PT  - JOURNAL ARTICLE
AU  - Sebastian J. Schreiber
AU  - Masato Yamamichi
AU  - Sharon Y. Strauss
TI  - When rarity has costs: coexistence under positive frequency-dependence and environmental stochasticity
AID  - 10.1101/161919
DP  - 2017 Jan 01
TA  - bioRxiv
PG  - 161919
4099  - http://biorxiv.org/content/early/2017/07/11/161919.short
4100  - http://biorxiv.org/content/early/2017/07/11/161919.full
AB  - Stable coexistence relies on negative frequency-dependence, in which rarer species invading a patch benefit from a lack of conspecific competition experienced by residents. In nature, however, rarity can have costs, resulting in positive frequency-dependence particularly when species are rare. Many processes can cause positive frequency-dependence, including a lack of mates, mutualist interactions, and reproductive interference. Reproductive interference (RI), fitness reduction due to interspecific matings, is widespread in plants and animals with sexual reproduction. Because species that are rare in the community can be overwhelmed by mating attempts from common species, RI results in positive frequency-dependence and possible extinction of species when they become rare. Understanding the role of RI and positive frequency-dependence is particularly important in systems experiencing environmental fluctuations. These fluctuations can drive species to low frequencies where they are then vulnerable to costs of rarity and positive frequency-dependence. Here we analyze deterministic and stochastic mathematical models of two species interacting through both RI and resource competition. Individual fitness in these models decomposes into two components, mating success and reproductive potential. Mating success always exhibits positive frequency-dependence. Consistent with classical coexistence theory, reproductive potential exhibits negative frequency-dependence when individuals experience greater intraspecific competition than interspecific competition. In the absence of environmental fluctuations, our analysis reveals that (1) a synergistic effect of RI and niche overlap that hastens exclusion, (2) trade-offs between susceptibility to RI and reproductive potential facilitate coexistence, and (3) coexistence, when it occurs, requires that neither species is initially rare. In the presence of environmental fluctuations, our analysis highlights that (1) environmental fluctuations are likely to drive one of the species extinct, and (2) this risk of species loss is marginalized with sufficiently positively correlated demographic responses of the two species to the environmental fluctuations. This dynamic works in opposition to another coexistence mechanism, the storage effect, which gets weaker as species exhibit more similar demographic responses to environmental fluctuations. These results highlight the need to develop a theory of coexistence accounting for positive frequency-dependent interactions and environmental fluctuations.