White J Wilson, Warner Robert R
Department of Ecology, Evolution, and Marine Biology, University of California, Santa Barbara, California 93106, USA.
Ecology. 2007 Dec;88(12):3044-54. doi: 10.1890/06-1949.1.
In coral reef fishes, density-dependent population regulation is commonly mediated via predation on juveniles that have recently settled from the plankton. All else being equal, strong density-dependent mortality should select against the formation of high-density aggregations, yet the juveniles of many reef fishes aggregate. In light of this apparent contradiction, we hypothesized that the form and intensity of density dependence vary with the spatial scale of measurement. Individual groups might enjoy safety in numbers, but predators could still produce density-dependent mortality at larger spatial scales. We investigated this possibility using recently settled juvenile bluehead wrasse, Thalassoma bifasciatum, a small, aggregating reef fish. An initial caging experiment demonstrated that juvenile bluehead wrasse settlers suffer high predation, and spatial settlement patterns indicated that bluehead wrasse juveniles preferentially settle in groups, although they are also found singly. We then monitored the mortality of recently settled juveniles at two spatial scales: microsites, occupied by individual fish or groups of fish and separated by centimeters, and sites, consisting of approximately 2400-m2 areas of reef and separated by kilometers. At the microsite scale, we measured group size and effective population density independently and found that per capita mortality decreased with group size but was not related to density. At the larger spatial scale, however, per capita mortality increased with settler density. This shift in the form of density dependence with spatial scale could reconcile the existence of small-scale aggregative behavior typical of many reef fishes with the population-scale density dependence that is essential to population stability and persistence.
在珊瑚礁鱼类中，密度依赖型种群调节通常是通过捕食刚从浮游生物中沉降下来的幼鱼来实现的。在其他条件相同的情况下，强烈的密度依赖型死亡率应该会抑制高密度聚集的形成，然而许多珊瑚礁鱼类的幼鱼却会聚集在一起。鉴于这一明显的矛盾，我们推测密度依赖的形式和强度会随着测量的空间尺度而变化。个体群体可能会因数量众多而获得安全，但在更大的空间尺度上，捕食者仍可能导致密度依赖型死亡率。我们使用刚沉降下来的小蓝头濑鱼（Thalassoma bifasciatum）进行了调查，这是一种小型的聚集性珊瑚礁鱼类。最初的网箱实验表明，小蓝头濑鱼幼鱼遭受着很高的捕食率，而且空间沉降模式表明，蓝头濑鱼幼鱼虽然也有单独出现的情况，但它们更倾向于成群沉降。然后，我们在两个空间尺度上监测了刚沉降下来的幼鱼的死亡率：微生境，由个体鱼或鱼群占据，间隔为厘米级；以及地点，由面积约2400平方米的珊瑚礁区域组成，间隔为千米级。在微生境尺度上，我们分别测量了群体大小和有效种群密度，发现人均死亡率随群体大小的增加而降低，但与密度无关。然而，在更大的空间尺度上，人均死亡率随定居者密度的增加而增加。密度依赖形式随空间尺度的这种转变，可能会使许多珊瑚礁鱼类典型的小规模聚集行为的存在，与种群稳定性和持久性所必需的种群尺度密度依赖相协调。
