Modelling the distribution of Bonelli's eagle in Spain: implications for conservation planning

Abstract

Bonelli's eagle, Hieraaetus fasciatus , has recently suffered a severe population decline and is currently endangered. Spain supports about 70% of the European population. We used stepwise logistic regression on a set of environmental, spatial and human variables to model Bonelli's eagle distribution in the 5167 UTM 10 × 10 km quadrats of peninsular Spain. We obtained a model based on 16 variables, which allowed us to identify favourable and unfavourable areas for this species in Spain, as well as intermediate favourability areas. We assessed the stepwise progression of the model by comparing the model's predictions in each step with those of the final model, and selected a parsimonious explanatory model based on three variables -slope, July temperature and precipitation -comprising 76% of the predictive capacity of the final model. The reported presences in favourable and unfavourable areas suggest a source-sink dynamics in Bonelli's eagle populations. The fragmented spatial structure of the favourable areas suggests the existence of a superimposed metapopulation dynamics. Previous LIFE (The Financial Instrument of the European Union for the Environment and Nature) projects for the conservation of this species have focused mainly on the northern limit of its range, where the sharpest population decline has been recorded. In these areas, favourability is low and Bonelli's eagle populations are probably maintained by the immigration of juveniles produced in more favourable zones. However, southern populations, although stable, show signs of reduction in productivity, which could menace the population sizes in the whole study area. We suggest that conservation efforts should focus also on known favourable areas, which might favour population persistence in unfavourable areas through dispersal.

Linking density, productivity and trends of an endangered species: The Bonelli's eagle in Spain

“…Reproductive performance was higher in the core provinces than in the peripheral ones, where high favourability areas for the species are scarce and dispersed and most of the territory either is unfavourable or has an intermediate adequacy (Muñoz et al 2005). Population trends were likewise positive in core provinces but negative in peripheral ones.…”

“…This is specially important in the Spanish metapopulation of the Bonelli's eagle as the decrease in pre-adult mortality during the first two years of life was more important for the stabilization of the metapopulation than similar decreases in the values of adult mortality (Soutullo et al 2008). Contrary to these views, international EU-funded conservation projects (LIFE programme) are being normally directed towards peripheral populations in low-suitability areas, and conservation plans typically do not pay enough attention to the dispersal process (Muñoz et al 2005). territorial pair (number of monitored territories in brackets).…”

“…, Figures 1b and 2; Lawton 1993, Sergio andNewton 2003; but see Lesica and Allendorf 1995;Nathan et al 1996) where human impacts need still to be corrected (see Ontiveros et al 2004 for a review of these impacts). The emigration of individuals from healthy populations in core areas would eventually be beneficial to those in the periphery (Muñoz et al 2005). However, a complete conservation strategy should also take into account factors at larger scales such as the presence of areas that attract juveniles during natal dispersal (Cadahia et al 2005;López-López et al 2007).…”

“…In our opinion the conservation efforts should not be restricted to those areas in the periphery of the distribution range following regional interests that are based on the relative rarity of the species within geopolitical units (see also Muñoz et al 2005), which, however, do not necessarily reflect its global rarity or conservation relevance (Rodrigues and Gaston 2002).…”

“…Population trends were likewise positive in core provinces but negative in peripheral ones. According to Muñoz et al (2005), the spatial variation in regional favourability for Bonelli's eagle could be explained by a balance between source-sink or metapopulation dynamics, where adult mortality is the key factor determining the result.…”