Unit 21: Ecology and the Environment
How energy and matter cycle through an ecosystem, and what happens to the whole web when humans change one part of it.
12 topics · 37 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
Levels of Ecological Organization
- How it worksHow do organisms, populations, communities, ecosystems, and the biosphere nest inside each other?
- Common mistakeWhy isn't a population the same thing as a community?
- Ties togetherHow does the hierarchy of ecological organization mirror the hierarchy of organization inside an organism's own body?
Your turn1 of 2
A wolf population's size in a forest depends not just on the number of deer available to hunt, but also on competing predators, plant abundance, and even soil conditions supporting the whole food web. Why does a single wolf population's size depend on every other species living in the same forest?Food Chains and Food Webs
- How it worksHow do arrows in a food web trace the flow of energy from producers up through several levels of consumers?
- How it worksWalk me through what happens to a food web's other species when one key predator is removed.
- Common mistakeWhy isn't a food chain realistic on its own since most real ecosystems form interconnected webs instead?
Your turn1 of 2
Removing sea otters from a coastal ecosystem can eventually devastate kelp forests and the many fish species that depend on that kelp, even though those fish never directly interacted with the otters. Why does removing one species sometimes collapse populations that never even interacted with it directly?Energy Flow and Trophic Pyramids
- How it worksHow does only about ten percent of energy transfer from one trophic level up to the next?
- Worked exampleFind how much energy reaches the third trophic level starting from 10,000 kilocalories of producer energy.
- Common mistakeWhy doesn't all the energy an organism eats get passed on to whatever eats it next?
Your turn1 of 2
Only about 10 percent of energy transfers from one trophic level up to the next. Starting from 36339 kilocalories of energy at the producer (first trophic) level, approximately how much energy reaches the third trophic level?The Carbon and Nitrogen Cycles
- How it worksHow does carbon cycle between the atmosphere, living organisms, and fossil fuels over long timescales?
- How it worksHow do nitrogen-fixing bacteria convert atmospheric nitrogen into a form plants can actually use?
- Common mistakeWhy can't plants and animals use the nitrogen gas that makes up most of the atmosphere directly?
Your turn1 of 2
A carbon atom exhaled by a dinosaur millions of years ago could, after cycling through the atmosphere, plants, and other organisms over an immense span of time, plausibly be part of a molecule in your body today. Why might a carbon atom in your body today have once been part of a dinosaur's exhaled breath?Population Growth: Exponential and Logistic
- How it worksHow does exponential growth produce a curve that keeps accelerating upward without limit?
- How it worksHow does logistic growth level off into an S-shaped curve once resources start running out?
- Worked exampleFind the population size after three doubling periods starting from 100 bacteria growing exponentially.
- Common mistakeWhy doesn't a real population keep growing exponentially forever the way the bacteria math predicts?
Your turn1 of 2
A bacteria population starts at 3485 cells and doubles with every doubling period under ideal exponential growth. What is the population size after 3 doubling periods?Carrying Capacity and Limiting Factors
- How it worksHow do limiting factors like food, water, and space set an ecosystem's carrying capacity?
- Going furtherWhat happens to a population's size when it temporarily overshoots its environment's carrying capacity?
- Common mistakeWhy isn't carrying capacity a fixed number that never changes for a given environment?
Your turn1 of 2
A deer population in a fixed forest habitat grows steadily for a while after being introduced, then levels off and stops increasing even though deer keep reproducing every year. Why does a deer population stop growing once it reaches a certain size in a fixed forest habitat?Predator-Prey Relationships
- How it worksHow does a predator population's growth lag slightly behind its prey population's growth and decline?
- How it worksHow does a predator-prey cycle graph show two populations chasing each other's changes over time?
- Common mistakeWhy doesn't a predator population wiping out all of its prey usually happen in a stable ecosystem?
Your turn1 of 2
Lynx and snowshoe hare populations, tracked over more than a century of fur trapping records, rise and fall together in a repeating cycle roughly every ten years, rather than settling at one stable number. Why do lynx and snowshoe hare populations rise and fall in a repeating cycle over the decades?Symbiosis: Mutualism, Commensalism, and Parasitism
- How it worksHow does mutualism benefit both species while parasitism benefits only one at the other's expense?
- Going furtherWhat happens with a barnacle attached to a whale that illustrates commensalism instead of mutualism?
- Common mistakeWhy isn't every close relationship between two species automatically classified as mutualism?
Your turn1 of 2
A clownfish swims safely among a sea anemone's stinging tentacles, tentacles that would seriously injure or kill most other fish that brushed against them, while the anemone benefits from the clownfish chasing off its own predators. Why do clownfish live safely inside a sea anemone's tentacles that would kill most other fish?Competition, Niches, and Ecological Succession
- How it worksHow does competitive exclusion push one of two competing species toward local extinction or a different niche?
- How it worksWalk me through how a bare patch of ground transforms through ecological succession into a mature forest.
- Common mistakeWhy isn't an organism's niche just the physical place where it lives?
Your turn1 of 2
Two closely related bird species requiring the exact same nesting sites, food source, and habitat conditions generally can't both maintain stable populations together in the same area indefinitely. Why can't two species that need the exact same resources coexist indefinitely in the same habitat?Biomes of the World
- How it worksHow do temperature and precipitation patterns determine which biome forms in a given region?
- Common mistakeWhy isn't a biome the same thing as a single specific ecosystem?
- Ties togetherHow does a biome's set of typical adaptations connect back to natural selection acting on its resident species?
Your turn1 of 2
A desert in Arizona and a desert in the Sahara sit on opposite sides of the planet, with different specific plant and animal species living in each, yet both support cactus-like succulents, nocturnal animals, and other similar water-conserving adaptations. Why do these two deserts support such similar kinds of adaptations?Human Impact: Habitat Loss and Biodiversity
- How it worksHow does habitat fragmentation isolate populations into patches too small to sustain themselves?
- Going furtherWhat happens to an ecosystem's stability as its overall biodiversity decreases?
- Common mistakeWhy doesn't protecting one single endangered species automatically protect the whole ecosystem it lives in?
Your turn1 of 2
A single acre of rainforest can host species found nowhere else on the planet, so clearing that one acre can permanently eliminate those species entirely rather than just reducing their numbers elsewhere. Why does clearing a single acre of rainforest sometimes eliminate species that live nowhere else on Earth?Human Impact: Pollution, Climate Change, and Invasive Species
- How it worksHow do greenhouse gases trap heat in the atmosphere that would otherwise radiate back into space?
- How it worksHow does an invasive species introduced to a new ecosystem disrupt food webs that evolved without it?
- Common mistakeWhy doesn't a single cold winter disprove the overall trend of long-term climate change?
Your turn1 of 2
Fossil fuel burning has occurred at large industrial scale for over a century, and average global temperature measurements over that same period show a clear long-term upward trend. Why does burning fossil fuels for over a century measurably raise the average temperature of the entire planet?