Population Size (Grade 11 NSC Matric Life Sciences): Revision Notes

Population Size

Introduction to population ecology

Population ecology is the scientific study of how groups of organisms interact with their environment and how their numbers change over time. Understanding population size is crucial for conservation efforts, wildlife management, and predicting how species respond to environmental changes.

A population consists of all the individuals of the same species that live in the same area at the same time and can breed with each other. This is different from a community (different species living together) or an ecosystem (all living and non-living components in an area).

Key terminology

Before exploring population size, it's important to understand these fundamental terms:

Term	Definition
Species	A group of organisms that can interbreed and produce fertile offspring
Population	Individuals of the same species living in the same habitat who can interbreed
Community	Different species populations that interact within a specific habitat
Ecosystem	A community of organisms plus their physical environment
Habitat	The specific place where an organism lives and completes its life cycle
Natality	The birth rate in a population, usually expressed as a percentage
Mortality	The death rate in a population, usually expressed as a percentage
Immigration	New individuals moving into a population from other areas
Emigration	Individuals leaving a population to move to other areas

Factors that influence population size

Population size constantly changes due to four main demographic processes. These work like a balance scale - some factors increase population size while others decrease it.

The equation for calculating population change is:

$\text{Population Change} = (\text{Births} + \text{Immigration}) - (\text{Deaths} + \text{Emigration})$

When this calculation gives a positive result, the population grows. When negative, the population shrinks.

Increasing population factors

Natality (birth rate): When environmental conditions are favourable, more offspring are born and survive to reproductive age. This increases population size.

Immigration: New individuals joining the population bring fresh genetic material and increase total numbers. Some may stay temporarily (like seasonal migrants), while others become permanent residents.

Decreasing population factors

Mortality (death rate): Deaths from disease, predation, accidents, or old age reduce population size. High mortality rates can quickly shrink populations.

Emigration: When individuals leave to find better resources or mates elsewhere, the original population decreases. This often happens when overcrowding occurs.

Environmental resistance and carrying capacity

Not all populations can grow indefinitely. Environmental resistance includes all the factors that prevent unlimited population growth.

Carrying capacity represents the maximum number of individuals that an environment can support sustainably over time. When populations approach this limit, growth slows dramatically.

In a stable population, numbers fluctuate around the carrying capacity but don't exceed it for long periods. However, if a population overshoots its carrying capacity, resources become depleted and the population may crash dramatically.

Types of environmental resistance factors

Environmental resistance comes from two main types of factors:

Density dependent factors have stronger effects when populations are large and crowded. Examples include:

• Competition for food, water, space, and shelter
• Increased predation (more prey attracts more predators)
• Disease spread (easier in crowded conditions)
• Territorial behaviour and stress
• Accumulation of waste products

Density independent factors affect populations regardless of their size. These are usually unpredictable natural events such as:

• Natural disasters (tsunamis, volcanic eruptions, floods)
• Extreme weather events (droughts, severe storms)
• Wildfires and landslides
• Human activities (pollution, habitat destruction)

Methods for determining population size

Scientists use different techniques to measure population size depending on the type of organism and practical constraints.

Direct counting methods

Direct counting gives the actual population size and works best for larger, slow-moving organisms that are easy to observe.

Aerial photography: Taking photographs from aircraft or satellites allows researchers to count large animals like elephants, whales, or herds of grazing animals.

Census: A complete count of all individuals in a population. This method is commonly used for human populations but can also work for some wildlife populations in controlled areas.

Indirect counting methods

When direct counting is impossible or impractical, scientists estimate population size using sampling techniques.

Mark-recapture method: This technique involves capturing a sample of animals, marking them harmlessly, releasing them, and later recapturing another sample to see how many marked individuals are found.

The calculation uses the Petersen Index formula:

$N = \frac{M \times C}{R}$

Where:

• $N$ = estimated total population size
• $M$ = number of individuals marked in first sample
• $C$ = total number captured in second sample
• $R$ = number of marked individuals recaptured in second sample

Quadrat sampling: Square frames are placed randomly in an area to count organisms within them. This works well for plants, slow-moving animals, and sessile organisms like mussels.

Population size calculation:

$\text{Population size} = \frac{\text{average number per quadrat} \times \text{total area}}{\text{quadrat area}}$

For reliable results:

• Use multiple random samples
• Calculate averages from several quadrat throws
• Ensure quadrats are placed randomly across the study area

Worked example: mussel population study

Population growth patterns

When population size is plotted against time, two main growth patterns emerge in nature.

Geometric (J-shaped) growth

Geometric growth produces a J-shaped curve and occurs when populations can reproduce rapidly without significant environmental limits. This pattern is common in:

• Bacteria and other microorganisms
• Insects with short generation times
• Populations colonising new habitats

The pattern shows three phases:

• Lag phase: Slow initial growth as individuals establish themselves
• Exponential phase: Rapid population growth with abundant resources
• Death/extinction phase: Sudden collapse when resources are exhausted

Logistic (S-shaped) growth

Logistic growth creates an S-shaped curve and is more common in larger organisms like mammals that reproduce more slowly. This sustainable growth pattern includes four main phases:

1. Lag phase: Slow initial growth as the population establishes
2. Exponential phase: Rapid growth when conditions are optimal
3. Decelerating growth phase: Growth slows as environmental resistance increases
4. Equilibrium phase: Population stabilises around carrying capacity

Logistic growth is more sustainable because it responds to environmental limits before resources are completely depleted.

Comparison of growth patterns

Feature	Geometric Growth	Logistic Growth
Shape	J-shaped curve	S-shaped curve
Common in	Microorganisms, insects	Larger animals, plants
Environmental limits	Ignored until collapse	Responded to gradually
Sustainability	Unsustainable	More sustainable
Outcome	Often ends in crash	Stabilises at carrying capacity