Temperature effects on intraspecific competition: an experimental study on a stored product pest

Background/Question/Methods

There is no question that the planet is warming at an alarming rate. The only question is whether organisms can acclimate and adapt to warming fast enough to avoid extinction. Elucidating how temperature affects population regulation is critical in determining population viability in the face of warming. The need is particularly acute for ectotherms, which constitute the majority of biodiversity on the planet. While there are competing theories on temperature effects on competition in ectotherms, few or no studies have tested comparative predictions for multiple hypotheses within the same experimental design. We seek to fill this key gap in our knowledge of how temperature affects population regulation.Here, we present results from laboratory experiments on the bean beetle Callosobruchus maculatus (Coleoptera: Chrysomelidae), a cosmopolitan pest of stored products. We measured life history trait responses to beetle density at seven temperatures ranging between 18-36 °C. We quantified density-dependence in birth, maturation, and juvenile mortality to investigate whether rising temperatures strengthen or weaken intraspecific competition. Based on previous theory, we hypothesize that the effects of temperature and competition will act antagonistically at the optimal temperature for reproduction, and synergistically at low and high temperature extremes.

Results/Conclusions

We find that warmer temperatures lead to a higher birth rate and lower adult lifespan. Juvenile mortality was high at both low and high temperature extremes, leading to low adult emergence. The highest density treatments yielded the fewest emerging adults, suggesting strong negative density-dependence in juvenile mortality. Ongoing experiments suggest that high temperatures may weaken population regulation in two ways. First, lower adult density due to higher mortality at high temperatures may predispose populations to extinction via demographic stochasticity. Second, higher fecundity at temperatures around the physiological optimum could lead stronger competition, which, combined with higher mortality, could also reduce densities to levels that predispose populations to stochastic extinction. We discuss the implications of these findings on insect population viability and pest control in a warming world.