Community characteristics: species richness, dominance, diversity indices, abundance

 

Community characteristics: species richness, dominance, diversity indices, abundance

Community is an organized ecological unit in which organism interact through various associations such a predation, competition, mutualism and parasitism, linked to each other via feeding relationships and are adapted to prevailing physical environmental surroundings.

These interactions, associations and adaptations by the organisms provide community its distinct structure and influence other characteristics such as growth and developments of the community, dominance and species diversity.

•      The basic structure of the community is divided into

•      1.  physical structure

•      2.  Biological structure.

•       

.Physical structure

•      The physical structure of community is defined by the growth forms and life forms.

•      Growth forms and Life forms- The structure and form of vegetation defines the differences between different terrestrial communities.

•      On the basis of the growth forms the vegetation of the community can be classified.

•      Plant community may exhibit different growth forms such as short or tall plants, woody or herbaceous plants or deciduous or evergreen plants.

•      The herbs, shrubs and trees are further sub-divided these categories into evergreen sclerophylls, needle-leafed evergreens, thorn trees, broad-leafed evergreen or broad-leafed deciduous trees, dwarf shrubs, shrubs, grasses, ferns, mosses, lichens and forbs.

 

Various growth form have different mode of arrangement classifying community into

•      (a) Horizontal Zonation

•      (b) Vertical stratification,

•      . Populations assembled to form communities and these populations are dispersed into definite vertical or horizontal strata.

 

(a) Horizontal Zonation

The spatial arrangement of community species exhibits patterns and based on these patterns the community is divided into sub-communities that are ecologically related. If the distribution pattern is horizontal it’s called zonation layering in the community. For example in lakes or deep ponds majorly three zones are recognized i.e. littoral, limnetic (Photic or open-water) and profundal zone (Aphotic or Deep-water).

(b) Vertical stratification,

•      Vertical change in the pattern of community structure is called stratification. Vertical Stratification is as simple as the horizontal zonation community of pond, where each zone has different vertical storey, or complex stratification.

Biological structure.

•      A community has the following characteristics:

•      (a) Structure: By virtue of understanding the structure of the community, the frequency, density, and abundance of different type of species are measured.

•       (b) Dominance: The community type is determined by the dominant species. These species of one or more type either occupy large space or occur in large number and called as dominant species.

•      (c) Diversity: The community show diversity which is composed of different species of plants and animals in different groups that may belong to different growth forms or life forms and are essentially prevailing in uniform environmental surroundings. Diverse communities are healthy and stable communities.

•      (d) Periodicity: The dominant species of the community are studied in various seasons of the year to determine various life processes such as reproduction, growth, and respiration. Periodicity is defined as the expression and reoccurrence of various life processes annually at regular intervals in nature.

•      (e) Stratification: Within ecological communities, the habitat arrangement in form of layering (either vertical or horizontal) is called Stratification.

•      The stratification of two different types of communities may differ such as the lake community represent horizontal stratification whereas mountain plant communities obey vertical stratification.

•      (g) Ecological Niche: Ecological niche is defined as the role or function of species it plays in its ecosystem. In the ecological complex, different plants and animals of different species differ in their function and their combined interactions with other species in its environment are called its ecological niche. In other words, it can also be defined as the small habitat of single species within a large habitat in which it survives. E.P Odum defines and differentiate ecological niche and habitat by saying that ecological niche is the profession of the species within the ecosystem whereas the habitat is its address.

•      (h) Community Productivity: Community productivity is defined as the net storage of energy and production of biomass per unit time by the community.

•      (i)Biotic Stability: Biotic stability is the ability of a community to regain its equilibrium followed by disturbances causing population fluctuations. The stability of the community is directly dependent on the diversity of the community.

 

In 1994, Krebs characterized communities into five characteristics that can be studied, namely

1.     Growth forms and life forms

2.     Species richness,

3.     Dominance,

4.     Abundance

5.     Trophic structure.

 

Life tables are used to describe age-specific mortality and survival rates for a population. When this information is combined with fecundity data, life-tables can be used to estimate rates of population change (e.g., r, lambda, and Ro).

I. Types of Life Tables

1.       Cohort or age-specific or dynamic life tables:

data are collected by following a cohort throughout its life. This is rarely possible with natural populations of animals. Note: a cohort is a group of individuals all born during the same time interval.

2.       Static or time-specific life tables:

Age-distribution data are collected from a cross-section of the population at one particular time or during a short segment of time, such as through mortality data. Resulting age-specific data are treated as if a cohort was followed through time (i.e., the number of animals alive in age class x must be less than alive in age class x-1). Because of variation caused by small samples, data-smoothing techniques may be required

3.       Composite - data are gathered over a number of years and generations using cohort or time-specific techniques. This method allows the natural variability in rates of survival to be monitored and assessed (Begon and Mortimer 1986).

II. Semelparity and Iteroparity

•        Semelparity - Individuals that have only a single, distinct period of reproductive output in their lives, prior to which they have largely ceased to grow, during which they invest little or nothing in survival to future reproductive events and after which they therefore die For annual species, this results in nonoverlapping generations. Examples other than annual plants and some insects?

•        Iteroparity - Individuals that normally experience several or many such reproductive events. During each period of reproductive activity the individual continues to invest in future survival and possibly growth, and beyond each it therefore has a reasonable chance of surviving to reproduce again results in overlapping generations.

1.       Birth pulse - reproductive activity is restricted to a specific breeding season. refer to this as "overlapping semelparity."

2.       Birth flow - reproductive events merge into a single extended period.

Species richness:

Species richness is the number of species within a community or area.

For example, if we have two plots of lands, A and B, and plot A has twenty four species of plants and plot B has eighty four species of plants, plot B has higher species richness.

Species richness does not take into account the distribution of species within the area or what is referred to as species evenness.

In the example above, if the majority of the individuals in plot B with eighty four different types of species all come from one or two different species, this plot would have low species evenness.

In the image below, both communities have identical species richness because they contain two species of trees. In terms of their evenness, community X is more even than community Z because there is an equal number of both tree species.

Dominance

In each community, a few overtopping species are present in greater bulk. By their greater number or biomass (living weight) the dominant species modify the habitat characteristic and influence the growth of other species in the community. In most communities only a single species, being particularly conspicuous, is dominant and in such case.the community is named after the dominant species, as for example spruce forest community. In some communities, however, there may be more than one dominant species, as in oak-fir forest in the western Himalayas.

Abundance

Relative species abundance

Relative species abundance and species richness describe key elements of biodiversity. Relative species abundance refers to how common or rare a species is relative to other species in a given location or community.

Usually relative species abundances are described for a single trophic level. Because such species occupy the same trophic level they will potentially or actually compete for similar resources.

 For example, relative species abundances might describe all terrestrial birds in a forest community or all planktonic copepods in a particular marine environment.

Relative species abundances follow very similar patterns over a wide range of ecological communities.

 

Diversity indices :

Types of Diversity Indices of Biodiversity

The two types are:

 (1) Dominance Indices

(2) Information-Statistic Indices.

1. Dominance Indices:

Dominance indices are weighted toward the abundance of the commonest species. A widely used dominance index is Simpson’s diversity index. It takes into account both richness and evenness.

 Simpson’s Diversity Indices:

The term “Simpson’s diversity index” can actually refer to any one of 3 closely related indices.

Simpson’s Index (D):

Simpson’s index measures the probability that any two individuals drawn at random from an infinitely large communities will belong to the same species. There are two versions of the formula for calculating D.

Either is Acceptable but is to be Consistent:

where,

n = the total number of individuals of each species,

N = the total number of organisms of all species.

The value of D ranges between 0 and 1.

With this index, 0 represents infinite diversity and 1, no diversity. That is, the bigger the value of D, the lower the diversity. This does not sound logical, so to get over this problem, D is often subtracted from 1 or the reciprocal of the index is taken.

Simpson’s Index of Diversity 1-D:

This index represents the probability that two individuals randomly selected from a community will belong to different species. The value of this index also ranges between 0 and 1, but here, the greater the value, the greater the diversity.

Simpson’s Reciprocal Index 1/D:

The value of this index starts with 1 as the lowest possible figure. This figure would represent a community containing only one species. The higher the value, the greater would be the diversity. The maximum value is the number of species in the sample. For example, if there are five species in the sample, then maximum value is 5.

The name Simpson’s diversity index is often very loosely applied and all three related indices described above (Simpson’s index, Simpson’s index of diversity and Simpson’s reciprocal index) have been quoted under term, depending on authors.

As an example, let us consider the following table:

Simpson's Reciprocal Index 1/D

Putting the values into the formula for Simpson’s index:

 

Then, Simpson’s index of diversity

1 – D = 0.7 and

Simpson’s reciprocal index 1/D = 3.3.

All these three values represent the same biodiversity. It is, therefore, important to ascertain which index has actually been used in any comparative studies of biodiversity. The disadvantage of Simpson’s index is that it is heavily weighed toward the most abundant species, as are in all dominance indices.

The addition of rare species with one individual will fail to change the index. As a result, Simpson’s index is of limited value in conservation biology if an area has many rare species with just one individual.

 

  

Shannon Index:

A widely used diversity index is Shannon index.

The Index is given by:

 

where,

pi is the proportion of individuals found in the ith species and In denotes natural logarithm.

The following table gives an example:

Shannon Index

Putting the values into the formula for Shannon index, Hs = 1.201

Even the rare species with one individual (species E) contributes some value to the Shannon index, so if an area has many rare species, their contributions would accommodate. Shannon index has a minus sign in the calculation, so the index actually becomes 1.201, not-1.201. Values of Shannon index for real communities are often found to fall between 1.5 and 3.5. The value obtained from a sample is in itself of no significance. The index becomes useful only while comparing two or more sites.

 

Ecotones

Ecosystems are almost always a patchwork of communities that exist at different successional stages. The sizes, frequencies, and intensities of disturbances differ among ecosystems, creating differences in what is called the patch dynamics of communities. Along the edges of each of the patches are areas called ecotones.

These junction zones often contain species of each of the overlapping communities as well as some species that have become adapted specifically for living in these zones.

In many cases, the number of species and the population density are greater within the ecotone than in the surrounding communities, a phenomenon known as the edge effect.

Forest birds whose nests are deep within the interior of mature forests are less likely to be attacked than those within ecotones. The cutting of mature forests has increased the extent of ecotones, concomitantly increasing the rate of cowbird parasitism across North America.

Ecological niches

An ecological niche encompasses the habits of a species..

As a species adapts to the physical parameters and biota within the community, natural selection favours the development of specialized features that allow the species to uniquely exploit the surrounding resources. Physical conditions of the region—such as temperature, terrain, or nutrient availability—help to mold the niche, and biological constraints such as predation, competition, or lack of resources limit the ways in which a species exploits its environment.

For example, plant species differ in their requirements for light, nutrients, and microorganisms, as well as in their ability to fend off competitors and herbivores. Herbivore species can eat only a subset of the plants available within a community, and predators can capture only some of the many potential prey species. Thus the species “carves out” a niche for itself in the community (see below The effects of competition).

The niche of a species evolves as physical and biological factors in the community change—provided that such changes are slow enough to allow species to adapt to them. The main constraint on this evolution is that no two species in a community can have the same niche. Specialized modes of existence thus provide a selective advantage to coexistent species, offsetting direct competition for available resources.