For my purposes here, the details and measures of biodiversity or not really necessary.
Conservation thinking indicates that biodiversity is a good thing. That is, a highly diverse system is worth preserving. The may be more than one reason for this - for example, shouldn’t we preserve biodiversity because we want to share this planet with as many organisms as possible? BUT biodiversity is/was thought to be good for the ecosystem? That’s not very specific; Good means what? It turns out the good here means good for maintaining ecosystem functioning; for example, one benefit of an ecosystem is that it processes energy - that is energy flows through the system. In turn, this flow provides energy for organisms at a variety of trophic levels, and not to be left out this energy flow through trophic levels is the only reason we (human) are on this planet. Thus a system with higher biodiversity will have ‘better’ energy flow. Ecosystems might maintain a good flow of energy under changing conditions (drought, increases in predators, seasonal changes, other climatic changes) this aspect of an ecosystem is usually referred to as stability. Stability being, in the case of energy, a steady flow of energy through the system. The term resilience is often used in ecological systems (well at least since Holling (1973). A resilient system maintains integrity (maintains energy flow for example) in the face of disturbance (the aforementioned changes).
This is what has been assumed but, is this true? Does more biodiversity lead to steady energy flow? Does more biodiversity mean a more stable/steady/resilient system? Does biodiversity = good?
There is some evidence that in small systems more biodiversity enables a more steady system. But it turns out, there is actual very little evidence for this idea in larger systems. This may be of particular interest since conservation actions often tout biodiversity as good. Keep in mind large systems are,…well,…large and complex and difficult to study.
There is evidence that stability of a system is influenced not by biodiversity itself but functional diversity. These two are not mutually exclusive. Species in different functional groups could be species that eat different things - remember food webs from 6th grade science class? This idea suggest that a system made up of one carnivore, and four herbivores that all feed on grasses, is less stable than a system made up of grass eaters, and leaf eaters, and algae eaters, and carnivores of three sizes that each feed on prey of different sizes.
Ecological stability appears to be generated by functional diversity. More specifically, overlap in functional diversity (niche overlap) generates stability. If there are two species that eat some of the same things, or at least take up the same trophic level, and one of them goes extinct, the second species fills the role - or part of the role- making that particular system more resilient to change.
To put this another way: “Ecosystems are resilient when ecological interactions reinforce one another and dampen disruptions”
Lets get a little more confused, because, of course, there are alternative definitions of resilience. One is a measure of the magnitude of the disruption that will cause change (this comes initially from Holling (1973)) and is termed Ecological Resilience. This suggest that there is a threshold, some level or limit that, once passed, makes alteration inevitable.
The other definition of resilience is a measure of time. It is the time it takes to rebound back to where a system was (this comes from conservation master Stuart Pimm (Pimm 1984). This has been called Engineering Resilience.
Lets look at two illustrations of these resiliences (and like biodiversity and functional diversity these two types of resilience are not mutually exclusive). Figure 1 is ecological resilience and Figure 2 is Engineering resilience.
Figure 1. Ecological Resilience - Magnitude to transform to another state
Figure 2. Engineering Resilience - Time to return
Some of the work by Holling and others suggest systems are more resilient when species with overlapping function are on different scales - temporal or spatial: When you are setting up your field study to test these ideas, (you are setting up a study of this right?), you might be able to be approximate scale differences by using large and small species that eat similar things.
As mentioned already the biodiversity and functional diversity are not excluded from each other, in fact they are connected - as biodiversity increases, functional diversity is likely to increase. So as species richness (the easiest measure of biodiversity) grows (Figure 3, left, below), species functions overlap more (Figure 3, right section). In a real system it would probably not be so clean looking.
Take-Home Message:
- Biodiversity = good
- Better when it includes high functional diversity
- Even better when there is functional diversity overlap
- Even, even, better when the functional diversity overlap is of species working at different scales (time or space)
- A system can change and…
- exist in an altered steady state.
- and/or rebound back to previous state.
If it rebounds quickly then it has high engineering resilience, if it tough to change in the first place it has high ecological resilience – they are not mutually exclusive
Some readings:
DeAngelis DL. 1980. Energy flow, nutrient cycling and ecosystem resilience. Ecology 61:764-71.
Holling CS. 1973. Resilience and stability of ecological systems. Annual Review of Ecology and Systematics 4: 1-23.
Peterson G, Allen C, and Holling CS. 1998. Ecological resilience, biodiversity, and scale. Ecosystems 1: 6-18.
The figures stolen and used above are from this publication.
Pimm SL. 1991. The Balance of Nature? Chicago:Univ. ChicagoPress. 434 pp.