What controls spatial niches? Testing theory with tarweeds

Traditional spatial niche theory holds that species coexist because they are outcompeted in or cannot tolerate the abiotic conditions in one another’s preferred locations.. Newer theories by Tilman (stochastic niches),  Chesson (spatial storage effect), and Hubbell (neutral theory) propose that propagule limitation is key to coexistence.  We tested these contrasting theories with three late-flowering annual plants whose distributions form sharp boundaries along continuous soil gradients in a California grassland. We asked if Holocarpha virgata (HV) is excluded from Calycadenia pauciflora (CP), and Hemizonia congesta (HC) habitats by pure propagule limitation; by competition at the seed, seedling, or adult stage; or by unsuitable abiotic conditions. Our design used four disturbance treatments in CP and HC habitats (none, fall clearing, fall + spring clearing, and fall + spring + summer clearing) and two seeding treatments (none, and enough HV seeds to “saturate” the site). We also established natural controls in HV habitat. Seedling density was nearly twice as high in CP as HC habitats, but clearing had no effect on seedling density in either habitat. Plant height and flower number were significantly higher in HC than in CP habitats, but only in plots cleared in the fall + spring or the fall + spring + summer. Adult density was higher in HC than CV habitat and was unaffected by clearing. Thus, habitat had a significant effect on seedling and adult density and habitat + competition had a significant effect on growth and fecundity. Compared with the controls, all of our experimental plots yielded high seedling densities but low survival, growth, and seed production. So far, plant densities in experimental plots in the second year of the experiment are very low in all treatments. These preliminary results support traditional niche partitioning due to abiotic unsuitability and competition.