Indirect effects of climate on mountain pine beetle eruption, spread, and invasive potential

Background/Question/Methods

Climate change is often invoked ambiguously to explain range expansions in plants and animals. We contend that in order to elucidate how climate drives species spread and distribution, mechanistic models must be confronted with data. In this research we illustrate how this can be done by formulating and testing several integrodifference equation models that connect climate to the population dynamics and spread of the mountain pine beetle (Dendroctonus ponderosae Hopkins). In this system, consistently warmer and drier summers stress the host trees of the mountain pine beetle making them easier to attack. Warmer winters also decrease overwinter mortality in mountain pine beetle juveniles. By constructing competing models we evaluate the relative importance of the indirect effect of climate on host tree susceptibility to the importance of the direct effect of winter temperature on the overwinter survival of juvenile beetles.

Results/Conclusions

Our models indicate that the long-term effects of consistently drier growing conditions on pine trees is a more important determinant of the spatial distribution of mountain pine beetle populations than yearly fluctuations in winter temperatures. Thus, in some cases, the indirect effects of climate may have greater impacts on species distributions than direct effects. Based on our model comparisons we also argue that a realistic representation of dispersal is critical for models that predict the impact of climate on the distributions of mobile species because the effects of climate can be non-local in moving populations. This is especially true for very vagile species like the mountain pine beetle.