By eating the plants they receive energy that was stored in the plants. This energy originally came from the sun. It was captured by the plants through the process of photosynthesis. Shrews are primary consumers in this food chain. They eat the producers to obtain energy.
Hawks are secondary consumers because they eat the primary consumers.
When pesticides or herbicides enter the food chain they sometimes have long reaching and unintentional effects. Thus, their effects can be cumulative as you go up the food chain. This is the process of bio-accumulation. A grasshopper may eat a nonlethal dose of a pesticide and then get eaten by a shrew. Because the chemical does not decompose it is stored in the shrew's body.