Chapter 10 of 10 - AP Biology Course
Ecology & Populations
Ecology examines interactions between organisms and their environment at every scale, from individual physiology to global biogeochemical cycles. This chapter covers population dynamics, community interactions, energy flow, nutrient cycling, and the conservation challenges facing modern ecosystems.
Levels of Ecological Organization
Ecology is organized into hierarchical levels. An organism interacts with its abiotic environment. A population is a group of individuals of the same species in a defined area. A community includes all the populations of different species in that area. An ecosystem adds abiotic factors (energy, nutrients, climate) to the community. The biosphere encompasses all ecosystems on Earth.
Population ecology focuses on how population size changes over time. Key factors include birth rate, death rate, immigration, and emigration. Population density (individuals per unit area) and dispersion patterns (clumped, uniform, random) describe spatial distribution within a habitat.
Population Growth Models
Exponential growth follows the equation dN/dt = rN, where r is the intrinsic rate of increase. This produces a J-shaped curve and occurs when resources are abundant, as in bacteria colonizing fresh media. No population sustains exponential growth indefinitely because resources are finite.
Logistic growth adds a carrying capacity term: dN/dt = rN(K - N)/K. As the population (N) approaches carrying capacity (K), growth rate slows and eventually stabilizes, producing an S-shaped (sigmoid) curve. Density-dependent factors such as competition, disease, and predation intensify as population size increases.
| Feature | Exponential growth | Logistic growth |
|---|---|---|
| Equation | dN/dt = rN | dN/dt = rN(K - N)/K |
| Curve shape | J-shaped | S-shaped (sigmoid) |
| Carrying capacity | Not incorporated | Growth slows as N approaches K |
| Limiting factors | None assumed | Density-dependent regulation |
| Real-world example | Bacteria in fresh nutrient broth | Deer population in a bounded forest |
Quick Check
At what population size does logistic growth reach its maximum growth rate?
Fill in the Blank
Organisms that invest heavily in producing many small offspring with little parental care are called________-selected species, while those producing few large offspring with extensive parental investment are K-selected.
Community Ecology and Species Interactions
Species within a community interact in ways that affect fitness and population dynamics. Interspecific competition (-/-) occurs when two species use the same limited resource, as described by the competitive exclusion principle - no two species can occupy the same niche indefinitely. Resource partitioning reduces competition by dividing the niche.
Predation (+/-) drives coevolutionary arms races between predator and prey, producing adaptations such as camouflage, warning coloration, and mimicry (Batesian and Mullerian). Mutualism (+/+) benefits both partners, as in mycorrhizal fungi and plant roots. Commensalism (+/0) benefits one species without affecting the other. Parasitism (+/-) benefits the parasite at the host's expense.
Energy Flow and Trophic Levels
Energy enters ecosystems primarily through primary producers (autotrophs) that convert solar energy to chemical energy via photosynthesis. Gross primary productivity (GPP) is the total energy fixed; net primary productivity (NPP) equals GPP minus energy used in the producers' own respiration.
Only about 10% of energy at one trophic level transfers to the next - the rest is lost as heat through cellular respiration. This 10% rule explains why food chains rarely exceed four or five levels and why top predators are relatively rare. Energy flow is one-directional (producers to consumers to decomposers) and follows the laws of thermodynamics.
Energy decreases at each trophic level due to metabolic heat loss. Nutrients are recycled by decomposers.
Primary Producers
Autotrophs (plants, algae) fix solar energy via photosynthesis
Primary Consumers
Herbivores that feed on producers - receive about 10% of producer energy
Secondary Consumers
Carnivores that eat herbivores - receive about 10% of primary consumer energy
Tertiary Consumers
Top predators - receive about 10% of secondary consumer energy
Decomposers
Fungi and bacteria recycle nutrients from all trophic levels back to the soil
An ecological pyramid illustrating the decrease in available energy at each successive trophic level. Producers form the broad base; top predators occupy the narrow apex.
Quick Check
If primary producers in an ecosystem fix 10,000 kcal of energy, approximately how much energy is available to secondary consumers?
Biogeochemical Cycles
Unlike energy, matter is recycled through ecosystems. The water cycle moves water through evaporation, transpiration, condensation, and precipitation. The carbon cycle links photosynthesis (CO2 fixation) with cellular respiration (CO2 release), fossil fuel combustion, and ocean absorption. Rising atmospheric CO2 from human activity drives climate change by enhancing the greenhouse effect.
The nitrogen cycle involves nitrogen fixation (N2 to NH3 by bacteria), nitrification (NH3 to NO3-), assimilation by plants, and denitrification (NO3- back to N2). Nitrogen is often a limiting nutrient in terrestrial ecosystems, and excess nitrogen from fertilizer runoff causes eutrophication in aquatic systems.
Carbon Dioxide
carbon dioxide
Carbon dioxide is central to the carbon cycle. Plants fix CO2 during photosynthesis, while respiration and combustion release it back into the atmosphere. Rising CO2 levels from fossil fuel use drive global climate change through the greenhouse effect.
Formula
CO2
Mol. Weight
44.01 g/mol
Fill in the Blank
The process by which bacteria convert atmospheric nitrogen (N2) into ammonia (NH3) is called nitrogen________.
Ecological Succession and Conservation
Primary succession begins on bare substrate with no soil (lava flows, retreating glaciers). Pioneer species like lichens and mosses gradually build soil. Secondary succession occurs after a disturbance (fire, farming) destroys a community but leaves soil intact, allowing faster recovery. Both types trend toward a climax community, though modern ecologists recognize that disturbance regimes keep most ecosystems in dynamic flux.
Biodiversity encompasses species richness (number of species) and relative abundance (evenness). Ecosystem services - pollination, water purification, carbon sequestration, nutrient cycling - depend on biodiversity. Major threats include habitat destruction, invasive species, overexploitation, pollution, and climate change. Conservation strategies range from protected areas and habitat corridors to captive breeding and legal protections like the Endangered Species Act.
Quick Check
Which type of ecological succession would occur on a newly formed volcanic island?
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