Effects of contaminants on wildlife

Persistent organic pollutants, as well as mercury, tend to accumulate in aquatic ecosystems more than in terrestrial ones. These levels are magnified as they move up the food web. This means that the highest levels of these contaminants are found in top predators – especially marine mammals and fish-eating birds.

There is no evidence of current widespread effects of contaminants on Canadian Arctic wildlife, though polar bears of southern and western Hudson Bay, as well as some high Arctic seabirds, have contaminant levels that may be placing them at risk.3 However, what is known is based only on studies of a few species and is usually based on the effects of a single contaminant. Little is known about impacts of the contaminant mixtures that wildlife are exposed to, or about interactions of contaminants with other changes in ecosystems.3

Contaminant levels are much higher in some areas of southern Canada than they are in the Arctic (see graphs of contaminant trends earlier in this section). Levels of contaminants measured in wildlife often exceed thresholds beyond which biological effects are known to occur from laboratory studies (usually based on species other than those of concern in the wild). While direct evidence of impacts on wildlife populations is difficult to obtain, associations between high contaminant levels and observations of effects – like tumours, abnormal gonads, or poor reproductive success17, 28 – underscore conservation-level concerns for some populations. The clearest example of known impacts is that of DDT-associated egg-shell thinning in birds29 – but high levels of contaminants are suspected to contribute to declines in several wildlife populations, for example, herring gulls in the Great Lakes30 and beluga whales in the St. Lawrence Estuary.31, 32

Contaminants in killer whales off the Pacific Coast

Average levels in killer whale biopsy samples, mid-1990s, parts per million
Map and graphs: concentration of contaminants in three killer whale populations off the Pacific coast. Click for graphic description (new window).
Source: adapted from Ross, 200633

PCBs and PBDEs are known to adversely affect neurological development, reproductive development, and immune system function of marine mammals.33 Because they are long-lived top predators, killer whales accumulate high concentrations of persistent organic pollutants, including PCBs and PBDEs.29, 34, 35 The concentrations of PCBs in the three killer whale populations along the B.C. coast exceed levels known to affect the health of harbour seals,33 and the PCB levels of two populations are among the highest in marine mammals in the world.35

The large variation in contaminant concentrations among the populations is related to their feeding habits. Transient whales feed on marine mammals, placing them higher in the food web, while both resident populations feed largely on salmon that acquire contaminants from global sources in the North Pacific Ocean.29 Southern resident whales also feed on prey that pick up contaminants from the industrial coastal waters of southern B.C. and northwest Washington, leading to higher PCB and PBDE accumulation.29 These or other contaminants may be a factor in the decline of this endangered population of killer whales (see Marine Biome).36

Photo: killer whales ©
  Killer whales

Recovery of peregrine falcons in Canada

Number of sites occupied by peregrines, 1970 to 2005
Graph: number of sites occupied by peregrines. Click for graphic description (new window).
Source: data from COSEWIC, 20076

Photo: peregrine falcon © Gordon Court

The story of the peregrine falcon shows that contaminants can have major effects on biodiversity and that banning and restricting contaminants works. Peregrines in Canada declined dramatically from the 1950s to 1970s, mainly from egg-shell thinning caused by DDT and its breakdown products.6 With the banning of DDT in Canada in 1970, 1972 in the U.S., and 2000 in Mexico, DDT slowly declined in the environment. Conservation actions and reintroductions helped populations to increase once DDT levels were low enough for eggs to hatch successfully. Some parts of Canada such as the Okanagan Valley of British Columbia may still have too much legacy DDT for peregrine falcons to nest successfully.7

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