Increased unpredictability in spruce budworm outbreaks following habitat loss and landscape fragmentation

Abstract

The spruce budworm (SBW) is the insect defoliator responsible for the greatest tree mortality in North America. Fragmentation and loss of habitat are expected to affect the movement of SBW through forested landscapes during periods of dispersal thereby modifying outbreak severity and frequency.We developed a spatially explicit tri-trophic model by combining a forest landscape model and a system of difference equations to evaluate how the interactions between the SBW, its hosts and its parasitoids are disrupted by habitat loss and fragmentation. We designed landscapes with varying quantities of habitat and fragmentation levels to measure their influence on population dynamics and outbreak severity and frequency.Our model simulations show that landscapes with higher proportion of host tree habitat were also characterised by greater SBW average densities, average cumulative defoliation and outbreak severity, which agrees with earlier studies. However, the bottom-up control by tree hosts breaks down as their proportion in the landscape decreased or when they formed isolated patches. Indeed, we found that landscapes with a low proportion of habitat fragmented into relatively large distant patches produced greatest variability in average SBW abundance and cumulative defoliation between simulation runs. In landscapes with higher habitat proportion, increased isolation between patches produced asynchronous outbreaks. Moreover, in these landscapes, parasitoids occupy fewer habitat cells than the SBW, suggesting that they were not able to efficiently track their prey. Together, these results indicate that increased distance between habitat patches can disrupt trophic control of SBW leading to increased unpredictability and asynchrony in outbreak dynamics.For continued effective management of SBW using current deterministic decision-making tools, we recommend prioritising the maintenance of habitat connectivity in forested landscapes.