Abstract
Mammalian herbivores shape the structure and function of many nutrient-limited or low-productive terrestrial ecosystems through modification of plant communities and plant–soil feedbacks. In the tundra biome, mammalian herbivores may both accelerate and decelerate plant biomass growth, microbial activity and nutrient cycling, that is, ecosystem process rates. Selective foraging and associated declines of palatable species are known to be major drivers of plant–soil feedbacks. However, declines in dominant plants of low palatability often linked with high herbivore densities may also modify ecosystem process rates, yet have received little attention. We present data from an island experiment with a 10-year vole density manipulation, to test the hypothesis that herbivores accelerate process rates by decreasing the relative abundance of poorly palatable plants to palatable ones. We measured plant species abundances and community composition, nitrogen contents of green plant tissues and multiple soil and litter variables under high and low vole density. Corroborating our hypothesis, periodic high vole density increased ecosystem process rates in low-productive tundra. High vole density was associated with both increasing relative abundance of palatable forbs over unpalatable evergreen dwarf shrubs and higher plant N content both at species and at community level. Changes in plant community composition, in turn, explained variation in microbial activity in litter and soil inorganic nutrient availability. We propose a new conceptual model with two distinct vole–plant–soil feedback pathways. Voles may drive local plant–soil feedbacks that either increase or decrease ecosystem process rates, in turn promoting heterogeneity in vegetation and soils across tundra landscapes.
Highlights
Mammalian herbivores exert strong control on vegetation biomass and species composition in many low-productive and nutrient-limited ecosystems, such as the northern tundra biome (Oksanen and others 1981; Aunapuu and others 2008)
Principal component analysis (PCA) and subsequent linear mixed models on PC1 and PC2 scores indicated divergence in vegetation communities between high and low vole densities in transplants and transects (Figure 2A–C, Supplementary Table S3). For both transplants and transects, small values of PC1 were associated with high abundance of forbs C. suecica and R. chamaemorus, whereas large values were associated with high abundance of evergreen dwarf shrubs E. nigrum and V. vitis-idaea (Figure 2B, C)
PC1s seemed to represent a transition between forb and evergreen dwarf shrub abundance, PC2s were associated with vegetation patterns related to micro-topography, for example, exposure and moisture, and subsequent variation in abundance of deciduous dwarf shrubs B. nana and V. myrtillus (Supplementary Table S3)
Summary
Mammalian herbivores exert strong control on vegetation biomass and species composition in many low-productive and nutrient-limited ecosystems, such as the northern tundra biome (Oksanen and others 1981; Aunapuu and others 2008). In parallel with changes in vegetation and leaf chemistry, herbivory may both accelerate and decelerate ecosystem process rates, i.e., rates of primary production, litter decomposition and nutrient cycling in tundra (Olofsson and others 2004b; Brathen and others 2007), boreal (Pastor and Naiman 1992) and grassland systems (Sirotnak and Huntly 2000; Bakker and others 2004, 2009) It remains an open question if herbivoredriven acceleration of process rates can occur in nutrient-limited communities, and if so, under what conditions (Jefferies and others 1994; Augustine and McNaughton 1998; Bardgett and Wardle 2003). Urine and feces deposition may enhance nutrient cycling (Bakker and others 2004, 2009) locally in low-productive systems (Stark and others 2015)
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