Author: isabel

The effect of reindeer grazing on the assemblage of ground-dwelling spiders was studied along the Finnish-Russian border fence on lichen dominated pine heath. Changes in species richness, abundance and evenness of spider assemblages were analyzed in relation to changes in vegetation and environmental factors in long term grazed and ungrazed sites, as well as sites that had recently switched from grazed to ungrazed and vice versa. Grazing was found to have a significant impact on height and biomass of lichens and other ground vegetation. However, it seemed not to have an impact on the total abundance of spiders. This is likely caused by opposing family and species level responses of spiders to the grazing regime. Lycosid numbers were highest in grazed and linyphiid numbers in ungrazed areas. Lycosidae species richness was highest in ungrazed areas whereas Linyphiidae richness showed no response to grazing. Four Linyphiidae, one Thomisidae and one Lycosidae species showed strong preference for specific treatments. Sites that had recovered from grazing for nine years and the sites that were grazed for the last nine years but were previously ungrazed resembled the long term grazed sites. In the long term ungrazed sites we found both spider and plant species typical of peatlands living among the thick Cladonia lichen mat on the sandy soil.

The results emphasize the importance of reindeer as a modifier of boreal forest ecosystems. The sites with reversed grazing treatment demonstrate that recovery from strong grazing pressure at these high latitudes is a slow process whereas reindeer can rapidly change the conditions in previously ungrazed sites similar to long term heavily grazed conditions.

You can read the full article here.

Reference: Saikkonen T, Vahtera V, Koponen S, Suominen O (2019) Effects of reindeer grazing and recovery after cessation of grazing on the ground-dwelling spider assemblage in Finnish Lapland. PeerJ 7:e7330 

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Picture: Fence separating a grazed and an ungrazed site in Finnish Lapland (photo: Otso Suominen, University of Turku)

Almost all plant species, particularly when stressed, release a complex blend of volatile organic compounds (VOCs) into the atmosphere. These VOCs are a fundamental defense of plants, conferring enhanced tolerance/resistance against abiotic stresses and acting as communication cues among community members. They also play important roles in atmospheric chemistry as they contribute to the formation of ozone and secondary organic aerosols.

Multitude of abiotic and biotic factors such as temperature and insect feeding are known to modulate the synthesis and release of plant VOCs. In the Arctic, however, no empirical studies have so far evaluated how climate warming and its concomitant increase in herbivory pressure interactively affect VOC emission patterns. In this study, we examined the effect of climate warming and increased insect activity on the release of plant VOCs to the atmosphere in the Arctic. We measured VOC emissions from dwarf birch Betula nana growing naturally in a subarctic wet heath, where ecosystem plots were subjected to passive warming by open-top chambers for 8 or 18 years.

The results showed that under 2°C of canopy warming obtained by open-top chambers, the plant leaf area consumed by insect herbivores increased by 300%. We exogenously applied a plant hormone methyl jasmonate to mimic plant defense responses to insect herbivory. We found that warming increased monoterpene emissions about twofold, irrespective of exposure duration, and that mimicked herbivory alone increase monoterpene emissions fourfold, and induced de novo synthesis and release of homoterpenes. Interestingly, combined warming and mimicked herbivory increased monoterpene emissions 11 fold, suggesting that warming amplifies the effects of herbivory on VOC emissions. 

Our observation of the unexpected high release rates of VOCs under combined warming and herbivory conditions suggest that increased insect activities will have a tremendous impact on VOC emissions in the Arctic and may tip into a domino effect on VOC-mediated interactions between species as well as the atmospheric chemistry. Our study underlines the importance of a focus on biotic stress on VOC emissions during climate change.

You can read the full paper here.

Reference: Li T, Holst T, Michelsen A, Rinnan R (2019) Amplification of plant volatile defence against insect herbivory in a warming Arctic tundra. Nature Plants 5:568–574

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Picture: Assessing how climate warming and insect feeding affect plant volatile release from Arctic plants in Abisko, Sweden (photo: Riikka Rinnan, Copenhagen University).

In many parts of the world, wild herbivores coexist with domestic livestock. Direct interactions between wild and domestic herbivores such as competition for forage often occur, as do indirect interactions such as apparent competition through predator densities. However, environmental management of wild herbivores and livestock is usually completely disconnected.

In a newly published paper, James Speed and colleagues analyse densities of wild herbivores and livestock in space (across Norway’s rangelands) and time (from 1949 to 2015). They find that livestock densities have greatly reduced, while wild cervids have increased in density. This is especially strong in milder regions, while mountain and northern regions remain dominated by livestock today (Figure 1).

These changes signify that Norway’s herbivore communities have become wilder – a process known as passive rewilding. However, both livestock and wild herbivores remain under stringent environmental management today. Interactions between wild and domestic herbivores, through both vegetation and predation, indicate that management of livestock and wild herbivores should be more integrated.

You can read the full paper here.

Reference: Speed JDM, Austrheim G, Kolstad AL, Solberg EJ (2019) Long-term changes in northern large-herbivore communities reveal differential rewilding rates in space and time. PLoS ONE 14(5): e0217166

https://doi.org/10.1371/journal.pone.0217166

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Picture: Moose (Alces alces, European elk) is one of the species that has increased most in density across Norway from 1949 to 2015 (photo: James Speed, NTNU).

Herbivores can sometimes alter the trajectory of entire ecosystems. In the northern-boreal mountain birch forest of Northern Fennoscandia, outbreaks by geometrid moths have caused mass mortality of mountain birch across large areas since the early 2000s. The severity of the outbreaks has been linked to the climatically facilitated range-expansion of the winter moth (Operophtera brumata), that has recently established itself as a new outbreaking defoliator in many areas that originally experienced outbreaks only by the native autumnal moth (Epirrita autumnata).

In the current study – focused on the Varanger region in northeast Norway – we show that the cumulative defoliation inflicted by such a novel two-species outbreak has exceeded a critical tolerance threshold (tipping point) of the forest, resulting in an abrupt increase in the mortality rate of birch stems. We also show that this severe mortality event has resulted in the loss of internal positive feedback mechanisms in forest regrowth, as regeneration by both basal sprouting and sapling production are positively affected by the presence of surviving stems and trees. Thus, reforestation may be very slow or fail altogether in areas that have been forced across critical tolerance thresholds and suffered mass mortality of trees. Accordingly, climatically induced alteration of moth defoliation regimes in the mountain birch forest could cause this ecosystem to undergo rapid and potentially persistent changes of state.

You can read the full paper here.

Reference: Vindstad, OPL, Jepsen, JU, Ek, M, Pepi, A, Ims, RA (2019) Can novel pest outbreaks drive ecosystem transitions in northern‐boreal birch forest? Journal of Ecology 107: 1141–1153.

https://doi.org/10.1111/1365-2745.13093

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Picture: A conceptual model for how forest mortality induced by moth outbreaks may be followed by regeneration or a persistent transition to a treeless ecosystem state (photo: Jakob Iglhaut).

Large mammalian herbivores occur in many terrestrial ecosystems and have large impacts on plant communities. They can for example change the availability of growth-limiting nutrients for plants, such as nitrogen (N) and phosphorus (P). Through this, herbivores might change nutrient limitation of plant growth (between N and P limitation) and thereby impact plant productivity.

Reindeer (or caribou) are important herbivores in arctic tundra and they are known to affect soil nutrient availabilities. In this study we examined the effect of long-term light and heavy reindeer grazing on nutrient limitation of plant growth in a Scandinavian arctic tundra. We performed a fertilization experiment, whereby we added N and P in a full-factorial manner, across the two grazing regimes in heath and meadow vegetation.

The productivity of the plant communities showed contrasting responses to our fertilization treatments under light and heavy reindeer grazing. Under light grazing, productivity increased with N and P additions in both the heath and meadow vegetation. Under heavy grazing, productivity increased when N was added in the heath, and increased even more when N was added together with P. In the meadow, productivity increased when P was added, either alone or in combination with N.

These results clearly show that heavy and long-term grazing by reindeer promoted P limitation of plant communities in both the heath and meadow vegetation. The mechanisms behind the shift towards these conditions were not the same in both vegetation types. In the N-poor heath, reindeer increased soil N availability due to a shift towards more N-rich graminoids, while in the meadow reindeer decreased soil P availability. It remains unclear how reindeer decrease soil P, but they may simply export more P from the system than N due to their large P demand for the production of their antlers.

In this study, we have experimentally confirmed that heavy grazing by large mammalian herbivores for a long time has the potential to change nutrient limitation of plant growth. Such a change can have significant ecological impacts, including changes in plant species composition and success of alien plant species.

You can read the article here.

Reference: Sitters, J., Cherif, M., Egelkraut, D., Giesler, R., Olofsson, J. (2019) Long‐term heavy reindeer grazing promotes plant phosphorus limitation in arctic tundra. Functional Ecology

https://doi.org/10.1111/1365-2435.13342

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Picture: Herd of reindeer on the heavily grazed side of the fence in Reisadalen, Norway (photo: Judith Sitters)

Science is suffering a reproducibility crisis that is associated with the impacts of many biases on the outcomes of scientific research. Among these biases, those caused by the properties of the human mind are seldom appreciated by scientists working in the fields of ecology and environmental science. In the present paper, we asked to what extent studies of spatial patterns in insect herbivory suffer from various biases. We answered this question by comparing the outcomes of studies that have used methods of data collection which were potentially prone or not prone to particular biases. This comparison was conducted by meta-analysis of 125 scientific paper containing data on plant losses to insects in different environments. We found that biases emerged when scientists non-randomly selected one or a few plant species when assessing community-wide herbivory or when  scientists did not apply randomisation procedures when selecting leaves for measurements of herbivory. The reported effects were greater when the persons that collected samples and measured leaf damage were aware of the research hypothesis or sample origin than when the collectors/measurers were blinded to these factors. We conclude that our present-day knowledge regarding differences in insect herbivory between environments is considerably biased due to an unconscious tendency of researchers to find support for their hypotheses and expectations, as this generally leads to overestimation of the effects under study. These cognitive biases can be avoided by (i) applying randomisation procedures in sampling and (ii) blinding of the research hypotheses and sample origins. These measures should be seen as obligatory; otherwise, accumulation of biased results in primary studies can ultimately lead to false general conclusions in subsequent research syntheses.

Reference: Zvereva, E., Kozlov, M.V. (2019) Biases in studies of spatial patterns in insect herbivory. Ecological Monographs.

https://doi.org/10.1002/ecm.1361

You can read the full article here.

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Picture: A tropical herbivore (bush cricket) mimicking leaf with insect damage (photo: V. Zverev, 2019)

In this study, Lily Zhao and colleagues use carbon and nitrogen stable isotope analysis to infer summer dietary differences of Svalbard reindeer found in different valleys. Through regression models, they were able to predict how reindeer dietary intake depend on climatic variables, with July temperature being the strongest predictor.  Using stable isotope analysis, they found robust dietary differences between and within reindeer populations separated by less than 50 km in the proportions of non-mycorrhizal vascular plants and mosses. These localized differences in forage availability related to the onset of the growing season as shown through the onset of the growing season in each valley through time. Overall, this publication provides evidence that carbon and nitrogen stable isotope analysis can detect dietary shifts over short time periods and at fine spatial scales, which means that isotopic studies can help monitor herbivory in the changing Arctic.

Reference: Zhao, L.Z., Colman, A.S., Irvine, R.J. et al. (2019) Isotope ecology detects fine-scale variation in Svalbard reindeer diet: implications for monitoring herbivory in the changing Arctic. Polar Biology.

https://doi.org/10.1007/s00300-019-02474-8

You can read the full article here.

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Picture: Couple of Svalbard reindeer females and calf in April
(photo: Justin Irvine)