Species diversity is important because a community with more species is generally more stable and more productive. A stable community shows little year-to-year change in productivity, recovers from disturbances (natural or man-made), and resists invasion by alien species. Memory hook: "More species = more stable" (David Tilman's outdoor-plot experiments proved this for NEET).
Tilman's outdoor-plot idea: plots with more species (right) keep total biomass steady year to year (stable and more productive), while plots with few species (left) swing up and down. More species means more stability.
Your doubts, answered
What does 'stability' mean for a biological community?
NCERT gives three signs of a stable community. (1) It does not show too much change in productivity from year to year. (2) It is resistant or resilient to occasional disturbances, whether natural or man-made. (3) It is resistant to invasions by alien species. If a community shows all three, we call it stable.
Does a community with more species really become more stable?
Yes. For many decades ecologists believed that communities with more species tend to be more stable than communities with fewer species. David Tilman's long-term experiments gave the first solid evidence: plots with more species showed less year-to-year change in total biomass, which is a sign of stability.
What exactly did David Tilman find?
Tilman ran long-term ecosystem experiments using outdoor plots. Two clear results: (1) plots with more species showed less year-to-year variation in total biomass (more stable), and (2) increased diversity also led to higher productivity. So more species gave both more stability and more output.
What is the difference between 'resistant' and 'resilient'?
Resistant means the community does not change much when a disturbance hits it. Resilient means the community can return to its normal state quickly after it is disturbed. NCERT says a stable community must be either resistant or resilient to occasional disturbances.
Why should NEET students care about this small topic?
NEET repeatedly asks who did the outdoor-plot experiment (Tilman) and what stability means. It also links to the Rivet Popper Hypothesis (Ehrlich). One line to remember: rich biodiversity is essential for ecosystem health and imperative for the survival of the human race.
⚠️ The NEET trap ✗ Tilman proposed the Rivet Popper Hypothesis and the species-area relationship. ✓ David Tilman did the long-term outdoor-plot experiments on diversity and stability. The Rivet Popper Hypothesis is by Paul Ehrlich, and the species-area relationship is by Alexander von Humboldt. 🧠 One name, one contribution: Tilman = outdoor plots (stability + productivity). Do not mix him with Ehrlich or Humboldt in match-the-list questions.
Real NEET questions
2024
Match List I with List II. List I: A. Robert May, B. Alexander von Humboldt, C. Paul Ehrlich, D. David Tilman. List II: I. Species-Area relationship, II. Long term ecosystem experiment using outdoor plots, III. Global species diversity at about 7 million, IV. Rivet popper hypothesis.
A · A-III, B-I, C-IV, D-II ✓
B · A-I, B-III, C-II, D-IV
C · A-III, B-IV, C-II, D-I
D · A-II, B-III, C-I, D-IV
Solution: David Tilman did the long-term ecosystem experiments using outdoor plots (D-II), which showed more species gives more stability and higher productivity. Robert May = 7 million species estimate (A-III), Humboldt = species-area relationship (B-I), Ehrlich = rivet popper hypothesis (C-IV). Only option A matches all four.
2023
In 'rivet popper hypothesis', Paul Ehrlich compared the rivets in an airplane to
A · Ecosystem
B · Genera within a family
C · Species within a genus ✓
D · Genetic diversity
Solution: In Ehrlich's analogy the airplane is the ecosystem and each rivet is a species; popping a rivet is like losing a species. This links to the importance of species diversity: losing many species weakens the whole ecosystem, just as losing rivets weakens the plane. The keyed answer here is species (option C).
Solved Biodiversity and Conservation NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Who showed that more species means more stability?
David Tilman. His long-term outdoor-plot experiments showed that plots with more species had less year-to-year variation in total biomass and also higher productivity.
What are the three properties of a stable community?
Low year-to-year change in productivity, resistance or resilience to disturbances, and resistance to invasion by alien species.
Is high biodiversity only about ecosystem health?
No. NCERT says rich biodiversity is not only essential for ecosystem health but also imperative for the very survival of the human race on this planet.
How is this topic linked to the Rivet Popper Hypothesis?
Both show why every species matters. Tilman gives the experimental proof (more species = more stable), while Ehrlich's rivet analogy gives the picture (each species is a rivet holding the ecosystem together).
