When the “selfish herd” becomes the “frozen herd”: spatial dynamics and population persistence in a colonial seabird

Abstract

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 Ad elie 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 Ad elie 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.

The influence of subcolony-scale nesting habitat on the reproductive success of Adélie penguins

“…The sudden decrease in colony size at Cape Royds was likely accompanied by increased fragmentation of subcolonies 74 – 76 and an associated increase in the relative proportion of edge nests, driving a decline in subcolony quality and perhaps allowing increased skua predation to hinder recovery. High perimeter-area-ratios and slow growth at Cape Royds are consistent with a population “frozen” in a suboptimal arrangement of nests 74 , 75 . Using an individual-based model, McDowall and Lynch 75 demonstrated that even when penguins are “allowed” to occasionally change nest sites, the slow rate of movement resulting from high nest site fidelity and incomplete information leads to suboptimal nest arrangements: decisions about when and where to move are made by individuals with incomplete information as the quality of their final nest site depends on unknown movements of other penguins.…”

“…High perimeter-area-ratios and slow growth at Cape Royds are consistent with a population “frozen” in a suboptimal arrangement of nests 74 , 75 . Using an individual-based model, McDowall and Lynch 75 demonstrated that even when penguins are “allowed” to occasionally change nest sites, the slow rate of movement resulting from high nest site fidelity and incomplete information leads to suboptimal nest arrangements: decisions about when and where to move are made by individuals with incomplete information as the quality of their final nest site depends on unknown movements of other penguins. Thus, nest site fidelity, even if incomplete, can result in a persistently fragmented configuration of nests that inhibits growth.…”

Breeding Ecology of Adélie Penguins in Mid Victoria Land, Ross Sea Antarctica

“…Although the stressors hypothesized for penguins are inherently greater at EdPo than at AdCo (i.e., higher per capita skua predation pressure, persistent fast ice in front of the colony), our findings clearly support that local factors such as nest position and predation risk around the nest, and in turn, the quality of the nesting environment, seem more important than the general condition of the colony in determining the probability of chick survival. The number of neighboring skua pairs, which had the strongest effect size amongst other predictors, seems to be especially crucial for penguin breeding success [75]. In the long-term, skua numbers may thus be expected to be critical drivers of Adélie penguin population dynamics (see also [76] for southern rockhopper penguin Eudyptes chrysocome).…”

Protective geometry and reproductive anatomy as candidate determinants of clutch size variation in pentatomid bugs

“…There is evidence that the spatial arrangement of eggs within clusters can influence offspring survival, with individuals on the margins having lower survival, in a manner similar to 'selfish herd' effects observed across many taxa of both sessile and motile organisms that form aggregations (Hamilton 1971;McDowall and Lynch 2019). In insects, eggs on the insides of clusters may be less susceptible to predation and parasitism (Weseloh 1972;Friedlander 1985;Gross 1993;Hondo et al 1995;Mappes and Kaitala 1994;Mappes et al 1997;Kudo 2001;Kudo 2006;Deas and Hunter 2012) and may also suffer lower mortality due to desiccation (Clark and Faeth 1998).…”