Organisms and Populations

Key Concepts

Levels of Biological Organisation

Ecology is concerned with four levels: Organisms, Populations, Communities, and Biomes.

Population Attributes

A population is a group of individuals of the same species in a defined area, sharing resources and interbreeding.

Birth Rate & Death Rate: Expressed as per capita changes (e.g., 0.4 offspring per lotus per year).
Sex Ratio: Ratio of males to females in a population.
Age Pyramid: Graphic representation of age distribution. Shapes indicate:
- Growing: Broad base (more young individuals).
- Stable: Uniform distribution.
- Declining: Narrow base.
Population Density (N): Measured by number, biomass, or per cent cover. Indirect methods include pug marks and fecal pellets for tiger census.

Population Growth

Fluctuates based on:

Natality (B): Number of births.
Mortality (D): Number of deaths.
Immigration (I): Number of individuals entering.
Emigration (E): Number of individuals leaving.

Equation: Nt+1 = Nt + [(B + I) - (D + E)]

Growth Models

Exponential Growth: Occurs when resources are unlimited. Results in a J-shaped curve.

The 'r' Value The intrinsic rate of natural increase (r) is a very important parameter for assessing the impacts of any biotic or abiotic factor on population growth.

Equation: dN/dt = rN (where r is the intrinsic rate of natural increase).
Integral form: Nt = N0 ert.

Logistic Growth: More realistic; resources are limited. Results in a Sigmoid (S-shaped) curve.
- Carrying Capacity (K): Maximum number a habitat can support.
- Equation (Verhulst-Pearl): dN/dt = rN [(K-N)/K]

Population Interactions

Interaction	Species A	Species B	Example
Mutualism	+	+	Lichens (Algae + Fungus); Mycorrhiza
Competition	-	-	Flamingoes and Fish for zooplankton
Predation	+	-	Tiger and Deer; Sparrow and Seed
Parasitism	+	-	Ticks on dogs; Cuscuta on hedges
Commensalism	+	0	Orchid on mango branch; Cattle egret
Amensalism	-	0	Penicillium inhibiting bacteria

Predation: Predators act as conduits for energy transfer and keep prey populations under control. Prey defenses include camouflage, poisons (Monarch butterfly), and thorns (Cactus).
Competition:

Gause’s Competitive Exclusion Principle Two closely related species competing for the same limited resources cannot co-exist indefinitely; the competitively inferior one will eventually be eliminated.

Gause’s Competitive Exclusion Principle: Two closely related species competing for the same limited resources cannot co-exist.
Resource Partitioning: Avoiding competition by different feeding times or foraging patterns (e.g., MacArthur's warblers).
Parasitism:
- Ectoparasites: Live on the surface (Lice, Ticks).
- Endoparasites: Live inside the body (Liver fluke, Plasmodium).
- Brood Parasitism: Cuckoo laying eggs in a crow's nest.
Mutualism: Co-evolution is common (e.g., Fig tree and Wasp).

Sexual Deceit The orchid Ophrys employs sexual deceit to get pollination done by a species of bee. One petal of its flower bears an uncanny resemblance to the female of the bee in size, colour and markings.

Sexual Deceit is used by the Mediterranean orchid Ophrys to attract bee pollinators.

Key Concepts

Ecology is concerned with four levels: Organisms, Populations, Communities, and Biomes.

A population is a group of individuals of the same species in a defined area, sharing resources and interbreeding.

Birth Rate & Death Rate: Expressed as per capita changes (e.g., 0.4 offspring per lotus per year).

Sex Ratio: Ratio of males to females in a population.

Age Pyramid: Graphic representation of age distribution. Shapes indicate:

Growing: Broad base (more young individuals).
Stable: Uniform distribution.
Declining: Narrow base.

Population Density (N): Measured by number, biomass, or per cent cover. Indirect methods include pug marks and fecal pellets for tiger census.

Fluctuates based on:

Natality (B): Number of births.

Mortality (D): Number of deaths.

Immigration (I): Number of individuals entering.

Emigration (E): Number of individuals leaving.

Equation: Nt+1 = Nt + [(B + I) - (D + E)]

Exponential Growth: Occurs when resources are unlimited. Results in a J-shaped curve.

The 'r' Value The intrinsic rate of natural increase (r) is a very important parameter for assessing the impacts of any biotic or abiotic factor on population growth.

Equation: dN/dt = rN (where r is the intrinsic rate of natural increase).

Integral form: Nt = N0 ert.

Logistic Growth: More realistic; resources are limited. Results in a Sigmoid (S-shaped) curve.

Carrying Capacity (K): Maximum number a habitat can support.
Equation (Verhulst-Pearl): dN/dt = rN [(K-N)/K]

Interaction	Species A	Species B	Example
Mutualism	+	+	Lichens (Algae + Fungus); Mycorrhiza
Competition	-	-	Flamingoes and Fish for zooplankton
Predation	+	-	Tiger and Deer; Sparrow and Seed
Parasitism	+	-	Ticks on dogs; Cuscuta on hedges
Commensalism	+	0	Orchid on mango branch; Cattle egret
Amensalism	-	0	Penicillium inhibiting bacteria

Predation: Predators act as conduits for energy transfer and keep prey populations under control. Prey defenses include camouflage, poisons (Monarch butterfly), and thorns (Cactus).

Competition:

Gause’s Competitive Exclusion Principle: Two closely related species competing for the same limited resources cannot co-exist.

Resource Partitioning: Avoiding competition by different feeding times or foraging patterns (e.g., MacArthur's warblers).

Parasitism:

Ectoparasites: Live on the surface (Lice, Ticks).
Endoparasites: Live inside the body (Liver fluke, Plasmodium).
Brood Parasitism: Cuckoo laying eggs in a crow's nest.

Mutualism: Co-evolution is common (e.g., Fig tree and Wasp).

Sexual Deceit is used by the Mediterranean orchid Ophrys to attract bee pollinators.

Organisms and Populations

Organisms and Populations

Key Concepts

Levels of Biological Organisation

Population Attributes

Population Growth

Growth Models

Population Interactions

On this page

Organisms and Populations

Organisms and Populations

Key Concepts

Levels of Biological Organisation

Population Attributes

Population Growth

Growth Models

Population Interactions

On this page