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Master of Science (MS)
Peterson, Merrill A., 1965-
Hooper, David U., 1961-
The relentless spread of invasive plant species has illuminated their capacity for disrupting essential ecosystem services, including the pollination of native flowers. Invaders that are particularly showy, resource-laden and widespread appear to be the most likely to create pollinator competition. Native plants that are most likely to be impacted by the encroachment of such invaders are those that are sensitive to disturbance, locally rare and obligate-outcrossers. This study examined the effects of a widespread showy invader of the Pacific Northwest, Rubus armeniacus, on an imperiled endemic wildflower, Sidalcea hendersonii. These species are sympatric, have overlapping flowering phenologies and provide pollinators with similar resources: copious amounts of pollen and nectar. Pollinator observations revealed that 77% of species that visited S. hendersonii also visited the invader and that R. armeniacus flowers received more than three times as many total visits as S. hendersonii flowers. Further, three of the most common insect pollinators in the study system, Apis mellifera, Thymelicus lineola, (Lepidoptera: Hesperiidae) and Nacerdes melanura (Coleoptera: Oedemeridae) were non-native. Invasive pollen deposition on native stigmas was high, with 67% of the S. hendersonii stigmas containing at least one R. armeniacus pollen grain. However, there was no relationship between either invasive pollen deposition or S. hendersonii seed set and distance from the invader (1m, 15m and 50m). Across all distances, pollen supplementation revealed that S. hendersonii was pollen-limited, with an average increase in seed set of 37% in pollen-supplemented flowers. Thus, either R. armeniacus does not have a reproductive effect on S. hendersonii and the observed pollen limitation was a result of other environmental factors, or R. armeniacus creates a pollen-limiting force that is felt across the entire study area equally. Regardless, the first-year seed set of this rare, self-incompatible species in a highly invaded environment was on par with several wild S. hendersonii populations in British Columbia (Marshall 1997) and the high degree of visitor diversity suggests that the plant will be serviced even in highly invaded communities. This result, and the general vigor and health of all the transplanted study plants, sheds a positive light on the restoration potential of a rare endemic. Moreover, a main threat to the species, as reported in British Columbia and Oregon populations, is seed predation by weevils (Macrorhoptus sidalcea and Anthonomus melancholicus) (Marshall 1997, Marshall and Ganders 2001, Gisler and Love 2005), which were not found at the Ferndale, Wash., study site, suggesting that restored S. hendersonii populations may actually have greater reproductive success than remnant endemic populations, at least in the short term.
Western Washington University
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Johnson, Natasha S., "The effects of a widespread, showy invasive plant (Rubus armeniacus) on pollinator visitation rates, pollen deposition, and seed set in a rare native wildflower (Sidalcea hendersonii)" (2010). WWU Masters Thesis Collection. 70.