FORTY YEARS of CHANGE in SOUTHWESTERN BEE ASSEMBLAGES Catherine Cumberland University of New Mexico - Main Campus
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University of New Mexico UNM Digital Repository Biology ETDs Electronic Theses and Dissertations Summer 7-15-2019 FORTY YEARS OF CHANGE IN SOUTHWESTERN BEE ASSEMBLAGES Catherine Cumberland University of New Mexico - Main Campus Follow this and additional works at: https://digitalrepository.unm.edu/biol_etds Part of the Biology Commons Recommended Citation Cumberland, Catherine. "FORTY YEARS OF CHANGE IN SOUTHWESTERN BEE ASSEMBLAGES." (2019). https://digitalrepository.unm.edu/biol_etds/321 This Dissertation is brought to you for free and open access by the Electronic Theses and Dissertations at UNM Digital Repository. It has been accepted for inclusion in Biology ETDs by an authorized administrator of UNM Digital Repository. For more information, please contact [email protected]. Catherine Cumberland Candidate Biology Department This dissertation is approved, and it is acceptable in quality and form for publication: Approved by the Dissertation Committee: Kenneth Whitney, Ph.D., Chairperson Scott Collins, Ph.D. Paula Klientjes-Neff, Ph.D. Diane Marshall, Ph.D. Kelly Miller, Ph.D. i FORTY YEARS OF CHANGE IN SOUTHWESTERN BEE ASSEMBLAGES by CATHERINE CUMBERLAND B.A., Biology, Sonoma State University 2005 B.A., Environmental Studies, Sonoma State University 2005 M.S., Ecology, Colorado State University 2014 DISSERTATION Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy BIOLOGY The University of New Mexico Albuquerque, New Mexico July, 2019 ii FORTY YEARS OF CHANGE IN SOUTHWESTERN BEE ASSEMBLAGES by CATHERINE CUMBERLAND B.A., Biology B.A., Environmental Studies M.S., Ecology Ph.D., Biology ABSTRACT Changes in a regional bee assemblage were investigated by repeating a 1970s study from the U.S. Southwest of bees visiting native sunflower (Helianthus annuus). Results showed declines in abundance and species richness of native bees and increases in non- native Apis mellifera. Climate data indicate drought increased over the 40-year period, favoring introduced and generalist species. Experimental placement of A. mellifera in an area of low A. mellifera density in New Mexico reduced native bee visitation, but improved reproduction in H. annuus plants. Meta-analytic models comparing pollination effectiveness in specialist versus generalist, native versus non-native, and native pollinators versus introduced A. mellifera indicated no support for greater specialist effectiveness, but higher effectiveness of native bumble bees (Bombus spp.) compared to non-native pollinators, especially A. mellifera. Changes in pollinator species composition, particularly replacement of native pollinators by introduced A. mellifera, affect plant reproduction and may cascade to changes in plant community composition. iii TABLE OF CONTENTS CHAPTER 1: Declines in abundance and diversity of southwestern U.S. sunflower pollinators resurveyed after 40 years .........................................................................................1 Abstract ................................................................................................................................2 Introduction ..........................................................................................................................3 Materials and Methods .........................................................................................................8 Results ................................................................................................................................20 Discussion ..........................................................................................................................32 Conclusion .........................................................................................................................43 CHAPTER 2: Introduced honey bees (Apis mellifera) alter native bee visitation and seed set in a native plant species ......................................................................................................45 Abstract ..............................................................................................................................46 Introduction ........................................................................................................................47 Materials and Methods .......................................................................................................51 Results ................................................................................................................................59 Discussion ..........................................................................................................................65 Conclusion .........................................................................................................................72 CHAPTER 3: Are specialist and native pollinators more effective than generalists and non-natives? A meta-analysis ..................................................................................................74 Abstract ..............................................................................................................................75 Introduction ........................................................................................................................76 Materials and Methods .......................................................................................................80 Results ................................................................................................................................93 iv Discussion ........................................................................................................................105 Conclusion .......................................................................................................................111 APPENDICES .....................................................................................................................113 Appendix A ......................................................................................................................113 Appendix B ......................................................................................................................121 REFERENCES .....................................................................................................................124 v CHAPTER 1 Declines in abundance and diversity of southwestern US sunflower pollinators resurveyed after 40 years 1 Abstract Recent assessments of pollinator populations have revealed declines – a serious concern given the ecosystem services pollinators provide. Species-level datasets for tracking long- term changes in pollinator populations are therefore highly valuable. We investigated changes over the past 40 years in a regional bee assemblage by repeating a study conducted in the U.S. Southwest. Bees visiting sunflower (Helianthus) species were sampled from 1973–1977 by Hurd, LaBerge, and Linsley in Arizona, California, and New Mexico. We resampled the bee fauna at 11 of the original locations in 2015–2016. After accounting for sampling effort, we report significant declines in abundances of native bee species, and significant increases in introduced Apis mellifera abundance compared to the 1970s. Species richness decreased significantly at Arizona and New Mexico sites, driven mainly by declines in specialist bees, but richness at California sites did not differ from the 1970s. Overall, 83% of bee species collected in the 1970s were observed in 2015–2016. Generalist bee assemblage composition differed significantly from the 1970s at California sites, mostly due to changes in dominant species. Climatic data suggest drought conditions increased at all sites over the 40-year period, and further that bee population trends were not explained by chance sampling of years that were climatically extreme relative to decadal norms. We found significant correlations between six temperature/precipitation-related variables and changes in abundance in one or more of our bee categories (all bees, Apis mellifera, generalists, specialists). We conclude that sunflower bee abundance and diversity in the U.S. Southwest have decreased since the 1970s, and hypothesize that climate change has favored introduced and generalist species. 2 Introduction Insect extinction rates have not been comprehensively monitored at the global scale, but there is recent evidence of dramatic regional-level declines across a broad range of taxa [Hallmann et al. 2017; Powney et al. 2019]. Among orders, Hymenoptera may be especially vulnerable to declines and extinctions, partly for genetic reasons: Haplodiploid sex determination results in the production of sterile males and reduces effective population size [Zayed & Packer 2005]. Their role as pollinators in agriculture and natural ecosystems accords the Hymenoptera considerable economic and ecological importance [Potts et al. 2010]. Bees, the largest contributors to pollination, have experienced population crashes and local extirpations [NRC 2007; Steffan-Dewenter et al. 2005]. Their status has become a key scientific and public policy concern [Potts et al. 2011; USDA-EPA 2015]. Extinction rates are not equally distributed across all species. A key question is whether specialist species, which have narrow habitat or dietary tolerances, are more vulnerable to declines than generalists with wider tolerances. (For convenience we use the dichotomous terms “specialist” and “generalist,” though their ecological manifestations