Landscape configuration regulates intensity and frequency of spruce budworm outbreaks

Background/Question/Methods

Forestry activities are major disturbances to forest ecosystems modifying their community composition and structure. In particular, landscape fragmentation created by harvesting alter the ability of species to disperse within forests and hence the spatial setting in which species interactions take place. This study investigate the effect of forest fragmentation on the spatiotemporal dynamics of the spruce budworm (Choristoneura fumiferana, SBW), a major insect pest of Canadian boreal forests, whose periodic outbreaks are known to emerge from both bottom-up and town-down trophic interactions. We use a spatially-explicit model to determine the relative effects on structuring outbreak patterns of (1) differences in movement ability between the SBW and its parasitoid predators, and (2) the heterogeneity of the forest landscape. We developed a model that combines a cellular automaton approach with differential equations to represent the spruce budworm’s interactions with its food web in a range of spatially heterogeneous environments. The model is a tritrophic representation of the SBW centred food web containing: three tree species, a primary and secondary host and a non-host species; a herbivore species, the spruce budworm; and one or more parasitoid species. Two types of fragmentation patterns were tested: (1) in one we used square patches (of the same size) and we varied the distance between them (reminiscent of harvesting cut blocks), and in (2) we used neutral landscape models in which we varied both the amount of habitat and the level of aggregation of habitat patches. Through testing scenarios on different types of fragmented landscapes (square patches of habitat and random aggregated landscapes), with increasing distances between areas of suitable habitat, we can estimate a range of potential outbreak recurrences.

Results/Conclusions

We show that fragmentation alters outbreak dynamics, and that the effect varies depending on the type of fragmentation and distance between patches. In comparison to the homogeneous landscapes, fragmentation on the square patch landscape showed that an increase in distance between fragments corresponded to a decrease in intensity of outbreaks overall. The neutral landscape on the other hand showed increased intensity in outbreaks with greater aggregation, but with little influence on recurrence time. Our results suggest that forestry harvesting patterns can have variable effects on outbreak. Analysis of a wider variety of landscape composition and investigating forest composition may inform us of which harvesting patterns (spatial organization of cuts and replanting) are best to minimise the damage caused by SBW outbreaks.