A COMPARISON OF BEE FAUNA IN TWO NORTHERN CALIFORNIA COASTAL DUNE SYSTEMS By Laura Snyder Julian A Thesis Presented to The Faculty of Humboldt State University In Partial Fulfillment of the Requirements for the Degree Master of Science in Natural Resources: Biology Committee Membership Dr. Michael R. Mesler, Committee Chair Dr. Erik S. Jules, Committee Member Dr. Frank J. Shaughnessy, Committee Member Dr. Mahesh Rao, Committee Member Dr. Michael R. Mesler, Graduate Coordinator December, 2012 ABSTRACT COMPARISON OF BEE FAUNA IN TWO NORTHERN CALIFORNIA COASTAL DUNE SYSTEMS Laura Snyder Julian Baseline information is needed to protect pollinators in at-risk ecosystems. One such system, Tolowa Dunes (Crescent City, California), in northern California, is undergoing habitat fragmentation caused by an introduced, invasive plant, Ammophila arenaria . My study had two purposes: 1) to gather baseline data on the bee fauna at Tolowa, and 2) to explore possible relationships between bee fauna and available floral and nest resources in Northern California coastal dune systems. In particular, I assessed whether the presence of invasive non-native plants such as A. arenaria affected the bee fauna at Tolowa. Surveys were conducted at Tolowa from March through August 2007 to establish baseline information about the bee community, floral abundance, floral visitation, and bee nest substrate. Vegetation cover, determined from existing data, was used to explore the relationship between bee fauna and landscape at three spatial scales (100 m, 250 m and 1,050 m). Using existing data from previous studies, I compared the bee fauna I found at Tolowa to that of two other Northern California coastal dune ecosystems: the North Spit of Humboldt Bay (near Eureka, California) and Bodega Head (Bodega Marine Research Station at Bodega Bay, California). These three areas vary in the composition of their invasive non-native plant communities and bee resources, so comparing these areas can reveal relationships between bee fauna and these ii environmental factors. Bee abundance and species richness at Tolowa were positively correlated with intact, native dune vegetation, especially when the rare plant, Phacelia argentea was present. A negative correlation was found between cover of A. arenaria , coastal scrub, and total exotic vegetation versus the abundance and/or richness of all bees. NMS ordination found a scale-dependent relationship between bees and vegetation at 100 m and 250 m, where there was a positive relationship between solitary bees and intact, native dune vegetation. At all scales, solitary bees were negatively associated with A. arenaria , coastal scrub, and total exotic vegetation. Differences between the bee faunas at Tolowa, the North Spit, and Bodega Head are possibly explained by differences in nest substrate availability. For example, Melissodes pallidisignata is a bee that requires compacted sand for nesting. This bee was absent at the North Spit (where there is little of this nest substrate), but was present at Tolowa and Bodega Head, where compact sand is abundant. Differences in bee faunas at these locations are also possibly explained by differences in floral composition and abundance; while exotic vegetation was positively correlated with abundance of common Bombus spp. at the North Spit, a similar correlation was not found at Tolowa. Possibly, this is because invasive plants at the North Spit provide some floral resources, whereas at Tolowa the only invasive plant is A. arenaria , which provides no floral resources for bees. My results suggest that the diversity of the coastal bee community at Tolowa is at risk due to habitat fragmentation from A. arenaria invasion. iii ACKNOWLEDGEMENTS My special gratitude to Dr. Michael Mesler for sharing his enthusiasm for bees and to Susan Nyoka for sharing her love of coastal dunes. I extend special thanks to the members of my graduate committee, Drs. F. Shaughnessy, E. Jules and M. Rao, for their careful review of my work. Dr. R. Thorp generously shared his time and expertise in bee identification. Thank you to M. Wasbauer for sharing the preliminary results of his bee surveys at Tolowa. I am grateful to Sue Calla and Rick Hiser for their hospitality and for their work, and the work of the Tolowa Dune Stewards, to protect Tolowa. I am especially grateful for the love and sustained support I receive from my husband, Brian Julian. This is dedicated to my father, Phil Snyder. iv LIST OF TABLES 1. Plant associations and corresponding abbreviations used in vegetation analysis..33 2. Nest and floral resources found at study sites, south to north……....………..….35 3. Site characteristic percent cover for major vegetation types and summary groups at 100 m, 250 m and 1,050 m .................……………………………..….36 4. Spearman rank correlation between major vegetation categories......……………37 5. Distribution of bee species found at Tolowa during plot surveys.....................…40 6. Bee species relative abundance pooled across all sites at Tolowa.....……….…...43 7. Spearman-rank correlation between bee abundance and major vegetation categories. Values in bold indicate significant correlations after Bonferroni correction ( p ≤ .001). Values in bold italics indicate significant correlations after Holm’s correction ( p ≤ .025). ...................................................….….…......44 8. Spearman-rank correlation between bee richness and selected vegetation categories. Values in bold indicate significant correlations after Bonferroni correction ( p ≤ .002). Values in bold italics indicate significant correlations after Holm’s correction (p ≤ .05)..........................................................................47 9. Correlation coeffiecients between the abundance of vegetation and vegetation groups at 100 m, 250 m and 1,050 m and the first two ordination axes, from an NMS ordination of 19 bee species and 36 variables at 12 study sites, Tolowa Dunes. Values in bold are for r values ≥ ± 0.500………………………...……48 10. Correlation coefficients between the abundance of nest substrate at 100 m and the first two ordination axes, from an NMS ordination of 19 bee species and 36 variables at 12 study sites, Tolowa Dunes. Values in bold are for r values ≥ ± 0.500……………………………………………..................................................49 11. Correlation coefficients between the abundance of floral resources at 100 m and the first two ordination axes, from an NMS ordination of 19 bee species and 36 variables at 12 study sites, Tolowa Dunes. Values in bold are for r values ≥ ± 0.500.…...........................…………………………………………….…………49 v 12. Correlation coefficients between the abundance of bee species and bee functional groups at 100 m and the first two ordination axes, from an NMS ordination of 19 bee species and 36 variables at 12 study sites, Tolowa Dunes. Values in bold are r values ≥ ± 0.500……...…………………………………...50 13. Correlation coefficients between the abundance of other categories at 100 m, 250 m and 1,050 m and the first two ordination axes, from an NMS ordination of 19 bee species and 36 variables at 12 study sites, Tolowa Dunes. Values in bold are r values ≥ ± 0.500.........................…….……………………………………50 14. Relative abundance of bee species at Tolowa and the North Spit........................52 15. Summary statistics for NMS ordination axes for combined Tolowa and North Spit ordination. Values over 0.500 are bolded…………………..…….…………53 16. Bees that are common to abundant at one or more sites. Abundance scale; R = < 5; U = 5-10; O = 11-25; C = 26-50; A = > 50 …………………………....57 vi LIST OF FIGURES Figures Page 1. Study site locations: Tolowa, Del Norte County, California; North Spit, Humboldt County, California and Bodega Marine Research Station, Mendocino County, California. .........................................................................34 2. Proportional abundance of nest substrate measured in surveys at Tolowa, by plot......................................................................................39 3. Bee species richness at Tolowa, by site.................................................................41 4. Bee abundance, all species at Tolowa, by site.......................................................42 5. Bi-plot resulting from nonmetric multidimensional scaling of the bee fauna at Tolowa Dunes........................................................................................................51 6. Bi-plot resulting from nonmetric multidimensional scaling of the bee fauna at Tolowa Dunes and the North Spit..........................................................................54 7. Comparison of the bee fauna at Tolowa versus the North Spit: a) number of bees by species, per observation hour, at Tolowa and b) number of bees by species, per observation hour, at the North Spit..................................................................55 8. Comparison of plot vegetation at Tolowa and the North Spit: a) 1 hectare plot composition by vegetation type, Tolowa and b) 1 hectare plot composition by vegetation type, North Spit....................................................................................56 vii LIST OF APPENDICES Appendix Page A. Site characterization at 100 m, 250 m and 1,050 m...............................................58 B. Floral visitation and floral surveys........................................................................62 C. Floral visitation by bee species..............................................................................66 D. Bee