This can happen due to earthquakes, mudslides, hurricanes, toxic spills, bulldozers, etc. Harsh environments tend to have simpler sets of feeding relationships than environments less harsh, so food chains are more likely to be found in the arctic and the deserts, while complex food webs are more likely to be found in temperate and tropical areas. Simple food chains are liable to be upset more easily than complex food webs, since organisms don't have many choices of other organisms to feed on when their primary food dies off. In this example, in the Aleutian Islands, the population of humans fishing went up, so the fish stocks went down. The seal population, which feeds on fish, then went down.
Sea otters eat urchins, so with fewer otters, the urchin population increased.
Urchins eat kelp, so the kelp population almost disappeared. There are lots of organ systems in the body. Living organisms must maintain a relatively constant internal environment. If the pH, salt concentration, water concentration, temperature etc. are not constant, the chemistry of the organism is upset, and, if the condition is not corrected, it will die. The thalamus, in the center of the head, processes input from your sense organs.