Population Growth (4.2)

What is a population?

A population is a group of organisms of the same species living in the same area at the same time and capable of interbreeding.

What is population size?

The total number of individuals in a population.

Why do ecologists need to know population size?

To monitor species, manage resources, assess biodiversity, predict future population trends, identify conservation needs, and study ecological relationships.

What is a census?

A direct counting method in which every individual in a population is counted.

What are direct techniques for determining population size?

Methods that involve counting every individual in the population.

What are indirect techniques for determining population size?

Methods that estimate population size by counting only a sample of the population.

What is the mark-recapture technique?

An indirect sampling method used to estimate the size of mobile animal populations by marking individuals and later recapturing a second sample.

What is a quadrat?

A square frame of known dimensions used to estimate the population size of plants and sessile organisms.

What are sessile organisms?

Organisms that are permanently attached to a surface and cannot move from place to place.

What is population growth?

The increase in the number of individuals in a population over time.

What is geometric growth?

Population growth under ideal conditions where resources are unlimited, producing a J-shaped growth curve.

What is exponential growth?

Rapid population growth that occurs when resources are unlimited and environmental resistance is absent or very low.

What is logistic growth?

Population growth that slows and stabilises as environmental resistance increases and carrying capacity is reached.

What is carrying capacity (K)?

The maximum number of individuals that an environment can support indefinitely without degrading the habitat.

What is environmental resistance?

The combined effect of limiting factors that prevent a population from reaching its maximum reproductive potential.

What is a limiting factor?

Any factor that restricts population growth and prevents unlimited increase in population size.

What is a stable population?

A population whose numbers fluctuate around the carrying capacity.

What is an unstable population?

A population that greatly exceeds carrying capacity, causing habitat damage and a decrease in carrying capacity.

What are density-dependent factors?

Limiting factors whose effects become stronger as population density increases.

What are density-independent factors?

Factors that affect population size regardless of population density.

What is the intrinsic rate of natural increase (r)?

The potential growth rate of a population under ideal conditions, calculated as birth rate minus death rate.

What does a high value of r indicate?

A population capable of growing rapidly.

What does a low value of r indicate?

A population that grows slowly.

What are examples of direct population estimation methods?

Counting humans directly, counting small organisms directly, counting animals from vehicles or aircraft, using aerial photographs.

When are direct methods most suitable?

When organisms are highly visible, when organisms are stationary or slow-moving, when habitats have clear boundaries.

Give an example where direct counting may be used.

Counting coral colonies or sea anemones within a defined reef area.

Which organisms are best studied using mark-recapture?

Mobile animals such as fish, insects, birds, and mammals.

State the steps of the mark-recapture method in order.

Demarcate a study area, capture a first sample, count and mark individuals, release marked individuals, allow mixing with the population, capture a second sample, count total individuals and marked individuals, calculate population size.

What is the mark-recapture formula?

P=\frac{M\times S}{T} Where: P = estimated population size, M = first sample marked, S = second sample size, T = marked individuals recaptured.

Marine biologists mark 200 clownfish. Later they catch 150 fish, of which 30 are marked. Estimate the population size.

P = (200 × 150) ÷ 30, P = 1000 clownfish.

Why must marking not harm the organism?

Injury may affect survival and produce inaccurate results.

Why must marks be clearly visible?

So previously marked organisms can be accurately identified.

Why must marking not change behavior?

Behavioral changes may affect movement, feeding, or recapture probability.

Why must marked organisms be allowed time to mix?

To ensure the second sample is representative of the entire population.

Why must the population remain closed?

Immigration and emigration alter population size and reduce accuracy.

Why should sampling occur over a short period?

To minimise births and deaths affecting population size.

State all assumptions required for a valid mark-recapture investigation.

Marks remain visible, marks do not affect survival, marks do not affect behavior, marked individuals mix randomly, no migration occurs, no significant births or deaths occur, equal chance of capture for all individuals.

Which organisms are best studied using quadrats?

Plants and sessile organisms.

Describe the quadrat method.

Measure the study area, place quadrats randomly, count organisms in each quadrat, calculate the average number per quadrat, estimate the total population.

Why must quadrats be placed randomly?

To avoid sampling bias.

Why is quadrat size important?

The size must suit the organism being studied.

What determines the reliability of quadrat sampling?

Accurate counting, random placement, suitable quadrat size, adequate number of quadrats.

Starfish counts in five 2 m² quadrats are 3, 4, 2, 5 and 6. Calculate the average number per quadrat.

Average = (3 + 4 + 2 + 5 + 6) ÷ 5 = 4 starfish per quadrat.

What conditions are required for exponential growth?

Unlimited food, unlimited water, unlimited space, little competition, little predation, minimal disease.

What shape does exponential growth produce?

A J-shaped curve.

Why is growth initially slow in a J-shaped curve?

Few individuals are present to reproduce.

Why does growth become very rapid later?

Increasing numbers of breeding individuals reproduce simultaneously.

What birth and death rates occur during exponential growth?

High birth rate and low death rate.

In what situations is exponential growth commonly observed?

New habitats, newly introduced species, bacterial populations, rodents, insect outbreaks.

Why is exponential growth rarely maintained in nature?

Resources eventually become limited.

What may happen after rapid exponential growth?

A sudden population crash or decline due to resource depletion.

What shape does logistic growth produce?

An S-shaped (sigmoid) curve.

Why is logistic growth considered more realistic than exponential growth?

Natural populations experience limiting factors and environmental resistance.

What converts a J-shaped curve into an S-shaped curve?

Environmental resistance caused by limiting factors.

What happens as a population approaches carrying capacity?

Growth slows because resources become limiting.

What is the lag phase?

A period of slow growth when few individuals are reproducing, and the population is becoming established.

Why is growth slow during the lag phase?

Few breeding pairs, population establishing itself, reproduction takes time.

What is the exponential (accelerating) phase?

A period of rapid population increase due to abundant resources and many reproducing individuals.

Why does rapid growth occur during the exponential phase?

Resources are abundant, competition is low, birth rate exceeds death rate.

What is the decelerating (transitional) phase?

A phase where growth slows as resources become limited.

Why does growth slow during the decelerating phase?

Competition increases, food becomes limited, water becomes limited, space becomes limited, environmental resistance increases.

What is the equilibrium (stationary/plateau) phase?

A phase where population size fluctuates around carrying capacity.

Why is the population stable during the equilibrium phase?

Birth rate approximately equals death rate.

What does a plateau on a logistic graph indicate?

The population has reached carrying capacity.

What creates carrying capacity?

Environmental resistance.

Explain the relationship between environmental resistance and carrying capacity.

Environmental resistance is the process that limits growth, while carrying capacity is the maximum population size produced by those limits.

Can carrying capacity change?

Yes.

When can carrying capacity increase?

When resources, habitat or environmental conditions improve.

When can carrying capacity decrease?

When resources decline or habitat is damaged.

What are density-dependent factors?

Factors whose effects increase as population density increases.

Give examples of density-dependent factors.

Competition for food, water, space; limited shelter; increased predation; disease spread; waste accumulation.

Why do diseases spread more easily in dense populations?

Individuals have more frequent contact with one another.

What are density-independent factors?

Factors that affect populations regardless of their size or density.

Give examples of density-independent factors.

Floods, fires, droughts, extreme temperatures, natural disasters.

What is population overshoot?

When a population temporarily exceeds carrying capacity.

What usually happens after overshoot?

Population size declines because resources cannot support all individuals.

What happens if carrying capacity continues to decline?

The population may undergo a decline phase and eventually become extinct.

How can a bacterial population become extinct in a Petri dish?

Resources become exhausted, space becomes limited, waste accumulates, carrying capacity decreases, population crashes.

Give examples of events that may cause extinction following overshoot.

Resource depletion, disease outbreaks, floods, fires, severe habitat loss, climate change.

How do you identify an exponential growth graph?

A J-shaped curve showing rapid growth under ideal conditions.

How do you identify a logistic growth graph?

An S-shaped curve showing growth slowing near carrying capacity.

What must always be labelled on population growth graphs?

X-axis = Time, Y-axis = Population size.

What line is usually shown on a logistic growth graph?

Carrying capacity (K).

What are the four phases commonly labelled on a logistic growth graph?

Lag phase, exponential growth phase, decelerating growth phase, equilibrium (stationary) phase.

Which population growth model is most commonly found in nature?

Logistic growth, because resources are always limited.