Data from: Top predators constrain mesopredator distributions

Top predators can suppress mesopredators by killing them, competing for resources and instilling fear, but it is unclear how suppression of mesopredators varies with the distribution and abundance of top predators at large spatial scales and among different ecological contexts. We suggest that suppression of mesopredators will be strongest where top predators occur at high densities over large areas. These conditions are more likely to occur in the core than on the margins of top predator ranges. We propose the Enemy Constraint Hypothesis, which predicts weakened top-down effects on mesopredators towards the edge of top predators’ ranges. Using bounty data from North America, Europe and Australia we show that the effects of top predators on mesopredators increase from the margin towards the core of their ranges, as predicted. Continuing global contraction of top predator ranges could promote further release of mesopredator populations, altering ecosystem structure and contributing to biodiversity loss.

顶级捕食者（top predators）可通过捕杀中层捕食者（mesopredators）、争夺资源以及引发恐惧效应，抑制其中层捕食者种群。但目前尚不明确，在大空间尺度与不同生态背景下，顶级捕食者对中层捕食者的抑制作用如何随其分布范围与种群丰度发生变化。
我们提出，当顶级捕食者在大面积区域内维持高密度种群时，其对中层捕食者的抑制作用将达到最强。这类种群条件更易出现在顶级捕食者分布范围的核心区域，而非分布边缘地带。
据此我们提出天敌约束假说（Enemy Constraint Hypothesis），该假说预测：随着向顶级捕食者分布范围的边缘区域趋近，其对中层捕食者的下行调控效应（top-down effects）将逐渐减弱。
我们依托北美、欧洲与澳大利亚的赏金猎杀数据开展研究，结果表明，顶级捕食者对中层捕食者的调控效应如其预测一般，从其分布范围的边缘向核心区域逐渐增强。
全球范围内顶级捕食者分布范围的持续收缩，可能会进一步促进中层捕食者种群释放，进而改变生态系统结构并加剧生物多样性丧失。