Albatrosses hooked in the wind of change

Marine megafauna, a key component of ocean condition and functioning (1), is increasingly threatened by direct exploitation, incidental capture of nontarget species or bycatch, competition for forage fish by fisheries, pollution, and rapid ongoing climate change (2, 3). Seabirds are particularly at risk and their conservation status has deteriorated faster over recent decades (4). Currently, 31% of the world’s seabird species are listed as threatened on the International Union for the Conservation of Nature’s Red List and 45% of the species of albatrosses, petrels, and shearwaters are threatened (5). However, assessing seabird population status and trends, and proposing management actions require a good understanding of these threats and a quantification of their relative and combined impacts on demography and population dynamics. Although several studies have investigated the effect of individual environmental factors on seabird demographics, most only examined part of the demographic components, few investigated the additive effects of several environmental actors, and even fewer used a multiple species and comparative approach. In PNAS, Pardo et al. (6) address this gap by analyzing nearly all demographic rates (immigration and emigration rates are very challenging to estimate from single-site studies and were not estimated), and by identifying demographic, climate, and anthropogenic drivers of population growth rates, the rate at which the number of individuals in a population increases in a given time period expressed as a fraction of the initial population, of three sympatric albatross species since the early 1980s. The comparative approach and the results presented by Pardo et al. (6) significantly expand our view of ecological processes governing seabird populations and suggest the possibility of a new form of fisheries mitigation.

Several statistical methods exist to describe and explain demographic processes in populations (the demographic paradigm) and to quantify the impact of environmental factors on demographics (the ecological paradigm). Pardo et al. (6) used the more process-oriented approach based on estimation and analysis of demographic parameters, such as survival or breeding probabilities, assessment of the effect of environmental factors on these parameters, and joint modeling of these parameters using matrix population models (7, 8). Using high-quality data from ringed individuals collected over five decades as part of a remarkable long-term monitoring program of albatrosses on Bird Island, South Georgia, Southern Ocean, Pardo et al. (6) are able to disentangle the relative and additive influences of fisheries and climate on the population dynamics of albatrosses, including their effects on juvenile and immature classes. Estimating juvenile and immature survival is challenging in such long-lived species where individuals remain continuously at sea during their early life (9). By doing so, the effects of environmental drivers on population dynamics were examined over the entire life cycle, which, together with the wealth of information assembled, including at-sea distribution data to inform spatial exposure of individuals to environmental conditions, constitutes a particular strength of the Pardo et al. (6) study.

A full understanding of the effects of environmental drivers on the dynamics of wild populations requires estimating the relative contribution of each demographic rate to the observed changes in population growth rate. Pardo et al. (6) estimate these contributions and show that adult survival was, in average, the largest contributor in all species, but the contributions of the other demographic rates differed between species. Juvenile survival and recruitment were the second most important contributors for the wandering albatross Diomedea exulans, whereas for the gray-headed Thalassarche chrysostoma and black-browed albatrosses Thalassarche melanophris it was breeding success. Pardo et al. (6) then show that exposure to longline fisheries negatively affected adult survival for two of the three species, which given the extreme sensitivity of population growth rate to adult survival in these long-lived marine species, resulted in alarming population declines (from ∼43% to ∼65%) since the 1980s. Although positive effects of fisheries were found on some breeding parameters, their influence on population growth rate was too small to counteract the negative effects on adult survival.

Additionally to the effect of fisheries, climate (wind strength and direction, sea surface temperature, sea ice extent) also affected demographic parameters and population dynamics in all three species. Another strength of the Pardo et al. (6) study is to include krill density as an explanatory factor in their analyses, which constitutes a direct food resource for two of the three species (10) and which enhances adult survival of gray-headed albatrosses. Surprisingly in the wandering albatross, the longest-lived species studied, survival parameters were more affected by climate factors, and particularly by wind conditions, than were breeding parameters. Life history theory predicts that, in species with such extreme demographic strategies, adult survival should be buffered against climate variation (11). Perhaps the shift in the wind patterns in the Southern Ocean that occurred in the 1990s constitutes unprecedented climate conditions experienced by this wandering albatross population. Although not explicitly shown by Pardo et al. (6), it is reasonable to expect variations in age at first breeding in the wandering albatross given the relationships between population density and demographic parameters. Long-term ecological studies have demonstrated earlier maturation in response to an increase in adult mortality and a decrease in breeding numbers when density dependence occurs (12, 13) as predicted by theory (14).

Pardo et al. (6) also show that environmental factors and fisheries acted additively and sequentially in one of the studied species, the gray-headed albatross. An important population decline occurred in 2000 caused by an extreme crash in adult survival, due to poor food availability following an El Niño event during the preceding years, combined with a peak in longline fishing effort. Although the consequences of El Niño events seem more complex and variable than previously thought (15), such a scenario, if confirmed in other ecosystems, could be used to decrease bycatch mortality by reinforcing mitigation measures in years following such extreme climatic events.

The three albatross populations studied by Pardo et al. (6) inhabit ocean regions where some of the world’s most important and fastest climate changes have been observed during the past 50 y [sea temperature increase, sea ice extent decrease, changes in wind patterns, decrease in krill density (16, 17)], and where some of the world’s highest longline fishing efforts and bycatch rates have been recorded (2). Other, but not all, albatross populations face more favorable environmental conditions, and some are increasing or remain stable (18). The methodological approach developed by Pardo et al. (6) is applicable to stable or increasing populations provided sufficient data are available (Fig. 1), and should also prove useful for identifying factors causing the stability or growth of these populations, which is important for their conservation and management.

Fig. 1.

Framework designed to estimate the demographic rates, determine their contribution to population growth rate, assess population trends, and identify the environmental drivers of population dynamics of three sympatric albatross species. The blue boxes indicate original data, the orange boxes represent models, and the green boxes represent estimated quantities and inferences.

The work of Pardo et al. (6) serves as a convincing example of the complexities involved in how marine megafauna responds to warming climate and increasing fishing effort, both consequences of human population growth and unsustainable use of resources (19). Given the inertia and long-term response of the climate system, a priority for management and conservation of albatrosses and other marine megafauna should be to minimize bycatch and direct competition for resources by fisheries as much as possible, since other threats such as pollution and alien invasive predators also have additive effects on marine predator demography. Bycatch hot spots are relatively well known across the world’s oceans (2), and needed are robust observer data collection and directed mitigation implementation and enforcement.

Pardo et al.’s results and interpretation could never have been made without the extensive datasets available for the three species and collected by numerous dedicated fieldworkers. More comprehensive high-quality, long-term, individual-based monitoring programs such as this greatly enhance our understanding of marine ecosystems functioning and should be one of the concerns of institutions funding evolutionary ecology and conservation programs (20). Finally, the additive effects of other environmental factors such as pollutants and competition with other species on marine predator population dynamics deserve investigation and could help explain a proportion of the temporal variability in population growth rates.