Leaf stable isotopes suggest shared ancestry is an important driver of functional diversity

Scale dependent niche conservatism in fish communities of the largest freshwater lake in China

Two major theoretical concepts of niche evolution lead to conflicting predictions in ecological studies: the competitive exclusion principle (CEP) predicts that closely-related species should be sufficiently divergent to coexist, whereas niche conservatism (NC) suggests that closely-related species should be more ecologically similar. Here, we test this conundrum by employing stable isotope ratios (δ13C, δ15N) to estimate trophic niches and test niche evolution in fish communities of Poyang Lake, central China. At a broad phylogenetic scale involving 57 species, we examined the relationships between trophic niche differences along genetic distances and tested phylogenetic signals. We found that trophic differences were positively associated with genetic distances when genetic distances were less than 0.24, showing strong phylogenetic signal, but not when larger than 0.24. We then focused on seven Cultrinae species coexisting at a local scale and compared trophic niche differences within and between sister species, closely-related species, and distantly-related species. We found that trophic differences between distantly-related species were significantly larger than those between closely-related species at a broad spatial scale, supporting NC. However, trophic differences between sister species were larger than those between closely-related species at a small local scale (individual sampling sites), suggesting the importance of CEP not NC. Hence, our findings suggest that niche evolution operates in a scale-dependent manner: in a phylogenetic scale (time scale), NC predictions were met well below a certain range, not above that range; at a spatial scale, CEP predictions were met for coexisting sister species, however the other species followed the NC predictions.

Evolutionary history and root trait coordination predict nutrient strategy in tropical legume trees

Plants express diverse nutrient use and acquisition traits, but it is unclear how trait combinations at the species level are constrained by phylogeny, trait coordination, or trade-offs in resource investment. One trait - nitrogen (N) fixation - is assumed to correlate with other traits and used to define plant functional groups, despite potential confounding effects of phylogeny. We quantified growth, carbon metabolism, fixation rate, root phosphatase activity (RPA), mycorrhizal colonization, and leaf and root morphology/chemistry across 22 species of fixing and nonfixing tropical Fabaceae trees under common conditions. Belowground trait variation was high even among closely related species, and most traits displayed a phylogenetic signal, including N-fixation rate and nodule biomass. Across species, we observed strong positive correlations between physiological traits such as RPA and root respiration. RPA increased ~ fourfold per unit increase in fixation, supporting the debated hypothesis that N-fixers 'trade' N for phosphatases to enhance phosphorus acquisition. Specific root length and root N differed between functional groups, though for other traits, apparent differences became nonsignificant after accounting for phylogenetic nonindependence. We conclude that evolutionary history, trait coordination, and fixation ability contribute to nutrient trait expression at the species level, and recommend explicitly considering phylogeny in analyses of functional groupings.

Isotopic niches do not follow the expectations of niche conservatism in the bird genus Cinclodes

Phenotypic traits are expected to be more similar among closely related species than among species that diverged long ago (all else being equal). This pattern, known as phylogenetic niche conservatism, also applies to traits that are important to determine the niche of species. To test this hypothesis on ecological niches, we analysed isotopic data from 254 museum study skins from 12 of the 16 species of the bird genus Cinclodes and measured stable isotope ratios for four different elements: carbon, nitrogen, hydrogen and oxygen. We find that all traits, measured individually, or as a composite measurement, lack any phylogenetic signal, which in turn suggests a high level of lability in ecological niches. We compared these metrics to the measurements of morphological traits in the same genus and found that isotopic niches are uniquely evolutionarily labile compared to other traits. Our results suggest that, in Cinclodes, the realized niche evolves much faster than expected by the constraints of phylogenetic history and poses the question of whether this is a general pattern across the tree of life.

Higher local intra- than interspecific variability in water- and carbon-related leaf traits among Neotropical tree species

BACKGROUND AND AIMS

Intraspecific variability in leaf water-related traits remains little explored despite its potential importance in the context of increasing drought frequency and severity. Studies comparing intra- and interspecific variability of leaf traits often rely on inappropriate sampling designs that result in non-robust estimates, mainly owing to an excess of the species/individual ratio in community ecology or, on the contrary, to an excess of the individual/species ratio in population ecology.

METHODS

We carried out virtual testing of three strategies to compare intra- and interspecific trait variability. Guided by the results of our simulations, we carried out field sampling. We measured nine traits related to leaf water and carbon acquisition in 100 individuals from ten Neotropical tree species. We also assessed trait variation among leaves within individuals and among measurements within leaves to control for sources of intraspecific trait variability.

KEY RESULTS

The most robust sampling, based on the same number of species and individuals per species, revealed higher intraspecific variability than previously recognized, higher for carbon-related traits (47-92 and 4-33 % of relative and absolute variation, respectively) than for water-related traits (47-60 and 14-44 % of relative and absolute variation, respectively), which remained non-negligible. Nevertheless, part of the intraspecific trait variability was explained by variation of leaves within individuals (12-100 % of relative variation) or measurement variations within leaf (0-19 % of relative variation) and not only by individual ontogenetic stages and environmental conditions.

CONCLUSIONS

We conclude that robust sampling, based on the same number of species and individuals per species, is needed to explore global or local variation in leaf water- and carbon-related traits within and among tree species, because our study revealed higher intraspecific variation than previously recognized.

Is variation in inter-annual precipitation a mechanism for maintaining plant metabolic diversity?

In order for diverse species to coexist in ecological communities, they must vary in ways that reduce competition. Often, this is done by some form of spatial niche separation where small differences in environment allow for coexistence among species. However, temporal separation of resources could also be a factor in driving community diversity. Here, we ask whether inter-annual variation in growing season precipitation could provide sufficient variation in water availability to allow plant species with different intrinsic metabolism to co-occur. We hypothesized that species would differentially respond to soil water availability, and that species with a metabolic strategy to conserve water at the expense of carbon gain would grow better in dry conditions relative to species with a metabolic strategy to gain carbon at the expense of foliar water loss. We measured above-ground biomass and leaf-level metabolism using carbon and oxygen stable isotope ratios for seven Asteraceae species across five experimental water treatments. Species differentially responded to variation in growing season water availability and, importantly, how they responded could be explained by differences in metabolism. Water-conservative species grew best in the dry treatments and had lower growth in wet treatments. Carbon-acquisitive species displayed the opposite pattern, with maximal growth in wet treatments and steep declines in dry treatments. Metabolic differences among co-occurring species may help explain temporal variation in growth, and could provide an underlying physiological mechanism for long-term dynamics that promote biodiversity.

Trait coordination and environmental filters shape functional trait distributions of forest understory herbs

Understanding the drivers of trait selection is critical for resolving community assembly processes. Here, we test the importance of environmental filtering and trait covariance for structuring the functional traits of understory herbaceous communities distributed along a natural environmental resource gradient that varied in soil moisture, temperature, and nitrogen availability, produced by different topographic positions in the southern Appalachian Mountains.To uncover potential differences in community-level trait responses to the resource gradient, we quantified the averages and variances of both abundance-weighted and unweighted values for six functional traits (vegetative height, leaf area, specific leaf area, leaf dry matter content, leaf nitrogen, and leaf δ13C) using 15 individuals of each of the 108 species of understory herbs found at two sites in the southern Appalachians of western North Carolina, USA.Environmental variables were better predictors of weighted than unweighted community-level average trait values for all but height and leaf N, indicating strong environmental filtering of plant abundance. Community-level variance patterns also showed increased convergence of abundance-weighted traits as resource limitation became more severe.Functional trait covariance patterns based on weighted averages were uniform across the gradient, whereas coordination based on unweighted averages was inconsistent and varied with environmental context. In line with these results, structural equation modeling revealed that unweighted community-average traits responded directly to local environmental variation, whereas weighted community-average traits responded indirectly to local environmental variation through trait coordination.Our finding that trait coordination is more important for explaining the distribution of weighted than unweighted average trait values along the gradient indicates that environmental filtering acts on multiple traits simultaneously, with abundant species possessing more favorable combinations of traits for maximizing fitness in a given environment.