Broad-scale rather than fine-scale environmental variation drives body size in a wandering predator (Araneae, Lycosidae)

Abstract

Body size is one of the most important individual traits, determining various other life-history traits, including fitness. Both evolutionary and ecological factors shape the body size in arthropods, but the relative contribution of abiotic drivers acting at different spatial scales has been little investigated. We aimed to identify the importance of two broad-scale variables (study region and elevation) in shaping body size of the free-running and locally abundant wolf spider Pardosa palustris (LINNAEUS 1758), in contrast to the fine-scaled variable topographic position. Therefore, we set up transects along environmental gradients in the arctic-alpine ecosystems of Norway, which we analyzed using a random forest approach to identify the relative importance of topographic position, elevation, and study region on body size of P. palustris. Our approach revealed that research region was the best explanatory variable, followed by elevation and topographic position. Differences in body size were most likely a consequence of the pronounced differences in season length and the ability of P. palustris to avoid local unfavorable environmental conditions due to its high mobility.

Differences in Mobility and Dispersal Capacity Determine Body Size Clines in Two Common Alpine-Tundra Arthropods

“…This meets the assumption of our hypothesis. We thereby confirm the proposed effect of mobility at the observational level noted by Hein et al [27] and Bowden et al [26].…”

“…The advantage of the prolonged life-cycle at high elevations is thus to grow larger and have higher reproduction values, always under the disadvantage of a concomitant increased mortality rate [62]. So far, our previous studies on P. palustris in the research area studies could not unravel whether P. palustris shows a prolonged life-cycle at higher elevations or not [27,63,64]. In females, however, the production of egg sacs might be delayed at higher elevations and latitudes due to the timing of snow melt [63,64].…”

“…Preliminary investigations showed pronotum width at the base correlated strongly with pronotum length across the middle (R = 0.92, n = 50) and elytra length (R = 0.91, n = 50). In lycosid spiders, the unsclerotized abdomen is rather soft and prone to damage [27]. Damage is often unavoidable in species determination of spiders.…”

“…Especially female spiders are usually determined to species via dissection of their genitalia, which leads to damage to the abdomen. Measuring the width of the sclerotized prosoma is a viable proxy for overall body size [27,74]. For these reasons, we will use body size synonymously with both proxies when we describe our own data set.…”

“…Bowden et al [26] found that habitat heterogeneity at finer scales overrides effects at broader scales in species with low mobility. In turn, Hein et al [27] and Horne et al [28] recently proposed that high mobility in a species might obscure reactions to environmental factors at finer scales.…”