The Galapagos Islands

Introduced Animals and Plants


"As natural selection acts by competition, it adapts the inhabitants of each country only in relation to the degree of perfection of their associates; so that we need feel no surprise at the inhabitants of any one country, although on the ordinary view supposed to have been specially created and adapted for that country, being beaten and supplanted by the naturalised productions from another land".

Charles Darwin,

Voyage of the Beagle, 1835.



From the time that the first of the Galapagos Islands began to emerge from the ocean over five million years ago, plants and animals from different parts of the Pacific and Caribbean started arriving and establishing themselves on land and in the water within and around the Archipelago.


Those species that flourished in this new environment found a balance that led to a harmony within this particular group of animals and plants. With the arrival of each new species small changes occurred and with the passing of time the balance returned. This equilibrium was maintained until the arrival of another species, in 1535, that of man.


Not only was man himself to upset this fragile state by activities such as the over harvesting of species like the fur seals and giant tortoises; but the domesticated species that arrived with him, (as well as those animals that hitched rides on the ships that came to the islands), began to invade, wreaking havoc and causing irreparable damage to much of the native and endemic flora and fauna.


With the first visitors to the islands came black rats, and pigs, goats, cats, dogs, donkeys, cattle, and horses soon followed these. Nevertheless, introductions were not only limited to animals. Highly aggressive plant species, in particular, the fruit, Guayava, the Quinine tree, Elephant grass, Blackberry, and the Hedge bush, have also been responsible for habitat invasion and destruction, particularly on the four inhabited islands, Santa Cruz, Isabela, San Cristobal and Floreana.


Black rats and subsequently Norwegian rats have been responsible for the disappearance of several of the native and endemic populations. Rats are cannibalistic and are almost certainly the reason for the extinction of the endemic rice rat populations from some of the islands. They are also known to prey upon the eggs and young of the birds, as well as those of the reptiles, marine and land iguanas, and giant tortoises.


Cats prey upon the young of some seabirds, but more frequently feed on the fledglings of the finches, flycatchers, warblers, doves, and mockingbirds. In many areas, these land bird populations have adapted to the presence of cats and as a result are far more timid and difficult to approach than those populations living in areas free of such introduced species. Dogs have been responsible for the demise of great numbers of land and marine iguanas, killing both young and adults. Populations such as that of the land iguanas inhabiting the area of Cerro Dragon on the northwest coast of the island of Santa Cruz were almost wiped out until intervention by the National Park Service and Darwin Station personnel saved them. Pigs are perhaps the most destructive of all the introduced species, preying upon the eggs and young of seabirds, as well as of many reptile species. In the islands where pigs have been present or are still found today the population numbers of giant tortoises, land and marine iguanas and particularly marine turtles have been seriously affected. Hatching rates of green sea turtles from beaches in Santiago were less than 1%, as the nests were systematically dug up and the eggs eaten by pigs, until the National Park Service and Darwin Station personnel began protection programmes. Galapagos or Dark-rumped petrels, Pterodroma phaeophygia, were brought to the brink of extinction as their nesting burrows in the highlands of the islands were dug up and the eggs, hatchlings and adults eaten. As long ago as 1835 Charles Darwin noted the presence of both goats and pigs on Floreana.


Goats cause massive destruction of habitat, causing the decline in the populations of land iguanas and tortoises and changes in the nesting areas of some bird species. They reproduce rapidly, colonising any island where they are introduced. Having no natural predators in the Galapagos they spread unchecked, devouring all vegetable matter to a height of around 4 ft. / 120 cm. On the rim of Alcedo volcano, Isabela Island, where their numbers reached critical levels, there developed a great deal of concern for the tortoise population of this volcano. They had even been seen climbing on the shells of the tortoises so as to be able to reach higher up to the more succulent buds of the forest canopy.


Donkeys, horses and cattle, although non-native species, do not appear to cause such high levels of damage as the other species mentioned above. They are not as fast at breeding and their numbers can be controlled more easily. They do cause some erosion and are responsible for the dispersal of the seeds of introduced plants such as the "guava" that is spread in the dung of the cattle.


The problem does not end with the larger animals. Galapagos has many other introduced species such as smooth-billed anis, chickens, pigeons, frogs and geckos as well as numerous invertebrates, including spiders, cockroaches, fire ants and wasps. The latter are obviously virtually impossible to control and their entry into the islands goes undetected as large quantities of produce is shipped from the mainland to satisfy the needs of the local population. Moreover, once in the islands, their eradication is unlikely. With the recently National Park implemented system of quarantine, the numbers of invasive species arriving to the islands has been reduced. But if awareness is not created amongst the local population, as to the gravity of the problem of introduced species, the situation can only get worse.


Of the many plant species that have been brought into the islands, the four species referred to at the beginning of this section are perhaps considered the most damaging, for two reasons. One is their ability to disperse their seeds; the second is their resistance to attempts at eradicating them. The Quinine plant for example may produce up to 500 000 seeds at a time and the mechanism for dispersal is wind. When cut down by machete multiple shoots spring from the stump. Chemical controls such as herbicides are ineffectual unless injected directly into the plant, and this only works if the concentration of herbicide is very high. The only control that is both practical in terms of cost and effect is that of manually pulling the plants one by one. Even so, this is expensive in terms of money and manpower. 


Eradication, the Solution... the only real answer to the problem of introduced species is their control and eventual eradication. Whilst they remain within this environment the native and endemic species will never regain the balance that existed prior to these introductions. For the larger mammals, the only process that can be used at present is that of hunting. Chemical and biological controls are either unavailable or simply not desirable due to the complications these can cause. Even so, shooting the animals is both costly in time and money. By the mid 1990 the estimation for the eradication of goats from Isabela was estimated at over ten million dollars and rising. It wasn’t until the beginning of the new millennium that eradication work began in earnest. The “Isabela Project”, as it was termed, began, using helicopters to transport hunters to areas previously deemed inaccessible, and provided a platform for aerial hunting. These techniques, at first perfected on the island of Santiago, proved so successful that a mere five years after its conception the Isabela Project was completed and the islands of Santiago and Isabela declared “goat free”. This followed an earlier major victory, which was the elimination of pigs from Santiago, officially in 2001. Both these projects demonstrate that given adequate funding, the Galapagos National Park Service together with the Darwin Station is capable of great success in the eradication of introduced species.


In those areas where goats have been totally eradicated or the populations greatly reduced the recovery of the native and endemic vegetation species is a rapid one. To diminish the possibility of these plant species becoming extinct in those areas where goats are particularly prevalent, fenced quadrants have been set up, both to protect small areas from the goats and to act as a seed bank for the future, aiding the natural re - colonization of the island by the vegetation after the goats have finally been removed.


In the case of the rats, poisoning, using a mammal specific poison is the only option as trapping would be far too slow a method. Eradication on the larger islands is still not considered feasible, as the areas involved are too extensive. The only island so far from which rats have been eliminated is Bartolome, in 1976. For the time being control of rats has to be on a small, localized scale. Eradication programs are concentrated around nesting areas of reptiles and seabirds during the nesting season or all year round if there is no fixed seasonal nesting period. Thus eggs and later the hatchlings of those species most affected by rats can be afforded protection during these critical first few weeks or months of their life.


Cats and dogs are also controlled on a localized scale, although with a concerted effort dog numbers can be greatly reduced as was done on San Cristobal, Floreana, Northern Santa Cruz, and Southern Isabela. Both hunting and poison are used.


If the Galapagos Islands are to retain their uniqueness and the native and endemic species are to continue to survive and flourish then the problems caused by introduced species must be resolved. Eradication programs must continue and stricter control measures need to be implemented to prevent further spreading of these species and to prevent them being reintroduced into an island once they have been eradicated. Only then shall the balance return.

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