Spillover of organisms from rainforests affects local diversity of land-snail communities in the Akagera savanna in Rwanda

Laetoli, Tanzania: Extant terrestrial mollusc faunas shed new light on climate and palaeoecology at a Pliocene hominin site

“…Significant differences were found between the mollusc faunas of different habitat types ( Table 5 ), in species composition, richness and abundance (Tables 4 – 6 , Figs 4 and 5 ). That total mollusc species richness was highest in woodland/bushland habitats, rather than in the afromontane forests on Lemugurut, is somewhat unexpected because other studies have typically reported lower species richness in savanna than adjacent areas of forest [ 12 , 13 , 44 ]. (However, this may, in part, reflect the small number of sites (ie three) sampled on Lemugurut and the inclusion of the riverine forest sites 26.I and 26.II, which contain a mixture of forest and non-forest species, in the non-forest group).…”

“…But in some cases, the focus has been on historical or biogeographical factors [ 5 , 45 ], and this has identified variation and/or discontinuities that cannot be readily accounted for by current environmental conditions. The only detailed ecological study [ 12 ] of land-snails associated with an East African savanna ecosystem (in Rwanda) focussed on landscape scale factors and concluded that mollusc species richness was most strongly affected by distance from the remnants of rainforest occurring on an adjacent escarpment, with the secondary effects of annual precipitation (as reported here) and distance from the closest watercourse (or ‘next stream’). Wronski et al [ 12 ] hypothesised that the savanna snail fauna’s persistence was dependent, in part, on spillover from adjacent rainforests.…”

“…The only detailed ecological study [ 12 ] of land-snails associated with an East African savanna ecosystem (in Rwanda) focussed on landscape scale factors and concluded that mollusc species richness was most strongly affected by distance from the remnants of rainforest occurring on an adjacent escarpment, with the secondary effects of annual precipitation (as reported here) and distance from the closest watercourse (or ‘next stream’). Wronski et al [ 12 ] hypothesised that the savanna snail fauna’s persistence was dependent, in part, on spillover from adjacent rainforests. The present study did not examine landscape scale effects per se , but it did reveal strong compositional similarities between the faunas of the riverine forest sites (26.I and 26.II) and Afromontane forest fauna on Lemagurut, the former essentially being an impoverished version of the latter.…”

“…A small number of shells tentatively identified by Tattersfield [ 1 ] as a Cerastua species, which is regarded here as a weak indicator of closed forest conditions, might reflect temporal variation in conditions, or possibly the occurrence of patches of forest, during the Lower Laetolil. Some of these genera were also recorded in a savanna ecosystem in Rwanda [ 12 ], in particular, Succinea ( princei Preston), although only as 2 specimens, and two Gittenedouardia species, one recorded in moderately large numbers. However, [ 12 ] did not find Burtoa , despite Rwanda being closer to the centre of Burtoa ’s current geographical range, and Laetoli being on or close to the edge.…”

“…In contrast, and with the notable exception of an account of the molluscan fauna of a Rwandan savanna ecosystem [ 12 ], there have been no detailed studies on the faunas associated with East Africa’s other major vegetation types. Knowledge of the faunas of non-forested areas in East Africa has previously been based on anecdotal species lists and literature records; there is no review like that provided for South Africa [ 13 ].…”