V. NEGATIVE FEEDBACKS

Many of the reasons for the pesticide treadmill (needing to apply more and more pesticide just to keep in the same place control-wise) involve disruptions with natural controls over pests.

Pest control in nature generally relies on a system of negative feedbacks. This use of "negative" does not imply that these are "bad" feedbacks, but implies feedbacks that tend to maintain stability in a system

Resistance to drastic changes and thus stability in natural ecosystems are maintained in part by negative feedback systems. These are familiar to you through considering the system that regulates the heat in your home. When the temperature in your house decreases below some set point, the thermostat that senses temperature is activated to send a "turn on" message to your furnace. Then, once the temperature rises above another set point, the thermostat sends a message to the furnace, telling it to shut off. Thus, the temperature in your house is maintained within bounds. This illustrates a negative feedback system. Negative feedback systems act to maintain homeostasis within systems; that is, to keep them in a reasonably constant state.

For the case of pest controls in natural ecosystems, several systems similar to this operate. For example, consider two compartments as indicated in the sketch below:

BI300017.gif

BI390000.gif One represents the size of the pest population ("PE")

BI390000.gif One represents the size of a population of creatures that are predators on this pest ("PR")

An arrow runs from the pest population to the predator population and vice versa. The direction of each arrow indicates causal relationships. Thus, the arrow from the pest population to the predator population represents the effects of the pests on the predators, while the arrow from the predator population towards the pest population represents the effects of the predators on the pests.

Each arrow has associated with it either a "+" or a "-" sign. Whether the signs are + or - is deduced by examining the effects that increasing or decreasing the causal agent has on the one being affected.

For example, consider the arrow running from the pest to the predator, which indicates effects of the pest population on the predator population. Which sign (+ or -) should be attached to that arrow? Well:

BI390000.gif If the pest population increases, so will the predator population (because the pest is food for the predator).

BI390000.gif If the pest population decreases, so will the predator population (because it will have a lessened food supply).

These are "parallel" relationships, in that if the causal compartment increases, so does the compartment affected by it, while if the causal compartment decreases, so does the compartment affected by it. We symbolize parallel relationships like this with the "+" sign.

In contrast, consider the casual arrow running from the predator to the pest:

BI390000.gif If the predator population increases, the pest population will decrease (as it gets preyed on more heavily.)

BI390000.gif If the predator population decreases, the pest population will increase (as it is then preyed on less heavily).

These relationships are antiparallel in that if the causal compartment (the predator) decreases, the affected compartment (the pest) increases, while if the predator increases, the pest decreases. We symbolize this interaction with the "-" sign.

Now, what we want to deduce, is the nature of the overall system, not just the individual interactions. To deduce this, we multiply the signs of the two interactions (in this case a "+" times a "-" representing the effects of pest on predator and the effects of predator on pest, respectively). The product of these interactions (of this multiplication) is negative (a positive times a negative = a negative), indicating that this is a negative feedback system, which tends to maintain stability within the system.

Check yourself: Choose another pair of interactors involved in control of pests in natural ecosystems (such as host availability and pest population) and work out whether their interaction is stabilizing (negative feedback) or destabilizing (positive feedback, as would result when the signs on each arrow are both either negative or positive). Please feel free to submit your analysis to me if you wish (strictly optional).

What happens in many agroecosystems (or other systems treated regularly with pesticides) is that pesticides interfere with these negative feedback controls. We get the spray trying to regulate the pest instead, so that the natural feedback doesn't work well. For example, if predator populations are diminished by the spray as well (either directly or indirectly ), when chemical treatments are removed, the predator population will be small, favoring an increase in the pest -- i.e. the negative feedback system is weakened -- and the fairly typical and dramatic pest rebound occurs. Thus, pesticides interfere with the natural systems of checks and balances (negative feedback systems) and we become increasingly dependent on their use. Excessive reliance on pesticides can become another addiction and treadmill; we have to keep running faster and faster just to stay in the same place.

Check yourself: In what other area of agriculture have we seen the necessity for farmers to add more and more of an input just to stay in the same place? (You can click to see...)

The next section (">>" at the bottom of the page) discusses something that has been implicit throughout much of this discussion of pesticides in agriculture; that is, that there ARE many reasons to be concerned about high rates of pesticide application. (For general navigation reminders, click "Navigate ," here.)

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