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 consequences of changing weather and climate on insects.