Competition for space in a structured landscape: The effect of seed limitation on coexistence under a tolerance-fecundity trade-off

Life-history trade-offs are important coexistence mechanisms in plant communities. In particular, a trade-off between seed quality and seed output may explain coexistence among species with a wide variety of seed sizes in heterogeneous landscapes with varying degrees of local stresses such as shade, drought and browsing. Under spatially implicit formulations of the tolerance-fecundity trade-off, species diversity is only bounded by community size and environmental heterogeneity. However, these formulations leave unanswered the question of how dispersal limitation, an important aspect of real communities, impacts the ability of the tolerance-fecundity trade-off to support coexistence. Here we show using stochastic cellular automata that coexistence under the tolerance-fecundity trade-off in a spatially structured landscape is strongly modulated by the interactions between the spatial scales of species dispersal and environmental variability. Specifically, coexistence in a patchy landscape is higher under short dispersal scales relative to the scale of environmental variation. These conditions allow species to segregate spatially and thus avoid interspecific competition. In addition to dispersal limitation, coexistence under the tolerance-fecundity trade-off is promoted by a sharp drop in tolerance as local environmental stress increases, a high overall seed output across species, local homogeneity in environmental conditions and weak age structure in fertility. In particular, coexistence is much higher when species have full tolerance to stress levels below a threshold and zero tolerance above that threshold, with nothing in between. This steep gradient contributes to coexistence because it maximizes niche separation between similar species. Synthesis. When plants compete for space, dispersal limitation and landscape spatial structure can conspire to boost coexistence and diversity. Our results show that the tolerance-fecundity trade-off is a strong niche differentiation mechanism when species experience a well-defined distinction between suitable and unsuitable environments, and supports higher diversity when dispersal scales are shorter than the scales of variation in the environment. We conclude that the trade-off is more likely to be a relevant coexistence mechanism in the context of large-scale environmental variation, such as topography, than small-scale variation, such as in litter cover and canopy shading.