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Biological communities


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A community is a complex group of individuals interacting and sharing an environment.
Communities can be characterized by their structure (the types and numbers of species present) and dynamics (how communities change over time).

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Biological communities

  1. 1. Definition Community characteristics – diversity, stratification, dominance, structure, periodicity, fluctuations, stability, interdependence, interactions and succession. Biotic community – salient features, types. Concept of biocoenosis. Community concept, types and hypotheses. Community succession. Important species(Foundation, keystone, invasive) and concepts (ecotone, edge effect, Niche). summary
  2. 2. Definition -1 • Communities are assemblages of large number of species that occur together in space and time and all interact directly or indirectly with each other. Definition -2 • A community is defined as groups of different organisms living together in a particular space Definition -3 • A biotic community comprises plants and animals present in a particular area. • E.g. microbial community, plant community, animal community
  3. 3.      plants and animals belonging to different taxonomic groups are present in a community. The abundance of populations and number of species varies greatly in a community. Species richness = no. of species in a community. Species evenness = the distribution of individuals among the species. Species diversity is the product of richness and evenness.
  4. 4. All communities have a physical structure and biological pattern.  Stratification is a layering or series of separations that differently distributes the organisms occurring in an ecosystem.  Stratifications can be described in terms of separations between organisms in space (vertical or horizontal) or  In terms of separations between organisms in time (periodicity). 
  5. 5. An ecosystem can be stratified in space either vertically (layers) or horizontally (concentric circles).  Stratification of terrestrial communities reflects the life forms of plants.  Stratification results in economy of space and per unit area of the land surface.  Organisms in a community display daily, lunar, seasonal or irregular patterns that separate them in time. 
  6. 6. Tropical rain forest Coniferous forest Deciduous forest Thorn forest Thorn scrub Tall-grass Short-grass prairie prairie Desert scrub
  7. 7. All the organisms in a community are not equally important in determining the nature of that community. Only a few species generally exert the major controlling influence on the community due to their size, number or activities. These groups largely control the energy flow, which are known as ecological dominants. In pasture grass and cattle are ecological dominants.
  8. 8. A community is a composite biotic unit. The individual populations of different species depend on each other and on the abiotic environment for their food, shelter and reproduction. A community consists of mainly 3 groups. Producers= autotrophic organisms Consumers= herbivores and carnivores Decomposers= microbes
  9. 9.  The rhythmic changes occur in the activities of organisms. 1. Seasonal periodicity – temperature, rainfall and photoperiod determine the reproductive cycle of its inhabitants. 2. Dial periodicity – animals of terrestrial community active during the day and inactive during night.
  10. 10. Even within permanent and stable community, fluctuations in species abundance and numbers occur due to the interplay of biotic potential and environmental resistance.  It may be regular or cyclic.  E.g. host-parasite or predator- prey 
  11. 11.      Community stability is the degree of fluctuations in the size of the populations comprising the community. Environmental stability is the fluctuations in the abiotic factors of the ecosystem. Both the types of stability are closely related. The structure of community has a major role in determining the degree of environmental variability. For e.g. the organisms of a forest floor is protected from winds, evaporation and temperature variations by the thick forest canopy.
  12. 12. A community is a self-sustained unit.  The interdependence among different species of a community is found for food, reproduction and protection.  Most communities are interdependent, or share a mutual responsibility for group survival. 
  13. 13.  Competition occurs when organisms of the same or different species attempt to use an ecological resource(water, nutrients, light, food or space) in the same place at same time.  Predator –prey interactions  Symbiosis- different species rely on each other :3 kinds. – Mutualism- both partners benefit. – Commensalism- one partner benefits and the other is unaffected. – Parasitism- One benefits the other is harmed.
  14. 14.  Trophic structures are the feeding relationships within communities i.e. who's eating whom  The first trophic level is made up of the primary producers, the organisms that obtain the energy from inorganic sources.  Primary producers are photosynthetic organisms; more generally, primary producers are autotrophs (i.e., they fix CO2)  Consumers are the heterotrophs, i.e., organisms that obtain their carbon from other organisms.
  15. 15. It is the development of a community.  It is a directional change in the community structure as a function of time.  There two fundamental types of succession: primary and secondary. Primary succession is the series of community changes which occur on an entirely new habitat.  Secondary succession is the series of community changes which take place on a previously colonized, but disturbed or damaged habitat. 
  16. 16.  There is a balanced assemblage of autotrophic plants and heterotrophic animals in the trophic structure of a community.
  17. 17. A community is an aggregate of organisms, which form a distinct ecological unit. 2. The size of community unit may be large (forest community) or small (community of invertebrates). 3. Different community occurs in different habitats. 4. The composition and character of a community is an indicator of the type of environment that is present. 1.
  18. 18. 5.The structure of a community is dependent on: 1) the number of species.2) the relative number individuals in each species. 6.Species diversity indices give quantitative expression of community structure. 7.Communities are constantly changing. The replacement of one community by another is called community succession. The final stage in community succession is a climax community. On the whole a community is considered as a highly integrated self-contained organic unit.
  19. 19. Minor communities Major communities • Are those communities more or less dependent on neighboring communities for energy. • They are secondary aggregations within a major community. • Are those communities of sufficient size and completeness of organization. • They are relatively independent of resources from adjacent communities. • They receive only solar energy from the sun.
  20. 20. A biocoenosis is a biotic community, coined by Karl Mobius in 1877.  This concept defines community as the interacting organisms living together in a habitat(biotope).   Zoocoenosis refers to faunal community  Phytocoenosis refers to floral community  Microbiocoenosis refers to microbial community.
  21. 21. Community concept is one of the most important principles in Ecology.  This concept defines community as diverse organisms usually live together in an orderly manner.  Victor E. Shelford has defined “the community as an assemblage of populations living in a prescribed area or physical habitat with a definite trophic organization and metabolic pattern”. 
  22. 22.    The individualistic hypothesis was proposed by H.A. Gleason( American Ecologist),1917. This hypothesis depicted a community as a chance assemblage of species found in an area because they have similar abiotic requirements. In other words ―vegetation is continuously variable in response to a continuously varying environment‖.
  23. 23. The interactive hypothesis was proposed by F.E. Clements in 1916.  According to him, each community is an assemblage of closely linked species having mandatory biotic interactions that cause the community to function as an integrated unit. 
  24. 24. Communities are stable, integrated, and orderly entities.  a. Biologically they have a highlypredictable composition.  b. The diversity and abundance of a species in a particular community will be the same before and after a disturbance.  c. Communities develop by passing through a series of predictable stages, culminating in a stable climax community. 
  25. 25. Communities are neither stable nor predictable.  a. Plant and animal communities are ephemeral associations of species that just happen to share similar climatic requirements.  b. It is largely a matter of chance whether a similar community develops in the same area after a disturbance occurs. 
  26. 26. The number of species  The relative abundance of those species  The types of species  The number, types and strength of interactions among species (i.e., the structure of food webs) 
  27. 27.  1.Tropical        rain forests They found at equator. Temperatures and rainfall are high and variation is low. Plants grow all year long. High productivity, and high amounts of aboveground biomass. Highest amounts of BIODIVERSITY on Earth. Extraordinary structural diversity. A multilayered tree canopy is intermingled with vines, epiphytes, shrubs and herbs.
  28. 28.  2.Temperate      Forests Precipitation is high and relatively constant throughout the year. This abundance of moisture allows trees to dominate the landscape. Plants experience a seasonal period of dormancy. Productivity higher than deserts or grasslands, lower than tropical forests. Biodiversity is moderate.
  29. 29.  3. Grasslands These areas are also called prairies or steppes.  Precipitation conditions are quite dry.  Temperatures are moderate but highly SEASONAL.  Plant growth occurs in wet and warm months  Grasslands can develop in forested regions if recurring fires burn out  Productivity is lower than in forests.  Grassland soil is highly fertile. 
  30. 30.  4.Deserts Low annual precipitation  Low productivity  Individual plants are widely spaced due to the intense competition for water.  Desert species must cope with extreme temperatures and aridity. 
  31. 31.   Pond community – there may be populations of aquatic plants, herbivorous organisms, carnivores and bacteria at the bottom. They are interdependent for food supplies and form a specific group.
  32. 32.       Species diversity is a key feature of biological communities. Species diversity affects the stability and productivity of communities. A latitudinal gradient of species diversity exists for many taxa. Species diversity declines as latitude increases. High diversity in the tropics leads to high productivity. Diversity is positively correlated with stability.
  33. 33. Each community type experiences a characteristic type, frequency, and severity of disturbance. This is known as a community’s disturbance regime.  Disturbance patterns profoundly affect the composition of communities.  Examples—Fires in boreal forests and tree falls in temperate and tropical forests. 
  34. 34. Definition •Community succession is defined as a gradual, continuous and unidirectional change in the species composition of a natural community over a long time.
  35. 35. All ecosystems change over time  There are two types of ecosystem changes   Nondirectional directional changes-ReplacementFluctuations  Directional changes-Biogeographic change- Succession  All these types of changes may occur simultaneously within a single ecosystem
  36. 36. Primary Community succession succession Secondary succession It takes place in barren areas It takes place in disturbed areas
  37. 37. Early stages Middle stages Final stages • Low biomass • Low nutrient levels • Low Species diversity • Short food chain • High productivity • Rich biomass • Rich organic nutrients • More species diversity • Stable high biomass • High organic nutrients & low productivity • Complex food chain • High species diversity &competition
  38. 38. Pioneer Community/ Sere / intervening First community communities Climax community
  39. 39. Forest Succession Pioneer Comm. Climax Comm.
  40. 40. Increase in Species composition And species diversity Increase in species density And heterogeneity Increase in Organic nutrients Metabolic stability in Community metabolism
  41. 41. Foundation species are considered the ―base‖ of a community, having the greatest influence on its overall structure.  They are usually the primary producers:  Foundation species may physically modify the environment to produce and maintain habitats that benefit the other organisms that use them.  E.g. the photosynthetic corals of the coral reef. 
  42. 42.      A species whose presence is key to maintaining biodiversity within an ecosystem and to upholding an ecological community’s structure. The species may be a strong interactor or a weak interactor. Strong interactors are fewer in number and critical to the ecosystem function. Elimination of a keystone species may result in the loss of many other species in a community. E.g. beavers of Canada, Bengal tiger of India, African elephants.
  43. 43. Invasive species are non-indigenous species whose introduction can cause harm to the community and the environment.  Invasive species are often better competitors than native species, resulting in population explosions.  These new species usually overtake the native populations, driving them to localized extinctions. 
  44. 44. A niche is the role of an organism in a community (Charles Elton 1927)- functional niche = functional status of an organism in its community.  Joseph Grinnell (1917)referred niche as an organism’s physical environment- spatial niche  Niche is a property of a biotic community.  Niches includes all the physical, chemical and biological conditions.  The niche concept was formulated to study the role of organisms in community. 
  45. 45. An edge is where two or more different vegetational communities meet. e.g. forestgrass land.  An ecotone is where two or more communities not only meet but also intergrade.  Edges result from abrupt changes in soil type, topographic differences, geomorphic differences and microclimatic changes.  Ecotones arise from the blending of two or more vegetational types. 
  46. 46. A community is a complex group of individuals interacting and sharing an environment.  Communities can be characterized by their structure (the types and numbers of species present) and dynamics (how communities change over time). 
  47. 47.      Dr. B. Victor is a highly experienced postgraduate professor, retired from the reputed educational institution - St. Xavier’ s College(Autonomous), Palayamkottai, India-627001. He was the dean of sciences, assistant controller of examinations and coordinator several academic research workshops. He has more than 32 years of teaching and research experience He has taught a diversity of courses and published 45 research articles in reputed national and international journals. Send your comments to :