University of Alberta

Hilary Cooke

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Hilary Cooke

Ecology of the cavity-using community in mixedwood boreal forest


Ph.D. Research
I am studying the ecology of the cavity-using community in old mixedwood boreal forest of Alberta and Saskatchewan. This system has a rich community of birds and mammals that use cavities in trees for nesting and roosting. These species interact through their competition for suitable nest sites. Woodpeckers (primary excavators) excavate a new cavity each year and are limited by the availability of suitable excavation sites. The suitability of an excavation site varies by species, with some species (e.g. Yellow-bellied Sapsuckers and Pileated Woodpeckers ) preferring live Trembling Aspen with heartwood decay (caused primarily by the fungus Phellinus tremulae) and others (e.g. Northern Flickers) preferring already dead and decaying trees (snags). Chickadees and Nuthatches are described as weak excavators because they will reuse woodpecker holes or excavate their own cavity in soft, decaying snags. Secondary users (including Tree Swallows, Mountain Bluebirds, American Kestrels, Bufflehead, Common Goldeneye, Red Squirrels, owls, and bats) cannot excavate their own cavity. These species are limited by the availability of cavities formed naturally (e.g. by splits in tree trunks or the tops of broken-top trees) or cavities excavated by woodpeckers.

The functional relationships among the species creating and using cavities, and the flow of the cavity resource through the community, has been described as a nest web, a novel approach to studying communities based in food web theory (see Martin and Eadie 1999 Forest Eco & Man 115:243). In the interior coniferous forests of central B.C., Martin et al. (2004) found support for this bottom-up model of community organization, with the cavity resource flowing up from the primary excavators to a diverse group of secondary nesters. Northern Flickers were identified as a keystone excavator, creating the majority of cavities used by secondary nesters. Given differences in forest structure, and the composition of the cavity-using community, it is likely that the cavity nest web for old mixedwood boreal systems differs from that described for interior B.C.

Old mixedwood boreal forest stands (>100 yrs) are characterized by high densities of large trees, snags, and trees infected with heartrot fungi. Consequently, there is a richer and more abundant community of cavity nesters in old stands compared with young forest stands. The prevalence of old forest habitat will decrease across the landscape as forestry practices aim to minimize loss due to death and decay through short harvest rotation periods (~80 yrs). By decreasing the availability of old forest habitat, creating forest openings and edges, and increasing early successional habitat, timber harvest may modify the cavity nest web structure. Shifts in composition and densities of excavators may have cascading effects up through the nest web, affecting secondary nesters in turn, and thus changing functional relationships among cavity users.

Impacts of clearcut timber harvest have prompted forestry companies to adopt a "natural disturbance model" (NDM), which attempts to approximate fire and fire skips by harvesting larger areas and leaving larger patches of trees and snags. Under this approach, harvest effects may be ameliorated if retained patches act as "lifeboats", supporting the primary excavators and secondary users, and the functional relationships among them, found in old forest cavity communities. The relative value of retained forest patches to act as "lifeboats" may depend on the size, shape, and structure of retained patches.

My research objectives are 1) to describe the cavity nest web for oldmixedwood boreal forest and to identify keystone excavators; 2) to determine the value of retained forest patches as "lifeboats" for maintaining functional relationships among cavity users by describing the nest web for NDM harvested landscapes and contrasting it with the nest web for intact forest landscapes; and, 3) to characterize key features of occupied cavity trees, and the local habitat and landscape structure surrounding them.

Photos by C. Kolacz.

Professional Background
I have a B.Sc. in Zoology from the University of Guelph (Guelph, Ontario) and a Master of Science in Wildlife Management from Humboldt State University (Arcata, California).

My field research experiences include studying Neotropical migrant songbirds and habitat fragmentation with the University of Guelph, and Bald Eagles and landbirds with the Point Reyes Bird Observatory (Stinson Beach, California).

Before coming to the University of Alberta, I worked for five years with the Wildlife Conservation Society studying riparian birds in arid regions of California, Oregon, and Wyoming. My research focused on responses of songbirds to riparian restoration accomplished through alternative grazing practices and beaver reintroductions. I worked with private and public land managers to understand how birds responded to their management efforts, to identify key habitat needs of riparian obligate species, and to identify riparian bird indicator species.

My extra-curricular activities at the University of Alberta include the Biology Graduate Student Association Executive and cofounder of the Ecology Graduate Student Seminar Series.

Last Modified:2004-11-29