Thirteen benthic foraminiferal species dominate modern “monospecific” faunas (dead faunas with >80% of one species in >63-μm samples) in New Zealand. These faunas occur in sheltered, often brackish, intertidal or shallow-subtidal environments, never deeper than 25 m. None occurs along exposed coasts or in the open ocean. Seven agglutinated species (Entzia macrescens, Haplophragmoides wilberti, H. manilaensis, Miliammina fusca, M. obliqua, Trochammina inflata, Trochamminita salsa) dominate “monospecific” faunas in salt marshes with varying salinity and elevational ranges. All but H. manilaensis have been recorded comprising 99–100% of at least one fauna. A further six species (Ammobaculites exiguus, Ammonia aoteana, Ammotium fragile, Elphidium excavatum clavatum, E. williamsoni, E. gunteri) dominate “monospecific” faunas in unvegetated intertidal and shallow-subtidal (<3 m), sheltered estuary, inlet, or lagoon settings. “Monospecific” Amb. exiguus faunas are inferred to have been produced by dissolution of calcareous components. A further 17 species dominate modern near-monospecific faunas (50–80% of one species), 11 at depths <50 m and six in the open ocean at 50–4000-m depth. “Monospecific” and near-monospecific faunas are more common in higher latitudes, where overall species diversity is lower.

Six species dominate “monospecific” early Miocene faunas in northern New Zealand: Elphidium crispum in a sheltered gravel beach; Nonionella novozealandica in a deep-water (50–100 m), possibly dysoxic harbour; and three larger, more robust species (Amphistegina aucklandica, Lepidocyclina orakiensis, Miogypsina intermedia) in current- or wave-concentrated beach or shallow-marine deposits. The only bathyal or abyssal “monospecific” fauna is dominated by Amphimorphinella butonensis occurring in a fossil hydrocarbon seep setting.

Many of the modern “monospecific” faunas (especially those in salt marshes) are cosmopolitan, whilst most of the fossil and some of the modern faunas are endemic to the New Zealand region. These high-dominance faunas are produced by taphonomic and ecological processes. Taphonomic causes include wave or current concentration by winnowing or transport in high energy, shallow-marine environments and carbonate dissolution in low pH, brackish, salt marsh or deep-sea settings. Ecological drivers include highly specific adaptations that allow species to outcompete all others in stressful (intertidal), strongly variable (high-tidal brackish), or unusual (hydrocarbon seep) environments. Sometimes high test productivity of opportunistic species may result in near-monospecific faunas.

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