BIOTROPICA 44(1): 63–72 2012 10.1111/j.1744-7429.2011.00776.x Short‐term Effects of Single Species Browsing Release by Different‐sized Herbivores on Sand Forest Vegetation Community, South Africa Georgette Lagendijk1, Bruce R. Page, and Rob Slotow Amarula Elephant Research Programme, Biological and Conservation Sciences, Westville Campus, University of KwaZulu‐Natal, Private Bag X 54001, Durban 4000, South Africa ABSTRACT Manipulations of herbivores in protected areas may have profound effects on ecosystems. We examine short‐term effects on tree species assemblages and resource utilization by a mesoherbivore and small‐size herbivores (ungulates < 20 kg) in Sand Forest, after browsing release from a megaherbivore (elephant), or both a mega‐ and mesoherbivore (nyala), respectively. Effects were experimentally separated using replicated exclosures where all trees were counted, identified to species and browsing events recorded. Tree species assemblages were impacted by both elephant and nyala, and by each herbivore species individually. Tree turnover rates were higher where both herbi- vore species were present than in their combined absence. Diet was segregated among elephant, nyala and small‐size herbivores. Both resource specificity and browsing pressure by nyala increased in absence of elephant; small‐size herbivores increased resource specificity in absence of elephant, and increased browsing pressure in absence of both elephant and nyala. This implies interference competition with competitive release. The indirect effect of the manipulation of herbivore populations, through the removal of one or two herbivore species, caused a shift in tree species composition and diet of smaller‐size herbivores. These indirect effects, especially on tree species composition, can become critical as they affect vegetation dynamics, biodiversity and ecosystem processes. Therefore, in order to con- serve habitats and biodiversity across all trophic levels, conservation managers should consider the effects of: (1) the full herbivore assemblage present; and (2) any effects of altering the relative and absolute abundance of different herbivore species on other herbivore species and vegetation. Key words: elephant; exclosure; interference competition; Licuáti forest; Loxodonta africana; nyala; Tragelaphus angasii; vegetation change. THE CURRENT STATUS OF CONSERVATION RESULTS IN MANY MAMMAL one or two key herbivores on resource utilization by smaller her- SPECIES CO‐EXISTING AT HIGH densities within small, protected bivores, and the effects of the consequent browsing release by a areas (Chapin et al. 2000, Slotow et al. 2005). Different‐sized her- single herbivore species on tree communities, are less well‐known bivores can substantially impact conservation areas, and its eco- (Schmitz et al. 2000). Understanding the effects of browsing logical functioning, through their use of different food resources release is especially important in protected areas subject to active (Levick et al. 2009). Complex interactions between mechanisms herbivore management (e.g., population reductions, removal or such as predation, competition and facilitation promote co‐exis- introduction). tence of animal species (Pace et al. 1999), but disrupting these African ungulates provide a unique opportunity to test for has functional consequences such as the modification of ecosys- such within‐guild effects because of their diversity (Du Toit & tem processes (Hooper & Vitousek 1997, Tilman et al. 1997). Cumming 1999), different functional groups (Prins & Douglas‐ When the density of a particular herbivore species is reduced, Hamilton 1990), abundance and active conservation management competitive release occurs, as the constraint of the competing (e.g., Carruthers et al. 2008, Morgan et al. 2009). Here, we distin- herbivore species is removed (Kareiva 1982). The ‘released’ herbi- guish among mega‐ (species with a body mass 1000 kg vore species now uses different food resources compared to [Owen‐Smith 1988]), meso‐ (medium‐size herbivores 50–450 kg when the competitor was present. The effect of this competitive [Fritz et al. 2002, de Garine‐Wichatitsky et al. 2004]) and small‐ release can cascade into lower trophic levels as the plant species size herbivores (ungulates < 20 kg [Bothma et al. 2004]). composition shifts, in response to changed foraging behavior of We focus on Sand Forest, a deciduous dry forest endemic to the released herbivore species, which ultimately affects ecosystem northeastern South Africa and southern Mozambique (Kirkwood processes (Chapin et al. 2000). & Midgley 1999, Matthews et al. 2003, Siebert et al. 2004). In the The effects of competitive release of herbivores on vegeta- two main localities where Sand Forest is conserved in South tion have been studied extensively for groups of similar‐size her- Africa, both elephant Loxodonta africana, a megaherbivore ($: bivores through exclusion experiments (Young et al. 1998, Shaw 2500 kg; #: 5000 kg [Owen‐Smith 1988]), and nyala Tragelaphus et al. 2002, Goheen et al. 2004, 2007, Levick & Rogers 2008, angasii, a mesoherbivore ($: 65 kg; #: 110 kg [Kirby et al. 2008]), Moe et al. 2009). However, the effects of the selective removal of became locally abundant after fencing the protected area (i.e., Tembe Elephant Park), and reintroduction (i.e., Phinda Private Received 16 March 2010; revision accepted 11 January 2011. Game Reserve). Although few large mammal species utilize Sand 1Corresponding author; e‐mail: [email protected] Forest (Matthews 2005), both elephant and nyala do, and impact ª 2011 The Author(s) 63 Journal compilation ª 2011 by The Association for Tropical Biology and Conservation 64 Lagendijk, Page, and Slotow on the structural diversity while foraging (Matthews 2005, Kirby Phinda was created in 1991, after which game was intro- et al. 2008). In addition, it is expected that each herbivore species duced. Fifty‐eight elephant were released between 1992 and 1994 also affects tree species composition. The browsing herbivore (Druce et al. 2006). At the start of this study (2005), 75 elephant community within Sand Forest also includes small‐size herbi- were present, which increased to 98 individuals in 2007 (based vores, such as common duiker Sylvicapra grimmia, red duiker Ceph- on an individually identified and monitored elephant population alophus natalensis and suni Neotragus moschatus. The Sand Forest [e.g., Druce et al. 2008]). Nyala numbered approximately 1100 and ecosystem thus provides a relatively simplified large herbivore 1750 individuals in 2005 and 2007, respectively (based on annual browsing guild in terms of diversity, while being complete in aerial game counts). Other browsing ungulates on Phinda include terms of complexity, including the full spectrum of different‐size giraffe Giraffa camelopardalis (2007 annual helicopter game count herbivores (i.e., mega‐, meso‐ and small‐size herbivores). 154), kudu Tragelaphus strepsiceros (188), impala Aepyceros melampus By excluding either a megaherbivore, or both mega‐ and mes- (1690), red duiker (23), common duiker (no count available) and oherbivores, using a replicated exclosure experiment, we created suni (no count available). The only mega‐ and mesoherbivore uti- the opportunity to study competitive release when key elements lizing the Sand Forest patches in Phinda were elephant and nyala; (i.e., elephant and/or nyala) were artificially removed. While testing giraffe, kudu and impala did not use the forest (D. D. G. Lagen- for these effects, we focused firstly on changes in woody vegetation dijk, pers. obs.). communities, expecting the removal of a key herbivore species, Sand Forest is a dense vegetation type, with a closed canopy with consequential browsing release for other herbivores, to alter of 5–12 m in height and without a significant understory, grow- tree species assemblages. Secondly, we focused on dietary segrega- ing on acidic, sandy soils with very little clay (Matthews et al. tion between different herbivore groups, expecting diet overlap and 2003). Characteristic tree species include Balanites maughamii, Cleis- browsing pressure of nyala and small‐size herbivores to increase tanthus schlechteri, Cola greenwayi, Croton pseudopulchellus, Dialium due to browsing release after exclusion of their larger counterparts. schlechteri, Drypetes arguta, Hymenocardia ulmoides, Newtonia hilde- Resource availability for small‐size herbivores should be greater brandtii, Pteleopsis myrtifolia, Strychnos henningsii and Toddaliopsis brem- within their height reach due to browsing release by nyala (Lagen- ekampii (Moll 1980, Kirkwood & Midgley 1999, Matthews 2005). dijk et al. 2011), as they feed in overlapping height ranges, and potential competitive displacement by larger herbivores (i.e., ele- EXPERIMENTAL SETUP.—In November 2005, part of the Sand For- phant and/or nyala) is eliminated when these animals are removed. est was fenced from elephants using electrified strand wires as Therefore, the objectives were to determine the short‐term part of a long‐term ( > 10 yr) vegetation monitoring experiment effects of browsing release on: (1) tree species assemblages; and (Fig. 1). The fence consisted of two electrified (60 pulses of (2) resource utilization by a mesoherbivore and small‐size herbi- 7000 V/min) high tension galvanized wires (2.4 mm thick), vores after browsing release by their larger counterparts, i.e., (i) a approximately 1.8 m and 2 m above the ground, enclosing megaherbivore (elephant) or (ii) both a mega‐ and mesoherbivore