Study Notes on Organism and its environment

  • Ecology at the organismic level is essentially physiological ecology which tries to understand how different organisms are environments in terms of not only survival but also reproduction.
  • You may have learnt in earlier classes how the rotation of our the Sun and the tilt of its axis cause annual variations in the intensity and duration of temperature, resulting in distinct seasons.
  • These variations together with annual variation in precipitation (remember precipitation includes both rain and snow)
  • On account for the formation of major biomes such as desert, rain forest and tundra.


  • Over a natural selection, evolved its survival and reproduction in its habitat.
  • Temperature: Temperature is the most ecologically relevant environmental factor.
  • You are aware that the average temperature on land varies seasonally, decreases progressively from the equator towards the poles and from plains to the mountain tops.
  • It ranges from sub-zero levels in polar areas and high altitudes to >50°C in tropical deserts in summer.
  • There are, however, unique habitats such as thermal springs and deep-sea hydrothermal vents where average temperatures exceed 100° C.
  • It is general knowledge that mango trees do not and cannot grow in temperate countries like Canada and Germany.
  • The snow leopards are not found in Kerala forests and tuna fish are rarely caught beyond tropical latitude in the ocean.


  • Next to temperature, water is the most important factor influencing the life of organisms.
  • In fact, life on earth originated in water and is unsustainable without water.
  • Its availability is so limited in deserts that only special adaptations make it possible to live there.
  • The productiyity and distribution of plants is also heavily dependent on water.
  • You might think that organisms living in oceans, lakes and rivers should not face any water-related problems, but it is not true.
  • 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 in inland waters, 30-35 in the sea and > 100 per cent in some hypersaline lagoons.


  • Since plants produce food through photosynthesis, a process which is only possible when sunlight is available as a source of energy.
  • we can quickly understand the importance of light for living organisms, particularly autotrophs.
  • Many species of small plants (herbs and shrubs) growing in forests are adapted to photosynthesise optimally under very low light conditions because they are constantly overshadowed by tall, eg canopied trees.


  • 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.
  • Various characteristics of the soil such as soil composition, grain size and aggregation determine the percolation and water holding capacity of the soils.
  • These characteristics along with parameters such as pH, mineral composition and topography determine to a large extent the vegetation in any area.
  • This in turn dictates the type of animals that can be supported.
  • Similarly, in the aquatic environment, the sediment characteristics often determine the type of benthic animals that can thrive there.
  • Responses to Abiotic Factors Having realised that the abiotic conditions of many habitats may vary drastically in time.
  • But before attempting to answer this question, we should perhaps ask first why a highly variable.
  • External environment should bother organisms after all. One would expect that during the course of millions of years of their existence.
  • The species would have evolved a relatively constant internal (within the body) environment that permits all biochemical reactions and physiological functions to proceed with maximal Organisation.


  • Some organisms are able to maintain homeostasis by physiological (sometimes behavioural also) means which ensures constant body temperature, constant osmotic concentration.
  • All birds and mammals, and a very few lower vertebrate and invertebrate species are indeed capable of such regulation (thermoregulation and osmoregulation).
  • Evolutionary biologists believe that the 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.


  • An overwhelming majority (99 per cent) of animals and nearly all plants cannot maintain a constant internal environment.
  • Their body temperature changes with the ambient temperature.
  • 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. Considering the benefits of a constant internal environment to the organism.
  • we must ask why these conformers had not evolved to become regulators.
  • Thermoregulation is energetically expensive for many organisms .


  • The organism can move away temporarily from the stressful habitat to a more hospitable area and return when stressful period is over.
  • In human analogy, this strategy is like a person moving from Delhi to Shimla for the duration of summer.
  • Many animals, particularly birds, during winter undertake long-distance migrations to more hospitable areas.


  • While considering the various alternatives available to organisms for coping with extremes in their environment.
  • we have seen that some are able to respond through certain physiological adjustments while others do so behaviourally (migrating temporarily to a less stressful habitat).
  • These responses are also actually, their adaptations. So, we can say that adaptation is any attribute of the organism (morphological, physiological, behavioural) that enables the organism to survive and reproduce in its habitat.
  • Many adaptations have evolved over a long evolutionary time and are genetically fixed.
  • Mammals from colder climates generally have shorter ears and limbs to minimise heat loss. (This is called the Allen’s Rule.)
  • In the polar ses 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.


  • A population has certain attributes that an individual organism does An individual may have births and deaths, but a population has births and death rates.
  • In a population these rates refer to per capita births and deaths, respectively.
  • The rates, hence, expressed is change in numbers increase or decrease with respect to members of the population.
  • Here is an cumple, If in a pond there are 20 lotus plants last year and through reproduction 8 new plants are added, taking the current population to 28, we calculate the birth rate as 8/20 = 0.4 offspring per lotus per year.
  • If individuals in a laboratory population of 40 fruit flies died during a specified interval, say a week, the death rate in the population during that period
  • 34/40 -0.1 individuals per fruitfly per week.
  • Another attribute characteristic of a population is sex ratio. An individual is either a male or a female but a population has a sex ratio leg.. 60 per cent of the population are females and 40 per cent males).
  • The predator or the effect of a pesticide application, we always evaluate them in terms of any change in the population size.
  • be as low as <10 (Siberian cranes at Bharatpur wetlands in any year) go into millions (Chlamydomonas in a pond).
  • Population size, more 108 technically called population density (designated as N), need me necessarily be measured in numbers only.
  • Although total number is.generally the most appropriate measure of population density, it is in some cases either meaningless or difficult to determine.


  • The size of a population for any species is not a static parameter.
  • It keeps changing in time, depending on various factors including food availability, predation pressure and adverse weather.
  • In fact, it is these changes in population density that give us some idea of what is happening to the
  • population – whether it is flourishing or declining. Whatever might be the ultimate reasons.
  • the density of a population in a given habitat during a given period, fluctuates due to changes in four basic processes.
  • The two of which (natality and immigration) contribute to an increase in population density and two (mortality and emigration) to a decrease.

( Natality refers to the number of births during a given period in the population that are added to the initial density.

(1) Mortality is the number of deaths in the population during a given period

(HD Immigration is the number of individuals of the same species that have come into the habitat from elsewhere during the time period under consideration

(iv) Emigration is the number of individuals of the population who left the habitat and gone elsewhere during the time period under. Consideration.


  • Does the growth of a population with time show any specific and predictable pattern? We have been concerned about unbridled human population growth and problems created by it in our country and it is therefore natural for us to be curious if different animal.
  • The populations in nature behave the same way or show some restraints on growth.
  • Perhaps we can learn a lesson or two from nature on how to control population growth.


  • Resource (food and space) availability is 229 obviously essential for the unimpeded growth of a population.
  • Ideally, when resources in the habitat are unlimited, each species has the ability to realise fully its innate potential to grow in number, as Darwin observed while developing his theory of natural selection.


  • No population of any species in nature has at its space disposal unlimited resources to permit exponential growth.
  • This leads to competition between individuals for limited resources.
  • Eventually, the ‘fittest’ individual will survive and reproduce.
  • The governments of many countries have also realised this fact and introduced various restraints with a view to limit human population growth.
  • In nature, a given habitat has enough resources to support a maximum possible number, beyond which no further growth is possible.


  • Interspecific interactions arise from the interaction of populations of two different species.
  • They could be beneficial, detrimental or neutral (neither harm nor benefit) to one of the species or both. Assigning a’+’ sign for beneficial interaction.
  • sign for detrimental and 0 for neutral interaction, let us look at all the possible outcomes of interspecific.


  • 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 interactions detrimental to the other species (host and prey, respectively).
  • The interaction where one species is benefitted and the other is neither while others benefitted nor harmed is called commensalism.
  • In amensalisman mammals). So,. The other hand one species is harmed. Whereas the other is unaffected. Predation, parasitism and commensalism share a common characteristics the interacting species live closely together.