Megafaunal Communities in Rapidly Warming Fjords along the West Antarctic Peninsula: Hotspots of Abundance and Beta Diversity

Glacio-marine fjords occur widely at high latitudes and have been extensively studied in the Arctic, where heavy meltwater inputs and sedimentation yield low benthic faunal abundance and biodiversity in inner-middle fjords. Fjord benthic ecosystems remain poorly studied in the subpolar Antarctic, including those in extensive fjords along the West Antarctic Peninsula (WAP). Here we test ecosystem predictions from Arctic fjords on three subpolar, glacio-marine fjords along the WAP. With seafloor photographic surveys we evaluate benthic megafaunal abundance, community structure, and species diversity, as well as the abundance of demersal nekton and macroalgal detritus, in soft-sediment basins of Andvord, Flandres and Barilari Bays at depths of 436–725 m. We then contrast these fjord sites with three open shelf stations of similar depths. Contrary to Arctic predictions, WAP fjord basins exhibited 3 to 38-fold greater benthic megafaunal abundance than the open shelf, and local species diversity and trophic complexity remained high from outer to inner fjord basins. Furthermore, WAP fjords contained distinct species composition, substantially contributing to beta and gamma diversity at 400–700 m depths along the WAP. The abundance of demersal nekton and macroalgal detritus was also substantially higher in WAP fjords compared to the open shelf. We conclude that WAP fjords are important hotspots of benthic abundance and biodiversity as a consequence of weak meltwater influences, low sedimentation disturbance, and high, varied food inputs. We postulate that WAP fjords differ markedly from their Arctic counterparts because they are in earlier stages of climate warming, and that rapid warming along the WAP will increase meltwater and sediment inputs, deleteriously impacting these biodiversity hotspots. Because WAP fjords also provide important habitat and foraging areas for Antarctic krill and baleen whales, there is an urgent need to develop better understanding of the structure, dynamics and climate-sensitivity of WAP subpolar fjord ecosystems.     

Distinct or similar? Soft bottom polychaete diversity in Arctic and Antarctic glacial fjords

The main aim of this study was to compare the polychaete communities in two similar polar areas: an Arctic fjord, Hornsund (Svalbard) and an Antarctic fjord, Ezcurra Inlet (South Shetlands). This is the first attempt to compare Arctic and Antarctic diversity based on fully comparable datasets. Forty van Veen grab samples were collected in each fjord: twenty replicates were taken in each of two fjord areas characterized by a different level of glacial disturbance—in the inner (glacial bay) and outer (fjord mouth) region of both fjords, from depths of about 100 m in 2005 (Hornsund) and in 2007 (Ezcurra Inlet). In the glacial bays, species richness and diversity were significantly higher in Ezcurra Inlet than in Hornsund due to higher rate of glacial disturbance in the latter one. In the outer areas, species richness was similar in both fjords, although diversity values were higher in Ezcurra Inlet. Polychaete species richness in the habitats characterized by similar level of disturbance (outer areas of the fjords) was the same in both polar regions. At this small scale, where community drivers are very similar, the species richness seems to be independent from the local or regional species pool.     

Benthic infaunal community variability on the northern Svalbard shelf

Marine benthic communities are effective indicators of environmental change. Yet in the Arctic, there are few empirical tests of how sustained climatic change may influence community structure. Northern Svalbard is influenced by both warm Atlantic and cold Arctic water masses, providing an opportunity to assess potential effects of long-term environmental changes by examining spatial variation in community structure. We examined benthic macroinfaunal communities and sediment pigments under Atlantic and Arctic water masses on the northern shelf and fjords of Svalbard. We report on infaunal biomass, abundance, species composition, and diversity at 10 stations spanning 79°–81°N and ranging in depth from 200 to 500?m. Benthic biomass averaged 128?g?WW?m^?2 (48–253?g?WW?m^?2), mean density was 3,635?ind.?m^?2 (780–7,660?ind.?m^?2), and species richness varied from 45 to 136?taxa?stn.^?1. Abundance-based community structure clustered stations in groups related to water mass characteristics, with Atlantic and Arctic shelf stations being well distinguished from each other. Dominant taxa were different in Atlantic- and Arctic-influenced locations. Faunal biomass was highest in the Atlantic-influenced fjords, followed by Arctic fjords and Arctic shelf stations, with Atlantic shelf stations having the lowest biomass. Species richness and diversity were inversely related to biomass. Patterns in faunal biomass were strongly correlated with sedimentary pigments (R ²?=?0.74 for chl a and R ²?=?0.77 for phaeopigments), with large differences in sedimentary pigment concentration among stations. These relationships suggest that benthic fauna on the northern Svalbard shelf are food limited and dependent on predictable, albeit episodic, delivery of organic matter from the water column.     

The future of soil invertebrate communities in polar regions: different climate change responses in the Arctic and Antarctic?

The polar regions are experiencing rapid climate change with implications for terrestrial ecosystems. Here, despite limited knowledge, we make some early predictions on soil invertebrate community responses to predicted twenty‐first century climate change. Geographic and environmental differences suggest that climate change responses will differ between the Arctic and Antarctic. We predict significant, but different, belowground community changes in both regions. This change will be driven mainly by vegetation type changes in the Arctic, while communities in Antarctica will respond to climate amelioration directly and indirectly through changes in microbial community composition and activity, and the development of, and/or changes in, plant communities. Climate amelioration is likely to allow a greater influx of non‐native species into both the Arctic and Antarctic promoting landscape scale biodiversity change. Non‐native competitive species could, however, have negative effects on local biodiversity particularly in the Arctic where the communities are already species rich. Species ranges will shift in both areas as the climate changes potentially posing a problem for endemic species in the Arctic where options for northward migration are limited. Greater soil biotic activity may move the Arctic towards a trajectory of being a substantial carbon source, while Antarctica could become a carbon sink.     

Decadal change in macrobenthic soft-bottom community structure in a high Arctic fjord (Kongsfjorden, Svalbard)

Marine benthic macrofauna communities are considered a good indicator of subtle environmental long-term changes in an ecosystem. In 1997/1998 and 2006, soft-bottom fauna of an Arctic glacial fjord Kongsfjorden was extensively sampled and major communities were identified along the fjord axis, which were related to the diminishing influence of glacial activity. Spatial patterns in community structure and species diversity were significantly different in the central basin of Kongsfjorden between periods while there was no change in the inner part of the fjord. In 1997/98, three faunal associations were distinguished with significant differences in species richness and diversity (H′) while in 2006 only two faunal associations were identified and there were no differences any more between the two formerly distinct associations in the central fjord. The increased input of Atlantic water due to a stronger West Spitsbergen Current may be the reason for unification of previous clear faunal division. The faunal association in the inner, well separated glacial part of the fjord, characterized by strong glacier influence, was protected from Atlantic water inflow and, hence, the macrobenthic fauna essentially remained unaffected. Reduced abundance of species typical for glacial bays in the central part of the fjord in 2006 may result from the decreasing effect of Blomstrandbreen glacier, strong increase of input of Atlantic water into the fjord and increased temperature of West Spitsbergen Current. Higher values of POC in 2006 than in 1998 are likely the effect of increased primary production resulting from warmer water temperatures.     

Marine‐terminating glaciers sustain high productivity in Greenland fjords

Accelerated mass loss from the Greenland ice sheet leads to glacier retreat and an increasing input of glacial meltwater to the fjords and coastal waters around Greenland. These high latitude ecosystems are highly productive and sustain important fisheries, yet it remains uncertain how they will respond to future changes in the Arctic cryosphere. Here we show that marine‐terminating glaciers play a crucial role in sustaining high productivity of the fjord ecosystems. Hydrographic and biogeochemical data from two fjord systems adjacent to the Greenland ice sheet, suggest that marine ecosystem productivity is very differently regulated in fjords influenced by either land‐terminating or marine‐terminating glaciers. Rising subsurface meltwater plumes originating from marine‐terminating glaciers entrain large volumes of ambient deep water to the surface. The resulting upwelling of nutrient‐rich deep water sustains a high phytoplankton productivity throughout summer in the fjord with marine‐terminating glaciers. In contrast, the fjord with only land‐terminating glaciers lack this upwelling mechanism, and is characterized by lower productivity. Data on commercial halibut landings support that coastal regions influenced by large marine‐terminating glaciers have substantially higher marine productivity. These results suggest that a switch from marine‐terminating to land‐terminating glaciers can substantially alter the productivity in the coastal zone around Greenland with potentially large ecological and socio‐economic implications.     

Pre-adaptation of Arctic Plants to Climate Change*

Re-appraisal of high latitude ice cover during the Weichselian and the recent discovery of pre-Holocene plant communities in Novaya Zemlya and Northern Norway suggests that the contemporary Arctic flora may have had a longer time span in its occupation of high latitude sites than has hitherto been recognised. The present-day Arctic flora in some regions may therefore have derived at least some of its genetic inheritance from populations that survived in ice-free refugia in the high Arctic polar deserts during the latter stages of the Weichselian glaciation. Increasing evidence for sudden changes in climate at high latitudes in the past, suggests that Arctic vegetation would have been subjected to rapid climatic warming if it had maintained a presence in these regions during the late Weichselian. A review of what is known about plant survival in the high Arctic since pre-Holocene times, coupled with an examination of some of the physiological properties of modern populations in these regions is presented in an attempt to ascertain if high Arctic plants possess mechanisms which facilitate their capacity to respond either phenotypically or physiologically to climatic change.     

Spatio-temporal dynamics of species richness in coastal fish communities

Determining patterns of change in species richness and the processes underlying the dynamics of biodiversity are of key interest within the field of ecology, but few studies have investigated the dynamics of vertebrate communities at a decadal temporal scale. Here, we report findings on the spatio-temporal variability in the richness and composition of fish communities along the Norwegian Skagerrak coast having been surveyed for more than half a century. Using statistical models incorporating non-detection and associated sampling variance, we estimate local species richness and changes in species composition allowing us to compute temporal variability in species richness. We tested whether temporal variation could be related to distance to the open sea and to local levels of pollution. Clear differences in mean species richness and temporal variability are observed between fjords that were and were not exposed to the effects of pollution. Altogether this indicates that the fjord is an appropriate scale for studying changes in coastal fish communities in space and time. The year-to-year rates of local extinction and turnover were found to be smaller than spatial differences in community composition. At the regional level, exposure to the open sea plays a homogenizing role, possibly due to coastal currents and advection.     

Variability of protistan and bacterial communities in two Arctic fjords (Spitsbergen)

Krossfjorden and Kongsfjorden are Arctic fjords on the western side of Spitsbergen. These fjords share a common mouth to the open sea, and both are influenced by the input of sediment-rich glacial meltwater leading to decreased surface salinity, increased turbidity and decreased light penetration during summer. Earlier classical taxonomic studies had described the pelagic protistan composition of the Kongsfjorden during summer, revealing the dominance of flagellates of often unresolved taxonomic origin. Only little information existed on microbial eukaryote composition of the Krossfjorden as well as the bacterial composition of both fjords. The aim of the present study was to analyze and compare surface summertime protistan and bacterial communities in both fjords, using molecular approaches (16S and 18S rRNA DGGE, sequencing). Samples were collected three times a week from the central Kongsfjorden over a 1-month period. Additionally, 10 marine and 2 freshwater sites were sampled within a 1-week period in both Kongsfjorden and Krossfjorden. The central Kongsfjorden revealed a relatively stable protistan community over time with dinoflagellates, chlorophytes and small heterotrophs dominating. In contrast, the bacterial community varied over time and appeared to be correlated with the inflow of glacial meltwater. The Kongsfjorden and Krossfjorden were found to harbor distinctive bacterial and eukaryotic communities. We speculate that differences in glacial meltwater composition and fjord bathymetry affect the surface water properties and therefore the observed spatial variability in the community fingerprints.     

Nematode and macrofaunal diversity in central Arctic Ocean benthos

Deep-sea diversity studies have revealed intriguing patterns on both local and regional scales, but there is insufficient evidence with which to evaluate these trends in the Arctic Ocean basin. We collected data on the diversity of benthic macrofauna and meiofaunal nematodes along two transects from the shelf margin to the North Pole. Contrary to prevailing paradigms, there was no change in diversity with depth between 1000 and 4273 m. There was a trend, however, toward reduced taxonomic richness for both macrofauna and nematodes with increasing latitude. Regional (β-) diversity differences were not observed for nematodes, but significant contrasts in Bray-Curtis similarity-based community structure of macrofauna were seen between the Eurasian and Amerasian Basins, as well as between the Lomonosov and Mendeleev Ridges. Since fauna within the deep Arctic Ocean appear to represent a single species pool, we suggest that both local (α-) and β-diversity may be determined by ecological processes in the Arctic, and are not the consequence of historical or evolutionary processes. Furthermore, insights gained from studies of benthic-pelagic coupling, known to play a significant role in determining benthic community structure and function at high latitudes, may also be useful in investigations of Arctic biodiversity. This model may be valuable in designing future studies of biodiversity, and for predicting potential impacts of climate change on diversity patterns.     

Carbon cycling by seafloor communities on the eastern Beaufort Sea shelf

Tight pelagic-benthic coupling on Arctic shelves suggests that resident benthic communities may be particularly important in the cycling of carbon and regeneration of nutrients. We sampled 16 stations in the eastern Beaufort Sea during Autumn 2003 and Summer 2004 to determine spatial patterns in sediment community carbon demand, and the manner in which that demand was partitioned among epifauna, macroinfauna, and meio-/microfauna. Sediment carbon demand in this relatively oligotrophic area was similar to that measured in more productive Arctic shelf sites, and was largely related to the distribution of phytodetritus in surface sediments. Epibenthic megafaunal communities were dominated by echinoderms and exhibited peak abundance (up to 240 ind. m^− 2) and biomass at stations in the 60-90 m depth range. Partitioning of the carbon demand revealed the local importance of megafauna, accounting for up to 41% of the community demand. Macrofauna accounted for on average between 25 and 69% of the carbon demand, while meio-/microfauna were responsible for 31-75% of the demand. Total community carbon demand by the benthos is estimated to account for approximately 60% of the annual new production in the region, suggesting the great ecosystem importance of benthic communities on the Beaufort shelf, and potentially across the Arctic. Our study region is strongly influenced by the Mackenzie River, and ongoing climate change is likely to result in altered productivity regimes, changes in quality and quantity of available food, and higher levels of sediment deposition. Impacts of these events on benthic community structure and function will likely have repercussions throughout the ecosystem.     

Zooplankton in Svalbard fjords on the Atlantic–Arctic boundary

Zooplankton abundance and community structures were studied in three west Spitsbergen fjords at the beginning of the warm phase, which seem to have entered in 2006. Sampling was conducted in summer 2007 at stations distributed along transects in Hornsund, Isfjorden and Kongsfjorden. Variations in zooplankton standing stocks and community structures (assessing taxonomic diversity and zoogeographical affiliations) were analysed in relation to the environmental variables using multivariate techniques. The hydrographic conditions in Hornsund were influenced by the cold Arctic Water, whereas those in Isfjorden and especially in Kongsfjorden were, to a greater extent, under the influence of the warm Atlantic Water. High abundances of both meroplankton and holoplankton organisms were observed in Kongsfjorden, with high contributions of boreal and ubiquitous species (Calanus finmarchicus and Oithona similis, respectively). In Hornsund at the same time, the zooplankton consisted mainly of boreo-Arctic and Arctic species, the abundances of which were comparable along the West Spitsbergen Shelf. Our results indicate that the difference in hydrography had measurable effects on the zooplankton community in the study area. Furthermore, by comparing regions of contrasting oceanographic conditions, we present evidence as to how the zooplankton structure will change in the Arctic ecosystems if the warming trends continue to operate with the same dynamics. The advection of Atlantic waters to the Arctic seas may lead to changes in zooplankton structure, with increased abundance and contributions of boreal and small ubiquitous species. The ‘warmer Arctic fjords’ scenarios may also induce more rapid development of both holoplankton and meroplankton populations and, consequently, modify the trophic interactions in plankton communities.     

Site fidelity of Atlantic cod Gadus morhua L. as deduced from telemetry and stable isotope studies

A study was performed to test the hypothesis that Atlantic cod Gadus morhua captured within fjords do not migrate across the sill into outer fjord segments or coastal waters. An 18 month long telemetry experiment, corroborated by recapture patterns, showed that 40% of the tagged fish crossed the sill. The majority of the fish that crossed the sill, however, returned after short excursions of only a few days. Many fish remained stationary at or near the release location, and migrations up the fjord into brackish water appeared more frequent than migration out of the fjord. The stable isotope analyses showed significant differences between the inner and the outer fjord segments, and this was seen for both adults and juveniles. This suggests that the Atlantic cod in the two areas have different diets and probably utilize different foraging ranges. Thus, both the isotope studies and the telemetry experiment suggest limited cross-sill migration and rather high site fidelity. Marketing restrictions only apply to fish caught in the fjord proper, not the comparatively clean outer fjord system beyond the sill. Based on this study of migratory tendencies, it is concluded that contamination levels in Atlantic cod in both the Frierfjord proper and the outer fjords probably result from local exposure and appear little affected by inter-fjord migration.     

Landscape fragmentation affects responses of avian communities to climate change

Forecasting the consequences of climate change is contingent upon our understanding of the relationship between biodiversity patterns and climatic variability. While the impacts of climate change on individual species have been well‐documented, there is a paucity of studies on climate‐mediated changes in community dynamics. Our objectives were to investigate the relationship between temporal turnover in avian biodiversity and changes in climatic conditions and to assess the role of landscape fragmentation in affecting this relationship. We hypothesized that community turnover would be highest in regions experiencing the most pronounced changes in climate and that these patterns would be reduced in human‐dominated landscapes. To test this hypothesis, we quantified temporal turnover in avian communities over a 20‐year period using data from the New York State Breeding Atlases collected during 1980–1985 and 2000–2005. We applied Bayesian spatially varying intercept models to evaluate the relationship between temporal turnover and temporal trends in climatic conditions and landscape fragmentation. We found that models including interaction terms between climate change and landscape fragmentation were superior to models without the interaction terms, suggesting that the relationship between avian community turnover and changes in climatic conditions was affected by the level of landscape fragmentation. Specifically, we found weaker associations between temporal turnover and climatic change in regions with prevalent habitat fragmentation. We suggest that avian communities in fragmented landscapes are more robust to climate change than communities found in contiguous habitats because they are comprised of species with wider thermal niches and thus are less susceptible to shifts in climatic variability. We conclude that highly fragmented regions are likely to undergo less pronounced changes in composition and structure of faunal communities as a result of climate change, whereas those changes are likely to be greater in contiguous and unfragmented habitats.     

Size structure of benthic freshwater fish communities in relation to environmental gradients

The benthic fish communities of 161 Swedish lakes, sampled with multi-mesh gillnets, were examined by direct gradient analyses with ten environmental factors. A geographic gradient, correlated with climatic factors and productivity, was most important for ordination of species optima. By contrast, the distribution of biomass in different size-classes and trophic groups, was influenced more directly by local factors such as water quality (pH) and lake morphometry (area and maximum depth). The communities are not only structured at the species level, but also by size-related allocation of resources, within and between coexisting species. This national survey confirmed patterns observed in previous local field studies, as well as experiments designed for testing mechanisms acting on size-structured populations. However, it highlighted the need for more extensive studies on if and how large-scale environmental variation affects the cues for size-related ontogenetic niche shifts in facultative piscivores, because species such as perch Perca fluviatilis , brown trout Salmo trutta , and Arctic charr Salvelinus alpinus often dominate the benthic fish community.     

Genetic divergence at the synaptophysin (Syp I) locus among Norwegian coastal and north-east Arctic populations of Atlantic cod

Several nuclear RFLP loci have been discovered recently that exhibit extensive allele frequency variation among Norwegian coastal and north-east Arctic populations of Atlantic cod Gadus morhua. One of these polymorphisms was detected by hybridizing an anonymous cDNA clone (GM798) against genomic DNA digested with the restriction enzyme DraI. This cDNA clone has now been sequenced and identified as synaptophysin (Syp I), an integral synaptic vesicle membrane protein. Primers were constructed that amplify an intron of the Syp I gene that is polymorphic for the DraI site, thus making it possible to use a PCR-based assay to score the polymorphism. A total of 965 individuals sampled from the Barents Sea, coastal areas and fjords in northern Norway have been analysed for this polymorphism. The results confirm that highly significant differences exist between NE Arctic and coastal cod at the Syp I locus. A cluster analysis revealed a deep split between coastal and Arctic populations and hierarchical F-statistics indicated that about 40% of the total variation was attributable to differences between Arctic and coastal groups. The temporal stability of allele frequencies was assessed by comparing Syp I allele frequencies among samples of juveniles (0 group) captured at specific locations in fjords in consecutive years and among samples of adults and juveniles collected from the same fjord. Samples of juveniles collected in 1994 and 1995 in Malangen were genetically indistinguishable whereas juveniles sampled from Dønnesfjord and Ullsfjorden over the same 2-year period exhibited significant differences. Adults and 0-group individuals collected from the same fjord were found to be genetically indistinguishable in Malangen, but not in Balsfjorden. In addition to detecting large differences among Arctic and coastal groups, the Syp I locus suggests that genetic heterogeneity exists among resident populations of cod in different fjords and that gene flow among populations throughout northern Norway may be considerably lower than previously believed.     

Drivers of Collembola assemblages along an altitudinal gradient in northeast China

Altitudinal changes in the diversity of plants and animals have been well documented; however, soil animals received little attention in this context and it is unclear whether their diversity follows general altitudinal distribution patterns. Changbai Mountain is one of few well‐conserved mountain regions comprising natural ecosystems on the Eurasian continent. Here, we present a comprehensive analysis of the diversity and community composition of Collembola along ten altitudinal sites representing five vegetation types from forest to alpine tundra. Among 7834 Collembola individuals, 84 morphospecies were identified. Species richness varied marginally significant with altitude and generally followed a unimodal relationship with altitude. By contrast, the density of Collembola did not change in a consistent way with altitude. Collembola communities changed gradually with altitude, with local habitat‐related factors (soil and litter carbon‐to‐nitrogen ratio, litter carbon content, and soil pH) and climatic variables (precipitation seasonality) identified as major drivers of changes in Collembola community composition. Notably, local habitat‐related factors explained more variation in Collembola assemblages than climatic variables. The results suggest that local habitat‐related factors including precipitation and temperature are the main drivers of changes in Collembola communities with altitude. Specifically, soil and litter carbon‐to‐nitrogen ratio correlated positively with Collembola communities at high altitudes, whereas soil pH correlated positively at low altitudes. This documents that altitudinal gradients provide unique opportunities for identifying factors driving the community composition of not only above‐ but also belowground invertebrates.     

The benthic algal vegetation of the Mjóifjörđur,Eastern Iceland

The benthic algal vegetation of Mjóifjör?ur, situated in the central area of the East Icelandic coast, is described regarding the species distribution from the head to the mouth of the fjord, the main algal associations and zonation patterns. The vegetation gradient along the fjord coast is related to the hydrographic conditionsviz. the influence of freshwater discharge in the inner area and that of the cold water masses of the East Icelandic Current in the outer area. Comparisons are made with the vegetation of other Icelandic fjords investigated during surveys of the Icelandic algal vegetation as a whole.     

Alpha and beta diversity of connected benthic–subsurface invertebrate communities respond to drying in dynamic river ecosystems

Drying disturbances are the primary determinant of aquatic community biodiversity in dynamic river ecosystems. Research exploring how communities respond to disturbance has focused on benthic invertebrates in surface sediments, inadequately representing a connected community that extends into the subsurface. We compared subsurface and benthic invertebrate responses to drying, to identify common and context‐dependent spatial patterns. We characterized community composition, alpha diversity and beta diversity across a gradient of drying duration. Subsurface communities responded to drying, but these responses were typically less pronounced than those of benthic communities. Despite compositional changes and in contrast to reductions in benthic alpha diversity, the alpha diversity of subsurface communities remained stable except at long drying durations. Some primarily benthic taxa were among those whose subsurface frequency and abundance responded positively to drying. Collectively, changing composition, stable richness and taxon‐specific increases in occurrence provide evidence that subsurface sediments can support persistence of invertebrate communities during drying disturbances. Beta‐diversity patterns varied and no consistent patterns distinguished the total diversity, turnover or nestedness of subsurface compared to benthic communities. In response to increasing drying duration, beta diversity increased or remained stable for benthic communities, but remained stable or decreased for subsurface communities, likely reflecting contrasts in the influence of mass effects, priority effects and environmental filtering. Dissimilarity between subsurface and benthic communities remained stable or increased with drying duration, suggesting that subsurface communities maintain distinct biodiversity value while also supporting temporary influxes of benthic taxa during drying events. As temporary rivers increase in extent due to global change, we highlight that recognizing the connected communities that extend into the subsurface sediments can enable holistic understanding of ecological responses to drying, the key determinant of biodiversity in these dynamic ecosystems.     

Climatic and biotic extreme events moderate long‐term responses of above‐ and belowground sub‐Arctic heathland communities to climate change

Climate change impacts are not uniform across the Arctic region because interacting factors causes large variations in local ecosystem change. Extreme climatic events and population cycles of herbivores occur simultaneously against a background of gradual climate warming trends and can redirect ecosystem change along routes that are difficult to predict. Here, we present the results from sub‐Arctic heath vegetation and its belowground micro‐arthropod community in response to the two main drivers of vegetation damage in this region: extreme winter warming events and subsequent outbreaks of the defoliating autumnal moth caterpillar (Epirrita autumnata). Evergreen dwarf shrub biomass decreased (30%) following extreme winter warming events and again by moth caterpillar grazing. Deciduous shrubs that were previously exposed to an extreme winter warming event were not affected by the moth caterpillar grazing, while those that were not exposed to warming events (control plots) showed reduced (23%) biomass from grazing. Cryptogam cover increased irrespective of grazing or winter warming events. Micro‐arthropods declined (46%) following winter warming but did not respond to changes in plant community. Extreme winter warming and caterpillar grazing suppressed the CO₂ fluxes of the ecosystem. Evergreen dwarf shrubs are disadvantaged in a future sub‐Arctic with more stochastic climatic and biotic events. Given that summer warming may further benefit deciduous over evergreen shrubs, event and trend climate change may both act against evergreen shrubs and the ecosystem functions they provide. This is of particular concern given that Arctic heath vegetation is typically dominated by evergreen shrubs. Other components of the vegetation showed variable responses to abiotic and biotic events, and their interaction indicates that sub‐Arctic vegetation response to multiple pressures is not easy to predict from single‐factor responses. Therefore, while biotic and climatic events may have clear impacts, more work is needed to understand their net effect on Arctic ecosystems.