Category: new publications

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Herbivores affect the distribution of rare plants in alpine and Arctic tundra

Domestic sheep in Norway

Herbivores influence many aspects of ecosystems in Arctic and alpine tundra, from plant population dynamics and community composition to soil carbon storage and nutrient cycling. Herbivory is a multi-scale process, herbivores select their food at small spatial scales, but also at large spatial scales in terms of their ranges and distributions.

We have a good understanding of how herbivores affect site level dynamics, but not larger scale dynamics. Indeed, while climatic variables have been successfully used to model species distributions, biotic interactions including herbivory have been notably absent from such approaches. However, a recent study seeks to change this.

Herbivory Network members James Speed and Gunnar Austrheim from the NTNU University Museum at the Norwegian University of Science and Technology have published a paper in the journal Biological Conservation where they use a national-level data set mapping densities of all large herbivores to predict the distribution of rare plant species within Norway. The study focussed on seven arctic and alpine tundra plant species on the Norwegian Red List for species, and the main herbivores grazing in these ecosystems – domestic sheep along with both wild and semi-domestic reindeer.

Although climatic variables were the most important factor in controlling the distribution of all seven species, there was a clear signal of herbivore densities influencing the distributions. This was most notable for Primula scandinavica (Scandinavian primrose). Low densities of herbivores were associated with low habitat suitability for this species. Speed and Austrheim mapped the regions of Norway where herbivore density was the factor most limiting the distribution of each rare plant species. There was little overlap in these areas between the seven plant species. This suggests that the management of herbivore populations for the conservation of rare plant species needs to be specific for each location and plant species.

Reference: Speed, J.D.M. & Austrheim, G. (2017) The importance of herbivore density and management as determinants of the distribution of rare plant species. Biological Conservation, 205:77-84. http://dx.doi.org/10.1016/j.biocon.2016.11.030

Picture: Domestic sheep in Norway (photo: Atle Mysterud)

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Experimental warming increases herbivory by leaf‐chewing insects in an alpine plant community

caterpillar

Tone Birkemoe and collaborators have recently published a paper on the effects of experimental warming on insect herbivory. Here’s Tone’s summary of their paper:

Climate warming is predicted to affect species and trophic interactions worldwide, and alpine ecosystems are expected to be especially sensitive to changes. In this study, we used two ongoing climate warming (open-top chambers) experiments at Finse, southern Norway, to examine whether warming had an effect on herbivory by leaf-chewing insects in an alpine Dryas heath community.

We recorded feeding marks on the most common vascular plant species in warmed and control plots at two experimental sites at different elevations and carried out a brief inventory of insect herbivores. Experimental warming increased herbivory on Dryas octopetala and Bistorta vivipara. Dryas octopetala also experienced increased herbivory at the lower and warmer site, indicating an overall positive effect of warming, whereas B. vivipara experienced an increased herbivory at the colder and higher site indicating a mixed effect of warming.

The Lepidoptera Zygaena exulans and Sympistis nigrita were the two most common leaf-chewing insects in the Dryas heath. Based on the observed patterns of herbivory, the insects life cycles and feeding preferences, we argue that Z. exulans is the most important herbivore on B. vivipara, and S. nigrita the most important herbivore on D. octopetala. We conclude that if the degree of insect herbivory increases in a warmer world, as suggested by this study and others, complex interactions between plants, insects, and site-specific conditions make it hard to predict overall effects on plant communities.

A popular version of the paper is available in Norwegian from this web-page: http://blogg.nmbu.no/insektokologene/2016/09/spiser-insekter-mer-planter-i-en-varmere-verden/

Reference: Birkemoe T, Bergmann S, Hasle TE, Klanderud K. (2016) Experimental warming increases herbivory by leaf‐chewing insects in an alpine plant community. Ecology and Evolution 6(19):6955-6962

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Biotic interactions mediate patterns of herbivore diversity in the Arctic

herbivore diversity map

Understanding the forces that shape biodiversity is essential for improving our ability to predict the responses of ecosystems to rapid, ongoing environmental change.  In the Arctic, herbivores often play a key role in the functioning of terrestrial ecosystems.  The diversity of herbivores varies across the Arctic, and until now, no one knew whether this was shaped by physical environmental factors, like temperature, or biotic factors, such as plant productivity.

We collected information on the distribution of all 73 species of vertebrate herbivores that occur in the Arctic.  Vertebrate herbivores are particularly important as they affect the structure and dynamics of plant communities and provide food for higher trophic-level predators.  The results of this study showed that herbivore diversity in the Arctic is higher in areas with greater plant productivity and with higher diversity of predators.  The interactions between plants, herbivores and predators, occurred over large spatial scales across the Arctic, in ecosystems where patterns of biodiversity were supposed to be affected mainly by temperature variation.

Reference: Barrio, I.C., Bueno, C.G., Gartzia, M., Soininen, E.M., Christie, K.S., Speed, J.D., Ravolainen, V.T., Forbes, B.C., Gauthier, G., Horstkotte, T., Hoset, K.S., Høye, T.T., Jónsdóttir, I.S., Lévesque, E., Mörsdorf, M.A., Olofsson, J., Wookey, P.A. and Hik, D.S. (2016) Biotic interactions mediate patterns of herbivore diversity in the Arctic. Global Ecology and Biogeography 
25(9):1108-1118

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Quantifying muskox plant biomass removal and spatial relocation of nitrogen in a high arctic tundra ecosystem

The muskox (Ovibos moschatus), a key species in the arctic tundra, is the only large-bodied herbivore in Northeast Greenland. At Zackenberg, muskoxen can be found in some of the highest densities in the world. Consequently, a significant grazing pressure of muskoxen is expected there.

In addition, the impact on the nutrient budgets may be even larger, as herbivores tend to feed on plant parts that are rich in nutrients, and they may also redistribute nitrogen and other nutrients through defecation and urina­tion across the landscape.

This study quantify the biomass removal and nitrogen relocation by muskoxen during the snow-free period in the years 1996 to 2013 in the high arctic tundra ecosystem at Zackenberg, Northeast Greenland. By doing this, we aim at not only quantifying the muskox as a key herbivore, but also its role as redis­tributor of nutrients.

Muskoxen removed almost negligible amounts of the available forage, with under 1% during the summer. However, the muskoxen redistributed large amounts of nitrogen between vegetation types. Muskoxen at Zackenberg forage mainly in the graminoid-dominated areas, but defecate primarily in Salix snowbeds, resulting in net nitrogen transfers from the nitrogen-rich wet habitats to the nitrogen-poor, drier habitats in the same magnitude as the dissolved inorganic nitrogen pool in similar arctic soils. Hence, while the quantitative impact of muskox biomass removal seems negligible, its role as a nitrogen carrier in a nitrogen limited ecosystem is likely to have a significant impact on tundra communities. The results thus stress the central role of muskoxen in the tundra ecosystem.

Reference: Mosbacher JB, Kristensen DK, Michelsen A, Stelvig M, Schmidt NM.  (2016) Quantifying muskox plant biomass removal and spatial relocation of nitrogen in a high arctic tundra ecosystem. Arctic, Antarctic, and Alpine Research 48:229-240

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Picture: Muskox grazing (photo: Jesper Mosbacher)

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