Insects in the Web of Life
Insects constitute by far the largest amount of animal food available to flesh eaters both on dry land and in freshwater. The one quarter of the earth that is not covered by the oceans and seas is inhabited by an immense and not yet completely censused population of insects. The 900,000 currently known insect species (at least three million are yet to be discovered and named, according to reasonable estimates [Stephen Marshall]) constitute about 75 percent of the currently known 1,200,000 animal species on land, in freshwater, and in the oceans. The Canadian entomologist Brian Hocking made the daring but educated guess that the world population of insects is about one quintillion (1 followed by eighteen zeroes) individuals. Even if he overestimated by trillions, that would still be a stupendous population.
Although insects are small, they are generally so numerous in most terrestrial and freshwater ecosystems that, on a per-acre basis, they not only outnumber but also outweigh all the other animals-including deer and moose-combined. On the face of it, this is hard to believe. But keep in mind that a single acre of land may be home to many millions of insects of hundreds or even thousands of species. By contrast, the home territory of one small bird is likely to encompass as much as an acre, and that of a large mammal, such as a thousand-pound moose, several hundred or even thousand acres. Thus the biomass of an animal that weighs hundreds of pounds may be much less that one pound per acre. Also keep in mind that most people notice only a few of the many insects around them, perhaps a ladybird beetle or a large and beautiful butterfly but more often the insects that sting, bite, or otherwise annoy them. Yet the other insects, by far the vast majority in almost any ecosystem, go unnoticed. Not only are they small, but many are difficult to see because they are camouflaged, and many are out of sight because they live in the roots, stems, or other parts of plants; as parasites within the bodies of insects and many other animals; or in the soil or other cracks and crevices of the environment.
Insects are, either directly or indirectly, the most plentiful source of flesh for animals that don't eat plants. But they are important to these predators not just because of their abundance. Plant-feeding insects, estimated to be about 450,000 species, and the insects and other animals that eat them are by far the most important link between green plants and animals that don't eat plants, a conduit through which predators receive the energy of the sun, which green plants-and only green plants-can capture and make available to animals via photosynthesis, in the form of sugars. Insect-eating insects play another significant, although less important, role. By eating tiny organisms and incorporating their prey's nutrients in their own bodies, large insects become "nutrient packages" for large insectivores that cannot profitably pursue and eat tiny organisms themselves.
Data gathered by Eugene Odum and other ecologists show just how important a part of the food chain insects are in specific ecosystems. For example, in a field of herbaceous plants in North Carolina, the biomass of the plant-feeding insects alone-not including any predaceous, parasitic, or scavenging species-was nine times greater than that of sparrows and mice, the larger and more conspicuous and by far the most numerous of the vertebrates in that field. On an East African plain, just two species of ants-only those two, among hundreds of other kinds of insects-were about equal in weight, per acre, to the combined weight of the large grazing animals, such as wildebeests, zebras, and antelopes. In these two habitats and in almost all others, insects are by far the most abundant of the prey animals in both numbers and biomass. As is to be expected, and as we will see in the next chapter, hundreds of thousands of different kinds of animals exploit this nutritious, protein-rich food: spiders, scorpions, insects, frogs, toads, lizards, birds, mammals.
The insects almost certainly have more different lifestyles, ways of surviving and "making a living," than do any other group of animals. One species or another occupies every-or nearly every-ecological niche. An ecological niche is not just a place; it includes all of the resources, food, nesting sites, hiding places, and so on, required by an organism. Except for aquatic species, insects that undergo gradual metamorphosis occupy essentially the same niche throughout their lives. Those with complete metamorphosis often occupy two very different niches in their larval and adult stages.
Dragonflies, grasshoppers, cockroaches, mantises, true bugs (order Hemiptera), and lice are some insects that gradually metamorphose. A newly hatched grasshopper-a nymph-looks very much like its parents but lacks wings. As it grows, it molts several times, and its developing wings, which are external, can be seen gradually increasing in size until the hopper stops growing and molts for the last time to become an adult with flightworthy wings. Insects with gradual metamorphosis have only three life stages: the egg; the nymph, the growing stage; and the adult, the egg-laying reproductive stage. Nymphs look and behave much like adults, except in most aquatic species. For instance, adult dragonflies are aerial acrobats that pursue flying insects. But the nymphs are aquatic, don't look at all like the adults, and are fierce predators that eat aquatic insects and even small fish.
Beetles, fleas, flies, wasps, bees, moths, and butterflies are among the many insects that undergo a complete metamorphosis. The baby butterfly just hatched from the egg is a wormlike larva that does not at all resemble its parents. A biologist from another planet might think that the larva and the butterfly are two quite different kinds of animals, as dissimilar as birds and snakes. Complete metamorphosis proceeds in four life stages: the egg; the wingless larva, called a caterpillar in the case of butterflies and moths; the pupa, the transition stage in which the larva metamorphoses into the adult; and the winged reproductive stage. Larvae not only look different than their parents but also usually behave very differently. Caterpillars, for example, have chewing mouthparts, and most feed on plants, usually the leaves. The wingless pupae, with only a few exceptions, can squirm but cannot walk or crawl and are usually tucked away in a safe place, perhaps in the soil, under bark, or in a silken cocoon. The adult, a butterfly or moth for example, has large wings that developed internally in the pupa, as do its long, soda straw-like siphoning mouthparts, used for sucking nectar from blossoms.
The larvae and the adults are specialists, anatomically and behaviorally equipped to do particular tasks. The larvae eat, grow, and do their best to foil predators. The adults suck the sugary nectar that supplies energy to fuel their frequent flights as they seek mates and as the females distribute their eggs. Most butterflies and moths, as well as many other insects, glue their eggs only to one of the few plant species their fussy-host plant-specific-offspring will be willing to eat. The larvae are "eating machines," and the adults are flying gonads or, as Carroll Williams said, "flying machines devoted to sex." Because of the benefits of such specialization, complete metamorphosis has been an evolutionarily more successful strategy for survival than gradual metamorphosis, at least as judged by the number of known insect species on earth today-only about 135,000 (15 percent) with gradual metamorphosis, while 765,000 (85 percent) have complete metamorphosis.
Insects with either type of metamorphosis can occupy similar ecological niches. Grasshoppers, Japanese beetles, June beetles, and many other insects occupy fairly commonplace niches. The adults feed on foliage and lay their eggs in the soil. However, after hatching from the egg, nymphal grasshoppers immediately make their way to the surface and feed on the leaves of grasses or herbaceous plants, while the white C-shaped larvae, known as grubs, of the two beetle types, which both have complete metamorphosis, remain in the soil and feed on plant roots. They pupate in the soil, and the adults dig their way up to the surface after they have shed the pupal skin.
Other insects have more complex and elaborate lifestyles. For example, a male burying beetle whose search for a dead animal has been successful sits on the body of the small dead animal he has discovered and releases a scent, a sex-attractant pheromone. A female soon joins him, and, working together, they burrow back and forth beneath the dead body until it sinks deep enough into the ground so that they can cover it with soil. Then they create an open space underground around the buried carcass and cover the dead animal with a secretion that kills bacteria, thereby delaying decomposition. The female then lays as many as thirty eggs in the soil near the carrion. After the larvae hatch, they crawl to a nest prepared by their parents, who feed them by regurgitating predigested carrion. Eventually the larvae feed on their own. The father then leaves, but the mother guards her young until they are ready to pupate. In a few weeks her offspring emerge from the soil as adults and begin another cycle.
Elsewhere, a tiny female gall wasp inserts an egg into an oak leaf. As May Berenbaum wrote in Bugs in the System, gall makers commandeer "the plant's hormonal system in such a way that the plant is induced to produce bizarre and unusual growths [galls], which provide the insect with a place to live and with nice nutritious tissue on which to feed." The larva that hatches from her egg causes the oak leaf to form an abnormal growth, in this case an "oak apple," a light tan spherical gall that may be as large as a table tennis ball. Another gall maker, a moth, lays an egg in the stem of a growing goldenrod in spring, causing the plant to produce an egg-shaped thickening almost an inch in diameter. In summer the caterpillar grows to full size. The following spring it gnaws an exit hole through which, after it pupates, it will emerge as a moth. But it may not survive that long. In winter a hungry downy woodpecker may peck a hole in the gall, pull out the caterpillar, and make a meal of it.
These few examples give no more than an inkling of the many different ways in which insects conduct their lives. Insectivores must, of course, have the anatomical and behavioral adaptations required to catch their prey. A bird, for example, can snatch an adult grasshopper, beetle, gall wasp, or gallfly from the air with its beak, but only a tunneling animal or a bird that probes in the soil is likely to find subterranean eggs, grubs, or pupae. Only a woodpecker is likely to get at a larva in a gall or burrowing under the bark of a tree.
The evolution of the millions of different kinds of insects that live on earth now and the many extinct species that we know only as ancient fossils began about four hundred million years ago, when the first insects-to-be were gradually leaving the water, where life began, to move onto the land. They probably reached the shore via moist organic debris at the edges of freshwater ponds and once on land probably continued to feed on soft rotting organic matter, which they ate with their primitive, unspecialized mouthparts, the organs of ingestion. From these simple creatures evolved the diverse assortment of modern insects, as different from one another as grasshoppers with mouthparts specialized for chewing on plants, butterflies with tubelike mouthparts for sucking nectar from flowers, and mosquitoes with piercing-sucking mouthparts for consuming the blood of birds, mammals, or reptiles.
Plants and animals, of course, continue to evolve. But how does evolution work? Charles Darwin had the brilliant insight that natural selection is the driving force of evolution, producing new species just as breeders produce new dog breeds through artificial selection, by selecting animals with desirable traits to be the parents of the next generation. (Keep in mind that all breeds, from the tiny Chihuahuas to the huge Saint Bernards, are descended from the wolf.) Natural selection, while tending to cull poorly adapted individuals, favors those better adapted to avoid hazards and to take advantage of opportunities. For example, an individual with even slightly better camouflage than others will be somewhat less likely to be noticed by a predator and, consequently, somewhat more likely to survive and become a parent. Heritable adaptive traits are passed on to future generations and given enough time will spread to all members of a population. As the centuries or millennia pass, more favorable mutations accumulate in a population until those who have them are so different from the other members of their species that they become a separate, distinct, reproductively isolated species, one whose members do not breed with members of other species.
These new adaptive traits constantly arise as genetic mutations caused by means such as radioactivity, ultraviolet light, cosmic rays, or intrinsic factors in DNA, the genetic material itself. Mutations are random, some favorable and many unfavorable. However, evolution is by no means a random process; it is directed by natural selection, which tends to eliminate unfavorable mutations and generally perpetuates favorable mutations. Think of a prospector panning for gold. He scoops up a mixed assortment of sand, pebbles, and-with luck-a few bits of gold. But only the heavier flakes and nuggets of the valuable gold survive the panning. They are not, unlike the lighter, valueless mixture of sand and gravel, washed out of the pan as he swirls the water. In a similar way, natural selection preserves favorable genes and eliminates deleterious genes.
Some of the insects' most important adaptations are responses to insectivores, a numerous and pervasive threat to their survival. The ultimate goal of any organism is, of course, to reproduce itself, to pass its genes on to future generations, and to accomplish this it must survive long enough to attain sexual maturity. As the great English naturalist Henry Bates wrote in 1862 in "Contributions to an insect fauna of the Amazon Valley, Lepidoptera: Heliconidae":
Every species in nature may be looked upon as maintaining its existence by virtue of some endowment enabling it to withstand the host of adverse circumstances by which it is surrounded. The means are of endless diversity. Some are provided with special organs of offence, others have passive means of holding their own in the battle of life. Great fecundity is generally of much avail.... A great number have means of concealment from their enemies, of one sort or another. Many are enabled to escape extermination or obtain subsistence, by disguises of various kinds: amongst these must be reckoned the adaptive resemblance of an otherwise defenceless species to one whose flourishing race shows that it enjoys peculiar advantages.
The last sentence refers to the fascinating subject of the last chapter of this book, harmless insects, and a few other harmless animals, that foil predators by bluffing, mimicking the appearance and even the behavior of other insects or other animals that sting, are unpalatable, or are avoided by predators for other reasons.
Besides reproducing themselves, insects perform indispensable ecological services. As discussed above, they are the most important link between plants and animals that don't eat plants, and they have other important roles in virtually all terrestrial and freshwater ecosystems. One of their major functions, which we have all heard about, is to pollinate plants. Most of the green plants are flowering plants, called angiosperms (Greek for "a seed encased by an ovary"), and except for hummingbirds, bats, and just a few other animals, it is the insects that transport the sperm-containing pollen from the male parts of one flower to the female parts of another. Most flowers have coevolved with bees, butterflies, or other pollinators. Their colors and scents attract insects and reward them with nectar and pollen, which many insects eat and which are virtually the only foods consumed by the thousands of species of bees (at least 3,500 in North America alone). No one knows how many of the flowering plant species are pollinated by insects, but Stephen Buchmann and Gary Nabhan have reported that of the 94 major crop plants on earth, the wind pollinates 18 percent, insects 80 percent, and birds 2 percent.
Insects have many other functions in the web of life, only a few of which I will mention here. Plant-feeding insects help to keep plant populations from increasing to a size that would disrupt a stable ecosystem. For example, when the European Klamath weed, also known as St. John's wort or locoweed, reached California, its population exploded because it had no natural enemies there; it choked out grasses in pastures to the extent that they were useless for grazing cattle. After a European leaf beetle that eats Klamath weed was introduced into California, the weed became scarce, and grew mainly in shady places, where it was less likely to be attacked by the leaf beetle. An agricultural entomologist remarked that insects are their own worst enemies. And indeed they are. Thousands of insects, probably more than 300,000 of the known species, eat other insects. As Peter Price noted, insects, mostly ants, are the "world's premier soil turners," more so than earthworms, which are generally given credit for this. Without the scarabs and other dung-feeding insects, we might, to use a bit of hyperbole, be knee-deep in excrement. Furthermore, ants and other insects disperse the seeds of some plants.
In the next chapter we will meet a few of the many animals-spiders, scorpions, toads, birds, bats, mice, and even bears-that eat insects. The threat to the insects from these insectivores is enormous, but as we will see in following chapters, insects have evolved many, often amazing ways to avoid being eaten. But keep in mind that neither insects nor other organisms are completely immune to predation. If they were, their populations would probably explode, causing ecological havoc.