Every living thing needs to be in balance with its environment. In northeastern Ohio, there are tens of thousands of deer. In some places the deer population has gotten too big, meaning that it’s out of balance. When there are too many deer, then they start eating peoples’ bushes, shrubs and grass. This results in unhealthy deer and unhappy people! In other words, the deer are out of balance with their environment; there isn’t enough food to support them all.
Equilibrium is when there is a balance between two or more things. In biology, when we talk about equilibrium, we’re talking about living things being spread out so that they can get their four basic needs met: food, water, shelter and space. If they are unable to get any of these basic needs met, then they are out of equilibrium with the environment. Even more importantly, if other organisms are unable to get any of their basic needs met because of the deer, then the deer are out of equilibrium with their environment. If you take too much or get too little from your environment, then you’re not in balance with your surroundings!
The example between the deer and peoples’ trees, shrubs and grass is a little silly: the deer don’t prevent us from getting our basic needs met. But there are areas of Ohio where deer are responsible for the destruction of crops (plants farmed for food) which does threaten that most basic of our needs, food.
The reason that there are so many deer in the first place is actually our society’s fault. Wolves and cougars hunt deer, but we hunted so many wolves and cougars that by 1900 the only ones around had fled north to Canada. Over the last hundred or so years, deer have been able to survive and reproduce because they have no natural predators (other than us). When species become extinct or if they emigrate (leave a particular area), it often leaves a gap in the food web. Other species can benefit, while others can be harmed in this change in equilibrium.
The opposite change can be just as destructive: when a new species is introduced, it can often be disastrous. Take, for example, rabbits in Australia. Rabbits were never native to Australia; they were introduced by the British who brought rabbits there in 1788. They were kept in cages until 1859, when just 24 rabbits were released into the wild. Because there were no natural predators in Australia, and the climate was perfect for rabbit reproduction, they reproduced so rapidly that they spread throughout the entire country. Within 10 years, hunters were able to kill 2 million rabbits in the wild, and it had no noticeable effect on the rabbit population!
The rabbits in Australia is an example of an invasive species (or non-native species). When invasive species enter a particular area, they usually change the equilibrium in their favor by pushing out one or more native species. When humans use invasive species to solve problems, it can often cause even bigger problems.
Rats, not native to Hawaii, were accidentally introduced when they hitched a ride on boats headed to the islands. Since rats eat crops, some farmers had the idea to introduce an animal that could eat the rats: the mongoose. Well, mongooses do eat rats, but they also eat almost anything else; what’s more, the mongoose sleeps at night and rats sleep during the day. The mongooses didn’t end up hunting the rats, so the rat problem was not solved; it only created a new mongoose problem! Most of the Hawaiian native species (plant and animal) are now considered endangered species due to the destructiveness of the mongoose.
Species immigrate (move into a certain place) and emigrate quite often, but as we’ve learned, there are more than biotic factors in an ecosystem. Since the abiotic factors can often influence living things, climate changes often cause shifts in equilibrium. For the hundreds of millions of rabbits in Australia, it would be disastrous if the average temperature dropped significantly; suddenly, the rabbits would not be able to survive in such a cold climate, and Australia’s rabbit problem would disappear. However, Australia would end up with a lot of other problems!