Distribution Patterns of Land Snails in Ugandan Rain Forests Support The

Journal of Biogeography (J. Biogeogr.) (2008) 35, 1759–1768 ORIGINAL Distribution patterns of land snails in ARTICLE Ugandan rain forests support the existence of Pleistocene forest refugia Torsten Wronski and Bernhard Hausdorf* Zoological Museum, University Hamburg, ABSTRACT Hamburg, Germany Aim We investigated whether the biogeographical patterns expected if the East African fauna was affected by cycles of contraction to refugia and expansion of ranges, as has been previously hypothesized, can be found in the land snail fauna of rain forests in Uganda. Location The Albertine Rift region and the Lake Victoria forest belt in Uganda. Methods Snails and slugs were sampled in 60 plots in 13 rain forests, and small species were extracted from 5-L leaf litter samples. Relative species richness was calculated by rarefaction. The influence of putative determinants of species richness was examined by bivariate correlation and multiple regression. Clustering and nestedness were tested by Monte Carlo simulations with a null model that considers the range size distribution, the species richness distribution of the forests, and the spatial autocorrelation of the occurrences of each species. Biotic elements were determined by model-based Gaussian clustering. Results A total of 169 land snail species were recorded from 13 Ugandan rain forests. Relative species richness increases with rainfall and altitude, and decreases with evaporation and distance from the putative East Congolian refugia. Mean annual rainfall and distance from the putative East Congolian refugia were included in the best multiple regression model. The distribution areas of the Ugandan land snails are significantly clustered. Two montane, two lowland and a northern biotic element were found. The mean range extension increases with increasing distance from the putative East Congolian refugia. Moreover, the ranges of the Ugandan land snails are significantly nested. The centre of the sets of nested subsets is in the Virunga Mountains, close to the putative East Congolian refugia. Main conclusions The decrease of diversity with increasing distance from the putative East Congolian refugia, the clustering and nestedness of ranges, and the range size increase with increasing distance from the refugia indicate that the East African land snail fauna was affected by cycles of contraction to refugia and expansion of ranges. The significant clustering and nestedness cannot be explained by current environmental conditions. Given the environmental history, it can be supposed that the lowland elements expanded post-glacially, whereas the ranges of the montane species are *Correspondence: Bernhard Hausdorf, probably currently contracting. Zoological Museum, University Hamburg, Keywords Martin-Luther-King-Platz 3, 20146 Hamburg, Germany. Biogeography, biotic elements, land snails, nestedness, Rapoport effect, refuge E-mail: [email protected] theory, species diversity, Uganda, vicariance. ª 2008 The Authors www.blackwellpublishing.com/jbi 1759 Journal compilation ª 2008 Blackwell Publishing Ltd doi:10.1111/j.1365-2699.2008.01933.x T. Wronski and B. Hausdorf INTRODUCTION Refuge theory assumes that the recent distribution of organisms is influenced by past, usually Pleistocene, environmental changes that resulted in the contraction of ranges into refugia or the expansion of ranges from refugia (Haffer, 1969, 1982, 1997; Mayr & O’Hara, 1986). Refugia are areas that are less affected by environmental changes than the surrounding regions, so that organisms that become extinct elsewhere can survive there. Moreover, it has often been assumed that the contraction of ranges into refugia resulted in speciation by vicariance (Haffer, 1969, 1982, 1997; Mayr & O’Hara, 1986). The existence and importance of Pleistocene refugia in the northern continents, which were heavily affected by Pleisto- cene glaciations, is universally accepted. However, the existence and role of refugia in the tropics remains controversial (Nichol, 1999; Smith et al., 2005; Noonan & Wray, 2006). Usually the existence of refugia has been inferred only from Figure 1 Map of Uganda showing the locations of the 13 forests the recent distributions of species richness and endemic species. studied: 1, Mgahinga; 2, Echuya, 3, Bwindi, 4, Kalinzu; 5, Kasyoha- However, several other patterns in the distribution of organisms Kitomi; 6, Maramagambo; 7, Kibale; 8, Semliki; 9, Matiri; 10, are expected to emerge if retraction to refugia and expansion Bugoma; 11, Budongo, 12, Mpanga; 13, Mabira. Dashed line: from refugia are processes that affect recent biogeography. Such approximate eastern border of the putative East Congolian refugia patterns include the clustering of ranges (Hausdorf & Hennig, (modified from Hamilton, 1976: fig. 1; Maley, 1996: fig. 5, 2001: 2004), the nestedness of ranges (Hulteń, 1937; Daubenmire, fig. 5; Jolly et al., 1997). 1975; Hausdorf & Hennig, 2003a), and an increase of average range extension with increasing distance from the refugia cutting. However, the plots in the Mabira and Bugoma forests (a Rapoport effect: Pfenninger, 2004; Hausdorf, 2006). were situated in recreation forest and in heavily encroached We investigated whether the patterns expected if the fauna forest patches, respectively, and sampling in Maramagambo was affected by cycles of contraction to refugia and expansion of Forest was carried out in colonizing forest. Sampling was ranges can be found in the land snail fauna of rain forests in the carried out in March and April 2006, during the rainy season, Albertine rift region in Uganda. We did not consider the when snails and slugs are most active. A combination of visual potential role of refugia in speciation. Land snails are good searching and sorting a standardized volume of litter is the model organisms for historical biogeography because they are most efficient method for land snail inventories, if repeated poor active dispersers that might reflect historical patterns visits to sites are not possible (Emberton et al., 1996; Cameron better than mammals or flying insects. The Albertine rift region & Pokryszko, 2005). Thus, we collected all living slugs and is especially suitable for such a study because Pleistocene refugia snails as well as their empty shells for 2 h at each plot, and in and post-glacial recolonization of adjacent areas have been addition collected c. 5 L of surface leaf litter and soil into proposed for this region (Hamilton, 1976; Diamond & Ham- plastic bags for later sampling. The litter samples were dried, ilton, 1980; Struhsaker, 1981; Crowe & Crowe, 1982; Rodgers fractioned by sieving, and sorted. More details of the sampling et al., 1982; Kingdon, 1990; Maley, 1996, 2001; Tattersfield, plots and lists of the sampled species will be published 1996; Grubb, 2001; Linder, 2001) and because it is one of the elsewhere (Wronski & Hausdorf, in preparation). centres of high biodiversity in Africa (Pomeroy, 1993). The material will be kept in the Zoological Museum at the University Hamburg in Germany. A reference collection has been deposited at the Zoological Museum of Makerere MATERIALS AND METHODS University, Kampala, in Uganda. Study area and sampling Measuring species richness We sampled 60 plots, each 20 m · 20 m, in 13 forests within protected areas along the Albertine Rift Valley and in the forest In order to survey the land snail fauna of a forest exhaustively it belt around Lake Victoria in Uganda (Fig. 1, Table 1). Plots is certainly insufficient to sample a few plots (Cameron & were selected so that diverse aspects within a forest were Pokryszko, 2005). Therefore, we standardized species richness to covered and according to accessibility. The number of plots account for the number of sampled specimens in a forest. We varied among forests, as some sites considered in the sampling used the approach proposed by Prendergast et al. (1993) to plan could not be visited because of heavy rainfall during the calculate relative species richness. First, the number of speci- study period or because of political instability. Most plots were mens was rarefied for each pair of forests to the lower number in primary forest that had no more than limited selective collected in one of the forests using EcoSim ver. 7 (Gotelli & 1760 Journal of Biogeography 35, 1759–1768 ª 2008 The Authors. Journal compilation ª 2008 Blackwell Publishing Ltd Distribution patterns of land snails in Uganda Entsminger, 2006). A pairwise measure of relative species richness, corrected for sampling effort, of forest A compared with forest B is calculated as the number of species expected in A divided by that expected in B for the same number of specimens. Distance to East Congolian refugia (km) The overall index of relative species richness for a forest is then calculated as the geometric mean of all such pairwise values. Mean altitude (m) Putative determinants of species richness We tested seven abiotic variables as potential determinants of Mean soil pH species richness (Table 2), namely mean annual rainfall, mean monthly evaporation (averaged over the year; calculated ) 2 estimates from meteorological data and corrected evapora- Forest area (km tion-pan data), mean annual maximum temperature (the three climatic parameters were taken from the Atlas of Uganda, Uganda Department of Land and Surveys, 1967), forest area (mainly from Howard et al., 2000), soil pH (from Langdale- Brown et al., 1964), mean altitude (measured with GPS), and Mean monthly evaporation (mm) shortest distance from the putative

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