Intraspecific priority effects mediate population growth and trait change in a host-pathogen system

Background/Question/Methods

Intraspecific variation is a significant driver of ecological interactions in a variety of natural systems. Most studies of intraspecific variation, however, incorporate only a single habitat patch, though many populations and communities receive input from neighboring patches via dispersers. These metapopulations and metacommunities allow for repeated contact between individuals that are often adapted to different local conditions (i.e. species interactions such as predation or parasitism). Can these interactions between intraspecific variation, spatial structure, and species interactions affect population ecology and evolution? I tested this question using priority effects, which are generally studied between species. Theory suggests that intraspecific or “genetic” priority effects (GPE’s) can modify population growth and trait change, which directly influence competitive interactions and evolutionary potential; this, however, remains largely untested. I manipulated the order of arrival of individuals from two lake-dwelling zooplankton populations (Daphnia dentifera), creating treatment monocultures and polycultures, with and without priority effects. I crossed these order of arrival treatments with the presence or absence of a fungal pathogen (Metschnikowia bicusipidata) to explore context-dependence in the strength or direction of GPE’s. I examined population growth, infection status, and average body size, an important trait affecting both competitive ability and disease tolerance.

Results/Conclusions

Results indicate that polyculture populations exhibited differential growth depending on the order of arrival of individuals from two source lakes. When individuals from one source lake (C) were added before individuals from lake D, initial growth rates were higher than the opposite arrival order (D->C). This changed, however, for treatments inoculated with a fungal pathogen; D->C treatments grew faster than C->D. This context-dependence seems to be mediated by changes in body size, as average size only changed in response to pathogen pressure when individuals from D were added first. Changes in body size represent an evolutionary response to pathogens in natural settings, so these results document both ecological (growth rate) and evolutionary (trait change) effects of GPE’s. Here there is a clear link to the broader community, as changes in initial growth of a dominant competitor (like D. dentifera in temperate lakes) can affect competition with other primary consumers. Similarly, if priority effects limit the evolutionary trait response to pathogen pressure, disease epidemics may also depend on an interaction between intraspecific variation and spatial structure. GPE’s may thus represent an important driver of community assembly in natural systems due to their effects on both ecological and evolutionary processes.