Rural Sprawl May Threaten Wildlife in Reserve

New research suggests that ranchettes in the Yellowstone National Park area could degrade the best habitat for birds and so cut their population growth below sustainable levels.

"Our findings suggest that alteration and destruction of the remaining productive habitats outside nature reserves will pose dire threats to many wildlife populations," say Andrew Hansen and Jay Rotella of Montana State University Bozeman in the August issue of the journal Conservation Biology.

Globally, development is concentrated near biodiversity hotspots. This is partly because private lands are more likely to have productive habitats such as lowlands and coastal and riparian areas, which generally have moderate climates, ample water and fertile soil.

In contrast, reserves are more likely to be at higher elevations and have poor soil, which makes them less productive. This disparity means that species in a reserve may depend on the more productive habitat on nearby private lands, making the species vulnerable to development outside the reserve.

Hansen and Rotella studied the effects of land use and habitat type on bird populations in the Greater Yellowstone Ecosystem (GYE), where rural residential development has increased rapidly. Development has increased more than four times since 1970 in the Montana and Wyoming parts of the GYE. The 3,600-square mile study area ranged from a high-elevation plateau in Yellowstone National Park to privately owned lowlands.

The researchers assessed bird abundance and diversity at 100 sites that represented the study area's range of topography, climate and soils. They found 135 bird species in the study area, and sites estimated to have 60 percent or more of the maximum bird abundance and diversity were designated "bird hotspots."

The researchers also assessed the reproduction and population growth of American robins and yellow warblers at two types of sites: low-elevation cottonwood stands and high-elevation aspen stands.

Hansen and Rotella found that most of the bird hotspots were on or near private lands, which were mostly at low elevations. Only about 7 percent of the bird hotspots were in reserves, which were mostly at higher elevations. In addition, rural residential development was concentrated near the bird hotspots on private lands. Home densities were nearly 70 percent higher within about a mile of hotspots than elsewhere on private lands.

The proximity of development to the low-elevation hotspots could mean trouble for the birds. Hansen and Rotella found that robins may depend on the low-elevation hotspots: the results suggest that robin populations are increasing in the cottonwood but not in the aspen sites. The researchers attribute this to the fact that the robin nesting season is two weeks longer in the low-elevation cottonwoods than in the high elevation aspen, giving females more opportunity to renest.

The researchers also found that yellow warblers may already be threatened by development. The results suggest that even though more females are renesting in the cottonwoods, the warbler populations are still shrinking there. The researchers attribute this in part to the fact that the cottonwoods are surrounded by rural residential development, and areas with denser development also have more nest predators (such as magpies and crows) and cowbirds, parasites that lay their eggs in other birds' nests. For instance, the researchers found that cowbird parasitism of warbler nests was five times higher in cottonwoods than in aspens (about 44 percent versus 9 percent).

Cowbirds did not lay eggs in robin nests, presumably because robins chase these parasites away. Robins also chase potential nest predators.

The birds' dependence on hotspots on private lands and the yellow warbler's vulnerability to development suggest that reserves may not be enough to protect biodiversity.

"These trends cast doubt on the viability of current strategies that rely on nature reserves for wildlife conservation and ignore intervening lands," the authors wrote. "Conservation strategies to protect population source areas outside reserves are likely necessary to reduce rates of future extinction in nature reserves."

Their work could help managers figure out which lands outside of reserves should be protected.