An ecosystem is localized group of interdependent organisms together with the environment that they inhabit and depe

can be seen at all time-scales, from the shortest to the longest. Short-term changes, which are observable by people, are often cyclical and predictable: night and day, the monthly cycle of the tides, the annual change of the seasons, and the growth, reproduction, and death of individuals. Viewed at this level many ecosystems, when not disturbed by humans, appear superficially to be stable and unchanging, maintained in equilibrium by the “balance of nature”. The sun is the original source of energy in an ecosystem. Producers (plants) convert the light energy into chemical energy, storing it in their cells. When primary consumers (herbivores) eat the producers, the energy changes into a form that can be stored in animal cells. Secondary consumers (carnivores) transform the energy once again. Decomposers may occupy several positions in the pyramid, both receiving energy from decaying plants and animals and supplying it to detrivores and fungus-eaters. Since energy is lost in each step, populations are necessarily smaller at each higher level of the pyramid.