Abstract:
Although spiders are a diverse and ecologically important group of predators across the
sub-Antarctic islands, relatively little is known about their biology. Here we provide data on the abundance,
body size variation and sex ratio of an indigenous spider, Myro kerguelenensis, across an altitudinal
gradient on Marion Island. In so doing we test explicitly the hypotheses that density will decline with
declining resource availability at higher elevations, and that a converse Bergmann body size cline will be
found in this species. Density of M. kerguelenensis decreased with altitude and ranged from a mean density
of 5.3 (SD 3.42) individuals per m2 at 50m a.s.l. to a mean density of 0.83 (SD 1.15) individuals per m2
at 600m a.s.l. Mean female sternum width was 1.39mm (SD 0.44) and mean male sternum width was
1.40mm (SD 0.22). No evidence for Bergmann or converse Bergmann clines was found. At increasing
altitudes, sex ratios became increasingly female-biased with populations at 600m a.s.l. comprising 0.87
(SD 0.28) females, on a proportional basis, possibly as a result of resource limitation and an increase in the
prevalence of sexual cannibalism. The food web implications of this study are highlighted.
