In the 1960s the ecologist Robert Paine started an experiment on the rocky shore of Washington State. In one stretch of tide pools he kept removing the ochre sea stars, Pisaster ochraceus, and left a nearby stretch alone. Without the sea stars, which eat mussels, the mussels spread and crowded out many of the other species living on the rocks. The number of species fell. In 1969 Paine called species like this sea star keystone species. One predator was holding a whole community together. Let's look closer at how ecosystems work, from single organisms to whole biomes. This is Unit 8 of AP Biology.
Responses to the environment
Organisms respond to changes with behavior and physiology. A woodlouse that speeds up in spots that are too hot or too cold, and so tends to end up somewhere milder, is showing kinesis: undirected movement. A taxis is movement directed toward or away from a stimulus. Plants respond to day length (photoperiodism) and to light direction (phototropism). Animals signal visually, by sound, by touch, electrically or chemically, to warn of predators, mark territory or attract mates. Behaviors that raise survival and reproduction are favored by natural selection, including cooperation and kin selection.
Energy: how organisms get it and use it
- Endotherms use heat from their own metabolism to keep a steady body temperature. Ectotherms mostly regulate it by behavior, such as basking.
- A net gain of energy means growth, storage and more reproduction. A net loss means losing mass, fewer offspring and eventually death.
- Autotrophs capture energy: photosynthetic ones from sunlight, chemosynthetic ones from inorganic molecules, even without oxygen. Heterotrophs get it by eating organic matter.
Energy flows, matter cycles
Energy enters as sunlight and leaves as heat; it flows one way. Matter is recycled through biogeochemical cycles:
- Water: evaporation, transpiration, condensation, precipitation.
- Carbon: photosynthesis takes CO2 in; respiration, decomposition and combustion release it.
- Nitrogen: soil microorganisms carry out its steps, including fixation, ammonification, nitrification and denitrification. The atmosphere is the biggest reservoir.
- Phosphorus: released from weathering rock, passed along food chains and returned by decomposition.
Only about 10% of the energy at one trophic level is passed to the next, on average. This is the simple version; the real figure varies between ecosystems.
Populations
With no limits, a population grows exponentially (dN/dt = rmax N). Real resources run out, so growth usually becomes logistic, leveling off at the carrying capacity, K. Density-dependent factors like competition and disease bite harder in crowded populations; density-independent ones like a frost do not.
Communities
A community is described by which species it has and how diverse it is (Simpson's diversity index). Species interact through competition, predation and symbiosis: mutualism (+/+), commensalism (+/0) and parasitism (+/−). Similar species can coexist by niche partitioning.
Biodiversity and disruptions
Diverse ecosystems are usually more resilient. A field of genetically identical crops can be wiped out by one disease. Removing a keystone species can set off a trophic cascade. The return of wolves to Yellowstone from 1995 is often told as a cascade story, but researchers still debate how much the wolves alone changed the plants.
Humans speed up change. Invasive species such as kudzu and zebra mussels exploit new niches. Biomagnification concentrates toxins like DDT in top predators. Eutrophication from fertilizer runoff causes algal blooms that use up oxygen as they decay. Geological and weather events, from El Niño to the asteroid impact linked to the dinosaurs' extinction, also reshape ecosystems.
Common mistakes students make
- Saying energy cycles. Matter cycles; energy flows through and is lost as heat.
- Putting maximum growth at K. In the logistic model, growth is fastest around K/2 and zero at K.
- Thinking a keystone species must be common. Its effect is out of proportion to its numbers.
- Saying the environment causes helpful mutations. Mutations are random; the environment selects among them.
Frequently asked questions
What is on AP Biology Unit 8?
Responses to the environment, energy flow, population growth, density effects, community ecology, biodiversity and disruptions to ecosystems.
What is the difference between exponential and logistic growth?
Exponential growth has no limits and makes a J-shaped curve. Logistic growth slows as the population nears carrying capacity, making an S-shaped curve.
Why is only about 10% of energy passed up a food chain?
Most energy is used for cellular respiration and lost as heat, and some is in parts that aren't eaten or digested.
What is a keystone species?
A species with a much bigger effect on its community than its numbers suggest, like Paine's sea stars.
Try it
Say each answer before you flip the card. Then predict: what would you expect to see if someone removed the sea stars from a tide pool and came back a few years later?
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