Bee and ant burrows in Quaternary “coffee rock” and Holocene sand dunes, Kowhai Bay, Northland, New Zealand

Abstract

Curious, multi-coloured, and pelleted sand mounds (tumuli) constructed by solitary bees, together with similar, but loosely piled white sand mounds made by ants, are a striking feature of deflation and erosion surfaces in coastal sand dune territory of northernmost New Zealand. These native bee and ant mounds (to > 20 mounds/m 2) are found, respectively, on consolidated Quaternary “coffee rock” or atop shifting modern dune sands at Kowhai Bay, Aupouri Peninsula, across an area of several hundred square metres of bare and/or partly vegetated ground, lying some 800 m inland from the open ocean-facing Kowhai Beach. The vicinity is otherwise covered by scrubland vegetation dominated by the local ti tree, kanuka ( Kunzea ericoides, var. linaris), and the aggressive introduced alien, Acacia longifolium, which bloom in spring or summer, respectively, attracting bees, ants and other insects to the area. Many of the pelleted bee mounds are connected to simple and open, vertical to steeply inclined cylindrical shafts (cf . Skolithos) that may reach depths approaching one metre. The shafts sometimes terminate in a slightly ovoidal chamber (cell) that is lined with translucent, mucoidal, parchment-like layers and/or stuffed with pollen, and which occasionally contains a single white larva. These biogenic structures are created by solitary endemic colletid bees ( Leioproctus ( Leioproctus) metallicus) which we observed burrowing vertically through firm “coffee rock” at rates of up to 4 cm/hour. Many of these shafts are plugged near the surface by sand (uppermost 10 cm), or dead-end at shallow depths, suggesting concealment and decoy strategies used to avoid the bees’ natural predator, parasitic gasteruptid wasps. In contrast to the pelleted and multi-coloured bee mounds, those made by ants (e.g., Monomorium antarcticum) are uniformly pale grey or white in colour, with a granular and smooth, fine to medium sand surface (i.e ., they are non-pelleted). These ant mounds are crescent-shaped to circular, with a conspicuous central entrance hole that lies in a cratered depression, and which opens into irregularly branching burrows and passages that lack a lining other than some weak and discontinuous mucus-like coating (cf. Socialites). Quadrat sampling suggests that bee and ant mound distributions are oppositely related to substrate coherency. Field excavations indicate strong overprinting by bee excavations on consolidated dune fabrics. It is suggested that their burrows may have influenced groundwater movement in these iron-rich, paleosol-bearing strata. They also imply a paleoenvironmental shift from Kauri forest cover to deflationary sand dune episodes in the semi-tropical climate of the late Quaternary of northern New Zealand. The terrestrial deflationary setting can be likened to omission surfaces of marine environments.