Major disturbances can temporarily remove factors that otherwise constrain population abundance and distribution. During such windows of relaxed top-down and/or bottom-up control, ungulate populations can grow rapidly, eventually leading to resource depletion and density-dependent expansion into less-preferred habitats. Although many studies have explored the demographic outcomes and ecological impacts of these processes, fewer have examined the individual-level mechanisms by which they occur. We investigated these mechanisms in Gorongosa National Park, where the Mozambican Civil War devastated large-mammal populations between 1977 and 1992. Gorongosa’s recovery has been marked by proliferation of waterbuck (Kobus ellipsiprymnus), an historically marginal 200-kg antelope species, which is now roughly 20-fold more abundant than before the war. We show that after years of unrestricted population growth, waterbuck have depleted food availability in their historically preferred floodplain habitat and have increasingly expanded into historically avoided savanna habitat. This expansion was demographically skewed: mixed-sex groups of prime-age individuals remained more common in the floodplain, while bachelors, loners, and subadults populated the savanna. By coupling DNA metabarcoding and forage analysis, we show that waterbuck in these two habitats ate radically different diets, which were more digestible and protein-rich in the floodplain than in savanna; thus, although individuals in both habitats achieved positive net energy balance, energetic performance was higher in the floodplain. Analysis of daily activity patterns from high-resolution GPS-telemetry, accelerometry, and animal-borne video revealed that savanna waterbuck spent less time eating, perhaps to accommodate their tougher, lower-quality diets. Waterbuck in savanna also had more ectoparasites than those in the floodplain. Thus, plasticity in foraging behavior and diet selection enabled savanna waterbuck to tolerate the costs of density-dependent spillover, at least in the short term; however, the already poorer energetic performance of these individuals implies that savanna occupancy may become prohibitively costly as heterospecific competitors and predators continue to recover in Gorongosa. Our results suggest that behavior can provide a leading indicator of the onset of density-dependent limitation and the likelihood of subsequent population decline, but that reliable inference hinges on understanding the mechanistic basis of observed behavioral shifts.

中文翻译：

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恢复中的非洲有蹄类动物种群密度依赖栖息地扩张的生态和行为机制

重大干扰可以暂时消除限制人口丰度和分布的因素。在这种自上而下和/或自下而上控制的宽松窗口期间，有蹄类动物种群会迅速增长，最终导致资源枯竭和密度依赖型扩展到不太受欢迎的栖息地。尽管许多研究探索了这些过程的人口结果和生态影响，但很少有人研究它们发生的个体层面的机制。我们在戈龙戈萨国家公园调查了这些机制，莫桑比克内战在 1977 年至 1992 年期间摧毁了大型哺乳动物种群。戈龙戈萨的恢复以豺（Kobus ellipsiprymnus)，这是一种历史上边缘的 200 公斤羚羊物种，现在的数量大约是战前的 20 倍。我们表明，经过多年不受限制的人口增长，羚羊已经耗尽了其历史上首选的洪泛区栖息地的食物供应，并且越来越多地扩展到历史上避免的稀树草原栖息地。这种扩张在人口统计上是倾斜的：在洪泛区，壮年人的混合性别群体仍然更为普遍，而单身汉、独行者和亚成年人则居住在热带稀树草原。通过结合 DNA 元条形码和饲料分析，我们表明这两个栖息地的羚羊吃完全不同的饮食，它们在洪泛区比在稀树草原更易消化和富含蛋白质；因此，虽然两个栖息地的个体都实现了正的净能量平衡，洪泛区的能量表现更高。高分辨率 GPS 遥测、加速度计和动物传播视频的日常活动模式分析表明，稀树草原羚羊进食的时间更少，这可能是为了适应它们更严格、质量更低的饮食。稀树草原的羚羊也比洪泛区的羚羊有更多的体外寄生虫。因此，觅食行为和饮食选择的可塑性使稀树草原羚羊至少在短期内能够承受密度依赖性溢出的成本；然而，这些个体本已较差的精力表现意味着，随着异种竞争者和捕食者继续在戈龙戈萨恢复，稀树草原的占用可能会变得非常昂贵。