Insect pollinators and predators of Spalding’s catchfly (Silene spaldingii) of the Zumwalt Prairie, Oregon Final Report – January 2012 USFWS Agreement # F10AC00090 (old #13420-A-J002) Robert V. Taylor Sandra J. DeBano The Nature Conservancy Oregon State University 906 S River St Hermiston Agricultural Research & Extension Center Enterprise, OR 97828 PO Box 105 [email protected] Hermiston, OR 97838 [email protected] Abstract Spalding’s catchfly (Silene spaldingii) is a rare, threatened, perennial wildflower which depends on insect pollinators for reproduction. We studied the pollinators insect predators, and ungulate browse rates on S. spaldingii in 2010 and 2011 on the Zumwalt Prairie. We also examined fruit and seed production to assess reproductive output and conducted a greenhouse experiment to examine seed viability. We found that Bombus fervidus and B. appositus were the principal pollinators of S. spaldingii and confirmed that bees carried S. spaldingii pollen and that 34% of muli-plant visits involved 6 or more catchfly plants. Bees were also more likely to visit dense patches of S. spaldingii and areas having high numbers of blooming forbs. We found that 70% of S. spaldingii stems were browsed or had their flowers eaten by insect predators such as the noctuid moth Heliothis oregonica. Only 10% of stems produced mature fruits with an average of 119 (±42 SD) seeds per fruit. Seed germination rates were low (9%) compared to rates found in previous research on this plant; seeds produced later in the season had slightly higher rates (16%) than those produced earlier (6%). Although S. spaldingii on the Zumwalt Prairie appears to have adequate insect pollinators available, low seed production and viability may present a threat to this population of this federally-listed, threatened plant. Introduction Spalding’s catchfly (Silene spaldingii S Watson) is a perennial wildflower that once grew throughout the bunchgrass prairies of the inland Pacific Northwest. Due primarily to the plowing of its habitat, this plant is now very rare and was listed as a threatened species under the U.S. Endangered Species Act (ESA) in 2001(US Fish and Widlife Service 2007). Currently, this plant’s geographic range includes small portions of Washington, Oregon, Idaho and Montana, where populations are mostly small and fragmented (Figure 1). Fourteen populations occur in the Blue Mountains Basin ecoregion in Spalding’s s catchfly pollination and predation northeast Oregon, an area that encompasses both the Zumwalt Prairie and the adjacent Wallowa Valley and Wallowa Lake moraines, and varies in elevation from approximately 1100 to 1600 m. A large population of S. spaldingii (>10,000 individuals) inhabits rolling loess-influenced bunchgrass hills and plateaus on The Nature Conservancy’s Zumwalt Prairie Preserve and adjacent lands (Figure 1; Jansen and Taylor 2010). Due to its large size and distance from other catchfly populations, this area is significant for conservation of this species. With funding from the US Fish and Wildlife Service and other sources, we conducted a study to investigate how insect pollinators and insect predators affect reproductive success in this large population S. spaldingii. Our study addressed three key issues identified in the Recovery Plan relating to insect pollination, insect predation, and catchfly’s ability to successfully reproduce by seed (US Fish and Widlife Service 2007). First, S. spaldingii reproduces only via seed and has been shown to rely on insect pollinators for production of fertile seed (Hitchcock and Maguire 1947, Lesica 1993). One study observed an overall reduction in catchfly fitness of more than 95% in the absence of insect pollinators due to decreases in the proportion of fruits maturing, the number of seeds per fruit, rates of germination, and seedling growth and survival (Lesica 1993). Lesica and Heidel (1996) found that the rate of fruit abortion was negatively correlated with the rate of insect visitation. Thus, there is ample evidence that insect-mediated cross-pollination is critical to maintaining viable populations of this species. In recent years, however, many insects that perform important pollination services have been declining raising concerns that native plant species may be inadequately served by pollinator populations (Allen-Wardell et al. 1998). Although at least one thorough study of the pollinators of S. spaldingii has been done in the past (Lesica 1993), it did not include the Zumwalt Prairie as a study site and was done over two decades ago. In this study we sought to identify the principal pollinators, compare them to the available pool of pollinators in the area and document rates of visitation. Because the effectiveness of pollinators is also influenced by contextual factors (Kremen et al. 2007) we also evaluated two factors that might influence visitation rates: the density of S. spaldingii plants in the area and the abundance of other flowering plants. We use these data to test predictions made by resource concentration (Root, 1973 , facilitation (Waser, 1979), and competition (Pleasants, 1980) hypotheses which provide alternate explanations of how the abundance of plants influence the abundance of associated insects. Second, observers of S. spaldingii have documented various insect predators (e.g., weevils and other beetles, lepidopteran larvae), which prey on the flowers and fruits of S. spaldingii with rates as high as 90% reported in some studies (US Fish and Widlife Service 2007). Although, it is clear that insect predators can reduce the number of fruits and seeds produced by S. spaldingii, to date there have been no studies examining the 2 Taylor & Debano US FWS Agreement# F10AC00090 Spalding’s s catchfly pollination and predation extent to which they actually affect catchfly populations. As part of this study, we tracked all catchfly plants in six plots throughout the growing season and documented rates of predation by insects and other predators. By following plants until senescence we also were able to determine how many catchfly plants successfully produced fruits. Finally, the long-term viability of S. spaldingii populations depends ultimately on its ability to produce a sufficient number of viable seeds. Whereas lack of pollination services might be one cause for low seed production and/or low seed viability, other factors, including small population size, genetic isolation, and inbreeding, are other possibilities. To understand whether pollination or some other factor might be limiting reproductive output of S. spaldingii, we collected fruits and counted seeds from approximately 90 plants and tested the viability of the seeds. Using these data, along with observations we made on fruit production by catchfly plants, we present some rough, but intriguing estimates of total reproductive output of this rare plant on the Zumwalt Prairie. In the sections below we describe our methods and report the results of our study. We conclude this report with a discussion of our findings and how they relate to the conservation of S. spaldingii. By coupling the results of this study to recent investigations of the effects of cattle grazing on the grassland food web (including pollinators) we make recommendations to land managers and policy makers which, if followed, should serve to benefit populations of key pollinators and in doing so help secure the viability of S. spaldingii populations across its geographic range, thus significantly advancing efforts for recovery of the species. Methods Study Area The study was conducted at the Zumwalt Prairie Preserve (ZPP, lat 45º 3’ N, long 116º 6’ W) located in Wallowa County in northeastern Oregon (Figure 2 a,b). The 13,269-ha preserve is owned and managed by The Nature Conservancy (TNC) and lies in the southwestern portion of the Pacific Northwest Bunchgrass Prairie (Tisdale 1982). At 1,060-1,680 m elevation, the preserve is dominated by native bunchgrasses, including Idaho fescue (Festuca idahoensis Elmer), Sandberg bluegrass (Poa secunda J. Presl), prairie Junegrass (Koeleria macrantha [Ledeb.] Schult.), and bluebunch wheatgrass (Pseudoroegneria spicata [Pursh] A. Löve). It has a diverse assemblage of over 112 forb species including Aster L. spp., western yarrow (Achillea millefolium L.), lupines (Lupinus L. spp.), prairie smoke (Geum triflorum Pursh), and cinquefoil (Potentilla L. spp.) (Kennedy et al. 2009). S. spaldingii tends to have a patchy yet relatively 3 Taylor & Debano US FWS Agreement# F10AC00090 Spalding’s s catchfly pollination and predation substantial distribution within the preserve, with more than 50,000 plants estimated to occur across 112 ha (Jansen and Taylor 2009). Pollinator Observations Pollinator observations were conducted between 3-12 August 2010 at 30 randomly located sites in the southern portion of the preserve (Harsin pasture), which is known to have a large concentration of the species and has been excluded from livestock grazing since 2005 (Figure 2c; Jansen and Taylor 2009). Timing of the observations coincided with the peak blooming period; S. spaldingii was observed blooming in the area from 22 July-2 September and 85% of all blooms were observed before 13 August (R.V. Taylor, unpublished data). We made observations by walking from site to site recording all instances of potential pollinators visiting S. spaldingii, following pollinators to multiple plants, and capturing visitors when possible. A total of 91.25 person-hours were spent doing observations, including 7 person-hours