Invasive Species

One of the most signigicant threats to the wildlife of the Galapagos islands, particularly Santa Cruz and Santiago, is the introduction of non-native or invasive species. Such species can compete with native species for food or habitat, expose the ecosystem to foreign diseases, and also prey on native species (The Galapagos is especially vulnerable in this regard because of its natural absence of ground predators)¹ Since the early 1600s, when whalers first introduced rats, dogs, and cats to the Galapagos, humans have been responsible for introducing at least 11 new mammals and over 300 exotic plant species.² The following species are those that have had the most significant impact on Santa Cruz and Santiago Islands.

Goats: During the War of 1812, while sailing through the Galapagos, Captain David Porter on the Essex stopped at Santiago Island and released four goats to graz^2.5e. They all escaped and soon reproduced without control of man or natural predators. By 1984 the population already exceeded 100,000³. Goats on Santiago eat nearly all vegetation, including young tree saplings, and have been responsible for turning highland forests into pampa-like grasslands. With no natural predators, their population is only controlled by the size of the island and food availability (though goats are so adaptable they have even been observed drinking sea water in times of drought)³.

Dogs: Dogs were introduced to Santa Cruz Island around 1868, when settlements and convict colonies were abandoned and canine pets escaped into the wild (such species are considered feral when they are wild but originally descendants of domestic ancestors). In 1984 the feral population was between 25 and 70 dogs, which had formed two groups: Coastal and Highland dogs. Highland dogs live in packs of 7 or 8 and feed on feral cattle. Coastal dogs live in packs of 2 or 3 and are much more harmful to native wildlife, feeding on marine iguanas, fur seals, blue-footed boobies, young tortoises, and penguins. In fact, during several days in the late 1970s packs of Coastal dogs attacked and killed over 500 marine iguanas on Santa Cruz, leaving the uneaten bodies to rot⁴. Overall, coastal dogs have reduced the marine iguana population by over 15%, while completely eradicating marine iguanas from the southern portion of Santa Cruz³.

Pigs: Pigs, notable for causing the extinction of the dodo bird in the Galapagos in 1681, weren't introduced to Santiago and Santa Cruz until the 1920s and 1930s, though they have still had a significant negative impact. Studies have shown they are more likely than any other mammals to be vectors of contagious diseases³. They also destroy vegetation, dig up and eat tortoise/ground-bird eggs, and prey on native insect larvae². On Santiago Island pigs have actually been observed taking eggs of sea turtles as they were being laid, . Not surprisingly, turle egg hatching rates have dropped from 80% to 3% as a result. As of 1987 the pig population on Santiago was between 2,000 and 3,000 and, with no natural predators, unlikely to decrease.

Black Rats: Black rats have been on Santa Cruz since 1934 and Santiago since 1835, arriving as stowaways on merchant and cargo ships³. On Santa Cruz they have caused the extinction of endemic rice rats, and on both islands are known to kill and eat tortoise hatchlings and nesting dark-rumped petrels². Black Rats are especially harmful because they can survive in all seven life zones of Santa Cruz and Santiago.

Cats: Feral cats are widespread on Santa Cruz, feeding on native birds, insects, crabs, and lizards³. However, their primary food source is black rats, which makes them somewhat helpful in controlling invasive rat populations.

Fire Ant: On Santa Cruz Island the fire ant is not only a nuisance to people, but also harmful to native species: it feeds on native ants, snails, insect larvae, grasshoppers, beetles, and spiders³.

Cinchona: Cinchona, the source of quinine, is very abundant on Santa Cruz, covering 30%-40% of the highlands. As an invasive species it replaces native sedges and ferns, which provide habitat and protection for the Galapagos Rail. One study, conducted from 1986-2000, found that the probability of detecting rails on Santa Cruz fell from 36% to 27% as a result of the spread of Cinchona. The study predicts that the entire highland zone will be covered by the year 2020, leaving the Galapagos Rail and possibly other native species with little or no habitat⁵.

Other less significant invasive species on Santiago and Sana Cruz include feral donkeys and horses, which compete with tortoises and iguanas for food and trample nesting sites; frogs, which may reduce native insect populations and endanger native predators with poisonous skin⁶; feral cattle; feral sheep; mice; and Rock doves.

Several plans to deal with these invasive plans have already been implemented with partial success. The Isabella Project, initiated in 2000, established a quarantine system to prevent the introduction of new species, and took steps to eliminate existing invasive species. Hunters trained are in New Zealand and use their skills to begin shooting pigs on Santiago Island⁷. Also on Santiago, large areas are being fenced in to protect vegetation from feral goats.

In 2001 the Charles Darwin Reasearch Station on Santa Cruz Island began a six-year, $37 million project to eradicate alien species using the following techniques: Sharp-shooters use GPS systems to track goat herds and eliminate them. Cats are radio-collared to locate those that are hiding. High-powered air guns are used to kill rock doves (abundant on Santa Cruz) and Smooth-Billed Anises. Poisons and bush clearing are used to eradicate fire ants. Elevated bait stations are used to catch black rats but not native rice rats. Herbacides are sprayed to eradicate cinchona seedlings. Farmers are encouraged to grow more produce to reduce the amount of imported produce. Finally, a ban is placed on impoorted goods that could potentially introduce new species⁸.

Invasive species are still a significant threat to the native wildlife of the Galapagos Islands, especially Santiago and Santa Cruz, but their impact is being gradually mitigated by these new plans.