Forest or the trees: At what scale do elephants make foraging decisions?

Abstract

For herbivores, food is distributed spatially in a hierarchical manner ranging from plant parts to regions. Ultimately, utilisation of food is dependent on the scale at which herbivores make foraging decisions. A key factor that influences these decisions is body size, because selection inversely relates to body size. As a result, large animals can be less selective than small herbivores. Savanna elephants (Loxodonta africana) are the largest terrestrial herbivore. Thus, they represent a potential extreme with respect to unselective feeding. However, several studies have indicated that elephants prefer specific habitats and certain woody plant species. Thus, it is unclear at which scale elephants focus their foraging decisions. To determine this, we recorded the seasonal selection of habitats and woody plant species by elephants in the Ithala Game Reserve, South Africa. We expected that during the wet season, when both food quality and availability were high, that elephants would select primarily for habitats. This, however, does not mean that they would utilise plant species within these habitats in proportion to availability, but rather would show a stronger selection for habitats compared to plants. In contrast, during the dry season when food quality and availability declined, we expected that elephants would shift and select for the remaining high quality woody species across all habitats. Consistent with our predictions, elephants selected for the larger spatial scale (i.e. habitats) during the wet season. However, elephants did not increase their selection of woody species during the dry season, but rather increased their selection of habitats relative to woody plant selection. Unlike a number of earlier studies, we found that that neither palatability (i.e. crude protein, digestibility, and energy) alone nor tannin concentrations had a significant effect for determining the elephants’ selection of woody species. However, the palatability:tannin ratio was important for selection of woody species during the dry season. Ultimately, our results indicate that elephants make top-down foraging decisions by first selecting landscapes, then habitats within those landscapes and finally species within habitats. As a result, the impacts they can have across environments are likely the result of the selection of plant species within preferred habitats.

Introduction

Herbivores forage across environments where the distribution and quality of food varies both spatially and temporally (Senft et al., 1987). Spatially, food is distributed in a hierarchical manner ranging across plant parts, species, feeding stations, patches, foraging sites, habitats, landscapes and regions (Senft et al., 1987, Bailey et al., 1996, Owen-Smith, 2002). Temporally, food quality and availability varies between seasons. Within these seasons, the utilisation of forage is dependent on the scale at which herbivores make foraging decisions. Herbivores may focus initially at large scales (e.g. habitats or landscapes), which constrain smaller scale decisions (e.g. species and patches), or they may focus on small scales, such as individual bites of specific plant parts or species, which may ultimately lead to large-scale patterns (Senft et al., 1987, Shipley, 2007).

A key factor that influences the foraging decisions of mammalian herbivores is body size. Due to lower mass-specific metabolic requirements and a larger gut volume (i.e. potentially longer retention time), large herbivores are better able to survive on lower quality food compared to small herbivores (Bell, 1971, Jarman, 1974, Demment and Van Soest, 1985). As a result, food selectivity generally scales with body size with smaller herbivores needing to be more selective than larger herbivores (Jarman, 1974, Gordon and Illius, 1994). Thus, because larger herbivores are generally forced to feed less selectively to provide sufficient intake, it is possible that they will make foraging decisions at larger spatial scales. For example, they may initially select for areas that have high food availability irrespective of plant species composition. Once such large-scale foraging decisions have been made, however, large herbivores may focus their feeding on the most nutritious species available. In contrast, due to their higher nutritional requirements, small herbivores will likely initially select at smaller spatial scales such as plant species or patches, and feed extensively on high quality plants.

The largest living terrestrial herbivore is the savanna elephant (Loxodonta africana) (males 5500–6000 kg, females 2500–2800 kg; Owen-Smith, 1988). As a result, it represents the upper limit with regard to tolerance of low quality food and an ability to feed unselectively. In addition, savanna elephants are mixed-feeders and thus utilise a wide range of both grasses and woody plants throughout the year. During the wet season, elephants tend to have a higher intake of grass, but as the dry season progresses they shift away from grass and feed more extensively on woody plants (Cerling et al., 2007, Osborn, 2004). The extent to which elephants utilise grass and browse varies between populations (e.g. Cerling et al., 1999, Cerling et al., 2007). However, the overall intake of grasses by elephants across Africa is generally lower than their intake of woody vegetation (Van der Merwe et al., 1990, Cerling et al., 2007, Codron et al., 2006).

Previous studies have found that elephants feed selectively in habitats and on woody plant species (e.g. Guy, 1976, Jachmann and Bell, 1985, Viljoen, 1989). However, very few studies have attempted to determine the order in which elephants make foraging decisions between these spatial scales (Babaasa, 2000, Holdo, 2003, De Knegt et al., 2011). Ritchie and Olff (1999) suggested that large-bodied herbivores should not be able to detect fine-scale food patches and thus should make foraging decisions at large spatial scales only. If these authors are correct, then elephants likely select for habitats rather than species.

Consistent with the above-mentioned prediction, De Knegt et al. (2011) suggested that elephants initially select areas based on large-scale characteristics such as high forage availability and proximity to water, and then select habitats within these areas based on finer scale vegetation characteristics (e.g. tree cover and herbaceous biomass). In addition, Holdo (2003) found that elephants tended to select patches where the trees had similar nutrient levels. As soil quality is a key factor influencing species composition and food quality, Holdo (2003) suggested that elephants were likely selecting areas with high soil quality rather than individual plant species. In contrast, Babaasa (2000) suggested that in the Bwindi Impenetrable National Park, Uganda, seasonal habitat use of elephants is driven by the search for preferred food plants. This was especially true during the wet seasons when elephants responded to flushes of bamboo shoots. Finally, using a range of fertilised patches of mopane trees of different sizes, Pretorius et al. (2011) found that elephants focused their foraging decisions at small spatial scales such that they obtained high nutritional intakes.

To determine the spatial scale at which elephants make foraging decisions, we focused our study on the seasonal foraging of elephants in the Ithala Game Reserve, South Africa. If the benefits of their large body size are a key driver behind their foraging decisions (i.e. ability to survive on lower quality food), we predict that elephants will feed relatively unselectively and thus limit their selection to larger spatial scales (e.g. habitats). However, because large-bodied herbivores can tolerate low quality food does not mean that they prefer it to high quality food (Bell, 1971). As a result, an alternative expectation is that elephants will feed selectively. The spatial scale at which this selection occurs, however, may vary between seasons. For example, during the wet season when food quality and availability are highest, it is likely that elephants would select for habitats. By doing this, they would then focus their foraging in areas that contain large concentrations of high quality species. In contrast, during the dry season when food quality and availability decline, and only a few woody plant species retain nutrients (e.g. high crude protein and low concentrations of secondary compounds) (Owen-Smith, 1994, Scogings et al., 2004), elephants may shift their preferences and select for specific plant species. Furthermore, because these high quality woody species may be found in a number of different habitats, the elephants’ degree of habitat selection during the dry season should decline.