Pattern and Process in the Distributions of Non-diadromous Species 2: The ‘Pencil-Galaxias’ Species Group

Abstract

The ‘pencil galaxias’ species group includes five non-diadromous species that are widespread across central and southern New Zealand, with a centre of greatest diversity in the inland Mackenzie Basin of the eastern central South Island. They are small, very slender species of cold gravel rivers. Their geographical ranges relate closely to earth history, especially in the South Island, where some species’ ranges still reflect events during the Pleistocene glaciation, or reflect historical changes in river flow patterns. Absence of dwarf galaxias from South Westland reflects Pleistocene glaciation there, whereas absence in the northeastern North Island reflects Holocene to recent volcanism. Several pencil galaxias species are found widely in the inland, intermontane valleys of the eastern South Island, and probably penetrated them when Pleistocene glacial ice retreated. Dwarf galaxias is present on both sides of Cook Strait, probably a consequence of a land connection across the strait at lowered sea levels in the Pleistocene. Some species are present upstream of contemporary glacial lakes, and these distributions probably relate to riverine flow patterns during glacial retreat a few thousand years ago.