High-arctic plant-herbivore interactions under climate influence under Climate Influence

This chapter focuses on a 10-year data series from Zackenberg on the trophic interactions between two characteristic arctic plant species, arctic willow Salix arctica and mountain avens Dryas octopetala, and three herbivore species covering the very scale of size present at Zackenberg, namely, the moth Sympistis zetterstedtii, the collared lemming Dicrostonyx groenlandicus and the musk ox Ovibos moschatus. Data from Zackenberg show that timing of snowmelt, the length of the growing season and summer temperature are the basic variables that determine the phenology of flowering and primary production upon which the herbivores depend, and snow may be the most important climatic factor aVecting the diVerent trophic levels and the interactions between them. Hence, the spatio-temporal distribution of snow, as well as thawing events during winter, may have considerable eVects on the herbivores by influencing their access to forage in winter. During winter, musk oxen prefer areas with a thin snow-cover, where food is most easily accessible. In contrast, lemmings seek areas with thick snow-cover, which provide protection from the cold an d some predator s. Therefor e, lemm ings may be aVected directly by both the timing of onset an d the durati on of wi nter snow- co ver. Mu sk ox en significantl y reduced the pro ductiv ity of arcti c willow , while high densities of collared lemmings during win ter reduc ed the producti on of mo untain avens flower s in the followi ng su mmer. Un der a deep snow -layer scen ario, climate and the previous yearís de nsity of musk ox en had a ne gative e V ect on the present yearís produ ction of arctic willow. Pr evious yea rís pr imary prod uction of arctic willow , in turn, signi ficantl y aVected the present ye arís den sity of musk oxen positivel y. Clima tic facto rs that aV ect primary pr oduction of plants indirectl y, influenced the spati al distribut ion of he rbivo res. Additional ly, sno w distribut ion directly aVected the dist ribution of he rbivor es, an d hen ce, in turn, aVected the plant co mmuni ty by selec tive feedi ng and local ly reducing the standi ng bio mass of forage plants. Altho ugh only few moth larva e were obs erved at Zac kenb erg, these had in so me cases impor tant local e Vects owi ng to their foragin g on up to 60% of the flower stands on indivi dual moun tain aven s. UV -B radiation induces plants to prod uce secondary plant meta bolit es, whi ch pro tects tissues against UV - B damage. Thi s results in low er production of anti - herbivor e defense s and impr oves the nutritional quality of the foo d plants . Zacken berg data on the relationshi p between variation in de nsity of collared lemmings in win ter and UV -B radiat ion indir ectly sup ports this mech anism, which was origin ally proposed on the ba sis of a positive relations hip between UV - B radiat ion an d reprod uction in two su b- arctic spe cies of hares (Lep us timidu s and Lepus amer icanus ).