Island plants let down their defense.

Give a hungry sheep the choice of a leafy island shrub or a mainland version and the sheep will almost always graze on the island greenery. That's what biologists Lizabeth Bowen and Dirk Van Vuren of the University of California, Davis found in a study comparing how well six shrubs from Santa Cruz Island and their California counterparts 30 kilometers away defend themselves against predators.

There's good reason for the sheep's fondness for the island flora, the researchers report in the October Conservation Biology. In analyzing the specific defenses in the plants' arsenals, they found that island plants have significantly fewer and shorter spines per leaf area. The island subspecies of the bush poppy (Dendromecon rigida) lacks spines completely

Less striking were the differences in chemical defenses, represented by amounts of phenols and tannins. One of the island shrubs (Ceanothus) has significantly lower concentrations of the compounds than the mainland version, while the island Christmas berry (Heteromeles) actually has more tannin for part of the year.

The difference may be explained by the island's evolutionary history Santa Cruz Island has thousands of insect species, including many plant feeders, but lacks native mammalian grazers. The plants have largely maintained their insect-deterring chemical defenses but have lost the spines and other features that ward off animals like sheep.

Consequently, the researchers say, such island communities are susceptible to high rates of extinction when new plant-eating animals arrive. On Santa Cruz, introduced sheep that were culled in the 1980s chewed up the plant community. With their habitat destroyed, ground-nesting birds died out.