Ecosystem Services, Endemic and Keystone Species

Four categories of ecosystem services

Ecosystem services are the benefits humans obtain from functioning ecosystems, and the exam expects the four standard categories:

• Provisioning — physical goods taken out: timber, fish, fresh water, medicinal compounds.
• Regulating — processes that control conditions: wetlands filtering nitrogen out of runoff, forests storing carbon, mangroves absorbing storm surge, insects pollinating crops.
• Cultural — recreation, tourism, spiritual and aesthetic value.
• Supporting — the underlying processes that make the other three possible: soil formation, nutrient cycling, primary production.

The distinction that gets tested: water purification is regulating, nutrient cycling is supporting. Regulating services act on a condition people experience directly; supporting services are one step further back in the causal chain.

Endemic and keystone species

An endemic species is found in one place and nowhere else. Endemism is not about rarity within a site — an endemic bird can be abundant on its island — it is about the global range being small. That is exactly why endemics dominate recent extinction lists: one fire, one introduced rat, one clearing operation can remove $100\%$ of the world population.

A keystone species has an effect on the community far out of proportion to its biomass. Sea otters are under $2\%$ of the biomass of a kelp system, yet removing them lets sea urchins strip the kelp forest to bare rock. The test for keystone status is always a removal experiment, never abundance or body size.