Phenological shifts constitute one of the clearest manifestations of climate warming. Earlier emergence is widely reported in high-latitude species, but a significant number of species exhibit delayed, or no change in, emergence. We reconcile these disparate observations and predict population-level phenological patterns using a theoretical framework based solely on mechanistic temperature responses of the species’ underlying life history traits. We develop a variable delay differential equation model parameterized with data on reproduction, mortality, and maturation rates from insects at different latitudes. We use this framework to predict a latitudinal pattern of species’ responses to different scenarios of climate change: increases in mean annual temperature and increases in the amplitude of seasonal fluctuations.
Species’ phenologies shift in opposite directions under increases in the mean environmental temperature versus increases in the amplitude of seasonal fluctuations. Peak abundance occurs earlier when warming involves an increase in the mean, but is delayed when warming involves an increase in fluctuations. Low latitude species are disproportionately vulnerable to warming in the mean temperature, with decreases in abundance and extinctions. We find that warming does not necessarily lead to a longer activity period in high-latitude species because it elevates summer temperatures above the upper limit for reproduction and development, splitting the activity period into spring and fall parts. Our findings both confirm and confound expectations about warming: an increase in the mean temperature is more detrimental to low latitude species adapted to high mean temperatures and low-amplitude seasonal fluctuations; an increase in seasonal fluctuations is more detrimental to high-latitude species adapted to low means and high-amplitude fluctuations.