Answer:
Environment and competition is the leading cause of evolution. It's an instinct to want to live, but trying to survive is difficult when other animals have the same goal. An ecosystem is a cycle, but every generation of this ecosystem slightly changes, and so do the animals. Every bug that has fitness can start overpopulating the ecosystem, leading to a surge of resources for what preys on the bug. Now bird's population goes up, so do other animal populations. Or the bug becomes an invasive species, and birds are being hunted too often to decrease the population.
In a different scenario, bugs are evolving because of the selection pressure that the environment is giving them. The birds have become better over the generation, and now have a more suited beak to catch the bug off the ground. The bug population maybe be decreasing, but the ones who live through the crisis are populating with bugs that certain gene that helped them out. But it's not over, the bug population isn't declining but it wasn't as big as it was before. The bird will find more ways to find and capture the bugs. But then bugs will keep surviving through the selection pressure and become much harder to find since they figured out camouflage. The bug that was neon yellow was dying since they were easy to see. Now they can range from green to black to hide in the environment. Now the birds are facing the selection pressure, it's a cycle.
(look, im not good at gammar, so you should get this looked at before entering it. I put it through gammarly if that helps.
