July 12, 2002
A few extremely problematic introduced organisms threaten native and endemic species in several genera. Following closely behind the main threats are hundreds more increasingly invasive exotic species, all imported intentionally or accidentally by humans, these plants and animals are on their way to eradicating what has been called a "living laborator[y] of evolution" (Loope et al. 1988). Variably successful attempts are being undertaken to preserve the pre-human status of the islands for both its scientific usefulness and intrinsic value as an ecological anomaly. In light of the unusual evolutionary examples present on the Galápagos and our less-than-elementary understanding of the effects of species introduction, it is clear that conservation efforts should be redoubled and expanded in all respects.
The product of many concurrently colliding geological forces, the Galápagos are completely volcanic in origin (McBride 1918; Thornton 1971). Though the islands present today are all relatively young, there may have been island landforms in the location of the current Galápagos archipelago for as long as 90 million years (Cornell University 2000). Therefore, it is conceivable that wayward plants and animals have been evolving in and adapting to this unique environment for some or all of that time. This period of isolation is certainly enough time to establish endemic organisms, numerous examples of which exist on the islands.
Upwards of 500 native plant species, some 30% or more of which are endemic, have been identified in the various environs of the Galápagos (Mauchamp 1997; Schofield 1989). Divided strictly among six clearly defined vegetation zones, plant communities play a vital role in stabilizing the island ecology. The vegetation zones include the Littoral Zone (mangroves, shore and dune plants), the Dry Zone (arid region of cacti, shrubs, and lichens at low elevation), the Transition Zone (cacti mixed with less specialized plants at increasing elevations), the Scalesia Zone (dominated by endemic Scalesia pedunculata tree, rich soil and vegetation), the Miconia Zone (drier altitudinous region with Miconia robinsoniana shrubs and other low-lying vegetation), and finally the Fern-Sedge Zone (low-growing vegetation that occurs at tops of highest peaks) (Schofield 1989).
In pristine areas of the Galápagos, the several vegetation zones are home to many unique animal communities. There is no reason to expect a significantly lower rate of endemism among animals, though in sheer numbers there are certainly fewer endemic and native species (Mauchamp 1997). Only two endemic mammals currently exist, rice rats (Oryzomys bauri and Nesoryzomys narboroughii) that are the remainder of a once-diverse group of seven species (Clark 1980). Of the various land and sea birds found on and around the islands, many are completely unique to the archipelago. Also notable are the endemic lizards and reptiles, including the several large iguana species, the smaller lava lizards, and the severely threatened Galápagos tortoises. Existing among the plant communities and vertebrate animals are innumerable and often only briefly studied invertebrate forms, all of which are threatened by introduced and invasive species. It is clear that the Galápagos hold a wealth of endemic diversity and a unique island biogeography well worth an effort of conservation.
Unfortunately, the threats to this astonishing diversity are numerous and daunting. Seven particularly aggressive and damaging organisms, four species of animals and two species of plants, have been blamed for a large portion of the damage caused thus far (Schofield 1989). As the total numbers of exotic species run much higher, the threat is real and substantial.
Widespread, fecund, and especially adaptable to adverse conditions, goats (Capra hicus) thrive on and are by far the largest threat to the native flora and fauna of the Galápagos (Schofield 1989). Introduced first on Santiago in 1813, goats are present in significant numbers on this island and several others. The goats that inhabit the islands demonstrate a remarkable rate of growth, increasing in one case from as little as a single pair to at least 20,000 individuals in 15 years (Hamann 1975). Ranging over all of the vegetation zones, the goats can thrive in almost any environment with sufficient vegetation, often traveling great distances when a locality is fully exploited (Schofield 1989). Hardy creatures, they have been known to drink salt water in times of drought, a characteristic that makes them a threat to all species on the islands.
Goats have been observed feeding on many varieties of plants, including ferns, herbs, shrubs, and trees (Schofield 1989). Lower elevations saw the Dry and Transitory Zones stripped of low-growing foliage. In the hills, the endemic Scalesia pedunculata was eliminated except when impossible to reach. It seems that no variety of plant is safe; Opuntia echios cacti were found eaten through the trunk (Schofield 1989). Especially vulnerable are the seedlings of the unusual cactus tree. On smaller islands where goats were presents the young cacti were completely eliminated.
Essentially every environment in which goats have been released has suffered irreparable damage to its landscape and diversity. Describing Santa Fé, Eibl-Eibesfeldt (1976) writes "It was so over-run by goats that grass and herbs were eaten to the roots during the dry season, exposing the soil to erosion. Bushes were torn up and even the tree-cacti were attacked."
As a result of the severe damage the goats are causing, efforts at eradication were made. Successful elimination of most or all of the invasive goat population was achieved on South Plaza, Santa Fé, Española, Marchena, and Rábida (Schofield 1989). Even islands where goats have been eliminated as the result of the extensive and costly eradication programs continue to show the effects of the goat population. Some species are quick to regenerate, but others, despite the recovery of key dispersing animals and the rest of the local communities, are extremely slow growers. As a result, the plant communities are reestablished differently than they originally existed and have therefore been permanently disrupted, never to return to a truly pristine condition (Schofield 1989). Eradication is also being attempted on Pinta and Santiago, with the hope that the programs will be as successful if not more so than those already completed.
Another species that has proved particularly harmful to the Galápagos is the feral pig (Sus scrofa). Introduced about 150 years ago, Darwin himself mentioned their presence on several of the islands he visited in 1835 (Darwin 1845). They are currently known to inhabit Floreana, Isabela, San Cristóbal, Santa Cruz, and Santiago (Schofield 1989). Extensive studies have been made of the feral pig population on Santiago, and there is some dispute over the number of pigs actually living on the island. Counts range from a few thousand to well over 20,000, with a recent and thorough survey counting 3,000 individuals (Schofield 1989; Coblentz & Baber 1987).
Apparently preferring the higher altitudes where more succulent foliage is available, pigs are found primarily in and around the Miconia Zone (Coblentz & Baber 1987). They feed heavily on roots and low-growing vegetation, though they do not appear to substantially disrupt the plant community. The distribution of some plant species has been reduced, though plant communities are not severely threatened by the pigs, which seem to have replaced the giant tortoise as a major herbivore (Coblentz & Baber 1987). The more direct threat is to small populations of native lizards and birds whose eggs are subject to predation by foraging pigs. Among other problems, feral pigs may be to blame for the extinction of land iguanas on Santiago (Coblentz & Baber). As they are generalist feeders, most plants and animals are subject to consumption, and if the pig population is allowed to continue unchecked more detrimental impacts will undoubtedly take place.
Studies of possible control methods have been conducted with varying results. Proposed measures include trapping, shooting, snaring, and poisoning. Rated for efficiency and effectiveness, poisoning appears to be the most effective method (Coblentz & Baber 1987). However, there is some dispute over the possibility of non-target effects on the Galápagos hawk, and shooting continues to be used as a control measure, though populations still remain high in many locations (Schofield 1989).
Introduced cattle (Bos taurus) and donkeys (Equus asinus) also pose a significant threat to the island communities. Both species have been allowed to roam freely by their owners, and have lived in the Galápagos since the 1830's (Schofield 1989). Floreana, Isabela, and Santa Cruz are home to both cattle and donkeys, with cattle being exclusive to San Cristóbal and donkeys exclusive to Santiago.
Being large grazing herbivores, these animals have spread over a wide area and do substantial damage to undergrowth species (Schofield 1989). Also, environmental disruption threatens all species in a given area, as both of these animals leave noticeable trails and clearings, allowing the thin soil to erode away in many places. Among other plants, Miconia robinsoniana is especially threatened by cattle, and has been significantly reduced as they have expanded their range into higher altitudes (Schofield 1989). No coordinated effort has been made to control either species, although ranchers have been asked by the National Park Service to fence in their animals.
Other animal threats include the tramp ant Wasmannia auropunctata, feral cats (Felis catus), and black rats (Rattus rattus). The tramp ant, not common on Santa Cruz until the middle of the 20th century, is well on its way to exterminating or severely reducing the numbers of 17 native ants, four of which are endemic (Clark et al. 1982). Clark's (1982) study of the ant population on Santa Cruz shows that the tramp ant is substantially altering the invertebrate ecology of the island. Feral cats have been introduced to Isabela, San Salvador, Santa Cruz, San Cristóbal, and Floreana (Thornton 1971). Though they do not have a major impact on the native ecology of the islands, Stone et al. (1994) contends that the cats' presence is changing the behavior of endemic lava lizards. Statistically significant changes in wariness and sprint speed have been measured in populations of lava lizards exposed to feral cat predation (Stone et al. 1994). The first European visitors to the islands probably brought with them the verminous black rat some time in the last five centuries (Clark 1980). On islands where Rattus rattus is present, no endemic rodents remain, all having been replaced by the hardy invader. By no means accounting for all of the introduced animal species, these examples provide a mere sampling of the more harmful impacts invaders can have.
Not to be overlooked is the most damaging species of introduced plant, the guava tree (Psidium guajava). Covering some 40,000 hectares, guava trees are out of control on Floreana, San Cristóbal, Santa Cruz, and Isabela (Schofield 1989). Engaged in a symbiosis with fungal species that aid in nutrient gathering, the guava has an adaptive advantage over native species (Schofield 1989). Due to this advantage and the dissemination of seed by introduced cattle, guava trees have taken over entire regions on each of the four islands. Large populations have been found in four of the six vegetation zones as the guava has continued to expand its territory, displacing many endemic plants (Schofield 1989).
Despite its expansive coverage, the introduction of guava trees could be solved with a concerted effort of eradication. Cutting down trees followed by herbicide treatment and replanting with a fast-growing endemic species has been proven an effective method of eliminating unwanted stands of guava (Schofield 1989). Also, the control of introduced cattle would help reduce the further spread of guava trees.
Quinine (Cinchona succirubra) is also a major threat. Introduced to Santa Cruz in the 1940's as a potential crop, it has grown out of control into the Miconia and Fern-Sedge Zones. Taking advantage of the same adaptations as the guava, quinine is in some situations even more invasive because it can tolerate heavy shade (Schofield 1989). Despite intensive efforts to eradicate quinine, the lightweight seeds have been able to disperse enough to keep small populations scattered around Santa Cruz.
Several other species are becoming increasingly problematic as they naturalize to the Galápagos environment. These include, but are not limited to, lantana (Lantana camara), Citrus spp., avocado (Persea americana), grass species, and others (Schofield 1989). As introductions continue and species become established, cumulative damage to the native and endemic plants increases. Even though the most damaging species are being reduced, the high number of invaders daunts any effort at eradication (Mauchamp 1997).
Making matters worse is persisting ignorance of the problems cultivated plants are causing. Even when knowledge is disseminated, local growers often ignore recommendations, continuing to plant Pennisetum purpureum for pasture and the tree Cedrela odorata (Mauchamp 1997). The most telling statistics show that very few invasive species exist on uninhabited islands; the problematic introductions are in all cases but one plants that were or are still cultivated on the islands (Mauchamp 1997).
Clearly, the most harmful introductions of all are the human inhabitants of the Galápagos Islands. Unaware or unsympathetic to the plight of endemic organisms, local farmers continue to propagate invasive species of plants and animals, accelerating a process that began some 500 years ago. The extent of the damage to the environment has been directly correlated with the amount of people living on a given island; the most populated islands are by far the most affected (Mauchamp 1997).
If any real success is to be achieved in the restoration and preservation of the pre-human condition of the Galápagos ecosystem, a total and concerted effort must be made to eliminate further introductions from visitors, imported goods, and local agriculture. The best (but extremely unlikely) solution would be to relocate all people living on the islands that are not directly involved in research or conservation efforts. Surely a more hospitable home could be found for the few thousands that live there now.
And perhaps this extreme measure is necessary. Our understanding of the implications of even a single species introduction is elementary at best. We have yet to prove the operation of many theoretical evolutionary mechanisms (Abrahms 1996). As we continue to eliminate the integral parts of this natural laboratory, we eliminate one of our best opportunities to study these mechanisms at work. Until our knowledge is exponentially expanded, no result short of complete restoration is acceptable in the case of these true gifts of time and chance, the Galápagos Islands.
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