Effects of group size (local conspecific density) on individual performance can be substantial, yet it is unclear how these translate to larger-scale and longer-term outcomes. Effects of group size can be mediated by both top-down and bottom-up interactions, can change in type or direction across the life cycle, and can depend on the spatial scale at which group size is assessed. Only by determining how these different processes combine can we make predictions about how selection operates on group size or link hierarchical patterns of density-dependence with population dynamics. We manipulated the density of a leaf beetle, Leptinotarsa juncta, at three nested spatial scales (patch, plant within a patch, and leaf within plant) to investigate how conspecific density affects predator-mediated survival and resource-mediated growth during different life stages and across multiple spatial scales. We then used data from field predation experiments to assess how L. juncta densities at hierarchical scales affect different aspects of predation. Finally, we incorporated predator- and resource-mediated effects of density in a model to explore how changes in group size due to density-dependent predation might affect mass at pupation for survivors. The effects of L. juncta density on predation risk differed among scales. Per capita predation risk of both eggs and late-instar larvae was lowest at high patch-scale densities, but increased with plant-scale density. The final mass of late-instar larvae declined with increasing plant-scale larval density, potentially due to truncated development of high-density larvae. Predation incidence (i.e., group attack rate) increased with larval density at all spatial scales. A high coefficient of variation (i.e., greater aggregation) of L. juncta density was associated with lower predation incidence at some scales. Our model suggested that predator- and resource-mediated effects of density interact: lower survival at high larval density is mitigated by high final mass of larvae in the resulting smaller groups. Our results emphasize the importance of spatial scale and demonstrate that effects of top-down and bottom-up interactions are not necessarily independent. To understand how group size influences fitness, predator- and resource-mediated effects of density should be measured in their demographic and spatial context, and not in isolation.

中文翻译：

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整合跨尺度和复杂生命周期的局部密度的自上而下和自下而上的影响

群体规模（局部同种密度）对个人表现的影响可能很大，但尚不清楚这些影响如何转化为更大规模和更长期的结果。群体规模的影响可以通过自上而下和自下而上的相互作用来调节，可以在整个生命周期的类型或方向上发生变化，并且可能取决于评估群体规模的空间尺度。只有通过确定这些不同过程如何结合，我们才能预测选择如何影响群体规模或将密度依赖的层次模式与人口动态联系起来。我们在三个嵌套的空间尺度（斑块、斑块内的植物、和植物内的叶子）以研究同种密度如何影响不同生命阶段和跨多个空间尺度的捕食者介导的生存和资源介导的生长。然后，我们使用来自野外捕食实验的数据来评估等级尺度上的 L. juncta 密度如何影响捕食的不同方面。最后，我们将捕食者和资源介导的密度效应纳入模型，以探索由于密度依赖性捕食导致的群体规模变化如何影响幸存者的化蛹质量。L. juncta 密度对捕食风险的影响因尺度而异。卵和晚龄幼虫的人均捕食风险在高斑块规模密度下最低，但随着植物规模密度而增加。晚龄幼虫的最终质量随着植物规模幼虫密度的增加而下降，可能是由于高密度幼虫的发育被截断。在所有空间尺度上，捕食发生率（即群体攻击率）随着幼虫密度而增加。L. juncta 密度的高变异系数（即更大的聚集）与某些尺度上较低的捕食率有关。我们的模型表明，捕食者和资源介导的密度效应相互作用：在由此产生的较小群体中，幼虫的最终质量高，从而减轻了高幼虫密度下较低的存活率。我们的结果强调了空间尺度的重要性，并证明自上而下和自下而上相互作用的影响不一定是独立的。要了解群体规模如何影响健康，捕食者和资源介导的密度效应应在其人口统计和空间背景下进行衡量，而不是孤立地衡量。