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Mountain Pine Beetle Habitat Supply Modelling Project: Predicting Species Occurrences in Response to Large-scale Disturbances
In British Columbia, populations of the mountain pine beetle (Dendroctonus ponderosae; MPB) have already created the largest infestation
of a forest insect ever recorded in Canada. Even though large-scale disturbances in forests, such as insect outbreaks causing extensive
mortality of trees, are a natural phenomenon, they can have widespread and significant effects on wildlife. Developing appropriate responses
to the effects on wildlife populations and distributions poses significant challenges to current paradigms for management of wildlife habitat.
Current estimates suggest that up to 195 vertebrate species may be affected by the MPB outbreak and/or the associated management intended to control or mitigate the MPB effects. Along with the expected losses of habitat for foraging and reproduction, other potential effects upon wildlife populations may include: (1) degradation of remaining habitat below the quality needed to sustain reproduction, (2) altered community structure through shifts in ranges of other species, causing potential food-web shifts and altered predator prey interactions; and (3) changes in dispersal opportunity and success resulting in altered gene flows and potential failures to reoccupy parts of species ranges. The objective of this study is to develop models for projecting probabilities that species will occupy habitats altered by MPB-induced changes in forest structure and composition.
Based upon literature reviews and a compilation of species accounts, we focused on the types of ecological and management effects on terrestrial vertebrate species resulting from MPB-induced mortality at different scales: (1) the stand scale, including changes in overstory and understory species composition, within-stand structures, canopy closure, and amounts of standing and fallen deadwood; and (2) the landscape scale, including changes in size of habitat patches suitable for meeting different life requisites, seral stage composition, and proximity to roads (leading to other sources of disturbance). More indirect influences of MPB-induced changes include alteration of some key species interactions (e.g., displacement from sites for reproduction, increased risk of mortality through altered predator populations and/or increased access. We organized these relationships into a hierarchical model, reflecting the assumed order in which these relationships come into play in determining species occurrences.
Habitat-occupancy models were developed for 13 vertebrate species that we expected would demonstrate a gradient of responses to losses of lodgepole pine as a result of mortality from attack by mountain pine beetle. Results showed that the occurrence probability of some species was sensitive to the modeled effects of habitat quality changes, while others were less sensitive. Further improvements to the modeling protocol to improve the utility of the methods for strategic resource management planning and conservation assessments are described.
We used the results of this modeling exercise as a starting point to consider general design elements of habitat supply models capable of representing the effects of large-scale disturbances on future habitat structures and biodiversity issues. Approaches to habitat supply models have evolved over several decades, incorporating an ever-broadening range of ecological processes, such as classification of habitat types by life requisite function, individual-based and spatially-explicit habitat-population models, natural and human-disturbance models, ecological succession models and predator-prey food webs. They have been used in applications ranging from locating limiting habitats for populations of single species to risk assessments of ecosystem function in multi-valued land-use models of human activity. Current design frameworks are emphasizing the hierarchical nature of habitat-species-human interactions, and models are increasingly being built in a modular fashion.
We outlined the key design elements for future HSMs that operate at a wide variety of spatio-temporal scales. We focus on a protocol for HSM development and implementation that permits checking and testing of accuracy of model outcomes at several points.
