Ecology is the study of the relationships between organisms and the relationships between organisms and their environment. Ecology was first recognized as an academic subject in 1869 when German naturalist Ernst Haeckel (1834-1919) first coined the term ecology. The word is derived from the Greek words eco, meaning "house" and logy, meaning "to study," indicating that ecology is the study of organisms in their home.

Ecologists often distinguish between two parts of environment as a whole: the living or biotic part and the nonliving or abiotic part. The biotic part of the environment includes all organisms such as animals, plants, bacteria, and fungi.

The abiotic part includes all physical features like temperature, humidity, availability of light, as well as chemical components, such as the concentrations of salts, nutrients, and gases. Ecology, then, is the study of the relationships between and among the biotic and abiotic environments.

Ecology as part of the biological sciences

The components of the biological world are often organized along a spectrum. Assuming the spectrum is laid out from left to right with the left being the smallest, atoms are at the far left. When atoms combine together they organize into molecules, which are just to the right of atoms on the spectrum.

Moving right along the spectrum, next comes cells, which are the smallest unit of life. Next come tissues, such as muscle tissue and nervous tissue, which are collections of cells that work together to perform a function. These tissues are then organized into organs, such as the heart and the brain, and these are found to the right of tissues on the spectrum.

Organs work together as organ systems, such as the cardiac system, which includes the heart as well as the blood vessels that transport blood throughout the body. Farther to the right along the biological spectrum are organ systems, which come together to form an individual organism, such as a human, fish, or kelp.

Towards the center of the biological spectrum, individual organisms are grouped together into populations. These populations are all the members of a species that live together. Even farther to the right of the biological spectrum, populations are grouped into communities, which are all the organisms that are found in a specific location.

Communities include members of different species. Communities depend upon the nonliving world in order to survive, so an ecosystem, also called an ecological system, represents all the relationships between a community and the abiotic world.

All the communities of the world together make up the biosphere, which is found near the right side of the biological spectrum. The biosphere interacts with all of the abiotic parts of Earth, including the atmosphere, which are the gasses surrounding the planet; the hydrosphere, which is the water on Earth; and the lithosphere, which is the soil and rock on Earth. The biosphere and its relationships with the atmosphere, hydrosphere and lithosphere make up the ecosphere. The ecosphere is on the extreme right hand side of the biological spectrum.

Ecologists generally focus their research on the part of the biological spectrum that is to the right of the individual. For example, a population ecologist may study the ways that populations of sardines off the coast of California differ in their mating habits from populations of sardines off the coast of Chile.

On the next level, community ecologist will study the diet of the various populations of sardines. An ecosystem ecologist may study how the populations of sardines are affected by the changes in temperature associated with the warming or cooling of the oceans.

Subdivisions and important concepts of ecology

The field of ecology is often subdivided because it incorporates so many different disciplines. Two large groupings within ecology are autecology and synecology. Autecology is the study of the individual organism or an individual species. This part of ecology might focus on the life history of an animal or plant.

For example, one could study how the caddis fly grows from an egg into a larva (early stage of insect’s life) that builds a house of sand at the bottom of a river and then metamorphoses (changes form) into a fly. Autecology might also investigate how the caddis fly adapts to its environment.

For example, a study of how well the caddis fly larvae houses are hidden from predators (animals that hunt others for food) would be an aute-cological study. Synecology focuses on groups of organisms and how they work together. If a study estimated the amount of energy fish obtained by eating caddis fly larvae, it would be synecological. Synecology tends to ask questions that study the ecosystem on a large scale.

Another way to subdivide the subject of ecology is by the kind of environment. Commonly, environments are grouped into freshwater (lakes, rivers, and streams), marine (oceans), or terrestrial (land-based). Although the fundamental principles of ecology hold in all of these environments, the specific animals and plants vary and it is often convenient to study each type separately.

Finally, ecology can be divided into different types of organisms. This is called a taxonomical grouping. For example, one might study plant ecology, bacterial ecology, or insect ecology. This allows the study to be focused on a specific group and to use similar methods to study the different organisms in the group.

For example, in order to study the environmental factors that influence the growth of marine algae, one could develop several growth environments with different light and different concentrations of the nutrients phosphate and nitrate. These same growth environments could be used to grow several species of algae. The results might be useful in predicting where and when the rapid growth of algae might occur in the ocean.

The ecosystem. Every living thing has requirements in order to exist: food, water, gases, stable temperatures and a place to live. Living organisms depend the nonliving environment for many of these requirements. The relationships between the biotic and the abiotic are called an ecological system, or an ecosystem.

Inorganic (non-living) substances such as carbon, nitrogen, phosphorous, carbon dioxide, and oxygen are required for all organisms to produce the molecules in their bodies.

Autotrophs are organisms that use inorganic substances to make energy. (The root word auto means "self" and the root word troph means "to eat.") Most often plants are autotrophs, using sunlight, water, and carbon dioxide in a process called photosynthesis to produce energy in the form of carbohydrates that their cells need.

Heterotrophs are organisms that consume autotrophs in order to get their energy and grow. (The root word hetero means "other.") Heterotrophs include animals that eat plants as well as animals that eat other animals. Finally there are the decomposers or saprotrophs, such as bacteria and fungi. (The root word sapro means "to decompose.")

These organisms break down dead organisms into inorganic substances, which may then be used by autotrophs. Understanding the ways that substances and energy flow through ecosystems is one of the fundamental principles in ecology.

Homeostasis. Homeostasis is used to describe the tendency for a biological system to resist change. (The root word homeo means "the same" and the root word stasis means "standing.") A principle of ecology is that ecosystems generally remain homeostatic.

In other words, if there are no outside influences, the number of organisms that live in any given location will tend to remain the same, and they will have the same food supply and access to shelter over time. Even minor changes in the environment, such as temperature changes or changes in rainfall, will not greatly affect an ecosystem.

One of the ways that an ecosystem maintains homeostasis is through negative feedback mechanisms. For example, kelp forests grow off the coasts of California. Sea urchins eat the kelp, keeping its density relatively constant. In turn, sea otters eat sea urchins.

If the population of sea otters were to suddenly decrease, the population of sea urchins would grow because they would not have any predators. However, the sea urchins would eat a large amount of kelp removing their food supply. Many urchins would starve, decreasing the population of urchins and allowing the kelp to grow back to its former density.

Eventually the ecosystem would return to its former state. Of course, large disruptions to ecosystems can be very destructive. For example, hunting sea otters to extinction (as almost occurred during the early part of the twentieth century) would completely disrupt the homeostasis of the California kelp forest.

Energy in the ecosystem. One of the fundamental principles of physics is that energy cannot be created or destroyed. It can, however, be transformed from one form to another. Light is a form of energy. It can be transformed into heat or chemical energy that is stored in food. In fact, this is the basis for photosynthesis. Some of the energy in light is stored in the chemical bonds of the molecules in plants.

Another fundamental principle of physics states that when energy is transformed from one form to another, some of the energy is lost. In photosynthesis, some of the energy in light is lost as heat. This means that the transformations of energy within ecosystems are never 100% efficient. Energy is always lost as it flows from one organism to the next.

In any ecosystem, energy is transferred between organisms as they eat and are eaten by other organisms. Usually the autotrophs or primary producers (such as seaweed and algae) capture light energy from the Sun through photosynthesis. They store this energy in the chemical bonds of the molecules that make up their cells.

Herbivores (plant eaters such as urchins and snails) eat the autotrophs. These grazers convert the energy in the chemical bonds of the primary producers into energy that is stored in the chemical bonds of their own cells. Usually about 80-90% of the energy in the chemical bonds of the primary producer is lost during this process.

Next, carnivores (meat eaters such as frogs and fish) eat the herbivores. They convert the energy stored in the chemical bonds of the herbivores into energy stored in the chemical bonds of their own cells. Again, about 80-90% of the energy is lost in this transformation.

The result of the energy loss each time an organism is eaten results in what is called an ecological pyramid. At the base of the pyramid are the primary producers; in the middle are the herbivores and at the top the carnivores.

If one measures the weight of each of these groups after the water has been removed (called the dry weight), one gets an idea of how much energy is stored in chemical bonds at each level of the pyramid. For example, in a lake in Wisconsin, the dry weight of the primary producers is 96 grams per square meter. The dry weight of the herbivores is only 11 grams per square meter.

The dry weight of the carnivores is just 4 grams per square meter. The ecological pyramid demonstrates how about 80-90% of the energy stored in chemical bonds is lost every time an organism is eaten. It also shows that there can never be as many predators as there are prey; there is just not enough energy for that to occur.