Browsing intensity of herbaceous forbs across a semi-arid savanna catenal sequence

Abstract

a b s t r a c tEcological models to explain savanna heterogeneity and functioning weakly represent herbaceous forbs, which inevitably created knowledge gaps regarding the diversity and ecology of forbs. Forbs constitute over 70% of semi-arid savanna species richness. The aim of this study was therefore to (i) identify forb species that potentially form part of herbivore diet, and (ii) determine forb browsing intensity across a granitic savanna catenal sequence. Localized contrasts between nutrient-poor dystrophic uplands and nutrient-rich eutrophic bottomlands led us to predict that bottomlands would host forb assemblages dominated by browsed species. Forbs contributed to 78% of the total herbaceous layer species richness, of which 43% were browsed at varying intensities along the catenal sequence. Each topographic zone hosted a unique assemblage of browsed forbs, although browsing intensity was highest in the sodic bottomlands. Some browsed forbs formed prostrate, stoloniferous covers on the bottomlands, creating a lawn type dominated by forbs. Results presented here are the first to identify browsed forb species and to illustrate the turnover in browsing intensity of forbs across a semi-arid savanna catenal sequence. Our results add weight to the need to include herbaceous forbs in savanna management models to better understand savanna resource ecology and heterogeneity.

Forb ecology research in dry African savannas: Knowledge, gaps, and future perspectives

“…The long evolutionary history of large mammalian herbivores and fire events in the structuring and functioning of African savanna vegetation suggests that grazing effects would largely depend on the diversity of wild herbivore guilds (game) and their grazing intensity combined with fire intensity, fire frequency, and fire timing. Several herbivore exclusion experiments (Burkepile et al, 2013;Eby et al, 2014;Kimuyu et al, 2017;Koerner et al, 2014;Odadi et al, 2007;Siebert & Scogings, 2015;Veblen et al, 2016;Young et al, 2005) provide evidence that wild African herbivores affect forb communities invariably due to species-specific forage preferences at different spatial and temporal scales (see also section 3.4). Studies undertaken at the long-term herbivore exclusion plots in Kenya were particularly focused on the relationship between forb cover and different herbivore guild grazing (Kimuyu et al, 2017;Odadi et al, 2007;Riginos & Grace, 2008;Veblen et al, 2016;Young et al, 2005), which revealed negative effects imposed by cattle, eland, and megaherbivore (i.e., elephant) foraging on forb cover and abundance.…”

“…Forb cover alone is, however, a poor predictor of forage preference due to herbivore trade-offs between forage quantity and quality, predator avoidance, and/or interspecific competition (Burkepile et al, 2013;Kimuyu et al, 2017). Moreover, at species level and across spatial scales, there remains a poor understanding of the palatability of forbs (Siebert & Scogings, 2015) including their chemical defense mechanisms and other species-specific herbivore defense traits (Chikorowondo et al, 2017).…”

Elephants in the understory: opposing direct and indirect effects of consumption and ecosystem engineering by megaherbivores

“…Perhaps surprisingly given the attention to their effects on trees and other animals, elephants' interactions with understory plants—which can account for >70% of plant diversity in semi‐arid savannas (Seibert and Scogings )—remain little studied (but see, e.g., Augustine , Veldman et al. , Pringle et al.…”

“…Perhaps surprisingly given the attention to their effects on trees and other animals, elephants' interactions with understory plants-which can account for >70% of plant diversity in semi-arid savannas (Seibert and Scogings 2015)-remain little studied (but see, e.g., Augustine 2003, Veldman et al 2013, Louthan et al 2013. Moreover, although elephants are often cited as a threat to the conservation of endemic plants and the maintenance of pastoral lands (Glover 1963, Johnson et al 1999, Landman et al 2014), many such reports only consider the direct (i.e., consumptive) effects of elephant herbivory.…”

The impact of herbivore exclusion on forb diversity: Comparing species and functional responses during a drought

“…Despite being considered a particularly nutritious and high‐quality food class for browsers in South African savannahs (Du Toit, Rogers, & Biggs, ), and constituting an important part of ungulate diets at certain times of the year (Van Der Merwe & Marshal, ; Odadi, Karachi, Abdulrazak, & Young, ; Scholes, ), forbs are generally overlooked or lumped into a “non‐grass” category in range condition assessments (Scott‐Shaw & Morris, ). Forbs are widely recognised to contribute significantly to the total species richness of herbaceous layers in savannah and grassland systems (Van Coller, Siebert, & Siebert, ; Jacobs & Naiman, ; Van Oudtshoorn, ; Scott‐Shaw & Morris, ; Siebert & Scogings, ; Trollope, van Wilgen, Trollope, Govender, & Potgieter, ; Turner & Knapp, ; Uys, ); however, the functional contribution of this herbaceous life form and its response to direct drivers of herbaceous vegetation dynamics (i.e., herbivory and rainfall variability) remains elusive (Lettow, Brudvig, Bahlai, & Landis, ; Scott‐Shaw & Morris, ; Siebert & Scogings, ).…”