Organisms and Populations

Organisms and Populations

This chapter Organisms and Populations, focuses on the interactions between organisms and their environment, exploring key concepts such as abiotic factors, adaptations, population dynamics, and various types of species interactions.

Introduction

• The living world is remarkably diverse and organised in a hierarchical manner.
• Levels of Biological Organisation
  • Macromolecules
  • Cells
  • Tissues
  • Organs
  • Individual organism
  • Populations
  • Communities
  • Ecosystems
  • Biomes
• Individual organism forms the basic unit of ecological hierarchy.

Ecology

• Definition
  • Ecology is the branch of biology that studies interactions:
    1. Among organisms
    2. Between organisms and their physical (abiotic) environment
• The term “ecology” was first coined by Ernst Haeckel in 1866.

• Ecological Levels of Organisation
  1. Organism
  2. Population
  3. Community
  4. Biome

This chapter mainly focuses on organism and population levels.

Organism and Its Environment

Physiological Ecology

• Ecology at organismic level is often termed physiological ecology.
• It explains how organisms adapt to their environment in order to:
  1. Survive
  2. Reproduce successfully

Environmental Variations and Biome Formation

• Causes of Seasonal Variation
  • Earth’s rotation
  • Tilt of Earth’s axis
  • Variation in annual precipitation (rain and snow)
• These factors lead to formation of major biomes such as:
  • Desert
  • Rainforest
  • Tundra
• Regional and local variations within biomes create diverse habitats.

Habitats

• Definition
  • A habitat is the specific place where an organism lives.
  • It is characterised by a combination of:
    • Physical conditions
    • Chemical conditions
    • Biotic interactions

• Life exists in both favourable and extreme habitats such as:
  • Deserts (e.g., Rajasthan desert)
  • Rain-soaked forests (e.g., Meghalaya forests)
  • Deep oceans
  • Polar regions
  • Mountain tops
  • Thermal springs
  • Intestine of animals

Components of Habitat

1. Abiotic Components (main)
   • Temperature
   • Water
   • Light
   • Soil
2. Biotic Components
   • Pathogens
   • Parasites
   • Predators
   • Competitors

An organism constantly interacts with both components.

Adaptations

• Over evolutionary time, organisms develop adaptations through natural selection.
• Purpose of Adaptations:
  • Optimise survival
  • Ensure successful reproduction

Ecological Niche

• Definition
  • A niche is the functional role and position of a species within its habitat.
• It includes:
  • Range of environmental conditions tolerated
  • Resources utilised
  • Microclimate occupied
  • Timing of activity
  • Type of predators and competitors

Important Concepts

• Each species occupies a distinct niche.
• No two species occupy exactly the same niche.
• Sharing of resources between species is called niche overlap.
• Abundance of species within its habitat is referred to as niche density.

Major Abiotic Factors

1. Temperature

• Importance:
  • Temperature is one of the most ecologically significant environmental factors.
• Variation & Range:
  • Varies seasonally on land.
  • Decreases from the equator to poles and from plains to mountains.
  • Subzero levels in polar regions and high altitudes
  • Above 50°C in tropical deserts
  • Above 100°C in thermal springs and deep-sea hydrothermal vents

Effect of Temperature on Organisms

• Physiological Impact
  • Influences enzyme kinetics
  • Affects basal metabolic rate
  • Regulates activity and physiological functions

• In plants, temperature affects:
  • Photosynthesis
  • Respiration
  • Flowering
  • Mineral and water absorption
  • Growth and development

• In animals, temperature influences:
  • Metabolic activities
  • Distribution
  • Behaviour
  • Growth and development
  • Sex ratio and colouration

Geographical Distribution: Thermal tolerance largely determines species distribution.

Thermal Tolerance Categories

Based on tolerance range:

1. Eurythermal Organisms
   • Tolerate wide range of temperatures
   • Example: Most mammals and birds
2. Stenothermal Organisms
   • Survive within narrow temperature range
   • Example: Polar bear, amphibians, many plants

Regulation of Body Temperature

Based on ability to maintain internal temperature:

• Poikilothermal (Ectothermic) Organisms
  • Body temperature varies with environment
  • Example: Reptiles, amphibians
• Homoiothermal (Endothermic) Organisms
  • Maintain constant internal temperature
  • Example: Birds and mammals

2. Water

• Importance:
  • Water is essential for all life forms.
• It regulates:
  • Metabolic reactions
  • Nutrient transport
  • Temperature balance
  • Cellular processes

Availability

• Water availability varies greatly:
  • Scarce in deserts
  • Abundant in oceans, lakes and rivers
• For aquatic organisms, not only quantity but quality of water is important.

Water Quality and Salinity

Salinity

• Salt concentration is measured in parts per thousand (ppt).
  • Inland freshwater: < 5 ppt
  • Sea water: 30–35 ppt
  • Hypersaline lagoons: > 100 ppt
• Chemical composition and pH also influence survival.

Salinity Tolerance

1. Euryhaline Organisms
   • Tolerate wide range of salinity
   • Example: Salmon
2. Stenohaline Organisms
   • Tolerate narrow range of salinity
   • Example: Shark

Osmotic Problems

• Freshwater and marine environments differ greatly in salt concentration.
  • Freshwater animals placed in seawater lose water rapidly.
  • Marine animals placed in freshwater gain excess water.
• Such osmotic imbalance can be fatal.
• Therefore, many species are restricted to specific salinity ranges.

3. Light

• Importance in Plants:
  • Light is essential for photosynthesis.
  • Light characteristics affecting organisms:
    1. Intensity
    2. Duration
    3. Quality (wavelength)

• Photoperiod
  • Duration of daylight regulates flowering in many plants.
• Forest Adaptation
  • Small herbs and shrubs under tall canopy trees adapt to low light conditions.

• Importance in Animals:
  • Animals use light as a cue for:
    • Foraging
    • Reproduction
    • Migration
• Seasonal changes in light (photoperiod) regulate behavioural patterns.

Light and Aquatic Ecosystems

• Light penetration decreases with depth.
• Deep ocean regions (>500 m) remain permanently dark.
• Organisms there rely on:
  • Chemosynthesis
  • Organic matter sinking from upper layers

Light availability is often correlated with temperature gradients.

4. Soil

• Definition
  • Soil is the upper weathered layer of earth’s crust that supports terrestrial plant life.
  • The study of soil science is called pedology.

• Factors Affecting Soil Formation
  1. Climate
  2. Weathering of rocks
  3. Sedimentation
  4. Method of soil development

Soil Characteristics

1. Physical Properties

• Composition
• Grain size
• Aggregation

• These determine:
  1. Water percolation
  2. Water holding capacity

• Particle Size Order
  • Clay < Silt < Fine sand < Coarse sand < Gravel
• The proportion of these particles gives rise to soil types:
  1. Sandy soil
  2. Clayey soil
  3. Loamy soil

2. Chemical Properties

• pH
• Mineral composition

Topography also influences soil properties.

Soil Profile

• A vertical section from surface to parent rock is called a soil profile.
• It includes:
  • O-horizon (surface litter)
  • A-horizon (topsoil)
  • B-horizon (subsoil)
  • C-horizon (parent material)

Soil and Vegetation

• Soil characteristics largely determine:
  • Type of vegetation
  • Distribution of plant species
• In aquatic ecosystems, sediment properties determine the type of benthic animals present.

Conceptual Integration

• Water, light, and soil collectively determine:
  • Species distribution
  • Vegetation pattern
  • Adaptation strategies
  • Community structure
• These abiotic factors form the foundation of population dynamics and ecological interactions.

Responses to Abiotic Factors

• Organisms are constantly exposed to fluctuating environmental conditions.
• To survive stressful abiotic conditions, they have evolved different strategies.

Homeostasis

• Definition:
  • Homeostasis is the process by which an organism maintains a relatively constant internal environment despite changes in the external environment.
• Maintained Parameters
  • Body temperature
  • Osmotic concentration
  • pH
  • Metabolic balance
• Importance
  • Ensures optimal enzyme activity and maximum efficiency of physiological processes.
• Example
  • Humans use heaters and air conditioners to maintain comfortable temperature (~25°C).

Strategies for Coping with Environmental Stress

1. Regulation

• Definition
  • Organisms that maintain constant internal temperature or osmotic balance through physiological or behavioural mechanisms are called regulators.
• Examples
  • Birds
  • Mammals
  • Few lower vertebrates and invertebrates
• Mechanisms
  • Sweating in summer (cooling)
  • Shivering in winter (heat production)
  • Osmoregulation in aquatic animals

Note – Some organisms are partial regulators — they regulate within a limited range and conform beyond that.

• Energy Cost
  • Thermoregulation is energetically expensive.
  • Small animals lose heat rapidly due to:
    • Larger surface area relative to volume
  • Example:
    • Shrews and hummingbirds are absent in polar regions due to high energy demands.
    • Plants generally lack temperature regulation mechanisms.

2. Conform

• Definition
  • Organisms that cannot maintain constant internal conditions are called conformers.
• Characteristics
  • Body temperature changes with ambient temperature
  • Internal osmotic concentration varies with surroundings
• Statistics
  • About 99% of animals
  • Nearly all plants
• Example
  • Most invertebrates and aquatic organisms.

3. Migration

• Definition
  • Temporary movement from unfavourable habitat to more favourable conditions.
• Purpose
  • Avoid seasonal or extreme stress.
• Examples
  • Birds migrating to warmer regions during winter
  • Humans moving to cooler places during summer

4. Suspension

• Definition
  • Temporary escape from adverse conditions by reducing metabolic activity or entering dormancy.

Forms of Suspension

1. Hibernation
   • Winter dormancy
   • Example: Bears
2. Aestivation
   • Summer dormancy
   • Example: Snails and fish
3. Diapause
   • Stage of suspended development
   • Example: Zooplankton in lakes and ponds

• Plant Examples
  • Formation of thick-walled spores (bacteria, fungi, lower plants)
  • Seeds and vegetative parts remain dormant until favourable conditions return

Adaptations

• Definition:
  • Adaptation is any morphological, physiological, or behavioural attribute that enables an organism to survive and reproduce in its habitat.

Morphological Adaptations (examples)

1. Desert Plants (Xerophytes)

• Thick cuticle → reduces water loss
• Sunken stomata → minimises transpiration
• CAM pathway → stomata open at night
• Leaves reduced to spines (e.g., Opuntia)
• Flattened green stems perform photosynthesis

2. Cold Climate Mammals

• Allen’s Rule
  • Mammals in colder climates have shorter ears and limbs to minimise heat loss.
  • Example: Arctic fox compared to desert fox.
• Blubber
  • Thick fat layer in seals provides insulation.

Physiological Adaptations (examples)

• Kangaroo Rat (Desert Adaptation)
  • Rarely drinks water
  • 90% water obtained from metabolic oxidation of fat
  • Produces highly concentrated urine
  • Minimises water loss through faeces

• High Altitude Adaptation
  • Increased red blood cell count
  • Higher haemoglobin concentration
  • Increased breathing rate
  • Example: Himalayan populations (Tribes) show higher RBC counts.

• Microbial Adaptations
  • Archaebacteria survive in hot springs (>100°C)
  • Fish survive in Antarctic waters below freezing
  • Marine invertebrates tolerate high hydrostatic pressure

Behavioural Adaptations (examples)

• Desert Lizards
  • Bask in sunlight when cold
  • Seek shade during extreme heat
  • Burrow into soil to avoid temperature extremes

Conceptual Integration

• Organisms respond to abiotic stress through:
  • Regulation
  • Conformation
  • Migration
  • Dormancy
• Long-term survival depends on evolutionary adaptations.
• These responses determine species distribution, abundance, and ecological success.

Populations

• Definition
  • A population is a group of individuals of the same species living in a specific geographical area at a given time.
• Key Characteristics
  • Share or compete for similar resources
  • Potentially interbreed
  • Even asexually reproducing individuals are considered a population for ecological study
• Examples
  • Bacteria in a petri dish
  • Rats in a house
  • Teak trees in a forest
  • Lotus plants in a pond

Importance in Ecology

• Natural selection operates at the population level.
• Population is the unit where evolutionary change occurs.
• Links ecology with population genetics and evolution.

• Note:
  • Individuals have births and deaths.
  • Populations have birth rate and death rate.

Attributes of a Population

A population possesses characteristics that an individual organism does not.

1. Birth Rate (Natality)

• Definition
  • Number of births per individual (per capita) in a population during a given time.
• Example
  • If 20 lotus plants produce 8 new plants in one year:
  • Birth rate = 8 / 20 = 0.4 offspring per individual per year.
• Effect
  • Increases population density.

2. Death Rate (Mortality)

• Definition
  • Number of deaths per individual (per capita) during a given time.
• Example
  • If 4 out of 40 fruit flies die:
  • Death rate = 4 / 40 = 0.1 per individual.
• Effect
  • Decreases population density.

3. Sex Ratio

• Definition
  • Proportion of males and females in a population.
• Example
  • 60% females and 40% males.
• Importance
  • Determines reproductive potential and future population growth.

4. Age Distribution (Age Structure)

• Definition
  • Proportion of individuals in different age groups within a population.
• Ecological Age Groups
  1. Pre-reproductive (juvenile)
  2. Reproductive (adult)
  3. Post-reproductive (old)

Age Pyramid

Graphical representation of age distribution in a population.

Types of Age Pyramids

1. Triangular (Expanding Population)
   • Large pre-reproductive group
   • Rapid population growth
2. Bell-shaped (Stable Population)
   • Nearly equal proportion in all age groups
   • Zero or very slow growth
3. Urn-shaped (Declining Population)
   • Larger post-reproductive group
   • Negative population growth

Age structure directly influences natality and mortality.

5. Population Size and Density (N)

• Definition
  • Population density is the number of individuals of a species per unit area or volume at a given time.
• Importance
  • Indicates population status within a habitat.
  • Population size may range from a few individuals to millions.

Methods of Measuring Population Density

1. Total Count
   • Total number of individuals present.
   • Not practical for very large populations.
2. Per Cent Cover or Biomass
   • Used when counting individuals is impractical or misleading.
   • Example: One large banyan tree versus thousands of small weed plants.
3. Relative Density
   • Uses indirect indicators of abundance.
   • Examples:
     • Bacterial colonies in a petri dish
     • Number of fish caught per trap
4. Indirect Estimation
   • Used when direct counting is difficult.
   • Examples:
     • Pug marks for tiger census
     • Fecal pellets for wildlife estimation

Age distribution and population density together help determine growth trends and ecological stability.

Population Growth

Dynamic Nature

• Population size is not static; it changes over time.

Factors Influencing Population Size

• Food availability
• Predation pressure
• Weather conditions
• Habitat conditions

Population density helps determine whether a population is increasing or decreasing.

Four Basic Factors/Processes Affecting Population Density

Population density fluctuates due to four fundamental processes:

1. Natality (B)
   • Number of births during a given period.
   • Increases population density.
2. Mortality (D)
   • Number of deaths during a given period.
   • Decreases population density.
3. Immigration (I)
   • Individuals entering a habitat from elsewhere.
   • Increases population density.
4. Emigration (E)
   • Individuals leaving the habitat.
   • Decreases population density.

Population Density Equation

If $N_t$Nt is the population density at time t, then:

Nₜ₊₁ = Nₜ + [(B + I) − (D + E)]

• Where:
  • B = Number of births
  • I = Number of immigrants
  • D = Number of deaths
  • E = Number of emigrants

Interpretation

Population increases when:
(B + I) > (D + E)

Population decreases when:
(B + I) < (D + E)

• Note
  • Under normal conditions, natality and mortality are the primary determinants of population growth.
  • Immigration and emigration become important during:
    • Colonization of new habitats
    • Seasonal migration
    • Disturbance events

Conceptual Link

• Population attributes determine:
  • Growth pattern
  • Evolutionary change
  • Ecological stability
  • Species survival

The study of populations (Demography) forms a bridge between ecology and evolution.

Growth Models

Introduction to Population Growth

Population size changes over time in a predictable manner.

• Growth depends on:
  • Food availability
  • Predation pressure
  • Weather conditions
  • Space and other limiting resources
• Two major growth patterns are observed:
  1. Exponential growth
  2. Logistic growth

1. Exponential Growth

• Condition
  • Occurs when resources are unlimited and environmental resistance is absent.
• Principle
  • Under ideal conditions, population grows at its maximum intrinsic rate.

Mathematical Expression

$$\frac{dN}{dt} = (b – d) \times N$$

• Where:
  • N = Population size
  • b = Birth rate
  • d = Death rate

Let, r = (b − d)

Then,$$\frac{dN}{dt} = rN$$

Where r = Intrinsic rate of natural increase

Interpretation

• If r > 0 → Population increases
• If r = 0 → Population stable
• If r < 0 → Population declines

Examples of r Values

• Norway rat → r = 0.015
• Flour beetle → r = 0.12
• Humans in India (1981) → r = 0.0205

Growth Curve

• J-shaped growth curve
• Characteristics
  • Slow initial phase
  • Rapid acceleration
  • Sudden crash when environmental resistance appears

Classic Illustrations

• Chessboard Example:
  • Starting with one grain and doubling on each square results in enormous growth by the 64th square.
• Microbial Example:
  • A Paramecium doubling daily can reach extremely large numbers in a short time.

Limitation

• Exponential growth is not sustainable in nature due to limited resources.

2. Logistic Growth

• Condition
  • Occurs when resources are limited.
• Principle
  • As population size increases, competition intensifies and growth rate slows down due to resource limitation.

Carrying Capacity (K)

• Definition
  • Maximum population size that a habitat can support sustainably.

Growth Phases

1. Lag Phase
   • Slow initial growth.
2. Exponential (Acceleration) Phase
   • Rapid population increase.
3. Deceleration Phase
   • Growth slows due to limited resources and increasing competition.
4. Asymptote Phase
   • Population stabilizes at carrying capacity (K).

Mathematical Expression

$$\frac{dN}{dt} = rN \frac{(K – N)}{K}$$

• Where:
  • N = Population size
  • r = Intrinsic rate of increase
  • K = Carrying capacity

Growth Curve

• S-shaped (Sigmoid curve)

Why Logistic Model is More Realistic

• Resources are finite
• Environmental resistance increases with population density
• Most natural populations follow this pattern

Comparison: Exponential vs Logistic Growth

• Exponential Growth
  • Unlimited resources
  • No environmental resistance
  • J-shaped curve
  • Theoretical/ideal condition
• Logistic Growth
  • Limited resources
  • Environmental resistance present
  • S-shaped curve
  • Realistic natural condition

Life History Variation

• Concept
  • Populations evolve life history strategies to maximize reproductive success.

Darwinian Fitness

• Definition
  • Reproductive success of an organism in its natural habitat.
  • Higher fitness is generally associated with a higher intrinsic rate of increase (r).

Reproductive Strategies

1. Single Breeding Event (Semelparity)
   • Organisms reproduce once and then die.
   • Examples:
     • Pacific salmon
     • Bamboo
2. Multiple Breeding Events (Iteroparity)
   • Organisms reproduce several times during their lifetime.
   • Examples:
     • Most birds
     • Most mammals

Offspring Size vs Number

Strategy 1: Many Small Offspring

• Examples:
  • Oysters
  • Pelagic fishes
• Features:
  • High mortality rate
  • Little or no parental care
  • High intrinsic rate (r)

Strategy 2: Few Large Offspring

• Examples:
  • Birds
  • Mammals
• Features:
  • High parental care
  • Greater survival probability
  • Lower intrinsic rate (r)

Key Conceptual Link

• Population growth pattern depends on:
  • Resource availability
  • Environmental resistance
  • Life history traits
  • Abiotic and biotic constraints
• Understanding life history variation and growth models helps in:
  • Wildlife management
  • Pest control
  • Conservation biology
  • Human population management

Population Interactions

No Single-Species Habitat

• No habitat on Earth is occupied by only one species.
• Every organism depends directly or indirectly on others for survival.

Biological Community

• Plants, animals, and microbes interact to form biological communities.
• Even plants depend on:
  • Soil microbes for nutrient absorption
  • Animal agents for pollination and seed dispersal

Interspecific Interactions

• Definition
  • Interactions between populations of different species.
• Nature of Interaction
  • Interactions may be:
    • Positive (+)
    • Negative (–)
    • Neutral (0)
• Possible Outcomes
  • Mutualism (+/+) → Both species benefit
  • Competition (–/–) → Both species are harmed
  • Predation (+/–) → Predator benefits, prey harmed
  • Parasitism (+/–) → Parasite benefits, host harmed
  • Commensalism (+/0) → One benefits, other unaffected
  • Amensalism (–/0) → One harmed, other unaffected

Predation

• Definition
  • An interaction in which one species (predator) captures, kills, and consumes another species (prey).

Ecological Role

1. Energy Transfer

• Predation transfers energy fixed by plants to higher trophic levels.
• Examples:
  • Tiger eating deer
  • Sparrow eating seeds

(Herbivores are ecologically considered predators of plants.)

2. Population Regulation

• Controls prey population size
• Prevents ecosystem instability

In the absence of predators, prey populations may grow excessively.

Example – Invasive Species

• Exotic species may become invasive in the absence of natural predators.
• Example: Prickly pear cactus in Australia was controlled after introduction of a moth predator.

3. Biological Control

• Agricultural pest management is based on predator–prey regulation.

4. Maintaining Species Diversity

• Predators reduce competition among prey species.
• Classic example: Removal of starfish Pisaster from the American Pacific coast led to extinction of several competing species.

Prudent Predators

• Overexploitation of prey can cause extinction of both prey and predator.
• Thus, natural predators are generally “prudent” and avoid overexploitation.

Prey and Plant Defences

Prey Defences

• Camouflage (cryptic coloration)
• Poison production

• Example: Monarch butterfly becomes distasteful by feeding on poisonous weeds during larval stage.

Plant Defences (Against Herbivores)

• Since plants cannot escape, they evolve strong defence mechanisms.

• Morphological Defences
  • Thorns (Acacia, cactus)
• Chemical Defences
  • Toxic substances
  • Cardiac glycosides (e.g., Calotropis)
  • Commercial plant chemicals such as nicotine, caffeine, quinine

These adaptations reduce herbivore damage and increase survival.

Competition

• Definition
  • Competition is an interaction in which the fitness (intrinsic rate of increase, r) of one species is reduced due to the presence of another species.
• Darwin’s Concept
  • Darwin’s ideas of “struggle for existence” and “survival of the fittest” emphasize the role of competition in evolution.
  • Competition may occur even between unrelated species if they use the same resource.

Types of Competition

1. Intraspecific Competition
   • Occurs between individuals of the same species.
   • More severe because resource requirements are identical.
2. Interspecific Competition
   • Occurs between individuals of different species.

Based on Resource Use

• Resource Competition
  • Occurs when species compete for limited resources.
  • Example: Flamingoes and fishes competing for zooplankton.
• Interference Competition
  • One species directly reduces the feeding efficiency of another, even when resources are abundant.

Competitive Exclusion Principle

• Proposed by
  • G. F. Gause
• Statement
  • Two closely related species competing for the same limiting resource cannot coexist indefinitely.
  • The competitively inferior species will eventually be eliminated.
• Limitation
  • Applicable mainly when resources are limiting.
  • Not universally applicable under all ecological conditions.
  • Laboratory experiments support elimination under limiting resources.

Examples from Nature

• Connell’s Experiment
  • Joseph Connell showed that Balanus (larger barnacle) excluded Chthamalus (smaller barnacle) on Scottish coasts.
• Competitive Release
  • When a superior competitor is removed, the restricted species expands its distribution.
• Resource Partitioning
  • Species evolve mechanisms to reduce competition and coexist.
  • Example: Robert MacArthur demonstrated that five warbler species coexisted on the same tree by using different foraging zones.
• Impact
  • Competition strongly influences evolution, species distribution, and community structure.

Parasitism

• Definition
  • An interaction in which one organism (parasite) derives food and shelter from another organism (host), causing harm.
• Evolutionary Aspect
  • Parasites and hosts co-evolve.
  • Host develops resistance → Parasite evolves counter-adaptations.

Adaptive Features of Parasites

• Loss of unnecessary sense organs
• Adhesive organs or suckers
• Reduced or absent digestive system
• High reproductive capacity

Types of Parasites

1. Ectoparasites

• Live on the external surface of the host.
• Examples:
  • Lice on humans
  • Ticks on dogs
  • Cuscuta (parasitic plant)
• Cuscuta lacks chlorophyll and derives nutrition from host plants.

2. Endoparasites

• Live inside the host body (liver, lungs, intestine, RBCs, etc.).
• Features:
  • Complex life cycles
  • High reproductive output
  • Simplified body structure
• Examples:
  • Human liver fluke (uses snail and fish as intermediate hosts)
  • Malarial parasite (vector: mosquito)

Brood Parasitism

• Definition
  • A parasitic bird lays its eggs in the nest of another bird species.
• Mechanism
  • Host incubates and rears the parasitic chick.
• Evolutionary Adaptation
  • Parasitic eggs resemble host eggs to avoid detection.
  • Example: Cuckoo (koel) laying eggs in crow’s nest.

Ecological Significance of Parasitism

• Effects on Host
  • Reduced survival
  • Reduced growth
  • Lower reproduction
  • Increased vulnerability to predators

Parasites influence population density and natural selection.

Conceptual Integration

• Population interactions:
  • Regulate population size
  • Influence species diversity
  • Drive natural selection
  • Shape community structure
• Understanding these interactions is essential for:
  • Ecosystem management
  • Conservation biology
  • Pest control strategies
  • Evolutionary ecology

Commensalism

• Definition:
  • Commensalism is an interaction between two species in which one species benefits while the other is neither harmed nor benefited (+ / 0).
• Key Characteristics:
  • One-sided benefit
  • No measurable harm or benefit to the host species

Examples of Commensalism

1. Orchid on Mango Tree

• Mechanism:
  • Orchid grows as an epiphyte on the mango tree branch.
• Benefit:
  • Orchid gains physical support and better access to sunlight.
• Effect on Mango Tree:
  • No nutrient extraction → mango tree remains unaffected.
  • Such epiphytes are termed ectocommensals as they use the host only for attachment.

2. Barnacles on Whale

• Mechanism:
  • Barnacles attach to the whale’s body surface.
• Benefit:
  • Transport to nutrient-rich waters.
• Effect on Whale:
  • No significant harm or benefit.

3. Cattle Egret and Grazing Cattle

• Mechanism:
  • Cattle disturb insects while grazing.
• Benefit:
  • Egrets feed on insects flushed out.
• Effect on Cattle:
  • Unaffected.

4. Clown Fish and Sea Anemone

• Mechanism:
  • Clown fish lives among the stinging tentacles of sea anemone.
• Benefit:
  • Protection from predators.
• Effect on Sea Anemone:
  • No clear benefit or harm.

Mutualism

• Definition:
  • Mutualism is an interaction between two species in which both species benefit (+ / +).
• Nature of Association:
  • Often obligatory (partners cannot survive independently)
  • May involve exchange of food, shelter, protection, or reproductive services

Classic Examples of Mutualism

1. Lichens

• Partners:
  • Fungus + Alga/Cyanobacterium
• Role of Alga:
  • Performs photosynthesis → produces carbohydrates.
• Role of Fungus:
  • Provides protection, moisture retention, and minerals.
• This is an intimate and often obligatory mutualism.

2. Mycorrhizae

• Partners:
  • Fungi + Plant roots
• Role of Fungi:
  • Enhance absorption of water and minerals (especially phosphorus).
• Role of Plant:
  • Provides carbohydrates to fungi.
• This association improves plant growth and soil nutrient cycling.

Plant–Animal Mutualism

General Principle

• Plants depend on animals for:
  • Pollination
  • Seed dispersal
• In return, plants provide:
  • Nectar
  • Pollen
  • Fruits
• Such associations often lead to co-evolution.

Fig–Wasp Mutualism (One-to-One Specificity)

• Mechanism:
  • Each fig species has a specific pollinator wasp species.
  • Female wasp enters the fig inflorescence to lay eggs.
  • Pollination occurs during egg laying.
• Benefit:
  • To Wasp:
    • Fig fruit acts as oviposition site
    • Developing seeds nourish larvae
  • To Fig:
    • Assured pollination
• This represents a highly specific, co-evolved mutualism.

Orchid–Bee Interaction (Sexual Deceit)

• Example
  • Mediterranean orchid (Ophrys)
• Mechanism
  • One petal resembles a female bee in size, colour, and markings.
  • Male bee attempts pseudocopulation.
  • Pollination occurs during this act.
• Evolutionary Insight
  • Any morphological change in the bee must be matched by changes in orchid morphology.
  • This demonstrates co-evolution.

Amensalism (Conceptual Clarity)

• Definition
  • An interaction in which one species is harmed while the other remains unaffected (– / 0).
  • Example: Grevillea robusta releases chemicals that inhibit growth of nearby seedlings.

Protocooperation (Advanced Concept)

• Definition
  • Both species benefit, but the association is not obligatory.
  • Example: Bird removing ticks from rhinoceros.

Ecological Significance

• Commensalism
  • Enhances survival without harming host
  • Promotes species coexistence
• Mutualism
  • Increases reproductive success
  • Drives co-evolution
  • Maintains biodiversity

Many ecosystems depend heavily on mutualistic networks such as pollination webs.

Chapter Summary

Ecology – Core Concept

• Definition
  • Ecology is the study of interactions among organisms and between organisms and their abiotic (physical and chemical) environment.
• Levels of Ecological Organisation
  • Organism
  • Population
  • Community
  • Biome
• The individual organism is the basic unit of ecological hierarchy.

Organism and Environment

Abiotic Factors

• Major environmental factors influencing life:
  • Temperature
  • Water
  • Light
  • Soil
• These factors determine the distribution, survival, and reproduction of organisms.

Adaptations

• Organisms evolve morphological, physiological, and behavioural adaptations to survive in specific habitats.
• Examples include:
  • Desert plant modifications
  • Blubber in polar animals
  • CAM pathway in xerophytes

Responses to Abiotic Stress

• Homeostasis
  • Maintenance of stable internal conditions despite external fluctuations.

Coping Strategies

• Regulation
  • Some organisms maintain constant body temperature or osmotic balance.
• Conformation
  • Most organisms adjust internal conditions according to the environment.
• Migration
  • Temporary movement to favourable habitats.
• Suspension
  • Dormancy during unfavourable conditions (hibernation, aestivation, diapause).

Population Ecology

• Definition of Population
  • A population is a group of individuals of the same species living in a defined area and sharing or competing for similar resources.
• Population Attributes
  • Birth Rate (Natality)
  • Death Rate (Mortality)
  • Sex Ratio
  • Age Distribution (represented through age pyramids)
  • Population Density (N)

• Age Pyramid Types
  • Expanding (triangular)
  • Stable (bell-shaped)
  • Declining (urn-shaped)

Population Density

• Reflects ecological success and can be measured by:
  • Number of individuals
  • Biomass
  • Percent cover
  • Indirect estimation methods

Population Change Equation

• Population density changes due to:
  • Natality (B)
  • Mortality (D)
  • Immigration (I)
  • Emigration (E)
• Growth depends on the balance between (B + I) and (D + E).

Population Growth Models

• Exponential Growth
  • Occurs when resources are unlimited.
  • Described by equation: dN/dt = rN
  • Produces a J-shaped curve.
• Logistic Growth
  • Occurs when resources are limited.
  • Growth slows as population approaches carrying capacity (K).
  • Produces an S-shaped curve.

• Intrinsic Rate of Natural Increase (r)
  • Measures inherent growth potential of a population.

Life History Strategies

• Species evolve reproductive strategies to maximise Darwinian fitness.
• Examples
  • Single breeding event (e.g., Pacific salmon)
  • Multiple breeding events (e.g., mammals)
  • Many small offspring (r-selected tendency)
  • Few large offspring (K-selected tendency)
• Life history traits evolve under abiotic and biotic constraints.

Population Interactions

• No species exists in isolation.
• Interspecific interactions influence survival and evolution.

Types of Interactions

• Mutualism (+/+)
  • Both species benefit.
• Commensalism (+/0)
  • One benefits, other unaffected.
• Amensalism (–/0)
  • One harmed, other unaffected.
• Parasitism (+/–)
  • Parasite benefits, host harmed.
• Predation (+/–)
  • Predator kills and consumes prey.
• Competition (–/–)
  • Both species suffer due to shared resource use.

Ecological Significance of Interactions

• Predation
  • Transfers energy across trophic levels
  • Controls prey populations
  • Maintains species diversity
• Competition
  • Drives natural selection
  • May lead to competitive exclusion
  • Can result in resource partitioning
• Mutualism
  • Enhances survival and reproduction
  • Often leads to co-evolution
• Parasitism
  • Leads to host–parasite co-evolution
  • Reduces host fitness

Conceptual Integration

Ecology links:

Organism → Population → Evolution

• Natural selection operates at the population level.
• Environmental pressures shape adaptations and population dynamics.

• Understanding populations and their interactions helps explain:
  • Species distribution
  • Community structure
  • Biodiversity patterns
  • Ecosystem stability