Local environmental gradients on a catenal scale create ecological patterns from the crest to the stream of the hillslope. Bottom-up drivers interact with top-down controls to give rise to these patterns. A multidisciplinary project was conducted to study the processes that govern functioning, structure and heterogeneity on a catena in a third-order catchment in the Southern Granite Supersite in the Kruger National Park. The project included abiotic components (e.g. groundwater-surface water interactions, soil chemical and physical properties) as well as biotic components (e.g. soil microbes, small aquatic organisms in ephemeral pools, plant communities, vegetation structure and mammal diversity). Each of these components was investigated in detail along the catenal gradient and reported on in separate articles in this special issue. The drought of 2015–2016 occurred during the sampling period of the study and information on the response of vegetation and mammals to the drought were included. In this article, a synthesis of findings from the separate components or disciplines is provided to highlight the interactive functioning and ecological patterns of the catena. These findings were then used to develop a framework for multidisciplinary studies in similar environments. The framework highlights the interactive relationships between various components of the ecosystem and the importance of a multidisciplinary approach.
The findings of this study were used to develop a conceptual framework outlining how a range of biotic and abiotic patterns and processes interact along the catenal gradient. The framework highlights the importance of recognising these interactions in a multidisciplinary approach focused on one supersite.
One of the key concepts linking ecology and biodiversity management is environmental heterogeneity. In the words of Pickett, Cadenasso and Benning (
The general concept of heterogeneity becomes ecologically meaningful when it is given operational life by specifying what agents modify what substrates, what controls the modification, and what organisms or processes respond to the resulting spatial template. (p. 25)
The concept of savanna heterogeneity has been fundamental in how this has informed the management of the Kruger National Park (KNP), and has influenced, for example, how fire, artificial surface water and large herbivores are managed to maintain, mimic or, in some cases, restore inherent heterogeneity. For an overview of the concepts of heterogeneity in the savanna context on different levels from a fine to a broader scale, and how that has influenced thinking and management of the KNP, see Du Toit, Rogers and Biggs (ed.
At the scale of our study (in this instance, a third-order catchment in a granitic landscape), heterogeneity is the outcome of local environmental gradients along a hillslope (from the crest to the valley bottom). This spatial heterogeneity typically exhibits a common form of organisation and symmetry – termed the catena (
Bottom-up drivers (like climate and soils), together with top-down controls (like fire and herbivory), interact to give rise to the emergent heterogeneity and patterning. This is illustrated in the article by Sankaran et al. (
With this as background, a multidisciplinary project was conducted that specifically focused on hydrology, soil, microbial, small aquatic organisms (in ephemeral pans and mud wallows), vegetation, mammal diversity and heterogeneity linked to below ground–above ground relationships along the catena or hillslope. Each of these different components of the ecosystem was researched separately and published as individual articles in a special issue, which this article (as part of the same special issue) aims to integrate. The objectives of this article were (1) to provide a framework of ecological patterns and interactions linking certain abiotic and biotic components of the catena ecosystem and (2) to briefly synthesise the main findings from the individual multidisciplinary research articles published in this special issue. A brief overview of the study area is provided together with a concise report of methodological approaches followed during the multidisciplinary project. Detailed descriptions of materials and methods can be found in the individual articles.
The study was conducted in KNP on the Southern Granite Supersite (Stevenson-Hamilton Supersite) (Smit et al.
The multidisciplinary project continued on earlier work conducted by Riddell et al. (
Plants were classified based on species composition and abundance and grouped into communities for the same 49 plots in the belt transect where the soil observations were made (Theron, Van Aardt & Du Preez
Extreme drought conditions coincided with the study period during one of the strongest El Niño southern oscillations in the historical record (Hu & Fedorov
The framework to highlight the interconnectivity between the various components of this ecosystem was developed through several discussions and workshops between the various researchers. Where available, the literature on similar studies was consulted to identify relationships between various components. Although some of the main components of ecosystem connectivity were addressed in this study, the framework also identified other relationships which should be addressed in future research.
A catena is a soil sequence where each soil group occurs on similar parent material, but in arrays from the crest to the footslope along a hillslope (Brady & Weil
The southern granites of KNP can have well, moderately or poorly drained soil forms. The spatial distribution is dominantly associated with terrain morphology and depends on a combination of soil forming factors (e.g. parent material and topography). All of this results in varying types of fertility along the catena that forms different gradients of supply to the soil biota. The heterogeneity in soil fulfils different hydrological functions. Firstly, it controls infiltration rate that determines runoff. Secondly, it controls plant water-holding capacity. Thirdly, it regulates sub-surface lateral water flow in sandy crests with low clay content. Fourthly, it serves as a clay plug and creates surface water flow in clayey areas down the slope where permeability of soil restricts drainage and infiltration (Khomo et al.
The catena thus forms an ecosystem that consists of abiotic (gradient of soil types, soil properties and nutrients; ground water flow; surface water; topography; micro-climate; etc.) and biotic components (micro-organisms, plants and animals) that are connected and interact in the different zones present along the hillslope. The focus of this section is to describe the interconnectivity between some of the components of the catena ecosystem that formed part of a multidisciplinary study conducted on one catena or hillslope. This article summarises only the basic connections between soil, hydrology, micro-organisms in the rhizosphere and roots of dominant vegetation, vegetation structure, mammals, permanent and temporary water sources, and drought in the catena ecosystem. The readers are referred to the specific articles in the special issue for further detail.
Links between abiotic and biotic components of the savanna granitic catena ecosystem. Ecosystem components that did not form part of this study are indicated in dotted line blocks, but only fire has been linked to other components. A connection to soil or plants implies its sub-components as well.
Interconnectivity of components that formed part of this study on a granitic catena ecosystem in the savanna.
The following discussion refers to the numbered arrows in
Soil is a first-order control of hydrological processes as it governs infiltration rates, storage and drainage (2). The relationship between soil and hydrological processes is, however, interactive, as water plays a primary role in the formation of the morphological, chemical and biological properties of soil (2). These interactions between soil and water are known as hydropedology. The soil distribution pattern is determined by five soil-forming factors of which geology and topography are key factors at the hillslope scale (3). In combination with the soil distribution, the topography affects hydrological processes (4).
Nutrients, water and anchorage are key provisioning functions of soils to plants (5). Different soils have specific abilities to support plant species and vitality of growth; thus, the soil distribution pattern is a key determinant in the distribution of various vegetation communities at catenal or hillslope scale. Plants, on the contrary, influence soil processes like infiltration and provide soil with organic material and nutrients through their leaves and root systems. Mining of nutrients from the subsoil and fractured rock is an important biocycling process that brings nutrients from deep soil layers and returns it to the surface, which determines the chemical, physical and biological properties of soil. In combination with the soil distribution pattern (as impacted by topography, geology and hydrology), micro-climate and micro-topography can influence soil water availability that governs vegetation distribution in savannas (5).
The patchiness of the vegetation is a common occurrence in savannas and is observed at specific scales. Vegetation are grouped in patterns because of species composition and plant communities growing in the same local patch, presence of bare and vegetated patches, sub-canopied habitats that differ from open uncanopied areas, varying vegetation structure, etc. Spatial patterns can be described based on the size of plants and their canopies (vegetation structure), as an example. Sub-canopy shaded habitats are not only important for thermal regulation of animals, but also are preferred areas of grazing (
Large trees (> 5 m) are important in the environment, but declines in their numbers have been reported in KNP (Eckhardt, Van Wilgen & Biggs
Not only does vegetation influence herbivore spatio-temporal patterns, but herbivores also influence vegetation patterns. Number 8 in
The relationship between vegetation and mammals is also interactive (7 and 8,
Uprooting of trees by elephants would also impact the soil properties (9,
Spatial patterns consisting of vegetated patches that alternate with bare soil areas are characteristic in savannas (Augustine
Climate, especially temperature and moisture, has a direct impact on soil biochemical reactions (Whalen & Sampedro
Microbes, together with the vegetation in which root zones they occur, can be impacted by soil moisture and nitrogen availability. February and Higgins (
Vernal pools and mud wallows are important components of the savanna ecosystem. A vernal pool is a seasonal, shallow, usually small, ephemeral water body, with no permanent inlet or outlet. Their occurrence and frequency of occurrence are directly determined by the precipitation (15,
Climate (together with geology) is one of the main drivers of long-term vegetation patterns and processes in KNP (Scholtz et al.
The framework discussed in this section (
The focus of this section will be on providing a short, introductory synthesis of the findings published in the articles in this special issue that fits into the framework provided in the previous section. The catena in the study area consists of a low relief crest and upper midslope (slope = 1%), sodic site, shrubveld inside a riparian area (footslope), a small floodplain area and drainage line. The geology, different soil types, some soil properties, the hydrology, vegetation communities, the number of trees and mammals are summarised in
Hydropedological response model, topography, soil types and properties, plant communities, vegetation structure (percentages and numbers of individual plants per 100 m transect) and number of mammal species in each zone on the catena. Numbers of mammals may seem similar between catenal zones, but do not necessarily fully overlap in species composition (Janecke & Bolton
Hydrological processes included all three catchment orders, with a focus on the third-order catena, whereupon it was shown that lateral interflow along the hillslope at the first-order augment streamflows, whereas the second- and third-order stream reaches are conduits for groundwater recharge to the fractured rock aquifer and thereby show transmission loss from the stream network. Recharge soils are typical of the crests and footslopes through preferential vertical flow, whilst the midslope sodic regions are responsive in nature with strong lateral flow contributions at the surface and shallow sub-surface. Hillslope soil water balances revealed large variations in the riparian zones resulting from upslope contributions and evapotranspiration losses (Riddell et al.
Four main soil forms were found from the crest to the valley bottom, namely, Clovelly, Sterkspruit, Bonheim and Dundee (Bouwer et al.
Differences between the four main soil types (chemical and physical properties) led to different vegetation communities and sub-communities that are associated with each soil type (
Differences in soil properties, moisture content, nutrients and the depth of the hard rock bed can all play a role in the number of trees (excluding other factors such as the impact of animals), especially large trees in different catenal zones. In the study area (
Just before the drought conditions peaked, the shrubveld inside the riparian area on the catena studied had a higher density of shrubs and of grass plants than the other vegetation zones (Janecke
On a microscale the different ecotypes and soil conditions influence microbial communities in the rhizospheres of associated plants. Although the same plants may occur across vegetation zones, the microbes associated with the rhizospheres differ because of chemical differences (Vermeulen et al.
The vegetation creates habitat and provides food for herbivores. Available space for larger herds of herbivores and for larger sized mammals (mega-herbivores), as well as competition for food resources, predators, shelter and other factors, could influence animal densities in certain areas that can again affect their impact on vegetation and soil (
In the study area, the temporary water-filled depressions (mud wallows and vernal pools) were found on the midslope-sodic ecotone (close to the seepline) and next to the drainage line (
Various mammals used some of these temporal water bodies as mud wallows for thermal regulation, but it became especially heavily utilised during the intense heat of the extreme drought. The soils of the sodic site and riparian area have a higher clay content (
As is common during drought periods, isolated small-scale rainfall events occurred during the drought and the study area seemingly received more rain during the study period than the surrounding landscapes, resulting in some grass plants retaining above-ground biomass and experiencing limited flushing, with trees retaining green leaves for longer. Anecdotal evidence during the study period suggested occurrence of some precipitation on the watershed between the Sabie and Crocodile River, where the supersite is situated, which is likely to have contributed to the seeming ‘greenness’ experienced. The veld in the study area appeared like a green island in the surrounding drought-stricken area during March 2016 and might have acted as an important critical resource area to sustain herbivores. The severity of the drought eventually took its toll on the vegetation and during October 2016, almost no herbaceous vegetation was observed, only bare soil (pers. obs. October 2016.). The presence of temporary surface water in holes created by large mammals was used for drinking and as mud wallows during 2015 and 2016 drought period (Janecke & Bolton
Plant community and species composition changed in the crest and sodic site zones during the different sampling seasons as the drought progressed. These changes could not be seen in the richness and diversity of plants in these mentioned zones. Changes in the riparian areas were, however, not that clear and can be ascribed to the possible hydraulic lift from deeper soil layers, decreasing the impact of the drought (Van Aardt et al.
This study provides baseline data and biotic–abiotic associations that can be built upon and the hypotheses presented by our conceptual framework can be further tested in future. Future research can
Biodiversity and heterogeneity associated with the catena ecosystem processes are important components in understanding the dynamics of the savanna ecosystem as a whole. Most research projects focus on only one or a few of the biotic and abiotic components, and possibly one or two connections between these components, but studies considering a multitude of biotic and abiotic components and their interaction, feedback and integration are rare. This research aimed to contribute towards the knowledge gap by studying the biotic and abiotic components during the same period and in the same local area, to make the interactions, relationships and processes of the ecosystem more explicit and, hence, providing a systemic conceptual understanding of the patterns and connections within the study system. To better understand the complexity of these ecosystems, there will always be opportunity for research to gain more knowledge and a better understanding of these unique ecosystems.
Furthermore, we trust that this article, and all the studies forming part of the special issue, will contribute valuable baseline information and provide a useful first stab at a conceptual understanding of the patterns, processes and interactions on the KNP Southern Granite Supersite. We envisage that this will stimulate further research on the supersites to become pivotal research sites for multi-disciplinary studies of savanna ecosystems.
The authors would like to acknowledge the University of the Free State (UFS) Strategic Research Fund for funding the multidisciplinary research; SANParks Scientific Services and game guards for their friendly assistance; Dr Marcele Vermeulen from Microbial, Biochemical, and Food Biotechnology (UFS) for her contributions to the article; and the late Dr Fred Kruger (OTS & UFS) who was very enthusiastic and excited about this multidisciplinary project but passed away in 2017 before the publication of the special issue.
The authors declare that they have no financial or personal relationships that may have inappropriately influenced them in writing this article.
All authors directly participated in the study design and interpretation of the multidisciplinary research. The first three authors wrote the main part of the article, whilst B.B.J. and J.v.T. also created the figures with some input from the entire team. The other authors contributed mostly to their expert research field in the article. P.A.L.l.R. is acknowledged for the vision and concept of this research project. M.T.S. and A.C.v.A assisted with language editing.
Ethical approval for the multidisciplinary project as a whole was obtained from the Interfaculty Animal Ethics Committee at the University of the Free State (UFS-AED2019/0121). Declaration: For project number LROPAL1255 (LROPAL1345 extension addendum) as on the contract between the principal investigator and SANParks Scientific Services, the following permits were issued to collect and remove soil samples for laboratory analyses, vegetation samples for identification purposes, mud samples to identify small aquatic organisms and soil samples from rhizospheres for microbe analyses: SK069; SK2095; SK054.
The authors are grateful to the University of the Free State (UFS) Strategic Research Fund for largely funding this multidisciplinary research.
Data sharing is not applicable to this article as no new data were created or analysed during this study.
The views and opinions expressed in this article are the authors’ own and not an official position of the institution or funder.
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