Plant hybrid zones affect biodiversity: Tools for a genetic-based understanding of community structure

Plant hybrid zones are dynamic centers of ecological and evolutionary proce sses for plants and their associated communities. Studies in the wild and i n gardens with synthetic crosses showed that hybrid eucalypts supported the greatest species richness and abundances of insect and fungal taxa. In an updated review of 152 case studies of taxa associated with diverse hybridiz ing systems, there were 43 (28%) cases of hybrids being more susceptible th an their parent species, 7 (5%) resistant, 35 (23%) additive, 35 (23%) domi nant, and 32 (21%) showed no response to hybridization. Thus, most taxa res pond to hybrids in ways that result in equal or greater abundance, and hybr ids tend to accumulate the taxa of their parent species. These studies sugg est that genetic-based plant traits affect the distribution of many species and that the variation in hybrids can be used as tools to examine the gene tic components of community structure and biodiversity. Several patterns ha ve emerged thus far. (1) Genetic variation between classes of hybrids (e.g. , F-1's vs. backcrosses) may equal or even exceed that found between specie s. (2) As a reflection of this genetic variation, herbivores are more likel y to differentiate between hybrid classes than they are to differentiate be tween pure plant species. (3) The communities associated with different hyb rid classes can differ from one another as well as from their parental spec ies. (4) Generalist and specialist herbivores predictably vary in their res ponses to hybrids. (5) Plant hybrid zones may represent essential habitat f or some arthropod species. (6) Even nesting birds respond to hybridizing pl ants. (7) Including multiple trophic levels and taxa from microbes to verte brates, susceptible hybrid genotypes support greater biodiversity than resi stant genotypes. (8) The effects of hybridization on common or keystone spe cies can either positively or negatively affect biodiversity. The indirect impacts of hybridization on biodiversity may exceed the direct impacts and may result in "apparent" herbivore resistance or susceptibility at the comm unity level. (9) Although hybrids are often maligned, exotic or problem hyb rids generally result from human disturbances, whereas native hybrids are p art of natural ecosystems and should be conserved. Three predictions are made: (1) Intermediate genetic differences between th e parental species will result in the greatest genetic variation in the hyb rid zone, which in turn will have a positive effect on biodiversity. (2) Bi directional introgression enhances species richness on hybrids, whereas F-1 sterility and unidirectional introgression limit the accumulation of speci es on hybrids. (3) Although susceptible hybrids are likely to support the g reatest biodiversity, the impacts of hybridization on keystone species will be crucial in determining the overall effect.