Community Characteristics-III

4.     Dominance

In a community different species interacts among themselves and in some communities the interaction results into dominance by one species or by a prominent species in group. The organisms dominating others are referred to as dominants.

In ecology the proportion of biomass or abundance of one species or taxon than other interacting species or taxon in a community. It’s the dominant species that defines the ecological community. For example Alnus glutinosa (Alder) is the tree dominating in the Western Europe woodland areas. They are used to classify or identify the type of ecology.

In a community we can consider a community as dominant on the basis of following:

Either they are occupying maximum space of community habitat or have highest biomass or play critical role in nutrient cycling, contribute maximum to energy flow or regulate other community organisms.

Sometimes numerically abundant (means more in number) makes organisms superior and dominant but not necessarily always. Microclimate within the community also effect and complicate this system by contributing more dominant species per microclimate. Microclimates have local environment differences like nutrients levels, moisture, topographic location etc.

Its only how impactful and important functions a species play in shaping the structure and function of community which decides its dominance. Sometimes even the low density group of species or a single species can be dominant.

Keystone species: Dominant species (plants/animals) playing crucial and unique role and highly effect community structure and function in relative to its abundance. These keystone species have very intense inter species associations thus, controls the number and types of other species in community. Therefore, if we remove keystone species the community will shift to new form dramatically and vary from its original structure and function.

A classic example of keystone species is Pisaster ochraceus, a starfish. This starfish is a keystone predator and the only natural predator for mussels, sea urchins and many other shellfishes. So, if we remove starfish, the mussels or urchin population will proliferate in an uncontrolled manner shifting the community.

Another example includes a prey predator system where small predators like weevil E. lecontei which forage on herbaceous species called E. watermifoil. E. watermifoil can eliminate dominant plant species of the inhabited community but it’s the predator E. lecontei which control E. watermifoil from doing so by feeding on it. E. watermifoil (prey) number is less and thus requires low density of predator (E. watermifoil). But if predator is eliminated out of the community, the prey will outgrow in number dramatically thereby, vanishing the dominant species of community and thus, by eliminating the small number predator, the community character will now be altered without its actual dominant species.

This example indicates that dominant species directly control the community character but keystone species indirectly alter the community character.

Several approaches are used to determine the ecological dominance.

If a sample is collected from a large area than the individuals of a species found in large number represents the abundance of species and its distribution within ecosystem is called as relative species abundances.

a.     Relative abundance: When the total abundance of all organisms is compared to numerically abundant one species it is called as species relative abundance. If a sample is collected from a large area than the individuals of a species found in large number represents the abundance of species and its distribution within ecosystem is called as relative species abundances.

b.     Relative dominance: Dominance among same sized species can be measured by occupying by a species to the entire area of community.

c.      Relative frequency: Among different sized species, the dominance is measured by the relative frequency.

All these three measurements summed up to provide an important value to each species. These values of species ranked them in a list and index species are the species with high level of important value.

Sporadically/Locally abundant

The frequency of species occurrence in all samples is termed as incidence which relates to abundance. If the incidence or frequency is low but the abundance of species in sample is high it is called sporadically abundant.

How to measure and calculate Relative species abundance?

Sampling methods such as:

1.     Track count

2.     Spotlight count

3.     Monitoring point pressure

4.     Roadkill counts

5.     Plant cover for plant species etc.

Relative abundance of species = No. of species from one sampling/ Total no. of species of all sampling