Introduction to Ecosystems

An ecosystem is every organism in an area, plus the nonliving parts it interacts with: water, sunlight, soil nutrients, temperature. A pond is an ecosystem because the fish, algae, insects, and bacteria all depend on the water chemistry, the sunlight hitting the surface, and each other. Pull out any one piece and the rest shifts too. That's the test for whether something counts as one ecosystem: do the biotic and abiotic pieces actually exchange energy and matter with each other.

Inside that pond or forest, predator and prey numbers lock into a push-pull cycle. When prey like snowshoe hares are abundant, predators like lynx have plenty to eat, so lynx survive better and raise more young. But turning extra food into extra lynx takes time, a breeding season or two. By the time lynx numbers climb, hares are already being overhunted and start to crash. Fewer hares means lynx start starving and their numbers fall too, a year or so after the hares. With fewer lynx hunting them, hares rebound, and the whole cycle repeats. The lynx peak always lags the hare peak because lynx reproduction only catches up to food supply after a delay.

This lag is also why predators rarely wipe out their prey. As prey get scarce, predators decline right along with them, easing the pressure before extinction happens. Add a second predator hunting the same hares, and the hares face heavier combined pressure, deeper crashes, and the two predators now compete with each other too, which can desynchronize or dampen the original clean cycle. This feeds directly into later material on energy flow: every one of these population swings runs on energy passed up the food chain, and most of it is lost at each step.