Contrasting effects of competition on plant fitness and distributions along environmental gradients

Abstract

O_LISpecies distributions have long been understood to depend on the complex interplay between the abiotic environment and biotic interactions. Empirical work in ecological communities has increasingly revealed how competition can mediate species demographic responses to environmental variation, but understanding how the demographic consequences of spatially variable competition manifest in observed distribution patterns remains an important challenge. C_LIO_LIHere, we describe a conceptual framework for characterizing competition-driven mismatches between responses of fitness and occurrence to environmental gradients. We then explore these mismatches for eight annual plant species in an edaphically variable California grassland landscape. C_LIO_LIWe experimentally quantified how species demographic rates (germination rate, fecundity) and fitness respond to spatial variation in the soil environment, either in the presence or absence of naturally occurring neighbors. We also surveyed species occurrence across the study landscape. Combining these demographic and occurrence data, we asked whether observed occurrence patterns are congruent with responses of intrinsic fitness (i.e., fitness in the absence of competitors) to soil gradients. C_LIO_LIWe found that competition altered responses of fitness to the primary soil gradient (soil texture) for many (4/8) species. In turn, occurrence patterns were often poorly or even inversely related to responses of intrinsic fitness to this environmental axis. In contrast, we found that competition had relatively little effect on responses of fitness and occurrence to a secondary soil gradient (soil Ca:Mg). C_LIO_LISynthesis. We demonstrate that spatially variable competition can contribute to mismatched responses of fitness and occurrence to environmental variation. Importantly, these quantitative mismatches depend on the species and environmental gradient in question. Our results caution against assuming that variation in occurrence implies variation in intrinsic fitness (or vice versa) without first disentangling how the abiotic environment and competition impact the demographic processes that underlie species distributions. C_LI