Biodiversity is the variety of life: the number of different species, the genetic differences within each species, and the variety of ecosystems. You have met the ways species depend on each other in Competition, Predation and Symbiosis and how communities rebuild in Ecological Succession.
Key idea
Biodiversity counts kinds, not individuals. Healthy ecosystems provide ecosystem services people rely on: clean water, fertile soil, pollination of crops, food, wood and medicines. More diverse ecosystems often recover better after a disturbance.
Why it works
Diversity spreads the risk. If a disease kills one kind of grass in a prairie with thirty species, other plants fill the gap and the animals that need plants still have food. In a field of a single crop, the same disease can wipe out everything at once. Many species also do overlapping jobs, like pollinating or holding soil, so losing one does not stop the job.
Worked example: Compare two plots
Plot A has 500 plants of 3 species. Plot B has 300 plants of 12 species. Which has greater species diversity?
- Count kinds, not individuals.
- Plot A: 3 species. Plot B: 12 species.
Answer: Plot B, even though it has fewer plants.
Worked example: Name the service
Honeybees and wild bees visit apple blossoms. What ecosystem service is that?
- Apple trees need pollen moved from flower to flower to make apples.
- The bees do that work for free.
Answer: Pollination.
Common mistake
Students measure biodiversity by the number of individuals. A field with a thousand plants of one kind has very low diversity. Always ask: how many different species are there, and is any one species taking over?