Species co-occurrence and environmental factors and their effect on the distribution of forest birds in mature forests

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Abstract Measures of species co-occurrence, as a proxy of biotic interactions, may be as important as environmental factors when explaining the distribution of many species. We assessed the relative effect of both species co-occurrence and environmental factors in predicting the occurrence (presence/absence) of woodpeckers and passerines in mature forests of the Northern Iberian Peninsula. We hypothesized that woodpecker occurrence would affect the occurrence of passerines. We calculated the occurrence of woodpecker species using models that included environmental factors aggregated at three spatial scales (200, 500 and 1,000-m2). The occurrence of passerine species was then modelled by using either species co-occurrence (including the occurrence of woodpecker species) or environmental factors at the 200-m2 cell scale. The occurrence of five out of nine passerine species was best explained by the co-occurrence of woodpeckers alone, whereas the occurrence of the remaining passerines was best explained using a combination of species co-occurrence (i.e. woodpeckers and/or other passerines) and environmental factors. In Southern Europe, woodpeckers are expanding their distribution in line with forest maturation, and this could increase the diversity of bird communities in forest systems.