EFFECTS OF THE EXXON-VALDEZ OIL-SPILL ON HABITAT USE BY BIRDS IN PRINCE-WILLIAM SOUND, ALASKA

Oil spills may affect species through direct effects on population siz e and structure and direct and indirect (toxicological) effects on rep roduction. Spill effects on the habitats these organisms occupy have r eceived less attention, but they are no less important. For 2.5 yr fol lowing the Exxon Valdez oil spill in Prince William Sound, Alaska, we studied the use of oil-affected habitats by 42 species of marine-orien ted birds. On 11 survey cruises, we surveyed bays that had received di fferent levels of initial oiling, We related the abundance of individu al species in the bays to the oiling gradient, using regression models that included habitat measures to control for variations among the si tes in features other than oiling level. We defined a spill-induced im pact as a statistically significant relationship between the abundance of a species and values along the oiling gradient, after accounting f or the effects of variations in habitat features. We used among-year c omparisons of regressions between oiling levels and abundance, control led for season, to assess recovery. We concluded that recovery from a spill-induced impact had occurred when we no longer could detect a sig nificant relationship between a species' abundance and oiling levels. Overall, 23 (55%) of the 42 species exhibited no initial negative impa cts on their use of oil-affected habitats. Of the 19 species that did exhibit negative impacts, 13 (68%) showed evidence of recovery within 2.5 yr !the final survey in 1991). Six species (Horned Grebe, Red-neck ed Grebe, Barrow's Goldeneye, Bufflehead, Mew Gull, and Northwestern C row) showed no clear evidence of recovery by our final survey. The pro portion of species recorded on individual surveys that exhibited negat ive impacts at that time declined over the study, from 54% on the firs t survey after the spill in 1989 to 10% in late 1991. A principal comp onents analysis revealed extensive ecological overlap between species that were negatively impacted in their use of oil-affected habitats an d those that were not. The six species that had not recovered by late 1991 tended to be intertidal feeders and residents, but these traits a lso characterized some species that did not exhibit initial impacts an d some species that subsequently recovered from impacts, We detected n o obvious ecological differences between species that suffered spill i mpacts on habitat use and those that apparently were not affected, or between impacted species that later recovered in their use of habitats and species that had not yet recovered. These results indicate that t he Exxon Valdez oil spill had clear initial negative impacts on habita t use by nearly half of the species examined, suggesting substantial i nitial effects on habitat suitability for these species. These impacts persisted for <2.5 yr for most affected species, This rate of recover y in habitat use parallels the rapid recovery (usually <2 yr) document ed for other oil-affected communities (e.g., intertidal invertebrates, fishes, and birds) that have been studied in Alaska and elsewhere.