¶ … ecosystem" is used frequently in the popular media, and yet most people do not have a clear, working definition of an ecosystem. An ecosystem simply refers to all of the organisms and their physical environment that exist within a certain, specific area. This includes all plants, animals, and their environment, which can include water, dirt, rocks, and the air that surrounds them. For example, all of the organisms that live in a New England tide pool, plus their physical environment would make up an ecosystem. In addition, a deciduous forest and a salt pond marsh are also ecosystems. A biosphere is an aggregate of many ecosystems rather than a single ecosystem. The biosphere is made up of the entire portion of the earth that supports life. It encompasses many communities and ecosystems.

In the simplest terms, the biosphere is the surface of the earth, and all the organisms contained within. A more complex definition argues that a biosphere is stable, and capable of adaptation, and also capable of being a major geological force. Using this more complex definition, a biosphere could be said to exist on an asteroid, on the ice caps on Mars, or even under the oceans of Jupiter's moon, Europa.

A food web is the interconnected and elaborate feeding relationship between organisms in a single ecosystem. In contrast, a food chain simply refers to the pathway in which food (energy) moves from trophic level to trophic level. A food web allows us to show the diversity of an ecosystem's feeding relationships, as it allows that omnivores like humans can eat both consumers and producers.

In a deciduous forest, a food web can be a complex entity. Primary producers make photosynthetic products, and in the forest, they include the trees and grass. Primary consumers are usually herbivores that eat the primary producers, and include caterpillars, mice, ants, deer, and chipmunks. Secondary consumers eat the primary consumers, and include carnivores like wolves and bears that eat deer and chipmunks, and snakes that eat mice....

In the deciduous forest, humans can function as tertiary consumers, by eating secondary consumers like bears (although this is clearly uncommon).
In addition, snakes function as tertiary consumers when they eat mice. Humans, bears and snakes can act as primary consumers when they eat primary producers like berries.

An owl can eat both tertiary consumers like snakes and secondary consumers like mice.

A high-level consumer like the grizzly bear requires a large range to survive, which can encompass anywhere from 11 square miles to 1000 square miles of habitat. In the Brooks Range of Alaska, where there is minimal human interference, the average grizzly bear home range is 521 square miles. This need is based on the basic principles that underlie energy flow, trophic structure and ecological pyramids.

Trophic structure simply refers to the different feeding relationships within an ecosystem. It governs the route of energy flow in an ecosystem. Energy flow is the path that energy that is originally derived from the sun (through photosynthesis by primary producers). Energy cannot be recycled, so each ecosystem must be continually powered by the addition of energy through photosynthesis. Further, energy is lost at each trophic level, through the laws of thermodynamics.

An ecological pyramid is an illustration of the organisms within a specific ecosystem. The broad base of the ecological pyramid is made of primary producers that supply limited energy through photosynthesis. Increasingly smaller layers of herbivores, and ever-decreasing numbers of carnivores overlap this broad base. At the very top of the pyramid are tertiary consumers like the grizzly bear.

Given that energy is lost with each transfer of energy as it flows throughout the trophic structure, large animals like grizzly bears have access to a limited amount of energy. They are necessarily limited by the amount of energy that enters the ecosystem with the primary producers, and further limited by the energy loss at each trophic…