Glossary

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S

Salt pan
A basin or pool containing an accumulation of salty water or salt.
Saprophyte
An organism that carries out external digestion of non-living organic matter and absorbs the products across the plasma membrane of its cells (e.g. fungi).
Savannah
The tropical grassland biome.
Scramble competition
The most extreme form of overcompensating density dependence in the effect of intraspecific competition on survivorship where all competing individuals are so adversely affected that none of them survive.
Searching efficiency
The instantaneous probability that a given predator will consume a given prey (also called 'the attack rate').
Secondary productivity
The rate at which biomass is produced per unit area by heterotrophic organisms.
Seed bank
The population of viable dormant seeds that accumulates in and on soil and in sediments under water.
Selective pressure
A force acting on populations that determines that some individuals leave more descendants (or genes) than others to subsequent generations (and so gives direction to the process of evolution).
Self-limitation
A process where intraspecific competition leads to a reduction in reproduction and/or survival at higher densities.
Self-thinning
The progressive decline in density which accompanies and interacts with the increasing size of individuals in a population of growing individuals.
Semelparity
Where organisms produce all of their offspring in a single reproductive event over one relatively short period.
Serotiny
The retention by a plant (usually by trees) of seeds in hard enclosing structures, e.g. ovaries or cones so that they are not dispersed and free to germinate until after some disaster, especially forest fire.
Serpentine soil
Soil formed by the weathering of serpentine rock which contains high concentrations of various heavy metals. Serpentine soils are commonly localized and bear a specialized flora of species tolerant of these metals and of locally specialized 'ecotypes' of species that are also found elsewhere.
Sessile organism
Literally a 'seated' organism. One whose position is fixed in space except during a dispersal phase, e.g. a rooted plant, barnacles, mussels (Mytilus), corals.
Sexual recombination
The process by which DNA is exchanged between homologous chromosomes by chromosome pairing and crossing-over at meiosis during gamete formation.
Shredders
Aquatic animals that feed on coarse particles of organic matter.
Sigmoid curve
An 'S-shaped' curve in which there is an initial acceleration phase followed by a subsequent deceleration phase leading to a plateau.
Silurian
A geological era lasting from approximately 438 to 408 million years ago.
Social facilitation
An increase in consumption rate with increasing consumer density, resulting, for instance, from an increased time available for feeding when less time is required for vigilance against predators.
Somatic polymorphism
The presence on the same genetic individual (genet) of organs of two or more different forms, e.g. several different leaf shapes on the same plant, two distinct types of seed produced on the same flower or inflorescence, modules of very different form in the body of a salp.
Species - area relationship
A common pattern in which the number of species on islands decreases as island area decreases.
Species-deletion stability
Tendency in a model community for the remaining species to remain at locally stable equilibria after a species is made extinct.
Species diversity
An index of community diversity that takes into account both species richness and the relative abundance of species.
Species richness
The number of species present in a community.
Spiracle
A hole in the sides of insects through which the tracheal respiratory system connects with the exterior, and which can be opened and closed.
Stable equilibrium
In an ecological context, a level of a population, or populations, or of resources, which is returned to after slight displacements from that level.
Stable limit cycles
A regular fluctuation in abundance, the path of which is returned to after slight displacements from the path.
Standing crop
The biomass of living organisms within a unit area.
Static life table
A life table constructed from the age structure of a population at a single moment in time.
Steppe
Treeless plains of southeastern Europe and Siberia.
Stochastic forces
Random processes that affect community structure.
Stoloniferous
Bearing stolons.
Stolons
Horizontally growing, short-lived, stems that root at the nodes. Usually at or on the surface of the soil.
Stoma (pl. stomata)
Pore in the epidermis of plants through which gas exchange takes places; especially abundant on leaves.
Stress
Physics has a strict definition of 'a force per unit area' and producing 'strain' in the body to which the force is applied. Biology has a wide variety of meanings, e.g. any condition that results in reduced growth, any condition that prevents an organism from realizing its 'genetic potential'. The word is often redundant, e.g. the effects of drought stress/the effects of drought. The word is also often used confusingly in two senses, both to describe force and the condition induced in the organism by the force - a confusion of stimulus and response. We have tried not to use the word.
Structural diversity
Range of types of physical structure in a community that may provide habitats for species.
Stylet
Slender, elongated mouth part, usually of an insect, used for stinging or piercing prey or sucking sap.
Succession
The non-seasonal, directional and continuous pattern of colonization and extinction on a site by populations.
Succulents
Plants with fleshy or juicy tissues with high water content characteristic of desert and saline environments.
Superorganism concept
The view of communities as consisting of member species that are tightly bound together both now and in their common evolutionary history.
'Supertramp species'
Species with good colonizing ability that arrive earliest in a new habitat.
Surplus yield model
A simple model of the impact of harvesting on a population, in which the population is represented by its size or biomass, undifferentiated into any internal structure.
Surrogate resource
A resource, not in itself in limited supply, which is competed for because of the access it provides to some other resource, which is or may become limited in supply.
Survivorship
The probability of a representative newly born individual surviving to various ages.
Survivorship curve
A plot of the declining size of a cohort, or presumed cohort, as the individuals die, usually with time on the horizontal axis and log10 lx on the vertical axis (where lx is the proportion of the original cohort still alive).
Switching
Of a predator, the tendency to switch between prey categories according to their relative abundance in the environment.
Symbiosis
A close association between the individuals of pairs of species. The term 'mutualism' is reserved for symbioses for which there is evidence that the association brings mutual gains.
Sympatry
The presence of two or more species living in such proximity that breeding between them should be possible, though their continued existence as separate species indicates that it does not normally happen. This contrasts with allopatry in which regional or geographical isolation normally denies the possibility of interbreeding.
Synergism
The situation in which the combined effect of two forces, e.g. treatment with two drugs, is greater than the sum of their separate effects.

 
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