Whalers exploited and drastically reduced the numbers of these unique tortoises in the 19th century. Recently, tortoises suffered from new threats -- including fires caused by humans, habitat degradation due to invasions of introduced plants, overgrazing by goats, and predation from introduced dogs and cats. Together these pose immediate and long-term threats to the survival of many of the island's giant tortoises.

Introduced mammals

Until recently, these areas were not considered at risk. During the past decade, however, feral goats have invaded Alcedo from Sierra Negra to the south. Goat populations have exploded on Alcedo, causing deforestation, erosion, and competition with tortoises for food. Tens of thousands of goats are estimated to be on Alcedo, and scientists fear that feral dogs may follow. These introduced mammals may continue dispersing northward to Volcan Wolf, constituting an island-wide threat to tortoises and their habitats in areas where the animals were thought to be safe from the threat of extinction.

Introduced mammals such as feral goats and dogs pose direct threats to tortoises through destruction of habitat and predation, respectively, but indirect impacts of deforestation cause equally serious problems due to soil erosion, changes in micro climates, and variable success of nesting zones. For example, the sex of tortoise can be altered by changes in the temperature at which the eggs from which they hatch are incubated (i.e. they have temperature-dependent sex-determination). Destruction of nesting zones in some areas may result in a skewing of sex ratios or total reproductive failure at specific attitudinal belts.

Tortoise eggs develop unattended in subterranean nest chambers in areas with appropriate soil historically used by nesting females. Destruction of vegetation by goats and the accompanying soil erosion will have long-term impact on the reproductive success of giant tortoises. The survival of young tortoises will have long-term effects on population levels on Alcedo and the northern volcanoes of Isabela.

Poaching

Scientists and park personnel have found as many as 81 giant tortoises killed by humans, and some of these incidents were in areas where tortoises are already extremely rare and vulnerable to extinction. In particular, incidents involving individuals of Geochelone guntheri, which is known at present from a limited area on the southern slopes of Cerro Azul, are of primary concern.

The fact that the tortoise deaths due to humans are concentrated in five sites (at both inland and coastal localities) on Isabela that are visited by fishermen and recent settlers suggest that the root cause of the problem is the huge influx of immigrants who have arrived from the mainland during the past decade.

Almost 70% of the human population is new to the islands. These new immigrants did not grow up with an awareness of the National Park, the Charles Darwin Research Station, or with the tradition of sustained subsistence fishing. They are much less aware of the extraordinary biodiversity and natural resources at stake in the islands; many do not understand the concept of a protected area, or what activities and access are allowed in the Park. Thus, population pressures from the mainland are directly affecting the ecological integrity of the Gaálapagos. Excerpted from the Galápagos Conservation Trust autumn 1995 newsletter.

For more information visit the Galápagos Coalition.

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