Trophic phylogenetics: evolutionary influences on body size, feeding, and species associations in grassland arthropods

Influence of phylogenetic diversity of plant communities on tri-trophic interactions

Phylogenetic diversity of plant communities can influence the interaction between plants, herbivores, and their natural enemies. Plant communities with phylogenetically distant species tend to present a wide variety of functional traits and ecological niches, which in turn can influence competitive interactions among plants as well as food and habitat quality for herbivores and their natural enemies. To assess some different mechanisms by which phylogenetic diversity of plant communities can influence herbivores and their natural enemies, we established 12 experimental plots of tropical trees with two treatments: high and low phylogenetic diversity. We measured plant growth and anti-herbivore defenses, herbivore foliar damage, and predator activity in seven species that were present in both treatments. We found significant differences in the expression of plant traits as a function of species identity and their life history, but also depending on the phylogenetic context in which they grew. Pioneer species had higher growth and produced more phenolics in plots with high phylogenetic diversity versus plants in plots with low phylogenetic diversity. Accordingly, herbivore damage in these species was greater in plots with low phylogenetic diversity. Finally, predator activity on caterpillar clay models placed on plants was greater within the low phylogenetic diversity treatment, but only for non-myrmecophytic species. These results suggest that plant phylogenetic diversity can influence the expression of growth and defensive traits and further modify the interaction between plants, herbivores, and their natural enemies. However, such effects depend on plant life history and the presence of mutualistic interaction with ants.

Intraspecific variation in plant-associated herbivore communities is phylogenetically structured in Brassicaceae

As a result of co‐evolution between plants and herbivores, related plants often interact with similar herbivore communities. Variation in plant–herbivore interactions is determined by variation in underlying functional traits and by ecological and stochastic processes. Hence, typically, only a subset of possible interactions is realised on individual plants. We show that insect herbivore communities assembling on individual plants are structured by plant phylogeny among 12 species in two phylogenetic lineages of Brassicaceae. This community sorting to plant phylogeny was retained when splitting the community according to herbivore feeding guilds. Relative abundance of herbivores as well as the size of the community structured community dissimilarity among plant species. Importantly, the amount of intraspecific variation in realised plant–herbivore interactions is also phylogenetically structured. We argue that variability in realised interactions that are not directly structured by plant traits is ecologically relevant and must be considered in the evolution of plant defences.

Biodiversity enhances the multitrophic control of arthropod herbivory

Arthropod herbivores cause substantial economic costs that drive an increasing need to develop environmentally sustainable approaches to herbivore control. Increasing plant diversity is expected to limit herbivory by altering plant-herbivore and predator-herbivore interactions, but the simultaneous influence of these interactions on herbivore impacts remains unexplored. We compiled 487 arthropod food webs in two long-running grassland biodiversity experiments in Europe and North America to investigate whether and how increasing plant diversity can reduce the impacts of herbivores on plants. We show that plants lose just under half as much energy to arthropod herbivores when in high-diversity mixtures versus monocultures and reveal that plant diversity decreases effects of herbivores on plants by simultaneously benefiting predators and reducing average herbivore food quality. These findings demonstrate that conserving plant diversity is crucial for maintaining interactions in food webs that provide natural control of herbivore pests.

Can ancestry and morphology be used as surrogates for species niche relationships?

Species interactions are difficult to quantify, and, consequently, many studies have used species traits and phylogeny as proxies under an assumption of niche conservatism (i.e., closely related and morphologically similar species should have similar niches). However, few studies have investigated whether niches actually are conserved within and across diverse communities. Here, we tested the degree to which phylogenetic relatedness and morphological similarity predict diets and stable isotopic ratios (δ 15N and δ 13C), two common descriptors of the trophic niche, in fish assemblages of two small streams in the Neotropics. We also tested the strength of the association between isotopic ratios and diet composition and found significant correlations implying that isotopic signals reveal trophic structure despite error associated with estimates of trophic enrichment and variation associated with tissue preservation, metabolism, and other factors affecting isotopic ratios. Morphological traits yielded a significant phylogenetic signal, and both morphological traits and phylogeny were correlated with diet composition, with morphological traits being a stronger predictor. We infer that functionally relevant morphological traits of fish can be used to infer trophic niches for certain kinds of questions and analyses when trophic data are lacking. However, we highlight that using phylogenetic and morphological data in combination with dietary and/or isotopic data can improve resolution of assemblage trophic structure and niche diversification.

Related plants tend to share pollinators and herbivores, but strength of phylogenetic signal varies among plant families

Related plants are often hypothesized to interact with similar sets of pollinators and herbivores, but this idea has only mixed empirical support. This may be because plant families vary in their tendency to share interaction partners. We quantify overlap of interaction partners for all pairs of plants in 59 pollination and 11 herbivory networks based on the numbers of shared and unshared interaction partners (thereby capturing both proportional and absolute overlap). We test for relationships between phylogenetic distance and partner overlap within each network; whether these relationships varied with the composition of the plant community; and whether well-represented plant families showed different relationships. Across all networks, more closely related plants tended to have greater overlap. The strength of this relationship within a network was unrelated to the composition of the network's plant component, but, when considered separately, different plant families showed different relationships between phylogenetic distance and overlap of interaction partners. The variety of relationships between phylogenetic distance and partner overlap in different plant families probably reflects a comparable variety of ecological and evolutionary processes. Considering factors affecting particular species-rich groups within a community could be the key to understanding the distribution of interactions at the network level.

Contrasting effects of plant diversity on β- and γ-diversity of grassland invertebrates

The diversity of primary producers strongly affects the structure and diversity of species assemblages at other trophic levels. However, limited knowledge exists of how plant diversity effects at small spatial scales propagate to consumer communities at larger spatial scales. We assessed arthropod community β and γ-diversity in response to experimentally manipulated plant community richness in two long-term grassland biodiversity experiments (Jena, Germany and Cedar Creek, USA) replicated over two years. We calculated arthropod species turnover among all plot combinations (β-diversity), and accumulated number of arthropod species occurring on (1) all pairwise plot combinations and (2) 40 randomly selected six-plot combinations (γ-diversity). The components of arthropod diversity were tested against two measures of plant diversity, namely average plant α-diversity ( PSR ¯ ) and the average difference in plant α-diversity between plots (ΔPSR). Whereas PSR ¯ points to the overall importance of plant α-diversity for arthropod community turnover and diversity on a larger scale, ΔPSR represents the role of habitat heterogeneity. We demonstrate that arthropod γ-diversity is supported by high, homogeneous plant α-diversity, despite lower arthropod β-diversity among high- compared to low-diversity plant communities. We also show that, in six-plot combinations, average plant α-diversity has a positive influence on arthropod γ-diversity only when homogeneity in plant α-diversity is also high. Varying heterogeneity in six-plot combinations showed that combinations consisting solely of plots with an intermediate level of plant α-diversity support a higher number of arthropod species compared to combinations that contain a mix of high- and low-diversity plots. In fact, equal levels of arthropod diversity were found for six-plot combinations with only intermediate or high plant α-diversity, due to saturating benefits of local and larger-scale plant diversity for higher trophic levels. Our results, alongside those of recent observational studies, strongly suggest that maintaining high α-diversity in plant communities is important for conserving multiple components of arthropod diversity. As arthropods carry out a range of essential ecosystem functions, such as pollination and natural pest-control, our findings provide crucial insight for effective planning of human-dominated landscapes to maximize both ecological and economic benefits in grassland systems.

Agricultural land-use history causes persistent loss of plant phylogenetic diversity

Intensive land use activities, such as agriculture, are a leading cause of biodiversity loss and can have lasting impacts on ecological systems. Yet, few studies have investigated how land-use legacies impact phylogenetic diversity (the total amount of evolutionary history in a community) or how restoration activities might mitigate legacy effects on biodiversity. We studied ground-layer plant communities in 27 pairs of Remnant (no agricultural history) and Post-agricultural (agriculture abandoned >60 yr ago) longleaf pine savannas, half of which we restored by thinning trees to reinstate open savanna conditions. We found that agricultural history had no impact on species richness, but did alter community composition and reduce phylogenetic diversity by 566 million years/1,000 m² . This loss of phylogenetic diversity in post-agricultural savannas was due to, in part, a reduction in the average evolutionary distance between pairs of closely related species, that is, increased phylogenetic clustering. Habitat restoration increased species richness by 27% and phylogenetic diversity by 914 million years but did not eliminate the effects of agricultural land use on community composition and phylogenetic structure. These results demonstrate the persistence of agricultural legacies, even in the face of intensive restoration efforts, and the importance of considering biodiversity broadly when evaluating human impacts on ecosystems.

The role of diversification in community assembly of the oaks (Quercus L.) across the continental U.S

PREMISE OF THE STUDY

Evolutionary and biogeographic history, including past environmental change and diversification processes, are likely to have influenced the expansion, migration, and extinction of populations, creating evolutionary legacy effects that influence regional species pools and the composition of communities. We consider the consequences of the diversification process in shaping trait evolution and assembly of oak-dominated communities throughout the continental United States (U.S.).

METHODS

Within the U.S. oaks, we tested for phylogenetic and functional trait patterns at different spatial scales, taking advantage of a dated phylogenomic analysis of American oaks and the U.S. Forest Service (USFS) Forest Inventory and Analysis (FIA).

KEY RESULTS

We find (1) phylogenetic overdispersion at small grain sizes throughout the U.S. across all spatial extents and (2) a shift from overdispersion to clustering with increasing grain sizes. Leaf traits have evolved in a convergent manner, and these traits are clustered in communities at all spatial scales, except in the far west, where species with contrasting leaf types co-occur.

CONCLUSIONS

Our results support the hypotheses that (1) interspecific interactions were important in parallel adaptive radiation of the genus into a range of habitats across the continent and (2) that the diversification process is a critical driver of community assembly. Functional convergence of complementary species from distinct clades adapted to the same local habitats is a likely mechanism that allows distantly related species to coexist. Our findings contribute to an explanation of the long-term maintenance of high oak diversity and the dominance of the oak genus in North America.

Increased grassland arthropod production with mammalian herbivory and eutrophication: a test of mediation pathways

Increases in nutrient availability and alterations to mammalian herbivore communities are a hallmark of the Anthropocene, with consequences for the primary producer communities in many ecosystems. While progress has advanced understanding of plant community responses to these perturbations, the consequences for energy flow to higher trophic levels in the form of secondary production are less well understood. We quantified arthropod biomass after manipulating soil nutrient availability and wild mammalian herbivory, using identical methods across 13 temperate grasslands. Of experimental increases in nitrogen, phosphorus, and potassium, only treatments including nitrogen resulted in significantly increased arthropod biomass. Wild mammalian herbivore removal had a marginal, negative effect on arthropod biomass, with no interaction with nutrient availability. Path analysis including all sites implicated nutrient content of the primary producers as a driver of increased arthropod mean size, which we confirmed using 10 sites for which we had foliar nutrient data. Plant biomass and physical structure mediated the increase in arthropod abundance, while the nitrogen treatments accounted for additional variation not explained by our measured plant variables. The mean size of arthropod individuals was 2.5 times more influential on the plot-level total arthropod biomass than was the number of individuals. The eutrophication of grasslands through human activity, especially nitrogen deposition, thus may contribute to higher production of arthropod consumers through increases in nutrient availability across trophic levels.

Arthropod communities on hybrid and parental cottonwoods are phylogenetically structured by tree type: Implications for conservation of biodiversity in plant hybrid zones

Although hybridization in plants has been recognized as an important pathway in plant speciation, it may also affect the ecology and evolution of associated communities. Cottonwood species (Populus angustifolia and P. fremontii) and their naturally occurring hybrids are known to support different plant, animal, and microbial communities, but no studies have examined community structure within the context of phylogenetic history. Using a community composed of 199 arthropod species, we tested for differences in arthropod phylogenetic patterns within and among hybrid and parental tree types in a common garden. Three major patterns emerged. (1) Phylogenetic diversity (PD) was significantly different between arthropod communities on hybrids and Fremont cottonwood when pooled by tree type. (2) Mean phylogenetic distance (MPD) and net relatedness index (NRI) indicated that communities on hybrid trees were significantly more phylogenetically overdispersed than communities on either parental tree type. (3) Community distance (D_pw) indicated that communities on hybrids were significantly different than parental species. Our results show that arthropod communities on parental and hybrid cottonwoods exhibit significantly different patterns of phylogenetic structure. This suggests that arthropod community assembly is driven, in part, by plant-arthropod interactions at the level of cottonwood tree type. We discuss potential hypotheses to explain the effect of plant genetic dissimilarity on arthropod phylogenetic community structure, including the role of competition and environmental filtering. Our findings suggest that cottonwood species and their hybrids function as evolutionarily significant units (ESUs) that affect the assembly and composition of associated arthropod communities and deserve high priority for conservation.