1. Ecology
n It is a branch of science which deals with the interactions among organisms and between the organism and its physical (abiotic) environment.
2. Organisational Levels of Ecology
n Organism: Living component of the environment at individual level is called organism.
n Ecology at the organismic level is physiological ecology which reveals how different organisms are adapted to their environments. The organism is the smallest level of ecological hierarchy.
n Population: Population is defined as the sum total of all individuals of a species in a specific geographical area.
n Species: The species are the group of individuals of one or more populations which resemble each other and can interbreed among themselves.
n Biotic community: The assemblage of all the populations of different species present in an area that interact among themselves are called biotic community. It is of three types:
(i) Plant community
(ii) Animal community
(iii) Microbial community
n Ecosystem: The sum total of the biotic (living) and abiotic (non-living) components of a particular geographical area, being integrated through exchange of energy and recycling of nutrients are collectively called ecosystem.
n Biome: The large unit of environment consisting of a major vegetation type and its associated fauna in a specific climatic zone is called a biome.
n Biosphere: All the ecosystems of the world are collectively called biosphere.
n Niche: The ecological niche of an organism represents the range of conditions that it can tolerate, the resources it utilises and its functional role in the ecological system. Each species occupies a distinct niche and no two species occupy the same niche.
3. Environment
n Environment is referred to as the sum total of all the physical and biotic conditions which influence the organism in terms of survival and reproduction.
n Different seasons result due to
(i) rotation of earth around the sun.
(ii) tilting of the earth on its axis.
n The major biomes of the world include desert, grassland, rainforest and tundra.
n Formation of different biomes is due to
(i) annual variations in intensity and duration of temperature.
(ii) annual variations in precipitation.
n The above annual variations together with annual variation in precipitation (remember precipitation include both rain and snow) thus form major biomes.
n The biomes may be desert, rainforest and tundra.
n Regional and local variations within each biome lead to the formation of different habitats.
4. Components of Ecosystem
5. Major Abiotic Factors
(i) Temperature
n It is the most ecologically relevant environmental factor.
n It is observed that seasonally, the average temperature on land varies.
n The temperature decreases progressively from the equator to the poles and from plains to the mountain top.
n The range of temperature varies from subzero levels in polar areas to >50°C at high altitude in tropical deserts in summer.
n The temperature can affect the kinetics of enzymes and through it the basal metabolism and other physiological functions of the organisms.
n The organism tolerating the high range of temperature is called eurythermal e.g., cyclops, Artemisia, Toad lizard and the organism which can tolerate narrow range of temperature is called stenothermal e.g., palms, corals, snakes, some fishes.
n Level of thermal tolerance of different species determine their geographical distribution.
(ii) Water
n It is the next important factor as life is unsustainable without water.
n The amount of water in an environment determines the productivity and distribution of plants.
n For aquatic habitat, the quality of water becomes important like pH value, salinity and temperature of water.
n The organisms tolerating wide range of salinities are called euryhaline e.g., migratory fish like Hilsa, Salmon and the organisms that tolerate only narrow range of salinities are called stenohaline e.g., number of organism.
n Fresh water forms cannot live in sea water for long because of osmotic problems.
(iii) Light
n Light is important because autotrophs make food with the help of light (photosynthesis) and O2 is evolved during this process.
n The small plants like herbs and shrubs can perform photosynthesis under very low light conditions as they are overshadowed by tall trees.
n The plants depend on sunlight to meet their photoperiodic requirement for flowering.
n For many animals, 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.
n In deep sea, animals have special devices for life as many are luminescent like Angler fish.
(iv) Soil
n The nature and properties of soil varies with different places.
n The nature and properties of soil depend on the climate and weathering process.
n The characteristics of soil: soil-composition, grain size and aggregation, determine the percolation and water holding capacity of the soil.
n The vegetation in an area is determined by some soil parameters like pH, mineral composition and topography.
6. Responses to Abiotic Factors
n During the course of million years of existence, many species would have evolved a relatively constant internal (within the body) environment that permits all biochemical reactions and physiological functions to proceed with maximum efficiency and thus, enhances the overall fitness of the species.
n The organisms try to maintain the constancy of its internal environment (a process called homeostasis) despite varying external environmental conditions that tend to upset its homeostasis.
7. How do Living Organisms Cope with Environment?
(i) Regulate
n Some organisms maintain homeostasis by physiological and behavioural means, such organisms are called regulators. All birds and mammals and few lower vertebrate and invertebrate species maintain homeostasis by thermoregulation and osmoregulation.
n The success of mammals is largely due to their ability to maintain a constant body temperature.
n In summers, when outside temperature is more than our body temperature, we sweat profusely and the resulting evaporative cooling brings down the body temperature.
n In winters, when temperature is lower we shiver, a kind of exercise that produces heat and raises the body temperature.
n Plants do not have such mechanism to maintain internal temperatures.
(ii) Conform
n Majority (99%) of animals and nearly all plants cannot maintain a constant internal environment. Their body temperature is determined by ambient temperature.
n The osmotic concentration of the body fluids change with that of the ambient water osmotic concentration, such animals and plants are simply called conformers.
n Loss or gain of heat is a function of surface area. The small animals have larger surface area relative to their volume. They lose body heat very fast in low temperature. So, they expend energy to generate body heat through metabolism for adjusting. Therefore, very small animals like shrews and humming birds are rarely found in polar regions.
n During evolution, some species have evolved the ability to regulate but only over a limited range of environment conditions and beyond that limit they conform.
(iii) Migration
n The temporary movement of organisms from the stressful habitat to a more hospitable area and return when favourable conditions reappear, is called migration.
n The long distance migration is very common in birds. In winter, famous Keoladeo National Park (Bharatpur) in Rajasthan hosts thousands of migratory birds coming from Siberia and other extremely cold northern regions.
n Examples of migratory animals, are Siberian crane, Whale, Caribou, Lamprey, Eel, Salmon.
(iv) Suspend
n Some bacteria, fungi and lower plants, under unfavourable conditions slow down metabolic rate and form a thick-walled spore to overcome stressful conditions. These spores germinate under onset of suitable environment.
n The animals that fail to migrate, might avoid the stress by escaping in time, e.g., bear frog goes into hibernation during winter.
n Snail and fish go into aestivation to avoid summer.
n Zooplanktons under unfavourable conditions enter diapause, a stage of suspended development.
n Cyst formation in Amoeba.
8. Adaptations
n Any morphological, physiological and behavioural attribute of the organism that enables it to survive and reproduce in its habitat is called adaptation.
n Over a long period of time, many adaptations have evolved and are incorporated in the gene, thus becoming heritable.
(i) Adaptation in Kangaroo rat (Dipodomys merriami)
n The Kangaroo rat in North American deserts is capable to meet its internal water requirement by oxidation of fat where water is a by-product.
n It has the ability to concentrate its urine for minimum loss of water through excretory products.
n Prevents water loss by living in burrows during day.
n Solidification of faeces.
n Nasal counter current mechanism to retrieve moisture from air being exhaled.
(ii) Adaptation in desert plants
n Desert plants have thick waxy coating on leaves called cuticle, for minimum loss of water through transpiration.
n They have special photosynthetic pathway (CAM) that enables minimum loss of water during daytime because stomata remain closed.
n Some desert plants, e.g., Opuntia develop spines instead of leaves and photosynthetic function is carried out by the flattened stem.
n Stomata are arranged in deep pits to minimise loss through transpiration.
(iii) Adaptation in mammals in cold climate
n Mammals have shorter ears and limbs to minimise heat loss. This is called Allen’s rule.
n Seals (aquatic mammals) have a thick layer of fat (blubber) below their skin that acts as an insulator and reduces excessive loss of body heat.
(iv) Adaptation in desert lizards
n They absorb heat from the sun when the body temperature drops below the comfort zone.
n They move into shade when ambient temperature rises above the comfort levels.
n Some burrow into soil to escape above ground heat.
(v) Adaptation at high altitude in humans
n People at high altitudes (> 3,500 m like in Rohtang Pass, near Manali, Mansarovar in China occupied Tibet) experience altitude sickness.
n Symptoms: Nausea, fatigue, heart palpitations.
n Cause: The people living in high altitudes compensate low oxygen by increasing production of red blood cells (RBCs).
n The binding capacity of haemoglobin decreases and breathing rate increases.
n People travelling to high altitude get slowly acclimatized (adjust) and stop experiencing altitude sickness by:
(a) Increasing RBC production
(b) Decreasing Binding capacity of hemoglobin.
(c) Increasing breathing rate.
n People living at high altitudes of Himalayas have higher RBC count or total Hb than people living in plains.
(vi) Biochemical Adaptations
n Biochemical Adaptations in marine invertebrates and fish living at great depths in oceans where pressure is more than 100 times the normal atmospheric pressure.
(vii) Adaptations in fish found in Antarctic Waters (temperature below zero)
n These fishes show cold hardening i.e., physiological adaptation allowing animals to live comfortably in cold conditions. It is of two types:
(a) Freeze tolerance: Extracellular spaces contain ice nucleating proteins that form ice. Small amount of water is withdrawn from cells and thus cells have high solute concentration that protects them from freezing.
(b) Freeze avoiding animals: Body fluids of these animals contain antifreeze solutes like glycerol, antifreeze proteins which lower the freezing point of body fluids below 0°C and thus ice formation is prevented and animals remain active, e.g., Ice fish (Chaenocephalus).
(viii) Adaptations in Archaebacteria
n Most animals have metabolism and physiology functioning optimally in narrow temperature range (37°C for human beings) but Archaebacteria flourish in hot springs and deep sea hydrothermal vents where temperature exceeds 100°C because they have special enzymes and plasma membrane constituents that help them to metabolise comfortably at high temperature.
9. Population Attributes
n Population is defined as the total number of individuals of a species in a specific geographical area, sharing/ competing for similar resources which can interbreed under natural conditions to produce fertile offsprings and function as a unit of biotic community.
n Population ecology links ecology to population genetics and evolution.
n Characteristics of a population:
(i) Population size or density of a species is the number of individuals of a species per unit area or volume
(ii) Birth or natality rate: It is expressed as the number of births per 1,000 individuals of a population per year.
(iii) Death or mortality rate: It is expressed as the number of deaths per 1,000 individuals of a population per year.
(iv) Sex ratio: It is expressed as the number of females per 1,000 males of a population in given time.
n A population at any given time is composed of individuals of different ages. When the age distribution (per cent individuals of a given age or age group) is plotted for the population, the resulting structure is called age pyramid.
n For human population, the age pyramids generally show age distribution of males and females in a combined diagram.
n The shape of the pyramids reflects the growth status of the population and is of three types:
(a) Expanding (Triangular shaped pyramid): Number of prereproductive individuals is very large, reproductive individuals moderate in no. and postreproductive are fewer. Population is growing and show rapid increases.
(b) Stable (Bell shaped pyramid): Population size remains stable, neither growing nor diminishing i.e., all the age group are evenly balanced.
(c) Declining (Urn shaped pyramid): Population is declining or diminishing population showing negative growth.
n The pyramids also indicate the ratio of pre-reproductive, reproductive and post-reproductive individuals in a population.
10. Population Growth
n The size of a population depends on food availability, predation pressure and weather. Therefore, size of the population is not a static parameter.
Population Density (N) = Number of individuals or % cover or biomass.
n The population density depends on few basic processes:
(i) Natality: It is the number of births during a given period of time. It increases the population density.
So birth rate =
(ii) Mortality: It is the number of deaths in a given time period. It decreases the population density. e.g., if 4 individuals out of 40 fruit flies died during specified time interval.
Death rate =
(iii) Immigration: It is the number of individuals of same species added to a habitat in a given time period. It increases the population density.
(iv) Emigration: It is the number of individuals of same species that move to a different habitat in a given time period. It decreases the population density.
n The population density is given by the following equation:
Nt = N0 + [(B + I) − (D + E)]
where Nt = population density at time t, B = birth rate, I = immigration, D = death rate, E = emigration, and N0 = population in the beginning.
n This equation shows that the population density will increase, if the number of births plus the number of immigrants (B+I) is more than the number of deaths plus the number of emigrants, i.e., (D+E), otherwise it will decrease.
11. Population Growth Models
n There are two models of population growth:
(i) The exponential growth
(ii) Logistic growth
(i) Exponential Growth
n The exponential or geometric growth is common where the resources (food + space) are unlimited.
n Each species has the ability to realise fully its innate potential to grow in number.
n The equation for exponential growth can be derived as follows:
Integral form of exponential growth equation is
Nt = N0ert
where, N = population size,
Nt = population density after time t,
N0 = population density at time zero,
r = intrinsic rate of natural increase,
e = the base of natural logarithms (2.71828),
b = birth rate (per capita births)
d = death rate (per capita death rates).
n ‘r’ is an important parameter assessing impacts of biotic and abiotic factors on population growth.‘r’ for flour beetle was 0.12, for Norway rat was 0.015 and for human population in India was 0.0205 in 1981.
n In exponential growth, when N in relation to time is plotted on graph, the curve becomes J shaped.
(ii) Logistic growth
n The resources become limited at certain point of time, so no population can grow exponentially.
n This growth model is more realistic.
n Every ecosystem or environment or habitat has limited resources to support a particular maximum number of individuals called its carrying capacity (K).
n When N is plotted in relation to time t, the logistic growth show sigmoid curve and is also called Verhulst–Pearl logistic growth. It is given by the following equation:
where N = population density at time t
r = intrinsic rate of natural increase
K = carrying capacity.
n Graph shows lag phase, followed by phases of acceleration and deceleration and finally an asymptote when population density reaches the carrying capacity.
12. Life History Variation
n Darwinian fitness refers to the populations where they evolve to maximise their reproductive fitness, i.e., high ‘r’ value.
n Under selection pressures, organisms evolve towards the most efficient reproductive strategy.
n The rate of breeding varies from species to species, as some organisms breed once in their lifetime (Pacific salmon fish, bamboo), while others breed many times during their lifetime (most birds and mammals).
n Some organisms produce a large number of small-sized offsprings (oysters, pelagic fishes), while others produce a small number of large-sized offsprings (birds, mammals).
n Ecologists suggest that life history traits of organisms have evolved in relation to the constraints, imposed by the abiotic and biotic components of the habitat, in which they live.
13. Population Interaction
n Interspecific interactions are interactions of populations of two different species.
n The interactions may be
l beneficial/positive effect indicated by +.
l harmful/detrimental/negative effect indicated by –.
l nveutral interaction/no effect on the species indicated by 0.
Table 13.1 Population Interactions
Species A | Species B | Name of Interaction |
+ | + | Mutualism |
– | – | Competition |
+ | – | Predation |
+ | – | Parasitism |
+ | 0 | Commensalism |
– | 0 | Amensalism |
(i) Predation
n It is an interspecific interaction, where an animal, called predator, kills and consumes the other weaker animal called prey.
n Predation is nature’s way of transferring energy to higher trophic levels, e.g., a tiger (predator) eating a deer (prey), a sparrow (predator) eating fruit or seed (prey), etc.
n The role of predators:
(a) Predators keep prey population under control. This is called biological control.
(b) Predators also help in maintaining species diversity in a community, by reducing the intensity of competition among prey species.
(c) Besides acting as ‘conduits’ for energy transfer across trophic levels, predators play other important roles. In absence of predator species, prey species could achieve very high population densities and lead to ecosystem instability.
n When certain exotic species are introduced into a geographical area, they become invasive and start spreading fast because the invaded land does not have its natural predators, e.g. Prickly pear cactus introduced in 1920’s into Australia created havoc by spreading to millions of hectares and thus was brought under control only after a cactus feeding predator (a moth) was introduced into the country from its natural habitat.
n If a predator is too efficient and over-exploits its prey, then the prey might become extinct and following it, the predator will also become extinct due to the lack of food.
n They also help in maintaining species diversity in a community by reducing intensity of competition among competing prey species, e.g., In rocky intertidal communities of American Pacific Coast, star fish Pisaster is important predator. In the beginning when all starfishes were removed from an intertidal area more than 10 species of invertebrates became extinct in a year because of interspecific competition.
n The prey defence mechanisms
(a) To avoid being detected easily by the predators, some species of insects and frogs are cryptically coloured (camouflaged).
(b) The Monarch butterfly is highly distasteful to its predator (birds) because of a special chemical present in its body which is acquired by the butterfly by feeding on a poisonous weed in its caterpillar stage.
(c) 25% of insects are phytophagous, i.e., feed on plant sap and other parts of plants. So, some plants have thorns or spines for defence mechanism, e.g., Acacia, cactus.
(d) Some plants produce highly poisonous chemicals like cardiac glycosides, nicotine, caffeine, quinine, strychnine, opium, etc., are produced by plants actually as defences against grazers and browsers e.g. , Calotropis grows in abandoned fields.
(ii) Competition
n Competition is a type of interaction where both the species suffer. It may exist between some species (interspecific competition) or between individuals of same species (intraspecific competition).
n The competition occurs due to limited resources between closely related species.
n Some totally unrelated species could also compete for the same resource, e.g., in some shallow South American lakes, visiting flamingoes and resident fishes compete for their common food, zooplanktons.
n In interspecific competition, the feeding efficiency of one species might be reduced due to the interfering and inhibitory presence of the other species, although the resources are abundant.
n For example, after the introduction of goats in Galapagos Islands, the Abingdon tortoise became extinct within a decade due to greater browsing efficiency of the goats.
n A species whose distribution is restricted to small geographical area because of presence of competitively superior species is found to expend its distributional range when competing species is removed. Connell’s Elegant field experiment showed on rocky sea coasts of Scotland, larger and competitively superior barnacle Balanus dominates intertidal area and excludes smaller barnacle Chathamalus from that zone.
n Competitive release refers to the phenomenon of a species whose distribution is restricted to a small geographical area because of the presence of a competitively superior species, is found to expand its distributional range dramatically when the competing species is experimentally removed.
n Gause’s competitive exclusion principle states that two closely related species competing for the same resource cannot coexist indefinitely and the competitively inferior one will be eliminated eventually by the superior one.
n Resource partitioning: It refers to the phenomenon in which species facing competition might evolve mechanisms that promote coexistence rather than exclusion. MacArthur showed that five closely related species of warblers living on the same tree were able to avoid competition and coexist due to behavioural differences in their foraging activities.
n Herbivores are more adversely affected by competition than carnivores.
(iii) Parasitism
n It is the mode of interaction between two species in which one species (parasite) depends on the other species (host) for food and shelter, and in this process damages the host. In this process one organism is benefited (parasite) while the other is being harmed (host).
n Adaptation of parasite:
(a) The parasite have evolved to be host-specific in such a manner that both host and parasite tend to co-evolve.
(b) Loss of unnecessary sense organs as they do not interact with external environment [For example, eyes as they are found in an environment that lacks light.)
(c) Presence of adhesive organs or suckers to cling to host
(d) Loss of digestive system to absorb digested food from increase chances of survival.
(e) High reproductive capacity.
(f) Presence of adhesive.
(g) Loss of chlorophyll and leaves (e.g. cuscuta), to derive its nutrition from the host plant which it parasitises.
(h) Presence of more than one host to facilitate parasitisation of its primary host.
(i) Eggs resemble the host egg (e.g., crow) in size and colour to reduce the chances of host bird (i.e., Koel) detecting them.
n The life cycles of some parasites are complex, where one or more intermediate host or vectors to facilitate parasitisation are present.
(a) The human liver fluke depends on two intermediate hosts, a snail and a fish, to complete its life cycle.
(b) Malarial parasite (Plasmodium) needs a vector (mosquito) to complete its life cycle.
n Majority of parasites harm the host by reducing the survival, growth and reproduction of the host. They reduce its population density by making it physically weak.
n Parasites may be of two types: ectoparasites and endoparasites.
S. No. | Endoparasite | Ectoparasite |
(i) | These are the parasites which live inside the host’s body at different sites like liver, kidney, lungs, etc., for food and shelter. | These are the parasites which feed on the external surface of the host organism for food and shelter. |
(ii) | Example, tapeworm, liver fluke, Plasmodium. | Example, lice on humans, ticks on dogs, copepods, Cuscuta. |
n The phenomenon in which one organism (parasite) lays its eggs in the nest of another organism is called brood parasitism. Eggs of parasitic birds have evolved to resemble host’s eggs in size and colour to reduce the chance of host bird detecting foreign eggs and remove them from nest. e.g., Cuckoo lays eggs in Crow’s nest.
(iv) Commensalism
n Commensalism is referred to as the interaction between two species where one species is benefited and the other is neither harmed nor benefited.
n Few examples of commensalism:
(a) An orchid growing as an epiphyte on a mango tree. The orchid gets shelter and nutrition from mango tree while the mango tree is neither benefited nor harmed.
(b) Barnacles growing on the back of whale. Barnacles are benefited to move to location for food as well as shelter while the whales are neither benefited nor harmed.
(c) The egrets are in close association of grazing cattle. The cattle egrets are benefited by the cattle to detect insects because cattle stir up the bushes and insects are flushed out from the vegetation, to be detected by cattle egrets.
(d) Commensalism is also found between sea anemones (that has stinging tentacles) and the clown fish. The fish is protected from predators and sea anemones are neither benefited nor harmed.
(v) Amensalism
n Amensalism is referred to as the interaction between two different species, in which one species is harmed and the other is neither benefited nor harmed.
n For example, the mould Penicillium secretes penicillin which kills bacteria but the mould is unaffected.
(vi) Mutualism
n Mutualism is referred to as the interspecific interaction in which both the interacting species are benefited.
n Some examples of mutualism
(a) Lichens represent close association between fungus and photosynthetic algae or cyanobacteria, where the fungus helps in the absorption of nutrients and provides protection while algae or cyanobacterium prepares the food.
(b) Mycorrhizae are close mutual association between fungi and the roots of higher plants, where fungi helps the plant for absorption of nutrients while the plant provides food, & protection for the fungus.
(c) Mutualism are found in plant–animal relationships. Plants take the help of animals for pollination and dispersal of their seeds and animals are rewarded in the form of nectar or edible pollen or oviposition (site for laying egg).
(d) Orchids have evolved to attract right pollinator insect (bees and bumble bees). Mediterranean orchid. Ophrys muscifera employs sexual deceit to get pollinated by bee species. One petal of flower resembles female bee in size, color and markings and male bee is attracted and pseudocopulates with it. During this process of pseudocopulation, the pollen grains are dusted on the body of male bees. With such pollen dusts, male bee pseudocopulates to another flower of the same species and pollination takes place. Here we see co-evolution, i.e., if female bee’s colour patterns change during evolution, orchid flower also co-evolves to maintain resemblance of petal to female bee.
(e) Co-evolution is also seen in many species of fig trees which are pollinated by specific species of wasp. Female wasp uses fruit for oviposition and also uses developing seeds within fruit for nourishing its larvae. Wasp pollinates the fig inflorescence while searching for suitable egg laying sites. In return, the fig offers the wasp some of its developing seeds as food for the developing wasp larvae.
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