How Earth's Ecosystems Work

Ecosystems include both living and nonliving components. These living, or biotic, components include habitats and niches occupied by organisms. Nonliving, or abiotic, components include soil, water, light, inorganic nutrients, and weather. An organism's place of residence, where it can be found, is its habitat. A niche is is often viewed as the role of that organism in the community, factors limiting its life, and how it acquires food. 

Producers, a major niche in all ecosystems, are autotrophic, usually photosynthetic, organisms. In terrestrial ecosystems, producers are usually green plants. Freshwater and marine ecosystems frequently have algae as the dominant producers. 

Consumers are heterotrophic organisms that eat food produced by another organism. Herbivores are a type of consumer that feeds directly on green plants (or another type of autotroph). Since herbivores take their food directly from the producer level, we refer to them as primary consumers. Carnivores feed on other animals (or another type of consumer) and are secondary or tertiary consumers. Omnivores, the feeding method used by humans, feed on both plants and animals. Decomposers are organisms, mostly bacteria and fungi that recycle nutrients from decaying organic material. Decomposers break down detritus, nonliving organic matter, into inorganic matter. Small soil organisms are critical in helping bacteria and fungi shred leaf litter and form rich soil. 

Even if communities do differ in structure, they have some common uniting processes such as energy flow and matter cycling. Energy flows move through feeding relationships. The term ecological niche refers to how an organism functions in an ecosystem. Food webs, food chains, and food pyramids are three ways of representing energy flow.

Producers absorb solar energy and convert it to chemical bonds from inorganic nutrients taken from environment. Energy content of organic food passes up food chain; eventually all energy is lost as heat, therefore requiring continual input. Original inorganic elements are mostly returned to soil and producers; can be used again by producers and no new input is required.

The main source of energy for almost all forms of life is the Sun.   Scientists discovered an exception, communities of organisms around ocean vents where food chains begin with chemosynthetic bacteria.  These organisms oxidize hydrogen sulfide generated by inorganic chemical reactions inside the Earth's crust. Their source of energy is the internal heat of the Earth instead of the heat of the Sun. 

Food chains indicate who eats whom in an ecosystem. Natural ecosystems have numerous interconnected food chains, much like a complex web. Each level of producer and consumers is a trophic level. Some primary consumers feed on plants and make grazing food chains; other consumers like mushrooms feed on detritus.

The population size in an undisturbed ecosystem is limited by the food supply, competition, predation, and parasitism.

The trophic structure of an ecosystem forms an ecological pyramid. The base of this pyramid represents the producer trophic level. At the apex, or top, is the highest level consumer or top predator. The pyramid of biomass is calculated by multiplying the average weight for organisms times the number of organisms at each trophic level. We can use an energy pyramid illustrates the amounts of energy available at each successive trophic level. The energy pyramid always shows a decrease moving up trophic levels because: 

A food chain is a series of organisms each feeding on the one preceding it. 

There are two types of food chains: 

primary producer
primary consumer
secondary consumer
tertiary consumer
decomposer

Energy flow and the relative porportions of various levels in the food chain. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Food chains are simplifications of complex relationships. A food web is a more realistic and accurate representation of energy flow. Food webs are networks of feeding interactions within any given species.

The food pyramid provides a detailed view of energy flow in an ecosystem. The first level consists of the producers (usually plants). All higher levels are consumers. The shorter the food chain the more energy is available to organisms. 

herbivore
omnivore
carnivore
decomposer
scavenger

Most humans occupy a top carnivore role, about 2% of all calories available from producers ever reach the tissues of top carnivores. Leakage of energy occurs between each feeding level. Most natural ecosystems therefore do not have more than five levels to their food pyramids. Large carnivores are rare because there is so little energy available to them at the top of the pyramid.
 

Food generation by producers varies significantly between ecosystems. Net primary productivity (NPP) is the rate at which producer biomass is formed. Tropical forests and swamps are the most productive terrestrial ecosystems. Reefs and estuaries are the most productive aquatic ecosystems. All of these productive areas are in danger from human activity. Humans redirect nearly 40% of the net primary productivity and directly or indirectly use nearly 40% of all the land food pyramid.