Ward Darren M, Nislow Keith H, Folt Carol L
Department of Biological Sciences, Dartmouth College, Hanover, NH 03755, USA.
Oecologia. 2008 Jun;156(3):515-22. doi: 10.1007/s00442-008-1011-4. Epub 2008 Mar 4.
The effect of predators on prey populations depends on how predator-caused mortality changes with prey population density. Predators can enforce density-dependent prey mortality and contribute to population stability, but only if they have a positive numerical or behavioral response to increased prey density. Otherwise, predator saturation can result in inversely density-dependent mortality, destabilizing prey populations and increasing extinction risk. Juvenile salmon and trout provide some of the clearest empirical examples of density-dependent mortality in animal populations. However, although juvenile salmon are very vulnerable to predators, the demographic effects of predators on juvenile salmon are unknown. We tested the interactive effects of predators and population density on the mortality of juvenile Atlantic salmon (Salmo salar) using controlled releases of salmon in natural streams. We introduced newly hatched juvenile salmon at three population density treatments in six study streams, half of which contained slimy sculpin (Cottus cognatus), a common generalist predator (18 release sites in total, repeated over two summers). Sculpin reversed the direction of density dependence for juvenile salmon mortality. Salmon mortality was density dependent in streams with no sculpin, but inversely density dependent in streams where sculpin were abundant. Such predator-mediated inverse density dependence is especially problematic for prey populations suppressed by other factors, thereby presenting a fundamental challenge to persistence of rare populations and restoration of extirpated populations.
捕食者对猎物种群的影响取决于捕食导致的死亡率如何随猎物种群密度变化。捕食者可以强化密度依赖型的猎物死亡率，并有助于种群稳定，但前提是它们对增加的猎物密度有正向的数量或行为反应。否则，捕食者饱和会导致反向密度依赖型死亡率，使猎物种群不稳定，并增加灭绝风险。幼年鲑鱼和鳟鱼为动物种群中密度依赖型死亡率提供了一些最清晰的实证例子。然而，尽管幼年鲑鱼极易受到捕食者的攻击，但捕食者对幼年鲑鱼的种群统计学影响尚不清楚。我们通过在天然溪流中控制投放鲑鱼，测试了捕食者和种群密度对幼年大西洋鲑（Salmo salar）死亡率的交互作用。我们在六条研究溪流中的三种种群密度处理下投放新孵化的幼年鲑鱼，其中一半溪流中有粘背杜父鱼（Cottus cognatus），一种常见的广食性捕食者（总共18个投放点，在两个夏天重复进行）。杜父鱼改变了幼年鲑鱼死亡率的密度依赖方向。在没有杜父鱼的溪流中，鲑鱼死亡率是密度依赖型的，但在杜父鱼数量丰富的溪流中则是反向密度依赖型的。这种由捕食者介导的反向密度依赖对受其他因素抑制的猎物种群尤其成问题，从而对稀有种群的存续和灭绝种群的恢复构成了根本性挑战。
