Presence and Constancy - Synthetic Characters
Presence and constancy refer to how uniformly a species occurs in different stands in a community. For example, when a species is found in 15 out of the 20 stands in a community the presence or constancy is 75 per cent.
The terms presence and constancy are used in more or less in the same sense but they are however not synonyms. The term constancy is used when equal, measured sample areas are used for study, and presence is used when the area of sampling unit varies from stand to stand and especially when it is not measured. Many times, the sampling units do not have the same area, because of the nature of vegetation, as for example, small irregular stands in rock crevices or on sand deposits along a stream. Species in a community can be classified into five classes of constancy, according to the percentage of occurrence in sampling units or stands. These classes are:
Class I - 1-20% of the sampling units of a community
Class II - 21-40s of the sampling units of a community
Class III - 41-60% of the sampling units of a community
Class IV - 61-80% of the sampling units of a community
Class V - 81-100% of the sampling units of a community
You might have noticed that Classes IV and V include those species that occur in a large number of stands. The species that occur in over 80-90% or in more sampling units are called constant species. These species are important as they characterise and help to distinguish a community type. The species belonging to class IV and V indicate two possibilities:
- The species have a wide ecological amplitude and are therefore capable of growing in various micro-habitats, and
- The various sampling units are very similar in environmental conditions, so that species of narrow amplitude can grow in all of them.