Canadian Forest Service Publications

Too much of a good thing: landscape-scale facilitation eventually turns into competition between a lepidopteran defoliater and a bark beetle. 2015. Goodsman, D.; Goodsman, J.; McKenney, D.W.; Lieffers, V.; Erbiligin, N. Landscape Ecology 30:301-312.

Year: 2015

Issued by: Great Lakes Forestry Centre

Catalog ID: 35870

Language: English

Availability: PDF (request by e-mail)

Available from the Journal's Web site.
DOI: 10.1007/s10980-014-0139-3

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Species distributions are influenced by how individuals interact with conspecifics, how they interact with other species, and by abiotic environmental factors. Resolving the nature of interspecific interactions using the relative spatial distributions of multiple species can therefore be considered an inverse problem.

Objective: We wished to determine whether defoliation by a lepidopteran (Choristoneura biennis [Freeman]) facilitates subsequent spruce beetle (Dendroctonus rufipennis [Kirby]) attack using spatiotemporal infestation patterns.

Methods: We used convergent cross mapping to probe time series of historical outbreaks of C. biennis and D. rufipennis in British Columbia, Canada, for evidence of interspecific interactions. We then fitted mixed model logistic regressions to spatial outbreak data to determine whether the probability of D. rufipennis infestation is impacted by prior defoliation by C. biennis.

Results: Convergent cross-mapping suggested that prior defoliation by C. biennis impacts D. rufipennis populations but this method cannot give information on the nature of the interaction. Our logistic regressions, however, provided insight into the nature of interactions by showing that the odds of moderate D. rufipennis infestation increased after moderate C. biennis infestation but decreased after severe C. biennis outbreaks. Thus, interactions between C. biennis and D. rufipennis are facilitative at moderate severities of C. biennis defoliation, but increasingly competitive as C. biennis outbreak severity increases.

Conclusions: Interactions between our study insects shifted from facilitative to competitive depending on outbreak severity—a proxy for population density. Density-dependent shifts from facilitation to competition are likely common in the animal kingdom.

Plain Language Summary

In large forest insect outbreaks, the areas impacted by multiple outbreak species often overlap in space and time. Therefore, interactions between insects that share common host trees may be evident at large spatial scales. Our objective was to determine whether defoliation by a lepidopteran pest (Choristoneura biennis [Freeman]) facilitates subsequent spruce beetle (Dendroctonus rufipennis [Kirby]) outbreaks on a landscape-scale. We converted maps of historical outbreaks of C. biennis and D. rufipennis in British Columbia between 1978 and 2010 to lattice form. We then compared spatiotemporal logistic regression models of D. rufipennis dynamics that included interactions between the two outbreak species to models that excluded them. Models that included interactions between our study insects outperformed those that did not. Recent defoliation by C. biennis increased the probability of infestation by D. rufipennis. Moreover, variables relating to defoliation by C. biennis to subsequent infestation by D. ruffipennis were more important when D. rufipennis outbreaks were building than when they were declining. Defoliation by C. biennis appears to promote the establishment D. rufipennis in the initial stages of D. rufipennis outbreaks and is likely one of many factors that enable beetle populations to irrupt.