|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 ).|