The species richness pattern of vascular plants along a tropical elevational gradient and the test of elevational Rapoport's rule depend on different life‐forms and phytogeographic affinities

Abstract

The research about species richness pattern and elevational Rapoport's rule (ERR) have been carried out mostly in the temperate regions in the recent years and scarcely in the tropical mountains; meanwhile, it is unclear whether the ERR is consistent among different life‐forms and phytogeographic affinities. Here, we compiled a database of plant species of Mount Kenya, a tropical mountain of East Africa, and divided these species into twelve groups depending on the life‐form and phytogeographic affinity of each species. We inspected the species richness pattern of each group along the elevation gradient and also tested ERR of each group using Stevens' method. Our results showed that species richness of the total species showed a positively skewed (hump‐shaped) pattern along the elevation gradient and different life‐forms and phytogeographic affinities showed similar hump‐shaped patterns as the total species. The average elevation range size of the total species and herbaceous species showed increasing patterns along the elevation gradient, while lycophytes and ferns, and woody species showed an obvious downward trend after peaking in the high elevation regions. We concluded that the widely distributed herbaceous species which also have broad elevation range sizes are more applicable to ERR, while the narrowly distributed woody species with small elevation range sizes occurring in the higher elevations could reverse ERR. Therefore, we concluded that the ERR is not consistent among different organisms in the same region.

Latitudinal patterns of species richness and range size of ferns along elevational gradients at the transition from tropics to subtropics

“…Originally conceived for latitudinal gradients, the idea that range sizes may be determined by climatic seasonality was later extended to elevational gradients as well (Stevens, 1992, climatic variability hypothesis). While these patterns have been documented for a wide range of taxa in many regions (Addo‐Bediako, Chown, & Gaston, 2000: insects; Ribas & Schoereder, 2006: many groups; Morin & Lechowicz, 2011: trees; Pintor, Schwarzkopf, & Krockenberger, 2015: lizards; Tomašových et al, 2016: birds and marine bivalves), there are also a good number of studies, mainly along elevational gradients in animals but also in plants, that do not corroborate the rule or even reporting a reverse pattern or mixed results suggesting that it varies between taxa and continents (Bhattarai & Veetas, 2006; Pintor et al, 2015; Ribas & Schoereder, 2006; Rohde & Heap, 1996; Rohde, Heap, & Heap, 1993; Ruggiero, 1994; Zhou et al, 2019). Support for the rule is also scarce in the tropics (Blackburn & Gaston, 1996; Rhode, 1996).…”

A Likelihood Framework for Modeling Pairwise Beta Diversity Patterns Based on the Tradeoff Between Colonization and Extinction

“…FIGURE 3 | Relationships of the colonization rate and extinction rate with multi-site beta diversity indices inferred using the proposed likelihood model and species richness (labeled as "matrix size") in a numerical simulation. of high colonization rates of species in high latitudes (Figure 4) is useful in explaining Rapoport's rule, which hypothesizes that species inhabiting high-latitude areas usually have large range sizes (Stevens, 1989(Stevens, , 1992(Stevens, , 1996Chen and Srivastava, 2015;Xing and He, 2018), even though it is not confirmed or partially supported in many empirical studies (Kerr, 1999;Bhattarai and Vetaars, 2006;Feng et al, 2016;Zhou et al, 2019). In our likelihood model, extinction rate contributed to species richness difference between a pair of sites (that is, species turnover) (Figure 1).…”

“…For future research, it would be interesting and worthwhile comparing native and non-native species communities and differences in their colonization and extinction frequencies, to provide better understanding of how non-native species change biodiversity patterns of local native ecosystems. Moreover, it would also be interesting to compare the colonization and extinction rates of different functional groups with different traits (Zhou et al, 2019), and partition the proportional contributions of colonization and extinction in structuring community dissimilarity. Finally, inspired by spatial metapopulation theory (Hanski and Thomas, 1994;Hanski, 1998Hanski, , 1999, it would be valuable to develop a spatially explicit colonizationextinction model to quantify spatial beta diversity patterns and provide insights for systematic conservation planning (Hanski and Thomas, 1994) when more-detailed information about locations of detected species is available and incorporated into the employed model.…”

Area and environmental heterogeneity could shape the hump-shaped pattern of species richness along elevation gradient

“…1a) [38][39][40][41]. In Mount Kenya endemic species and different life forms i.e., trees, shrubs, lianas, climbers, shrubs, lycophytes and ferns also show hump shaped pattern as shown by a study conducted by Zhou et al [42]. The hump shaped pattern in Mount Kenya makes it possible to draw a conclusion that volcanic mountains in tropical East Africa for example, Mount Kilimanjaro have similar vegetation patterns [43].…”

“…The plant distribution data of Mount Kenya come from a comprehensive checklist of seed plants based on data from various scientific expeditions to Mount Kenya since the 1900s [14,42]. The species richness was defined as the total number of species present in each elevation band referred to as γ-diversity [1,35] and this was achieved by interpolation method by defining a species as being present in every 100-m elevation band between its lower and upper elevation limits [12,15,35].…”