There has been strong interest in how intraspecific variation ("trait
variation") affects coexistence, stability, productivity, and other
aspects of ecological dynamics. However even when all individuals are
identical, there is high variability in outcomes: some individuals are
lucky while others are not. Trait variation is therefore only
important if it adds substantially to the variability produced by
simple luck. Some empirical case studies suggest that in fact, the
effects of luck will dominate the effects of trait variation. Is this
inevitable? We ask when trait variation has a substantial effect on
the variability of lifetime reproductive success (LRS), using two
approaches. We partition the variation in LRS into contributions from
luck and trait variation, and we ask what can be inferred about an
individual's traits, and with what certainty, given an observed LRS.
We apply our methods to a simple two-stage life cycle model and to
empirical case studies, a matrix model for the sea gull Ryssa
tridactyla and IPMs for the shrub Artemisia tripartita and the
perennial grass Pseudoroegneria spicata.
Results/Conclusions
Luck usually dominates the variance of LRS. Trait variation only has
a substantial effect if opportunities for luck are suppressed ---
e.g., if there is little variation in annual number of offspring or
lifetime number of reproductive bouts. Luck also tends to obscure what
we can infer about an individual’s traits from its LRS. For our sea
gull case study, having above-average traits is necessary but not
sufficient to have exceptional (90th percentile) LRS: individuals have
to be lucky as well as special. For the shrub and grass models, the
"trait" is competition at birth. Having below-average competition at
birth is neither necessary nor sufficient to have exceptional
LRS. Individuals with especially low LRS probably had above-average
competition at birth, but this is the only signal we find. In all of
these cases, the dominance of luck is likely due to the variability of
adult lifespan (and hence of the number of reproductive bouts). LRS is
a measure of individual success within a single generation; it is
unclear whether traits may have larger cumulative importance over many
generations as the effects of luck average out, and how this depends
on the degree of trait inheritance.