Future Imperfect: Steller Sleuths Predict Probability of Extinction

As professions go, scientists and detectives are close cousins: both attempt to divine answers to complex problems, based (more often than not) on imperfect evidence. Such is the nature of the mysterious decline of western Alaska’s Steller sea lions over the past 25 years. Any number of possible causes might explain the plummeting population, and so dozens of scientific sleuths are examining past and present clues to identify and hunt down the ecological perpetrators.

Some forward-looking researchers, however, are focusing on what has not yet happened to sea lions: namely, extinction. Just as a crime prevention unit is concerned with identifying potential trends and activities, so are Arliss Winship and Andrew Trites from the University of British Columbia. Their new study, published in the journal Marine Mammal Science, attempts to forecast the likelihood of extinction for western Alaskan Steller sea lions based on past population trends.

Like most detective work, predicting extinction is not a precise science. There are almost always large uncertainties involved, such as what prompted the Steller sea lion decline in the first place. A number of hypotheses have alternately implicated human interference, predation by killer whales, and a reduction in prey due to fisheries and/or a changing ocean climate. As yet, however, there is no smoking gun: a lack of key evidence has made it difficult to conclusively rule out any one hypothesis.

Preparing the Case

Enter Winship & Trites, armed with a Population Viability Analysis (PVA), a statistical tool that attempts to predict the probability of a species or population going extinct – or reaching some critical number of individuals – over a specified period.

Given the considerable uncertainties surrounding the western Alaskan “crime scene”, Winship & Trites quickly recognized it would be almost impossible to arrive at one result that would conclusively described the probability of extirpation (localized extinction) of the species in the region. Thus, they explored three distinct ecological scenarios – essentially reconstructing three versions of the events leading up to the “crime” – and assessed the probability of extinction under each scenario.

In this way, they sought to quantify the uncertainty associated with the risk of extirpation for Steller sea lions. Furthermore, this approach would show how different hypotheses about the cause of the Steller sea lion decline can actually affect the predicted risk of extinction. In detective terms, this is equivalent to comparing how the conflicting testimonies of two eyewitnesses can influence the outcome of a trial.

The study’s verdict? If current population trends continue into the future, Winship & Trites predict that many subpopulations (rookeries) have a >5% chance of extinction within 100 years. However, the probability of overall extinction for the species during that time is low.

Back at the local precinct, this scientific detective work represents “just the facts, ma’am. Just the facts.” However, in the hands of managers, this information could be used to assess whether western Alaska’s Steller sea lions merit their current “endangered” listing, or to influence the way Steller sea lion populations are listed in the future.

Publication:
Risk of extirpation of Steller sea lions in the Gulf of Alaska and Aleutian Islands: a population viability analysis based on alternative hypotheses for why sea lions declined in western Alaska. Winship, A.J., and A.W. Trites. 2006. Marine Mammal Science 22(1):124-155. abstract We estimated the risk that the Steller sea lion will be extirpated in western Alaska using a population viability analysis (PVA) that combined simulations with statistically fitted models of historical population dynamics. Our analysis considered the roles that density-dependent and density-independent factors may have played in the past, and how they might influence future population dynamics. It also established functional relationships between population size, population growth rate and the risk of extinction under alternative hypotheses about population regulation and environmental variability. These functional relationships can be used to develop recovery criteria and guide research and management decisions. Life table parameters (e.g., birth and survival rates) operating during the population decline (1978?2002) were estimated by fitting simple age-structured models to time-series of pup and non-pup counts from 33 rookeries (subpopulations). The PVA was carried out by projecting all 33 subpopulations into the future using these estimated site-specific life tables (with associated uncertainties) and different assumptions about carrying capacities and the presence or absence of density-dependent population regulation. Results suggest that the overall predicted risk of extirpation of Stelsler sea lions as a species in western Alaska was low in the next 100 yr under all scenarios explored. However, most subpopulations of Steller sea lions had high probabilities of going extinct within the next 100 yr if trends observed during the 1990s were to continue. Two clusters of contiguous subpopulations occurring in the Unimak Pass area in the western Gulf of Alaska/eastern Aleutian Islands and the Seguam?Adak region in the central Aleutian Islands had relatively lower risks of extinction. Risks of extinction for a number of subpopulations in the Gulf of Alaska were reduced if the increases observed since the late 1990s continue into the fu ture. The risks of subpopulations going extinct were small whe n densit ydependent compensation in birth and survival rates were assumed, even when random stochasticity in these vital rates was introduced.

23 January 2006