Darwin's Backyard
Page 13
It was bad, but not disastrous. He followed Lubbock’s suggestion with a test case, the flora of New Zealand, dividing the plants into two groups: genera with four or more species (“large” genera), and those with one, two, or three species (“small” genera). He found 339 species in the large category and 323 in the small one. Of the 339 species, 51 presented varieties. Proportionally speaking, if the incidence of varieties was the same in the two groups then by proportion he should find 48.5 varieties in the small category. In fact, there were only 37 varieties within the smaller genera. That was completely consistent with Darwin’s overarching argument, and then some: he realized that by this method of proportional analysis larger genera could be shown to not only have more varieties, but disproportionately more. This was the smoking gun he was seeking. “The case goes as I want it,” he wrote cautiously to Lubbock, “but not strong enough, without it be general, for me to have much confidence in.”29 Prevailing upon Hooker to resend all of those botanical books he had long since returned, he threw himself into redoing his calculations. He generated some 200 pages of tables by the time he was done.30 Actually, he didn’t generate all those tables himself: Darwin employed the local school master, Ebenezer Norman, to do much of the laborious tabulating for him. He emerged from his growing mountain of tables with not only more robust evidence for divergence of species, but also a principle of divergence. He first used the term that August, reporting his results to Hooker and asking for still more botanical manuals: “If it will all hold good it is very important for me; for it explains, as I think, all classification, ie the quasi-branching & sub-branching of forms, as if from one root, big genera increasing & splitting up &c &c, as you will perceive.— But then comes, also, in what I call a principle of divergence, which I think I can explain, but which is too long & perhaps you would not care to hear.”31 It may have been too long to explain to Hooker, but a month later he sent Asa Gray, in Massachusetts, a long but concise explanation of his now all-important principle after defining natural selection. It’s worth quoting in full here, especially as he has worked in several lines of evidence such as the Woburn Abbey experiment and his biodiversity surveys:
One other principle, which may be called the principle of divergence plays, I believe, an important part in the origin of species. The same spot will support more life if occupied by very diverse forms: we see this in the many generic forms in a square yard of turf (I have counted 20 species belonging to 18 genera),—or in the plants and insects, on any little uniform islet, belonging almost to as many genera and families as to species.— We can understand this with the higher, animals whose habits we best understand. We know that it has been experimentally shown that a plot of land will yield a greater weight, if cropped with several species of grasses than with 2 or 3 species. Now every single organic being, by propagating so rapidly, may be said to be striving its utmost to increase in numbers. So it will be with the offspring of any species after it has broken into varieties or sub-species or true species. And it follows, I think, from the foregoing facts that the varying offspring of each species will try (only few will succeed) to seize on as many and as diverse places in the economy of nature, as possible. Each new variety or species, when formed will generally take the places of and so exterminate its less well-fitted parent. This, I believe, to be the origin of the classification or arrangement of all organic beings at all times. These always seem to branch and sub-branch like a tree from a common trunk; the flourishing twigs destroying the less vigorous,—the dead and lost branches rudely representing extinct genera and families.32
It was a grand ecological and evolutionary vision, and his new Principle of Divergence was the lynchpin.
Analysis of manual after botanical manual all through the remainder of 1857 yielded, with a few exceptions, the same result: species of larger genera had disproportionately more varieties than their counterparts in smaller genera. There was only one explanation, he thought: size of genus really is a direct indicator of evolutionary success in the recent past. Abundant and widespread, these species are even now rich in variation and yielding varieties, just as he thought. Thus Darwin’s “disgraceful blunder” had a silver lining. His Principle of Divergence brought together several key strands of thought: variation, struggle, ecological division of labor, extinction, classification, and diversity in small uniform areas. Selection favors distinctness among variant forms of a given species, since by virtue of that distinctness those variant forms can more stably coexist. It is a de facto ecological division of labor, stemming from a selection dynamic that promotes branching and rebranching of lineages over time. That is, the Tree of Life grows and spreads not simply through natural selection, but through natural selection’s divergence dynamic that forces branching.
The sole figure in On the Origin of Species, illustrating the outcome of Darwin’s Principle of Divergence. Time, reckoned in thousands or tens of thousands of generations, is represented on the vertical axis, and morphological and ecological divergence is represented on the horizontal. The diagram illustrates how descendant species become increasingly more different (divergent) from ancestral parental forms (capital letters along the bottom) over time. Darwin envisioned this divergence process giving rise to branching and re-branching lineages, forming the Tree of Life. From Darwin (1859), p. 117.
Darwin’s Principle of Divergence is a concept as sobering as it is enthralling. The reality of it is that destruction and extinction are the inevitable flip sides to growth, adaptation, and diversification. It must have been jarring to him though, realizing that it wasn’t simply that larger, growing groups take the place of smaller, declining ones. No, more poignantly the growing groups displace the declining groups—the offspring varieties and species drive their parents to extinction. He well knew how that would be received. But it is what it is, he recognized; there was no going back or softening the blow. Most people then, as now, could only “behold the face of nature bright with gladness,” as he put it in Origin. “Nothing is easier than to admit in words the truth of the universal struggle for life, or more difficult . . . than constantly to bear this conclusion in mind.” Failing this, he cautioned, “the whole economy of nature, with every fact on distribution, rarity, abundance, extinction, and variation, will be dimly seen or quite misunderstood.”33
Paley, Darwin’s sometime hero and fellow son of Christ’s College, would have been appalled. But in the spirit of Cardinal Cesare Baronio’s admission to Galileo that “The Bible teaches us how to go to heaven, not how the heavens go,” Darwin held true to his conviction that scientific truths must be followed however unwelcome or uncomfortable they may be to received wisdom, or presumed scriptural truth. So it was that Darwin’s taste for field botany and little experimental plots helped introduce principles that eventually became central concepts of the science of ecology, namely, competition, competitive exclusion, niche partitioning, and even character displacement (mutual adaptation by divergence in morphology between two species with similar ecological needs that overlap in their ranges). In a case of giants standing on giants’ shoulders, these concepts were elaborated and refined by several twentieth-century pioneers of mathematical ecology, including Charles Elton, G. Evelyn Hutchinson, Robert MacArthur, and others.
British-born Yale ecologist Hutchinson in particular brought fresh perspectives both empirical and theoretical to Darwin’s dual interests in competition and diversification. In his classic 1959 paper “Homage to Santa Rosalia, or why are there so many kinds of animals?”—given, fittingly enough, on the occasion of his presidential address to the American Society of Naturalists 100 years after the reading of the Darwin and Wallace papers at the Linnean Society—Hutchinson refined the niche concept, now called the “Hutchinsonian niche,” and stimulated interest in the central role of energy flow in food chains and food webs, and how these relate to community stability and the maintenance of biodiversity. Darwin would have appreciated how Hutchinson’s insights, like his own, had come from extrapolati
ng the local to the global, in this case observations made at Lindley Pond, not far from the Yale campus, and a small pond on Mount Pellegrino in Sicily where the Shrine of Santa Rosalia is found. And, given Darwin’s sense of humor, he would have been tickled to know that this renowned ecologist who helped extend and formalize Darwin’s nascent “ecological” ideas had had an inauspicious first encounter with the Darwin family as a youth growing up in Cambridge, where his father was a lecturer in mineralogy. Hutchinson, then 11 years old, and his 9-year-old brother Leslie played a prank on Darwin’s son George (by then Sir George) and his wife when they came for a dinner party one evening. The impish boys locked their parents and distinguished guests in the dining room, tossing the keys into the garden and cutting off the dining room lights from the main breaker. If that was an “experiment” to see what would happen the results were likely less than happy (“The subsequent parts of the story,” Hutchinson concluded, “are repressed”), and the same was true of at least one other ill conceived “experiment” of Hutchinson’s youth: he pushed his friend Christina into a pond to see if she would float (“an early interest in the hydrodynamics of organisms of which I am not proud.”34). Thank goodness Hutchinson turned his energies to pursuing the power of Darwinian experimentising to untangle the bewilderingly tangled bank of nature, building upon Darwin to found the modern field of evolutionary ecology in the process.
Experimentising: A Taste for Botany
“I believe a leaf of grass is no less than the journey-work of the stars.” Those immortal words in Leaves of Grass by Walt Whitman were published just a year before Darwin established his Lawn Plot experiment. Darwin would have agreed, and saw even greater significance in a whole patch of grass blades. To him a little plot of lawn might as well have been all of teeming Amazonia: the same principles of competition, selection, and adaptation applied no matter how small the land area.
I. Darwin’s Lawn Plot
We’ll make similar observations in our own little plots, whether with lawn or some other ground cover. It may be difficult to fully replicate Darwin’s experiment, insofar as a history of mowing was important for his lawn plots. This is because he thought that if plant growth is limited by grazing or mowing, a greater diversity will result, whereas unchecked growth will eventually result in some plants outcompeting the others, lowering overall diversity. If you know your plot’s mowing history, your experiment will be more comparable to Darwin’s. Regardless, you can learn much by looking closely at the number and frequency of species found in a small area of uniform habitat.
A. Materials
• Notebook and pencil
• Tape measure
• String or cord
• Garden stakes or sticks (4 per plot, to mark corners)
• Basic plant identification book, such as the Weeds and Wildflowers Golden Guides (St. Martin’s Press, New York), the Peterson’s Field Guide to Wildflowers, or online plant identification guides like eNature, searchable by geographical area (www.enature.com/fieldguides).
B. Procedure
1. Measure a 3 × 4-ft (0.9 × 1.2-m) plot in a grassy or weedy area, ideally untreated with pesticides or fertilizer, marking the corner of each with a garden stake and fencing off with the string or cord (attaching or looping around the stakes). You now have a “small and uniform” plot, just like the one Darwin established in his back yard in March of 1856. (Make sure you contact anyone who mows this area to let them know to avoid your plot.)
2. Every 2–3 weeks, census your plot by identifying its plants and counting how many of each species there are. Identify plants as best you can to genus or species. This can be difficult for plants lacking flowers, and for some groups like grasses, even when you do have the flowers. In a pinch simply identify to “morphospecies,” or “kinds” based on obviously different morphology. If you are able to identify to genus or species it will be easy to determine the taxonomic family or order to which the plant belongs. For example, sites such as Wikipedia (en.wikipedia.org), Tropicos (www.tropicos.org), and efloras (www.efloras.org) list the full classification for plant genera or species. It might be best to tag or label your plants to help keep track of what you have identified and what’s new at your next census.
3. Compare your results with Darwin’s: over the course of the growing season he found 20 species belonging to 18 genera and 8 orders in his lawn plot. Is your plot as diverse as his, or more or less so? Your findings will likely differ from Darwin’s, maybe dramatically. Since many factors can influence local diversity (soil characteristics, local climate, time since last site disturbance, etc.) it would be difficult to say why your results are similar or different from Darwin’s. In fact, even replications at Down House, where Darwin did the experiment, give varying results (see McLauchlin below). Such are the vagaries of ecology.
Sketch of Darwin’s “Lawn Plot.”
II. Darwin’s Weed Garden Experiment
Darwin recognized the difficulty in conveying to the average person the organisms’ ceaseless struggle for existence and the creative role this plays through selection. Comprehending this point is all the more difficult because struggle is experienced in different ways for different organisms, seasons, times of day, life stages, location, and many other factors. When we observe nature we often miss the struggle, seeing only peace and harmony, and mistake this for the natural condition of the living world. Darwin’s aim in the Weed Garden Experiment was to focus on one group of organisms (plants) at just one life stage (seedling). He thought if he could remove one set of pressures, namely crowding by other plants, he could identify the other destructive forces acting on them (e.g., frost, drought, insects, slugs) and measure their cumulative effects over time. His experiment—a pioneering investigation in population ecology—is easily replicated. See if you can measure seedling mortality and compare your results with his.
A. Materials
• Notebook and pencil
• Tape measure and ruler
• String or cord
• Garden stakes or sticks (4 per plot)
• Chicken wire or aluminum flashing, to erect a barrier around the plot 1 ft (~ 0.3 m) high
• Wood dowels 0.125 in. (~3 mm) in diameter, galvanized wire, or twist ties in 4 in. (~10 cm) lengths
• Permanent marker
• Scissors
• Paper, cut into ~½ in. (1.3 cm) squares
B. Procedure
1. In late winter measure one or more 3 × 2-ft (0.9 × 0.6-m) plots, marking the corners with a garden stake or stick and marking the perimeter with string or cord (attaching the cord to the stakes or looping around them).
2. Clear the plot by removing all existing plants; dig the soil as necessary to remove the roots or rhizomes of any perennials.
3. Place the chicken wire around the plot, embedding it into the ground as best you can, as a barrier to protect the area from animals digging or trampling the emerging seedlings.
4. Check the plot on a regular schedule, at least every 2 or 3 days. As seedlings first appear, mark their location by inserting a dowel or wire bearing a number into the ground adjacent to the plant, taking care not to contact the seedling, and record the number in the notebook. Label each seedling individually using the paper squares numbered with indelible marker; use scissors or a sharp pencil to make a small slit or hole on opposite sides of each numbered square, and insert the dowel or wire through these to affix the square as above.
5. While you continue marking the position of each seedling as it appears, observe and note any evidence of activity or presence of slugs, snails, insects, birds, etc. that may harm the seedlings. At appropriate intervals, perhaps once or twice weekly, keep a log of the climate and soil conditions, noting average temperature, rainfall and moisture of soil. If, as the plants grow, it is possible to identify any of them, note that, too.
6. Census days: Once or twice monthly, approximately 2–4 weeks apart depending on the schedule you settle upon, remove the numbered wires without a seed
ling, indicating that the seedling has been destroyed. Count and record the number of numbered wires. Record any notes about climate or other factors that may explain the numbers of lost and surviving plants.
C. Analysis
1. Following Darwin’s lead, for each census day record (1) the number of seedlings killed since the previous count, (2) the number of surviving seedlings, and (3) the total number of seedlings that have emerged to date.
2. Use your data to produce a survivorship table, showing the percentage of seedlings surviving out of the cumulative number emerging over time. For example, on March 31, 1857, Darwin recorded that out of 55 seedlings total to that date, 30 survived and 25 had died. That gave a survivorship of 30/55 = 54 percent. At his June 1, 1857, census Darwin found just 80 surviving and 277 that had died, giving 80/357 = 22 percent survivorship. Survivorship in his experiment dropped to 19 percent by July 1st, and 17 percent by August 1st.