A cosmic view of ‘tundra gardens’: satellite imagery provides a landscape-scale perspective of Arctic fox ecosystem engineering

Abstract

Animal ecology has benefitted greatly from advancements in remote sensing technology and data availability in recent decades. Most animal ecology studies using remote sensing data have focused on assessing how environmental characteristics shape animal abundance, distribution, or behavior. But the growing availability of high-resolution remote sensing data offers new opportunities to study how animals, in turn, shape ecosystems. We use high-spatiotemporal resolution Sentinel-2 satellite imagery to evaluate the effects of Arctic fox (Vulpes lagopus) denning activity on vegetation. Arctic fox dens are characterized with unique vegetation relative to the surrounding area, presumably due to decades of nutrient accumulation and bioturbation. We use an imagery-derived metric (NDVI) to compare maximum plant productivity and plant phenology patterns on Arctic fox dens vs. reference sites, i.e., points generated within areas of preferred denning habitat as predicted from a habitat selection analysis. We show that high-resolution satellite imagery can be used effectively to quantify the effects of Arctic fox denning activity on vegetation. Plant productivity and the rate of green up were both greater on fox dens compared to reference sites. Productivity on these preferred-habitat (reference) sites was lower than average productivity on the tundra (i.e., random sites), indicating that foxes primarily establish dens in low-productivity areas. Our findings support previous studies that proposed Arctic foxes function as ecosystem engineers in low Arctic ecosystems by converting sites of low productivity into sites of high productivity through their denning activity. Plant productivity was unrelated to recent den occupancy patterns, indicating fox denning activity has long-term legacy effects on plants that last well beyond the lifetime of foxes. We add to the growing body of literature that recognizes predators can be drivers of landscape heterogeneity and influence ecosystem dynamics through patch-scale pathways, such as by concentrating nutrients into localized areas. Our study demonstrates the efficacy of using remote sensing technologies to advance our understanding of the functional roles that predators specifically, and animals generally, occupy in ecosystems.

Patchy indirect effects: predators contribute to landscape heterogeneity and ecosystem function via localized pathways

“…2A), earning them the nickname 'gardens of the tundra.' Arctic fox dens are large enough that the productivity and phenology of den vegetation can be studied from satellite imagery (Johnson-Bice et al 2022b).…”

“…For instance, characteristics of local invertebrates and plants can be directly compared at a carcass or nutrient accumulation patch with nearby reference sites (e.g., (Barry et al 2019;Barton et al 2016;Ben-David et al 1998;Bump et al 2009b;Gharajehdaghipour et al 2016). Alternatively, quantitative models based on characteristics of carcass or nutrient patches could be used to generate reference locations matching those characteristics (Johnson-Bice et al 2022b). Manipulative experiments or remotely sensed trail cameras can also be used to evaluate food web-related effects that occur in and around patches (Cortés-Avizanda et al 2009a;Ruprecht et al 2021;Spencer et al 2021;Zhao et al 2022).…”

“…Surveys are likely one of the easiest ways to quantify home site density. For instance, census survey methods have been used to quantify Arctic fox den density (Johnson-Bice et al 2022b), and thus the landscape-scale impact fox denning behavior has on other tundra species. If predator home sites (or colonies, haul-out sites) vary significantly in size, these size-dependent effects can be accounted for when assessing the broader scale impacts of these patches (Bokhorst et al 2019).…”

Patchy indirect effects of predation: predators contribute to landscape heterogeneity and ecosystem function via localized pathways

“…They instead re‐use the same dens for decades – even centuries – to raise pups (Macpherson 1969). For instance, in Manitoba, Canada, Arctic foxes re‐use the same dens year after year presumably because suitable den sites are restricted to elevated beach ridges that are easy to excavate and prevent the den from flooding during snowmelt (Johnson‐Bice et al 2023). As foxes repeatedly occupy the dens, the decay of accumulated prey remains and fox excrement increases soil nutrient concentration and plant production (Smith et al 1992, Gharajehdaghipour et al 2016), resulting in nutritionally enriched, and locally diverse, plant communities on Arctic fox dens (Bruun et al 2005, Fafard et al 2020).…”

Patchy indirect effects: how predators drive landscape heterogeneity and influence ecosystem dynamics via localized pathways

“…2A), earning them the nickname 'gardens of the tundra.' Arctic fox dens are large enough that the productivity and phenology of vegetation on the dens can be monitored and studied from satellite imagery, including revealing that plants on dens green-up faster relative to reference sites in similar habitats (Johnson-Bice et al 2022b).…”