211

Conservation Biology, Pages 211–212Volume 13, No. 1, February 1999

Reduced Defenses in Insular Endemic Plants:An Evolutionary Time Frame

DIRK VAN VUREN* AND LIZABETH BOWEN

Department of Wildlife, Fish, and Conservation Biology, University of California, Davis, CA 95616, U.S.A.

Recently we reported reduced defenses against her-bivory in island plants that had evolved in the absence ofmammalian herbivores (Bowen & Van Vuren 1997).Dudley (this issue) has emphasized the importance ofdrawing upon paleobiogeographic information to estab-lish an evolutionary time frame for studies such as ours.We agree, and we welcome the opportunity to elaborateupon our findings accordingly. We disagree, however,with Dudley’s claim that the presence of dwarf mam-moths (

Mammuthus exilis

) during the Pleistocene in-validates the possibility of recent evolutionary change inisland plants. The evidence suggests that mammoths onSanta Cruz Island have been extinct for about 10,000years, ample time for plants to evolve reduced defensesin the absence of selection pressure by mammalian her-bivores.

The existence of mammoths on Santa Cruz Island hasbeen known for decades (Cushing et al. 1984), but thefew specimens recovered have been inadequate for radi-ometric dating. Further, dating based on associated or-ganic material may be inaccurate (Cushing et al. 1984;Roth 1996), in part because fossil deposits may havebeen altered by the impacts of exotic herbivores (Brum-baugh 1980; Van Vuren 1982). Nonetheless, four lines ofevidence suggest that mammoths vanished from SantaCruz Island by about 10,000 years before the present(BP): (1) wood associated with a mammoth tusk foundon Santa Cruz Island was dated at 10,290

6

110 years BP(Cushing 1993); (2) the youngest reliable age for mam-moth specimens or associated material from adjacentSanta Rosa Island is 11,800

6

800 years BP (Roth 1996);(3) there is no evidence that humans were contempo-rary with mammoths in the Channel Islands (Cushing etal. 1986), and radiometric dating suggests that humanshave inhabited the northern Channel Islands, which in-clude Santa Cruz, since about 10,000 years BP (Olson &

Broecker 1961; Guthrie 1993); and (4) mammoths be-came extinct worldwide by around 9500 years BP, ex-cept on Wrangel Island in the Arctic Ocean (Vartanyanet al. 1993).

Evidence of rapid evolution in island species suggeststhat 10,000 years without mammalian herbivores ismore than enough time for plants to evolve reduced de-fenses. Dwarf mammoths on Wrangel Island evolvedtheir small stature in just 5000 years (Vartanyan et al.1993), and red deer

(Cervus elaphus

) on the island ofJersey became reduced to one-sixth of their body sizein less than 6000 years (Lister 1989). Dwarfism wasevident in black rats (

Rattus rattus

) on Lavezzi Islandwithin 500 years (Vigne et al. 1993). Evolution in islandplants may occur even more quickly: several herbaceousspecies on islands off the coast of British Columbiaevolved traits for reduced dispersal ability, a commoncharacteristic of insular taxa, in just a few generations(Cody & Overton 1996). Given the potentially strong se-lection pressure on plants from the fitness cost of main-taining expensive defenses in the absence of herbivory(Rhoades 1979; Marquis 1991), it seems likely that re-duced defenses against herbivores could evolve quicklyas well.

Thus, the evidence suggests that shrubs on Santa CruzIsland evolved reduced defenses within the last 10,000years in response to the absence of mammalian herbi-vores. Sheep were introduced in the 1850s and num-bered at least 50,000 by 1890 (Van Vuren & Coblentz1989), for an average density exceeding 2/ha. Sheepbrowsing likely resulted in strong selection pressure forincreased defenses in shrubs, but their relatively longgeneration time, exacerbated by the consumption ofpractically all seedlings by sheep (Hobbs 1980), proba-bly precluded an evolutionary response.

Acknowledgments

We thank K. Crooks, D. Kelt, and M. Meyer for com-ments on the manuscript.

*

email dhvanvuren@ucdavis.eduPaper submitted July 31, 1998; revised manuscript accepted Septem-ber 3, 1998.

212

Recent Evolution in Island Plants Van Vuren & Bowen

Conservation BiologyVolume 13, No. 1, February 1999

Literature Cited

Bowen, L., and D. Van Vuren. 1997. Insular endemic plants lack de-fenses against herbivores. Conservation Biology

11:

1249–1254.Brumbaugh, R. W. 1980. Recent geomorphic and vegetal dynamics on

Santa Cruz Island, California. Pages 139–158 in D. M. Power, editor.The California Islands: proceedings of a multidisciplinary symposium.Santa Barbara Museum of Natural History, Santa Barbara, California.

Cody, M. L., and J. M. Overton. 1996. Short-term evolution of reduceddispersal in island plant populations. Journal of Ecology

84:

53–61.Cushing, J. E. 1993. The carbonization of vegetation associated with

“fire areas,” mammoth remains and hypothesized activities of earlyman on the Northern Channel Islands. Pages 551–556 in F. G.Hochberg, editor. Third California Islands symposium: recent ad-vances in research on the California Islands. Santa Barbara Museumof Natural History, Santa Barbara, California.

Cushing, J., M. Daily, E. Noble, V. L. Roth, and A. Wenner. 1984. Fossilmammoths from Santa Cruz Island, California. Quaternary Research

21:

376–384.Cushing, J., A. M. Wenner, E. Noble, and M. Daily. 1986. A groundwa-

ter hypothesis for the origin of “fire areas” on the Northern Chan-nel Islands, California. Quaternary Research

26:

207–217.Dudley, J. P. 1999. Coevolutionary implications of an endemic Pleis-

tocene megaherbivore fauna for insular floras of the CaliforniaChannel Islands. Conservation Biology

13:

209–210.Guthrie, D. A. 1993. New information on the prehistoric fauna of San

Miguel Island, California. Pages 405–416 in F. G. Hochberg, editor.Third California Islands symposium: recent advances in researchon the California Islands. Santa Barbara Museum of Natural History,Santa Barbara, California.

Hobbs, E. 1980. Effects of grazing on the northern population of

Pinus

muricata

on Santa Cruz Island, California. Pages 159–165 in D. M.Power, editor. The California Islands: proceedings of a multidisci-plinary symposium. Santa Barbara Museum of Natural History,Santa Barbara, California.

Lister, A. M. 1989. Rapid dwarfing of red deer on Jersey in the Last In-terglacial. Nature

342:

539–542.Marquis, R. J. 1991. Evolution of resistance in plants to herbivores.

Evolutionary Trends in Plants

5:

23–29.Olson, E. A., and W. S. Broecker. 1961. Lamont natural radiocarbon

measurements VII. Radiocarbon

3:

141–175.Rhoades, D. F. 1979. Evolution of plant chemical defense against herbi-

vores. Pages 3–54 in G. A. Rosenthal and D. H. Janzen, editors. Her-bivores: their interaction with secondary plant metabolites. Aca-demic Press, New York.

Roth, V. L. 1996. Pleistocene dwarf elephants of the California Islands.Pages 249–253 in J. Shoshani and P. Tassy, editors. The Probos-cidea: evolution and palaeoecology of the elephants and their rela-tives. Oxford University Press, Oxford, United Kingdom.

Van Vuren, D. 1982. Effects of feral sheep on the spatial distribution ofartifacts on Santa Cruz Island. Bulletin of the Southern CaliforniaAcademy of Sciences

81:

148–151.Van Vuren, D., and B. E. Coblentz. 1989. Population characteristics of

feral sheep on Santa Cruz Island. Journal of Wildlife Management

53:

306–313.Vartanyan, S. L., V. E. Garutt, and A. V. Sher. 1993. Holocene dwarf

mammoths from Wrangel Island in the Siberian Arctic. Nature

362:

337–340.Vigne, J. D., G. Cheylan, L. Granjon, and J. C. Auffray. 1993. Evolution

ostéométrique de

Rattus rattus

et de

Mus musculus

sur de petitesîles: comparaison de populations médiévales et actuelles des îlesLavezzi (Corse) et de Corse. Mammalia

57:

85–98.