Density per cent; E = 81-100 per

Density gives an idea of competition. If density is more, it means there is more degree of competition between the indivi­duals of the species. Density is expressed as number of individuals per unit area and is calculated as follows:

Density = Total number of individuals of the species in all the sampling units / Total number of sampling units studied

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2. Frequency:

Frequency is expressed as the percentage occurrence of individuals of a species in a number of observations. It indicates the homogeneity of dispersion of the individuals of a species in the community and is determined as follows:

% frequency = Number of sampling units in which the species occurred / Total number of sampling units studied ? 100

Thus, the estimate of frequency depends upon the size of the sampling unit. A very small sampling unit underestimates the fre­quency of widely spaced individuals, while a large sampling unit overestimates the frequency.

After determining the percentage frequency of the various species recorded, these are distributed among Raunkiaer’s (1934) five frequency classes-—A, B, C, D and E, depending upon their percentage frequency as follows :

A = 0 —20 per cent;

B = 21 —40 percent;

C = 41 —60 per cent;

D = 61 — 80 per cent;

E = 81-100 per cent.

Raunkiaer proposed a “law of frequency” which states that the percentage of species falling under five classes of frequency will be in order A>B>C>D

3. Species dominance:

The nature of biotic communities is controlled either by physical or abiotic conditions such as substrate, the lack of moisture, and wave action, or by some biological mechanism. Biologically regulated communities are often influen­ced by a single species or by a group of species that modify the environment. These organisms are called dominants, e.g., in forest community large-sized trees are dominants, while shrubs and herbs are mostly subdominants.

It is not easy to describe a dominant or to determine the dominant species. The dominants in a community may be the most numerous, possess the highest biomass, preempt the most space, make the largest contribution to energy flow or nutrient cycling, or by some other means control or influence the rest of the community.

Thus, in a forest community, trees are dominant. They decrease light intensity, increase the relative humidity, inter­cept precipitation, monopolize most of the moisture and nutrients in the soil, decrease wind velocity and furnish shelter and food for animals.

Grasses play a similar, though less conspicuous, role in prairie communities; sedges, rushes and cattails in marsh commu­nities; sagebrush in the arid habitat of the Great Basin; mussels and barnacles on a rocky seashore; and so forth.

Though some ecologists have given the role of the dominant organisms to those that are numerically superior, but numerical abundance alone is not sufficient. For example, a species of plant can be widely distributed over the area and yet exert little influence on the community as a whole.

In the forest the small or under- story trees can be numerically superior, yet the nature of the community is controlled by a few large trees that overshadow the smaller ones. In such a situation the dominant organisms are those with the greatest biomass or that preempt most of the canopy space and thus, regulate the distribution of light. Ecologists measure such dominants by biomass or basal area (i.e., cover).

(i) Community mass:

The community mass is biomass which refers to the weight of organic material, living or dead. In terres­trial communities the biomass is correlated with the weight and complexity of strata of the community. Maximum biomass is observed in dense tropical forests growing on river flood plains.

These forests may weigh up to 1000 metric tons dry weight per hectare (Vyas and Golley, 1975). R. Mistra (1972) has estimated the biomass of an Indian 60-year old forest near Varanasi as 239 metric tons per hectare. Grassland at Varanasi had a biomass as high as 33 metric tons per hectare. Biomass declines from the optimum for the community with decreased moisture and/or temperature and with increased biotic disturbance.

In terrestrial communities the animal biomass is usually less than 10 per cent of the plant biomass. However, in aquatic communities the bio­mass of the plants may be much smaller than that of the animals, where the production is in the form of algae or where the animals feed on imported organic matter.

(ii) Basal cover or canopy cover:

Cover is the area occupied by a plant and can be expressed as basal cover or canopy cover. The basal cover is the land area occupied by the cross section of the stem. The canopy cover is the total land area under the canopy of a plant. In fact, the basal area can only be a small fraction of the total land area in a community but the canopy cover of a single species, the dominant species, may be several times the total ‘and area because of overlapping canopies.

Further, the dominant organism may be relatively scarce yet by its activity control the nature of the community. For example, the predatory, starfish Piaster preys on a number of associated species and reduce competitive interaction among them, so a num­ber of different prey species are able to coexist (Payne, 1966).

If the predator is removed, a number of prey species disappear and one becomes dominant In effect, thus, the predator regulates the structure of the community and must be regarded as the dominant.

Moreover, the dominant species may not be the most essential species in the community from the standpoint of energy flow and nutrient cycling. Dominant species achieve their status by occu­pying niche space that might potentially be occupied by other species of the community.

For example, when the American chest­nut was eliminated by blight from oak-chestnut forest, the chest­nut’s position was taken over by other oaks and hickories. Al­though dominant species frequently shape populations of other trophic levels in the community, dominance necessarily relates to the species occupying the same tropic level.

If a species or small group of species is to achieve dominance, it must relate to a total population of species, all of which have similar ecological require­ments. One or several become dominants because they are able to exploit the range of environmental requirements more efficiently than other species in the same tropic level.

The subdominant species exist because they are able to occupy a niche or portions of it that the dominants cannot effectively occupy. Dominant organ­isms thus are generalists capable of utilizing a wide range of phy­siological tolerances, while subdominant organisms are specialized in their environmental requirements and more limited in their physiological tolerances.

Lastly, the degree of dominance expressed by any one species appears to depend in part on the position the community occupies on a physical or chemical gradient. For example, at one particular point on a moisture gradient, species A and species B may be the dominants. As the gradient becomes drier, species B may assume a subdominant position in the community, its place as a dominant may be taken by a third species, C. Nutrient enrichment too, can change the structure of the community.

Lakes receiving excessive sewage discharges shift from a diverse assemblage of nutrient- thrifty diatoms to a few blue-green algae that are able to exploit a nutrient-rich system (see Smith, 1977).