Trends in woody vegetation cover in the Kruger National Park, South Africa, between 1940 and 1998

Eckhardt, H.C.; Wilgen, Brian W. van; Biggs, Harry

doi:10.1046/j.1365-2028.2000.00217.x

Cited by 159 publications

(175 citation statements)

References 21 publications

(11 reference statements)

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“…Indeed, the decrease of the large-tree vegetation component in the Kruger National Park (South Africa) has been attributed to increased elephant densities as a consequence of the large number of AWPs here. Sequential photography revealed that the density of trees >5 m decreased in the Kruger Park by up to 38% on basalt-derived soils within a 12-year study period (Eckhardt et al 2000). While the authors of this study were cautious to attribute the observed tree density decrease to the growing elephant population alone, they suggested the closing of AWPs as a possible management measure to be taken, should large-tree density further decrease.…”

Section: Introductionmentioning

confidence: 91%

Tree cover and biomass increase in a southern African savanna despite growing elephant population

Kalwij

Boer

Mucina

et al. 2010

Ecological Applications

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Abstract. The growing elephant populations in many parts of southern Africa raise concerns of a detrimental loss of trees, resulting in overall reduction of biodiversity and ecosystem functioning. Elephant distribution and density can be steered through artificial water points (AWPs). However, this leaves resident vegetation no relief during dry seasons. We studied how the introduction of eight AWPs in 1996 affected the spatiotemporal tree-structure dynamics in central Chobe National Park, an unfenced savanna area in northern Botswana with a dry-season elephant density of ;3.34 individuals per square kilometer. We hypothesized that the impact of these AWPs amplified over time and expanded in space, resulting in a decrease in average tree density, tree height, and canopy volume. We measured height and canopy dimensions of all woody plants around eight artificial and two seasonal water points for 172 plots in 1997, 2000, and 2008. Plots, consisting of 50 3 2 m transects for small trees (0.20-3.00 m tall) nested within 50 3 20 m transects for large trees (≥3.0 m tall), were located at 100, 500, 1000, 2000, and 5000 m distance classes. A repeated-measures mixed effect model showed that tree density, cover, and volume had increased over time throughout the area, caused by a combination of an increase of trees in lower size classes and a decrease in larger size classes. Our results indicate that the decrease of large trees can be attributed to a growing elephant population. Decrease or loss of particular tree size classes may have been caused by a loss of browser-preferred species while facilitating the competitiveness of less preferred species. In spite of 12 years of artificial water supply and an annual elephant population growth of 6%, we found no evidence that the eight AWPs had a negative effect on tree biomass or tree structure. The decreasing large-tree component could be a remainder of a depleted but currently restoring elephant population.

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Section: Introductionmentioning

confidence: 91%

Tree cover and biomass increase in a southern African savanna despite growing elephant population

Kalwij

Boer

Mucina

et al. 2010

Ecological Applications

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“…Low recruitment rates of large tree species have also been reported by Holdo (2003) and Caro et al (2005). In addition, open savannahs become denser due to bush encroachment which is often the result of landscapes being over-grazed (Jeltsch et al 1997;Tobler et al 2003) and because of poor fire management (Eckhardt et al 2000). Since bush-encroached areas are mostly dominated by small and densely growing trees, tree size might be important in influencing grass quality and structure.…”

Section: Introductionmentioning

confidence: 94%

Tree size and herbivory determine below-canopy grass quality and species composition in savannahs

2009

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Large single-standing trees are rapidly declining in savannahs, ecosystems supporting a high diversity of large herbivorous mammals. Savannah trees are important as they support both a unique flora and fauna. The herbaceous layer in particular responds to the structural and functional properties of a tree. As shrubland expands stem thickening occurs and large trees are replaced by smaller trees. Here we examine whether small trees are as effective in providing advantages for grasses growing beneath their crowns as large trees are. The role of herbivory in this positive tree-grass interaction is also investigated. We assessed soil and grass nutrient content, structural properties, and herbaceous species composition beneath trees of three size classes and under two grazing regimes in a South African savannah. We found that grass leaf content (N and P) beneath the crowns of particularly large (ca. 3.5 m) and very large trees (ca. 9 m) was as much as 40% greater than the same grass species not growing under a tree canopy, whereas nutrient contents of grasses did not differ beneath small trees (\2.3 m). Moderate herbivory enhanced these effects slightly. Grass species composition differed beneath and beyond the tree canopy but not between tree size classes. As large trees significantly improve the grass nutrient quality for grazers in contrast to smaller trees, the decline of the former should be halted. The presence of trees further increases grass species diversity and patchiness by favouring shade-tolerant species. Both grazing wildlife and livestock will benefit from the presence of large trees because of their structural and functional importance for savannahs.

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“…2 Relationship between the elephant density (N/km 2 ) and the overall tree stem density (N/km 2 ) (a), the number of stems per tree (b) and the estimated total consumption of browse (filled squares) and grass (open squares) of the relevant herbivores depicted as a percentage of the total consumption excluding elephants in Sabi Sand WildtuinMalaMala complex (c) (Fritz et al 2002;Kohi et al 2011;Valeix et al 2011;Kohi 2013). The impact of elephant may be site-or contextdependent (Guldemond and Van Aarde 2008) and varies spatially within KNP (Eckhardt et al 2000). Elephant numbers in the Sabi Sand Wildtuin-MalaMala complex were strongly negatively correlated with densities of both small and large trees.…”

Section: Discussionmentioning

confidence: 90%

“…In the proximity of large perennial rivers, current elephant densities in the KNP exceed 1.5 animals/km 2 (Smit and Ferreira 2010). Elephant impacts can also vary spatially and can be site-or context-dependent (Eckhardt et al 2000;Guldemond and Van Aarde 2008). Communicated by M. Scandura This impact of elephants on trees in the system has raised the concern of scientists and managers alike (Whyte et al 1998;Van Aarde et al 1999;Van Aarde et al 2006;Kerley et al 2008), as the decline in woody cover could negatively affect the browsing species in the system.…”

Section: Introductionmentioning

confidence: 99%

Elephant-mediated habitat modifications and changes in herbivore species assemblages in Sabi Sand, South Africa

Boer

Oort

Grover³

et al. 2015

Eur J Wildl Res

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Elephant Loxodonta africana conservation might indirectly influence the wider herbivore community structure, as elephants have the ability to significantly modify the savanna habitat. Uncertainty remains as to the consequences of these effects, as elephants might either compete with other species or facilitate foraging especially for grazers and smaller browsing species by increasing the amount of grass or the amount of browse at lower feeding heights. We studied these potential cascading effects of elephants by using 16 years of data (1992-2011) from the Sabi Sand Wildtuin, South Africa, which showed a steady increase in elephant densities from 0.12 to 2.03 elephants/km 2 over this period. We demonstrate that tree densities, and browse availability at feeding heights below 2 m, decreased with increasing elephant densities, and that there was no positive effect of elephants on browse availability. The changes in elephant densities were good predictors (R 2 adj >0.50) in explaining population fluctuations of other herbivore species. The total body mass of grazers increased more than that of the browsers, shifting the community toward a grazer and megaherbivore-dominated community. An increasing density of elephants changes the composition of the herbivore community, as mesobrowsers are unable to benefit from the impact of elephants on trees, but megagrazers show strong positive responses. Hence, changes in elephant densities as a result of poaching or conservation may trigger cascading community effects. These are neglected but important consequences of (negative or positive) human impacts on elephant numbers, especially in restricted areas such as reserves and national parks.

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Trends in woody vegetation cover in the Kruger National Park, South Africa, between 1940 and 1998

Cited by 159 publications

References 21 publications

Tree cover and biomass increase in a southern African savanna despite growing elephant population

Tree cover and biomass increase in a southern African savanna despite growing elephant population

Tree size and herbivory determine below-canopy grass quality and species composition in savannahs

Elephant-mediated habitat modifications and changes in herbivore species assemblages in Sabi Sand, South Africa

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