Timing isn’t everything: impacts of maximum abundance and duration of a seasonal resource on consumer fitness

Kirsty H. Macphie, Joel Pick, James Pearce-Higgins, Jarrod D Hadfield, Albert B Phillimore

Phenological advances are among the most apparent biotic responses to warmer springs in mid- to high-latitude regions, with evidence that consumers are advancing less than the resources they rely on. Here, we extend the match/mismatch hypothesis to predict how the mean timing, maximum abundance and duration of the resource phenological distribution impacts on consumer fitness. Using data from 44 sites and 11 years on the phenological distributions of spring arboreal caterpillars and blue tits (Cyanistes caeruleus), we find that (1) an earlier caterpillar mean timing is associated with an earlier optimum hatch date, (2) a higher caterpillar abundance peak is associated with higher maximum breeding success and (3) a wider caterpillar distribution is associated with weaker stabilising selection around the optimal hatch date, though the evidence for this third effect is weaker. We then demonstrate that variation in caterpillar distribution height has pronounced effects on blue tit mean breeding success and on the strength of selection for earlier hatch dates. Our analytical approach provides a general framework for examining how phenological distributions of interacting species translate into impacts on mean fitness and selection, key parameters for predicting the fates of populations in a changing climate.