Diversity-stability relationships with exploitative competition for variable resources: Diversity and niche breadth are stabilizing

Background/Question/Methods

Biological diversity is known to impact the temporal stability of populations and communities, and recent theoretical advances have focused on two mechanisms behind these effects. First, species diversity allows differences among species-environment interactions to generate the so-called portfolio effect of diversity. Second, species diversity allows compensatory dynamics as populations are released from interspecific competition. Untangling these two mechanisms has been the primary aim of a large body of theory. Recent syntheses conclude that species-environment interactions are the predominate control over the variability of total community abundance. Population fluctuations generated by compensatory dynamics do not affect summed abundance because of their negative covariance. However, these conclusions derive from models that treat both competition and environmental variability phenomenologically, lacking a mechanism. A common source of competition among consumers is their reliance on shared resources. Variation in those resources also yields a mechanistic source of consumer variability. We evaluated the robustness of current stability theory using models systems with explicit exploitative competition and environmental variability at the resource level. The two models analyzed are a classic damped Lotka-Volterra predator-prey system and a simple model of competition for light among phytoplankton.

Results/Conclusions

We used spectral analysis and Fourier transforms of ordinary differential equations linearized about average consumer and resource levels to develop analytical relationships between resource variability and the resulting consumer variability. In line with current theory, we observed from the Lotka-Volterra model that consumer diversity stabilizes total consumer abundance whenever consumers utilize somewhat different resources. Our results from the phytoplankton model yield an important insight about the relationship between competition and consumer stability. Competition was manipulated as the breadth of the fluctuating light spectrum which a phytoplankter can absorb with its array of photopigments. In isolation, generalists were more stable than specialists because of their larger resource portfolio. While generalist consumers were also stronger competitors, their large niche breadth dampened the fluctuations from compensatory dynamics. Therefore, increased competition (generalists verse specialists) was associated with more stable consumer populations. Additionally, since isolated generalists are more stable than isolated specialists, the effect of diversity on total consumer abundance was more pronounced among specialist communities. Our study advances diversity-stability theory along two lines. First, increased niche overlap does not necessarily destabilize consumer populations. Second, when resources are variable, there is a stronger effect of diversity on population stability among specialist consumers than among generalist consumers.