Conservation of Grassland Birds in North America: Understanding Ecological Processes in Different Regions: "Report of the AOU Committee on Conservation"

Abstract

—Many species of birds that depend on grassland or savanna habitats have shown substantial overall population declines in North America. To understand the causes of these declines, we examined the habitat requirements of birds in six types of grassland in diff erent regions of the continent. Open habitats were originally maintained by ecological drivers (continual and pervasive ecological processes) such as drought, grazing, and fi re in tallgrass prairie, mixed-grass prairie, shortgrass prairie, desert grassland, and longleaf pine savanna. By contrast, grasslands were created by occasional disturbances (e.g., fi res or beaver [Castor canadensis] activity) in much of northeastern North America. The relative importance of particular drivers or disturbances diff ered among regions. Keystone mammal species—grazers such as prairie-dogs (Cynomys spp.) and bison (Bison bison) in western prairies, and dam-building beavers in eastern deciduous forests—played a crucial, and frequently unappreciated, role in maintaining many grassland systems. Although fi re was important in preventing invasion of woody plants in the tallgrass and moist mixed prairies, grazing played a more important role in maintaining the typical grassland vegetation of shortgrass prairies and desert grasslands. Heavy grazing by prairiedogs or bison created a low “grazing lawn” that is the preferred habitat for many grassland bird species that are restricted to the shortgrass prairie and desert grasslands. Ultimately, many species of grassland birds are vulnerable because people destroyed their breeding, migratory, and wintering habitat, either directly by converting it to farmland and building lots, or indirectly by modifying grazing patt erns, suppressing fi res, or interfering with other ecological processes that originally sustained open grassland. Understanding the ecological processes that originally maintained grassland systems is critically important for eff orts to improve, restore, or create habitat for grassland birds and other grassland organisms. Consequently, preservation of large areas of natural or seminatural grassland, where these processes can be studied and core populations of grassland birds can fl ourish, should be a high priority. However, some grassland birds now primarily depend on artifi cial habitats that are managed to maximize production of livestock, timber, or other products. With a sound understanding of the habitat requirements of grassland birds and the processes that originally shaped their habitats, it should be possible to manage populations sustainably on “working land” such as catt le ranches, farms, and pine plantations. Proper management of private land will be critical for preserving adequate breeding, migratory, and winter habitat for grassland and savanna species. Received 12 December 2006, accepted 24 April 2007. Resumen.—Muchas especies de aves que dependen de habitats de pastizal o savana han mostrado disminuciones signifi cativas en sus poblaciones en Norte America. Para poder entender las causas de estas disminuciones examinamos los requerimientos de habitat de aves 8E-mail: raask@conncoll.edu. Coauthors are listed alphabetically. 9U.S. Department of Interior (retired). ORNITHOLOGICAL MONOGRAPHS NO. 64 2 Bird species that depend on grassland and shrubland have declined in many regions in eastern and central North America during the past century, and open-country species frequently outnumber woodland species on state lists of endangered and threatened species (Askins 1993). Grassland birds, in particular, appear to be in trouble; during the past 25 years they have shown “steeper, more consistent, and more geographically widespread declines than any other behavioral or ecological guild” of North American birds (Knopf 1994:251). An analysis of continent-wide population trends on Breeding Bird Survey (BBS) routes between 1966 and 2002 showed that only 3 of 28 species of grassland specialists increased signifi cantly, whereas 17 species decreased signifi cantly (Sauer et al. 2003). Although some species that showed a continent-wide population decline were increasing in particular regions, many species (including Bobolink [Dolichonyx oryzivorus] and Eastern Meadowlark [Sturnella magna]) declined throughout almost their entire breeding range. Population declines of grassland birds have occurred not only in the northeastern United States (Vickery 1992), where regenerating forest has replaced much of the farmland that dominated the landscape in the 18th and 19th centuries (Norment 2002), but also in the Midwest (Herkert 1995) and the Great Plains (Knopf 1994), the historical centers of abundance and diversity of grassland birds. Population declines have also occurred in grassland birds in South America (Vickery et al. 1999b), Europe (Newton 1998), and other parts of the world (Goriup 1988). Although declines in particular grassland bird populations can be att ributed to a wide variety of factors, such as habitat fragmentation, nest parasitism, pesticides, and invasion by woody vegetation (Peterjohn and Sauer 1999), an overriding cause of regional declines appears to be agricultural intensifi cation. Because most natural grasslands were converted to farmland or are used as ranchland, grassland birds now largely depend on habitats that are managed for agricultural production. Although farmland and pasture may provide good breeding or wintering habitat for some grassland bird species (e.g., Wunder and Knopf 2003), their suitability oft en declines as agriculture becomes more effi cient. Agricultural intensifi cation involves a shift toward monocultures that support fewer natural species (Matson et en seis regiones del continente. Habitats abiertos originalmente se mantenían por conductores ecológicos (procesos ecológicos continuos y perdurables), como sequía, pastoreo, y/o fuego como en praderas de pastizal alto, mediano, y corto, pastizal desértico y sabana de pino de hoja larga. En contraste, los pastizales se crearon por disturbios ocasionales (fuego o actividad de castores) en el noreste de Norte America. Especies claves de mamíferos (como perrito de las praderas y bisonte en las praderas del oeste y castores en bosque deciduos del este) jugaron un papel crucial, y frecuentemente no apreciado, manteniendo sistemas de pastizales. Mientras el fuego fue importante en prevenir la invasión de especies leñosas en praderas de pastizal alto y mediano, el pastoreo jugo un papel mas importante en mantener la vegetación típica de pastizales cortos y desérticos. Alta presión de pastoreo por perrito de las praderas y bisontes crearon una capa de “césped pastoreado” que es el habitat preferido por algunas especies de aves de pastizal que están restringidas a las Grandes Planicies y pastizales desérticos. Muchas especies de aves de pastizal estan vulnerables porque la actividad humana ha destruido sus habitats de anidacion, migración e invernacion directamente mediante la conversión a áreas de cultivo o construcción, o indirectamente mediante la modifi cación de patrones de pastoreo, supresión de fuego, o interfi riendo con otros procesos ecológicos que originalmente mantenían el pastizal abierto. El entendimiento de los procesos ecológicos que mantenían el sistema de pastizal es sumamente importante para esfuerzos de mejoramiento, restauración, o creación de habitats para aves de pastizal y otros organismos. Consecuentemente, de alta prioridad debería de ser la preservación de grandes áreas de pastizal natural o seminatural donde estos procesos se podrían estudiar. Sin embargo, muchas especies de pastizal ahora dependen principalmente de habitats artifi ciales manejados para maximizar la producción de ganado, madera, u otros productos. Con un claro entendimiento de los requerimientos de habitat de aves de pastizal y los procesos que originalmente moldearon sus habitats seria posible mantener sus poblaciones en terrenos manejados como ranchos ganaderos, granjas, y plantaciones de pino. El manejo apropiado de tierras privadas será critico para la preservación adecuada de areas de habitat de anidacion, migración e invernacion para aves de pastizal y savana. CONSERVATION OF GRASSLAND BIRDS 3 al. 1997), and the channeling of more primary production toward food or fi ber. In a broad sense, intensifi cation would include conversion of rangeland to cropland, and the shift to the use of exotic grasses and forbs in pastures and hay meadows. A suite of factors associated with agricultural intensifi cation tend to degrade grassland bird habitat: these include increased use of pesticides, removal of natural fi eld edges, spring plowing, land drainage, replacement of mixed farms with farms dominated by one crop, harvesting or mowing earlier in the season when birds are still nesting, and higher stocking rates for livestock (Newton 1998). Murphy (2003) showed that population changes in grassland birds in eastern and central North America between 1980 and 1998 were highly correlated with changes in agricultural land use in their breeding areas. The most important factor was loss of rangeland, which was associated with negative population trends for 12 species of grassland birds. Rangeland is used for livestock production, but it generally is not managed as intensively as cropland or pasture and it is dominated by native species of plants, providing habitat for a diverse group of grassland birds (Peterjohn 2003). In the Midwest, population declines in several species of grassland birds are highly correlated with declines in the combined area of pasture and hay meadow (Herkert et al. 1996). Our goal is to provide recommendations for halting and reversing the decline in grassland birds. In some cases, this may be accomplished by restoring natural grasslands, but in other cases it is more realistic to try to promote farming and ranching methods that make the land both economically productive and biologically diverse. In either case, it is impo