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.