Spatially explicit analysis of gastropod biodiversity in ancient Lake Ohrid

Abstract

Abstract. The quality of spatial analyses of biodiversity is improved by (i) utilizing study areas with well defined physiogeographical boundaries, (ii) limiting the impact of widespread species, and (iii) using taxa with heterogeneous distributions. These conditions are typically met by ecosystems such as oceanic islands or ancient lakes and their biota. While research on ancient lakes has contributed significantly to our understanding of evolutionary processes, statistically sound studies of spatial variation of extant biodiversity have been hampered by the frequently vast size of ancient lakes, their limited accessibility, and the lack of scientific infrastructure. The European ancient Lake Ohrid provides a rare opportunity for such a reliable spatial study. The comprehensive horizontal and vertical sampling of a speciesrich taxon, the Gastropoda, presented here, revealed interesting patterns of biodiversity, which, in part, have not been shown before for other ancient lakes.In a total of 284 samples from 224 different locations throughout the Ohrid Basin, 68 gastropod species, with 50 of them (= 73.5%) being endemic, could be reported. The spatial distribution of these species shows the following characteristics: (i) within Lake Ohrid, the most frequent species are endemic taxa with a wide depth range, (ii) widespread species (i.e. those occurring throughout the Balkans or beyond) are rare and mainly occur in the upper layer of the lake, (iii) while the total number of species decreases with water depth, the proportion of endemics increases, and (iv) the deeper layers of Lake Ohrid appear to have a higher spatial homogeneity of biodiversity. Moreover, gastropod communities of Lake Ohrid and its feeder springs are both distinct from each other and from the surrounding waters. The analysis also shows that community similarity of Lake Correspondence to: T. Hauffe (torsten.hauffe@bio.uni-giessen.de) Ohrid is mainly driven by niche processes (e.g. environmental factors), but also by neutral processes (e.g. dispersal limitation and evolutionary histories of species). For niche-based mechanisms it is shown that large scale effects such as type of water body or water depth are mainly responsible for the similarity of gastropod communities, whereas small scale effects like environmental gradients affect gastropod compositions only marginally. In fact, neutral processes appear to be more important than the small scale environmental factors, thus emphasizing the importance of dispersal capacities and evolutionary histories of species.

Latitudinal gradients of freshwater gastropods from the Western Palearctic

“…As an exception, the rate of addition of new species in lakes and ponds is exceptionally high between 40°N and 41°N. This especially abrupt change in species richness of lotic habitats is due to the unusual increase of species in lacustrine basins of the Balkan Peninsula, especially in that of Lake Ohrid with over 68 species (Hauffe et al 2011;Darwall et al 2014). In addition, this study demonstrates that, regardless of the habitat, the addition of new species gradually slows at higher latitudes until it reaches asymptotic values.…”

Constant diversification rates of endemic gastropods in ancient Lake Ohrid: ecosystem resilience likely buffers environmental fluctuations

“…The majority of the animal groups form relatively old species flocks in several higher taxa, including crustaceans (Wysocka et al, 2008(Wysocka et al, , 2014, leeches (Trajanovski et al, 2010), and gastropods (Albrecht et al, 2006;Wilke et al, 2007Wilke et al, , 2009. In fact, gastropods represent the most speciose animal group in Lake Ohrid, with 74 species described, 56 of which are endemic to the lake and its catchment Albrecht et al, 2009Albrecht et al, , 2014Hauffe et al, 2011;Radoman, 1985). The largest share of this diversity is held by snails of the family Hydrobiidae (Caenogastropoda: Truncatelloidea), including 13 pyrgulinid and 27 other endemic species (Radoman, 1983).…”

Diversity and Spatial Distribution of Extant Freshwater Ostracodes (Crustacea) in Ancient Lake Ohrid (Macedonia/Albania)

“…Ostracodes in Lake Ohrid show a distribution pattern similar to that for Gastropoda [32], in that the most abundant snail species are endemic and non-endemic species are relatively rare. Endemic snail species displayed a broad water-depth range, whereas non-endemic species were largely restricted to shallow waters, down to only ~5 m. Furthermore, most of the widespread species, which were previously unknown from the lake, occur mainly at the north end where the two largest Macedonian cities are located (Ohrid and Struga), and where much land is in agriculture.…”

“…The maximum water depth (293 m) is found in the southeast part of the basin. The lake has a surface area of 360 km 2 and a volume of 50.7 km 3 [31,32]. The steep-sided basin is located in a tectonic graben system of Pliocene to Pleistocene age [11] and is surrounded by mountains that reach ~1500 m a.s.l.…”

“…It has been suggested that the subaqueous springs create a high number of ecological niches and spring water has been important in the evolution of the Lake Ohrid ecosystem [32,38]. Furthermore, small-scale differences in sediment composition may be responsible for the patchiness in sub-structuring of benthic communities.…”