Effects of temporal variation in temperature and density dependence on insect population dynamics

C V Haridas,Blair D Siegfried,Bruce E Hibbard,Lance J Meinke,Brigitte Tenhumberg

doi:10.1002/ecs2.1287

C V Haridas, Blair D Siegfried + Show 3 more

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https://doi.org/10.1002/ecs2.1287

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Abstract

AbstractUnderstanding the effects of environmental variation on insect populations is important in light of predictions about increasing climatic variability. This paper uses the univoltine western corn rootworm (WCR, Diabrotica virgifera virgifera LeConte) as a case study and employs deterministic and stochastic modeling to evaluate how insect population dynamics is shaped by density‐dependent survival and annual variation in temperature, which are key in regulating insect populations. Field data showed that larval survival varied significantly between years but was constant for a range of densities. Survival dropped only beyond a threshold density, a feature resembling generalized Ricker functions used in modeling density‐dependent survival due to scramble competition for resources. We used soil temperature data for 20 yr to model annual variation in developmental time and survival. The deterministic model, where the developmental time was same across years, showed that though survival was high and did not change for a range of densities (i.e., density‐independent survival), predicted densities were large enough that strong density dependence could occur in the field (i.e., predicted densities fall in the region where survival drops sharply) and that populations could exhibit stable equilibrium, cycles, etc. Interestingly, populations with lower density‐independent survival were less likely to produce stable equilibrium compared to populations with higher density‐independent survival. We found that population densities were at stable equilibrium when both mean developmental time and fertility were relatively low or when developmental time and fertility were relatively high. This in turn implies that, in warmer regions, where mean developmental time will be lower, stability is more likely for insect populations with low fertility; species in warmer regions will experience cyclical and unstable dynamics when fertility is high. While increase in the mean developmental time reduces overall survival, increasing variation in developmental time could increase mean survival, a consequence of the Jensen's inequality, since survival was a concave decreasing function of developmental time. Hence, both mean and variability in temperature affect the dynamics of insect populations. Finally, we found that stochastic variation in soil temperature produced large variation in predicted population densities that could potentially enhance or diminish the effect of density dependence.

Highlights

Two important factors influencing population dynamics of insects and other poikilothermic organisms are temperature and density dependence in vital rates such as fertility, survival, and developmental time (Varley et al 1973)
Understanding the effects of environmental variation on insect population dynamics is important in light of predictions of increasing climatic variability (Meehl and Tebaldi 2004)
Even though stochastic models are increasingly being used in ecology to understand the persistence of several species (Boyce et al 2006), predictions about insect population dynamics are mostly based on deterministic models using laboratory measurements in simplified environmental conditions (Kingsolver et al 2011)

Summary

Introduction

Two important factors influencing population dynamics of insects and other poikilothermic organisms are temperature and density dependence in vital rates such as fertility, survival, and developmental time (Varley et al 1973). While degree-d ay models work well for predicting developmental time under constant temperatures, temporal variation in temperature can produce variation in developmental time and can have important consequences for population dynamics (Boyce et al 2006). While the effects of demographic stochasticity and individual-level variation in developmental time due to availability and quality of resources on population dynamics have been analyzed in some studies (e.g., Blythe et al 1984, Plant and Wilson 1986, Briggs et al 1993, Wearing et al 2004), effects of abiotic factors, especially the effects of variation in temperature on insect population dynamics have only rarely been explicitly incorporated in population models. It is critical to understand the population level effects of stochastic temperature variation because climate models predict changes in means and variability in temperature (Helmuth et al 2002)

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