stuart_pimm's picture
Doris Duke Chair of Conservation Ecology, Duke University; Author, The World According to Pimm: a Scientist Audits the Earth

Mother Nature's Laws

"Deep, elegant, or beautiful explanation" requires something equally singular to explain. That means laws, by which I simply mean generalities or patterns. Thus, the "law of gravity" is general enough to describe pendulums and planets. Pendulums do not quicken my heart, but living things fascinate me, even if I have yet to express affection for nematodes.Writing from Sarawak, Alfred Wallace nailed the most important law of living things in a crisp 18 words:

Every species has come into existence coincident both in space and time with a pre-existing closely allied species.

With judicious editing, Wallace could have fit his 1855 "laws of evolution" paperinto today's word limits of PNAS or Nature. We don't find trilobites scattered in the Devonian, Jurassic, and Eocene with nothing in between. Nor are polar bears only in Greenland, Patagonia, and Tibet. The paper screams for an explanation of these bundled generalities of palaeontology and biogeography.

The scientific community were asleep at the wheel and barely noticed. A few years later, that forced Wallace to send his deep, elegant and very beautiful explanation to Darwin for moral support. Darwin had the same explanation of course. The resulting and familiar history is not where I want to go with this.

What other laws has Mother Nature given us for biological diversity?

The average geographical range size of a group of species is very much larger than the median range.

The average of the geographical ranges of 1684 species of mammals in the New World is 1.8 million km2, but 50% of those species have ranges smaller than 250,000 km2—a seven to one ratio. For the region's three main bird groups, the ratio is five and eight times and for amphibians, forty times. There are many species with small ranges and few with large ranges.

There are more species in the tropics than temperate regions.

The first explorers to reach the tropics uncovered this law. Rembrandt was painting birds of paradise and marine cone shells in the early 1600s. Wallace went first to the Amazon because collecting novel species was how he earned a living.

Species with small ranges concentrate in places that typically are not where the largest numbers of species live.

This just doesn't make sense. Surely, with more species, one should have more species with large ranges, small ranges, and everything in between. It isn't so. Small-ranged species concentrate in some very special places. About half of all species live in a couple dozen places that together constitute about 10% of the ice-free parts of the planet.

Species with small ranges are rare within those ranges, while those with large ranges are common.

Pardon the language, but Mother Nature is a bitch. You'd think she'd give species with small ranges a break—and make them locally common. Not so. Widespread species tend to be common everywhere, while local ones are rare even where you find them.

What inspired Darwin and Wallace were encounters with places rich in birds and mammals found nowhere else—the Galapagos and the islands in southeast Asia. There are no such places in Europe. Darwin spent most of HMS Beagle's voyage too far south in South America, while Wallace's first trip was to the Amazon. The Amazon is very rich in species, but it is a striking example of the law that such places rarely have many species with small ranges. (I suspect this cost Wallace dearly because his sponsors wanted novelty. He found that on his next trip, to the East.)

Scientists found widespread species first. Darwin and Wallace were among the first explorers to encounter the majority of species—those with small, geographical ranges concentrated in a few places. Even for well-known groups of species, those with the smallest ranges have only been discovered in the last decades.

What deep, elegant, and beautiful explanation underpins these ineluctably connected laws? There isn't one.

Given the observed distribution of range sizes, the tropics have to have more species simply because there are in the middle. Sufficiently large ranges must span the middle—that's the only way to fit them in. But, they need not be at the ends—which geographically tend to be arctic or temperate places. Yet, middles have more species than ends even when the middles are not tropical. There are more species in the middle of Madagascar's wet forests, though it's the northern end (with fewer species) that is closer to the equator, for example.

In addition, warm, wet middles—tropical moist forests—have more species than hotter and drier middles. The correlation of species with warmth and wetness is compelling, but a compelling mechanism is illusive.

Small-ranged species can be anywhere—near middles or near ends. They are not. They tend to be on islands (Galapagos, southeast Asia), and on "habitat islands"—mountaintops—(the Andes). This fits our ideas on how species form. Alas, they are not on temperate islands and mountains, so Darwin and Wallace had to leave home to be inspired. Except for salamanders: the Appalachians of the Eastern USA seem to have different species under every rock, forming an a theoretically obstinate temperate centre of endemism that isn't matched by birds, mammals, plants, or indeed other amphibians.

To make matters worse, all this assumes that we know why some species have large ranges and more have small. We do not.

In short, we have correlations, special cases, and some special pleading, but elegance is missing. A deep explanation need not be there, of course. Our ignorance hurts.

Concentrations of local, rare species are where human actions drive species to extinction one hundred to one thousand times faster than the natural rate. Yes, we can map birds and mammals and so know where we need to act to save them. But not butterflies, which people love, let alone nematodes. Without explanations, we cannot tell whether the places we protect birds will also protect butterflies. Unless we understand Mother Nature's Laws and can extend them to the great majority of species still unknown to science, we may never know what we destroyed.