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?