THE ECOLOGICAL IMPACTS OF SIGNAL CRAYFISH IN UPLAND STREAM ECOSYSTEMS

Abstract

Non-native species are an important driver of global biodiversity loss. Worldwide, crayfishes are one of the prominent groups of non-native species. In this study, the American signal crayfish Pacifastacus leniusculus, the most widespread non-native species in Europe, was used as a model invasive crayfish species to determine the impacts and factors driving the dispersal of non-native species in upland stream ecosystems of northeast England.

Strong impacts of signal crayfish on stream biota over short (~7 weeks), medium (7 years) and long (28 years) timescales was evident through a combination of controlled mesocosm study, field surveys of a large number of streams and historical data. Density-dependent impacts of crayfish on multiple components of ecosystems including algal growth, leaf litter decomposition, macroinvertebrates and benthic indigenous fish were revealed. Stable isotope analyses showed a significant change in the trophic position of benthic fish in relation to crayfish density but it remained unchanged for crayfish. Decreased abundance of benthic fishes and young-of-year salmonids were recorded over time in crayfish-invaded streams whereas an opposite trend was recorded in uninvaded streams. Benthic fish disappeared in two invaded streams. Three uninvaded streams were invaded between 2011 and 2018. Dramatic declines in macroinvertebrate abundance and taxonomic richness were recorded in invaded streams and stream reaches compared to uninvaded controls.

This thesis also identified the factors driving the dispersal of invading crayfish in upland streams through the analysis of crayfish personality, propagule pressure and habitat suitability. Study of three population conditions (fully-established, newly-established and invasion front) revealed that crayfish dispersal in invaded habitats is context dependent. Personality traits played an important role in dispersal, especially at the invasion front but other factors including local population density and availability of refuges also play a key role. Apart from conventional personality traits (e.g. activity, distance moved and exploration), climbing ability, a trait that has received less attention in behavioural studies, was found to influence crayfish dispersal at newly-established and invasion front sites.

Currently, no single method is effective in controlling the spread of non-native crayfish to new sites, and at locations where invasive crayfish already exist. Therefore, improvement of existing legislative measures and raising awareness through education are very much needed to reduce intentional and unintentional introductions. In invaded habitats, if early detection is possible, damage can, potentially, be minimised through existing control methods. In-stream barriers may offer promise in controlling crayfish invasion in streams but this requires further research to validate and optimise designs. Findings of this thesis have contributed to our understanding of biological invasion, especially in upland stream ecosystems and underline the importance of managing crayfish invasion.