Wood Bison Have An Effect On Vegetative Species Biodiversity

The Herd May Be Improving Their Forage Opportunity

Graminoids, Browse, and Forb species represent distinct plant communities that Bison forage on, graminoids being the most preferred. While graminoid coverage is highest in the recovering loaf treatment, browse species coverage is highest in the meadow treatment and forb species coverage is similar in all treatments besides forest, in which it is lower (Figure 18). The meadow treatment also has considerable browse species presence which, while not the most important forage item for the herd, is still relevant given its higher protein content (Hecker et al., 2020). This gives further credence to improved forage opportunities, as the herd is more active in meadow treatments than surrounding forest treatments, indicating their activity again may be increasing their forage opportunity. 

Well we cannot say for sure if the herd is improving their forage opportunities simply through their own activities, our analysis does point in this direction. More work exploring this area of study would be needed to confirm this.

The Herd May Be Affecting Successional Pathways

With regards to succession,  it seems as though the recovering loaf may becoming its own distinct relative composition of species with a separate plant cover composition, both the meadow and forest are dominated by woody browse species, while the recovering loaf has a far more homogenous distribution of cover types (Figure 18). The successional pathways at play here are one of two:

1. Meadow -> disturbance by bison -> loaf -> recovering loaf

2. Forest -> disturbance by bison -> loaf -> recovering loaf 

The loaf treatment has a much higher amount of bare earth, which seems to come at the expense of the browse species in the meadow and forest treatments. Particularly in the meadow to loaf comparison, it appears that the increase in bare earth from the meadow section is entirely made up for by the reduction in browse cover; while the levels of forbs is slightly higher and the level of graminoids is remarkably similar. It is when the recovering loaf is factored in that the successional changes become apparent, as all 4 cover types range from between 10% for the browse species and 32% for the forb species, this narrow range (20%) of percent cover indicates a very even ecosystem which differs substantially from the meadow and forest which are dominated by the browse type. 

Conclusion & Implications

We found that the recovering loaf treatment did show an increase in species richness relative to the loaf treatment, indicating that as succession progresses species diversity may increase too, which falls in line with previous evidence suggesting bison increase species richness (Gottlieb et al. 2024, Ratajczak et al. 2022, Knapp et al. 1999). However, species richness is still below that of the meadow treatment. We also found that areas of high bison presence, the meadow, had higher species richness than those of lower bison presence, the forest treatment. It remains to be seen if the species richness of the recovering loaf will reach that of the meadow or surpass it. As the meadow is theorized to be a direct result of bison presence(Hillman, personal communication, 2024), long term study of this habitat is essential.

It is commonly accepted that bison have an effect on the vegetation composition of their environments, and this is particularly true here, as the disturbance by bison is what creates the loafs in the first place. Specifically, we found evidence that the herd may be affecting successional pathways in their habitat. When examining the relative proportions of vegetative cover types between treatments, we found that recovering loaf had a more even distribution of vegetation cover types than any other treatment (Figure 17). This indicates that bison activity may actually be affecting the successional pathways of their environment, and that the recovering loaf may be transitioning to a plant community that is distinct from the meadow. The loaf and meadow treatments show reduced graminoid presence consistent with foraging by bison, with a reduction in browse species in the loaf being compensated by an increase in bare soil.

The recovering loafs hold approximately three times more graminoid cover than the next closest treatments of  the meadow and loaf. Although, the recovering loaf also has low occupancy by the bison given that they spend most of their time in the meadow and loaf treatments. The high presence graminoids in the recovering indicate that, as the herd allows these areas to recover, graminoids begin to take over at higher levels than previous to bison activity. As graminoids are the preferred forage of the herd (Strong and Gates 2009, Jung 2015), this provides evidence that the disturbance caused by the herd is improving forage conditions, although if the bison are not foraging in the recovering loaves they do not gain this benefit, therefore more study is required to look at bison occupancy of the treatments. The Ronald lake herd does also feed on the browse species, particularly in the summer, therefore a variety of vegetation types could be beneficial (Hecker et al. 2021, Hecker et al. 2020).

This herd has been designated by both provincial and federal governments as important to maintain (ECC Canada, 2018). As one of the few free ranging disease free herds of wood bison in Canada, it it critical to the continued survival of the species to maintain multiple large herds that do not interact with each other in order to guard against potential shocks, like disease and harsh winters, and to maintain genetic diversity (COSEWIC, 2014). Climate change will only make these shocks worse, therefore increased herd size is highly important, as the Ronald Lake herd is still below the target size in management plans. The Canadian government's plan for maintaining the wood bison relies on several large herds acting as redundancies in case a herd is extirpated. Conservation decision makers must understand how this herd forages and the effect it has on forage species in order to plan for the future growth of the herd, an understanding which this analysis has focused on.