A probability-based analysis of temporal and spatial co-occurrence in grassland birds

Aim To test for non-random co-occurrence in 36 species of grassland birds using a new metric and the C-score. The analysis used presence-absence data of birds distributed among 305 sites (or landscapes) over a period of 35 years. This analysis departs from traditional analyses of species co-occurrence in its use of temporal data and of individual species' probabilities of occurrence to derive analytically the expected co-occurrence between paired species. Location Great Plains region, USA. Methods Presence-absence data for the bird species were obtained from the North American Breeding Bird Survey. The analysis was restricted to species pairs whose geographic ranges overlapped. Each of 541 species pairs was classified as a positive, negative, or non-significant association depending on the mean difference between the observed and expected frequencies of co-occurrence over the 35-year time-span. Results Of the 541 species pairs that were examined, 202 to 293 (37-54%) were positively associated, depending on which of two null models was used. However, only a few species pairs (< 5%) were negatively associated. An additional 89 species pairs did not have overlapping ranges and hence represented de facto negative associations. The results from analyses based on C-scores generally agreed with the analyses based on the difference between observed and expected co-occurrence, although the latter analyses were slightly more powerful. Main conclusions Grassland birds within the Great Plains region are primarily distributed among landscapes either independently or in conjunction with one another. Only a few species pairs exhibited repulsed or segregated distributions. This indicates that the shared preference for grassland habitat may be more important in producing coexistence than are negative species interactions in preventing it. The large number of non-significant associations may represent random associations and thereby indicate that the presence/absence of other grassland bird species may have little effect on whether a given focal species is also found within the landscape. In a broader context, the probability-based approach used in this study may be useful in future studies of species co-occurrence.