Johnson Darren W
Department of Ecology and Evolutionary Biology, University of California, Santa Cruz 95060, USA.
Ecology. 2006 May;87(5):1179-88. doi: 10.1890/0012-9658(2006)87[1179:phcavi]2.0.co;2.
Density dependence in demographic rates can strongly affect the dynamics of populations. However, the mechanisms generating density dependence (e.g., predation) are also dynamic processes and may be influenced by local conditions. Understanding the manner in which local habitat features affect the occurrence and/or strength of density dependence will increase our understanding of population dynamics in heterogeneous environments. In this study I conducted two separate field experiments to investigate how local predator density and habitat complexity affect the occurrence and form of density-dependent mortality of juvenile rockfishes (Sebastes spp.). I also used yearly censuses of rockfish populations on nearshore reefs throughout central California to evaluate mortality of juvenile rockfish at large spatial scales. Manipulations of predators (juvenile bocaccio, S. paucispinus) and prey (kelp, gopher, and black-and-yellow [KGB] rockfish, Sebastes spp.) demonstrated that increasing the density of predators altered their functional response and thus altered patterns of density dependence in mortality of their prey. At low densities of predators, the number of prey consumed per predator was a decelerating function, and mortality of prey was inversely density dependent. However, at high densities of predators, the number of prey killed per predator became an accelerating response, and prey mortality was directly density dependent. Results of field experiments and large-scale surveys both indicated that the strength of density-dependent mortality may also be affected by the structural complexity of the habitat. In small-scale field experiments, increased habitat complexity increased the strength of density-dependent mortality. However, at large scales, increasing complexity resulted in a decrease in the strength of density dependence. I suggest that these differences resulted from scale-dependent changes in the predatory response that generated mortality. Whether increased habitat complexity leads to an increase or a decrease in the strength of density-dependent mortality may depend on how specific predatory responses (e.g., functional or aggregative) are altered by habitat complexity. Overall, the findings of this study suggest that rates of demographic density dependence and the resulting dynamics of local populations may largely depend upon attributes of the local habitat.
种群统计学速率中的密度依赖性会强烈影响种群动态。然而，产生密度依赖性的机制（如捕食）也是动态过程，可能会受到当地条件的影响。了解当地栖息地特征影响密度依赖性的发生和/或强度的方式，将增进我们对异质环境中种群动态的理解。在本研究中，我进行了两项独立的野外实验，以探究当地捕食者密度和栖息地复杂性如何影响幼年岩鱼（Sebastes spp.）密度依赖性死亡的发生和形式。我还利用对加利福尼亚中部近岸礁石上岩鱼种群的年度普查，来评估大空间尺度上幼年岩鱼的死亡率。对捕食者（幼年博氏岩鱼，S. paucispinus）和猎物（海带、囊型和黑黄[KGB]岩鱼，Sebastes spp.）的操控表明，增加捕食者密度会改变其功能反应，从而改变其猎物死亡率的密度依赖性模式。在捕食者低密度时，每个捕食者消耗的猎物数量是一个减速函数，猎物死亡率呈反向密度依赖性。然而，在捕食者高密度时，每个捕食者杀死的猎物数量变成了加速反应，猎物死亡率呈直接密度依赖性。野外实验和大规模调查的结果均表明，密度依赖性死亡的强度也可能受到栖息地结构复杂性的影响。在小规模野外实验中，栖息地复杂性增加会增强密度依赖性死亡的强度。然而，在大尺度上，复杂性增加会导致密度依赖性强度降低。我认为这些差异是由产生死亡率的捕食反应中与尺度相关的变化导致的。栖息地复杂性增加是导致密度依赖性死亡强度增加还是降低，可能取决于栖息地复杂性如何改变特定的捕食反应（如功能或聚集反应）。总体而言，本研究结果表明，种群统计学密度依赖性速率以及由此产生的当地种群动态可能在很大程度上取决于当地栖息地的属性。
