Consider this food web. It is rich with animal species that fill a variety with niches within the ecosystem. Imagine that a local farmer has sprayed the area with insecticide. Predict how this might "initially" impact the food web and the roles each organism plays within it.
Although the farmer's spray would wipe out the insects, the other herbivores, rabbits, squirrels, mice and birds, would take over the empty niches and the food web would continue with very little change.
One section of the food web would be greatly impacted right away: the section containing the insects and insect-eating animals. Without insects and possibly spiders, there would be no food for the frogs, shrews and insectivorous birds.
The entire food web would be decimated. The food web would collapse without this niche being filled due to the pesticides used.
One section of the food web would be greatly impacted: the section containing the hawk and snake. They would quickly disappear due to the pesticide use.
The loss of insects and insect-eating animals would have a cascading effect throughout the food web. Without insects as a food source, frogs, shrews, and insectivorous birds would struggle to find enough food, leading to a decline in their populations. This would ultimately affect the predators higher up in the food chain, such as hawks and snakes, as they lose their source of prey.
The absence of insects could also impact the plant community. Many plants rely on insects for pollination, so the decrease in the insect population could result in reduced pollination and lower plant reproductive success. This could have further implications for animals that rely on plants for food or habitat, potentially leading to a decline in those species as well.
Overall, the initial impact of the pesticide use would be the disruption and loss of the insect and insect-eating animal populations. This would cause a decline in species that rely on them for food, ultimately leading to a collapse of the food web and a reduction in biodiversity within the ecosystem.