Published online 15 December 2000 | Nature | doi:10.1038/news001221-2

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Still waters run species out

For water-borne life, turbulence can make the difference between life and death. .

Stirring things up allows the disadvantaged to survive in watery ecosystems. Species that might otherwise be eradicated by those more advantaged can beat the odds, thanks to turbulence, researchers report in the Proceedings of the National Academy of Sciences USA1. This effect might even explain how a diversity of primitive replicating molecules arose on the early Earth, providing the variety that allowed the evolution of more complex life.

Intuitively, one might expect the opposite. If one species of, say, aquatic bacteria or plankton competes more successfully than another for the available food resources, stirring the water might be expected to mix the species more and ensure that less successful species do not escape domination.

But it is not that simple, áron Péntek of the Scripps Institution of Oceanography in La Jolla, California, and co-workers now show. The turbulence that ensues when a river flows past a rock or when an ocean current circumvents an island does not guarantee good mixing. It can draw the flow into fine filaments. Without mixing with one another, these filaments may weave complex webs where surprising things can happen.

The researchers conducted computer simulations of a model ecosystem in which two species, A and B, compete for the same food or nutrient supply, carried in the water. When A or B consumes the food, they multiply; but they also die off at a steady rate. In this model, A is more successful because it multiplies faster than B.

Péntek's group added 'droplets' of A and B to the food-permeated water as it flowed past a turbulence-inducing obstacle. Species A initially increased faster than B. But when the flow drew out into thin filaments, the weaker species gained an advantage.

Why? Because the more abundant species passed out of the elongated, convoluted filaments more readily, just as heat is lost more quickly from a segmented radiator than from a solid slab of the same amount of metal. This gave the less abundant species, B, more chance to replicate within the filament.

Although they were convoluted, the filaments remained separate in the wake of the obstacle rather than mixing. So a filament rich in B could become isolated from one rich in A, and hence the weaker species could continue to dominate some filaments. Thus, both A and B survived, even though in still waters A would rapidly push B to extinction.

The researchers suggest that this effect probably operates in the oceans, enabling various species of phytoplankton (plant-like microorganisms) to coexist in diverse ecosystems, where otherwise just one type would grab all the nutrients in the water. 

  • References

    1. Károlyi,G., Péntek, Á., Scheuring, I., Tél, T. & Toroczkai, Z. Chaotic flow: the physics of species coexistence. Proceedings of the National Academy of Sciences USA 97, 13661 - 13665 2000. | Article |