Great truths are sometimes so enveloping and exist in such plain view as to be invisible. One of them is the dominance on the land of flowering plants and insects. In addition to their overwhelming biomass, nearly a quarter million species of plants and three-quarters of a million insects have been described by biologists to date, together composing a full two-thirds of all kinds of organisms known to exist on the planet. Other terrestrial and freshwater groups, from protozoans to vertebrates, pale by comparison.
The joint hegemony of these two great groups is not an accident. It is the result of coevolution, the process in natural selection during which species adapt to one another and thereby build rich ecosystems. The interaction between plants and insects has been going on a long time. It began over 100 million years ago with the origin of the flowering plants and accelerated with their ascendancy in the world’s vegetation during the early Cenozoic era some 40 million years later. Much of their coevolution was mutualistic: Species—more often whole complexes of species——came to seal obligatory partnerships with their insect counterparts. Such relationships, construed broadly, are among the central topics of ecology. Ants, for example, among the most abundant of insects, spread the seeds of plants, protect them from herbivores, and enrich the soil in which they grow. Insect detritovores such as termites and woodboring beetles convert dead vegetation into nutrients that can be reabsorbed by the roots of living plants. And, as Stephen Buchmann and Gary Nabhan so clearly and evocatively remind us, a majority of flowering plants must have insects to reproduce.
There is a welded chain of causal events that leads directly to our species: if plants, including many food and forage crops, as well as natural floras, must have insects to exist, then human beings must have insects to exist. And not just one or two kinds of insects, such as the friendly and lovable honeybees, but lots of insect species, vast numbers of them. The reason is that millions of years of coevolution have finely tuned the relations between particular plants and their special pollinators. The shapes and colors of the flowers, their scent, their location on the stalks, the season and daily schedule of their pollen and nectar offerings, as well as other qualities we admire but seldom understand, are adjusted precisely to attract particular species of insects; and those specialists in turn, whether beetles, butterflies, bees, or some other group, are genetically adapted to respond to certain kinds of flowers. In lesser numbers the same is true of the interactions between plants and species of birds, bats, and other vertebrates dependent on diets of pollen and nectar.
Nature, we learn, is kept productive and flexible by uncounted thousands of such partnerships. The connections are fragile, and we are reminded by melancholy case histories reviewed by Buchmann and Nabhan that when one partner is extinguished, the other is at the very least put at risk—and sometimes doomed, if it happens to be adapted to no other partner. No phenomenon in nature illustrates more vividly the principle that conservation measures must be directed at ecosystems, not just individual species. If the last pollinator species adapted to a plant is erased by pesticides, or habitat disturbance, the plant will soon follow. And as these and other populations decline or disappear, the consequences spread through the remainder of the food net, weakening other interspecific relationships.
Those unconcerned about the natural world, and I hope their numbers are dwindling by persuasion, will do well to consider the consequences for humanity of the decline of pollinator complexes. Eighty percent of the species of our food plants worldwide, we are informed, depend on pollination by animals, almost all of which are insects. One of every three mouthfuls of food we eat, and of the beverages we drink, are delivered to us roundabout by a volant bestiary of pollinators.
The evidence is overwhelming that wild pollinators are declining around the world. Most have already experienced a shrinking of range. Some have already suffered or face the imminent risk of total extinction. Their ranks are being thinned not just by habitat reduction and other familiar agents of impoverishment, but also by the disruption of the delicate “biofabric” of interactions that bind ecosystems together. Humanity, for its own sake, must attend to the forgotten pollinators and their countless dependent plant species. In the following chapters Buchmann and Nabhan make a compelling case for more focused research on pollinator complexes and increased attention to their status as an integral part of future conservation planning and restoration ecology.
Edward O. Wilson
MUSEUM OF COMPARATIVE ZOOLOGY
HARVARD UNIVERSITY