COURSE NAME: ENVIRONMENTAL STUDIES FOR UNDERGRADUATES FOR ALL COURSES
PAPER TITLE: ENVIRONMENTAL STUDIES FOR UNDERGRADUATES
UNIT: 3 – ECOSYSTEM
TOPIC: ECOLOGICAL SUCCESSION, FOOD CHAINS, FOOD WEBS AND ECOLOGICAL PYRAMIDS
OBJECTIVE: -
By the end of the session, the students will be able to know:-
Objective – By The end of the session students will be able to -
1. Know about concept of Ecological Succession.
2. Learn about causes of Ecological Succession.
3. Understand about types of Ecological Succession.
4. Get information about process of Succession
5. Get details of Hydrosere and Xerosere.
6. Know about Food Chain
7. Know about Food Web
8. Know about Ecological Pyramids
Introduction & Causes of Ecological Succession
Nature is dynamic, an ever changing complex. Many observations of natural changes long ago have resulted in the concept of Ecological Succession. Bare land slowly becomes covered with vegetation and its associated animal life. The vegetation that initially develops in the bare land is gradually replaced in the course of time by a second type, which crowds out the first. But even the second type of vegetation is replaced by the third and so on. In the areas where these stages of replacement occur, it has been found that communities of living organisms (plants & animals) replace one another in a very systematic, predictable and orderly sequence. This process of community change is known as Ecological Succession. In other words it can also be referred as “Development of different communities of living organisms one after the other over the same area in the course of time.”
The term ‘Ecological Succession’ was first of all used by Hult (1855) during the study of a community of southern Sweden. A clear cut concept of Succession was developed by the botanists Warming (1896) and Cowles (1899). In India studies of Ecological Succession have been made by ecologists like Mishra, Rangnathan, Champion, Suri, Pandey and Sharma.
Important causes of Ecological Succession
Important causes for succession include Climatic, Topographic and Biotic.
Fluctuating or unfavorable climatic conditions cause partial or total destruction of the vegetation and as a result bare area develops. Drought, heavy snowfall and lightening are some of the important factors of climate resulting in destruction of vegetation. Among the topographic causes erosion of soil and deposition of soil are important factors.
Sometimes upper fertile soil is removed by the action of wind and water. This is called soil erosion. As a result of soil erosion bare area is exposed where succession starts.
Soil deposition results through heavy storms, glaciers, snowfall and landslides. If deposition of soil takes place over an area covered with vegetation, the plants occurring there may be destroyed or suppressed. This soil deposition results in a new bare area on which succession initiates.
Many biological agencies also affect the vegetation in many ways. Grazing, Cutting, Cultivation, Harvesting and Deforestation, all are caused by biological or living agencies and directly responsible for either vegetation change or destruction. Many parasitic plants and animals also destroy the vegetation; as a result bare area is formed.
Types & Process of Ecological Succession
Types of Ecological Succession
Succession occurring over a bare area which was not previously occupied by vegetation is called Primary Succession. Succession occurring on rocky surfaces, sandy layer and in ponds and lakes are common examples of Primary Succession. However, when succession occurs over an area initially colonized by vegetation but destroyed due to some reason it is called Secondary Succession. Succession takes place on abandoned farmland, strip mines, roadsides and landfill areas represent Secondary Succession.
Succession characterized by early and continued dominance of autotrophic organisms is called by Autotrophic, but when Heterotrophic organisms are dominating then it is referred to as Heterotrophic type.
In succession, when organisms react with the environment and change it, causing its own replacement by new communities it is called Autogenic Succession. However, when the replacement of existing community is caused by external conditions and not by the existing community, then it is named as Allogenic.
Process of Succession
The process of Ecological succession involves the sequential stages called Nudation, Invasion, Competition and reaction and stabilization or climax.
Nudation is the basic process of succession and here formation of bare area takes place. There are many reasons responsible for bare area formation like landslides, soil erosion, flooding, long and continuous drought, volcanic eruption, deposition, fire and diseases.
During invasion, plants and animals from surrounding areas reach the bare area, establish there and aggregate by the way of reproduction. The seeds, spores and other Propogules of organisms reach the bare area by agency of air, water and animals and the whole process is called Migration or Dispersal. Migrated plant species try to establish in new area by the way of germination of seed, seedling growth, vegetative growth, flowering and fruiting. Plant species that are successful in fruiting establish there, while unsuccessful species disappear.
The successful establishment of species in bare areas is called as Ecesis. Ecesis involves germination, growth and reproduction. After successful establishment, species increase their number by reproduction forming large population and the phenomenon is called Aggregation.
When population increases in limited bare area, the competition or struggle for nutrients and space starts among them. The struggle may be intra or inter species. Due to struggle and other type of abiotic and biotic interactions, the environment is modified and gradually becomes unsuitable to the existing community, which sooner or later starts getting replaced by a new community. In modified environmental conditions more suitable species enter the area and compete with the previously present species. Due to the new suitable species, previously present species either become sub-dominant or are eliminated.
The addition of organic matter, moisture and minerals by small plants in the area, again makes it suitable for large plant species. Increased availability of food material allows various carnivore animals and birds to join the community. In this way various communities develop successively, called ‘serial communities’, till a final stable community is developed, known as climax community. Ecologists are of the opinions that climax community is relatively more stable.
The rate of succession changes is rapid or fast initially and gradually becomes slow. The process and early appearing species have a short life cycle while species of later stages have long life span.
Ecological Succession: Hydrosere
Types of Succession:
A. Hydrosere: Succession in the Hydrosere has the following stages:
(i) Phyto-plankton Stage: Depth of water is 15-20 inches. Algae like chlormydomorous and spirogyra are common.
(ii) Submerged Stage: Depth of water is almost 10 inches. Common plants are hydrilla, vallisneria are to be seen. When they die the depth decreases due to their debris getting deposited at the bottom.
(iii) Floating Stage: Depth of water is 2-5 inches. Plants like pistia and eichhorata grow here. On their death and deposition at the bottom of the lake, the depth decreases, making way for another stage, i.e. reed swamp stage or amphibious stage
(iv) Reed Swamp or Ampbibious Stage: In the reed swamp or amphibious stage, depth of water is upto 2 inches. Common plants are typha, phragmites. When they die they are succeeded by marsh meadow.
(v) Marsh Meadow Stage: Marsh meadow stage occurs when water is hardly1 to 2 inches deep. Here the soil is marshy and is invaded by many plant species like Juncus, Carex, Polygonum and Eleocharis. All these plants react with the habitat and make it suitable for forthcoming Woodland stage.
(vi) Woodland Stage: Woodland stage has shrubs and trees. Those plants that can tolerate water logged condition invade here and survive. Examples of plant species present at this stage are Salix, Cornus, Cephalanthus, Alnus and Populus. These plants by their reaction make the soil unsuitable for woodland plants themselves and suitable for forest trees and shrubs.
(vii) Climax Stage: The moist soil of the woodland stage, enriched with Bacteria, Fungi and other Organisms, is invaded by forest trees and shrubs, forming the Climax stage. After a few generations only the most tolerant species survive and gradually pure forest is established. Thus, the area once covered by deep water is now changed into a forest.
During the Hydrosere the changing plant stages also see simultaneous changes in animal types. Protozoans like Paramaecium, Euglenas and Amoeba form the pioneer stage. Abundant plankton growth support Blue gill fish (Lepomis) & sun fish (Micropterus).
With submerged vegetation dragonflies and few crustaceans inhabit the plants. Hydra, gill breathing snails, frogs, diving beetles, whirling beetles and other insects which utilize the undersurface of leaves, associate with floating stage. Different kinds of Turtles and snakes are also found with this stage.
Animals that can live with emergent plants appear with Reed swamp stage. Different species of many flies and dragonflies spend their nymphal stage on submerged plant stem and climb to the surface when they are ready to emerge as adult. Gill breathing organisms are replaced by lung breathing organisms. Among insects, water scorpion, giant water bug and scavenger beetles are also present. Amnelid worms colonize the bottom. Some birds also become common to the area.
In marsh meadow stage the pond becomes a temporary pond which may dry up in summer. So, only those animals are present which can withstand drying in summer and freezing in winter. Like snails, beavers and flashes are gradually excluded.
Plants of marsh meadow stage are replaced by woody forest and accordingly there is a change in animal life also. Terrestrial forms now occupy the habitat.
Thus, a lake gets converted to forest through succession. That is why there is a popular saying, “A lake begins to die the moment it is born”.
Ecological Succession: Xerosere
B. Xerosere
When succession begins on a dry place where there is extreme shortage of water it is called xerarch and with different stages of development it is referred as Xerosere. Xerosere is completed with pioneer lichen, foliose lichen, moss, herbaceous plants, shrub and climax stages.
Over rocky surface, where most conditions are unsuitable for the growth of most plants, few blue green algae and crustose lichens inhabit forming Pioneer stage. Blue green algae like Scytonema are found to adhere to the rocky surface by their mucilaginous sheath. The algae tolerate all extreme conditions and utilize atmospheric Nitrogen. Usually at high altitude where climate is cooler, crustose lichens like Rhizocarpon and Rinodina are common pioneers.
Lichens produce some acids which cause weathering of rocks. Weathering of rocks form small particles, which mix with dead remains of algae and lichen, forming a thin moist layer on the rocky surface. Over this moist layer foliose lichens can grow successfully constituting second stage. As soon as little soil is accumulated on rocky surface some foliose lichens like Dermatocarpon, Parmelia and Umbilicaria grow there. Foliose lichens slowly replace the crustose lichens by making an overshadow over crustose lichen by their thalli.
By death & decay of Thalli of foliose lichens, weathering of rock and collection of dust particles by wind, more soil is formed over which moss stage occurs. Mostly Xerophytic mosses like Polytrichum, Tortula and Grimmia make appearance.
Sometimes foliose lichen and mosses exist together but later on mosses completely replace the foliose lichens. Mosses also increase the amount of soil, and by their death and decay a mat like cover is formed over rocky surface. This cover can hold sufficient amount of water and with soil particles it is suitable for growth and development of herbaceous plants. Under Herbaceous stage, annual, biennial and perennial herbs make their appearance like Poa and Festuca.
Process of rock disintegration and accumulation of humus and nutrients are greatly accelerated. Herbaceous plants, now shaded, tend to disappear and these plants are replaced by shrubs plants of shrub stage. These shrubs may start from seeds or invade from surrounding area by rhizomes. These make the conditions unsuitable for herbs and over shadow them.
Shade of shrubby plants also controls the evaporation of water from soil and wind movement. Humus and Humidity increases, which favors the growth of tree seedlings that form climax stage. First some xeric species of tree appear which later on give place to mesophytic tree species. These mesophytic tree species increase both in number and vigour and ultimately a forest is formed.
Food Chains & Food Webs
The Ecosystem consists of two parts-
A- Abiotic or Non Living Part – Including Soil, Atmosphere, Climate, Water and Natural forces.
B- Biotic-Part or Living Part – Including Organisms – Plants and Animals
In the living part of ecosystem all organisms have different feeding status, known as Trophic Levels. There are five trophic-levels.
Trophic level 1 – It includes all green plants.
Trophic level 2 – Includes all types of Herbivores.
Trophic level 3 –Includes carnivores which eat herbivores
Trophic level 4 –Carnivores which eat carnivores of lower tropic level, hence they are called top carnivores
Trophic level 5 – All decomposers constitute this level
Arrangement of all trophic levels is represented by Food Chains, Food Web, and Ecological Pyramids.
Food Chain:-
Food chain is the linear arrangement of Trophic-levels, where a series of organisms pass energy from one to another, as each provides food to the next level.
Food Chains are classified into two types-
Herbivorous grazing chain & Detritus grazing chain
A. Herbivorous grazing Chain – It always starts with green plants where whole plant or plant part is used as food. Different types of habitats have different types of organisms, so the food chain is also of various types. Forest, Crop-lands, Grass-lands, Deserts, Rivers, Lakes and Oceans, all have different types of food chains.
Some of the common examples are-
Forest –
Ground Plants feed the Deer, which in turn are food for the Fox. The foxes are eaten by Lions.
Ground Plants are eaten by Rabbits. Rabbits are food for the Jackal. The jackal may be eaten by Lions.
Croplands –
Crop Plants are consumed by Rodents, which are eaten by Snakes. In turn, snakes may be eaten by various kinds of birds like a Peacock, a Kite or the Hawk.