New Zealand terrestrial molluscs
New Zealand’s land snail fauna is globally distinctive, and illustrative of many of the evolutionary and community patterns seen in the New Zealand invertebrate fauna as a whole.
Low family diversity
Of the 80 or more land snail families known globally, only 11 families are represented. This low rate of representation at the family level is not unusual for regional faunas – continental and archipelagic alike – and simply reflects the largely sub-global distributions of families.
A feature of the New Zealand land snail fauna is the absences of families that tend to dominate land snail faunas elsewhere; examples include Camaenidae, Helicidae, Hygromiidae, Streptaxidae, Subulinidae and Zonitidae.
This has many parallels in the New Zealand biota. For example, among New Zealand invertebrates there is remarkably low diversity of species that feed on land snails. Among Coleoptera (beetles) that may prey on land snails, only the Carabidae (ground beetles) are species rich. Specialised snail-eaters such as Drilidae, Lampyridae, and Silphidae are entirely absent from the New Zealand beetle fauna. Among Diptera (flies), the families Calliphoridae (blow flies), Phoridae (shuttle flies), Sacrophagidae (flesh flies), and especially Sciomyzidae (marsh flies) include important predators and parasitoids of land snails in many areas of the world yet are poorly represented in New Zealand. Formicidae (ants), which include important generalist predators in most areas of the world, are similarly poorly represented in New Zealand native ecosystems.
High genetic and species diversity
In contrast to low family diversity, the New Zealand land snail fauna is extraordinarily diverse at the generic and species levels. Spectacular evolutionary diversification has occurred in those families with recognised Gondwanian affinities, namely Pupinidae, Rhytididae, Athoracophoridae, and, in particular, the Punctidae and Charopidae. The result is a fauna estimated to comprise about 1400 species and thus is among the most speciose in the world for the land area. All but five of these species are endemic to New Zealand.
Much of this New Zealand species diversity has yet to be formally described. For example, collections at the Museum of New Zealand Te Papa Tongarewa and Auckland War Memorial Museum presently house at least 450 species-level taxa of the family Charopidae from New Zealand. Only 210 of these species have been formally described. Similarly, 90 punctid species have been described and currently recognised as valid, distinct entities within the New Zealand region, but collections presently house at least 286 additional undescribed New Zealand species-level taxa.
Shell shape and diversification
The New Zealand land snail fauna is globally distinctive in that taxonomic diversification has occurred with little apparent ecological diversification. Studies from various parts of the world have shown that variation in shell shape among species tends to exhibit a bimodal distribution, with species possessing either a tall shell or a subglobular to flat shell. Often, different phylogenetic clades have divergent shell morphologies when they coexist in a region, but display greater variation in shell form when they occur alone. Shell shape is evidently evolutionarily tied to niche characteristics – where and how species seek out a living – and begs the question of how families come together and share out the ecological opportunities. The New Zealand land snails depart from the common pattern in that the fauna is strongly dominated by species with flat shells, and thus exhibits little tendency to evolve tall shells despite the apparent absence of competitors.
The majority of New Zealand land snails are detritivores. They occur on the forest floor (and in humus suspended in epiphytes and other arboreal sites) and feed preferentially on the micro-organisms associated with plant decay, rather than on the dead plant material per se. Some are arboreal, and are primarily grazers of phylloplane micro-organisms. Others are carnivores, feeding on earthworms, amphipods, and other snails. A notable feature of the New Zealand fauna, relative to many continental areas, is the absence of herbivores that feed primarily on green plant tissues.
New Zealand land snails are also remarkable for the extraordinarily high numbers of species that can occur at individual sites. In many northern North Island forests, and some South Island sites, communities of 30 to 70 snail species can occur within areas of only a few square metres. This number of species in communities – often referred to as alpha or sympatric diversity – contrasts markedly with the 5–10 species typical of land snail communities elsewhere in the world.
Only in some temperate forests of Europe (e.g. Caucasus Mountains) and North America (Appalachian Mountains) and tropical rainforests of Africa, Madagascar, Japan and the West Indies is this level of diversity approached, with 20–45 sympatric species occasionally recorded. Some areas within New Zealand support rather modest land snail diversity and thus in terms of species richness approach conditions more typical of communities worldwide.