Compound Specific Trends of Chemical Defences in Ficus Along an Elevational Gradient Reflect a Complex Selective Landscape

Elevation drives intraspecific metabolomic differentiation in natural and experimental populations

A recent paradigm shift in ecology supports that the classic functional trait space should be extended to include the metabolome. Accordingly, metabolomic signatures differ between plant species or genotypes depending on where they grow along ecological gradients. Yet, it remains unclear whether environmental gradients alone induce intraspecific phytochemical shifts in natural populations, and if these differences can be replicated experimentally. To address this gap, we combined an observational and experimental study to explore how elevation drives differences in the diversity, endemism and composition of specialized metabolites within the widely distributed perennial grass Festuca rubra. We show that both natural and experimental populations (consisting of identical sets of unique genotypes) exhibit distinct metabolic profiles, unique to each elevation. While natural populations growing in alpine environments displayed lower phytochemical diversity and endemism than those inhabiting lower elevations, phytochemical diversity and endemism peaked at intermediate elevation for the experimental populations. In addition, elevation caused shifts in overall composition of metabolomic groups for experimental populations, reflected in alterations in the over- and under-representation of metabolites within specific superclasses. Our study demonstrates that elevation plays an important role in shaping the metabolome. Rapid shifts in abiotic and biotic environments following climate change may act to alter the plant metabolome, which can impact future ecosystem-level dynamics.

Testing the role of biotic interactions in shaping elevational diversity gradients: An ecological metabolomics approach

Seminal hypotheses in ecology and evolution postulate that stronger and more specialized biotic interactions contribute to higher species diversity at lower elevations and latitudes. Plant-chemical defenses mediate biotic interactions between plants and their natural enemies and provide a highly dimensional trait space in which chemically mediated niches may facilitate plant species coexistence. However, the role of chemically mediated biotic interactions in shaping plant communities remains largely untested across large-scale ecological gradients. Here, we used ecological metabolomics to quantify the chemical dissimilarity of foliar metabolomes among 473 tree species in 16 tropical tree communities along an elevational gradient in the Bolivian Andes. We predicted that tree species diversity would be higher in communities and climates where co-occurring tree species are more chemically dissimilar and exhibit faster evolution of secondary metabolites (lower chemical phylogenetic signal). Further, we predicted that these relationships should be especially pronounced for secondary metabolites known to include antiherbivore and antimicrobial defenses relative to primary metabolites. Using structural equation models, we quantified the direct effects of rarefied median chemical dissimilarity and chemical phylogenetic signal on tree species diversity, as well as the indirect effects of climate. We found that chemical dissimilarity among tree species with respect to all metabolites and secondary metabolites had positive direct effects on tree species diversity, and that climate (higher temperature and precipitation, and lower temperature seasonality) had positive indirect effects on species diversity by increasing chemical dissimilarity. In contrast, chemical dissimilarity of primary metabolites was unrelated to species diversity and climate. Chemical phylogenetic signal of all metabolite classes had negative direct effects on tree species diversity, indicating faster evolution of metabolites in more diverse communities. Climate had a direct effect on species diversity but did not indirectly affect diversity through chemical phylogenetic signal. Our results support the hypothesis that chemically mediated biotic interactions shape elevational diversity gradients by imposing stronger selection for chemical divergence in more diverse communities and maintaining higher chemical dissimilarity among species in warmer, wetter, and more stable climates. Our study also illustrates the promise of ecological metabolomics in the study of biogeography, community ecology, and complex species interactions in high-diversity ecosystems.

Birds and bats reduce herbivory damage in Papua New Guinean highland forests

Insectivorous predators, including birds and bats, play crucial roles in trophic cascades. However, previous research on these cascades has often relied on permanent predator exclosures, which prevent the isolation of specific effects of birds and bats, given their different activity patterns throughout the day. Moreover, limited knowledge exists regarding the variations in individual effects of these predators under different biotic and abiotic conditions, such as changes in elevation. To address these uncertainties, our study aimed to investigate the distinct effects of bats and birds on arthropod densities in foliage and herbivory damage in lowland and highland rainforests of Papua New Guinea (PNG). Predator exclosures were established for one month to exclude diurnal or nocturnal predators across 120 saplings (ca. 2.5-4 m tall) selected from two lowland and two highland forests (i.e., 30 saplings per study site) along the Mt. Wilhelm transect in PNG. Arthropods were collected and measured, and herbivory damage was analyzed at the end of the experiment. Birds significantly reduced arthropod densities by 30%, particularly in arthropods longer than 10 mm, regardless of elevation. Additionally, both birds and bats appeared to mitigate herbivory damage in highland forests, with protected saplings displaying up to 189% more herbivory. Our results support previous studies that have demonstrated the ability of insectivorous predators to reduce leaf damage through the control of arthropods. Furthermore, our approach highlights the importance and necessity of further research on the role of seasons and elevations in trophic cascades.

Phytochemical diversity and their adaptations to abiotic and biotic pressures in fine roots across a climatic gradient

Phytochemicals and their ecological significance are long ignored in trait-based ecology. Moreover, the adaptations of phytochemicals produced by fine roots to abiotic and biotic pressures are less understood. Here, we explored the fine roots metabolomes of 315 tree species and their rhizosphere microbiome in southwestern China spanning tropical, subtropical, and subalpine forest ecosystems, to explore phytochemical diversity and endemism patterns of various metabolic pathways and phytochemical-microorganism interactions. We found that subalpine species showed higher phytochemical diversity but lower interspecific variation than tropical species, which favors coping with high abiotic pressures. Tropical species harbored higher interspecific phytochemical variation and phytochemical endemism, which favors greater species coexistence and adaptation to complex biotic pressures. Moreover, there was evidence of widespread chemical niche partitioning of closely related species in all regions, and phytochemicals showed a weak phylogenetic signal, but were regulated by abiotic and biotic pressures. Our findings support the Latitudinal Biotic Interaction Hypothesis, i.e., the intensity of phytochemical-microorganism interactions decreases from tropical to subalpine regions, which promotes greater microbial community turnover and phytochemical niche partitioning of host plants in the tropics than in higher latitude forests. Our study reveals the convergent phytochemical diversity patterns of various pathways and their interactions with microorganism, thus promoting species coexistence.

Contrasting levels of β-diversity and underlying phylogenetic trends indicate different paths to chemical diversity in highland and lowland willow species

Diverse specialised metabolites contributed to the success of vascular plants in colonising most terrestrial habitats. Understanding how distinct aspects of chemical diversity arise through heterogeneous environmental pressures can help us understand the effects of abiotic and biotic stress on plant evolution and community assembly. We examined highland and lowland willow species within a phylogenetic framework to test for trends in their chemical α-diversity (richness) and β-diversity (variation among species sympatric in elevation). We show that differences in chemistry among willows growing at different elevations occur mainly through shifts in chemical β-diversity and due to convergence or divergence among species sharing their elevation level. We also detect contrasting phylogenetic trends in concentration and α-diversity of metabolites in highland and lowland willow species. The resulting elevational patterns contribute to the chemical diversity of willows and suggest that variable selective pressure across ecological gradients may, more generally, underpin complex changes in plant chemistry.

What Goes in Must Come Out? The Metabolic Profile of Plants and Caterpillars, Frass, And Adults of Asota (Erebidae: Aganainae) Feeding on Ficus (Moraceae) in New Guinea

Insect herbivores have evolved a broad spectrum of adaptations in response to the diversity of chemical defences employed by plants. Here we focus on two species of New Guinean Asota and determine how these specialist moths deal with the leaf alkaloids of their fig (Ficus) hosts. As each focal Asota species is restricted to one of three chemically distinct species of Ficus, we also test whether these specialized interactions lead to similar alkaloid profiles in both Asota species. We reared Asota caterpillars on their respective Ficus hosts in natural conditions and analyzed the alkaloid profiles of leaf, frass, caterpillar, and adult moth samples using UHPLC-MS/MS analyses. We identified 43 alkaloids in our samples. Leaf alkaloids showed various fates. Some were excreted in frass or found in caterpillars and adult moths. We also found two apparently novel indole alkaloids-likely synthesized de novo by the moths or their microbiota-in both caterpillar and adult tissue but not in leaves or frass. Overall, alkaloids unique or largely restricted to insect tissue were shared across moth species despite feeding on different hosts. This indicates that a limited number of plant compounds have a direct ecological function that is conserved among the studied species. Our results provide evidence for the importance of phytochemistry and metabolic strategies in the formation of plant-insect interactions and food webs in general. Furthermore, we provide a new potential example of insects acquiring chemicals for their benefit in an ecologically relevant insect genus.

Chemical Similarity of Co-occurring Trees Decreases With Precipitation and Temperature in North American Forests

Plant diversity varies immensely over large-scale gradients in temperature, precipitation, and seasonality at global and regional scales. This relationship may be driven in part by climatic variation in the relative importance of abiotic and biotic interactions to the diversity and composition of plant communities. In particular, biotic interactions may become stronger and more host specific with increasing precipitation and temperature, resulting in greater plant species richness in wetter and warmer environments. This hypothesis predicts that the many defensive compounds found in plants’ metabolomes should increase in richness and decrease in interspecific similarity with precipitation, temperature, and plant diversity. To test this prediction, we compared patterns of chemical and morphological trait diversity of 140 woody plant species among seven temperate forests in North America representing 16.2°C variation in mean annual temperature (MAT), 2,115 mm variation in mean annual precipitation (MAP), and from 10 to 68 co-occurring species. We used untargeted metabolomics methods based on data generated with liquid chromatography-tandem mass spectrometry to identify, classify, and compare 13,480 unique foliar metabolites and to quantify the metabolomic similarity of species in each community with respect to the whole metabolome and each of five broad classes of metabolites. In addition, we compiled morphological trait data from existing databases and field surveys for three commonly measured traits (specific leaf area [SLA], wood density, and seed mass) for comparison with foliar metabolomes. We found that chemical defense strategies and growth and allocation strategies reflected by these traits largely represented orthogonal axes of variation. In addition, functional dispersion of SLA increased with MAP, whereas functional richness of wood density and seed mass increased with MAT. In contrast, chemical similarity of co-occurring species decreased with both MAT and MAP, and metabolite richness increased with MAT. Variation in metabolite richness among communities was positively correlated with species richness, but variation in mean chemical similarity was not. Our results are consistent with the hypothesis that plant metabolomes play a more important role in community assembly in wetter and warmer climates, even at temperate latitudes, and suggest that metabolomic traits can provide unique insight to studies of trait-based community assembly.

Biomass, abundances, and abundance and geographical range size relationship of birds along a rainforest elevational gradient in Papua New Guinea

The usually positive inter-specific relationship between geographical range size and the abundance of local bird populations comes with exceptions. On continents, the majority of these exceptions have been described from tropical montane areas in Africa, where geographically-restricted bird species are unusually abundant. We asked how the local abundances of passerine and non-passerine bird species along an elevational gradient on Mt. Wilhelm, Papua New Guinea relate to their geographical range size. We collected data on bird assemblages at eight elevations (200–3,700 m, at 500 m elevational increments). We used a standardized point-counts at 16 points at each elevational study site. We partitioned the birds into feeding guilds, and we obtained data on geographical range sizes from the Bird-Life International data zone. We observed a positive relationship between abundance and geographical range size in the lowlands. This trend changed to a negative one towards higher elevations. The total abundances of the assemblage showed a hump-shaped pattern along the elevational gradient, with passerine birds, namely passerine insectivores, driving the observed pattern. In contrast to abundances, the mean biomass of the bird assemblages decreased with increasing elevation. Our results show that montane bird species maintain dense populations which compensate for the decreased available area near the top of the mountain.