Adélie Penguin Colony Size and Local Population Extinction: A Predation-Induced Tipping Point

Adélie penguin research in the Palmer LTER (PALTER) region has historically focused on five island rookeries that at the inception of investigations in 1974 held 15,202 breeding pairs.  During the 2011/2012 field season, these rookeries held 2,511 breeding pairs, an 83% decrease in abundance relative to original estimates.  Changes in these populations, however, have not been symmetrical, but instead encompass trends that are island-specific.  Most noteworthy among these, is that of the five original populations, only four remain, as the Litchfield Island population went extinct in 2007.  Based on the dynamics associated with this event, we have long-hypothesized that observed island-specific population trends may be driven at least in part by threshold changes in the proportions of penguins distributed between large (> 50 breeding pairs) and small (< 50 breeding pairs) colonies, but had insufficient data to test this idea.  Here we show that colony size is indeed associated with island-specific Adélie penguin population changes.  Thus, where large breeding colonies still encompass much of an island’s population, penguin numbers have remained stable since the 2007 extinction event.  In contrast, where the opposite is observed and small breeding colonies predominate, populations have continued to decline over the same time period.  As causal mechanisms, we suspect that increasing vulnerability of smaller breeding groups to predation by Brown skuas is driving these dynamics, which suggests a population extinction source that is independent of—but works in tandem with—other undesirable effects on demography induced by marine variability.  Based on similarities between trends evident on Litchfield Island nearly a decade ago and patterns currently evolving, we are predicting that two additional island rookeries will be extinct in the PALTER region within 3-5 years.