2024-02-01 21:00:00
For a novice bird watcher it is actually quite useful: you only need to know a few percent of the species and then you can name half of all the random birds you encounter. The same goes for plant lovers, mushroom researchers and even microbiologists. In each case, only a few percent of the species make up more than half of the biomass of a group.
This law surrounding general and special species has been known in biology for a long time and scientists have been wondering about it for just as long. To date, no one really has a conclusive explanation for it. Why is it that a handful of species always manage to rule the roost and the rest remain relatively special? This week, a group of Wageningen ecologists propose a new solution to the riddle in the journal of the American Academy of Sciences, PNAS. Despite their background, the biologists’ explanation is strictly mathematical: once a species becomes very common, or very rare, it is computationally extremely difficult to escape that, they argue.
Critical balances
One of the authors is aquatic ecologist Marten Scheffer, who won the Spinoza Prize in 2009 for his work on critical balances in biology and beyond. In biological systems, Scheffer was one of the first to argue at the time, there are generally stable equilibria, which can only be brought into a new state by a major disturbance. A cloudy ditch covered with duckweed or a clear ditch with aquatic plants on the bottom? One situation will never flow smoothly into the other, but can only change with a strong impulse from outside.
For this new work, Scheffer has revived a metaphor from that old work: he sees the different balances as different bowls on a mountainside. “A ball placed in such a bowl represents a balance that cannot simply be disturbed,” he explains. “If you tap the ball, sooner or later it will end up in the bottom of the same bowl again. Unless you really give it a very firm tap. Then the ball can shoot over the edge and end up higher or lower in the next bowl, or in a new equilibrium.”
According to Scheffer and colleagues, both the very general and the very special species in any species group are ‘caught like balls in bowls’. Only by very drastically shaking up the environment can they escape from it and subsequently become special or general. “And that has more to do with blunt probability calculation than with biological logic,” Scheffer adds. “In our opinion, there are no biological rules that dictate which species should become very successful. That is mainly luck.”
A dice game
Scheffer and colleagues addressed this issue in various ways. “The simplest way is to imagine it as a kind of dice game in a group of people, who all start with the same number of points. The total number of points always remains the same, just like the carrying capacity of nature. But the roll of the dice determines how much your points are multiplied or divided. If you keep this up for a while, you will see that the participants who start with a profit after one or two throws see their winnings increase very quickly and that the losers just as quickly end up with almost nothing. That may seem counter-intuitive, since it is a dice game. But as soon as profit is multiplied by what you already have, things can still go fast.”
Something similar occurs in nature, says Scheffer. “Profit in terms of biology means: more offspring. And if those descendants also share in the profits, things will go fast there too.”
According to Scheffer, the effects of the natural game of dice can be seen from the Amazon rainforest to our own intestines: an enormous dominance of just a few species, trees or bacteria respectively. “In the case of those trees, it is difficult to prove that process experimentally,” says Scheffer. “After all, it takes a very long time for a seed to form a new tree, which can itself form new seeds. This is different with bacteria. They have a generation time of hours instead of decades, where you see dominance of a few species occurring very quickly.”
A life insurance policy for the system
The fact that so few species are in charge in nature is certainly no reason to neglect the rest, the researchers say. The bulk of species is not useless, but serves as a kind of life insurance for the system if, for whatever reason, the few dominant species suddenly have a hard time. “If conditions change drastically, a few new ones always emerge from the dormant, gray mass of scarce species that can then take over the dominant role,” says Scheffer.
According to Scheffer, a professional group that has often experienced these laws personally are cheese makers. “Cheese makers who tried to work very efficiently and try to keep a limited mix of only the most dominant bacteria from a cheese culture in a jar were sooner or later disappointed. For example, due to an infection with bacteriophages, a type of virus, their culture sometimes collapsed and they suddenly found themselves empty-handed in the cheese factory. But when they kept a rich mix of both common and more special bacteria in a mother culture, they had a much more stable process.”
Thinking along the same lines, according to Scheffer you should certainly not conclude that the bulk of biodiversity is superfluous. “Certainly, a limited group of trees, birds, butterflies or bacteria are in charge numerically, but that certainly does not make the rest useless in the longer term.”
As he did earlier in his Spinoza work, Scheffer also extends the discovered laws beyond biology, for example to economics. “Stable equilibria and critical transitions also apply there. And there too you will find a small group of people who control the bulk of the resources. Contrary to what those people themselves usually claim, we believe this has nothing to do with their own merits or hard work. The Elon Musks and Jeff Bezossen of this world were mainly just lucky that the ball rolled their way at the right time. And unfortunately it is true that most people who are born for a dime do not simply become a quarter.”
Simulation of the dice game over 30 roundsImage Egbert van Nes, WUR
Rolling dice for dominance
To get an impression of how crucial a little early luck is for the course of evolution, the authors’ dice game can also be literally replayed. Players start with a pot of 100 points each and take turns rolling a die. The outcome is processed as follows:
1: you lose 80% of your pot
2: you lose 40% of your pot
3: you lose 20% of your pot
4: you win 20% of your pot
5: you win 40% of your pot
6: you win 80% of your pot
Scheffer and Van Nes kept the total number of points constant in this game, because in nature a population cannot grow indefinitely. This requires some extra calculations, whereby a deficit or surplus is added or subtracted pro rata from the players’ score after each round. In this example with 5 players, 500 points must always remain in the game. For this purpose, the score achieved in each round is divided by the new total and then multiplied by 500.
The graph below shows a game progression over thirty rounds, as generated by a computer. In this case it is player two who suddenly takes a huge lead over the rest after fifteen rounds. You see something similar happening in most simulations, says Van Nes. Although it can also take shorter or longer, sometimes even much longer. He regularly experiences that the winner changes after thousands of rounds due to bad luck.
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#Winners #nature #luck #wisdom