Animals are important sentinels, or forewarners, of ecosystem change. Both dramatic and subtle changes in wild populations may forewarn us of alterations in ecosystems that may ultimately affect humans. Animal sentinels may predict changes in ecosystems resulting from habitat loss, fragmentation, degradation, or contamination.
Loss of habitat can result directly in loss of species.
As marine mammals are top predators in the marine ecosystem, they are potentially ideal sentinels of the aquatic ecosystem. However, as many of these species are pelagic, access to these animals is usually limited to stranded animals, which are far from a representative sample of the free-living population. But, as stranded animals are more likely to be the diseased segment of the population, they may yield valuable information on pathology in the population, which would be difficult to detect if a random sample were examined. However, during extensive investigations of immune function in large rookeries of sea lions on the Channel Islands off California, only one case of carcinoma was documented. Carcinomas in laboratory rodents and fish have been associated with exposure to contaminants, such as PAHs and PCBs.