Introduction
Prairie dogs (Cynomys sp .) have evolved together with grasslands (Goodwin, 1995; Castellanos et al. 2016; Seersholm et al. 2020) and provide key ecosystem engineering activities which make them valuable for grassland conservation (Davidson, Ponce, Lightfoot et al. 2010; Davidson et al. 2012; Martinez-Estevez et al. 2013). Their grazing and burrowing activities (from here on summarized as disturbances) directly and indirectly alter habitat structure crucial for other species’ presence (Davidson & Lightfoot, 2007; Duchardt et al. 2019), prevent shrub encroachment (Weltzin et al. 1997; Ceballos et al. 2010; Ponce-Guevara et al. 2016), maintain landscape heterogeneity (Bangert and Slobodchikoff, 2000; Davidson and Lightfoot, 2006; Gervin et al. 2019) and increase fodder quality for cattle by reducing leaf age, which increases the plants nitrogen intake (Sierra-Corona et al. 2015). Nevertheless, their disturbances have also been shown to have negative effects on vegetation structure and characteristics that are prioritized by ranchers, e.g. by reducing biomass and cover of grasses as well as by increasing forb and annual species cover, which reduces fodder quantity (Connell et al. 2019). All this has led to the belief that prairie dogs are degrading grassland vegetation, altering agricultural fields and competing with livestock, resulting in the species being threatened by recreational shooting and poisoning by ranchers and farmers (Miller et al. 2007). Although some conservation measures have been taken to preserve them (e.g. through agri-environmental schemes and the designation of conservation areas), these have not been able to change the socio-ecological views of local communities (Miller, Ceballos & Reading, 1994; SEMARNAT, 2018).
Many grasslands are disturbance-adapted ecosystems (Gibson, 2009), on which small-scale disturbances by herbivorous burrowing mammals have played a fundamental role for vegetation structure (Davidson et al. 2012). As climate changes and its variability increases, the mechanisms underlying organisms’ interactions with their habitat will likely shift. Understanding the mediating role of prairie dog disturbance on vegetation structure, and its interaction with environmental conditions through time, will increase knowledge on the risks and vulnerability of grasslands, allowing for nature-based solutions that can be applied to grassland management (Pörtner, 2021). Despite this, it is only recently that studies have started to include interactions between disturbance and multiple environmental conditions like soil, precipitation and temperature (Buzhdygan et al 2020; Jaschke et al. 2020; Ahlborn et al. 2021), and very few have explored the role burrowing herbivorous mammals have on these changes (Coggan et al. 2018).
Plant functional traits, i.e. physiological, phenological and morphological features, mediate between habitat disturbances and ecosystem functions and hence call for exploring trait variations within communities (Mouillot et al. 2013; Hanisch et al. 2020). For instance, plant responses to grazing have shown to directly alter the distribution and variation of specific leaf area (SLA) as well as leaf nitrogen content (van der Plas et al. 2016). However, impacts on these traits would easily be ignored by looking solely into taxonomic diversity, which in most cases is not comparable between communities that are dissimilar or not complementary to each other, making generalization difficult (Chao et al. 2000). Whereas, functional diversity indices summarize species’ traits and their abundances via their distribution within the functional space, allowing to explore complementary characteristics between communities (Mouchet et al. 2010). Furthermore, the distribution of trait variations can be captured through environmental filtering (the act of environmental conditions acting as a filter that allow only certain traits to persist; Zobel et al. 1997) allowing for the identification of niche processes (Mason et al. 2005; Villéger et al. 2008; Mouillot et al. 2013). Paired with community-weighted means (CWM), we can analyse community composition and their trait-environment relationships (Funk et al. 2017; Miller et al. 2019).
Here, we focus on traits that have proven useful to identify vegetation responses to grazing, semi-arid habitats and seasonality effects. Namely, specific leaf area (SLA) and vegetative height are proxies for multiple ecosystem functions like biomass production, erosion control, fodder quality, soil fertility, water regulation and competitive ability, and traits such as photosynthetic pathway, life history and growth form relate to temperature, CO² levels, available nutrients, and water efficiency, as well as timing and duration of survival (Moles et al. 2009; Hanisch et al. 2020). Furthermore, the selected traits are key to relate vegetation structure dynamics with ecosystem conditions within semiarid grasslands which have evolved through droughts and disturbance regimes since the Pleistocene, developing high numbers of C4, perennial and shrub species (Gibson, 2009). To the best of our knowledge, no other study has yet examined the functional relationship between different grassland types and the response of vegetation to disturbance by prairie dogs. Furthermore, no study that we know of has analysed how these relationships change over the seasons.
Our aim here was to identify vegetation responses to disturbance by the prairie dog species Cynomys mexicanus (endemic to northeastern Mexico) in different grassland types and seasons using taxonomic and functional diversity as well as CWM traits. We assume that functional diversity metrics will be more sensitive and will help to provide an in-depth understanding of the mechanisms or patterns of community changes. Understanding these complex ecosystem interactions will help inform future management and conservation strategies to protect prairie dogs and to maintain vegetation diversity and the essential functions of semi-arid grassland under future environmental changes. We therefore used the traits mentioned above and calculated plant functional diversity and CWM traits for the prairie dog-dominated grasslands within GPCA El Tokio, to answer the following questions: 1) Is there an effect of prairie dog disturbance on taxonomical and functional plant diversity, and how are CWM traits being filtered? 2) Is the effect constant across different grassland types? 3) Does season influence these effects?