Abstract
Herbivores may have extensive top-down effects in open grazing ecosystems, generating vegetation changes by grazing and trampling. Trampling effects are understudied, but may be a major ecological factor. In cold alpine-Arctic ecosystems grazing and trampling by wild tundra reindeer (Rangifer tarandus) may be particularly important in lichen-dominated heaths. Dry lichen are crushed by trampling, and it is estimated that volume loss of lichen trampled may be considerably larger than lichen volume eaten by reindeer. Humidity affects lichen pliability and elasticity, and thereby resilience to trampling. Although crucial for estimating lichen vegetation trampling loss, the relationship between humidity and lichen elasticity is not well known. We collected samples of three lichen species in natura and in factorial experiments tested effects of species, levels of humidity (25, 70, 80, 90 and 100% RH) and temperatures (5 and 25 °C), on resilience to trampling (pressure resistance). The humidity:species interaction was the strongest factor increasing pressure resilience with increasing humidity, whereas temperature had small or no effects. Lichen elasticity increased rapidly above 70% RH. Consequently, when estimating lichen resources and potential trampling loss, number of dry days (less than 70% RH) should be estimated. This also has important ramifications for effects of climate change on the sustainability of reindeer populations.
Highlights
Herbivores may have extensive top-down effects in opengrazing ecosystems, e.g. alpine and tundra areas, generating vegetation responses and changes via their grazing and trampling (e.g. Cumming and Cumming 2003; Rosenthal et al 2012; Holtmeier 2015; Heggenes et al 2017)
At 25 °C, F. nivalis tended to be more variable than C. rangiferinea and C. stellaris, which responded (Fig. 1), except that C. stellaris tended to be slightly less elastic than C. rangiferina at 25 to 80% relative air humidity (RH)
At higher RH, at or more than 80%, the increased elasticity conveyed by humidity prevented breakage
Summary
Herbivores may have extensive top-down effects in opengrazing ecosystems, e.g. alpine and tundra areas, generating vegetation responses and changes via their grazing and trampling (e.g. Cumming and Cumming 2003; Rosenthal et al 2012; Holtmeier 2015; Heggenes et al 2017). Herbivores may have extensive top-down effects in opengrazing ecosystems, e.g. alpine and tundra areas, generating vegetation responses and changes via their grazing and trampling The effects of trampling on vegetation may differ from those associated with grazing, with respect to the magnitude and quality of vegetation changes. In cold alpine-Arctic ecosystems negative effects of trampling on vegetation are likely to be effected by the large, mammalian herbivore wild tundra reindeer (Rangifer tarandus) when both grazing—and trampling—the different species of lichen (Danell et al 1994; Kumpula 2001; Heggenes et al 2017) typically important in alpine-Artic ecosystems (Crittenden 2000; Odland et al 2018).
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