Niche and neutral controls over the coexistence of serpentine annual plants

Coexistence results when niche mechanisms, those that cause species to limit themselves more than others, overcome the fitness inequality among species.  Yet the degree to which coexistence in natural systems results from strong niche mechanisms overcoming large fitness inequality versus weak niche mechanisms operating on species of similar fitness, is completely unknown.  These alternatives can be evaluated by examining how species per capita growth rates change as their relative frequencies within communities change, an approach we took with serpentine annual plant communities in California.  Specifically, we seeded experimental communities in the field such that each of ten species ranged in relative frequency from 1% to 100% of the community.  Results showed species-specific responses to relative frequency.  Some species, such as Micropus californicus showed very weak niche stabilization.  This result emerged because Micropus germinated a very small fraction of its seeds, exposing little of its population to frequency-dependent processes.  Other species, by contrast, showed declining per capita growth as their relative frequency increased, the signature of stabilizing, niche processes.  Our work suggests that both niche and neutral processes contribute to coexistence in serpentine annual plant communities.