Population Dynamics and Interactions

Overview

This lecture covers the Organisms and Population chapter, focusing on population attributes, population density, growth models, and population interactions, with emphasis on definitions, equations, graphs, and examples important for exams.

Levels of Biological Organization

• Organism: Single living individual performing all life processes.
• Population: Group of organisms of the same species in a given area using similar resources.
• Community: Group of different populations living and interacting in an area.
• Ecosystem: Interaction of communities with their physical environment.
• Biosphere: Global sum of all ecosystems.

Population: Definition and Attributes

• Population definition: Group of individuals of the same species living in a given geographical area, sharing or competing for similar resources.
• Population ecology: Study of populations, linked with genetics and evolution.

Main Population Attributes

• Attributes belong to population, not individual (e.g., birth rate, death rate, sex ratio, age distribution).

Birth rate (Natality)

• Number of births per unit population per unit time.
• Expressed as per capita birth rate (b).
• Example:
  • Initial lotus plants = 20.
  • New plants added by reproduction = 8.
  • Birth rate = 8/20 = 0.4 offspring per lotus per year.

Death rate (Mortality)

• Number of deaths per unit population per unit time.
• Expressed as per capita death rate (d).
• Example:
  • Initial lotus plants = 20.
  • Deaths = 5.
  • Remaining = 15.
  • Death rate = 5/20 per unit time.

Sex ratio

• Ratio of males to females in a population.
• Helps predict reproductive potential of population.
• Can be applied to animals, humans, or male–female flowers in plants.

Age distribution and Age groups

• Percentage of individuals in different age groups in a population.
• Three age groups:
  • Pre-reproductive: Children, not yet capable of reproduction.
  • Reproductive: Adults and middle-aged, actively reproducing.
  • Post-reproductive: Old individuals, reproduction ended.

Age Pyramids

• Graphical (pyramidal) representation of age distribution of a population.
• Based on percentages in pre-reproductive, reproductive, post-reproductive groups.

• Expanding: Wide base, many children; growth expected.
• Stable: Pre-reproductive and reproductive similar; size stable.
• Declining: Narrow base; fewer children; size will shrink.

Population Size and Population Density

• Population size / density (N): Number of individuals of a species per unit area or volume.
• Represented as N.
• Example drawing: Large circle (area) containing several small circles (individuals), count small circles to get N.

Measuring Population Density

• Direct count (number of individuals) when feasible (e.g., fish in small pond, bacteria on slide).
• Biomass (total weight of living organisms) used when counting individuals is difficult (e.g., huge banyan tree, dense grassland).
• Indirect signs when organisms are hard to see:
  • Fecal pellets (droppings).
  • Pug marks (footprints).
  • Nests, burrows.

Factors Causing Fluctuations in Population Density

Four basic processes change population density:

• Processes increasing density: natality, immigration.
• Processes decreasing density: mortality, emigration.

Population Density Equation

• If Nₜ is population density at time t, Nₜ₊₁ at time t+1:

  • Nₜ₊₁ = Nₜ + B + I − D − E
    (B = total births, I = total immigrants, D = total deaths, E = total emigrants)

• In symbolic per capita form: Nₜ₊₁ depends on Nₜ plus gains minus losses.

Population Growth Models

Two idealised models:

• Exponential growth model.
• Logistic growth model.

Exponential Growth Model

• Condition: Unlimited resources (food, space, etc.).
• Under unlimited resources, population grows very rapidly.

Graph

• Axes:
  • X-axis: Time.
  • Y-axis: Population density.
• Shape: J-shaped curve.
• Population increases slowly at first, then very rapidly (explosive growth).

Equation

• dN/dt = rN
  • N = population size or density.
  • r = intrinsic rate of natural increase (rate at which population grows under ideal conditions).

Features

• Unrealistic in nature because unlimited resources do not exist.
• If checks are absent, even slow-growing animals (like elephants) could reach enormous numbers.

Logistic Growth Model (Verhulst–Pearl Model)

• More realistic; resources are limited.
• Nature has a carrying capacity (K): maximum population size that environment can support.

Graph

• Axes:
  • X-axis: Time.
  • Y-axis: Population density.
• Shape: Sigmoid or S-shaped curve.
• Phases:
  • Lag phase: Slow initial growth.
  • Log (exponential) phase: Rapid growth.
  • Stationary phase: Growth slows and levels off near K.

Equation

• dN/dt = rN (K − N) / K
  • N = population density.
  • r = intrinsic rate of natural increase.
  • K = carrying capacity of environment.

Features

• Growth slows as N approaches K because resources become limiting.
• Describes how carrying capacity regulates population size.
• Considered realistic for natural populations.

Comparison Table: Exponential vs Logistic Growth

Population Interactions

• Two broad interaction types:
  • Interspecific: Between individuals of different species.
  • Intraspecific: Among individuals of same species.

Effect Notation

• Positive (+): Beneficial effect.
• Negative (−): Harmful effect.
• Zero (0): No effect.

Predation

• Predator: Organism that kills and eats another.
• Prey: Organism that is eaten.

Ecological Roles of Predators

• Help transfer energy to higher trophic levels in food chains.
• Keep prey populations under control.
• Maintain species diversity in communities.

Examples

• Grass → Grasshopper → Lizard → Snake:
  • Each consumer is predator of lower trophic level.
• Herbivores are predators of plants:
  • About 25% of all insects are phytophagous (plant-eating).

Competition

• Both species negatively affected.
• Occurs when they compete for the same limiting resource.

Gauss’s Competitive Exclusion Principle

• When two closely related species compete for the same resource:
  • They cannot coexist indefinitely.
  • Competitively superior species survives.
  • Inferior species is eliminated.

Examples

• Abingdon tortoise vs goats in Galapagos Island:
  • Goats introduced by humans.
  • Goats grazed vegetation faster.
  • Abingdon tortoise lost food, became extinct locally.
  • Superior competitor (goat) persists, inferior (tortoise) eliminated.
• Connell’s barnacle experiment (Scotland intertidal zone):
  • Larger barnacle species dominates and excludes smaller species from part of the shore.

Competitive Release

• When competing species is removed, the restricted species can expand its distribution range significantly.

Resource Partitioning

• Competing species avoid direct competition by using different parts or aspects of the same resource.

Example

• Warblers (birds) on a tree:
  • Different species feed in different parts of the same tree (top, middle, lower branches, bark).
  • Each species uses different microhabitat or food, reducing competition.

Parasitism

• Parasite lives on or in host and derives nutrients, harming host.
• Host–parasite often co-evolve.

General Adaptations of Parasites

• Loss of sense organs.
• Presence of suckers or adhesive organs to cling to host.
• Loss or reduction of digestive system (absorbs pre-digested food).
• Very high reproductive capacity.

Types of Parasites and Examples

• Brood parasitism:
  • Cuckoo lays eggs in crow’s nest.
  • Cuckoo chick raised by crow, cuckoo parents avoid parental care.

Commensalism

• One species benefits, other neither harmed nor benefited.

Key Examples

• Orchid on mango tree:
  • Orchid gains support and better light.
  • Mango tree unaffected.
• Barnacles on whales:
  • Barnacles gain transport and access to food-rich water.
  • Whale unaffected.
• Cattle and cattle egret:
  • Egrets eat insects flushed out by grazing cattle.
  • Cattle unaffected.
• Sea anemone and clownfish:
  • Clownfish gains protection among stinging tentacles and access to food scraps.
  • Sea anemone not significantly affected.

Mutualism

• Both species benefit from association.

Examples

• Fig tree and wasp:
  • Wasp lays eggs inside fig inflorescence; protected site for larvae.
  • Wasp helps in pollination of fig flowers.
• Lichens:
  • Symbiosis between algae (or cyanobacteria) and fungus.
  • Algae provide food via photosynthesis.
  • Fungus provides water, minerals, and protection.
• Mycorrhizae:
  • Association between fungus and plant roots.
  • Fungus helps roots absorb water and minerals (especially phosphorus).
  • Plant provides organic nutrients to fungus.
• Sexual deceit in Ophrys orchid (ferocious orchid):
  • Flower mimics female bee appearance and scent.
  • Male bees attempt pseudo-copulation, get deceived.
  • Result: Pollination of orchid (benefit), bee gains temporary sexual stimulus.
  • This is mutualistic at the level of pollination but involves sexual deceit.
  • Term: Pseudocopulation.

Example–Interaction Matching (Exam Style)

• Ophrys orchid and bees → Mutualism (sexual deceit and pollination).
• Cattle and cattle egret → Commensalism.
• Sea anemone and clownfish → Commensalism.
• Ticks and dogs → Parasitism.
• Cuscuta and host plant → Parasitism (brood/plant parasite type).
• Tiger and deer → Predation.

Key Terms & Definitions

• Organism: Individual living being.
• Population: Group of same species individuals in area sharing resources.
• Population density (N): Number of individuals per unit area or volume.
• Natality (birth rate, b): Number of births per unit population per time.
• Mortality (death rate, d): Number of deaths per unit population per time.
• Immigration (i): Individuals entering a population from elsewhere.
• Emigration (e): Individuals leaving a population to another area.
• Age distribution: Percentage of individuals in different age groups.
• Pre-reproductive: Age group before reproductive capability.
• Reproductive: Age group actively reproducing.
• Post-reproductive: Age group after reproductive period.
• Age pyramid: Graphical representation of age distribution.
• Exponential growth: Unlimited resource growth, J-shaped curve.
• Logistic growth: Growth with carrying capacity, S-shaped curve.
• Carrying capacity (K): Maximum population size environment can support.
• Intrinsic rate of natural increase (r): Potential growth rate under ideal conditions.
• Interspecific interaction: Between different species.
• Intraspecific interaction: Among same species.
• Mutualism (+,+): Both species benefit.
• Competition (−,−): Both harmed by shared resource use.
• Predation (+,−): Predator benefits, prey harmed.
• Parasitism (+,−): Parasite benefits, host harmed.
• Commensalism (+,0): One benefits, other unaffected.
• Amensalism (−,0): One harmed, other unaffected.
• Resource partitioning: Division of resources to reduce competition.
• Competitive exclusion principle: Inferior competitor eliminated when competing for same resource.
• Brood parasitism: One species uses another to incubate eggs and raise young.
• Pseudocopulation: False mating behaviour caused by mimicry, leading to pollination.

Action Items / Next Steps

• Revise NCERT text for “Organisms and Population” thoroughly, focusing on definitions and diagrams.
• Memorise:
  • Four population attributes and examples.
  • Population density equation Nₜ₊₁ = Nₜ + B + I − D − E.
  • Exponential and logistic growth equations and graph shapes (J vs S).
  • Definitions and symbols for r and K.
  • Gauss’s competitive exclusion principle and examples (Abingdon tortoise, barnacles).
• Learn and be able to reproduce:
  • Age pyramids (expanding, stable, declining) and their interpretations.
  • Interaction table with sign combinations (+, −, 0) and correct examples.
• Practice previous exam questions on:
  • Population attributes.
  • Population growth models.
  • Population interactions (matching interactions with examples).