Plant functional traits and the multidimensional nature of species coexistence

Abstract

Understanding the processes maintaining species diversity is a central problem in ecology, with implications for the conservation and management of ecosystems. Although biologists often assume that trait differences between competitors promote diversity, empirical evidence connecting functional traits to the niche differences that stabilize species coexistence is rare. Obtaining such evidence is critical because traits also underlie the average fitness differences driving competitive exclusion, and this complicates efforts to infer community dynamics from phenotypic patterns. We coupled field-parameterized mathematical models of competition between 102 pairs of annual plants with detailed sampling of leaf, seed, root, and whole-plant functional traits to relate phenotypic differences to stabilizing niche and average fitness differences. Single functional traits were often well correlated with average fitness differences between species, indicating that competitive dominance was associated with late phenology, deep rooting, and several other traits. In contrast, single functional traits were poorly correlated with the stabilizing niche differences that promote coexistence. Niche differences could only be described by combinations of traits, corresponding to differentiation between species in multiple ecological dimensions. In addition, several traits were associated with both fitness differences and stabilizing niche differences. These complex relationships between phenotypic differences and the dynamics of competing species argue against the simple use of single functional traits to infer community assembly processes but lay the groundwork for a theoretically justified trait-based community ecology.

Keywords: coexistence; community assembly; competition; functional traits.

Fig. 1.

Functional trait correlates of stabilizing niche (A) and average fitness (B) differences among 18 annual plants. Because average fitness and stabilizing niche differences are pairwise measures, correlations are calculated with Mantel tests. (C and D) Trait correlations with the two components of fitness differences, the demographic ratio and the competitive response ratio. Colored lines show correlations calculated from the Mantel test, ranging from −1 at the center of the plot to 1 at the margin. Central band of gray denotes the central 95% of null correlation values from the mantel permutations. See Table 2 for trait abbreviations. Results that are significant following Benjamini–Hochberg correction for multiple comparisons (Table S2) are marked in bold with an asterisk.

Fig. 2.

Average fitness and stabilizing niche differences for each pair of species (denoted by a single point) in the experiment. The shaded gray area represents the region where the condition for coexistence is met ($\rho <{\kappa }_i/{\kappa }_j$, where species j is the fitness superior). Twelve species pairs fall in this region; in all other cases fitness differences exceed niche differences and one species is predicted to eventually exclude the other. Note that our experiment focused on interactions at a neighborhood spatial scale over a single generation and therefore does not capture the spatial and temporal heterogeneity that allows these pairs to coexist at the landscape scale.

Fig. 3.

Trait differences between coexisting and noncoexisting species pairs (as identified in Fig. 2). Pairs predicted to coexist are significantly more similar in leaf area and phenology (Wilcoxon signed-rank test P < 0.05) than species pairs in which one is predicted to exclude the other; all other trait differences are not significant (see Fig. S4).