State of the Greater Fundy Ecosystem

GREATER FUNDY ECOSYSTEM RESEARCH PROJECT

UNB Faculty of Forestry and Environmental Management

State of the Greater Fundy Ecosystem

Effects of Habitat Fragmentation on the
Pairing Success of a Forest Interior
Bird
Dwayne L. Sabine
University of New Brunswick
P.O. Box 44555, Fredericton, NB, E3B 6C2

The impacts of forest fragmentation on bird populations in North America have attracted considerable attention. There is evidence that forest fragments may represent marginal habitat for forest interior birds for a variety of reasons (see Whitcomb et al. 1981). Some species are absent or occur in low densities in forest fragments, and those individuals which are present may experience low levels of reproductive success.

Forests in New Brunswick are currently undergoing fragmentation due to intensive harvest levels. This is a dynamic process, with forest fragments being imbedded in a matrix of regenerating forest. Most research to date has been conducted in areas dominated by agriculture and/or urbanization; effects of fragmentation in a largely forested landscape are poorly understood.

Ovenbird; immature plumage
(Photo: J. Brownlie)

GOALS

This study evaluated the hypothesis that fragments of mature forest in a managed-forest landscape are suboptimal habitat for forest-interior birds compared to large tracts of similar forest. To test the hypothesis, we predicted that pairing success of male Ovenbirds would be lower on fragments than on contiguous forest sites.

METHODS

The study was conducted in the GFE using forests in Fundy NP as contiguous forest sites, and the harvested areas around the park as fragmented forest sites. A total of 7 fragment sites were selected, ranging in size from 2.5 to 38.5 ha. Within Fundy NP, 4 hardwood stands ranging in size from 14.9 to 27.0 ha were selected for study. Mating status was determined for 27 male ovenbirds in 1992 and for 20 males in 1993.

RESULTS

There was no difference in either the mating success or cumulative densities of male ovenbirds between fragments and contiguous forest sites (Table 1). However, when the largest fragment (38.5 ha) is removed from analysis, mating success (40%) and cumulative density (0.95/10 ha) of male ovenbirds on the remaining 6 fragments (2.5 - 16.3 ha) are somewhat lower.

Results of this study contrast those found in similar studies conducted in the eastern U.S., which reported marked differences in ovenbird mating success between fragment and large forest sites. However, these studies were conducted in areas where forest fragments are imbedded in a matrix of agricultural land.

Although the smaller GFE fragments contained few ovenbirds, density and pairing success for male ovenbirds in the largest fragment (38.5 ha) were similar to values recorded for the Fundy NP plots. In contrast, several of the other studies reported reduced densities and rates of pairing success in fragments much larger than our largest fragment.

For example, Van Horn et al. (1995) recorded pairing successes ranging from 0.0% to 27.7% in 5 fragments between 150 ha and 350 ha in size. These results suggest that the threshold for negative impacts of fragmentation might occur at a much lower fragment size in forested landscapes than in the more modified agricultural landscapes.

IMPLICATIONS FOR MANAGEMENT

It appears that fragmentation of mature forest within a forested landscape may not cause the deleterious effects reported for ovenbirds elsewhere where forest fragmentation is caused by non-forested landscapes. Alternatively, negative effects may occur but at a much lower fragment size. The current forest harvesting pattern in New Brunswick results in relatively large, irregular, interconnected blocks of unharvested, mature forest. Small, distinct, mature forest fragments are uncommon, and represent a small proportion of the available mature forest. If this pattern continues, then fragmentation of mature forest as it occurs within the GFE may not be detrimental to all forest-interior birds.

Further reading:

Sabine, D.L., A.H. Boer, and W.B. Ballard. 1996. Impacts of habitat fragmentation on pairing success of male Ovenbirds, Seirus aurocapillus, in southern New Brunswick. Canadian Field-Naturalist 110: 688-693.

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