DOI: 10.5061/dryad.18931zd1c
title: Early snow melt and diverging thermal constraints control body size in arctic-alpine spiders

description: 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.
description: 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.