Patterns and Predictors of Small Mammal Phylogenetic and Functional Diversity in Contrasting Elevational Gradients in Kenya

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 mitochondrial Cytochrome b gene 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 PDNTI and 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.

Decomposing niche components reveals simultaneous effects of opposite deterministic processes structuring alpine small mammal assembly

“…In this study, the elevational range were narrow (≤400 m). Thus, climatic and historical events, which have a substantial effect on a large spatial scale (Peters et al, 2016;Onditi et al, 2022), may not be the predominant driver of the species richness pattern. Given that the total height of each mountain varied vastly (3,600 to 6,740 m a.s.l.…”

“…In contrast, Ding et al (2021) found that phylogenetic structure, but not functional structure, was more overdispersed in bird assemblages at high elevations in the central Chinese Himalayas. The tendency of small mammal assemblages to be phylogenetically and functionally clustered in high elevations also showed contrasting patterns in three elevational gradients in Kenya (Musila et al, 2019;Onditi et al, 2022). Overall, these works have prompted more studies into the complexity of community assembly dynamics in the alpine area.…”

Elevation filters bat, rodent and shrew communities differently by morphological traits

“…These areas are known to be of high conservation importance harbouring many rare and threatened species (Farooq et al, 2020). This is particularly true of the Afrotropical mountains (Onditi et al, 2022), which is a globally important region in terms of threatened species (Ceballos & Ehrlich, 2006). Furthermore, mountains will play an increasingly important role in the long-term persistence of biodiversity in a climatically uncertain future (Payne et al, 2017).…”

“…These areas are known to be of high conservation importance harbouring many rare and threatened species (Farooq et al., 2020). This is particularly true of the Afrotropical mountains (Onditi et al., 2022), which is a globally important region in terms of threatened species (Ceballos & Ehrlich, 2006).…”

“…These are frequently viewed as ‘filters’ that exclude certain species from a community, and there is continued debate about which determines patterns of species occurrence (Ausprey et al., 2023; Cadotte & Tucker, 2017). Due to the rapid changes in environmental conditions associated with mountains (Graham et al., 2014) environmental filtering plays an important role in elevational gradients (Chakravarty et al., 2021; Onditi et al., 2022), which in turn may affect functional diversity, as only species with specific traits are able to cope with the extreme conditions at high elevations (Montaño‐Centellas et al., 2021).…”