Population Growth Curves
- Populations don’t grow indefinitely.
- Over time, they follow a sigmoid growth curve, which consists of three distinct phases: rapid growth, slowed growth, and stabilization.
- This curve explains how populations interact with their environment and the limits imposed by resources, competition, predators, and diseases.
Recall that a population is a group of organisms of the same species living in a specific area.
Phases of the Sigmoid Growth Curve
1. Exponential Growth Phase:
Population grows rapidly due to:
- Abundant resources (e.g., food, space).
- Few limiting factors like predators or diseases.
Bacteria in a nutrient-rich petri dish double every few hours during this phase
- Think of a population of rabbits introduced to a new area with plenty of food and no predators.
- The population will grow quickly as each pair of rabbits produces offspring.
2. Transitional Phase
Growth slows as resources become limited, leading to:
- Increased competition for food, space, and water.
- Higher mortality rates as some individuals cannot survive or reproduce.
This phase marks the beginning of environmental resistance, where factors like competition and predation start to limit growth.
3. Plateau Phase
Population size stabilizes near the carrying capacity because:
- Birth and death rates are roughly equal.
- Resources are insufficient to support further growth.
Recall that carrying capacity refers to the maximum population size that an environment can sustainably support.
- A forest can only support a certain number of deer based on the availability of food and space.
- Once this limit is reached, the population stabilizes.
Factors Affecting Population Growth Curves
1. Predation
- Impact: Predators control prey populations by hunting them.
- Effect on Curve: The curve fluctuates as prey population declines, followed by predator decline, then recovery.
In predator-prey cycles, an increase in foxes leads to fewer rabbits, which in turn reduces fox numbers.
2. Competition
- Impact: Individuals compete for limited resources like food, water, and space.
- Effect on Curve: Growth slows earlier, reducing the carrying capacity.
3. Disease
- Impact: High population density makes it easier for diseases to spread, reducing population size.
- Effect on Curve: Rapid decline during outbreaks, followed by stabilization as the population recovers.
Disease outbreaks in crowded rat populations can cause sudden declines.
Why Exponential Growth Occurs in the Initial Phase
Exponential growth happens when:
- Resources are plentiful.
- There are few limiting factors like predators or diseases.
- The population is small, allowing rapid reproduction.
When yeast is added to a sugar-rich environment, it reproduces rapidly until the sugar is depleted or waste products accumulate.
Remember that the sigmoid curve is an idealized model. Real populations may not follow it perfectly due to factors like sudden environmental changes.
Testing Population Growth Against Models
- Scientists use population growth graphs to:
- Compare real populations to theoretical models.
- Identify how factors like predation or resource scarcity impact growth.
- Graph Setup:
- Logarithmic scale: Population size on the vertical axis.
- Linear scale: Time on the horizontal axis.
- Imagine tracking the growth of a bacterial population in a lab.
- By plotting the data on a graph, you can see how closely it follows the exponential growth model.
- How do population growth models help us make decisions about resource management and conservation?
- What are the ethical implications of controlling population sizes?
When analyzing population growth curves, always:
- Describe the three phases (exponential, transitional, plateau).
- Explain how factors like predation, competition, and disease modify the curve.
Reflection and Review
- How do density-dependent factors (e.g., predation, competition, and disease) shape population growth curves?
- Why does exponential growth never last indefinitely in real populations?
- What are some real-world examples of populations that follow a sigmoid growth curve?
Try sketching a population growth curve and labeling key phases and factors that influence it. Can you describe each phase in your own words?
