GED Science Practice Test: Spraying of Pesticides and Herbicides

Another impact that humans have on ecosystems is through the use of chemicals that kill weeds and undesirable animal species.  Herbicides are chemicals that kill undesirable plants, or weeds.  Pesticides are chemicals that kill unwanted animal species.  As with invasive species, the effects of these chemicals are not just on the immediate population of undesired organisms, but the chemicals can have effects at all levels of the food chain, due to changes in food sources.  The elimination of one plant species with a herbicide could allow another  plant species to flourish.  The consumers that either the other plants species, could increase in number.  If that consumer is an undesirable species, the herbicide that was originally sprayed solved one problem and created another.  Furthermore, herbicides can alter the characteristics of the soil upon which it is sprayed.  Such changes to the soil, such as the pH, can affect a number of other plant and insect species.  Additionally, herbicides and insecticides can percolate through the soil and enter the groundwater supply.  In this way, these chemicals can end up in our water supply and also affect the organisms in aquatic ecosystems as well.

Particularly troublesome from an ecological perspective are pesticides and herbicides that are indiscriminate killers.  For example, many insecticides kill a number of insect species, not just the one that is considered the pest by humans.  Consider a situation in which a farmer’s plant crop is being ravaged by an insect species.  The plant crop, however, has a mutualistic relationship with another insect species.  The question in such a situation is:  which is worse—the loss of the undesirable insect species or the loss of the beneficial insect species?  Honeybees are one such population negatively impacted by insecticides meant to kill a different species.  Honeybees help in the pollination of plant species.  Beekeepers are concerned about changes in honeybee populations linked to insecticide use. The map below shows the states that report “honeybee colony collapse disorder,” which is expected to be a result of pesticide application. Such questions come from an ecological perspective that considers interactions between and among species and their environments.


Another problem with the use of herbicides and pesticides is biological magnification.  Remember the structure of the food chain; a tertiary consumer, for example, eats secondary consumers, which ate primary consumers, which ate producers.  Thus, a herbicide in a plant ends up in the primary consumer and so on.  Herbicides can be found in extremely high concentrations in organisms occupying higher levels of the food chain.  While the herbicide may not have been toxic to a tertiary consumer in the concentrations with which it was applied to the undesirable plant species, it might very well be toxic in the concentrations within the tertiary consumer.  This idea that a toxin can increase in concentration as it travels up the food chain is referred to as biological magnification. The following diagram visually represents biological magnification.  Notice how the chemical pollutant, the dots, become more concentrated as you move up the food chain:


There are many possible alternatives to controlling undesirable plant and animal populations.   One involves completely eliminating toxic chemicals, and either manually removing the pest, or using naturally-occurring substances to create environments that are inhospitable to the undesirable organisms.  Food crops produced through such an approach are called organic.  The term organic refers to the natural growth of plants without chemicals, including chemical fertilizers.  For this reason, organic plants often do not produce the size and quantity of food that non-organic plants do. (See the discussion on overpopulation in the following section).  Another solution is to encourage the growth of, or introduce, a population of organisms that is a predator or at least a competitor with the undesirable species.  This approach is relatively inexpensive, and often successful.  However, there are always ecological concerns with the introduction of new species into ecosystems, as you read in the previous section.  Yet another approach is to engineer chemicals that more specifically target the undesirable species.  Some interesting work with this technique has been done with the use of pheromones.  Pheromones are chemicals specific to a species that can alter the behavior of organisms of those species.  For example, sex pheromones can draw insects to a location to breed.  If that location also housed an insecticide, just the species of animal attracted to the pheromone would be killed by the insecticide.


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