Week 1: What is an ecosystem?

Introduction

Throughout this course you will be considering these overarching questions:

• What is the importance of understanding ecosystems?
• How do they work?
• How crucial is their conservation?

But first, what constitutes an ecosystem and how is an ecosystem defined? Consider the habitats shown in Figure 1. Are they individual ecosystems or groups of ecosystems? Could there be different ways of defining ecosystems?

Figure 1 A montage of ecosystems

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There are four photos of possible ecosystems or groups of ecosystems to illustrate the problenmss of defining an ecosystem. The images are: the Jurassic coast in the UK, a river system in South Korea, plants floating on a pond and a butterfly feeding from a single flower.

Figure 1 A montage of ecosystems

By the end of this first week you will be able to explain how an ecosystem is defined, in terms of energy flow, and be able to define and use terms which are introduced in the videos and text and apply them to new situations and examples where appropriate.

There is more than one way in which an ecosystem can be defined. In the following video Dr Mike Gillman highlights the difference between two schools of thought. One definition is that an ecosystem is an area where groups of organisms experience similar conditions.

Alternatively, an ecosystem is a living system of energy transfer, a whole complex of organisms living together, linked by energy transfer. The key difference is that ‘area’ defines one, whereas ‘energy relationships’ define the other.

Before we can begin to tackle the larger issues of ecosystems and how they have been compromised by human intervention, we must understand what is meant by ‘ecosystem’. As you watch the video ‘What is an ecosystem?’, consider the following questions, which we will discuss in the next section:

• What is the key difference between the definitions?
• What is the working definition of ‘ecosystem’ that we are going to use in this course?

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What is an ecosystem?

DR MICHAEL GILLMAN

Ecosystem-- what is it? What does it mean? How can we define what an ecosystem actually is? Well, there are two ways of doing it-- two definitions, two schools of thought. In the red corner, supporters of a geographical definition. They would say that an ecosystem is an area where groups of organisms experience similar conditions. These guys talk about the rainforest ecosystem, the Arctic ecosystem, that sort of thing. Defined like this, ecosystems tend to be, well, big. It's a widely used definition. You'll have heard politicians talk about ecosystems like that. And conservation groups tend to do the same thing. On the other hand, over here in the blue corner are people who prefer a different definition. These people say, it's not just an area, it's a living system of energy transfer, of nutrients being passed up and passed on. It's all about the system itself, not the box it comes in. This is the approach that would have been favoured by one of the founding fathers of the science of ecology, Sir Arthur Tansley. His view of an ecosystem revolved around his concept of the biome, which we would today call a community. He saw a community as being—

NARRATOR

"the whole complex of organisms naturally living together, whose life must be considered and studied as a whole."

DR MICHAEL GILLMAN:

Tansley's main interest was in plants, so his work and ideas tend to focus on them. He defined plant communities as--

NARRATOR

"--any collection of plants growing together which has a whole certain individuality."

DR MICHAEL GILLMAN

Tansley's ideas were controversial and much debated. But they found favour in North America and Europe and, today, form the basis of the British National Vegetation Classification. The key thing is Tansley's ideas of communities were not just about geographical location. They were about how species interact together. And so was born Tansley's definition of an ecosystem.

NARRATOR

"A wider conception still is to include, with the biome, all the physical and chemical factors of the biome's environment, or habitat, as parts of one physical system which we may call an ecosystem, because it's based on the oikos, or home, of a particular biome. All the parts of such an ecosystem, organic and inorganic, biome and habitat, may be regarded as interacting factors."

DR MICHAEL GILLMAN

So there it is. That's what we mean by an ecosystem. And for the purposes of this course, we'll define ecosystem as--

NARRATOR

"--a set of organisms and abiotic components, linked by processes of energy transfer and cycling of materials."

DR MICHAEL GILLMAN

And that, I reckon, is a win for the blue corner.

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1 Define an ecosystem

Figure 2

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Close up image of a variety of plants in the same habitat.

Figure 2

You should now be able to provide definitions of the key terms you have encountered in the previous video. Begin to construct your personal glossary of them, which you can keep by you as you study.

Building your own glossary is an effective way of consolidating your learning. We have provided a Glossary template  which you can use to build your own if you choose. There are already a few entries in the Glossary, to give you a feel for the sort of definitions that you should be including. But whatever method you choose for collecting together terms and definitions, by the end of your study of ecosystems you should have a working document that you can add to if you study other science subjects.

To formulate a working definition of an ecosystem you may have to use some of the terms you have added to your glossary. Is the following definition a suitable one to work with?

2 Investigating ecosystems

Let’s examine the concept of an ecosystem in more detail using an example that is familiar to many people: a pool on a rocky beach, in Britain (Figure 3). As you work through, consider what defines the limits of a particular ecosystem.

• How does the tide change the nature of a rock pool ecosystem?
• What role does the Sun take in sustaining the ecosystem in the rock pool?

Also think about how can this be extrapolated and applied to other ecosystems with variable physical environments.

Maximise

Figure 3 A rock pool with examples of the organisms that you might find. The ‘shanny’ is also known as ‘the common blenny’.

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Diagram of a sea shore rock pool showing some of the organisms that you might find in one. The examples are bacteria, plankton, sea anemones, whelks, shrimps and fish.

Figure 3 A rock pool with examples of the organisms that you might find. The ‘shanny’ is also known as ‘the common blenny’....

2.1 The rockpool

Figure 4

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A photo of a typical rock pool on the seashore with a large starfish in the foreground.

Figure 4

Like all ecosystems, a rock pool is linked to the wider world and to other ecosystems. This link is most apparent in the shape of the tides every day, which change the sea water in the pool and bring in new organisms from the open ocean (as well as allowing others to escape back into the sea). The tides also change the physical characteristics of the pool and its surroundings. When the tide is out, the rock pool is a collection of organisms living together in a fairly clearly defined place. When the tide is in, the pool may become no more than a small depression on the rocky sea bed.

Now let’s look at the components of our rock pool ecosystem. Remember, any ecosystem contains living things, a physical environment and a source of energy.

The most obvious living things will tend to be the largest ones: seaweeds, sea anemones, whelks, shrimps, fish and so on. But this shouldn’t blind us to the importance of the organisms we can’t see. For example, the water itself is full of tiny plants and animals – called ‘plankton’ – that are food for many of the larger creatures. And the water and rocks contain huge numbers of the simple single-celled organisms called bacteria, and other microscopic forms of life, that play an important part in the working of the ecosystem.

The most obvious components of the physical environment of the pool are the rock that surrounds it, the sea water in it, and the air above it (when the tide is out). But these physical factors are far from fixed. The tides, and the effect of sunlight on the exposed pool, mean that the organisms that live in it must be able to withstand changes and extremes of, for example, temperature or salt content. Some small pools dry up altogether in the summer or ice over in winter when the tide is out.

Almost all of the energy that supports the life in the pool arrives in the form of light from the sun. Some of this energy is captured by seaweeds attached to the rocks and microscopic plants (types of plankton). These plants are eaten by animals, which are eaten by other animals, and so on. But the sun is not the only source of energy involved in the workings of this particular ecosystem: the tides that sweep across the pool are driven up and down the beach by the gravitational pull of the moon.

Remember that it is the observer – you or I – who defines an ecosystem, and selects which organisms or aspects of the environment to study. In much the same way, you have to decide what is important when you make notes.

However big or small our ecosystem, and whichever aspect of the system we choose to study, it is ecology that provides the framework that allows us to investigate how it works – and to wonder at the beauty and strangeness of it all.

3 Study a habitat

It is possible to observe ecosystems physically, mapping out observations to determine the network that exists in a given habitat. These observations provide a valuable baseline for understanding a given ecosystem, making it easier to determine the possible negative factors that might be influencing it.

In this video, we join a group of students who are learning how to classify habitats, in the field. After you watch the video, add a definition of an ‘indicator species’ to your glossary.

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Study a habitat

SPEAKER 1

What the students are doing at the moment is learning how to do a phase one habitat survey. They're trying to summarise their observations on a very coarse scale using a map.

SPEAKER 2

There's a complete description in here of what you're supposed to do, how you're supposed to do it, how you're supposed to undertake your fieldwork, how you're supposed to prepare your final maps.

SPEAKER 1

They have to look at the different types of vegetation and classify them into broad groups like grassland, woodland, scrub, fen, bog, mire, fairly broad categories. And then, by looking at the plants, it enables them to decide which one of those categories the vegetation falls into. And we have a colour coding scheme that they have to learn from a reference book with very clearly defined nationally agreed colour codes. And then they can further map and shade it in, depending on which habitat they're looking at.

SPEAKER 2

So you have these coloured maps produced. And the idea is those colours are uniform, if you like. So it uses the same colours for the same type of vegetation anywhere in the country. And generally, it's sort of green for woodlands, orange is for grasslands, purple is for mires.And if you identify a piece of land as being, for instance,-- what should we choose? Marsh or marshy grassland? We've got to decide what type of grasslands. And it could be an acid grassland, a neutral grassland, a calcareous grassland. It could be improved grassland. There's a range of possibilities. As an example, see the fields which are a much sort of richer green colour? Those are fields which have been improved, and you can improve fields by adding lots of fertiliser to it. Or you can sometimes sort of partially improve land by having a very high stocking rate. This stuff we're on here, would you classify this as being improved or semi-improved? Do you think there's any chemical fertilisers put onto it? Any sort of huge stocking rate, lots of sheep dung and stuff? OK. So we've got a piece of grass which is probably unimproved. And what we're looking for are indicator species which sort of help us make that decision. You've already seen the mat grass. Remember the map grass with the tillers? So lots of tillers there. So there's masses of mat-grass here. So we've got quite a few plants here which are all indicating acid conditions. So we can classify this area then as being an acid grassland.

SPEAKER 1

It's enabled us to take a big picture. Mapping exercises like this have been going on for the last 15 years or so, and we've been trying to cover the whole country.

SPEAKER 2

Don't worry now about everybody trying to mark down everything. We'll run through it partially together when we get back in. So have a look at the map and maybe make a few notes on it.Almost all of the area between where we are now and the eastern boundary-- and the eastern boundary is that long sloping line which follows the track coming up from the road that eventually heads to the centre-- all of that is going to be mapped as being unimproved acid grassland.

SPEAKER 1

It helps us to identify areas where you are likely to find certain rare species. So we can decide on areas that need protecting. Where you have areas of development it helps you to decide on whether a developer should go ahead, what kind of development should go ahead, and any mitigation factors you might have to bring in, any habitat creation you could do while you're doing it.

SPEAKER 3

It all came together when I climbed to the top of that hill because you're sort of looking at areas on the ground and start looking at what vegetation there is and trying to identify the underlying geology by what size or what plants there are. And when you actually go up there and have a look, it actually makes it quite apparent the different patches, brighter green areas of the grasslands and the fen areas are obviously a different colour. So it sort of brings it all together.

[MUSIC PLAYING]

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4 The wetlands of Wicken Fen

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The wetlands of Wicken Fen

[MUSIC PLAYING]

NARRATOR

The wetlands of Wicken Fen-- Britain's oldest nature reserve. Some 7,000 different species of mammals, birds, plants, insects, and other invertebrates have been recorded here, making this one of the most biologically diverse places in the UK. Why is Wicken Fen so rich in species? What is the relationship between habitat diversity and species diversity? And what role does conservation play in promoting biodiversity at this very special reserve? Wicken Fen, this is one of the last surviving fens in Western Europe. For much of the year, this low-lying land is waterlogged, creating the unique fen environment of waterways, marshes, meadows, and woodland.

[MUSIC PLAYING]

A remnant of the great fen that once covered six and a half thousand square kilometres, Wicken Fen is situated in Eastern England, just below The Wash in the county of Cambridgeshire. Today, 99.9% of what was the great fen has been replaced by farmland, and Woodwalton, Holme, Chippenham, and Wicken are the only remaining areas of fen. Wicken Fen, though it covers less than 4 squarer kilometres, is now the largest fen in the region. A nature reserve for just over 100 years, the fen is home to a tremendous diversity of species. Near the woods, a herd of deer, one of the fen's 29 mammal species, graze the scrub. Among the grasses and wildflowers of the fen meadows, Ringlet butterflies flutter, one of a thousand species of butterflies and moth recorded here. There are 200 species of bird on the fen, including reed warbler. Counting all of the different species of Wicken Fen, including animals, plants, and fungi, is one way of determining its level of biodiversity.

JOANNA FREELAND

Biodiversity is a contraction of the terms biological and diversity, and it basically just refers to the number of species and the number of individuals within each species, living within a particular place and a particular time. The easiest way to measure biodiversity is just to go out and count, first of all, how many species there are and second, how many individuals there are within each species.

NARRATOR

But biodiversity is more complex than just a headcount. It's influenced by a number of factors. One of these is the range of habitats within an area. A habitat is an environment in which a species, be it plant or animal, can live.

JOANNA FREELAND

Within the UK, Wicken Fen would definitely be considered to be a biodiversity hotspot. There's an estimated 7,000 species of insects, plants, mammals, all other different types of taxa at Wicken Fen. And of course, going along with that, there's a huge range of different types of habitats.

NARRATOR

The open water is just one of many habitats at Wicken, each with its own characteristic species. Because Wicken is a fen, its multiplicity of habitats depends in part on moisture levels. The wettest areas are the waterways, ditches, and ponds. These contain aquatic plant species like water lilies and pond weeds.Adjacent to these areas are shallow waters which are suitable for reeds. In areas where these waters dry in summer, fields of sedge dominate. Dryer again are the fen meadows, which are home to wildflowers and grasses. Dryer areas may also be covered by woodland and bushes, like birch and gelder rose. In terms of biodiversity, habitat and species are closely linked to each other.

JOANNA FREELAND

The more habitats that you have within an area, the more species, as a rule, you will find within that area. That is because, for the first part, habitats are defined, to a fairly large extent, by the species living within them.

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Wicken Fen is a conservation area in the east of England, near Cambridge. It is one of Britain’s oldest nature reserves and home to around 7,000 species of mammals, birds, plants, insects and other invertebrates.

Wicken Fen has been described as ‘biodiversity hotspot’, and offers the opportunity to observe multiple habitats, each with its own ecosystem, interacting with each other and forming larger ecosystems. The wide range of habitats, the diversity of species, and the complex food webs present in Wicken Fen are all factors in determining biodiversity.

Take this opportunity to explore the area around Wicken Fen.

4.1 Following the food web in Wicken Fen

Figure 5 Aerial view of Wicken Fen

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An aerial view of Wicken Fen in Cambridgeshire, UK showing the fen and wetlands with farms and cultivated land in the distance abutting the fen.

Figure 5 Aerial view of Wicken Fen

The definition of an ecosystem can be framed in terms of energy flow. You need to know what organisms are living in it – the biodiversity of the system. Next, you can work out how the organisms are linked.

The sum of the nutritional links between organisms in an ecosystem is known as the food web.

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Following the Food Web in Wicken Fen

NARRATOR

The sedge-field habitat is made up of many different species. The saw-sedge predominates but other plants also thrive. These include marsh bedstraw and bindweed. Invertebrate species found here include snails and beetles.

ADRIAN CALSTON

Wicken is so rich in biodiversity today because it has such a range of habitats from the scrub through to the sedge fields, the reeds, and all these ditches that run across. So there's a lot of different habitats and they can all support different species.

NARRATOR

This can be clearly seen by comparing the species of two neighbouring fen habitats-- a fen meadow and woodland. The meadow is filled with grass and wildflower species. There's the marsh pea, for instance and thistles. These plants are home to a wide variety of invertebrate species.

ADRIAN CALSTON

Got a variety of very, very small little flies in here, several different varieties of spider. That little tiny one running across is a money spider. There's one generically known as a garden spider there. And there's a caterpillar. Two different species there. And one or two hoverflies and a lacewing.

NARRATOR

The woodland habitat beside the meadow includes tree and shrub species like hawthorn. This habitat is suitable for a different group of invertebrates.

ADRIAN CALSTON

We're now in the area of scrub about 20 yards from the meadow we've just come from. And in here there are different species. As you can see, they're very, very small. There's a different species of spider there, another one there, another one there, another one there, five or six different species.

What we can see here is that we've got quite a few different species from the ones that we found in the meadow. And it shows that with a lot of species, particularly insects, they're very dependent on the specific type of habitat. So we've got two different habitats really quite close together containing different species. And as a result of that, we can see that a site which has got a variety of different habitats is going to have a much higher biodiversity than a site with just a single habitat.

NARRATOR

Biodiversity doesn't simply depend on the range of habitats. It's also influenced by the way species interact with one another. This is the Lode, the main water channel running through Wicken Fen. One way its plants and animals interact is through the food web. Each species is food for other species. The more species there are to eat, the more there will be eating them. The food web underpins biodiversity because it means many different organisms can coexist.At the base of a food web are plants. The slow-flowing clear waters of the Lode offer the perfect growing conditions for submerged aquatic plants. Plants use energy from the sun to create new tissue through photosynthesis.

JOANNA FREELAND

When there is plant tissue available, then there is food for herbivores, which are organisms that eat plants. When plants die, they provide food for detritivores, which are small organisms, often microorganisms, that eat dead and decaying material. And of course, when the herbivores die, they in turn provide more food for detritivores. And herbivores also provide food for carnivores, which are animals and occasionally plants that feed on animal tissue.

NARRATOR

In the Lode waters, herbivores like water snails nibble the aquatic vegetation. Small carnivorous fish, including roach and minnows, are prey for larger, freshwater carnivores like pike. While some of these species spend their entire lives under water, others may move between habitats at different stages of their development and so become part of different food webs. Dragonflies and damselflies, for instance, spend much of their life under water as larvae and emerge from the water when they mature. In the process, they're part of two food webs-- the aquatic and the terrestrial. Rory McKenzie-Dodds is from the Dragonfly Project and regularly visits the Fen to observe the 20 species of dragonfly and damselfly found here.

RORY MCKENZIE-DODDS

Can we go nearer this bank on port side? Dragonflies are particularly interesting from the point of view of food webs. Underwater dragonflies will be eating midge larvae and mosquito larvae. At this larval stage they tend to get eaten by insectivorous fish.

NARRATOR

Underwater the larvae are carnivorous. This one is pursuing a water snail. But at the same time they are food for other carnivores. In this case a carnivorous insect, the water boatman, is feeding on a damselfly larva. Within the aquatic food web, the larvae are both predator and prey.

RORY MCKENZIE-DODDS

Probably the most dangerous time is the moment when they actually climb out of the water and then they start to pump their wings out. And really they can't move at that stage and they're ideal meals for, particularly, birds. Blackbirds make a great habit of munching them, so do wagtails. In the adult stage they continue to be carnivorous. They eat midges and gnats and mosquitoes, butterflies, bees, very often each other. That's what they eat. And they're eaten in turn by birds. Hobbies, for example, make a particular speciality of catching dragonflies.

NARRATOR

Now part of the terrestrial food web, adult damselflies and dragonflies are also prey for a diversity of carnivores including spiders, wasps, frogs, and small mammals. This example demonstrates how complex food webs support many species within the habitat. So the range of habitats, the diversity of species, and the complicated way in which parts of the food web interact, are all factors in determining biodiversity.

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4.2 Managing habitats in Wicken Fen

Figure 6 Cutting the sedge on the fen, part of the management programme.

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A tractor cutting the sedge on Wicken Fen. This is part of the National Trust management programme for the fen.

Figure 6 Cutting the sedge on the fen, part of the management programme.

The diversity of life in Wicken Fen is influenced by the way in which humans use the habitats that make up the ecosystem, as Professor Gowing explains. It is possible to determine the best management programme, by setting up an experiment.

Whilst watching the video, consider the following:

• Why does Wicken Fen have to be managed?
• What were the key features of the experiment that was set up at Wicken Fen in the 1920s?

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Managing Habitats in Wicken Fen

NARRATOR

Key to the maintenance of this diversity is the management of habitats, a human intervention that has been happening for centuries. At the edge of the fen, the Cottage Museum depicts how life may have been for workers on the fen before it became a nature reserve. The vegetation was cut for thatching local houses, and as bedding and feed for domestic animals. Tools such as this were used to cut peat for fuel. The result is a landscape stamped with centuries of rural culture-- physically, in the peat diggings, paths, ditches, and dykes, and ecologically, in the plant and animal communities that have developed over time.

JOANNA FREELAND

The reason why Wicken Fen has to be managed is because humans have been cutting it back, and tending the ditches, and so on, and creating, as a result of this, certain habitats. And within these habitats, different sets of species co-exist with each other.

NARRATOR

Wicken Fen was given to the National Trust in 1900, since when it's been run as a nature reserve. Tasks, such as hay cutting, used be part of people's livelihood. Today, they are continued as a means of conservation. The driving force behind much of the Trust's activity is managing a natural process known as succession.

JOANNA FREELAND

Succession is the natural way in which habitats change over time. If you start with a habitat that has essentially nothing living there, the first species that come in will be weedy plant species, because they can survive on soil that has low nutrients. And they can take energy from the sun, and therefore, form the basis for a food web to begin there. Associated with these plants-- these first plants coming in-- will be a range of invertebrates. And when the plants and the invertebrates die, the detritivores will help to decompose them. Their nutrients will be released into the soil. And as the soil becomes richer, a wider range of plant species can now come in and grow there. And over time, you'll get the progression from weeds to grassy species to shrubs, hedges. And eventually, you'll end up with forests there.

NARRATOR

If woodland were the only habitat here, then biodiversity would plummet. And if succession were allowed to proceed naturally, much of the fen would be covered by bushes or trees. It's only by halting the process of succession, at various stages, that so many habitats can be maintained. This idea of conserving habitats by managing succession was first explored in the early 1900s. Wicken was one of the first sites at which the effects of management on habitat diversity were explored. An early exponent of this was Harry Godwin, a pioneer of plant ecology.

ADRIAN CALSTON

These are the Godwin plots. And they're an experiment that was set up in the 1920s to look at the impact of management on vegetation. There are four plots. And then, down at the far end, there is a control. And the idea is to see what happens with different frequency of cutting, and the impact of that on vegetation. And what we've found out, also since then, is that obviously has an impact, also, on the types of animals that live here as well. So where we are now, we are in the first year. And this vegetation is cut every year. And it's dominated by grasses. And this is the purple moor grass. In addition to grass being in here, we can see that there are a number of flowering plants. This one here is purple loosestrife. And in this kind of community, we'll find that there are butterflies, such as the large skipper and the small skipper, and moths, such as the silver bard, which occur. And if we just move a couple of yards over to here, we're now in the area that's cut every two years. And here, a new plant has come in. This one here is called the saw-sedge. It has a very jagged edge along the back of it.

NARRATOR

Even as little as one year difference between cutting periods has a significant impact on the species that are able to grow. This difference can be attributed to succession.

ADRIAN CALSTON

Well, I'm now moving from the area which is cut every two years into this one, which is cut every three years. And immediately, you can see that this is much denser and much taller than the previous area. It's almost dominated, now, by the saw-sedge. But we can begin to see, now, the beginnings of seedlings of small shrubs. This one here is alder buckthorn, for example. Then, moving along over here, we can see the reed is quite dominant. And this one, obviously, is a couple of years old, as it's got a dead flower head on the top. Not only is the vegetation different with this longer cutting regime, but this is also reflected within the animals. Now some animals live as little, tiny larvae, for two or three years, within side the stems of these grasses. And of course, in the plots over there where they're cut every year, those animals have got no chance of maturing and becoming adult, whereas here, they actually can. And one animal that lives in here is a moth called the reed leopard. It lives as a larvae, inside here, for two to three years, and then emerges as the adult. And here, it's able to do that. What you can also see within this plot, as well, is there are an awful lot of spiders webs. As there's no annual disturbance, these animals can build these complicated webs, and then hunt around, in this longer grass, upon the things that also live within here. This is the compartment that's cut every four years. And again, we can see very tall, very coarse vegetation dominated by the reeds and the sedge, but also, increasingly now, by areas of scrub. Here-- this plant here is a creeping willow, a little bush that's beginning to come in. It's coming in because it's been cut so infrequently it's able to shoot and actually get growing, and grow reasonably tall. And then finally, moving on over to here, this is the area which is not cut at all. And we can see, now, it's dominated by bushes, which are growing quite tall now. This plant here is the gelder rose. Over there is privet, which is now coming into flower. And above us is a birch tree.

NARRATOR

The trees and bushes of the control plot, which hasn't been cut at all, represent the final stage of succession in this area. The plots illustrate the process of succession from an open meadow to woodland. The different stages are entirely a result of how frequently each plot is cut.

ADRIAN CALSTON

The Godwin plots at Wicken show a number of different things, really. They show the role of management-- that if you manage things in one type of way, you can create one type of habitat. If you manage it differently, a different habitat can be created. But what they also show, if you look at them all in sequence, it shows you a succession from, basically, fen meadows through to scrub.

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5 Week 1 quiz

This quiz is about defining the term ecosystem and applying your knowledge to a simple ecosystem.

Open the quiz in a new window or tab then come back here when you're done.

6 Summary of Week 1

In the final video of this week, Dr David Robinson, Senior Lecturer in Biological Science at The Open University, discusses what you have learned so far in this course and then introduces some of the ideas and examples explored in Week 2.

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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.

End transcript

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There are different ways that an ecosystem can be defined and you reach a working definition of an ecosystem. You examined the core concept of an ecosystem through the example of a rock pool. Guidance as to how to study ecosystems was provided through the case study of Wicken Fen.

The next part of our learning journey starts with carbon and the capture of energy from light by plants. Plants are at the base of the food chain for all the animals in an ecosystem. The familiar woodland is an ideal place to begin to understand ecosystems in more depth.

If you would like a short break or to find out more about coastal habitats and Wicken Fen visit our Ecosystems area on OpenLearn.

You can now go to Week 2.

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

Figure 2: Courtesy of Shawndra Hayes-Budgen

Figure 3: magnetcreative; iStockphoto.com

Figure 5: Adrian Warren; Last refuge

Figure 6: taken from: http://wickenvision.blogspot.com/2014/10/let-cutting-commence.html

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The Wetlands of Wicken Fen © The Open University, contains BBC clips © BBC

Summary Week 1 © The Open University, contains BBC clips © BBC

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Week 2