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?