The Loom of Life: Unravelling Ecosystems

  • Menno Schilthuizen
Springer: 2008. 220 pp. £37.50 9783540680512 | ISBN: 978-3-5406-8051-2

In the summer of 1966, Harvard biologist Edward O. Wilson and his student Daniel Simberloff undertook a classic ecology experiment in Florida. They identified a number of minuscule mangrove islands, took a census of their mostly insect fauna, and then paid an exterminator to kill all the animals on the islands with methyl bromide. Observing the islets as they became repopulated, they found that, after eight months, nearly all had regained the same number of species as they had hosted before the extermination. But most of these inhabitants were not of the same species as before.

The scientists' interventions confirmed the theory of island biogeography that Wilson and Robert MacArthur had published some years earlier. It states that the number of species on an island depends on the rates of immigration and extinction, which are influenced by the distance of an island from the mainland and the island's size. Notably, the number does not depend on what the species are, nor on the roles they have in their ecosystems, known as niches.

Studies of Florida's mangrove islands have helped researchers to understand how biodiversity develops. Credit: TBKMEDIA.DE/ALAMY

Evolutionary biologist and science writer Menno Schilthuizen retells the story of the Wilson–Simberloff experiment at some length in The Loom of Life. His book is a readable, anecdotal introduction to the ecology of diversity, addressing basic questions about why there are so many different species and why some species are rare and others common.

Yet the book is hard to get into. It lacks a straightforward introduction and, with uninformative chapter titles such as 'The more the merrier', a reader who idly picks it up would need to read 50 pages to understand where all the stories are going. It is not until this point in the book — after touring through the scientific usefulness of isolated ecosystems, the history of the idea of food chains, the staggering amount of global biodiversity, and the idea that each species inhabits a slightly different niche — that the first hints of dispute enter the plot.

Schilthuizen sets out the idea that two species can coexist in the same ecosystem only if they have different niches. Evolutionary theory predicts that two species with the same living requirements and appetites would go head-to-head, and if one is even infinitesimally more efficient at exploiting their shared niche, it will, over the course of generations, outcompete and exterminate the other. Then he drops the bombshell. What about phytoplankton? Any one patch of water contains hundreds of different species, all swimming in the same water, all competing for the same energy source: sunlight. Even with the odd difference in mineral uptake, how could so many niches exist in the uniform surface of the sea? Ecologists could go mad trying to find the minute niche differences at work; maybe there is an easier way.

After launching into the history of island biogeography, Schilthuizen introduces Stephen Hubbell's unified neutral theory of biodiversity, which Schilthuizen describes as aiming “to mimic and predict real, broad brush biodiversity patterns while ignoring the niches that undeniably existed”. The theory puts forth a simple predictor of the abundance of various species in an ecosystem using only the rate of migration and a fundamental biodiversity number that takes into account the rate of speciation. In general, the theory shows how abundances of each species drift up and down. Sometimes new species enter the system, occasionally one will go extinct, and now and then one will speciate into two.

It is odd that the book does not give a precise definition of 'neutral' in the neutral theory, namely the assumption that any differences between the behaviour of the various species being considered — all the trees in a forest, say, or all the corals on a reef — have no overall effect in the model.

Clearly a fan of the theory, Schilthuizen makes time for its opponents, even if he uncharitably calls their negative reactions “kneejerks”. He sets out the evidence from both sides but avoids hashing it out, jumping on instead to ecosystem stability. All these questions are related, but the book counters expectations by becoming a survey of the great questions about the ecology of diversity instead of an argument in favour of the neutral theory.

The Loom of Life is useful. Much of the public — and even some of the professional environmental movement — knows little about the rules ecologists have posited for creating and maintaining biodiversity. They might read this slender book for a bearing on how to tackle environmental problems.