Principles

Robert MacArthur [10] proposed measuring the stability of an ecosystem by measuring the number of alternative pathways it contains through which energy can flow. He justified this measure by arguing that an ecosystem with many pathways, representing an abundance of species organized in a complex food web, tends to equilibrate fluctuations in population as predators will switch from less abundant to more abundant prey species, lowering population densities of the more common and allowing the density of the less common to increase.

Charles Elton [5, pp. 146-150] gave six reasons for thinking the hypothesis to be true:

1. Evidence from mathematical models suggests that those with few species are inherently unstable.

2. Laboratory experiments are consistent with the mathematical models.

3. Habitats on small islands are more susceptible to invasion than are those on continents.

4. Less diverse habitats of cultivated or planted land are more susceptible to invasion than undisturbed habitat.

5. Highly diverse tropical forest ecosystems are relatively resistant to pest invasion.

6. Orchard spraying, which simplifies ecological relationships, tends to increase the likelihood of severe oscillations in pest populations.

In 1975, Daniel Goodman summarized the mounting evidence against the diversity-stability hypothesis by responding to each of Elton's arguments for it [6]

1,2

Models of more complex communities showed just the opposite of what Elton asserted. The more species that interact the less likely the system is to be stable.

3

The data suggesting vulnerability of islands to invasions by pest species may result from accidents of distribution or other special characteristics of islands.⁴

4,6

Crops and orchard tree planted in pure stands do not represent equilibrium low diversity systems. It is difficult to find evidence that naturally low diversity communities are more susceptible to invasion than naturally high diversity communities.⁵

5

The tropical biota is so diverse and complex that large fluctuations might go unnoticed. Furthermore, there is evidence that even highly diverse systems can be dramatically altered by invaders, e.g., the impact of the crown-of-thorns starfish on coral reefs.