Development and Parameterization of a Rain- and Fire-driven Model for Exploring Elephant Effects in African Savannas

Abstract

We describe the development and parameterization of a grid-based model of African savanna vegetation processes. The model was developed with the objective of exploring elephant effects on the diversity of savanna species and structure, and in this formulation concentrates on the relative cover of grass and woody plants, the vertical structure of the woody plant community, and the distribution of these over space. Grid cells are linked by seed dispersal and fire, and environmental variability is included in the form of stochastic rainfall and fire events. The model was parameterized from an extensive review of the African savanna literature; when available, parameter values varied widely. The most plausible set of parameters produced long-term coexistence between woody plants and grass, with the tree-grass balance being more sensitive to changes in parameters influencing demographic processes and drought incidence and response, while less sensitive to fire regime. There was considerable diversity in the woody structure of savanna systems within the range of uncertainty in tree growth rate parameters. Thus, given the paucity of height growth data regarding woody plant species in southern African savannas, managers of natural areas should be cognizant of different tree species growth and damage response attributes when considering whether to act on perceived elephant threats to vegetation.

Elephant spatial use in wet and dry savannas of southern Africa

“…African savannas can be divided into dry, climatically driven and wet, disturbance‐driven systems (Sankaran et al , 2005). The nature and strength of interactions between species in savanna ecosystems therefore differ according to rainfall (see Sankaran et al , 2005; Baxter & Getz, 2008). Because elephant spatial utilization is influenced by rainfall (Western & Lindsay, 1984; Thouless, 1998; Verlinden & Gavor, 1998), differences in elephant spatial use between wet and dry savannas may partially explain differences in the impacts of elephants among places.…”

Precipitation and elephants, not fire, shape tree community composition in Serengeti National Park, Tanzania

“…Many savannas are also prone to high rates of disturbance (notably fire), and the ability of trees to survive or escape disturbance events is strongly size dependent (Pellew 1983, Holdo 2005). There has therefore been a significant interest on tree woody cover dynamics in relation to disturbance regimes (Baxter & Getz 2008, Holdo et al 2009a, Wakeling et al 2011. However, less effort has been devoted to understanding the processes that drive tree community composition in these systems, despite the fact that savanna tree communities can be highly diverse and variable across space (Higgins et al 2012, Scholtz et al 2014).…”

“…Many savannas are also prone to high rates of disturbance (notably fire), and the ability of trees to survive or escape disturbance events is strongly size dependent (Pellew , Holdo ). There has therefore been a significant interest on tree woody cover dynamics in relation to disturbance regimes (Baxter & Getz , Holdo et al . , Wakeling et al .…”

Seasonal Selection Preferences for Woody Plants by Breeding Herds of African Elephants (Loxodonta africana) in a Woodland Savanna

“…Elephant feeding behaviour in combination with natural processes, for example, climate and fire, is the architects in different African landscapes and has been the focus of numerous studies describing elephant diet [6][7][8][9][10][11]. However, many of these studies failed to define selection preferences at the plant species level and only describe preferences at the broad level of growth forms [12,13].…”