Theoretical Population Biology 92 (2014) 36–50 Contents lists available at ScienceDirect Theoretical Population Biology journal homepage: www.elsevier.com/locate/tpb Variation in moisture duration as a driver of coexistence by the storage effect in desert annual plants Galen Holt ∗ , Peter Chesson Department of Ecology and Evolutionary Biology, University of Arizona, 1041 E Lowell St. Tucson, AZ 85721, United States article info Article history: Received 7 May 2013 Available online 14 November 2013 Keywords: Species coexistence Storage effect Environmental variation Germination Desert annual plants Soil moisture abstract Temporal environmental variation is a leading hypothesis for the coexistence of desert annual plants. Environmental variation is hypothesized to cause species-specific patterns of variation in germination, which then generates the storage effect coexistence mechanism. However, it has never been shown how sufficient species differences in germination patterns for multispecies coexistence can arise from a shared fluctuating environment. Here we show that nonlinear germination responses to a single fluctuating physical environmental factor can lead to sufficient differences between species in germination pattern for the storage effect to yield coexistence of multiple species. We derive these nonlinear germination responses from experimental data on the effects of varying soil moisture duration. Although these nonlinearities lead to strong species asymmetries in germination patterns, the relative nonlinearity coexistence mechanism is minor compared with the storage effect. However, these asymmetries mean that the storage effect can be negative for some species, which then only persist in the face of interspecific competition through average fitness advantages. This work shows how a low dimensional physical environment can nevertheless stabilize multispecies coexistence when the species have different nonlinear responses to common conditions, as supported by our experimental data. © 2013 Elsevier Inc. All rights reserved. 1. Introduction Annual plant communities have figured prominently in both theoretical and empirical studies of the contribution of envi- ronmental variation to the maintenance of species diversity. Theoretical work illustrates how environmentally sensitive ger- mination generates the storage effect coexistence mechanism in a temporally variable environment (Chesson, 1994, 2003; Ches- son et al., 2004). This theory has primarily focused on the coexis- tence of species competing for resources (e.g. Chesson, 1994, 2003), although more recent work has extended these results to the storage effect acting through apparent competition (Kuang and Chesson, 2008; Chesson and Kuang, 2010). Several field studies have provided empirical support for the storage effect in annual plants arising from variable germination, variable growth, and competition between species (Facelli et al., 2005; Sears and Ches- son, 2007; Angert et al., 2009a). Theoretical models of coexistence in a temporally variable environment normally do not model the physical environment directly, but instead model fluctuations in population parameters, such as germination fraction, which are assumed to be driven ∗ Corresponding author. E-mail addresses: gholt@email.arizona.edu (G. Holt), pchesson@u.arizona.edu (P. Chesson). by fluctuations in the physical environment (Chesson et al., 2004; Kuang and Chesson, 2009). The probability distributions of fluctuations in these parameters (environmental responses) are the inputs to the models. These studies have provided general understanding of the ability of environmental fluctuations to promote coexistence in terms of the statistical properties of the environmentally dependent parameters (Chesson, 1994). In particular, the strength of the storage effect is determined by the variances and correlations of these fluctuating environmental responses (Chesson, 1994, 2003; Angert et al., 2009b). However, a key problem is determining what these variances and correlations are for input to the models. Environmentally dependent germination rates provide the clearest example of fluctuating environmental responses in annual plants, although reproductive output also depends on environmental conditions during growth in ways that can promote the storage effect (Pake and Venable, 1995; Angert et al., 2009a). Although variances and correlations of germination fractions have been measured from field data (Angert et al., 2009a; Chesson et al., 2014) it is difficult to obtain long enough sequences of observations for much precision. In the absence of good estimates of species differences in these respects, the phenomenological germination fractions used in most theoretical studies assume a great deal of symmetry between species in the variances and correlations of germination fractions (Chesson et al., 2004; Kuang and Chesson, 2009). However, Angert et al. (2009a) provide good 0040-5809/$ – see front matter © 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.tpb.2013.10.007