Abstract
Various niche hypotheses have been formulated to explain the success of nonnative species within a novel community. The vacant niche hypothesis (VNH) predicts that nonnative species, upon arrival to a new area, will be successful by occupying a niche and exploiting resources which are not being utilized locally by native species. The niche competition hypothesis (NCH) states that nonnative species may occupy a niche which overlaps with a niche being utilized by native species. The trophic disruption hypothesis (TDH) is an extension of the NCH which predicts that nonnative species, following their introduction, will result in the trophic and isotopic displacement of native species, or of the native community as a whole. The aquatic community of Rainey Slough, located in Glades County, Florida, USA, is a relatively undisturbed wetland dominated by Water Hyacinth that provides an ideal setting to examine the VNH, NCH, and TDH within the context of a long-term dataset which has been procured since the 1970s. Multiple nonnative fishes have proliferated at Rainey Slough over the past 50 years, including Walking Catfish (Clarias batrachus), Brown Hoplos (Hoplosternum littorale), African Jewelfish (Hemichromis bimaculatus/Rubricatochromis letourneuxi), and Asian Swamp Eels (Monopterus albus/javanensis). Stable isotopes have been useful across multiple studies which examine long-term change in aquatic communities across the δ13C and δ15N niche axes, and fluid-preserved museum specimens provide the opportunity to examine isotopic change over long periods of time. However, fluid-preservation methods have been reported to cause either depletion or enrichment of δ13C and δ15N in tissues. In this thesis, I (1) conducted a meta-analysis of the effects of ethanol and formalin preservation on isotopic ratios in preserved fish and invertebrate tissues, and (2) used an extensive collection of preserved specimens and contemporary samples to construct isotopic niches of aquatic species collected from Rainey Slough from 1967-2025. My results suggest that nonnative fishes do not follow the VNH or NCH consistently, although their impacts on native species, particular sirenids, are evident. I found evidence of the TDH, as historical sirenids may have been isotopically displaced through prolonged competitive interactions with Walking Catfish, and the influx of new invaders, including Asian Swamp Eels, likely prevented the sirenid population from recovering. My investigation of isotopic change within the Rainey Slough aquatic ectotherm community provides an example of the potential application of isotopic corrections to account for fluid-preservation, demonstrates the usefulness of natural history collections within the context of stable isotope analysis, and highlights the importance of scientific collection of nonnative species within the early stages of ecological invasion.
