Regional maintenance of local priority effects by eco-evolutionary trade-offs
Species can competitively exclude others from local communities simply by virtue of early arrival. Such priority effects reduce local diversity, but can enhance variation in species composition among local communities. However, local priority effects can also scale up to result in regional dominance by a single species, unless species are continuously supplied from an external species pool, as assumed in MacArthur and Wilson's theory of island biogeography. This assumption is unrealistic in many cases, and it is unknown how local priority effects can be maintained in the absence of an external species pool. To address this question, we developed an individual-based simulation model in which two species interacted within local patches that were linked by dispersal. Individuals in the model were characterized by two traits, competitive ability and sensitivity to toxins produced by heterospecific individuals, parameterized to cause local priority effects. In addition, we allowed species to evolve by mutation and selection under a trade-off: an individual could be either competitive or toxin-tolerant, but could not be both at the same time. For comparison, we also used a neutral model where individuals did not differ in competitive ability and were insensitive to toxins.
In the neutral model, one species eventually went extinct due to drift. When individuals had fixed traits, local priority effects led to regional dominance by a single species, as expected. In contrast, when individuals had evolving traits, regional coexistence was more stable. As one species became more common, it evolved to become more competitive, but less tolerant to toxins produced by heterospecifics. In the meantime, the other, rarer species remained tolerant to toxins and took advantage of the other species' toxin susceptibility to invade some patches. These eco-evolutionary dynamics resulted in complex frequency dependence at the regional scale. When one species was regionally rare, it had a higher growth rate than the more common species, despite the presence of positive frequency dependence when both species were regionally common. We found, however, that this mechanism of regional coexistence depended on a number of factors including mutation rate, dispersal rate, local disturbance rate, local community size, and metacommunity size. We also found that regional abundances were greatly influenced by the type of dispersal assumed ("migrant pool" vs. "propagule pool" models). Taken together, our results suggest that eco-evolutionary dynamics can allow local priority effects to persist, thereby contributing to regional coexistence of species.