HYBRIDIZATION AND THE EXTINCTION OF RARE PLANT-SPECIES

Much has been written about the role of interspecific competition, dis ease, herbivory, and the loss of key mutualisms in the extinction of r are plant species. Interspecific hybridization rarely is considered am ong the biotic interactions that promote extinction. We show how hybri dization may contribute to the demise of rare plant species through de mographic swamping and genetic assimilation by an abundant congener. W e contend that the growth of the hybrid subpopulation is the key to ra re species assimilation, and we show how the production of hybrid seed , the fitness of hybrids and pest pressure affect hybrid proliferation . We also discuss how habitat disturbance, unspecialized pollinators, nod weak crossing barriers promote hybridization, and how the negative consequences of hybridization are unlikely to he compensated for by i mmigration from conspecific populations. We also illustrate stages In the demise of species in island floras. We suggest that hybridization is nn increasing threat to mw species because ecological barriers are being disrupted by human activities.