(18) Environmental Science

Components of the Ecosystem

ES18

What Are the Major Components of an Ecosystem?

CONCEPT 3-3 Some organisms produce the nutrients they need, others get the nutrients they need by consuming other organisms, and some recycle nutrients back to producers by decomposing the wastes and remains of organisms.

Ecosystems Have Living and Nonliving Components

Two types of components make up the biosphere and its ecosystems: One type, called abiotic, consists of nonliving components such as water, air, nutrients, and solar energy. The other type, called biotic, consists of biological components-plants, animals, and microbes.

Different species and their populations thrive under different physical and chemical conditions. Some need bright sunlight; others flourish in shade. Some need a hot environment; others prefer a cool or cold one. Some do best under wet conditions; others thrive under dry conditions.

Each population in an ecosystem has a range of tolerance to variations in its physical and chemical environment. Individuals within a population may also have slightly different tolerance ranges for temperature or other factors because of small differences in genetic makeup, health, and age. For example, a trout population may do best within a narrow band of temperatures (optimum level or range), but a few individuals can survive above and below that band. Of course, if the water becomes much too hot or too cold, none of the trout can survive.

Several Abiotic Factors Can Limit Population Growth

A variety of abiotic factors can affect the number of organisms in a population. Sometimes one or more factors, known as limiting factors, are more important in regulating population growth than other factors.

This ecological principle is called the limiting factor principle: Too much or too little of any abiotic factor can limit or prevent growth of a population, even if all other factors are at or near the optimal range of tolerance. This principle describes one way in which population control-a scientific principle of sustainability.

On land, precipitation often is the limiting abiotic factor. Lack of water in a desert limits plant growth.

Soil nutrients also can act as a limiting factor on land. Suppose a farmer plants corn in phosphorus poor soil. Even if water, nitrogen, potassium, and other nutrients are at optimal levels, the corn will stop growing when it uses up the available phosphorus. Too much of an abiotic factor can also be limiting. For example, too much water or fertilizer can kill plants. Temperature can also be a limiting factor. Both high and low temperatures can limit the survival and population sizes of various terrestrial species, especially plants.

Important limiting abiotic factors for aquatic life zones include temperature, sunlight, nutrient availability, and the low solubility of oxygen gas in water (dissolved oxygen content). Another limiting abiotic factor in aquatic life zones is salinity-the amounts of various inorganic minerals or salts dissolved in a given volume of water.

Producers, Consumers, and Decomposers Are the Living Components of Ecosystems

Ecologists assign every organism in an ecosystem to a feeding level, or trophic level, depending on its source of food or nutrients. Producers, sometimes called autotrophs (self-feeders), make the nutrients they need from compounds and energy obtained from their environment.

On land, most producers are green plants, which generally capture about 1% of the energy that falls on their leaves and convert it to chemical energy stored in organic molecules such as carbohydrates. In freshwater and marine ecosystems, algae and plants are the major producers near shorelines. In open water, the dominant producers are phytoplankton-mostly microscopic organisms that float or drift in the water.

Most producers capture sunlight to produce energy-rich carbohydrates (such as glucose, C6H12O6) by photosynthesis, which is the way energy enters most ecosystems. Although hundreds of chemical changes take place during photosynthesis, the overall reaction can be summarized as follows:

carbon dioxide + water + solar energy glucose + oxygen

6 CO2 + 6 H2O + solar energy C6H12O6 + 6 O2

All other organisms in an ecosystem are consumers, or heterotrophs (“other-feeders”) that cannot produce the nutrients they need through photosynthesis or other processes and must obtain their energy storing organic molecules and many other nutrients by feeding on other organisms or their remains. Primary consumers, or herbivores (plant eaters), are animals such as rabbits and zooplankton that eat producers, mostly by feeding on green plants. Secondary consumers, or carnivores (meat eaters), are animals such as deer and some fish that feed on the flesh of herbivores.

Third and higher-level consumers are carnivores such as tigers and wolves that feed on the flesh of other carnivores. Omnivores such as pigs, foxes, cockroaches, and humans, play dual roles by feeding on both plants and animals.

THINKING ABOUT

What You Eat

When you had your most recent meal, were you an herbivore, a carnivore, or an omnivore?

Other consumer organisms called decomposers and detritus feeders complete the cycling of nutrients by releasing nutrients from the dead bodies of plants and animals for reuse by producers. Decomposers, primarily certain types of bacteria and fungi, are specialized consumer organisms that recycle nutrients in ecosystems.

They digest food outside of their bodies by secreting enzymes that break down the bodies of dead organisms into compounds such as water, carbon dioxide, minerals, and simpler organic compounds that producers can take up from the soil, water, and atmosphere and use as nutrients.

Other consumers, called detritus feeders, or detritivores, feed on the wastes or dead bodies of other organisms, called detritus (“di-TRI-tus,” meaning debris). Examples include small organisms such as mites and earthworms, some insects, and larger scavenger organisms such as vultures. These organisms extract some of the chemical energy stored in dead organic matter, and their bodies and wastes in turn serve as food for other detritus feeders and decomposers. Hordes of these organisms can transform a fallen tree trunk into a powder and finally into simple inorganic molecules that plants can absorb as nutrients. Thus, in natural ecosystems the wastes and dead bodies of organisms serve as resources for other organisms, as the nutrients that make life possible are recycled again and again-another scientific principle of sustainability.

In summary, some organisms produce the nutrients they need, others get the nutrients they need by consuming other organisms, and some recycle the nutrients in the wastes and remains of organisms so that producers can use them again (Concept 3-3).

THINKING ABOUT

Insects

How would your life be changed if such insects disappeared? Why do timber companies want to eliminate many of these insects?

Producers, consumers, and decomposers use the chemical energy stored in glucose and other organic compounds to fuel their life processes. In most cells this energy is released by aerobic respiration, which uses oxygen to convert glucose (or other organic nutrient molecules) back into carbon dioxide and water. 

Although the detailed steps differ, the net chemical change for aerobic respiration is the opposite of that for photosynthesis.

Some decomposers get the energy they need by breaking down glucose (or other organic compounds) in the absence of oxygen. This form of cellular respiration is called anaerobic respiration, or fermentation.

The end products of this process are compounds such as methane gas (CH4), ethyl alcohol (C2H6O), acetic acid (C2H4O2, the key component of vinegar), and hydrogen sulfide (H2S, when sulfur compounds are broken down). Note that all organisms get their energy from aerobic or anaerobic respiration, but only plants carry out photosynthesis.

THINKING ABOUT

Chemical Cycling and the Law of Conservation of Matter

Explain the relationship between chemical cycling in ecosystems and the biosphere and the law of conservation of matter

Energy Flow and Nutrient Recycling Sustain Ecosystems

Ecosystems and the biosphere are sustained through a combination of one-way energy flow from the sun and nutrient recycling. These two scientific principles of sustainability arise from the structure and function of natural ecosystems, the law of conservation of matter, and the two laws of thermodynamics.

Decomposers and detritus feeders are absolutely essential to life on the earth. They complete the cycling of matter by breaking down organic matter into simpler nutrients that can be reused by producers. Without decomposers and detritus feeders, the planet would be overwhelmed with plant litter, dead animal bodies, animal wastes, and garbage. In addition, most life as we know it could not exist because the nutrients stored in such wastes and dead bodies would be locked up and unavailable for use by other organisms.

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