Abstract
Empirical patterns that emerge from an examination of food webs over gradients of environmental variation can help to predict the implications of anthropogenic disturbance on ecosystems. This “dynamic food web approach” is rarely applied at the coastal margin where aquatic and terrestrial systems are coupled and human development activities are often concentrated. We propose a simple model of ghost crab (Ocypode quadrata) feeding that predicts changing dominant prey (Emerita talpoida, Talorchestia sp., Donax variablis) along a gradient of beach morphology and test this model using a suite of 16 beaches along the Florida, USA coast. Assessment of beaches included quantification of morphological features (width, sediments, slope), macrophyte wrack, macro-invertebrate prey and active ghost crab burrows. Stable isotope analysis of carbon (13C/12C) and nitrogen (15N/14N) and the SIAR mixing model were used to determine dietary composition of ghost crabs at each beach. The variation in habitat conditions displayed with increasing beach width was accompanied by quantifiable shifts in ghost crab diet and trophic position. Patterns of ghost crab diet were consistent with differences recorded across the beach width gradient with respect to the availability of preferred micro-habitats of principal macro-invertebrate prey. Values obtained for trophic position also suggests that the generalist ghost crab assembles and augments its diet in fundamentally different ways as habitat morphology varies across a highly dynamic ecosystem. Our results offer support for a functional response in the trophic architecture of a common food web compartment (ghost crabs, macro-invertebrate prey) across well-known beach morphologies. More importantly, our “dynamic food web approach” serves as a basis for evaluating how globally wide-spread sandy beach ecosystems should respond to a variety of anthropogenic impacts including beach grooming, beach re-nourishment, introduction of non-native or feral predators and human traffic on beaches.
Highlights
Ecologists have long searched for general topological patterns in food webs across disparate ecological systems with the goal of identifying structure that mediates the stability of nature’s complex networks [1,2,3,4]
Researchers utilizing the dynamic food web approach have employed stable isotopes to show that changes in food web structure appear to be most dramatically driven by mobile consumers of a high trophic level, which are capable of behaviorally responding to spatial variation in prey and environmental conditions
Mole crab density was highest at the two narrowest sites (CC, IND), macrophyte wrack biomass was highest at two intermediate width sites (ANC, SGSP) and coquina clam density was highest at two of the wider intermediate sites (PRD, BOW) (Fig 4)
Summary
Ecologists have long searched for general topological patterns in food webs across disparate ecological systems with the goal of identifying structure that mediates the stability of nature’s complex networks [1,2,3,4]. Changing Habitat Morphology Modifies Predator Diet metrics (e.g., number of trophic links) using a “static food web approach” While such efforts have produced a powerful and informative literature, an emerging body of research, instead, considers how a specific food web (e.g., lake, rocky intertidal, sandy beach, mudflat) changes, or adapts, across environmental/physical gradients [5,6,7,8]. Though, increased prey densities (i.e., resource availability) in different habitats can influence the foraging decisions of key mobile and upper trophic level consumers [11, 12] When such documented changes in food web structure are mechanistically understood, they can act as powerful tools to predict the implications of altered resource accessibility or availability which is often the consequence of human impact on ecosystems
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