In this article we will discuss about the origin and characteristics of biotic community.
Origin of Biotic Community:
All the living things make the biotic system of the earth. Neither organisms nor species populations exist by themselves in nature, but are always part of an assemblage of different species populations living together in the same area. Any assemblage of populations of living organisms in the prescribed area or habitat is termed biotic community.
For example, the different species of organisms occurring in a pond constitute the pond community. Some other communities such as swamps, deserts, large lakes, grass lands and many others of different dimensions.
Similarly, the interdependent species in any environment, such as a forest, a lawn or a desert track, constitute biotic communities. A community may be composed primarily of animals, or primarily of plants, but most communities consist of both animals and plants.
The factors that influence the living organisms are known as biotic factors. In the natural world there is interdependence of one form of life on another. A population of single species is not a viable entity by itself. It will not be able to survive for long.
For example, the sparrows require worms, insects or seeds or some other kind of food. Thus the most appropriate definition of biotic community— “a biotic community is a naturally occurring assemblage of plants and animals that live in the same environment, are mutually sustaining and interdependent, and are constantly fixing and dissipating energy.”
However, the different species in a biotic community share a common environment and their relationships are based on direct or indirect functional interactions. The nature of relationship is determined by the requirements of the members of the community.
Biotic community organisation results from interdependence and interaction among populations of different species in a habitat. Large number of biotic communities found in nature due to two specific reasons.
They are as follows:
(i) Existence of diverse habitats with characteristic environmental conditions and
(ii) Co-occurrence of different species whose tolerance ranges overlap with the environmental condition obtained in that habitat. When similar habitat conditions are repeated at another location, the same biotic community gets established there.
Characteristics of Biotic Community:
Each biotic community consists of very diverse organisms belonging to different kingdoms of living things. The number of species and abundance of population in communities also vary greatly. The organisms in a community depend upon each other as well as upon the non-living environment for food, shelter and reproduction.
Species Composition:
The kinds of plants and other organisms present in a community indicate its species composition, which differs from one community to another. Sometimes, in the same community, there may be seasonal variation in plant species.
Each species of community has got definite range of tolerance towards the physical and biological environmental conditions of the habitat. The range of environment a species can tolerate is called its ecological amplitude. The nature of community of a particular habitat is determined by the species, and physical and biotic influences prevailing in the locale of community.
Dominance:
A biotic community may have major categories of growth forms, such as trees, shrubs, herbs and mosses. Out of hundreds of species present in the community, relatively only a few exert a major controlling influence due to their large size, numbers of activities.
The phenomenon is called dominance. “Dominant species are those which are highly successful ecologically and which determine to a considerable extent the conditions under which the associated species must grow.”
The dominance in the community may be the result of co-action between two or more species. Different communities are generally recognized and named on the basis of dominant species occurring in them. For example, a forest community in which pine trees are dominant is called pine forest.
Grassland represents a community which has grass species dominating over the other herbs. Sometimes, communities are named after environmental factors, such as desert community, marine community, mangrove vegetation, etc.
Physiognomy:
General appearance of vegetation is referred to as physiognomy. It constitutes general stature, shape and life-forms of the species comprising the vegetation and actually the classification of vegetation types has been done on the basis of physiognomy.
The species of a community can be grouped into several life-forms on the basis of general appearance and growth. The physiognomy is the total effect created by the combination of vertical structure and architecture of dominant species of vegetation.
For example, the high physiognomy of a forest differs distinctly from a low physiognomy of a grassland. However, several communities though possess similar physiognomy, yet differ markedly on the basis of species composition and dominants, e.g., different types of forests.
Stratification:
Every biotic community has a vertical layering or stratification of organisms or environmental conditions. A number of examples can be cited to support the concept of community stratification from different habitats.
In grassland community three strata, namely:
(a) Subterranean,
(b) Floor and
(c) Herbaceous may be recognized.
The subterranean stratum contains the roots of the principal vegetation and provides permanent shelter to bacteria, fungi, protozoans, nematodes, earthworms, ringworms and several other invertebrates. The floor stratum consists of basal parts of the vegetation, including the rhizomes of grass plants.
In this stratum, generally the insects, spiders, reptiles and rodents are present. The herbaceous stratum of grassland community is represented by upper parts of grasses and herbs. Several types of insects, birds and grazing mammals are included in this stratum.
In a forest community, five vertical strata are present. They are:
(a) Overstorey stratum,
(b) Understorey stratum,
(c) Transgressive stratum,
(d) Seedling stratum, and
(e) Subterranean stratum.
A typical forest may have an overstorey stratum comprised of trees that are more than forty feet in height. These trees make a canopy. Just beneath this canopy there is an understory stratum that extends from twenty feet in height to a short distance below the overstorey. A transgressive stratum extends from four feet to twenty feet or more and comprises of shorter shade loving species.
The seedling stratum begins at the soil level and extends to the lower limit of the transgressive strautm. The subterranean stratum in the forest community is moist and contains a large amount of humus. It is very prominent up-to a depth of two to three meters. Each strautm has its own group of animals.
However, most animals can move from one stratum to another in search of food or in response to many biotic factors, such as insects, snails, birds and squirrels.
The animals found above the soil include turtles, snakes, some birds, insects and a variety of mammals, such as rabbits, deer, wolves and foxes. The animals living in humus soil are beetles, fly larvae, spiders, annelids, antipodes, mites, protozoans, nematodes and some springtails.
In pond community, vertical stratification is very little. However, in deep ponds and lakes three strata:
(a) Littoral zone,
(b) Limnetic zone and
(c) Profundal zone can be recognized.
The littoral zone comprises shallow water region and is occupied by rooted plants. The limnetic zone occupies the depth up-to which the light penetrates and inhabited by planktons, nektons andneustons. In profundal zone, there is no penetration of light, and therefore, it has no photosynthetic organisms.
Species Diversity:
The biotic community is a natural assemblage of a large number of plant and animal species in an area. However, in any particular habitat there is no considerable variation in environmental condition, the plants growing together in a community show unique uniformity in their behaviour.
Vegetation, therefore, is reflection of a climate and, in general, widely separated areas having similar climate have similar aspects of landscape.
Some communities, for example, tropical rain forest and coral reef community, show high species diversity with many different kinds of species living at each trophic level.
Some community areas have limits but more often the community boundaries are hard to define. A clearly distinguished area or a type of area with uniform habitat conditions and supporting characteristic type of vegetation is termed biotype.
Ecotone and Edge Effect:
The zone where two or more different communities meet and integrate, is called transition zone or ecotone. This zone of integration may be narrow or wide, local (e.g., a zone between field and a forest) or regional (e.g., the transition between forest and grass land). Ecotone contains few species from both communities. The total number of species is often greater in the ecotone than in the adjoining communities.
The ecotone or transition zone exhibits a shift in dominance of the conspicuous species of both sides. It may also include a number of highly adaptable species that tend to colonize such transitional areas. Because of this, the variety (i.e., species diversity) and density of life is often greatest in such areas.
This potential for the ecotone to act as a habitat for species found in neither major community is called edge effect. Thus the tendency of increased variety and density of some organisms at the community border is known as edge effect.
The organisms that occur primarily, or most abundantly, or spend the greatest amount of their time in junctions between communities are called edge species. A common example of the edge effect in action can be seen in those species of owl that live in or near ecotones between forests and grasslands. They depend on forest trees for nesting and do their hunting in the grassland, where they depend on field rodents for food.
In man-made communities such as agricultural fields, the ecotone between the field and the forest act as refuge for species formerly found in the ploughed area, as well as for other plants such as weeds. Ecotones of this type are also the prime habitat of may species of insects, game birds, and mammals.
Keystone Species:
The species, which have much greater influence on community characteristics, relative to their low abundance or biomass, are known as keystone species. Such species play a vital role in controlling the relative abundance of other species.
When keystone species is removed, it causes serious disruption in the functioning of the community. For example, in the tropical rain forests, the different species of figs are the keystone species as they produce large quantity of fruits. During the time of food scarcity, these fruits are consumed by monkeys, bats, birds, etc. Thus, by protecting the fig trees, the animals dependent on them are also conserved.
Link Species:
However, only a few species act as keystone species, while others act as link species. For example, mycorrhizal fungi in soil are critical link species as they establish essential links in the absorption of nutrients from the soil and other organic substances.
Some critical link species are responsible for providing food to the network species, while others act as pollinators of flowers, and some act as agents for dispersal of seeds and fruits. Tropical rain forests are rich in critical link species due to high degree of animal dependent pollination and dispersal.