What effects can a disturbance have on a community?

A) Fogging with insecticides is likely to have directly reduced the number of insects. However, the reduction in the number of birds and new growths of orchids may not have been directly caused by the pesticides.

B) Many pesticides are fat-soluble. The amount of pesticide directly encountered by the birds may have been within their limits of tolerance. As insect eaters, however, they may have accumulated much more of the pesticide over time. Any one insect contains only a small amount of the pesticide, but over time, the birds consume many insects and the pesticide in each accumulates in the birds’ adipose (fat) tissues.

Most orchids are pollinated by insects. General fogging for insect pests would also kill the insects that pollinate the orchids and result in less new growth in the orchid population. Many plants have coevolved with specific insect species. If their pollinator dies out, the orchid species dies out as well.

C) Different types of experiments could be set up. One possible experiment would test the effect of direct spraying on a population of birds that had no prior exposure to the pesticides. Groups of birds would need to be maintained in a facility that allows you to control what they eat and to observe their behavior. At the beginning of the experiment, all the birds would be directly sprayed with pesticides at a level consistent with possible exposure in the wild. For the remainder of the experiment, half of the birds would be fed insects that contained no pesticides. The other half would be fed insects sprayed with pesticide. Immediately before spraying and immediately after spraying, you would measure the level of pesticides in specific tissues in the birds. You would continue to measure the levels of pesticide in these tissues at set time intervals. The behavior and levels of pesticides in both sets of captive birds could be compared to each other and to those of birds in the wild.

D) If direct spraying caused the deaths in the birds, you would expect to see equal death rates in both populations of captive birds. If bioaccumulation caused the deaths, you would expect to see higher numbers of deaths in the birds fed insects sprayed with pesticide. You would also expect to see a relatively constant level of pesticide in the tissues of the birds fed unsprayed insects and increasing levels of pesticide in the tissues of birds fed sprayed insects. When you compared the levels of pesticides in the tissues, you would expect to see a strong correlation between the levels of pesticides in the birds fed sprayed insects and in the birds from the wild.