1. Bioaccumulation
Bioaccumulation is what happens when the concentration of pesticides first sprayed on the soil builds up as it passes through the food chain due to animals eating plants and each other.

2. How does this happen?
The concentration rises because each trophic level has to feed on increasing amounts of biomass as you rise up the levels.
This is due to the fact that each needs to consume more to gain enough energy for survival and reproduction.
This is further magnified by their solubility in fatty tissues. This also means that they can be stored.

3. Why is this a problem?
The pesticides themselves are a problem due to the fact that no enzyme, micro organism or chemical process can degrade them. This means that they remain permanently in the soil, plant or body of any plant or mammal that eats another that is contaminated.
This means that the toxic substance is easily passed along the food chain.

4. What does this mean for animals?
The problem lies in the fact that when the pesticide reaches a certain level, it becomes toxic. This can have mixed effects for the top consumers in the food chain.
E.g. birds can have reduced fertility.
At its worse, the high toxicity can also lead to death.

5. What can be done about it?
Instead of spraying MORE of a pesticide on the crop, farmers should spray DIFFERENT pesticides on the crop. This means that the concentration of a particular toxin can not become so harmful.
Stop using pesticides all together, and breed plants that are naturally resistant to pests.