Species diversity and the rivet popper hypothesis

This article reviews the rivet popper hypothesis, and looks into why species diversity matters.

Does it make a difference if an ecosystem has $10$‍ species of insects, or $20$‍? Will it matter if a species of frog dies out in the next ten years?

Ecologists believe the answer is yes, it does matter! One popular theory that these scientists have is that - the higher the variety of species in an ecosystem, the more stable that ecosystem is. Let's look at an example to understand what this means.

David Tilman ran a twenty year-long experiment to study the importance of species diversity. He studied a large grassland that was divided into plots with different mixtures of plant species. He found that plots with more diverse plants survived better long term, had higher productivity, and were therefore more stable systems. This was because within these plots, some plants would provide the nutrients that others needed, while others could create more optimal environments by fixing nitrogen from the surroundings and so on.

Thus, this theory says that increased biodiversity allows species to interact with each other in a useful manner, which makes the ecosystem more stable overall.

The stability theory offers one explanation, but it still does not tell us whether it matters if we have $100$‍ species of frogs or $50$‍. While we still don't have all the answers, another ecologist named Paul Ehrlich came up with another way to look at this question.

He compared an ecosystem to an airplane, and each species within it to the rivets that hold the plane together. The image below depicts rivets on the wing and body of a plane; you might have seen rivets on other objects as well (even your jeans pockets have flat, metal rivets on them!)

In this hypothesis, Ehrlich asks us to imagine a plane that is fully functional. However, each time it flies, every passenger on the plane removes (or pops) one rivet. The first few missing rivets do not affect the plane, and it still flies well. However, as more and more rivets get removed, parts of the plane become loose since they are no longer held together, and they start falling off. As a result, the plane becomes unstable, and can no longer fly.

How does this relate to an ecosystem? Similar to the plane, the ecosystem can continue to be stable as it loses a few species. But as more and more species are lost, their individual contributions to the ecosystem are also lost, and the community starts to fall apart as a result.