Local environmental effects and spatial effects in macroecological studies using mapped abundance classes: the case of the rookCorvus frugilegusin Scotland

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Abstract
Summary: The study of the spatial pattern of species abundance is complicated by statistical problems, such as spatial autocorrelation of the abundance data, which lead to the confusion of environmental effects and dispersal. Atlas‐derived data for the rook in Scotland are used as a case study to propose an approach for assessing the likely contribution of dispersal and local environmental effects, based on a Bayesian Conditional Autoregressive (CAR) approach. The availability of moist grasslands is a key factor explaining the spatial pattern of abundance. This is influenced by a combination of climatic and soil‐related factors. A direct link to soil properties is for the first time reported for the wide‐scale distribution of a bird species. In addition, for this species, dispersal seems to contribute significantly to the spatial pattern and produces a smoother than expected decline in abundance at the north‐western edge of its distribution range. Areas where dispersal is most likely to be important are highlighted. The approach described can help ecologists make more efficient use of atlas data for the investigation of the structure of species abundance, and can highlight potential sink areas at the landscape and regional scale. Bayesian spatial models can deal with data autocorrelation in atlas‐type data, while clearly communicating uncertainty through the estimation of the full posterior probability distribution of all parameters.

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