Forest fragmentation and birds

For much of the last ten years, many migratory birds in the US seemed to be in trouble. Forest fragmentation, an increasingly widespread phenomenon, seemed to be causing alarming declines among birds. But numbers from surveys puzzled scientists. Not all forest-dwelling species were declining in number. Some species even seemed to be on the increase. Two ecologists now offer an explanation for this seeming conflict in data. Long term populations of many fragmentation sensitive birds depend upon the proportion of the species which actually occupy fragmented landscapes. When documenting bird numbers, they say, regional numbers, regional landscape patterns, and dispersal should be considered as important factors and scientists should be careful to not assume that local bird activities will mirror regional or global trends.

Therese Donovan from the US Geological Society Vermont Cooperative Fish and Wildlife Research Unit and Curtis Flather from the USDA Forest Service Rocky Mountain Research Station analyzed ten years’ worth of data regarding ten species of birds which had been previously deemed sensitive to forest fragmentation. The results of Donovan and Flather’s work appear in the April issue of Ecological Applications, a peer-reviewed journal published by the Ecological Society of America.

Long-term population trends among the birds were variable, with the populations of some species increasing while others declined. By closely analyzing the available data on all ten species, Donovan and Flather discovered that there was a significant connection between the areas which birds chose to nest in and their population trends. Species which were more likely to occupy fragmented landscapes, such as the Wood Thrush (Hylocichla mustelina) and the Northern Cardinal (Cardinalis cardinalis), were more likely to experience region-wide declines. Species such as the Red-eyed Vireo(Vireo olivaceus) and the Scarlet Tanager (Piranga olivacea), however, experienced population increases. On the whole, Tanagers and other “area-sensitive” species appeared less likely to nest in fragmented landscapes which are poor for reproduction; most of the individuals in these species nested in unfragmented forest landscapes, allowing populations to increase regionally. For instance, between 1970 and 1980, Tanagers actually increased region-wide by a rate of 3.11 percent.

Past studies of forest fragmentation and birds focused on the size of a given forest patch and its relationship to the density of a bird species. Some studies also examined how patch size affected a bird’s ability to reproduce. These studies demonstrated that small patches could have a negative impact on the fitness of a bird and its ability to produce large numbers of young.

For this study, however, Donovan and Flather examined their data with an eye for the issue of scale. Instead of making an intensive study of what was happening on the local level, they examined data from an area extending southward from Minnesota to Louisiana, and eastward all the way to the East Coast. Although certain birds might do poorly on a local level, the researchers hypothesized that a species’ tendency to increase or decrease in response to fragmentation would depend upon more region-wide factors.

Overall, Donovan and Flather found that regional population declines were uncommon among the bird species examined. Only two of the ten investigated, Wood Thrushes and Northern Cardinals, showed range-wide population declines over the period of time included in the study.

“Our findings caution against using localized data to make generalizations about broader geographic scales,” says Donovan. Data from a local level may not account for certain tendencies among birds.

Some species, for instance, may experience what is known as a “source-sink” population structure. In some areas where that species of bird nests, reproductive rates may exceed mortality rates, creating a “source.” Other areas may act as a “sink” for that species, with mortality exceeding the number of young produced each year. Local sink populations can be rescued from extinction by immigration from the source populations, if the birds disperse accordingly across a given region. Hence, evaluation of trends at a local scale may not reflect their tendency to disperse to other areas and reproduce in other locations.

Importantly, however, the researchers note that their findings reinforce the important negative connection between fragmentation and population levels.

“This study provides additional support for the hypothesis that fragmentation of breeding habitat throughout the breeding range could spur range-wide population declines,” says Flather. “But in order to do a better job of conserving species, we need to improve our understanding of the dispersal of species throughout regions, and we need to enhance our understanding of what drives birds to inhabit one landscape versus another.”

Founded in 1915, the Ecological Society of America (ESA) is a scientific, non-profit, organization with over 7500 members. Through ESA reports, journals, membership research, and expert testimony to Congress, ESA seeks to promote the responsible application of ecological data and principles to the solution of environmental problems. For more information about the Society and its activities, access ESA's web site at: http://www.esa.org.