Abstract
Understanding how herbivores shape plant biomass and distribution is a core challenge in ecology. Yet, the lack of suitable remote sensing technology limits our knowledge of temporal and spatial impacts of mammal herbivores in the Earth system. The regular interannual density fluctuations of voles and lemmings are exceptional with their large reduction of plant biomass in Arctic landscapes during peak years (12–24%) as previously shown at large spatial scales using satellites. This provides evidence that herbivores are important drivers of observed global changes in vegetation productivity. Here, we use a novel approach with repeated unmanned aerial vehicle (UAV) flights, to map vegetation impact by rodents, indicating that many important aspects of vegetation dynamics otherwise hidden by the coarse resolution of satellite images, including plant–herbivore interactions, can be revealed using UAVs. We quantify areas impacted by rodents at four complex Arctic landscapes with very high spatial resolution UAV imagery to get a new perspective on how herbivores shape Arctic ecosystems. The area impacted by voles and lemmings is indeed substantial, larger at higher altitude tundra environments, varies between habitats depending on local snow cover and plant community composition, and is heterogeneous even within habitats at submeter scales. Coupling this with spectral reflectance of vegetation (NDVI), we can show that the impact on central ecosystem properties like GPP and biomass is stronger than currently accounted for in Arctic ecosystems. As an emerging technology, UAVs will allow us to better disentangle important information on how herbivores maintain spatial heterogeneity, function and diversity in natural ecosystems.
Highlights
Quantifying the heterogeneity at scales relevant to mammalian herbivores
One spectacular example of such fluctuations are vole and lemming cycles that are a common feature of Arctic ecosystems[22], and drive synchronous interannual fluctuations in biomass of field-layer vegetation with reductions of (12–24%) during cycle peak years, which are visible as a reduced normalized difference vegetation index (NDVI) in satellite images in the following y ear[23]
We used repeated Unmanned aerial vehicles (UAVs) flights equipped with RGB and multispectral sensors in four 15–21 hectare large areas during (2018) and after (2019) a vole and lemming peak in the tundra in northernmost Sweden to survey the impact of rodent peaks with a high spatial resolution at landscape scale (Fig. 1)
Summary
Quantifying the heterogeneity at scales relevant to mammalian herbivores. The strong population fluctuation of many herbivores in northern ecosystems[19] provides a unique opportunity to study the impact of herbivores at large spatial scales by comparing conditions before and after a population peak using remote sensing techniques. The fairly small vole and lemming peak in 2018 resulted in a relatively small reduction in plant biomass (4–16%), GPP (3–12%) and NDVI (0.7–4%) compared to previous peaks during the last two decades (Table 1)[23], but rodents still impacted between 4.5 and 18.9% of the landscape according to our best estimate from the UAV data (Fig. 3, Table 1) This supports previous studies identifying voles and lemmings as drivers of Arctic vegetation[23,33,35,36,37], and indicate that even small peaks have substantial impacts on ecosystems it would be hard to record using traditional methods linking field plots or s atellites[23]. The level of mapped rodent impact differed across the different
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