Toke T. Høye

Ecological Dynamics Across the Arctic Associated with Recent Climate Change

Assessing the Arctic The Arctic is experiencing some of the most rapid climate change currently under way across the globe, but consequent ecological responses have not been widely reported. At the close of the Fourth International Polar Year, Post et al. (p. 1355) review observations on ecological impacts in this sensitive region. The widespread changes occurring in terrestrial, freshwater, and marine systems, presage changes at lower latitudes that will affect natural resources, food production, and future climate buffering. At the close of the Fourth International Polar Year, we take stock of the ecological consequences of recent climate change in the Arctic, focusing on effects at population, community, and ecosystem scales. Despite the buffering effect of landscape heterogeneity, Arctic ecosystems and the trophic relationships that structure them have been severely perturbed. These rapid changes may be a bellwether of changes to come at lower latitudes and have the potential to affect ecosystem services related to natural resources, food production, climate regulation, and cultural integrity. We highlight areas of ecological research that deserve priority as the Arctic continues to warm.

The effects of phenological mismatches on demography

Climate change is altering the phenology of species across the world, but what are the consequences of these phenological changes for the demography and population dynamics of species? Time-sensitive relationships, such as migration, breeding and predation, may be disrupted or altered, which may in turn alter the rates of reproduction and survival, leading some populations to decline and others to increase in abundance. However, finding evidence for disrupted relationships, or lack thereof, and their demographic effects, is difficult because the necessary detailed observational data are rare. Moreover, we do not know how sensitive species will generally be to phenological mismatches when they occur. Existing long-term studies provide preliminary data for analysing the phenology and demography of species in several locations. In many instances, though, observational protocols may need to be optimized to characterize timing-based multi-trophic interactions. As a basis for future research, we outline some of the key questions and approaches to improving our understanding of the relationships among phenology, demography and climate in a multi-trophic context. There are many challenges associated with this line of research, not the least of which is the need for detailed, long-term data on many organisms in a single system. However, we identify key questions that can be addressed with data that already exist and propose approaches that could guide future research.

Rapid advancement of spring in the High Arctic

Summary Despite uncertainties in the magnitude of expected global warming over the next century, one consistent feature of extant and projected changes is that Arctic environments are and will be exposed to the greatest warming [1]. Concomitant with such large abiotic changes, biological responses to warming at high northern latitudes are also expected to outpace those at lower latitudes. One of the clearest and most rapid signals of biological response to rising temperatures across an array of biomes has been shifts in species phenology [2–4], yet to date evidence for phenological responses to climate change has been presented from most biomes except the High Arctic [3]. Given the well-established consequences for population dynamics of shifts in the timing of life history events [5,6], it is essential that the High Arctic be represented in assessments of phenological response to climate change. Using the most comprehensive data set available from this region, we document extremely rapid climate-induced advancement of flowering, emergence and egg-laying in a wide array of species in a high-arctic ecosystem. The strong responses and the large variability within species and taxa illustrate how easily biological interactions may be disrupted by abiotic forcing, and how dramatic responses to climatic changes can be for arctic ecosystems.

Phenological sensitivity to climate across taxa and trophic levels

Differences in phenological responses to climate change among species can desynchronise ecological interactions and thereby threaten ecosystem function. To assess these threats, we must quantify the relative impact of climate change on species at different trophic levels. Here, we apply a Climate Sensitivity Profile approach to 10,003 terrestrial and aquatic phenological data sets, spatially matched to temperature and precipitation data, to quantify variation in climate sensitivity. The direction, magnitude and timing of climate sensitivity varied markedly among organisms within taxonomic and trophic groups. Despite this variability, we detected systematic variation in the direction and magnitude of phenological climate sensitivity. Secondary consumers showed consistently lower climate sensitivity than other groups. We used mid-century climate change projections to estimate that the timing of phenological events could change more for primary consumers than for species in other trophic levels (6.2 versus 2.5–2.9 days earlier on average), with substantial taxonomic variation (1.1–14.8 days earlier on average).

Phenology of High-Arctic Arthropods: Effects of Climate on Spatial, Seasonal, and Inter-Annual Variation

Publisher Summary This chapter discusses the way climate affects the terrestrial arthropod assemblage in a single ecosystem in Zackenbergdalen in high-arctic Northeast Greenland, based on data from the Zackenberg Basic monitoring program. The chapter focuses on the effects of snow and temperature on the phenology of the most abundant taxa of arthropods and on their spatial variations in emergence patterns. The chapter describes the arthropod fauna and relative differences between sampling plots and years. The chapter provides analyses on the phenology of three Diptera families (Chironomidae, Muscidae, and Sciaridae), one family of parasitoid wasps ( Ichneumonidae ), one family of Lepidoptera ( Nymphalidae ), two families of spiders ( Lycosidae and Linyphiidae ), collembolans, and mites. The chapter statistically analyzes the importance of temperature and plot-specific dates of snowmelt for interannual variation in phenological events. The chapter estimates the spatial synchrony of capture numbers among traps within trapping plots and between pairs of plots. The chapter discusses the way in which differences in phenological sensitivity to climate across taxa can lead to trophic mismatch within the arthropod food web and its ecological repercussions.

Nonlinear flowering responses to climate: are species approaching their limits of phenological change?

Many alpine and subalpine plant species exhibit phenological advancements in association with earlier snowmelt. While the phenology of some plant species does not advance beyond a threshold snowmelt date, the prevalence of such threshold phenological responses within plant communities is largely unknown. We therefore examined the shape of flowering phenology responses (linear versus nonlinear) to climate using two long-term datasets from plant communities in snow-dominated environments: Gothic, CO, USA (1974–2011) and Zackenberg, Greenland (1996–2011). For a total of 64 species, we determined whether a linear or nonlinear regression model best explained interannual variation in flowering phenology in response to increasing temperatures and advancing snowmelt dates. The most common nonlinear trend was for species to flower earlier as snowmelt advanced, with either no change or a slower rate of change when snowmelt was early (average 20% of cases). By contrast, some species advanced their flowering at a faster rate over the warmest temperatures relative to cooler temperatures (average 5% of cases). Thus, some species seem to be approaching their limits of phenological change in response to snowmelt but not temperature. Such phenological thresholds could either be a result of minimum springtime photoperiod cues for flowering or a slower rate of adaptive change in flowering time relative to changing climatic conditions.

Phenological response of tundra plants to background climate variation tested using the International Tundra Experiment

The rapidly warming temperatures in high-latitude and alpine regions have the potential to alter the phenology of Arctic and alpine plants, affecting processes ranging from food webs to ecosystem trace gas fluxes. The International Tundra Experiment (ITEX) was initiated in 1990 to evaluate the effects of expected rapid changes in temperature on tundra plant phenology, growth and community changes using experimental warming. Here, we used the ITEX control data to test the phenological responses to background temperature variation across sites spanning latitudinal and moisture gradients. The dataset overall did not show an advance in phenology; instead, temperature variability during the years sampled and an absence of warming at some sites resulted in mixed responses. Phenological transitions of high Arctic plants clearly occurred at lower heat sum thresholds than those of low Arctic and alpine plants. However, sensitivity to temperature change was similar among plants from the different climate zones. Plants of different communities and growth forms differed for some phenological responses. Heat sums associated with flowering and greening appear to have increased over time. These results point to a complex suite of changes in plant communities and ecosystem function in high latitudes and elevations as the climate warms.

Response of an arctic predator guild to collapsing lemming cycles

Alpine and arctic lemming populations appear to be highly sensitive to climate change, and when faced with warmer and shorter winters, their well-known high-amplitude population cycles may collapse. Being keystone species in tundra ecosystems, changed lemming dynamics may convey significant knock-on effects on trophically linked species. Here, we analyse long-term (1988–2010), community-wide monitoring data from two sites in high-arctic Greenland and document how a collapse in collared lemming cyclicity affects the population dynamics of the predator guild. Dramatic changes were observed in two highly specialized lemming predators: snowy owl and stoat. Following the lemming cycle collapse, snowy owl fledgling production declined by 98 per cent, and there was indication of a severe population decline of stoats at one site. The less specialized long-tailed skua and the generalist arctic fox were more loosely coupled to the lemming dynamics. Still, the lemming collapse had noticeable effects on their reproductive performance. Predator responses differed somewhat between sites in all species and could arise from site-specific differences in lemming dynamics, intra-guild interactions or subsidies from other resources. Nevertheless, population extinctions and community restructuring of this arctic endemic predator guild are likely if the lemming dynamics are maintained at the current non-cyclic, low-density state.

Differences in food abundance cause inter-annual variation in the breeding phenology of High Arctic waders

Previous work has shown that High Arctic waders in Greenland are “income breeders”, i.e. the resources used for egg formation are based almost entirely on biomass obtained on the breeding grounds. Thus, their breeding phenology is expected to be highly sensitive to inter-annual variation in food abundance during the pre-laying period. Early spring snow-cover may also influence timing of egg-laying either directly or mediated through food resources. Here, we report on the inter-annual variation in clutch initiation of three wader species breeding in High Arctic Greenland, Sanderling (Calidris alba), Dunlin (Calidris alpina) and Ruddy Turnstone (Arenaria interpres), in relation to spring snow-cover and spring arthropod abundance over ten breeding seasons at Zackenberg Research Station 1995–2005. Food abundance had the strongest effect on timing of clutch initiation, while the proportion of snow-free land had a weaker but still significant effect, i.e. more food and more snow-free land both result in earlier egg-laying. We hypothesize that food is most important when there is sufficient snow-free land to nest on, while snow-cover is of increasing importance in years with late snowmelt.

A greener Greenland? Climatic potential and long-term constraints on future expansions of trees and shrubs.

Warming-induced expansion of trees and shrubs into tundra vegetation will strongly impact Arctic ecosystems. Today, a small subset of the boreal woody flora found during certain Plio-Pleistocene warm periods inhabits Greenland. Whether the twenty-first century warming will induce a re-colonization of a rich woody flora depends on the roles of climate and migration limitations in shaping species ranges. Using potential treeline and climatic niche modelling, we project shifts in areas climatically suitable for tree growth and 56 Greenlandic, North American and European tree and shrub species from the Last Glacial Maximum through the present and into the future. In combination with observed tree plantings, our modelling highlights that a majority of the non-native species find climatically suitable conditions in certain parts of Greenland today, even in areas harbouring no native trees. Analyses of analogous climates indicate that these conditions are widespread outside Greenland, thus increasing the likelihood of woody invasions. Nonetheless, we find a substantial migration lag for Greenlands current and future woody flora. In conclusion, the projected climatic scope for future expansions is strongly limited by dispersal, soil development and other disequilibrium dynamics, with plantings and unintentional seed dispersal by humans having potentially large impacts on spread rates.