[show abstract][hide abstract] ABSTRACT: Advancing phenology in response to global warming has been reported
across biomes, raising concerns about the temporal uncoupling of trophic
interactions. Concurrently, widely reported flower visitor declines have
been linked to resource limitations. Phenological responses in the
Arctic have been shown to outpace responses from lower latitudes and
recent studies suggest that differences between such responses for
plants and their flower visitors could be particularly pronounced in the
Arctic. The evidence for phenological uncoupling is scant because
relevant data sets are lacking or not available at a relevant spatial
scale. Here, we present evidence of a climate-associated shortening of
the flowering season and a concomitant decline in flower visitor
abundance based on a long-term, spatially replicated (1996-2009) data
set from high-Arctic Greenland. A unique feature of the data set is the
spatial and temporal overlap of independent observations of plant and
insect phenology. The shortening of the flowering season arose through
spatial variation in phenological responses to warming. The shorter
flowering seasons may have played a role in the observed decline in
flower visitor abundance. Our results demonstrate that the dramatic
climatic changes currently taking place in the Arctic are strongly
affecting individual species and ecological communities, with
implications for trophic interactions.
[show abstract][hide abstract] ABSTRACT: Animals often alternate between searching for food locally and moving over larger distances depending on the amount of food they find. This ability to switch movement strategy can have large implications on the fate of individuals and populations, and a mechanism that allows animals to find the optimal balance between alternative movement strategies is therefore selectively advantageous. Recent theory suggests that animals are capable of switching movement mode depending on heterogeneities in the landscape, and that different modes may predominate at different temporal scales. Here we develop a conceptual model that enables animals to use either an area-concentrated food search behavior or undirected random movements. The model enables animals to increase their food intake by fine-tuning the relative contribution of the two types of behavior. In contrast to most models of optimal foraging, our model does not assume food to be distributed in large, well-defined patches, and our focus is on how animals should move rather than on which patch is most profitable. We demonstrate how the model, which builds on the animals’ ability to remember the profitability and location of previously visited areas, is capable of producing home ranges and of generating realistic movement patterns for the harbor porpoise, both at fine and intermediate temporal scales.
[show abstract][hide abstract] ABSTRACT: Predicting which species will occur together in the future, and where, remains one of the greatest challenges in ecology, and requires a sound understanding of how the abiotic and biotic environments interact with dispersal processes and history across scales. Biotic interactions and their dynamics influence species' relationships to climate, and this also has important implications for predicting future distributions of species. It is already well accepted that biotic interactions shape species' spatial distributions at local spatial extents, but the role of these interactions beyond local extents (e.g. 10 km(2) to global extents) are usually dismissed as unimportant. In this review we consolidate evidence for how biotic interactions shape species distributions beyond local extents and review methods for integrating biotic interactions into species distribution modelling tools. Drawing upon evidence from contemporary and palaeoecological studies of individual species ranges, functional groups, and species richness patterns, we show that biotic interactions have clearly left their mark on species distributions and realised assemblages of species across all spatial extents. We demonstrate this with examples from within and across trophic groups. A range of species distribution modelling tools is available to quantify species environmental relationships and predict species occurrence, such as: (i) integrating pairwise dependencies, (ii) using integrative predictors, and (iii) hybridising species distribution models (SDMs) with dynamic models. These methods have typically only been applied to interacting pairs of species at a single time, require a priori ecological knowledge about which species interact, and due to data paucity must assume that biotic interactions are constant in space and time. To better inform the future development of these models across spatial scales, we call for accelerated collection of spatially and temporally explicit species data. Ideally, these data should be sampled to reflect variation in the underlying environment across large spatial extents, and at fine spatial resolution. Simplified ecosystems where there are relatively few interacting species and sometimes a wealth of existing ecosystem monitoring data (e.g. arctic, alpine or island habitats) offer settings where the development of modelling tools that account for biotic interactions may be less difficult than elsewhere.
[show abstract][hide abstract] ABSTRACT: 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.
Proceedings of the Royal Society B: Biological Sciences 09/2012; 279(1746):4417-4422. · 5.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: The study of phenology is a unifying discipline in ecology. Although
studied most commonly as a response to environmental variation such as
climate change, phenology is a conceptually powerful integrator of
ecological dynamics across levels of biological organization. Here, we
will present data from two long-term study sites in Greenland, where
spring is advancing rapidly due to recent warming. Our results highlight
important features of phenology as an integrator of ecological dynamics
in the Arctic, from variation at the plant species level in time and
space, to species interactions across trophic levels between plants and
herbivores, to the importance of phenology in ecosystem carbon exchange.
Beyond its role as a response variable and integrator of ecological
dynamics, we suggest phenology is an important driver of ecological
response to climate change.
[show abstract][hide abstract] ABSTRACT: The use of stable isotopes in diet analysis usually relies on the different photosynthetic pathways of C3 and C4 plants, and the resulting difference in carbon isotope signature. In the Arctic, however, plant species are exclusively C3, and carbon isotopes alone are therefore not suitable for studying arctic herbivore diets. In this study, we examined the potential of both stable carbon and nitrogen isotopes to reconstruct the diet of an arctic herbivore, here the muskox (Ovibos moschatus (Zimmermann, 1780)), in northeast Greenland. The isotope composition of plant communities and functional plant groups was compared with those of muskox faeces and shed wool, as this is a noninvasive approach to obtain dietary information on different temporal scales. Plants with different root mycorrhizal status were found to have different δ15N values, whereas differences in δ13C, as expected, were less distinct. As a result, our examination mainly relied on stable nitrogen isotopes. The interpretation of stable isotopes from faeces was difficult because of the large uncertainty in diet–faeces fractionation, whereas isotope signatures from wool suggested that the muskox summer diet consists of around 80% graminoids and up to 20% willows. In conclusion, the diet composition of an arctic herbivore can indeed be inferred from stable isotopes in arctic areas, despite the lack of C4 plants.
Canadian Journal of Zoology 09/2011; 89(10):892-899. · 1.50 Impact Factor
[show abstract][hide abstract] ABSTRACT: Snow cover plays a major role in the climate, hydrological and ecological systems of the Arctic and other regions through
its influence on the surface energy balance (e.g. reflectivity), water balance (e.g. water storage and release), thermal regimes
(e.g. insulation), vegetation and trace gas fluxes. Feedbacks to the climate system have global consequences. The livelihoods
and well-being of Arctic residents and many services for the wider population depend on snow conditions so changes have important
consequences. Already, changing snow conditions, particularly reduced summer soil moisture, winter thaw events and rain-on-snow
conditions have negatively affected commercial forestry, reindeer herding, some wild animal populations and vegetation. Reductions
in snow cover are also adversely impacting indigenous peoples’ access to traditional foods with negative impacts on human
health and well-being. However, there are likely to be some benefits from a changing Arctic snow regime such as more even
run-off from melting snow that favours hydropower operations.
KeywordsSnow–Arctic–Climate–Albedo–Hydrology–Ecology–Biogeochemical cycling–Geochemical processes–Forestry–Infrastructure–Tourism–Indigenous cultures–Human health
AMBIO A Journal of the Human Environment 01/2011; 40:32-45. · 2.30 Impact Factor
[show abstract][hide abstract] ABSTRACT: The effects of landscape modifications on the long-term persistence of wild animal populations is of crucial importance to wildlife managers and conservation biologists, but obtaining experimental evidence using real landscapes is usually impossible. To circumvent this problem we used individual-based models (IBMs) of interacting animals in experimental modifications of a real Danish landscape. The models incorporate as much as possible of the behaviour and ecology of four species with contrasting life-history characteristics: skylark (Alauda arvensis), vole (Microtus agrestis), a ground beetle (Bembidion lampros) and a linyphiid spider (Erigone atra). This allows us to quantify the population implications of experimental modifications of landscape configuration and composition.
Starting with a real agricultural landscape, we progressively reduced landscape complexity by (i) homogenizing habitat patch shapes, (ii) randomizing the locations of the patches, and (iii) randomizing the size of the patches. The first two steps increased landscape fragmentation. We assessed the effects of these manipulations on the long-term persistence of animal populations by measuring equilibrium population sizes and time to recovery after disturbance. Patch rearrangement and the presence of corridors had a large effect on the population dynamics of species whose local success depends on the surrounding terrain. Landscape modifications that reduced population sizes increased recovery times in the short-dispersing species, making small populations vulnerable to increasing disturbance. The species that were most strongly affected by large disturbances fluctuated little in population sizes in years when no perturbations took place.
Traditional approaches to the management and conservation of populations use either classical methods of population analysis, which fail to adequately account for the spatial configurations of landscapes, or landscape ecology, which accounts for landscape structure but has difficulty predicting the dynamics of populations living in them. Here we show how realistic and replicable individual-based models can bridge the gap between non-spatial population theory and non-dynamic landscape ecology. A major strength of the approach is its ability to identify population vulnerabilities not detected by standard population viability analyses.
PLoS ONE 01/2010; 5(1):e8932. · 3.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: The climate effects research program in Zackenberg in high arctic Greenland got a counterpart in Nuuk in low arctic West Greenland in 2007. The programme NuukBasic is described and, for the first time, results will presented from several of the monitoring components (Table 1). In particular, we focus on changes in plant phenology, vegetation greenness, graded effects of UVB radiation and lake ecology. Results are compared and contrasted concurrent changes at the high arctic site Zackenberg in Northeast Greenland.Biological Monitoring elements in NuukBasis
[show abstract][hide abstract] ABSTRACT: Seasonal timing of reproduction (phenology) is highly responsive to global warming, especially in the Arctic. Here, we present a comparative analysis of multi-annual observational data on phenological dynamics across trophic levels from Zackenberg, North-East Greenland (a High Arctic site) and Kangerlussuaq, West Greenland (a Low Arctic site). Both sites have experienced considerable warming and our analyses indicate that rates of change in plant phenological responses may differ between sites, related to different proximal drivers at the two sites. We also present parallel data on interacting organisms (pollinators and mammalian herbivores) to evaluate the risks and effects of trophic mismatch at these two sites.
[show abstract][hide abstract] ABSTRACT: It is well established that climate affects organisms in a wide range of different ways, including their distributions and, indeed, their performance (i.e. changes in growth, survival and reproduction). However, establishing whether species respond to changes in climate is not necessarily equal to establishing whether species performance is also affected. This argument also applies when climate effects are considered across trophic levels, such as consumer-resources interactions. For example, a phenological response of a plant may have no effect on its performance but may indeed have significant effects on herbivores. Here we emphasize and exemplify how the choice of a ``biological currency'' of climate change effects may be highly informative in one aspect but apparently useless in another. We do this by using the comprehensive data collected on plant species, large herbivore and their interactions at Zackenberg and Kangerlussuaq in Greenland.
[show abstract][hide abstract] ABSTRACT: 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.
[show abstract][hide abstract] ABSTRACT: Variation in carrying capacity and population return rates is generally ignored in traditional studies of population dynamics. Variation is hard to study in the field because of difficulties controlling the environment in order to obtain statistical replicates, and because of the scale and expense of experimenting on populations. There may also be ethical issues. To circumvent these problems we used detailed simulations of the simultaneous behaviours of interacting animals in an accurate facsimile of a real Danish landscape. The models incorporate as much as possible of the behaviour and ecology of skylarks Alauda arvensis, voles Microtus agrestis, a ground beetle Bembidion lampros and a linyphiid spider Erigone atra. This allows us to quantify and evaluate the importance of spatial and temporal heterogeneity on the population dynamics of the four species.
Both spatial and temporal heterogeneity affected the relationship between population growth rate and population density in all four species. Spatial heterogeneity accounted for 23-30% of the variance in population growth rate after accounting for the effects of density, reflecting big differences in local carrying capacity associated with the landscape features important to individual species. Temporal heterogeneity accounted for 3-13% of the variance in vole, skylark and spider, but 43% in beetles. The associated temporal variation in carrying capacity would be problematic in traditional analyses of density dependence. Return rates were less than one in all species and essentially invariant in skylarks, spiders and beetles. Return rates varied over the landscape in voles, being slower where there were larger fluctuations in local population sizes.
Our analyses estimated the traditional parameters of carrying capacities and return rates, but these are now seen as varying continuously over the landscape depending on habitat quality and the mechanisms of density dependence. The importance of our results lies in our demonstration that the effects of spatial and temporal heterogeneity must be accounted for if we are to have accurate predictive models for use in management and conservation. This is an area which until now has lacked an adequate theoretical framework and methodology.
[show abstract][hide abstract] ABSTRACT: Statistical autoregressive analyses of direct and delayed density dependence are widespread in ecological research. The models suggest that changes in ecological factors affecting density dependence, like predation and landscape heterogeneity are directly portrayed in the first and second order autoregressive parameters, and the models are therefore used to decipher complex biological patterns. However, independent tests of model predictions are complicated by the inherent variability of natural populations, where differences in landscape structure, climate or species composition prevent controlled repeated analyses. To circumvent this problem, we applied second-order autoregressive time series analyses to data generated by a realistic agent-based computer model. The model simulated life history decisions of individual field voles under controlled variations in predator pressure and landscape fragmentation. Analyses were made on three levels: comparisons between predated and non-predated populations, between populations exposed to different types of predators and between populations experiencing different degrees of habitat fragmentation.
The results are unambiguous: Changes in landscape fragmentation and the numerical response of predators are clearly portrayed in the statistical time series structure as predicted by the autoregressive model. Populations without predators displayed significantly stronger negative direct density dependence than did those exposed to predators, where direct density dependence was only moderately negative. The effects of predation versus no predation had an even stronger effect on the delayed density dependence of the simulated prey populations. In non-predated prey populations, the coefficients of delayed density dependence were distinctly positive, whereas they were negative in predated populations. Similarly, increasing the degree of fragmentation of optimal habitat available to the prey was accompanied with a shift in the delayed density dependence, from strongly negative to gradually becoming less negative.
We conclude that statistical second-order autoregressive time series analyses are capable of deciphering interactions within and across trophic levels and their effect on direct and delayed density dependence.
[show abstract][hide abstract] ABSTRACT: One of the ecological consequences of rising temperatures in the Arctic is an advancement of the timing of spring events such as the onset of the plant growing season. Here, we present data from Greenland illustrating that, while plant growth is occurring earlier there, the timing of arrival on breeding grounds by species such as caribou and muskoxen that are dependent upon those plants has not advanced. The result is a developing "trophic mismatch" between the timing of resource demand by reproducing females and the timing of resource availability by newly emergent forage plants. We will discuss the consequences this warming-induced trophic mismatch may pose for the persistence of mammalian herbivores in the Arctic.
[show abstract][hide abstract] ABSTRACT: Temporal advancement of resource availability by warming in seasonal environments can reduce reproductive success of vertebrates if their own reproductive phenology does not also advance with warming. Indirect evidence from large-scale analyses suggests, however, that migratory vertebrates might compensate for this by tracking phenological variation across landscapes. Results from our two-year warming experiment combined with seven years of observations of plant phenology and offspring production by caribou (Rangifer tarandus) in Greenland, however, contradict evidence from large-scale analyses. At spatial scales relevant to the foraging horizon of individual herbivores, spatial variability in plant phenology was reduced--not increased--by both experimental and observed warming. Concurrently, offspring production by female caribou declined with reductions in spatial variability in plant phenology. By highlighting the spatial dimension of trophic mismatch, these results reveal heretofore unexpected adverse consequences of climatic warming for herbivore population ecology.
Proceedings of the Royal Society B: Biological Sciences 10/2008; 275(1646):2005-13. · 5.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: In highly seasonal environments, offspring production by vertebrates is timed to coincide with the annual peak of resource availability. For herbivores, this resource peak is represented by the annual onset and progression of the plant growth season. As plant phenology advances in response to climatic warming, there is potential for development of a mismatch between the peak of resource demands by reproducing herbivores and the peak of resource availability. For migratory herbivores, such as caribou, development of a trophic mismatch is particularly likely because the timing of their seasonal migration to summer ranges, where calves are born, is cued by changes in day length, while onset of the plant-growing season on the same ranges is cued by local temperatures. Using data collected since 1993 on timing of calving by caribou and timing of plant growth in West Greenland, we document the consequences for reproductive success of a developing trophic mismatch between caribou and their forage plants. As mean spring temperatures at our study site have risen by more than 4 degrees C, caribou have not kept pace with advancement of the plant-growing season on their calving range. As a consequence, offspring mortality has risen and offspring production has dropped fourfold.
Philosophical Transactions of The Royal Society B Biological Sciences 08/2008; 363(1501):2369-75. · 6.23 Impact Factor
[show abstract][hide abstract] ABSTRACT: In this paper we investigate whether Ellenberg's Climate Quotient (EQ, defined as the mean temperature of the warmest month divided by annual precipitation, multiplied by IOOO), can be used to predict the distribution of Carpinus betulus in Denmark. It has been suggested that the competitive relationship between the two tree species Fagus sylvatica and C.betulus is related to EQ in central Europe. Areas with low EQ have dominance of E. sylvatica, whereas higher EQs are associated with dryer and warmer climates that favour C.betulus. To determine if this holds true, also in northern Europe, we investigate the present distribution of C.betulus in Denmark, based on a comprehensive dataset with presence-absence information of the species. We relate the distribution of C.betulus to 12 climate parameters and indices (including EQ), analyse it in a geographical information system and compare the ecology of C.betulus and E sylvatica in four Danish forests, located in different climatic and floristic regions. The highest density of C.betulus was found in eastern Denmark where EQ is high, i.e. summer temperature is relatively high and precipitation low. In the western and south-western parts of the country, where the climate is slightly more wet and more oceanic, there are fewer populations of C.betulus. Based on present climatic data it seems that the climate of Denmark does not limit the occurrence of C.betulus, except perhaps for a small area in western Jutland. We believe that climate changes in the late Holocene cannot alone account for the changes in the distribution of C.befulus in Denmark. And future climate change is likely to affect the distribution of C.betulus with generally better conditions.
[show abstract][hide abstract] ABSTRACT: Nonlinear and irregular population dynamics may arise as a result of phase dependence and coexistence of multiple attractors. Here we explore effects of climate and density in the dynamics of a highly fluctuating population of wild reindeer (Rangifer tarandus platyrhynchus) on Svalbard observed over a period of 29 years. Time series analyses revealed that density dependence and the effects of local climate (measured as the degree of ablation [melting] of snow during winter) on numbers were both highly nonlinear: direct negative density dependence was found when the population was growing (Rt > 0) and during phases of the North Atlantic Oscillation (NAO) characterized by winters with generally high (1979-1995) and low (1996-2007) indices, respectively. A growth-phase-dependent model explained the dynamics of the population best and revealed the influence of density-independent processes on numbers that a linear autoregressive model missed altogether. In particular, the abundance of reindeer was enhanced by ablation during phases of growth (Rt > 0), an observation that contrasts with the view that periods of mild weather in winter are normally deleterious for reindeer owing to icing of the snowpack. Analyses of vital rates corroborated the nonlinearity described in the population time series and showed that both starvation mortality in winter and fecundity were nonlinearly related to fluctuations in density and the level of ablation. The erratic pattern of growth of the population of reindeer in Adventdalen seems, therefore, to result from a combination of the effects of nonlinear density dependence, strong density-dependent mortality, and variable density independence related to ablation in winter.