Concept and Dynamics of Community—Plant Succession

A community, or biocenose, is an aggregate of organisms which form a distinct ecological unit. Such a unit may be defined in terms of flora, fauna, or both. Community units may be very small like the community of invertebrates and fungi in a decaying log. The extent of a community is limited only by the requirement of a more or less uniform species composition.

A different community occurs in each different habitat and environmental unit of larger size, and in fact the composition and character of the community is an excellent indicator of the type of environment that is present. Since plants and animals, bacteria and fungi, all occur together in the same habitat and have many interrelations, they can scarcely be considered independently of each other. Together they make up the biotic community.

Communities may be distinguished as major or minor. Major communities are those which, together with their habitats, form more or less complete and self sustaining units or ecosystems, except for the indispensable input of solar energy. Minor communities, often called societies, are secondary aggregations within a major community and are not, therefore, completely independent units as far as circulation of energy is concerned.

The biotic community along with its habitat is called an ecosystem. The term “ecosystem” has been loosely applied to units of various sizes and characteristics but had best he limited to distinctive combinations of air, soil, water conditions with vegetative, animal and microbic life that possess functional unity.

In short one can say that biotic community is a grouping of different but interacting populations of different species which live harmoniously in a given locality, e.g. pond community, lawn community may further be split into animal community, plant community and microbe community. Plants provide food, shelter and oxygen to animals. The latter provide CO₂, plough the soil, help in pollination and dispersal. Microbes take part in decomposition of dead bodies and release of minerals.

(A) Interactions:

They may be neutral, positive or negative. Various kinds of interactions are as follows:

(i) Scavenging:

Feeding on dead bodies. Animals feeding on flesh of dead animals are called ‘scavengers’ e.g. Vulture, Jackal, Hyena (eat left over from lion/tiger’s kill), Crow (actually omnivorous) Ant (actually omnivorous). Scavenging keeps the earth clean.

(ii) Commensalism:

One is benefitted without the other being harmed e.g. E. coli in human intestine and epiphytes perched on other plants for space only.

(iii) Mutalism:

It is a mutually beneficial relationship/interaction between individuals of two different species.

Mutualism and symbiosis are synonyms though symbiosis was formerly considered to be any interaction between two different organisms, e.g. (i) Association between fungus (protection, fixation and absorption of water) and alga (food manufacture) in lichen. (ii) Rhizobium (nitrogen fixation) and root (shelter, food) in nodulated legumes etc.

(iv) Protocooperation:

It is non-obligatory beneficial relationship that develops when two different organisms get associated. Cattle Egret feeds on lice and ticks of cattle.

(v) Amensalism:

Without deriving much benefits, one inhibits the other by secreting allochemics, e.g. Pencillium on staphylococus (Flemming, 1928) by Penicillin. Juglans (walnut) on other nearby plants by Juglone, Tagetes on soil nematodes. Chlorella vulgaris against diatom Nitzschia frustrulum.

(vi) Competition:

It is rivalry for obtaining the same resource. Interspecific competition reduces the size of population. Interspecific competition causes one to be eliminated (Cause hypothesis/principle of competitive exclusion) or undergo different adaptations (e.g. 14 species of Finches in Galapagos

Islands or 20 species of Antelopes in Serngeti plains, roots of different depths).

(vii) Mimicry:

Mimicry is the resembling to another organism, structure or surroundings (camoulflage).

(a) Protective:

Concealing type provides protection from predators, e.g. Stick Insect, Leaf Insect, Dead leaf Butterfly. In warming type, the mimic resembles a ferocious/poisonous/distasteful organism, e.g. Vicerory Butterfly mimics Monarch Butterfly.

(b) Aggressive:

In concealing type, a predator resembles the surroundings, e.g. Praying Mantis. In alluring type, the predator has resemblance with a favoured article, e.g. mouth of African Lizard resembles a flower. Flower of Ophyrs resemble female wasp (Colpa) and are pollinated by male wasp.

(viii) Parasitism:

One organism called parasite obtains nourishment from another organism called host.

(a) Non pathogenic (e.g. Entamoeba coli) and pathogenic (e.g. Entamoeba histolytica).

(b) Temporary /Intermittent (e.g. female mosquito) and permanent (e.g. Ascaris).

(c) Ectoparasite (e.g. Aphids, Lice) and endoparasite (e.g. intracellular Plasmodium, tissue parasite Trichinella).

(d) Holoparasite (complete parasite, e.g. animal parasites) or hemiparasite (semiparasite e.g. Lovanthus).

(e) Stem parasite (e.g. Cuscuta, stem borer) root parasite (e.g. Rajflesia, rootnematodes).

(f) Hyperparasite: Parasite over parasite e.g. Cicinnobolus cesatii on powdery mildew.

At times the parasite spread in epidemic form and causes large scale misery.

(ix) Predation:

One organism called predator captures, kills and death another organism called prey. Predators are carnivorous but some herbivores which destroy whole plants/seeds are also called predators. A predator may become prey for another.

(x) Negative Human Interactions:

They include primitive shifting agriculture (jhoom), felling of trees, hunting, overgrazing, scarping and litter removal, introduction of weeds, forest clearing and human induced forest fires.

(B) Biotic Stability:

It is the absence of major fluctuations and the ability of a biotic community to undergo homeostasis or achieving equilibrium after a disturbance. It depends upon species diversity, presence of biological control, population density and carrying capacity of environment, biogeo- chemical cycling, seeding growth, degree of disturbance, climax/seral community, etc. The larger the number of species present in a biotic community, the more stable it is.

Maintenance of almost same number of different types of organisms within an ecosystem or biotic community is called ‘balance of nature.’

(C) Biotic/Ecological Succession:

It is the successive development of different biotic communities at the same site till a climax community develops there.

Even when the climate of an area remains stable year after year, ecosystems have a tendency to change from simple to complex in a process known as succession. This process is familiar to anyone who has seen a vacant lot or cleared woods slowly become occupied by an increasing number of plants, or a pond become dry land as it is filled with vegetation encroaching from the sides.

(i) Primary Succession:

Primary succession occurs on base, lifeless substrate, such as rocks, or in open water, where organisms gradually move into an area and change its nature. Primary succession occurs in lakes left behind after the retereat of glaciers, on volcanic islands that rise above the sea, and on land exposed by retreating glaciers. Primary succession that occurs on land in called XERARCH succession, to distinguish it from the hydrarch primary succession that occurs in open water.

Primary succession on glacial moraines provides an example. On bare, mineral-poor soils, lichens grow first, forming small pockets of soil. Acidic secretions from the lichens help to break down the substrate and add to the accumulation of soil. Mossess then colonize there pockets of soil, eventually building to take hold. Over a hundred years, the alders build up the soil nitrogen levels until spruce are able to thrive, eventually crowding out the older and forming a dense spruce forest.

In a similar example involving Hydrach succession, an oligotrophic lake (poor in nutrients)- may gradually, by the accumulation of organic matter, become eutrophic-(rich in nutrients).

Primary succession in different habitats may over the long term arrive at the same kinds of vegetation- vegetation characteristic of the region as a whole. This relationship led American ecologist F.E. Clements, at about the turn of the century, to propose the concept of final climax vegetation (and the related term ‘climax community’). With an increasing realization that (1) the climate keeps changing, (2) The process of succession is often very slow, and (3) the nature of a region’s vegetation is being determined to an increasing extent by human activities, ecologists do not consider the concept of ‘climax vegetation’ to be as useful as they once did.

(ii) Secondary Succession:

If a wooded area is cleared and left alone, plants will slowly reclaim the area. Eventually, traces of the clearing will disappear and the area will again be woods. This kind of succession, which occurs in area where an existing community has been disturbed, is called secondary succession. Humans are often responsible for initiating secondary succession, but it may also take place when a fire has berned off an area, or in abandoned agricultural fields.

Why Succession Happens:

“Communities evolve to have greater-total biomass and species richness in a process called succession.” In other words, succession happens because species alter the habitat and the resources available in it, often in ways that favour other species. There dynamic concepts are of critical importance in the process : tolerance, inhibition and facilitation.

(A) Tolerance:

Early successioal stages are characterized by weedy-y-selected species that do not complete well in established communities but are tolerant of the harsh, abiotic conditions in barren areas.

(B) Facilitation:

The weedy early successional stages introduce local changes in the habitat that favour other, less weedy species. Thus, the mosses in the Glacier Bay succession fix nitrogen that allows alders to invade. The alders in turn lower soil pH as their fallen leaves decompose, and spruce and hemlock, which require acidic soil, are able to invade.

(C) Inhibition:

Sometimes the changes in the habitat caused by one species, while favouring other species, inhibit the growth of the species that caused them. Alders, for example, do not grow as well in acidic soil as the spruce and hemlock that replace them.

As ecosystem mature and more K-selected species replace y -selected one, species richness and total biomass increase but net productivity decreases. Because earlier successional stages are more productive than later ones, agricultural systems are internationally maintained in early successional stage to keep net productivity high.

Primary succession takes long time of 1000 years or more while secondary succession takes 50-100 years for grass and 100-200 years for forest. Ferns are generally the first to grow after forest fire because of their underground rhizomes. Secondary succession is not completed if invaded by weeds e.g. Sphagnum, Eichhornia, Lantana, and Eupatorium.

Pioneer Community:

It is the first biotic community on a base area.

Seral Community: Transitional community appearing in an area during biotic succession.

Climax Community:

Stable, self-perpetuating and final community appearing in an area and at the end of biotic succession which is in perfect harmony with climate of the area-hence climate climax community.

Dominant Species:

It is the most numerous and generally the largest of the species present in a seral/ climax community which exerts the maximum influence.

Sere:

It is the sequence of developmental stages from pioneer to climax communities, e.g. Lithosere (on rock), psammosere (on sand), hydrosere (in water). Xerarch/Hydrarch : series of developmental stages in an area/water body. Biotic succession involves changes to long-lived plants, greater species diversity, niche specialisation, increase in biomass, higher soil humus, food webs, stable biotic community and mesic conditions.

So one can say that causes of succession are as follow:

(i) Baring of an area

(ii) Changes brought about by pioneer and seral communities not favourable to them but favourable to next ones.

Lithosere-Xerosere (Succession on Bare Rocks):

(A) Lichen Stage:

Bare rock is invaded first by crustose lichens (e.g. Graphis, Rhizocarpon). They corrode the rock at places causing foliose lichens (e.g. Parmelia, Dermatocarpon) to invade, eliminate crustose lichens and creating conditions for invasion by mosses. In tropics, blue green algae are pioneers instead of lichens.

(B) Moss Stage:

Mosses capable of tolerating drough invade the human rich holes created by foliose lichens, e.g. Tortula, Grimmia, Polytrichum. They create more humus and shade to eliminate lichens.

(C) Annual Grass Stage:

Annual grasses are slowly replaced by perennial grasses with runners and rhizomes, e.g. Heteropogon, Cymbopogon. Several small animals begin to reside. Herbivorous animals visit the site.

(D) Shrub Stage:

Shrubs begin to grow in area occupied by perennial grasses. They increase soil and humus contents besides moisture, e.g. Rubus, Rhus, Capparis, Zyzyphus.

(E) Climax Community:

Initially hardy, light demanding small trees invade the area. They make the habitat shadier and move moist. Ultimately, trees shrubs and herbs representing the climax community begin to grow in the area.