ORGANISMS AND POPULATIONS

Ramdeo Misra – Father of Ecology in India
Full Name	Ramdeo Misra
Title	Father of Ecology in India
Life Span	1908 – 1998
Date of Birth	26 August 1908
Field	Ecology, Botany
Ph.D.	Ph.D. in Ecology (1937)
Ph.D. Supervisor	Prof. W. H. Pearsall, FRS
University (Ph.D.)	Leeds University, United Kingdom
Institutional Contribution	Established teaching and research in ecology
Department	Department of Botany
University (India)	Banaras Hindu University (BHU), Varanasi
Major Research Areas	Tropical plant communities, ecological succession, plant population responses, productivity, nutrient cycling
Ecosystems Studied	Tropical forests and grasslands
Academic Contribution	Formulated the first postgraduate course in Ecology in India
Research Guidance	Supervised 50+ Ph.D. scholars
National Impact	His students spread ecology teaching and research across India
Major Awards & Honors	Fellow of Indian National Science Academy (INSA); Fellow of World Academy of Arts and Science
Prestigious Award	Sanjay Gandhi Award for Environment and Ecology
Policy Contribution	Instrumental in establishment of National Committee for Environmental Planning and Coordination (1972)
Long-term Legacy	Laid foundation for Ministry of Environment and Forests (established in 1984)
Overall Legacy	Pioneer of ecological education, research, and environmental policy in India

Understanding Biological Complexity & Ecological Questions
Concept	Living World
Explanation / Key Points	Highly diverse and complex
Approach to Study	Complexity understood by studying different levels of biological organisation
Levels of Biological Organisation	Macromolecules → Cells → Tissues → Organs → Organism → Population → Community → Ecosystem → Biome
Types of Scientific Questions	Two types: How-type and Why-type
How-type Questions	Deal with mechanism (structure and function)
Why-type Questions	Deal with significance / adaptive value
Example (Bird Singing)	How: Voice box and vibrating bones work Why: Communication with mate during breeding
Scientific Observation	Nature should be observed with a questioning, analytical mindset
Examples of Ecological Questions	Why night-blooming flowers are white? How bees locate nectar? Why cactus has thorns? How chicks recognize their mother?

Ecology: Organisms and Populations
Definition of Ecology	Study of interactions among organisms and between organisms and their abiotic environment
Major Levels Studied in Ecology	Organisms, Populations, Communities, Biomes
Focus of the Chapter	Population level ecology
Population Ecology	Studies characteristics, growth, distribution, and interactions of populations

11.1.1 Population Attributes
Population attributes are not applicable to individuals
1. Birth Rate (Natality)
Definition	Number of births per individual per unit time
Expression	Per capita birth rate
Example	20 lotus plants → 8 new plants Birth Rate = 8 / 20 = 0.4 offspring/lotus/year

2. Death Rate (Mortality)
Definition	Number of deaths per individual per unit time
Expression	Per capita death rate
Example	40 fruit flies → 4 deaths/week Death Rate = 4 / 40 = 0.1 deaths/fruit fly/week

3. Sex Ratio
Definition	Proportion of males and females in a population
Individual vs Population	Individual is male or female; population has a sex ratio
Example	60% females : 40% males

Age Distribution & Age Pyramid
Age Distribution	Percentage of individuals in different age groups
Graphical Representation	Age Pyramid
Human Age Pyramid	Shows male and female age distribution
Significance	Indicates growth status of population
Types of Age Pyramids
Triangular	Growing population
Bell-shaped	Stable population
Urn-shaped	Declining population

Population Size / Population Density (N)
Definition	Number of individuals of a population per unit area
Symbol	N
Importance	Indicates population status in habitat
Range	<10 (Siberian cranes) to millions (Chlamydomonas)

Methods of Measuring Population Density
Method	When Used	Example
Total Number	When counting is feasible	Small populations
Biomass	When number is misleading	Banyan tree vs carrot grass
Percent Cover	For plant populations	Grasslands
Relative Density	When absolute number not required	Fish caught per trap
Indirect Estimation	Large or elusive animals	Tiger census using pug marks, fecal pellets
Cell Density	Microorganisms	Bacteria in petri dish

11.1.2 POPULATION GROWTH – Overview
Population Size	Not static; changes with time
Factors Affecting Growth	Food availability, predation pressure, climate/weather
Significance	Changes indicate whether population is flourishing or declining
Population Density	Fluctuates due to four basic biological processes

Four Basic Processes Affecting Population Density
Process	Definition	Effect on Population Density
Natality (B)	Number of births during a given period	Increases population
Mortality (D)	Number of deaths during a given period	Decreases population
Immigration (I)	Individuals entering population from outside	Increases population
Emigration (E)	Individuals leaving population	Decreases population

Population Growth Equation
Equation	Nt+1 = Nt + [(B + I) − (D + E)]
Explanation	Population density at time t+1 depends on births, deaths, immigration and emigration
Condition for Growth	(B + I) > (D + E)
Major Contributors (Normally)	Births and deaths
Special Conditions	Immigration important during colonisation of new habitats

Growth Models in Populations
Growth Model	Resource Availability	Curve Shape
Exponential Growth	Unlimited	J-shaped
Logistic Growth	Limited	S-shaped (Sigmoid)

(i) Exponential Growth Model
Condition	Unlimited food and space
Observation by	Charles Darwin
Growth Pattern	Exponential / Geometric
Curve	J-shaped curve
Equation	dN/dt = rN
Where r =	(b − d)
r	Intrinsic rate of natural increase
Significance of r	Measures impact of biotic and abiotic factors

Integral Form of Exponential Growth
Formula	Nt = N₀ eʳᵗ
Meaning	Population grows exponentially with time
Nt	Population density at time t
N₀	Initial population
e	Base of natural logarithm (2.71828)

Examples of r Values
Norway rat	0.015
Flour beetle	0.12
Human population (India, 1981)	0.0205

Significance of Exponential Growth
Rapid increase	Even slow-growing species can reach huge numbers
Chessboard anecdote	Demonstrates power of exponential growth
Example	Paramecium doubling daily for 64 days

(ii) Logistic Growth Model
Condition	Limited resources
Nature of Growth	Initially slow, then fast, finally stabilises
Curve Shape	Sigmoid (S-shaped)
Maximum Limit	Carrying capacity (K)
Growth Phases	Lag → Acceleration → Deceleration → Asymptote
Realism	More realistic for natural populations

Logistic Growth Equation (Verhulst–Pearl Model)
Equation	dN/dt = rN (K − N)/K
Explanation	Growth rate decreases as population approaches K
N	Population density
r	Intrinsic rate of natural increase
K	Carrying capacity

Comparison: Exponential vs Logistic Growth
Feature	Exponential Growth	Logistic Growth
Resources	Unlimited	Limited
Curve	J-shaped	S-shaped
Carrying capacity	Not considered	Considered (K)
Applicability	Ideal conditions	Natural ecosystems
Realism	Less realistic	More realistic

11.1.3 LIFE HISTORY VARIATION
Goal of Population Evolution	To maximise reproductive fitness (Darwinian fitness / high r value)
Darwinian Fitness	Ability to leave maximum viable offspring
Driving Force	Natural selection under specific environmental pressures
Life History Traits	Age at reproduction, number of offspring, size of offspring, frequency of reproduction
Reason for Variation	Constraints imposed by abiotic and biotic factors of habitat
Research Status	Active area of ecological and evolutionary research

Examples of Life History Strategies
Trait	Strategy 1	Strategy 2
Breeding Frequency	Breed once in lifetime (Pacific salmon, bamboo)	Breed many times (birds, mammals)
Offspring Number	Many small offspring (oysters, pelagic fishes)	Few large offspring (birds, mammals)
Fitness Maximisation	Depends on habitat conditions	No single strategy is universally superior
Life history traits evolve in response to environmental constraints, not by choice.

11.1.4 POPULATION INTERACTIONS
Community Structure	No species exists in isolation
Requirement	At least one interacting species
Plant Dependence	Soil microbes, pollinators, seed dispersers
Interaction Type	Interspecific (between different species)
Nature of Interaction	Beneficial (+), harmful (–), or neutral (0)

Table 11.1: Types of Population Interactions
Species A	Species B	Interaction Type
+	+	Mutualism
–	–	Competition
+	–	Predation
+	–	Parasitism
+	0	Commensalism
–	0	Amensalism

(i) Predation
Definition	One species (predator) benefits, other (prey) is harmed
Ecological Role	Energy transfer across trophic levels
Population Control	Keeps prey population under check
Ecosystem Stability	Prevents prey overpopulation
Biological Control	Use of predators to control pests

Examples of Predation
Tiger–Deer	Classic predator–prey
Sparrow–Seed	Herbivory = predation
Prickly pear in Australia	Controlled by moth predator
Starfish (Pisaster)	Maintains species diversity

Anti-Predator Adaptations
Insects, frogs	Camouflage
Monarch butterfly	Chemical toxicity
Plants (Acacia, cactus)	Thorns
Calotropis	Cardiac glycosides
Plant chemicals	Nicotine, caffeine, quinine etc.
Predators are prudent; overexploitation leads to extinction of both prey and predator.

(ii) Competition
Definition	Fitness (r value) of one species reduced in presence of another
Resource Limitation	Not always required
Types	Exploitative and interference competition
Affected Groups	More severe in plants and herbivores

Evidence of Competition
Flamingoes vs fishes	Compete for zooplankton
Abingdon tortoise	Extinct due to goats
Barnacles (Connell)	Balanus excludes Chthamalus
Competitive Release	Expansion after removal of competitor

Gause’s Competitive Exclusion Principle
Statement	Two closely related species competing for the same limited resources cannot coexist indefinitely

Avoiding Competition: Resource Partitioning
Mechanism	Different feeding times/areas
Example	Warblers on same tree
Behavioural differentiation	MacArthur’s warbler study

(iii) Parasitism
Definition	Parasite benefits, host is harmed
Host Specificity	Many parasites are host-specific
Co-evolution	Host resistance ↔ parasite counter-adaptation
Impact on Host	Reduced growth, reproduction, survival

Parasite Adaptations
Loss of sense organs	Reduced need
Suckers/adhesive organs	Attachment
High reproduction	Survival
Complex life cycle	Transmission

Types of Parasites
Ectoparasites	Lice, ticks, copepods
Endoparasites	Liver fluke, malarial parasite
Parasitic plant	Cuscuta
Brood parasite	Cuckoo (koel)
Female mosquito is not a parasite → interaction is temporary, not living on host.

(iv) Commensalism
Definition	One benefits, other unaffected
Examples	Orchid on mango, barnacles on whale
Cattle egret	Benefits from grazing cattle
Clown fish	Protected by sea anemone

(v) Mutualism
Definition	Both species benefit
Classic Examples	Lichen, mycorrhiza
Plant–Animal Mutualism	Pollination, seed dispersal
Reward System	Nectar, pollen, fruits

Co-evolution Examples
Fig–Wasp	One-to-one species specificity
Orchid–Bee (Ophrys)	Sexual deceit
Co-evolution	Linked evolution of mutualists

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