Relationship between Diversity and Stability of an ecosystem

The blog below is a summary of ecosystem stability and diversity in Basics of Environmental Science by Michael Allaby.

Let's imagine the invasion of pests in an agricultural ecosystem (raising a single crop; monoculture) versus a forest ecosystem (with diverse species). The former appears more vulnerable to invasion than the latter. It is easy to imagine pests establishing themselves at the expense of single species in monoculture farms, versus many species in the forest ecosystem. The former appears less stable versus the latter with more stability. From these observations, we may say that diversity brings stability, and its opposite simplicity confers instability, which is true. However, ecologists have reported stable ecosystems yet with fewer species. For instance, several tropical freshwater swamp ecosystems having fewer species, exhibit fewer or no change in many years. These examples of ecosystems comprising relatively few species yet remaining highly stable contrast the previously concluded relationship between diversity and stability. In the 1970s, ecologists began to develop mathematical models that could establish the relationship between diversity and stability with certainty. 

American ecologist E. P. Odum suggested that it is essential to define stability. Stability is clearly the action of the ecosystems in response to the change brought by external stimuli, such as extreme weather, fires, invasions by pests, and diseases. The ecosystems would respond to the odds either by being resistant i.e. preserving their essential features or by being resilient i.e. quickly resuming their former states, after a disruption. Thus, stability could either mean ‘resistance stability’ or ‘resilience stability’. After stability had been defined more clearly, scientists found that theoretically increasing the diversity of an ecosystem appears to reduce overall stability, however with important qualifications. Increasing the species reduces the resistance and resilience but increasing the number and strength of interactions or connections between species, increases it. A few experimental field studies conducted by David Tilman of the University of Minnesota found that species richness increased the resistance and resilience of the ecosystems in experiments. This offers support to an earlier view. In either case, jumping to any conclusion should be avoided.