Predator and prey communities exhibit a common but unexpected organizational structure.
(Amaury Laporte)

In nature, the relationship between predators and their prey seems like it should be simple: The more prey that’s available to be eaten, the more predators there should be to eat them.

If a prey population doubles, for instance, we would logically expect its predators to double too. But a new study, published Thursday in the journal Science, turns this idea on its head with a strange discovery: There aren’t as many predators in the world as we expect there to be. And scientists aren’t sure why.

By conducting an analysis of more than a thousand studies worldwide, researchers found a common theme in just about every ecosystem across the globe: Predators don’t increase in numbers at the same rate as their prey. In fact, the faster you add prey to an ecosystem, the slower predators’ numbers grow.

“When you double your prey, you also increase your predators, but not to the same extent,” says Ian Hatton, a biologist and the study’s lead author. “Instead they grow at a much diminished rate in comparison to prey.” This was true for large carnivores on the African savanna all the way down to the tiniest microbe-munching fish in the ocean.

Even more intriguing, the researchers noticed that the ratio of predators to prey in all of these ecosystems could be predicted by the same mathematical function — in other words, the way predator and prey numbers relate to each other is the same for different species all over the world.

“That’s what was very surprising to us, to see this same pattern come up over and over,” Hatton says. But what’s actually driving the pattern remains something of a mystery.

Hatton and his colleagues suspect that different aspects of different ecosystems may drive the predator-prey ratio: For example, Hatton says, competition for space might be a major factor controlling animal populations, but changes in the nutrients used and produced by plankton might have more of an effect on some marine ecosystems.

The thing that’s puzzling is that the same mathematical function can be used to predict all of these ecosystems’ responses. And that’s not all: In a strange twist, the researchers observed that the same function can also be used to predict several other natural processes as well.

One of these is the reproduction rates of prey species. If you remove predators from an ecosystem, prey populations start to increase, since there’s nothing eating them. But there’s a catch: As their populations continue to grow, they reproduce at lower and lower rates — in other words, they continue to increase their numbers, but more and more slowly. And their growth rate can be predicted by the same mathematical function used to predict the way predators increase in response to their prey.

Even more fascinating is that the same function applies to certain processes in individual organisms’ bodies. One phenomenon observed consistently in nature is that smaller animals, like mice, tend to be faster, have higher metabolisms, live shorter lives and reproduce at higher rates, while large animals, like elephants, are slower in all aspects. So as size increases, the rate at which bodily functions are performed changes. And the pattern in these changes is governed by — you guessed it — that same mathematical function.  

The recurrence of this function in many levels of the natural world indicates “that there might some kind of process that exists at multiple levels of organization,” Hatton says. “The cell, the tissue, the body, the community: Those are all levels of organization in ecology-speak. I think that this suggests that there could be processes that sort of recur, recapitulate, across different levels.”  

The next step would be to develop a theory that explains this pattern at all the different levels on which it occurs. But until that happens, just observing that the pattern exists might be able to help scientists make some practical decisions when it comes to wildlife conservation.

“What we have is a baseline,” Hatton says. “We know that if there’s this many prey, we should be able to explain how many predators there are.” Conservationists could use these predictions to look at predator populations and observe whether there are fewer individuals than the formula would suggest there should be, which might indicate that an ecosystem is in trouble.

In the meantime, for researchers who like a good puzzle, the paper provides another mystery to chew on — one that, once unlocked, could reveal many secrets about how the natural world works.

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