ZOOGEOMORPHIC CASCADES AND CONVERGENCE OF BIOTURBATION STRUCTURES ALONG TEMPERATE AND TROPICAL VEGETATED BIOTOPE MARGINS

Multiple controls on burrow geometry and extent, including the optimization of the energy expended on excavation, may result in convergent gross burrow characteristics in a variety of semi-fossorial taxa. In temperate riparian settings, soil reworking by groundhogs (Marmota monax), while localized, exerts a major impact on densely vegetated stream margins. In southeastern Pennsylvania, multiple burrow complexes with up to 5 surface openings (entrances/exits) occur proximal to active cutbanks and have average inclinations of 30°. Most spoil mounds have been reworked by surface processes or partially collapsed into abandoned shafts. In tropical carbonates, extensive bioturbation by blue land crabs (Cardisoma guanhumi) is under investigation on the Bahamian Islands of Eleuthera and San Salvador. In supratidal mangrove fringe, this decapod excavates galleries with multiple openings (diameter: 8 to >20 cm) and associated spoil piles. Similar to groundhog burrows, these mega-invertebrate structures have relatively steep inclinations, but their termini may be submerged more frequently by tidally mediated water table fluctuations. Due to the magnitude of substrate reworking, both tracemakers have substantial influence on local microtopography and sediment budget (through sediment trapping and spoil dispersal). Their large diameters also make these structures ideal for high-resolution imaging with 400-800 MHz georadar, allowing visualization of partially filled burrow segments. Vertical penetration of >1 m causes either avoidance or extraction of large clasts (including artifacts), which otherwise remain inert beneath stable soil profiles due to the absence of advective processes common in permafrost regions. In both settings, the length of time needed to excavate and maintain burrow complexes, as well as their effect on surrounding vegetation and commensal or secondary inhabitants, generate diagnostic geomorphic cascades that must have analogs in the geologic record.