All organisms in a well-functioning ecosystem exist in a natural balance with each other and with their physical surroundings. This means that all the organisms play a part in the ecosystem, have a home, and have enough food (energy) to survive.

     In every ecosystem, regardless of the type, the sun is the initial source of energy. Plants use sunlight directly to produce their own food (producers). Some consumers eat the plants to get energy (herbivores), while other consumers eat animals to get the energy they need to live (carnivores). The flow of energy from one organism to another in an ecosystem is called a food chain. In a healthy food chain the number of producers is much greater than the number of the higher order consumers. The amount of energy available at each level in a food chain can be expressed as a food pyramid with plants forming its base and each successive level having less energy available to it. Each time an organism at a higher level in the food chain consumes another organism at a lower level of the food chain, a transfer of energy takes place.

     When a food chain is disrupted, the balance of the ecosystem is upset. Not all members of some species have a place to live or can get enough to eat. When a whole species is in immediate danger of no longer existing, we call it an endangered species. A population in which all the individuals have died out and will never again exist on earth is an extinct species.

     Extinction is not a new phenomenon on earth. Species of plants and animals have evolved and died out for as long as life has been here. However, as the human population has drastically increased in the last 200 years, the rate of extinction of other species has increased as well. Today plant and animal species are disappearing at a rate at least 1000 times faster than at any other time in the last 65 million years. Some scientists estimate that we are losing one species per day, others predict that the annual rate of species loss may accelerate to 50,000 by the year 2000. There are several reasons for this alarming loss of species, most of which stem from human impact on the environment.

     Although habitat alteration accounts for the largest single cause of extinction, disruptions in the food chain are also major factors. As human population increases, its need for food increases as well. In developing countries with no wildlife protection laws, hunting threatens many species. Commercial hunting provides food and other animal products that people need or want. However, unchecked hunting of animals for food or other human use accounts for 21 % of extinctions. Examples of animals hunted to extinction include the Caribbean monk seal, the Steller’s sea cow, the Mauritian giant tortoise, and the passenger pigeon.

     Extinction of species such as these affects not only the species themselves, but also the species they prey on and the species that prey on them. Whenever one link in a food chain is removed, the entire balance of energy flow in an ecosystem is threatened. Another human activity that disrupts natural food chains and leads to endangerment and extinction is the introduction of exotic species into an area. Introduction of exotic species accounts for approximately 16% of extinctions.

     An exotic species is one that is not native to a particular region. When an exotic species is introduced into a balanced ecosystem, it can threaten native species that have no natural defenses to it. Exotic species are especially dangerous because they can affect all parts of the food chain. Plants can crowd out and eliminate other plants and predators can wipe out a whole species of prey. Examples of animals that have gone extinct due to the introduction of exotic species include the Jamaican wood rail, the Eastern hare-wallaby, and the Round Island boa constrictor.

     Finally the use of toxins in the environment by humans to control pests endangers some species and upsets the balance of natural food chains. A pest is an organism which lives where it is unwanted by humans because it impacts human health or the food supply (e.g. cockroaches and mosquitoes). However, toxins used to control pests do not break down easily and build up in the environment. As they get absorbed into the ecosystem, they become concentrated in organisms higher up in the food chain. Thus toxins meant for pests also cause problems for higher order consumers, can threaten their existence and cause problems for humans. Examples of animals that are endangered at least partly due to the use of toxins to control pests include the Schaus’ swallowtail butterfly, the white pelican, and the black-footed ferret.

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