Rarefaction, Relative Abundance, and Diversity of Avian Communities

--The common practice of expressing community structure in terms of indices of diversity and evenness involves a serious loss of information. Differences attributable to the accumulation of species with increasing area are ignored, differences in the density of individuals are often masked by other factors, and many combinations of species richness and relative abundance can produce the same value of the index. As an alternative we suggest (1) comparing species richness by standardizing samples either to equal numbers of individuals or to the number of individuals expected on equal-sized plots, and (2) expressing the relative abundance of species as a graph of their relative abundances arranged in a decreasing array. We present an analysis of bird census data based on the proposed methods, and we include comparisons with applications of four indices commonly used in ecology, the Shannon-Weaver index of diversity, the J' evenness index, the inverse of Simpson's measure of concentration, and Hill's evenness index. For 37 Breeding Bird Censuses taken in various terrestrial habitats across the United States and Canada, the proposed methods reveal some very general relationships about the organization of bird communities in different habitats. Equal-sized areas of mature deciduous forest and secondgrowth habitats may be equally species rich (14-24 species with •> 1 breeding territory per 6 ha); the density of individuals (territorial pairs) is generally higher in deciduous forest habitats, and the relative abundance of bird species shows more dominance (less evenness) in the deciduous forest. Mixed coniferous-deciduous forests and dense young deciduous forests have fewer species than mature eastern deciduous forests or second-growth abitats (9-16 and 7-10 species per 6 ha, respectively), although the density of individuals is approximately equal to that in second-growth habitats. Coniferous forests are species-poor (5-8 per 6 ha), and the density of territorial pairs is low (8-12 per 6 ha compared with 40-70 in deciduous forests). Although the proposed methods require assumptions that need to be evaluated carefully, we are optimistic that they will have other useful applications in the analysis of arian communities. Received 8 October 1980, accepted 15 April 1981. A large literature has developed in ecology presenting descriptive analyses of biotic assemblages (Dennis et al. 1979, Patil and Taille 1979). Although one should not infer mechanisms of community regulation from such studies (Pielou 1975), certain patterns recur in vertebrate communities in different habitats (Palmgren 1930, Udvardy 1957, MacArthur and MacArthur 1961, Williams 1964, MacArthur 1964, Karr and Roth 1971, Wiens 1973, Willson 1974), climates (Bock and Lepthien 1974, Rotenberry 1978), seasons (Rotenberry et al. 1979), and geographic areas (Pianka 1966, Recher 1969, Karr 1971, Tramer 1974, Cody 1975, Short 1979). One methodological problem with much of this literature and with community ecology generally since the early 1960's is the expression of community structure in terms of indices of diversity and evenness. Indices such as H' [-5; p/log Pi] (Shannon and Weaver 1949, Margalef 1958) and J' [H'/log s] (Pielou 1966a) confound important parameters that should be defined as precisely as possible and examined separately before communities are compared. These are (1) the number of species (species richness), (2) their relative abundance (evenness), (3) the number of individuals or territorial pairs, and (4) the area sampled. To combine any of these variables into a single statistic assures that the relative effects of the contributing parameters cannot be determined. The same value of the index can result from various com785 The Auk 98: 785-800. October 1981 786 JAMES AND RATHBUN [Auk, Vol. 98 binations of values of the parameters (Pielou 1975). Here, we recommend that data be standardized to either equal numbers of individuals (in this case, territorial pairs) or equal-sized areas before comparisons are attempted. We propose that rarefaction and relative abundance curves be used as an alternative to diversity indices. In addition to clarifying components of biological interest, these methods avoid many of the mathematical deficiencies of the application of indices. We will examine the community structure of a large set of breeding bird censuses on the basis of the traditional methods and then present four graphic displays of the results of rarefactions (Figs. 1-3) and relative abundance curves (Fig. 4).