Predator Diet and Trophic Position Modified with Altered Habitat Morphology

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.通过探究环境变化梯度下的食物网所揭示的经验模式，有助于预测人为干扰对生态系统的影响。这种“动态食物网方法”极少应用于水陆系统耦合且人类开发活动高度集中的海岸带边缘区域。我们提出了一个关于鬼蟹（Ocypode quadrata）摄食的简单模型，该模型可预测沿海滩形态梯度变化的优势猎物（Emerita talpoida、Talorchestia sp.、Donax variablis），并利用美国佛罗里达沿岸的16处海滩对该模型进行验证。海滩评估包括对形态特征（宽度、沉积物类型、坡度）、大型海藻枯落物、大型无脊椎动物猎物以及活跃鬼蟹洞穴的量化分析。采用碳（¹³C/¹²C）、氮（¹⁵N/¹⁴N）稳定同位素分析及SIAR混合模型，测定各海滩鬼蟹的饮食组成。随着海滩宽度增加，生境条件发生变化，鬼蟹的饮食与营养位也随之出现可量化的变化。鬼蟹的饮食模式与海滩宽度梯度上主要大型无脊椎动物猎物的偏好微生境可利用性差异相一致。营养位的测定结果还表明，作为广食性物种的鬼蟹，会在高度动态的生态系统中，随生境形态的变化以本质上不同的方式组合并调整其饮食结构。本研究结果证实，在不同典型海滩形态下，常见食物网模块（鬼蟹、大型无脊椎动物猎物）的营养结构存在功能响应。更为重要的是，我们提出的“动态食物网方法”可作为评估全球广泛分布的沙滩生态系统如何响应各类人为干扰的基础框架，这些干扰包括海滩清理、海滩补沙、外来或野生捕食者引入以及海滩人类活动。