CHANGING HABITAT AND HABITAT USE BY BIRDS AFTER THEEXXON VALDEZOIL SPILL, 1989–2001

Abstract

Evaluations of the ecological consequences of environmental accidents can benefit from a long‐term perspective. To assess the effects of the Exxon Valdez oil spill on habitat use and occupancy during midsummer by marine‐oriented birds over a 12‐yr period following the spill in 1989, we conducted surveys in 10 study bays in Prince William Sound, Alaska, USA. These bays varied from completely unoiled to some of the most heavily oiled locations in the spill zone. We used oiling as a quantitative variable with and without habitat measures as covariates to assess spill effects and their changes over time. We also used oiling as a categorical variable (“oiled” vs. “unoiled” bays) to conduct between‐year, repeated‐measures analyses that used 1984 data as a baseline and to plot abundance trends by oiling category. We combined the results of these analyses in a “weight‐of‐evidence” approach to determine overall impacts and recovery for each species. To assess changes in habitat condition following the oil spill, we also measured several habitat features in 1991, 1998, and 2001. There were significant increases in the proportion of shoreline covered by rockweed (Fucus) and mussels in oiled bays between 1991 and 1998–2001, suggesting recovery of oiled habitats. Of the 25 bird species whose status we evaluated, 12 (48%) exhibited no evidence of any spill impacts, 10 (40%) exhibited negative impacts that subsequently disappeared, and three (12%) exhibited positive relationships with oiling that later disappeared. No species provided clear evidence of any delayed impacts. In comparison with our earlier analysis of a more extensive data set collected from these same study bays during 1989–1991, we detected slightly more initial spill impacts in the present study but also found that several species whose habitat occupancy had not recovered by the end of our earlier study had now recovered. Our conclusions differ from those of some other studies of the effects of the Exxon Valdez oil spill on birds, perhaps because of differences in study design, in the criteria used to assess impact and recovery, and in the definition of “recovery.” These differences illustrate the difficulties of evaluating the consequences of environmental accidents, even in such a dramatic and well‐studied situation as the Exxon Valdez oil spill. Long‐term studies of environmental accidents create a paradox: the long‐term perspective is needed to see how impact and recovery dynamics are played out, yet background natural variation can make it increasingly difficult to detect clear patterns. With the passage of time after an environmental accident, more things happen, making it more difficult to attribute observed patterns to the event alone. Consequently, long‐term assessments of ecological impacts may be restricted to documenting only large effects; the use of α levels greater than the traditional 0.05 can facilitate such analyses and reduce the likelihood of committing type II errors.

Applying Ecological Risk Assessment to Environmental Accidents: Harlequin Ducks and the Exxon Valdez Oil Spill

“…These areas differed in habitat features other than oiling, however; moderately and heavily oiled bays had less complex (convoluted) shorelines, more rocky cliffs, less intertidal coverage of Fucus and seagrass, fewer streams supporting salmon runs, and less open conifer woodland in the adjacent supratidal habitats than did unoiled or lightly oiled reference bays (Wiens et al 2001). When these and other habitat variables were included as covariates in the analyses, the significant negative relationships with oiling level disappeared (Wiens et al 2004). Integral Consulting, Inc.…”

“…Abundances were also lower in western PWS than in eastern PWS well before the spill in 1984, however, suggesting inherent differences in habitat suitability beyond which the spill had only short-term effects. Analyses conducted at a finer scale of resolution (Day et al 1997, Wiens et al 2004 indicated that the population densities of harlequins were substantially lower in oiled than in unoiled bays both before and after the spill, although densities increased proportionately more in the previously oiled bays from 1997 to 2001 than in the unoiled bays. Collectively, these analyses indicate that habitat plays a greater role in determining variations in harlequin abundance in PWS than does oiling history or any lingering effect of the spill.…”

“…The cumulative effects of unrelated environmental changes since 1989 (Wiens et al 2004) and disagreements about how to define and measure "recovery" Wiens 2005, Integral Consulting, Inc. 2006) complicate assessments of the status of harlequin ducks in areas affected by the spill. For example, sea-surface temperatures and ocean salinity in the North Pacific are influenced not only by annual phenomena such as ENSO (El Niño-Southern Oscillation) events but also by decadal-scale shifts in climate and oceanography that are superimposed on longer-term trends in ocean conditions related to global warming (Mantua and Hare 2002, Peterson and Schwing 2003, Litzow 2006, Royer and Grosch 2006.…”

“…Extrapolating from these samples, Short and colleagues (2004) concluded that some 11 ha of PWS shoreline continued to be contaminated by surface or subsurface Exxon Valdez oil residues in 2001. Using the annual loss rate of beached oil volume of 20 to 26 percent derived by Short and colleagues (2004), Harwell and Gentile (2006) Initially, the spill oiled many shoreline areas potentially frequented by harlequin ducks, and early analyses indicated that the use of moderately or heavily oiled areas by harlequins was significantly lower than that of unoiled areas (Day et al 1997, Wiens et al 2004). These areas differed in habitat features other than oiling, however; moderately and heavily oiled bays had less complex (convoluted) shorelines, more rocky cliffs, less intertidal coverage of Fucus and seagrass, fewer streams supporting salmon runs, and less open conifer woodland in the adjacent supratidal habitats than did unoiled or lightly oiled reference bays (Wiens et al 2001).…”

Top-down and bottom-up control of infauna varies across the saltmarsh landscape

“…One assumption using this type of analysis is that although response variables at different sites may differ spatially, those differences track each other over time. This assumption, however, may be violated, reducing confidence in results (Wiens et al, 2004). To bolster confidence and to identify the direction of changes for significant interactions, we visually inspected graphs of data in pre-treatment and treatment periods.…”