The Fragmented Forest
What is Forest Fragmentation?
Forest fragmentation occurs when large, contiguous forests are divided into smaller patches by residential and commercial development, roads, agriculture, and, in some cases, timber harvesting (Figure 3). Clear-cutting can temporarily fragment mature, contiguous forest until the clear-cut area has regenerated to a successional stage in which it no longer poses an ecological barrier to forest-interior birds. As mature forests become fragmented, less habitat is available for breeding birds, and a variety of factors, such as increased brood parasitism and nest predation, result in lower reproductive success in the habitat that remains (Brittingham & Temple 1983; Wilcove 1985; Martin 1988; Robinson et al. 1995). Thus, forest fragmentation not only causes a net loss of habitat, it can also reduce the suitability of remaining habitat in a region. Characteristics of a forest that can determine its quality as bird habitat include the size and shape of a forest patch, how isolated the patch is from other forests, how much forest remains in the surrounding landscape, the land-use matrix, and how much edge habitat exists nearby. Most of these characteristics are interrelated, so it's difficult to change one without affecting another.
It is important to distinguish between a forest that is fragmented by agricultural or urban development and a forested landscape composed of a mosaic of mature and regenerating stands that result from timber harvesting. The first situation typically is more damaging to forest bird populations and may represent permanent habitat loss, whereas the latter situation may only cause a temporary reduction in habitat for forest-interior species that rely on mature forests. Furthermore, early successional forests do provide habitat for many bird species, including some Neotropical migrants that are declining. Nevertheless, forest-interior species that require mature forests are affected by both sources of fragmentation. In most large landscapes the needs of early successional species can be met quickly through various sources of disturbance, including timber harvesting. Much more time, however, is required to develop suitable habitat for species that require mature forest. Effective conservation strategies must focus on maintaining adequate amounts of mature forest at any point in time.
The Scarlet Tanager is one of at least 20 species shown to be negatively affected by forest fragmentation in various studies throughout the eastern United States. For example, Robbins et al. (1989) found that in Maryland the probability of finding a Scarlet Tanager increased as the size of a forest patch increased or became less isolated. Results from the Cornell Lab of Ornithology's Project Tanager (see Project Tanager) also indicate that both forest-patch size and degree of isolation are important but the way these two factors affect Scarlet Tanagers varies in a complex fashion across this species' range (Rosenberg et al. 1999). We will take a closer look at these regional effects in later sections of this guide.