Storage effect

When species populations encounter a period of time when resources are limiting the ability for a species to sustain itself and recover in size to a stronger and more abundant population is the basis of the storage effect. Knowing how a particular species deals with resource limitations is important for conservation efforts dealing with aquatic and terrestrial species. Depending on a particular species' life cycle a population can recover from low numbers even though it has been reduced to low levels whereas other species may need large population size every year to maintain.

What is the storage effect?
The storage effect hypothesizes that recruitment fluctuations promote species coexistence in communities of long-lived organisms. In an environment where recruitment is occasionally low, a long life stage buffers competitors against exclusion in a way that is not possible in models with no overlapping generations. The idea of a “storage effect” attempts to explain that the temporal recruitment fluctuations among species can lead to stable coexistence of competitors. Coexistence will occur when there are constant or varying environments. Warner and Chesson explain that there are three essential conditions that would permit patterns of stable coexistence in systems where recruitment was space limited: (1) early vital rates and larval supply among species is variable across years; (2) adults survive over long periods of poor recruitments; and (3) episodic recruitments (high rates of settlement) are matched by generation time. The theory originally applied to two species that compete for a common limiting resource such as attachment sites or territories. Strict resource limitation means that one species increases at the expense of the other and competitive exclusion is favored. Exclusion can be prevented however if two conditions are met. One, environmental conditions must vary, resulting in fluctuating recruitment rates. This can allow low-density species a high potential for occasional relative increases in population size. Second, adults must be able to survive over periods of poor recruitment, such that the population declines only slowly during these periods. Under these conditions, a species is able to recover from low densities, and exclusion can be prevented.

A strong recruitment produces a supply of individuals that survives over a number of potential reproductive periods. This enables survival of species when recruitment is low. In the presence of overlapping generations and fluctuating recruitment rates, the average population growth rate is more strongly affected by the benefits of favorable periods than by the costs of unfavorable periods. This is referred to as the storage effect because strong recruitments are figuratively stored in the adult population, and are capable of contributing to reproduction when favorable conditions return.

Examples of the storage effect
In some organisms such as perennial plants, fish, and marine invertebrates, long-lived adults establish overlapping generations, and the competition is among juveniles. In other organisms such as annual plants, insects, zooplankton, and phytoplankton, competition exists among short-lived active individuals, and the long-lived stage is a dormant seed, egg, or cyst. In these systems, overlapping generations result from the repeated germination or hatching over a number of seasons of the seeds or eggs produced in any single year.

Split cohorts (seasonal pulses of eggs and larvae) and contingent behavior dispersive and retentive patterns in early dispersal) are two life history behaviors that contribute to the storage effect in a study of marine fishes. Many factors contribute to intra-population spawning modalities, size and age structure of adults play an important role. For diadromous fishes, time of spawning and early life history energetic thresholds is hypothesized to lead to alternative life cycles. Therefore, time of spawning may lead to the storage effect by protecting against spatial variance in early vital rates.

A specific example of the storage effect is the recovery of the Chesapeake Bay striped bass, Morone saxatalis, a large temperate sea bass. Due to depression of spawning stock abundance in the early 1980s, strong fishing controls were implemented during the late 1980s. Consequently, a population boom occurred during the 1990s, stimulated by strong year classes that predated 1970. Demographic analysis showed that egg production stored in spawners greater than 15 years in age was a key element in their rapid recovery.

The long lifespans of tree species relative to the scale of temporal variability may provide a buffer or storage effect that promotes species diversity by allowing tree species to persist through periods of low recruitment, when years of high fecundity or survivorship do not coincide with periods of gap formation, and thus maintain a positive long-term population growth rate.

A study performed on Sonoran Desert annuals by Pake & Venable demonstrated the principle of overlapping generations from repeated germination of seeds over a number of seasons occurring from the production of seeds within a single year. Environmental fluctuations and a between-year seed bank mediate the coexistence of competing species. Desert annuals require high water availability and have adaptations that allow them to time their germination to the most favorable, and often more predictable, part of the year. Emergence during opportune environmental conditions such as water, light, and temperature demonstrate the ability of seeds to store themselves until environmental conditions are right. The length of time seeds can stay viable in the soil and the fraction of seeds that germinate determine how well a species can persist in an arid environment.