The Part You Cannot Remove

Five ecosystems, one environment getting worse

Each system has a set of jobs that must keep being done. A shock kills every species that cannot tolerate it, and the system keeps working as long as every job still has somebody doing it. Three of these systems have exactly the same number of species.

All five, side by side

Read the first two columns against the last two. Species count does not order this table, and neither does the number of species per job.

Take one species out and see what follows

A twelve-species web. Two rules decide what happens next: a consumer with nothing left to eat dies, and a competitive dominant with nothing holding it down takes the space and crowds out everything else that needs it. Pick a species to remove.

Plot the damage against the abundance

Every species in the web, with how many individuals it has along the bottom and how many other species its removal costs up the side. If abundance told you which species mattered, these points would line up.

Leave one species out at a time

The correlation between abundance and impact is recomputed twelve times, each time with one species held out. A relationship that survives this is real. One that does not is a single data point wearing a coefficient.

What you would see, and what you would see later

Consequences take time to arrive. The first species to go are the ones that depended directly on what was removed; the ones that depended on those follow in the round after.

Adding counts too

The objective says addition or removal. Adding a species changes the structure just as surely — in one of these cases it protects the ecosystem, and in the other it dismantles it without anything being removed at all.

Check yourself

Answer these in your own words before you open the answers.

1. State the relationship between an ecosystem's diversity and its resilience, as the CED words it, and identify the two separate conditions in that sentence.
2. Two ecosystems have the same number of species and one is far more resilient. Explain how that is possible.
3. Explain why having two species do a job protects an ecosystem only under a particular condition, and state the condition.
4. Define a keystone species.
5. Explain why counting individuals cannot tell you which species in an ecosystem is the keystone.
6. Describe the mechanism by which the keystone in this web has an effect out of all proportion to its numbers.
7. Producers are also listed as maintaining the diversity of an ecosystem. Explain why they are not therefore keystone species.
8. Explain why the full effect of removing a species is not visible at the time it is removed.
9. Explain how adding a component to an ecosystem can change its structure, and give one example that helps and one that harms.
10. A survey finds that the abundance of a species correlates with how much its loss would cost. Explain what you would want to check before believing that.