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Unit 2
6–8% of examThis unit is about the variety of life and why it matters. You'll learn how biodiversity helps ecosystems recover from disruptions, what services nature provides to people, and how ecosystems change over time through natural disasters, adaptation and succession.
Longer videos that cover the whole unit. Good for a first pass or a final review.
Biodiversity includes genetic diversity, species diversity and habitat diversity. A population with more genetic variety is better able to survive changes in its environment, and a sudden crash in population size (a bottleneck) can wipe out much of that variety.
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Ecosystem services are the benefits people get from nature, grouped into four types: provisioning (goods like food, wood and medicine), regulating (like climate control and flood protection), cultural (like recreation) and supporting (like soil formation and nutrient cycling). When people damage ecosystems, losing these services can cost money and harm health.
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Island biogeography studies why some islands, or island-like habitats such as a forest patch surrounded by farms, hold more species than others. Bigger islands and islands closer to the mainland usually have more species, and many island species evolve into specialists, which makes them vulnerable to invasive species.
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Ecological tolerance is the range of conditions, like temperature, salinity or sunlight, that an organism can survive. Inside its optimal range it thrives, in the zones of physiological stress it struggles, and beyond its limits it dies.
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Natural events like hurricanes, floods, wildfires, droughts and volcanic eruptions can disrupt an ecosystem as much as people do, or more. They can be periodic (repeating on a regular cycle, like wet and dry seasons), episodic (coming now and then, like droughts) or random (like earthquakes). Over long stretches of time, Earth's climate and sea level have also shifted many times, and animals often migrate to cope.
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An adaptation is an inherited trait that helps an organism survive and reproduce in its environment. Natural selection spreads these traits through a population over many generations, so if the environment changes faster than a population can adapt, individuals must change their behavior, move or die.
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Succession is the gradual change in which species live in an area. Primary succession starts on bare rock with no soil, led by pioneer species like lichens and mosses, while secondary succession is faster because soil is left after a disturbance like a fire or abandoned farm. Keystone and indicator species also tell you a lot about how an ecosystem is doing.
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