An ecosystem is a community of different species (living things--Biotic) interacting with each other and with the chemical and physical factors making up the nonliving (Abiotic) environment. The nonliving parts of an ecosystem include--water, air, nutrients, and solar energy. Physical factors like sunlight, temperature, precipitation, wind, latitude (distance from the equator), altitude (distance above sea level), frequency of fire, and the nature of the soil affect lan ecosystems. Water currents, concentrations of dissolved nutrients (such as phosphorus and nitrogen), level of dissolved oxygen and the amount of suspended material are physical factors that affect aquatic ecosystems. In both landand aquatic ecosystems, the amount of water, air, supply of nutrients or toxic substances are important chemical factors that impact interactions. Together these physical and chemical factors help determine which species will thrive and survive under varying conditions. Some living things require a hot environment, other a cold one. Some species do best under dry conditions while others do best in wet conditions. The chemical and physical factors in an ecosystem can limit or prevent the growth of populations within the ecosystem.

Every organism in an ecosystem is directly or indirectly linked with other organisms in the ecosystem. Understanding how the changes in the environment affect one group of organisms could lead to predicting impact on other populations within the ecosystem. Like other living systems, ecosystems are highly organized. The organization is illustrated by the role organisms play in the ecosystem. Organisms are divided into:

1) producers--organisms that make their own food from compound obtained from the environment

2) consumers--
organisms that depend on other organisms for food
a) herbivores--plant eaters; primary consumers that feed directly on producers
b) carnivores--meat eaters; secondary consumers feed only on primary consumers
c) Omnivores--eat both plants and pigs, rats, foxes, bears, cockroaches, YOU

3) decomposers--organisms that break down dead plants and animals and return nutrients to the soil and bacteria and fungi are consumers that breakdown and recycle wastes and remains of all organisms.

Organisms within each ecosystem compete for food, space, light, heat, water, air, and shelter. When different species in an ecosystem need similar requirements in order to live, they may interact with one another. As result of this interaction, many different kinds of relationships (symbiosis) are formed between living things in an ecosystem. The relationships include:

1) predator--feeds on all or part of a living organism of another species--prey
2) mutualism--two species interact in ways so that both benefit
3) commensalism--two species interact in ways so one benefits and the other is not affected
4) parasitism--when one species feeds on another organism (host) by living in or on the host...the parasite benefits by gaining nutrients, shelter etc and the host suffers sometimes death