zoogeomorphology Zoogeomorphology is the study of the geomorphological effects of animals, including both vertebrates and invertebrates. The geomorphological effects encompass the processes of erosion, transport, and deposition of rock, soil, and unconsolidated sediments. Zoogeomorphological effects specifically involve the movement of rock, soil, or unconsolidated sediment from one location to another, whereas faunal bioturbation involves mixing of rocks and sediment without net transport from a site. The geomorphological effects of animals can be categorized into direct effects involving actual excavational or contructional activities accomplished by the processes of digging for and caching of food, burrowing and nest-building, mound-building (usually in association with burrowing), wallowing, geophagy and lithophagy (animal consumption of soil or rocks), and dam-building by beavers (genus Castor); and indirect effects arising through animal influences (trampling, tunnelling, burrowing, vegetation removal) on surface and subsurface infiltration, soil creep, surface wash, and rainsplash detachment.
Burrowing encompasses features ranging from shallow day-beds and spawning redds to extensive underground tunnel complexes. Burrowing bioturbates surface and subsurface sediment, leads to deposition of sediment in the form of surface spoil heaps and related distinctive landforms such as termite and ant mounds, and can cause surface collapse through complete undermining. Some spoil heaps and burrow complexes are at scales sufficiently large to be discernible on aerial photographs and satellite imagery (e.g. wombat warrens in Australia). Burrowing and excavation of nest cavities by Atlantic puffins led to the undermining and destruction of the island of Grassholm, an 8.9 ha island, off the south-western coast of Wales. Indirectly, burrowing produces profound geomorphological results through its influence on soil texture and structure, fertility, and infiltration capacity; and the resulting changes, both in production of vegetation cover and in surface run-off and erosion.
Mima mounds are earth mounds, approximately 20–30 m in diameter and 2 m high, found throughout the western two-thirds of North America. Seen from the air, they resemble hundreds to thousands of spheres partly buried in the ground. Mima-like mounds have also been reported from the highlands of Kenya in eastern Africa, and Argentina. The mounds are typically composed of unconsolidated fine soil, varying in texture from loamy sands to clay loams. Small stones are more concentrated in mound soils than in intermound soils, but larger stones ranging in size up to cobbles and boulders are commonly exposed in the intermound zones. Cross-sections through mounds reveal a lenticular (biconvex) outline. Five scientific hypotheses for the origin of Mima-type mounds have emerged. These include wind or water erosion, wind deposition, periglacial freeze–thaw dynamics, seismic activity, and soil translocation by fossorial rodents or other animals. The fossorial rodent, or Dalquest–Scheffer, hypothesis states that Mima mounds are the result of soil translocation by burrowing rodents. Objections to the fossorial rodent hypothesis include observations that gophers are widely distributed but mounds are not, and gophers are missing from many mound sites, including those of the type area at Mima prairie, Washington State.
By far the most controversial, and unlikely, geomorphological features attributed to burrowing are the Carolina Bays of the south-eastern Coastal Plain of the United States. These ‘bays’ are shallow, elliptical, wetland depressions, typically characterized by a long axis orientation from north-west to south-east and a prominent sand rim. Many bays are at least 1km long, and over 500 000 bays exist from southern Georgia to Maryland. Although current thinking suggests that they were created by south-west–north-east prevailing winds during the Pleistocene, one school of thought attributed their creation to vast quantities of schooling, spawning fish, creating tidal-flat excavations during the Pleistocene.
Trampling by animals can, directly or indirectly, lead to erosion. It is a direct agent of erosion when trampling along the edge of a stream, pond, turf terrace, or erosion pan causes hoof or paw chiselling and sloughing and erosion of banks. Natural animal trails produced by trampling are widespread; examples include trails produced by elephants and hippopotamuses in tropical grasslands, bison in the semi-arid grasslands of the Great Plains, mountain goats in the Rocky Mountains, and caribou in the Arctic tundra of the Barren Grounds of Canada's Northwest Territories. The indirect effects of trampling are difficult to quantify, but can be categorized as the reduction or complete removal of vegetation, and an attendent increase in the bulk density of the soil, leading to lowered infiltration capacity and increased surface run-off.
Animal excavations in search of food sources can result in pit-and-mound topography and the removal of large quantities of sediment. Grey whales feeding on the shelf of the Bering Sea ‘slurp’ large quantities of ocean-floor sediment from which they extract benthic invertebrates, leaving behind a pitted ocean floor. Sediment is expelled near the surface as large mud plumes. Approximately 16 000 whales feed in the Bering Sea annually, and resuspend a minimum of 120million m
3 of sediment, which is nearly three times the annual load of suspended sediment discharged into the Bering Sea by the Yukon River.
Wallowing in mud or dust is a common activity among many animals, ranging from small rodents to elephants, African buffalo, seals, and beluga whales. Quantitative data on the effects of wallowing are extremely limited. However, it has been pointed out that nearly every natural depression in the American and Canadian Great Plains has at one time or another been referred to, sometimes in jest and sometimes in all seriousness, as a ‘buffalo wallow’ resulting from wallowing by American bison. The abrupt cessation of this geomorphological impact, owing to the near-extinction of the bison through overkilling following European contact, must stand as one of the most striking examples of the creation of a disequilibrium landscape anywhere on Earth.
Probably more than any other animal except humans, beavers geomorphologically alter the landscape through their dam-building and related activities. Damming of streams creates appropriate habitat for the animals and may completely alter the local drainage pattern. Hydrological effects of beaver dams include the creation of ponds, diversion channels, and multiple surface-flow paths; the reduction of downstream discharge during dry periods below dams; alteration of discharge during high flow; and alteration of the overall water balance. Beavers may act as agents of karstification; in Ontario, Canada, beaver ponds acted to concentrate water that subsequently disappeared through karstic sink-holes in the base of the ponds. Dams and canals were built by beavers to combat lowering water levels, but only served to further concentrate water and exacerbate the karstification.
Indirect effects associated with animals, particularly with burrowing and tunnelling, include lowering of soil bulk density and increasing soil porosity (with accompanying increases in the moisture-retentive capability of a soil, increasing levels of organic matter, alteration of soil pH, and increasing nutrient levels). The soils of South African ant mounds, for example, have higher infiltrability, and significantly more organic matter, phosphorus, potassium, and nitrogen than soils of intermound areas. Similar results have been described for mound soils of ants in the western USA. The presence of Quaternary stone lines (a differentiated zone in the upper part of soils) has also been attributed directly to pedoturbation by ants.
D. R. Butler
Bibliography
Butler, D. R. (1995) Zoogeomorphology – animals as geomorphic agents. Cambridge University Press.
