Gradient analysis of vegetation

The confusion comes from use of the same word, "niche," for different concepts. We shall distinguish three senses of the word as: (a) the niche as the position or role of a species within a given community-the functional concept of niche; (b) the niche as the distributional relation of a species to a range of environments and communities-the niche as habitat, or the place niche concept; and (c) the niche as an amalgam of both these ideas, and thus defined by both intracommunity and intercommunity factors. Of these (b) makes "'niche'" synonymous with "'habitat"; for the present, we term (c) the "'habitat + niche" concept. Grinnell (1917, p. 433) first proposed "niche" to designate the place in an association occupied by a single species, emphasizing for the California thrasher not only food relations but dependence on cover, and adaptation thereto in physical structure and temperament. The thrasher's "ultimate associational niche . . . is one of the minor niches which with their occupants all together make the chaparral association." Grinnell (1924, p. 227) later called the ecologic niche the ultimate unit of habitat, and still later (1928, p. 435) defined it as the "ultimate distributional unit, within which each species is held by its structural and functional limitations." Grinnell's intention was probably closest to concept (c) of the habitat

Niche, Habitat, and Ecotope

Conservation of birds requires an understanding of their nesting requirements, including area as well as structural characteristics of the habitat Previous studies have shown that many neotropical migrant bird species seem to depend on extensive forested areas, but the specific area requirements of individual species have not been clarified sufflciently to aid in design and management of effective preserves For this S-year study, bird and vegetation data were obtained at 469 points in forests ranging in area from 01 ha to more than 3,000 ha in Maryland and adjacent states Data were analyzed first by stepwise regression to identify habitat factors that had the greatest influence on relative abundance of each bird species In the relatively undisturbed mature forests studied, degree of isolation and area were significant predictors of relative abundance for more bird species than were any habitat variables For species for which forest area was a significant predictor of abundance} we used logistic regression to examine the relationship between fcxrest area aIld the probability of detecting the species In managing forest lands for wildlifel top priority should go toward providing for the needs of areasensitive or rare species rather than increasing species diversity per se Avian species that occur in small and disturbed forests are generalists that are adapted to survival under edge conditions and need no special assistance frorn man Forest reserves with thousands of hectares are required to have the highest probability of providing for the least common species of forest birds in a region However, if preservation of large contiguous forest tracts is not a realistic optionS results of this study suggest 2 alternative approaches First, if other habitat attributes also are eonsidered smaller forests may provide suitable breeding sites for relatively rare species Second7 smaller tracts in close proximity to other forests may serve to attract or retain area-

Habitat area requirements of breeding forest birds of the middle Atlantic states

Bird species diversity was linearly correlated with foliage height diversity and curvilinearly with total percent vegetation cover. The addition of trees in a vegetational series has a disproportionate effect on the addition of species, primarily by the addition rather than the expansion of guilds. No basic relationship of species-packing within guilds is associated with bill or body size except frequently within two-member guilds. Estimated abundance and biomass of birds does not appear to be related to productivity of the habitats, in contrast to reports from the literature. Similarity of bird species composition is not related to similarity of foliage distribution, when like study areas are compared. Avifaunas of grasslands generally differed more among themselves than did those of forests. Bird species overlaps were cor- related with foliage height overlaps only for part of the variational range for two- and three- layered habitats, and little if at all for grasslands. Width of "habitat-niche" is not related to numerical dominance, taxonomic or ecological categories. Because many of these results do not coincide with previous attempts at ecological general- ization, great care in such attempts seems strongly indicated. Studies should be made of meticulously delineated subcommunities, resource measurement, and reproductive success, as well as events in the nonbreeding season.

Avian Community Organization and Habitat Structure

Brown-headed cowbird populations and their rate of brood parasitism on forest songbirds in eastern North America have increased since 1900. Brood parasitism of forest songbirds is highest near open habitat. High brood parasitism rates within isolated fragments of forest habitat reduce reproductive success of certain forest songbirds and may be responsible for their recent declines. (Accepted for publication 10 July 1982)

Richard R. Bond

Papers

Ecological Distribution of Breeding Birds in the Upland Forests of Southern Wisconsin