Plant reproductive susceptibility to habitat fragmentation

AGUILAR, R., ASHWORTH, L., GALETTO, L., AIZEN, M.A.

Bariloche, Rio Negro

Taller; A workshop on Forest Fragmentation in South America; 2006

The loss and fragmentation of natural habitats by human activities are pervasive phenomena in terrestrial ecosystems across the Earth and the main drivers of current biodiversity loss. Throughout the last two decades fragmentation studies on plant populations have mainly focused on demographic processes, with particular emphasis in evaluating the effects of fragmentation on plant fecundity (Aizen et al. 2002, Hobbs & Yates 2003, Ghazoul 2005). Animal-mediated pollination is a key process for the sexual reproduction of most extant flowering plants, thus the pollinator fauna and pollination process have equally been studied in relation to habitat fragmentation (Aizen & Feinsinger 2003). By means of a meta-analysis we quantitatively reviewed the results from independent fragmentation studies, with the aim of testing whether pollination and reproduction of plant species may be differentially susceptible to habitat fragmentation depending on certain reproductive traits that typify the relationship with and the degree of dependence on their pollinators. We found an overall large and negative effect of fragmentation on pollination and on plant reproduction. The compatibility system of plants, which reflects the degree of dependence on pollinator mutualism, was the only reproductive trait that explained the differences among the species’ effect sizes. In agreement with our stated hypothesis, self-incompatible plants showed a strong negative effect of habitat fragmentation on reproduction, whereas self-compatible plants were not significantly affected. Contrary with the expectations based on the classical theoretical concepts, which hold that reproduction of specialist species should be more negatively affected by fragmentation than generalist species, the effect sizes of plants with different degree of pollination specialization did not differ significantly between them. For both, pollination specialist and generalist plants, the weighted-mean effect sizes were large, negative, and significantly different from zero. We propose a possible explanation for this unexpected response pattern based on Ashworth et al. (2004). Furthermore, a highly significant correlation between the effect sizes of fragmentation on pollination and reproductive success suggests that the most proximate cause of reproductive impairment in fragmented habitats may be pollination limitation. We discuss the conservation implications of these findings and give some suggestions for future research on this area