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Subsampling Herbarium Collections to Assess Geographic Diversity Gradients: A Case Study with Endemic Orchidaceae and Rubiaceae in Cameroon

Authors

  • Vincent Droissart,

    1. Laboratoire de Complexité et Dynamique des Systèmes Tropicaux, Université Libre de Bruxelles – ULB, 50 Av. F. Roosevelt, CP 169, 1050 Bruxelles, Belgium
    2. Institut de Recherche pour le Développement (IRD), Unité Mixte de Recherche AMAP (Botanique et Bioinformatique de l'Architecture des Plantes), Boulevard de la Lironde, TA A-51/PS2, F-34398 Montpellier Cedex 5, France
    3. Herbarium et Bibliothèque de Botanique africaine, Université Libre de Bruxelles – ULB, 50 Av. F. Roosevelt, CP 169, 1050 Bruxelles, Belgium
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  • Olivier J. Hardy,

    1. Service Evolution Biologique et Ecologie, Université Libre de Bruxelles – ULB, 50 Av. F. Roosevelt, CP160/12, 1050 Bruxelles, Belgium
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  • Bonaventure Sonké,

    1. Service Evolution Biologique et Ecologie, Université Libre de Bruxelles – ULB, 50 Av. F. Roosevelt, CP160/12, 1050 Bruxelles, Belgium
    2. Laboratoire de Botanique systématique et d'Ecologie, Département des Sciences Biologiques, Ecole Normale Supérieure, Université de Yaoundé I, B.P. 047, Yaoundé, Cameroon
    3. Missouri Botanical Garden, Africa and Madagascar Department, PO Box 299, St. Louis, Missouri 63166-0299, U.S.A.
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  • Farid Dahdouh-Guebas,

    1. Laboratoire de Complexité et Dynamique des Systèmes Tropicaux, Université Libre de Bruxelles – ULB, 50 Av. F. Roosevelt, CP 169, 1050 Bruxelles, Belgium
    2. Herbarium et Bibliothèque de Botanique africaine, Université Libre de Bruxelles – ULB, 50 Av. F. Roosevelt, CP 169, 1050 Bruxelles, Belgium
    3. Laboratory of Plant Biology and Nature Management, Vrije Universiteit Brussel – VUB, Pleinlaan 2, B-1050 Brussels, Belgium
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  • Tariq Stévart

    1. Herbarium et Bibliothèque de Botanique africaine, Université Libre de Bruxelles – ULB, 50 Av. F. Roosevelt, CP 169, 1050 Bruxelles, Belgium
    2. Missouri Botanical Garden, Africa and Madagascar Department, PO Box 299, St. Louis, Missouri 63166-0299, U.S.A.
    3. National Botanic Garden of Belgium, Domein van Bouchout, Nieuwelaan 38, B-1860 Meise, Belgium
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ABSTRACT

We compiled herbarium specimen data to provide an improved characterization of geographic patterns of diversity using indices of species diversity and floristic similarity based on rarefaction principles. A dataset of 3650 georeferenced plant specimens belonging to Orchidaceae and Rubiaceae endemic to Atlantic Central Africa was assembled to assess species composition per half-degree or one-degree grid cells. Local diversity was measured by the expected number of species (Sk) per grid cell found in subsamples of increasing size and compared with raw species richness (SR). A nearly unbiased estimator of the effective number of species per grid cell was also used, allowing quantification of ratios of ‘true diversity’ between grid cells. Species turnover was measured using a presence/absence-based similarity index (Sørensen) and an abundance-based index that corrects for sampling bias (NNESS). Our results confirm that the coastal region of Cameroon is more diverse in endemic species than those more inland. The southern part of this coastal forest is, however, as diverse as the more intensively inventoried northern part, and should also be recognized as an important center of endemism. A strong congruence between Sørensen and NNESS similarity matrices lead to similar delimitations of floristic units. Hence, heterogeneous sampling seems to confer more bias when measuring patterns of local diversity using raw species richness than species turnover using Sørensen index. Overall, we argue that subsampling methods represent a useful way to assess diversity gradients using herbarium specimens while correcting for heterogeneous sampling effort.

Abstract in French is available in the online version of this article.

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