Competition-defense trade-off increases the diversity of microbial plankton communities and dampens trophic cascades

5 February 2019

Cadier, Mathilde; Andersen, Ken Haste; Visser, Andre; Kiørboe , Thomas

The competition/defense trade-off is a significant source of functional diversity in ecological communities. Here, we present a theoretical framework to describe the competition/defense trade-off and apply it to a size-based model of a unicellular plankton community. Specifically, we investigate how the emergent community structure depends on the shape of the trade-off, and on whether the cost of defense is paid for by a lowered resource affinity or by an elevated metabolic rate. The inclusion of defense affects the size distribution and trophic strategies of the emerging community dependent on environmental conditions (eutrophic vs. oligotrophic) and leads to increased diversity in size and trophic strategy under eutrophic conditions. Eutrophic conditions allow for better-defended organisms than oligotrophic conditions. In most scenarios, competition/defense trade-offs dampen trophic cascades in the seasonal cycle simulations, and increase the abundance of mixotrophs. We further demonstrate that it matters how the cost of defense is manifest (decreased affinity vs. increased metabolic rate), and that it has a significant effect on the resulting plankton community (overall biomass, size and feeding strategy diversity), particularly when the efficiency of the defense increases in direct proportion to the investment. Our results demonstrate that the structure of the ecosystem crucially depends on details of the defense trade-off. This finding highlights the importance of a mechanistic understanding of defense trade-offs, e.g., obtained through experimental measurements of specific defense mechanisms.