The evolution of directed connectance under stress
Stressors have the potential to indirectly affect biodiversity by influencing trophic interactions, which are realized in the form of directed links between trophic levels. For instance, stressors could affect interactions between communities, exposed to several, simultaneously influential biodiversity drivers, by eliminating certain key species. However, studies focusing on the evolution of connectance between stressed producer and consumer communities are lacking.
Under the influence of biotic (predation, food limitation) and abiotic (stressors that affect fecundity or survival in different trophic levels) drivers we purposed to explore how directed connectance between predators and prey in a planktonic community evolves over time.
We present a food web model that simulates biodiversity at two trophic levels, taking into account several biotic and abiotic drivers, to simulate change in connectance over time between stressed producer and consumer communities. We employed the model in a full factorial design by varying the stressor type (stressor affecting prey, stressor affecting predator), control (top-down, bottom-up), and low and high initial connectance values. We defined connectance as the number of realized links divided by the total number of possible links.
High and low realized connectances departed from their corresponding initial connectance values. Connectance decreased with time in top-down controlled food-webs but increased under bottom-up control. When prey were stressed, and a decrease in prey diversity led to a decrease in predator diversity, bottom-up control resulted in fairly constant connectances over time for both low and high initial connectance values (following an initial increase from initial connectance values). In contrast, when predators were stressed, and a decrease in predator diversity led to an increase in prey diversity for low initial connectance values, top-down control increased to levels close to initial connectance values. However, connectance remained fairly constant over time for high initial connectance values when predators were stressed and top-down control was exerted (following an initial decrease from initial connectance values).
We conclude that connectance in communities under stress evolves contingent on the indirect effects of stressors on predator-prey interactions and this phenomenon is more evident at lower initial connectance values.