Environmental stochasticity in dispersal areas can explain the ‘mysterious’ disappearance of breeding populations

Abstract

We present the results of an individual-based simulation model, showing that increasing the mortality of non-breeding dispersers within settlement areas can lead to the extinction of species and (meta)populations in a subtle way. This is because the areas where dispersers settle are generally unknown or difficult to detect. Consequently, fewer efforts are devoted to the conservation of these sites than to the conservation of breeding territories. Additionally, high mortality rates affecting the floater sector of a population become evident in the breeding sector only after several of years, when it is too difficult or too late to halt the decline. As a result, because most conservation projects on endangered species and populations mainly focus on breeding areas, many current efforts may be wasted in locations other than those in which conservation would be really necessary and effective.

Floater Dynamics Can Explain Positive Patterns of Density‐Dependent Fecundity in Animal Populations

“…A possible scenario accounting for a positive relationship between breeding pair density and fecundity (i.e., the Allee effect; Allee et al 1949) could be the one in which breeder density decreases well below the saturation density of the population due to high levels of floater mortality (see Courchamp et al 1999 for a review of scenarios and factors involved in dependent endogenous factors, that is, dynamic feedbacks affecting population numbers and involving time lags. In fact, the effects of floater mortality appear in the breeding areas only after several decades (Penteriani et al 2005a). Although negative density-dependent fecundity has been previously described for our eagle population (Ferrer and Doná zar 1996), some periods of positive relationship between fecundity and density have been also detected (M. Ferrer and V. Penteriani, unpublished data).…”

“…Using the same procedures as Penteriani et al (2005aPenteriani et al ( , 2005b, we analyzed the effects of floater mortality (ranging from 5% to 30%) on the mean fecundity (i.e., the mean number of fledglings per year, averaged over all pairs in the population) of a breeding population under two different scenarios: (1) when the gradient of the age at first reproduction varied from 3 to 5 years and (2) for saturation levels of 10, 15, and 20 breeding pairs. We explored these ranges of values for age at first reproduction and saturation levels of the population because they best reflect the real scenarios that we observed for the Spanish imperial eagle in Doñ ana (southwestern Spain).…”

“…Because in previous analyses (Penteriani et al 2005a(Penteriani et al , 2005b we highlighted that environmental stochasticity has a stronger influence on the whole population when variations in environmental conditions occur in a synchronous way (i.e., in both the settlement and breeding areas at the same time), the simulations were performed under the less adverse scenario, that is, when the variations in environmental conditions occur asynchronously. An increase in floater mortality could be due to environmental stochasticity (e.g., natural catastrophes and habitat destruction or fragmentation) or human-induced deaths (e.g., poisoning, electrocution).…”

“…Factors affecting the survival of floaters (i.e., dispersing individuals able to enter as breeders into the reproductive population when a breeding territory or a potential matethe owner of a suitable breeding territory-becomes available) also influence the dynamics of the whole population due to their effects on the breeding segment of the population in reproductive areas (Penteriani et al 2005b). Furthermore, an increase in floater mortality within settlement areas can explain puzzling declines or extinctions of breeding populations (Penteriani et al 2005a). In fact, we previously showed that (1) the number of floaters in a population is a function of the fecundity in the breeding territories (i.e., the higher the fecundity, the sooner an equilibrium is attained) and is limited by the availability of settlement areas, (2) environmental stochasticity has a lower impact on breeders when it takes place in a temporally and spatially disjunct way (in either settlement or breeding areas) and therefore affects only a portion of a population, and (3) an increase in floater mortality within settlement areas negatively affects the breeding population, reducing both the mean number of pairs and mean fecundity.…”

Status and habitat changes in the endangered Spanish Imperial Eagle Aquila adalberti population during 1974–2004: implications for its recovery

“…It must also be emphasized that in recent years, a lot of these private lands have made management agreements with NGOs (covering 18.9% of eagle territories found in 2004), and habitat management there has focused on meeting eagles' requirements (González and San Miguel 2004). Promoting this type of management is therefore very important for the survival of non-breeding individuals (Penteriani et al 2005), and also for those breeding individuals that use these areas as secondary communal hunting zones (unpubl. data).…”