A sustainable environment is an environment in which all of the plants, animals, and other forms of life in it are able to exist in the ecosystem without any exterior aid or interference. Granted, this doesn’t mean that every single animal will live to die of old age, but it means that the natural order of things is preserved and tucked away from human intrusion. If organisms can thrive, get their own food and water, and inhabit their own shelters, then the environment has provided resources for at least one population of organisms, making the environment sustainable.
Let’s take a look at a lake. Beneath the surface of the lake, there are fish and other forms of algae. The fish can feed off of the algae in the lake; however, if something else is introduced into the lake that eats the algae, then the fish population will not have anything to eat unless it adapts surprisingly quickly, and it will decline unless it can compete for the essential resource of food with the invaders. Oddly enough, this is a sustainable environment: organisms have their own lives, and the natural order of competition is still in effect.
Now, let’s say some humans go to the lake and start catching all of the fish. Within a month or so, the lake is completely devoid of fish, and the algae population has grown. While it is probable that the algae can continue to survive until it hits its astronomically high carrying capacity, what about the fish population? When humans entered the ecosystem, they disrupted the balance of nature by completely removing a population from its habitat; although the environment was sustainable, humans came in and ruined it.
A sustainable environment can be compared to a self-sufficient community: nothing is needed that exists outside of the ecosystem. For example, budding towns in developing countries strive to become self-sufficient, and they do this by producing their own crops, brining in cattle to breed and take milk and meat from, use trees to build houses, and survive just by using what they have. A sustainable environment is an area in which the organisms can live by themselves without relying on humans to come in, label them as endangered, and save their lives.
Additionally, an environment needs to have such resources in order to be labeled as sustainable. Adequate food, water, shelter, and space for at least one population of organisms is required in order for the environment to be able to sustain anything. It is of paramount importance that the environment isn’t too harsh for anything to survive in; although certain types of creatures can live in extremely cold climates, most cannot, so the environment needs to be able to support at least one species in order to be a sustainable environment.