ORGANISMS AND POPULATION - Class 12 Karnataka Board History

Organisms and Populations

Ecology: Definition and Scope

Ecology is a branch of biology that studies the interactions among living organisms and between organisms and their physical (abiotic) environment. These interactions include those among living organisms (biotic factors) and between living and non-living (abiotic) factors such as sunlight, water, soil, and temperature.

• Ecology: Study of interactions among organisms and between organisms and their environment.
• Biotic factors: Living components (plants, animals, microbes).
• Abiotic factors: Non-living components (sunlight, water, soil, temperature).

Levels of Biological Organization

Biological organization is structured in hierarchical levels, each with increasing complexity. The…

Organisms and Populations

Ecology: Definition and Scope

Ecology is a branch of biology that studies the interactions among living organisms and between organisms and their physical (abiotic) environment. These interactions include those among living organisms (biotic factors) and between living and non-living (abiotic) factors such as sunlight, water, soil, and temperature.

• Ecology: Study of interactions among organisms and between organisms and their environment.
• Biotic factors: Living components (plants, animals, microbes).
• Abiotic factors: Non-living components (sunlight, water, soil, temperature).

Levels of Biological Organization

Biological organization is structured in hierarchical levels, each with increasing complexity. The main levels relevant to ecology are organism, population, community, ecosystem, biome, and biosphere.

• Cell → Tissue → Organ → Organ System → Organism → Population → Community → Ecosystem → Biome → Biosphere
• Population: Group of individuals of the same species occupying a specific area at a specific time.
• Community: Different populations interacting in a given area.
• Ecosystem: Community plus abiotic factors.
• Biome: Large geographic area with characteristic climate, flora, and fauna.
• Biosphere: Sum of all biomes on Earth.

Population and Its Attributes

A population is defined as a group of individuals of the same species living in a defined geographical area at a particular time. Populations have attributes that are not found in individual organisms, such as birth rate, death rate, and sex ratio.

• Population: Group of same species in a specific area and time.
• Attributes unique to populations:
  • Birth rate (natality): Number of births per unit time.
  • Death rate (mortality): Number of deaths per unit time.
  • Sex ratio: Proportion of males to females.
• Population size: Number of individuals present in a population.

Population Growth and Age Pyramids

Population growth is influenced by birth rate, death rate, immigration, and emigration. Age pyramids graphically represent the age distribution of a population and can predict future growth trends.

• Age groups:
  • Pre-reproductive: Individuals not yet capable of reproduction.
  • Reproductive: Individuals capable of reproduction.
  • Post-reproductive: Individuals past reproductive age.
• Types of age pyramids:
  • Expanding (triangular): Broad base, high proportion of young; indicates growing population.
  • Stable (bell-shaped): Similar proportions in pre-reproductive and reproductive; stable population.
  • Declining (urn-shaped): Narrow base, fewer young; declining population.

Population Density and Measurement

Population density refers to the number of individuals per unit area or volume. It is commonly measured by counting individuals, but alternative methods like biomass or percent cover are used when counting is impractical.

• Population density (N): Number of individuals per unit area.
• Measurement methods:
  • Absolute density: Direct count of all individuals.
  • Relative density: Estimate based on sampling or indirect evidence (e.g., pug marks, fecal pellets).
  • Biomass: Total mass of living matter.
  • Percent cover: Area occupied by a species.

Factors Affecting Population Density

Population density is dynamic and changes due to four main processes: natality, mortality, immigration, and emigration. These processes determine the increase or decrease in population size over time.

• Natality (B): Number of births added to the population.
• Mortality (D): Number of deaths removing individuals.
• Immigration (I): Individuals entering from other populations.
• Emigration (E): Individuals leaving to other populations.
• Population density at time t+1:$$N_{t+1} = N_t + (B + I) - (D + E)$$
• If $(B + I) > (D + E)$, population increases; if $(D + E) > (B + I)$, population decreases.

Population Growth Models

Population growth can follow predictable patterns, mainly exponential or logistic growth, depending on resource availability.

• Exponential growth: Occurs when resources are unlimited.
  • Equation: $dn/dt = rN$
  • $r$ = birth rate $-$ death rate
  • Integrated form: $N_t = N_0 e^{rt}$
  • J-shaped curve: Rapid, unchecked growth.
• Logistic growth: Occurs when resources are limited.
  • Equation: $dn/dt = rN \left(\frac{K-N}{K}\right)$
  • $K$ = carrying capacity (maximum population size environment can support)
  • S-shaped (sigmoid) curve: Growth slows as population approaches $K$.

Life History Variations

Organisms exhibit different reproductive strategies based on evolutionary pressures. Some breed only once in their lifetime (semelparity), while others breed multiple times (iteroparity). The number and size of offspring also vary.

• Semelparity: Single reproductive event (e.g., bamboo, salmon).
• Iteroparity: Multiple reproductive events (e.g., birds, mammals).
• Many small offspring: Little parental care, high mortality (e.g., oysters, pelagic fish).
• Few large offspring: High parental care, higher survival (e.g., humans, birds).

Population Interactions

Populations interact in various ways, affecting each other's survival and reproduction. The main types of interactions are predation, competition, parasitism, commensalism, and mutualism.

• Predation: One species (predator) kills and eats another (prey). Maintains energy flow and population control.
• Competition: Two species compete for the same resource, both are negatively affected.
• Parasitism: One species (parasite) benefits at the expense of another (host).
• Commensalism: One species benefits, the other is unaffected.
• Mutualism: Both species benefit.

Examples and Significance of Population Interactions

Population interactions have ecological significance, influencing energy transfer, population control, and species diversity.

• Predation:
  • Maintains prey population under control.
  • Example: Introduction of natural predator (Cactoblastis moth) to control invasive cactus in Australia.
  • Predators are prudent; overexploitation can lead to extinction of both prey and predator.
• Competition:
  • Can occur between closely or distantly related species.
  • Example: Flamingos and resident fish competing for zooplankton.
  • Competitive exclusion principle: Two closely related species competing for the same resource cannot coexist indefinitely; the superior competitor will exclude the other.
  • Resource partitioning: Coexistence by utilizing different resources or niches (e.g., warbler birds feeding at different parts of the same tree).
• Parasitism:
  • Parasites evolve adaptations for survival (loss of unnecessary organs, adhesive organs, high reproductive capacity).
  • Life cycles often involve multiple hosts (e.g., human liver fluke, Plasmodium).
  • Types: Ectoparasites (outside body), endoparasites (inside body), brood parasitism (e.g., cuckoo laying eggs in crow's nest).
• Commensalism:
  • Examples: Orchids on mango branches, barnacles on whales, cattle egrets with cattle, clownfish with sea anemone.
• Mutualism:
  • Examples: Fig and wasp (both depend on each other for reproduction), orchid and bee (pseudo-copulation for pollination).
  • Both partners benefit and may co-evolve.

Key Formulas and Representations

Population growth and density changes are represented mathematically as follows:

• Exponential growth:$$dn/dt = rN$$$$N_t = N_0 e^{rt}$$
• Logistic growth:$$dn/dt = rN \left(\frac{K-N}{K}\right)$$
• Population density change:$$N_{t+1} = N_t + (B + I) - (D + E)$$
• Birth rate: Number of births divided by initial population.
• Death rate: Number of deaths divided by initial population.

Exam-Focused Takeaways

Understanding population attributes, growth models, and interactions is crucial for analyzing ecological dynamics and predicting changes in population size and structure. Key points include:

• Population attributes (birth rate, death rate, sex ratio) are unique to populations, not individuals.
• Age pyramids help predict future population trends.
• Population density can be measured by number, biomass, or percent cover.
• Four processes (natality, mortality, immigration, emigration) control population size.
• Exponential growth occurs with unlimited resources; logistic growth with limited resources and carrying capacity.
• Population interactions (predation, competition, parasitism, commensalism, mutualism) shape community structure and ecosystem stability.
• Examples and case studies illustrate the ecological significance and exam relevance of these concepts.