Studies on the diversity of land molluscs raise a number of crucial questions that remain unanswered, or in some cases have not been asked. At the most fundamental level, we need to question the comparability of species concepts used in different studies, which influence our assessment of overall diversity and of regional differentiation. Current estimates of global diversity are likely to be too low. Beyond that, the assumption that all species can be represented by equivalent digits needs to be challenged: the range of size, habits and trophic levels needs to be taken into account. While higher taxonomic categories can be used as rough proxies for similar ecology, we have very little detailed information other than for shell size and shape, where the huge range suggests some radical differences within and among faunas. The range of niches occupied may differ among faunas: are we dealing with comparable taxonomic entities? Our knowledge of microhabitat requirements is severely deficient.
Regional species richness varies much more than maximum local (site) richness across faunas. The poor regional faunas of areas subject to glacial or periglacial Pleistocene conditions are not always impoverished at local level. While this might suggest the presence of competitive interactions, the evidence for competition is scarce. There are many cases where closely-related species live in sympatry at the smallest scales. Nevertheless, there are cases of parapatric or microallopatric distributions of congeners, and metapopulation dynamics can facilitate the co-existence of potential competitors. There may be no general rule; cases of both character displacement and convergence in sympatry are reported.
While most studies accept the traditional model of allopatric speciation to account for the development of diversity, the balance between vicariance and dispersal as driving forces remains undetermined, and clearly varies from case to case. Poor powers of active dispersal may permit genetic differentiation over very short distances, but there are a number of spectacular cases of long-distance passive dispersal that suggest that our direct estimates of dispersal are questionable. Better knowledge of actual dispersal and of the ecological requirements of land mollusc species should be priorities for the future.