The relative contribution of allochthonous and autochthonous production in zooplankton nutrition has been of interest since the net heterotrophy of lakes was recognised to be common. I measured the 13C signature of epilimnetic CO2, particulate organic carbon (POC), and zooplankton in 27 north-temperate lakes in late summer and used the relationships between the POC and zooplankton 13C signatures and the CO2 signature to estimate the autochthonous contribution to these fractions of the plankton. My hypothesis was that POC and zooplankton signature would reflect the 13CO2 signature if they were autochthonous. Conversely, increasing allochthonous C would result in a 13C signature of POC or zooplankton that is increasingly influenced by the allochthonous 13C signature (δ-28‰) and decreasingly dependent on the CO2 signature. The average autochthonous contribution to epilimnetic POC was estimated to be between 62 and 75%. Epilimnetic zooplankton were, on average, between 77 and 91% autochthonous, indicating that zooplankton bias their feeding towards the autochthonous fraction of POC. On average, zooplankton were 1.2‰ enriched in 13C relative to POC, but their biased feeding on phytoplankton means that they can be depleted relative to POC in lakes where POC is highly depleted in 13C. The relationship between 13C POC and 13CO2 allowed us to estimate average photosynthetic fraction as -15.9‰. This estimate is independent of how much allochthonous C contributes to POC. Variation in photosynthetic fractionation was not a major contributor to differences among lakes in POC and zooplankton 13C signature. Allochthonous C is an important, although clearly secondary, source of C to zooplankton of these lakes in late summer. I expanded the above analysis by culling the literature for 13C stable isotope data of lake CO2, POC, and zooplankton. I found that, similar to the lakes that I had sampled, POC signature showed a strong influence of allochthonous C, and inferred that it was close to 50% allochthonous on average. I calculated an autochthonous fractionation of -14.1‰ for the metadata, which was similar to that of the lakes I sampled. While POC had a considerable allochthonous contribution, zooplankton signatures were strongly related to the CO2 signatures, suggesting that their carbon was mostly autochthonous. Therefore, while terrestrial inputs form a major portion of POC, zooplankton C, on average, was largely autochthonous. I also examined the differences in 13C/15N among zooplankton taxa, and differences in 13CO2, 13C/15N of POM, and 13C/15N of zooplankton with depth. There were small differences among the 15N of various taxa, and I did not detect differences in 13C amongst taxa. I found vertical heterogeneity was most marked in 13CO2 signatures, which generally depleted appreciably with increasing lake depth. The signatures of 13C POM and 13C zooplankton also generally depleted with depth, but much less so than did 13CO2. I interpret this as indicating that a large portion of POM and zooplankton C in the metalimnia and hypolimnia of these lakes is derived from C fixed in the epilimnia.

The relative contribution of allochthonous and autochthonous production in zooplankton nutrition has been of interest since the net heterotrophy of lakes was recognised to be common. I measured the 13C signature of epilimnetic CO2, particulate organic carbon (POC), and zooplankton in 27 north-temperate lakes in late summer and used the relationships between the POC and zooplankton 13C signatures and the CO2 signature to estimate the autochthonous contribution to these fractions of the plankton. My hypothesis was that POC and zooplankton signature would reflect the 13CO2 signature if they were autochthonous. Conversely, increasing allochthonous C would result in a 13C signature of POC or zooplankton that is increasingly influenced by the allochthonous 13C signature (δ-28‰) and decreasingly dependent on the CO2 signature. The average autochthonous contribution to epilimnetic POC was estimated to be between 62 and 75%. Epilimnetic zooplankton were, on average, between 77 and 91% autochthonous, indicating that zooplankton bias their feeding towards the autochthonous fraction of POC. On average, zooplankton were 1.2‰ enriched in 13C relative to POC, but their biased feeding on phytoplankton means that they can be depleted relative to POC in lakes where POC is highly depleted in 13C. The relationship between 13C POC and 13CO2 allowed us to estimate average photosynthetic fraction as -15.9‰. This estimate is independent of how much allochthonous C contributes to POC. Variation in photosynthetic fractionation was not a major contributor to differences among lakes in POC and zooplankton 13C signature. Allochthonous C is an important, although clearly secondary, source of C to zooplankton of these lakes in late summer. I expanded the above analysis by culling the literature for 13C stable isotope data of lake CO2, POC, and zooplankton. I found that, similar to the lakes that I had sampled, POC signature showed a strong influence of allochthonous C, and inferred that it was close to 50% allochthonous on average. I calculated an autochthonous fractionation of -14.1‰ for the metadata, which was similar to that of the lakes I sampled. While POC had a considerable allochthonous contribution, zooplankton signatures were strongly related to the CO2 signatures, suggesting that their carbon was mostly autochthonous. Therefore, while terrestrial inputs form a major portion of POC, zooplankton C, on average, was largely autochthonous. I also examined the differences in 13C/15N among zooplankton taxa, and differences in 13CO2, 13C/15N of POM, and 13C/15N of zooplankton with depth. There were small differences among the 15N of various taxa, and I did not detect differences in 13C amongst taxa. I found vertical heterogeneity was most marked in 13CO2 signatures, which generally depleted appreciably with increasing lake depth. The signatures of 13C POM and 13C zooplankton also generally depleted with depth, but much less so than did 13CO2. I interpret this as indicating that a large portion of POM and zooplankton C in the metalimnia and hypolimnia of these lakes is derived from C fixed in the epilimnia.

Ecological communities – groups of interacting species – are subject to a variety of disturbances. Understanding responses to these disturbances is a primary goal of community ecology. The structural complexity of the community and the traits of the community’s constituent species are both known to have a significant impact on a community’s response to a disturbance. In this thesis, we investigated how these two scales – the community level and the species level – interactively affected community responses to both short and long term disturbances. Our first hypothesis was that interaction strength would be weaker in species with many interactions when compared to species with fewer interactions. To test this hypothesis, we used simulated food webs and found that, in locally stable food webs, species with many interactions tended predominantly to have interactions with predators or with prey. While these many predator or prey interactions were weak, they tended to be balanced by a few interactions of the opposite type (with prey or predators) which were stronger than average. The structure of the network, where species had predominantly one type of interaction, was essential for this relationship between the number and strength of interactions to arise. Our second study investigated how food webs of varying size and connectance respond to press and pulse disturbances. Many studies of food web stability only focus on the response to short term or “pulse” disturbances, however, as anthropogenic impacts on food webs increase, it is important to increase our understanding of food web responses to long term or “press” disturbances and determine whether they follow the same pattern as pulse disturbances. We found that more species rich and connected food webs were less stable to both types of disturbance and the more stable a food web was to a pulse disturbance, the more stable it was to a press disturbance as well. We also found that the traits – trophic level and number of interactions – of the disturbed species impacted a food web’s resistance to a press disturbance. Food webs were less resistant to the disturbance of species with many interactions or low trophic level than species with few interactions or high trophic level. The strength of species’ effects on stability was also moderated by the structural complexity of the food web. Together the work that makes up this thesis suggests that, to understand the stability of food webs to any kind of disturbance, we should consider both the structure of the network and the traits of the species embedded within it. While we found that networks were more vulnerable to disturbance of certain species than others, this observation also depended on the structure and complexity of the community they existed in. This has important implications for communities subject to disturbances, especially those disturbances which alter the way in which communities are structured and species interact.

Invasive species are a known stressor on aquatic ecosystems, particularly in the waters of the Great Lakes basin. A recent invader, Hemimysis anomala, has had significant impacts on the food webs of Europe, where it invaded previous to its spread to North America. However, despite the fact that Hemimysis is now widespread in the Great Lakes basin, no analysis has been done on the trophic position of Hemimysis in North America invaded sites. This thesis used carbon (??13C) and nitrogen (??15N) stable isotopes to examine spatial and temporal patterns in Hemimysis trophic niche use in invaded North American sites in an attempt to broaden the knowledge base on this invader and to examine potential impacts this invader may have on the food webs of the Great Lakes. A spatial comparison of trophic niche use by Hemimysis among 13 sites in Lake Erie, Lake Ontario, and the St. Lawrence River was conducted between late July and mid-September of 2011. Main sources of carbon (benthic versus pelagic production) and trophic offset, or trophic distance from basal food web items, of Hemimysis were quantified using Hemimysis ??13C and ??15N values. Results indicated that: 1) Hemimysis relied predominantly on pelagic carbon sources at the majority of sites, and isotopic differences between life-stages existed at two of the 13 sites examined, 2) the trophic offset and reliance on pelagic food sources did not differ significantly between lotic and lentic sites, and 3) the isotopic niche width of Hemimysis was spatially heterogeneous, varying by an order of magnitude among sites, but was unrelated to the degree of isotopic variation in the basal food web at each site. Observed ranges in trophic offset and the pelagic fraction of dietary carbon indicate that Hemimysis derives carbon from both benthic and water column sources, as well as at multiple trophic levels. Results support the notion that Hemimysis is an opportunistic omnivore that displays significant dietary flexibility. To test the relative importance of key biotic and abiotic factors, taken from the literature, in driving Hemimysis isotopic variation, a temporal analysis was conducted at two North American sites, one in Lake Ontario and one in the St. Lawrence River, which were repeatedly sampled for Hemimysis and related food web items between September 2008 and January 2012. Seasonal patterns of winter enrichment ??? summer depletion were found in Hemimysis ??15N in Lake Ontario, but a similar pattern was not seen in the St. Lawrence River. Multiple regression models were used to determine the importance of water temperature, Hemimysis C:N ratios, Hemimysis length, and the isotopic values of basal food web components in explaining observed variation in Hemimysis ??13C and ??15N values. Significant relationships were found between Hemimysis isotopic values and water temperature, but relationships with the isotopic signatures of the pelagic basal food web were weak or nonexistent. Hemimysis ??13C values were significantly correlated with C:N ratios. Strong evidence of an ontogenetic dietary shift was found in Lake Ontario, with length showing a significant positive correlation with Hemimysis ??15N. All together the factors included in the models explained little of the observed variation in Hemimysis isotopic values, with approximately 20 % of the observed variation in Hemimysis ??13C, and just under half of Hemimysis ??15N variation, being explained by the included factors. As such, Hemimysis isotopic variation must be explained by factors not included in this study and may include factors such as species composition of the invaded site and availability of prey. Overall, the results of this thesis highlight the opportunistic and flexible nature of Hemimysis diet, and demonstrate the need for future work to determine the main drivers of isotopic variability and trophic niche selection of Hemimysis. The degree of trophic flexibility seen in Hemimysis implies that potential food web impacts will be site specific and heavily reliant on food web dynamics and environmental characteristics of the invaded site.

In this study of energy pathways in karst groundwater the first chapter examines spatial and temporal patterns of bacterial density and activity in the Dorvan-Cleyzieu karst aquifer, France. During baseflow, bacterial density and activity in the water column was similar in upper and lower zones of the aquifer. Floods apparently scoured inactive bacteria from the aquifer matrix but had little effect on respiring cells. Dissolved organic carbon was more abundant at the base of the aquifer, probably because of patchy distribution of particulate organic matter in upper aquifer zones that leached dissolved organic carbon. The temporal sequence of flooding and drying in the aquifer appears to play an important role in the maintenance of biofilms which should be important energy sources to higher trophic levels in the aquifer. The ecosystem expansion and contraction model, originally developed to describe surface streams, may be a good descriptor of spatial and temporal patterns of microbial films in karst aquifers. The process of leaf and wood breakdown in cave streams in Organ Cave, West Virginia is examined in Chapter 2. Leaf and wood breakdown rates and microbial biomass and respiration on leaves and wood were compared between cave streams with and without coarse particulate organic matter (CPOM) input from the surface to examine the role of CPOM input in leaf and wood breakdown. Breakdown rate and pattern of microbial colonization of leaves and wood were typical of results reported for surface streams. Unlike in surface streams, CPOM input did not influence breakdown rate or microbial colonization on leaves and wood, apparently because nutrients are not limiting in cave streams. Nutrient addition had little effect on microbial films on wood in either stream type. Gammarus minus is an important shredder in Organ Cave streams and G. minus colonization accelerated leaf breakdown rates. Leaf and wood transport rates were low and, when combined with breakdown rates, suggest that CPOM will be retained and transformed to fine particles near its entry point to the subsurface. In chapter 3 I examine cave stream food web structure and the role various organic matter sources in stream trophic dynamics. I used stable isotope (13C and 15N) natural abundance analysis and a 13C-acetate tracer release to establish feeding relationships and to trace the use and importance of bacterial carbon in cave streams with and without CPOM input. Cave streams contained three trophic levels consisting of organic matter sources, primary consumers, and predators. Patterns of 13C labeling in the stream were similar to that in similar studies of surface streams. 13C acetate was incorporated into epilithic biofilms and fine benthic organic matter (FBOM). Some primary consumers, Fontigens tartarea, Gyraulus parvus, and Physa were highly labeled and showed a longitudinal labeling pattern consistent with the consumption of epilithic biofilms. An epigean caddisfly, Dolophilodes, was highly labeled and probably feeds on suspended organic matter. Other primary consumers, Gammarus minus and Caecidotea holsingeri, feed on FBOM and epilithon. Two amphipods, Stygobromus emarginatus and S. spinatus, and a planarian, Macrocotyla hoffmasteri, are predators in the streams. Leaves and wood were not major energy sources directly used by stream animals. Dissolved organic matter (DOM) originating from soils appears to be the primary energy source for stream food webs by fueling bacterial production that is then used by higher trophic levels. Because epilithon C turnover times were relatively long (12.7 - 17 days), DOM can be immobilized in cave stream biofilms, enhancing the efficiency with which the microbial loop may transfer energy to higher trophic levels. / Ph. D.

A food web was constructed for a boreal-forest dominated national park in south-eastern Sweden, called Tyresta national park and its surrounding nature reserve. Due to increasing threats to mammalian species and their critical role in the food web mammals were used as the outset of the food web, and from there branching out to taxa trophically interacting with mammals directly and indirectly. The outset of the food web was constructed using available inventory data, focusing on mammals. From inventory data and 53 interaction sources, consisting of mostly primary literature a food web was created that includes 32 mammalian taxa, six bird families, five invertebrate taxa, fish, amphibians, fungi, reptiles, detritus and 9 different basal plant taxa. The results of the literature review was that the most consumed staple taxon of all were hexapods, with it being a staple food for 15 taxa, six of them being bats preying mainly on dipterans. There are a wide variety of ways that interspecific competition could come to affect taxa in the study area, for example through intraguild predation, increased dietary breadth or avoidance, or a combination of these. This method could serve as a complement to existing food webs or the construction of new ones in a cheaper, less intrusive and less time-consuming way, albeit missing local adaptations. The constructed food web structure and findings of interspecific competition in Tyresta can be used for further analyzing certain links or branching the web out even further, with increased depth or quantification of certain taxon.

Habitat loss and fragmentation have been identified as the main drivers of biodiversity loss. These drivers increase the proportion of habitat edges and change the configuration of landscapes. Habitat edges are known to affect ecological patterns and processes, however, is still unknown how these boundaries affect the assemblage of interactions among species within a community, and particularly its structure. Food webs depict not only the composition of the community, but also the feeding links, which represent a measure of energy flow. Therefore, they can inform about the relationships among community diversity, stability, and ecosystem functions. This thesis explores the effects of habitat edges across native vs. managed forests on the food web of a tri-trophic system comprising plants, herbivores (Lepidoptera larvae) and predators (parasitoids). Particularly, it addresses three main objectives: 1) how food webs at habitat edges are assembled from the species and interactions present in the adjoining habitats; 2) how phylogenetic diversity and the coevolutionary signal among interacting species change across a habitat edge gradient; and 3) whether the mechanisms driving community-wide consumption rates and the ecosystem service of pest control are related to structural characteristics of the food webs. The key findings of this thesis are that, despite the composition of species and interactions of native and managed habitats merging at their interface, food-web structure did not arise as a simple combination of its adjacent habitat webs, potentially due to differential responses of organisms to habitat edges. Moreover, beyond taxonomic composition, the phylogenetic diversity and signal of coevolution among interacting species also change between habitat types, even though this did not translate to changes in consumption rates. Consumption rates and their stability increased with complementarity and redundancy in resource-use among predators. This reflects how environmental changes such as habitat fragmentation can have an effect beyond composition per se, affecting the assemblage of species interactions and even potentially interfering with natural evolutionary processes. Therefore, using interaction-network approaches for determining the impacts of changes may shed light on the underlying mechanisms driving such changes, and help to develop landscape management plans that reduce negative effects on species assemblages.

Climate change and fisheries have affected marine environments worldwide leading to impacts on ecosystem structure and functioning. However there is clear evidence of spatial variability in the response of these impacts both within and among marine ecosystems. Although several studies have tried to explain the effect of these impacts on marine food webs, it is unclear how they interact, and how they may affect marine ecosystems remains an important unanswered question. This suggests the urgent need for multiple-trophic level and ecosystem-based management approaches to account for both fisheries and climate change impacts at ocean basins across the globe. Marine apex predators, such as seabirds, are vulnerable to the effects of both climate and fishing impacts, and can be used as reliable and sensitive bio-indicators of the status of the marine ecosystem. The Celtic Sea ecosystem is a productive shelf region in the Northeast Atlantic. It is characterized by high fish and invertebrate biodiversity. In addition, internationally important numbers of seabirds, such as Northern gannet Morus bassanus (L.), Manx shearwater Puffinus puffinus (B.), Common guillemot Uria aalge (P.) and Black-legged kittiwake Rissa tridactyla (L.), breed along the Celtic Sea coasts. In recent years, fisheries from across Europe have intensively exploited the Celtic Sea, leading to changes in stock structure. Moreover, the increase in annual average Sea Surface Temperature by 0.67 oC over the past two decades has altered the composition of plankton communities. These impacts, independently and in tandem, are likely to have had dramatic effects upon the Celtic Sea food web emphasizing the need to enhance our understanding of this important marine ecosystem. In this thesis the effects of climate change and fisheries on the Celtic Sea pelagic food web are evaluated, in particular focussing on the response of seabird populations. This is in part because of recent declines in the breeding success of many seabird colonies in the northeast Atlantic, particularly around the North Sea. Long-term data across four trophic levels (phytoplankton, zooplankton, mid-trophic level fish and seabirds) and different modelling approaches are used to determine factors influencing seabird productivity at different geographical scales. First, I review the direct and indirect effects of climate change and fisheries upon marine ecosystems, as well as their impacts upon marine birds. Second, I use data collected during 1986-2007 from a single seabird colony, across four trophic levels, to investigate long-term direct and indirect climate effects. The results suggest only a weak climate signal in the Celtic Sea, and this is only evident between mid-trophic level fish and certain species of seabird. Third, a similar multi-trophic level approach across three nearby regions in the southwest UK (Irish Sea, Celtic Sea, and English Channel) reveal no evidence of a bottom-up signal during the period 1991-2007. These findings are in contrast with the nearby North Sea region, where a strong bottom-up effect was found to affect seabird populations, highlighting the importance of regional-based studies across multiple trophic levels. Finally, to provide a more complete picture of the Celtic Sea, and how it might respond to changes in fisheries management and climatic variation, I use the complex tropho-dynamic ecosystem model Ecopath with Ecosim. The main focus is on how seabird biomass changes in response to the application of different fisheries regimes likely to be implemented under forthcoming reforms to the Common Fisheries Policy (e.g. the application of quotas and discard bans), as well as future climate change scenarios, in order to provide guideline support for resource management and seabird conservation in the Celtic Sea. The results suggest that some seabird guilds (gulls and some other scavengers) may be negatively affected by a reduction in discards, while other species (offshore divers) will benefit from a decrease in the fishing of pelagic fish species. Climate change is likely to have a negative impact across all trophic levels with a strong negative impact upon seabird populations. Therefore seabirds are likely to show species-specific responses to both climate variation (bottom-up effect) and changes in fishing practices, in particular our findings suggest that for some species climate may outweigh the fisheries impacts even when fisheries pressure is reduced by 50%. In summary, this study suggests that despite the generally negative impact of climate described for some regions in the Northeast Atlantic, the Celtic Sea ecosystem seems to be more resilient. However, both climate and fisheries and the interactions between these factors should be taken into account in the formulation of future management plans for the Celtic Sea ecosystem. The use of multiple-trophic level and ecosystem-based approaches over multiple spatial and temporal scales has helped to elucidate possible trophic mechanisms that are the response to future fishing and climate impacts in the Celtic Sea. The results of this study could have implications for both management plans and conservation policy.

Doctor of Philosophy / Division of Biology / Anthony Joern / John Blair / Ecological communities are comprised both of species and their interactions. The importance of species interactions is embraced by ecological network analysis, a framework used to identify non-random patterns in species interactions, and the consequences of these patterns for maintaining species diversity. Here, I investigated environmental drivers of the structure of plant-pollinator and plant-herbivore networks. Specifically, I asked: (1) Do global-scale climate gradients shape mutualistic and antagonistic networks? (2) At a landscape scale (within a 3,487 ha research site), how do contrasting regimes of major grassland disturbances - fire frequency and grazing by bison (Bison bison) - shape plant-pollinator network structure? (3) How do fire and grazing affect plant-grasshopper network structure? And, (4) What is the role of plant species diversity in determining plant-herbivore network structure? At the global scale, variability in temperature was the key climatic factor regulating both antagonistic and mutualistic network structural properties. At the landscape scale, fire and grazing had major consequences for plant-pollinator and plant-herbivore communities. In particular, bison grazing increased network complexity and resistance to species loss for both plant-pollinator and plant-herbivore systems. Results from an experimental grassland restoration that manipulated plant diversity suggest that plant diversity directly affects plant-herbivore structure and increases network stability. Collectively, these results suggest that environmental gradients and plant species diversity regulate the network structure of ecological communities. Determining how the structure of ecological interactions change with environmental conditions and species diversity improves our ability to identify vulnerable communities, and to predict responses of biodiversity to global change.

<p>Environmental changes, such as climate change, can have differential effects on species, with important consequences for community structure and ultimately, for ecosystem functioning. In the Arctic, where ecosystems are experiencing warming at twice the rate as elsewhere, these effects are expected to be particularly strong. A proper characterization of the link between warming and biotic interactions in these particular communities is of global importance because the tundra's permafrost stores a vast amount of carbon that could be released through decomposition as greenhouse gases and alter the global rate of climate change. In this dissertation, I examine how arthropod communities are responding to warming in the Arctic and how these responses might be affecting ecosystem functioning. </p><p>I first address the question of whether and how long-term changes in climate are affecting individual groups and overall community structure in a high-arctic arthropod food web. I find that increasingly warm springs and summers between 1996-2011 differentially affected some arthropod groups and that this led to major changes in the relative abundances of different trophic groups within the arthropod community. Specifically, spring and summer warming are associated with relatively more herbivores and parasitoids and fewer detritivores within the community. These changes are particularly pronounced in heath sites, suggesting that arthropod communities in dry habitats are more responsive to climate change than those in wet habitats. I also show that herbivores and parasitoids are sensitive to conditions at subzero temperatures, even during periods of diapause, and that all trophic groups benefit from a longer transition period between summer and winter. These results suggest that the projected winter and springtime warming in Greenland may have unexpected consequences for northern arthropod communities. Moreover, the relative increase in herbivores and loss of detritivores may be changing the influence of the arthropod community over key ecosystem processes such as decomposition, nutrient cycling, and primary productivity in the tundra. </p><p>Predator-induced trophic cascades have been shown to impact both community structure and ecosystem processes, yet it is unclear how climate change may exacerbate or dampen predator effects on ecosystems. In the second chapter of my dissertation, I investigate the role of one of the dominant tundra predators within the arctic ecosystem, wolf spiders, and how their impact might be changing with warming. Using results from a two-year-long field experiment, I test the influence of wolf spider density over the structure of soil microarthropod communities and decomposition rates under both ambient and artificially warmed temperatures. I find that predator effects on soil microarthropods change in response to warming and that these changes translate into context-specific indirect effects of predators on decomposition. Specifically, while high densities of wolf spiders lead to faster decomposition rates at ambient temperatures, they are associated with slower decomposition rates in experimentally warmed plots. My results suggest that if warming causes an increase in arctic wolf spider densities, these spiders may buffer the rate at which the massive pool of stored carbon is lost from the tundra. </p><p>Wolf spiders in the Arctic are expected to become larger with warming, but it is unclear how this change in body size will affect spider populations or the role of wolf spiders within arctic food webs. In the third chapter of my dissertation, I explore wolf spider population structure and juvenile recruitment at three sites of the Alaskan Arctic that naturally differ in mean spider body size. I find that there are fewer juveniles in sites where female body sizes are larger and that this pattern is likely driven by a size-related increase in the rate of intraspecific cannibalism. These findings suggest that across the tundra landscape, there is substantial variation in the population structure and trophic position of wolf spiders, which is driven by differences in female spider body sizes. </p><p>Overall, this dissertation demonstrates that arctic arthropod communities are changing as a result of warming. In the long-term, warming is causing a shift in arthropod community structure that is likely altering the functional role of these animals within the ecosystem. However even in the short-term, warming can alter species interactions and community structure, with important consequences for ecosystem function. Arthropods are not typically considered to be major players in arctic ecosystems, but I provide evidence that this assumption should be questioned. Considering that they are the largest source of animal biomass across much of the tundra, it is likely that their activities have important consequences for regional and global carbon dynamics.</p> / Dissertation