Differential arthropod responses to warming are altering the structure of Arctic communities

The Arctic is experiencing some of the fastest rates of warming on the planet. Although many studies have documented responses to such warming by individual species, the idiosyncratic nature of these findings has prevented us from extrapolating them to community-level predictions. Here, we leverage the availability of a long-term dataset from Zackenberg, Greenland (593 700 specimens collected between 1996 and 2014), to investigate how climate parameters influence the abundance of different arthropod groups and overall community composition. We find that variation in mean seasonal temperatures, winter duration and winter freeze–thaw events is correlated with taxon-specific and habitat-dependent changes in arthropod abundances. In addition, we find that arthropod communities have exhibited compositional changes consistent with the expected effects of recent shifts towards warmer active seasons and fewer freeze–thaw events in NE Greenland. Changes in community composition are up to five times more extreme in drier than wet habitats, with herbivores and parasitoids generally increasing in abundance, while the opposite is true for surface detritivores. These results suggest that species interactions and food web dynamics are changing in the Arctic, with potential implications for key ecosystem processes such as decomposition, nutrient cycling and primary productivity.

The Arctic is experiencing some of the fastest rates of warming on the planet. Although many studies have documented responses to such warming by individual species, the idiosyncratic nature of these findings has prevented us from extrapolating them to community-level predictions. Here, we leverage the availability of a long-term dataset from Zackenberg, Greenland (593 700 specimens collected between 1996 and 2014), to investigate how climate parameters influence the abundance of different arthropod groups and overall community composition. We find that variation in mean seasonal temperatures, winter duration and winter freeze-thaw events is correlated with taxon-specific and habitat-dependent changes in arthropod abundances. In addition, we find that arthropod communities have exhibited compositional changes consistent with the expected effects of recent shifts towards warmer active seasons and fewer freeze-thaw events in NE Greenland. Changes in community composition are up to five times more extreme in drier than wet habitats, with herbivores and parasitoids generally increasing in abundance, while the opposite is true for surface detritivores. These results suggest that species interactions and food web dynamics are changing in the Arctic, with potential implications for key ecosystem processes such as decomposition, nutrient cycling and primary productivity.
2018 The Authors. Published by the Royal Society under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/4.0/, which permits unrestricted use, provided the original author and source are credited.

Background
The rapid warming of the Arctic provides us with a valuable opportunity to learn about the general implications of climate change for the structure and function of biological communities. Temperatures in the Arctic have increased almost twice as fast as the average global increase over the past 100 years [1][2][3][4], and its ecosystems are proving to be both sensitive and responsive to such warming [5][6][7][8][9]. Although an abundance of studies has demonstrated the strong effects of climate change on a range of arctic organisms (e.g. [5,[9][10][11][12][13]), it is becoming increasingly clear that species responses can be fairly idiosyncratic [14], and that our ability to extrapolate species-level findings to the level of communities may thus be limited [15]. However, a better understanding of community-level responses to climate change is essential, because the structure and composition of communities contribute to how ecosystems function [16].
In spite of the dramatic change in climate that the Arctic has experienced in the recent past, the studies that have explored general responses to this phenomenon have yielded limited evidence of major shifts in community structure. Although warming experiments have shown some site-specific community-wide changes in plant diversity [17,18], a surprising number of long-term experimental [19,20] and observational studies [21][22][23][24] have found little-to-no overall change in arctic and alpine plant communities. In particular, compared to some of the compositional changes being observed in low-arctic communities [25][26][27][28][29][30][31], ecosystems in the High Arctic still appear to be relatively stable [18,19,22,32]. However, this apparent stability of the High Arctic could simply be an artefact. For example, the typically long lifespans and slow developmental times of many arctic species [33][34][35], often a product of selection imposed by extremely short growing seasons, pose challenges in detecting changes in population turnover and in identifying the specific environmental stressors responsible for those changes. Similarly, the large inter-annual climatic variability that characterizes extreme environments, such as the Arctic, may obscure temporal trends in population dynamics and complicate the statistical detection of population changes. The latter issue is particularly challenging in arctic datasets because there is some evidence that communities in extreme environments tend to be less dynamic and to exhibit very slow succession rates [36,37]. The use of long-term datasets, which are rare for arctic ecosystems [38], could aid in resolving these issues [39].
Arthropods have long been recognized as a model group for detecting organismal responses to climate change because of their short lifespans and the strong effects that temperature can have on their life histories. As a point of comparison, while tundra plants often live from decades to centuries [33][34][35]40], the lifespans of most arctic arthropods are much shorter, typically spanning from a few months to several years [41,42]. Thus, while long-lived species such as plants may accumulate subtle changes over the years, arthropods are more likely to show stronger responses to inter-and intra-annual variability in temperature. Nevertheless, there are conflicting predictions on how arthropods will respond to warming in cold environments [43,44]. On the one hand, higher temperatures could reduce population numbers through heat stress [45], desiccation [44], phenological mismatches [46,47], or forced relocation to cooler habitats [48]. On the other hand, warmer temperatures could benefit arthropod species by alleviating thermal cold stress [49] and lengthening the active season, which can facilitate accelerated growth or maturation and potentially result in additional cohorts within the year [41,49,50]. In addition, depending on the ecosystem or habitat-specific responses to warming, increasing temperatures could indirectly facilitate resource acquisition through changes in plant community composition (i.e. food quality) [51], increased rates of primary productivity (i.e. food quantity) (e.g. [52]), or altered availability of substrate and habitat space [53].
The Zackenberg Basic Monitoring Programme at Zackenberg Research Station has consistently monitored climate variables and surface-active arthropod communities in northeastern Greenland since 1996 [54]. This dataset is uniquely suited for the study of community-wide responses to climate change, because it contains information on all trophic levels within the local arthropod community and because the area has undergone extreme summertime warming over recent years (e.g. [47,55,56]). Here, we quantify changes in seasonal temperature patterns at Zackenberg for the period from 1996 to 2014, explore how these changes relate to changes in abundances of different functional groups of surfaceactive arthropods, and evaluate the extent to which arthropod communities in the three main habitat types (wet fen, mesic heath and arid heath; see [57]) have changed in the recent past. Based on previous short-term studies of arthropod responses to warming (e.g. [44,58,59]), we expected that detritivores would be the most sensitive to a warming climate and that the greatest changes in overall arthropod community composition would occur in those habitats with lower moisture availability (arid and mesic heath). The climate parameters we consider in our analyses include the mean temperature for the summer in which a sample was collected as well as the mean temperatures for the previous spring, winter, fall and summer. In addition, we consider the duration of winter, because both the temperature and the length of time of exposure to extreme cold can be important determinants of arthropod survival [60][61][62]. Finally, we also consider the annual number of winter freeze-thaw events, which are another source of temperature-related stress for these animals [49,63].

Study site
Zackenberg Research Station is located in high-arctic northeast Greenland (74°28' N; 20°34' W). The area is characterized by a continental climate with cold winters and generally dry conditions, and there is continuous permafrost with a maximum active layer depth of 20-100 cm. Mean summertime air temperatures typically vary between 3°C and 7°C [55]. Over the study period of 1996-2014, Zackenberg had a mean annual temperature of −9°C, with the majority of positive average temperatures falling within the summer months of June, July and August [56].

Environmental data
We used hourly data on ambient air temperature measured 200 cm above the soil surface from the Zackenberg climate station (downloaded from data.g-e-m.dk/, accessed 7 October 2015) to calculate conditions for each summer and its preceding spring, winter, fall and summer. We tested for effects of average summer temperature from both the year in which arthropod sampling occurred and one year prior, because the current abundance of arthropods may be partially determined by the conditions during the prior breeding season. While we acknowledge that the most relevant temperature measures for much of the arthropod community are likely to be at the soil surface [64], those data were unfortunately not available for our study plots. However, we note that soil surface temperatures and those at 200 cm above ground were highly correlated over the 19-year study period at the nearby local climate station (Pearson's product-moment correlation: r 161 710 = 0.859), indicating that the metric used here is a reasonable approximation of the temperature experienced by the animals sampled in this study and that the patterns of change over time that we detected at 200 cm are indicative of the changes observed at the surface level. Because air temperature is less likely to vary by microhabitat differences in snow cover or soil moisture, we use this metric in the analyses below. Summer air temperature data were not available from the Zackenberg climate station for 1995 (the year prior to the start of our study), so it was imputed using data from the Climate Research Unit (CRU) TS3.23 Dataset (downloaded from crudata.uea.ac.uk/cru/data/hrg/cru_ts_3.23/, accessed 25 February 2016; see electronic supplementary material, figure S1). Electronic supplementary material, figure S2 and table S2 show the variation in average seasonal temperatures (and winter-related variables; see below) at Zackenberg during the study period.
Operationally, we define winter as the months of November through March, a period with almost exclusively negative temperatures throughout our time-series data. Based on annual seasonal patterns of freeze-thaw cycles, the transition months of April/May and September/October were designated as spring and fall, respectively, and summer was subsequently defined as June, July and August. A more precise measure for the duration of winter was derived using standardized temperature thresholds associated with seasonal freeze-thaw cycles (as in [65]). Specifically, we designated the onset of winter as the last fall date in which there was an hourly temperature measure of 2°C (i.e. start of winter) and the end of the winter as the first spring date at which the temperature reached 2°C (i.e. end of winter). The number of freeze-thaw events per winter was calculated as the number of times that the temperature crossed 0°C within that same period. In measuring winter duration, we ignored rare instances where temperatures rose above 2°C in the middle of the winter and immediately returned to seasonably cold temperatures [56].

Arthropod data collection
Arthropods were monitored weekly from late May to the end of August from 1996 to 2014 (samples from 2010 were lost in transit from Greenland) in three main habitat types (wet fen, mesic heath and arid heath), which differ in plant community composition, soil moisture and the timing of snow melt fall, winter and spring) exhibit high levels of correlation, we avoided statistical artefacts associated with multicollinearity by reducing them to a smaller number of composite predictors via principal components analysis (PCA) in R [78,79]. Variables were centred, scaled and transformed for normality (if needed) prior to PCA. Next, we used linear regression models to explore changes in the underlying components of our climate variables over the 19-year study period. Changes over time for each of the raw climate variables can be seen in electronic supplementary material, figure S2.
We tested for potential habitat-specific effects of our climate variables on arthropod abundances (i.e. cumulative annual catches corrected by trapping days) by fitting separate linear mixed effects models for each taxonomic group. Interactions between habitat type and each of the composite climate parameters derived from PCA were included as fixed predictors. Sampling plot and study year were included as random effects. These mixed models were fitted using the package lmerTest [80,81]. Abundance data for the different taxonomic groups were log transformed in order to conform to the assumptions of linear models [82].
We also performed non-metric multidimensional scaling (NMDS) on the arthropod abundance data to explore how the structure of these communities varied in response to the climate variables (i.e. composite predictors of climate). NMDS is a technique for community analysis that finds an n-dimensional ordination of the taxonomic groups that best describes the Bray-Curtis dissimilarity in community composition between plots while minimizing 'stress', a measure of badness of fit [83]. Kruskal's stress values of less than 0.2 are generally considered appropriate [84,85]. We ran the NMDS with the Vegan package for R [86] using the metamds function with random starting configurations (maximum of 200 random starts to reach a convergent solution), and we selected the number of ordination axes to keep via visual inspection of a scree plot of stress values [83]. We then tested whether compositional variation at the community level was related to our environmental predictors by fitting the ordination scores to habitat type and the composite climate parameters with Vegan's envfit function [86]. This function is explicitly designed to fit both environmental vectors and factors onto ordination scores [86].

Environmental trends over the study period
The principal component analysis with varimax rotation revealed three underlying components of climate at Zackenberg. We found a significant change in PC1 over the study period, indicating that active seasons (summers and falls) became progressively warmer and that winters in later years had fewer freeze-thaw events (F 1,16 = 16.98, p < 0.001, r 2 = 0.51). This finding is consistent with previous reports of summertime warming at Zackenberg over recent years (e.g. [47,55,56]). There was no evidence of significant changes in the temperature of the non-active season (winters and springs; PC2; p = 0.43) or in winter duration (PC3; p = 0.44) over the study period. The relative contributions of each climate variable to the different components derived from the PCA (i.e. 'warmer active seasons and fewer freeze-thaw events', 'warmer non-active seasons' and 'longer winters') are presented in table 1.

Links between arthropod abundances and environmental predictors
In total, our analyses included 593 788 arthropods captured during June-August from 1996 to 2014. An average of 9185 (arthropod) specimens were caught per year in the single wet fen plot, 10 308 specimens in the two mesic heath plots, and 13 495 specimens in the two arid heath plots. While pitfall traps may lead, in some instances, to biased sampling with overrepresentation of large-bodied and/or more active arthropods, we note that smaller-bodied arthropods and groups that are often under-sampled with this method (like detritivores) were nevertheless notably well represented in our traps (electronic supplementary material, figure S3). Electronic supplementary material, figure S3 also shows the variation in annual abundances of each taxonomic group during the study period.
We found that habitat type is an important predictor of the abundance of most taxonomic groups. In particular, predators (Araneae) and flies (Diptera) were more abundant in the wet fen than in the heath sites (table 2), whereas the opposite was true for herbivores (Hemiptera and Lepidoptera). There were also fewer Acari in the mesic heath than in the arid heath sites (table 2).
We also found that the magnitude and direction of climate-related responses varied among taxa and were highly dependent on habitat type (table 2). For example, when active seasons were warmer and winters had fewer freeze-thaw events (higher PC1 scores), surface detritivores were present in lower abundances (Acari only in arid heath, Collembola in all habitats: figure 1a,b). By contrast, Hemiptera and     parasitoid Hymenoptera were found to be more abundant in relation to higher PC1 scores (figure 1c,d), particularly in arid heath (Hemiptera) and mesic heath (Hymenoptera) habitats. Similarly, warmer nonactive seasons (higher PC2 scores) were found to correlate with lower Collembola abundance (figure 2a) and lower spider abundance in wet fen and arid heath (figure 2b) but higher abundance of parasitic Hymenoptera in mesic heath (figure 2c). Longer winters (higher PC3 scores) were correlated with lower abundances of surface detritivores in both heath habitats (figure 3a,b) and with habitat-specific changes in spiders (figure 3c) and Hemiptera (figure 3d). No climate effects were detected on Diptera or Lepidoptera, but the abundances of both of these groups varied significantly between habitats (table 2). Overall, these results indicate that certain functional groups (e.g. detritivores such as Collembola and Acari) may be more sensitive to climatic variation than others.

Links between variability in community structure and environmental predictors
Non-metric multidimensional ordination (NMDS) of arthropod community composition resulted in a two-axis solution and a Kruskal's stress value of 0.195 (figure 4a). This ordination analysis confirms that arthropod community composition differs according to habitat type (p < 0.001; r 2 = 0.29), with communities within mesic and arid heath habitats being more similar to one another than to those within wet fen habitat. NMDS also indicates that community composition within these high-arctic communities during our 19-year study period was significantly related to variation in PC1 (warmer active seasons and fewer freeze-thaw events; p = 0.004; r 2 = 0.13) and PC2 (warmer non-active seasons; p = 0.003; r 2 = 0.12). The nature and direction of these associations are depicted graphically in figure 4a through correlation vectors [86]. Owing to the observed shift towards warmer active seasons and fewer freeze-thaw events over our study period (see section on Environmental trends above), we can infer that those communities present in years with cooler active seasons and more freeze-thaw events (lower values of PC1) were communities that occurred in the beginning of the study period while those present in years with warmer active seasons and fewer freeze-thaw events (higher values of PC1) occurred later. Although variability in arthropod community composition was also significantly linked to warmer non-active seasons, we note that there is currently little evidence for significant changes in non-active season temperatures at Zackenberg in the recent past.
Overall, our ordination analysis paints a picture that is consistent with our analyses on individual arthropod groups: as active seasons have become warmer and winters have brought about fewer freezethaw events, Zackenberg's arthropod communities have seen an increase in the number of herbivores (Hemiptera) and parasitoids (Hymenoptera) and a decrease in detritivores (Collembola and Acari).  Table 2. Mixed effects model results of summertime abundances of the most common arthropods at Zackenberg, as predicted by habitat type and the three principal components summarized in table 1. Higher values of PC1 are indicative of warmer active seasons and fewer winter freeze-thaw events; higher values of PC2 represent warmer non-active seasons, and higher values of PC3 are indicative of longer winters. Interactions between habitat type and each of the composite environmental variables were included in all models as fixed effects, whereas Plot and Year were included as random effects. Arid heath is the reference category for habitat type. Individual models were simplified by removing non-significant predictors one by one. Results shown here are from the most parsimonious models. * p < 0.05, * * p < 0.001, * * * p < 0.0001.         . The habitat types are wet fen (black circles), mesic heath (blue filled circles) and arid heath (red triangles). Grey trend lines show significant relationships between average daily animal abundances and PC1, whereas the coloured trend lines signify a significant interaction between PC1 and habitat type. Results from these mixed effects models and those from the other arthropod groups can be found in table 2.
NMDS also indicates that temporal shifts in community composition have been more pronounced in arid and mesic heath habitats than in the wet fen. For example, between the first third (1996-2001) and the last third (2008-2014) of the study period, changes in the average NMDS scores of arid and mesic heath communities were respectively about five and three times larger than those from the wet fen community (figure 4b), an observation which is robust to comparisons between different 6-year time windows. Considering our findings on taxon-specific responses to climate variation, it seems likely that the slight differences in community-level changes between the arid and mesic heath (figure 4b) result from differential habitat-specific responses by Hemiptera and Hymenoptera to warmer active seasons and fewer freeze-thaw events (table 2 and figure 1c,d). Specifically, the finding that abundances of Hemiptera increased more in the arid heath than in the mesic heath (figure 1c), whereas those of Hymenoptera showed the opposite pattern (figure 1d) is consistent with the way in which these groups are expected to respond to increasing temperatures and fewer freeze-thaw events (changes in PC1 and PC2; figure 4a,b).

Discussion
We have shown that over a 19-year period in the recent past, the Zackenberg area of NE Greenland has experienced significant warming of active seasons and progressively fewer winter freeze-thaw events. In addition, our analyses demonstrate that these changes in climate are associated with major axes of change in the composition of local arthropod communities and are correlated with significant changes in community structure over time. Specifically, abundances of surface detritivores are declining with the increasingly warmer active seasons and reduced number of freeze-thaw events, whereas abundances of parasitoids and certain herbivores are increasing. . Habitat types are denoted by colour and symbol type: wet fen (black circles), mesic heath (blue filled circles) and arid heath (red triangles). Solid coloured lines signify a significant interaction between PC2 and habitat type in relation to abundance. Collembola abundances were negatively related to variability in PC2, independent of habitat type (depicted by single grey trend line). Results from these mixed effects models and those from the other arthropod groups can be found in table 2.    Although the long-term dataset that our study is based upon was not originally designed to explore differences among habitat types (i.e. replication within habitat types is low), our analyses were nevertheless still able to uncover that taxon-specific and community-level responses to changing climatic conditions at Zackenberg can be habitat-specific and are likely to vary across small spatial scales (e.g. such as the short distances between our research plots). For example, we observed much larger changes in arthropod community composition in association with active season warming and winter freezethaw events in the arid and mesic heath sites than in the wet fen habitat. These results are consistent with earlier claims that climatic changes can elicit differential responses in habitats that vary in moisture availability [59,87,88], as soil moisture has direct effects on desiccation-susceptible arthropods and can also have indirect effects via changes in plant palatability [89]. In addition, water availability plays an important role in the overwintering strategies of many arctic arthropods [90]. The potential for habitatspecific responses could also be driven by differences in temperature and other microhabitat conditions at the soil surface, which can vary depending on vegetation type and snow cover (e.g. [27,88,[91][92][93]). For example, one possible reason why mesic heath communities exhibit slightly different changes than those we observe in arid heath or wet fen (figure 4b) is that mesic heath sites experience later snow melt than the other habitats [66] and therefore may select for different phenologies [92], temperature-associated responses or species interactions. The latter can be seen in the way in which warmer active seasons in mesic heath habitats are associated with higher abundances of parasitoid wasps but not of their Lepidoptera prey. This difference suggests that parasitoid pressure on Lepidoptera may be increasing over time in mesic heath. Similarly, the higher abundances of Hemiptera in arid heath but no change in potential spider predators indicate that predator pressure could be declining in those areas. While arthropod communities that inhabit heath versus fen sites are compositionally distinct from one another (figure 4a), it is possible that movement by individuals across the mosaic landscape could somewhat buffer communities from the altered species interactions caused by changing abundances. However, our results suggest that overall, differences in habitat-specific responses by arthropod groups to climatic changes may be altering the strength of some predator-prey interactions, particularly in heath habitats.
Notably, although arthropod communities in drier habitats appear to be more responsive to climate change than those in wet habitats (figure 4b), the opposite appears to be the case for plants [24,26,57]. This difference in habitat-specific responses by the plant and arthropod communities suggests that increasing temperatures could magnify the existing spatial heterogeneity across the landscape. For example, while we might have expected herbivores (i.e. Hemiptera) to respond positively to the increase in plant productivity in wet fen habitat that occurred during the course of our study [94,95], we found instead that the herbivore abundance remained largely stable there. Assuming that rates of herbivory scale with the abundance of herbivores, this finding suggests a proportional decrease in invertebrate herbivory in wetter habitats. Conversely, although plant communities in drier heath habitats have been relatively resistant to warming [24,26], herbivore abundances have increased in arid heath (figures 1c, 4a), potentially leading to higher rates of herbivory in those areas. This mismatch between plants and herbivores supports the hypothesis that northern herbivores are more limited by abiotic conditions than by resource availability [96]. A number of studies have shown how vertebrate herbivores can influence how warming affects arctic plant communities with important consequences for carbon exchange [97][98][99][100][101][102][103]. Less attention has been paid to changes in the lower baseline herbivory rates by arthropods, but our findings are consistent with results from previous modelling efforts [104] and observations on Betula nana and B. glandulosa [105] that suggest that impacts on vegetation by non-outbreak herbivores will increase with the predicted increases in temperature. Moreover, we may have even underestimated the herbivore response to warming in all habitats, because our trapping method was not focused on sampling the foliage community, which has a higher proportion of herbivores than the surface-dwelling community [67]. The increases in herbivore abundances that we observed in the arid heath are probably reducing the small, albeit increasing amount of carbon that is annually fixed and retained in this arctic ecosystem ( [94], but see [106]).
Our results are also consistent with prior claims that above-ground and soil-dwelling arthropods might respond differently to warming and that while warming may benefit herbivores (e.g. [107,108]), it will be detrimental to detritivores (e.g. [44,58,59]). We found that detritivores in this system, particularly Collembola, are extremely sensitive to temperature-related changes in their environment. A number of previous studies on experimental climate warming have shown that Collembola respond negatively to increasing temperatures but that these responses are more likely due to changes in moisture associated with warming than the warming itself ( [44] and references therein). Detritivores with more heavily sclerotized forms (e.g. mites such as Oribatida and Mesostigmata) are more resistant to desiccation and warming [44,109], which may explain why we only observed a decline in Acari in the arid heath but not the mesic heath or wet fen (figure 1a). The ecosystem implications of having relatively fewer surface-active detritivores in these arthropod communities are unclear (but see [67]). Soil animals are commonly found to enhance decomposition [110][111][112][113] and nutrient cycling [114][115][116] through their consumption of litter and interactions with the microbial community. In this case, lower detritivore densities in the litter may translate to slower decomposition on the tundra, which could slow the rate at which the large amounts of stored carbon are released from permafrost soils. At the same time, if lower abundances of detritivores result in fewer plant-available nutrients, like nitrogen, there could be limitations imposed on primary productivity [114] which would slow the rate at which carbon is fixed. However, predicting future impacts of warming on detritivore-mediated processes are inherently difficult, because they largely depend on the specific composition of local detritivore and microbial communities [117]. Furthermore, while our results demonstrate the negative effects of longterm warming on both groups of surface-active detritivores, these findings may not always extend to communities further below ground (e.g. [118,119]), where differences between Collembola and mite responses to experimental warming have been recorded (e.g. [59,[120][121][122]). Overall, the uncertainty surrounding the sensitivity of arctic detritivores and their influence over ecosystem processes under different environmental conditions highlights the demand for more studies focused on the functional role of these animals [123].
Even though summer and early fall are the active season for most arctic arthropods, we found that conditions during the preceding winter were also related to arthropod abundances and community structure. In particular, abundances of all except two arthropod groups (Diptera and Lepidoptera) were associated with either the length of winter (PC3) or the mean temperature during the non-active season (PC2). These patterns are consistent with earlier claims that cold-adapted polar arthropods are sensitive to winter conditions, even during periods of diapause [49,65,[124][125][126][127][128]. While we did not observe longterm changes in mean temperature in the non-active season at Zackenberg, climate projections for Greenland suggest both winter and springtime will eventually be warmer [129]. Thus, our results indicate that predictions regarding the potential type and magnitude of future changes in northern arthropod communities will require accounting for warming in both the active and non-active seasons.
In conclusion, we have shown that long-term warming has differentially affected arthropod groups in high-arctic Greenland, leading to major changes in the relative abundances of different trophic groups within the arthropod community. These changes were especially pronounced in the dry habitats, suggesting that the strength of organismal responses to warming is likely to be habitat-specific and linked to moisture availability. Potential avenues to expand on these findings include increasing taxonomic resolution and replication within habitat types, as well as explicitly assessing how changes in biotic interactions driven by warming-associated shifts in community composition are altering ecosystem functions in the Arctic. In particular, the changes that we observed in the overall structure of the arthropod community (namely the increase in herbivores and reduction in detritivores) suggest that climate-related effects on high-arctic arthropods could ultimately affect ecosystem processes such as decomposition, nutrient cycling and primary productivity [67]. Given that the Arctic is a reservoir for approximately half of the global pool of soil organic carbon [130][131][132], the patterns we document here could have important consequences not only for regional but also for global carbon dynamics.
Ethics. This work was conducted with arthropods and thus there were no animal welfare issues; all fieldwork was carried out through Zackenberg Ecological Research Operations (ZERO) under the Greenland Ecosystem Monitoring (GEM) organization.
Data accessibility. All data used here, including the arthropod community composition data and the climate (temperature) data are publically available at http://data.g-e-m.dk/.