Long-term population dynamics of an insect in a simple food web under a changing environment

Weather conditions are important for the population dynamics of "cold-blooded" animals like insects, with both direct and indirect effects (via the food web). How weather, in combination with other factors, generates population change, and how such effects change over time, are important questions in times of climate change. We monitored an insect seed predator population during a 36-year period of changing weather patterns. The insect is part of a simple food web dominated by seed consumption and lacking natural enemies. Environmental conditions were relatively stable during the first half of the study, but patterns changed during the latter half. Areas of host plant patches increased and seed production entered a strong bi-annual pattern. Insect abundance was measured twice during the yearly life cycle, before and after summer reproduction, and seed resources and competitor densities were measured at the end of the summer. We fit a population model to abundance data to investigate the population dynamics of the insect in relation to changing patterns in weather conditions and food resources. There were both direct and indirect effects of weather, operating at multiple time scales. Abundant sunshine during summer resulted in increased population growth during the same period, but it also resulted in increased survival the following winter. Population growth further depends on seed set the previous summer, which in turn depends on summer rainfall and is likely affected by climate change. This implies indirect weather effects at both short-term and decadal time scales. The new pattern of seed production seems to have led to increased average insect abundance but did not otherwise lead to clear changes in the dynamics of the population. This can be explained by weak regulation of the dynamics of the insect such that short-term environmental variation leads to long unstable population fluctuations. Our study illustrates how insect responses to drastic changes in their environment can be subtle, slow, and hard to detect, manifested by long-term fluctuations. This highlights the importance of long-term data and mechanistic understandings of population dynamics to assess the consequences of changing weather and climate on insects.