Spatial variation in the environment and associated changes in species composition are ubiquitous features of ecological communities. Ecologists have long sought to understand how such joint variation in the abiotic and biotic environment shapes species populations and communities. This endeavor has taken on renewed importance as environmental change increasingly threatens to disrupt species interactions. However, characterizing the causes and consequences of species interactions across disparate environments remains a major empirical challenge. Here, I leverage an experimentally tractable California annual grassland system to study how spatial variation in the environment and competition shapes plant coexistence and distributions.
In Chapter 1, I ask how spatially variable competition drives mismatches between plant fitness and occurrence along environmental gradients. By experimentally quantifying the demography of eight annual plant species along edaphic gradients in a serpentine grassland, either with or without competitors, I demonstrate that competitors can modify species' demographic responses to environmental gradients. Crucially, observed occurrence patterns were often poorly related to these demographic responses. These findings caution against assuming that variation in occurrence implies variation in fitness, or vice versa.
In Chapter 2, I turn my attention to how spatial heterogeneity promotes species coexistence via the spatial storage effect. I studied 24 annual plant species in the grassland from Chapter 1 and found that they exhibit variable responses to spatial variation in the environment and competition that depend on their functional traits. Additionally, these trait-based demographic responses contributed to elevated competition at otherwise favorable sites. These results are consistent with the storage effect and demonstrate how functional traits can modulate coexistence in spatially variable environments.
In Chapter 3, I conducted a greenhouse experiment to characterize competition between an annual grass and forb under watering treatments emulating range-wide rainfall variation. I found that although per capita competition was strongest in arid conditions, community-wide competition was stronger in moister environments where fitness asymmetries were large. These contrasting results emphasize the importance of distinguishing between competition at the individual and community level. Together, this dissertation combines experiments with modern coexistence theory to offer an integrative account of plant coexistence and distributions in a spatially variable world.
