Consumer versus resource control and the importance of habitat heterogeneity for estuarine bivalves

The relative influence of consumers (top down) and resources (bottom up) on the distribution and abundance of organisms remains a key question in ecology. We examined the relationships between consumer and resource variables along a productivity gradient for a dominant predator–prey interaction in a marine soft-sediment system. We 1) quantified density and size of the clam Macoma balthica (prey species) in six replicate sites at each of four habitat types (shallow mud, deep mud, muddy sand and detrital mud) in the Rhode River, Chesapeake Bay. We selected one habitat type of high food availability and clam density (shallow mud) and another of low food availability and clam density (muddy sand) for manipulative experiments. Then, we 2) measured M. balthica survival and growth through transplants, 3) measured food availability as sedimentary organic carbon content, 4) quantified predator density, and 5) calculated predator foraging efficiency in the two habitat types. Clam density in the four habitat types differed and was related to sedimentary carbon availability and predator density. One of the habitats, detrital mud, appeared to be a population sink because it only held juvenile Macoma that never survived to reproductive age. Macoma size and growth, and predator (mainly blue crab Callinectes sapidus) densities were positively correlated with productivity and were higher in shallow mud than muddy sand. In contrast, Macoma mortality, local ‘interaction strength’, and predator foraging efficiency were lower in the productive habitat (shallow mud). Thus, predation intensity was inversely correlated with productivity (food availability); consumer and resource effects differed by habitat type; and, at a relatively small spatial scale, consumer and resource forces jointly determined population dynamics in this soft-sediment marine system.

消费者下行控制（top-down）与资源上行控制（bottom-up）对生物分布与种群丰度的相对影响，始终是生态学领域的核心科学问题之一。本研究以海洋软底质生境中的优势捕食者-猎物互作系统为研究对象，沿生产力梯度探究了消费者与资源变量间的关联。具体研究内容如下：1）在切萨皮克湾罗德岛河的四种生境类型（浅泥滩、深泥滩、泥砂滩与腐泥滩）的各6个重复样地中，定量测定了猎物物种白樱蛤（Macoma balthica）的种群密度与个体大小；2）选取食物可利用性与蛤类密度均较高的生境（浅泥滩），以及二者均较低的生境（泥砂滩）开展控制性野外实验，通过移植实验测定白樱蛤的存活率与生长速率；3）以沉积物有机碳含量作为指标量化食物可利用性；4）统计捕食者的种群密度；5）计算捕食者的觅食效率。

研究结果表明：四种生境中的蛤类密度存在显著差异，且与沉积物有机碳含量及捕食者密度密切相关。其中腐泥滩生境表现为种群汇（population sink），该生境中仅存在幼年白樱蛤，且这些个体无法存活至繁殖年龄。白樱蛤的个体大小、生长速率以及捕食者（主要为蓝蟹Callinectes sapidus）的种群密度均与生产力呈正相关，且在浅泥滩中的数值显著高于泥砂滩。与之相反，高生产力生境（浅泥滩）中的白樱蛤死亡率、局域互作强度与捕食者觅食效率均更低。综上，捕食压力与生产力（食物可利用性）呈负相关；消费者与资源的调控效应因生境类型而异；在较小的空间尺度下，消费者与资源的共同作用决定了该海洋软底质生境中的种群动态。