Introduction
Burrowing mammals play an important role in their ecosystem (Beca et al., 2021; Davidson et al., 2012; Root-Bernstein & Ebensperger, 2013), and have been described as keystone species (Whitford & Steinberger, 2010) and ecosystem engineers (Wesche et al., 2007). Burrows form microhabitats that are used as shelter by other vertebrates (Ewacha et al., 2016; Murdoch et al., 2009) and invertebrates (Sorokina & Pont, 2011; Yoshihara et al., 2010). Burrowing mammals can occur in high densities, therefore they also impact their environment through non-engineering effects (Prugh & Brashares, 2012) and can be important prey for predators (Davidson et al., 2012). But probably the most affected group of organisms are plants. The forming of mounds of burrow material and exposing soils through trampling and herbivory alters the competition between plants, promoting pioneer species. Additionally, mounds mainly consist of soil from beyond the surface, which has different chemical properties. Thus, it promotes different plant species than in the surrounding area. Hence, an increase in plant biodiversity and species richness can occur (Ballová et al., 2019; La et al., 2003; Lindtner et al., 2018; Niu et al., 2020; Valkó et al., 2020). Colonies and single burrows offer varied microhabitats and each can support a different plant community (Ballová et al., 2019; Sasaki et al., 2013). Faeces, urine and animal carcasses (in the case of burrows of predators) increase soil nutrient content around the burrow. The vegetation around the burrow profits from this, increasing its nutritional status. In the proximity of burrows, the content of nitrogen and phosphorus in plant biomass is usually 20-30% higher than in the surrounding area (Villarreal et al., 2008; Whicker & Detling, 1988), but some studies show an increase of up to 100% in N and 200% in P (Van Staalduinen & Werger, 2007). On the other hand, the positive effect on plant growth fades above some level of burrow density (Guo et al., 2012), as the high level of disturbance (grazing, digging and trampling) results in the creation of bare soil patches with no vegetation, which are more prone to water evaporation and N loss (Pang et al., 2020). Therefore, the relationship between burrowing animal activity and vegetation cover can be manifold. In habitats where undisturbed areas are completely covered with vegetation, the presence of burrows and mounds obviously decreases total vegetation cover (Louw et al., 2019). However, burrowing animals can also cause an increase in vegetation cover, but it is limited to a certain magnitude of disturbance, above which cover drops (Tang et al., 2019). This type of unimodal relationship is also found for the impact of burrowing animals on species richness or aboveground biomass (English & Bowers, 1994; Guo et al., 2012; Pang & Guo, 2017; Whicker & Detling, 1988) and is considered a classical example to support the intermediate disturbance hypothesis (Connell, 1978; Grime, 1973). Because of the reasons mentioned above, burrowing animals (mainly mammals) were often considered and treated as pests: an approach which has been criticized by scientists due to the need for biodiversity preservation and the role of burrowing species in the environment (Delibes-Mateos et al., 2011; Smith & Foggin, 1999), but also because their negative impact on plant yield and pasture productivity is doubtful (Bagchi et al., 2006; Guričeva, 1985).
Studies on the impact of burrowing animals on vegetation are numerous, but the majority of them were conducted on steppes, arid and semi-arid plains, including pastures or alpine meadows, as well as hot deserts (Beca et al., 2021; Davidson et al., 2012; Mallen-Cooper et al., 2019; Platt et al., 2016; Root-Bernstein & Ebensperger, 2013). Almost no studies show the ecological roles of burrowing mammals in extremely arid high-altitude habitats in an early stage of soil formation and plant succession, where plant productivity is limited by water shortage and low temperatures, rather than nutrient availability and competition for space. High-altitude habitats are among the regions where climate change is the most rapid (Kohler et al., 2010). Melting glaciers are exposing mineral soils, which are slowly being colonised by microorganisms and plants (Egli et al., 2012). Whether this process can be facilitated by burrowing ecosystem engineers is unknown. The first step to answering this question should be a study of the impact of burrowing mammals on plant communities already established in glacier forelands.
We studied the response of vegetation to the activity of long-tailed marmots (Marmota caudata, hereafter referred to as marmots, as they are the only species from this genus in the study area) inhabiting the valley of the Koksoy river, which flows from the Ujsu glacier in the Eastern Pamir mountains – an extremely dry habitat with hampered soil development and plant succession (Kabala et al., 2021). We used an unmanned aerial vehicle to take images of the area and map vegetation. Given the initial character of our study, this method would allow us to preliminarily assess the relationship between the presence of marmot burrows and vegetation cover in a high number of replicates (burrows), without indicating the precise cause of this relationship. We also measured N and P content in aboveground green plant biomass in a distance gradient from burrow entrances, to check whether plants are utilizing the nutrient input from marmot faeces. We also used stable isotope composition as a marker of animal-derived N. Preliminary field observations of vegetation in the study area suggested, that the impact of marmots on plants is not pronounced, as there was no visible increase in vegetation properties around burrows (Suska-Malawska et al. unpubl.), unlike in many other studies (Ballová et al., 2019; Fafard et al., 2019). Also, faeces were seen mainly on mounds, close to the burrow entrance. Therefore, we planned our study on a small scale, in 20-meter buffers around burrow entrances.