Organism and population

• Ecology


  The study of interactions among organisms and between the organism and its physical (abiotic) environment
• Levels of biological organization in ecology

  • Organisms
  • Populations
  • Communities
  • Biomes
• Physiological ecology


  The study of adaptation of an organism to environments in terms of survival and reproduction
• The rotation of earth and the tilt of its axis cause annual variations in temperature & seasons. Major biomes (desert, rain forest, tundra etc.) are formed due to these variations & precipitation (rain & snow)
• Regional and local variations within a biome lead to the formation of different habitats
• Life exists even in extreme & harsh habitats
• Niche


  The distinct role and position of an organism in its environment
• Abiotic factors


  • Temperature
  • Water
  • Light
  • Soil
• Eurythermal


  Organisms that can tolerate a wide range of temperatures
• Stenothermal


  Organisms that can tolerate only a narrow range of temperatures
• Euryhaline


  Organisms that tolerate a wide range of salinities
• Stenohaline


  Organisms that tolerate only a narrow range of salinity
• Regulate


  The maintenance of homeostasis by physiological & behavioural means
• Conform


  Organisms that cannot maintain a constant internal environment, their body temperature or osmotic concentration change with the surrounding conditions
• Migrate


  Animals that move away temporarily from stressful habitat to a more hospitable area and return when stressful period is over
• Suspend


  Organisms that reduce their metabolic activity to survive unfavourable conditions
• Adaptation


  The morphological, physiological & behavioural attribute that enables an organism to survive and reproduce in its habitat
• Adaptations of kangaroo rat in North American deserts

  • Internal fat oxidation gives water as byproduct if there is no external source of water
  • Ability to concentrate urine so that minimal volume of water is used to remove excretory products
• Adaptations of desert plants

  • Presence of thick cuticle on leaf surfaces
  • Sunken stomata minimise water loss due to transpiration
  • CAM photosynthetic pathway enables their stomata to remain closed during day time
  • Desert plants like Opuntia have no leaves (they are reduced to spines), photosynthesis is done by stems
• Adaptations of mammals


  • Mammals from colder climates have shorter ears and limbs to reduce heat loss (Allen's Rule)
  • 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
• Physiological and biochemical adaptations

  • Archaebacteria are found in hot springs & deep-sea hydrothermal vents where temperature is >100??C
  • Many marine invertebrates & fishes live at great depths in the ocean where the pressure is >100 times the normal atmospheric pressure
  • At a high-altitude place (>3,500 m) the body compensates low O2 availability by increasing RBC & breathing rate and decreasing the binding capacity of hemoglobin
• Behavioural adaptations


  • Desert lizards bask in the sun and absorb heat when their body temperature is low, but move into shade when the ambient temperature starts increasing
  • Some species burrow into the soil to hide and escape from the above-ground heat
• Population


  A group of individuals of same species that live in a given geographical area, share or compete for similar resources and potentially reproduce
• Birth rate


  Per capita births
• Death rate


  Per capita deaths
• Sex ratio


  The proportion of males and females in a population
• Age pyramid


  The structure obtained when the age distribution (% individuals of a given age or age group) is plotted for the population
• Population size or population density (N)


  The number of individuals of a species per unit area or volume
• Natality (B)


  The number of births in a population during a given period
• Mortality (D)


  The number of deaths in a population during a given period
• Immigration (I)


  The number of individuals of the same species that have come into the habitat from elsewhere during a given time period
• Emigration (E)


  The number of individuals of the population who left the habitat and gone elsewhere during a given time period
• Population growth


  Nt+1 = Nt + [(B + I) - (D + E)]
• Exponential growth


  If resources are unlimited, each species shows its full innate potential to grow in number, and the population grows in an exponential or geometric fashion
• Intrinsic rate of natural increase (r)

  An important parameter for assessing impacts of any biotic or abiotic factor on population growth
• When colonizing habitat, immigration may be more significant to population growth than birth rates
• Exponential growth


  • Resources (food & space) are essential for the unimpeded population growth
  • If resources are unlimited, each species shows its full innate potential to grow in number. Then the population grows in an exponential or geometric fashion
  • If population size = N, birth rates (per capita births) = b and death rates (per capita deaths) = d, then the increase or decrease in N during a unit time period t (dN/dt) will be dN/dt = (b – d) × N
  • Let (b–d) = r, then dN/dt = rN
  • The r ('intrinsic rate of natural increase') is an important parameter for assessing impacts of any biotic or abiotic factor on population growth
• Logistic growth


  • There is no population in nature having unlimited resources for exponential growth. This leads to competition among individuals for limited resources
  • Eventually, the 'fittest' individuals survive and reproduce
  • In nature, a given habitat has enough resources to support a maximum possible number, beyond which no further growth is possible. It is called carrying capacity (K)
  • A population with limited resources shows initially a lag phase, phases of acceleration & deceleration and finally an asymptote. This type of population growth is called Verhulst-Pearl Logistic Growth. It is described by following equation: N = K / (1 + (K-N0)/N0 * e^(-rt))
• Life History Variation


  • Populations evolve to maximise their reproductive fitness or Darwinian fitness (high r value)
  • Some organisms breed only once in their lifetime (Pacific salmon fish, bamboo) while others breed many times (most birds and mammals)
  • Some produce a large number of small-sized offspring (Oysters, pelagic fishes) while others produce a small number of large-sized offspring (birds, mammals)
  • These facts indicate that life history traits of organisms have evolved due to limited abiotic and biotic components of the habitat
• Population Interactions


  • Mutualism
  • Competition
  • Predation
  • Parasitism
  • Commensalism
  • Amensalism