  The storage effect is a general explanation for coexistence in a variable
environment. The generality of the storage effect is both a strength - it can
be quantified in many systems - and a challenge - there is not a clear
relationship between the abstract conditions for storage effect and species'
life-history traits (e.g., dormancy, stage-structure, non-overlapping
generations), thus precluding a simple ecological interpretation of the storage
effect. Our goal here is to provide a clearer understanding of the conditions
for the storage effect as a step towards a better general explanation for
coexistence in a variable environment. Our approach focuses on dividing one of
the key conditions for the storage effect, covariance between environment and
competition, into two pieces, namely that there must be a causal relationship
between environment and competition, and that the effects of the environment do
not change too quickly. This finer-grained definition can explain a number of
previous results, including 1) that the storage effect promotes annual plant
coexistence when the germination rate fluctuates, but not when the seed yield
fluctuates, 2) that the storage effect is more likely to be induced by resource
competition than apparent competition, and 3) that the spatial storage effect
is more probable than the temporal storage effect. Additionally, our expanded
definition suggests two novel mechanisms by which the temporal storage effect
can arise: transgenerational plasticity, and causal chains of environmental
variables. These mechanisms produce coexistence via the storage effect without
any need for stage structure or a temporally autocorrelated environment.
