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Complexity
The Evolution of Earth's Biodiversity and the Future of Humanity
By William C. Burger Prometheus Books
Copyright © 2016 William C. Burger
All rights reserved.
ISBN: 978-1-63388-194-5
CHAPTER 1
BILLIONS OF BEETLES
There is a bit of old gossip that continues to be retold amongst biologists. According to this tale, the eminent British biologist, J. B. S. Haldane, was attending a cocktail party when he was confronted by a prelate with a rather unusual question. Repeated ever since the late 1930s, this brief encounter has become a staple when discussing the diversity of life on our planet. The religious gentleman's question was: "As a student of biology, Dr. Haldane, what can you tell us about the nature of God?" Unfazed by the gravity of the question, the eminent biologist is said to have responded simply and directly: "An inordinate fondness for beetles!"
Yes indeed! The Creator seems to have a fondness for beetles. Why else would there be so many of them? At latest count, beetles number around 380,000 described species. What this means is that a student of insects has gone to the trouble of providing the description for a new species, given it a Latin name, designated the specimens to which the name applies, and published all this in a scientific text or periodical. Nowadays, students of insects (entomologists) like to have an illustration published along with the description of a new species, and they insist that a designated specimen — the type specimen — be deposited in a public museum or a university collection. All this effort, all over the world, has given us around 380,000 scientific names for beetles! Compare that with about 5,500 species of described living mammals, or a bit more than 10,000 species of birds. Shifting to the plant world, there are over 310,000 described species of land plants on our resplendent planet. This is a conservative number, including mosses, liverworts, ferns and their allies, all the conifers, as well as more than a quarter million flowering plants. Add all these green plants together and they still don't reach a number of species that can match the beetles. Returning to animals, there seem to be more species of beetles than there are species of all the non-microscopic animals living in all the oceans. This aquatic crowd includes jellyfish, many kinds of worms, mollusks of all sorts, lots of crabs, and thousands of fish species. Three-hundred and eighty thousand described species are a whole lot of species! And, as if that weren't trouble enough, their numbers keep growing as scientists discover new and undescribed species living high in rainforest trees, and even little ones hiding in the soil not far from where many of us live.
WHY ARE THERE SO MANY KINDS OF BEETLES?
Beyond God's pleasure, what might be the reasons for beetles being so numerous? Like so many puzzles in biology, this question elicits many answers. Surely, the first of many factors is the most obvious: beetles are little. The largest weigh about as much as a mouse, the smallest less than a fly. In strong contrast: there are fewer than a dozen species of rhinos and elephants alive in the world today. These are the largest and heaviest land animals, each requiring a range of many square kilometers in which to live and prosper. Beetles, on the other hand, can live their short lives within only a few square meters. Here we have a pattern seen throughout the living world: numbers of both individuals and species decline as the sizes of animals increase. Clearly, being small makes it possible to pack a lot of beetles into the same small area.
Small size is just one factor helping us understand the diversity of beetles. Beetles are found in many diverse habitats, carrying on a grand variety of life activities. Huge numbers live in both temperate and tropical regions; a few can be found in the tundra and on high mountains, but none are resident in Antarctica. We know how flour beetles can invade our kitchens, carpet beetles damage wool rugs, and Japanese beetles chew on garden plants. In a warm June evening, june bugs (really beetles) fly into our screens, while fireflies (also beetles) flash signals in the dark of early evening. In addition to a grand number of species, some beetle species have populations made up of millions of individuals.
In the American West, ladybird beetles (also called ladybugs) move high into the mountainsides and congregate together to spend the winter. During the early 1900s, collectors would go into the Sierra Madre highlands of California to collect these slumbering beetles, selling them to farmers in their battle against aphids. (Ladybird beetles, both larvae and adults, feed on aphids.) This collecting activity, in the early part of the last century, saw the gathering of several tons — that's right tons — of beetles each winter. When you calculate that there may be 44,344,000 ladybird beetles in a ton of 'em, you've got a real lot of beetles. The take-home message is clear: by combining 380,000 species of beetles, with populations that may reach millions of individuals, it is no exaggeration to claim that our planet does indeed support billions of beetles. But what is it about beetles that have made them such a grand success?
Extinction is a constant erosive force in the history of life. A long fossil record tells us that perhaps 98 percent of all the species that ever lived are already long gone. Surviving changing climates, droughts, disease pandemics, and the occasional geological upheaval is a challenge a great majority of plants and animals have failed. Thus, when we see a lineage as successful as the beetles, we know they've got to be a well-adapted crowd. In fact, a simple glance at most any beetle will reveal their primary survival attribute: they are tough. Trekking over the leafy forest floor you can step on nearly any beetle, only to have it scurry away as you lift your boot. A hard exoskeleton amply protects a delicate interior! The key innovation here is that what was once a forward pair of wings became modified into hard wing covers (elytra) protecting the top of the entire abdomen. Their front and bottom sides are also armor encased. In fact, many species look like miniature tanks. The "Volkswagen beetle" got its moniker because this automobile really did resemble some beetles. Best of all, the tough wing covers of beetles can be raised up, allowing the actual wings to unfold and fly into the distance. Being rather solid means that, while most beetles can fly, they are neither fast nor agile. And that's why the larger ones fly only at night, avoiding the pursuit of predatory birds in the light of day. Being well protected, and with the ability to fly, gives them two significant advantages in the struggle for survival. But beetles have another trait that is even more significant.
A DIVIDED LIFE-HISTORY
Beetles belong to a large assemblage of insects with a four-stage life cycle; they undergo a complete metamorphosis (and are called holometabolous). Such insects have their lives divided into four separate and distinct stages. The first is the egg, which may be a short-lived stage, or it may be a means of surviving a harsh winter or a long dry season. At the appropriate time, the egg is broken open by its little occupant, a small larva. The larval stage is a significant part of the life cycle, being devoted entirely to eating and growing. Beetle larvae range from minute wormlike beings to the larger grubs we often find in soil and rotten wood, while others lead an active predatory life. Whether sedentary or active, this part of the life cycle is devoted to accumulating the energy needed to build the adult insect.
Becoming transformed from a fat grub to an elegant adult requires a separate stage: the pupa. While you may be more familiar with the pupae (plural of pupa) of butterflies and moths, all insects with a four-stage life cycle have this stage. It is within the pupal skin that much of the insect dissolves! That's correct: almost everything within their outer skin becomes liquefied. This fluid, reorganized by several special regions within the pupa, then develops into the fully adult beetle. A miraculous transformation, when you think about it, this is a significant factor in the success of many insect species. The adult can develop into a form completely distinct from its larva, able to fly off to find a mate and lead an entirely new and different life. In effect, beetles have two lives: a larval eating phase, and a travelling/reproductive mature phase. That's the good news. The bad news is that this life trajectory comes with a serious limitation: the adult can grow no more! The new adult is as big as it will ever become.
SCARABS AND BURYING BEETLES
Another significant aspect of beetle diversity is their many differing lifestyles. This allows different species of beetles to live in the same environment without getting in each other's way. For example, scarab or dung beetles spend much of their time looking for fresh dung. When they find it, a pair of beetles — mom and dad — will energetically create a spherical dung ball. The ball is rolled to a convenient site for burial, where the ball will house and nourish one of their larvae. Scarab beetles aren't likely to get in the way of burying beetles who busy themselves looking for the bodies of dead mice and birds. Burying beetles, like their scarab relatives, have complex behavioral traits to live their special lifestyles — all geared to rearing another generation of beetles.
As a boy and a collector of almost anything biological, I chanced upon a dead mouse in the woods bordering our country cabin. Here, I figured, was a chance to get a little mouse skeleton for my collections. So I tied the mouse carcass to a nearby stem with thin wire, making sure that no scavenging animal would cart away my prize. I then covered the body with leaves to further protect it. Busy ants, I thought, would clean the carcass and leave me with a fine little skeleton. Coming back only a day later to check on the deceased, I found that my mouse had disappeared. It was no longer under the covering of leaves. But the metal wire was in place, and it led into the ground. My mouse had been buried! Sure enough, after watching carefully, I noticed a pair of burying beetles digging to lower the dead mouse even deeper into the soil. As in the case of the scarab beetles, the buried mouse would provide nourishment for a new generation of beetles.
The burying beetle pair will continue feeding on their carcass and watching their young develop. They even use antibiotics to reduce bacterial activity on the carcasses they feed upon. In contrast, scarabs lay only one egg within the dung ball, bury it, and then go on to fashion more balls for more offspring. That's why the African savanna isn't covered with dung, despite herds of zebras and many species of antelope. In fact, Africa is home to over two thousand species of scarab beetles. By burying huge amounts of dung, these beetles perform important ecological services. Dung (cow patties are an example) can depress the growth of plants beneath them, serve as nourishment for hordes of flies, and help spread parasites and disease. In addition to removing dung from the soil surface, busy scarabs effectively fertilize the soil and help aerate compacted soil.
The importance of dung beetles became clearly apparent in Australia. Cattle were brought to Australia to develop a beef industry — great for producing hamburgers, but not so good for the landscape. It turned out there were no larger dung beetles in Australia to deal with the cow pies produced by our large domestic cattle. Soon pasture lands were bedecked with bone-dry, rock-hard cow pies. This new groundcover just sat there, like so many concrete platters, reducing the absorption of sparse rain into the soil, suppressing the growth of grass and forbs, and dramatically reducing the productivity of the land. Clearly, bigger species of scarab beetles were the missing element in the Australian outback.
Beetle variety is awesome, and not just in numbers. Diving beetles are voracious little predators in lakes and ponds, both in their larval and adult phases. Since they devour aphids, ladybird beetles are welcome in our gardens. Fireflies, or lightning bugs, fascinate us with their ability to produce flashes of yellow-green light. But there are multitudes of other kinds of insects as well. Dangerous wasps with potent poison and the stingers to deliver it; elegant dragonflies along the shores of lakes and streams; smelly stink bugs; bumblebees working one flower after another; the pesky flies, and lots more. Let's move beyond beetles and consider all the insects.
THE NUMBERS OF INSECTS
As we search for additional lineages with huge numbers of species, we find that these are also insects. The flies, order Diptera, number about 150,000 species, while butterflies and moths (Lepidoptera) have over 120,000 species. Put these all together, add in the beetles and lesser groups, and we probably have around one million species of described insects. And that's just at our current state of knowledge. Ants, bees, and wasps (the order Hymenoptera) currently number around 115,000 named species. However, one expert thinks the real number inhabiting our planet is closer to a million!
These are huge species numbers in the world of living things. In addition, some species include large numbers of individuals. Surely one of the most spectacular expressions of insect numbers is that of locust swarms. Not only impressive in their vast numbers, these far-flying grasshoppers can devastate agricultural crops, threatening people with starvation. The most notorious of these is the desert locust (Schistocera gregaria), ranging from northern Africa to India. These are the creatures mentioned with foreboding in the Bible. My students and I witnessed a swarm of these migrating pests in eastern Ethiopia during a February field trip. Having stopped along a high mountain road, we looked into a distant valley and noticed an odd "cloud." It was morning, and I assumed it was persisting early fog. But it did look strange; the moisture of early morning should have been long gone. The next day that very same cloud — a horde of migratory locusts — flew over our college campus. Probably about two kilometers long and half a kilometer wide (1.2 by 0.3 miles), this cloud took almost an hour to stream by. Standing under the passing horde, I couldn't begin to comprehend the numbers passing overhead. Fortunately, they weren't stopping to eat; they were in full travel mode. A few days later we saw this same swarm flying over the region's highest mountain, Gara Mulatta. To get across that mountain, these locusts were flying at an altitude of 3,400 meters (11,000 ft.)! Estimating the numbers of a particular swarm in East Africa, researchers came up with a total figure of about fifty billion individuals. And with each locust weighing around 2.7 grams (a tenth of an ounce), that swarm added up to 115,100 tons of locusts. That's a lot of grasshoppers! Nowadays, with global weather satellites showing rainfall patterns, locust outbreaks can be predicted, and measures can be taken to arrest their reproduction.
Another insect outbreak of extraordinary numbers occurs exclusively in the eastern United States. Only, in this case, the explosion of numbers is both predictable and long-delayed. Once every seventeen years (thirteen years in southern broods), our eastern woodlands are assaulted by a cacophony of deafening decibels. These are the calls of periodical cicadas that lived their long lives underground, sucking sap from the roots of trees and shrubs. Lacking the four-stage life cycle of holometabolous insects, cicadas — like grasshoppers — grow through a series of nymphal stages. Emerging together in the late spring of their seventeenth year, the nymphs crawl out of the ground and up stems and trunks. Once securely positioned, their skin splits open along the back, and the soft-bodied adult emerges. Hanging quietly for a few hours, wings expand and their exoskeleton hardens; they are now ready to fly. Soon the males begin their ear-splitting choruses; hoping to attract the ladies. Indeed, hiking through a forest harboring thousands of these tree-top choristers can be very uncomfortable! (Fortunately, they stop calling at nightfall.) Three different species of cicadas make up most of these singing swarms, and they are divided into a number of distinct geographical broods, each emerging on its own seventeen-year or thirteen0year schedule. While cicada species are found throughout the tropical and temperate world, only these particular species in the eastern United States display a uniquely synchronous emergence.
(Continues...)
Excerpted from Complexity by William C. Burger. Copyright © 2016 William C. Burger. Excerpted by permission of Prometheus Books.
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