Phylogenies and traits provide distinct insights about the historical and contemporary assembly of aquatic insect communities

Abstract

The assumption that traits and phylogenies can be used as proxies of species niche has faced criticisms. Evidence suggested that phylogenic relatedness is a weak proxy of trait similarity. Moreover, different processes can select different traits, giving opposing signals in null model analyses. To circumvent these criticisms, we separated traits of stream insects based on the concept of α and β niches, which should give clues about assembling pressures expected to act independently of each other. We investigated the congruence between the phylogenetic structure and trait structure of communities using all available traits and all possible combinations of traits (4095 combinations). To account for hierarchical assembling processes, we analyzed patterns on two spatial scales with three pools of genera. Beta niche traits selected a priori - i.e., traits related to environmental variation (e.g., respiration type) - were consistently clustered on the smaller scale, suggesting environmental filtering, while α niche traits - i.e., traits related to resource use (e.g., trophic position) - did not display the expected overdispersion, suggesting a weak role of competition. Using all traits together provided random patterns and the analysis of all possible combinations of traits provided scenarios ranging from strong clustering to overdispersion. Communities were phylogenetically overdispersed, a pattern previously interpreted as phylogenetic limiting similarity. However, our results likely reflect the co-occurrence of ancient clades due to the stability of stream habitats along the evolutionary scale. We advise ecologists to avoid using combinations of all available traits but rather carefully traits based on the objective under consideration. Both trait and phylogenetic approaches should be kept in the ecologist toolbox, but phylogenetic distances should not be used as proxies of traits differences. Although the phylogenetic structure revealed processes operating at the evolutionary scale, only specific traits explained local processes operating in our communities.

Keywords: Assembly rules; community assembly; ecophylogenetics; habitat filtering; niche complementarity; trait structure.

Figure 1

Schematic view of study design and pools of genera used in different null models. (A) On riffle micro‐scale, each sample (30 × 30 cm) was considered an entire community, while, on the stream scale each community was the sum of abundances over the 10 samples collected. (B) We used three different pools of genera in our study. The stream pool is composed of all genera from the same stream as the analyzed sample. It thus considers that interacting genera are only those that inhabit the same stream. The river basin pool considers all genera from the Itanhaém river basin. It does not consider dispersal limitation on the river basin scale. The regional pool considers taxa sampled in the whole São Paulo State region. This last pool is more prone to reveal patterns due to large‐scale processes.

Figure 2

Box plots of values of Nearest taxon index (NTI) and Net relatedness index (NRI) on riffle micro‐scale and on stream scale calculated with trait and phylogenetic distances. The pool of genera used in the null model was composed of taxa found in the 13 streams of the Itanhaém river basin. Trait distances were calculated in three different ways: using all traits, using α niche traits and using β niche traits. Median values significantly different from zero according to two‐tailed Wilcoxon test have “*” for P < 0.01.

Figure 3

Barplot showing the ordered median value of Nearest taxon index (NTI) and Net relatedness index (NRI) on the riffle micro‐scale and on stream scale using all combinations of traits (n = 4095 combinations) and abundance data and considering all genera from the data set as the pool of taxa for the null model. NTI and NRI results are ordered from highest overdispersion to highest clustering.

Figure 4

Coordinates in the first axis of redundancy analysis. The response matrix is the result of all indices using all combinations of traits (P.A. means presence–absence data). The explanatory variables were the identities of the traits in the 4095 combinations. The first axis summarizes 87% of variation in the data. Negative coordinates of NRI and NTI on the first axis correspond to high NRI and NTI values and were thus interpreted as trait clustering. NRI, Net relatedness index; NTI, Nearest taxon index.