Author: isabel

In the long-term, herbivores can alter nutrient dynamics in tundra ecosystems by changing the functional composition of the vegetation. Yet, herbivores may also alter nutrient dynamics by modifying plant-community nutrient levels, and these changes are likely to happen much faster than structural changes. Here, by using novel Near Infrared-Reflectance Spectroscopy (NIRS) methodology, we quantified the effects of small rodents and reindeer on tundra-grassland plant-community nitrogen and phosphorus levels throughout a single growing season. Herbivores were immediate, positive modifiers of plant-community nutrient levels. Plant-community nutrient contents in herbivore-affected tundra were still much higher in senescent leaves at the end of the summer, suggesting that herbivory is accelerating short-term tundra-grassland nutrient dynamics. Overall, our findings from tundra-grasslands align with theoretical expectations of positive herbivore effects on nutri­ent cycling in relatively productive ecosystems, when domi­nant, nutrient-rich growth forms benefit from herbivory.

Reference: Petit Bon, M., Inga, K. G., Jónsdóttir, I. S., Utsi, T. A., Soininen, E. M., & Bråthen, K. A. (2020). Interactions between winter and summer herbivory affect spatial and temporal plant nutrient dynamics in tundra grassland communities. Oikos. https://doi.org/10.1111/oik.07074

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Picture: experimental sites at Ifjordfellet, Finnmark, northern Norway (photo: Matteo Petit Bon)

Arctic ecosystems are especially sensitive to climate change. As the Arctic warms, shrubs have been observed to expand. Such changes in vegetation could have major implications on how Arctic ecosystems respond to and feedback on climate change. However, it remains unclear why such shrub expansion remains spatially uneven across the Arctic, with herbivory suggested as a key regulating factor. This study mapped the changes in shrub cover in Yamal Peninsula over three decades using satellite imagery and related these changes to summer temperature and reindeer population size. Surprisingly, we found no evidence that shrubs had expanded, despite the increasing temperatures. At the same time, local reindeer population size grew by over 75%. The results indicate that herbivory pressures could counteract the climate-induced shrub expansion in the region. This suggests that the strategic management of semi-domesticated reindeer husbandry could be a promising tool to maintain tundra landscapes in a warming Arctic. 

Reference: Verma, M., to Bühne, H.S., Lopes, M., Ehrich, D., Sokovnina, S., Hofhuis, S.P. and Pettorelli, N., 2020. Can reindeer husbandry management slow down the shrubification of the Arctic?. Journal of Environmental Management, 267:110636
https://doi.org/10.1016/j.jenvman.2020.110636

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Image credit: @Photo by Steve Morgan/GreenPeace

During the last century, moose have taken advantage of increasing shrubs in the tundra and expanded their northern range into Arctic Alaska. As the Arctic warms up, shrub patches along rivers are aggregating and coalescing to form larger and more connected patches, creating more moose habitat. Models predict that moose habitat will likely more than double by 2099 in Arctic Alaska. Current research focuses on how this expected increase in moose habitat will affect future opportunities for moose hunting in remote communities of the region.

Reference: Zhou, J., Tape, K.D., Prugh, L., Kofinas, G., Carroll, G., Kielland, K. (2020) Enhanced shrub growth in the Arctic increases habitat connectivity for browsing herbivores. Global Change Biology
https://doi.org/10.1111/gcb.15104

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Picture: Shrub expansion enhances habitat connectivity for browsing herbivores like moose, Alces alces (Photo: Ken Tape, University of Alaska Fairbanks)

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)