The context-dependent spread and impacts of invasive marine crabs

Following the establishment of a non-native species, there is often speculation about the potential impacts to the native ecosystem. While these early predictions may be necessary for management, they are often based on a general understanding of invasion ecology rather than context-specific research. The unique nature of each introduction event means these generalizations are prone to over- or under-estimating invasive species impacts. This thesis predicts the impacts of invasive marine true crabs (infraorder Brachyura), with a focus on the invasive European green crab (Carcinus maenas), using both general ‘rules of thumb’ and context-specific research. In Chapter 2, I conduct a meta-analysis to demonstrate that while native and invasive crabs typically have a similar overall impact on prey species, some combinations of prey type and experimental design can favour invasive crabs. In Chapter 3, I examine the geographical variability of green crab impacts worldwide. Using green crabs collected from invasive (South Africa and Canada) and native (Northern Ireland) populations, I conduct a comparative functional response experiment to show how the foraging behaviour of an invasive species varies among regions. In Chapter 4, I use an enclosure experiment to determine how the impact of green crabs on eelgrass (Zostera marina) ecosystems changes with crab density, and conclude that there is the potential for extensive loss of habitat-forming eelgrass in the presence of high densities of green crabs. In Chapter 5, I explore the issue of site-level variability in the abundance, and therefore potential impact, of green crabs on the west coast of Vancouver Island, British Columbia. I develop a species distribution model to identify small-scale biotic and abiotic predictors of ‘hyper-abundant’ populations of green crab. The thesis as a whole explores the generalizations often used to predict invasive impacts and prioritize impact mitigation efforts. I find that, for green crabs, generalizations that rely on the origin or specific invasion history of an invasive species are prone to over-estimating impact. However, measures of density or abundance, paired with an understanding of context-specific behaviours, are more likely to produce reliable impact predictions for these invasive species.