Wednesday, May 26, 2021

Wonderful Diversity: The Shifting Lives of Plants

I just finished reading Jonathan Sauer’s Plant Migration: The Dynamics of Geographic Patterning in Seed Plant Species, published in 1988. In this book, the author was manifestly attempting to explain large-scale patterns and principles that governed the way in which plant species moved from one place to another. He tried to classify the different processes and kinds of habitat. He failed spectacularly. In some cases, he ended up with very, very short chapters when he found almost no examples of what should have been a general principle. The closest he came to a general law was the (trivially obvious) fact that dispersal is centrifugal (outward from a seed source) while natural selection is centripetal (getting rid of plants from places to which they have dispersed but cannot survive). Duh. He admitted that the conclusion he reached is that there are no general rules of plant migration. Each plant species has migrated during its history, and that every plant species has its own story. And he said he was pleased to have failed in his quest. I, also, am pleased. What a beautiful, diverse, quirky world this is, as illustrated by the history of each different plant species.

I enjoyed this book because I love stories. I read the Bible for its stories, not for the theological frameworks that have been imposed upon it. I read history because of its stories of unpredictable directions, not for some overarching theories.

Here are some examples from Sauer’s book that I found interesting.

  • The true cinnamon tree (Cinnamomum zeylandicum) is native to Sri Lanka (formerly Ceylon; hence the name zeylandicum) and southern India. Then why is there a patch of it in Mauritius, an island off the coast of Madagascar, about 3000 km away from Sri Lanka? A botanist might try to think of a way that cinnamon seeds might float on ocean currents and end up on Mauritius. But this is the way it actually happened. The Dutch had a monopoly on the spice trade, including cinnamon. The French wanted in on the trade, and managed to get some cinnamon plants to Mauritius, an island that they controlled. They planted some groves there. But in the face of an impending British invasion, they destroyed the garden. Too late; the birds had already spread the seeds from the garden to the surrounding forest.
  • Matricaria discoides is the little pineappleweed that grows in disturbed soils in Siberia but is also “native to” North America. Ever since I first saw this tiny plant growing in the compacted dirt of our junior high track in California, I have loved it: you can step on it, but it keeps growing. Its response to maltreatment is to release a beautiful pineapple scent. One might think that this plant was brought from Siberia to North America the same way as many other invasive plants such as Russian thistle (tumbleweed), by industrial transportation. But archaeological studies show that Matricaria was present in western North America before the arrival of Europeans. This indicates that it might have dispersed naturally or might have been carried by Native Americans when they first crossed the Bering land bridge.
  • Fennel is usually a small garden herb, but now that it has escaped from gardens into the disturbed grasslands around Santa Barbara (and on Santa Cruz island) they grow two or three meters tall and are almost the only large plants around.
  • The story of eastern Australia being taken over by Opuntia cactus from America in the early twentieth century, and their subsequent biological control, is well known to nature-lovers. What is less well known is that the Australian government blamed the spread of the cactus on emus, a large flightless native bird, eating the fruits. So, they set bounties on emus, which resulted in 335,000 emus being killed. The cactus only spread faster, up to 100 hectares per hour.
  • One of the ways that cactus spread in Australia was by the cactus pads sticking to livestock, falling off in a new location, and rooting.
  • Dam construction in the American southwest, at fever pace from the 1930s and 1960s, has led to the spread of the invasive tamarisk (salt cedar) in two ways. First, there are no longer downstream floods that would wash away the salt cedar thickets. Second, sand accumulation above the dam created new habitat for the tamarisks.

One theme that does recur over and over is that nearly all of the changes that humans have inflicted on the natural world have been disruptive. The natural world is incredibly complex, and whatever happens in one part causes a change in another part. Humans have tried to impose simplicity on a system that was never meant to have it, whether it was sheep ranches on the Santa Barbara Channel Islands or farms in the Midwest. To force nature to stay within the conceptual boundaries we have set for it is like getting a two-year-old to always do exactly what you want. Only it’s worse, because every species is like a squirming toddler. Just about the only example of humans doing something constructive was that the goats which overran Santa Catalina Island off the California coast stayed away from the resort town of Avalon, perhaps scared of the people, and this allowed the native chaparral and sage to grow back.

A major example of human interference has been fire suppression, about which I have previously written in this blog. Foresters wanted to control all wildfires but, Sauer said on page 130, they “quickly realized…that they had grabbed a tiger by the tail.” In the absence of small fires, dead wood builds up, with the result that any fires that do start will be infernos.

Another recurring theme is that colonial powers have repeatedly kicked out native peoples who were living sustainably, whether Native American or native New Caledonian people, and then impose a system unsuited to the land and which inevitably fails. Fire suppression is a good example. David Douglas, after whom the Douglas fir is named, saw Natives burning savannas in 1826.

Sauer brought part of the confusion on himself. He included migration in space, whether it was migrations of just a few meters or migrations halfway around the world. He included migration in time, whether just since the last ice age or since the beginning of plant life on dry land. The result was a mishmashed compendium of fascinating stories. But I liked it anyway. Long live diversity!

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