Several species of small mammals are permanent residents of the alpine region. Some of these hibernate, while others are active throughout the year and live on the soil surface under the snow during winter months. In arctic alpine regions, many small mammals show high amplitude population cycles and in some years they may attain very large population sizes. Thus, it is not surprising that small mammals often have a severe impact.

Most small mammals have distinct habitat preferences. For example, mountain voles (Microtus montanus) usually inhabit wet places rich in grasses and herbs, while other voles (Clethrionomys spp.) typically inhabit drier places with many boulders and stones. Some species of small herbivorous mammals are restricted to special habitats such as boulder fields, so that their impact is focused very locally.

In addition to consumption of plant material some small mammals exhibit a strong mechanical impact by burrowing, which may initiate further disturbance by abiotic factors.

While the patterns and intensity of herbivory by small mammals may be similar to that in lowlands, their relative impact in alpine ecosystems may be much higher (unlike that of invertebrate herbivores). As a consequence, plants that are affected more by the belowground activities of small mammalian herbivores may be at greater disadvantage in higher mountains as in lowlands.

1 - Peromyscus maniculatus

Rodents

In Fennoscandian alpine tundra, small rodents occur cyclically at high concentrations every 3-4 years. Average density of small rodents is estimated at 18000 individuals km^-2 at Hardangervidda (Norway) in peak years. In these years their direct impact, especially upon preferred food plants, is very high, while in years of low population density, removal of plant biomass is negligible. In addition to the plants they take as food, all small rodents clip considerable amounts of plant material, e.g. for nest building, and disturb the ground with their burrowing activities. They also cause local accumulation of nutrients around their holes and pathways which comes from their faeces and from clipped plant material. In alpine regions at lower latitudes, population cycles are less pronounced than at higher latitudes.

2 - Ochotona princeps

Pika

Pikas (Ochotona princeps) are common in the subalpine and alpine regions of North America. As they are not equipped to defend themselves physically against predators, they are restricted to talus slopes where they can hide between the rocks. Pikas are territorial and do not hibernate. They are generalized herbivores, but nonetheless they feed selectively. During the summer they harvest plant material and store it as "haypiles" to meet their food needs in the winter.
Near talus inhabited by pikas, the vegetation is dominated by unpalatable and grazing-tolerant plants, especially graminoids and cushion plants. As herbivory decreases with distance from the talus, forbs become more abundant and plant biomass and competition among plants increases.

29 August 2011
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