A1.1.4 Continued…


Diversity-Stability Theory

Theoretical models suggest that there could be multiple relationships between diversity and stability, depending on how we define stability (reviewed by Ives & Carpenter 2007). Stability can be defined at the ecosystem level — for example, a rancher might be interested in the ability of a grassland ecosystem to maintain primary production for cattle forage across several years that may vary in their average temperature and precipitation.

Figure 1 shows how having multiple species present in a plant community can stabilize ecosystem processes if species vary in their responses to environmental fluctuations such that an increased abundance of one species can compensate for the decreased abundance of another.

Biologically diverse communities are also more likely to contain species that confer resilience to that ecosystem because as a community accumulates species, there is a higher chance of any one of them having traits that enable them to adapt to a changing environment. Such species could buffer the system against the loss of other species.

Figure 1: Conceptual diagram showing how increasing diversity can stabilize ecosystem functioning

Each rectangle represents a plant community containing individuals of either blue or green species and the total number of individuals corresponds to the productivity of the ecosystem.

Green species increase in abundance in warm years, whereas blue species increase in abundance in cold years such that a community containing only blue or green species will fluctuate in biomass when there is interannual climate variability.

In contrast, in the community containing both green and blue individuals, the decrease in one species is compensated for by an increase in the other species, thus creating stability in ecosystem productivity between years.

Note also that, on average, the diverse community exhibits higher productivity than either single-species community. This pattern could occur if blue or green species are active at slightly different times, such that competition between the two species is reduced. This difference in when species are active leads to complimentary resource utilization and can increase total productivity of the ecosystem.

In contrast, if stability is defined at the species level, then more diverse assemblages can actually have lower species-level stability. This is because there is a limit to the number of individuals that can be packed into a particular community, such that as the number of species in the community goes up, the average population sizes of the species in the community goes down.

For example, in Figure 2, simple communities are represented by a box; in this case, these communities are so small that they can only contain 3 individuals. For example, this could be the case for a small pocket of soil on a rocky hillslope.

There are 3 potential species that can colonize these communities — blue, dark green, and light green — and for the sake of this example let’s assume that the blue species has traits that allow it to survive prolonged drought.

Looking at all possible combinations of communities containing 1, 2 or 3 species, we see that, as the number of species goes up, the probability of containing the blue species also goes up. Thus, if hillslopes in this region were to experience a prolonged drought, the more diverse communities would be more likely to maintain primary productivity, because of the increased probability of having the blue species present.

Each of the simple communities can only contain three individuals, so as the number of species in the community goes up, the probability of having a large number of individuals of any given species goes down.

The smaller the population size of a particular species, the more likely it is to go extinct locally, due to random — stochastic — fluctuations, so at higher species richness levels there should be a greater risk of local extinctions. Thus, if stability is defined in terms of maintaining specific populations or species in a community, then increasing diversity in randomly assembled communities should confer a greater chance of destabilizing the system.

Figure 2: Conceptual model illustrating the insurance hypothesis

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