Category: new publications

Elevational shifts in the distribution of species can have large consequences to high-elevation ecosystem. More so, when we are talking about herbivores.

In a field experiment, Patrice Descombes and collaborators translocated various species of grasshoppers from medium elevations (1,400 m a.s.l.) to three alpine grassland sites at 1,800, 2,070 and 2,270 m a.s.l. in the Anzeindaz region in the Vaud Alps. They measured how the biomass, structure and composition of the alpine plant communities changed under the influence of the herbivorous insects. They also investigated whether some plant species were more susceptible to herbivory, for instance plants with tougher leaves, or those containing more silica or other constituents such as phenols or tannins.

The grasshoppers’ feeding behaviour had a clear influence on the vegetation structure and composition of the alpine flora. Alpine communities display clear structure in the organisation of the canopy, with plants with tough leaves at the top, and more shade-​tolerant plants with softer leaves at the bottom. But this natural organisation was disturbed, because the translocated grasshoppers preferred to feed on taller and tougher alpine plants, that were more similar to their lower-​elevation food plants. As a result, the insects reduced the biomass of dominant alpine plants, favouring the growth of small-​statured plants. The overall plant diversity thus increased in the short term.

Global warming can disrupt the ecological balance of biotic communities because mobile animals, including many herbivorous insects, can expand their habitat to higher elevations more rapidly than sedentary plants. Herbivorous insects from lower altitudes could therefore have an easy time in alpine habitats with resident plants that are insufficiently prepared to defend themselves against those new herbivores. This could change the current structure and functioning of alpine plant communities as a whole. Climate change would thus have an indirect impact on ecosystems, in addition to the direct consequences of rising temperatures.

Their study has been published in Science. You can find the full article here.

Reference: Descombes, P., Pitteloud, C., Glauser, G., Defossez, E., Kergunteuil, A., Allard, P.M., Rasmann, S. and Pellissier, L. (2020) Novel trophic interactions under climate change promote alpine plant coexistence. Science, 370(6523):1469-1473.

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Picture: Research site at 1,800 m in the Chablais region. In the background: Vallon de Nant and the Grand Muveran (photo: P. Descombes)

Imagine a bird taking flight. Feathers, hollow bones, and relatively small organs help them overcome gravity and reduce the high costs of flying. Now imagine an herbivorous bird in a cold, snowy winter where thermoregulation and larger guts needed to digest fibrous plants add to energetic costs. Our team of Americans, Swedes, Icelanders, and Norwegians wanted to understand how non-migratory, herbivorous grouse maximize digestion when consuming woody and chemically defended plants during the winter compared to grazing geese and more omnivorous birds. We compared the activity of enzymes responsible for digesting sugars (maltase and sucrase) and protein (aminopeptidase-N) in the small intestinal and cecal tissues of Black Grouse (Tetrao tetrix), Capercaillie (T. urogallus), and Willow Ptarmigan (Lagopus lagopus) from Sweden and Rock Ptarmigan (Lagopus muta) from Iceland collected between February and April 2017. We found that enzyme activity in the small intestine was higher than in the ceca indicating the relatively larger role of host digestion in the upper gut and larger role of the microbiome in digestion in the ceca. In addition, we found that grouse had relatively lower activities of sugar-digesting enzymes when compared to geese and omnivores and that both grouse and geese had lower activities of protein-digesting enzymes than omnivores. Within the grouse, Black Grouse had relatively higher enzyme activity for sugars in the small intestine than Ptarmigan which may reflect greater access to sugar-containing foods than Ptarmigan. Results indicate that relatively longer guts observed in grouse in winter serve to increase retention time to maximize yield of digestion and may not require high activity of digestive enzymes. Results also underscore the need to investigate links among diet quality, morphology, and physiology relative to season, geographic location, and taxonomy to better understand the behavior, ecology, and evolution of avian herbivores.

This research has been accepted for publication in the Journal of Ornithology and has recently been published on-line here: https://doi.org/10.1007/s10336-020-01835-z.

The research was made possible through the support of Henrik Andrén and staff at the Grimsö Wildlife Research Station as well as Geir Rune Rauset, Þorkell Lindberg Þórarinsson, Friðrik Jónasson, and Onni – the barking bird dog. Funding was provided by the Fulbright-Hays Grant and the National Science Foundation [DEB-1146194 and OIA-1826801].

Reference: Newman, J., Maurer, M., Forbey, J.S., Brittas, R., Johansson, Ö., Nielsen, Ó.K., Willebrand, T. and Kohl, K.D., 2020. Low activities of digestive enzymes in the guts of herbivorous grouse (Aves: Tetraoninae). Journal of Ornithology, pp.1-9.

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Text: Jennifer Forbey, Boise State University

Picture: Willow ptarmigan (photo credit: Lawrence Hislop, Norwegian Polar Institute)

Asian montane rangelands are a part of the largest contiguous grassland system in the world, and support the region’s primarily livestock based economies and lifestyles. Despite this and the fact that they are facing some of the most rapid changes in climatic conditions, they are still relatively understudied. To fill this gap, we setup a grazing x climate change (warming, drought and increased rainfall) experiment at two montane rangeland sites in Spiti valley, India and Tost in the Gobi-Altai mountains of Mongolia.

We found that climate and clipping interactively reduced vegetation cover and biomass but did not affect species richness. Treatment effects and their interactions varied between sites. In ungrazed plots, vegetation cover and biomass declined sharply in response to warming (18%–35%) and drought (20%–50%) at the two sites, and, surprisingly also declined slightly in response to increased rainfall (20%) at Tost. While the effects of climate treatments were largely similar in the grazed and ungrazed plots in Tost, they were larger in the ungrazed plots in Spiti.

Taken together our findings show that future changes in climate, including just over 1°C of warming, could undermine the sustainability of pastoral economies and the persistence of wildlife across Asian montane rangelands. Furthermore, grazing by herbivores will play an important role in mediating rangeland responses to climate change; thus, pasture management in concert with local pastoralists will be crucial in mitigating the adverse effects of climate change on rangelands, pastoral livelihoods and wildlife populations.

You can read the full article here.

Reference: Kohli, M., Mijiddorj, T.N., Suryawanshi, K.R., Mishra, C., Boldgiv, B. and Sankaran, M., 2020. Grazing and climate change have site‐dependent interactive effects on vegetation in Asian montane rangelands. Journal of Applied Ecology.

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Picture: Montane rangelands provide livelihoods to pastoralists (photo credit: M. Kohli, University of Minnesota)

Climate change in Arctic regions is linked to an expansion of woody-taxa (shrubification) and an increase in biomass as tundra becomes greener. Herbivores like reindeer and caribou are often considered able to suppress this vegetative greening through trampling and grazing. Many studies have quantified the effect of herbivory on woody taxa in local experimental sites, but there are still few studies quantifying reindeer use at the landscape scale that can shed light on their potential to suppress Arctic vegetative greening.

This study has measured reindeer habitat use within the low Arctic tundra zone of Yamal, West Siberia using reindeer faecal pellet-group counts. Active layer thickness was also measured and related to the reindeer use, as intense use of for example denuded land also could favour deeper thawing of the permafrost. These measures can help us to understand herbivore impact on the growth and recruitment of deciduous shrubs, many of which serve as fodder (e.g. Salix spp.), in favourable habitats, such as naturally denuded landslides in permafrost areas.

The results confirm intensive use by reindeer of areas with higher productivity, low erect shrub tundra and upper slopes, ridges and valleys, while the use of tall Salix areas was less intense. This suggest that reindeer are unlikely to suppress growth of already tall-erect woody taxa, while they may suppress shrubs low in stature. The study did not find any direct relationship between permafrost thawing and reindeer use, but intensively used sites seemed to have a shallower active layer. The study concludes that reindeer use of the landscape and hence their effects on the landscape correlates with the landscape structure and that future research is needed to evaluate the role and efficiency of reindeer as ecosystem engineers capable of mediating the effects of climate change.

Reference: Skarin, A., Verdonen, M., Kumpula, T., Macias-Fauria, M., Alam, M., Kerby, J.T. and Forbes, B.C., 2020. Reindeer use of low Arctic tundra correlates with landscape structure. Environmental Research Letters.
https://doi.org/10.1088/1748-9326/abbf15

You can find the full paper here, and learn more about the study in the beautiful video summary below:

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Picture: Reindeer in Yamal, Western Siberia (photo © Hannes Skarin).

The original ideas and stimulus for the study came up from observations of heavy ptarmigan browsing on tundra willows at several arctic sites (H. Roininen and R. Virtanen during the Swedish-Russian tundra expedition to arctic Eurasia in the mid 1990s). These observations were at odds with the dominant idea that primarily abiotic stresses constrained tundra willow abundance and growth. We found it necessary to establish ptarmigan exclusion experiments to measure the browser impacts on tundra willows.

We thus established experiments at a tundra landscape in NW Finnish Lapland close to Kilpisjärvi Biological Station. By setting up some pilot experiments, we soon realized that reindeer summer browsing was pretty strong on willows, and this had to be considered too. Because many willow genets in that area had stunted growth-form due to browsing, we thought it would be of interest to rejuvenate willows by initially cutting a subset of experimental genets, and let them develop vigorously growing ramets without or with browsing. The eventual experimental design thus included small mesh cages excluding ptarmigan and reindeer and large mesh allowing ptarmigan browsing but excluding reindeer, and these treatments for both rejuvenated and old genets. Indeed, our initial interest and experimental design was set up to find out how ptarmigan and reindeer effects on willow growth and survival of willows and insect herbivores.

In the first years of the experiment, reindeer effects on willow shrubs were pretty strong and have been published earlier (den Herder et al. 2004 [1]). Because of rather slow growth of willows (just c. 2-3 cm /year) the intention was to continue monitoring over several years. In that region, autumnal moth outbreaks had not been seen for decades and we did not expect them to happen. However, a couple of years after the  initiation of the experiment things changed: the first autumnal moth (Epirrita autumnata) outbreak took place in 2004-2005 (as most intense) which caused virtually complete defoliations of willow (and nearby mountain birch forests), and obviously negative effects on growth. The second outbreak came in 2011-2013 (a bit less severe than the first) with nearly as strong effects on willow as during the first outbreak. Some background insect herbivory occurred every year, and there were marked among-year variations in key climate variables thought to impact willow growth. We thought that the long-term experimental monitoring data would allow us to look at how insect herbivory, summer thermal and precipitation conditions influence annual shoot growth and even how these depend on browsing pressure.

The analyses of relations between annual shoot growth increments, climatic and biotic drivers turned out to be challenging, and we found empirical dynamic modelling (EDM) to be the most powerful approach for the purpose. The EDM analyses on the 20-year time series showed that negative effects of insect herbivory on shoot growth became stronger in warmer summers, whereas precipitation effects tended to become less negative effects with increasing precipitation, and this latter effect seems to moderate warming effect. These effects further depended on browsing pressure and ramet age.

The results establish that local growth dynamics of willows at a forest tundra ecotone is impacted by multiple herbivores that moderate growth responses to climate. The research continues to look the long-term cumulative impacts of treatments, and also the effects of changes in reindeer management, on the survival and growth of willows.

You can find the full text here.

Reference: Virtanen, R., Clark, A.T., den Herder, M., Roininen, H. (2020) Dynamic effects of insect herbivory and climate on tundra shrub growth: roles of browsing and ramet age. Journal of Ecology 109(3):1250-1262
https://doi.org/10.1111/1365-2745.13551

[1] den Herder, M., Virtanen, R., & Roininen, H. (2004). Effects of reindeer browsing on tundra willow and its associated insect herbivores. Journal of Applied Ecology, 41(5), 870-879. [link to publisher]

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Text by Risto Virtanen, University of Oulu

Picture: An experimental exclosure with reindeer fence and a willow genet (Salix phylicifolia) inside the fence (taken in 2019 N of Lake Haukijärvi, tundra like habitat with other species Betula nana, Salix glauca, and close to Betula pubescens ssp. pumila treeline) (photo: Risto Virtanen, University of Oulu, 2020)

Diet composition in herbivores differs greatly from season to season because they eat many types of grasses, shrubs, and forbs. This flexibility allows arctic-alpine species like mountain hares (Lepus timidus) to cope with changing environmental conditions over the seasons and the corresponding changes in food quantity and quality. Understanding how, when, and why animals use certain areas is critical to their conservation and management.  

Rehnus and Bollmann investigated the distribution of pellets and of individual core activity areas of mountain hares in the Swiss Alps and compared differences between spring and autumn. Pellets were collected from 2014 to 2019; individuals were determined using an established, non-invasive genetic technique.

Their study presents details about altitudinal shifts in the feeding sites and activity areas of mountain hares in the Alps. They conclude that the annual “wave” of greening from lower to higher elevations, which correlates with forage availability and quality, explains this shift. However, further research is needed on how these shifts are affected by climate change and how they interact with nutritional needs and thermoregulatory mechanisms.

Reference: Rehnus, M., Bollmann, K. (2020) Mountain hares Lepus timidus follow the green-up wave in the pursuit of high-quality food. Wildlife Biology, 2020, 3: 00720. doi: 10.2981/wlb.00720

You can find the full paper here.

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Picture: Mountain hare during grooming in early-summer (photo © Rolf Giger).

Herbivory can modify vegetation responses to climatic changes, but climate and herbivory also affect each other and rarely work in isolation from other ecological factors. Thus, it is challenging to predict how herbivory counteracts, amplifies and interacts with climate impacts. We used experimental exclosures in Norway and Canada to investigate how moose browsing can modify climatic responses of boreal trees. By modeling complex causal pathways including several climatic factors, multiple tree species, competition, tree height, time, food availability, and herbivore presence, density and browsing intensity, we found that moose can indeed counteract positive temperature responses of trees. However, this effect varied between tree species depending on moose foraging preferences. Furthermore, we found that moose can contribute positively to the growth of some trees by constraining growth of competing trees. This “cooling power” of herbivory appears to be highly context-dependent, so in order to understand its potential to prevent changes induced by warming climate, ecological complexity needs to be acknowledged.

Reference: Vuorinen, Katariina E. M., Anders L. Kolstad, Laurent De Vriendt, Gunnar Austrheim, Jean‐Pierre Tremblay, Erling J. Solberg, and James D. M. Speed (2020) Cool as a moose: How can browsing counteract climate warming effects across boreal forest ecosystems? Ecology: e03159.

You can find the full paper here.

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Picture: A field site in Gaspesie (Photo by Katariina E. M. Vuorinen).

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