Interplay between nitrogen deposition and grazing causes habitat degradation

Abstract

Increased atmospheric nitrogen (N) deposition has been held responsible for the large‐scale invasion of graminoids (grasses, sedges and rushes) in a wide range of habitats from forests to upland heaths, causing dramatic changes in plant species composition. Concurrently with an increase in N deposition over the last century, livestock grazing has intensified in many parts of the world following policy reform, leading to large‐scale degradation of natural and seminatural ecosystems. On the basis of a series of experiments conducted in a Scottish montane ecosystem, we discovered that grazing and N deposition do not operate independently, and the interplay between them is leading to the replacement of valuable moss‐dominated habitat by grasses and sedges. Our study indicates that in setting ‘critical loads’ of N, widely used to minimize habitat degradation, it is necessary to account for substantial amplification of N‐deposition effects by grazing.

Short‐ and long‐term effects of changes in reindeer grazing pressure on tundra heath vegetation

“…Grazing by large herbivores and nitrogen deposition are causing a transition from dwarf shrub-and moss-dominated vegetation to grass-dominated vegetation in a number of tundra, mountain and boreal ecosystems (Thing 1984;Alonso et al 2001;Strengbom et al 2002;Van der Wal et al 2003;Van der Wal & Brooker 2004;Croll et al 2005). My results indicate that the reindeer-induced transition from dwarf shrub-to graminoid-dominated ecosystems (Olofsson et al , 2004) is reversible, but that the recovery of dwarf shrubs is a slower process than the increase of graminoids.…”

“…The transition from dwarf shrubto graminoid-dominated ecosystems probably increases the quality of summer pasture for reindeer (Olofsson et al 2004). However, this vegetation shift has been regarded as habitat degradation in many other ecosystems, as the graminoid dominated vegetation stage is regarded as unnatural or less valuable (Alonso et al 2001;Strengbom et al 2001Strengbom et al , 2002Van der Wal et al 2003).…”

Sheep grazing in the North Atlantic region: A long-term perspective on environmental sustainability

“…They are necessarily limited by the carrying capacity of the vegetation, i.e. the forage quantity and quality, but can also vary as the result of events in the physical environment, changing economic conditions, and the state of agricultural subsidy systems (van der Wal et al 2011). In Iceland, sheep numbers have fluctuated considerably due to diseases (1761,1855,1878,1932) and volcanic eruption (1783) (Thórhallsdóttir 2003).…”

“…Continued overgrazing of heathland can lead to its conversion to grassland in many cases, facilitating the dominance of graminoids that can sustain higher levels of offtake. This creates a positive feedback loop for grazing, as more grazing leads to more resources becoming available van der Wal et al 2003). Furthermore, sheep grazing may change bryophyte community composition (Jónsdóttir 1984;Austrheim et al 2007) and reduce abundance of mosses (Jóns-dóttir 1991; Magnússon and Magnússon 1 992;Fosaa 2015) and also, in some cases, lichens (Mysterud and Austrheim 2008).…”

“…Both spatially and temporally, impacts of grazing on plants, animals and ecosystems, as well as interactions between sheep grazing, other herbivore species and environmental change drivers, have been highly variable (Hartley and Mitchell 2005;DeGabriel et al 2011). Nevertheless, sheep grazing has been maintained by state subsidies and remains, from an ecological perspective, an important land use with marked effects on biodiversity, ecosystem function, plant biomass quantity and quality and soil stability, with important implications for the economy of rural communities where sheep grazing is locally important (van der Wal et al 2011). …”