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

Abstract

Elevational gradients affect the production of plant secondary metabolites through changes in both biotic and abiotic conditions. Previous studies have suggested both elevational increases and decreases in host-plant chemical defences. We analysed the correlation of alkaloids and polyphenols with elevation in a community of nine Ficus species along a continuously forested elevational gradient in Papua New Guinea. We sampled 204 insect species feeding on the leaves of these hosts and correlated their community structure to the focal compounds. Additionally, we explored species richness of folivorous mammals along the gradient. When we accounted for Ficus species identity, we found a general increase in flavonoids and alkaloids. Elevational trends in non-flavonol polyphenols were less pronounced or showed non-linear correlations with elevation. The abundance of insect herbivores decreased with elevation, while the species richness of folivorous mammals showed an elevational increase. Insect community structure was affected mainly by alkaloid concentration and diversity. Although our results show an elevational increase in several groups of metabolites, the drivers behind these trends likely differ. Flavonoids probably provide figs with protection against abiotic stressors, such as UV-irradiation. In contrast, alkaloids affect insect herbivores and may provide protection against mammalian herbivores and pathogens. Concurrent analysis of multiple compound groups alongside ecological data is an important approach for understanding the selective landscape that shapes plant defences.

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

“…Ehrlich and Raven (1964) proposed that such coevolution of plants and their natural enemies is a major driver of diversification in both groups. Hence, variation over climatic gradients in the relative importance of natural enemies as agents of selection on plants may contribute to the parallel diversity gradients in woody plants and their natural enemies (Dyer et al, 2007;Schemske et al, 2009), with consequences for variation in plant metabolomes (Defossez et al, 2018;Moreira et al, 2018;Sedio et al, 2018b;Volf et al, 2020).…”

“…However, at regional scales, plants tend to invest more in defense in environments in which the cost of tissue loss to natural enemies is greater (Givnish, 1999;Fine et al, 2006;Defossez et al, 2018;Volf et al, 2020). Hence, recent studies suggest that the cost of tissue loss to herbivores or pathogens runs counter to that of pest and pathogen abundance, and hence decreases with precipitation, temperature and edaphic resources (Fine et al, 2006;Sam et al, 2020;Volf et al, 2020).…”

“…However, at regional scales, plants tend to invest more in defense in environments in which the cost of tissue loss to natural enemies is greater (Givnish, 1999;Fine et al, 2006;Defossez et al, 2018;Volf et al, 2020). Hence, recent studies suggest that the cost of tissue loss to herbivores or pathogens runs counter to that of pest and pathogen abundance, and hence decreases with precipitation, temperature and edaphic resources (Fine et al, 2006;Sam et al, 2020;Volf et al, 2020). The fitness landscape of plant secondary metabolites likely varies with multiple biotic and abiotic gradients over broad climatic gradients at global and regional scales, and plant responses may favor greater investment in stressful abiotic environments where tissue loss is costly, yet greater diversity or interspecific divergence in chemical defense composition in less stressful abiotic environments with high herbivore and pathogen pressure (Volf et al, 2020).…”

“…Hence, recent studies suggest that the cost of tissue loss to herbivores or pathogens runs counter to that of pest and pathogen abundance, and hence decreases with precipitation, temperature and edaphic resources (Fine et al, 2006;Sam et al, 2020;Volf et al, 2020). The fitness landscape of plant secondary metabolites likely varies with multiple biotic and abiotic gradients over broad climatic gradients at global and regional scales, and plant responses may favor greater investment in stressful abiotic environments where tissue loss is costly, yet greater diversity or interspecific divergence in chemical defense composition in less stressful abiotic environments with high herbivore and pathogen pressure (Volf et al, 2020). Few studies have examined the relationship between plant metabolomic composition and major climatic or environmental gradients for specific woody plant lineages (Fine et al, 2006;Sam et al, 2020;Volf et al, 2020) and even fewer have done so at the community scale (Sedio et al, 2018b).…”

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

“…Wilhelm, abundances of arthropods followed a humped-shaped pattern with a peak at ca. 1,700 m (Sam et al, 2020;Supriya et al, 2019;Volf et al, 2020), providing the best food resources for insectivorous passerines in the middle of the gradient. In contrast, abundance as well as biomass of fruits decreased steeply with increasing elevation (Hazel, 2019;Segar et al, 2017).…”