Earlier springs enable High-Arctic wolf spiders to produce a second clutch - supplementary data

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 program at Zackenberg, Northeast Greenland. We dissected
individual egg sacs and counted the number of eggs and partially developed
juveniles, and measured carapace width of the mothers. Upon 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. This is likely a result of female spiders producing
their first clutches earlier in those years and allowing 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.