Discussion
Effects of prairie dog disturbance on diversity and CWM meansDisturbance by prairie dogs has been shown to affect multiple
vegetation parameters (Connell et al. 2019; Duchardt et al. 2021). The
main objective of this study was to understand the effects prairie
dogs have on the GPCA El Tokio grasslands, using functional and
taxonomic diversity measures. Contrary to our expectations, prairie
dog disturbance showed an effect mostly when it was moderated by
grassland type or season. In fact, only cover had an independent
effect that was slightly higher without prairie dog disturbance. These
findings are in contrast to studies stating that prairie dogs have
negative effects on cattle due to their disturbance activities, e.g.
forage consumption (Vermiere et al. 2004; Derner et al. 2006). Our
findings in fact add to the literature of how environmental variables
play a stronger role than grazing and animal disturbances on
vegetation (Török et al. 2018; Grinath et al. 2019; Jäschke et al.
2020). It is also important to note that there were no independent
effects with at least moderate evidence of prairie dog disturbance on
the CWM height and leaf area, traits that are usually associated with
grazing pressure (Diaz et al. 2007). In fact, CWM leaf area was only
dependent on season and there was no evidence that CWM height was
affected. We found moderate to weak evidence that prairie dog
disturbance did filter C4 cover, which was higher in sites without
prairie dog disturbances. This can be explained by the fact that
prairie dogs prefer to feed on grasses (Mellado et al. 2005). Most
grasses present in the study area are C4, specifically the grasses
with highest cover such as Muhlenbergia villiflora in almost
all grassland types, Sporobolus cryptandrus in agricultural
grasslands, Aristida pansa in calcareous grasslands andBouteloua dactyloides in mountain grasslands, and so a lower
cover would be expected. However, it is important to mention that a
recent study, covering a period of 72 years (Augustine et al. 2017),
showed that some of these C4 species are being outcompeted by C3
species in the long term, and have a larger decline without grazing
effects.
In addition, we found that functional diversity, but not species
diversity, responded to the joint effects of grassland type and
seasonality with prairie dog disturbance, confirming not only the need
of including multiple environmental variables and their interactions to
identify ecosystem complexity (Dainese et al. 2015), but also the
importance of considering functional diversity to further understand the
instances of these patterns (Cadotte et al. 2011). Prairie dog
disturbance moderated FSpe in agricultural grasslands, possibly
explained by the suppression of rapid growing species with extreme
traits (eg. Salsola kali, Machaeranthera tanacetifolia, Kochia
scoparia ) that have higher LA and height and are able to grow and
dominate in agricultural grasslands without prairie dog disturbance.
These species grow despite the lack of ideal water availability and soil
conditions, because they benefit from the gain of resources due to
nutrients from fertilization that remain after abandonment (Laliberte et
al. 2012). Prairie dogs need short vegetation for predator avoidance
(Hoogland, 1995), and their suppressing effect has been shown in
previous literature (Ponce-Guevara et al. 2016; Hale et al. 2020).
However, since the mechanism behind this suppression is unclear, further
studies are needed to determine whether prairie dogs colonize
agricultural grasslands before or after rapid-growing species have a
chance to grow, or whether other mechanisms are at work (e.g. drought
avoidance; Blumenthal et al. 2020). We further found no evidence of
positive or negative effects on FEve and FDiv between grassland types,
nor between any other of the measured conditions. This indicates that
traits were mostly unchanged in their distribution and abundance between
communities in the functional space volume. We can assume that the
redundancy of traits is most likely increasing due to the restrictive
environmental conditions that only well-adapted species can withstand
(Villéger et al. 2008, Mouillot et al.
2013).2) Grassland types as major effect
drivers
We found that grassland types explained most of the effects on plant
functional and taxonomic diversity as well as CWM of traits. There was
very strong evidence that mountain grasslands were positively affected
in almost all measures, usually followed by arid, calcareous and
agricultural grasslands, respectively. This can be explained by the
tendency of mountain grasslands to have leptosol soils, highly variable
slopes as well as higher elevation and lower atmospheric pressure,
leading to higher precipitation and lower temperatures (Gommes, 2002;
Anjos et al. 2015). These conditions are known to often cause an
increase in plant species richness and cover (Speed et al. 2013;
Buzhdygan et al. 2020). In addition, Pando-Moreno et al. (2013) found
that many of the sites in mountain grasslands within GPCA El Tokio had a
lower level of electrical conductivity and absence of gypsum, while
sites that fall within calcareous, arid and agricultural grassland types
had at least some percentage of gypsum in them. The presence of gypsum
most likely acts as a habitat filter for CWM traits, as gypsum soils are
known to limit plant life due to their chemical and physical properties
which restrict plant growth (Escudero et al. 2015). Calcareous and
mountain grasslands have similar filtering effects on perennial and C4
cover; the effect is most likely due to calcareous soils having lower
gypsum levels. Although gypsum soils have been classified as calcareous
soils and vice versa in many soil classification systems (Herrero,
2004), they have distinct effects on biodiversity, and it is even
possible that a mix of both soil types could lead to higher biodiversity
(Meyer et al. 1992; Luzuriaga et al. 2015). Arid grasslands in this
study are also dominated by gypsum soils and have higher temperatures
which, together with low precipitation, result in higher level of
aridity which can act as a strong filter for most CWM traits
(Vicente-Serrano et al. 2012; Munson et al. 2013). Similarly, the strong
filtering effect of agricultural grasslands is most likely due to the
lack of evolutionary history of vegetation, where surrounding landscape
factors might play a greater role in species establishment due to
continuous land use changes over many years (Gustavsson et al. 2007),
paired with the restrictive conditions faced by vegetation on gypsum and
calcareous soils (Meyer et al. 1992). Future studies, disentangling the
climatic and edaphic effects of these grasslands types are needed to
properly understand these patterns (Le Bagousse-Pinguet et al. 2017). On
the other hand, the Inverse Simpson evenness showed an opposite result.
It was higher for agricultural grasslands, which may be explained by the
fact that the index assigns a higher evenness value to communities with
an almost equal amount of rare and dominant species (Smith and Wilson,
1996). Hence, the higher the amount of rare species is, the lower is the
Inverse Simpson evenness (Magurran, 2004).