Some species can be taken out of an ecosystem with little effect and some cannot, and abundance is not what tells them apart.
Learning objective 8.6.A Describe the relationship between ecosystem diversity and its resilience to changes in the environment, and 8.6.B explain how the addition or removal of any component of an ecosystem will affect its overall short-term and long-term structure.
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.
Species in the system—
Species doing each job—
First job lost at a shock of—
Worst capacity lost in one step—
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.
System
Species
Per job
Do they differ?
Breaks at
Worst single drop
What buys resilience, and what only looks as though it does
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.
Removed—
Its share of all individuals—
Other species lost—
Rounds for it to finish—
What a keystone species is, and how this one works
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.
Impact against abundance—
… with one species left out—
Impact against structural role—
Abundance rank of the worst loss—
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.
Species left out
Its abundance
Correlation without it
Change
Why abundance fails, and what works instead
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.
Removed
Lost after one round
Lost once settled
Arrives late
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.
Addition
What it is
Species lost
Compared with
Short term against long term, and why additions matter
Check yourself
Answer each question before you open the answers.
(a) Describe the relationship between the diversity of an ecosystem and its resilience to changes in the environment. (b) Identify the two separate conditions that make an ecosystem less resilient.
Two ecosystems have the same number of species, but one is far more resilient. Explain how this is possible.
(a) Identify the condition under which having two species perform the same job protects an ecosystem. (b) Explain why having two species perform each job does not protect an ecosystem when this condition is not met.
Describe a keystone species.
Explain why the number of individuals of a species cannot be used to identify the keystone species in an ecosystem.
Describe how the keystone species in the twelve-species food web has an effect out of proportion to its numbers.
Producers also maintain the diversity of an ecosystem. Explain why producers are not keystone species.
Explain why the full effect of removing a species is not visible at the time it is removed.
(a) Describe how adding a component to an ecosystem can protect the ecosystem. (b) Describe how adding a component to an ecosystem can harm the ecosystem.
A survey reports that the abundance of a species is correlated with the number of species lost when it is removed. Explain what the leave-one-out analysis in this tool shows about the reliability of this correlation.