COS 123-5
Intrapopulation heterogeneity in territorial quality reduces extinction risk in an endangered territorial bird predator
Variation in the characteristics of individuals that compose populations (i.e. intrapopulaton heterogeneity) is a ubiquitous phenomenon in nature. Such variation may affect key aspects of organisms such as their survival and reproduction rates and it may also dramatically affect population dynamics. Most population models account for intrapopulation heterogeneity according to apparent features of individuals such as sex, age or developmental stage. However, there are a wide range of mechanisms derived from variation in morphology, physiology, behavior and environmental factors that, being more subtle, may generate remarkable intrapopulation heterogeneity and that are not usually incorporated into population models. Consequently, their demographic consequences are still poorly understood. Here, we perform a population viability analysis of the Bonelli’s eagle in NE Iberian Peninsula, a territorial predator endangered in Europe. We explore the consequences on viability predictions depending on whether the models accounted or not for heterogeneity in survival and fertility of territorial birds. Territories showed a high predictability in their location and in their values of local survival and reproductive rates, meaning that overall demographic variance of these rates was lower when heterogeneity was considered. Consequently, we predict that models accounting for territory heterogeneity should predict a more optimistic fate for the population.
Results/Conclusions
We found considerable heterogeneity in local survival and reproductive rates among territories over the period 1990-2013. Overall annual survival was estimated at 0.879 (range: 0.596-0.953; n=58 territories; minimum of 10 individual-years of observations). Overall productivity was estimated at 1.22 fledglings per pair (range: 0.40-1.80; n=59 territories; minimum of 5 territory-years). In 2013, our population was composed by approximately 70 territorial pairs. As expected, predictions on the viability of the population were more optimistic under the scenario of territory heterogeneity than when assuming that territories were equivalent (0.977, CI95=0.961-0.989 vs. 0.967, CI95=0.938-0.982). The probability of quasi-extinction (reaching 5 pairs) in the next 50 years was very low under the two scenarios but ten-fold lower under territory heterogeneity. Temporal variances of both adult survival and productivity were lower under territory heterogeneity. In our study area, heterogeneity in the performance of territorial birds could be caused by differences in either local environmental features, individual quality of eagles that hold territories or both. We suggest that accounting for intrapopulation heterogeneity of apparently equivalent individuals may contribute to a better understanding of the processes shaping natural populations as well as to sharpen the predictions of viability assessments of populations or species of conservation concern.