Introduced fauna: how abandoned cats on the island decimate entire populations of endemic birds, reptiles, and rodents

22/07/2021

The gull takes off from the rocky reefs of the island. Down below, the flamingos chirp insistently, as it hurries to search for food in the clear waters of the Gulf of California. A fish is soon caught and gobbled up by the bird; later she will regurgitate it to feed her hungry young. This has always been the case, but within this natural food chain, rats, cats, goats and other species introduced by man are a threatening artificial link.

More than half of Mexico's islands are located in the Pacific Ocean, mainly in the Gulf of California. These insular territories are home to many seabirds, including pelicans, frigates, gulls, cormorants, boobies, and gull-fishers, among others, and are also a refuge for migratory species that use the place during the winter season, or as a resting and feeding site en route, or during nesting periods to lay their eggs on rocky surfaces, vegetation or other substrates, depending on their reproductive habits.

Birds are not the only members of these environments. In the islands of the Gulf of California, there is a great diversity of reptiles, and this taxonomic group is considered to have the highest percentage of endemism in the area, as well as mollusks, crustaceans, insects, amphibians, and mammals that form part of the island's riches and play a fundamental role in the balance of these fragile ecosystems.

The presence of xerophytic scrub as the dominant vegetation is explained by the influence of various factors related to its location in arid regions, although the vegetation can also be green and lusher in places where precipitation is higher, as is the case in the region south of the gulf. Among the most common plants on the islands are choya, biznaga, nopal, sweet pitahaya, as well as other cacti, leguminous plants, and agavaceae. It is estimated that there are between 600 and 650 different species of plants in these territories.

The xerophytic scrub is known for providing shelter to a considerable number of Mesoamerican vertebrates, which places it among the main types of vegetation inhabited by this zoological group. In fact, in Mexico, this vegetation occupies the third place among the habitats of endemic vertebrates and the second place among the habitats of vertebrate species that live in very restricted areas.

Environmental education for fishermen, tourists, and other visitors to the islands is essential to preserve the biodiversity of such special environments.

The marine waters surrounding the islands are extremely productive, and their abundance of fish, shrimp, and other organisms is what attracts such a large concentration of bird populations. These waters are home to mollusks, balans, anemones, starfish, crabs, sea urchins, sponges, sea cucumbers, polychaete worms, and brown, green, and red algae that, together with other organisms, make up the aquatic environment interrelated with the terrestrial ecosystem. Furthermore, the region is home to 35% of the world's known cetacean species, and more than 60% of the North American population of marine mammals.

The geographic location of the islands in the Gulf of California - far from large human populations and shipping routes - and their lack of freshwater, has meant that they have relatively little human disturbance compared to other islands on the planet and, for this reason, their relief and biodiversity have not been greatly transformed. However, some alterations can be worrying, and others have already caused irrevocable damage. Perhaps the worst of all is precisely the introduction of exotic animals, i.e. species that are not native to the area, generally domestic mammals that accompany humans.

There are many examples of extinctions of native animals caused by introduced animals. According to some specialists in the field, in the case of birds alone, these animals are responsible for the disappearance of more than half of the 176 species and subspecies that have become extinct in the world since the 17th century. On the Mexican islands, the situation is alarming. For example, on the island of Guadeloupe, there is a huge population of goats, which were initially introduced at the beginning of the last century. As the goats had no predators on these islands, their population increased, and in a short time, the number of goats was much greater than the island could support.

Another animal introduced to the island is the common cat, which attacks the birds that nest on the ground, some of them due to the loss of forests. It is estimated that these felines have killed at least five species of birds typical of the island, among them two endemic species that are considered extinct: the Guadeloupean scarlet macaw and the Guadeloupean petrel. Mice and dogs have also been introduced to the island, which also have a negative impact on the ecosystem. In 1922 the government officially declared Guadeloupe Island a Sanctuary of Wildlife, but this measure has not been enough to stop the serious biological deterioration of this magnificent place.

With the destruction of island ecosystems, in addition to the extinction of endemic species, important socioeconomic values are lost.

During their evolution, the seabirds, reptiles, mammals, and plants of most of the islands of the Gulf of California were not besieged by predatory mammals and therefore did not develop behaviors to defend themselves against them. Seabirds, which generally have low reproductive rates, late sexual maturation, and long life spans, are very sensitive to these attacks, and nesting times are a time of risk, as their eggs may end up as food for a hungry rat or mouse, or be attacked while in their nests. Few species nest in almost every island, but with huge populations, so the disappearance of a population in a given area is detrimental to the species in general, and if this or any other disturbance is repeated in all the places where the species exists, it can lead to the total extinction of the species.

A strategy to prevent local extinction of seabirds and reptiles

In the islands of the Gulf of California, the exotic animals most frequently introduced by humans are rats, mice, cats, and goats. To eradicate these mammals and protect the native flora and fauna, it is necessary to eliminate in the short term the greatest number of introduced individuals, maintain the eradication program until the last individual is eliminated, carry out the program when the natural food sources of these animals are scarce and generate environmental education programs that disseminate the importance of this eradication, as well as the conservation of the fauna. 

The black rat and house mouse-introduced to Isla Rasa more than a century ago during the extraction of guano-occasionally fed on seabird eggs, dying chicks, crustaceans, ants, beetles, and other insects, vegetation, etc. These mammals eliminated the nesting colony of the Craveri night duck on the island, and preyed on the eggs of the reddish egret, preventing its multiplication. To eradicate these rodents, the toxicant known commercially as Talon was used, which is lethal to small mammals, and which has been used effectively in New Zealand. 

The toxicant was placed in tubes where the rodents could be introduced, but so that it was not directly exposed to the birds and outdoors. At the beginning of the poisoning campaign, the consumption of Talon was high, and by the third week it began to decrease; by the sixth week, no signs of rodents were found in the traps. In subsequent monitoring, no signs of rodent activity were found either, so presumably, the eradication was a success.

It is common to consider islands as a tourist resource, but uncontrolled visits to them can constitute a risk of animal introductions, collection of endemic species, indiscriminate hunting, pollution, etc. The planning of tourist activities, awareness and education programs, and the restoration of damaged ecosystems, together with adequate eradication plans for existing exotic species, will lead to the conservation of these territories and their high biological diversity. The islands are also natural laboratories for education and research to learn about nature. The future of the species that inhabit the islands depends, to a great extent, on the sensitivity and respect that humans now show for such special environments.

By Emma Romeu, Source: Biodiversitas (4) Mexico: CONABIO