Fluctuations related to rainfall in richness and abundance of birds of steppic Mediterranean habitats
Published: Volume 52(1), June 2005. Pages 53-66.
Original Title: Fluctuaciones relacionadas con la precipitación en la riqueza y abundancia de aves de medios esteparios mediterrÁneos
Aims: The existence of interannual variations in the richness and abundance of bird species breeding in pseudo-steppic Mediterranean areas is analysed and related to the important fluctuations that take place in rainfall, which in turn deeply influence vegetation growth. The hypothesis of food limitation as a key factor in determining population sizes, allows the prediction of the existence of positive correlations between these parameters.
Location: The study was carried out in the Extremadura region, south-west Spain. Seven areas of pseudo-steppes were selected, characterized by large treeless plains devoted to un-irrigated cereal cultivations and/or extensive grazing grasslands. Climate is typically Mediterranean and semi-arid, with annual rainfall around 400-600 mm, mild and relatively wet winters, and very dry and hot summers.
Methods: Bird communities were surveyed by means of point counts, placed along fixed routes which were repeated every year in the same spring dates (14 to 27 point counts in each locality). Surveys were conducted in five different years, three of them consecutive. Data regarding changes in rainfall were obtained from meteorological stations placed in or very close to the study areas.
Results: Cumulative rainfall during the months preceding breeding bird censuses (January to April) showed significant inter-annual variations. Densities of several breeding bird species also exhibited important changes along the study period, especially noticeable among grassland or steppic species (those nesting in the ground), where they were statistically significant for most of the common species. Fluctuations among years of bird densities usually took place in parallel to those of rainfall. Average richness was positively correlated to rainfall in the whole of the breeding bird community, as well as in the steppic birds group, but it was not correlated to rainfall in the non-steppic birds group (species usually nesting in buildings or trees). Analysis of the variance by species allowed us to differentiate in the inter-annual variations the effects of years and localities. In only three steppic species there were significant changes mainly explained by differences between years and at the same time, showing a positive significant correlation with rainfall (Quail Coturnix coturnix, Fan-tailed Warbler Cisticola juncidis and Corn Bunting Emberiza calandra). On the other hand, there were species apparently not affected by rainfall, among them all or most belonging to the non-steppic group.
Conclusions: The parallel and spatially synchronic fluctuations of cumulative rainfall, indices of vegetation growth, and densities of several grassland bird species, agree with a limiting role of the resources linked to vegetation, whose marked reduction during dry years would impose reductions in the population sizes of those species. Nevertheless, changes in rainfall and plant productivity do not affect many other bird species, among them all or most in the non-steppic group, making difficult to assume that the limiting factors are directly related to food (although insectivorous species may have fluctuated on average more than seed-eating species). An alternative or complementary explanation may link the decreases in abundances of certain species to the simplification taking place during dry years in the habitat structural complexity.