Author: isabel

This paper highlights the key role arctic herbivores play in arctic vegetation and nutrient dynamics, and reveal the potential strong ecosystem responses that may happen if changes in large herbivore abundances occur. In this study the authors examined the impacts of a large dominant herbivore in the tundra ecosystem, the muskox, Ovibos moschatus, using exclosures established in a fen in high Arctic Greenland. Using weekly sampling throughout the growing season, they quantified the effects of muskox exclusion on aboveground plant biomass, and on concentrations and pools of carbon and nitrogen, and explored the timing of peak biomass and nutrient pools. Excluding muskoxen profoundly changed plant abundances and dynamics of the fen, with more than a doubling of aboveground plant biomass and carbon and nitrogen pool sizes. Specifically, large increases in mosses and litter were observed, while graminoid biomass did not change significantly with exclusion of muskoxen. Excluding muskoxen advanced the peak of plant biomass, but also led to a more rapid decline, resulting in an earlier and shorter period of maximal plant biomass inside the exclosures. The largest impact of muskoxen on the arctic fen ecosystem seems to be through their trampling effects on the moss layer, which likely mediated most of the observed changes. This study demonstrates how quickly an ecosystem may respond to changes in the abundance of large herbivores, and highlights their pivotal role in modifying vegetation dynamics and nutrient cycling in tundra ecosystems.

Reference: Mosbacher, J.B., Michelsen, A., Stelvig, M., Hjermstad-Sollerud, H., Schmidt, N.M. (2018) Muskoxen modify plant abundance, phenology and nitrogen dynamics in a High Arctic fen. Ecosystems https://doi.org/10.1007/s10021-018-0323-4

You can read the full article here.

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Picture: Tundra ecosystems can respond rapidly to changes in the abundance of large herbivores (photo credit: Lars Holst Hansen, Aarhus University)

Muskox grazing can drive large changes in vegetation in the long term, but these changes do not seem to influence the composition of arthropod communities nor the structure of their predator –prey interactions. This surprising result implies that high Arctic arthropod communities might be highly resistant to changes in their habitat, possibly due to the high connectance of their food webs.

You can find more about this interesting study here.

Reference: Schmidt, N.M., Mosbacher, J.B., Eitzinger, B., Vesterinen, E.J., Roslin, T. (2018) High resistance towards herbivore-induced habitat change in a high Arctic arthropod community. Biology Letters 14: 20180054.

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Picture: Muskox grazing in Greenland (photo: Jesper Mosbacher)

Predictive modelling is increasingly common in ecology, and statistical models created in one context are often used to predict the state of the system in other contexts. However, the fast development of predictive ecology calls for caution,as it is not always clear whether the current understanding of ecological processes is comprehensive enough to warrant predictions. The amplitude of rodent population cycles (i.e., peak-phase abundances) has been hypothesized to be determined by vegetation properties in tundra ecosystems. We assessed the spatial and temporal predictability of food and shelter plants effects on peak-phase small rodent abundance during two consecutive rodent population peaks.

During the rodent population peak in 2007 at Varanger peninsula, rodent abundance was related to the abundance of food and shelter plants. Still, plants alone were not enough to predict how high the rodent peak in 2011 was. The role of plants was then likely to be over driven by other food web components(like predators) and weather. This study highlights two challenges for predicting food web functioning to the future: i) gathering long enough time-series to include different situations in the predictive models and ii) including several key interactions instead of isolating one of them from the remaining food web.

You can access the full publication here.

Reference: Soininen, E.M., Henden, J.A., Ravolainen, V.T., Yoccoz, N.G., Brathen, K.A., Killengreen, S.T., Ims, R.A. (2018) Transferability of biotic interactions: temporal consistency of arctic plant-rodent relationships is poor. Ecology and Evolution 8(19):9697-9711.

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Picture: Lemming grazing in a snowbed in Finnmark (photo: Eeva Soininen)

Many old stories include warnings to beware of nasty surprises that can appear harmless, like the wolf disguised as a sheep to better sneak up on the unknowing flock.  But what if the sheep themselves are actually the nasty surprise? In some parts of Iceland, extensive land degradation has been linked to overgrazing by livestock, as well as land clearing and natural processes including a harsh climate and frequent volcanic activity.  Identifying the factors responsible for ecosystem changes in these environments is essential for making good management decisions. In this paper, we use state-and-transition models, a simple representation using boxes and arrows, to describe ecosystem changes in Iceland over three periods with different human influence, from pre-settlement times to present days. The results suggest increasing complexity in recent decades where the models include stronger human influences, particularly sheep grazing.  These models can be used to make predictions about possible management interventions, so they can help farmers, managers and researchers identify realistic and achievable restoration goals.  

You can access the full publication here.

Reference: Barrio, I.C., Hik, D.S., Thórsson, J., Svavarsdóttir, K., Marteinsdóttir, B., Jónsdóttir, I.S. (2018) The sheep in wolf’s clothing? Recognizing threats for land degradation in Iceland using state‐and‐transition models. Land Degradation and Development 29:1714-1725.

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Picture: Icelandic sheep (photo: David Hik)

Herbivores can have large impacts on the composition and functioning of plant communities and may even drive the vegetation towards a different state. For example, grazing by reindeer can result in a shift from relatively unproductive dwarf shrub vegetation to a more nutrient rich vegetation type dominated by herbaceous plants. We studied so-called historical milking grounds (HMGs) in northern Sweden: old reindeer herding sites that were subject to high numbers of reindeer for many centuries, up to a century ago. The high reindeer concentrations in the past caused the vegetation to shift locally to small meadow-like patches, which are still clearly discernible in the otherwise heath-and shrub dominated tundra, a century after their active use.One possible mechanism for the long-term stability of HMG vegetation is the interaction with biotic and abiotic components of the soil. More specifically, a positive interaction between the vegetation and associated soil biota would help stabilize the current vegetation and slow down invasion rates of other plant species. We tested this assumption by growing seedlings representative of HMG vegetation (Potentilla crantzii) and control vegetation (Betula nana) in soils from HMGs and control sites, in a climate chamber. By sterilizing the soils, we could separate the biotic from abiotic drivers, to try and understand the different growth responses of plants in different soils. We found that,although both the typical control- and HMG plant species grew well in HMG soils due to the increased nutrient availability, microorganisms in the HMG soil provided an additional benefit for the typical HMG plant species. This was mostly the case when soils from nutrient poor habitats (heath) were considered.In summary, we conclude that positive interactions between plants, soils and soil organisms can explain the long-term stability of vegetation shifts that were caused by reindeer many centuries ago.

You can access the full publication here.

Reference: Egelkraut, D., Kardol, P., De Long J.R., Olofsson, J. (2018) 
The role of plant–soil feedbacks in stabilizing a reindeer‐induced vegetation shift in subarctic tundra. Functional Ecology 32:1959-1971.

Snow can limit access to food during winter for resident Arctic herbivores, forcing them to aggregate in the few patches with limited snow. This study used molecular analysis and stable isotope ratios of carbon and nitrogen of fecal samples of Arctic hare, rock ptarmigan and muskox in High Arctic Greenland to assess their diet overlap, and if this overlap increased as winter progressed. The results show that despite foraging in the same areas and generally feeding on the same plant taxa, the quantitative dietary overlap between these three herbivores was limited. This may be attributable to species-specific consumption rates of plant taxa.

You can read the full article here.

Reference: Schmidt, N.M., Mosbacher, J.B., Vesterinen, E.J., Roslin, T., Michelsen, A. (2018) Limited dietary overlap amongst resident Arctic herbivores in winter: complementary insights from complementary methods. Oecologia 187:689–699.

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Picture: Fecal pellet (photo: Lawrence Hislop, Norwegian Polar Institute)