Egg Size vs. Egg Number Trade-Off in the Alpine-Tundra Wolf Spider

The effect of environmental conditions on reproductive traits in spiders is not completely understood. We studied the trade-off between the egg number and egg size of a common spider species along an elevational gradient in Norway. Life history theory predicts that egg size should decrease and clutch size increase as temperatures rise. In 2006, 2010, and 2014, female lycosid spiders (Pardosa palustris) carrying first egg sacs were hand sampled from 690 to 1460 m above sea level (a.s.l.). The eggs were counted, and the body and egg sizes for each female were individually estimated using digital photography. An analysis of covariance was performed using linear mixed-effects models to test for trade-off differences between sampling years, and along the elevational gradient. Unexpectedly, the egg size versus number trade-off was consistent along the elevational gradient, and thus appeared to be independent of elevation-induced temperature changes. However, this trade-off varied considerably between years. Egg-size variations in relation to body size appeared to be independent of year and did not vary along the elevational gradient. Our results revealed that the trade-off between egg number and egg size does not always hold and might be more plastic than assumed. This suggests that P. palustris, which has a broad habitat niche and a wide geographic distribution, will easily cope with temperature-regime shifts in cold environments. Consequently, this might lead to advantages regarding the offspring survival rate relative to coexisting species, and thus to changes in the terrestrial arthropod community of alpine-tundra ecosystems.