Biology: Class 12 — Chapter 11: Organisms and Populations

• Revered as the Father of Ecology in India.
• Established teaching and research in ecology at the Department of Botany, Banaras Hindu University (BHU), Varanasi.
• Laid foundations for understanding tropical communities, succession, environmental responses of plant populations, and nutrient cycling in tropical forest and grassland ecosystems.
• Formulated the first postgraduate course in ecology in India.
• His efforts paved the way for the establishment of the National Committee for Environmental Planning and Coordination (1972), which later led to the Ministry of Environment and Forests (1984).

While an individual organism has births and deaths, a population has birth rates and death rates. These rates are expressed as per capita changes in numbers relative to the population members.

• Birth Rate Example: If 20 lotus plants in a pond increase to 28 in a year, the birth rate is calculated as: $$ \frac{8}{20} = 0.4 \text{ offspring per lotus per year} $$
• Death Rate Example: If 4 fruitflies in a laboratory population of 40 die in a week, the death rate is: $$ \frac{4}{40} = 0.1 \text{ individuals per fruitfly per week} $$
• Sex Ratio: A population has a sex ratio (e.g., 60% females and 40% males), whereas an individual is either male or female.
• Age Pyramids: Graphical representation of the age distribution (percent individuals of a given age group) plotted for a population. For human populations, these show the distribution of males and females:
  • Expanding: High proportion of pre-reproductive individuals.
  • Stable: Pre-reproductive and reproductive individuals are nearly equal.
  • Declining: Fewer pre-reproductive individuals than reproductive ones.
• Population Density (N): The size of the population in a given habitat. It can be measured by total number, percent cover, biomass, or relative density (e.g., fish caught per trap, tiger pug marks, and fecal pellets).

The size of a population fluctuates due to changes in four basic processes:

1. Natality (B): Number of births in the population during a given period.
2. Mortality (D): Number of deaths in the population during a given period.
3. Immigration (I): Number of individuals of the same species that have come into the habitat from elsewhere.
4. Emigration (E): Number of individuals of the population who left the habitat.

The population density at time $t + 1$ is given by the equation:

$$N_{t+1} = N_t + [(B + I) - (D + E)]$$

Growth Models:

• Exponential Growth: Occurs when resources (food and space) are unlimited. The population grows in a geometric fashion, resulting in a J-shaped curve.
  • Equation: $$\frac{dN}{dt} = (b - d) \times N = rN$$ where $r$ is the intrinsic rate of natural increase.
  • Integral form: $$N_t = N_0 e^{rt}$$ where $N_t$ is population density after time $t$, $N_0$ is density at time zero, $r$ is the intrinsic rate of natural increase, and $e$ is the base of natural logarithms (2.71828).
  • Historical $r$ values: Norway rat ($0.015$), flour beetle ($0.12$), human population in India in 1981 ($0.0205$).
• Logistic Growth: More realistic model as resources are finite. A population growing in a habitat with limited resources shows a lag phase, phases of acceleration and deceleration, and finally an asymptote when it reaches the carrying capacity (K). This results in a sigmoid (S-shaped) curve, known as Verhulst-Pearl Logistic Growth.
  • Equation: $$\frac{dN}{dt} = rN \left(\frac{K - N}{K}\right)$$

Populations evolve to maximise their reproductive fitness, also called Darwinian fitness (high $r$ value):

• Some organisms breed only once in their lifetime (e.g., Pacific salmon fish, bamboo).
• Others breed many times during their lifetime (e.g., most birds and mammals).
• Some produce a large number of small-sized offspring (e.g., Oysters, pelagic fishes).
• Others produce a small number of large-sized offspring (e.g., birds, mammals).

In nature, different species interact to form biological communities. These interspecific interactions can be beneficial (+), detrimental (-), or neutral (0):

Detailed Interactions:

• (i) Predation: Nature's way of transferring energy fixed by plants to higher trophic levels.
  • Predators keep prey populations under control. For example, the invasive prickly pear cactus in Australia was controlled only after its predator (a moth) was introduced.
  • Predators maintain species diversity by reducing competition among prey species (e.g., starfish Pisaster in rocky intertidal communities).
  • Prey defenses: Camouflage (insects, frogs), chemical defenses (Monarch butterfly accumulates toxic chemicals from a poisonous weed during its caterpillar stage), and morphological defenses in plants (thorns of Acacia and Cactus, cardiac glycosides produced by Calotropis).
• (ii) Competition: Occurs when species compete for limiting resources, though totally unrelated species can also compete (e.g., visiting flamingoes and resident fishes in South American lakes competing for zooplankton).
  • Competitive Release: A species expands its range when its competitively superior competitor is experimentally removed (e.g., Connell's field experiments where the superior barnacle Balanus excluded Chathamalus).
  • Gause's Competitive Exclusion Principle: Two closely related species competing for the same resources cannot co-exist indefinitely; the competitively inferior one will eventually be eliminated.
  • Resource Partitioning: Co-existence mechanism where species choose different times for feeding or different foraging patterns (e.g., MacArthur showed five species of warblers co-existing on the same tree).
• (iii) Parasitism: One species (parasite) benefits by obtaining free lodging and meals at the expense of the host.
  • Parasites are often host-specific and co-evolve with their hosts.
  • Adaptations: Loss of unnecessary sense organs, presence of suckers/hooks, loss of digestive system, high reproductive capacity.
  • Complex life cycles: Human liver fluke (requires snail and fish as intermediate hosts); malarial parasite (requires mosquito vector).
  • Ectoparasites: Feed on external surface (e.g., lice on humans, ticks on dogs, marine copepods, and the parasitic plant Cuscuta).
  • Endoparasites: Live inside the host's body (e.g., in liver, kidney, lungs).
  • Brood Parasitism: Parasitic birds lay eggs in the host's nest for incubation (e.g., cuckoo/koel laying eggs in a crow's nest).
• (iv) Commensalism: One species benefits, and the other is unaffected.
  • An orchid growing as an epiphyte on a mango branch.
  • Barnacles growing on the back of a whale.
  • Cattle egrets foraging close to grazing cattle (cattle flush out insects from vegetation).
  • Clown fish living among stinging tentacles of sea anemones (fish gets protection from predators).
• (v) Mutualism: Both interacting species benefit.
  • Lichens: Mutualistic relationship between a fungus and photosynthesising algae or cyanobacteria.
  • Mycorrhizae: Association between fungi and roots of higher plants. Fungi help in nutrient absorption, while the plant provides carbohydrates.
  • Plant-Animal Mutualism: Fig trees and pollinator wasps (one-to-one relationship where the wasp pollinates the fig inflorescence and uses the fig fruit for egg-laying and larval nourishment).
  • Sexual Deceit: The Mediterranean orchid Ophrys employs sexual deceit to get pollinated by a species of bee. One petal of its flower resembles a female bee, prompting male bees to "pseudocopulate" and transfer pollen.