biome distribution
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ORGANISMS AND POPULATIONS
  • ORGANISM AND ITS ENVIRONMENT
    • the rotation of our planet around the Sun and the tilt of its axis cause annual variations in the intensity and duration of temperature, resulting in distinct seasons
    • Regional and local variations within each biome lead to the formation of a wide variety of habitats.
    • key elements that lead to so much variation in the physical and chemical conditions of different habitats
      • temperature, water , light , soil.
  • Major Abiotic Factors
  • Temperature:
    • Temperature is the most ecologically relevant environmental factor
    • the average temperature on land varies seasonally, decreases progressively from the equator towards the poles and from plains to the mountain tops.
    • There are, however, unique habitats such as thermal springs and deep-sea hydrothermal vents where average temperatures exceed 1000 C.
    • A few organisms can tolerate and thrive in a wide range of temperatures (they are called eurythermal), but, a vast majority of them are restricted to a narrow range of temperatures (such organisms are called stenothermal)
  • Water:
    • For aquatic organisms the quality (chemical composition, pH) of water becomes important.
    • The salt concentration (measured as salinity in parts per thousand), is less than 5 per cent in inland waters, 30-35 per cent the sea and > 100 per cent in some hypersaline lagoons
    • Some organisms are tolerant of a wide range of salinities (euryhaline) but others are restricted to a narrow range (stenohaline).
  • Light:
    • Many plants are also dependent on sunlight to meet their photoperiodic requirement for flowering. For many animals too, light is important in that they use the diurnal and seasonal variations in light intensity and duration (photoperiod) as cues for timing their foraging, reproductive and migratory activities.
    • The UV component of the spectrum is harmful to many organisms while not all the colour components of the visible spectrum are available for marine plants living at different depths of the ocean
  • Soil:
    • The nature and properties of soil in different places vary; it is dependent on the climate, the weathering process, whether soil is transported or sedimentary and how soil development occurred.
    • the organism should try to maintain the constancy of its internal environment (a process called homeostasis) despite varying external environmental conditions that tend to upset its homeostasis.
  • Distribution of biomes
  • Regulate: Some organisms are able to maintain homeostasis by physiological (sometimes behavioural also) means which ensures constant body temperature, constant osmotic concentration, etc. All birds and mammals, and a very few lower vertebrate and invertebrate species are indeed capable of such regulation (thermoregulation and osmoregulation)
    • ‘success’ of mammals is largely due to their ability to maintain a constant body temperature and thrive whether they live in Antarctica or in the Sahara desert
  • Plants, on the other hand, do not have such mechanisms to maintain internal temperatures
  • Conform: An overwhelming majority (99 per cent) of animals and nearly all plants cannot maintain a constant internal environment.
    • In aquatic animals, the osmotic concentration of the body fluids change with that of the ambient water osmotic concentration.
    • These animals and plants are simply conformers
  • Thermoregulation is energetically expensive for many organisms. This is particularly true for small animals like shrews and humming birds. Heat loss or heat gain is a function of surface area. Since small animals have a larger surface area relative to their volume, they tend to lose body heat very fast when it is cold outside; then they have to expend much energy to generate body heat through metabolism. This is the main reason why very small animals are rarely found in polar regions.
  • Migrate : The organism can move away temporarily from the stressful habitat to a more hospitable area and return when stressful period is over
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  • Every winter the famous Keolado National Park (Bhartpur) in Rajasthan host thousands of migratory birds coming from Siberia and other extremely cold northern regions
  • Suspend: In bacteria, fungi and lower plants, various kinds of thickwalled spores are formed which help them to survive unfavourable conditions – these germinate on availability of suitable environment
  • In animals, the organism, if unable to migrate, might avoid the stress by escaping in time. The familiar case of bears going into hibernation during winter is an example of escape in time. Some snails and fish go into aestivation to avoid summer–related problems-heat and desiccation
  • Under unfavourable conditions many zooplankton species in lakes and ponds are known to enter diapause, a stage of suspended development.
  • Adaptations
    • Many desert plants have a thick cuticle on their leaf surfaces and have their stomata arranged in deep pits to minimise water loss through transpiration.
    • They also have a special photosynthetic pathway (CAM) that enables their stomata to remain closed during day time.
    • Some desert plants like Opuntia, have no leaves – they are reduced to spines–and the photosynthetic function is taken over by the flattened stems
    • Mammals from colder climates generally have shorter ears and limbs to minimise heat loss. (This is called the Allen’s Rule.) In the polar seas aquatic mammals like seals have a thick layer of fat (blubber) below their skin that acts as an insulator and reduces loss of body heat.
    • Some organisms possess adaptations that are physiological which allow them to respond quickly to a stressful situation.
  • Example: If you had ever been to any high altitude place (>3,500m Rohtang Pass near Manali and Mansarovar, in China occupied Tibet) you must have experienced what is called altitude sickness. Its symptoms include nausea, fatigue and heart palpitations. This is because in the low atmospheric pressure of high altitudes, the body does not get enough oxygen. But, gradually you get acclimatised and stop experiencing altitude sickness. How did your body solve this problem? The body compensates low oxygen availability by increasing red blood cell production, decreasing the binding capacity of hemoglobin and by increasing breathing rate.
  • Desert lizards lack the physiological ability that mammals have to deal with the high temperatures of their habitat, but manage to keep their body temperature fairly constant by behavioural means. They bask in the sun and absorb heat when their body temperature drops below the comfort zone, but move into shade when the ambient temperature starts increasing.
  • POPULATIONS
  • Population Attributes
    • In nature, we rarely find isolated, single individuals of any species; majority of them live in groups in a well defined geographical area, share or compete for similar resources, potentially interbreed and thus constitute a population
    • Both the species benefit in mutualism and both lose in competition in their interactions with each other. In both parasitism and Predation only one species benefits (parasite and predator, respectively) and the interaction
    • interaction where one species is benefitted and the other is neither benefitted nor harmed is called commensalism
    • In amensalism, on the other hand, one species is harmed whereas the other is unaffected.
  • Besides acting as ‘conduits’ for energy transfer across trophic levels, predators play other important roles. They keep prey populations under control. But for predators, prey species could achieve very high population densities and cause ecosystem instability.
  • Biological control methods adopted in agricultural pest control are based on the ability of the predator to regulate prey population
  • The Monarch butterfly is highly distasteful to its predator (bird) because of a special chemical present in its body. Interestingly, the butterfly acquires this chemical during its caterpillar stage by feeding on a poisonous weed
  • Competition:
    • Firstly, totally unrelated species could also compete for the same resource. For instance, in some shallow South American lakes visiting flamingoes and resident fishes compete for their common food, the zooplankton in the lake.
    • Secondly, resources need not be limiting for competition to occur; in interference competition, the feeding efficiency of one species might be reduced due to the interfering and inhibitory presence of the other species, even if resources (food and space) are abundant.
  • Parasitism
    • parasitic mode of life ensures free lodging and meals,
    • Many parasites have evolved to be host-specific (they can parasitise only a single species of host) in such a way that both host and the parasite tend to co-evolve;
    •  The human liver fluke (a trematode parasite) depends on two intermediate hosts (a snail and a fish) to complete its life cycle.
      • Parasites that feed on the external surface of the host organism are called ectoparasites
    •  Cuscuta, a parasitic plant that is commonly found growing on hedge plants, has lost its chlorophyll and leaves in the course of evolution. It derives its nutrition from the host plant which it parasitises.
    • In contrast, endoparasites are those that live inside the host body at different sites (liver, kidney, lungs, red blood cells, etc.). The life cycles of endoparasites are more complex because of their extreme specialisation.
    • Brood parasitism in birds is a fascinating example of parasitism in which the parasitic bird lays its eggs in the nest of its host and lets the host incubate them.
  • Commensalism:
    • This is the interaction in which one species benefits and the other is neither harmed nor benefited
    • An orchid growing as an epiphyte on a mango branch, and barnacles growing on the back of a whale benefit while neither the mango tree nor the whale derives any apparent benefit.
    •  cattle egret and grazing cattle in close association, a sight you are most likely to catch if you live in farmed rural areas, is a classic example of commensalism.
    • Another example of commensalism is the interaction between sea anemone that has stinging tentacles and the clown fish that lives among them. The fish gets protection from predators which stay away from the stinging tentacles. The anemone does not appear to derive any benefit by hosting the clown fish.
  • Mutualism
    • This interaction confers benefits on both the interacting species.
    • Lichens represent an intimate mutualistic relationship between a fungus and photosynthesising algae or cyanobacteria.
    • mycorrhizae are associations between fungi and the roots of higher plants
    • Some of the most fascinating cases of mutualism in nature are seen in plant-pollinator interactions.
ECOSYSTEM
  • An ecosystem can be visualised as a functional unit of nature, where living organisms interact among themselves and also with the surrounding physical environment.
  • Ecosystem varies greatly in size from a small pond to a large forest or a sea
  • ECOSYSTEM – STRUCTURE AND FUNCTION
    • Interaction of biotic and abiotic components result in a physical structure that is characteristic for each type of ecosystem.
    • Vertical distribution of different species occupying different levels is called stratification. For example, trees occupy top vertical strata or layer of a forest, shrubs the second and herbs and grasses occupy the bottom layers.
    • There is a unidirectional movement of energy towards the higher trophic levels and its dissipation and loss as heat to the environment.
  • PRODUCTIVITY
    • A constant input of solar energy is the basic requirement for any ecosystem to function and sustain
  •  Primary production is defined as the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis.
  • The rate of biomass production is called productivity
  •  Gross primary productivity of an ecosystem is the rate of production of organic matter during photosynthesis. A considerable amount of GPP is utilised by plants in respiration. Gross primary productivity minus respiration losses (R), is the net primary productivity (NPP).
  • Net primary productivity is the available biomass for the consumption to heterotrophs (herbivores and decomposers)
  • Secondary productivity is defined as the rate of formation of new organic matter by consumers.  
  • DECOMPOSITION
    • decomposers break down complex organic matter into inorganic substances like carbon dioxide, water and nutrients and the process is called decomposition.
    • Dead plant remains such as leaves, bark, flowers and dead remains of animals, including fecal matter, constitute detritus, which is the raw material for decomposition.
    • humus that is highly resistant to microbial action and undergoes decomposition at an extremely slow rate.
    • Being colloidal in nature it serves as a reservoir of nutrients. The humus is further degraded by some microbes and release of inorganic nutrients occur by the process known as mineralisation.
    • Temperature and soil moisture are the most important climatic factors that regulate decomposition through their effects on the activities of soil microbes. Warm and moist environment favour decomposition whereas low temperature and anaerobiosis inhibit decomposition resulting in build up of organic materials.
  • ENERGY FLOW
    • Except for the deep sea hydrothermal ecosystem, the sun is the only source of energy for all ecosystems on Earth.
    • Of the incident solar radiation, less than 50 percent of it is photosynthetically active radiation (PAR).
    • ecosystems are not exempt from the Second Law of thermodynamics. They need a constant supply of energy to synthesise the molecules they require, to counteract the universal tendency toward increasing disorderliness.
    • No energy that is trapped into an organism remains in it for ever. The energy trapped by the producer, hence, is either passed on to a consumer or the organism dies. Death of organism is the beginning of the detritus food chain/web.
    • The consumers that feed on these herbivores are carnivores, or more correctly primary carnivores (though secondary consumers). Those animals that depend on the primary carnivores for food are labelled secondary carnivores.
    • The detritus food chain (DFC) begins with the dead organic matter. It is made up of decomposers which are heterotrophic organisms, mainly fungi and bacteria. They meet their energy and nutrient requirements by degrading dead organic matter or detritus. These are also known as saprotrophs
    • In an aquatic ecosystem, GFC is the major conduit for energy flow. As against this, in a terrestrial ecosystem, a much larger fraction of energy flows through the detritus food chain than through the GFC.
    • Organisms occupy a place in the natural surroundings or in a community according to their feeding relationship with other organisms. Based on the source of their nutrition or food, organisms occupy a specific place in the food chain that is known as their trophic level.
    • Each trophic level has a certain mass of living material at a particular time called as the standing crop. The standing crop is measured as the mass of living organisms (biomass) or the number in a unit area
    • The number of trophic levels in the grazing food chain is restricted as the transfer of energy follows 10 percent law – only 10 percent of the energy is transferred to each trophic level from the lower trophic level
    • There is no limitation in detritus food chain for the number of trophic levels because 10% rule is not applicable, and has a high magnitude of energy also.
    • In detritus food chain the energy flow remains as a continuous passage rather than as a stepwise flow between discrete entities. The organisms in the detritus food chain are many and include algae, fungi, bacteria, slime moulds, actinomycetes, protozoa, etc. Detritus organisms ingest pieces of partially decomposed organic matter, digest them partially and after extracting some of the chemical energy in the food to run their metabolism, excrete the remainder in the form of simpler organic molecules.Therefore, it doesn’t follow a stepwise 10% law.
  • ECOLOGICAL PYRAMIDS
    • the trophic level represents a functional level, not a species as such. A given species may occupy more than one trophic level in the same ecosystem at the same time;
    • In most ecosystems, all the pyramids, of number, of energy and biomass are upright, i.e., producers are more in number and biomass than the herbivores, and herbivores are more in number and biomass than the carnivores. Also, energy at a lower trophic level is always more than at a higher level.
    • Pyramid of energy is always upright, can never be inverted, because when energy flows from a particular trophic level to the next trophic level, some energy is always lost as heat at each step. Each bar in the energy pyramid indicates the amount of energy present at each trophic level in a given time or annually per unit area
    • However, there are certain limitations of ecological pyramids such as it does not take into account the same species belonging to two or more trophic levels. It assumes a simple food chain, something that almost never exists in nature; it does not accommodate a food web. Moreover, saprophytes are not given any place in ecological pyramids even though they play a vital role in the ecosystem.
  • ECOLOGICAL SUCCESSION
    • An important characteristic of all communities is that composition and structure constantly change in response to the changing environmental conditions.
    •  These changes lead finally to a community that is in near equilibrium with the environment and that is called a climax community
    •  The gradual and fairly predictable change in the species composition of a given area is called ecological succession. During succession, some species colonise an area and their populations become more numerous, whereas populations of other species decline and even disappear.
    • The entire sequence of communities that successively change in a given area are called sere(s)
    •  The individual transitional communities are termed seral stages or seral communities
    • In the successive seral stages, there is a change in the diversity of species of organisms, increase in the number of species and organisms as well as an increase in the total biomass.
    • Succession is hence a process that starts where no living organisms are there – these could be areas where no living organisms ever existed, say bare rock; or in areas that somehow, lost all the living organisms that existed there. The former is called primary succession, while the latter is termed secondary succession.
    • Examples of areas where primary succession occurs are newly cooled lava, bare rock, newly created pond or reservoir
    • Before a biotic community of diverse organisms can become established, there must be soil. Depending mostly on the climate, it takes natural processes several hundred to several thousand years to produce fertile soil on bare rock.
    • Secondary succession begins in areas where natural biotic communities have been destroyed such as in abandoned farm lands, burned or cut forests, lands that have been flooded.
    • Since some soil or sediment is present, succession is faster than primary succession.
    • At any time during primary or secondary succession, natural or human induced disturbances (fire, deforestation, etc.), can convert a particular seral stage of succession to an earlier stage. Also such disturbances create new conditions that encourage some species and discourage or eliminate other species.
  • Succession of Plants
    • Hydrarch succession takes place in wetter areas and the successional series progress from hydric to the mesic conditions.
    • xerarch succession takes place in dry areas and the series progress from xeric to mesic conditions. Hence, both hydrarch and xerach successions lead to medium water conditions (mesic) – neither too dry (xeric) nor too wet (hydric).
    • The species that invade a bare area are called pioneer species.
    • The climax community remains stable as long as the environment remains unchanged.
  • NUTRIENT CYCLING
    • The amount of nutrients, such as carbon, nitrogen, phosphorus, calcium, etc., present in the soil at any given time, is referred to as the standing state.
    • The movement of nutrient elements through the various components of an ecosystem is called nutrient cycling. Another name of nutrient cycling is biogeochemical cycles
    • Nutrient cycles are of two types: (a) gaseous and (b) sedimentary.
    • reservoir for the gaseous type of nutrient cycle (e.g., nitrogen, carbon cycle) exists in the atmosphere and for the sedimentary cycle (e.g., sulphur and phosphorus cycle), the reservoir is located in Earth’s crust
    •  Environmental factors, e.g., soil, moisture, pH, temperature etc., regulate the rate of release of nutrients into the atmosphere.
  • Ecosystem – Carbon Cycle
    •  the composition of living organisms, carbon constitutes 49 per cent of dry weight of organisms and is next only to water
    •  71 per cent carbon is found dissolved in oceans. This oceanic reservoir regulates the amount of carbon dioxide in the atmosphere
    • atmosphere only contains about 1per cent of total global carbon
    • Fossil fuel also represent a reservoir of carbon. Carbon cycling occurs through atmosphere, ocean and through living and dead organisms.
    • Fossil fuel also represent a reservoir of carbon. Carbon cycling occurs through atmosphere, ocean and through living and dead organisms.
    • Some amount of the fixed carbon is lost to sediments and removed from circulation. Burning of wood, forest fire and combustion of organic matter, fossil fuel, volcanic activity are additional sources for releasing CO2 in the atmosphere
    • Human activities have significantly influenced the carbon cycle. Rapid deforestation and massive burning of fossil fuel for energy and transport have significantly increased the rate of release of carbon dioxide into the atmosphere
  • Ecosystem – Phosphorus Cycle
    • Phosphorus is a major constituent of biological membranes, nucleic acids and cellular energy transfer systems.
    •  The natural reservoir of phosphorus is rock, which contains phosphorus in the form of phosphates. When rocks are weathered, minute amounts of these phosphates dissolve in soil solution and are absorbed by the roots of the plants
    • Herbivores and other animals obtain this element from plants. The waste products and the dead organisms are decomposed by phosphate-solubilising bacteria releasing phosphorus. Unlike carbon cycle, there is no respiratory release of phosphorus into atmosphere.
    • The other two major and important differences between carbon and phosphorus cycle are firstly, atmospheric inputs of phosphorus through rainfall are much smaller than carbon inputs,
    • secondly, gaseous exchanges of phosphorus between organism and environment are negligible.

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