Food Pyramid and Biocides
Last Review/Updated: May 31, 2002
A
food pyramid is like a food chain, and represents some of the energy
relationships between members of a natural community. However, a food
pyramid illustrates the relative number or weights of the different
kinds of animals, as they relate to food habits.
In terms of numbers, there are always fewer carnivores (meat eaters) than herbivores (plant eaters), and fewer herbivores than plants in any given community. Biologists have determined that it generally takes about 1000 kg of plants to feed and maintain 100 kg of herbivores. Likewise, that 100 kg of herbivores can only feed and support about 10 kg of carnivores.
For example, a field may grow 1000 kg of grasses, which supports 100 kg of ground squirrels, mice, grouse and insects. These in turn support 10 kg of weasels and songbirds, which in turn may support 1 kg of hawk or eagle.
At each step on the food pyramid, the plants and animals spend about 90% of the energy they consume for their own purpose, e.g. growing, moving around, feeding, digesting, maintaining body heat, and reproducing. Only about 10% of the energy consumed at one step is transferred to the next step. That is why there are fewer and fewer animals with each succeeding step on the food pyramid.
Raptors are at the top of the food pyramid of most communities. Raptors eat both herbivores and carnivores, and thus receive energy from more than one level of the pyramid.
Because of their position at the top of the food pyramid, raptors have been especially susceptible to the effects of certain biocides. Biocides are chemicals widely used in agriculture and forestry to destroy or repel animal pests or weeds. Some of these biocides are very selective, acting only on one particular pest species. Others are nonselective, and can affect many organisms.
Insecticides used to control insect pests, and fungicides used as seed dressings can also kill small birds and rodents, reducing a raptor's food supply. The indiscriminate use of herbicides (weed killers) destroys large areas of vegetation.
However, it wasn't until the development of certain "persistent pesticides" that drastic reductions in the numbers of some wildlife species were noticed. The first of these pesticides, DDT, appeared in the mid-1940's. DDT is non-selective, and affects many forms of animal life. DDT was used throughout the world umtil the late 1960's when major declines in populations of many bird species became evident. (For example, bald eagle, peregrine falcon, brown pelican.)
Unlike other pesticides, persistent pesticides have a very stable chemical structure that resists breakdown, and remain in the environment long after their application. Indeed, their long-lasting effect was one reason they were developed. However, wind or water can transport them to areas far from the site of application. For example, pesticides have been found in animals living in the Arctic, thousands of kilometres from the nearest site of application.
Persistent
pesticides are not easily excreted from the bodies of animals that
ingest them. The pesticide is usually sprayed on plants and is eaten
by plant-eating animals (insects, rodents, birds, etc.). If the chemical
does not kill the animal outright, it is eventually broken down and
stored in the animal's body. Unfortunately, some of these byproducts
are also harmful. Some are stored in the fatty tissue of the body,
and remain until the animal is eaten or dies from other causes. The
longer the animal eats food contaminated with pesticide, the greater
the concentration of pesticide in its fatty tissues. A predator feeding
on a number of these animals then consumes the pesticide in a higher
concentration than did the prey species. Thus, the concentration of
pesticide in the predator becomes several times larger than that in
its prey species. This is called "biological magnification."
Because raptors are at the top of the food pyramid, they can receive large doses of poison with every animal they eat.
Although the slow accumulation of biocides in a raptor's body may not kill the animal outright, it can affect the bird's ability to reproduce. Many populations of raptors have been drastically reduced by DDT because the birds laid infertile, or thin-shelled eggs that were broken before the young could hatch.
Although the use of DDT was banned in Alberta during the early 1970s, other persistent pesticides are still used. We must seriously question the value of pesticides that affect the entire food chain. A reduction in numbers of members of any one link on a food chain can be a warning that something is wrong in our environment. We must never forget that we too are members of a food chain, and what is affecting other species may also be affecting us.
