Biology Key Terms grade 12

Biodiversity

The variety of living organisms in an area, and it is measured at three levels: genetic diversity, species diversity and ecosystem diversity.

Genetic diversity

The variety of genetic characteristics found within a species, and it matters because it allows a population to adapt when the environment changes.

Species diversity

The number of different species present together with their relative abundance, so a count of species alone is not enough to describe it.

Ecosystem diversity

The variety of habitats, biological communities and ecological processes within a region, and a wider range of ecosystems supports more niches and therefore more species.

Biological species concept

A group of similar organisms capable of sexually interbreeding to produce fertile offspring, so a horse and a donkey remain separate species because a mule is infertile.

Limits of the species concept

It cannot be applied to extinct species, asexual organisms, species that hybridise, or populations that are geographically isolated, and it ignores evolutionary relationships entirely.

Binomial name

The two-part scientific name of a species, written with a capitalised genus and a lowercase specific epithet, and italicised when typed or underlined when handwritten.

Taxonomy

The scientific study of the diversity of living organisms and the way they are classified, and its systems are fluid because DNA analysis continues to reclassify species.

Taxon

Any group within a system of classification, and an organism belongs to exactly one taxon at each rank rather than to two.

Linnaean system

A hierarchical classification running domain, kingdom, phylum, class, order, family, genus and species, in which groups become smaller and members more similar at each lower rank.

Dichotomous key

An identification tool that offers two contrasting choices at each step, so that one statement is always false and the correct one leads either to another step or to a species name.

Microhabitat

A small specialised area within a larger habitat that has its own distinct conditions, such as the underside of a rock, a tree hollow or a tide pool.

Holdridge life zone system

Classifies ecosystems using mean annual biotemperature, annual precipitation and the potential evapotranspiration ratio, so a site at 4 degrees with 180 millimetres of rain is classified as boreal desert.

Specht's classification

Classifies vegetation using the life form and height of the tallest stratum together with projective foliage cover, so eucalypts of 5 to 10 metres at 39 per cent cover are a low open-forest.

Random sampling

Selects sample locations using a random number generator so that every location in the study area has an equal chance of being chosen, which removes observer bias.

Systematic sampling

Collects data at fixed regular intervals using a structured layout, which suits environmental gradients because it shows how species and conditions shift with distance.

Stratified sampling

Divides a habitat into distinct sections that differ in an important way and samples each one independently, which ensures that every key habitat type is represented in the data.

Sampling method vs technique

A method is how the sample locations are chosen, being random, systematic or stratified, whereas a technique is the tool used to collect the data once the location is fixed, such as a quadrat or a transect.

Quadrat

A defined sample area, usually square, within which the organisms present are counted or estimated, and its size, number and placement must all be justified before sampling begins.

Line transect

A sampling line that records only the organisms actually touching it, which produces a very small sample and can easily miss patchy or clumped species.

Belt transect

Places a quadrat at each set interval along a sampling line, which produces far more usable data than a line transect because abundance and cover can be measured at every station.

Lincoln index

Estimates the size of a mobile population using the formula N equals M multiplied by n, divided by m, and it assumes no births, deaths or migration occur between the two samples.

Species richness

The number of different species present in an ecosystem, and it takes no account of abundance, so a species with two individuals counts the same as one with two thousand.

Species evenness

A measure of how evenly individuals are spread across the species present, and a community dominated by one or two species is considered less diverse than one where several are similarly abundant.

Percentage cover

The proportion of a quadrat's area that a species occupies, and because it is estimated by eye, different observers record different values and overlapping layers can push the total above 100 per cent.

Percentage frequency

The probability that a species is found within a single quadrat, calculated as the number of quadrats containing the species divided by the total number of quadrats, multiplied by 100.

Simpson's Diversity Index

Accounts for both richness and evenness using the formula SDI equals 1 minus the sum of n(n-1) divided by N(N-1), and on this form a value closer to 1 indicates high diversity.

Validity

Whether a sampling method actually measures the true characteristics of the ecosystem, and it is improved by stratifying the site so that every habitat type is represented.

Reliability

Whether repeated samples taken in the same way produce consistent results, and it is improved by using a fixed quadrat size, set intervals and repeating the survey on several days.

Distribution

Where a species is found within an ecosystem, which forms in response to the availability of resources and to environmental conditions.

Abundance

How many individuals of a species occupy a given space, reported as a density, a percentage cover or a count per quadrat.

Clumped distribution

Where resources collect in patches, so individuals gather in those areas, and it is the most common pattern found in nature.

Uniform distribution

Where individuals compete strongly for space or resources and therefore space themselves evenly, as penguins do when nesting.

Limiting factor

A biotic or abiotic factor that restricts the number of individuals in a population, such as food availability, competition, predation, disease, water, space or environmental conditions.

Density-dependent factor

A factor that influences population size more strongly as density increases, so competition, predation and disease all bite harder in a crowded population.

Density-independent factor

A factor that affects a population regardless of its density and can cause sudden declines whatever the population size, as fire, flood, drought and cyclones do.

Intraspecific competition

Between members of the same species and limits population growth when too many individuals need the same food or territory.

Interspecific competition

Between members of different species and can push one species out of an area entirely.

Eutrophication

An excess of nutrients causes algal blooms whose decomposition depletes dissolved oxygen, producing low-oxygen dead zones in which fish and other organisms cannot survive.

Tolerance range

The range of an abiotic factor within which a species can survive, and outside that range individuals either die or migrate, as corals do when temperatures exceed their limit.

Carrying capacity

The size of population that can be supported indefinitely on the available resources and services of an ecosystem, and it changes as biotic and abiotic conditions change.

Exponential growth

The J-shaped curve produced when resources are unlimited and no limiting factor operates, so the rate of increase continues to rise as the population grows.

Logistic growth

The S-shaped curve produced when environmental resistance slows growth as the population approaches its carrying capacity, giving a lag phase, an exponential phase, a deceleration and a plateau.

Environmental resistance

The combined effect of all the limiting factors that slow population growth as a population approaches its carrying capacity.

r-strategist

An organism that produces many small offspring with little parental care and has a short life, is typically a pioneer or generalist, and is regulated mainly by density-independent factors.

K-strategist

An organism that produces few large offspring with heavy parental care and has a long life, appears in later succession as a specialist, and is regulated mainly by density-dependent factors.

Population growth rate

The change in population size expressed over time, calculated as births plus immigration minus deaths and emigration, converted to a percentage and then divided by the time period.

Energy flow and matter cycling

Energy flows through an ecosystem in one direction and is eventually lost as heat that cannot be reused, whereas matter cycles endlessly between the atmosphere, the soil and living tissue.

Producer

An autotroph that converts light energy into chemical energy through photosynthesis, forming the base of every food web and fixing carbon from the atmosphere.

Decomposer

A bacterium or fungus that breaks down dead biomass and waste, returning nutrients to the soil and making the constant recycling of matter possible.

Trophic level

An organism's feeding position in a food chain, counted upward from the producers, and ecosystems rarely support more than four or five levels.

Food chain

A linear series of feeding relationships in which the arrows show the direction of energy flow, pointing from the organism eaten to the organism eating it.

Food web

A complex network of interrelated food chains, formed because most organisms feed on a variety of resources rather than on a single species.

Pyramid of numbers

Represents each trophic level with an area proportional to the number of organisms present, and it can invert where one large producer supports many small consumers.

Pyramid of biomass

Represents each trophic level by the total dry mass of organisms per square metre at one point in time, which accounts for organism size but only captures a single moment.

Pyramid of energy

Represents each trophic level by its energy content over a period of time, usually in kilojoules per square metre per year, and it is the most accurate representation of the energetics.

Gross primary productivity

The rate at which solar energy is captured in sugar molecules during photosynthesis, measured per unit area per unit time and before any losses are subtracted.

Net primary productivity

The energy stored as biomass by producers once respiratory loss has been subtracted from GPP, and it represents the energy actually available to herbivores.

Secondary production

The energy from food that is incorporated into new consumer tissue, calculated as N equals I minus the sum of F and R, where F is waste and R is respiration.

Ecological efficiency

The net production of new biomass at a trophic level expressed as a percentage of the energy flowing through that level, and it is generally about ten per cent.

Why energy is lost

It is used in respiration and released as heat, lost in faeces and urine, and not all individuals are eaten and not all parts are digestible.

Transfer

The movement of matter between parts of the environment with no chemical change occurring, such as precipitation, runoff or the diffusion of carbon dioxide into a leaf.

Transformation

A change in the chemical form of matter, such as photosynthesis or nitrification, although in the water cycle only the physical state changes.

Photosynthesis in the carbon cycle

Converts carbon dioxide and water into glucose and oxygen, and it is the only process in the carbon cycle that removes carbon dioxide from the atmosphere.

Nitrogen fixation

Converts unreactive nitrogen gas into ammonia, ammonium or nitrates, and it is carried out by nitrogen-fixing bacteria living free in the soil or in the root nodules of legumes, and also by lightning.

Ammonification

The breakdown of organic nitrogen in dead matter, urea and faeces into ammonia and ammonium, and it is carried out by saprophytic bacteria and fungi.

Nitrification

The oxidation of ammonium first into nitrite and then into nitrate by nitrifying bacteria, and because it requires oxygen it only occurs in well-aerated soil.

Denitrification

The conversion of nitrate back into nitrogen gas by denitrifying bacteria, and it occurs in waterlogged anaerobic soils, which is why moorland soils are low in nitrogen.

The four nitrogen bacteria

Nitrogen-fixing bacteria, saprophytic bacteria acting as decomposers, nitrifying bacteria and denitrifying bacteria.

Predation

An interaction in which one organism consumes all or part of another, benefiting the predator and harming the prey, and it affects where prey live as well as how many there are.

Mutualism

A symbiotic relationship in which both species benefit, as when an anemone protects a clownfish and the clownfish in turn provides it with food.

Commensalism

A relationship in which one species benefits while the other is unaffected, as when barnacles are carried to plankton-rich waters by a whale.

Parasitism

A relationship in which one species benefits at the expense of its host without normally killing it, as ticks and fleas do when feeding on a dog.

Symbiosis

A close long-term interaction between two species, covering mutualism, commensalism and parasitism, and it may be obligate or merely advantageous.

Ecological niche

The role and space that an organism fills in an ecosystem, including all of its interactions with the biotic and abiotic factors of its environment.

Fundamental niche

Every resource and condition a species could potentially use in its environment, and it requires the complete absence of competition.

Realised niche

The narrower portion of the fundamental niche that a species actually occupies once competitors are present, and it is sometimes described as a competitive refuge.

Competitive exclusion principle

Two species competing for the exact same limited resource cannot stably coexist, because one will always hold a slight advantage over the other.

Niche partitioning

The division of a niche so that competing species use the environment differently and can therefore coexist, and it occurs through space, time, resource type or morphology.

Keystone species

One whose influence on the structure and function of a community is disproportionately large relative to its abundance, so its removal causes significant change across the ecosystem.

Overexploitation

The harvesting of species from the wild faster than their populations can recover, which collapses populations and often triggers trophic cascades through the whole community.

Habitat destruction

A natural habitat can no longer support the species present, which removes feeding grounds, shelter and breeding sites and fragments the populations that remain.

Monoculture

The practice of growing a single crop or variety in a field at one time, which simplifies food webs, removes the natural enemies of pests and allows pest outbreaks to spread rapidly.

Bioaccumulation

The build-up of a pollutant within a single organism across its lifetime, occurring because the substance is absorbed faster than it can be excreted.

Biomagnification

The increase in concentration of a pollutant at each successive trophic level, so that top predators carry the highest concentrations of all.

Trophic cascade

A change at one trophic level ripples through the levels above and below it, as removing sea otters does to an entire kelp community.

Spatial variation

The differences between ecosystems found in different locations, which is why a rainforest, a desert and a temperate woodland differ in productivity and species composition.

Temporal variation

Changes occurring within the same ecosystem over time, including seasonal change, successional stages and long-term shifts in climate or fire regime.

Succession

The process by which communities of plants and animals colonise an area and are then progressively replaced by other, more varied organisms over time.

Primary succession

Begins on bare rock or water where there is no soil, no vegetation and no organisms, and lichens act as the pioneer species that create the first thin layer of soil.

Secondary succession

Where vegetation existed previously and was abruptly removed, and it proceeds much faster than primary succession because soil and a seed bank are already present.

Pioneer species

The first coloniser of a new area, and it tolerates extreme conditions, grows and reproduces quickly, disperses easily and improves the soil for the species that follow.

Sere

An intermediate stage in the process of succession, sitting between the pioneer community and the final climax community, also called a seral community.

Climax community

The final stable self-sustaining stage of succession, holding the highest biomass and biodiversity, deep fertile soil and long-lived K-selected species.

Successional trends

Biomass, biodiversity and the complexity of interactions all increase, abiotic conditions stabilise, and r-selected species are progressively replaced by K-selected species.

Nucleotide

The repeating unit of a nucleic acid and consists of a pentose sugar, a phosphate group and a nitrogenous base, with deoxyribose and thymine in DNA and ribose and uracil in RNA.

Purines and pyrimidines

Purines are adenine and guanine and have a double ring structure, whereas pyrimidines are thymine and cytosine and have a single ring, and a purine always pairs with a pyrimidine.

Complementary base pairing

Joins adenine to thymine with two hydrogen bonds and cytosine to guanine with three, and because these bonds are weak the strands can be separated for replication.

5' and 3' ends

The 5' end carries a free phosphate group attached to the fifth carbon of the sugar, whereas the 3' end carries a free hydroxyl group attached to the third carbon.

Antiparallel

That the two strands of a double helix run in opposite directions, so the 5' end of one strand is paired with the 3' end of the other.