The biosphere is organized in a hierarchical structure in which individual organisms are organized into populations. Several interacting populations form biotic communities. And a distinctive community living in a certain physical environment forms an ecosystem. An ecosystem is a group of animals, plants, and microbes interacting with each other and with their physical environment in such a way as to ensure their existence. Hence an ecosystem is the functional unit of sustainable life on Earth. No individual cell, population, or biotic community forms a sufficient entity to support life. Ecological populations are groups of interbreeding organisms of the same species living in close proximity with one another. Species represent a group of individual organisms in nature that have strong similarities to one another because of close genetic kinship (no more than 2% genetic differences), and can interbreed to produce an offspring that is fertile. The mule is the offspring of a horse and a donkey, two separate species. As a result, they are infertile.

In general, ecosystems do not have sharp boundaries. Instead there are gradational or transitional communities referred to as ecotones. Several related or similar groups of ecosystems are known as biomes. Biomes are grouped into two distinct categories: terrestrial and aquatic. Terrestrial biomes consist of ecosystems inhabiting land environments such as tundra, temperate grasslands, tropical forests or hot deserts; whereas aquatic biomes consist of water-dwelling ecosystems such as pelagic, and benthic freshwater ecosystems. Aquatic biomes are typically distinguished by salinity, climate, and water depth.

(Map showing major terrestrial biomes)


Human Analogy to Natural Biotic Structure


The parts of the biosphere occupied by humans are called society. In societies, we can identify parallels with the way the biosphere is organized, particularly in Amercan cities. One can consider humans living in a particular "development" as "natural populations." Several contiguous developments in a city constitutes a suburb (or neighborhood), which is the human version of "biotic communities." Several suburbs/neighborhoods, when incorporated, becomes a city or town. Cities are the human equivalents of natural "ecosystems." A state can be considered as consisting of several cities, and can be compared to "biomes."


Common Ecosystem Functions

Despite the variations among ecosystems, they all share a common structure due to common functions of basic processes within each ecosystem.

         They all engage in energy transfer within the community , and energy in an ecosystem flows in one direction.

         They all must recycle various chemical substances required for growth, reproduction, and protection.

As a result, the basic biotic structure is based on feeding relationships. The producers, consumers, and detritus feeders and decomposers are the basic feeding categories that allow energy and matter to flow through ecosystems. All organisms must feed to satisfy the basic processes listed above.

Producers are mainly green plants that manufacture their own food through the process of photosynthesis, using simple inorganic substances from the environment, and sunlight as a source of energy.

Carbon dioxide + water + sunlight ---------------> sugar + oxygen

Chlorophyll is the green dye in plants used to capture light energy to support the reaction. Some bacteria use a purple pigment for photosynthesis but by far, the dominant group of producers are green plants.

Consumers, on the other hand, feed on available organic matter to obtain energy and nutrients. Three kinds of consumers exist: primary,secondary, and tertiary consumers.


Primary consumers are herbivores and consume vegetation, or primary producers.

Secondary consumers include:

carnivores (consumes meat foods).

Detritivores are the third group of feeders in a biotic community. Primary detritivores feed directly on organic wastes (detritus), and are also known as decomposers. Examples are bacteria, fungus etc, that decompose dead leaves, twigs, animal droppings. Secondary detritivores, such as beetles, ants, termites, and earthworms, feed on primary detritivores. Large animals such as hyenas, jackals, or vultures are also examples of secondary detritivores called scavengers.

An established pathway of feeding in any ecosystem is called the food chain. Terrestrial food chains are tyupically shorter than aquatic ones. But food chains rarely exist in isolation. A herbivore population may consume several different types of plant daily and the herbivore population may be preyed upon by several types of carnivores. Consequently, all food chains are interconnected into a complex web called a food web. This is even clearer when you consider decomposers such as bacteria that may feed on several individual food chains.


(Terrestrial food web figure)

Trophic Levels

As a result of the complexity of feeding relationships, a simple overall pattern of feeding relationships does not exist in each ecosystem. But one can simplify this by assigning each feeding relationship into several levels called trophic (or feeding) levels. All producers belong to the first trophic level, and all primary consumers (herbivores) belong to the second trophic level, followed by carnivores at the third trophic level (or fourth trophic levels) and parasites and decomposers at the first, second, third, fourth, etc., trophic levels. It is much less complex to refer to the food chain or web using these generic terms. Hence all feeding groups, based on the trophic level concept, can be lumped into two categories - autotrophs (self feeders or plants) and heterotrophs (other feeders including consumers and detritivores).

Now, one can address energy flow from one trophic level to another using the biomass pyramid. The biomass is the net dry weight of all organic tissue at each trophic level. The biomass pyramid is a food chain concept that explains the numerical limits of the amount of energy available at each trophic level in an ecosystem. Typically, there is no more than five trophic levels in a terrestrial ecosystem (Some aquatic biomes may have more than five trophic levels). There is a dramatic decrease in biomass from the first through to the fifth trophic levels, which suggests that the available energy transferred from one level to another should likewise decrease dramatically. In terrestrial ecosystems, the the biomass transfer from one trophic level to another is no more than 20 percent. This observation is a result of the fact that most food consumed at one trophic level is unavailable to the next level because it is used up in body growth/repair, operational energy, and lost as waste in excreta. Hence it takes no more than four or five trophic levels for the available feeding biomass to approach zero pounds or kilograms. Therefore, the population of all heterotrophs are limited by what green plants produce. If the production capacity of autotrophs decrease, all other organisms at higher trophic level will decrease accordingly.

Decomposers are particularly important in energy flow, consuming the largest part of the energy flow in all ecosystems. Humans now divert, for their own use, about 40 percent of the net primary productivity of all land plants on Earth.


(Biomass pyramid figure)


Abiotic Factors

In addition to biological factors such as the food web, there are physical and chemical factors that determine the survival of an organism in an ecosystem, and can also affect structure of the ecosystem. The abiotic (nonbiological) factors in an aquatic environment would include salinity, temperature, light, chemical nutrients, bottom substrate, water depth, water clarity or turbidity, and currents. Coral reefs, for example, require very low turbidity (maximum light), moderate to high temperatures (tropical climates), shallow depths (for effective light penetration and temperature limitations), and moderate to high salinity (30 - 40 ppt) to survive. A coral reef ecosystem will not develop if all of these abiotic factors are not met.

In general, every organism has an optimum range, zones of stress, and limit of tolerance for each abiotic factor. A factor that is outside the optimal range causes stress and, therefore, limits growth, reproduction, and even survival. The maximum range of the zone of stress is called the limit of tolerance. A factor that is beyond the zone of stress causes death and sometimes, the extinction (or extirpation of an ecological population) of a species.

Biotic factors can also be limiting factors in an ecosystem. Competition and predation, are examples of the biotic factors that limit or regulate populations.