Date of Award

Winter 2022


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Document Type


Degree Name

Master of Science (MS) in Biology




This study seeks to determine the individual effects of two non-native fish, the brook stickleback, Culaea inconstans, and pumpkinseed, Lepomis gibbosus, on the aquatic invertebrates and food webs of Turnbull National Wildlife Refuge (TNWR). Ten ponds were analyzed and compared with stable isotope analysis and invertebrate abundance data. Three ponds were chosen to represent each of the following categories: fishless, brook stickleback invaded (BS), and coinvaded, along with a single pumpkinseed (PS) pond. Overall, δ[superscript]13C and δ[superscript]15N values increased in invaded ponds, indicating shifts in the zooplankton community structure and the carbon source of predatory invertebrates. Mean δ[superscript]13C was the highest among coinvaded ponds at 6.35% greater than mean fishless δ[superscript]13C, reflecting a shift towards benthic carbon sources. Mean δ[superscript]15N of BS ponds was 2.85% greater than fishless, which may indicate a greater amount of omnivory among invertebrate predators within BS ponds. Mean zooplankton length decreased by 104% in the presence of BS ponds and 79% in coinvaded ponds. The addition of pumpkinseed was found to be the primary factor effecting zooplankton abundance and biomass, where abundance was 662.3% greater in coinvaded ponds than in fishless ponds and biomass was 328.6% greater than fishless ponds. Stickleback alone was not found to be a significant factor in abundance or biomass. The increase in abundance and biomass in coinvaded ponds were a result of an increase in small zooplankton species, especially Chydoridae species (17-fold greater abundance), and reduction of larger branchiopods. Invertebrate predators such as Hirudinae and Coleoptera demonstrated enriched δ[superscript]13C and δ[superscript]15N, likely due to increased omnivory in the presence of fish. The largest amount of variation in isotopic values and zooplankton community structure was seen among coinvaded ponds and the possible explanatory factor of relative fish abundance (CPUE) was explored but undetermined due to the inability to rule out other factors such as seasonal effects. Further research is needed to differentiate the effects of these fish, particularly that of pumpkinseed. These insights will aid managers at TNWR in implementing effective management plans designed to address the specific effects of both species.