Ecological implications
Our results suggest that adult insects will emerge earlier under climate change scenarios due to warming temperatures, but the termination of insect activity will be less sensitive to changing climates. This may lead to an overall lengthening of insect duration in response to global warming, particularly in areas with high precipitation. Longer insect activity periods may buffer against phenological mismatch of insects interacting with other trophic levels, as long as insect abundance is sufficiently high. However, mounting evidence suggests widespread terrestrial insect declines (van Klink et al., 2020; Wagner et al., 2021; Warren et al., 2021), which raises the threat of reduced ecological services regardless of how much synchrony occurs between interacting species.
One of the fundamental unanswered questions in understanding insect response to global change is which species will thrive — the winners, and which will be most negatively impacted — the losers. Phenology may be a key indicator of winners versus losers given recent work demonstrating that changes in insect population sizes correlate with phenological lability. Some insects may be able to adjust and thrive in warmer environments if additional land use changes are not occurring (Michielini et al., 2021). For example, multivoltine Lepidoptera with early adult emergence in warm years showed increased within- and between-year population growth in Britain (Macgregor et al., 2019). Elongated adult activity periods were also the best predictor of increases in relative abundance of Massachusetts butterflies (Michielini et al., 2021).
Our results point to two life history traits that may predict winners and losers in the face of future climate change. Detritivores and insects with larval habitats in freshwater exhibit a stronger response of activity period to temperature than do other insects, indicating these species may be relatively better suited to persist in novel climate scenarios. Conversely, species that have underground larval habitats may be more at risk, as duration for these species remains relatively fixed across temperature gradients. We note that these conclusions must be interpreted with care, as our models are primarily fit across a spatial gradient. Space-for-time models of ecological change are controversial because ecological processes are often nonstationary (Damgaard, 2019). Still, some empirical studies support these inferences. A recent meta-analysis found broad declines in terrestrial insects but increases in freshwater insect populations (van Klink et al., 2020). No net declines in detritivores were detected across five long term ecological research sites (Crossley et al., 2020), although these results have been questioned (Welti et al., 2020).