The Diversity of Life

I’ve been reading Edward O. Wilson’s The Diversity of Life recently*, and was struck by some of his  turns of phrase, and thought I’d share a couple of them with you. I’ve also been reading two of his three books co-authored with Bert Holldöbbler (Journey to the Ants and Superorganism), where the writing is much less dynamic. In this more personal book, though, Wilson writes lyrically and movingly about the “big picture” biology that so many of us need to remind ourselves about.

On mental puzzles that verge on obsession

Wilson opens the book with a vivid scene of walking through the rain forest at night, at leisure after a very long day; he is pondering new possibilities to explain the incredible profusion of life in the tropics, a weighty topic that he is extremely well suited to ponder (from Old French, ponderer, from Latin ponderāre, from pondus, ponder, weight). After all, he co-authored the theory of island biogeography, and had only been studying the natural world in the tropics for over forty years at the time! His thoughts kept returning to this idea, which he describes as

…the kind of favorite puzzle that keeps forcing its way back because its very intractability makes it perversely pleasant, like an overly familiar melody intruding into the relaxed mind because it loves you and will not leave you (5).

The puzzle Wilson is writing about is how to explain the existence of biological diversity rather than simpler systems dominated by one or a few organisms. In other words, why don’t dominant species dominate globally? Why would a successful organism not be able to succeed in every habitat without bothering to go through the process of speciation?

Wilson’s answer is quite long, and I won’t give it away here; suffice to say that it generally aligns with the explanation given in a recent article in Wired, inspired by a tree researcher from Duke University, who has suggested that, at least for trees, we’ve been studying things at the wrong scale; lopping off the extremes to come up with averages:

The traditional explanation — every organism has its niche, competing not with other species but its own — sounds nice, but has holes. According to the tree study, that’s because ecologists haven’t looked for the right niches.

“We take this very complex, high-dimensional thing called the environment, and average out all the variation that organisms really require,” said Jim Clark, a Duke University biologist and author of the study, published Feb. 25 in Science. “Biodiversity is very much a niche response, but it’s just not evident at the species level.”

[Clark] has spent the last 18 years studying trees in the southeastern United States and has assembled 22,000 detailed individual accounts, spanning 11 forests and three regions. For each tree, Clark has recorded its precise, on-the-ground (and in-the-ground and above-the-ground) exposure to moisture and nutrients and light, its response, and its proximity to other plants.

Ecologists usually aggregate this information, turning it into average. By going tree-by-tree, Clark found that there are, in fact, enough niches to go around. They’re filled when competition in a species drives individuals to fill them. Biodiversity — or, from another perspective, configurations of organisms that don’t need to compete against each other — is the result of this fierce race for resources.

The niches could only be seen at a fine-grained level, not in the coarse analyses typically used by ecologists. “We take environmental variation and project it down to a very small set of indices. Light becomes average light per year. Moisture becomes average moisture per year. It’s not just light and water and nitrogen — it’s variations of each of those things, in different dimensions,” said Clark.

The theory of island biogeography deals with relatively simple ecosystems, where every new arrival, in  effect, has a pretty good chance of finding a niche. Clark’s research explains biodiversity in larger, more established and more complex ecosystems, an important step forward.

But, let’s get back to Wilson.

Two general categories of living things

Wilson provides a fascinating description of two categories of  living things that I had never thought about before; appropriate, I suppose, in a book celebrating the diversity of life:

  1. Aeolian plankton. Wilson introduces the term when discussing the spider on the island of Rakata (the remnants of the exploded island Krakatau): “Ballooning spiders are members of what ecologists, with the accidental felicity that sometimes pops out of Greek and Latin sources, have delightfully called the aeolian plankton. In ordinary parlance, plankton is the vast swarm of algae and small animals carried passively by water currents; aeolian refers to the wind” (19-20).
  2. Abyssal benthos. This “community of organisms living on or close to the ocean floor” was discovered, according to Wilson, “during the Challenger expedition of 1872-1876, whose mud samples disclosed a wide array of previously unknown organisms” (141).  The abyssal benthos contains “swarms of polychaete worms, peracarid crustaceans, mollusks, and other animals found nowhere else on earth.”

What other whole realms of life fall outside our experience? (For a possible answer, try looking into Margulis & Chapman’s illustrated catalog of life on Earth, about which I’ve written earlier.) Whatever they are, you can bet they’re not, on average, flimsy fragile things, at least on a population level.

On the resilience of nature

The greatest powers of the physical environment slam into the resilient forces of life, and nothing much happens (9).

This reminds me of the newscasters’ descent into hyperbole with the recent “Snowpocalypse” or “Snowicane,” or my all-time favorite, their perennial description of wildfires: “thousands of acres ravaged/destroyed/charred today as Fire X rages out of control.” Anyone who understands the nature of ecosystems, particularly the fire-adapted ecosystems of south Florida, would rather speak of acres being “rejuvenated” by fire. Slash pine, coastal scrub, even Everglades ecosystems evolved to handle fire, depend on fire to maintain themselves. Without fire to reset the system, for example, pinewoods or coastal scrub would succeed to hammocks. Fire is part of the natural order, not a natural “disaster.” Or, to emphasize it differently, fire is a disaster for humans and the built environment, not for ecosystems. [On disaster more generally, think back to Rousseau, in his famous letter to Voltaire, in which he blames human society, not God, for the destruction caused by the Lisbon earthquake in 1755; response to this anti-Enlightenment view was swift and punishing. But was Rousseau wrong?]

As Wilson puts it about the ecosystem of the tropical rain forest that prompted the quote, “For a very long time, 150 million years, the species within the rain forest evolved to absorb precisely this form and magnitude of violence” (9). Even hurricanes, devastating as they are to human habitations, are taken into account by tropical species. The Gumbo-limbo tree (Bursera simaruba, below) simply breaks, and its fallen limbs sprout where they land!

Paradise Trees (Simarouba glauca) have a similar strategy.

Black Ironwood trees (Krugiodendron ferreum) have taken the opposite approach, and evolved to be so dense, so hard, that a “normal” hurricane wind won’t do much to them. A strong enough wind, of course, will destroy the tree, so it’s a high-stakes gamble.

Both strategies produce resilient trees; other organisms have similar adaptations to survive the extremes of nature. The southern longleaf pine is adapted to frequent fire; it grows slowly underground for a while, building up its reserves while it’s still a very small tree, then it grows extremely quickly, to get the vulnerable crown high enough above the surrounding grass that it won’t catch fire in an ordinary blaze. It’s all part of the accommodation of the natural world to natural events.

Fires don’t destroy the natural world; they are a part of it. Hurricanes don’t, either; they are part of the natural world. Not even volcanic explosions on the order of Krakatau (Krakatoa in the popular memory) can destroy life permanently; it’s simply too resilient.

For those who don’t remember, the explosion occurred on August 27, 1883, and in May of 1884—according to a French ship’s naturalist who visited the remnant island, Rakata, at that time—the only sign of life was a microscopic spider, “busy spinning its web” (19). Within a year, though, grasses had colonized the island, and full-scale “tropical islandnesss” resumed in a surprisingly short period of time.

So tropical storms, hurricanes, not even volcanic explosions powerful enough to destroy islands, “are not enough…to break the crucible of life” (23). What, then, could be powerful enough? Well, you guessed it.

We are.

On extinction

There have been 5 great prehistoric extinction events:

  1. Ordovician (440 mya)
  2. Devonian (365 mya)
  3. Permian (245 mya)
  4. Triassic (210 mya)
  5. Cretaceous (66 mya)

The fossil record favors those species that lived in the ocean or shallow seas, because their bodies are far more likely to have been fossilized than those of species that lived in dry areas. Nevertheless, scientists have been able to estimate the percentage of species loss from the genera and families that disappeared in each of these events. According to Wilson, writing back in 1993, the current rate of extinction exceeds that of all previous episodes. We truly are in the sixth mass extinction of life on the planet, and we are causing it.

To recover from each of the previous five required tens of millions of years. According to Wilson, 25my for the Ordovician, 30my for the Devonian, Permian and Triassic (combined because they were so close together in geologic time) 100my, and only 20my for the Cretaceous event.

This does not bode well for the planet’s chances of recovering biodiversity within a human lifetime, or even within the lifetime of the human species.

Better hit the museums now, while you still can…

*My copy is the 1993 paperback edition; the book has been reissued and updated since then. Advances in DNA sequencing, computer technology, and other forms of bioassay will have affected nearly every chapter; I need to get my hands on a fresh copy to see what’s changed…