Determinants of seasonal changes in availability of food patches for elephants (Loxodonta africana) in a semi-arid African savanna

Abstract

Loss of biodiversity caused by impact of elephants (Loxodonta africana) on African woodlands may require a management response, but any action should be based on an understanding of why elephants choose to utilise trees destructively. Comprehension of elephant feeding behaviour requires consideration of the relative value of the plant groups they may potentially consume. Profitability of available food is partly determined by the time to locate a food patch and, therefore, as a foundation for understanding the influence of food availability on diet selection, key controls on the density of grass, forb, and browse patches were investigated across space and time in a semi-arid African savanna. Density of food patches changed seasonally because plant life-forms required different volumes of soil water to produce green forage; and woody plants and forbs responded to long-term changes in soil moisture, while grasses responded to short-term moisture pulses. Soil texture, structure of woody vegetation and fire added further complexity by altering the soil water thresholds required for production of green forage. Interpolating between regularly-timed, ground-based measurements of food density by using modelled soil water as the predictor in regression equations may be a feasible method of quantifying food available to elephants in complex savanna environments.

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

“… Superscripts refer to selected studies only. 1 Buitenwerf et al (), 2 O'Connor (), 3 Gilo and Kelkay (), 4 Jacobs and Naiman () , 5 Odadi et al (), 6 Young et al (), 7 Zerbo et al (), 8 Dreber and Esler (), 9 O'Connor, , 10 Shackleton (), 11 Walker and Knoop (), 12 Belsky et al (), 13 Augustine (); 14 Ludwig, De Kroon, Berendse, & Prins, (), 15 van der Waal et al (), 16 Linstädter et al (), 17 Muvengwi et al (), 18 Masunga et al () , 19 Smit (), 20 Dreber et al (), 21 Rutherford et al (), 22 Hejcmanová et al (), 23 Odadi et al (), 24 Tessema et al (), 25 Angassa and Oba (), 26 Keya (), 27 Metzger, Coughenour, Reich, and Boone (), 28 Hanke et al (), 29 Tessema et al (), 30 Wesuls et al (), 31 Riginos et al (), 32 Riginos and Grace (), 33 Koerner et al (), 34 Belsky et al (), 35 Linstädter et al (), 36 Britz and Ward (), 37 Burkepile et al (), 38 Eby et al (), 39 Mlambo et al (), 40 Chikorowondo et al (), 41 Clegg and O'Connor (), 42 Traill (). …”

“…Similarly, grasses with the ability of abrupt responses to soil water pulses have an advantage over forbs when rainfall conditions are favorable (Clegg & O'Connor, ; Masunga et al, ; O'Connor, ). Many forbs respond rather to medium‐term, seasonal soil water fluctuations (Clegg & O'Connor, ; Walker & Knoop, ) and thus become outcompeted by the growing grass. Nonetheless, forbs can recover well following sustained periods of drought (O'Connor, ) due to a variety of drought‐tolerant traits.…”

“…Fire performs varying effects on forb communities. Their divergent responses to grazing and fire on different substrates (Clegg & O'Connor, ; Eby et al, ; Masunga et al, ; Nepolo & Mapaure, ), by different herbivore guilds (Koerner et al, ), and fire return intervals (Burkepile et al, ) make forb community responses unpredictable in most African savannas. Fire, in its various forms and interactions, may result in reduced forb richness as unpalatable, perennial forb species become abundant and dominate over grasses (Eby et al, ; Masunga et al, ).…”

“…Fire, in its various forms and interactions, may result in reduced forb richness as unpalatable, perennial forb species become abundant and dominate over grasses (Eby et al, ; Masunga et al, ). Fire‐induced forb community changes are therefore suggested to be controlled by species‐specific traits since fire is known to inhibit the establishment of certain forb species, while promoting the growth, germination, or seed set of others (Clegg & O'Connor, ).…”

“…They also include a high proportion of toxic species, at least for humans and livestock (e.g., forbs contribute to over 60% of the most common toxic plants in South Africa; Van Wyk, Van Heerden, & Van Oudtshoorn, ), whose population dynamics can be an important factor in range management. Forbs provide forage for several herbivore guilds—from insects (Andersen & Lonsdale, ) to megafauna (Clegg & O'Connor, ; Landman, Kerley, & Schoeman, )—as they are a nutritious food class for browsers and mixed feeders in savannas (Du Toit, ), and may constitute an important part of ungulate and cattle diet at certain times of the year (Odadi, Karachi, Abdulrazak, & Young, ; Odadi, Young, & Okeyo‐Owuor, ; Veblen, Porensky, Riginos, & Young, ). Furthermore, forbs constitute the largest component of herbaceous species richness in grassland (Bond & Parr, ; Koerner et al, ; Pokorny, Sheley, Svejcar, & Engel, ; Scott‐Shaw & Morris, ; Zaloumis & Bond, ) and savanna ecosystems (Van Coller, Siebert, & Siebert, ; Pavlovic, Leicht‐Young, & Grundel, ; Shackleton, ; Uys, ), which may vary little across gradients of tree and shrub cover (Dreber, Van Rooyen, & Kellner, ) or grazing intensities (Hanke et al, ; Rutherford, Powrie, & Thompson, ).…”