Are Boreal Birds Resilient to Forest Fragmentation? An Experimental Study of Short-Term Community Responses

Abstract

We studied the effect of habitat fragmentation on the richness, diversity, turnover, and abundance of breeding bird communities in old, boreal mixed‐wood forest by creating isolated and connected forest fragments of 1, 10, 40, and 100 ha. Connected fragments were linked by 100 m wide riparian buffer strips. Each size class within treatments and controls was replicated three times. We sampled the passerine community using point counts before, and in each of two years after, forest harvesting, accumulating 21340 records representing 59 species. We detected no significant change in species richness as a result of the harvesting, except in the 1‐ha connected fragments, where the number of species increased two years after isolation. This increase was accounted for by transient species, suggesting that the adjacent buffer strips were being used as movement corridors. Diversity (log series alpha index) was dependent on area in the isolated fragments only after cutting, having decreased in the smaller areas. Turnover rates in the isolated fragments were significantly higher than in similar connected or control areas, due to species replacement. Crowding occurred in the isolated fragments immediately after cutting, but two years after fragmentation, the responses in abundance of species varied with migratory strategy. Numbers of Neotropical migrants declined in both connected and isolated fragments, and resident species declined in isolated fragments. Most species in these groups require older forest, many favoring interior areas. Abundance of short‐distance migrants, most of which are habitat generalists, did not change. Overall, although there was no decrease in species richness from our recently fragmented areas, community structure was altered; maintaining connections between fragments helped to mitigate these effects. Nevertheless, the magnitude of the fragmentation effects we documented is small compared with those observed elsewhere. Birds breeding in the boreal forest, where frequent small‐ and large‐scale natural disturbances have occurred historically, may be more resilient to human‐induced habitat changes, such as those caused by forest harvesting. However, these results should be interpreted with caution. First, they are short‐term and address only broad‐scale community responses based on species richness and relative abundance. Second, the study area was embedded in a landscape where large areas of old, mixed forest are still available, potentially dampening any local‐scale impacts of fragmentation.

Benefits of aggregate green tree retention to boreal forest birds

“…Bird communities from large patches have been found to be more similar to old forest communities than bird communities from small patches (Schieck and Hobson, 2000). However, patch sizes from 0.5 to 14 ha have been found to contain most species present in continuous forest (Seip and Parker, 1997;Schmiegelow et al, 1997). In our examination of patch size we found a suggestion of its effects on bird community composition based on a visual inspection of the ordination but no statistical evidence for this effect.…”

“…Bird communities in patches were a combination of intact forest species and open habitat species, and intermediate in composition between cuts and forests. Species (Schmiegelow et al, 1997;Seip and Parker, 1997;Merrill et al, 1998;Schieck and Hobson, 2000;Preston and Harestad, 2007;Söderström, 2009). Contrary to this pattern recent Michigan study did not find a positive response of forest songbirds to green-tree retention (Otto and Roloff, 2012), however, they did not examine individual patches but looked for patterns over entire harvested stands with a combination of aggregate and dispersed retention, which could potentially dampen the effects.…”

“…However, studies have also demonstrated that forest patches can provide habitat for at least some forest species (Hansen et al, 1995;Schmiegelow et al, 1997;Seip and Parker, 1997;Merrill et al, 1998;Schieck and Hobson, 2000) although in all of these studies only occupancy or abundance were examined. In this study, we compared boreal bird abundance and breeding in residual forest patches with plots of dispersed retention and intact forest during the breeding season.…”

Effect of forest management on a rare habitat specialist, the Bicknell's Thrush (Catharus bicknelli)

“…Indeed, relative to habitat generalists, forest-dwelling specialists have shown disproportionate declines in response to habitat changes caused by forestry (Schmiegelow et al 1997). For example, the decline of the Kirtland's Warbler (Dendroica kirtlandii (S.F.…”

The usefulness of top predators as biodiversity surrogates indicated by the relationship between the reproductive outputs of raptors and other bird species

“…Nevertheless, it is difficult to evaluate the reproductive output and survival of many species within large areas (i.e., landscape or regional scale; but see Robinson et al, 1995;Peak et al, 2004;Lloyd et al, 2005). Hence, the question of how to easily measure the reproductive output and survival of many species simultaneously has long been an issue (Vickery et al, 1992;Schmiegelow et al, 1997;Bonifait et al, 2006).…”