PS 39-66 - Range-restricted bee species are vulnerable to extirpation in urban habitat fragments

Abstract: Background/Question/Methods

Insect declines are now reported from numerous locations worldwide, but the scope and causes of this phenomenon are debated. Given the diversity and importance of insects in terrestrial ecosystems, however, identifying why insects may be declining should be an important research focus. Range size represents a species-level trait that is inversely associated with extinction risk for a wide array of organisms. Here we examine the relationship between range size and risk of local extinction for bees in scrub ecosystems in coastal southern California. Previous work in this system demonstrates that scrub fragments support a lower diversity of native bees compared to large expanses of the same habitat. Using four years (2011, 2012, 2015 & 2016) of data, we test the hypothesis that range-restricted bee species exhibit greater vulnerability to extirpation in habitat fragments compared to species with large range sizes. Study sites were 1-hectare plots in two categories: (i) habitat fragments (< 120 ha, n = 7) surrounded by urban development, and (ii) natural reserves ( > 500 ha, n = 7) in western San Diego County, California. In each year we surveyed every plot every two weeks between 8:00 am – 3:00 pm from March – July with pan traps painted white, fluorescent yellow, and blue in equal proportion.

Results/Conclusions

In four years of sampling, we found 167 native bee species (n = 14,306 individuals) across all plots. We classified species as “range-restricted” if their global range falls in the bottom quartile (≤ 164,000 km², roughly the area of Washington state) of the range size distribution including all species. Range-restricted bees represent 14% of all bee individuals captured in reserves but < 3% of all bee individuals captured in fragments. On average, habitat fragments support 3 range-restricted species, whereas reserves host on average 6. These findings support the hypothesis that range-restricted bee species exhibit greater vulnerability to extirpation in habitat fragments compared to species with large range sizes. Given that range sizes are often incompletely documented for many insect species, especially in comparison to plants and vertebrates, our findings demonstrate the importance of documenting insect range sizes. Museum specimens and community science platforms represent existing but disparate data sets that should be integrated to clarify range size for poorly studied insect groups.