DENSITY-DEPENDENT factors become more limiting as population DENSITY increases:
Food shortages — more individuals compete for the same food supply.
Disease — pathogens spread more easily in dense populations.
Predation — predators focus on areas of high prey density.
Waste accumulation — toxins from metabolic waste build up.
DENSITY-INDEPENDENT factors affect populations regardless of their density:
Severe weather — frost, drought, flood, fire.
Natural disasters — volcanic eruption, tsunami.
Human activity — habitat destruction, pollution.
Human impacts on population:
Hunting and fishing can reduce populations below viable levels.
Deforestation reduces carrying capacity for forest species.
Pollution increases death rates.
Introduction of invasive species can cause collapse of native species populations.
⚠️ Common Mistake
In predator-prey cycles, the PREDATOR population FOLLOWS the prey — with a time lag. Predator numbers rise AFTER prey numbers rise (because it takes time to breed). Predator numbers fall AFTER prey decline. Students often draw them rising and falling at the same time — they should be slightly out of phase.
📌 Key Note
Population limited by resources (food, water, space), predation and disease = carrying capacity. Predator-prey cycles are out of phase — predator follows prey with a lag. Intraspecific (same species) competition is most intense. Interspecific = between different species.
🎯 Matching Activity — Match the Population Ecology Concept
Match each term to its correct description. — drag the symbols on the right to match the component names on the left.
Carrying capacity
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Intraspecific competition
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Interspecific competition
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Predator-prey cycle
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Density-dependent factor
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Predator and prey populations fluctuate — predator follows prey with a time lag
Becomes more limiting as population density increases — e.g. food shortage, disease
Maximum population size that a habitat can sustainably support
Competition between individuals of DIFFERENT species for the same resource
Competition between individuals of the SAME species — most intense
⭐ Higher Tier Only
Students should be able to interpret predator-prey population graphs — explaining the time lag between predator and prey peaks, and predicting the effects of changing one population on others in a food web. The carrying capacity is the maximum population that resources can sustain. Students should understand the concept of intraspecific competition as the primary density-dependent control on population size.
🎯 Test Yourself
Question 1 of 2
1. In a predator-prey graph, the predator population peaks AFTER the prey population peaks. Why?
2. Why is intraspecific competition more intense than interspecific competition?
⭐ How Well Do You Understand This Topic?
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