May sympatric Lesser Spotted Eagles and Black Storks compete for nesting sites in spatially varying environments

Sympatric species are likely to compete with one another unless there is a low degree of overlap in their resource use, in which case these species are able to coexist. Disclosing of biotic interactions between sympatric species is important from both theoretical and practical perspectives, especially when the species are of conservation concern.  However, environmental heterogeneity may introduce variation in the intensity of biotic interactions due to differential a varying species responses to the environmental gradient. In this study, we analysed the overlap in nesting sites between the internationally protected, mature, forest-dwelling Lesser Spotted Eagle Clanga pomarina and the Black Stork Ciconia nigra. The importance of landscape heterogeneity for habitat segregation between these species was also assessed. The nesting sites of 123 pairs of Lesser Spotted Eagles and 78 pairs of Black Storks, located across different landscapes of the Central, Central-Eastern and Eastern Lithuanian ecoregions were described. A series of discriminant analyses were performed to explore the pattern of habitat differentiation between species nesting in the above-mentioned regions. The habitat differentiation was estimated by niche overlap values (range 0-1, with a value 0.6 suggested to be the threshold between coexistence and competition). Comparison of nesting sites of mature forest-dwellers resulted in the niche overlap values for Central, Central-Eastern and Eastern regions being 0.5, 0.63 and 0.55, respectively. These results indicated relatively high niche overdispersion between nesting sites occupied by eagles and storks. Different variables and/or their combinations resulted in habitat differences in each ecoregion. Our data indicate that biotic interaction between species is mediated by environmental heterogeneity. Although our data tend to support the coexistence of the Black Stork and the Lesser Spotted Eagle, in certain regions these mature, forest-dwelling predators may use similar habitats and compete for prime sites under specific landscape structures. We, therefore, propose the necessity of the importance of a spatially-segregated estimation on biotic interactions when developing conservation programmes and allocating conservation actions within the target region.