Competition

Organisms compete for things in short supply. Plants compete for light, water, minerals and space. Animals compete for food, water, space (territory) and mates. Members of the same species compete hardest, because they need exactly the same things.

Losing a competition means growing less, having fewer young, moving away or dying. Some organisms dodge it: bluebells flower in spring, before the trees above them come into full leaf.

Predator–prey cycles. When prey increase, the predators have more food, so they increase too, a little later. More predators eat more prey, so the prey fall. Then the predators go hungry and fall, and the prey recover. Canada lynx and snowshoe hares do this roughly every ten years.

Winners and losers. Grey squirrels outcompete red squirrels: they are bigger, digest acorns better and carry squirrelpox. Japanese knotweed crowds out native plants. A cuckoo chick pushes out its foster brothers and sisters and gets all the food.