Phenological shifts are a ubiquitous response to climate change and have the potential to disrupt natural communities by changing the timing of species interactions. Typically, phenological shifts are measured by change in the first or mean phenological event of a population, and we therefore know little about how individuals are distributed around these summary metrics, or the consequences of this distribution. However, this information could be critical for understanding and predicting how phenological shifts affect ecological communities. Here, I use a combination of long-term observation, manipulative experiments, and theoretical modelling to elucidate the patterns of shifts in phenological distributions and their consequences on ecological communities.
First, by using a novel approach to analyze a 15-year record of 66 amphibian species pairs, we demonstrate that phenological shifts significantly increase the interaction potential of coexisting competitors, thereby altering temporal community structure and potentially increasing interspecific competition. Importantly, these increases in interaction potential were mediated by non-uniform and species-specific shifts in entire phenological distributions and consequently could not be captured by metrics traditionally used to quantify phenological shifts.
Next, to understand the consequences of these shifts in phenological distributions, I experimentally manipulated the hatching phenology of competing amphibian species and measured outcomes on population demography and heterospecific competition. Results indicate that the shape of the phenological distribution strongly influences intraspecific competition by changing the density of individuals and relative strength of early vs. late hatching individuals.
Finally, I developed a theoretical framework to explore effects of shifts in phenological distributions in different biological systems and identify factors that modify these effects. Using an agent-based model, I found that intraspecific competitive asymmetry and competitive context (mean arrival timing relative to heterospecific competitor) both strongly mediate the ecological role of phenological distribution. Specifically, the effects of phenological distribution on demographic rates were strengthened when individuals competed asymmetrically and when heterospecific competitor had first access to the resource. Together, these results demonstrate the potential of phenological shifts to reshape temporal structure of natural communities and show under which conditions it is most important to consider entire phenological distributions of natural populations.