Staff Project Update:

Conserving and Restoring Pollinator Populations on Farms

by Claire Kremen, Center for Conservation Biology, Stanford and Robert L. Bugg, SAREP

Organic farms depend on biodiversity for many functions including soil fertility maintenance, control of pest organisms, and pollination. Previous research on biodiversity in agriculture has mainly been concerned with biological control and soil fertility, but pollination has received little attention. Without pollinators some fruit, vegetable, and seed crops might yield less or fail altogether. Scientists have predicted serious declines in managed and wild colonies of the honey bee (Apis mellifera, introduced from Eurasia), as well as in populations of other pollinating insects, including native species of bees (e.g., bumble bees, carpenter bees, leaf cutting bees, sweat bees). Such declines could have ecological and economic consequences. In addition to a decline in crop yields, loss of pollinators could result in loss of native plant species, with cascading effects throughout the ecosystem. Despite their importance, little is known about wild pollinator populations or the consequences of their decline.

This research addresses whether wildlands contribute insects that pollinate vegetable crops and the value of such “pollination services.” Since habitat loss and fragmentation are the suspected causes of a reduction in pollinator populations, this information could then be used as an argument for conserving wildlands and restoring native plants on field borders.

It was challenging to find plants that relied entirely on insect pollination and were likely to benefit from a diversity of native pollinators. Crops selected for our first field season included tomato, eggplant, peppers, melons, and cucumbers. We added strawberries, because their flowering unexpectedly extended into mid-June 1999, when our study began.

After many field visits, we settled on 20 farms and six wildlands sites (representing a total of 37 study sites when multiple parcels were included). These sites included both organic and conventional farms across a gradient of proximity to native vegetation and other natural habitats. The study is a true landscape-scale experiment, relying heavily on Geographic Information Systems (GIS) technology to map not only the crops, but also various wild nectar and pollen sources that may support native pollinators. Using aerial photos provided by the California Department of Water Resources, we can assess the aerial extent of different habitat types (riparian, chaparral, or oak woodland) in the vicinity of farms.

Simply visiting a flower does not constitute pollination. So far, we have observed heavy visitation by several species of native bees to flowers of strawberries, watermelon, muskmelons, eggplant, and cucumber. At times, visits by native bees exceeded those by honey bees, even on farms with managed hives. Surprisingly, native bees were found on all the farms studied, regardless of management type (organic or conventional) or the proximity to wildlands. Our detailed studies have shown that native bees not only visit, but definitely pollinate watermelon.

Although native bees are found on all farms, we do not yet know whether abundance or diversity varies among sites, nor the influence of various environmental factors. We will statistically test these influences, assessing the relative importance of on-farm plant diversity (including crops, weeds, and other plantings) versus diversity and types of habitats surrounding the farms as predictors of pollinator diversity and abundance. In the next two years we will further refine and apply our methods and remain alert to new opportunities.

Funding for this project was provided by the National Fish and Wildlife Foundation, Mead Foundation, The Nature Conservancy, The Organic Farming Research Foundation, the Stanford University Field Studies Internship Program, and the Wildlife Conservation Society. Special thanks to Robbin Thorp, professor emeritus, UC Davis entomology department and noted expert in bee systematics and ecology, for invaluable assistance in defining the scope and focus of the project, and identifying the bees from among the dazzling array of flower visitors.

For more information contact Claire Kremen, Center for Conservation Biology, Stanford University, ckremen@leland.Stanford.edu or Robert L. Bugg, SAREP, University of California, Davis, rlbugg@ucdavis.edu.