Effects of habitat fragmentation on native bee diversity and community composition in San Diego County

Scientists estimate that over 80% of all flowering plants are pollinated by animals. Insects, in particular, provide a large portion of the pollination services in many ecosystems. Thus, global declines in pollinator diversity, driven largely by habitat loss and degradation, have elicited considerable attention. Although studies show that pollinators tend to become less diverse in fragmented landscapes, responses of the pollinator community to anthropogenic disturbance often differ across pollinator species and across ecosystems. Thus, in order to forecast the consequences of anthropogenic disturbance for pollination services, or make informed management plans to protect remaining natural habitats, ecologists and conservationists must document how the local pollinator community responds to disturbance in each specific ecosystem of interest.

Pictured below: A typical view of Mediterranean scrub ecosystem in San Diego, CA.

San Diego County, along with the rest of the American southwest, is home to one of the world’s richest assemblages of bees, which are critical pollinators in most ecosystems. Unfortunately, human development has led to the destruction of vast portions of the habitat unique to this region. A sizable portion of the remaining, undeveloped habitat exists as fragments surrounded by urban landscapes. Although these fragmented habitats often appear to host relatively healthy populations of native plants, they are known to harbor predictably fewer species of birds and mammals compared to relatively large, intact natural reserves.

We surveyed native bees in coastal sage scrub ecosystems in a series of habitat fragments bound by urban landscapes, large natural reserves, and gardened residential roadsides. Consistent with findings from other parts of the world, we found that habitat fragments harbored fewer bee species and genera compared to natural reserves. Additionally, bee assemblages in fragments contained higher proportions of “weedy” generalist bee species that forage from a wide breadth of plant species, including ornamental and invasive plants not native to the local ecosystem. Intuitively, residential roadsides harbored even fewer bee species and genera.

Pictured above:Some examples of specialist (top row) and generalist bee species (bottom row) found in San Diego County, clockwise from top left. 1) Diadasia opuntiae (cactus bee) in the flower of Opuntia littoralis (coast prickly pear cactus). 2) Tetraloniella pomonae (long-horned bee) resting on the flower of Chrysanthemum coronarium (garland chrysanthemum). 3) Dieunomia nevadensis (Nomiine sweat bee) on inflorescence of Isocoma menziesii (coast goldenbush). 4) Halictus tripartitus (small sweat bee, top), Ceratina acantha (small carpenter bee, middle), and Lasioglossum sp. (small sweat bee, bottom) all foraging in the flower of Carpobrotus edulis (Hottentot fig). 5) Lasioglossum sisymbrii (large sweat bee) foraging in the flower of Malacothamnus fasciculatus (chaparral mallow). 6) Agapostemon texanus (ultra-green sweat bee) foraging in the flower of Calystegia sp. (morning glory).

Given our findings, we are currently examining whether this erosion of bee species richness in fragmented habitats leads to a decrease in the quality of pollination services received by plants in these habitats. Presently, the spectrum predicting the consequences of pollinator species loss on pollination services is marked by two alternative hypotheses. On one extreme, theory predicts that each pollinator species extirpated means the loss of a high-quality pollinator (and thus a portion of pollination services) for certain plant species. On the other extreme, theory predicts that abundant, disturbance-tolerant generalist pollinator species are responsible for the bulk of the pollination services provided to most plant species and will thus buffer the loss of other, more specialized pollinator species extirpated by anthropogenic disturbance. Our research aims to place the actual scenario of pollination services in San Diego on this hypothetical continuum.

Funding provided by:
Mildred E. Mathias UC Natural Reserve System Grant (2013)
Sigma Xi Grant in Aid of Research (2012-2013)
National Geographic Society Young Explorer Grant (2011-2013)
UC Academic Senate Bridge Fund Grant (2011-2012)
Jean Marie Messier Memorial Fund Award (2011)