Quantifying the role of environmental variation in individual success

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Robin Snyder, Biology, Case Western Reserve University, Cleveland, OH and Stephen Ellner, Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, NY

Background/Question/Methods

How much of an individual's success --- lifetime reproductive output,
lifespan, probability of reaching the canopy, etc. --- comes from
having good traits vs lucky environmental conditions vs lucky
demographic transitions? Previous work has shown that in a constant
environment, success is determined more by demographic luck than by
traits, but it has been unclear how much influence environmental
variation has on individual success. In this talk we present and
apply new methods that let us fully quantify the contributions of
traits, environmental variation, and demographic transitions to the variance of individual success among individuals in a
population. Furthermore, we can see how these contributions add up over age, size, or other
stages, thereby identifying critical windows when surviving/growing or
getting a favorable environment has an outsize influence.
Results/Conclusions

We apply our methods to a size-structured model of two tropical trees
(Simarouba amara and Minquartia guianensis) in a variable light
environment. Partitioning variance in the number of years in the
canopy, a proxy for lifetime reproductive success, we find that
environmental luck peaks very early in life, which we suspect is about
needing some high light years early on to escape the high mortality
seedling stages. The more light sensitive species has an additional
peak for trees that are large but still sub-canopy --- likely because
high light can boost an individual into the canopy whereas low light
can hold it back. However, demographic luck is far more important
than environmental luck for both species. Like environmental luck,
demographic luck has a very early peak, but its secondary peak occurs
as or after individuals enter the canopy and is present for
both species. Previous work suggests that the first peak is about
surviving long enough to reproduce and the later peak is about
transitions to states with high expected success.