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

5.1 Organism and its Environment

Responses to Abiotic Factors

Organisms must maintain a constant internal environment (homeostasis) despite varying external environmental conditions. They do this through various responses:

1. Regulate (Regulators):

   • Some organisms maintain homeostasis by physiological (sometimes behavioral) means, ensuring constant body temperature (thermoregulation) and constant osmotic concentration (osmoregulation).
   • Examples: All birds and mammals, and a very few lower vertebrate and invertebrate species.
   • On a graph plotting internal level vs. external level, a regulator is represented by a flat, horizontal line.

2. Conform (Conformers):

   • The overwhelming majority (99%) 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 changes with that of the ambient water osmotic concentration.
   • Examples: Most fishes, amphibians, reptiles, and invertebrates.
   • On a graph, a conformer is represented by a steep diagonal line, showing internal levels changing directly with external levels.

3. Partial Regulators:

   • These species have the ability to regulate, but only over a limited range of environmental conditions. Beyond that limit, they simply conform.
   • On a graph, their curve initially rises diagonally (conforming) and then flattens out horizontally (regulating), or vice versa.

4. Migrate:

   • The organism can move away temporarily from the stressful habitat to a more hospitable area and return when the stressful period is over.
   • Example: Many animals, particularly birds, during winter undertake long-distance migrations to more hospitable areas (e.g., Siberian cranes coming to Keoladeo National Park in Rajasthan).

5. Suspend:

   • In bacteria, fungi and lower plants, various kinds of thick-walled spores are formed which help them to survive unfavorable conditions.
   • In animals, the organism, if unable to migrate, might avoid the stress by escaping in time.
   • Hibernation: Winter sleep to avoid cold (e.g., bears).
   • Aestivation: Summer sleep to avoid heat and desiccation (e.g., some snails and fish).
   • Diapause: A stage of suspended development under unfavorable conditions (e.g., many zooplankton species in lakes and ponds).

Adaptations (Important Ecological Rules)

Adaptation is any attribute of the organism (morphological, physiological, behavioral) that enables the organism to survive and reproduce in its habitat. Many adaptations have evolved over a long evolutionary time and are genetically fixed.

5.2 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 adds new individuals to the population.
   • Potential Natality (Biotic Potential): The maximum possible reproductive output of a population under ideal, unlimiting environmental conditions.
   • Realized Natality: The actual reproductive output, which is always lower than potential natality due to environmental resistance (limited food, space, predation, disease, and competition).

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.

7. Survivorship Curves:

   • Graphical representation of the number of individuals alive at each age.
   • Type I (Highly convex): Low mortality in early and middle life, rapid drop in old age (e.g., Humans, large mammals).
   • Type II (Diagonal): Constant mortality rate regardless of age (e.g., Birds, some lizards).
   • Type III (Highly concave): High mortality in early life, low mortality for the few that survive (e.g., Oysters, pelagic fishes).

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.