Department of Ecology and Evolution, Stony Brook University, 610 Life Sciences Building, Stony Brook, New York, 11794, USA.
Ecology. 2019 Oct;100(10):e02823. doi: 10.1002/ecy.2823. Epub 2019 Aug 14.
Aggregations are common in ecological systems at a range of scales and may be driven by exogenous constraints such as environmental heterogeneity and resource availability or by "self-organizing" interactions among individuals. One mechanism leading to self-organized animal aggregations is captured by Hamilton's "selfish herd" hypothesis, which suggests that aggregations may be driven by an individual's effort to minimize their risk of predation by surrounding themselves with conspecifics. We demonstrate that aggregations observed in Adélie Penguin (Pygoscelis adeliae) colonies are a convolution of both self-organized dynamics and external forcing arising from landscape terrain. In fluid, highly mobile aggregations, individuals are constantly moving in response to changing environmental conditions, the locations of predators, or the movements of conspecifics. However, when the ability to rearrange is limited and spatial reconfiguration occurs on slower time scales than changes in population size, systems may become trapped in suboptimal arrangements. We use simulated annealing to demonstrate that Adélie Penguin colonies are frozen in suboptimal spatial arrangements, and employ an individual-based modeling approach to demonstrate that this suboptimal spatial configuration is driven by a convolution of nest site fidelity and stochastic events at the level of individual nests. The resulting spatial dynamics are responsible for a hysteretic response to long-term changes in abundance. We find that declining abundance leads to fragmentation even in a homogeneous environment, which has population-level consequences for reproductive success because predation is biased towards colony edges. Strong edge effects from heterogeneous predation coupled with fragmentation in response to population declines create a positive feedback cycle that can accelerate population decline. This work provides a mechanistic understanding of complex spatial structuring in penguin colonies, provides a link between current spatial patterning and past dynamics, and suggests the possibility of critical collapse in seabird populations.
聚集在生态系统中是很常见的，其范围涉及多个尺度，可能是由环境异质性和资源可利用性等外部约束，或者是个体之间的“自组织”相互作用所驱动。导致自组织动物聚集的一种机制是由 Hamilton 的“自私羊群”假说捕捉到的，该假说表明，聚集可能是由个体努力最小化其被周围同种个体捕食的风险所驱动的。我们证明，在阿德利企鹅（Pygoscelis adeliae）群体中观察到的聚集是自组织动态和由景观地形引起的外部强制的卷积。在流体、高度移动的聚集中，个体不断地移动以响应不断变化的环境条件、捕食者的位置或同种个体的运动。然而，当重新排列的能力受到限制，并且空间重新配置的时间尺度比种群大小的变化慢时，系统可能会被困在次优的安排中。我们使用模拟退火来证明阿德利企鹅群体被冻结在次优的空间排列中，并采用基于个体的建模方法来证明这种次优的空间配置是由巢址保真度和个体巢水平上的随机事件的卷积所驱动的。由此产生的空间动态是对长期丰度变化产生滞后响应的原因。我们发现，即使在同质环境中，数量减少也会导致碎片化，这对生殖成功率产生了种群水平的影响，因为捕食偏向于群体边缘。来自异质捕食的强烈边缘效应加上对种群减少的响应的碎片化，形成了一个正反馈循环，可能会加速种群减少。这项工作提供了对企鹅群体中复杂空间结构的机制理解，为当前空间模式和过去动态之间建立了联系，并表明海鸟种群可能会发生临界崩溃。
