Shima Jeffrey S
Department of Ecology, Evolution, and Marine Biology and the Marine Science Institute, University of California, 93106, Santa Barbara, CA, USA.
Oecologia. 2001 Jan;126(1):58-65. doi: 10.1007/s004420000486. Epub 2001 Jan 1.
Density-dependent mortality can regulate local populations - effectively minimizing the likelihood of local extinctions and unchecked population growth. It is considered particularly important for many marine reef organisms with demographically open populations that lack potential regulatory mechanisms tied to local reproduction. While density-dependent mortality has been documented frequently for reef fishes, few studies have explored how the strength of density-dependence varies with density, or how density-dependence may be modified by numerical effects (i.e., number-dependent mortality). Both issues can have profound effects on spatial patterns of abundance and the regulation of local populations. I address these issues through empirical studies in Moorea, French Polynesia, of the six bar wrasse (Thalassoma hardwicke), a reef fish that settles to isolated patch reefs. Per capita mortality rates of newly settled wrasse increased as a function of density and were well approximated by the Beverton-Holt function for both naturally formed and experimentally generated juvenile cohorts. Average instantaneous mortality rates were a decelerating function of initial densities, indicating the per capita strength of density-dependence decreased with density. Results of a factorial manipulation of density and group size indicate that per capita mortality rates were simultaneously density- and number-dependent; fish at higher densities and/or in groups had higher probabilities of disappearing from patch reefs compared with fish that were solitary and/or at lower densities. Mortality rates were ~30% higher for fish at densities of 0.5 fish/m than at 0.25 fish/m. Similarly, mortality rates increased by ~45% when group size was increased from 1 to 2 individuals per patch, even when density was kept constant. These observations suggest that the number of interacting individuals, independent of patch size (i.e., density-independent effects) can contribute to regulation of local populations. Overall, this work highlights a greater need to consider numerical effects in addition to density effects when exploring sources of population regulation.
密度依赖型死亡率能够调节局部种群——有效降低局部灭绝的可能性以及不受控制的种群增长。对于许多具有开放种群结构、缺乏与本地繁殖相关潜在调节机制的海洋珊瑚礁生物而言，这一点被认为尤为重要。虽然密度依赖型死亡率在珊瑚礁鱼类中经常被记录到，但很少有研究探讨密度依赖的强度如何随密度变化，或者密度依赖如何因数量效应（即与数量相关的死亡率）而改变。这两个问题都可能对丰度的空间格局和局部种群的调节产生深远影响。我通过在法属波利尼西亚的莫雷阿岛对六带裂唇鱼（Thalassoma hardwicke）进行实证研究来解决这些问题，六带裂唇鱼是一种定居在孤立斑块礁上的珊瑚礁鱼类。新定居的裂唇鱼的人均死亡率随密度增加而上升，对于自然形成的和实验产生的幼鱼群体，均能很好地用贝弗顿 - 霍尔特函数来近似。平均瞬时死亡率是初始密度的递减函数，表明密度依赖的人均强度随密度降低。密度和群体大小的析因操纵结果表明，人均死亡率同时依赖于密度和数量；与单独或低密度的鱼相比，处于较高密度和/或群体中的鱼从斑块礁消失的概率更高。密度为0.5条鱼/平方米的鱼的死亡率比密度为0.25条鱼/平方米的鱼高约30%。同样，即使密度保持不变，当每个斑块的群体大小从1增加到2个个体时，死亡率增加了约45%。这些观察结果表明，相互作用个体的数量，独立于斑块大小（即密度独立效应），可以对局部种群的调节做出贡献。总体而言，这项工作强调了在探索种群调节来源时，除了密度效应外，更有必要考虑数量效应。
