A Population Checked: Competition, Climate & Predation

Darwin had the handy Parslow as butler, a cook, gardeners — servants of all kinds to keep Down House running. Still, it’s impressive how he used his own time. When you read The Origin of Species, and think of how you spend your own days, watching Downton Abbey or The Hour seems a shocking waste of time. “There must be some worms or weeds I can study!”

Following Darwin’s Malthusian-inspired musings on populations, he turns to what keeps a species in check. At different stages of life, some species are particularly vulnerable — as eggs, for example. Plants seeds, too, suffer enormously, mostly because they’re choked by the vegetation already on the ground. They have no hope with the competing growth. And this is where you think what would have happened had Darwin grown up with video games?

Ever curious, Darwin digs (or probably a gardener digs) a patch three feet by two feet to get rid of any competing vegetation and watches what happens. Each of the 357 weeds that come up, he identifies and marks, and waits to see what happens. Will they all live? Slugs and insects are the chief nemesis of 295 of the weeds.

Darwin also keeps a three feet by four feet patch of turf unmowed or not browsed by animals. Out of 20 plant species, nine species never make it.

Lest his readers start musing on the vulnerabilities of the young in obtaining food, and how food determines populations, Darwin reminds us that plants and animals are some species’ meal ticket. He’s setting the stage for the concept of a food web. Before that, he has to explain another factor: climate, which sometimes has a direct bearing on population: the bitter winter of 1884-5 destroyed four-fifths of the birds on his grounds. But hang on, says Darwin. “Climate brings on the most severe struggle between individuals, whether of the same or distinct species, which subsist on the same kind of food.”

So let’s say its a bad year for salmon because when juveniles headed to the ocean to get big and strong before migrating back to their natal streams, the climate was not in their favour. They had less food. Fewer salmon come back, which is bad for bears. Bad for humans too, but which species will ultimately get more salmon: humans or bears?

Darwin, Strawberries, and Downton (Abbey?)

Charles Darwin ends Chapter One with “Circumstances Favourable to Man’s Power Over Selection.” For all of Darwin’s clarity, his writing is formal, and a disembodied English voice from a British period drama seems to be narrating this blog. (Right now the voice sounds like Hugh Bonneville. A pity considering Maggie Smith has all the best lines.)

Darwin focuses on variability in this subsection on domestication. The most important circumstance for successful selection, Darwin writes, is the sheer number of individuals in a population.

Plant and animal breeders have valued variability for a long time. In part, it made them wealthy. As Darwin remarks, the poor people in Yorkshire can never improve their sheep as they only own small lots. Another circumstance favourable, in regard to animal selection, is the ability to enclose them. And, of course, you need money and land to enclose grazing animals. It’s easier to play God with pigeons, since they generally mate for life.

At Down House, Darwin kept pigeons and a large garden for his selection experiments. Darwin manipulated garden plants — notably potatoes — but probably strawberry plants too. He wrote about strawberries in The Variation of Animals and Plants Under Domestication. He uses the fruit (not truly a berry) in The Origin of Species to show how quickly humans will domesticate a crop if they value it. Only three kinds of strawberry varieties were known, in France, in 1746. Twenty years later, there were five varieties, and by the time Darwin writes, strawberry varieties were “inumerable.” Part of the reason, Darwin writes, lies with the promiscuity of American strawberries as noted by horticulturist T.A. Knight writing in 1818. “…there is abundant and additional evidence of the extent to which the American forms spontaneously cross. We owe indeed to such crosses most of our choicest existing varieties.” (pp. 351/352  The Variation of Animals and Plants Under Domestication.)

The question begging to be asked: were the “wild” strawberries of North America "unconsciously selected" by people living there for thousands of years?

Knight successfully crossed dozens of strawberry varieties. One selection was named “Downton” and he published a colour plate of the delectable fruit, described as “exquisitely rich far excelling any other ever tasted.” Which is also a good description of Downton Abbey (not so much the series itself, but the setting. No idea if the creator and writer of the series Julian Fellowes knows that, and from the Wikipedia page, there is no indication that he has horticultural leanings.)

Darwin ends acknowledging that variability is governed by many unknown laws. By this time, scientists know variability is crucial to selection and that they’re slightly clueless as to what ensures variability, aside from great numbers of individuals within a population. It’s all the more tragic that today, knowing what we know, we fail to protect variability in wild food through habitat protection — salmon on the West Coast, being a prime example.