Rodents have long been recognized as ecosystem engineers, (e.g., Dickman 1999; Legagneux et al. 2012) important for regulating biospheric activity. They are key species, affecting plant and predator abundances (Krebs 2011). They aerate and increase groundwater recharge through soil turbation, aid in decomposition and nutrient cycling, and provide habitats for other species. They control plant productivity, species richness and composition, and promote ecological succession (e.g., Zhang et al. 2003; Prugh and Brashares 2012; Tschumi et al. 2018; Ballová et al. 2019).
Rodent impact is especially predominant in the Arctic, where lemmings (Dicrostonyx spp. and Lemmus spp.) and voles (Microtus spp. and Myodes spp.) modify tundra plant communities influencing plant diversity, structure, and nutrient dynamics (Tuomi et al. 2019; Lindén et al. 2021). Indeed, small rodents often consume even more plant materials than larger herbivores, and they can have dramatic impacts on vegetation – especially during population peaks (Petit Bon et al. 2020). Not only do rodents eat tundra plants, but they also alter vegetation to create structures such as winter nest-storages, runways, and latrines (Roy et al. 2022). An understanding of high-latitude terrestrial ecosystems thus demands study of impacts of rodent activity on plant communities.
There have been hundreds of studies of the effects of rodents on vegetation in northern ecosystems (see Soininen and Neby, 2024 for review). Many of these focus on rodent population cycle dynamics and the complex interactions between these herbivores and the plant communities they rely on (Andreassen et al. 2021) – notwithstanding the role of predators (e.g., Hanski et al., 2001). In particular, reduced plant quality has been hypothesized to lead to rodent population cycles if high abundance of rodents leads to lower-quality foods, either through fallback on lower quality plant species or by decreased quality of the foods they prefer.
It has been long understood that northern latitude plants defend against herbivory by decreasing nutritional quality to herbivores (Schultz 1964; Batzli et al. 1980). In some plants, this happens through induction of defenses. Graminoids, for example, often respond to herbivory by increasing their silica content (Massey and Hartley 2006; Soininen et al. 2013a). This can wear teeth(Calandra et al. 2016) and lead to decreased digestive efficiency and calorie assimilation, which might explain decreased body mass, and increased mortality (Huitu et al. 2014). Phenolics have also been suggested as plant defenses against rodent activity (Oksanen et al. 1987). These defensive compounds bind to a herbivore’s digestive enzymes and inactivate them (Velickovic and Stanic-Vucinic 2018); though support for induction of phenolics in response to herbivory by rodents is not clear (Saetnan and Batzli 2009; Huitu et al. 2014).
An alternative mechanism leading to reduced plant quality due to rodent herbivory is changes in the nutritional content. For instance, structural compounds, such as lignin and cellulose, reduce plant digestibility and the energy that an herbivore can extract from a plant (e.g., Distel et al. 2005). In this light, there has been surprisingly little work done to consider the impact of herbivory by rodents on plant fiber content (but see Bergeron & Jodoin 1989 and Hambäck et al 2002). Alternatively, plant quality may increase due to rodent herbivory, as plants may contain more protein where they have been fertilized by small rodent feces. See, for example, Tuomi et al. (2019), who found high rodent density associated with higher plant N content.
Our study presents a first look at the impacts of rodent herbivory on crude protein, and acid detergent fiber (ADF - cellulose and lignin) on common tundra graminoids in the tundra meadows of the Varanger Peninsula, Norway. We selected the graminoid species based on growth form: the mat grass, Calamagrostis sp., the sedge, Carex nigra, and the tussock grass, Deschampsia cespitosa.