3. Disjunct distribution of bryophytes in the Neotropics and Africa
A feature of the biogeography of many groups is disjunction, where related taxa are found in separate, often widely separated geographic areas with no populations in between (despite seemingly suitable habitat). Studying disjunctions offers an opportunity to better understand historical connections between taxa in distant regions and how species dispersal mechanisms contribute to distribution patterns across scales. Among plants, disjunctions between the Neotropical region and the African region (including its associated islands) have received attention but the drivers of observed patterns remain poorly understood (Davis et al., 2002; Lopes et al., 2023).
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Examples of Neotropical-African disjunctions include the fern Asplenium auritum from the Neotropics where populations of the species are also reported in scattered localities in East Africa, Madagascar and the Mascarenes (Moran and Smith, 2001). Likewise, the leafy liverworts Symbiezidium barbiflorum are only known from two type localities in Madagascar, but were first reported and are widespread in South America (Figure 1) (Sass-Gyarmati and Pócs, 2002). Distribution patterns such as these are puzzling given Madagascar and South America were last connected 180 million years ago and the absence of intermediate lineages in comparable habitats in the more proximate and more recently connected landmasses of India and mainland Africa.
​​​​​​​​​​​Disjunct distribution patterns are hypothesized to result from various processes such as historical land connections, the fragmentation of a once more continuous distribution, and long-distance dispersal. Previous analyses, however, have suffered from inadequate representation of specimens from important geographic areas and taxonomic uncertainty, leading to limited confidence in the validity of many supposedly disjunct taxa.​​
Figure 1: The disjunct Neotropical-African distribution of exemplars: (right) Symbiezidium barbiflorum (Lindenb. et Gottsche) A.Evans and (left) Leiomela (Mitt.) Broth
​​Bryophytes provide an opportunity to better characterize the relationships between taxa shared between the Neotropics and-Africa. Notably for the bryophytes, disjunct distributions are common and often observed at the infra-specific level, with subpopulations of a single species observed in both the Neotropics and Africa. Reflecting this, endemism rates for bryophyte species are reported to be substantially lower than for other taxa: in Madagascar approximately 90% of vascular species are endemic, whereas only 30% of bryophytes are endemic (though current estimates are based mainly on outdated literature review and morphology based species description that lack resolution). In Brazil, estimates of endemism among vascular plant species are approximately 57% but only 19.6% for bryophytes (Costa et al., 2015; Forzza et al., 2012). More accurate taxonomy and synthesis are needed to determine whether these patterns are due to bryophytes’ noted efficient dispersal (e.g., due to their air-borne diaspores), remnant populations in refugia from long ago geographic connections, or are artefacts of morphologic similarity leading to misleading species identifications.
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The aims of this study can be clustered in three points, where I seek to characterize the Neotropics-Africa disjunction using bryophytes as a model system:
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Resolving taxonomic uncertainty by conducting taxonomic revision and identification of erroneously and unidentified specimens
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Synthesizing data on the geographic connections between the bryophyte floras of the Neotropical and African regions using museum and complementary databases
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Documenting the morphological differentiation among intercontinental populations and assessing the rate of morphological change over time: changes that can be assessed by examining specimens in herbarium from different decades
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Bryophytes provide rich material for addressing these aims. A screening of the literature revealed 35 species and 8 genera or infrageneric liverwort taxa with Neotropical-African ranges. Delgadillo (1993) reported 334 shared moss species between the Americas and Africa, of which 45 are exclusively from South America and Africa. Previous analyses of the Neotropical-African disjunct distribution of bryophytes were limited due to inadequate representation of the diversity of bryophytes in the two regions (Delgadillo, 1993; Gradstein et al., 1983). Additionally, the existing collections for bryophytes will benefit from updated identifications in this difficult to identify, often cryptic group.
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