Ecological filters constrain recruitment in four introduced annual plants in a California grassland
Spatial variation in biotic and abiotic filters can promote coexistence and limit competitive exclusion by allowing species to increase when rare on some parts of the landscape. Covariance between the environment and intraspecific competition leads to stronger competition between conspecifics in favorable locations than unfavorable locations. In this project, we quantified the strength of intraspecific competition experimentally by adding seeds at varying densities and evaluating the success of recruitment as a function of seed supply. Using four species, we incorporate abiotic and biotic filters into recruitment functions to ask whether spatial variability in soil water resources and grass cover creates variation in the relationship between seed supply and recruitment. We decompose recruitment limitation to evaluate the relative strength of resources and biotic factors as sources of mortality, and we ask whether the strength of density dependent limitation varies with abiotic and biotic filters.
Resources and biotic filters constrained recruitment for all species considered. The relative strengths of resources and biotic filters varied by species. High grass cover, associated with low species richness and high productivity, was either neutral or positive for recruitment in two grasses, and negative in two thistles. Soil water availability promoted recruitment for two of four species, and the timing at which soil moisture was important was also species specific. Consistent with spatial models of coexistence, the strength of density dependence varied with resources and competition. In previous work, recruitment has primarily been evaluated under static environmental conditions; a constraint that is unrealistic given that abiotic and biotic filters are spatially variable. Here, we show that recruitment varies spatially with resources and competition, and density dependent mortality is highest in locations that are most favorable for recruitment.