Role of Invasive Green Crabs in the Food Web of an Intertidal Sand Flat Determined from Field Observations and a Dynamic Simulation Model

Abstract

A benthic focussed food web model of intermediate complexity was developed for the intertidal sand flat in the Kejimkujik National Park Seaside, NS, Canada. The goal was to understand the role of invasive green crabs and to investigate potential effects of its removal on the ecosystem. Inputs of biomass, production, consumption and diet composition for each model compartment were obtained through targeted field collections in summers 2008–2010. Simulation (via Ecosim) was used to develop a data-driven baseline steady-state food web of summer conditions and to evaluate effects of ecosystem perturbations that included moderate and extreme removal of crab biomass, in the presence and absence of migrating shorebirds. In the baseline model, green crabs were important benthic predators and also prey for fishes and birds. Ecosystem properties (e.g. size, energetics, organisation) suggested a stable ecosystem. Scenario analyses predicted that crab removal caused only slight perturbations to overall ecosystem properties, but affected some trophic components. Predators that consumed mainly crabs (e.g. gulls) decreased in biomass with crab removal because diet changes could not compensate for lost crab prey. Predators that consumed crabs and also other prey (e.g. plovers) increased in biomass with crab removal because shared benthic prey increased. Migrating shorebirds had a greater impact on the food web than crab removal. Our study suggests that removal of invasive green crabs will likely not drastically change food web biomass and productivity. This study illustrates how field data can be synthesised with food web models for effective management of coastal ecosystems.