ISC11>Content>U-3-Organisms & environment>3_Community ecology
SCOPE OF SYLLABUS
- Ecology : Greek origin
- Study of the “house/environment” in which we live.
- Term proposed by Reiter. (1865)
- Used & defined by Haeckel (1866)
- The study of relationships between living organisms and between living organisms and their environment.
- A group of organisms that can interbreed to produce fertile offspring.
- A group of organisms of the same species living in the same area at the same time.
- All the populations of the different species living and interacting in the same ecosystem.
- A Community (Biotic) interacting with its abiotic environment.
- A major regional or global biotic community, characterized chiefly by the dominant forms of plant life and the prevailing climate.
- A biome contains many ecosystem.
- The biosphere is the collective interaction of all the biomes on the Earth.
- Community : In an environment or habitat, different types of plants and animals exist in close association and show interdependence. An actively interacting group of a number of different populations of several species in a common environment is known as biotic community.
- A mature, self-perpetuating biotic community contains only those species which are successfully adapted to that environment and to other species of that area.
- The assemblage of plants species in a habitat is called plant community, of animals species animal community and of microbes as microbial community.
- Community Ecology : Study of interactions among all populations in a common environment.
- Examples of biotic communities: Pond community, Forest community, Lake community etc.
BIOTIC COMMUNITY: BASIC FEATURES
CHARACTERISTICS OF BIOTIC COMMUNTY
1. Interdependence of animals and plants.:
- Communities are not a random mixture of species. The animal, plant and microbial community of a biotic community show fundamental dependence on each other.
- Nutritional interdependence
- Carbon dioxide-oxygen interdependence
- Interdependence in pollination
- Interdependence in dispersal of seeds and fruit
2. Trophic organisation
- In large terrestrial and aquatic communities, population of each species occupies a particular strata. This is called stratification.
- In a forest community, the tree tops, the branches, the leaf litter and the soil bottom are occupied by different species.
- A pond community has surface dwellers, bottom dwellers and those living in the intermediate depths.
- In any community, one or few species dominate either in numbers or in physical characteristics or in both over the species.
- In Grassland : Grasses
- Pine forest : Pine
5. Variety of species
- A community is formed of many species. These vary from community-to-community. A great variety of species are found in the tropical rainforest whereas only a few species are found in a polar community.
- In a community, there is interaction among the organisms of the same species, with other organisms of their own community or among the organisms of different communities. The organisms are related to one another through cooperative or competitive actions so that they maintain a balance. This relationship between organisms is classified into following two types:
- Intraspecific relationship
- Interspecific relationship
1. INTRASPECIFIC RELATIONSHIP
- It is the relationship among the individuals of the same species.
- The cooperative interactions in the members of the same species or population include mating behaviour, parental care, family formation, group formation, altruism, dominance-subordinate behaviour, animal societies and communication.
- The competitive interactions are dominance subordinate behaviour, leadership and home range or territoriality.
- The cooperative relationships are beneficial for the species.
2. INTER-SPECIFIC INTERACTION
I) POSITIVE INTERACTION
- Definition: Mutualism is an interaction between two organisms of different species where both the partners are benefited with none of the two capable of living separately.
3. Zoochlorellae and animals
4. Symbiotic Nitrogen Fixation:
5. Termites and Trichonympha:
- Termites feed on wood through cellulose-digesting flagellate, Trichomympha. Flagellate is unable to live independently while termites fail to utilise cellulose in the absence of flagellate.
6. Ruminants and Cellulose Digestion:
- Cellulose digesting bacteria live in the rumen of stomacn and in appendix of ruminant mammals. Other herbivores also possess cellulose digesting bacteria. The products of digestion are shared both by the animal and bacteria
7. Pollinators and Dispersers:
- The most spectacular and fascinating examples of mutualism with evolutionary significance are found in plant-animal relationships.
- Plants need the help of animals for pollinating their flowers and dispersing the seeds.
- Pollinators (animals) get edible pollen and nectar for pollination and dispersers get juicy and nutritious fruits for dispersing the seeds.
- Insects are most common pollinators.
B. PROTOCOORPORATION (+ +)
- Definition: Protocooperation is an association between organisms of different species in which both are mutually benefited but they can live without each other. It means protocooperation is equally beneficial for both but is not obligatory.
- Definition: Commensalism is an association between two different organisms or species in which one is always benefited but the other is neither benefited nor harmed. The species that derives benefit is called commensal and the other is called host.
- Benefits: Commensals derive benefit from the host in the form of food, protection, shelter, living space or transportation.
D. SCAVENGING (+0)
- Definition: Scavenging is the feeding by an animal on the remains or carcases of dead animals and on the refuse of living animals. Scavenging is a food relationship between a dead animal and its eater. The eater is called scavenger.
- Examples: Animals such as foxes, hyenas, vultures etc. are the animals, which are natural scavengers. Dogs, crows, ants are occasionally seen to do the work of scavengers.
II) NEGATIVE INTERACTION
- Definition: Predation is the direct food relation between two organisms of different species in which one animal (predator) captures, kills and feeds upon other animal (the prey).
- Examples: Familiar examples of predator-prey relationship are between Tiger and deer, Hawk and small Bird, Snake and Rat, Frog and Insects. Species like Frog may be both a prey and predator.It is predator for insects but a prey for other animals.
3. Predatory plants
OUTCOMES OF PREDATOR -PREY INTERACTION
- Offset oscillations in the population sizes of the predator and prey
- Defense mechanism
1. Offset oscillations in the population sizes of the predator and prey
2. Defense mechanism (in animals)
2. Defense mechanism (in plants)
- Definition: Parasitism is a food relationship between organisms of two different species in which the smaller one lives on or within the larger one and obtains its food from it. The former which obtains food is known as parasite and the latter which provides food and shelter to the parasite is called host.
- In this host-parasite relationship, the parasite is benefited but host harmed or not affected.
- Parasitology is the science that deals with various aspects like life-cycle, mode of infection, effect on host, adaptations, etc. of parasites.
- Types of parasites:
- Temporary parasitism and permanent parasitism
- Pathogenic and non-pathogenic parasites
- Facultative and obligate parasites
- Holoparasites and Hemiparasites
1. Temporary parasitism and permanent parasitism:
- Temporary parasites visit host for a short period for feeding as Bedbugs, Leeches and Mosquitoes. These are called intermittent parasites. However, Mosquito is not a parasite because it transfers parasite to the human beings. So it is a vector.
- Permanent parasites live in contact with host throughout their life. Eg. Ascaris, Taenia, Entmoeba
2. Ectoparasites and Endoparasites:
- The parasites which live on the surface of host’s body are called ectoparasites, while the parasites which live inside of the host’s body are called endoparasites. Ectoparasites either suck blood (in case of animal parasites) or juices (in case of plant parasites) or feed on living tissue. Some common human parasites are:
- Louse (Pediculus) that lives on human body and among the hair. It sucks blood, causes irritation and spreads diseases.
- Rat Flea (Xenopsylla) is ectoparasite on rats and spreads germs of plague from rat to man.
- Ticks (Dermacentor) are ectoparasites on dogs and cattle and cause tick bite fever or tick typhus in man.
- Endoparasites may be intracellular or extracellular. Common endoparasites of man are:
- Entamoeba histolytica is a protozoan that lives in the human intestine and causes amoebic dysentery.
- Malarial Parasite (Plasmodium) is also a protozoan that lives in RBCs and causes malaria fever.
- Tapeworm (Taenia) and Roundworm (Ascaris) live in alimentary canal.
- Filarialworm (Wuchereria bancrofti) lives in lymphatic glands and lymph vessels and causes elephantiasis.
- Guinea Worm (Dracunculus medinensis) is a roundworm that lives in the subcutaneous tissue and causes itching.
- Schistosoma lives in blood and Trypanosoma in cerebrospinal fluid.
3. Pathogenic and Nonpathogenic Parasites:
- The parasites whose presence produces diseases in the host are called pathogenic parasites.
- Example of some pathogenic parasites are bacteria (Vibrio cholera that causes cholera, Cornybacterium, diphtheriae which causes diphtheria and Mycobacteium leprae causing leprosy), a large number of fungi, some protozoans (Entamoeba histolytica, Trypanosoma, Leishmania), helminths (Fasciola, Taenia, Ascaris, Filaria, Hookworm and Pinworm, etc.) and Puccinia that causes rust in cereals.
- Nonpathogenic parasites do not harm the host. Entamoeba coli in human intestine is nonpathogenic.
4. Holoparasites and Hemiparasites:
- Holoparasites are totally dependent on the host for all their requirements. All animal endoparasites are holoparasites. In plants, Rafflesia and Cuscuta are holoparasites. Cuscuta depends on its host even for flower-inducing hormone or florigen. It has lost its chlorophyll and leaves in the course of evolution.
- Hemiparasites receive only a part of their nourishment from the host and manufacture part of their food, e.g., Viscum (Mistletoe) and Loranthus.
- These are parasites that live within or on another parasite, e.g., some bacteriophages are hyperparasites of bacteriophages, bacterium Pasteurella pestis of Xenopsylla chaeopsis (rat flea) and Cicinnobolus cesatti of powdery mildew.
- Amensalism is an association between two organisms of different species where one species is inhibited or killed and the other is unaffected.
- In simple words, in amensalism, one organism does not allow other organism to grow or live near it. It is also called antibiosis and the affected species is called amensal and the affecting species is called inhibitor.
- Such inhibition is achieved through the secretion of certain chemicals called allochemics or allelopathic substances.
- Examples:Most common phenomenon of antibiosis is formation of antibiotics that are antagonistic to the microbes.
- Penicillium notatum releases the antibiotic substance Penicillin, which inhibits the growth of variety of bacteria.
- Streptomyces griseus produce antibiotic Streptomycin, which again inhibits the growth of many bacteria.
- Roots of certain plants produce allochemic substances which check the growth of other plants to conserve resources, such as, Convolvulus arvensis, a weed inhibits the germination and growth of wheat.
- Definition: Competition is the interaction which involves struggle among organisms for the same resources, like water, nutrients space, sunlight or food, etc.
- Types: Competition is of two types. When both the competitors belong to the same species it is called intraspecific competition (intranecine) and when competition is among the individuals of different species, it is termed interspecific competition (internecine).
- Intraspecific Competition: Intraspecific competition is more acute than interspecific because all the members of a species have similar basic requirements and possess similar body structures. Establishment of territories by animals is meant for ensuring availability of their needs. Likewise, plants of the same species survive and grow favourably only when they are well-spaced. Intraspecific competition controls the size of population through decrease in the birth rate and increase in death rate. In an experiment, when Flour beetles kept in pairs of 1, 8, 40 and 80 showed a corresponding decrease in the number of eggs from 10, 6, 2 to 1.
- Interspecific Competition: In this type of competition, two or more populations belonging to the same trophic level, i.e., feeding habit, compete with one another for the same natural resources. For example, in a forest, trees, shrubs, herbs and vines compete with one another for sunlight, nutrients, water, pollinators and dispersal agents. Carnivorous animals like tigers and leopards may compete for the same prey. However, individuals of different species do not have similar types of adaptations, rather they may have different types of adaptations. Different types of individuals adapt themselves differently to survive and obtain the same resources. In case, the resource is in short supply, the superior individuals exclude or eliminate the inferior ones. Competition also exists in totally unrelated species. For example, in some shallow South American lakes, visiting flamingoes and resident fishes compete for the common food, the zooplankton in the lake. Secondly, resources need not be limited for competition to occur. There is another type of competition known as interference competition. In this type of competition, the feeding efficiency of one species is reduced due to the interfering and inhibitory presence of the other species, even if resources of food and space are abundant Therefore, competition may also be defined as a process in which fitness of one species (measured in terms of ‘r’ the inherent rate of increase) is significantly lowered in the presence of another species.
- One of the principle of nature is stability amidst diversity. The larger the number of diverse forms present in a community, the more stable that community will be. It means that the stability of a community depends not on the large populations of a few species but upon the number of populations of different species.
- A large population of a single species of Eucalyptus is likely to be totally wiped out by a fungal disease or insect attack. When there are many species of trees, only one species may be affected by a disease or a pest whereas the rest would survive.
- The stability of a community is displayed by the large population of wild animals in Africa. In Serengeti plains of Africa, about 20 species of antelopes live together in the area. Each species of antelope eats a different kind of grass or shrub. Some may even feed on the same kind of shrub but at different stages of its growth. This division of food preference enables all animals to get adequate nutrition and also keeps the habitat productive. But it has been observed that destruction of wildlife and introduction of domestic animals for grazing in an area destroys the productivity of land.
- Definition: An orderly and progressive replacement of one community by another till the development of a stable or climax community in a particular area is termed biotic or ecological succession.
- A biotic community normally undergoes continuous changes under the influence of various biotic and abiotic factors. The communities at any place modify the environment due to their activities.
- The modified environment becomes less suitable to the existing community which is replaced by some better suited more complex new community.
- The process is repeated leading to serial succession of previously existing community by the new one.
- The entire series of communities is known as sere, and the individual transitional communities as seral communities or seral stages.
- The first community to inhabit an area is called pioneer community. It has low biomass and nutrient level.
- The succeeding communities are called the transitional communities and the last stable community, the climax community The climax community has maximum diversity and niche specialisation.
TYPES OF ECOLOGICAL SUCCESSION
- Based on the habitat, ecological succession can be of following four types:
- Xerosere: The sequence of developmental stages of biotic succession on dry habitat like desert is termed as xerosere.
- Lithosere: It is the sequence of successional stages on a bare rock.
- Psammosere: It is the sequence of successional stages on coastal sand.
- Hydrosere: It is the sequence in biotic succession taking place in a waterbody-pond, pooi or lake. Such a succession is known as hydrarch succession.
TYPES OF SUCCESSIONAL COMMUNITIES
- Pioneer Community: It is the first community developing over a bare area. This community has a little diversity and takes a lot of time in converting the bare area suitable for the second or seral community. Members of pioneer community have a high rate of growth, a short lifespan, low biomass and low nutrient level. Therefore, they produce lots of organic matters.
- Seral or Transitional Communities: These communities develop in the area after the pioneer community but before the climax communities make their appearance. Seral or transitional communities have high biomass and high levels of organic nutrients. They slowly build soil, increase in biodiversity, niche specialisation and biogeochemical cycling. Successional species that develop latter are slow growing and long lived.
- Climax Community: It is a stable, self-perpetuating biotic community which establishes itself after the seral community. It shows a high niche specialisation, complex food webs and stratification. It is in perfect harmony with the climate of the region. In a climax community species diversity is the maximum.
KINDS OF SUCCESSION
- Primary and Secondary Succession
- Primary Succession or Prisere: When succession begins on primarily bare area which has not been previously occupied by a community (e.g., a newly exposed rock area, sand dunes, new island, deltas, shore or recent lava flow), it is known as primary succession or prisere. The first group of organisms (plants or animals) which become established in such an area is termed the pioneer community.
- Secondary Succession or Subsere: When community development occurs in a secondarily bare area from where community was removed and where nutrients and conditions for existence are already favourable, e.g., cutover forest, abandoned cropland and ploughed field, it is termed secondary succession. The destruction of previous community can occur due to forest cutting, forest fire, submergence due to floods, severe drought for several successive years, landslide or earthquake, heavy overgrazing or harvested crop fields left uncultivated for several years.
- The secondary succession is usually more rapid because some seeds and underground parts of plants are already present which quickly give rise to a new community as soon as conditions become favourable. The previously occupied territory is more receptive to biotic community development.
EVENTS IN THE PROCESS OF BIOTIC SUCCESSION
The process of succession always starts on a bare land and involves following sequential events:
- Migration: It is transport of spores, seeds or other structures of propagation to the bare area
- Ecesis: It is successful germination of propagules into the bare area. The germination of seeds and spores produce new seedlings or new plants. As a result, some individuals of species are established in the bare area.
- Colonization and Aggregation: In this stage, the plants that had established themselves multiply and form colonies or aggregate into small or large groups.
- Competition: When number of these individuals increases many-fold in a given area, they enter into intraspecific and interspecific struggle. This results in the elimination of unsuitable or weak plants and animals. They start showing various types of interactions.
- Invasion: New varieties of plants and animals migrate into this area and estabhsthemselves in spaces created due to elimination of weaker plants. This is called invasion.
- Reaction: New migrants interact with the existing plants. This is called reaction. This leads to a changed environment. The changed environment becomes unsuitable for the existing community. Under environmental pressure, the community also changes or gets replaced by better adjusted species.
- Stabilisation: This is the final stage of biotic succession. The terminal community becomes more or less stabilised in the environment and remains in equilibrium with climatic and other environmental conditions of that area. The stabiised community is called climax community.