The ageing of animals jeopardises the conservation objectives of zoos

It is a very high-quality article. It uses very accurate population pyramid data (since the age of animals in zoos is known with absolute certainty) and analyses a large number of species, making the results very reliable.

It is empirical evidence of what has been widely known and debated for several decades in the zoological sector. In zoological environments, animals live much longer, and this creates an altered population pyramid (since the population is controlled so as not to compromise the welfare of the animals due to lack of space). In nature, predators, parasites, lack of resources, and disease keep the population in balance and prevent a high proportion of geriatric animals. The problem is that this altered population pyramid in zoos reduces their conservation potential and jeopardises their positive impact on conservation. The constant increase in species with conservation problems puts further strain on the situation and means that the space available in zoos is increasingly in demand for new species.

From the point of view of biodiversity conservation, the role of zoos is very clear. Both the International Union for Conservation of Nature (IUCN) and the Convention on Biological Diversity have recognised that zoos and captive breeding play an essential role in combating the biodiversity loss crisis. The IUCN has even called on zoos and aquariums to redouble their efforts in the fight against what is already being called the sixth great mass extinction. On the other hand, the European Zoo Directive and the Spanish Zoo Law make the role of zoos and aquariums in biodiversity conservation crystal clear.

This article highlights the biodiversity conservation problems that can arise if indiscriminate bans on population control in zoos become widespread due to the extreme ageing of their populations. In some countries, such as Spain, regulations prohibiting euthanasia for population control have been enacted, driven by animal welfare organisations with an urban philosophy. This situation is viewed with great concern in countries such as Germany, Denmark and the Scandinavian countries, as it will jeopardise the ability of zoos to maintain genetically diverse populations capable of producing individuals for reintroduction.

The real problem is reconciling the needs of animal population management to maximise their capacity for biodiversity conservation with the demands of animal welfare organisations, whose philosophy is opposed to captivity. This article provides clear evidence supporting the need for zoo population management based on biodiversity conservation objectives.

Zoos must act as key players in conservation efforts. However, the ageing of the animals that inhabit them is a problem that hinders this work. Many zoos act as reservoirs for species that are threatened in the wild, as well as being key to numerous research and educational tasks. In this sense, the article is part of the current debate on the role of zoos as providers of a public conservation service, beyond their recreational function. However, because animals in captivity tend to live longer than those in the wild, due to the success of animal care, the high number of animals in zoos is beginning to jeopardise their ability to accommodate new ones. For this reason, population control is sometimes carried out through reproductive measures such as castration or sex segregation, leading to an inverted pyramid where older individuals predominate, to the detriment of younger ones. The study shows that this strategy, although understandable from a logistical and individual welfare point of view, is problematic in the long term from a conservationist perspective.

This study provides strong evidence based on data from a total of 361 and 413 mammal populations in North America and Europe, respectively, over 53 years (1970 to 2023), with the aim of studying their demographic trends. This lends great robustness to the study, as monitoring of this calibre is rare in demographic studies. The use of an internationally standardised database (Species360) and consistent analysis of multiple taxonomic groups reinforce the validity of the patterns observed. It is noted that as time progresses, the population pyramid takes on a diamond shape, with an increase in the average age of the population over time. Furthermore, this change occurs regardless of the conservation status of the species or the conservation strategy. This is also accelerated in North America, probably due to the smaller size of zoos in that area, although a pyramid shape is observed in years when conservation strategies have been implemented. However, the authors' predictions are based on theoretical scenarios and do not take into account causes of mortality, genetic information or the actual capacity of each zoo.

The authors do not merely expose the problem, but also provide a broad discussion of future management. They observe a decline in reproduction, leading to the disappearance of various populations studied. In addition, historical trends indicate that zoos have not managed species according to their priority based on their conservation status. As a result, ageing populations lose their capacity for renewal and resilience, calling into question their role as self-sustaining populations within ex situ conservation programmes.

The authors point out how harmful it is to prevent births without taking into account the need for reproductive replacement. For zoos to play a key role in species conservation, the promotion of reproduction and the management of older animals must be taken into account. This brings with it the need to broaden the debate about the strategies that should be implemented to achieve conservation objectives, even when these strategies may be controversial. Keeping animals in captivity requires a purpose that transcends mere exhibition, based on clear conservation and population sustainability objectives.