- Native bees play an important role as pollinators of natural vegetation and agricultural
crops. Yet many pollinators, including some native bees, are declining in numbers.
Some of the potential causes of these declines are habitat destruction and degradation
by various human land uses, including urban development and sprawl, construction of
roadways, and habitat conversion to agriculture such as crop production and livestock
grazing. Livestock grazing is one of the most common land uses in western North
America and it can impact floral and nesting resources that are important to native
bees. These effects are likely manifested through grazing's effect on vegetation and
soil characteristics. However, few studies have investigated how livestock grazing
impacts native bees in North America. As a result, the overall goal of this research was
to determine how a gradient of livestock grazing intensities impacts native bee
communities in a threatened and poorly studied grassland of western North America,
the Pacific Northwest Bunchgrass Prairie. Because no studies have examined the bee fauna of this grassland habitat, our study had two objectives: 1) describe the native bee
community in the Zumwalt Prairie in northeastern Oregon, which is one of the largest
remaining remnants of this unique grassland type, and how it changes through time,
and 2) investigate how livestock grazing affects that native bee fauna.
To address these objectives, we sampled pollinators during the summer of
2007 and 2008 in 16 40-ha pastures on a plateau in the Zumwalt Prairie using blue
vane traps. Each pasture was assigned one of four cattle stocking rates (high, medium
low, and no cattle), and grazing intensity was quantified by measuring utilization.
Grazing treatments were applied in the early summer for two years. We measured soil
and vegetation characteristics that related to floral and nesting resources of bees as
well as several metrics of the bee community, including diversity, richness, abundance, and community composition.
We found 92 species and 119 morphospecies of native bees in 27 genera. This
diverse community of native bees showed strong interseasonal and interannual
variation that appears to be related to weather and plant phenology. We also found that
even after exposure to just two years of grazing, some effects on vegetation and soils
were evident. For example, increased grazing intensity significantly reduced
vegetation structure, the abundance of certain blooming plants, surface soil stability,
and the amount of soil surface covered by herbaceous litter. In addition, increased
grazing intensity significantly increased soil compaction and the amount of bare
ground. Native bee communities responded grazing intensity through changes in
abundance, richness, diversity and community composition. Different bee taxa responded to grazing intensity differently and this response varied temporally. For
example, bumble bees were sensitive to grazing intensity early in the season, showing
reduced abundance, diversity, and/or richness with increased grazing intensity. In
contrast, halictid bees did not respond to grazing intensity in any season. However,
even within a genus or family, different species responded to grazing intensity in
different manners, potentially because of variation in life histories. This research
suggests that maintaining land with a mixture of livestock grazing intensities may be
the best way to conserve this important and diverse pollinator group in the Pacific
Northwest Bunchgrass Prairie.