Year of Award

2015

Document Type

Thesis

Degree Type

Master of Science (MS)

Degree Name

Wildlife Biology

Department or School/College

College of Forestry and Conservation

Committee Chair

Lisa Eby

Commitee Members

Winsor Lowe, Brian Steele

Keywords

Aquatic Ecology, Spatial Subsidies, Invasive Species, New Zealand Mudsnails, Phase Shift

Publisher

University of Montana

Subject Categories

Terrestrial and Aquatic Ecology

Abstract

Invasive species have the potential to alter the ecosystems within which they establish as well as adjacent but uninvaded ecosystems by altering the flow of nutrients or biota across system boundaries. New Zealand mudsnails (Potamopyrgus antipodarum; NZMS) are often very abundant where they invade, sequestering the majority of stream primary production. This sequestering of production may reduce aquatic insect production, subsequent insect emergence and subsidies to riparian ecosystems. Tetragnathid spiders, an insectivorous riparian specialist, are dependent on the emergence of aquatic insects and are typical indicators of aquatic insect subsidies to riparian zones. In addition, NZMS invasions can result in persistent community shifts in aquatic insect composition, potentially resulting in lasting impacts to subsidies. We hypothesized that in river sections where they are currently very abundant NZMS would have negative impacts on benthic aquatic insect biomass, reducing the emergence of aquatic insects and spiders that feed on them. We sampled benthic and emerging aquatic invertebrates and tetragnathid spider densities at five locations with various densities of NZMS on two invaded rivers in the Western United States. In the Portneuf River, ID NZMS are currently very abundant while in the Madison River, MT they were abundant fifteen years ago but have since declined. We also analyzed 17 years of benthic community data taken from one location on the Madison River during the NZMS invasion to examine how benthic community composition (and emergent taxa) shifted during peak NZMS densities. In contrast with our expectations, NZMS were positively associated with non-NZMS benthic invertebrate biomass and emergent biomass. Similar to our expectations, spiders had a positive association with emergent biomass and riparian habitat availability. But because of a 200- fold difference in secondary production among sites (sites with high NZMS biomass also had high non-NZMS invertebrate biomass) comparing sites with different densities potentially confounded NZMS density with potential productivity of the site. Examination of time series data of aquatic invertebrate community highlighted that NZMS were associated with lasting changes in benthic community composition marked by reductions in emerging taxa, including Ephmeroptera and Odonata and increases in Baetidae and Tubificidae. Pre-invasion data rarely exist therefore multiple, complementary means of investigation are often necessary to fully evaluate potential impacts of invaders.

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© Copyright 2015 Eric H. Richins