Variability in krill biomass links harvesting and climate warming to penguin population changes in Antarctica

Abstract

The West Antarctic Peninsula (WAP) and adjacent Scotia Sea support abundant wildlife populations, many of which were nearly extirpated by humans. This region is also among the fastestwarming areas on the planet, with 5-6°C increases in mean winter air temperatures and associated decreases in winter sea-ice cover. These biological and physical perturbations have affected the ecosystem profoundly. One hypothesis guiding ecological interpretations of changes in top predator populations in this region, the "sea-ice hypothesis," proposes that reductions in winter sea ice have led directly to declines in "ice-loving" species by decreasing their winter habitat, while populations of "ice-avoiding" species have increased. However, 30 y of field studies and recent surveys of penguins throughout the WAP and Scotia Sea demonstrate this mechanism is not controlling penguin populations; populations of both ice-loving Adélie and ice-avoiding chinstrap penguins have declined significantly. We argue in favor of an alternative, more robust hypothesis that attributes both increases and decreases in penguin populations to changes in the abundance of their main prey, Antarctic krill. Unlike many other predators in this region, Adélie and chinstrap penguins were never directly harvested by man; thus, their population trajectories track the impacts of biological and environmental changes in this ecosystem. Linking trends in penguin abundance with trends in krill biomass explains why populations of Adélie and chinstrap penguins increased after competitors (fur seals, baleen whales, and some fishes) were nearly extirpated in the 19th to mid-20th centuries and currently are decreasing in response to climate change.S ea ice plays a critical role in structuring ecosystem dynamics throughout the West Antarctic Peninsula (WAP) and Scotia Sea, and variations in sea-ice extent are hypothesized to affect penguin populations directly. As seasonal sea-ice extent and duration declines in this region, the Adélie penguin (Pygoscelis adeliae), which favors pack-ice habitat in winter, should decline in population size, whereas the closely related chinstrap penguin (Pygoscelis antarctica), which forages in ice-free water during winter, should increase (1-5). The foundation for this hypothesis is based on a short (7-y) series of simultaneously observed decreases in nesting populations of Adélie penguins and increases in chinstrap penguins following winters with low sea ice in the South Shetland Islands during the 1970s and 1980s (1). However there now is overwhelming evidence that, in contrast to expectations, both Adélie and chinstrap penguin populations are declining throughout the WAP and broader Scotia Sea region. Results and DiscussionAdélie and chinstrap penguin populations have declined more than 50% during the last 30 y at study colonies in the South Shetland Islands (Fig. 1A). Moreover, since 1987, interannual changes in Adélie and chinstrap breeding populations have been positively correlated (Pearson's r = 0.7; P < 0.001; n = 21; Fig. 1B). T...

Adélie penguin (Pygoscelis adeliae) survival rates and their relationship to environmental indices in the South Shetland Islands, Antarctica

“…Satellite-based observations of ocean color have revealed potential declines in marine primary production (Montes-Hugo et al 2009), while long-term net tow survey data suggest a possibly 80 % decline in the abundance of Antarctic krill (Euphausia superba; Atkinson et al 2004), the main food item for most seabirds, marine mammals, and fishes in the WAP region. As ice-obligate, krill-dependent predators that require snow-free ground for successful breeding, Adélie penguin populations would be expected to decline under such changing environmental conditions (Fraser et al 1992;Trivelpiece et al 2011). Monitoring data in the WAP region support that expectation (Hinke et al 2007;Lynch et al 2008).…”

“…Consequently, seabirds typically exhibit low variability in adult survival rates (Saether and Bakke 2000). During a period of population decline among Adélie penguins in the South Shetland Islands (Lynch et al 2008;Trivelpiece et al 2011), survival rates of banded Adélie penguins declined among juveniles, while both juvenile and adult age classes were characterized by strong variability, ranging from survival rates from\30 to[70 %. On average, adult survival rates were equivalent to average juvenile survival rates (ca.…”

“…Such low ice conditions are thought to negatively affect the distribution, fecundity, and survival of krill and may have precipitated the episodic recruitment and subsequent senescence of single krill cohorts that are evident throughout the 1990s and 2000s (Fraser and Hofmann 2003;Reiss et al 2008). Reduced krill availability at a time with relatively high krilldependent Adélie populations in the late 1970s and early 1980s (Trivelpiece et al 2011) may have set the stage for a downward adjustment to population size via higher mortality rates among penguins. However, such food web linkages can be enormously difficult to quantify, particularly on the spatial scales relevant to the foraging and overwinter distributions of ice-obligate Adélie penguins.…”

“…One possible result of the loss of critical sea ice habitat in the northern part of the Adélie range is an elevated importance of alternative drivers that affect variation in survival rates. For example, food web changes in the southern Scotia Sea that are associated with commercial krill fisheries, increased competition for prey due to the recovery of once-depleted marine mammal populations, and overall declines in preferred prey (Atkinson et al 2004;Trivelpiece et al 2011) may interact with ENSO events, reductions in sea ice extent, or local weather conditions to affect survival rates in nonlinear, complex ways. Such interactions could dampen the signal of a single index of environmental conditions, but are difficult to identify with standard CMR models.…”

“…Such interactions could dampen the signal of a single index of environmental conditions, but are difficult to identify with standard CMR models. Trivelpiece et al (2011) proposed that the changes in the Adélie penguin populations in the WAP region derived from the dynamic linkage between krill biomass and the variable winter sea ice conditions that directly affect krill recruitment dynamics. With respect to the survival rates reported here, the early period of decline in juvenile penguin survival rates in the 1980s may be linked with declines of krill biomass in the early 1980s that resulted from a decreased frequency of years with extensive winter sea ice.…”