Indirect and Cumulative Impacts

In addition to direct disturbances of the ground surface, other, less visible, impacts can accumulate over time. These may occur independently of each other, or may be exacerbated through synergy among various proximal effects. These indirect or cumulative impacts are now well documented and, while they were formerly unforeseen, scientists can predict them in many cases.

In vehicle tracks, for example, plant and soil nutrients can become significantly different than in undisturbed areas, with increases and decreases variable among species, growth forms, and soil types. Although the actual ruts may be small to begin with, the shift from scale-of-impact to scale-of-response can be several orders of magnitude, as in the case of drained wetlands. Even shallow ruts from as little as a singlepass vehicle track are capable of effectively diverting runoff from spring snowmelt away from wet and mesic meadows that depend upon this source of moisture. Such desiccation of wet tundra has resulted in the local extinction of aquatic sedges, Sphagnum spp. (mosses), and other hydric bryophytes, as well as an increase in surface albedo. Similarly, as little as a single passage of a vehicle in summer is sufficient to significantly reduce the abundance of soil arthropods. In areas with substantial ground-ice, thermokarst activity can expand appreciably. In northern Alaska, some disturbances on silty sediments covered at least twice the original area of impact after 30 years.

Gravel roads and sand quarries are subject to wind erosion and can spread sand and dust up to 1 km from the source. Road dust is alkaline and is capable of rapidly smothering bryophytes, lichens, and mushrooms at the surface as far as 35 m from the road. Dust significantly increases the pH of soils and surface waters, and alters the nutrient contents of abundant vascular plants and mosses in as few as four years. During the same time period, blowing sand can bury all mosses and lichens, and many vascular plants, up to a distance of 250 m from the source.

The above points raise an important question that has yet to be properly addressed in areas undergoing large-scale industrial development in tundra regions: that is, when do the gradually increasing number of anthropogenic patches begin to affect landscape-level patterns and processes? Empirical data from Arctic North America and Russia suggest that this is already happening in some ecosystems and that the effects are long-lasting. Furthermore, certain "natural" disturbances (i.e., thermokarst erosion, ungulate grazing) may work to increase the amount of habitat suitable for fugitive species that were uncommon or absent prior to the spread of human impact.

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