Author: isabel

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.

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.

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.

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.

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

In this paper published in Functional Ecology, Henni Ylänne and collaborators investigate the impact of reindeer on the carbon storage at two sub-arctic tundra sites in Northern Norway. The sites represent typical examples of herbivory-induced alternative ecosystem states, where high grazing pressure has led to a conversion of tundra shrublands or heaths to tundra meadows with higher rates of nutrient turnover. This study makes use of two 60-year-old reindeer fences that separate these alternative ecosystem states, and compares recent changes in above- and belowground carbon storage.

The paper reports that in the past 14 years, the increased reindeer numbers and higher trampling intensity have led to a larger areal extent of grass-dominated vegetation, leading also to higher soil nutrient availability. This demonstrates the capacity of tundra systems to adjust to changes in grazing pressure.

Further, the paper shows that the “grassification” of tundra shrublands, whether occurred recently or decades earlier, reduces carbon stored aboveground. However, the impacts of grazing belowground varied between the two study sites. At one site, the grass dominated, grazed area stored equal amounts of carbon compared to the tundra heath. At the other site, there was more carbon in the organic soil under the grass-dominated, grazed area compared to the shrub tundra. The consequences of grazing depend on the characteristics of the vegetation under light grazing.

Reference: Ylänne, H., Olofsson, J., Oksanen, L., Stark, S. (2018) Consequences of grazer‐induced vegetation transitions on ecosystem carbon storage in the tundra. Functional Ecology 32: 1091– 1102.

https://doi.org/10.1111/1365-2435.13029

Picture: Differences in vegetation across the reindeer fence in
Čearro (photo: Henni Ylänne, Lund University, 2018)