Ecology Terms Index
How ecologists turn a species list into numbers — and what those numbers can and cannot say.
Biodiversity measurement starts with two counts. Richness is the number of species found; abundance is the number of individuals per species. Everything else — every index, every comparison — is arithmetic on those two columns.
The Shannon index borrows from information theory: it treats each species as a symbol and asks how uncertain the identity of the next sampled individual is. High uncertainty means many species in similar proportions. Values for temperate plant communities typically fall between 1.5 and 3.5, but the absolute number matters less than the comparison it enables between matched samples.
Simpson's concentration asks a different question: what is the probability that two randomly drawn individuals are the same species? The answer runs from 0 to 1 and rises with dominance, so many field guides flip it to 1 − D. The two indices usually agree on ranking sites, but they weight rare species differently, which is why reports must name the index used.
Whittaker's three scales organise the comparison. Alpha diversity describes one site; beta diversity describes the turnover between sites; gamma diversity totals the region. A landscape of identical meadows can show high alpha and low beta, while a mosaic of bog, wood, and pasture can show the reverse.
Sampling decides what any of these numbers can see. Quadrats of known area give densities; transects along gradients give change; and the species-accumulation curve — new species plotted against sampling effort — tells the surveyor when to stop. A diversity value reported without its method is a number without a scale.
Further reading