Estimates of Species Richness and Geographic Distribution

In the 254 quadrats sampled in 2001, volunteer observers detected a total of 134 different avian species. Estimates of mean probabilities of occurrence and detection of these species indicate that the community is dominated by species that are rare, not simply difficult to detect (Figure 12.4). For example, average probabilities of occurrence were less than 0.10 for more than half of the 134 species observed in the sample. Given this result, it is not surprising that total richness N of all avian species in Switzerland is estimated to have been 170 (95 per cent credible interval = 151-195), well in excess of the number of species actually observed (Figure 12.5).

An important benefit of our occurrence-based model of the community is that estimates of species richness for individual sample locations are relatively easy to compute. Moreover, these estimates automatically account for the habitat characteristics observed at each location. Figure 12.6 illustrates the posterior distribution of the number of species present at each of 3 different sample locations. At one location, the estimated number of species nearly equals the number of species actually detected; however, at the other two locations, many more species are estimated to be present than were observed in the sample. We can summarize these results by plotting the posterior-predictive mean species richness for each of the 41365 quadrats in Switzerland (Figure 12.7). This map reveals the importance of elevation and forest cover to estimates of species occurrence and richness. Evidently, higher numbers of species are present in forested locations than in unforested ones, and the richness of species also appears to be lowest at highest elevations.

However, these results reflect only general distributional patterns. The estimated relationships between species occurrence and habitat covariates vary greatly among species. As an illustration, Figure 12.8 shows how the estimated average probability of occurrence changes with elevation for each of the 134 species observed in the survey and for Anthus trivialis (tree pipit), Buteo buteo (common buzzard), and Oenanthe oenanthe (northern wheatear) individually. Evidently, common buzzards are more common at low elevations, northern wheatears are more common at high elevations, and tree pipits prefer intermediate elevations.

These results are indicative of the great variety of scientifically relevant quantities that can be estimated quite easily given our conceptualization of the community (Table 12.2). Quantities that are relevant to the entire community of species, to subsets of the community (e.g., species richness at particular locations), or simply to individual species may all be estimated using the matrix of site-specific species occurrences Z and the vector of community membership w.

Figure 12.4. Histograms of average probabilities of occurrence and detection for the 134 breeding bird species observed in the 2001 survey of Switzerland.
Figure 12.5. Posterior distribution of species richness N for breeding birds sampled in the 2001 survey of Switzerland.

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