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.

低纬度地区的蜘蛛通常可产下多窝卵，但在活动季短得多的高纬度地区，这一现象尚未被观测到。然而北极地区的融雪时间正不断提前，这或许能让部分蜘蛛物种额外多产一窝卵。
为验证这一现象是否已在北极发生，我们采用了格陵兰东北部扎科本（Zackenberg）长期监测项目（1996-2014年）中，通过陷阱诱捕获得的冰川狼蛛（Pardosa glacialis）标本。我们对单个卵囊进行解剖，统计卵粒与部分发育幼体的数量，并测量母蛛的头胸甲宽度。
当发现窝卵数呈现双峰频率分布（这与低纬度地区狼蛛产下第二窝卵的特征一致）时，我们根据窝卵数将卵囊划分为首窝卵与第二窝卵。我们开展了三项检验：第一，首窝与第二窝卵的蜘蛛捕获中位日期是否存在差异；第二，窝卵数是否与雌蛛体型相关；第三，单季内第二窝卵的占比是否与气候因素相关。
研究结果显示，被归类为第二窝卵的个体，其捕获中位日期显著晚于首窝卵个体。在融雪更早的年份，首窝卵的出现时间更早，且第二窝卵的占比也更高——这大概率是因为雌蛛在这些年份能更早产出首窝卵，从而获得足够时间产下第二窝卵。首窝卵的窝卵数与雌蛛体型呈显著相关，而第二窝卵则未表现出这一相关性。
本研究首次提供证据，证明北极无脊椎动物可响应气候变暖而额外多产一窝卵。由于北极地区长期收集生活史数据存在诸多困难，这一现象或许普遍存在却长期被忽视。此外，鉴于狼蛛是分布广泛且极为重要的苔原捕食者，其繁殖率提升可能会对种群动态与食物网结构产生连锁影响。