[1] One strength of this theory is that, unlike most coexistence mechanisms, the storage effect can be measured and quantified, with units of per-capita growth rate (offspring per adult per generation).
This variation can come from a large degree of factors, including resource availability, temperature, and predation levels.
[citation needed] The storage effect mixes three essential ingredients to assemble a community of competing species that fulfill the requirement.
[5] In statistics, such correlation means that there will be a non-zero covariance between the change of population density in response to the environment and that to the competition.
For example, plant species have different preferred levels of light and water availability, which affect their germination and physical growth rates.
[1] Marvelously, in nature organisms are often able to slow down the rate of population decline in a hostile environment by alleviating the impact of competition.
Under the temporal storage effect, it can be accomplished by the adults of a species having long life spans, which are relatively unaffected by environmental stressors.
For example, an adult tree is unlikely to be killed by a few weeks of drought or a single night of freezing temperatures, whereas a seedling may not survive these conditions.
Namely, when multiple generations do not overlap (such as Labord's chameleon) or when adults have a high mortality rate (such as many aquatic insects, or some populations of the Eastern Fence Lizard[9]), buffered growth does not occur.
After this decoupling, buffered population growth limits the impact of interspecific competition when a species is not favored by the environment.
[1] Moreover, thanks to the buffered population growth, the rare species is able to survive the bad years or patches by "storing" the gains from the good years/patches.
[1] Since the fluctuation in recruitment rate is an indicator of covariance between environment and competition, and since species-specific environmental response and buffered population growth can normally be assumed in nature, finding much stronger fluctuation in recruitment rates in rare and low-density species provides a strong indication that the storage effect is operating within a community.
The fitness of an individual, as well as expected growth rate, can be measured in terms of the average number of offspring it will leave during its lifetime.
For ease of calculation, standard parameters E(t) and C(t) are defined, such that Both E and C represent the effect of deviations in environmental response from equilibrium.
As stated above, for the storage effect to contribute to species coexistence, we must have buffered population growth (i.e. it must be the case that γ < 0).
The first term of the expression is covariance between environment and competition (Cov(E C)), scaled by a factor representing buffered population growth (γ).
This conclusion follows the general trend that the introduction of a generalist predator will often weaken other competition-based coexistence mechanisms, and which result in competitive exclusion.
The first empirical study that tested the requirements of the storage effect was done by Pake and Venable,[2] who looked at three desert annual plants.
This, combined with the buffered population growth that is a product of a long-lived seed bank, showed that a temporal storage effect was probably an important factor in mediating coexistence.
This study was also important, because it showed that variation in germination conditions could be a major factor promoting species coexistence.
She was unable to measure certain important parameters (such as the rate of egg predation), but found that her results were robust to a wide range of estimates.
[3] The first test of the spatial storage effect was done by Sears and Chesson [10] in the desert area east of Portal, Arizona.
Using a common neighbor-removal experiment, they examined whether coexistence between two annual plants, Erodium cicutarium and Phacelia popeii, was due to the spatial storage effect or resource partitioning.
The storage effect was quantified in terms of number of inflorescences (a proxy for fitness) instead of actual population growth rate.
This suggests that P. popeii is able to avoid strong interspecific competition in some good patches, and that this may be enough to compensate for losses in areas favorable to E.
Initial studies were of 12 species of trees coexisting in a tropical deciduous forest at the Chamela Biological Station in Jalisco, Mexico.
Since species-specific environmental response and buffered population growth can be naturally assumed, their finding strongly suggests that the storage effect operates in this tropical deciduous forest so as to maintain the coexistence between different tree species.
[17] Angert and colleagues demonstrated the temporal storage effect occurring in the desert annual plant community on Tumamoc Hill, Arizona.