Causality of the relationship between grasses and herbivores and
methodological issues
When interpreting our results, it needs to be borne in mind that the
positive relation between herbivore species richness and grass species
richness can act in both directions. First, herbivores may support grass
richness by acting as a dominance control mechanism, suppressing
potentially dominant species, and increasing microsite heterogeneity
(e.g., Chaneton and Facelli 1991, Ritchie and Olff 1999, Jacobs and
Naiman 2008, La Plante and Souza 2018). This is further supported by the
positive effect of herbivore species richness manifest on basalts and
crests, where the most intense competition among grass species can be
assumed because this habitat/bedrock combination is where grasses reach
the highest dominance; this also holds for their relative dominance with
regards to other species (Hejda et al. 2022). Further, crests host fewer
grazers than both seasonal and especially perennial rivers, and this may
also contribute to their positive effect on grass species richness –
presuming a unimodal relation between grazing intensity and grass
richness (e.g. Frank 2005), the herbivores-grasses relationship can be
expected to be positive under the conditions of relatively low herbivore
abundances. Lastly, we believe that the presented causality of the
relationships also applies to perennial and seasonal rivers due to
abundant herbivores, such as elephants (MacFadyen 2019), that spend more
time by the rivers, and their grazing selectivity is likely to decrease
with grazing intensity (Cornelissen and Vulink 2015).
Alternatively, the richness of grasses can affect the richness of
herbivores by providing a more diverse food supply, following the logic
of the predator-prey relationship (e.g., Kallay and Cohen 2008, Malard
et al. 2020). It is impossible to solve this dilemma using comparative
data, and even exclosure experiments tend to give ambiguous results
(Chikorowondo et al. 2017, Li et al. 2017, Fenetahun et al. 2021).
Moreover, it is likely that both mechanisms with opposite directions are
in play with differing importance depending on the specific
environmental settings. It is probably more important in crests, where
grazing selectivity is expected to be higher under low grazing
intensities. In general, we suggest that the relationships between
grasses and herbivores may work in both directions, but in environments
with negative relationships, the effects of herbivores on vegetation
prevail.