Terms, indices, units
Ecology runs on measured words: a population, a trophic level, a diversity index each has a precise meaning and a way to count it. This index collects those terms, the arithmetic behind the common diversity measures, and the units field ecologists actually record. Every entry is written to be checked against a textbook or a field manual.
A term earns its place here only if it names something an ecologist can observe, count, or delimit.
Ecology is a young science with an old vocabulary problem. Words like 'population' sound ordinary, yet each carries a boundary: a population is all individuals of one species in a defined area at a defined time, and the definition fails the moment the area or the date goes unstated.
The entries here follow one rule: a term is included when it can be tied to a measurement, a delimitable set, or a process with recognisable stages. Succession qualifies because its stages can be recorded in the field; vague phrases like 'the balance of nature' do not.
Each definition is written to be checked. Where a textbook and a field manual phrase a term differently, the entry records the difference instead of choosing a winner.
By counting species and weighing how evenly individuals are shared among them.
Two counts sit beneath every diversity index. Richness is how many species are present; evenness is whether individuals are spread across them or piled into one or two. Two meadows can share a richness of twenty species and still differ sharply in evenness.
The Shannon index (H′) merges both counts into one value that rises with richness and evenness; temperate vegetation commonly scores between 1.5 and 3.5. Simpson's concentration (D) runs from 0 to 1 and climbs as a single species dominates, so many guides report 1 − D to keep 'higher means more diverse'.
No index is neutral. Each compresses a species list into a number and discards information, so a value is meaningful only beside the index's name, the sample size, and the method that produced it.
Because a frame of known area turns an impression into a density.
The quadrat — classically a one-square-metre frame for herbaceous vegetation — is the oldest sampling device in ecology. Everything inside is counted, everything outside ignored; repeat the frame and the counts become a density, individuals per square metre, comparable between sites and years.
A transect is the line along which quadrats are placed. Lines suit gradients such as shorelines or forest edges; random placement suits uniform ground. The choice is not cosmetic — placing frames only where the vegetation looks interesting is the most common way to bias a survey.
Sample size decides what a count can see. Ten quadrats may characterise a lawn; a species-rich meadow can need fifty before the species-accumulation curve flattens and new frames stop adding species.
Dates mark when key terms entered general scientific use, not when the underlying ideas were first noticed.
Each plate redraws a standard diagram — quadrat, transect, pyramid — in simplified form for quick reference.
The reading list names the standard textbooks, field manuals, and public reference works used to verify the entries on this page.