Author: isabel

We use a simple model of coupled carbon and nitrogen cycles in terrestrial ecosystems to examine how explicitly representing grazers versus having grazer effects implicitly aggregated in with other biogeochemical processes in the model alters predicted responses to elevated carbon dioxide and warming. The aggregated approach can affect model predictions because grazer-mediated processes can respond differently to changes in climate from the processes with which they are typically aggregated. We use small-mammal grazers in arctic tundra as an example. We conclude that implicitly aggregating the effects of small-mammal grazers with other processes results in an underestimation of ecosystem response to climate change relative to estimations in which the grazer effects are explicitly represented. The magnitude of this underestimation increases with grazer density. We therefore recommend that grazing effects be incorporated explicitly when applying models of ecosystem response to global change.

Reference: Rastetter, E.B., Griffin, K.L., Rowe, R.J., Gough, L., McLaren, J.R. and Boelman, N.T., 2022. Model responses to CO2 and warming are underestimated without explicit representation of Arctic small‐mammal grazing. Ecological Applications, 32(1), p.e02478. Link to publisher

In arctic tundra, large and small mammalian herbivores have substantial impacts on the vegetation community and consequently can affect the magnitude of carbon cycling. However, herbivores are often absent from modern carbon cycle models, partly because relatively few field studies focus on herbivore impacts on carbon cycling. Our objectives were to quantify the impact of 21 years of large herbivore and large and small herbivore exclusion on carbon cycling during peak growing season in a dry heath tundra community.

When herbivores were excluded, we observed a significantly greater leaf area index as well as greater vascular plant abundance. While we did not observe significant differences in deciduous dwarf shrub abundance across treatments, evergreen dwarf shrub abundance was greater where large and small herbivores were excluded. Both foliose and fruticose lichen abundance were higher in the large herbivore, but not the small and large herbivore exclosures. Net ecosystem exchange (NEE) likewise indicated the highest carbon uptake in the exclosure treatments and lowest uptake in the control (CT), suggesting that herbivory decreased the capacity of dry heath tundra to take up carbon. Moreover, our calculated NEE for average light and temperature conditions for July 2017, when our measurements were taken, indicated that the tundra was a carbon source in CT, but was a carbon sink in both exclosure treatments, indicating removal of grazing pressure can change the carbon balance of dry heath tundra. Collectively, these findings suggest that herbivore absence can lead to changes in plant community structure of dry heath tundra that in turn can increase its capacity to take up carbon.

Reference: Min, E., Wilcots, M.E., Naeem, S., Gough, L., McLaren, J.R., Rowe, R.J., Rastetter, E.B., Boelman, N.T. and Griffin, K.L., 2021. Herbivore absence can shift dry heath tundra from carbon source to sink during peak growing season. Environmental Research Letters, 16(2), p.024027. Link to publisher

While typically thought of as having top-down roles in ecosystems, small mammal herbivores bottom-up roles are often underappreciated. As structure builders, small mammals can impact ecosystem function through alterations of soil nutrient cycling. We examined the effects of small mammal-built structure types (hay piles, runways, latrines) on soil and plant biogeochemical cycling across three tundra ecosystems in northern Alaska.

We found that structures play important roles in regulating soil nitrogen levels, regardless of tundra ecosystem. However, different structures influence soil nutrients in different ways. Hay piles increased soil N and plant P, while latrines influenced soil C, N, and P, and runways mainly affected soil N. We also found strong changes in the cover of structures on tundra during different phases of the small mammal population cycle.

Structures built by small mammals have the ability to increase or decrease biogeochemical cycling rates and arctic ecosystem function. Additionally, changes in the abundance of these structures may create pulses of resource availability during the high phase of the population cycle or may aid in maintaining ecosystem function when animals are rare on the landscape. Our work highlights the roles that small mammals play in the bottom-up regulation of tundra ecosystems and the need to include small mammals in our understanding of ecosystem function.   

You can read the full paper here: https://doi.org/10.1111/1365-2435.14127

Reference: Roy, A., Gough, L., Boelman, N.T., Rowe, R.J., Griffin, K.L. and McLaren, J.R., 2022. Small but mighty: Impacts of rodent‐herbivore structures on carbon and nutrient cycling in arctic tundra. Functional Ecology.

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Text: Austin Roy, University of Texas at El Paso. This paper is a contribution of Team Vole, a group of collaborative scientists seeking to understand the impacts of small herbivores on carbon and nutrient cycling in tundra ecosystems.

Photo credit: Luke Johnson

Northern rangelands are changing fast, and these changes can have profound consequences to species coexistence and management.

In their recent paper, Noémie Boulanger-Lapointe and collaborators compiled occurrence data for the main vertebrate herbivore species present in the highlands of Iceland (sheep, reindeer, pink-footed goose and rock ptarmigan). They used an ensemble model workflow to analyse their distribution and its drivers and produced the first high-resolution national open-source and open-access models for Iceland.

The analyses show that vegetation productivity and soil type were the main drivers of herbivore species diversity across Iceland. The overlapping distributions of sheep and geese point out the potential for wildlife-livestock conflicts and for continued ecosystem degradation at higher elevations even under declining livestock abundance.

Compiling data on herbivore occurrence across Iceland was not a trivial task. Data sources ranged from GPS collar data to citizen science observations and span a long period of time (1861-2021). The data compilation provides the most extensive database on herbivore distribution in Iceland. Together with the open-access modelling workflow, such dataset provides a framework for transparent and repeatable science-based management decisions.

You can read the full paper here: https://www.sciencedirect.com/science/article/pii/S0048969722042371

Reference: Boulanger-Lapointe, N., Ágústsdóttir, K., Barrio, I.C., Defourneaux, M., Finnsdóttir, R., Jónsdóttir, I.S., Marteinsdóttir, B., Mitchell, C., Möller, M., Nielsen, Ó.K. and Sigfússon, A.Þ., 2022. Herbivore species coexistence in changing rangeland ecosystems: First high resolution national open-source and open-access ensemble models for Iceland. Science of The Total Environment, p.157140.

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This paper was part of Noémie’s postdoc project funded by the University of Iceland and is a contribution to the TUNDRAsalad project

Photograph: Sheep and pink-footed geese in Iceland (photo: Maite Gartzia)