Revisiting Janzen's mountain passes hypothesis from a microbial perspective: why tropical mountains are so biodiverse?

Abstract

Background: Although it is well known that microbial communities are extremely diverse, how and why levels of biodiversity are generated and maintained are enduring ecological questions. Janzen’s mountain passes hypothesis assumes that the relatively stable climate regimes of tropical mountains means that species have narrower ranges than montane regions in cooler climates, allowing disproportionately high levels of species coexistence and diversity. We test this hypothesis for soil bacteria and fungi, to explore the relative importance of niche and neutral dynamics in governing of microbial diversity on mountains. Results: We found that the tropical mountain showed greater climatic barriers that effectively limited the dispersal of microbes but had no similar soil barriers. Our results show that tropical mountains have higher β-diversity and greater species turnover rates along elevations than subtropical and subalpine mountains both for fungi and bacteria. β-diversity patterns along elevational gradients are mainly shaped by dispersal limitation coupled with ecological drift and homogeneous selection, particularly in the tropics. Conclusions: Therefore, stronger effects of climatic barriers in tropical mountains in conjunction with biogeographic species distributions and dispersal limitation permit greater biological diversity of soil bacteria and fungi, which supports the predictions of Janzen’s hypothesis. This study advances our understanding of mechanisms underpinning the community assembly of soil microorganisms by demonstrating that climatic-dependent dispersal limitations also operate in tropical soil microbial communities.

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

“…The latitudinal patterns of beta-level interactions also fit the prediction of LBIH, reflecting the changing intensity of biotic interactions along climatic gradients. It also supports the ecological significance of phytochemicals in fine roots for multi-trophic species coexistence (i.e., host plants and rhizosphere microorganisms), especially in low latitudes, that is, greater biotic interactions can further promote phytochemical niche partitioning of host plants and greater variation in rhizosphere microbial community composition (Zhang et al, 2022a).…”

“…5). The intense phytochemical-microorganism interactions in the tropics promote greater variation in microbial community composition (greater beta diversity or species turnover, Zhang et al, 2022a) and phytochemical niche partitioning of host plants (higher phytochemical endemism) than that of higher latitudes, which shapes the enormous multi-trophic coexistence in the tropics. Our study reveals phytochemical diversity patterns and their ecological significance in fine roots and provides novel insights into biotic interactions and species coexistence.…”

“…We calculated the Shannon -Wiener diversity index of the rhizosphere sample of each tree species for total fungi, fungal pathogen, total bacteria, and bacterial pathogen using the "vegan" package (i.e., alpha diversity). Based on rhizosphere total fungi, fungal pathogen, total bacteria, and bacterial pathogen data, we calculated the paired Bray-Curtis dissimilarity matrix of tree species to represent the dissimilarity of microbial community composition (i.e., beta diversity) (Zhang et al, 2022a) using the "betapart" package (Baselga et al, 2021).…”