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Biology Unit 8: Ecology

Ecology studies how organisms interact with each other and their environment. Energy flows through ecosystems while matter cycles.

Covers: Energy flow and food webs, population growth, community interactions, biogeochemical cycles, biodiversity

Study guide

Ecology studies interactions among organisms and their environment. Energy flows one way through ecosystems: producers capture sunlight through photosynthesis, and consumers eat producers or other consumers. Food chains and food webs show who eats whom, organized into trophic levels. Only about 10% of the energy at one level passes to the next, because most is lost as heat or used for life processes. That limits food chains to a few levels and makes energy pyramids narrow at the top. Decomposers break down dead matter and recycle nutrients.

Population growth can be exponential (J-shaped, when resources are unlimited) or logistic (S-shaped, leveling off at the carrying capacity K, the largest population the environment can support). Density-dependent factors like competition, predation and disease intensify as populations grow, while density-independent factors like weather affect them regardless of size.

Community interactions include competition (both harmed), predation and herbivory, and symbiosis: mutualism (both benefit), commensalism (one benefits, the other is unaffected) and parasitism (one benefits at the other's expense). Keystone species have outsized effects on their communities. Ecological succession rebuilds communities after disturbance, starting from bare rock (primary) or from soil left behind (secondary).

Biogeochemical cycles recycle matter. In the carbon cycle, photosynthesis removes CO₂ and respiration and combustion return it. In the nitrogen cycle, bacteria fix atmospheric N₂ into forms plants can use. The water and phosphorus cycles move those essential materials too.

Biodiversity, the variety of genes, species and ecosystems, makes ecosystems more stable and resilient. Habitat loss, invasive species, pollution, overharvesting and climate change are its main threats.

In the nitrogen cycle, nitrogen fixation turns N₂ into ammonia, decomposers return nitrogen from wastes as ammonium (ammonification), and nitrifying bacteria convert ammonium to the nitrates plants absorb. Denitrifying bacteria convert nitrates back to N₂ gas, which returns it to the atmosphere. Biotic factors are the living parts of an ecosystem, such as predators, prey and pathogens. Abiotic factors are the nonliving parts, such as temperature, light, water, pH and soil minerals. Both kinds of factors limit where species can live and how large their populations grow.

Organisms respond to environmental cues with changes in behavior and physiology. Day length, temperature or moonlight can trigger migration, flowering, mating or hiding from predators. Breeding is often timed so that young are born when food is most plentiful, which improves their survival. Removing or adding a predator can ripple down the food web in a trophic cascade. Without wolves, for instance, deer overgraze plants. Invasive species often spread quickly because their new range has none of their natural predators, competitors or diseases, so they outcompete native species.

Gross primary productivity is the total energy producers capture, and net primary productivity is what remains after they use some for their own respiration. NPP is the energy available to consumers. Simpson's diversity index (1 − Σ(n/N)²) rises when there are more species and when their populations are more evenly balanced.

Key ideas

• Producers capture energy; consumers eat; decomposers recycle matter.
• Only about 10% of energy passes to the next trophic level.
• Exponential growth is J-shaped; logistic growth levels off at carrying capacity (K).
• Interactions: competition, predation, mutualism, commensalism, parasitism.
• Carbon, nitrogen and water cycle through living and nonliving parts of ecosystems.

Formulas and rules

Common mistakes

• Saying energy cycles. Energy flows; matter cycles.
• Mixing up commensalism (+/0) and mutualism (+/+).

Words to know

10% rule

About 10% of energy passes to the next trophic level.

Carrying capacity (K)

The largest population an environment can support long-term.

Mutualism

A relationship where both species benefit.

Decomposer

An organism that breaks down dead matter and recycles nutrients.

Nitrogen fixation

Bacteria converting N₂ gas into usable ammonia.

Logistic growth

Population growth that levels off at carrying capacity K.

Producer

An organism that makes food from sunlight (autotroph).

Trophic level

A feeding level in a food chain.

Food web

Connected food chains in an ecosystem.

Exponential growth

J-shaped growth with unlimited resources.

Density-dependent factor

A limit that intensifies as populations grow, like competition or disease.

Commensalism

One species benefits, the other is unaffected.

Parasitism

One species benefits at the other's expense.

Keystone species

A species with an outsized effect on its community.

Primary vs. secondary succession

Starting from bare rock vs. from existing soil.

Biodiversity

The variety of genes, species and ecosystems.

Sample practice questions

13 more questions like these, from level 1 to 5, are in the practice set.