Theories focusing on the relationship between local abundance and regional occupancy are among the oldest and most well studied in ecology (Brown 1995; Gaston et al. 2000). Charles Darwin (1859) first noted that ‘... the dominant species – those which range widely, are the most diffused in their own country, and are the most numerous in individuals ...’. Over the past two decades, Darwin's observation has been restated as the abundance–occupancy rule and predicts that species of low local abundance are the most localized throughout a region and those of high abundance are the most widespread (Gaston et al. 2000). Although abundance–occupancy relationships are generally positive across diverse taxa (Blackburn, Cassey & Gaston 2006), recent studies have documented counter-examples and variations where individual species and entire assemblages do not always exhibit positive abundance–occupancy relationships (Fuller et al. 1995; Chamberlain & Fuller 2001; Blackburn et al. 2006; Reif et al. 2006; Symonds & Johnson 2006; Van Turnhout et al. 2007; Webb, Noble & Freckleton 2007). These counter-examples raise questions about the consistency of abundance–occupancy relationships for various species, communities and geographical regions.
The abundance–occupancy relationship exists in two forms: the interspecific and intraspecific comparison. Interspecific studies involve an examination of abundance and occupancy for many species during a single time period (Gaston 1996; Gaston, Blackburn & Gregory 1997b; Gaston et al. 1997c), while intraspecific studies focus on a single species or group of species, over time (Cade & Woods 1997; Blackburn et al. 1998; Gaston, Gregory & Blackburn 1999; Webb et al. 2007). Although intraspecific relationships are often positive, studies focusing on changes in large-scale population dynamics have found a high degree of variability among species and assemblages (Gaston et al. 1999). This variability is often attributed to the narrow range of variation observed in annual estimates of abundance and occupancy over limited time periods (Gaston et al. 2000). Less common are intraspecific examinations involving large groups of species over long periods of time (e.g., decades). In these types of studies, annual estimates can be replaced by long-term trend estimates. These trend estimates should more effectively describe intraspecific relationships because they mask the annual variation in abundance and occupancy data and counteract the effect of time-lags. A primary constraint in conducting such studies, however, has been the availability of regional data bases for many species spanning multiple years or decades.
Atlas surveys yield one type of regional occupancy data used for documenting the distribution of species and offer an opportunity for examining abundance–occupancy relationships in large-scale population dynamics (Donald & Fuller 1998; Gibbons et al. 2007). Atlas data are increasingly used for purposes of conservation planning and policy (Bishop & Myers 2005), assessing species-habitat associations (Trzcinski, Fahrig & Merriam 1999; Gates & Donald 2000), selecting areas for preserves (Araújo & Williams 2000; Araújo, Williams & Fuller 2002), and documenting range and population changes and their possible causes (Chamberlain & Fuller 2001; Donald, Green & Heath 2001; Donald et al. 2006; Van Turnhout et al. 2007). Given the increasing importance of atlases in ecology, there is a need to examine the consistency and temporal stability of abundance–occupancy relationships in measuring long-term, large-scale population change.
Although the strength and form of the abundance–occupancy has been found to vary across different ecological systems (Blackburn et al. 2006), relatively few studies have addressed this relationship using atlas data due to the rarity of repeat atlases (Gibbons et al. 2007). Those that have reported inconsistent results and offer a diversity of recommendations for using occupancy data in assessing large-scale population dynamics (Fuller et al. 1995; Böhning-Gaese & Bauer 1996; Donald & Fuller 1998; Chamberlain & Fuller 2001; Symonds & Johnson 2006; Van Turnhout et al. 2007). Böhning-Gaese & Bauer (1996) found that although changes in the occupancy and abundance of 151 species were highly correlated over a 10-year period, atlas data alone would not have identified large declines in abundance for entire assemblages of species (i.e., migratory groups). Similarly, in their analysis of Britain's two Breeding Bird Atlases, Chamberlain & Fuller (2001) found that 20 species of farmland birds showed no evidence of range contraction despite significant declines in abundance and further warned that a reliance on occupancy data alone could provide a misleading impression of large-scale population dynamics. In contrast, a study of two national atlases in the Netherlands found that 80% of 157 bird species showed similar changes in their abundance and occupancy over a 25-year period and concluded that, given adequate coverage, it is possible to assess population change using atlas data (Van Turnhout et al. 2007). The prominence of abundance–occupancy relationships provides a theoretical underpinning for using atlases to describe population dynamics, but as the preceding studies suggest, examples where no positive abundance–occupancy relationships were found do exist.
Our objective was to test the predictions of the abundance–occupancy rule using two independent data sets, the New York State Breeding Bird Atlas and the North American Breeding Bird Survey. The use of these two independent surveys offers a test of concurrence for detecting large-scale and long-term avian population change in New York State. We predicted that: (i) a positive interspecific relationship existed between abundance and occupancy in two separate time periods (1980–85 and 2000–05), (ii) a positive intraspecific relationship existed between changes in occupancy and abundance over the 20-year sampling period, and (iii) these relationships were stable over time and consistent between breeding habitat and migratory groups. The predictions of the abundance–occupancy rule provide a theoretical foundation for examining these patterns and evaluating the usefulness of occupancy data in describing regional population dynamics.