Herbivores are a main driver of ecosystem patterns and processes in semi-arid savannas, with their effects clearly observed when they are excluded from landscapes. Starting in the 1960s, various herbivore exclosures have been erected in the Kruger National Park (KNP), for research and management purposes. These exclosures vary from very small (1 m2) to relatively large (almost 900 ha), from short-term (single growing season) to long-term (e.g. some of the exclosures were erected more than 60 years ago), and are located on different geologies and across a rainfall gradient. We provide a summary of the history and specifications of various exclosures. This is followed by a systematic overview of mostly peer-reviewed literature resulting from using KNP exclosures as research sites. These 75 articles cover research on soils, vegetation dynamics, herbivore exclusion on other faunal groups and disease. We provide general patterns and mechanisms in a synthesis section, and end with recommendations to increase research outputs and productivity for future exclosure experiments.
Herbivore exclosures in the KNP have become global research platforms, that have helped in the training of ecologists, veterinarians and field biologists, and have provided valuable insights into savanna dynamics that would otherwise have been hard to gain. In an age of dwindling conservation funding, we make the case for the value added by exclosures and make recommendations for their continued use as learning tools in complex African savannas.
The earliest research exclosures were erected in the Kruger National Park (KNP) in the late 1960s and early 1970s (e.g. Makhohlola exclosure) with the intention ‘to provide areas where the grazing effects of animals could be excluded’ (Joubert
After the detection of widespread tuberculosis in buffalo (
During the early 2000s, several external research programmes which included exclosures in their experiments, were established in KNP. The University of Cape Town (UCT) erected two electrified exclosures, one on the October triennial fire treatment of the Shabeni EBP string at Pretoriuskop and another in the north-west corner of the Satara buffalo exclosure (hereafter UCT Satara exclosure). These exclosures formed part of the broader Tree Grass Programme, with the main objective to investigate hypotheses predicting tree-grass coexistence in savannas (February & Higgins
In 2018, researchers from several institutions (Universities of Florida and Eswatini being prominent), erected an exclosure experiment (called
Although exclosures can be valuable research sites, they are expensive research infrastructure to install and maintain, especially in areas with high densities of elephants (Grant et al.
Map of the Kruger National Park showing the locations and status of past and present exclosures (map produced in December 2021).
Here, we provide a review of exclosure-based research in KNP. Additionally, we were interested to evaluate the realised impact of these exclosures from the perspective of research output. Research published in peer-reviewed literature was reviewed under five different groupings, based on the history and objectives of the respective exclosures; that is, (1) the exclosures with the longest history (Hlangwini, N’waswitshumbe and Makhohlola), (2) the Nkhuhlu and Letaba exclosures, (3) disease work in the Satara (Buffalo) exclosure, (4) the Tree Grass Programme (UCT Shabeni and UCT Satara), and (5) a grouping capturing all the smaller exclosure work around Satara. Online Appendix 1 provides information about the exclosures, including age, size, experimental design and current status. Furthermore, Online Appendix 2 provides details of the sources used during this review, including the exclosures used and the main variables measured. Considering all these fenced areas as ‘herbivore exclosures’, even when some of them have enclosed selected herbivores for certain periods, stems from the fact that (1) elephant, an important landscape engineer in the park, was fenced out of structures, (2) herbivore densities in the exclosures were generally lower than areas outside the exclosures, (3) a few of them have been used both as enclosures and exclosures, and (4) fire has been excluded or carefully managed in many of the enclosures and exclosures.
This review examines the utility of small to large (1 m2 – 870 ha) and temporary (one growing season) to long-term (≥ 15 years) herbivore exclosures in KNP as research infrastructure. To gather all relevant literature, we conducted a Google Scholar database search using keywords including various combinations of ‘exclosure OR enclosure #name’, ‘enclosure KNP’ or ‘exclosure KNP’, as well as the names of the sections of KNP in which the exclosures are located. There are various spellings of the same exclosure name in the literature and all the spelling variations were included in the search. A single standardised spelling for each is used here. We focussed on published work; however, where work was not published, we referred to the corresponding thesis, if available. Work done within exclosures was included regardless of whether the objectives were related directly to herbivory or not. Research on the breeding of rare antelope was not included, as this information is poorly documented in the formal literature. Much of the history and lessons learned are captured in unofficial reports, meeting minutes, correspondence and ranger diaries, but not formally curated (i.e. not listed in the SANParks literature bibliography). Although this history and the associated lessons learned are important and worthy of publication, it requires a dedicated study beyond the scope of the current review.
A total of 75 peer-reviewed published works were retrieved during the search which ended on 31 August 2021.
Eighteen published studies include the Hlangwini (
A fence-line contrast of the southern edge of the Hlangwini exclosure showing the relatively similar woody densities inside (right) and outside (left) the fence.
A tall woodland establishes over time (right side) when herbivores are excluded on the southern basalts in Kruger National Park, here shown in the Makhohlola exclosure.
An aerial view of the eastern boundary fence of the N’waswitshumbe exclosure, showing higher woody density where herbivores are excluded.
The lower (eastern) corner of the N’waswitshumbe exclosure in (a) 1986 and (b) 2015 showing the thickening and expansion of woody vegetation inside of the exclosure compared to the persistent open landscape outside the exclosure.
Airborne LiDAR (Light Detection and Ranging) technology has shown that herbivores increase tree-fall rates, decreases woody cover, and alter 3-D structure of woody vegetation and canopy height (Asner et al.
Herbivores impact the abundance of certain species. For instance,
Work from elsewhere in KNP using small 1 m2 exclosures showed that the recruitment bottleneck for some tree species such as
These exclosures have hardly been used to study the impacts of herbivores on other faunal groups; only four publications were found that addressed it. One study at N’waswitshumbe exclosure found no differences in small mammal species composition in and out of the exclosure (MacFadyen et al.
The Nkhuhlu exclosures (full exclosure shown in
Fence-line contrast of a sodic site between full exclosure (left) and area open to herbivores (right) at the Nkhuhlu exclosures.
Aerial view of the Nkhuhlu exclosure showing the same sodic site as in
Aerial view of the Nkhuhlu exclosures comparing the crest without elephants and giraffe (partial exclosure, left) and the control open to all herbivores (right).
Thorough soil classification was done when the exclosures were erected (Paterson & Steenekamp
The earliest vegetation work from the two exclosures described and mapped their plant communities (Siebert & Eckhardt
The woody species composition was slow to react to the exclusion of herbivores at Nkhuhlu, with no change detected after 5 years (Scogings et al.
Although herbivores may negatively impact woody vegetation survival, severe droughts can be more acute in the short-term. During the 2015–2016 drought, Case et al. (
Work from KNP exclosures, including Nkhuhlu, found little consistent responses of C-based secondary metabolites (CBSMs) to herbivore exclusion (Scogings, Hjältén & Skarpe
The utilisation of sodic patches is very high when accessible to herbivores at the Nkhuhlu exclosure (
Generally, mammalian herbivores had negative effects on the total invertebrate abundance at the two exclosures (Jonsson et al.
The Satara (Buffalo) exclosure was erected to breed specific-pathogen-free buffalo that could be translocated out of the Foot and Mouth Disease (FMD) endemic area in KNP, and into the FMD-free zone of South Africa. This involved setting up founder herds of Tuberculosis (BTb)-negative and Brucellosis-negative animals caught in northern KNP and transferring them to the Satara (Buffalo) exclosure, to produce calves that would go through a series of quarantine procedures after weaning. The project produced over 400 BTb-, Brucellosis-, FMD- and Corridor disease-free calves that were used to restock Addo Elephant and Mokala National Parks in South Africa and Gorongosa National Park in Mozambique (J. Malan [SANParks], pers. comm., 17 September 2021).
From 2015 to 2017, the remaining animals in the Satara exclosure were used in an Oregon State University (OSU) Disease Ecology Research Programme that produced 8 publications and 10 postgraduate qualifications. The unique, longitudinal nature of the OSU study allowed for several diseases to be tracked over time, in individual animals and through the population, revealing temporal patterns not discoverable through typical cross-sectional studies. The Satara exclosure enabled this research, since studying a high number of buffalo, subject to environmental and seasonal disease dynamics, at bi-monthly intervals over 3 years, would otherwise not have been possible. The study was unique in that it used a large social animal in a natural setting; most other epidemiology studies of this kind are laboratory-based. The work helped establish that buffalo could serve as an ideal wild model system, linking individual immunology, environmental change (linked to metabiome health), and parasite transmission dynamics (Combrink et al.
The UCT Tree Grass Programme was established in KNP in 2005 and operated for 10 years. The two exclosures (UCT Shabeni and UCT Satara) were part of a broader programme which delivered more than 30 publications and had > 20 postgraduate students involved (E. February [UCT], pers. comm., 03 September 2021). The main objective of the programme was to investigate tree-grass coexistence in terms of two dominant hypotheses, namely, resource-based and disturbance-based. Much of the research that took place in the exclosures addressed the issues around allocation of resources between trees and grasses (February & Higgins
The results show a high degree of overlap of tree and grass roots, which gives rise to high potential competition (February et al.
During 2005–2006, a USA-SA consortium of researchers initiated a programme with the main objective to broadly test whether general rules can be formulated in terms of drivers, responses and functioning, across grassy ecosystems in North America and South Africa (Knapp et al.
Full (left) and partial (right) 7 m2 exclosures used by the USA-SA consortium on the Satara and N’wanetsi experimental burn plots and affiliated high-low productivity sites. A smaller 1 m2 exclosure for quantifying ANPP is visible in the middle.
Additionally, in 2013, a research group from the Universities of the Witwatersrand (WITS) and Liverpool (hereafter the Fire-Grazer Research Programme) established an experiment north of the N’wanetsi EBP string to test the influence of fire season and fire size on grazers, and the effect of post-burn herbivory on the grass layer (Donaldson et al.
The initial research investigated the drivers of annual herbaceous ANPP, using the exclosures on the EBPs and in the Satara (Buffalo) exclosure. Grass and forb ANPP were unaffected by fire regime or whether recent grazing was by a single species (i.e. buffalo) or a mixed suite of species (Buis et al.
The research which took place during drought (either 2015–2016 or experimentally induced) supported emerging patterns elsewhere that show that the herbaceous layer of grasslands and savannas have low resistance but high resilience to drought (Koerner & Collins
A low severity 3-year drought was experimentally imposed by Koerner and Collins (
Similar to the Nkhuhlu exclosures, the exclusion of herbivores in general decreases herbaceous plant diversity (Burkepile et al.
Removing mammalian herbivory drives a change in species composition and the accumulation of biomass in the herbaceous layer inside exclosures.
Although species richness and diversity decreased with herbivore removal during some years on the infrequently burned and unburned areas, there was no definitive change in species or functional community structure after 7 years of herbivore removal at any fire frequency (Forrestel, Donoghue & Smith
Where smaller-bodied herbivores, primarily the numerically dominant impala, are excluded (a, full exclosure), there is substantial increase in herbaceous biomass compared to where they have access to grazing (b, partial exclosure). This is apparent even in early season growth.
Furthermore, the effect of herbivore exclusion on species richness was shown to depend on site productivity (catenal position) and herbivore size (Burkepile et al.
Exclosures (left) clearly show the impact of herbivory on this productive, heavily-grazed foot-slope even in the first year after establishment.
Both the USA-SA consortium and the Fire-Grazer Research Programme showed that grazers respond positively to areas burned recently, and also prefer more frequently burned areas in general (Burkepile et al.
The results from research that included the Satara (Buffalo) exclosure as a single grazer site with less intense utilisation, found that grasses are higher in N and P in areas with a combination of wildlife species present, as opposed to areas with only a single species present (Baumgartner et al.
Within the context of a large protected area such as KNP, the herbivore exclusion sites enable intensive study of one of the main drivers in disturbance-driven savanna systems, that is herbivory by mammalian herbivores. They allow the partitioning of effects of those disturbances that managers can partly control (e.g. fire regimes, herbivore densities and disease management) from those that cannot be manipulated easily (e.g. droughts, floods and atmospheric CO2 concentrations). The understanding and knowledge that result from this research inform management decisions and policies, allowing SANParks to learn and adapt in an ever-changing environment.
Realising the value of exclosures as research infrastructure in KNP has been varied. The largest exclosures, which also have the longest histories, have been relatively underutilised as research sites (e.g. Hlangwini and N’waswitshumbe). This may be partly because of the lack of replicated treatments and the problem of pseudoreplication (Holdo & Mack
The two areas of savanna ecology in which a significant body of work has been produced, across multiple exclosures in KNP, are the woody layer and herbaceous layer dynamics. Most of the woody work was done in the Nkhuhlu exclosure, with additional work from the Hlangwini, N’waswitshumbe and Makhohlola exclosures. Multiple studies from the Nkhuhlu exclosure and the Satara-based programmes focused on the interactions between herbivores and fire on the herbaceous layer, with some additional work addressing potential competition between trees and grasses.
It is clear that herbivory decreases woody cover and simplifies vegetation structure (
Woody layer responses to herbivore exclusion in the Kruger National Park.
Variables | Effect of herbivore exclusion | Reference |
---|---|---|
Woody cover and structure | Structure (measured as height distribution) and cover increase, especially on basalts | (Asner & Levick |
Biomass and aboveground carbon increase, more so on basalt | (Colgan et al. |
|
Tree-fall rates decrease, especially on soils with higher clay content (i.e. basalt soils and lower lying granite soils) | (Asner & Levick |
|
Species abundance and richness | Abundances of some species increase; for eample |
(Combrink |
Growth rates | Disturbance-adapted species such as |
(Case et al. |
Defence | Carbon-based secondary metabolites do not respond in consistent ways to herbivore exclusion. Physical defences such as spines tend to decrease. | (Scogings, Hjältén & Skarpe |
Herbaceous layer responses to herbivore exclusion in the Kruger National Park.
Variables | Effect of herbivore exclusion | Reference |
---|---|---|
Herbaceous cover and productivity with fire | Increases in herbaceous biomass and cover | (Burkepile et al. |
Herbaceous cover and productivity without fire | Increases in herbaceous biomass, especially in riparian and sodic sites | (Jacobs & Naiman |
Species richness and diversity with fire | Declines in total species richness when all herbivores excluded | (Burns et al. |
Declines in total species richness when a single herbivore excluded | (Eby et al. |
|
No directional change in species richness but modest/transient declines in species richness/diversity when all herbivores excluded, effects of smaller bodied mammal herbivores pronounced | (Burkepile et al. |
|
During drought, forb richness and diversity, but not that of grass, were strongly decreased by all herbivore exclusion | (Van Coller & Siebert |
|
Species richness and diversity without fire | Reduced total species richness and diversity but dependent on body size and productivity of site; Exclusion of large herbivores increased richness at low productivity sites, exclusion of smaller herbivores decreased richness at high productivity sites | (Burkepile et al. |
Herbaceous productivity is mostly maintained with high herbivory and frequent fire, and is only impacted during extreme droughts, but recovers rapidly post-drought (Buis et al.
The initial dominance of
Apart from an interaction between fire regime, grazer exclusion and herbaceous dynamics, effects of herbivore exclusion depend on the types and sizes of animals and their interactions with different fire regimes (Burkepile et al.
Lastly, little work has compared mammalian herbivory with invertebrate herbivory impacts. The only study on this topic compared termite herbivory when mammals were excluded, and found that although both types of herbivory were more intense on termite mounds than off, termite herbivory removed more biomass than mammal herbivores during the wet season (Davies et al.
Given the experimental design and obvious effects of herbivory on savanna systems, exclosures have provided excellent learning opportunities for students across the field of education. We have highlighted the number of graduate students who were involved in exclosure research in KNP. Yet, these exclosures have also played a major role in undergraduate training. For example, the Organization of Tropical Studies (OTS) has been working in KNP since 2004 and has conducted student projects in multiple exclosures across the park. To date about 35 undergraduate student research projects from OTS have used SANParks exclosures (Unpublished data; OTS) to understand the effect of mega-herbivores on woody plant demography, changes in community composition of woody plants and grasses, the consequences of mutualism breakdown for plant demography, the effect of invertebrate versus mammalian herbivory on termite mounds, phenotypic variation in physical defences of woody plants and the influence of vegetation structure on invertebrate (butterflies and dung beetles) and small vertebrate communities. Many of these smaller research projects served as pilot projects for graduate studies and some of these have been published (e.g. Haba et al.
The lessons learned from setting up and managing exclosures, and increasing the research uptake thereof, include the following:
Give consideration to logistics such as ease of access, including distance from research camps and presence of all-weather access roads in selecting exclosure sites.
Ensure a budget and maintenance plan that stretches beyond the establishment of the exclosure, and which is linked to the time frame for which the exclosure is needed (e.g. 20 years). Build collaborations and joint ownership of exclosures with land managers during the planning phase.
Purposefully design the exclosure experiment with research questions in mind, for example variable fire treatments, across catenal positions, allowing selective exclusion of different-size herbivores.
Adapt the size of the exclosures to fit the research questions asked; for example the USA-SA consortium exclosures have been very productive in terms of research outputs. Although these exclosures cover a relatively small area, they are numerous and large enough to capture dynamic herbaceous layer changes.
Employing ‘clever’ design may save much money in the long-term; for instance, electrified 2- or 3-strand wire exclosures with electrified wires positioned horizontally up to 1 m, such as those used at Mpala in Kenya and Grumeti in Tanzania (A. Uys [Maritaba], pers. comm. 20 September 2021), are cost effective and extremely efficient as a barrier against megafauna movement. Full exclosures inside partial exclosures allow for some level of protection of the full exclosure against elephant damage and can decrease overall costs of maintenance.
Canvas the broader research community to solicit baseline information which will add value to the exclosure and stimulate further research (e.g. floristic composition, pre-treatment conditions, aerial photographic and LiDAR surveys, etc.). Make baseline information available as a ‘starter pack’ for researchers and students – this can include information pertaining to the year of establishment, size, spatially-explicit treatment history (e.g. fire treatments), long-term average and yearly rainfall, estimates of herbivore density outside exclosures, and so on. Collaboration beyond the initial group and objectives have made exclosures such as the USA-SA consortium exclosures very productive and maintains high energy and interest regarding the exclosures.
Prepare a closure or hand-over plan for the exclosure, which may include a review of research (i.e. knowledge generated, publications, students graduated, etc.), and a long-term maintenance plan including costing. Do not assume that the land manager or owner will be in the position to maintain ageing infrastructure beyond the lifespan of the programme.
Fence-line contrasts between exclosures and the surrounding landscape are very effective in demonstrating the significant effects and interactions of herbivores in savanna ecosystems to both managers and the public alike, and for stimulating new research ideas for scientists. In addition, exclosures are excellent outdoor classrooms and laboratories for training the next generation of ecologists. With a renewed interest in the implications of the mass extinction of mammal herbivores (e.g. Rowan & Faith
The loss of exclosure infrastructure in KNP represents a missed opportunity, in that it may form a global research platform similar to the EBPs. This fire experiment has been running uninterrupted for seven decades and have become one of the primary sites for studying fire effects globally (e.g. Pellegrini et al.
The authors would like to thank all the South African National Parks (SANParks) staff including section rangers, research assistants and technicians who have worked so hard for many years to service, maintain, and survey exclosures in the KNP. The authors also wish to thank Dr. Anthony Swemmer and South African Environment Observation Network (SAEON) for all the support, both logistically and financially, in maintaining and surveying some of the exclosures, especially the Letaba exclosure.
The authors declare that they have no financial or personal relationships that may have inappropriately influenced them in writing this article.
C.W-C., T.S., D.G., D.I.T., L.K. and I.P.S. contributed to the text; C.W-C., T.S., C.G., N.G., D.G. and I.P.S. conceptualised the work; J.B., A.M., L.K., and J.V. provided stats on exclosure maintenance; C.W-C. and T.S. analysed literature and data and C.S. provided the map.
This article followed all ethical standards for research without direct contact with human or animal subjects.
This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.
All data, in this case works of literature reviewed have been summarised in Online Appendix 2.
The views and opinions expressed in this article are those of the authors and do not necessarily reflect the official policy or position of any affiliated agency of the authors.