Degree of Threat: Medium
Comments: This species was nearly extirpated from North America between the late 1950s and early 1970s when pesticides entering the marine food web caused major population declines. The pesticide endrin killed pelicans directly, whereas DDT reduced reproductive success by causing pelicans to lay thin-shelled eggs that broke during incubation.
Populations are extremely vulnerable to chemical/pesticide pollution, which can result in eggshell thinning (reproductive failure) (Anderson and Hickey 1970, Blus et al. 1974), and presumably lethal poisoning. Populations (especially in California, Texas, and Louisiana) were decimated in the U.S. by pesticides (DDT and related compounds) in the 1950s and 60s. In the U.S. Caribbean, 7% of the pelican population in 1982 died as a result of fish die-offs in connection to chemical runoffs (e.g., organophosphates). Other threats include disturbance of nesting birds by humans (reduces reproductive success), declining fish (food) populations, increased turbidity (e.g., from dredging, resulting in reduced visibility of prey); oil and other chemical spills, entanglement in fishing gear, shooting, extreme weather conditions (freezing of soft parts, destruction of nest sites by hurricanes, storms), disease, and parasitism.
Human disturbance (e.g., recreational boating, poaching) not only disrupts reproductive success (Anderson and Keith 1980; Schreiber 1979), but may affect distribution patterns and age structure of pelicans using roosting sites during the nonbreeding season (Jaques and Anderson 1987). Habitat degradation affects both roosting and nesting patterns. On the Gulf Coast, nesting efforts have failed because nesting sites are susceptible to flooding as a result of continued site erosion (McNease et al. 1992).
Subspecies CALIFORNICUS: Declined greatly due to effects of pesticide contamination in the 1950s and 1960s. In Southern California threatened by pollution (primarily pesticides in food fishes, also oil), human disturbance of breeding colonies, loss or serious decline of food fishes due to human over-fishing (e.g., of anchovies); loss of post-breeding roost sites, fishing gear entanglement, and bacterial infection resulting from overcrowding at fish disposal areas in harbors (California Department of Fish and Game 1990). Human disturbance has decreased nesting success on Islas los Coronados, Mexico (Anderson 1988) and virtually extirpated the breeding colony (California Department of Fish and Game 1990). Southern populations in Mexico have faced problems associated with human disturbance and overexploitation of prey (e.g., sardines), yet they remain stable (D. W. Anderson, pers. comm.).
U.S. Caribbean: contaminant levels and availability of nesting habitat are not limiting or affecting the population at present. See Williams et al. (1992) for an account of die-offs that have been observed in Puerto Rico and the Virgin Islands; apparent causes include pesticides, botulism, and unknown factors. In the tropics and subtropics, coastal development and incidental take (e.g., artisanal fishing) is a problem and represents a major threat to the continued availability of mangrove habitat. Close to 91% of all roosting and nesting habitat utilized in the U.S. Caribbean are fringe and overwash mangroves. Fringe mangroves are particularly important to the feeding ecology of pelicans because they provide nutrient inputs and cover for the associated marine community, including food fishes. Both mangrove types are very sensitive to human-created stress such as deforestation, filling and extractions in the salt flats, sedimentation, and oil spills (Cintron and Schaeffer-Novelli 1983). Siltation caused by erosion could be adversely impacting coral reefs, seagrass beds, and mangrove forests (Cintron and Schaeffer-Novelli 1983, Velazco et al. 1985).