Are the species of management concern randomly distributed over the landscape, or are they found in certain habitats or physiographic provinces in greater numbers than expected? This question is addressed by comparing the distribution of listed species in various habitat classifications with the distribution of all other nongame species. Specifically, the following paragraphs compare the 47 forest and non-forest cover types identified by DeGraaf et al. (1991: 531-575), the 69 physiographic regions analyzed in the BBS database (Robbins et al. 1986, as modified; see Butcher 1990), and the 41 USFWS watershed-based ecosystem units located in the contiguous United States (USFWS 1995).
(1) Forest and Non-Forest Cover Types. (a) Absolute Numbers. The number of listed species found in each habitat (based on Tables 1-5 of DeGraaf et al. 1991) ranges from 4-35, with the central one-half of the values falling in the range of 12-23. Habitats in the top 20% (i.e., those supporting 25 or more listed species) include 1 Eastern Forest cover type (oak-hickory), 5 Great Plains cover types (Gulf prairies & marshes, south Texas shrub grassland, southern plains, central plains, and wetland and riparian), and 3 Western & Southwestern Non-Forest cover types (desert/riparian deciduous woodlands/marshes, annual grasslands/farms, and river/riparian woodlands/subalpine meadows). (b) Relative Numbers. The proportion of listed species (expressed as a percentage of the total number of nongame bird species found in each habitat) ranges from 7-75%, with the central one-half of the values falling in the range of 12-19%. Habitats in the top 20% (i.e., those in which the nongame bird community is composed of at least 20% listed species) include 1 Eastern Forest cover type (oak-hickory), 2 Eastern Non-Forest cover types (pasture/wet or sedge meadow, and everglades/mangroves/tropical hardwoods), 3 Great Plains cover types (southern plains, central plains, and northern plains), and 4 Western & Southwestern Non-Forest cover types (Sonoran desert scrub, Chihuahuan desert scrub, Mohave desert scrub, and annual grasslands/farms). Species of concern are not proportionally distributed among cover types (P <0.05). Listed species are represented in higher-than-expected numbers in Great Plains (P < 0.05) and Western & Southwestern Non-Forest (P < 0.05) cover types, and in lower-than-expected numbers in Western Forest (P < 0.01) cover types.
(2) BBS Physiographic Regions. (a) Absolute Numbers. The number of listed species found in each physiographic region (and detected on at least 1 BBS route) ranges from 5-32, with the central one-half of the values falling in the range of 14-22. Regions in the top 10% (i.e., those supporting 26 or more listed species) include 1 in the Atlantic Coastal Plain province (Upper Coastal Plain), 3 in the Eastern Piedmont Plateau province (Ridge & Valley, St. Lawrence River Plain, and Great Lakes Transition), 1 in the Appalachian Mountains & Boreal Forest province (Northern Spruce-Hardwoods), and 3 in the Great Plains province (Drift Prairie, Glaciated Missouri Plateau, and Great Plains Roughlands). (b) Relative Numbers. The proportion of listed species (expressed as a percentage of the total number of nongame breeding bird species found in each region) ranges from 6-29%, with the central one-half of the values falling in the range of 15-21%. Regions in the top 10% (i.e., those in which the nongame breeding bird community is composed of at least 23% listed species) include 1 in the Atlantic Coastal Plain province (South Texas Brushlands) and 6 in the Great Plains (High Plains Border, High Plains, Drift Prairie, Glaciated Missouri Plateau, Great Plains Roughlands, and Staked Plains). Species of concern are not proportionally distributed among physiographic regions (P < 0.01). Listed species are present in higher-than-expected numbers in physiographic regions of the Eastern Piedmont Plateau (P <0.05) and Great Plains (P < 0.01) provinces; and in lower-than-expected numbers in physiographic regions of the Western Mountains (P <0.01) and Pacific Slope (P < 0.01) provinces.
(3) USFWS Ecosystem Units. (a) Absolute Numbers. The number of listed species found in each ecosystem unit (and detected on at least 14 BBS routes) ranges from 2-25, with the central one-half of the values falling in the range of 8-15. Ecosystems in the top 10% (i.e., those supporting 23 or more listed species) include the Arkansas/Red rivers, Upper Mississippi River/Tallgrass Prairie, Great Lakes, and Ohio River Valley. (b) Relative Numbers. The proportion of listed species ranges from 6-36%, with the central one-half of the values falling in the range of 14-20%. Ecosystems in the top 10% (i.e., those in which the nongame breeding bird community is composed of at least 23% listed species) include the Lower Rio Grande, Pecos River, Edwards Plateau, and Texas Gulf Coast. When ecosystems are grouped into six major geographical regions corresponding approximately to FWS administrative regions, listed species are present in higher-than-expected numbers in Southwest (P <0.05) ecosystems; and in lower-than-expected numbers in West Coast (P < 0.01) ecosystems.
To summarize, over-represented habitats include the Great Plains, and Western & Southwestern Non-Forest cover types (sensu DeGraaf 1991); the physiographic regions of the Eastern Piedmont Plateau and Great Plains provinces (sensu Robbins et al. 1986, as modified; see Butcher 1990); and ecosystems in the Southwest (sensu USFWS 1995). Under-represented habitats include the Western Forest cover types (sensu DeGraaf 1991); the physiographic regions of the Western Mountains and Pacific Slope provinces (sensu Robbins et al. 1986, as modified; see Butcher 1990); and the ecosystems of the West Coast (sensu USFWS 1995).
The reader is cautioned against placing too much importance on the results of these preliminary analyses. We present this information merely as an interesting way of looking at the list, and hope that it will encourage others to undertake similar, but more rigorous, exercises to identify and prioritize avian habitats needing management attention.
[Discussion Contents]