Pritchard, Eleri Gwenllian; (2022) New approaches for exploring signal crayfish invasion biology and ecological impacts in headwater streams. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The global spread of invasive alien species has had severe ecological, social and economic implications, with freshwater systems proving particularly vulnerable to invasion. Freshwater crayfish are exceptionally successful invaders, and 90% of species introduced to Europe have become established in the wild. As ecosystem engineers, crayfish present a significant threat to aquatic ecosystems. However, methodological constraints have thus far limited our understanding of invasive crayfish ecology and environmental impacts. This project aims to better our understanding of the spatio-temporal ecological impact invasive crayfish have on native biota, using Bookill Gill Beck and Long Preston Beck in the Yorkshire Dales as a case study. A novel method was developed to produce quantitative data on crayfish populations along an invasion gradient from well-established sites to the invasion front, where native crayfish still persist. The method was rigorously tested to evaluate efficiency and optimal deployment time for both crayfish and benthic fish. Following proof-of-concept, the method was then used to explore the invasion ecology and ecological impacts of signal crayfish within the headwater system. Population demographics of signal crayfish were investigated along the invasion gradient over three subsequent field seasons (2018 - 2020), focussing on population density, size structure and biomass, including relationships with substrate composition. Subsequently, density- dependent impacts of signal crayfish on macroinvertebrate and fish communities were explored, comparing sites along the invasion gradient and temporal changes at individual sites over the three-year timeframe. Notable changes in macroinvertebrate community composition and severe declines of native fish were observed, with European bullhead the most affected fish species. The results of this research can be used to inform conservation and management decisions by greatly enhancing our understanding of the invasion biology and ecological impacts of invasive crayfish, whilst also offering a novel method to be used in quantitative population assessments in future research and monitoring.

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