Organisms in one ecosystem are called populations. Populations living with other populations are called a community. Another part of ecosystems is the physical environment, which could be the sun, air, water, weather, and dirt. Populations interact with the dirt and sun and water. But to live, each population needs a specific amount of this stuff. The need to get these things leads to competition. To compete in habitats, animals have adaptations.
Sometimes different populations relate with each other, which is called symbiosis. One kind of symbiosis is called mutualism, where both organisms are helped. Another kind is called commensalism, where one organism benefits and the other isn’t hurt. The last symbiosis is called parasitism, where an organism thrives whiles the other is affected. To survive, prey have to killed by predators (meaning that animals have to be killed). As it is easy to know that predators need prey, it is harder to see that prey need predators. But they do, because if predators would be taken out of the food chain, then the prey would get bigger, and then starve because all the food would be eaten.
A human population is like a neighborhood or town. But a animal population is a group of animals living in a habitat. A animal community is lots of groups of animals living in, say, a savannah. A human community is a county or state. Also, human competition is when humans and animals fight over land and trees. Animal competition is over water or food.