Are boreal birds resilient to forest fragmentation? An experimental study of short-term community responses

Ecology, Sept, 1997 by Fiona K.A. Schmiegelow, Craig S. Machtans, Susan J. Hannon

INTRODUCTION

Habitat loss and concomitant fragmentation are concerns central to the conservation of biodiversity. Despite efforts to slow rates of habitat destruction, most ecosystems are becoming increasingly fragmented. In North America, much attention has focused on fragmentation of forested habitat, particularly where there has been permanent forest loss due to agricultural and urban expansion. Although recurrent negative impacts, such as some edge effects, increased predation and herbivory, and failure of metapopulation dynamics, have been identified (Simberloff 1994), there have been few studies of fragmentation in managed landscapes where patches of older forest are surrounded by regenerating forest (McGarigal and McComb 1995, Hagan et al. 1996).

Both the theory of island biogeography (MacArthur and Wilson 1963, 1967) and metapopulation theory (Levins 1970) predict species loss from habitat fragments, because of higher extinction and lower recolonization probabilities in isolated habitats. Island biogeography theory also predicts that extinction probability will vary inversely, and recolonization directly, with island area, and that species-area curves from fragmented habitats will have steeper slopes and lower intercepts than curves from continuous habitat. Metapopulation theory places greater emphasis on the role of the intervening matrix in mediating these rates. Connecting areas with corridors to facilitate movement among patches has been suggested many times as a conservation solution (e.g., Mann and Plummer 1993), but few empirical studies have supported this recommendation (Hobbs 1992, Simberloff et al. 1992). Although recent evidence suggests that birds use corridors for adult and juvenile dispersal (Haas 1995, Machtans et al. 1996), whether this reduces species loss in adjoining habitat fragments is not known.

The boreal forest is the most extensive forest ecosystem in North America, and probably the least understood. The mixed-wood region of the boreal forest in Canada extends from northeastern British Columbia into southern portions of the Northwest Territories, and through Alberta and Saskatchewan into southwestern Manitoba (Rowe 1972). Until recently, the boreal mixed-wood remained in a relatively natural state, where large, natural disturbances, such as fire and insect outbreaks, and smaller scale disturbances, such as treefalls, created a mosaic of stand types and successional stages. This naturally patchy habitat has one of the highest levels of bird species diversity in North America (Robbins et al. 1986). However, increasing pressure from industrial forestry is resulting in widespread habitat fragmentation and changes in forest composition (Schmiegelow and Hannon 1993). Older forests are being harvested first, and the structural and compositional complexity of these areas has been correlated with high species diversity and specialization (Stelfox 1995), leading to predictable conflicts between timber production and habitat conservation (Cumming et al. 1994).

The goal of our analyses was to estimate the severity of fragmentation effects on breeding boreal bird communities in old, mixed stands. We present the results from a replicated experiment in which boreal forest was harvested to leave older forest fragments of different sizes: some completely isolated and some connected by riparian buffer strips. We used birds to monitor the effects of fragmentation because they are relatively easy to census, have known sensitivities to habitat fragmentation elsewhere, and are good biological indicators for this system (see Schmiegelow and Hannon 1993). We tested the following predictions:

1) Species loss from fragmented areas will result in species-area curves with steeper slopes and lower intercepts relative to similar-sized areas within continuous forest.

2) Small fragments, in particular, will experience higher species turnover than large fragments, and will lose old-forest specialists and area-sensitive species.

3) Abundances of certain species will temporarily increase in recently fragmented areas, because of displacement of individuals from adjacent harvested areas.

4) Bird communities in connected fragments will be less affected by adjacent harvesting than will those in completely isolated fragments.

STUDY AREA

We conducted this research near Calling Lake, in north-central Alberta, Canada (55 [degrees] N, 113 [degrees] W). Our study area encompassed [approximately equal to]140 [km.sup.2] of boreal mixedwood forest, within the moist mixed-wood subregion. Trembling aspen (Populus tremuloides), balsam poplar (Populus balsamifera), and white spruce (Picea glauca) are the most abundant upland tree species in this region, often occurring together in old, mixed stands, whereas black spruce (Picea mariana) characterizes wetter sites (Strong and Leggat 1981). The dominant understory shrubs are alder species (Alnus tenulfolia, A. crispa), with lesser amounts of willow (Salix spp.). Various fruiting shrubs (Rubus, Ribes, Rosa spp.), sarsaparilla (Aralia nudicaulis), and other herbaceous plants dominate the lower strata.