The following varieties of ecological diversity are recognized (Singh, 2002, Singh and Kumar 2003).
The number of species that occurs in a particular area is called its species richness (Donovan and Welden, 2002). It means the species richness in any habitat and is common currency of the study of biodiversity.
Species richness index is essentially a measure of the number of species in a defined sampling unit (Magurran, 1988).
Species richness is a function of sample size. However, it should not be confused with species abundance. Each natural habitat has a variety of species, which differ in their relative abundance. No community consists of species of equal abundance; some species are rare, others are common and still others may be abundant.
Species diversity measures are often more informative than species counts alone. According to Harper, (1977), there is “importance of taking an organism’s eye view of community diversity”. This comment is relevant to structural diversity as it is to species composition.
It means the diversity of resources that an organism (species) utilizes. For example, some fish species in the hill-streams have a wide trophic niche and depend on zooplankton, insects, and algae and diatoms for their food (Singh and Bahuguna 1983). In many cases food resources consumed by an organism differ during different stages of the life cycle, such as fry, fingerling and adult stages in case of fish. Thus, niche width is the measure of the diversity of resources utilized by a species.
The usual approach is to use the Simpson index or the Shannon index to calculate the niche width. The number of resource types observed (e.g., types of food items eaten, varieties of habitat utilized, kinds of behaviour employed) replace number of species in the equation. A separate value must be calculated for each type of resource and measures of abundance will depend on the way in which the index is being used.
If the niche width of a particular species is under consideration then abundance may be measured as the number of individuals either eating each type of food, living in each sort of habitat, or adopting each kind of behaviour. However, if we wish to measure the niche width of an individual, then abundance can be taken as the amount of each food type eaten, the time spent in each habitat or the frequency with which each behaviour is performed (Magurran, 1988).
It is the number of habitat types in a defined geographical area. This is an index, which measures the structural complexity of the habitat. This structural complexity of environment, in turn, is responsible for the presence of a wide variety of spatial and trophic niches. This means that if any habitat supports more microhabitats its biological diversity will be more as compared to a habitat which has less number of microhabitats. More studies on habitat diversity have been made for terrestrial environments.
The number of substrate types has been related to species diversity for aquatic insects, molluscs and benthic macro-invertebrates. Gorman and Karr (1978) have taken bottom type, depth and current into account to investigate the link between habitat diversity in streams and fish species diversity. The author found that habitat diversity was more in small hill streams and some tributaries than large, snow-fed rivers in the Garhwal Himalayas (Dobriyal and Singh 1988, Kumar 1992).
It is also called beta diversity. It means degree of change in species composition between sites or communities or along gradients. A number of studies on faunal diversity of fish and insects have clearly indicated that their distribution and abundance is governed by gradient and altitude, among other factors (Singh et al, 1994, Singh and Nautiyal, 1990). For example, stoneflies and rheophilic fish species in the rhithron parts of hill-streams are characteristic and are absent from their potamon parts.
One study indicated that there is zonation of animals within a river; Simulium monticole occurs from source to 12 km, Simulium variegatum from 12 to 35 km., and Simulium equinum from 20 to 50 km. All these studies indicate that greater diversity of species and habitats means greater ecological quality.