ENVIS BULLETIN - ENVIS Centre on Himalayan Ecology

PLANT POLLINATOR RELATIONSHIPJennersten (1988) found that the perennial herb, Dianthis deltoides, shows pollinationlimitation of seed set in meadow fragments but not in continuous habitat. In Argentinean dry thornforest, Aizen and Feinsinger (1994) found that the number of small bee species caught in traps waslower in two small forest fragments than in continuous forest. It is not necessary that species becomelocally extinct. The composition of the pollinator community could change if floral densities ordistributions are altered with fragmentation. In another example, it was found that long-tonguedbumblebees were more common in large, dense patches of flowers whereas short-tongued bumblebeeswere more common on plants in small, scattered patches. Fragmentation can also restrict pollinatormovement, which may reduce gene flow and result in increased inbreeding.Plant pollinator relationship, which is an example of a vital interaction, gets tremendouslyaffected by forest fragmentation. All available evidences show that pollinator abundance and diversitydeclines with fragmentation. The loss of plants or pollinators can cause cascading extinctionthroughout the community (Rathcke and Jules, 1993). Additional genetic processes in smallpopulation, like random genetic drift and inbreeding, may increase the probability of extinction.Habitat fragmentation locally changes plant species richness, in most cases this is followed by adecreases in species richness of the assemblage of pollinators and consequently pollinationeffectiveness changes. Therefore an immense loss of bio diversity will occur due to fragmentation.Pollinators that are specialists on one or few plant species, like euglossine bees and solitarybees, may be especially vulnerable to habitat fragmentation for many reasons. Because specialistpollinators often exist in small, patchy populations, small fragments are more likely to exclude them,and environmental or demographic stochasticity is more likely to cause their extinction. For aspecialist, the loss of its host plant will cause extinction. For specialists that exploit sparse, scatteredfloral resources, a reduction in floral abundance caused by fragmentation could also be threatening.Bronstein (Bronstein et al., 1990) presented a striking example from figs and their specialist fig wasppollinators. Because a tree, releasing wasps must be synchronized with another tree receptive to waspsin order to be pollinated, tree density is critical to wasp success and to tree pollination. They estimatedthat at least 95 tree individuals were necessary to maintain wasp populations for four years. Becausefigs can act as keystone species that provide food for many frugivores during periods of resourcescarcity, the loss of figs could generate cascading extinction throughout the animal and plantcommunity far beyond the fig pollinator wasp population. In contrast, generalist pollinators may beless vulnerable to fragmentation than specialists. Generalists often originally exist in widespreadpopulations and thus are less likely to be excluded in small fragments. Furthermore, because manyplant species can act as substitute or interchangeable resources for these pollinators, the loss of one orfew plant species should not directly threaten their survival. However, Bowers (1985) found thatfloral composition of meadows was a major component determining the probability of local extinctionof bumblebees, which are generalized floral visitors. On the other hand, many floral-speciesgeneralists are actually specialists for high-density resources. If fragmentation reduces the abundanceof these resources, these generalists may be highly vulnerable and experience high extinction rates asPimm and Pimm (1982) have suggested for Hawaiian honeycreepers. Even if species do not becomelocally extinct, the composition of the pollinator community could change if floral densities ordistributions are altered with fragmentation. For example, it was found that long-tongued bumblebeeswere more common in large dense patches of flowers whereas short-tongued bumblebees were morecommon on plants in small, scattered patches.The risks posed by fragmentation will also depend upon other non-food requirements andinteractions with other species. For example, small bee species often have specialized nestingrequirements, whereas most honey bee species have relatively generalized requirements. Aizen andFeinsinger (1994) suggested that the prevalence of honeybees in a fragment might partly reflect their24E N V IS C entre, G B P IH E D