3) Seasonal effects
Season independently affected species richness, annual and forb cover,
as well as CWM leaf area. Leaf area and other leaf traits are considered
to be directly related to the amount of water plants receive, especially
in dry habitats (Sack and Holbrook, 2006; Wellstein et al. 2017). Most
plants thus have higher leaf area during the wet season. Additionally,
multiple studies have shown that annual and forb species strongly
respond to increased precipitation levels (Yan et al. 2015; Spence et
al. 2016). This is most likely due to their high germination rates and
seed innate and water-controlled dormancy, as well as specific dispersal
adaptations (Freas & Kemp, 1983; Miranda et al. 2009) which together
allow them to grow when the best conditions occur. Our study supports
this body of literature and shows that, in fact, prairie dog disturbance
has no effect on annual plant species and forb cover (Baker et al. 2012;
Perez-Camacho et al. 2012).
Moreover, season modulated the effect of prairie dog disturbance on
RaoQ. We found strong evidence that this index was different between
prairie dog disturbance conditions during the dry season, where
disturbed sites had a lower RaoQ. No difference, however, was found
between disturbance conditions during the wet season. This result is
consistent with a recent three-year study conducted in northeastern
Wyoming (Connell et al. 2019) which showed that overall comparisons of
aboveground biomass were not different between sites with and without
prairie dogs, but that there was strong evidence of lower aboveground
biomass on sites with prairie dogs during years with dry springs, and a
reverse effect during wet springs. Furthermore, we found no differences
in graminoid cover for sites with prairie dog disturbance between the
dry and the wet season. Additionally, there was strong evidence that
sites without prairie dog disturbances increased graminoid cover during
the dry season. This is most likely due to prairie dogs feeding on
graminoids after the wet season, which reduces the grass cover that
could remain in the dry season but allows to maintain an overall stable
graminoid cover throughout the year (Mellado et al. 2005). However, due
to the nature of drylands to have variable precipitation (D’Odorico &
Battachan, 2012), further long-term studies are needed to monitor these
interactions, especially in light of future climate change projections
for the area, which predict an increase in rainfall variability
(Baez-Gonzalez et al. 2018). Likewise, although our results show
interactions between seasonality and disturbance, these effects only
show short-term trends. Sampling multiple years and seasons is necessary
to obtain an overall pattern and identify the mechanisms behind it, as
so many variables are interdependent and most likely have non-linear
effects (Paruelo et al. 2008).
Conclusion To the best of our knowledge, this is the first study to examine the
effects of prairie dog disturbance on vegetation using functional
diversity indices. Like other research, we support the idea that
community trait-based measures are more closely associated with
competitive interactions and environmental filtering, compared to
taxonomy-based approaches. Additionally, we found that prairie dogs had
only a minor negative effect on vegetation cover, even though our study
design focused on burrows and surrounding disturbance, favoring stronger
differences between conditions with and without active prairie dog
colonies. The effects of prairie dogs on C4 and graminoid cover were
particularly demonstrated in the dry season, with the latter negatively
affecting functional diversity only in the dry season, while offsetting
it in the wet season. Together with these previous results, this study
provides further evidence of the large impact environmental conditions
have on these grasslands. Tailored management strategies focusing on
climate change mitigation would be key to conserving and restoring this
threatened, semi-arid ecosystem. As climate change scenarios predict
more consecutive dry days and irregular, more intense rainfall
(Baez-Gonzalez et al. 2018), this could further aggravate plant cover
and functional diversity responses and thus reduce resilience. Hence,
longer-term interannual variation studies combining both types of
diversity measures should be undertaken. Future studies in GPCA El Tokio
can take advantage of the fixed location of prairie dog disturbance, as
well as varying environmental conditions within the relatively small
area, to assess responses of different grasslands to disturbance and
environmental change.