Soutenance mémoire
Arthropod sampling
Arthropods were sampled from June to August using a rectangular pitfall trap (38 cm x 5 cm and 7 cm deep). Above the pitfall trap, a 40 cm by 40 cm mesh screen was set vertically. Above the screen, a plastic cone funneled flying insects into a collecting jar (Fig.1). Traps were placed with the mesh perpendicular to prevailing winds, and their design was similar to traps used by Schekkerman et al. (2003). These passive traps provided a combined measure of abundance and activity levels of arthropods, and so a proxy for arthropod availability to foraging birds, and data from such methods have been correlated to chick growth rate (McKinnon et al. 2012; Schekkerman et al. 2003). Traps were used on four different Arctic Islands across the Canadian Arctic: Southampton (63°59’N, 81°40’W; me an summer temperature = 7.1 OC) from 2006 to 2008, Herschel (69°35’N,138°55′ W;10.6 oC) in 2007 and 2008, Bylot (73°8’N, 79°58’W; 5.8°C) from 2005 to 2008 and Ellesmere (Alert) (82°29’N, 62°21 ‘W; 3.8°C) in 2007 and 2008. At each site, five traps located 20 met ers apart from each other on a straight line were set in both dry upland (mesic) or low wetland (wet) tundra, the main foraging habitats for the dominant insectivorous bird species (passerines and shorebirds) during their brood rearing period. Site-specifie habitat descriptions are available in Smith et al. (2007) (Southampton), Ale et al. (20 Il) (Herschel), Gauthier et al. (20 Il) (ByIot), and Morrison et al. (2005) (Ellesmere). Traps were emptied at approximately two day intervals, and arthropods were stored in ethanol (70%) until sorting and identification in the laboratory. Insects were sorted into families, and spiders were grouped together. Springtails and mites were not inc1uded in theanalyses because of their very low contribution to total arthropod biomass. Butterflies and bumblebees were also exc1uded because few individuals were collected due to the design ofthe traps and because these few heavy specimens had a strong influence on daily variation in biomass. Moreover, adults of these taxa are not key prey for or passerines. Sorting and identification was conducted on a sub-sample three to five traps for each habitat and site. To standardize data order to ca1culate a daily index of arthropod availability (mg/trap), the arthropod biomass (dry mass) was divided by the number of traps sorted and number of days between sampling events. transformarthropod counts into dry mass, we used length to dry mass equations derived from our samples (Picotin 2008) or from the literature (Hodar 1996; Rogers et al. 1977; Sage 1982; Sample et al. 1993). For sorne of the dominant groups, we dried and weighed specimens and ca1culated a mean individual dry mass (Picotin 2008). When individual variation in size was high, individuals were grouped within size categories and mean dry mass was obtained for each category. A list of equations is provided in the supplementary materials of McKinnon et al. (2012).
Discussion
The objective of our study was to generate predictive models of daily arthropod availability in the Canadian Arctic. Based on data coUected at four different sites that varied in terms of their climate and arthropod communities, we found part of the variation to be dependent on climatic variables measured daily such as precipitation or wind. The other part of the variation was explained by daily temperature and a larger time scale measure of weather (thaw degree-day). lndeed, with mean daily temperature, thaw degreeday, and its quadratic form, more than 70% of the daily variation in arthropod availability was explained by our models. This is a substantial portion of the deviance, emphasizing the overarching importance of both temperature and variables measured over larger time scales in determining seasonal change in arthropod availability. Our research results complement work conducted in other Arctic regions such as the Taymir Peninsula in Siberia, where cumulative degree-days was a better predictor of the number of arthropods caught than the combination of date and temperature (Schekkerman et al. 2004). Together with our results, this highlights the importance of including temperature, cumulative temperature and its quadratic form in studies aimed at forecasting arthropod abundance rather than focusing mostIy on daily temperature variation as is often the case (Deutsch et al. 2008).As ectotherms, arthropods are highly sensitive to climate variation (BaIe et al. 2002; Danks 1981 ; Hodkinson et al. 1998). Population growth rates and development of many arthropod species is linked to temperature (Frazier et al. 2006; Huey and Berrigan 2001). As expected, we found higher dry mass (and diversity) of arthropods for sites with a warmer summer. However, predicting the effect of global warming also requires further information on possible lagged effects (like density dependence and previous summer temperature ).There was considerable variation in seasonal trends of arthropod availability in terms of timing, duration, and magnitude of peaks in total biomass both between sites and between habitats within sites. Arthropod availability was higher in wet habitats than in mesic habitats and this was consistent for aU our study sites; we uncovered no significant interaction between habitat and site. Interestingly, for a specific site, climatic variables had similar effects on the abundance and phenology of a particular family for both habitats. Knowing the habitat characteristic of a site and its temperature thus appear to be an important covariate to model Arctic arthropod availability. Across the sites, different patterns of abundance and phenology were evident among the different arthropod families that were sampled. For example, Tipulidae (crane flies) showed very synchronized peaks of abundance whereas Araneae (spiders) were present throughout the study period. This is likely related to the biology of the different families. In certain species of crane flies, for example, aIl adults that emerge in a given summer overwintered in the prepupal stage (MacLean 1973). As a result, there is no feeding requirement before their emergence as adults and a more synchronized emergence is therefore possible. Studies of crane flies in the Arctic, including ours (Fig. 3), have revealed weIl defined peaks of abundance (MacLean and Pitelka 1971; Tulp and Schekkerman 2008) supporting the concept of a synchronized emergence. As a consequence, our climatic models for crane flies had a good fit to the data. This life-history strategy has also been observed in a number of Arctic Chironomidae (non-biting midges) species (Danks and Oliver 1972). However, the timing of emergence of non-biting midges is directly linked to the temperature of the pond in which the prepupal stage resides (Danks and Oliver 1972). Since ponds can warrn up at different rates (based on depth, for example), emergence can be synchronized within ponds rather than across broad spatial scales. In our observations, there is more than one peak of non-biting midges in any given year and the peaks are not as weIl defined as for crane flies (see Fig. 3). Numerous peaks may also be due to several species emerging at different times considering that non-biting midges contributes a high proportion of species to the tundra insect fauna (MacLe an and Pitelka 1971). In spiders, we did not observe a synchronized peak of abundance (see Fig.3). They are present through the season and usuaIly are active at the very beginning of snow melt (Meltofte and H0ye 2007; this study). These contrasting patterns of arthropod phenology may be important in their role as food for birds. Both spiders and crane flies are important to successful reproduction of insectivorous birds but they likely play different roles. Spiders are active at the very beginning of snow melt and are present throughout the season (Meltofte and H0ye 2007; this study). It has been found that the abundance of wolf spider Pardosa glacialis (Thorell, 1872) (Araneae: Lycosidae) was probably the only variable influencing the timing of reproduction of jaegers Stercorarius longicaudus Vieillot 1819 in Greenland (Meltofte and H0ye 2007). Arthropods that are present early in the season may be especially important for the long distant migrants that arrive at their Arctic breeding grounds with little to no stored energy and must rely upon early emerging arthropods in order to rebuild fat reserves and produce eggs (Danks 1971; Klaassen et al. 2001; Meltofte et al. 2008). On the other hand,arthropods such as crane flies, which exhibit peaks later in the season may be more important for the growth and survival of offspring (Pearce-Higgins and Yalden 2004). Arthropods with an availability that is limited in time (synchronized emergence) can provide a great source of food for chicks if hatching is synchronized with emergence. But achieving this synchrony can be challenging, especially in the context of climate change (Both and Visser 2001; Thomas et al. 2001). For example, studies have now indicated that an asynchrony between hatch of shorebird chicks and peaks in crane flies can reduce chick growth rates (McKinnon et al. 2012) and even lead to potential population declines in sorne shorebird species (Pearce-Higgins et al. 2005). However, abundance of arthropods rather than asynchrony may better predict population decline in different shorebirds species (Pearce-Higgins et al. 2009; Pearce-Higgins 2010). Changes in climate are likely to induce changes in the patterns of arthropod availability with the potential to affect several trophic levels within the tundra food web. The unique data we have collected as part of this pan-Canadian effort, and the models we have tested will help us to forecast and/or hindcast arthropod availability over time, so that we can gain greater insight into the potential effects of changing arthropod availability for Arctic insectivores. Future research should continue to refine our understanding of seasonal variation in arthropod availability and attempt to study variation at lower taxonomic levels such as genus and, if possible, species. However, with over 2000 species of arthropods in arctic North America (Danks 1992), this could prove to a be a very interesting, though rather challenging task.
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Table des matières
REMERCIEMENTS
RÉSUMÉ
ABSTRACT
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INTRODUCTION GÉNÉRALE
CHAPITRE 1 ABONDANCE ET PHÉNOLOGIE DES ARTHROPODES TERRESTRES DE L’ARCTIQUE CANADIEN: MODÉLISATION DE LA DISPONIBILITÉ DES RESSOURCES ALIMENTAIRES POUR LES OISEAUX INSECTIVORES
1.1 RESUME EN FRANÇAIS DE L’ ARTICLE
1.2 TERRESTRIAL ARTHROPOD ABUNDANCE AND PHENOLOGY IN THE CANADIAN ARCTIC : MODELING RESSOURCE A V AILABILITY FOR ARCTIC-NESTING INSECTIVOROUS BIRDS
CONCLUSION
RÉFÉRENCES BIBLIOGRAPHIQUES
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