Earlier springs enable high-Arctic wolf spiders to produce a second clutch

Abstract

Spiders at southern latitudes commonly produce multiple clutches, but this has not been observed at high latitudes where activity seasons are much shorter. Yet the timing of snowmelt is advancing in the Arctic, which may allow some species to produce an additional clutch. To determine if this is already happening, we used specimens of the wolf spider Pardosa glacialis caught by pitfall traps from the long-term (1996-2014) monitoring programme at Zackenberg, NE Greenland. We dissected individual egg sacs and counted the number of eggs and partially developed juveniles, and measured carapace width of the mothers. Upon the discovery of a bimodal frequency distribution of clutch sizes, as is typical for wolf spiders at lower latitudes producing a second clutch, we assigned egg sacs to being a first or second clutch depending on clutch size. We tested whether the median capture date differed among first and second clutches, whether clutch size was correlated to female size, and whether the proportion of second clutches produced within a season was related to climate. We found that assigned second clutches appeared significantly later in the season than first clutches. In years with earlier snowmelt, first clutches occurred earlier and the proportion of second clutches produced was larger. Likely, females produce their first clutch earlier in those years which allow them time to produce another clutch. Clutch size for first clutches was correlated to female size, while this was not the case for second clutches. Our results provide the first evidence for Arctic invertebrates producing additional clutches in response to warming. This could be a common but overlooked phenomenon due to the challenges associated with long-term collection of life-history data in the Arctic. Moreover, given that wolf spiders are a widely distributed, important tundra predator, we may expect to see population and food web consequences of their increased reproductive rates.

Keywords: arthropods; climate change; life-history variation; phenology; reproduction.

Figure 1.

Frequency distribution of clutch sizes in the wolf spider Pardosa glacialis across 1069 egg sacs collected in pitfall traps during 1996–2014 at Zackenberg, NE Greenland. The solid line represents a locally weighted smoothing with a span parameter = 0.2 and identifies a local minimum at a clutch size of 47 eggs used to separate first and second clutches, as indicated by the vertical hatched line.

Figure 2.

Seasonal variation in total egg sac collection across the study period (grouped by 7 day periods). Dates are presented as day of year since 1 January. The white section of each bar indicates clutches with more than 47 eggs (first clutches) and the black sections are for those clutches with 47 or less eggs (second clutches).

Figure 3.

The proportion of second clutches out of total number of clutches in a given year regressed against (a) average date of snowmelt (day of year since January 1) across the study plots (slope = −0.0150 ± 0.0038, t₁₆ = −3.949, p = 0.0012) and (b) sampling year (slope = 0.0182 ± 0.0075, t₁₆ = 2.418, p = 0.028).

Figure 4.

Relationship between clutch size and body size of the mother for each habitat type (wet, mesic and arid) for (a) first and (b) second clutches. The relationships are significant for first clutches, but not for second clutches (see text for details). Body size of the mother was estimated as width of the carapace (in mm).