Where must niches differ, if differ they must? Spatial scaling of climatic, topographic, and soil control of distribution and performance in closely related annual plants

Background/Question/Methods

Whether closely related species can coexist is expected to depend on their evolutionary divergence in niches, relative to their evolutionary divergence in competitive ability. Also critical is the spatial scale at which niche differences appear. Even if close relatives exhibit strongly conserved environmental responses at large geographic scales, they may be able to coexist if their niches differ sufficiently at smaller (i.e., within-community) scales where neighborhood competition and population processes play out. To characterize the spatial scales at which niche differences appear between close relatives, we carried out hierarchical spatial analysis of environmental variation, distribution, and performance in the California-endemic annual plants Clarkia xantiana ssp. xantiana and C. speciosa ssp. polyantha, which have broadly overlapping geographic ranges and sometimes co-occur on smaller scales.

Results/Conclusions

At the scale of their entire ranges, the distributions of the two species are determined very similarly by variation in climate but differ somewhat in their associations with topography and soils, C. speciosa ssp. polyantha’s presence being associated with shallower slopes, more southerly aspects, and a broader range of substrates. More distinct species-presence associations with topography and substrate appeared at the scale of hundreds of meters, a pattern mirrored by species differences in relationships between abundance, reproductive performance, topography, and soil texture at scales of meters to tens of meters. At the neighborhood scale (centimeters to decimeters), however, individual distribution and performance (water status and biomass) and were spatially structured but not detectably associated with environmental variation. We hypothesize that species differences in plant-soil relations across many scales underlie some degree of spatial separation and allow species coexistence, despite strong similarity in climatic niche, while stochastic processes play a larger role in individual distribution at the smallest scales.