When there are groups of species that live in one area, those species make a community. A community is made up of many levels. First, there is the producer. Producers are pretty much plants like grass and algae. Producers are eaten by herbivores, and then the herbivore is consumed by the carnivore. There can be many carnivores that eat each other until you get to the biggest animal in the community.
The number of organisms, animals, and plants is called a population. To maintain the stability of a population, the combined number of immigrations (moving in) and births must equal the combined number of emigrations (moving out) and deaths. If the number of births and immigrations is larger than the number of deaths and emigrations, the population increases. Conversely, if the number of deaths and emigrations is greater than the number of births and immigrations, the population decreases.
If there is a tiny change in a population of one of the organisms in the community, all the other species will react. These reactions normally help maintain the balance of the community and the biodiversity.
When overpopulation occurs, many species must fight for food and shelter. For instance, when there gets to be too many rabbits to live in a burrow, they must fight each other for shelter and food.
Maintaining the Balance
Predators can often maintain the balance of nature. If the prey species grows rapidly, the predators would have more food, and would decrease the population of the prey. Also, the predators only attack the weak prey so the strong ones can breed and keep the species strong.
Parasites and diseases can reduce a population as well. A bad thing, though, is that most animals adapt to living with these pathogens (diseases and parasites). Stress can happen by being very crowded and can get animals to be very aggressive and fight with each other. Not as many of the species breed and ones that do breed produce minimal litters. Pathogens also spread quicker throughout crowded animals.
Upsetting the Balance
Besides natural factors, human factors also can upset the balance of nature. Humans upset the balance through deforestation, pollution, overhunting, and livestock grazing.
In the early 1900’s, a stable population of deer lived on the Kaibab Plateau in Arizona. Hunters came in and killed all the predators of the deer, and the deer grew at an astounding rate. There was not enough food and many starved. Balance did not return for a long time.
Another story that occurred in 1998 was that the population of kelp in the northern Pacific Ocean dropped massively because orcas were raiding sea otters in the kelp beds. This made a boom in the population of sea urchins, the otters, favorite food. The sea urchins ended up eating the kelp, which decreased the population of the kelp.