Density‐dependent effects on reproductive performance in a recovering population of White‐tailed Eagles Haliaeetus albicilla

Abstract

Understanding the mechanisms that shape density‐dependent processes and population dynamics is often essential for species conservation. Two key mechanisms of density‐dependent reductions in reproductive performance are a limited access to foraging habitats (the habitat heterogeneity hypothesis) and territorial aggression towards conspecifics (the interference competition hypothesis) at high population densities. Disentangling the relative importance of these mechanisms within populations below their carrying capacity is important for the evaluation of the success of conservation measures. However, relatively few studies have attempted to quantify the relative importance of both mechanisms for the reproductive performance of a population. Many raptor populations are ideal model systems to investigate density‐dependent effects because they are currently recovering from human‐induced reductions during the last decades. Using a 14‐year dataset, we combined analyses of individual reproductive performance with a mechanistic population model to investigate early signs of density‐dependent regulation in a population of White‐tailed Eagles Haliaeetus albicilla in north‐east Germany. We found a negative effect of the number of neighbouring breeding pairs and a positive effect of water surface area (as a proxy for the availability of favourable foraging habitat) on breeding success and on the average number of nestlings. The mean nearest neighbour distance between breeding pairs has decreased, and the mean distance of nests to the nearest water body has increased over the last 14 years. Moreover, the population model indicates that even though the population is still growing, carrying capacity could be reached at about 500–950 territorial pairs. These results suggest that the selection of nesting sites is determined by a trade‐off between the distance to favourable foraging habitat and the distance to neighbouring breeding pairs. To avoid increasing competition with conspecifics, due to continued population growth, breeding pairs seem to select increasingly suboptimal habitats. Therefore, our results suggest that the habitat heterogeneity and interference competition hypotheses are not necessarily mutually exclusive as mechanisms of density‐dependent population regulation, but can determine the reproductive performance of a raptor population simultaneously. Thus, a future decline in breeding success does not necessarily reflect a decrease in habitat quality but may rather be a consequence of density‐dependent mechanisms. This information may be useful for the interpretation of population trends and for the development of appropriate management strategies for recovering raptor populations.

Factors affecting population regulation of a colonial vulture

“…Studies on similar systems have mainly reported regulation through IAH (Fern andez et al 1998, Oppel et al 2015, Fern andez-Bellon et al 2016) with a few studies on colonial species supporting the HHH (Kokko et al 2004). In agreement with Heuck et al (2017), our results suggest that the habitat heterogeneity and interference competition hypotheses are not necessarily mutually exclusive, as mechanisms of density-dependent population regulation can determine the reproductive performance of a raptor population simultaneously. The rainy climate of Bizkaia imposed conditions that allowed the HHH to act, whereas in less rainy climates its importance may decrease, leading to the IAH being more important.…”

“…This effect should be considered when modelling future predictions of population trends of Griffon Vultures and implementing actions related to the management of this species. In agreement with Heuck et al (2017), our results suggest that the habitat heterogeneity and interference competition hypotheses are not necessarily mutually exclusive, as mechanisms of density-dependent population regulation can determine the reproductive performance of a raptor population simultaneously. It would be interesting to conduct similar analyses in populations under different weather conditions to elucidate the importance of environmental variability on IAH and HHH interactions.…”

Nest boxes buffer the effects of climate on breeding performance in an African urban raptor

“…We included nearest neighbour distance (NND, in meters, log transformed) as the distances to the closest present neighbour per year (see e.g. [61,62] for recent literature on density dependence in other raptor populations). We chose NND because studies exploring different measurements found the distance to nearest neighbours to be the best predictor for density dependence (e.g.…”

Neighboring group density is more important than forest stand age to a threatened social woodpecker population

“…Positive group-size effects on productivity are so large in many cooperatively breeding birds they can offset negative effects of competition on productivity (Courchamp et al 1999, Meade et al 2010, even in low quality habitat (Cusick et al 2018). This could have important implications for conservation of cooperative breeders with narrow habitat requirements because future declines in productivity may reflect density-dependent competition and small group sizes, rather than habitat degradation (Heuck et al 2017). Therefore, in addition to long-term data on territory quality, identifying factors moderating effects of density on group size and productivity trends for cooperative breeders requires long-term data at the scale of individual territories where groups interact and compete (Brouwer et al 2006).…”