Fragmention by Agriculture Influences Reproductive Success of Birds in a Shrubsteppe Landscape

Abstract

Shrubsteppe communities are among the most imperiled ecosystems in North America as a result of conversion to agriculture and other anthropogenic changes. In the Intermountain West of the United States, these communities support a unique avifauna, including several species that are declining and numerous others that are of conservation concern. Extensive research in the eastern and central United States and in Scandinavia suggests that fragmentation of formerly continuous forests and grasslands adversely affects reproductive success of birds, yet little is known of the potential effects on avian communities in Western shrublands. I used multi-model inference to evaluate the potential effects of local and landscape variables on nest predation and brood parasitism, and behavioral observations of color-banded birds to evaluate the potential effects of habitat fragmentation on seasonal reproductive success of passerines in the shrubsteppe of eastern Washington State, USA. Reproductive success of shrubsteppe-obligate passerines was lower in landscapes fragmented by agriculture than in continuous shrubsteppe landscapes. Daily survival rates for nests of Brewer's Sparrows (Spizella breweri; n=496) and Sage Thrashers (Oreoscoptes montanus; n=128) were lower in fragmented landscapes, and seasonal reproductive success (percentage of pairs fledging young) of Sage Sparrows (Amphispiza belli; n=146) and Brewer's Sparrows (n=59) was lower in fragmented landscapes. Rates of parasitism by Brown-headed Cowbirds (Molothrus ater) overall were low (4%) but were significantly greater in fragmented landscapes for Brewer's Sparrows, and parasitism resulted in fewer young fledged from successful nests. Simple models of population growth using landscape-specific fecundity and estimates of adult survival derived from return rates of banded male Sage Sparrows and Brewer's Sparrows suggest that fragmented shrubsteppe in Washington may be acting as a population sink for some species. Immediate conservation needs include halting further fragmentation of shrubsteppe, restoring low-productivity agricultural lands and annual grasslands to shrubsteppe where possible, and convincing the public of the intrinsic value of these imperiled ecosystems.

Crop type influences edge effects on the reproduction of songbirds in sagebrush habitat near agriculture

“…Similarity between the daily nest survival rate found here and daily nest survival rates of songbirds in sagebrush habitat calculated in other fragmented landscapes with negative population growth (Vander Haegen 2007) suggests that reproduction in the Okanagan area is insufficient to maintain songbird populations in sagebrush habitat. Negative population trends have been detected in the study region for all four songbird species studied here, and edge effects on reproduction may be contributing to these declines (Sauer et al 2014).…”

“…The edge effect on nest abundance in orchard edge habitat appears to be due to edge avoidance: individuals, particularly Vesper and Brewer's Sparrows, do not occupy orchard edge habitat as frequently as interior habitat, and so the relative abundance of nests found in orchard edge habitat is also lower. Grassland songbirds have varied responses to the presence of edges (Helzer andJelinski 1999, Ribic et al 2009), but birds in sagebrush shrubsteppe generally show sensitivity to habitat fragmentation (Knick and Rotenberry 1995, Ingelfinger and Anderson 2004, Vander Haegen 2007. Songbirds likely avoid edges with stronger vegetative contrast, and songbirds in sagebrush habitat in the study region have previously been shown to avoid areas with high tree density (Krannitz 2007).…”

“…Fragmentation of sagebrush landscapes has been suggested to contribute to observed declines of songbirds in sagebrush habitat (Knick and Rotenberry 2002, Vander Haegen 2007. Songbirds in fragmented sagebrush landscapes experience higher rates of nest predation and modeling suggests this contributes to negative population growth in fragmented landscapes (Vander Haegen et al 2002, Vander Haegen 2007.…”

“…Songbirds in fragmented sagebrush landscapes experience higher rates of nest predation and modeling suggests this contributes to negative population growth in fragmented landscapes (Vander Haegen et al 2002, Vander Haegen 2007. Several authors have suggested these types of fragmentation effects detected at large scales are caused by the cumulative impact of edge effects across the landscape (Bender et al 1998, Ries et al 2004, Fletcher et al 2007).…”

“…Many of the species that inhabit the sagebrush shrubsteppe are also imperilled and show apparent population declines , Wisdom et al 2003, Sauer et al 2014). Habitat fragmentation is considered a contributing factor to these declines because predation rates of songbird nests are generally higher in shrubsteppe landscapes fragmented by agriculture than unfragmented landscapes (Vander Haegen et al 2002, Vander Haegen 2007. This landscape level pattern may arise because of edge effects on nest predation in fragmented landscapes (Bender et al 1998, Ries et al 2004, Fletcher et al 2007.…”