A unified theory of compensatory dynamics: The direct and indirect impacts of multiple stressors on communities across timescales

Franz Simon, Alaska, The Nature Conservancy, Ithaca, NY

Background/Question/Methods
Ecosystems are exposed to multiple stressors, e.g., heatwaves and droughts, that can have synergistic and antagonistic impacts on communities. However, even a stressor directly impacting a single species may influence the whole community due to species interactions. However, one species' negative stressor may be an opportunity for another due to stress tolerance or reduced competition. Thus species can compensate for each other, which will stabilize communities. Most theories on multiple stressors are based on equilibrium assumptions despite species interactions being time scale-dependent. However, empirical studies have revealed that compensatory dynamics are limited to specific time scales. Using frequency response theory, I develop an approach for isolating environmental stressors' direct and indirect effects. I then combine these effects to show the time scales and community properties that will generate compensatory dynamics. Using a Lotka-Volterra model for two interacting species, I derive a general theory of how multiple stressors combine and which at different time scales communities will undergo compensatory dynamics.
Results/Conclusions
I find that at short time scales, direct effects dominate. At intermediate and slow time scales, communities become dominated by indirect effects. However, the time scale, relative growth rate, and per capita species interactions are all critical determinates of communities' stability. Furthermore, it is only at intermediate or long time scales with populations will compensatory dynamics emerge. Strong compensatory dynamics, at intermediate time scales, require populations to have similar growth rates. In agreement with most empirical studies, I find that under most conditions, population dynamics are synchronized. In summary, this theory provides a critical new approach for conceptualizing how multiple stressors impact communities across time scales and how they combine to generate the stability of a community.