Rapid, landscape scale responses in riparian tundra vegetation to exclusion of small and large mammalian herbivores

Abstract

21Productive tundra plant communities composed of a variety of fast growing herbaceous and 22 woody plants are likely to attract mammalian herbivores. Such vegetation is likely to respond 23 to different-sized herbivores more rapidly than currently acknowledged from the tundra. 24Accentuated by currently changing populations of arctic mammals there is a need to 25 understand impacts of different-sized herbivores on the dynamics of productive tundra plant 26 communities. Here we assess the differential effects of ungulate (reindeer) and small rodent 27 herbivores (voles and lemmings) on high productive tundra vegetation. A spatially extensive 28 exclosure experiment was run for three years on river sediment plains along two river 29 catchments in low-arctic Norway. The river catchments were similar in species pools but 30 differed in species abundance composition of both plants and vertebrate herbivores. Biomass 31 of forbs, deciduous shrubs and silica-poor grasses increased by 40-50 % in response to release 32 from herbivory, whereas biomass of silica-rich grasses decreased by 50-75%. Hence both 33 additive and compensatory effects of small rodents and reindeer exclusion caused these 34 significant changes in abundance composition of the plant communities. Changes were also 35 rapid, evident after only one growing season, and are among the fastest and strongest ever 36 documented in Arctic vegetation. The rate of changes indicate a tight link between the 37 dynamics of productive tundra vegetation and both small and large herbivores. Responses 38 were however not spatially consistent, being highly different between the catchments. We 39 conclude that despite similar species pools, variation in plant species abundance and 40 herbivore species dynamics give different prerequisites for change. 41 42

Collapse of an arctic bird community resulting from ungulate-induced loss of erect shrubs

“…We suspect that the negative responses in these insectivorous birds result from halted spatial spillover of food resources from the rich fauna of insects dwelling in the willow thickets (den Herder et al, 2004) onto the surrounding open habitats. However, we cannot exclude that even changed composition of the non-woody components of the meadow vegetation resulting from intense reindeer grazing (Ravolainen et al, 2011) (Post andPedersen, 2008, Olofsson et al, 2009). This proposal is motivated by the expectation that large-scale shrub encroachment will cause positive feedbacks on global climate warming though biophysical processes (Chapin et al, 2005).…”

“…1C). A recent exclosure experiment has shown that such willow ramets in riparian meadows in this study region are severely suppressed due to intense browsing (Ravolainen et al, 2011). The level of browsing even in the riparian habitats with willow thickets was high enough to cause substantial fragmentation of the willow thickets (Fig.…”

Spatial distribution in Norwegian lemming Lemmus lemmus in relation to the phase of the cycle

“…Although overgrazing can occur even during low population density (Virtanen et al 1997), lemmings have mainly been observed to overgraze their habitats at high population density (Oksanen and Oksanen 1981;Moen et al 1993): for instance, Moen et al (1993) found that lemming grazing decreased the cover of graminoids by 33%, and mosses by 66% during a peak year. Moreover, plant biomass increases substantially over time in exclosures without rodent grazing (Virtanen et al 1997;Ravolainen et al 2011) and a study using satellite imagery in northern Sweden showed that the impact of lemmings on plant biomass is strong enough to be visible from space (Olofsson et al 2012).…”

“…Snow affects the micro-climate which creates ecological niches for different plants (Ravolainen et al 2011). For example, snow bed habitat has a high richness of plants, similar to that in locally more productive habitats (Bruun et al 2006).…”

“…Lemmings feed mainly on mosses, graminoids, and sedges (Tast 1991;Soininen et al 2013, but see Soininen et al 2017) and the distribution of food depends partly on snow conditions as snow have a big impact on plant distribution in the mountain tundra (Ravolainen et al 2011) due to the long winters during which snow covers the ground (about 8 months). Snow affects the micro-climate which creates ecological niches for different plants (Ravolainen et al 2011).…”