Abstract
Ancestral adaptations to tropical-like climates drive most multicellular biogeography and macroecology. Observational studies suggest that this niche conservatism could also be shaping unicellular biogeography and macroecology, although evidence is limited to Acidobacteria and testate amoebae. We tracked the phylogenetic signal of this niche conservatism in far related and functionally contrasted groups of common soil protists (Bacillariophyta, Cercomonadida, Ciliophora, Euglyphida and Kinetoplastida) along a humid but increasingly cold elevational gradient in Switzerland. Protist diversity decreased, and the size of the geographic ranges of taxa increased with elevation and associated decreasing temperature (climate), which is consistent with a macroecological pattern known as the Rapoport effect. Bacillariophyta exhibited phylogenetically overdispersed communities assembled by competitive exclusion of closely related taxa with shared (conserved) niches. By contrast, Cercomonadida, Ciliophora, Euglyphida and Kinetoplastida exhibited phylogenetically clustered communities assembled by habitat filtering, revealing the coexistence of closely related taxa with shared (conserved) adaptations to cope with the humid but temperate to cold climate of the study site. Phylobetadiversity revealed that soil protists exhibit a strong phylogenetic turnover among elevational sites, suggesting that most taxa have evolutionary constraints that prevent them from colonizing the colder and higher sites of the elevation gradient. Our results suggest that evolutionary constraints determine how soil protists colonize climates departing from warm and humid conditions. We posit that these evolutionary constraints are linked to an ancestral adaptation to tropical-like climates, which limits their survival in exceedingly cold sites. This niche conservatism possibly drives their biogeography and macroecology along latitudinal and altitudinal climatic gradients.
Highlights
Biological diversity varies over spatial gradients, giving rise to biogeographical and macroecological diversity patterns [1]
Others predictions of the Phylogenetic niche conservatism (PNC) include a positive correlation between diversity and temperature [4, 5, 7], an increase in the size of taxa distribution ranges towards the colder end of an environmental gradient [10, 11], giving place to a macroecological pattern known as the Rapoport effect [12]; and local communities structured by competition in warm sites and structured by habitat filtering in cold sites [3, 13]
We did not find support for a mid-domain effect (Fig. 3, Prediction 2.2.1.). This outcome confirmed the role of habitat filtering in structuring soil protist communities along the elevational gradient investigated
Summary
Biological diversity varies over spatial gradients, giving rise to biogeographical and macroecological diversity patterns [1]. Phylogenetic niche conservatism (PNC) is a holistic, parsimonious and increasingly accepted hypothesis that integrates the processes involved in the formation of diversity patterns This hypothesis proposes that taxa retain their ecophysiological traits (e.g., thermal tolerance) over evolutionary time and that, they tend to remain in environments that exhibit the abiotic conditions within which they evolved [3]. Others predictions of the PNC include a positive correlation between diversity and temperature [4, 5, 7], an increase in the size of taxa distribution ranges towards the colder end of an environmental gradient [10, 11], giving place to a macroecological pattern known as the Rapoport effect [12]; and local communities structured by competition in warm sites and structured by habitat filtering in cold sites [3, 13]
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