Why are there so many kinds of phytoplankton?

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There is a long-standing conundrum in ecology called the plankton paradox. Famous by conservationist George Evelyn Hutchinson in 1961, The Paradox explores how strange it is that there are thousands of species of phytoplankton in the upper parts of the ocean. The first few feet of water is essentially a well-mixed soup, which means that all of these species of phytoplankton depend on the same nutrients. The competitive exclusion theory says that one of these species should be a bit stronger and should outperform the others. But none did. Why?

Hutchinson published the paradox at the height of the Cold War, when the air was charged with debates about the values ​​of competition and the sharing of resources. Ecological thinking was itself dominated by the idea that competition caused some species to prosper and others to disappear. But Hutchinson saw this thinking as an oversimplification, and he cited phytoplankton as an example of how there must be additional forces shaping biodiversity.

Over the past decades, ecologists have suggested many explanations for the persistence of several phytoplankton species, including the effects of rapid environmental change, the existence of co-dependencies between species, the uneven distribution of phytoplankton species, and the fact that some phytoplankton release toxins which can give them an edge over the competition. But a new study by Oregon State University ecologist Michael Behrenfeld and colleagues seeks to resolve the dilemma by taking a different perspective: that of plankton.

The phytoplankton are so small, and the distances between them so vast – from their perspective – that it’s likely the phytoplankton don’t compete at all, says Behrenfeld. If you imagine a phytoplankton is about the size of a tree’s root ball, he says, the next closest phytoplankton would be miles away.

The small size of a phytoplankton also means that it feels water as a thick substance, perhaps similar to what honey feels to us. When an individual phytoplankton moves, a layer of water called the boundary layer moves with it. This means that phytoplankton spend most of their time firmly separated from each other.

“When you think of it that way, it’s like, well, how can phytoplankton that is physically distant actually compete directly with each other? Said Behrenfeld.

Inspired by this idea, Behrenfeld decided to model the biodiversity of phytoplankton using an approach called neutral theory. Rather than modeling ecosystem dynamics as fueled by competition, this framework says that a community only loses species when, by chance, too many members die at the same time, and only gains species when they do. immigrate or when genetic mutations create them again.

For about a thimble of water, the neutral theory worked very well: The number of species that Behrenfeld’s model predicts to be present matches what scientists have observed in surveys at sea. enlarged the model to represent a larger body of water, a crack began to form.

“We have to remember that the water is mixed continuously,” says Behrenfeld. In a world dictated by neutral theory, phytoplankton would have to die at an unreasonable rate to make room for all the new plankton from other parts of the ocean. Instead of explaining why there is more than one species of phytoplankton, Behrenfeld’s neutral theoretical model predicted that there should in fact be an astronomical number of species of phytoplankton.

Behrenfeld and his colleagues therefore envisioned other forces that could limit the number of plankton species, even in an uncompetitive utopia, such as the attractiveness of phytoplankton to predators, the speed at which they reproduce, and how whose asexual reproduction affects genetic variation within a species. Their work paid off: Adding these elements to their model gave them roughly the same number of species that scientists have observed in the ocean.

Nick Record, a computer ecologist at the Bigelow Laboratory for Ocean Sciences in Maine, says Behrenfeld’s results show how the constant churning of the ocean is forcing scientists to find new ways of thinking about relationships between species. “Marine systems are really different” from those on earth, he says. “And they behave in these very different ways.”

Yet Record takes a different view of the plankton paradox. “It’s not really a paradox to resolve,” Record says. “It’s part of a story.

Rather than assuming that some solutions are good while others are wrong, Record believes that all of the proposed solutions to the paradox point to a greater truth about marine ecosystems – that they are complex enough that conservationists never can. find a unique model. to describe how they work.

Perhaps the next 60 years will see as many proposed solutions to the paradox as the last. And maybe that’s exactly how it should be when it comes to a good paradox.

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