Author: isabel

In African savanna wild herbivores are increasingly displaced by cattle and that is not without consequences. Where many cows graze, the soil becomes poorer because the cows do not sufficiently fertilise the soil where they graze. They mainly drop their dung at night when they are kept fenced-in to protect them from lions. This impoverishes the soil, which reduces the productivity and quality of the grass. However, when elephants are present, this soil depletion does not occur; the soil is even enriched. Elephants bring down trees in the savanna, which accelerates the return of nutrients to the soil. But more importantly, elephants compete with cattle for the same available food, which results in less grazing by cattle. This also means that cows export less dung away from the place where they graze, while dung deposition of wild herbivores is stimulated, which in turn feeds the soil. The combination of domestic cattle and elephants might therefore be a form of sustainable management for African savanna.

You can find the paper here, and read the news item by Mark E. Ritchie in Nature Sustainability and the highlight in Science.

Reference: Sitters, J., Kimuyu, D.M., Young, T.P., Claeys, P., Venterink, H.O.. (2020) Negative effects of cattle on soil carbon and nutrient pools reversed by megaherbivores. Nature Sustainability https://doi.org/10.1038/s41893-020-0490-0

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Picture: Elephants and cattle have contrasting effects on soil fertility at Mpala Research Centre (Photo: Dino J. Martins)

A new paper by Marc Macias-Fauria and collaborators published in January 2020 in Philosophical Transactions B reviews the environmental context of megafaunal ecological engineering in the Arctic, and suggests that the wide-scale introduction of large herbivores to the Arctic tundra to restore the ‘mammoth steppe’ grassland ecosystem and mitigate global warming could be economically viable.

In this thought-provoking paper, the authors use Pleistocene Park as a model and estimate how many herbivores would it take to revert tundra into a grassy mammoth steppe that could delay permafrost melt and reduce carbon emissions. They calculate what the costs of such a project would be and conclude that the idea is “reasonably viable economically”, but the challenge would be to implement it at a large enough scale to have a significant impact on global climate change mitigation because, in the authors own words, “the numbers of large herbivores required for such an undertaking do not exist”.

You can find the paper here, and read the news item on the University of Oxford’s website.

Reference: Macias-Fauria, M., Jepson, P., Zimov, N., Malhi, Y. (2020) Pleistocene Arctic megafaunal ecological engineering as a natural climate solution? Phil. Trans. R. Soc. B 375 http://doi.org/10.1098/rstb.2019.0122

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Picture: Woolly mammoths were driven to extinction by climate change and human impacts (Mauricio Antón © 2008 Public Library of Science)

Climate change is altering the timing of spring green-up and the timing of migratory herbivores, especially in northern latitudes. Shifts in timing of plant-herbivore interactions have the potential to alter nitrogen (N) cycling and availability, a critical resource in Arctic ecosystems. While earlier springs and earlier goose herbivory are expected to increase N resources, the relative impact of timing on soil resources is not well understood.

Ryan Choi and colleagues conducted a 3-year experiment in coastal western Alaska that simulated different timings of arrival by Pacific black brant (Brant bernicla nigricans) (3 weeks early, typical arrival, 3 weeks late, or no-arrival) and timings of the growing season for Carex subspathacea (3 week advancement and ambient conditions). They measured changes in soil-adsorbed and mobile inorganic (NH₄⁺-N, NO₃^–-N) and mobile organic N (amino acids).

Soil N increased with early goose grazing and decreased with late grazing. While both early goose arrival and season advancement lead to greater soil N, the timing of goose grazing had a greater influence on N availability than the advanced growing season. Because N is a limiting resource in northern ecosystems, even short-term impacts on soil N pools can have lasting effects on forage quality, plant community composition and productivity, and increase the likelihood of N leaching and loss in Arctic coastal wetlands.

The article is available at Ecosystems.

Reference: Choi, R.T., Beard, K.H., Kelsey, K.C., Leffler, A.J., Schmutz, J.A., Welker, J.M. (2020) Early Goose Arrival Increases Soil Nitrogen Availability More Than an Advancing Spring in Coastal Western Alaska. Ecosystems doi:10.1007/s10021-019-00472-9

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Text by Ryan Choi, PhD student at Utah State University

Picture: Flock of Pacific black brant (Brant bernicla nigricans) (photo: Ryan Choi, Utah State University, 2019)

Finding sustainable grazing management in tundra ecosystems is becoming increasingly important for northern agriculture. In the Icelandic highlands, past grazing practices have played a major role in the extensive soil erosion, leaving many parts of this tundra rangeland in poor condition. However, following a reduction in sheep numbers, stricter grazing management policies, and a progressive turn to fertilisers for combating soil erosion, the current impacts of grazing practices are not well understood.

In this study we measured the short-term effect of current sheep grazing by comparing the amount of bare ground and aboveground biomass in grazed and ungrazed plots, with and without fertiliser application, in adjacent dwarf-shrub heaths and sparsely vegetated areas, at two sites within and outside the volcanic active zone.

After three years, grazing did not affect the amount of bare ground, but in the fertilised plots grazing consistently reduced plant biomass (mainly graminoids). Fertiliser application can increase grazing intensity, which can counteract the additional biomass accumulation. Consequently, the removal of biomass by sheep needs to be considered when fertilisers are used as part of a strategy to combat erosion.

The article is open access and you can find the full text here.

Reference: Mulloy, T.A., Barrio, I.C., Björnsdóttir, K., Jónsdóttir, I.S., Hik, D.S. (2019) Fertilisers mediate the short-term effects of sheep grazing in the Icelandic highlands. Icelandic Agricultural Sciences, 32: 75-85.

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Text by Tara Mulloy, MSc student at Simon Fraser University

Picture: The study addressed the combined effects of fertiliser application and sheep grazing in the highlands of Iceland (photo: Tara Mulloy, Simon Fraser University, 2019)

Voles and lemmings are known for their high-amplitude population fluctuations and strong effects on dwarf shrub-dominated vegetation communities across the Fennoscandian tundra. Despite their key role in above-ground trophic interactions, there has been a dearth of research on how voles and lemmings affect ecosystem process rates, including ecosystem nutrient cycling and soil microbial activity. To address this gap, Maria Tuomi and colleagues studied how contrasting vole density regimes affect both vegetation and soil characteristics in a long-term island experiment located in West-Finnmark, Norway.

The study found that over the course of three population cycles, peak vole densities associated with increased abundance of palatable over non-palatable plants as well as increased nitrogen content in plant tissues. These changes in plant community characteristics link with increases in soil inorganic nitrogen content and microbial activity in litter. While these findings are in line with well-documented decimation of non-palatable shrubs during vole and lemming population peaks in Fennoscandia, they contrast predictions based on prevailing conceptual models on herbivore-plant-soil interactions. The study thus challenges the assumption that in low-productive ecosystems, herbivores slow down process rates by selectively eating nutritious plants, and proposes a new conceptual model with two alternative vole-plant-soil interaction pathways.

Reference: Tuomi, M., Stark, S., Hoset, K.S., Maria Väisänen, M., Oksanen, L., Murguzur, F.J.A., Tuomisto, H., Dahlgren, J., Bråthen, K.A. (2019) Herbivore effects on ecosystem process rates in a low-productive system. Ecosystems 22: 827-843

https://doi.org/10.1007/s10021-018-0307-4

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Picture: In their study, high peak vole densities over multiple population cycles increased the relative abundance of forbs like Rubus chamaemorus (front) (photo: Maria Tuomi, University of Turku, 2018)

Shrubification of arctic tundra is a well-recognized phenomenon, and it can be particularly rapid in moist habitats. Reindeer grazing can inhibit shrubification, but grazing impacts on mire vegetation have been overlooked. We studied grazing effects on plant communities and Salix lapponum in oroarctic mires at the border of Finland and Norway. We compared plant community structure and S. lapponum abundance and traits between (1) grazed fens (Finland); (2) experimental exclosures (Finland), where reindeer have been kept out for 13 years; and (3) non-grazed fens (Norway). Grazing effect on shrubification was assessed using the Normalized Difference Vegetation Index (NDVI) and leaf area index (LAI).

We did not find a uniform direction of vegetation change connected to the exclosure treatment, and grazing treatments were overlapping in multivariate ordination. Neither NDVI nor LAI indicated clear differences. Instead, significant results were revealed in total abundance of species groups and in S. lapponum traits. The cover of bryophytes was significantly lower under free grazing. Reindeer grazing reduced the abundance, height, and flowering and increased leaf N concentration of S. lapponum. We conclude that reindeer grazing controls willows and affects total abundance of important species groups, and plant community structure is resistant to grazing effects in oroarctic mires. 

Reference: Kolari THM, Kumpula T, Verdonen M, Forbes BC, Tahvanainen T (2019) Reindeer grazing controls willows but has only minor effects on plant communities in Fennoscandian oroarctic mires. Arctic, Antarctic, and Alpine Research, 51(1):506-520

https://doi.org/10.1080/15230430.2019.1679940

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Picture: Heidi Kitti, then PhD student, making ITEX-style tundra vegetation measurements in summer 2002 when the fenced reindeer exclosures were established in Näkkälä reindeer herding district, Finland (photo credit: Bruce Forbes, University of Lapland)

This new paper by David Anderson and collaborators reviews archaeological and palaeoecological evidence for the nature of occupation by humans (Nenets) and both wild and semi-domesticated reindeer at a low arctic tundra site on the Yamal Peninsula.

Yarte-6 on the Yuribei River is probably the most important Late Holocene archaeological site in West Siberia, which lies along the time continuum during the transition from hunting to herding reindeer. Yarte-6 is situated on a prominent headland overlooking the river valley and two large lakes. Although disused since the Middle Ages, the former camp site is covered by a lush grassy meadow, easily distinguished from the prevailing dwarf-shrub heath tundra vegetation.

When reindeer graze at modern Yamal tundra Nenets camps, their combined grazing and trampling creates a radial pattern. This has the effect of either killing or stunting the growth of shrubs within the grass/herb-rich area, while confining the growth of taller shrubs to the low-lying ravines encircling the site – which, as we will see, may have been an important feature for wild reindeer hunters. At low densities, Rangifer do not have a marked effect on either erect shrubs or ground-level vegetation. However, at higher densities, the negative impacts of more intensive grazing and trampling can become clear within as little as six years. While herbivores can help to preserve meadows by grazing the shrubs, their grazing is not essential when the graminoid-herb cover is so dense – as it is at Yarte-6 – that it prevents the germination of shrub seedlings. The establishment and long-term persistence of this grassy feature amongst well-grazed shrubs does indicate, however, that the activities of people and Rangifer had once been intense.

The question of what that shrub cover might have been like 1000 years ago is a key factor in interpreting past livelihoods. The Yarte promontory has few shrubs today, and indeed it would be hard to imagine how the existing willow copses could conceal a campsite or hide a hunter. However, the palynological evidence suggests that the shrub cover was thicker in the centuries after 800AD. During slightly warmer periods, willows grow taller and copses become more dense, as exemplified by the contemporary “shrubification” of Yamal, which is associated with the 21st-century warmer temperatures. The growth of shrubs during warm periods in the past might also have been encouraged by deeper snow cover, which also seems to be associated with warmer periods.

Reference: Anderson DG, Harrault L, Milek KB, Forbes BC, Kuoppamaa M, Plekhanov AV (2019) Animal domestication in the high Arctic: Hunting and holding reindeer on the I͡Amal peninsula, northwest Siberia. Journal of Anthropological Archaeology 55: 101079

https://doi.org/10.1016/j.jaa.2019.101079

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Picture: The Yarte promontory in Yamal (photo: Bruce Forbes, University of Lapland)