To predict species’ responses to a rapidly changing environment, it is necessary to detect current clines of life-history traits and understand their drivers. We studied body size variation, a key trait in evolutionary biology, of two arctic-alpine lycosid spiders and underlying mechanisms controlling this variation. We used long time-series data of body size sampled in Norway, augmented with museum data. Individuals of both species sampled in areas and years with longer snow-free periods grew larger than individuals in areas and years with shorter snow-free periods. Interestingly, temperatures under 0° C led to a larger body size in Pardosa palustris, while temperatures above 0 °C led to a larger body size in Pardosa hyperborea. We assume that P. palustris, as the generally larger species, is less sensitive to environmental variability and cold temperatures, because it can retain more energy than a smaller species can and, therefore, can invest more resources in its offspring. With rising temperatures, both species might profit from a higher resource availability. In a rapidly changing arctic-alpine environment, alterations in the life-history traits and adaptation strategies of spiders are expected, which, regarding body size, seem to be highly influenced by early snowmelt and diverging thermal constraints.

We collected individuals of P. hyperborea and P. palustris between 2002 and 2019 and extended the time-series of body size variation for the two species of focus by including material stored in the museum collection at the University Museum in Bergen, Norway (Department of Natural History) from the years 1971, 1972, and 1989. 

To measure carapace width, we photographed each individual through a Zeiss Discovery.v8 microscope and measured to the nearest 0.001 mm from those digital photos. 

Geiranger (62°03´N, 7°15´E; Møre og Romsdal), here referred to as “oceanic alpine”, is located in the western part of southern Norway and has an oceanic climate (Moen, 1998). The study area belongs to the low-alpine belt, characterised by dwarf shrubs and heather communities (Dahl, 1986). Vågå (61°53´N, 9°15´E; Innlandet), here referred to as “continental alpine”, is also located in the low-alpine belt, dominated by dwarf shrubs and heather communities (Dahl, 1986). Vassfaret (60°54´N, 9°47´E; Innlandet/Viken), here referred to as “continental montane”, lies approximately 200 km to the south of Vågå. The main valley is located at about 600 m a.s.l., in that part of the pre-montane region (Hauge & Wiger, 1980). The vegetation is characterised as coniferous forest dominated by spruce (Picea abies) and pine (Pinus sylvestris). Lichens and mosses dominate the ground vegetation (Aune, 1978; Hauge & Wiger, 1980).

Pitfall traps, as described in Hauge & Wiger (1980), were used as a sampling technique for the museum material. The traps were collected after snowmelt in summer every ten to 20 days. Individuals from our own collections were both trapped in pitfall traps following Naujok & Finch (2004) and hand-collected. We installed the traps immediately after snowmelt and emptied them every two weeks (every four weeks in 2003) until the end of the snow-free period. Hand-collections were carried out on warm, sunny days, when wolf spiders of the genus Pardosa are most active (Steigen, 1975). At each sampling site, we collected individuals for about one hour.