AP Biology 8.4 Effect of Density of Populations Study Notes - New Syllabus Effective 2025
AP Biology 8.4 Effect of Density of Populations Study Notes- New syllabus
AP Biology 8.4 Effect of Density of Populations Study Notes – AP Biology – per latest AP Biology Syllabus.
LEARNING OBJECTIVE
Explain how the density of a population affects and is determined by resource availability in the environment.
Key Concepts:
- Effect of Density of Populations
8.4.A – Effect of Population Density on Resources
🧠 Key Idea:
Population density (how many individuals are in a given area) is closely linked to resources. More resources = more organisms can survive; fewer resources = population declines.
🌾 1. What Determines Population Density?
- Resource availability:
- More food, space, water, shelter = supports higher population density
- Scarcity of resources = limits how many individuals can survive or reproduce
- Environment conditions:
- Harsh environments may support only low density populations
- Stable, rich environments may support high density populations
🔁 2. How Population Density Affects Resources
As density increases, individuals compete more for limited resources
- Too many individuals can deplete food, water, space, etc.
- This can lead to lower reproduction or higher mortality
📉 3. Density-Dependent Factors (Regulate Growth)
Factors that become stronger as population increases:
- Competition for food/mates
- Disease spreads faster
- Predation increases
- Waste accumulation harms health
These factors help stabilize the population and prevent overgrowth.
📈 4. Density-Independent Factors (Unrelated to Density)
These affect a population regardless of size:
- Natural disasters (floods, fires)
- Extreme weather (drought, freezing)
- Pollution
These can suddenly reduce population, even if it’s small.
🔎 Summary:
Resource availability and population density are closely linked – resources limit how many individuals can survive, and population size can also affect the availability and use of resources.
8.4.A.1 – Carrying Capacity: Nature’s Population Limit
🎯 Key Concept:
Carrying capacity (K) is the maximum number of individuals of a species that an ecosystem can sustain over time without degrading the environment.
🌱 What Determines Carrying Capacity?
Resources available in the ecosystem:
- Food
- Water
- Shelter
- Space
- Climate conditions
These limit how many organisms the environment can support long term.
📉 What Happens as a Population Approaches K?
Growth slows down due to:
- Increased competition
- Less available resources per individual
- More stress and disease
Population may fluctuate around the carrying capacity:
- Slight overshoots followed by diebacks
- Stabilizing over time
📊 Growth Patterns:
- Exponential Growth: Occurs when resources are unlimited (J-shaped curve)
- Logistic Growth: Population grows fast, then slows as it nears K (S-shaped curve)
🧠 Remember:
Carrying capacity is not fixed. It can change with:
- Environmental changes (e.g., drought)
- Human activity (e.g., pollution or conservation)
- Resource depletion or restoration
📝 Summary:
The carrying capacity is nature’s built-in limit – when populations grow beyond the resources available, natural forces push them back down.
8.4.A.2 – Logistic Growth & Population Limits
🎯 Key Concept:
As a population grows, it eventually hits limits. These limits can be due to:
- Density-dependent factors (affected by population size)
- Density-independent factors (random events)
When limits kick in, the population no longer grows exponentially — instead, it follows a logistic growth model.
📉 Logistic Growth: The S-Shaped Curve
Logistic growth happens when:
- A population grows rapidly at first (like exponential growth)
- Then slows down as it approaches the carrying capacity (K)
- Finally, it levels off and stabilizes near K
🧠 The shape = S-curve
✏️ Logistic Growth Equation:
\( \frac{dN}{dt} = r_{max} N \left( \frac{K – N}{K} \right) \)
Where:
\( \frac{dN}{dt} \) = change in population size over time
\( r_{max} \) = max growth rate per individual
N = current population size
K = carrying capacity
🧮 When N is small → fast growth
📉 When N gets closer to K → growth slows
⏸️ When N = K → growth = 0 (stable population)
📦 Factors That Limit Growth:
- Density-Dependent Factors
These increase in effect as population grows:- Limited food
- Limited space
- Disease
- Predation
- Competition
- Density-Independent Factors
Affect population regardless of its size:- Natural disasters
- Climate events
- Human impact (deforestation, pollution)
📝 Summary:
When a population grows, it doesn’t grow forever. As resources become limited or external events occur, logistic growth takes over, balancing the population around carrying capacity (K).