A radially symmetrical marine invertebrate with an endoskeleton, a water-vascular system, and tube feet, such as a sea star, a sea urchin, or a sea cucumber.


A method of navigation similar to sonar in which the sound produced bounces off an object used by bats and cetaceans.

Ecological efficiency

The percentage of energy in biomass produced by one trophic level that is incorporated into biomass by the next highest trophic level.

Ecological succession

Process in which communities of plant and animal species in a particular area are replaced over time by a series of different and often more complex communities.

Ecological zones

Characteristic areas within the terrestrial, aquatic (fresh and marine) and aerial habitats that have specific ecosystems .

Ecologically sustainable development

Development in which the total human population size and resource use in the world (or in a region) are limited to a level that does not exceed the carrying capacity or the existing natural capital and is therefore sustainable.


The study of the relationships between organisms and their environments, including, interactions of living organisms with one another and with their non-living surroundings, the flow of matter and energy in an environment, and the structure and functions of nature.



The study of the relationship between organisms and their environment.


All the biotic and abiotic components of an environment.


A basic fundamental unit that includes both living and non-living environment, each influencing the other and each necessary for the maintenance of life.

Ecosystem approach

The Ecosystem Approach is a strategy for the integrated management of land, water and living resources that promotes conservation and sustainable use in an equitable way. The Ecosystem Approach places human needs at the centre of biodiversity management. It aims to manage the ecosystem, based on the multiple functions that ecosystems perform and the multiple uses that are made of these functions. The ecosystem approach does not aim for short-term economic gains, but aims to optimize the use of an ecosystem without damaging it.

Ecosystem diversity

The variety of ecosystems that occurs within a larger landscape, ranging from biome (the largest ecological unit) to microhabitat.


Ecosystem services

Services, vital to the support of human life, provided by intact natural ecosystems. These include the purification of air and water, detoxification and decomposition of wastes, regulation of climate, regeneration of soil fertility, and production and maintenance of biodiversity, from which key ingredients of our agricultural, pharmaceutical, and industrial enterprises are derived. Historically, the nature and value of Earth’s life support systems have largely been ignored until their disruption or loss highlighted their importance.


Travel undertaken to witness sites or regions of unique natural or ecologic quality, or the provision of services to facilitate such travel.


A form of tourism that helps local people make money from an intact ecosystem; tourists pay for nature guides, food, and lodging to tour the ecosystem.


A study of the impact of toxins upon the ecosystems.


The outermost of the three germ layers of the gastrula that develops into the epidermis and epidermal tissues, the nervous system, external sense organs, and the mucous membranes lining the mouth and anus.

EctoparasiteA parasite that lives on a host but does not enter the host’s body.


The region in the cytoplasm located directly inside the cell membranes.


Referring to an animal whose body temperature is determined by the environment.

Efferent Neuron

A neuron that conducts impulses away from the central nervous system.


Contractions of the smooth muscles surrounding the urethra by which semen is forcefully expelled.

El Niño

A disruption of the normal air and water circulation patterns in the Pacific Ocean, leading to unusual weather in many parts of the world.


A venomous snake with two small, fixed fangs in the front of the mouth.


A particle with a negative electric charge that orbits the nucleus of an atom.

Electron Microscope

An instrument that uses a beam of electrons rather than a beam of light to enlarge the image of an extremely small object so that it can be seen.

Electron Transport Chain

Molecules in the thylakoid membrane or inner mitochondrial membrane that use some of the energy in electrons to pump protons across the membrane.

ElementA substance that ordinarily cannot be broken down chemically to form simpler kinds of matter.


A substance that cannot be broken down into simpler substances by chemical means. An element is composed of atoms that have the same atomic number, that is, each atom has the same number of protons in its nucleus as all other atoms of that element.

ElephantiasisA condition of swollen lymphatic vessels in the limbs caused by a parasitic filarial worm.

Embryo SacIn plants, a megagametophyte containing seven cells and eight nuclei.


 A tree with a canopy that forms about the general upper most continuous canopy.

EmigrationThe movement of individuals out of a population.

EmphysemaA degenerative lung disease.

Endangered species

Wild species with so few individual survivors that the species could soon become extinct in all or most of its natural range. IUCN The World Conservation Union (1994) definition, defines species as endangered if the factors causing their vulnerability or decline continue to operate.


Restricted to a specified region or locality.


The occurrence of a species in a particular locality or region.

Endocrine GlandA ductless gland that secretes hormones into the blood.

Endocrine System

A system of glands that transmit chemical messages throughout the body.


The process by which a cell surrounds and engulfs substances.


The innermost of the three germ layers of the gastrula; develops into the epithelium of the pharynx, respiratory tract, digestive tract, bladder, and urethra.


In plants, a specialized layer of cells that regulates substances entering the center of the root.


A parasite that lives inside the host’s body.


The region in the cytoplasm found in the interior of the cell.

Endoplasmic Reticulum

A system of membranous tubules and sacs in eukaryotic cells that functions as a path along which molecules move from one part of the cell to another.


An internal skeleton.


A tissue in angiosperms that provides food for the developing embryo.


A dormant bacterial cell enclosed by a tough coating.


A mutually beneficial relationship between one organism and another that lives within it; the hypothesis of endosymbiosis relating to the evolution of eukaryotes that holds that chloroplasts and mitochondria evolved from endosymbiotic bacteria.


An animal that generates its own body heat through metabolism.


A substance that causes a poison reaction; a compound that makes up part of the cell wall of Gram-negative bacteria.


A region adjacent to a eukaryotic gene that must be activated if the gene is to be expressed.


A neurotransmitter that blocks pain messages to the brain.

Enteric Bacteria

Gram-negative heterotrophic bacteria that inhabit animal intestinal tracts.


The process of mesoderm formation in deuterostomes in which the coelom forms in folded mesoderm.


The study of insects.


All external conditions and factors, living and nonliving (chemicals and energy), that affect an organism or other specified system during its lifetime; the earth's life-support systems for us and for all other forms of life - in effect another term for describing solar capital and earth capital.

Environmental audit

A detailed assessment to check if an organization is following the law, its environmental policies and its Environmental Management System (EMS). The results of the audit help the organization to improve its environmental policies and management system.

Environmental contaminant


A compound present in soil, water or air.

Environmental degradation

Depletion or destruction of a potentially renewable resource such as soil, grassland, forest, or wildlife by using it at a faster rate than it is naturally replenished. If such use is continued, the resource can become nonrenewable (on a human timescale) or non-existent.

Environmental Impact Assessment (EIA)

A method of analysis which attempts to predict the likely repercussions of a proposed major development (usually industrial) upon the social and physical environment of the surrounding area.

Environmental Science

A field of study that uses biological principles to look at the relationships between humans and the Earth.


A catalyst, usually a protein, in living systems.


A decongestant drug derived from Ephedra, a genus of desert shrubs.


A layer of cells that forms a continuous sheet over the outer surface of a plant or animal.


Coiled tube on each testis where sperm complete their development.


A flap of tissue that covers the trachea.


The loss of soil that is caused by wind or water.


A red blood cell.


A tube connecting the mouth or pharynx to the stomach or crop.


Hormone that helps regulate the menstrual cycle.


An aquatic biome found where freshwater streams and rivers flow into the sea; where the tides meet a river current.


A synthetic chemical that breaks down to release ethylene gas; used to ripen fruit.


The study of indigenous knowledge bases regarding plants and their uses.


A gaseous hormone produced by various parts of plants and released into the air.


The lineage of prokaryotes that includes all contemporary bacteria except archaebacteria.


The uncoiled form of chromatin.


Flagellated unicellular algae; many are photosynthetic.


A cell that contains a nucleus and membrane-bound organelles.

Eustachian Tube

A canal that connects the middle ear to the mouth cavity.


The increase of nutrients in an environment.


The process whereby nutrients (e.g. phosphates or nitrates) accumulate in a body of water.


The loss of water from the soil both by evaporation and by transpiration from the plants growing in the soil.  unintentionally (?)


Any gradual change. Organic evolution is any genetic change in organisms from generation to generation.


All the changes that have formed life on Earth from its earliest beginnings to the diversity that characterizes it today.

Excess land

Irrigable land, other than exempt land, owned by any landowner in excess of the maximum acreage limitation (ownership entitlement) under the applicable provision of reclamation law


The process of removing metabolic wastes.

Excurrent Siphon

A tube through which water exits the mantle cavity of a bivalve.

Exergonic Reaction

A chemical reaction that involves a net release of free energy.

Exocrine Gland

A gland that secretes non-hormonal chemicals through a duct.


A process in which a vesicle inside a cell fuses with the cell membrane and releases its contents to the external environment.


Coding region of a eukaryotic gene.


The hard external covering of some invertebrates that provides protection and support.

Exotic species

An organism that exists in the Free State in an area but is not native to that area. Also refers to animals from outside the country in which they are held in captive or free-ranging populations.


A toxic protein secreted by pathogenic bacteria.


The process in which air is forced out of the lungs.

Exponential Growth

A model of population growth in which the birth and death rates are constant.


Pertaining to study or maintenance of an organism or groups of organisms away from the place where they naturally occur. Commonly associated with collections of plants and animals in storage facilities, botanic gardens or zoos

Ex-situ conservation

The conservation of components of biological diversity outside their natural habitats.


Species are those whose members are living and reproducing at the present time.

Extensive agriculture

Maximizing the amount of land use for agriculture.

External Fertilization

The union of gametes outside the bodies of the parents, as in many fishes and amphibians.

External Respiration

The exchange of gases between the atmosphere and the blood.


As defined by the IUCN, extinct taxa are species or other taxa that are no longer known to exist in the wild after repeated search of their type of locality and other locations where they were known or likely to have occurred.


The evolutionary termination of a species caused by the failure to reproduce and the death of all remaining members of the species; the natural failure to adapt to environmental change.


The dying out of a species.

Extreme Halophile

An archaebacterium that lives in very high salt concentrations.


A localized region of pigment in some invertebrates and protozoa that detects changes in the quantity and quality of light.