Ecology: Key Terms and Glossary

How many individuals are present, or an index of that number, as distinct from the mere fact that a species occurs.

A reversible shift, within one lifetime, of how performance depends on a condition, as distinct from a genetic change across generations.

Acid reaching ecosystems from sulfur dioxide and nitrogen oxides, which damaged lakes and forests where the input was high and eased where that input later fell.

An increase in per-capita growth as density increases, while density is still low, for example because mates are scarce. An effect on one component of fitness need not put a threshold into the population's overall rate of increase.

Local richness, turnover of composition among sites, and regional richness. The region is local richness plus that turnover, so saving one rich patch does not save the whole.

Two different persistent states under the same external conditions, either of which a large shock can leave behind.

Anaerobic formation of N2 from ammonium plus nitrite, a return of nitrogen to the atmosphere that matters in oxygen-poor waters and in sediments.

A negative link between two species that has the sign of competition but is caused by a shared enemy, not by a shared resource.

Resemblance of an edible species to a defended model, an advantage that shrinks as the mimic becomes common relative to the honest signal.

The sinking of organic particles that carries carbon out of the surface ocean.

A rise in concentration, towards the top of a food web, of a substance that persists, dissolves in fat, and is poorly excreted.

A vegetation class set by climate and recognised by growth form, so the same biome on different continents shares a structure rather than a species list.

Every ecosystem taken together, plus the air and the seas to the extent that living things exchange gases with them.

Loss of a coral's symbiotic dinoflagellates, commonly under heat, after which reef production falls and, if the coral dies, the structure those species built is lost.

The population size at which logistic growth stops in a stated environment, changed if food, enemies, or climate change, and not a fixed property of the species.

The land that drains to one outlet, the unit on which a water or nutrient budget can be closed by measuring that outlet.

Retention of nutrient cations on the negative charges of clay and humus. Nitrate, being an anion, does not stick to those charges in most temperate soils, so water carries it away.

Climate associated with the present geographic range, which misleads when the range is not at equilibrium with climate or when another species sets the edge.

The set of populations that can affect one another inside an area the study itself delimits. What it has is a species list and their abundances, not a physiology of its own.

In Lotka–Volterra competition, the factor that converts an individual of the other species into equivalents of one's own. Where the carrying capacities are equal, coexistence needs each coefficient to be less than 1, and a product of the two below 1 is necessary but not sufficient, because one coefficient can still exceed 1.

The limiting-case claim that two species sharing one limiting resource cannot both persist if the habitat stays constant and well mixed. In the Lotka–Volterra model the stable outcome is mutual invasion: each species must still grow where its rival already sits at carrying capacity.

An abiotic factor an organism experiences but does not consume, such as temperature or pH, and therefore not something individuals compete to use up.

Grime's triangle for plants: competitors where stress and disturbance are both low, stress-tolerators where stress is high and disturbance low, and ruderals where disturbance is high and stress low. The corner in which both are high supports no strategy.

Chance in the births and deaths of particular individuals, strong when the population is small and averaging out as numbers grow.

Microbial reduction of nitrate to nitrous oxide and N2 when oxygen is absent, which returns nitrogen to the air and, if the path stops early, leaks a greenhouse gas.

A change in the per-capita birth rate or death rate as population size changes.

The path of energy and matter through dead organic matter, which on land often carries more of the flux than the chain that starts with living herbivores.

Where a species occurs, one of the two patterns the subject has to explain.

Chance loss and replacement of individuals, analogous in form to genetic drift but a sampling of organisms rather than of gene copies.

The living things of a stated place, together with the non-living stores of matter and the rates at which matter and energy move.

A species that changes the physical state of a site, as a beaver does by flooding a valley.

Benefits people draw from ecosystems, classed as provisioning, regulating, cultural, and in some schemes supporting. The classification is not a list of prices.

A population whose distinct form in a particular habitat is genetic, which a transplant or a common garden can show and a field appearance alone cannot.

A change in light, wind, predators, or other factors that reaches inward from the boundary of a habitat fragment, the distance depending on which factor is measured.

The warm phase of a Pacific oscillation: slackened trades let the warm pool shift eastward, so the cold water that normally rises, rich in nutrients, rises less along the eastern equator.

Restriction of a species to one place, a different quantity from richness, so a species-poor island can still hold organisms found nowhere else.

Higher performance of an introduced species attributed to specialist enemies left behind, a result that holds in some introductions and not in others.

Of a lake, high in nutrients and in production, a trophic state that people often intensify but that is not a synonym of polluted.

The community consequence of extra nutrients: algal production rises, decay of the bloom consumes oxygen, and animals die or leave the hypoxic water.

Water loss from a landscape by evaporation from surfaces plus transpiration from plants.

How equally abundance is shared among the species that are present.

Competition by depletion of a resource that both parties use.

Increase at a constant per-capita rate, with no positive equilibrium, which can describe a population still small relative to its resources and cannot continue in a finite world.

Species still living where the habitat can no longer support them over the long term, because the pieces are too small or too cut off, so a count made just after isolation overstates what will remain.

A successional mechanism in which the first occupants change the site so that later species establish more readily.

The regular spread of a river onto its floodplain, which accounts for more of the production of many large tropical rivers than a simple downstream continuum does.

A rate at which matter or energy moves. Atoms of a nutrient can cycle among pools; energy cannot, and leaves the ecosystem as heat.

A species that creates the habitat structure others use, such as kelp, a reef-building coral, or a canopy tree.

Breaking habitat into separate pieces, which increases edge and isolation even when the total area is unchanged.

How much a consumer eats as prey density rises. In the decelerating form, each remaining prey is at higher risk when prey are rare, because the consumer is searching rather than busy handling a meal.

The environments in which a population can persist without harmful interactions with other species; measured, wherever its intrinsic rate of increase is zero or positive.

The genetic individual that came from one zygote, as distinct from each module that may be counted separately.

Gross primary production, the carbon fixed by autotrophs, with no respiratory subtraction yet made.

Industrial fixation of N2 as ammonia, one of the human processes that have raised creation of reactive nitrogen to the same order as natural terrestrial fixation.

The tropical atmospheric circuit in which air rises near the equator, loses moisture, and descends in the subtropics, which is why many hot deserts lie near 30 degrees of latitude.

Dependence of a system's state on the path taken, so returning a driver to the value at which the system switched does not necessarily restore the earlier state.

Uptake of an inorganic nutrient into microbial biomass, which can lower the free supply in soil while carbon-rich litter is being broken down.

Gleason's claim that each species has its own tolerances, so a community is an overlap of distributions rather than a developing superorganism.

A successional mechanism in which early occupants hold the site until they die or are damaged.

Competition by direct harm, such as territorial defence or overgrowth, rather than by depletion of a shared resource alone.

The proposal that richness peaks when disturbance is neither rare enough for exclusion nor so frequent that few species tolerate it. Support is mixed, and the proposal is not a law.

Release of phosphorus from sediment, especially once the water above it is anoxic, so cutting the external input need not clear a lake at once.

A life history with more than one bout of reproduction.

The Mauna Loa record of atmospheric CO2, seasonal uptake and release in the Northern Hemisphere set on a rise from a pre-industrial level near 280 ppm to an annual mean above about 420 ppm by the mid-2020s.

A species whose effect on the community is large relative to its abundance, as distinct from a dominant whose effect is large because it is abundant.

Survivorship: the chance that a newborn is still living at age x.

A region made of several ecosystems, arranged as habitat patches, linking corridors, and the ground around them.

The higher richness of many taxa in the tropics than at higher latitudes, for which several explanations remain live and no single cause is agreed.

Liebig's claim that the scarcest required resource, judged against demand, is what limits growth, a first picture that fails if two resources limit together or if one becomes toxic.

Shelford's statement that an organism is excluded both by too little of a factor and by too much.

A mark–recapture estimate of numbers from the marked fraction of a later sample, for a closed population in which marks persist and every individual is equally catchable.

Growth in which the per-capita rate falls linearly as numbers approach a carrying capacity, so the absolute increase is greatest at half that capacity. The parameter of May's discrete map is not this continuous per-capita rate.

Fecundity at age x, usually counted as daughters when a life table follows females.

Leave a feeding patch when the intake rate there falls to the average for the whole habitat, travel time included. The argument assumes the forager knows that average, and it ignores danger unless danger is put into the currency.

The land between habitat patches. If a species can travel through it, the patches are not sea-surrounded islands; if it cannot, the isolation is real.

In the logistic model, the largest surplus catch, taken at half the carrying capacity. That peak has no spare margin: if numbers are pushed below it while effort stays high, surplus production falls as the stock falls, and the model supplies no quota for a named fishery.

Local populations joined by migration. The set persists only while empty patches are colonised faster than occupied patches go extinct, at least while the species is rare.

The route by which dissolved organic matter in the sea is used by bacteria and those bacteria are eaten by protozoa, omitted from a chain drawn only as phytoplankton, zooplankton, and fish.

The climate an organism actually meets, set by aspect, shade, and the air against a surface, rather than by a weather-station average.

Release of an inorganic nutrient, such as ammonium, from organic matter during decomposition.

A loss of timing between interacting species that use different cues and therefore shift their seasons by different amounts.

An interaction in which each partner leaves more offspring than it would without the other, in the conditions actually measured, and which a cheat can still exploit.

Convergence of two unpalatable species on one warning signal, so that they share the cost of teaching predators.

Net ecosystem production, gross primary production minus the respiration of the whole ecosystem, equal to net primary production minus heterotroph respiration.

A null model for trophically similar species that gives them identical per-capita birth, death, and dispersal, so diversity is maintained by speciation and ecological drift. It does not claim that niches are imaginary.

A predator–prey oscillation of the simplest Lotka–Volterra equations, whose amplitude is fixed by the starting numbers, so a disturbance moves the system onto another orbit instead of restoring the old one. The boreal lynx and snowshoe hare records are not that orbit: the hare is limited by its food as well as by the cat, and the equations include no refuge in space.

Shared use of a resource axis, which is required for exploitation competition on that resource and is not enough to prove that competition is occurring.

Microbial oxidation of ammonium to nitrite and then to nitrate, which produces acidity and a mobile anion.

A system that will not return to its historical composition under practical management. The term records that conclusion; it is not a reason to skip testing whether a threshold has in fact been crossed.

Net primary production, gross primary production minus the respiration of the autotrophs, the new plant or algal biomass available to other organisms.

The pattern expected if the process of interest is absent, which an observed pattern has to beat before that process is credited.

How predator abundance rises or falls after prey abundance does. Because it depends on births and deaths, it lags.

A fall of about 0.1 pH units in surface seawater since pre-industrial times, the water remaining a base near pH 8. The word names that fall, not a crossing into acid, and the hydrogen-ion concentration rises by about 26 percent because pH is a negative logarithm.

Of a lake, low in nutrients and in production, and usually still oxygenated in deep water.

In the Rosenzweig–MacArthur model, prey growth is capped and predator intake saturates; lifting that cap can turn a steady point into a wide oscillation. The result is a warning about that model, not a rule for every enriched habitat and not a case against nutrients everywhere.

Why so many phytoplankton species keep coexisting although the water seems mixed and the resources are few, a difficulty for the simplest exclusion model and not a proof that exclusion never occurs.

A consumer, typically an insect, that lives on or in a host and kills that host as a necessary part of its own development.

Removal of a pressure followed by waiting for dispersal and succession, contrasted with putting species or physical structure back. Either fails if the pressure remains.

The seasonal timing of life-cycle events. Many temperate series show earlier spring events as climate warms, and interacting species need not shift by the same amount.

The sunlit surface layer of a lake or the sea. By a teaching convention it ends near the depth that still receives about 1 percent of the light at the surface, a rough guide to where net photosynthesis remains possible, not a fixed physiological threshold.

The structure of vegetation, including stature, layers, and the balance of woody plants and grasses, by which a biome is recognised.

A quantity of a substance in a stated place, such as carbon in leaves or in soil, as distinct from the rate at which it moves.

The individuals of one species in an area stated for a particular question.

Directional community change on a new surface that lacks soil and a local seed bank, contrasted with change where soil, seeds, or living roots remain.

How many individuals of an introduced species arrive, and how often, which raises the chance of establishment without making establishment certain.

Treating many samples from one treated unit and one control unit as if the treatment itself had been repeated many times.

The resource concentration at which a population's per-capita growth is zero, so that on a single resource the species able to persist at the lower level draws the resource down and excludes the other.

The drier lee of a mountain range, where air that has already dropped moisture on the windward side descends warmer and drier.

A module, such as a stem or a polyp, that can live physiologically on its own, so tallying those modules does not tally the genetic individuals.

A comparison of richness reduced to a common sample size or a common coverage, without which a larger sample looks richer only because more effort was spent.

Nitrogen in forms organisms can use, as distinct from atmospheric N2, whose global creation by human activity is of the same order as natural terrestrial fixation.

The environments actually occupied once interactions are included. Negative interactions usually place it inside the fundamental niche, while facilitation can allow persistence beyond a niche defined on abiotic axes alone.

About 106 carbon : 16 nitrogen : 1 phosphorus by atoms in average marine plankton. It is not a chemical formula for every cell or every leaf.

Negative feedback that tends to return a population towards an equilibrium, as distinct from any factor that merely reduces the growth rate.

The decline of richness, over time, from the list a fragment inherits towards the lower equilibrium its new area can hold.

The size of a pool divided by its output flux, when the pool is near a steady state.

A factor that is consumed, so that use by one individual leaves less for another, unlike a condition such as temperature.

The number of species in a stated sample, which cannot be compared across unequal effort until sampling is standardised.

The proposed gradient from a shaded, leaf-fed headwater to a large river fed by fine particles from upstream, which a dam, a tributary, or a floodplain can reset.

The demographic net reproductive rate, the sum of l_x m_x, which says whether a female replaces herself over a generation. It is not the intrinsic rate of increase, and it is not the epidemiological R₀ that counts secondary infections.

A life history with a single bout of reproduction.

The habit of judging present abundance against what one first saw, which hides a longer decline that records, photographs, or a less exploited reference would show.

A layer in a developed soil, from surface litter through mineral soil, sometimes a leached layer and a layer of accumulation, down to weathered parent material.

Physical transfer of carbon to the deep ocean because cold water holds more dissolved CO2 and sinks at high latitudes.

A spatial structure in which a source has a positive local rate of increase and exports individuals, while a sink has a negative local rate and persists by immigration. A sink can hold most of the animals and still not be the site that sustains the population.

The rise of equilibrium species number with area, often summarised by a power law. A ratio calculated from the slope is not a forecast of which species a named reserve will keep, or of when any loss will occur.

Coexistence made stable when species favour different conditions, compete more strongly in their good periods, and keep the gains of a good period in a long-lived stage such as a seed bank.

Living together, in de Bary's sense, a term that includes parasites as well as mutualists and so does not by itself mean mutual benefit.

The layer in a stratified lake or sea across which temperature changes sharply between warm surface water and cold deep water.

A threshold beyond which positive feedback runs a system towards a different state. The phrase is not a name for every large change, and a harmful gradual trend is still real if no such threshold is known.

In succession, later species that establish even while the pioneers are still present, either by outlasting them or by drawing more growth from a resource that is already scarce, without needing the pioneers to improve the site.

Performance plotted against a condition, typically with a lower lethal limit, an optimum, and an upper lethal limit, the optima for survival, growth, and reproduction not necessarily coinciding.

An indirect effect transmitted down a food web, so that suppressing a predator can increase herbivores and decrease plants.

The fraction of production at one feeding level that reappears as production at the next. Teaching examples often put it near a tenth; many endotherms fall short of that, some aquatic grazing chains exceed it, and the fraction is not fixed.

Replacement of which species are present while the number of species on an island can stay steady.

Rise of deeper, colder, nutrient-rich water where surface water is driven offshore or diverges.

Appearance of a barrier across a range that used to be continuous, as distinct from dispersal across a barrier that was already there.