Rucete ✏ AP Biology In a Nutshell
21. Population Ecology, Community Ecology, and Biodiversity
This chapter explores how populations grow, how species interact within communities, and how biodiversity contributes to ecosystem resilience. It also introduces key ecological models and formulas used on the AP Biology exam.
Population Ecology
• Population growth depends on: – Population size (N) – Birth rate (B) – Death rate (D)
• Population growth = ΔN / Δt = B – D
Exponential Growth:
• Occurs when no limiting factors exist (abundant resources, no predators).
• J-shaped curve; faster growth as population increases.
• Equation: dN/dt = rmax × N
Logistic Growth:
• Occurs when resource limitations exist.
• S-shaped curve; stabilizes near carrying capacity (K).
• Equation: dN/dt = rmax × N × (K – N)/K
• Density-dependent factors: disease, predation, competition.
• Density-independent factors: temperature, weather, natural disasters.
K-Selected vs. r-Selected Populations
K-Selected:
• Stable environments • Few offspring • Reproduce more than once • High parental investment • Logistic growth
• Examples: birds, mammals
r-Selected:
• Unstable environments • Many offspring • Reproduce once • Low survival and parental care • Boom-bust cycles
• Examples: fish, amphibians
Community Ecology and Simpson’s Diversity Index
• Communities = multiple populations interacting in one habitat.
• Species composition: number of species present.
• Species diversity: richness + evenness.
• Simpson’s Diversity Index: – D = 1 – Σ(n/N)² – Higher value = more diverse community.
Ecological Relationships
Competition: individuals compete for limited resources (light, space, food).
– Intraspecific: within the same species – Interspecific: between different species
Predation: one species (predator) kills and eats another (prey).
– Prey defenses evolve: camouflage, toxins, warning coloration, mimicry
Herbivory: animals eat plants; plants evolve defenses (spines, toxins)
Symbiosis: close long-term relationships between species – Mutualism: both benefit (e.g., bees and flowers) – Commensalism: one benefits, other unaffected (e.g., barnacles on whales) – Parasitism: one benefits, one harmed (e.g., tapeworms in humans)
Invasive Species
• Invasive species are nonnative and outcompete native species.
• Often have no natural predators or competitors in new environment.
• Examples: – Zebra mussels (North America) – Cane toads (Australia) – Kudzu vine (U.S. South)
Biodiversity and Ecosystem Stability
• Biodiversity = variety of species, genes, and ecosystems.
• Higher biodiversity → greater ecosystem stability and resilience.
• Ecosystems recover faster from disruptions when biodiversity is high.
Keystone and Indicator Species
Keystone species: disproportionately affect ecosystem structure and stability.
• Example: Sea otters eat sea urchins → protect kelp forests.
Indicator species: sensitive to environmental changes; signal ecosystem health.
• Example: amphibians (permeable skin), lichens (air pollution)
Disruptions to Ecosystems
• Can be natural (floods, fires, droughts) or human-caused (pollution, habitat loss, invasive species).
• Disruptions may alter carrying capacity and cause species declines or extinctions.
• Human impact is currently the dominant driver of biodiversity loss.
In a Nutshell
Population and community ecology explain how species grow, interact, and form complex systems. Biodiversity supports ecosystem resilience, while ecological relationships—like predation and mutualism—shape species distributions. Invasive species, habitat destruction, and climate change threaten ecological balance worldwide.