Week 2: Understanding ecosystems

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.

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.

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.

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.

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.

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.

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.

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.

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.