Ecosystem Functioning and Stability
Ecological functions can be defined as involving "…ecological and evolutionary processes, including gene flow, disturbance, and nutrient cycling" (Noss 1990 ). This is the study of how components such as energy and types of species in the ecosystem change over time. It differs from the study of structure, which investigates how the components of ecosystems change over space and is covered in the Patterns section. Ecologists have long examined the ecological function (or role) for individual species, but the study of the ecological function of biodiversity itself is very recent. It is an extremely complex field, and is rapidly growing.
Species in an ecosystem can be functionally equivalent, meaning that they do much the same thing (i.e.have similar niches). This partly depends on how finely you choose to define their niches; they can be as general as "herbivores" or as specific as "well-disguised endophytic insect herbivores". Functionally equivalent species can be grouped together as functional types; these groupings are given different names, such as guilds or functional types, depending on exactly how they are put together.
Functionally equivalent species are considered to compete with one another, but this also depends on how finely they are described. In a simple model of a system all herbivores may compete with each other for the plants they eat. A more realistic and finely detailed model will be less likely to have species compete with one another, as herbivorous species tend to specialize on different plant species or even different parts of the same plant.
Species of different functional types don't compete against one another for resources. Carnivores and herbivores, for instance, don't compete with one another. Instead, the carnivores eat the herbivores.
Both the different functional types and number of functionally equivalent species in each functional type in an ecosystem contributes to the functional diversity of the ecosystem. Functional diversity is the variety of responses by species in the ecosystem to environmental change, or how many ways the ecosystem can respond to change. A larger functional diversity can mean that the ecosystem is more stable, as some species will react well to environmental stress, while low functional diversity means that the community as a whole can react poorly to change. See Does diversity lead to stability?, below, for more details.
There are many theories about how the number of species affects ecosystem functions. One of these is the redundancy hypothesis, shown to the right, which assumes that the rate of ecosystem functions increases as more species are present, but only up to a point. After this point, more species are redundant and do not have any additional effect on ecosystem functions. In this theory the loss of species has no initial effect, but after a certain point functions begin to suffer. In the figure to the right, three possible patterns of increasing function as related to diversity are shown.
Another theory, the rivet hypothesis, shown to the right, claims that each species added to an ecosystem increases ecosystem functions, although the increase in function may increase more slowly as more species are included. In this model any loss of diversity should be noticeable.
Opposed to theories that assume a definite relationship between diversity and ecosystem functions is the idea that there is no fixed relationship, and that the functions of an ecosystem are the result of what the interactions between species are. In this case what is important is not how many species are present but which species are present together and what environment they are in. The figure to the right shows just one of the possible relationships between ecosystem function and diversity according to this hypothesis.
Which of these theories is most accurate is not certain, given the problems of scale and the complexity of the measurements. The Rivet Hypothesis looks like a small scale (on the figures above, the leftmost part) of the Redundancy Hypothesis, so it may be difficult to tell the two apart at the relatively small scale that most studies are done.
Although it is a key question, the relationship between diversity and stability is still being resolved. As with many topics in biodiversity, there are different ways of expressing stability. One way is to define it as the ability of a system to return to its original state after being disturbed, so how quickly it can return and how large a disturbance it can return from are key variables. Another definition is how resistant to change the system is in the first place. No matter what the definition used, however, there are definite trends that appear.
If either the redundancy or rivet theories (see above) are correct, then more species means more stability. Current consensus is that greater diversity does lead to greater stability, for three general reasons:
The structure of a food web also affects the stability of the system. Food webs describe the flow of energy through the system, basically who eats whom and how often. Different levels exist, such as producers (usually plants), primary consumers (herbivores i.e. who eat plants), secondary consumers (who eat herbivores), and so on. The food web used to be called the food chain, but the amount of cross-links makes the whole thing more properly resemble a web than a simple linear chain.
Most of the links in the food web are weak, meaning that the consumer doesn't depend excessively on what it consumes. As long as the links are weak, no species will be greatly affected by a predator or prey whose population changes. Strong links means that species are greatly affected by changes in the populations of species they're linked to; if there are many strong links in the system, drastic changes in one species spread through the system along the strong links, destabilizing it.