The role of age of first breeding in modeling raptor reintroductions

Abstract

The present biodiversity crisis has led to an increasing number of reintroduction programs, and this conservation method is likely to be increasingly used in the future, especially in the face of climate change. Many fundamental questions in population ecology are focused on the mechanisms through which populations escape extinction. Population viability analysis (PVA) is the most common procedure for analyzing extinction risk. In the use of PVA to model the trajectories of reintroduced populations, demographic values are sometimes taken from other existing wild populations or even from individuals in captivity. Density dependence in productivity is usually considered in viability models, but density‐dependent variation in age of first breeding is usually ignored. Nevertheless, age of first breeding has a buffering effect on population fluctuations and in consequence on population persistence. We simulated the viability of Spanish Imperial Eagle ( Aquila adalberti ) and Osprey ( Pandion haliaetus ) populations using data from established and reintroduced populations in southern Spain. Our results show that reduction in the age of first breeding is critical in the success of reintroductions of such long‐lived birds. Additionally, increases in productivity allow populations to growth at maximum rate. However, without considering variation in age of breeding, and the associated increasing overall productivity, reintroduced populations seem nonviable. To ignore density dependence in age of breeding in PVA means that we are seriously limiting the potential of the model population to respond to fluctuations in density, thereby reducing its resilience and viability. Variation in age of first breeding is an important factor that must be considered and included in any simulation model involving long‐lived birds with deferred maturity.

Density‐dependent age of first reproduction as a key factor for population dynamics: stable breeding populations mask strong floater declines in a long‐lived raptor

“…Despite its clear relevance, empirical data about the variation in age of first reproduction in animal populations are often severely limited and therefore this aspect is also often ignored in demographic simulations and population viability analyses (PVAs) (cf. Ferrer, Otalora, and García‐Ruiz, 2004; Morandini et al., 2019; this study). The reliability and robustness of PVAs, however, depends critically on the assumptions made and many PVAs need to be improved in this regard (Chaudhary and Oli, 2020; Pe’er et al., 2013).…”

“…Antor et al., 2007; Furness, 2015; Langvatn et al., 1996). Especially for long‐lived vertebrates with age‐structured populations, variation in the age of first reproduction can substantially affect the total number of reproducing individuals and thus overall productivity (Ferrer, Otalora, and García‐Ruiz, 2004; Morandini et al., 2019). We hypothesize that density‐dependent variation in age of first reproduction can buffer breeding population size, potentially masking total population declines.…”

“…Especially for larger birds, like raptors, it is evident that a density dependence in the age of first reproduction is also directly linked to the size of the non‐breeding (‘floater’) part of the population (Antor et al., 2007; Elliott et al., 2011; Ferrer, Otalora, and García‐Ruiz, 2004; Morandini et al., 2019; Ortega et al., 2009; Penteriani, Otalora, and Ferrer, 2006), and a better understanding of floater dynamics is essential for conservation and management of these bird populations (Carrete, Donázar, and Margalida, 2006; Hunt, 1998; Penteriani, Ferrer, and Delgado, 2011). Despite its clear relevance, empirical data about the variation in age of first reproduction in animal populations are often severely limited and therefore this aspect is also often ignored in demographic simulations and population viability analyses (PVAs) (cf.…”

Raptor reintroductions: Cost‐effective alternatives to captive breeding

“…As predicted, recruitment probabilities were considerably higher for WildTrans eagles than for hacking-released conspecifics, to the extent that half of the WildTrans birds joined the pool of potential breeders each year during the first 2 years following release. As such, shortening the time until reproduction of released birds accelerates a shift towards a population largely composed of wild individuals born in the target area, which can contribute greatly to reintroduction success (Evans et al;Morandini et al, 2019). As shown by Sarrazin and Legendre (2000), this can be achieved by releasing adult captive-bred individuals, but according to our results, translocating wild individuals arises as an effective alternative that avoids the high economic costs of captive breeding until adulthood.…”

Age at first reproduction in wolves: different patterns of density dependence for females and males

“…When the population approaches carrying capacity and resources become limited for survival and/or reproduction, fitness decreases, with age at first reproduction being positively related to population density. Thus, the success of recolonization or reintroduction of species with initially low densities, possibly experiencing Allee effects, depends partly on age at first reproduction [ 16 ].…”