Abstract

Simple SummaryGlobal climate change is causing more frequent and severe droughts, which can have serious impacts on our environment. To examine how a severe drought in 2014 impacted wild bees in scrub habitats of San Diego, California, we compared bee samples collected before and after the drought. We also investigated whether habitat loss and fragmentation worsened the impacts of drought on wild bees by comparing samples collected from large natural reserves to those from small fragments of scrub habitat embedded in urban areas. Samples collected after the drought contained fewer bee species and fewer individual bees of most species, indicating that bee populations suffered losses during the drought. However, after-drought samples contained large numbers of Dialictus sweat bees, indicating that some bee species benefitted from environmental conditions present during the drought. The impact of drought on the composition of bee samples was three fold higher than the impact of habitat fragmentation, and habitat fragmentation did not appear to have exacerbated the impacts of drought. Our findings highlight the importance of studying how impacts of climate change compare with impacts of habitat loss and other threats to biodiversity conservation.Global climate change is causing more frequent and severe droughts, which could have serious repercussions for the maintenance of biodiversity. Here, we compare native bee assemblages collected via bowl traps before and after a severe drought event in 2014 in San Diego, California, and examine the relative magnitude of impacts from drought in fragmented habitat patches versus unfragmented natural reserves. Bee richness and diversity were higher in assemblages surveyed before the drought compared to those surveyed after the drought. However, bees belonging to the Lasioglossum subgenus Dialictus increased in abundance after the drought, driving increased representation by small-bodied, primitively eusocial, and generalist bees in post-drought assemblages. Conversely, among non-Dialictus bees, post-drought years were associated with decreased abundance and reduced representation by eusocial species. Drought effects were consistently greater in reserves, which supported more bee species, than in fragments, suggesting that fragmentation either had redundant impacts with drought, or ameliorated effects of drought by enhancing bees’ access to floral resources in irrigated urban environments. Shifts in assemblage composition associated with drought were three times greater compared to those associated with habitat fragmentation, highlighting the importance of understanding the impacts of large-scale climatic events relative to those associated with land use change.

Highlights

  • Climate change is considered one of the leading threats to biodiversity worldwide [1,2,3], with impacts associated with both climate warming and increased frequency and severity of extreme climate events [4], such as droughts

  • Plot-level species richness of plants in bloom did not differ between any pair of treatment combinations; there was a richness, except significant drought status and fragmentation status

  • Our analyses revealed strong impacts of drought on bee assemblages in the fragmented coastal sage scrub of San Diego

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Summary

Introduction

Climate change is considered one of the leading threats to biodiversity worldwide [1,2,3], with impacts associated with both climate warming and increased frequency and severity of extreme climate events [4], such as droughts. Plant-pollinator mutualisms are an example of a key biological interaction that may be vulnerable to the effects of drought (reviewed in [13]). Annual plants may remain dormant in the seed bank, while perennial plants may reduce their reproductive investment by producing fewer flowers [9,14]. Even if flowers do bloom, reduced nectar and pollen production caused by drought could result in the breakdown of interactions among plant and pollinator mutualists [14,15]. To our knowledge, no published study to date has investigated the joint impacts of drought and other anthropogenic stressors such as habitat loss and habitat intensification on pollinator assemblages

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