On Bees, Crops and Pollination

madagascar butterflyPollination is an important ecosystem service at risk. Habitat loss and agricultural intensification are the main threats to pollinators in Europe and America. This ‘pollinator crisis’ is being driven by declines in honeybees in North America and butterflies and bumblebees in Europe. In 11 European countries more than a quarter of bee species have been listed as threatened or endangered. Trends have shown that out-crossing plants reliant on insect pollination are declining in Britain, while plants reliant on wind dispersal are increasing, and self-pollinating plants remain intermediate.

Pollinators vary in their life strategies and so does their vulnerability to threats. Specialized pollinators, with specific habitat or dietary requirements, or with slower development or mobility tend to decline more readily than generalists. Generalists rely on a broader range of dietary and habitat conditions and are more robust in the face of change. Some scientists assume that a few remaining generalist species will ensure pollination services; however experiments have shown that for plant communities, even those with more generalized pollination systems, to remain in their most productive state, require high levels of pollinator diversity, as well as density. For this reason, it is wise to maintain the functional diversity of pollinators through conservation efforts.

Some thirty-five percent of crop volume production and seventy percent of global major crops rely on pollination services. Free (1993), through an extensive literature survey, found that the majority of crops that provide fruits and seeds for human consumption rely on pollination. Furthermore, vegetative crops like carrots and forage crops, such as alfalfa and clover, require animal pollination to exist. In the past, lower harvests of dates, figs and oil palm have been attributed to low abundances of pollinators. In Europe and America the landscape design relates heavily to the pollination services available in a given area. Crop visitation rates decline with increasing distance from pollinator refugia. Natural and semi-natural areas act as these refugia and as the distance from these habitats changes the diversity and abundance of pollinators varies. A varied landscape, with high habitat heterogeneity and plentiful edge habitats would provide a rich community of pollinators offering a supreme service. Unfortunately however, in many agricultural landscapes around the world the recent intensification of farming practises has reduced the amount of natural refugia and edge habitats. The use of pesticides directly reduces pollinating insect densities and reduces the likelihood of nests to form nearby.

In unfarmed areas the fragmentation of wild plant habitats limits the abundance and diversity of pollinators present. Spill-over effects from agricultural habitats might inundate natural areas with generalist pollinators and their associated predators. On the other hand, mass flowering crops might out compete populations of wild plants for pollinators, an especially serious problem if the plants are threatened or endangered within a conservation area. The interactions for gene flow in plant populations are highly relevant for the long term survival and adaptation of plant populations to today’s changing environments.

The economic value of pollination services for crops remains controversial despite a proven reduction in visitation of pollinators in many areas. The quantification of reductions in yield quantity, quality or net revenues is still in its infancy, requiring research to establish the direct impacts on humankinds survival into the future.

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