Contrasting environmental and regional effects on global pteridophyte and seed plant diversity

Authors

  • Holger Kreft,

  • Walter Jetz,

  • Jens Mutke,

  • Wilhelm Barthlott


H. Kreft (hkreft@uni-goettingen.de), Biodiversity, Macroecology and Conservation Biogeography Group, Fac. of Forest Sciences and Forest Ecology, Univ. of Göttingen, Büsgenweg 2, DE-37077 Göttingen, Germany, and Nees Inst. for Biodiversity of Plants, Univ. of Bonn, Meckenheimer Allee 170, DE-53115 Bonn, Germany, and Div. of Biological Sciences, Univ. of California San Diego, 9500 Gilman Drive MC 0116, La Jolla, CA 92093-0116, USA. – W. Jetz, Div. of Biological Sciences, Univ. of California San Diego, 9500 Gilman Drive MC 0116, La Jolla, CA 92093-0116, USA, and Dept of Ecology and Evolutionary Biology, Yale Univ., 165 Prospect Street, New Haven, CT 06520-8106, USA. – J. Mutke and W. Barthlott, Nees Inst. for Biodiversity of Plants, Univ. of Bonn, Meckenheimer Allee 170, DE-53115 Bonn, Germany.

Abstract

Pteridophytes (ferns and fern-allies) represent the second-largest group of vascular plants, but their global biogeography remains poorly studied. Given their functional biology, pteridophytes are expected to show a more pronounced relation to water availability and a higher dispersal ability compared to seed plants. We test these assertions and document the global pattern of pteridophyte richness across 195 mainland and 106 island regions. Using non-spatial and spatial simple and multiple regression models, we analyze geographic trends in pteridophyte and seed plant richness as well as pteridophyte proportions in relation to environmental and regional variables. We find that pteridophyte and seed plant richness are geographically strongly correlated (all floras: r=0.68, mainland: r=0.82, island floras: r=0.77), but that the proportions of pteridophytes in vascular plant floras vary considerably (0–70%). Islands (mean=15.3%) have significantly higher proportions of pteridophytes than mainland regions (mean=3.6%). While the relative proportions of pteridophytes on islands show a positive relationship with geographic isolation, proportions in mainland floras increase most strongly along gradients of water availability. Pteridophyte richness peaks in humid tropical mountainous regions and is lowest in deserts, arctic regions, and on remote oceanic islands. Regions with Mediterranean climate, outstanding extra-tropical centres of seed plant richness, are comparatively poor in pteridophytes. Overall, water-energy variables and topographical complexity are core predictors of both mainland pteridophyte and seed plant richness. Significant residual richness across biogeographic regions points to an important role of idiosyncratic regional effects. Although the same variables emerge as core predictors of pteridophyte and seed plant richness, water availability is clearly a much stronger constraint of pteridophyte richness. We discuss the different limitations of gametophytes and sporophytes that might have limited the ability of pteridophytes to extensively diversify under harsh environmental conditions. Our results point to an important role of taxon-specific functional traits in defining global richness gradients.

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