We immediately search for a pesticide to kill the insect or a fungicide to get rid of the tomato virus. Mass spraying of large areas with pesticides can destroy an entire ecosystem. Larger insects that prey on smaller ones may also be killed. So humans make the problem worse. Insects become immune to pesticides and some generations of insects may develop immunity to them.
Some pesticides remain in the food chain and their effects become magnified.
In nature, certain kinds of plants grow best in certain areas. A given area can support a certain amount of plant life because of space and nutrients needed by the plants. One limiting factor is the amount of water and minerals in the ground. Another factor is space available for plants to grow and flourish. Another factor is the amount of light that reaches the plant for photosynthesis to occur.