

Figure 1

Ecosystems comprise more than relationships between organisms in the habitat. They are affected by factors such as light, water, carbon dioxide and nutrients and, of course, human activity.

It is nearly impossible to understand all the interactions occurring in a given ecosystem at any one time, but it is possible to observe the types of interactions that are present – and there are six, described by Dr Mike Gillman in the following video. Analysing interactions in terms of these types can help to define the boundaries of a system, though this is not an easy task. Dr Vincent Gauci considers the routes of energy loss in ecosystems.

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Interactions

DR. MICHAEL GILLMAN

There are two main ways of defining what we mean by an ecosystem. Some people talk of organisms that share similar conditions. But a more useful definition is to talk about how organisms interact, how they work as a system. This is what Arthur Townsley had in mind when he coined the phrase. So ecosystems are all about interactions. And if we're going to get to grips with ecosystems, we've got to get to grips with those different interactions. But hang on, there's billions and billions of different interactions between millions of different species. Now obviously, we're not going to be able to analyse every single last one of those, but what we can do is look at the different types of interactions. Luckily, there's not so many of those. It's all about energy and nutrients. The ultimate energy source is the sun, and most plants interact with it using photosynthesis to turn the sun's energy into their own chemical energy. That energy is then passed on through a range of different feeding interactions. One way to classify all the different types of interactions is according to whether organisms have a net gain or a net loss from any interaction, whether they win or lose. Any win or loss of nutrients or energy may be vital. Winning or losing can affect whether any one organism survives or dies. There are six possible outcomes that are observed in natural systems. Commensalism, in which one species gains and there is no effect on the other species. Mutualism, in which both species gain. Parasitism, predation, and herbivory, where one species loses and one gains. And competition, where both species lose, or both individuals within a species lose. Analysing interactions like this helps us to understand what's going on in the ecosystem. It can also help us define an ecosystem's boundaries, which aren't always as clear-cut as you might think.

DR. VINCENT GAUCI

You could have a very simple ecosystem that has an apparent boundary in a pond or a lake. But the situation then gets more complicated because you then have runoff from the surrounding hills, and that brings nutrients into the lake. And so, it's not quite as easy as you think it might be.

DR. MICHAEL GILLMAN

So interactions help us to define the boundaries of an ecosystem. But in order to understand the functioning, we need much more detailed information. We need to know about energy. We need to know about the rate of transfer, we need to know about the route of transfer, and we need to know about the efficiency of transfer. And that brings us back to the primary source of energy, the sun.

DR. VINCENT GAUCI

We have a waveform of energy coming through the atmosphere from the sun and light, and that gets converted to a chemical form of energy. Now that chemical form of energy uses carbon, so you're making sugars and starches. Of all the sunlight that comes into the Earth's atmosphere, about 8% is trapped by green plants through photosynthesis, and we call that gross primary productivity. Now, of that 8%, about 50% is immediately respired out, so the carbon that's been fixed then leaves the plant, the remaining 50% goes into building the plant tissues. That can be leaves and stems. But also into leaving a little extra for a bad day, winter time. There'll be some that'll be stored away in roots and rhizomes, and a little bit will also go into reproduction. So, the manufacture of seeds.

DR. MICHAEL GILLMAN

There are four ways in which the plant's energy can be passed on within the ecosystem. It could be stored as perennial biomass. It can pass as dead tissue to decomposers. It can get eaten with its energy passing on to herbivores. Or it could be passed on through what's called soluble losses.

DR. VINCENT GAUCI

Some of this carbon that has gone into the plant then leaks out through its roots. This form of carbon could be considered a loss from the plant, but actually the plant is investing in a process that actually assists it.

DR. MICHAEL GILLMAN

It may be a loss to the individual plant, but it can be thought of as an investment in the whole ecosystem, a kind of plant tax if you like, and it can make a huge difference.

DR. VINCENT GAUCI

If you consider trees and forests, they develop these interactions with what we call mycorrhizae. Now these mycorrhizae do something that the plant can't. They're particularly good at taking nutrients out of the soil. In giving those nutrients to the plant, in return, they will get a source of carbohydrate or sugar, which sustains them. About a quarter or carbon loss from the plant goes into this mycorrhizae, and this interaction is actually responsible for a huge amount of what we call soil respiration; that is the CO2 that is lost from the forest floor. So it's tremendously important in terms of the cycling of carbon in an ecosystem.

DR. MICHAEL GILLMAN

It's not always a win-win scenario. Some net production is simply lost, washed away, or leached out. No system is perfectly efficient, and ecosystems are no different. Some are more efficient than others. Understanding the types of interactions and the flow of energy and nutrients is vital to understanding how ecosystems work.

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1 The carbon cycle

Dr Vince Gauci describes how carbon that plants have fixed from the atmosphere moves through an ecosystem and eventually is returned to the atmosphere. Carbon can be stored for long periods in the natural environment.

When you've watched the video think of some examples of places where carbon is stored.

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The peatbog problem

DR. FRED WORRALL

Our largest store of carbon in the country are not our forests, it's our peat bogs. The peat bogs of the UK store more carbon than the forests of Britain and France combined.

JULIETTE MORRIS

How significant a part can peat bogs play in helping to tackle global warming?

DR. FRED WORRALL

Oh, oh tremendous. The amount of carbon stored in our peat lands in the UK is the equivalent of 21 years of total UK CO2 output. So that's all the CO2 from cars, power stations, everything.

JULIETTE MORRIS

Gosh.

DR. FRED WORRALL

There's 21 years worth. And that's a relatively conservative estimate. So if we damage these areas, we're going to be contributing to our CO2. But also, this stuff has been growing here for thousands of years, and there's no reason why it couldn't keep growing for another 6,000, 8,000 years. This has been growing here since the last ice age, so it can keep on growing. And that means it can keep on storing carbon and keep on taking carbon out of the atmosphere. So if we manage these well, they will actually help us solve our problem. If we manage them badly, they will contribute to our problem.

JULIETTE MORRIS

Keeping across all these carbon movements is Fred's colleague, Bob Baxter.

JULIETTE MORRIS

So how does it work?

DR. BOB BAXTER

Well, what it's got is two major components that you can see here. One is simply just measuring. As air passes through the prongs of the system, the concentration of carbon dioxide in the atmosphere at 10 times per second. So very rapidly. And coupled with that, we have basically wind coming across the landscape, bouncing across the landscape if you like. We need to know the wind speed, so we use something called a sonic anemometer, which are the prongs that you can see on that system there. People at home may recognise the cup anemometer, which you see on weather stations often spinning around, telling wind speed. But that's just in one direction, just in the horizontal. We need to know whether the air is moving up from the land or down to the land.

JULIETTE MORRIS

And that then obviously tells you which direction the carbon's going in.

DR. BOB BAXTER

Yes, exactly.

JULIETTE MORRIS

Ultimately, what's going to happen to all your research?

DR. BOB BAXTER

So, what we're trying to do here at the present time is get a long run of information day by day. Early days at the moment in terms of what is happening certainly, but through modelling, through trying to predict into the future, then we're trying to use this as a baseline information of a number of years trying to predict what is happening in terms of global warming.

DR. VINCENT GAUCI

At the Open University, we're also researching greenhouse gas emissions from carbon-rich wetland ecosystems. Bob Baxter uses a micro meteorological tower to integrate over very large scales. But if you're interested in the very fine scale, like we are, all you need is this. It's called a chamber and it's used to trap emissions from the soil so we can analyse what they are. So you place a chamber on the soil's surface, and here we've got a nice soggy, peaty soil surface, which should be producing lots of methane, and you define the volume of the chamber by placing a lid onto the top there and sealing it. It's simple but effective. With the chamber in place, we can take an initial sample. This should show a composition that is similar to the ambient local atmosphere. It's our baseline sample. And then we wait, taking samples over time. 20 minutes later, I come back and take my third and final sample of the volume. I would expect to see a much larger concentration of methane in the chamber. The samples are analysed in the lab so we can calculate the rate at which methane is being produced and get an idea of what's happening to an important carbon store. This kind of field work looks at methane emissions as they are today. But in the lab, we can travel back in time and that's what Ph.D. student Carl Boardman is doing. Inside these sealed units, Carl has a selection of peat cores from a bog and a fen, two different types of peat land ecosystems that produce methane. The units are linked to gas cylinders so Carl can precisely control the makeup of the air inside. And he's particularly interested in the level of CO2.

CARL BOARDMAN

An experimental CO2 level is approximately half modern day concentration. And now that's significant because approximately a half modern-day concentration is equivalent to what was present 21,000 years ago, which was the last glacial maximum. So what we're trying to do is trying to recreate the CO2 concentration in the atmosphere back them.

DR. VINCENT GAUCI

So the air in Carl's experimental cabinet is the same mix of gases as the air would've been at the height of the last ice age, so we can find out how the availability of CO2 would've effected methane emissions back then. The way he samples and records methane emissions is similar to what I was doing out in the field, but a little more high tech. With the chamber in place, Carl can see a readout of the methane emissions immediately.

CARL BOARDMAN

What we're looking at now is a continuous readout of methane emissions coming from the peat core. On the X-axis, we've got time. On the Y-axis, we've got methane concentration in parts per million. The flat line before 800 seconds is the ambient methane concentration. About 800 seconds is when the chamber was put onto the peat core. When the chamber has been put onto the peat core, what you can see now is a linear increase in methane concentration with time that's coming from the peat core. The main reason why we're doing it is because current research is based upon modern day parameters; so when these studies or these models try to extrapolate and go back in time, they're actually constrained by the fact that they're using these modern day relationships. Well, hopefully, the results that we get from this experiment will constrain the models that are currently out there.

DR. VINCENT GAUCI

So, in a modern lab, like the one where Carl is conducting his experiments, you can really push or manipulate or constrain the system you're investigating to find out how it works. Same thing's happening with earth's climate system, where the carbon balance is being perturbed by human emissions. Now, we can really mimic these cycles and these perturbations in the laboratory, and that really helps us to find out what's going on out there in the real world. The cycle of carbon is the key to life on Earth. Plants absorb carbon as CO2 through photosynthesis, and its re-released over time through decomposition. But the balance of carbon is also important in regulating climate. So, as our climate changes, it becomes more and more important for us to understand both the balance and cycle of carbon. And that will help us to understand what will happen in the future.

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Life on earth is carbon based. A key feature of ecosystems is the passage of carbon through the system as part of the carbon cycle. Solar energy is captured in the leaves of plants and drives the incorporation of carbon into organic molecules. Carbon dioxide, in effect, combines with water to produce simple molecules. The process is called photosynthesis and in this video Sir David Attenborough describes it as the very basis of life.

How does the availability of light, water, carbon dioxide and nutrients affect the productivity of an ecosystem?

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How plants make food

DR. VINCENT GAUCI

The fundamental material of all living things on our planet is carbon. Now this starts out as an inorganic, molecular gas in the atmosphere, carbon dioxide or CO2. So to get the carbon into an ecosystem you need the process of photosynthesis. It's a process that is unique to plants and certain micro-organisms, but it benefits almost every living thing on earth. Photosynthesis is how plants make their food, using a simple set of ingredients. Sir David Attenborough described it as the very basis of life. So let's leave it to him to explain how it works.

SIR DAVID ATTENBOROUGH

Air seeps into the leaves from pores on their surface. It circulates within them and reaches tiny granules that contain a green substance, chlorophyll. This is the key facilitator that uses the energy of the sun to bond carbon dioxide to hydrogen derived from water and produces carbohydrate - sugars and starches. These dissolved in sap are then carried from the leaf into the body of the plant, even during the night when the leaf factory has shut down. Come the dawn, the sun re-appears and the process starts up again.

DR. VINCENT GAUCI

So photosynthesis is the fundamental process driving the production of material in ecosystems. Light, water, nutrients, and CO2 are all key ingredients in driving that level of productivity. If you were to reduce any one of those key ingredients, that would result in a loss of productivity in a plant, like this tree here. Those four key ingredients, together with temperature, are known as environmental variables. Each one of them can affect photosynthesis and as they are unevenly spread through space and time, there can be dramatic differences in productivity across the globe and over different time scales. The availability of light varies throughout the day as the earth spins on its axis. At the Poles there can be almost constant daylight in the summer months, but most of the planet experiences a diurnal cycle - night and day, darkness and light.

PROFESSOR DAVID GOWING

Well, light is key to photosynthesis because it's the source of energy, and therefore it determines the rate of which photosynthesis can proceed. This means that productive ecosystems need to be in full sun, like a forest canopy. Beneath the canopy where light is attenuated by the canopy above it, then plants can operate at a much lower rate and take in much less carbon per unit time. Too much light can be a problem because if leaves can't access carbon dioxide quick enough to use that energy in photosynthesis, the excess energy becomes a problem for the leaf. It may even damage the photosynthetic apparatus. So this is an issue at the top of the forest canopy, where the leaves are in full sun. And plants have come up with a whole range of adaptations to cope with that, including pigments to try and absorb the excess light and photo respiration, where they actually respire the carbon they have just fixed to produce carbon dioxide to soak up that excess energy.

DR. VINCENT GAUCI

At the equator, light is available for twelve hours a day, all year around. That steady supply of energy, combined with high levels of rainfall, make the tropical rain forests highly productive. Temperatures are high all year, so water is available in liquid form. It doesn't get locked away as ice during the winter. In other parts of the word there is a different mix and cycle of environmental variables. In temperate regions, where the seasons are more pronounced, production takes place in spring and summer, when there's the most sunlight and the warmest temperatures. This cycle of production through the seasons can vary year by year. And we can see a record of variations in tree rings, with the wider rings showing warmer summers. In dry, arid areas of the tropics it's not light that's the issue. It's the availability of water that limits production. Perhaps the most powerful way to show how water limits production is to look at what happens when a desert gets wet. And that happens on a huge scale in the area of the Kalahari Desert known as the Okovango Delta. These remarkable images were filmed by the BBC programme "Plant Earth". As water flows into the delta, the landscape is transformed. With the water comes life. Plants can once again produce carbohydrates through photosynthesis, converting CO2 and water. What was once a barren desert is now a wildlife oasis. This is an extreme example of water limitation and the relief water can bring to a dry ecosystem. But to fully understand the effect of water on productivity, we need to understand the biochemistry of photosynthesis.

PROFESSOR DAVID GOWING

Water is essential for taking carbon dioxide out of the air because plants exchange water vapour for carbon dioxide when their stomata are open. And so a lack of water can really limit the amount of carbon a plant takes in. In environments where you have a real lack of water then plants have come up with alternative photosynthetic pathways to cope with the problem. A group called the C4 plants concentrate carbon dioxide by having specialist cells that take it in, store it, and pass it to photosynthetic cells which are located deeper within the leaf. And by doing that, they're supplying those specialist cells with enough carbon dioxide that they can utilise light to its full without the risk of excess light damaging the apparatus. C4 plants tend to occur in dry environments in high light, so savannahs would be the most typical natural environment for them. And plants that have evolved in that sort of savannah environment are quite often used as crop plants such as maize, and sugar cane, and sorghum. They all use the C4 photosynthetic pathway, which means they are extremely productive if supplied with light. Taking that one step further, an even more extreme adaptation is cacti and succulents that only take up carbon at night. And they store the carbon as organic acids in their big, fleshy cells, for later use during the following day where it's re-released as carbon dioxide and normal photosynthetic pathways take over.

DR. VINCENT GAUCI

So light and water can influence and limit productivity. The same is true of CO2, which is vital for photosynthesis. Now the concentration of CO2 doesn't vary over the planet. On a fine scale you can have concentration differences that are sufficient to affect productivity. In a dense canopy, CO2 can be used up faster than it's being replenished from the atmosphere. Manmade emissions can also have an effect. So light and water, carbon dioxide and nutrients, all directly influence the productivity of any one ecosystem. These limiting factors, together with temperature, are known as environmental variables. But that's only half of the cycle. In the next film we will look at the other half - decomposition.

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2 Exploring oak woodland

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Deciduous woodland in autumn with fallen leaves carpeting the ground.

Figure 2

Woodlands provide a large variety of habitats, which are occupied by a huge variety of organisms. As an example, consider an oak wood, and the food chains and webs that exist in it.

In ‘Touring an oak woodland’, Professor David Streeter introduces you to a complex ecosystem. As you watch the video, recall the concept of indicator species, which you saw in the first week and consider the following questions.

• Why are the inter-relationships in ecosystems like woodlands so complex?
• What can food chains tell us about the ecosystems that exist in a particular woodland area?
• How have woodlands been managed?

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Touring an oak woodland

PROFESSOR DAVID STREETER

Oak woodland occupies a very special place in British natural history, because many people think that it represents the natural vegetation of much of lowland Britain before man became a significant influence. However, as no examples of that original wildwood survive, nobody really knows what it looked like. In fact, the traditional idea that the whole of lowland Britain was once covered by a continuous sea of oak woodland has now had to be quite seriously revised. Because modern pollen analysis has told us that the original woodland the predominant tree was not actually the oak but was the lime with hazel, and some elm, and of course oak as well. And what has happened is that during pre-history the lime, and to some extent the elm, has all being selectively removed. In Britain we've got two species of oak - the common or pedunculate oak, and the dumast or alternatively named sessile oak. And they have slightly different soil requirements. But in some of the sandy soils of the Weald of Sussex, like here, we have both species growing together. Historically woodlands were managed in one of two traditional ways - either as wood pasture or as coppice. Wood pasture was a method of managing woodland where tall, mature trees were allowed to grow and mixed up with grazing animals between them. Whilst coppice was a method of management whereby the trees were cut to ground level on a regular basis. And the purposes for which oak coppice was used was either for tannin, where the bark was stripped off the young shoots and the tannin was extracted from them, or as charcoal, or perhaps in some parts of the country for pit props. And what we've got here is the rather scrubby re-growth which hasn't been properly managed. On the deeper soils of the valley sides the oaks would have done much better. They would have grown into much finer specimens. So here the trees would have been allowed to mature and they would have been harvested as mature timber. Woodlands are complicated places. They occupy an enormous space from the soil to the top of the tree canopies. And that space is occupied by the trees, and the shrubs, and the herbaceous vegetation. And they generate a huge number of different kinds of habitats. And that, in turn, produces an enormous diversity of woodland organisms. Let's just have a look at some of the common woodland plants that are growing here by the edge of the path. What have we got? Well, obviously first there's bluebell. And with bluebell, what have I got down here? That's honeysuckle, and bramble or blackberry, whichever you like to call it. And bracken - the bracken fern. And if you gave that list of plants to a group of Japanese or Chinese ecologists and you said, where in the world was this list made? If they were any good they would have to say you were somewhere in Western Europe, in a woodland, on well-drained, acid soil. How would they know that? Well, firstly from the bluebell, which is one of the rarest plants in the world, because it's only found in Europe, west of the Black Forest, south of Holland and north of the Pyrenees - nowhere else. And the soil information would have come from the bracken. It only grows on acid soils and where those soils are well-drained. Honeysuckle has got a very similar distribution to the bluebell. And of course is not flowering here where I'm sitting. But behind me, where it's growing over the birches, it'll be in full bloom in a week or two's time. The tree canopies themselves support a much wider diversity of species than any other tree in Western Europe, many of them specific to the oak. And if you add to that the enormous numbers of organisms which are dependent upon dead and decaying wood, then you have the most species-rich habitat in the country.

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2.1 Following a food chain

In ‘Following a food chain’, Professor David Streeter and Professor Chris Perrins show how you can study one particular food chain in a complex ecosystem.

Each individual food chain tells only part of the story of the oak woodland. What would a diagram of the food chain in the oak wood featuring the winter moth look like?

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Following a food chain

PROFESSOR DAVID STREETER: Woods are typical ecosystems, a combination of biological communities occupying a physical environment. However, in many ways, woods are difficult ecosystems to study, because they are more complex than most. The size that they occupy is large, from the soil surface to the top of the tree canopy. And this space is occupied by trees, shrubs, herbaceous vegetation, and the ground layer, producing a huge variety of habitats, generating an enormous diversity of organisms. When trying to understand something as complex as the interrelationships between the different species in an ecosystem like a wood, it's helpful to focus on a single species in order to find out how it manages to maintain itself and survive as part of the community.

NARRATOR: Many oak woods contain breeding pairs of sparrowhawks. They are the commonest woodland birds of prey.

[BIRDS CHIRPING]

NARRATOR: Providing food for a nest full of sparrowhawk chicks is a full-time job.

PROFESSOR DAVID STREETER: Many woodland species breed in the spring, and food supplies are crucial throughout the breeding period. And they determine the success or failure of the next generation.

[BIRDS CHIRPING]

PROFESSOR CHRIS PERRINS: When the male is feeding the brood, it's quite noticeable. If they've got their timing right, they pretty well seem to specialise on tits. The trouble is the male tends to pluck them and pull their head off before they're brought in, so we don't have a good field guide for plucked and headless birds. And you're dependent, really, on identifying them from the legs.

[BIRDS CHIRPING]

NARRATOR: The tits must find enough food to raise their young.

[BIRDS CHIRPING]

PROFESSOR CHRIS PERRINS: We can easily fit up a camera behind the nest that's designed to take a shot each time the tit comes in with a caterpillar in its beak. You have to realise that the tits have these very large broods, and if they're to raise 10 or so young, they've got to be able to find food very easily. And the parents, actually, bring in 700 or 800 meals a day - 700 or 800 caterpillars during a day - and they can't waste time if they're to do that.

[BIRDS CHIRPING]

NARRATOR: Winter moth caterpillars have to find their own supply of food. Oak leaves form the final link in our food chain. How do oak trees obtain the energy and nutrients they need for growth and for making leaves? Like all green plants, oak trees use carbon dioxide and water to make vital organic compounds. This process is called photosynthesis. Photosynthesis takes place inside the oak leaves in tiny green structures called chloroplasts, which capture light energy from the sun. What happens next is a complex chain of reactions that can be summarised fairly simply. Water and carbon dioxide are converted using the sun's energy into simple sugars called carbohydrates. The oxygen released in the reaction diffuses from the leaves into the surrounding air for use by other organisms.

PROFESSOR DAVID STREETER: Individual food chains tell only part of the story. Woods contain many species of animals and plants, each with their own particular food chains. And considering the wood as a whole reveals many important ecological patterns and ideas.

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3 Fungi and the woodwide web

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The fruiting bodies of fungi on a tree branch.

Figure 3

Fungi are an important component of ecosystems, especially in forests or woodlands, as they are valuable for decomposition. Decomposition breaks down dead organic matter, releasing nutrients, which can then be reabsorbed.

In this audio, Dr David Robinson talks about how fungi also have an intimate relationship with trees, which extends the woodland ecosystem underground.

Reflect on the chain of interactions occurring between trees and fungi, starting with the photosynthesis in the tree canopy and ending with fungus in the tree’s roots absorbing nutrients.

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Transcript: Investigating symbiotic relationships

Investigating symbiotic relationships

NARRATOR

Fungi are an important part of any forest or woodland ecosystem. They are the major agents by which twigs and leaves are broken down, releasing nutrients for reabsorption by plants. And we know fungi also form a constructive partnership with living trees. David Robinson from the Open University's Life Sciences Department explains that although we have known about this partnership and relationship for some time, we are now learning more about the nature of that relationship.

DR. DAVID ROBINSON

You can go back nearly, I think, 400 million years and look at fossils, and you can actually determine that this relationship existed in fossils that length of time ago. So it isn't it biologically a new idea. It's only more recently that the precise nature of that relationship has been worked on. For example, it's become possible to use radioactive isotopes to track movement of molecules between fungi and tree roots. And then, even more recently, has come the applications of this knowledge, whereby horticulturalists and agriculturalists can make use of cultures of fungi to set up these relationships for themselves in areas that they're trying to plant. You can go and look at websites from people who supply mycelium - that's the fungal culture - for use in a whole range of applications.

NARRATOR

In Malaysia, scientists are now finding ways to apply the knowledge of the partnership between fungi and trees in order to ease a problematic relationship between economics and ecology.

DR. DAVID ROBINSON

I think the Interest about the Malaysian example is that they have a very particular problem that they are trying to solve there, and they're solving it with the use of fundamental research. And they're trying to pioneer techniques not only for changing logging practices in the country, but also for reclaiming areas which have been lost to forest and perhaps industrial areas of which now, with the aid of this research, they can hope to reclaim.

NARRATOR

In Malaysia, Dr. Lee Su See is trying to establish hardwood trees in a barren area of land using her knowledge of the way in which the relationship between fungi and trees works.

DR. DAVID ROBINSON

I was very struck by the experiment that Dr. Lee Su See was carrying out in reclamation. I mean, the area of land that she was working on looked absolutely impossible for plants to grow. Although, of course, there were one or two acacia plants that had managed to get established there. Of course, she was going to work on a large scale, so she couldn't just put plants there and hope for the best. She had to inoculate them with the fungus so as to get the web of mycelium and roots established. And it was obvious also that she needed to add other things, notably quite a lot of water. But it really didn't look the sort of place where you would expect to grow plants. And she clearly has had some success, and the success undoubtedly will continue. It's a long-term project, even so.

NARRATOR

The larger aim of Dr. Lee Su See's work is to get a sustainable source of hardwood in order to avoid the logging of untouched rain forest. In so doing, she is combining research with direct application.

DR. DAVID ROBINSON

So she's trying to get, really, quite large-scale production of hardwood, that is, really quite a number of trees in the same area, and get them established and growing away as quickly as possible. And this was difficult without the knowledge of the way in which the link with fungi worked.

NARRATOR

This growing knowledge of the large-scale web of relationships going on underground is changing scientists' views of the relationship between larger trees and saplings.

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Investigating symbiotic relationships

3.1 Unearthing the woodwide web

In nature, most trees form fungal connections. The health of the forest depends on fungus – decaying branches and leaf litter are rich with nutrients, and fungi can ferry these back to living plants.

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Unearthing the woodwide web

NARRATOR

Luxurious timber in the luxuriant rain forest. Economics inextricably bound with ecology. Now, biologists are unearthing a new set of relationships fundamental to the forest. Lessons from nature could help with man-made problems and literally turn our understanding of forests upside down. The first clue can be found almost anywhere there are trees. Even in a wet Yorkshire wood.

SPEAKER 1

If you come down and have a look, you can see there are cap scales. If you excavate around the fruit body, you might be able to find the remnants of a bag.

NARRATOR

Fungi seem unlikely candidates to start a revolution.

SPEAKER 1

And you can see this membranous ring underneath. Here we have a very distinctive fruit body. It's Phallus impudicus, a stinkhorn.

NARRATOR

But most of the action goes on beneath the soil. This fungus is digesting a dead piece of wood. Wood decomposers are the forest's recycling service. Nothing breaks down branches better. Look carefully in the leaf litter, and there are telltale signs of other decomposers. Skeleton leaves are caused by fungi. Other leaves are bleached when fungi attack.

PROFESSOR IAN ALEXANDER

Fungi are important components of the decomposer system in any ecosystem and particularly so in forests. They're one of the major agents by which the leaves and twigs which fall to the forest floor are broken down and the nutrients within them released for reabsorbtion by the plants.

NARRATOR

In the heat and humidity of the Malaysian rain forest, this happens up to five times faster than in a British oakwood. Ian takes up the trail with forest pathologist Dr. Lee Su See. As in the British woodland, decomposers deal with death. The health of the forest depends on them. Branches and leaf litter are a treasure trove of nutrients. Fungi feed them back to living plants. Again, to get to the business end, you have to get your hands dirty.

LEE SU SEE

Oh, wow, look at that.

PROFESSOR IAN ALEXANDER

Many of the fungi that occupy this part of the forest ecosystem form these long fungal strands. So the individual fungus can colonise quite a large area of the forest floor. And this serves as a sort of plumbing system for it to conduct carbohydrates and nutrients and water.

NARRATOR

This is part of an extraordinary network. Not all fungi get nutrients from breaking things down. Some of them form constructive partnerships with living trees. The budding mycologists are about to log onto a "woodwide" web.

SPEAKER 2

Is this it?

PROFESSOR IAN ALEXANDER

Yes, these are tree roots which are mycorrhizal. Some of these root tips will be infected by this fungus here.

NARRATOR

Mycorrhizal means the tree roots have teamed up with the fungus.

PROFESSOR IAN ALEXANDER

Oh, yes, look.

LEE SU SEE

Oh , yeah, wow. Look at that.

NARRATOR

And the fungus is part of a hidden underground community. It's quite interesting, the way that the mychorrhizae and the decomposers occupy the same bit of space, don't they, on the forest floor. So they must be interacting quite significantly.

JONATHAN LEAKE

I think below ground we have aspects of competition. But we also have a lot of interlinking between organisms. So the complexity of the below-ground linkages is something which is quite unique and different to what we see above ground, where we typically think much more about individual plants competing with each other or animals and plants interacting.

NARRATOR

These mycorrhizal pines have been cultivated for closer inspection. The fine threads are part of the fungus, collectively called the mycelium.

JONATHAN LEAKE

If you look here, you can see units that are much thicker, more robust. And these are joining together, forming an interconnected web connected back to the plant, and then extending out into the soil. At the same time, you can see there are finer mycelia extending off beyond the tips of these thicker structures.

NARRATOR

Plant and fungus connect in the bulbous tips. They become a single structure that looks different from either partner alone. Here, the hairs at the growing tip are replaced by mychorrhizas further up the root. It must be a mutually beneficial arrangement. In nature, most trees form fungal connections.

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3.2 Mutual benefits and competition

Figure 4 Tropical forests

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Two photos of a tropical forest. One shows the huge buttresses of one of the trees. The other shows dense vegetation and trees covering the ground up to the top of a distant hill.

Figure 4 Tropical forests

There is a close relationship between fungi and trees. As you watch the video note how this close relationship is being used to artificially reinvigorate ecosystems.

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Mutual benefits

DR. LEE SU SEE

Well, the dipterocarps are one of the most important family of timber trees in Malaysia and Southeast Asia. This family consists of a very large number of species, many of which have very important value as commercial timber trees.

NARRATOR

Behind every great dipterocarp lies a team of tiny fungi.

PROFESSOR IAN ALEXANDER

Underneath this big dipterocarp. Oh, yeah. Look at those. Oh yeah. The mycorrhizas are very active in competing for water and nutrients and providing benefit to the tree. In fact, the trees on these poor tropical soils just wouldn't survive without them

NARRATOR

So the tree taps the fungus for nutrients and water. By climbing to the top of the forest, Su See can get to the other side of the bargain.

DR. LEE SU SEE

Up in the canopy, the trees are photosynthesising. They're capturing the energy from the sunshine and with the carbon dioxide up here, they're going to produce sugars, which are then transported down the tree.

PROFESSOR IAN ALEXANDER

Down here on the forest floor is where the trees have to capture their water and their nutrients. So a proportion of the sugars, which are made up there in the canopy, come down here below ground to fuel the root system to capture nutrients. And some of that sugar finds its way out into the mycorrhizal fungus.

NARRATOR

With a Geiger counter and a mycorrhizal seedling, Jonathan can track the process.

JONATHAN

This shoot has received radioactive carbon dioxide in this box for just two hours. The shoot has photosynthesised and fixed some of that carbon. We'll check now and see where it is and where it's gone. You can hear the large amount of carbon that's being fixed by the shoots. And then, that carbon also is being transferred on to the root system. And then from the roots through the mycorrhizal root tips onto the external mycorrhizal mycelium that you can see here.

NARRATOR

So carbon passes not only from the shoot to the roots, but out of the plant to a totally different organism. A more sophisticated technique reveals the fungal side of the bargain. The patch contains nutrient rich leaf litter. Within weeks, the fungus will grow towards it and start to take up phosphorus and other nutrients.

JONATHAN

Phosphorus is one of the key nutrients in forest ecosystems that controls plant growth. And it's the one major nutrient that these mycorrhizal systems are very important in terms of acquiring from the soil.

NARRATOR

Adding radioactively labelled phosphorus reveals what happens next.

JONATHAN

JONATHAN: You can see straight away that we've got the radioactive phosphorus that we've added, some of it has already moved up. And this is where the plant is just outside of the imaging area, so it's moving up towards the plant. And if we look in a bit more detail, we can see firstly that it's moved through the fungus. This is the fungus connecting up to the plant. And then secondly, you can see that there are root tips here, mycorrhizal root tips, which have already acquired quite high concentrations of the phosphorus. This is the same system five days later. We can see the main pathways becoming even more evident now. And you could see the accumulation of phosphate, particularly in the root tips. Large amounts accumulating there. And some of it being transferred then on throughout the root system of the plant. The other part that's very exciting here is you see the distribution of phosphate that's being transferred around by the fungus. And particularly, to the growing tips where the demand is greatest as the fungus is growing.

NARRATOR

This all fits in to an intricate larger system. Trees and mycorrhizal fungi create a natural network with far-reaching connections.

DR. LEE SU SEE

Well, as you can see, the canopy consists of many layers of trees and the light filters down and the light intensity gets less and less. It gets darker as you get down to the forest floor. And when you have little seedlings down in the forest floor, most of them get very little light.

NARRATOR

Dipterocarp seedlings are often overshadowed by their parents. They're in very deep shade, where many kinds of tree wouldn't grow at all. In effect, they're waiting for dead men's shoes. If an older tree dies or falls, a gap in the canopy suddenly appears. Light floods in to fuel growth. Eventually, one of the seedlings will take the place of the dead tree in the canopy. But while they're waiting, the seedlings might be dependent on others.

PROFESSOR IAN ALEXANDER

Or when they germinate, they're going to encounter a web, a woodwide web if you like, of mycorrhizal fungi.

NARRATOR

As the seedlings root grows down into the soil, it releases a cocktail of chemicals. Sugars, amino acids and nutrients, an attractive meal for soil bacteria and fungi. The mycorrhizal fungus is just one of a crowd. But the plant root also releases chemicals called flavonoids. They act as a signal, and the mycorrhizal fungus is more sensitive to its message than other organisms. Under the influence of flavonoids, the fungus grows towards the root. A subtle, molecular conversation starts to take place. Closer in, a new chemical vocabulary comes into play. Cytokinins tell the fungus to branch. Now it's the turn of the fungus to release a chemical. It in turn communicates with the plant. In response, the plant switches off its natural defences and the root hairs, now redundant, disappear. The fungus has now been recognised. The stage is set for the formation of a fully fledged mycorrhizal partnership. In microcosm, this is what happens in a forest. A baby plant joins a larger one, which already has a mycorrhizal network.

JONATHAN

This mycorrhizal network is being supported by the larger and established plant, and the seedling growing here will become part of that network when it becomes infected and will gain the benefits of being part of that network, in terms of the uptake of nutrients.

PROFESSOR IAN ALEXANDER

That means that they have a ready-made system for capturing water and nutrients. And they may be getting that cheap because their parents upstairs in the sunlight up there, are producing the carbon to support this fungal web. And all these guys have to do is tap into it.

NARRATOR

It could be a case of parental care for a nursery full of seedlings.

JONATHAN

I think the big question outstanding is to what extent does this actually mean that plants no longer compete with each other in the conventional sense? To what extent are individual plants supporting perhaps individuals of other species, or even juveniles or seedlings of their own type through this network.

NARRATOR

If this idea is true, the seedlings are subsidised by the mycorrhizal network. The network is supported by the canopy trees. Even more controversial, something may pass from parents to offspring via the fungal web.

PROFESSOR IAN ALEXANDER

It's possible that some of the carbon, which comes from the over story trees and out into the web of mycorrhizal fungi, some of that may in fact find its way into these seedlings. And that means that that gives them an extra chance of surviving down here in these low light conditions until such time as a gap in the canopy opens and then, off they go.

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Now that you have viewed the video

• Think about the implications of the fungal web for competition.
• How does the shading effect of the canopy influence seedling development?

4 Life in trees

The previous section highlighted the complex links between trees and fungi. In this section you will turn your attention to animals that live in trees and the adaptations to an arboreal life that they exhibit. When you were exploring food chains in an oak wood you encountered animals adapted to a life in trees. Now you will look specifically at mammals.

Figure 5 A red squirrel

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A red squirrel sitting on the branch of a tree.

Figure 5 A red squirrel

Woods and forests present a number of problems for mammals that inhabit them. The habitat stretches vertically for a substantial distance yet for tree-dwellers to travel any horizontal distance they must either go down to the ground each time or jump from sometimes flimsy branches over large gaps. Sir David Attenborough describes how squirrels have overcome the problem.

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Life in Trees: squirrels

SIR DAVID ATTENBOROUGH

Squirrels deal with the problem with dazzling ease. They're such lightweights that they can race along the thin twigs at the very far end of the branches, and they're spectacular jumpers. Their powerful hind legs provide the thrust. Their long tail acts as a rudder. And their shorter front legs serve as shock absorbers to cushion the landing. Superb sight enables them to judge distance with great accuracy, an essential ability when racing along this three dimensional highway. They're at their most acrobatic during the mating season, when males start to pursue the females. One male may begin the chase, but others quickly join in. Eventually, one wins, but as soon as he's claimed his prize, the chase will start all over again, and the female may mate with up to eight different males in a single day. But a gap this size is just too big, so a grey squirrel, like a tamandua, often has to come to the ground if it's to visit all the trees in its range. A grey squirrel can leap eight feet, but there's another tree dweller that can leap much farther than that. Although it's no bigger than my hand, it can jump from this tree to that tree over there, more than 50 feet away, an astonishing distance. But to see how it does it, we'll have to come back at night. Since they have an acute sense of smell and love seeds and nuts, maybe these will tempt one down from the treetops. They are flying squirrels. How do they fly? Just watch. Maybe gliding squirrel would be a more accurate name. They're nonetheless astonishing. That furry membrane stretching between wrist and ankle makes a most efficient aerofoil. Flying squirrels are not territorial, and as many as half a dozen can be foraging in the same area of woodland. Although this little squirrel may have travelled a very long distance in order to get this valuable source of food, it's such an expert glider, it's done so with a minimum of effort. And in forests like this one, where food sources are often very widely dispersed, the ability to travel fast and far, but with very little effort, is a very valuable ability indeed. There are few gaps in these forests that defeat them, but to cross really long distances, they do need height. They steer partly with their tail and partly by moving their outstretched legs so that they vary the tension of their gliding membrane. And you can see that they can steer when one squirrel uses the same takeoff point, but glides away to land on different trees. Even so, they're not agile enough in the air to escape birds of prey, so during the day, they sleep in holes and only emerge when it's dark.

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4.1 The colugo

Figure 6 An adult colugo and its offspring

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An adult colugo with a young one. The adult is clinging to the vertical tree trunk and has spread the flaps of skin on either side of its body. The flaps enable the colugo to glide, as described in the text that follows.

Figure 6 An adult colugo and its offspring

An animal that is not closely related to the flying squirrel but shares common features is the colugo. The colugo is a bit of a mystery and the historical confusion is evident from its common name – the flying lemur. It neither flies (in that it doesn’t flap its limbs) nor is it a lemur.

The colugo is not a monkey either, despite the fact that its main predator is the monkey-eating eagle. Having once been placed with insectivores and then with bats, it’s now in a mammalian order of its own (the Dermoptera, i.e. ‘skinwings’), recognising its ancient and distinct evolutionary beginnings. This ancient origin is why it is such an interesting animal as it early on became adapted to a tree-dwelling life. As you read about the colugo, think about adaptations that have hidden ‘costs’ to the animal.

One particular evolutionary development associated with tree dwelling is taking to the air. The gliding habit evolved independently in different mammalian and reptilian lineages and yet the anatomical modifications that allow it are similar in, for example, flying squirrels and the colugo. In particular, the ‘sail of skin’, technically termed a patagium, stretches between the limbs – and a good deal further in the colugo, acting as an effective (and to some degree manoeuvrable) gliding membrane.

Colugos are sizeable mammals (about the size of a domestic cat) and entirely arboreal. Their record-breaking glides (in excess of 70 m) are achieved without great loss of height. But in trees, they move about rather awkwardly. The patagium is then an encumbrance and there’s a limited ability to grasp effectively – the colugo lacks the opposable thumb of primates. So the benefits of a gliding lifestyle are achieved at a ‘cost’. The resulting vulnerability – especially to the Philippine monkey-eating eagle (a species under threat, as are colugos) – may help explain why the colugo is nocturnal.

4.2 Flying squirrels

Flying squirrels are not closely related to the colugos but they have features in common. You have seen squirrels and read about the colugo. As you watch the video, think about how flying squirrels steer during their glides. Note the advantages of the gliding habit.

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The similarities between colugos and flying squirrels

CHRIS PACKHAM: The best time to see them is in the first couple of hours after dark. What I'm hoping is that if I stand here and stay really quiet, I'll be in for a real treat. It's a creature I've waited all my life to see, but they move so fast.

[MUSIC PLAYING]

Oh, did you see that? That was amazing. Went right past my face. Flying squirrel.

[MUSIC PLAYING]

They really are expert gliders - they can glide for up to 200 metres.

[MUSIC PLAYING]

When I was a kid, I was obsessed with things that were not meant to fly - flying fish, flying frogs, flying lizards, flying squirrels - and this is the first time I've ever seen them. It was worth a 45-year wait, honestly.

[MUSIC PLAYING]

Ah, did you? Ah, did you see that? I felt it, it went right through my hair, seriously, centre parting. It was like having a sheet of A4 coming right at my face. And as soon as they hit the tree, they are running - and up they go.

[MUSIC PLAYING]

They're just crisscrossing all the trees. And they immediately scamper up to the top and then take off and glide again. And sometimes, I've noticed, they can even change direction during flight.

[MUSIC PLAYING] Ah, hit me in the chest. It doesn't come better than that, does it?

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Figure 7 Flying squirrel

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A drawing of a flying squirrel gliding. The flaps of skin on either side of the body are extended and the tail is raised.

Figure 7 Flying squirrel

4.3 Flying foxes

Many species of flying fox (fruit bat) have important roles in ecosystems, dispersing seeds, pollinating flowers or providing food for predators. As they have evolved not only have they acquired adaptations that enable them to exploit aerial and forest habitats, but they have also evolved alongside plants in a process called co-evolution.

What are the likely advantages to flying foxes of their particular form of roosting, taking into account vulnerability to predators, the location of food and temperature regulation?

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Watching flying foxes

SIR DAVID ATTENBOROUGH

Gliding from branch to branch was a comparatively small step for tree living mammals, but there was one group of them that made a truly gigantic leap. Their arms changed into wings. The shoulders, the elbow, the wrist remain much the same, but the hand and the fingers changed dramatically. Flying foxes, fruit bats in Australia, they and their insect eating cousins are the only mammals that have developed true powered flight. They're so big that they can't roost in holes. Instead, they sleep out in the open, in colonies that may be hundreds of thousands strong. The thumb on each hand is free of the wing and has a hooked claw. Using that and the claws on the toes, fruit bats are surprisingly nimble, clambering about in the branches. Wings may have solved the problem of getting from one tree to another, but landing is still a challenge. As a fruit bat approaches its chosen perch, it goes into a glide. Then it lowers its toes and hooks them onto a branch. This is a textbook example of how it's supposed to be done. But some perches are more difficult to reach than others. Wings need regular grooming. They're also very delicate. But small tears quickly heal. The wing membrane is among the fastest growing of all mammalian tissues. They also fan their wings to keep themselves cool. It can be very hot hanging unprotected in the baking sun. Takeoff, too, requires a special technique. Two or three wing beats lift the body to the horizontal, and only then should the feet be unlatched. That way, you don't lose too much height. It's hard work, particularly if you're carrying a baby which is a third of your own weight. Once in the air, however, fruit bats are extremely strong flyers. They can travel great distances, as much as 30 miles, 50 kilometres, in a single night, if that's necessary to find food. They may have lost a lot of moisture hanging around in the midday sun, so their first call is often to a nearby lake, to get a drink. They do this in a rather unusual way. First, they dip their chests in the water. Then, they return to their roost and lick the moisture from their fur. But there are hazards - crocodiles. The bats only touch the water for less than a second, and usually, the crocodiles are just not quick enough to catch them. But if one miscalculates and comes down on the water, it's a different matter. They're surprisingly good swimmers. The worst danger comes when they get to land. Without being able to drop into space as they can from a perch, they find it very difficult to get airborne. Now, the crocodiles have the advantage. But a few individuals lost to crocodiles makes little impact on the bat colony. This roost alone contains a staggering five million. Living together in these vast numbers brings several important advantages. Flying foxes collect fruit and nectar of many different kinds. But knowing which species of fruit tree is in season at any particular time is not easy, and some are very unpredictable. If a few individual bats return smelling of a particular fruit, the news that this food has just come on the market spreads quickly through the whole colony. Each bat knows where trees of the various species can be found. So the next night, it'll go to its own favourite patch to collect the new fruit. That is why the whole five million don't follow one another to the same tree. Huge wings may be good for long distance flying, but they don't give great manoeuvrability in the air. And when the bats return in the dawn, hunters are awaiting them. Eagles know exactly where the bats' blind spots are and attack from below. Powerful though eagles are, fruit bats are big animals, and a hit isn't necessarily a kill. Raids like these are another reason why an individual bat finds it an advantage to roost in a colony. Since it's surrounded by tens of thousands of others, there's a good chance that an eagle will pounce on someone else. Most colonies have a resident pair of eagles that nest nearby. A breeding pair will take half a dozen or so bats a day, but that still makes little impact on bat numbers. Skilled though the eagles are in taking bats on the wing, their most successful strategy is to snatch them as they hang on the branches.

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Colonies of flying foxes may comprise as many as a few million individuals (five million is David Attenborough’s estimate), each with a wingspan of about 1.4 m, with the entire ‘camp’ perched on often denuded trees and engaged in intense social activity.

It’s little wonder that witnessing such a site has been described as a ‘memorable auditory and olfactory experience’. Such concentrations of flying foxes are ‘visible, audible and smellable for miles’ and therefore inevitably attract predators. But congregations of this type may decrease the likelihood of any one individual falling prey to predators, such as eagles. Communication between members of the camp may also increase the efficiency of locating suitable food. But the fact that food sources are depleted so comprehensively when visited en masse raises questions as to the degree of benefit of group living.

Another possible benefit of roosting is that foliage might be protective, shading these mammals from wind, rain and sun, though trees that become camps lose many of their leaves. Fruit bats, for instance, regulate their body temperature, partly by behavioural means. Huddling together in groups should in theory reduce the rate of heat loss in cooler conditions, and decrease the rate of warming when it’s very hot. In both circumstances, the surface area that each individual exposes is lessened by contact.

As you saw in the video, eagles (and owls) take a toll of flying foxes in transit, and the largely nocturnal habit of these species once again probably reflects selection pressure of this type. Flying foxes living on islands (more than 60 per cent of species do so) tend to venture forth in the daylight and in such environments predators are often less evident. Flying foxes can devastate crops, but they can also maintain ‘the fertility of the rain forest’. Flying foxes can certainly help disperse trees by transporting their seeds to new locations, either through their messy eating of fruits or by seeds passing intact through the gut. The seeds of the commercially important West African iroko tree depend on the straw-coloured flying fox for their dispersal. Flying foxes also help in the recolonization of deforested areas and in the establishment of plants on land newly formed or recently devastated by volcanic eruption.

Flying foxes are also important pollinators; many island species occupy the ecological niches taken over elsewhere by insects or hummingbirds, for example. The transfer of pollen from one flower to another on a different tree (i.e. cross-pollination) can confer a significant advantage to the species because it promotes genetic diversity of the next generation. So, the development of mechanisms that promote cross-pollination are very advantageous to trees. In Australia, pollination of some eucalyptus species depends almost entirely on visits from flying foxes. The flowering process of the kampong tree from Malaysia is intimately geared to the feeding habits of the dawn bat. Its flowers open just two hours or so after dusk and drop before dawn, coincident with the bat’s feeding time. The size and shape of the flower opening ensure that only the dawn bat can enter; as its long tongue reaches down to access the nectar, the position of the pollen-producing parts of the flower (the stamens) is such that pollen is deposited on the animal’s fur.

This is a further demonstration of the way in which the evolution of one species can increase its dependence on another, often reflecting some form of mutual advantage. This phenomenon is known as coevolution.

Figure 8 Flying fox

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Photograph of a flying fox hanging upside down in a tree.

Figure 8 Flying fox

5 Week 2 quiz

This test is about energy sources, the flow of energy through a terrestrial ecosystem and the relationships between organisms within that ecosystem.

6 Summary of Week 2

In this look back at the week, Dr David Robinson from The Open University discusses what you have learned so far in this course. The next week focuses on the adaptations of animals to the challenges posed by different types of ecosystems.

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Summary of week 2

NARRATOR

Ecosystems comprise more than habitat, inhabitants, and relationships between organisms, and learning about ecosystems in oak woodland demonstrates how complex ecosystems can be.

DR. DAVID ROBINSON

In woodlands like this, we can see parts of the ecosystem, but there are intricate and complex relationships between the organisms here - far more than we initially see when we walk into the woods. There's layer upon layer of interrelationships between the organisms. For example, like the woodwide web that links the trees here with the fungi under the ground. Understanding ecosystems transforms our view of the natural world, and it makes our own relationship with the natural world much more meaningful. In the next part of our learning journey, we'll be looking at ecosystems in different parts of the world, and in particular, how some organisms survive in extreme conditions through physiological adaptations. Understanding physiological adaptations is part of the process of making sense of ecosystems.

NARRATOR

The learning material in this section explores physiological adaptations like evaporators in the desert and adaptations to fluctuating food supply in the Arctic. In this video, Professor Paul Tett investigates how phytoplankton travel and survive in the sea.

PAUL TETT

The problem for phytoplankton is that they can very rarely get light and nutrients at the same time, because light is at the surface of the sea, and the nutrients are found deep down where organic matter decays in this cold quarter at the bottom of the sea.

NARRATOR

Here, Professor Mimi Koehl demonstrates how some suspension feeders eat.

MIMI KOEHL

3/4 of the Earth's surface is covered with water, and that water is full of particles, and a vast array of different kinds of creatures make their living by filtering those particles out of the water. Some of them, like anchovies and whales, swim around and let the water move through their filters as they go. Other animals, like sea fans and feather duster worms and feather stars sit on the bottom and put their filters up in the current.

NARRATOR

In this audio, Professor Aaron Bernstein describes some of the wonders of the microbial world and how it redefines our understanding of life.

AARON BERNSTEIN

The diversity of genes in the microbial world - we know that that is far greater than the diversity of genes in the rest of the living world. But really, we're utterly ignorant about the microbial world. In fact, it is the last great unexplored frontier.

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Woodlands are a good example of an intricate and complex ecosystem and you have now seen that there is a fascinating web of relationships beneath the ground that forms the wood-wide web. Above the ground there are animals that are well adapted to glide through the trees and you explored the comparisons between the adaptations of squirrels, colugos and bats.

Next week we’ll be looking at ecosystems in different parts of the world, and in particular, how some organisms survive in extreme conditions through physiological adaptations. Understanding physiological adaptations is part of the process of making sense of ecosystems. Examples will be taken from extreme habitats – deserts and the polar regions.

If you would like a short break, or to find out more about studying with The Open University, take alook at our online prospectus.

You can now go to Week 3.

Acknowledgements

Except for third party materials and otherwise stated in the acknowledgements section, this content is made available under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 Licence.

The material acknowledged below is Proprietary and used under licence (not subject to Creative Commons Licence). Grateful acknowledgement is made to the following sources for permission to reproduce material in this course:

Course image: courtesy of Dr. David J. Robinson, The Open University

Figure 1: fotoVoyager; iStockphoto.com

Figure 2: sborisov/iStockphoto.com

Figure 3: keiichihiki; iStockphoto.com

Figure 4: courtesy of Dr. David J. Robinson, The Open University

Figure 5: sakakawea7/iStockphoto.com

Figure 6: Valmol48; iStockphoto.com

Figure 7: Dorling Kindersley; iStockphoto.com

Figure 8: Smithore/iStockphoto.com

Audio/Video

Photosynthesis © The Open University, contains BBC clips © BBC

Following a food chain: © The Open University, contains BBC clips © BBC

How plants make food © The Open University, contains BBC clips © BBC

Life in trees: Squirrels © BBC

Watching Flying Foxes: © BBC

Summary video: © The Open University, contains BBC clips © BBC

Every effort has been made to contact copyright owners. If any have been inadvertently overlooked, the publishers will be pleased to make the necessary arrangements at the first opportunity.

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