Land use, soil and climatic factors associated with the demographic fragmentation of the rook (Corvus frugilegus L.) population in Scotland
This study investigates the influence of autoecological factors both on the spatial pattern of abundance, and on the demographic fragmentation, of the breeding population of the Rook (Corvus frugilegus L.) in Scotland. Two scales of analysis are considered, namely the regional scale (Scotland) and the landscape scale (Ythan catchment area). The study focuses on the investigation of how land use factors interact with other environmental variables. Patterns of association of the breeding density with land cover, climatic and soil factors are searched for and statistically examined. Soil factors, in particular, have been investigated in detail for the first time, because of their potential to influence the population of earthworms, the main prey for the Rook during the breeding season. A geostatistical simulation is used to address the problem of data spatial autocorrelation. Several models are integrated in a GIS environment to develop an index of potential energy intake (IPI), aimed at reflecting the long term seasonal availability of invertebrate prey (mainly earthworms) on grasslands. The results are scale dependent. At the regional scale, they indicate that the most important variables influencing the demographic fragmentation are the availability of grasslands and soil moisture. It is shown that this translates into potential availability of spring and summer food resources, in the form of soil invertebrates. The results indicates that, at least in Scotland, the decline in quality of grasslands as a habitat for soil invertebrates, rather than that in winter cereal stubble, might have been associated with the past Rook population decline. At the landscape scale, no difference was found between IPI values around colonies, versus IPI values around wooded areas devoid of colonies, or between IPI and number of nests. It is hypothesised that conspecific attraction contributes to decoupling between resources and breeding density at the landscape scale. The results of this study support the hypothesis of Brown et al. (1995) that the distribution of a species might be understood in terms of the extent to which local environmental conditions meet the niche requirements, but highlight the importance to account for the scale at which such requirements are assessed.