The Tree (42 page)

As a relative of the custard apple, the Amazonian tree
Annona sericea
is pretty primitive; and it is pollinated mainly by beetles, which as insects go are primitive too. But “primitive” does not mean “simple,” or merely “prototype”; and the degree of coadaptation between
A. sericea
and its pollinators is extraordinary. To be sure, the flowers of
A. sericea
are simple: three fleshy petals that never fully open, grouped around a central conical knob that bears both the stigma (female) and the stamens (male).

At about seven o’clock in the evening—soon after dark in the tropics—the flowers begin to warm up, to about 6°C (11°F) above ambient. You may well find this surprising: after all, we all learn at school that only mammals and birds are warm-blooded, able to raise their body temperatures just for the sake of it. In truth, though, many creatures can do this—probably including some dinosaurs, and certainly including some modern insects and some sharks. Some flowers can do it too. The rise in temperature helps intensify their odor. The flowers of
A. sericea
do not breathe out the sweet smell of violets and honeysuckle that entranced lovers in Shakespeare’s comedies but (says Ghillean Prance) a perfume more “like chloroform and ether.” Nevertheless, it serves its purpose. Beetles (of the particular kind known as chrysomelids), and also some flies, come flocking in.

The beetles squeeze their way past the fleshy petals to the cone of male and female organs within. This is a common device among insect-pollinated flowers: provide an obstacle that only the desirable insects can overcome. The stigmas at this time are ready to receive pollen, but the anthers, which provide pollen, are still closed. Any pollen that the beetle has about its person may thus be transferred to the stigmas. But the beetle, at this stage, cannot obtain pollen from the same flower that it is pollinating; so there can be no self-fertilization. The beetles often stay to copulate within the flowers, and as they mill about they transfer even more pollen.

When the beetle has transferred its pollen, the stigmas at the top of the central cone fall off. Then the anthers become erect and release their pollen, and so the beetles become coated in it. Then the stamens drop off. The beetles eat the bases of the petals, and then the petals fall off. Then the beetles can escape (they could escape before the petals fall off, but they generally do not) and fly to another flower—now carrying pollen from the flower they have just pollinated. The flies that may visit also serve as pollinators in passing, and may lay their eggs on the sepals, but the beetles are the main players. Note, in this account, that the flower is seriously damaged, not to say destroyed by the beetle: the flower sacrifices bits of itself to bribe the beetle. But so what? The flower is only a lure. Once pollination is effected, its job is done.

Flies are only bit players in the life of
Annona sericea,
but flies including midges are the prime pollinators of many a plant, including many a tree—and including the cacao tree,
Theobroma cacao.
Again, the cacao flower sacrifices part of itself—sterile parts of the flower—to encourage the midges; and, again, the flower is organized in such a way that as the midge feeds it is brushed with pollen. The flowers are produced on the trunks and branches (this is “cauliflory,” so typical of tropical-forest trees), and the midges breed mainly in the fruit pods that fall to the ground and decay. If the cacao grower is too tidy and clears away the pods, the cacao loses its pollinators. Here, as in all of life, too much hygiene doesn’t pay. Many flowers that are pollinated by flies smell horrible, incidentally, imitating the rotting flesh that flies prefer; and some of them heat up to make it worse. (The most famous is the world’s biggest flower,
Rafflesia,
from Indonesia.)

But the best-known insect pollinators, and probably the most important, are the bees. Some bees are very small. Some are extremely large, like the carpenter bees and bumblebees. Others are in between, like honeybees and the long-tongued (“euglossine”) bees. Many are solitary, some live in small colonies like the bumblebee, and some in very large colonies like the honeybee. Many are generalist pollinators, but some—especially among solitary species—are adapted to pollinate particular flowers, which, in turn, are highly adapted to them. Bees are strong fliers: studies in the Amazon in the early 1970s showed bees of the genus
Euplasia
returning to their nests when released from a distance of fourteen miles. In the normal course of foraging they commonly fly many miles in a day, following a regular route from flowering tree to flowering tree according to the strategy known as “traplining.”

Some trees attract many species of bees: one study in Costa Rica in the 1970s showed that one leguminous tree
(Andira inermis)
attracted seventy different kinds, from middle-sized long-tongued bees to big carpenter bees. By the same token, many species of bees visit many different species of plant. But a few—particularly solitary bees—do have very close, specific relationships with particular trees, requiring a great deal of coevolution between the two.

If a bee, on any one foraging trip, visited many different kinds of plant indiscriminately, it would not be much good as a pollinator. It wouldn’t help England’s wayside roses, for instance, if visiting bees flew off and distributed their pollen among the local clover. But it turns out that on any one trip, most bees (more than 60 percent in one study in the Amazon) focus on only one plant species and very few (only 15 percent) visit three or more species per trip—and this, of course, makes them far more efficient as pollinators. Perhaps this focus reflects an “optimum foraging strategy”: a method by which feeding efficiency is maximized. Optimum foraging strategy has been studied most closely in birds. For example, if a pigeon is given a lot of barley with a few peas, it ignores the peas altogether. Confronted with a lot a peas and a few grains of barley, it ignores the barley. It pays a forager to concentrate on whatever is likely to prove most rewarding on a particular day. By focusing on whatever food source is commonest, and is known to be reliable, the pigeon does not have to waste time in wondering whether any one item is a pea, or a barley grain, or a pebble. By the same token, if a bee once establishes that roses are in bloom, it tends to stick to roses. Let others focus on clover.

Furthermore, other studies have shown that once back in the hive, colonial bees exchange pollen with one another—not deliberately, but just as they mill about. So a bee that picks up pollen three miles to the east of the hive may pass some pollen to another that is foraging up to three miles to the west—and trees that are six miles apart may thereby find themselves exchanging pollen. Of course, at different times of year the generalist bees switch from one species to another, as each comes into bloom, and thus ensure a year-round supply (or season-round, in temperate latitudes). Then again, although the bees may be generalists, individual species of trees seem to adjust their flowering strategy to the needs of particular types. Thus, in the Amazon, some trees flower in a “big bang” fashion—a brilliant show of flowers in a short season—which ensures that bees in general will notice them. Others, however, favor the “steady state” approach. They produce only one or a few inflorescences per day, over a long period—and this tends to attract the kinds of bees, like the carpenter bees, that habitually fly long distances and follow the same kinds of routes every day. This, then, seems to be a particularly good strategy for trees that are very widely spaced; but, of course, it relies on the regular habits and industry of a few species of bees.

In the forests of Amazonia the pristine ecology has been much interrupted and to a large extent preempted these past few decades by bees imported from Africa: the so-called killer bees. These are simply a hybrid between an African bee and the familiar European honeybee; and very good honey makers they are too, favored by many beekeepers. They got into South America in the 1970s from a research laboratory in São Paulo, in the south of Brazil, and spread at more than 125 miles per year. By 1982 they were already crossing Colombia, thousands of miles to the north of São Paulo. “Killer” is well over the top, but they are certainly aggressive both to people and to other insects. Thus in 1973 Ghillean Prance, near Manaus, observed the insects that came to pollinate
Couroupita subsessilis,
a relative of the Brazil nut. The visitors included wasps and bees. The only visitor the following year was the honeybee—not necessarily, but very probably, the African interloper. Presumably what Professor Prance saw in Manaus is common all over South America. Presumably, too, the Africanized bees will sometimes do a good job; but in general, it seems unlikely that any one generalist, however aggressive, can pollinate the trees of the neotropics as efficiently as the droves of insects that have evolved specifically to the task.

Yet the Brazil nut itself might well be a beneficiary. Brazil nut trees are wonderful. They are an emergent species, half as tall again as most canopy trees. Of course, too, their nuts are extremely valuable, and so the Brazil nut is on a short list of Amazonian trees that it is forbidden to fell. So it is that when forests are cleared, the pastureland that is sown or grows up in its wake is punctuated by isolated Brazil nut trees, rather like the big solitary oaks in England’s stately parks, although the parkland oak trees are to the manner born and spread themselves most opulently, while the Brazil nut trees, deprived in middle age of their companions, seem forlorn, magnificent but somewhat haggard. Furthermore, although the Brazil nut trees have been conserved mainly because of their nuts, when they are in the middle of nowhere they are liable to remain unfertilized. Most of their pollinators won’t fly over big open spaces. But isolated oaks in English parks, served by wind that’s laden with pollen from surrounding woodlands, have no comparable problem.

Enter, though, the Africanized bee. Its aggression is matched by its energy, and it does apparently make the long trek out to the isolated Brazil nut trees. Introduced species on the whole are a bad thing—in fact, “exotics” seem to be the chief cause of extinction of native species, apart, of course, from gross loss of habitat. But here at least, just for once, there is some compensation. “It’s an ill wind,” as the adage has it.

Birds are great pollinators too—notably, but by no means exclusively, hummingbirds. Like butterflies, they typically prefer red, which they see best. Mammals also. Some, like the honey possums in Australia or the desert rats in South Africa, are highly specific. Others, like the capuchin monkeys of Amazonia or giraffes in Africa, may pollinate their prey trees inadvertently (but nonetheless usefully) as they forage. But among the mammals, as with all creatures, the most efficient pollinators are the fliers; and with mammals that means bats.

There are well over eight hundred species of bats—among mammals, only rodents have more species. They are of two main kinds. The microbats live all over the world. Many—like the familiar bats of temperate lands—eat insects; but others live on mammalian blood (the vampires), on frogs, or on nectar and fruit. The megabats, which include the flying foxes, live exclusively in the Old World. For the most part they are big and live largely or exclusively on nectar and/or fruit. They rely heavily on scent to find their food, as most mammals do. By contrast, the microbats locate their prey, whatever it may be, by echolocation: sending out a high-pitched squeak and analyzing the echo. Both groups of bats are nocturnal. If they fly by day—as they sometimes are obliged to do, particularly in cold weather, when there are too few insects flying at night—they are quickly picked off by hawks. Some studies have shown that day-flying bats rarely survive for more than a few hours. Mammals and birds both flourished in the wake of the dinosaurs and flying reptiles. But while the mammals came to dominate the ground, birds very definitely dominate the air. Bats are very successful—ubiquitous and various—but they are advised to stick to the hours of darkness. Even then, they must avoid the owls.

Trees (including many cacti) hold their flowers high for bat pollination.

Trees that would be pollinated by bats must adapt to them—just as they must to specialist bees, butterflies, or moths. None has adapted to them more spectacularly than a relative of the mimosa: the Amazonian tree
Parkia. Parkia
has been studied in particular by Dr. Helen Hopkins, but it is everybody’s favorite, like a favored sister, endlessly endowed. It is beautifully shaped, like a great flat-topped umbrella. Its leaves are doubly compound, like feathers. Its bark is smooth but not too smooth, in some species dotted with red. But most glorious of all are the flowers, pompoms of stamens and styles, bright red or pale yellow or bronze, depending on species, that hang from on high on long, thin threads like Christmas baubles.

The topmost flowers in each inflorescence are sterile, with copious nectar, which begins to flow early in the morning. By midmorning the first flowers begin to open, and all of them are open by midafternoon. At dusk, the flowers begin to release their pollen. Then the bats come. By morning all the nectar has gone, the filaments that bear the anthers have wilted, and the flowers have faded. One of nature’s most glorious shows lasts only one night. Showbiz is not the point. Replication and multiplication is the point. One night is enough: as we have seen already, animal pollination can be remarkably efficient. The would-be pollinators are constantly alert.