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A land manager's guide to conserving habitat for forest birds in ...

A land manager's guide to conserving habitat for forest birds in ...


Photo: Scott Gillingwater How does woodland shape alter edge effects? 50.4 ha 50.4 ha 50.4 ha 49.7 ha 28.3 ha 26.0 ha 710 m 8 ha 850 m 400.5 m 710 m 14.8 ha 1680 m 790 m A woodland’s shape affects the amount of forest interior it contains, as shown here with an approximately 50 hectare sized woodland. Woodlands may be quite large but lack forest interior because of their linear or irregular shapes. Woodlands shaped more like circles or squares have the greatest amount of forest interior proportional to their size and can have nearly double the interior space of narrow, rectangular woodlots of similar total area. 300 m Artwork: OMNR conditions at the edge (edge specialists); others, like the Ovenbird, are vulnerable to edge conditions and avoid or experience reduced reproductive success at edges. These are called forest interior specialists. Size and Shape The ecological characteristics of small fragments are different from those of larger areas, principally owing to the edge effect and the inability of small areas to support viable populations of species that have large territories or home ranges. Small fragments of a habitat usually support fewer species than large areas. Species whose occurrence or reproductive success is reduced in smaller fragments are called area sensitive species. Many forest birds are known to be area sensitive, including the Cerulean Warbler that will only nest in large forest tracts (often greater than 500 hectares [ha]). Other area sensitive species, like Ovenbird may nest in smaller forest fragments but they are often unable to raise young successfully because of high rates of predation and parasitism. These smaller fragments often do not offer enough interior habitat protected from edge effects to sustain breeding populations. Generally, forests less than 50 ha will support only the occasional area sensitive species, while forests 100–200 ha will support most area sensitive species. The shape of a forest patch is very important because it influences how much edge and interior habitat a patch contains. Forest patches that are square or circular in shape have less edge and more interior than long rectangular patches of the same size (see figure). Patches with less interior forest habitat may be less valuable to area sensitive species, consequently shape should be an important consideration in landscape planning. 26 The Fragmented Forest

Photo: Scott Gillingwater Source and Sink Dynamics Many songbird populations are composed of a network of sources and sinks linked by dispersal. Across the landscape, sources are high quality habitats where populations tend to increase. The other patches, the sinks, are low quality habitats that, on their own, are unable to sustain populations. However, sink populations can persist indefinitely, if excess individuals produced in the source frequently move to the sink. For a considerable number of forest breeding birds, productivity is much higher in large forest fragments than in small fragments and within these large patches the populations tend to increase (sources). Thus large forest fragments can provide excess individuals to neighbouring small fragments where productivity is much lower and not adequate for population growth or stability (sinks). The value of large forests to bird conservation is generally, although not consistently, accepted, as these may be critical in maintaining regional populations of some songbird species. In agricultural landscapes, nest predation and brood parasitism levels can be so high that many forest habitats are likely to be sinks. Yet, area sensitive birds may persist in small woodlot islands if they are able to disperse to these patches from nearby, large source populations (i.e. large fragments greater than or equal to 500 ha). Small woodlots that are isolated from adjacent large, source woodlots will tend to have impoverished bird communities, and support fewer species and fewer individuals than the large woodlot. Loss or degradation of nearby large source habitats will lead to population declines at larger scales, within surrounding small woodlots, as these sources typically rescue populations on surrounding sink habitats from extinction. Permanent changes to the large forests through land conversion or degradation of these fragments could lead to regional collapse of the area sensitive bird communities. Veery nestlings —Photo: OMNR Connectivity The proximity of a forest patch to nearby patches may be an important factor in the long-term maintenance of bird populations. Dispersal of young birds is important in source-sink dynamics (see Source and Sink Dynamics above) and can be limited when patches are isolated or the intervening habitat is difficult to cross (i.e., major highway). The degree to which forest patches are connected or their connectivity may be especially important for nonmigratory species like many woodpeckers, owls, and species with limited dispersal abilities such as salamanders. Regional Forest Cover The loss of forest habitat may have the most significant effect on the distribution and abundance of birds. As forest cover in a landscape decreases, the number of forest bird species supported in that landscape also decreases. Areas of high forest cover are more likely to incorporate a wide variety of habitats and thus a greater diversity of specialized species. In the Ottawa-Carleton region, which is approximately 30 percent forested, 100 percent of the species that should occur in this landscape are present. In contrast, almost 40 percent of the suite of forest birds has been lost from Essex region, where only five percent forest cover remains. Many studies have identified a minimum threshold of 20–35 percent forest cover. When forest cover is above this threshold, habitat configuration (edge, patch size, and connectivity) will have little or no effect on the number of species or abundance of birds. However, below the threshold, habitat configuration plays a critical role in determining species composition. The Fragmented Forest 27

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