Population dynamics of Folsomia qundrioculata (Tullberg) and Hypogastr
ura tullbergi (Schaffer) have been studied during foul years in a Cass
iope-heath close to Ny-Alesund, Svalbard. The study area was sampled a
t three different sites of similar vegetation. The density of F. quadr
ioculata appeared to fluctuate less between years th;ln did the densit
y of H. tullbergi. The latter species showed significant winter mortal
ity and a reproductive period in early summer only. The coldest summer
appeared to have the largest effect on the densities of H. tullbergi.
F. qundrioculata did not show significant winter mortality and reprod
uced throughout the two summers when reproductive activity was investi
gated. Individual specimens of F. quadrioculata apparently reproduce d
uring one summer only since adult specimen decreased in density from m
id-to late summer and high recruitment coincided with high adult morta
lity. H. tullbergi most likely requires two summers to complete develo
pment from egg to adult. However, due to the synchronised reproduction
in early summer, individuals of this species will not reproduce befor
e their third summer. The life cycles and dynamics of the species are
discussed in relation to their distribution and abundance at Svalbard.