Deterministic niche processes are responsible for the patchy distribution of the native annual Lupinus nanus in California grasslands

Propagule limitation is assumed to be a key population constraint if a species’ spatial distribution or abundance increases after propagule addition. The “niche opportunity" concept develops the role of propagule limitation further, by stating that species can expand their distributions if propagules arrive at the same time as disturbances that reduce the effects of competition on recruitment. I tested the niche opportunity hypothesis through an experiment that quantified the influences of propagule availability, competition, and abiotic constraints on the local distribution of Lupinus nanus.  This native annual legume grows in dense patches within California grasslands, and has heavy seeds with no dispersal mechanisms, suggesting short range dispersal.  I added seeds to competitor removal and control plots inside and outside of dense patches of Lupinus across two years that differed sharply in weather.  In the wet year reproductive fitness of seeded individuals outside of patches was >>1.0, comparable to fitness inside patches, but it was low (<1.0) when both years were considered.  Fitness was not limited by competition outside of patches in either year, rejecting the possibility that niche opportunity limits this species at this scale.  Instead, Lupinus fitness and density varied between the study years, and with site productivity, phosphorus, soil moisture. My results demonstrate strong effects of abiotic niche constraints on spatial distribution and fitness, and no consistent effects of propagule limitation or competition.  These findings suggest that although species may exhibit propagule limitation in a subset of years and locations, ultimately distributions can based on a deterministic niche.