Ecology

Population Dynamics

5th Year · 6th Year (Leaving Cert)

✓By the end of this lesson students will be able to describe and apply sampling techniques (quadrat and transect) for estimating population size and distribution.
✓By the end of this lesson students will be able to explain the factors influencing population growth and interpret different population growth curves.
✓By the end of this lesson students will be able to define and explain the concept of carrying capacity in an ecosystem.
✓By the end of this lesson students will be able to discuss the roles of competition and predation in regulating population dynamics.

Key concepts

Sampling Techniques: Quadrat

A quadrat is a square frame of a known area (e.g., 0.25 m² or 1 m²) used to sample plants or slow-moving animals in a habitat. It is used to estimate population density (number per unit area) and frequency (percentage of quadrats containing the species). For accurate results, quadrats should be placed randomly within the study area.

Estimated population = (Average number per quadrat / Area of quadrat) × Total area of habitat

Sampling Techniques: Transect

A transect is a line or strip used to study the distribution of organisms in relation to an environmental gradient (e.g., distance from the sea, changes in soil type). * A **line transect** involves laying a tape measure along the gradient and recording species touching the line at regular intervals. * A **belt transect** involves laying a tape measure and placing quadrats at regular intervals along the line, recording species within each quadrat.

Population Growth Curves

These graphs illustrate how population size changes over time. * **S-shaped (Sigmoidal) Curve**: Represents growth in an environment with limiting factors. It typically shows a lag phase (slow initial growth), a log/exponential phase (rapid growth), and a stationary phase (growth slows and stabilises at carrying capacity). * **J-shaped Curve**: Represents exponential growth in an environment with unlimited resources, often followed by a sudden population crash when resources are depleted or waste accumulates.

Carrying Capacity (K)

Carrying capacity is the maximum population size of a particular species that an environment can sustainably support indefinitely, given the available resources (food, water, shelter), habitat, and waste removal capacity. When a population reaches its carrying capacity, the birth rate equals the death rate, and population growth ceases.

Competition

Competition occurs when organisms require the same limited resources (e.g., food, water, space, light, mates). * **Intraspecific competition** occurs between individuals of the same species. It is generally more intense because individuals have identical resource requirements. * **Interspecific competition** occurs between individuals of different species.

Predation

Predation is an interaction where one organism (the predator) kills and consumes another organism (the prey). Predator-prey relationships often lead to cyclical fluctuations in the populations of both species, with a time lag between the peaks of prey and predator populations.

Key facts to remember

1Population dynamics refers to the study of how populations change in size and structure over time.
2Quadrats are used for estimating population density and frequency, especially for plants and sessile animals.
3Transects are used to study the distribution of organisms along an environmental gradient.
4Carrying capacity (K) is the maximum population size an environment can sustain, determined by limiting factors.
5S-shaped growth curves show populations reaching carrying capacity; J-shaped curves show exponential growth followed by a crash.
6Competition can be intraspecific (within a species) or interspecific (between different species).
7Predator-prey interactions often lead to cyclical fluctuations in both populations.
8Limiting factors (e.g., food, water, space, disease, predation) regulate population growth.

Worked examples

Example 1

A student used 10 quadrats, each measuring 0.5 m², to sample buttercups in a field with a total area of 200 m². The total number of buttercups counted across all 10 quadrats was 80. Estimate the total population of buttercups in the field.

I1. Calculate the total area sampled by the quadrats: 10 quadrats × 0.5 m²/quadrat = 5 m².

II2. Calculate the average density of buttercups per square metre: 80 buttercups / 5 m² = 16 buttercups/m².

III3. Estimate the total population in the field: 16 buttercups/m² × 200 m² (total field area) = 3,200 buttercups.

Answer

The estimated total population of buttercups in the field is 3,200.

Ensure random placement of quadrats to avoid bias and obtain a representative sample.

Example 2

Describe the three main phases of an S-shaped population growth curve, explaining what happens to the population size and growth rate in each phase.

I1. **Lag Phase**: In this initial phase, the population size is small, and the growth rate is slow. Organisms are adapting to the new environment, maturing, and reproducing at a low rate.

II2. **Log (Exponential) Phase**: During this phase, the population experiences rapid, accelerating growth. Resources are abundant, and the birth rate significantly exceeds the death rate. The population size increases exponentially.

III3. **Stationary Phase**: In this phase, the population growth rate slows down and eventually stabilises. The birth rate approximately equals the death rate, and the population size remains relatively constant. This occurs as the population approaches the carrying capacity of the environment due to limiting factors such as food scarcity, increased predation, disease, or accumulation of waste products.

Answer

The three main phases are Lag Phase (slow growth, adaptation), Log Phase (rapid exponential growth, abundant resources), and Stationary Phase (growth stabilises at carrying capacity due to limiting factors).

Be able to sketch and label an S-shaped curve, indicating the phases and carrying capacity.

Example 3

Explain how a predator-prey relationship typically influences the population dynamics of both the predator and the prey species, using an example.

I1. **Prey population increases**: When the prey population is abundant, there is plenty of food for predators. This allows the predator population to increase, but typically with a time lag.

II2. **Predator population increases**: As the predator population grows, more prey are consumed, leading to a decline in the prey population.

III3. **Prey population decreases**: With fewer prey available, the predator population experiences food scarcity, leading to a decline in predator numbers.

IV4. **Predator population decreases**: With fewer predators, the pressure on the prey population is reduced, allowing the prey population to recover and increase, restarting the cycle.

V5. **Example**: The classic example is the relationship between the Canadian lynx (predator) and the snowshoe hare (prey). Fluctuations in their populations show a cyclical pattern, with the lynx population peaks lagging behind the hare population peaks.

Answer

Predator-prey relationships result in cyclical fluctuations in both populations. An increase in prey leads to an increase in predators (with a lag), which then causes a decrease in prey, followed by a decrease in predators, allowing the prey to recover, thus repeating the cycle. For example, the lynx and snowshoe hare populations exhibit this pattern.

Remember that the predator population peak usually lags behind the prey population peak.

Common mistakes

✗Confusing the purpose of a quadrat (density/frequency) with a transect (distribution along a gradient).
✗Not explaining the role of limiting factors when discussing carrying capacity or the stationary phase of an S-shaped curve.
✗Assuming predator and prey populations fluctuate in perfect synchrony; there is always a time lag.
✗Failing to distinguish between intraspecific and interspecific competition, and their relative intensities.
✗Incorrectly labelling the axes or phases on population growth curve diagrams.

Exam tips

★When asked to describe sampling techniques, always include the purpose, method, and how the data is used (e.g., calculation of density).
★Be prepared to sketch and label both S-shaped and J-shaped population growth curves, clearly indicating the axes and phases.
★Use specific biological examples (e.g., yeast in a culture for S-curve, lynx and hare for predator-prey) to illustrate your explanations.
★Define all key terms precisely and accurately as per NCCA curriculum standards.

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