Abstract
Mountains of the Afrotropics are global biodiversity hotspots and centers of speciation and endemism; however, very few studies have focused on the phylogenetic and functional dimensions of Afromontane small mammals. We investigated the patterns and mechanisms of small mammal phylogenetic and functional diversity and assembly along elevational gradients in Mount Kenya, the second highest mountain in Africa, and a contrasting low mountain range, Chyulu Hills. We sampled 24 200-m interval transects in both sites; 18 in Mt. Kenya (9 each in the windward side, Chogoria, and the leeward side, Sirimon) and 6 in Chyulu. We extracted the mitochondrialCytochrome bgene to reconstruct a time-calibrated species tree for estimating phylogenetic diversity indices [phylogenetic richness (PD), mean nearest taxon distance (PDMNTD), and nearest taxon index (PDNTI)]. A functional trait data set was compiled from the field-recorded measurements and published data sets for estimating functional diversity indices [functional richness (FD), mean nearest taxon distance (FDMNTD), and nearest taxon index (FDNTI)]. Several environmental variables representing water-energy availability, primary habitat productivity, and topographic heterogeneity were used to estimate the predictive power of abiotic conditions on diversity variances using generalized linear and generalized additive regression models. The PD and FD peaked around mid-elevations in Mt. Kenya, unimodally increased or decreased in Chogoria and Sirimon, and monotonically increased in Chyulu. The divergence and community structure indices—PDMNTD, FDMNTD, and PDNTIand FDNTI—were relatively weakly associated with elevation. Overall, the tendency of assemblages to be phylogenetically and functionally closely related than expected by chance decreased with elevation in Mt. Kenya but increased in Chyulu. Across the indices, the annual precipitation and topographic ruggedness were the strongest predictors in Mt. Kenya, evapotranspiration and temperature seasonality were the strongest predictors in Chyulu, while temperature seasonality and terrain ruggedness overlapped as the strongest predictors in Chogoria and Sirimon in addition to annual precipitation in the latter and normalized difference vegetation index in the former. The observed contrasting trends in diversity distribution and the strongest predictors between elevational gradients are integral to the sustainable management of the high faunal biodiversity in tropical Afromontane ecosystems.
Highlights
Understanding species ecological and evolutionary spatial patterns and mechanisms underlying assembling patterns remains of heightened ecological interest in mountain ecosystems, where sharp variations in climate and space along elevational gradients dynamically structure communities over short distances (Rahbek et al, 2019a; Perrigo et al, 2020)
Kenya and the Chuylu Hills mountain range, our analyses offer a first-of-its-kind insight into how the phylogenetic and functional diversity of small mammals vary along tropical mountain elevational gradients and how patterns are associated with environmental predictors
Our results suggest that the habitat productivity hypothesis (Rosenzweig, 1992; McCain et al, 2018) might be a weaker predictor of PD and FD variances along elevational gradients in the tropics, the actual evapotranspiration (AET), PAM, and temperature seasonality (TAS) control of primary habitat productivity, in terms of vegetation diversity and abundance (Hawkins et al, 2003), which has more direct influences on small mammal ecological strategies and the association may confound the predictive influence of normalized difference vegetation index (NDVI) and vegetation types on diversity variances
Summary
Understanding species ecological and evolutionary spatial patterns and mechanisms underlying assembling patterns remains of heightened ecological interest in mountain ecosystems, where sharp variations in climate and space along elevational gradients dynamically structure communities over short distances (Rahbek et al, 2019a; Perrigo et al, 2020). While taxonomic diversity distribution patterns of mountains are considerably well studied in the Afrotropics, phylogenetic and functional dimensions have been of much less interest (McCain and Grytnes, 2010; Graham et al, 2014; Quintero and Jetz, 2018). There is a need for more studies on the Afrotropic mountains based on species evolutionary and ecosystem functioning dimensions to refine the integrated understanding of how environmental changes drive species assembling to enable effective biodiversity conservation actions. Mountain diversity-elevation associations are dominated by unimodal trends where the highest species richness occurs around the middle elevations (McCain and Grytnes, 2010; Guo et al, 2013; Quintero and Jetz, 2018). There is a need for more studies of poorly documented Afromontane mountains to disentangle the phylogenetic and functional diversity trends along elevational gradients, which can inform more ecologically informed biodiversity management actions (Brum et al, 2017)
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