Does the Number of Species Matter?

Here is a question ecologists have long wrestled with: does the number of species in a community really matter to the functioning of the ecosystem? A definitive answer has been hard to reach, but for many decades ecologists have believed that communities with more species tend to be more stable than those with fewer species.

The idea is intuitive — a richer community has more ways of keeping itself running when conditions change. But to test it, we first need to be clear about what stability actually means for a biological community.

What Makes a Community Stable

A stable community is expected to show three features. First, it should not show too much variation in productivity from year to year — its output stays fairly steady over time. Second, it must be either resistant or resilient to occasional disturbances, whether natural or man-made, so that it either withstands a shock or bounces back after one. Third, it must be resistant to invasions by alien species, so that outsiders cannot easily establish and take over.

Exactly how these attributes are linked to species richness is not fully understood, but experiments have begun to supply some tentative answers.

Tilman's Experiments

Some of the clearest evidence comes from the long-term ecosystem experiments of David Tilman, who used carefully monitored outdoor plots over many years.

Tilman found that plots with more species showed less year-to-year variation in total biomass — that is, greater diversity went hand in hand with greater stability. He also showed that in his experiments, increased diversity contributed to higher productivity. So richer plots were both steadier and more productive, lending real support to the old belief that diversity underpins stability.

The Rivet-Popper Hypothesis

Even if we do not fully understand how species richness supports an ecosystem, we can grasp why it matters through a famous analogy from the Stanford ecologist Paul Ehrlich — the rivet-popper hypothesis.

Imagine an ecosystem as an airplane, in which all the parts are joined together by thousands of rivets, each rivet standing for a species. If every passenger on board starts popping a rivet to take home — each rivet lost representing a species going extinct — flight safety may not be affected at first. But as more and more rivets are removed, the plane grows dangerously weak over time.

And which rivet is removed also matters. Losing rivets on the wings — the key species that drive major ecosystem functions — is a far more serious threat than losing a few rivets on the seats or windows inside the plane.

Quick Recap

  • Ecologists have long held that communities with more species tend to be more stable.
  • A stable community: shows little year-to-year variation in productivity, is resistant or resilient to disturbances, and is resistant to invasions by alien species.
  • David Tilman's outdoor-plot experiments showed that plots with more species had less year-to-year variation in biomass and that increased diversity raised productivity.
  • Paul Ehrlich's rivet-popper hypothesis: an ecosystem is like an airplane held together by rivets (species); losing a few may not matter at first, but losing more weakens it dangerously.
  • The position of the rivet matters — losing key species on the 'wings' (those driving major ecosystem functions) is far more damaging than losing minor ones.

Solved Examples — Section 5

Q1. What have ecologists long believed about the relationship between species richness and stability?

Answer: That communities with more species generally tend to be more stable than those with fewer species.

Q2. List the three features of a stable community.

Answer: It shows little year-to-year variation in productivity, it is resistant or resilient to disturbances, and it is resistant to invasions by alien species.

Q3. What did David Tilman find about biomass variation in his plots?

Answer: Plots with more species showed less year-to-year variation in total biomass.

Q4. What did Tilman's experiments show about diversity and productivity?

Answer: Increased diversity contributed to higher productivity.

Q5. Explain the rivet-popper hypothesis and who proposed it.

Answer: Proposed by Paul Ehrlich, it compares an ecosystem to an airplane held together by rivets (species); popping rivets one by one (species extinctions) may not matter at first, but removing more and more makes the plane dangerously weak.

Q6. In the rivet-popper analogy, why is the loss of rivets on the wings more serious?

Answer: Wing rivets represent key species that drive major ecosystem functions, so losing them threatens the whole system far more than losing minor rivets on seats or windows.