  The transition to turbulence exhibits remarkable spatio-temporal behavior
that continues to defy detailed understanding. Near the onset to turbulence in
pipes, transient turbulent regions decay either directly or, at higher Reynolds
numbers through splitting, with characteristic time-scales that exhibit a
super-exponential dependence on Reynolds number. Here we report numerical
simulations of transitional pipe flow, showing that a zonal flow emerges at
large scales, activated by anisotropic turbulent fluctuations; in turn, the
zonal flow suppresses the small-scale turbulence leading to stochastic
predator-prey dynamics. We show that this "ecological" model of transitional
turbulence reproduces the super-exponential lifetime statistics and
phenomenology of pipe flow experiments. Our work demonstrates that a fluid on
the edge of turbulence is mathematically analogous to an ecosystem on the edge
of extinction, and provides an unbroken link between the equations of fluid
dynamics and the directed percolation universality class.
