Is evolution faster at ecotones? A test using rates and tempo of diet transitions in Neotropical Sigmodontinae (Rodentia, Cricetidae).

We evaluated whether evolution is faster at ecotones as niche shifts may be needed to persist under unstable environment. We mapped diet evolution along the evolutionary history of 350 sigmodontine species. Mapping was used in three new tip-based metrics of trait evolution - Transition Rates, Stasis Time, and Last Transition Time - which were spatialized at the assemblage level (aTR, aST, aTL). Assemblages were obtained by superimposing range maps on points located at core and ecotone of the 93 South American ecoregions. Using Linear Mixed Models, we tested whether ecotones have species with more changes from the ancestral diet (higher aTR), have maintained the current diet for a shorter time (lower aST), and have more recent transitions to the current diet (lower aLT) than cores. We found lower aTR, and higher aST and aLT at ecotones than at cores. Although ecotones are more heterogeneous, both environmentally and in relation to selection pressures they exert on organisms, ecotone species change little from the ancestral diet as generalist habits are necessary toward feeding in ephemeral environments. The need to incorporate phylogenetic uncertainty in tip-based metrics was evident from large uncertainty detected. Our study integrates ecology and evolution by analyzing how fast trait evolution is across space.

Analyzing community-weighted trait means across environmental gradients: should phylogeny stay or should it go?

Functional traits mediate ecological responses of organisms to the environment, determining community structure. Community-weighted trait means (CWM) are often used to characterize communities by combining information on species traits and distribution. Relating CWM variation to environmental gradients allows for evaluating species sorting across the metacommunity, either based on correlation tests or ordinary least squares (OLS) models. Yet, it is not clear if phylogenetic signal in both traits and species distribution affect those analyses. On one hand, phylogenetic signal might indicate niche conservatism along clade evolution, reinforcing the environmental signal in trait assembly patterns. On the other hand, it might introduce phylogenetic autocorrelation to mean trait variation among communities. Under this latter scenario, phylogenetic signal might inflate type I error in analysis relating CWM variation to environmental gradients. We explore multiple ways phylogenetic history may influence analysis relating CWM to environmental gradients. We propose the concept of neutral trait diffusion, which predicts that for a functional trait x, CWM variation among local communities does not deviate from the expectation that x evolved according to a neutral evolutionary process. Based on this framework we introduce a graphical tool called neutral trait diffusion representation (NTDR) that allows for the evaluation of whether it is necessary to carry out phylogenetic correction in the trait prior to analyzing the association between CWM and environmental gradients. We illustrate the NTDR approach using simulated traits, phylogenies and metacommunities. We show that even under moderate phylogenetic signal in both the trait used to define CWM and species distribution across communities, OLS models relating CWM variation to environmental gradients lead to inflated type I error when testing the null hypothesis of no association between CWM and environmental gradient. To overcome this issue, we propose a phylogenetic correction for OLS models and evaluate its statistical performance (type I error and power). Phylogeny-corrected OLS models successfully control for type I error in analysis relating CWM variation to environmental gradients but may show decreased power. Combining the exploratory tool of NTDR and phylogenetic correction in traits, when necessary, guarantees more precise inferences about the environmental forces driving trait-mediated species sorting across metacommunities.

Insect herbivory fluctuations through geological time.

Arthropods and land plants are the major macroscopic sources of biodiversity on the planet. Knowledge of the organization and specialization of plant-herbivore interactions, such as their roles in food webs is important for understanding the processes for maintaining biodiversity. A limited number of studies have examined herbivory through geological time. The most have analyzed localities from one restricted interval within a geological period, or a time transition such as the Paleocene-Eocene boundary interval. In the present study, we analyzed the frequency of herbivory and density of damage type (DT) from the Middle Devonian to the early Miocene. The data were compiled from literature sources and focused on studies that describe occurrences of leaves with DTs indicating herbivore consumption as a proportion of the total number of leaves analyzed. The data were standardized based on the DT categories in the Damage Type Guide, and the age of each locality was updated based on the most recent geochronological standard and expressed in millions of years. Temperature and geological age were the best descriptors of the variation in herbivory frequency, which tended to increase at higher temperatures. Two models were equivalent to explain DT density: the interaction between CO2 levels and geological age, and O2 levels and geological age had the same predictive power. The density of DT tended to increase with higher content of atmospheric CO2 and O2 compared to modern values. The frequency of herbivory and the density of DTs appear to be influenced by long-term atmospheric variables.

A global meta-analysis of the relative extent of intraspecific trait variation in plant communities.

Recent studies have shown that accounting for intraspecific trait variation (ITV) may better address major questions in community ecology. However, a general picture of the relative extent of ITV compared to interspecific trait variation in plant communities is still missing. Here, we conducted a meta-analysis of the relative extent of ITV within and among plant communities worldwide, using a data set encompassing 629 communities (plots) and 36 functional traits. Overall, ITV accounted for 25% of the total trait variation within communities and 32% of the total trait variation among communities on average. The relative extent of ITV tended to be greater for whole-plant (e.g. plant height) vs. organ-level traits and for leaf chemical (e.g. leaf N and P concentration) vs. leaf morphological (e.g. leaf area and thickness) traits. The relative amount of ITV decreased with increasing species richness and spatial extent, but did not vary with plant growth form or climate. These results highlight global patterns in the relative importance of ITV in plant communities, providing practical guidelines for when researchers should include ITV in trait-based community and ecosystem studies.

A framework for metacommunity analysis of phylogenetic structure.

It is well known that species evolutionary history plays a crucial role in community assembly. Here, we offer a formal analytical framework to integrate in metacommunity analysis the species' phylogeny with their functional traits and abundances. We define phylogenetic structure of a community as phylogenetically weighted species composition. This is used to reveal patterns of phylogenetic community variation and to measure and test by specified null models the phylogenetic signal at the metacommunity level, which we distinguish from phylogenetic signal at the species pool level. The former indicates that communities more similar in their phylogenetic structure are also similar in their average trait values, which may indicate species' niche conservation for the given traits. We apply this framework to an example from grassland communities and find that traits with significant phylogenetic signal at the metacommunity level exhibit ecological filtering along the resource gradient, but since both mechanisms act independently on traits, niche conservatism is not supported.