Simulated Shrub Encroachment Impacts Function of Arctic Spider Communities
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Simulated Shrub Encroachment Impacts Function Of Arctic Spider Communities by Geoffrey Boyd Legault A thesis submitted in conformity with the requirements for the degree of Master of Science Department of Ecology & Evolutionary Biology University of Toronto © Copyright by Geoffrey B. Legault 2011 ii Simulated Shrub Encroachment Impacts Function Of Arctic Spider Communities Geoffrey Legault Master of Science Department of Ecology & Evolutionary Biology University of Toronto 2011 Abstract The projected increase in shrub abundance across sub-Arctic zones is expected to alter patterns of snow cover during the winter. As the amount of snow cover in an area impacts both melt date and winter snow pack, these changes may affect the phenology and survival of overwintering arthropods, such as spiders (Araneae). In this field study, we used snow fences to simulate shrub encroachment on a series of large (375 m2) tundra plots and examined the effects on the local spider assemblages during the following growing season. Snow fences increased winter snow cover and delayed snow melt in the treatment plots, paralleling the conditions of nearby shrub sites. Although our simulated shrub treatment did not affect the abundance or composition of spider communities over the season, adults from the dominant genus Pardosa (Lycosidae) had significantly higher body mass on treatment plots. This difference in mass was observed immediately following snow melt and persisted until halfway through the growing season. Given the importance of spiders as arthropod predators and as food sources for breeding birds, such a change in summer body mass could represent a significant shift in spiders’ functional contributions to Arctic ecosystems. ii iii Table of Contents List of Figures ................................................................................................................................ iv List of Tables ................................................................................................................................. iv Introduction ..................................................................................................................................... 1 Methods ........................................................................................................................................... 5 Results ........................................................................................................................................... 11 Discussion ..................................................................................................................................... 13 References ..................................................................................................................................... 31 Appendix 1: Supplementary Figures and Tables .......................................................................... 39 iii iv List of Figures 1 (a) Photographs of snow fences and (b) a typical sampling plot ............................................. 19 2 Aerial view (partial) of study site ............................................................................................. 20 3 Relationship between (log) spiders body mass and (log) abdomen length .............................. 21 4 Effectiveness of small-scare trenches at excluding spiders ..................................................... 22 5 Daily abundances of spiders over the season by treatment ...................................................... 23 6 ANOSIM comparison of spider communities over the season ................................................ 24 7 Penultimate and adult spider masses over the season by treatment ......................................... 25 8 Pardosa mass over the season by treatment ............................................................................ 26 9 Pardosa peak mass by site-specific winter snow depths ......................................................... 27 Appendix 1 (a) Average air and (b) surface temperatures over the season by treatment ............................ 39 2 Genera accumulation curves over the season ........................................................................... 40 3 Generic richness over the season by treatment ........................................................................ 41 4 May air temperatures for Churchill, Manitoba from 1945-2010 ............................................. 42 iv v List of Tables 1 Season-wide catches of spider genera on heath tundra sites .................................................... 28 2 Repeated-measures ANOVA on Pardosa body mass .............................................................. 29 3 ANOVA comparison of spider masses by genera over the season .......................................... 30 Appendix 1 Shapiro-Wilk normality scores for daily spider abundances by treatment .............................. 43 2 Results of repeated-measures ANOVAs on seasonal spider abundances by genera ............... 44 3 Results of Wilcoxon rank-sum tests on season spider abundances by genera ......................... 45 4 Similarity percentages analysis (SIMPER) of spider communities ......................................... 46 5 Results of Wilcoxon rank-sum tests on seasonal spider masses by genera ............................. 47 v 1 Introduction Recent environmental changes across the Arctic are well documented (Serreze et al., 2000; Peterson et al., 2002; Hinzman et al., 2005) and have been associated with changes to the surface vegetation (Stow et al., 2004; Johansson et al., 2006; Olthof et al., 2009; Forbes et al., 2010; Hill & Henry, 2011). The expansion of deciduous shrubs (in particular, Betula, Salix, and Aldus species) has been especially significant on the tundra and boreal forest-tundra ecotone (Sturm et al., 2001a; Tape et al., 2006; Hudson & Henry, 2009), with an average increase in landscape coverage of approximately 5-15% over the past half century. Experimental work suggests that such shrub encroachment is driven largely by increased fertilization (Press et al., 1998; Dormann & Woodin, 2002; van Wijk et al., 2004) as a result of temperature-induced shifts in litter decomposition (Cornelissen et al., 2007), mineralization (Rustad et al., 2001; Rinnan et al., 2007), spring melt date (Stone et al., 2002) and other factors (Wookey et al., 1993; Wahren et al., 2005; Walker et al., 2006; Hudson et al., 2011). Moreover, the presence of shrubs may facilitate intraspecific recruitment and growth by increasing winter nutrient release (Sturm et al., 2001b; Sturm et al., 2005), which would create a feedback mechanism where shrub coverage could continue to increase without an associated increase in air temperatures. Among the consequences of increased shrub cover in the Arctic is higher winter snow pack due partly to a reduction in the wind-driven sublimation of snow directly upwind and downwind of the shrub stems. The tendency of tall shrubs to act as ‘wind breaks’ on the otherwise flat tundra region can also affect how wind distributes snow across the landscape, with areas of high shrub cover typically acquiring higher amounts of snow. Using simulations, Liston et al. (2002) found that increased shrub cover could decrease snow sublimation by 20% or more across a landscape and produce a roughly equivalent increase in winter snow cover. In the field, Pomeroy et al. (2006) found that the influence of shrub coverage on snow cover was even more 2 pronounced, with snow depths in shrub dominated tundra sites on average twice that of nearby shrub-free sites receiving similar levels of snowfall. Increased snow pack during the winter as a result of shrub encroachment would likely delay spring melt date, due to the larger amount of snow requiring melting (Liston et al., 2002). There is some evidence that differences in the albedo and heat absorption of shrubs could accelerate melt rate (Strack et al., 2007), however, whether such differences could compensate for the higher initial snow pack would likely depend on other local factors, such as slope and total annual precipitation (Marsh et al., 2010). In any case, since shrubs alter the distribution of snow not only around their stems, but upwind and downwind of them, their presence on the landscape can have a substantial impact on how quickly a patch will melt out in the spring relative to neighboring shrub-free sites. The impact of shrub encroachment on Arctic systems through increased winter snow cover and/or earlier spring snow melt, could be particularly significant for terrestrial arthropod communities, which account for the majority of all animal life in the Arctic (Danks, 1981) and for which overwintering conditions are perhaps the most significant abiotic component of survival (review in Strathdee & Bale, 1998; Danks, 2004). The Arctic beetle, Pytho americanus Kirby, for instance, must overwinter under the bark of decaying spruce (Picea) trees, a habitat that enables gradual acclimation to low temperatures and allows them to supercool to temperatures well below zero (Ring & Tesar 1980). Similarly, Dendroctonus rufipenis (Coleoptera) demonstrates little freeze tolerance in the lab and appears to require shelter in fallen timber in order to survive the winter (Miller, 1982). In a recent study on antifreeze proteins in Arctic arthropods, Duman et al. (2004) found that many tundra insects and spiders exhibited little to no freeze tolerance, suggesting that the thermal character of overwintering sites may be especially important for