Island ecosystems world-wide are well known for their vulnerability to human activity, and particularly for endemic species' susceptibility to extinction in the presence of environmental disturbances (Olson, 1989).  Taxa endemic to islands from which herbivorous and predatory vertebrates were absent during their evolutionary histories often lack mechanisms that would allow them to defend themselves against mainland animals, particularly mammals, and the diseases and habitat degradation that can accompany them (Atkinson, 1989).  Evolution in isolation and taxonomic overturn among island flora and fauna are the foundations of endemism in island ecosystems (Case & Cody, 1983).  The rate at which human-caused disturbances (including species introductions) alter ecosystem properties, however, can swamp native species' abilities to acclimatize or adapt and, consequently, have species- and ecosystem-level effects that are measurable on the scale of centuries and even decades.  The few dozen islands of the Gulf of California, simultaneously desert and marine, may have figured into human occupation of the region for thousands of years, yet only in the latter half of the twentieth century have human activities resulted in indelible changes to island ecology (Bahre, 1983).

In many ecosystems, invasive plants constitute the majority of invasive species and lie at the root of changes to ecosystem structure and function.  On the islands in the Gulf of California, although invasive plant species can affect native vegetation and herbivores, the greatest ecological damage has resulted from intentionally or accidentally introduced vertebrates.  These introductions have occurred inadvertently through human use of the islands and island resources, and have also been made intentionally where the islands have been used to keep livestock. Introduced consumers in island ecosystems where native consumer fauna are sparse, as is the case with the Gulf islands (Bahre, 1983), can alter ecosystem dynamics and cause disturbances that interrupt succession and possibly favor other introductions (Vitousek, 1986; Atkinson, 1989).  On the Gulf islands, animals such as "house" mice (Mus musculus) and rats (Rattus sp.), compete with seabirds for the holes and crevices in which they nest and with native herbivores for food.  Feral domestic animals, including pigs, goats, and rabbits, compete with native herbivores for food and cause habitat destruction through trampling and erosion. Feral cats impact native vertebrates (rodents, lizards) and migratory or nesting seabird populations through predation (see Tershy & Croll, n.d.).
 
Different Gulf islands have encountered different invaders, to different degrees, and for varying periods of time.  Particularly on smaller islands, long-term ecological effects of invasions may be less severe than on larger islands with more complex biota, including more endemic species.  Where conspicuous mammal invaders are involved, as is the case with many Gulf island invasions, eradication efforts may prove, and in some cases have been, successful.   Where non-native competitors and predators are eliminated, reintroduction efforts may be possible (see Tershy & Croll, n.d.).  Future threats to island biodiversity lie in increased fishing in the Gulf of California and increased tourism to the Gulf region and the islands, activities which could increase species introductions (or reintroductions) as well as impact native flora and fauna (Bahre, 1983).  Efforts to protect the Gulf islands include increasing public awareness of introduced species problems through traditional education and on-site signage, establishing preserves for native biota on islands free from non-native taxa, collaborations between government agencies and academic groups in developing management and use strategies, and campaigns to change perceptions of these islands from exploitable to renewable resources (see Tershy & Croll, n.d.).

Photos:

• view toward Flecha and Jorobado Islands from Ventana Island, Bahia de los Angeles, Baja California, México (©1998,Patty West)
• Isla San Esteban (Sonora, México) or blotched chuckwalla, Sauromalus varius - an island endemic and endangered species (Ralph Crane/©1978,ASDM/ASDM archive)
• view toward Llave and Smith Islands from Cerraja Island, Bahia de los Angeles, Baja California, México (©1998,Patty West)

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Island Conservation
a comprehensive Web site dedicated to the Sea of Cortez, with publications, species lists, current projects, links, and contact information (requires at least Netscape 6 or IE 5)

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Atkinson, I. (1989)  Introduced animals and extinctions. In Conservation for the Twenty-first Century, edited by D. Western and M. C. Pearl, pp. 54-75.  Oxford University Press, New York.

Bahre, C. (1983) Human Impact: The Midriff Islands. In Island Biogeography in the Sea of Cortez, edited by T. J. Case and M. L. Cody, pp. 290-306. University of California Press, Berkeley.

Case, T. & M. Cody.  (1983) Synthesis: Pattern and Processes in Island Biogeography. In Island Biogeography in the Sea of Cortez, edited by T. J. Case and M. L. Cody, pp. 307-341. University of California Press, Berkeley.

Olson, S. L.  (1989)  Extinction on islands: man as a catastrophe.  In Conservation for the Twenty-first Century, edited by D. Western and M. C. Pearl, pp. 50-53.  Oxford University Press, New York.

Tershy, B. R. and D. A. Croll.  (n.d.)  Island conservation and introduced vertebrate eradications in Northwestern Mexico. Island Conservation and Ecology Group, University of California - Santa Cruz. Internet source: <http://herb.bio.nau.edu/~cortez/IslandCon.htm>.

Vitousek, P. M.  (1986)  Biological Invasions and Ecosystem Properties: Can Species Make a Difference?  In Ecology of Biological Invasions of North America and Hawaii, edited by H. A. Mooney and J. A. Drake, pp. 163-178. Springer-Verlag, New York.

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