Abstract:
In savannas, ruminant herbivores can have divergent impacts on tree recruitment and subsequent woody cover. Whereas heavy grazing by cattle results in woody thickening, intensive grazing by wildlife instead tends to be associated with lower woody cover.
To disentangle why woody cover is low in areas heavily grazed by wildlife, we investigated (a) whether grazing lawns attract indigenous mammalian browsers, and if a preference for short-grass habitat decreases with browser body mass as predator susceptibility decreases; and (b) whether browser attraction to grazing lawns translates into the suppression of woody plants, including seedlings and saplings, thus maintaining the openness of heavily grazed short-grass areas.
In Kruger National Park, South Africa, we contrasted browser abundance (using dung counts) on grazing lawns and on low-herbivory sites characterised by tall grass. Additionally, a herbivore exclosure experiment was set up to investigate the combined impact of browser removal and grass height habitat type on seedling survival and sapling growth of a dominant woody plant species. Finally, in Hluhluwe-iMfolozi Park (HiP), we examined the long-term (10 years) impact of browser removal on the growth rates of a range of woody species, monitored across ten sites along a gradient of herbivory ranging from grazing lawn to tall grass.
Steenbok and impala selected short- over tall grass as preferred browsing sites, while elephant preferred tall grass. Browser abundance on short grass decreased with browser body mass, indicating that predator avoidance might be a key factor driving mesoherbivores to utilise grazing lawns.
Seedling survival was lowest on grazing lawns when browsers were present, with mortality occurring in two out of every three seedlings. Similarly, sapling growth was lowest on grazing lawns, although browser removal had no significant effect. Evidence for increased browser impact on grazing lawns was clearest from our long-term herbivore exclosure experiment in HiP, which demonstrated that browsers strongly modify the growth rates of woody plants in short-grass habitats.
Synthesis. These results provide support for the hypothesis that browsers, particularly browsing mesoherbivores and mixed feeders, are attracted to short-grass habitats, and that they help maintain grazing lawn openness by suppressing seedling survival and woody plant growth where grass is kept short by grazers.
Description:
SUPPORTING INFORMATION: Table S1. Proportion short grass calculated as the mean proportion of grass height below 10 cm, recorded at ten sites in HiP (Hluhluwe and iMfolozi Game Reserves). The mean number of grazer dung piles per year (species: buffalo, blue wildebeest, impala, warthog, white rhino and zebra) are shown for each site. Values in brackets represent standard errors.
Figure S1. The effect of grass height and exclosure status (fenced vs. unfenced) on tree height gain (mean ± SE). Browsing impact on tree height gain was higher in areas with short grass i.e. browsers utilised trees in short-grass ecosystems more than in tall-grass ecosystems. Results of linear regression analyses are displayed for the unfenced treatment of two dominant woody plant species: a) D. cinerea (R2 = 0.38, p = 0.05) and b) A. nilotica (R2 = 0.44, p = 0.11), and for two woody plant functional types: c) fine-leaved (R2 = 0.33, p = 0.09) and d) broad-leaved (R2 = 0.60, p = 0.02), across 10 sites in Hluhluwe-iMfolozi Park for the period 2000-2009.
DATA AVAILABILITY STATEMENT: Data are available from the Dryad Digital Repository https://doi-org.uplib.idm.oclc.org/10.5061/dryad.76hdr7st3 (Voysey et al., 2020).