Selective pressure for increased demographic variability in a perennial plant community

Background/Question/Methods

Because variation in year-to-year population growth depresses long-run
growth, single-species life history theory has traditionally assumed
that there will be selective pressure for organisms to minimize
variation in the parameters that most influence population growth (the
demographic buffering hypothesis).  However, when growth is a
nonlinear function of demographic parameters, nonlinear averaging can
allow variation to increase mean population growth, potentially
producing pressure to increase demographic variability.  Furthermore,
in a multi-species context, demographic parameters can covary in a way
that allows species to reduce competition with each other (e.g. a
storage effect), producing further pressure to increase variability.
The question of which pressures dominate in a given situation is
important not only because of the implications for life history theory
but because fluctuation-dependent mechanisms such as the storage
effect are thought to enable coexistence in a number of systems.  Here
we estimate selective pressure for increased variability in growth,
recruitment, and survival in four co-occurring perennials using a
multi-species integral projection model fit to a 22 year data set.

Results/Conclusions

There is weak selective pressure for increased variability in
recruitment and for decreased variability in growth and survival.  All
of these effects are due to nonlinear averaging: while the presence of
competitors changes the intensity of the nonlinearity, the
contributions of mechanisms that rely on variable competition, such as
the storage effect, are negligible.  Furthermore, a storage effect
seems unlikely to evolve.  Even if selection were able to produce
increased recruitment variability, simulations show that this would
not be enough to create a storage effect.