The successful reintroduction of Bonelli’s eagle on the island of Mallorca
Bonelli’s Eagle -Banded eagle- It is an emblematic bird of prey of the entire Mediterranean that is in decline in central and northern areas of the Iberian Peninsula. On the island of Mallorca, it disappeared in the 1970s as a result of intense human persecution, despite being a key species in the natural environment as a predator of birds and mammals and essential for maintaining the balance of the ecosystem.
In 2011, the government of the Balearic Islands, with the support of the LIFE project, started the reintroduction program for this eagle on the island of Mallorca. Since its inception, the plan has had the scientific collaboration of the Conservation Biology Group of the University of Barcelona and the Biodiversity Research Institute -IRBio-, a team that has developed demographic analyzes to validate the most efficient strategies for reintroduction. The initiative has been a great success and today it is estimated that around 40 Bonelli’s eagles live in Mallorca, including 9 breeding pairs.
The study, published in the journal Animal Conservation, from the Zoological Society of London under the title Raptor reintroductions: Cost-effective alternatives to captive breeding, examines the demographic and economic efficacy of different strategies based on the release of birds of various ages and origins, that is, from captive breeding or free-born. In this sense, the teacher Joan Real, head of the UB Conservation Biology team, explains that “the success of the reintroduction of threatened species into the natural environment depends on several factors, such as the quantity and age of the reintroduced individuals, the quality of the target habitat , the reintroduction methods used, the origin of the reintroduced individuals and, obviously, the biological and ecological characteristics of each species “.
Thus, through a cost-benefit balance, with biological and economic indicators, the experts have compared three methodologies: the breeding of eagles in captivity and the subsequent release of the young specimens in the environment, better known as CaptHack; the release of young specimens born in the wild and later translocated, NestHack; and the release of non-juvenile eagles from rehabilitation centers, WildTrans.
In search of the best strategy
The non-juvenile eagle release system -WildTrans- would have been the most successful strategy in the reintroduction of Bonelli’s eagle in Mallorca. “This system allows the release of sexually mature, or almost mature, and more experienced individuals. This makes it easier for them to reproduce and grow the population more quickly in the first years of reintroduction, which is the most critical period,” he says. Jaume Badia-Boher, member of the UB Department of Evolutionary Biology, Ecology and Environmental Sciences and first author of the article.
The non-juvenile eagle release system-WildTrans- would have been the most successful strategy in the reintroduction of Bonelli’s eagle in Mallorca
Compared to the rest of the alternatives tested, lhe release of non-juvenile specimens is also the most economical strategy in terms of financial and human resources. Furthermore, it is the only option that guarantees the long-term persistence of the eagle population in the contexts of limited logistical resources evaluated.
The island environment of Mallorca could also have favored the results of the aforementioned strategy: “The natural barrier of the sea, in this case, could have limited the tendency of the released eagles to return to the populations of origin, a fact that can occur in species with a high dispersal capacity “, comments the IRBio professor, Antonio Hernández-Matías.
Aquila fasciata, a species particularly threatened by human action
In addition to the aforementioned factors, the complex equation to achieve a successful reintroduction includes good knowledge of the biology and ecology of the species, as well as experience in the management and availability of resources to breed animals in captivity successfully for release them later.
Like most diurnal raptors, these birds reach sexual maturity relatively late and lay few eggs, between one and two chicks per pair per year. The low annual productivity of chicks in turn causes wild populations, as well as reintroduced ones, with a very small number of individuals, to have a very slow growth.
As a consequence, raptor populations are highly sensitive to both natural and human impacts. “All these factors make raptors one of the most threatened animal groups in the world. Therefore, in order to achieve positive results, the reintroductions of these species require years of effort, persistence and sustained releases over time “, the authors point out.
Mortality caused by human action, caused, for example, by accidents with power lines, persecution, accidental or provoked poisonings, etc., further hinders the chances of survival of eagles reintroduced into the environment. “Therefore, before reintroducing a species, it is necessary to carefully evaluate whether the conditions are adequate, which may require the application of measures that reduce the magnitude of the sources of danger caused by man,” they warn from the team.
Conserve and protect biodiversity as a priority action
Biodiversity is reduced day by day in different parts of the entire planet. In this context of biological crisis, improving natural habitats is the first strategy to conserve biodiversity. As a precondition, however, it is necessary to eliminate the factors that affect the viability of the species, the causes of their mortality, food availability, among others.
Releasing animals into a new environment is an ex situ conservation methodology that should be applied only when there are no in situ alternatives to protect biodiversity
Releasing animals into a new environment is an ex situ conservation methodology that should be applied only when there are no in situ alternatives to protect biodiversity. This environmental solution requires meeting several requirements before putting it into practice. Among them, that the species in question has become extinct in the area where it originally inhabited and that the reintroduction process does not alter or endanger indigenous species or key ecological processes.
One of the factors that makes it difficult to assess species reintroduction plans is the lack of long-term monitoring programs for reintroduced populations. Promoting these studies will be decisive to know the population dynamics of the new individuals, evaluate the effectiveness of the measures taken and determine the decisive components in the success or failure of reintroductions in different settings.
And in this sense, the study published in the journal Animal Conservation brings an unprecedented perspective to a field of ecology on which there are still few studies in the natural environment. “Based on our results, we believe that this strategy could be more effective with species in which captive breeding is very expensive, and on islands or with species without high dispersal capacity. However, research in this area and science it is still rare, “the authors conclude.
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