Dr. Alan Tye, Head of the Department of Plants and Invertebrates
Charles Darwin Research Station
World Conservation 4/97-1/98, Invaders from Planet Earth
Although the problems of introduced mammals in Galapagos is widely known, the effects of other introduced organisms may, in the long term, be even more serious. The number of introduced plant species recorded from the islands increased from 77 (1971) to more than 460 (1997), while the native flora comprises only some 560 species. Data on introduced species of invertebrates are much less precise. Most of the alien plants were introduced deliberately, as useful or ornamental species, whereas the majority of alien insects were introduced accidentally, in shipments of produce, or otherwise in boats and planes. Both accidental and deliberate introduction rates have risen in line with the rapidly expanding human population.
Wholesale replacement
The National Park and the agricultural community suffer equally from the effects of many of the most serious invasions. Guava Psidium guajava currently occupies over 40,000 ha of land on all four of the large inhabited islands, including large parts of the agricultural zone as well as the National Park. Farmers dislike it, as it invades their pastures and fields and is costly in time, effort and money to remove.
In the National Park, it is replacing all three of the unique highland vegetation zones, the endemic Scalesia forest, the endemic Miconia robinsoniana shrub zone, and the highest, fern-sedge zone. In all of these zones it shades out, or otherwise out-competes the native species and produces a forest in which it is the sole dominant. Similarly, a species of blackberry Rubus niveus, also introduced for its fruit, is now heavily invading farm and Park areas on several islands, and farmers as well as park managers would like to see it eradicated. The benefits of such species, in terms of the fruit they produce, are not worth the problems created by their aggressive tendencies.
Potential for conflict
Although in these cases conflict between local communities and park management is minimal, in other instances the potential for disagreement is greater. The most important introduced timber tree, cigar-box "cedar" Cedrela odorata, is becoming a serious invader of the National Park, by means of its wind-borne seeds. It is not yet considered problematic by farmers, and is valued as the main source of high-quality timber available within the islands. If eradication of Cedrela is to be attempted, alternative timber sources have to be found. The Charles Darwin Research Station (CDRS) has therefore established a program to evaluate and promote "safe" timber species (with heavy seeds and no other known aggressive characteristics), in the hope that these can eventually replace Cedrela and other aggressive species.
In the case of introduced invertebrates, there is generally no conflict at all between the desires of local people and National Park management. All readily appreciate the health threats, economic costs and nuisance value of introduce ants, wasps, mosquitoes, blackfly, scale insects, woodborers and others.
Control is another matter
CDRS is now beginning serious efforts to investigate control of the introduced insects, with a program examining the potential for biological control of the cottony cushion scale Icerya purcchasi. Another program in collaboration with the National Park Service, to eradicate the little fire ant Wasmannia auropunctata from uninhabited islands. The history of plant control is longer, with several attempts having been made in the past to test control methods or to apply them in limited areas.
Unfortunately, there has been little continuity in past efforts and trials have not been properly monitored, nor control efforts maintained for long enough to achieve lasting results. This should change in the future, as realization grows of the severe effects of introduced plants on Galapagos ecosystems. At the moment, the battle against introduced species in Galapagos is being lost, but with increased awareness of the problems, and sustained action to prevent further introductions and to control the worst pests, the tide may yet be turned.