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Understanding Large-scale Population Effects

Nadav Nur, Ph.D.


A Common Murre leads its chick in their foraging area. If they dive for fish through floating oil slicks, seabirds may not only foul their plumage but also lose their natural insulation. Many succumb to hypothermia. Photo © Sophie Webb
Marine bird populations have been subject to repeated oiling and oil spills for the past 100 years. For people and agencies responding, the primary focus is usually the "body count"—the numbers of dead oiled birds that are found and of oiled but still living birds that are treated, rehabilitated, and (if all goes well) released back into the wild. But simply counting bodies does not tell enough about the larger-scale impact of oil spill incidents. For example, contact with oil can cause substantial non-lethal impacts, including reproductive failure, for a bird that survives.

The need to determine the impacts of oil spills on wildlife has required that we look not only at the "body count" but also try and understand the longer-term response of bird populations. For each species involved, we also consider its population biology and the ecological factors that influence its population. As mentioned by Gary Page in this Observer, PRBO's scientific assessments have helped guide effective management action to promote the long-term viability of affected populations.
Common Murres in their colony: An oil spill can affect populations of seabirds over time. Photo © David Gardner

The Common Murre, heavily involved in the 1986 Apex Houston spill, is also a study species in PRBO's long-term research on the Farallon Islands. Murres are both very numerous near the Farallones and highly subject to mortality from contact with floating oil slicks. To estimate the impact to murres of what began as a widespread "mystery spill," we used all the available information, including the characteristics of currents that may have carried oiled birds out to sea. We were able to conservatively estimate that 7,500 murres had been killed by the Apex Houston spill; only 56% of these were recovered as dead oiled birds on shore.

The crucial step in assessing the damage was then to predict this spill's impact over the long term, up to 50 years. From PRBO's data on the Farallon breeding population, we summarized the murre's reproductive success, survival, how long they take to reach reproductive maturity, and other vital rates. A modeling team assembled by the Department of Justice then used our findings to help assess impacts on natural resources. Their conclusion: for the foreseeable future, the Central California population of Common Murres might not be able to recover on its own to levels that would have existed without the Apex Houston spill. Full recovery would require the assistance of humans. The resulting damage settlement supported novel work by U.S. Fish and Wildlife Service to reestablish coastal murre colonies. PRBO emphasized the relevance of the Farallon Island National Wildlife Refuge in this effort: with the largest murre colony in Central California just 35 miles away from the main restoration site, the Farallones would provide a source of birds to recolonize nearby areas, helping ensure the recovery of murre populations affected by the oil spill.

Predicting Wildlife Recovery

Building upon this landmark, we helped develop a population modeling tool to enable wildlife managers to project an oil spill's impact and the chances for populations to recover. The model, a custom computer program that PRBO developed with the Gulf of the Farallones National Marine Sanctuary, showed the importance not just of "body count" but also of ecological context. For top predators like seabirds, recovery depends on highly variable environmental conditions. Over years or decades, the ocean shifts—from warm-water conditions with low productivity (e.g., El Niño years), to cold-water conditions with high productivity (e.g., La Niña years). The model showed that in warm-water phases, full recovery might be impossible or at least unlikely. In cold-water phases, a population could recover to pre-spill levels in ten years or less, even after a major oil spill.
Cormorants (here a Double-crested, left, and a Brandt's, right) have a life history advantage over Common Murres, when it comes to population recovery from catastrophic mortality. Photo © Tom Grey

At about the same time, the 1990s, we showed that a seabird's ability to recover from catastrophic mortality also depends very much on its life-history characteristics. David Ainley and I worked alongside many other scientists to provide information about the likely recovery of marine birds affected by the huge Exxon Valdez spill in Alaska. After surveying studies of seabird species throughout the world, we concluded that the Common Murre faced particular recovery challenges related to its breeding biology. Murres can rear at best only one chick per pair each year, and they are slow to mature reproductively (on the Farallones, the average age of first breeding is seven years). This strategy may be well suited to conditions of natural mortality, but it puts the species at added risk from mortality caused by oil spills. In contrast, Brandt's Cormorants can rear several chicks per breeding attempt, and they begin to breed at only two to four years of age—a life-history advantage for a recovering population.

Ultimately, discussions of oil spill impacts must stress that recovery is not assured for any bird species affected. Favorable environmental conditions—adequate prey, especially—to support high reproductive and survival rates are critical, as is a pool of mature individuals near enough to recolonize or repopulate depleted rookeries.