A diversity index says how many species a community has and how evenly; it does not say what holds the community together. You met keystone species before. Here we look at the forces that shape a community from above and from below.
- Top-down control: predators limit herbivores, which lets plants thrive. Remove the predator, and the effect cascades down: a trophic cascade. Where sea otters were hunted out, sea urchins boomed and grazed kelp forests down to bare rock; where otters returned, kelp recovered.
- Bottom-up control: nutrients limit producers, which limit everything that eats them. Add nutrients and producers grow.
- Keystone species: a species with an effect far larger than its numbers. When the ecologist Robert Paine removed the sea star Pisaster from tide pools, mussels took over and many other species disappeared.
- Ecosystem engineers: species that build or change habitat, such as beavers, whose dams turn streams into ponds and wetlands.
- Invasive species: species introduced to a new place where they spread and harm the community, often because their natural predators and diseases are missing.
Key idea
Communities are shaped from the top down (predators) and the bottom up (nutrients). Keystone species, ecosystem engineers and invasive species can restructure them. More nutrients do not always mean a healthier community.
Why it works
Why can adding nutrients hurt a lake? Nitrogen and phosphorus from fertilizer runoff or sewage feed a bloom of algae. The algae soon die, and bacteria decomposing them use up the dissolved oxygen through respiration. Fish and other animals suffocate. This process is called eutrophication. The bottom of the food web grew; the community collapsed.
Worked examples
Worked example: A trophic cascade
In a kelp forest, otters eat urchins and urchins eat kelp. Predict what happens to urchins and kelp if otters disappear.
- Otters gone: urchins are no longer eaten, so urchins increase.
- More urchins graze more kelp, so kelp decreases.
Answer: Urchins up, kelp down: a cascade two levels below the predator.
Worked example: Reading the lake data
In the graph, why does oxygen rise first and then fall far below where it started?
- Weeks 2 to 4: the algae bloom, and their photosynthesis adds oxygen.
- After week 4: the algae die, and decomposers respire as they break them down.
- Respiration uses more oxygen than the shrinking algae can make.
Answer: The bloom first adds oxygen; its decomposition then uses it up.
Common mistake
Thinking that adding nutrients always makes a community healthier. Extra nitrogen and phosphorus can cause algal blooms that use up the oxygen. And remember that effects spread through the web: removing one species changes its prey, its predators and, through them, species it never touched.