Decoupling the fundamental and realized components of niche conservatism

Background/Question/Methods

Recent developments in species distribution modeling, GIS, and phylogenetic methods have generated a renewed interest in examining patterns of niche evolution among closely related taxa. Collectively, these approaches quantify the environmental correlates of species ranges using occurrence data, and evaluate how these correlates vary among relatives. An ongoing debate challenges the degree to which these approaches characterize species niches, largely because they confound abiotic (e.g., climate) and biotic (e.g., competition) drivers of distribution patterns. We addressed this controversy by experimentally manipulating a niche axis that appears to be phylogenetically conserved in Lasthenia (Asteraceae), a genus of herbaceous plants that occupy a variety of osmotically stressful habitats, including vernal pool wetlands in California (USA). Previous analyses of Lasthenia distribution patterns found that the microelevation position (depth) at which populations occur in vernal pools is relatively conserved in the genus. Here, we conducted a controlled growth chamber experiment to isolate the role of hydrological responses in driving this pattern. We estimated the response curves of 13 Lasthenia species – 7 of which occur in vernal pool habitats – across 5 hydrological treatments to test the hypothesis that conservatism of distribution patterns within vernal pools is driven by similar hydrological tolerances among taxa.

Results/Conclusions

Although different Lasthenia species are associated with very different hydrological conditions in their natural habitats, we observed strikingly similar hydrological response curves among all 13 taxa in our controlled experiment. While the overall effects of hydrological treatment significantly influenced above-ground biomass, species did not vary in their mean responses to those treatments. All species – even those restricted to hydrologically stressful habitats in the field, such as seasonally flooded wetlands or drought-prone grasslands – exhibited optimum growth in the intermediate treatment. Species that occupy deeper positions within vernal pools tended to have broader hydrological tolerance, indicating that species restricted to vernal pools have adapted to the highly variable flooding conditions that characterize these wetlands. Finally, preliminary comparative analyses suggest that the fundamental hydrological niche in Lasthenia is actually more phylogenetically conserved than their distributions across flooding gradients.