Consequences of phenological shifts for species interactions: Testing the importance of size-mediated priority effects

Background/Question/Methods

Inter-annual variation in seasonal weather patterns causes shifts in the relative timing of phenological events of species, and this affects which ontogenetic stages of species interact. Despite the prevalence of this phenological variation, we currently lack a mechanistic understanding of how it affects these stage-structured interactions. Determining the consequences for species interactions, however, is critical to understanding and predicting how environmental variation affects demographic rates of populations and structure of communities. To address this problem, it is useful to consider the consequences of phenological timing in the framework of community assembly dynamics. This is because many aspects of the seasonal cycles of community development represent bouts of habitat re-colonization, including annual pulses of offspring. After colonizing, offspring of many organisms grow rapidly, so differences in arrival time between offspring pulses lead to differences in body size. We hypothesize that this could result in size-mediated priority effects, in which these size differences between offspring pulses alter interaction strength, direction, and type. In this study, we experimentally manipulated colonization time of naiads of the dragonfly, Pantala flavescens, relative to that of naiads of another, Tramea carolina, to determine the effects of phenological shifts on the dynamics of intraguild predation systems.  

Results/Conclusions

We found that when arrival time differences were large, the early-arriving Tramea completely eliminated the late-arriving Pantala from nearly all habitats via predation. This was likely due to the large body size differences between species when Pantala arrived and the long window of time during which Pantala remained relatively small and vulnerable to predation. However, Pantala exhibited faster growth rates relative to Tramea. Therefore, as differences in arrival time became smaller, the vulnerability window for Pantala decreased and its survival increased.  When arrival time differences were smallest, Pantala “outgrew” Tramea and became the predator, thus switching the direction of the interaction. A supplementary laboratory experiment demonstrated the importance of relative body size for these interactions, with even small size differences resulting in the larger dragonfly consuming the smaller one. Together, these results demonstrate how realistic levels of variation in relative arrival time can alter species interactions in ways that substantially affect survival and ultimately recruitment of offspring into adult populations. Considering that increases in size during ontogeny are ubiquitous, size-mediated priority effects are likely an important mechanism underlying phenology-driven changes to the structure in many communities, especially those that receive large seasonal pulses of colonizing offspring.