The impacts of trampling and ground disturbances on Antarctic soils

Most of Antarctica is covered in ice, but isolated sites with exposed soils exist as cliffs, coastal margins, nunataks, and seasonally snow- and ice-free ground. The largest ice-free region in Antarctica is the McMurdo Dry Valleys with an area of c. 15,000 km^{2 1}. Terrestrial life is concentrated in soils developed in these ice-free areas, which have a combined area of c. 49,500 km², and are mainly confined to the Antarctica Peninsula, the Transantarctic Mountains, MacRobertson Land and Dronning Maud Land². Most of these soils are characterised by a general lack of structural development and coherence, low organic matter, biomass and primary production, low moisture availability, slow decomposition rates, and limited soil biota³. These characteristics, combined with the general absence of higher vegetation (vascular plant species) and prevailing low temperatures, result in a greater vulnerability to human trampling⁴. Most human activities are concentrated in ice-free areas with easy access and mild climates (e.g. Antarctic Peninsula and associated archipelagos). The most intense ground disturbances occur in the vicinity of research stations or field camps and at key tourist sites. Other affected sites include areas of scientific importance, historical sites, lookout points for spectacular landscapes and coastal wildlife colonies (Fig. 1).

Trampling can lead to changes in soil properties and surface features including increases in track width, penetration resistance and bulk density^4-8. Trampling usually produces visible micro-relief changes^{4, 9-11} (Figs. 1 and 2), in addition to albedo alterations in some specific sites.

Several impacts on flora have also been identified, mainly in the Antarctic Peninsula where there is more extensive vegetation, the most obvious being reduction in vegetation cover and biomass around paths¹² (Fig. 3). Soil animals are directly affected through increased mortality and, indirectly, by the decrease of habitat quality affecting fecundity, abundance, composition and structure of the soil community^{6, 7, 13}. Certain microbiological parameters can be modified by foot traffic, including enzymatic activity and soil respiration^{13, 14}. Trampling reduces the amount of available nutrients in Antarctic moss communities¹². Additionally, it has been suggested that non-indigenous species establishment may be facilitated as a direct result of the foot traffic associated with human presence^{14, 15}, although additional evidence is needed to determine the relative importance of this mechanism.

The measured severity of disturbances depends on soil type, regional climate, mode and intensity of disturbance (foot versus vehicle), how dynamic the landscape is, and what component of the ecosystem is being investigated. Disturbances resulting from foot traffic and field camps usually cover a small area, but are often clearly visible¹⁶. Foot tracks form readily in certain vulnerable soils and may remain visible for many years after the event^{4, 10}. Vehicular traffic also results in ground disturbances which are often much more extensive and persistent¹⁷. Ground disturbance is often greatest where the overlying desert pavement is disturbed and underlying fine material exposed^{4, 13, 18}. In the McMurdo Dry Valleys, distinct walking tracks formed in soft material after as few as 20 pedestrian transits and are still visible up to 23 years after disturbance⁴ (Fig. 2, A and B). Non-cohesive soils with sandy pebble-gravel textures are also vulnerable to trampling, and damage is immediate⁷. In contrast, soils with a high surface-boulder cover and/or a large particle-size fraction are the least susceptible⁴ (Fig. 2, C and D). Other areas with aeolian sand dunes or coarse volcanic soils are readily disturbed, but the physical effects of regular foot traffic can disappear after one year due to the freeze-thaw activity and wind action^{7, 10} (Fig. 1, D). Experimental manipulations in soils located in the maritime Antarctic demonstrated that the effects of soil compaction could be completely reversed within 3-5 years if the area was closed to any human traffic during this period⁷. The same interval of time has been suggested for bryophyte and associated invertebrate communities to develop on previously bare soil¹⁹.

There are several instruments to manage the impacts of pedestrian traffic in Antarctica²⁰. The Scientific Committee on Antarctic Research (SCAR) has developed the ‘‘Environmental code of conduct for terrestrial scientific field research in Antarctica’’. This proposes two measures with reference to trampling: (1) to stay on established trails when available, and (2) to avoid walking on areas that are especially vulnerable to disturbance (e.g. peat soils, moss carpets, desert pavement, or muddy areas). Apart from these general recommendations, the Antarctic Treaty Parties have developed a collection of ‘‘Site guidelines for visitors’’ to provide specific instructions on the conduct of activities at the most heavily visited Antarctic sites, taking into account the environmental values and sensitivities specific to each site. Some measures for controlling the effects of trampling are mentioned, including the demarcation of closed areas to protect vulnerable features and the establishment of walking routes to avoid vegetation trampling. Finally, the management plans for some Antarctic Specially Managed Areas (ASMA) and the Antarctic Specially Protected Areas (ASPA) include instructions for protecting the environment during fieldwork or visits which help to limit impacts to soil. All these existing codes of conduct have to date contributed to controlling the scale of many of the potential impacts generated by trampling.

These recommendations require regular assessment and, where necessary, revision to ensure their continued effectiveness in the face of the predicted increases in the intensity of human activities. Future work could usefully address the variability seen in responses of different soil types to trampling impacts. For example, the effectiveness of the use of established paths that cross vegetation-free soils, is highly dependent on context^{7, 10}. At some sites of low intensity trampling, small changes at the soil surface recover relatively rapidly, in less than one annual cycle, suggesting that sometimes the dispersal of activity across wider corridors may be the most appropriate option rather than formation of a well-defined and long-lasting track. However, research has shown, for paths with high intensity use and those located in steep-sloped sites, that constraining use to a single well-defined track, on stony or bouldery surfaces wherever possible and avoiding muddy areas, keeps disturbance to a minimum^{7, 9, 18}. It is clear that both environmental conditions and expected use levels must be taken into account in determining when and where it is more appropriate to concentrate or disperse human activities⁷. A coordinated approach using an agreed suite of biophysical or chemical indicators to assess the vulnerability and recoverability of different Antarctic soil surfaces to human trampling would assist environmental managers and the tourism industry in choosing the most appropriate, site specific, strategy to minimize physical and biological impacts.