Ecology Terms Index
Indicator species, the quantities ecologists record, and the limits built into each measure.
An indicator species is one whose presence, absence, or abundance says something about conditions that are harder to measure directly. Lichens that fail where sulphur dioxide is high, or mayfly larvae that vanish from organically polluted streams, are the classic cases: the organism integrates conditions over time in a way a single instrument reading cannot.
The concept has firm limits. An indicator reflects the conditions it evolved to tolerate, in the place where it was calibrated; moving the logic to a different region or a different stressor without checking is the standard misuse. Indicators are also correlational — a missing species suggests a cause but does not prove one.
Behind the indicators sit the measured quantities, each with a conventional unit. Density is individuals per square metre; cover is a percentage of ground; biomass is dry mass per area; productivity adds time and becomes a rate. Most errors in ecological reporting are unit errors, so each quantity is printed here beside its unit and its usual method.
Some quantities are deliberately dimensionless. pH is a logarithmic scale; diversity indices are pure numbers; evenness is a ratio between 0 and 1. Dimensionless does not mean context-free — a Shannon value still needs its sample size, and a pH reading still needs its temperature and its method.
The rule of this index is simple: every quantity is printed with its unit, and every unit is printed with its limits. A measure that cannot say where it stops being valid has not been fully defined.
Further reading