Academic journal article The American Midland Naturalist

Local-Scale Habitat Associations of Grassland Birds in Southwestern Minnesota

Academic journal article The American Midland Naturalist

Local-Scale Habitat Associations of Grassland Birds in Southwestern Minnesota

Article excerpt

INTRODUCTION

The pressures of a growing global human population have increased rates of land conversion from native ecosystems to human use while the rate of habitat protection remains static (Hoekstra et al., 2005). Land conversion has been particularly prevalent in temperate grasslands, resulting in precipitous declines in the extent of this habitat type across much of North America, Europe, and South America (Askins et al, 2007). More than 50% of native temperate grasslands in North America have been converted to human use (Hoekstra et al, 2005). In particular row-crop agriculture has replaced temperate grasslands in much of North America (Askins et al, 2007). For example more than 84% of western Minnesota is dedicated to agricultural production (Minnesota Department of Natural Resources 2006).

The loss of grassland habitat has caused corresponding declines in the populations of grassland-specific species (Samson & Knopf, 1994). Obligate grassland-breeding birds, which require grassland for all aspects of their life history (Vickery et al, 1999), have experienced declines among the most severe seen for any avian group (North American Bird Conservation Initiative U.S. Committee, 2014).

The issue of habitat loss is further compounded in remnant grasslands by habitat degradation. Barriers to visibility (Eason & Stamps, 1992) or movement decrease the suitability of potential habitat for these grassland specialists (Harris & Reed, 2002; Hamer et al., 2006), which are adapted to open habitats characterized by lack of vertical stratification (Mengel, 1970). For instance the vertical complexity added to the habitat by woody vegetation has a negative effect on obligate grassland bird occurrences (Grant et al., 2004; Thompson et al., 2014). Trees and shrubs have been introduced to the grassland landscape through intentional planting and unintended encroachment (Fuhlendorf et al., 2002) due to fire suppression and modified grazing patterns (Samson & Knopf, 1994; Askins et al, 2007). Such interference with natural disturbance regimes that have historically maintained grassland also has allowed the accumulation of litter, shrubs, and taller, denser vegetation through the process of succession (McCracken and Rowan, 2005; Holimon ä al., 2012). In addition some invasive species such as smooth brome (Bromus inermis) compound the structural issues of succession both through dense growth patterns that decrease the heterogeneity of the habitat and by reducing litter decomposition rates (Askins et al, 2007). In particular these structural changes reduce the suitability of habitat for those species that require shorter, sparser vegetation (Powell, 2006).

For migratory grassland obligates, the threats of habitat loss and degradation in breeding regions have been exacerbated by similar threats experienced during other periods of the annual cycle (Faaborg et al, 2010; Renfrew et al, 2013). For example in Chihuahua, Mexico-a wintering ground for over 90% of migratory Great Plains grassland birds-a combination of agricultural conversion, desertification, and shrub encroachment has drastically reduced habitat extent and predicted carrying capacity (Pool et al, 2014). Similarly, grasslands of southeastern South America have been severely impacted by the expansion of livestock, agriculture, and forestry (Azpiroz et al, 2012). Additional threats on wintering grounds, staging grounds, and migratory routes include exposure to toxic agrichemicals, illegal capture, and lethal control (Azpiroz et al, 2012; Renfrew et al., 2013).

To conserve prairie biodiversity in general and sensitive grassland bird species in particular, researchers and management agencies frequently determine that large natural areas of open grassland need to be protected (Askins et al, 2007). Given the vulnerability of grassland bird species throughout their annual cycle and their troubling population declines, we wonder if these protected breeding habitats are continuing to support the species. …

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