Here is the food web again. What would happen if the population of one of the organisms changed?
The grass is the producer. If it died, the consumers that feed on it - rabbits, insects and slugs - would have no food. They would starve and die unless they could move to another habitat. All the other animals in the food web would die too, because their food supplies would have gone. The populations of the consumers would fall as the population of the producer fell.
Slugs, rabbits and insects all eat grass. If there were fewer slugs there would be more grass for the rabbits and insects to eat. With more food, the populations of rabbits and insects would increase. However, the thrushes would have to eat more insects to maintain their population, so it is also possible that the population of insects could decrease. This in turn may reduce the populations of voles and frogs.
There would be more food for the rabbits and slugs, so their populations would increase. However, there would be less food for the frogs and voles, so their populations would decrease. This means less food for the foxes and hawks. However, there are likely to be more rabbits and thrushes for them to eat, so their populations might stay the same.
All the food we eat relies on plants. This includes meat because animals such as pigs, sheep and cattle eat plants. Grasses such as wheat, barley and rice use the wind for pollination. Vegetables and plants that produce fruit rely on insects and other animals for their pollination. If the population of pollinating insects goes down, it reduces the amount of fruit for us to eat, and also the number of seeds for new crop plants to grow.