Unit 5: Ecology and Environment - Chapter 1: Organisms and Populations

5.1 Organisms and Populations

Definition of Population

A population is defined as a group of individuals of the same species living in a well-defined geographical area, sharing or competing for similar resources, and potentially interbreeding. For example, all the teakwood trees in a forest, all the rats in an abandoned dwelling, or all the lotus plants in a pond constitute a population.

Population Attributes: Density, Sex Ratio, Natality, Mortality, Growth Models (Exponential, Logistic), Age Pyramids

Populations have certain characteristics or attributes that an individual organism does not. These include:

1. Population Density:

   • The number of individuals of a species per unit area or volume at a given time.
   • It is not always measured in absolute numbers. For example, for a single banyan tree with a large canopy, counting the number of individuals of a bird species living on it might be more meaningful than just the number of trees.
   • Formula: Population Density (D) = Total number of individuals (N) / Space (S) or Volume (V).

2. Sex Ratio:

   • The ratio of males to females in a population.
   • Expressed as the number of females per 1000 males or vice versa.
   • For example, 60% females and 40% males in a population.

3. Natality (Birth Rate):

   • The number of births per thousand individuals in a population per unit time.
   • It is the increase in the number of individuals in a population under given environmental conditions.
   • It adds new individuals to the population.

4. Mortality (Death Rate):

   • The number of deaths per thousand individuals in a population per unit time.
   • It is the loss of individuals in a population due to death.
   • It decreases the population size.

5. Growth Models:

   • The size of a population changes depending on food availability, predation pressure, and adverse weather conditions.

   • The change in population density (N) over time (t) is given by: dN/dt = (b + i) - (d + e) Where: b = birth rate, i = immigration, d = death rate, e = emigration.

   • a. Exponential Growth Model:

     • Occurs when resources (food, space) are unlimited.
     • Each species has the ability to realize its full innate potential to grow (r = intrinsic rate of natural increase).
     • Equation: dN/dt = rN
     • Where: N = Population size, t = Time, r = Intrinsic rate of natural increase.
     • The r value is a measure of the maximum potential for population growth under ideal conditions.
     • The curve obtained is J-shaped.
     • Nt = N0 * e^(rt)
     • Where: Nt = Population density after time t, N0 = Population density at time 0, r = Intrinsic rate of natural increase, e = Base of natural logarithms (2.71828).

   • b. Logistic Growth Model:

     • Occurs when resources are limited, which is a more realistic scenario in nature.

• No population can grow exponentially indefinitely.
• The environment has a carrying capacity (K), which is the maximum population size that the environment can sustain indefinitely given the available resources.
• Equation: dN/dt = rN * ((K - N) / K)
• Where: K = Carrying capacity.
• The curve obtained is S-shaped (Sigmoid curve).
• Phases:
  • Lag phase: Slow initial growth.
  • Log (exponential) phase: Rapid growth.
  • Stationary phase: Population density reaches carrying capacity (K), and growth slows down and eventually stops.

6. Age Pyramids (Age Distribution):
   • The proportion of individuals of different age groups in a population.
   • Usually, three age groups are considered: pre-reproductive, reproductive, and post-reproductive.
   • The shape of the age pyramid indicates the growth status of the population:
     • Expanding (Triangular): High proportion of young individuals, indicating a growing population.
     • Stable (Bell-shaped): Even distribution of pre-reproductive and reproductive individuals, indicating a stable population.
     • Declining (Urn-shaped): Low proportion of young individuals, indicating a declining population.

Population Interactions:

Species interact with each other in various ways within a community. These interactions can be beneficial, detrimental, or neutral to one or both species involved.

1. Mutualism (+/+):

   • Both interacting species benefit.
   • Examples:
     • Lichens: Association between a fungus and photosynthetic algae/cyanobacteria. Fungus provides protection and absorption, algae/cyanobacteria provide food.
     • Mycorrhizae: Association between fungi and roots of higher plants. Fungi help in nutrient absorption, plants provide carbohydrates.
     • Pollination: Animals (e.g., bees, birds) get nectar/pollen, plants get pollinated.
     • Fig and Wasp: Wasp pollinates the fig flower, and the fig provides food and a place for the wasp to lay eggs.
     • Rhizobium in leguminous plants: Bacteria fix atmospheric nitrogen for the plant, and the plant provides shelter and nutrients.

2. Competition (-/-):

   • Both interacting species are harmed due to competition for limited resources.
   • Types:
     • Interference Competition: Direct aggressive interaction between individuals.
     • Exploitative Competition: Indirect competition where one species consumes a shared resource, making it unavailable for others.
   • Competitive Exclusion Principle (Gause's Principle): Two closely related species competing for the same limited resources cannot coexist indefinitely; the competitively superior one will eventually eliminate the other.
   • Resource Partitioning: Species competing for the same resources can avoid competition by choosing different times for feeding or different foraging patterns.

3. Predation (+/-):

   • One species (predator) kills and consumes another species (prey).
   • Importance:
     • Transfer energy across trophic levels.
     • Keep prey populations under control.
     • Maintain species diversity in a community by reducing the intensity of competition among competing prey species.
   • Adaptations in Prey to avoid Predation:
     • Camouflage (Cryptic coloration): Blending with surroundings (e.g., stick insects, chameleons).
     • Mimicry: Resembling another species that is dangerous or unpalatable (e.g., Monarch butterfly is unpalatable due to chemicals, Viceroy butterfly mimics it).
     • Chemical defenses: Producing poisonous or unpalatable chemicals (e.g., Monarch butterfly, certain plants like Calotropis).
     • Thorns and Spines: In plants (e.g., Acacia, cactus).

4. Parasitism (+/-):

   • One species (parasite) lives on or in another species (host) and derives nourishment from it, harming the host in the process.
   • Types:
     • Ectoparasites: Live on the external surface of the host (e.g., lice on humans, ticks on dogs, Cuscuta on plants).
     • Endoparasites: Live inside the host body (e.g., tapeworm, liver fluke, Plasmodium).
     • Brood Parasitism: A special type of parasitism where the parasitic bird lays its eggs in the nest of another bird and lets the host bird incubate them (e.g., cuckoo and crow).
   • Adaptations in Parasites: Loss of unnecessary sense organs, presence of adhesive organs/suckers, loss of digestive system, high reproductive capacity.

5. Commensalism (+/0):

   • One species benefits, and the other is neither harmed nor benefited.
   • Examples:
     • Orchids growing on a mango branch: Orchid gets support, mango tree is unaffected.
     • Barnacles on the back of a whale: Barnacles get transport and food, whale is unaffected.
     • Cattle Egret and grazing cattle: Egret feeds on insects disturbed by the cattle, cattle are unaffected.
     • Sea Anemone and Clownfish: Clownfish gets protection from predators by living among the stinging tentacles of the anemone, anemone is unaffected.

6. Amensalism (-/0):

   • One species is harmed, and the other is unaffected.
   • Example:
     • Penicillium and bacteria: Penicillium secretes penicillin, which inhibits the growth of bacteria, but Penicillium itself is unaffected.
     • Black walnut tree and other plants: Black walnut releases juglone, a chemical that inhibits the growth of other plants around it.