Most classical theory on species coexistence has been based on species-level competitive trade-offs. However, it is rapidly becoming apparent that plant species display high levels of trait plasticity, such that their fitness and competitive ability may be highly variable across a landscape. The implications of this plasticity are almost completely unknown for most coexistence theory. Here, we model a competition-colonization trade-off and incorporate species-level trait plasticity to evaluate its effects on coexistence. Following empirical observations, we allow for plasticity in seed size as function of habitat quality (both increasing and decreasing) across a landscape. Seed size in turn determines both dispersal ability and establishment probability, shifting an individual’s position on the competitor-colonizer gradient. In order to evaluate the effect of plasticity on coexistence we simulated competition between plant species with and without plasticity across a variety of conditions including trade-off strength, disturbance frequency, and resource levels. In each simulation we evaluated whether species were able to coexist and quantified the range of conditions that supported coexistence across plasticity scenarios.
Results/Conclusions
Our simulations show that the classic competition-colonization trade-off is highly sensitive to environmental context, and coexistence only occurs in narrow ranges of conditions. In contrast, plasticity in seed size lead to coexistence across a much broader range of competitive and environmental conditions. While plasticity alone led to much broader coexistence, details of the plastic response (e.g., whether seed size and seed number or only seed size were variable) determined aspects of the equilibrium community such as which species was dominant. Additionally, a traditional competition-colonization trade-off, without plasticity, only produced coexistence when seed number was constrained. The inclusion of plasticity potentially supports coexistence by functionally allowing segregation along a resource gradient offering a niche stabilization mechanism that reduces competitive differences between species. Developing theory that can scale from a small number of species to the levels of diversity seen in many real ecosystems has been a longstanding challenge in community ecology. Incorporating complexities such as plasticity that can make classic coexistence mechanisms more robust may be a key step in bridging that divide.