“…Whether these species' ranges have always been small or have contracted into relictual patches remains an open question. The old endemic species' tendency to aggregate may reflect historical range contractions into refugia [37,38]. In accordance with this idea, we found that young and old and endemic species had distinct habitat preferences (figure 1; Fisher's exact, p = 0.026; electronic supplementary material, table S3).…”
Section: Resultssupporting
confidence: 80%
“…Among Furnariidae birds in South America, the transition to secondary sympatry following a speciation event is initially slow, which coincides with expectations of interspecific competition limiting range overlap between recently derived lineages [36]. Alternatively, a slow initial rate of secondary sympatry could reflect young highland species dispersing vertically to lower elevations, instead of horizontally [37], following the increased availability of surface area and habitat connectivity downslope [13,14,37], or cyclic phases of glaciation and inter-glaciation periods [16,27]. Figure 3Richness-frequency distribution of young and old endemic species.…”
Section: Resultsmentioning
confidence: 76%
“…B 289: 20221102 overlap between recently derived lineages [36]. Alternatively, a slow initial rate of secondary sympatry could reflect young highland species dispersing vertically to lower elevations, instead of horizontally [37], following the increased availability of surface area and habitat connectivity downslope [13,14,37], or cyclic phases of glaciation and inter-glaciation periods [16,27].…”
The immense concentrations of vertebrate species in tropical mountains remain a prominent but unexplained pattern in biogeography. A long-standing hypothesis suggests that montane biodiversity hotspots result from endemic species aggregating within ecologically stable localities. Here, the persistence of ancient lineages coincides with frequent speciation events, making such areas both ‘cradles’ (where new species arise) and ‘museums’ (where old species survive). Although this hypothesis refers to processes operating at the scale of valleys, it remains supported primarily by patterns generated from coarse-scale distribution data. Using high-resolution occurrence and phylogenetic data on Andean hummingbirds, we find that old and young endemic species are not spatially aggregated. The young endemic species tend to have non-overlapping distributions scattered along the Andean treeline, a long and narrow habitat where populations easily become fragmented. By contrast, the old endemic species have more aggregated distributions, but mainly within pockets of cloud forests at lower elevations than the young endemic species. These findings contradict the premise that biogeographical cradles and museums should overlap in valley systems where pockets of stable climate persist through periods of climate change. Instead, Andean biodiversity hotspots may derive from large-scale fluctuating climate complexity in conjunction with local-scale variability in available area and habitat connectivity.
“…Whether these species' ranges have always been small or have contracted into relictual patches remains an open question. The old endemic species' tendency to aggregate may reflect historical range contractions into refugia [37,38]. In accordance with this idea, we found that young and old and endemic species had distinct habitat preferences (figure 1; Fisher's exact, p = 0.026; electronic supplementary material, table S3).…”
Section: Resultssupporting
confidence: 80%
“…Among Furnariidae birds in South America, the transition to secondary sympatry following a speciation event is initially slow, which coincides with expectations of interspecific competition limiting range overlap between recently derived lineages [36]. Alternatively, a slow initial rate of secondary sympatry could reflect young highland species dispersing vertically to lower elevations, instead of horizontally [37], following the increased availability of surface area and habitat connectivity downslope [13,14,37], or cyclic phases of glaciation and inter-glaciation periods [16,27]. Figure 3Richness-frequency distribution of young and old endemic species.…”
Section: Resultsmentioning
confidence: 76%
“…B 289: 20221102 overlap between recently derived lineages [36]. Alternatively, a slow initial rate of secondary sympatry could reflect young highland species dispersing vertically to lower elevations, instead of horizontally [37], following the increased availability of surface area and habitat connectivity downslope [13,14,37], or cyclic phases of glaciation and inter-glaciation periods [16,27].…”
The immense concentrations of vertebrate species in tropical mountains remain a prominent but unexplained pattern in biogeography. A long-standing hypothesis suggests that montane biodiversity hotspots result from endemic species aggregating within ecologically stable localities. Here, the persistence of ancient lineages coincides with frequent speciation events, making such areas both ‘cradles’ (where new species arise) and ‘museums’ (where old species survive). Although this hypothesis refers to processes operating at the scale of valleys, it remains supported primarily by patterns generated from coarse-scale distribution data. Using high-resolution occurrence and phylogenetic data on Andean hummingbirds, we find that old and young endemic species are not spatially aggregated. The young endemic species tend to have non-overlapping distributions scattered along the Andean treeline, a long and narrow habitat where populations easily become fragmented. By contrast, the old endemic species have more aggregated distributions, but mainly within pockets of cloud forests at lower elevations than the young endemic species. These findings contradict the premise that biogeographical cradles and museums should overlap in valley systems where pockets of stable climate persist through periods of climate change. Instead, Andean biodiversity hotspots may derive from large-scale fluctuating climate complexity in conjunction with local-scale variability in available area and habitat connectivity.
“…In a location where the species is indeed present on a 0.2 × 0.2 degree scale, it should be captured by this method. Our approach follows procedures of other studies that have resolved distributions using published elevational and geographical ranges (e.g., Quintero & Jetz, 2018;van Els et al, 2021). In our case, geographical distributions are only resolved to the level of state or country (Figure 1).…”
Aim: At the macroscale, climate strongly correlates with species richness gradients, resulting from differences in in-situ diversification and dispersal. One historical explanation for the pattern is that regions spanning temperate climates contain few species because past disturbances have generated high extinction rates, and species from tropical regions are unable to easily colonize temperate regions. We test these postulates for Himalayan plants, which span subtropical to temperate climates over steep elevational gradients.Location: Himalaya.
Time period: Present day.Major taxa studied: Angiosperms.
Methods:We use a comprehensive survey of 31 floras to document the elevational and geographical distributions of native Himalayan plants, augmented by field studies of trees in both the east and west Himalaya. We use grade of membership models to cluster species according to locations shared and phylogenetic analysis to evaluate diversification rates.Results: Species fall into four cohesive biotas, organized by climate. Points of turnover between biotas occur where the mean minimum temperature of the coldest month is approximately 0 °C (2,000-2,500 m), and at the point of occasional annual freezing (1,000-1,500 m); these boundaries run the length of the Himalaya. The patterns are retained when we consider whole clades rather than species. All plants (and the subsets trees, herbs and shrubs) belonging to the biota above the 2,000-2,500 m line have higher recent speciation rates than those lower down.
Main conclusions:We attribute the high rate of recent speciation in temperate climates to high rates of turnover, creating ecological and geographical opportunity.The high elevation biota has few species, but spans the largest area, implying species numbers are far from any carrying capacity, at least with respect to accumulation of allopatric forms. This study thus links climatic restrictions of clades to differences in diversification rates, and by inference species numbers.
“…In this case, our model may underestimate transition rates to sympatry. While we think it is unlikely that such biases would also apply to elevation overlap, developing models that incorporate the reciprocal evolution between geographical/elevation overlap and diversification dynamics is an important avenue for future research (Goldberg et al, 2011; Landis et al, 2022; van Els et al, 2021).…”
AimThe exceptional turnover in biota with elevation and number of species coexisting at any elevation makes tropical mountains hotspots of biodiversity. However, understanding the historical processes through which species arising in geographical isolation (i.e. allopatry) assemble along the same mountain slope (i.e. sympatry) remains a major challenge. Multiple models have been proposed including (1) the sorting of already elevationally divergent species, (2) the displacement of elevation upon secondary contact, potentially followed by convergence, or (3) elevational conservatism, in which ancestral elevational ranges are retained. However, the relative contribution of these processes to generating patterns of elevational overlap and turnover is unknown.LocationTropical mountains of Central‐ and South‐America.Time PeriodThe last 12 myr.Major Taxa StudiedBirds.MethodsWe collate a dataset of 165 avian sister pairs containing estimates of phylogenetic age, geographical and regional elevational range overlap. We develop a framework based on continuous‐time Markov models to infer the relative frequency of different historical pathways in explaining present‐day overlap and turnover of sympatric species along elevational gradients.ResultsWe show that turnover of closely related bird species across elevation can predominantly be explained by displacement of elevation ranges upon contact (81%) rather than elevational divergence in allopatry (19%). In contrast, overlap along elevation gradients is primarily (88%) explained by conservatism of elevational ranges rather than displacement followed by elevational expansion (12%).Main ConclusionsBird communities across elevation gradients are assembled through a mix of processes, including the sorting, displacement and conservatism of species elevation ranges. The dominant role of conservatism in explaining co‐occurrence of species on mountain slopes rejects more complex scenarios requiring displacement followed by expansion. The ability of closely related species to coexist without elevational divergence provides a direct and faster pathway to sympatry and helps explain the exceptional species richness of tropical mountains.
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