One such aviatrix lands on a herbaceous plant—a young climbing bean, say, in someone’s garden—and the pattern repeats. She plugs into the sap ducts on the underside of a new leaf, commences feasting, robbing the plant of its vital juices, and then delivers by parthenogenesis a great brood of wingless daughters. The daughters beget more daughters, those daughters beget still more, and so on, until the poor bean plant is encrusted with a dense population of these fat little sisters. Then again, neatly triggered by the crowded conditions, a generation of daughters are born with wings. Away they fly, looking for prospects, and one of them lights on, say, a sugar beet. (The switch from bean to beet is possible for our species of typical aphid, because it is not a dietary specialist committed to only one plant.) The sugar beet before long is covered, sucked upon mercilessly, victimized by a horde of mothers and nieces and granddaughters. Still not a single male aphid has appeared anywhere in the lineage.
The lurching from one plant to another continues; the alternation between wingless and winged daughters continues. Then, in September, with fresh and tender plant growth increasingly hard to find, there comes another change.
Flying daughters are born who have a different destiny: They wing back to the poplar tree, where they give birth to a crop of wingless females unlike any so far. These latest girls know the meaning of sex! Meanwhile, at long last, the starving survivors back on that final bedraggled sugar beet have brought forth a generation of males. The males too have wings. They take to the air in search of poplar trees and first love. Et voilà. The mated females lay eggs that will wait out the winter near bud sites on that poplar tree, and the circle is closed. One single aphid hatchling—call her the matriarch—in this way can give rise in the course of a year, from her own ovaries exclusively, to roughly a zillion aphids.
Good for her, you say. But what’s the point of it?
The point, for aphids as for most other parthenogenetic animals, is 1) exceptionally fast reproduction that allows 2) maximal exploitation of temporary resource abundance and unstable environmental conditions, while 3) facilitating the successful colonization of unfamiliar habitats. In other words, the aphid, like the gall midge and the weaver ant and the rest of their fellow parthenogens, is by its evolved character a hasty opportunist.
This is a term of science, not of abuse. Population ecologists make an illuminating distinction between what they label “equilibrium species” and “opportunistic species.” According to William Birky and John Gilbert, from a paper in the journal American Zoologist: “Equilibrium species, exemplified by many vertebrates, maintain relatively constant population sizes, in part by being adapted to reproduce, at least slowly, in most of the environmental conditions which they meet. Opportunistic species, on the other hand, show extreme population fluctuations; they are adapted to reproduce only in a relatively narrow range of conditions, but make up for this by reproducing extremely rapidly in favorable circumstances. At least in some cases, opportunistic organisms can also be categorized as colonizing organisms.” Birky and Gilbert emphasize that the potential for such rapid reproduction is “the essential evolutionary ticket for entry into the opportunistic life style.”
And parthenogenesis, in turn, is the greatest time-saving trick in the history of animal reproduction. No hours or days are wasted while a female looks for a mate; no minutes lost to the act of mating itself. The female aphid attains sexual maturity and, bang, she becomes automatically pregnant. No waiting, no courtship, no fooling around. She delivers her brood of daughters, they grow to puberty and, zap, another generation immediately. The time saved by a parthenogenetic species may seem trivial, but it is not. It adds up dizzyingly: In the same duration required for a sexually reproducing insect to complete three generations for a total of 1,200 off-spring, an aphid can progress through six generations (assuming the same maturation rate and the same number of progeny per litter) to yield an extended family of 318,000,000.
Even this isn’t speedy enough for some restless opportunists. That matricidal gall midge Miastor metraloas, whose larvae feed on fleeting eruptions of fungus under the bark of trees, has developed a startling way to cut further time from the cycle of procreation. Far from waiting for a mate, M. metraloas does not even wait for maturity. When food is abundant, it is the larva, not the adult female fly, who is eaten alive from inside by her own daughters. And as those voracious daughters burst free of the husk that was their mother, each of them already contains further larval daughters taking shape ominously within its own ovaries. While the food lasts, while opportunity endures, no Miastor metraloas female can live to adulthood without dying of motherhood.
The implicit principle behind all this nonsexual reproduction, all this hurry, is simple: Don’t pause to fix what isn’t broken. Don’t tinker with a genetic blueprint that works. Unmated female aphids, and gall midges, pass on their own genotypes virtually unaltered (except for the occasional mutation) to their daughters. Sexual reproduction, on the other hand, exists to allow genetic change. The whole purpose of joining sperm with egg is to shuffle the genes of both parents and come up with a new combination that might perhaps be more advantageous. Give the kid some potent new mix of possibilities, based on a fortuitous selection from what Mom and Pop individually had. Parthenogenetic species, during their hurried phases at least, dispense with this genetic shuffle. They stick stubbornly to the genotype that seems to be working. They produce (with certain complicated exceptions) natural clones of themselves.
But what they gain thereby in reproductive rate, in great explosions of population, they lose in flexibility. They minimize their genetic variability—that is, their options. They lessen their chances of adapting to unforeseen changes of circumstance.
Which is why more than one biologist has drawn the same conclusion as M.J.D. White: “Parthenogenetic forms seem to be frequently successful in the particular ecological niche which they occupy, but sooner or later the inherent disadvantages of their genetic systems must be expected to lead to a lack of adaptability, followed by eventual extinction, or perhaps in some cases by a return to sexuality.”
So it is necessary, even for aphids, this thing called sex. At least intermittently. A hedge against change and oblivion. As you and I knew it must be. Otherwise, surely by now we mammals and dragonflies would have come up with something more dignified.
Desert Sanitaire
THE ENGLISHMAN T. E. LAWRENCE, he of Arabian fame, was supposedly once asked why he so loved the desert. Skeptical historians have treated Lawrence far less kindly than Peter O’Toole and David Lean did, suggesting that far from being the reluctant demigod and charismatic catalyst of Arab revolt, as so appealingly pictured, he was more on the lines of a conniving, ambitious, and perpetually mendacious poseur. Also, unlike O’Toole, he stood only five foot three. To hear it from some of his biographers, the truth was not in Lawrence. He colluded with Lowell Thomas (in those days a young showman, more interested in dashing romance than journalistic fact) toward inventing and spreading the “Lawrence of Arabia” legend. He was a sadomasochistic neurotic whose entire life, say the critics, was “an enacted lie.” He liked costumes and he invented his own heroism.
This revisionist view, sound in principle, may in fact be a little too harsh. Lawrence certainly had something, and maybe that something was almost as valuable as the habit of veracity or full mental health. He had panache. He had high style. He had the gift for capturing, if not strict autobiographical truth, at least the human imagination. Why do you love the desert? they asked him later, when he was languishing through his self-imposed obscurity back in soggy, dreary England. Reportedly he said: “Because it’s clean.”
At least, I hope he did.
Of course, by a literal reading the notion is nonsense. Clean of what, dirt? Not if dust and perspiration and a week’s funky unwashed body grime can be counted. Clean of microbial infestation and many-legged vermin? Hardly. Perhaps clean of human infestation? That’s more plausible as a guess of what he meant, give
n the misanthropic side of his disposition. Anyhow, if you have ever spent time out there—not in Arabia, necessarily, but in the desert—down on the very ground, crunching off the miles with your boots, maybe you understand something of what poor troubled Lawrence was getting at.
It’s clean. It’s austere. It’s ascetic. Harshly infertile and fatally inhospitable. Solitary. Unconnected. It’s notable chiefly for what it lacks. America’s own preeminent desert anchorite seems to agree: Wherever his head and feet may go, says Ed Abbey, his heart and guts linger loyally “here on the clean, true, comfortable rock, under the black sun of God’s forsaken country.” It’s clean.
But what is it, this thing of such noteworthy cleanliness? “There is no single criterion,” according to the renowned desert botanist Forrest Shreve, “by which a desert may be recognized and defined.” Still, we have to start somewhere. And a desert is one of those entities, like virginity and sans serif typeface, of which the definition must begin with negatives.
In this case, lack of water. Not enough rain. Less than ten inches of precipitation through the average year. A desert is not, most essentially, a hot place or a sandy place or a place filled with reptiles and cacti and dark-skinned people wearing strange headgear. Fact number one is that it’s a dry place. Joseph Wood Krutch has written that “in desert country everything from the color of a mouse or the shape of a leaf up to the largest features of the mountains themselves is more likely than not to have the same explanation: dryness.” From such a simple starting point, things get more complicated immediately.
The matter of sheer dryness, for instance, is less crucial than the matter of aridity, which is a measure of how much or how little water remains available on a particular landscape surface for how long. Ten inches of rain distributed evenly throughout a lengthy cool season will support plants and animals in modest profusion; ten inches dumped from a great cloudburst on one summer afternoon, then not another drop for the rest of the year, will produce a few hours of wild flooding and leave behind a typical parched desert, with wide empty arroyos and a scattering of peculiarly specialized creatures. Whatever water there may be comes and goes quickly in a desert, erratically, never remaining available over time. It abides not. It pours off the slopes of treeless mountains. It gathers volume in drywashes and roars peremptorily away. It soaks down fast through the sandy soil and is gone. Most of all, it evaporates.
That’s the other prerequisite for any desert environment, lesser partner to dryness: evaporation, as wrought by heat and wind. A little rain falls occasionally, yes, but coming as it does in prodigal storms during the warmest months, burned off by direct sunshine and sucked away by the winds, the stuff disappears again almost at once. A system of land classification devised by Vladimir Köppen takes this into account, with a mathematical formula by which temperature and precipitation are together converted to an index of aridity. According to the Köppen method, any region where potential evaporation exceeds actual precipitation by a certain margin can be considered a desert. This rules out frigid locales with scant annual precipitation but plenty of permanent ice, such as Antarctica. Most of our own Southwest qualifies resoundingly.
But what, in the first place, makes a spot like Death Valley or Organ Pipe Monument so all-fired dry? Or a huge region like the Sahara? Or the Kalahari? Or the Taklimakan Desert of western China? Is it purely fortuitous that one geographical area—say, the Amazon basin—should receive buckets of moisture while another area not far away—the Atacama Desert in northern Chile—gets so little? The answer to that is no: not at all fortuitous. Three different geophysical factors combine, generally at least two in each case, to produce the world’s various zones of drastic and permanent drought: 1) high-pressure systems of air in the horse latitudes, 2) shadowing mountains, and 3) cool ocean currents. Together those three cast a tidy pattern, north and south, girdling our planet with deserts like a fat woman in a hot red bikini.
Don’t take my word for this: look at a globe. Spin it and follow the Tropic of Cancer with your finger as it passes through, or very near, every great desert of the northern hemisphere: the Sahara, the Arabian, the Turkestan, the Dasht-i-Lut of Iran, the Thar of India, the Taklimakan, the Gobi, and back around to the coast of Baja. Now spin again and trace the Tropic of Capricorn, circling down there below the equator: through the Namib and the Kalahari in southwestern Africa, straight across to the big desert that constitutes central Australia, on around again to the Atacama and the Monte-Patagonian of South America. This arrangement is no coincidence. It’s a result, first, of that high-pressure air in the horse latitudes.
The horse latitudes (traditionally so called for tenuous and uninteresting reasons) encircle Earth in a pair of wide bands, one north of the equator and one south, along those two lines, Cancer and Capricorn. The northern band spans roughly the area between latitudes 20° N and 35° N, and the counterpart covers a similar area of southern latitudes. Between the two bands is that zone loosely called “the tropics,” very hot and very wet, where most rainforest is located. This is also the zone of terrestrial surface that—because of its distance from the poles of rotation—is moving with greatest velocity as our planet spins through space. (The equator rolls around at better than 1,000 mph, while a point near the North Pole travels much slower.) For physical reasons only slightly less obscure than Thomistic metaphysics, the difference in surface velocity produces trade winds, variations in barometric pressure, and a consistent trend of rising air over the tropics. As the air rises, it grows cooler, therefore releasing its moisture (as cooling air always does) in generous deluge upon the tropical rainforests. Now those air systems are high and dry: far aloft in the atmosphere and emptied of their water. In that condition they slide out to the horse latitudes, north and south some hundreds of miles, and then again descend. Coming down, they get compacted into high pressure systems of surpassing dryness. And as the pressure of this falling air increases, so does its temperature. The consequence is extreme permanent aridity along the two latitudinal bands and a first cause for all the world’s major deserts.
The second cause is mountains—long ranges of mountains, sprawling out across the path of prevailing winds. These ranges block the movement of moist air, forcing it to ascend over them like a water-skier taking a jump. In the process, that air is cooled to the point where it releases its water. The mountains get deep snow on their peaks and the land to leeward gets what is left: almost nothing. Such a “rain shadow” of dryness may stretch for hundreds of miles downwind, depending on the height of the range. It’s no accident, then, that the Sahara is bordered along its northwestern rim by the Atlas Mountains, that the Taklimakan stares up at the Himalayas, that the Patagonian Desert is overshadowed by the Andes.
Ocean currents out of the polar regions work much the same way, sweeping along the windward coastlines of certain continents and putting a chill into the oncoming weather systems before those systems quite reach the land. Abruptly cooled, the air masses drop their water off the coast and arrive inland with little to offer. For instance, the Benguela Current, curling up from Antarctica to lap the southwestern edge of Africa, steals moisture that might otherwise reach the Namib. The Humboldt Current, running cold up the west coast of South America, keeps the Atacama similarly deprived. The California Current, flowing down from Alaska along the Pacific coast as far south as Baja, does its share to promote all-season baseball in Arizona.
Beyond all these causes of dryness, another important factor is wind, helping to shape desert not only through evaporation but also—and more drastically than in any other type of climate zone—by erosion. Powerful winds blow almost constantly into and across any desert, with heavier cold air charging forward to fill the vacuum as hot light air rises away off the desert floor. Desert mountains tend to increase this gustiness, and in some cases to focus winds through canyons and passes for still more extreme effect. In deserts of southwestern North America, they call the wind chubasco if it’s a fierce rotary hurricane of a thing, w
hirling up wet and mean out of the tropics and tearing into the hot southern drylands with velocities up to 100 mph, sometimes delivering more than a year’s average rainfall in just an afternoon. More innocent little whirlwinds, localized twisters and dust devils, are known as tornillos. The steadiest and driest wind out of northern Africa is known as sirocco, from an Italian word with an Arabic precursor; the sirocco is what gives southern Europe a sniff of Saharan desiccation. Besides raking away moisture and making life tough for plants and animals, winds work at dismantling mountains, grinding rock fragments into sand, piling the sand into dunes and moving them off like a herd of sheep. The writer and photographer Uwe George has called desert wind “the greatest sandblasting machine on earth,” and there is vivid evidence for that notion in any number of desert formations.
The winds and the flash floods are further abetted, in punishing the terrain, by huge fluctuations in surface temperature. A desert thermometer doesn’t just go up, way up; it goes wildly up and down, by day and by night, because the clear skies and the lack of vegetation allow so much of the day’s solar energy to radiate away after dark. Easy come, easy go, since there’s no insulation to slow the transfer of heat. The temperature of the land surface, furthermore, fluctuates even more radically than the air temperature; a dark stone heated to 175°F in the afternoon may cool to 50°F overnight. The result is a constant process of fragmentation—rocks splitting noisily, as though from sheer exasperation.
Natural Acts Page 12
