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1. To test hypotheses concerning the applicability of the Rapoport effect (RE: "species that occur at higher latitudes tend to have greater geographical range-size than species which have ranges limited to latitudes closer to the equator") to aquatic macrophytes at global scale, we analysed the world latitudinal distribu

Murphy, Kevin J.. 2016. Spatial and environmental drivers of macrophyte diversity and community composition in temperate and tropical calcareous rivers. Aquatic Botany, 132.The NERC and CEH trademarks and logos ('the Trademarks') are registered trademarks of NERC in the UK and other countries, and may not be used without the prior written consent of the Trademark owner. Spatial and environmental drivers of macrophyte diversity and community 1 composition in temperate and tropical calcareous rivers 2 3Abstract 30The hypothesis was examined that sources of variation in macrophyte species 31 richness (alpha-diversity: S) and community composition ("species-set"), attributable to 32 spatial and environmental, variables, may differ in importance between tropical and 33 temperate calcareous rivers (>10 mg CaCO 3 L -1 ). To test this hypothesis geographic, 34 environmental, and aquatic vegetation data were acquired for 1151 sites on calcareous 35 rivers within the British Isles, supporting 106 macrophyte species (mean S: 3.1 species 36 per sample), and 203 sites from Zambian calcareous rivers, supporting 255 macrophyte 37 species (mean S: 8.3 species per sample). The data were analysed using an 38 eigenfunction spatial analysis procedure, Moran's Eigenvector Maps (MEM), to assess 39 spatial variation of species richness and community composition at large regional scale 40 (>10 5 km 2 : British Isles and Zambia); and at medium catchment scale (10 4 -10 5 km 2 : 41 British Isles only). Variation-partitioning was undertaken using multiple regression for 42 species richness data, and partial redundancy analysis (pRDA) for community data. For 43 the British Isles, spatial and environmental variables both significantly contributed to 44 explaining variation in both species richness and community composition. In addition, a 45 substantial amount of the variation in community composition, for the British Isles as a 46 whole and for some RBUs, was accounted for by spatially-structured environmental 47 variables. In Zambia, species richness was explained only by pure spatial variables, but 48 environmental and spatially-structured environmental variables also explained a 49 significant part of the variation for community composition. At medium-scale, in the 50 British Isles, species richness was explained by spatial variables, and only for four of 51 the six RBUs. 52 53

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This study assessed the hypothesis that spatial and environmental drivers of river macrophyte diversity and community composition differ in relative importance in calcareous river systems located in warm regions of America versus Africa. We collected aquatic vegetation and spatio‐environmental data, during 2006–11, from >200 hardwater rivers, and associated floodplain waterbodies, located up to 30° north or south of the Equator, in México, Trinidad, Brazil, Argentina, U.S.A. (Florida), South Africa, Botswana and Zambia. Species rarefaction procedures were used to assess the impacts of differing sampling effort in the two continents upon estimation of γ‐diversity (“species pool”). We then used a cluster analysis approach (two‐way indicator species analysis: TWINSPAN) to classify samples into groups based upon species composition. Variation in species richness, community composition and six spatial and environmental variables, among samples making up these groups, were compared using ANOVA and Kruskal–Wallis procedures. Regression trees and redundancy analysis were used to infer the relative importance of spatial and environmental drivers in explaining variation in local species richness and species community composition between the two continents. Sorensen's index (Cs) was calculated to estimate species turnover (β‐diversity) between African and American samples. In total, 378 macrophyte taxa were recorded, with no significant difference in mean macrophyte α‐diversity between African and American sites, but with evidence for high species turnover between the two continents (Cs = 0.17). Rarefaction analysis confirmed the existence of a larger macrophyte species pool in the hardwater rivers sampled in Africa compared to America. TWINSPAN classification identified seven sample end‐groups, only one of which contained a mix of sites from both continents. PERMANOVA and non‐metric multidimensional scaling ordination analysis confirmed significant differences in community composition present in these sample‐groups. There were substantial differences between the sample‐groups for α‐diversity, and for spatial and environmental variables. The high species turnover between Africa and America may be accounted for by geographical segregation, along with differences in aquatic habitat characteristics, and varying long‐distance dispersal capacities of individual species. The relative importance of spatial and physicochemical drivers (latitude, pH, altitude, alkalinity and electrical conductivity but not flow) differed between the continents in influencing variation in both macrophyte diversity and community composition. Latitude was a significant, although nonlinear and rather complex, spatial driver of macrophyte α‐diversity in both American and African hardwater rivers. Water chemistry variables varied in relative importance as drivers of macrophyte α‐diversity for African and American sites individually, and for all sites combined, but pH and/or electrical conductivity were more important than alkalinity in each cas...