The competition‐tolerance tradeoff as a cause of species turnover along environmental gradients: the role of population differentiation

Abstract

One of the latest advances in studying species distribution proposes that the universal occurrence of a tradeoff between competition and environmental tolerance explains the segregated distribution of closely related species along environmental gradients. Given that this proposition applies to species that are closely related, here we moved one step down and explored the possibility that a tradeoff between competition and tolerance also controls the distribution of populations within species that occupy different envi- ronments. We tested our hypothesis with populations of the annual herb Centaurea solstitialis that grow at low versus high elevation (hereafter, Mediterranean versus steppe populations, respectively) in Anatolia. We characterized climatic conditions in which these populations grow and conducted a common garden, where we measured traits predicted to affect environmental tolerance, and an intraspecific competition experi- ment, that included different densities, with Mediterranean and steppe populations. We found that Mediterranean populations experience climatic conditions that appear more favorable for early growth than those experienced by steppe populations. Specifically, conditions are warmer and wetter upon this species emergence in the Mediterranean than steppe region. In addition, Mediterranean populations exhibited trait attributes typically associated with benign environments, whereas steppe populations displayed attributes thought to cope with harsh environments. These attributes included smaller seeds, lower dormancy, faster emergence and seedling growth, and higher specific leaf area in Mediterranean than steppe populations. Finally, individuals from Mediterranean populations were stronger competitors than those from steppe populations. Together, these findings suggest that adaptation to harsh environments comes at the expense of competitive ability in our study populations, and that this compromise may control their separated distribution along elevational/climatic gradients. Population differentia- tion may thus precede the later observed turnover of related species along environmen- tal gradients proposed to be caused by the competition–tolerance tradeoff.