Impact of - IDL-BNC @ IDRC - International Development Research ...

Using target-specific products minimizes the effect on desirable nontarget
species in the crop as well as in areas adjacent to it. As agriculture in developing
areas becomes increasingly intensive, wildlife species become relegated to
remnants of natural habitat next to farm land, such as small wetlands or
woodlots in the middle of cultivated fields, drainage ditches, fence lines,
hedgerows, or even small rock piles. Inadvertently, even these habitats can be
affected through the direct application of pesticides, spray drift, or runoff.
An example of this situation in Canada can be found in the Prairies where
small wetlands used by nesting ducks are surrounded by agricultural fields.
Most of the insecticides in use have the potential to reduce the number of
aquatic invertebrates in these wetlands, thereby reducing the food supply for
ducklings. The effect is greatest when insecticides are applied aerially because
the small wetland areas are sprayed directly. In addition, the use of herbicides
in the Prairies is also likely to reduce the quantity and quality of cover available
for nesting ducks (Mineau et al. 1987; Sheehan et al. 1987).
Long-term contamination of the wildlife environment is the most lasting and
widespread problem associated with pesticide use. Bioaccumulation of substances
that are poorly metabolized and excreted by wildlife species and that
accumulate in some tissue, such as fat in the case of lipophilic substances,
causes particular concern. The most notorious and best documented examples
are a number of organochiorine insecticides such as dichlorodiphenyltnchloroethane
(DDT) and dieldnn.
Although death is the surest sign of pesticide impact, sublethal and delayed
mortality effects are also significant. Many pesticides can affect the normal
functioning of exposed individuals at doses insufficient to cause death. At
high doses, the organophosphates and carbamates cause respiratory failure
and death. However, wild birds exposed to these agents in lesser amounts
have experienced impaired coordination and loss of appetite. Other consequences
of sublethal exposure to these agents include birds spending less time
at the nest, providing less food for their brood, being less able to escape
predation, and being more aggressive with their mates. Notable effects of
sublethal pesticide ingestion also include reproductive failure through
reduced hatching or fledging success. Several pesticide products can cause
embryonic mortality when sprayed directly onto eggs.
It is difficult to assess the extent to which environmental factors combine with
sublethal pesticide exposure to cause delayed mOrtality. Exposure to some
pesticides makes wildlife more vulnerable to predation. It also causes weight
loss and an inability to maintain body temperature, both of which increase the
chances of a small bird dying in inclement weather. Finally, exposure to
pesticides may be linked with reduced resistance to disease.
Although intuitively evident to the ecologist, it is most difficult to demonstrate
the indirect effect of pesticides on wildlife. In one of the best examples
available to date (Potts 1980), long-term data on population numbers and
pesticide use, as well as a great deal of fundamental research, were required
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