Organisms and Populations

Organisms and Populations

**Definition:** A **population** is a group of individuals of the same species living in a well-defined geographical area, sharing or competing for similar resources, and potentially interbreeding.

Examples: All cormorants in a wetland, rats in an abandoned dwelling, teakwood trees in a forest tract, bacteria in a culture plate, lotus plants in a pond.

**Key Point:** Population ecology links ecology to population genetics and evolution, as natural selection operates at the population level, not the individual level.

---

Population Attributes

**Population attributes** are characteristics that exist only at the population level, not for individual organisms. An individual is born and dies, but a population has **birth rates** and **death rates**.

1. Birth Rate (Natality) and Death Rate (Mortality)

**Birth rate (b):** Per capita number of births in a population during a given period; expressed as offspring per individual per unit time

Example: If 20 lotus plants in a pond produce 8 new plants, birth rate = 8/20 = 0.4 offspring per lotus per year

**Death rate (d):** Per capita number of deaths in a population during a given period

Example: If 4 out of 40 fruitflies die in a week, death rate = 4/40 = 0.1 individuals per fruitfly per week

2. Sex Ratio

An individual organism is either male or female, but a **population has a sex ratio** (e.g., 60% female, 40% male)

This ratio varies among populations and has implications for reproductive capacity

3. Age Structure (Age Pyramid)

**Age pyramid** or **age distribution** is the percentage of individuals in different age groups within a population

**Shape reflects population growth status:**

**Expanding pyramid** → Growing population (wide base, narrow top)

**Stable pyramid** → Stable population (relatively uniform width)

**Declining pyramid** → Declining population (narrow base, wide top)

Used to predict future population trends

In humans, age pyramids show separate distributions for males and females

4. Population Density (N)

**Population density** (designated as N) is the size of the population; may be measured as:

**Total number:** Most straightforward measure when feasible

Not appropriate when organisms vary greatly in size (e.g., one huge banyan tree vs. 200 carrot grass plants)

**Biomass:** Mass of all individuals; more meaningful when organisms vary greatly in size

**Per cent cover:** Used for organisms with large area coverage (plants, corals)

**Relative density:** Used when absolute numbers are impossible to count (e.g., fish caught per trap in a lake; tiger census based on pug marks and fecal pellets)

**Important Point for Exams:** Different measures serve different ecological investigations; the choice depends on the study objectives.

---

Population Growth: Basic Components

Population density at any time is determined by four factors:

1. **Natality (B):** Number of births added to initial density

2. **Mortality (D):** Number of deaths during the period

3. **Immigration (I):** Number of individuals of the same species entering the habitat from elsewhere

4. **Emigration (E):** Number of individuals leaving the habitat

Population Growth Equation:

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

Where:

$N_{t+1}$ = Population density at time t+1

$N_t$ = Population density at time t

B + I = Factors increasing population

D + E = Factors decreasing population

**Key Point:** Under normal conditions, **births and deaths** are most important; immigration and emigration become significant only during habitat colonisation.

---

Population Growth Models

Population growth follows predictable patterns based on resource availability. Two main models explain population dynamics:

1. Exponential Growth (J-shaped Curve)

**Definition:** Growth that occurs when resources are unlimited and the population increases at a constant rate per individual.

**Mathematical Expression:**

$$\frac{dN}{dt} = (b - d) \times N$$

Or, where **r = intrinsic rate of natural increase** (b - d):

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

**Integral Form:**

$$N_t = N_0 e^{rt}$$

Where:

$N_t$ = Population density after time t

$N_0$ = Population density at time zero

**r** = Intrinsic rate of natural increase (per capita rate of change)

e = 2.71828 (base of natural logarithm)

t = Time

**Characteristics:**

**r value examples:** Norway rat (r = 0.015), flour beetle (r = 0.12), India human population in 1981 (r = 0.0205)

Results in **J-shaped curve** when N is plotted against time

Population reaches enormous densities in short time under unlimited resources

**Darwin's example:** Even slow-growing animals like elephants could reach enormous numbers in absence of checks

**Famous Example — Chess Board Anecdote:** Doubling of wheat grains on each successive square of a chessboard demonstrates exponential growth; by the 32nd square, resources would be exhausted—illustrating that exponential growth cannot continue indefinitely.

2. Logistic Growth (S-shaped or Sigmoid Curve)

**Definition:** Population growth that accounts for limited resources; growth initially accelerates, then decelerates as population approaches **carrying capacity (K)**.

**Carrying Capacity (K):** The maximum population size that a habitat can sustain indefinitely, determined by limiting resources (food, space, water).

**Mathematical Expression (Verhulst-Pearl Logistic Growth):**

$$\frac{dN}{dt} = rN\left(\frac{K-N}{K}\right)$$

Where:

N = Population density at time t

r = Intrinsic rate of natural increase

K = Carrying capacity

$(K-N)/K$ = Fraction of resources available

**Phases of Logistic Growth:**

1. **Lag phase:** Slow initial growth as population adjusts to environment

2. **Log/Acceleration phase:** Rapid exponential-like growth when resources abundant

3. **Deceleration phase:** Growth slows as resources become limiting

4. **Asymptote/Stationary phase:** Population stabilises at K; N approaches K (growth rate ≈ 0)

**Characteristics:**

Results in **S-shaped (sigmoid) curve**

More realistic for natural populations than exponential model

Most animal populations have finite resources that eventually limit growth

Growth slows when N approaches K because competition for resources intensifies

**Key Difference:** Exponential growth assumes unlimited resources (unrealistic in nature); logistic growth incorporates environmental resistance and resource limitation (more realistic).

---

Life History Variation

**Life history traits** are reproductive and survival characteristics that organisms have evolved to maximise **Darwinian fitness** (reproductive success) in their specific habitat.

**Types of Life History Strategies:**

Breeding Pattern Variation:

**Semelparous (breed once):** Pacific salmon, bamboo (produce one massive reproductive event, then die)

**Iteroparous (breed many times):** Most birds and mammals (multiple breeding events throughout life)

Offspring Strategy Variation:

**r-strategy (high fecundity, low parental care):** Oysters, pelagic fishes (produce many small offspring with low individual survival)

**K-strategy (low fecundity, high parental care):** Birds, mammals (produce few large offspring with high individual survival)

**Key Concept:** Evolution of life history traits is driven by selection pressures from abiotic and biotic components of habitat. Different species maximise fitness through different strategies suited to their environmental constraints.

---

Population Interactions (Interspecific Interactions)

**Definition:** **Interspecific interactions** are interactions between populations of two different species living in the same community.

**Classification System:** Using +/- notation:

**+** = Beneficial to that species

**–** = Detrimental to that species

**0** = Neutral (neither benefit nor harm)

Types of Population Interactions:

| Interaction Type | Effect on Species 1 | Effect on Species 2 | Example |

|---|---|---|---|

| **Mutualism** | + | + | Flowering plants and pollinators, mycorrhizae |

| **Competition** | – | – | Two plant species for same resource |

| **Predation** | + | – | Tiger eats deer |

| **Parasitism** | + | – | Parasite feeds on host; host harmed |

| **Commensalism** | + | 0 | Orchid grows on tree (tree unaffected) |

| **Amensalism** | – | 0 | Penicillin produced by fungus kills bacteria |

**Key Point:** Predation, parasitism, and commensalism involve **intimate association** (interacting species live closely together), whereas competition may be indirect.

---

1. Predation

**Definition:** An interaction where one species (predator) feeds on another species (prey); predator benefits (+), prey harmed (–).

**Ecological Significance:**

**Energy transfer:** Transfers energy fixed by plants (autotrophs) to higher trophic levels

**Population control:** Keeps prey populations under control; prevents prey from reaching excessive densities and destabilising ecosystem

**Biodiversity maintenance:** Prevents any single prey species from monopolising resources

**Scope of Predation:**

Includes both carnivores (tiger eating deer) and herbivores (sparrow eating seeds)

Ecologically, herbivory is a form of predation on plants

**Invasive Species Example:** Prickly pear cactus introduced to Australia in 1920s spread rapidly into millions of hectares because the introduced habitat lacked its natural predators. Control was achieved only when cactus-feeding moth predator from the cactus's native habitat was introduced—demonstrating predator's role in population control.

**Exam Point:** Students must understand predation as an ecological process regulating both prey and predator populations, not just as feeding behaviour.

---

2. Parasitism

**Definition:** An interaction where **parasite** benefits by living on or in the **host** body and harming the host (parasite +, host –).

**Characteristics:**

Parasite is smaller than host and depends entirely on host for nutrition and habitat

Host usually survives but is weakened/harmed

Relationship is relatively long-term (compared to predation)

Parasites have evolved to avoid killing host immediately (reduces probability of transmission to new host)

**Examples:** Plasmodium (malaria parasite) in humans, tapeworms, blood-sucking insects.

---

3. Competition

**Definition:** Interaction where two species compete for same limiting resource (both species –); one or both may be harmed.

**Key Principle — Gause's Competitive Exclusion Principle (Law of Competitive Exclusion):**

**"Two species competing for the same limiting resource cannot coexist indefinitely in the same habitat; one species will be eliminated."**

**Mechanism:**

Species with higher competitive ability (better adapted to utilise the resource, faster reproduction, etc.) outcompetes the other

The competitively inferior species either emigrates, evolves to use different resource, or becomes extinct locally

Niche differentiation allows species to coexist by specialising on different resources or habitats

**Types:**

**Interspecific competition:** Between different species (competitive exclusion)

**Intraspecific competition:** Within same species (leads to population regulation)

---

4. Commensalism

**Definition:** Interaction where one species **benefits (+)** while the other species is **neither benefitted nor harmed (0)**.

**Characteristics:**

Commensal species depends on host species but does not harm it

Close physical association between species

Host is unaffected by commensal's presence

**Examples:**

Orchids growing on tree branches (absorb moisture, obtain better light; tree unaffected)

Cattle egret following grazing herds (catches disturbed insects; cattle unaffected)

Remora fish attaching to shark (transport and food scraps; shark unaffected)

---

5. Amensalism

**Definition:** Interaction where one species is **harmed (–)** while the other is **unaffected (0)**.

**Characteristics:**

One species produces substance or condition inhibiting other species

No physical contact or direct feeding relationship required

Harm is often chemical (antibiotic production, allelopathy)

**Examples:**

Penicillium fungus producing penicillin antibiotic kills nearby bacteria (bacteria harmed; fungus unaffected)

Allelopathic plants releasing inhibitory chemicals that prevent other plant species from growing nearby

Microorganism producing toxins that inhibit competitors

---

6. Mutualism

**Definition:** Interaction where **both species benefit (+, +)** from interaction; often obligatory for one or both species.

**Characteristics:**

Symbiotic association with reciprocal benefits

May be obligatory (one or both cannot survive without other) or facultative (beneficial but not essential)

Often involves coevolution of interacting species

**Examples:**

**Flowering plants and animal pollinators:** Plants get pollination/reproduction; animals get nectar/food and pollen (protein source)

**Mycorrhizae (fungi and plant roots):** Fungi obtain photosynthetic products from plant; plant obtains mineral nutrients and water from fungal hyphae (obligatory for many plants)

**Nitrogen-fixing bacteria (Rhizobium) and legume plants:** Bacteria obtain carbohydrates from plant; plant obtains usable nitrogen (NH₃) for protein synthesis

**Ruminants and gut bacteria:** Bacteria break down cellulose; animal obtains energy; bacteria obtain ideal habitat and nutrients

**Exam Important:** Mutualism demonstrates how interspecific interactions can be evolutionarily stable and mutually beneficial, unlike antagonistic interactions.

---

Exam-Important Summary Points

1. **Population density (N)** can be measured by total number, biomass, per cent cover, or relative density depending on study objectives and organism type

2. **Population growth equation:** $N_{t+1} = N_t + [(B + I) - (D + E)]$

3. **Exponential growth:** $\frac{dN}{dt} = rN$ produces **J-shaped curve**; assumes unlimited resources (unrealistic)

4. **Logistic growth:** $\frac{dN}{dt} = rN\left(\frac{K-N}{K}\right)$ produces **S-shaped curve**; assumes limited resources (realistic for natural populations)

5. **Carrying capacity (K):** Maximum population size sustainable indefinitely in a habitat

6. **Intrinsic rate of increase (r):** Per capita rate of population change; varies among species

7. **Interspecific interactions classification:** Use +/– notation to distinguish mutualism, competition, predation, parasitism, commensalism, amensalism

8. **Gause's principle:** Two species competing for same limiting resource cannot coexist indefinitely; competitive exclusion occurs

9. **Predation significance:** Transfers energy across trophic levels; controls prey populations; prevents ecosystem instability; invasive species lack natural predators

10. **Life history variation:** Species evolve different reproductive strategies (semelparous vs. iteroparous; r-strategy vs. K-strategy) based on selection pressures in their habitat