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Ecology: Organisms and Their Environments
Ecology: Organisms and Their Environments
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Ecology: Organisms and Their Environments Terms

Get down with the lingo


Anything that is not, nor has ever been, alive. Some examples of abiotic factors in an environment include precipitation, sunlight, and minerals. Abiotic is the opposite of biotic. Life or no life, that is the question.


Any living organism that makes its own food by converting simple inorganic molecules into complex organic compounds like carbohydrates, fats, and proteins. Autotrophs are often the "producers" in a food chain or web. Photoautotrophs use energy from light to make food, while chemoautotrophs obtain their energy from chemical reactions.


The process by which some substances increase in concentration in a food chain or food web. Biomagnification occurs because certain substances, including some pesticides and heavy metals, are not easily degraded and can accumulate in organisms’ tissues or internal organs. Pregnant women are advised not to eat certain kinds of fish because of the potential for biomagnification of mercury, a heavy metal, in aquatic systems. No, you cannot use biomagnification to light ants on fire.


Anything that is, or has ever been, alive. Examples of biotic factors in an environment include organisms, organic molecules, and cells. Biotic is the opposite of abiotic.


An organism that only eats animal tissue. Most predators and scavengers are exclusively carnivorous. Some examples of carnivores include members of the feline family, like lions, tigers, and house cats, and birds of prey, like eagles, hawks, and owls. Beef: It's what's for dinner.

Carrying Capacity

The maximum number of individuals in a single population that a given environment can sustain at a given time. The carrying capacity for a population can change when fluctuations occur in resources, like increases in nutrients or water; habitat structure, like deforestation; biotic interactions, like the introduction of a new predator or competitor into the environment; and other local factors. The carrying capacity is a characteristic of a single population of a single species at a single time in a single place. Therefore, it is a difficult concept to measure or even fully understand. The carrying capacity of the human population is an extremely important, yet controversial, topic in ecology.

Climax Community

A community of plants and animals that is considered stable, or at equilibrium, because the members of that community are the best adapted for the average conditions in their environment. The climax community is so named because it is considered the final and most stable condition in the process of ecological succession.


The evolution of one species in relation or response to the evolution of another closely interacting species or set of species. Examples of coevolution include the interactions of hosts and parasites, like bird lice and birds, and obligate mutualists, like bees and the flowers they pollinate. The human species has most certainly coevolved with several different species of bacteria living in the colon and other areas of the digestive system. Mmm, yummy.

Colonizer Species

A species that arrives early in the ecological succession of an environment. A little bit like the first Vikings to reach North America, except without the longboats and fuzzy hats. Colonizer species are usually able to disperse offspring quickly and over long distances, allowing more of their ilk to arrive before members of other species. However, colonizers do not often compete well with other species once a community begins to mature, and are often replaced by more stable persister species. Colonizer species are also sometimes called pioneer species. Davy Crockett-style.


A symbiotic (read: long-term and partly beneficial) relationship between organisms, where one organism benefits from the relationship, and the other receives no harm or benefit. A common example is the clown fish, or anemonefish, and the sea anemone (think Finding Nemo), where the fish receives protection and other services from the anemone, but the anemone receives no measurable benefit from the fish. Sounds inappropriate.


A group of two or more populations of organisms from different species inhabiting the same location at the same time. While humans often refer to their "community" as being a part of a group of other humans who live in the same small geographic location, a human population’s true ecological community includes all of the other organisms from other species in the area as well. Communities are composed only of biotic factors, aka living organisms. Abiotic factors like sunlight, temperature, and terrain are not considered part of a community; these factors are part of the ecosystem, which can contain one or more communities of organisms.


An interaction where individuals of different species—interspecific competition—or the same species—intraspecific competition—vie for limited resources. Examples of interspecific competition include trees of different species vying for limited sunlight in a rainforest, birds of different species vying for limited prey in a prairie, and even bacteria of different species vying for limited oxygen in your large intestine. Examples of intraspecific competition include lions vying for limited antelope in the Savannah; piglets vying for limited milk from their sow, or mom-pig; and even humans vying for limited space to build a home. Move it or lose it, buddy.

Competitive Exclusion

The process by which one species "wins" a competitive interaction by driving the other species to extinction. This can occur in either a local or a general sense. Humans are competitively excluding many species currently on the endangered species list. Nice job, us.


See primary consumer, secondary consumer, and tertiary consumer below. That's right—nothing to see here. Move along.


An organism that feeds on and breaks down dead or decaying matter in the process of ecological decomposition. Examples of decomposers include fungi, like mushrooms and molds; worms, like earthworms and some nematodes; and some bacteria. Decomposers are also called saprotrophs, meaning "putrid eaters."

Density-dependent Factor

A restriction on the growth of a population related to or affected by the number of organisms in a given space, or density, of that population. An example of a density-dependent factor in human populations is the flu virus. Because this killer bug is spread by coughing and sneezing, it will spread more quickly in denser populations, like in big cities, than in less dense populations, like in the rural countryside. You are our density.

Density-independent Factor

A restriction on the growth of a population not related to or affected by the density, or number of organisms in a given space, of that population. Examples of density-independent factors in human populations include major natural disasters like earthquakes or tsunamis, where the effect on the population, or proportion of individuals killed, is not intensified by an increase in the size of the population.


An organism that consumes detritus, aka decomposing organic matter, to obtain nutrients. All decomposers are detritivores, including fungi, worms, and some bacteria.


The study of interactions between organisms and their environments. Ecology includes the study of individuals, populations, communities, and ecosystems.


A term describing all the living and nonliving things in a certain location. Ecosystem studies in ecology explore the interactions between organisms, like individuals, populations, or communities, and the abiotic components in the environment, like chemicals, landscapes, and the like.

Energy Pyramid

A triangularly shaped drawing showing how energy from the Sun moves through the biological components of an ecosystem. Producers, like plants and algae, are at the bottom of the pyramid, and tertiary consumers, like carnivores that eat carnivores, and detritivores, or decomposers, are at the top.

Exponential Growth

An increase in population size at a rate proportional to the size of the population. During exponential growth, the rate of growth constantly increases. That is, the larger the population, the faster it grows. Exponential growth will only occur when density-dependent factors (see definition) are either nonexistent or not in force. A good example of exponential growth is in certain types of bacteria, where the population doubles with every generation as long as there are enough resources, like nutrients and space.


The buildup of nitrogen and phosphorous in an aquatic environment. Eutrophication usually leads to an exponential growth in bacteria and algae. These organisms quickly use all the available oxygen, which kills off all other species in the environment. Sucks to be them.

Food Chain

A simple, direct, and trophic, or eating, relationship among a group of organisms, where one organism, like a plant, is the food source for the next organism, like a cow, which in turn is the food source for the next organism, like a human, and so on and so forth.

Food Web

A complex trophic relationship among a group of organisms, consisting of interactions among multiple food chains (see definition above). A food web describes how multiple producers and consumers directly or indirectly interact in an ecosystem.

Growth Rate

A characteristic of a population describing the change in the size of the population over time. In its simplest form, a growth rate is calculated as the birth rate minus the death rate of a population. It can also be thought of in terms of the following equation: the growth rate (r) equals the population size at the end of a period of time (Nt+1) minus the population size at the beginning of a period of time (Nt), all divided by the population size at the beginning of a period of time (Nt). In symbols, the equation looks like this:


The physical environment where a population of a single species lives, or inhabits. A habitat consists of all the abiotic, or nonliving, resources influencing the population. A habitat is only understood in terms of the population it describes. For instance, we say "the black bear habitat" or "the whale habitat." It doesn’t describe the entire ecosystem, or a community of organisms, or even the home of a single individual. Habitats of different species can and almost always do overlap.


An organism that only eats tissue from autotrophic organisms, like plants and algae. Some examples of herbivores include members of the bovine family, like cows, bison, antelope, and sheep; members of the deer family, like moose, reindeer, and elk; and many insects, like leaf beetles, lady bugs, and aphids.


An organism that cannot convert sunlight or chemicals into "food" (and by food, we mean carbohydrates). Heterotrophs must obtain their nutrients by consuming other organisms. All animals, all fungi, and some kinds of bacteria are heterotrophs. This means that all carnivores, herbivores, and omnivores are also heterotrophs. And yes, you're a member of that not-so-exclusive club, too.


An organism on, or in, which a parasite lives or feeds. For example, dogs are known to be good hosts for fleas. Frontline Plus: it does wonders.

Iteroparous Reproduction

A reproductive strategy where an organism reproduces multiple times throughout its life but only produces a small number of offspring (usually just one) per reproductive event. Humans and other large mammals are iteroparous, except for mutant celebrities like the Octomom.

Keystone Species

A species in a community that has an effect on the community larger than one would expect based on the population size of that species. If a keystone species were to go extinct in a given environment, such extinction might lead to drastic consequences for a large number of other species in that environment.

Life History

A description of a species’ vital statistics, including age at first reproduction, survival probabilities, reproduction probabilities, clutch or litter size, reproductive strategy, and longevity. Life insurance companies rely on life histories, especially survival probabilities, to calculate insurance rates for people at different ages. Calculating how much people are worth. Sounds like a fun job.

Life Table

A chart for a species that shows, for each age, the probability that an organism will live to the next year. The period does not necessarily need to be a year, and can be any relevant generational time period. Life tables also usually include information on reproductive probabilities and other life history characteristics. Life insurance companies use massive life tables to compute insurance rates. Again, buckets of fun.

Logistic Growth

A type of population growth characterized by initial exponential growth, with a gradual slowing in growth rate as the population nears its carrying capacity (see definition). Since biologists don’t know of any populations that grow exponentially forever, logistic growth is thought to be the most common pattern of growth in the world.


A symbiotic (read: long-term and partly beneficial) relationship where both organisms benefit. A common example of mutualism is the relationship between pollinators, like honeybees, moths, or hummingbirds, and the plants they pollinate. In this situation, the pollinator receives the benefit of nutrition in the form of nectar, and the plant receives the benefit of having its pollen, equivalent to sperm, spread to other plants, increasing the probability of reproductive success. Honeybees are so cute; who knew that their real jobs were sperm-spreading?

Natural Selection

The process by which certain traits in a population of organisms become more common through the carriers' continued reproduction. The genetic variation in a population of organisms will result in the passing of certain traits from one generation to the next. However, the traits that are passed are not just any old traits; they are usually the ones that enhance or otherwise increase the likelihood of the organism's survival. Natural selection is a vital contributor to evolution and the emergence of new species. Basically, certain organisms adapt better to the environment, resulting in successful reproduction and survival. "Survival of the fittest." Planet of the Apes, anyone?


An organism's role in an environment, including how it uses its resources, relates to other organisms, and times its reproduction. Each individual organism has a niche in its population, community, and ecosystem, but niches are flexible and change depending on circumstances.


An organism that eats tissue from both plants and animals. Some examples of omnivores include members of the hominid family, like humans, chimpanzees, and orangutans, and many bird species, like hummingbirds, ducks, and woodpeckers.


An organism that can live on or in another organism, known as a host. Parasites, by nature, cause damage to their hosts. Some parasites, like mites, fleas, and lice, live on the outside of their hosts and are called ectoparasites. Other parasites, like malaria, hookworms, and tapeworms, live on the inside of their hosts and are called endoparasites. The amount of harm a parasite does to its host varies from parasite to parasite and from host to host. Some parasitic relationships result in death for the host, like the "head-splitting fungus" parasite of ants, while other hosts are completely unaware that they even have a parasite. 95% of humans play host to parasitic nematodes without ever knowing it. Fooled you, didn't they?


A type of symbiotic relationship (read: long-term and partly beneficial) where one organism, the parasite, benefits, and one organism, the host, is harmed. A common example of parasitism is the relationship between the malaria protozoan and humans. In this situation, the malaria parasite gets into the human blood stream, usually through a mosquito bite, and receives the benefit of nutrients and a place to live, grow, and reproduce. Sounds just dandy for the parasite. The human host, on the other hand, is unfortunately harmed because the malaria parasite damages red blood cells and liver cells.

Persister Species

A species that usually arrives later in the ecological succession (change of a community over a period) of an environment. Persister species are not usually able to disperse offspring quickly or over long distances. However, persisters compensate for this deficiency by competing well with other species once a community begins to mature. Using these sneaky tactics, they often replace the less stable colonizer species (see definition).


A group of organisms of the same species living in the same geographic area, like a swamp, a lake, a mountaintop, or an island. Ecologists sometimes debate the concept of a population, but in general, they accept that a population consists of individuals that all have a greater chance of breeding with each other than with individuals inhabiting areas beyond the population’s geographic "borders." Keep in mind, though, that population "borders" are often fuzzy and may overlap with other populations. Hey, we told you that there was a debate.

Population Density

The number of individuals of a species in an environment divided by the total land area, or water volume, of that environment. Population density is analogous to the concept of concentration in chemistry. In fact, population density is simply the concentration of organisms in a given area. Take that, Chemistry.

Population Ecology

A subtopic in ecology that deals with the dynamics of individuals within populations and how the populations interact with the environment.


A type of species interaction where one organism, aka the predator, consumes, in part or in whole, another organism, aka the prey. Examples of predators include snakes and members of the big cat family, such as lynx. The difference between parasitism and predation is not always clear, but in general, predators kill their prey almost immediately while parasites live in or on their hosts for an extended period of time. And then, just maybe, kill them.

Primary Consumer

A heterotrophic organism near the base of a food chain or food web (see definitions) that obtains all or some of its energy by consuming primary producers (see definition). Examples of primary consumers include all the strict herbivores, like bovines, deer, and most insects, and all the omnivores, like most humans, many birds, and some monkeys. Strict herbivores are always primary consumers and are only eaten by secondary consumers. Omnivores can be primary, secondary, and even tertiary consumers, depending on what they are eating. They, themselves, can be eaten by secondary or, rarely, tertiary consumers. Yep, definitely webby.

Primary Producer

An autotrophic (food-making) organism at the base of a food chain or food web that obtains all of its energy from the nonliving environment. Examples of primary producers include plants, algae, and some types of bacteria. Primary producers obtain their energy either from the Sun, through photosynthesis, or from the surrounding abiotic environment, through chemosynthesis. Primary producers are always the first biotic (read: living) sources of energy for an ecosystem. That’s why we call them "primary." They are eaten by primary consumers.


See primary producer above.

Reproductive Investment

The energy invested by an organism to create, feed, nurture, and protect offspring. Reproductive investment is also called parental investment. Any energy invested in reproduction, or parenting, is energy taken away from other needs of an organism, including survival and growth. You can thank your parents later. Or now. We'll wait.

Resource Partitioning

A situation that occurs when organisms occupying the same niche find a way to divide their niche and coexist. Resource partitioning usually occurs when species have different body plans or behaviors that allow for a successful division of a niche. An excellent example of this is seen in the cichlid fishes of the African Great Lakes. In this environment, cichlids from different species are known to live in the exact same location and even feed on the exact same prey without outcompeting each other. One of the most dramatic examples is of two similar cichlid species that feed on the scales of a single prey fish species. These cichlid species have opposite facing mouths—in one cichlid, the mouth is pointed toward the left side of the body, in the other, toward the right. Because of this difference, one species can only attack the prey from behind and to the left side while the other species needs to attack from behind and to the right. Peace and harmony through physical deformity.

Secondary Consumer

A heterotrophic organism, like you, that obtains some or all of its energy by eating primary consumers. Examples include all of the omnivores, like most humans, many birds, and some monkeys, and some of the carnivores, like bears, lions, and eagles. Secondary consumers do not eat other carnivores. If they do, they are behaving as tertiary consumers.

Semelparous Reproduction

A reproductive strategy where an organism reproduces once in its lifetime and often produces many offspring. Many fish and insects are semelparous. It is also known as "Big Bang Reproduction." A possible spinoff for The Big Bang Theory?


The compositional change in an ecological community over time. Succession occurs after some major, or minor, disturbance has disrupted the previous composition of the community. Complete disturbance, where all life is eradicated, like during a volcanic eruption, or the formation of a new island, results in a long-term successional process. Usually, disturbances are less complete, like forest fires, and succession to a climax community (see definition) occurs more quickly.

Survivorship Curve

A graphical drawing showing the percentage, or proportion, of individuals in a population that are expected to survive through a given age interval, for example, from age 1 to age 2. Based on the different population studies, a survivorship curve can be used for organisms that reproduce many eggs and die early, such as sea turtles or salmon. Survivorship curves could also be used by life insurance companies—though rarely are—to visually display the survival portion of the human life histories (see definition).


An interaction between individuals of different species. Symbiotic relationships include mutualism, parasitism, and commensalism. They do not include predator-prey interactions.

Tertiary Consumer

A heterotrophic organism that obtains some or all its energy by eating secondary consumers. An example of a tertiary consumer is a snake that eats a bird that ate a rodent. Most consumers, even strict carnivores, are not exclusively tertiary and will eat primary consumers as well as secondary consumers. So, watch out. Quaternary consumers do exist but are more rare than other types of consumers. You have been a quaternary consumer if you have ever eaten shark meat because shark eat big fish, that eat smaller fish, that feed on algae.

Trophic Level

A description of the position occupied by an organism in a food chain or food web. Simply put, an organism’s trophic level is defined by what it eats and what eats it. Examples of trophic levels include producers, consumers, and decomposers.
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