Papers by Ursula Scharler
Whole food webs studies – Mangroves
FlowCAr
FlowCAr R package Flow Network Construction and Assessment in R Network modelling requires a coge... more FlowCAr R package Flow Network Construction and Assessment in R Network modelling requires a cogent, standardised methodology for the vital construction phase to facilitate appropriate interpretation of outcomes from the final analysis. Different approaches to network construction or single solutions have been commonplace in network modelling over the past decades and could be improved with multi-solution analysis of a single system to account for the inherent variability of input data. Linear inverse modelling (LIM) can be used to quantify complex flow networks by calculating network link values from under sampled data, typical for ecological networks. The resultant networks can be analysed using Ecological Network Analysis (ENA), a branch of network ecology used to holistically analyse the structure and dynamics of interactions in networks. The FlowCAr package provides a standardised methodology for the construction phase of network modelling, producing a range of mathematically a...
Globally estuaries are under increasing pressure from human development impacts as well as the gr... more Globally estuaries are under increasing pressure from human development impacts as well as the growing effects of climate change that is already, and increasingly so, causing major changes in events such as droughts. Although droughts are regular occurrences and are projected to become progressively more significant in many parts of the world, their potential impacts on estuaries requires better understanding and quantification. This study aims to quantify environmental and phytoplankton parameter changes in two contrasting subtropical estuary types in South Africa; the temporarily closed uMdloti Estuary and the predominantly open to the sea uMlalazi Estuary during a hydrological drought event and subsequent non-drought period. Drought caused lower salinities, almost exclusively freshwater, throughout the temporarily closed estuarine type uMdloti. The marine-estuarine salinity environment of the predominantly open uMlalazi during drought expanded to include lower salinities in the n...
African Journal of Aquatic Science, 2020
Prolonged mouth closure in estuaries is often associated with hypersalinity, mainly as a result o... more Prolonged mouth closure in estuaries is often associated with hypersalinity, mainly as a result of reduced freshwater inflow. However, on the KwaZulu-Natal coast, South Africa, temporarily open/closed estuaries have a tendency towards hyposalinity during closed mouth conditions, primarily because of their perched nature reducing saltwater intrusion, but also due to persistent freshwater inputs, together with seepage of saline water through the sand berm into the sea. We investigated a system, the uMdloti Estuary, experiencing extended periods of mouth closure over two years with the aim of documenting the response of invertebrates to such conditions. Specifically, estuarine invertebrate biodiversity was severely diminished, which was attributed to persistently low salinity, rather than poor water quality due to the absence of flushing. Secondly, a distinct brackish/freshwater community established itself within the mesozooplankton and benthic invertebrate assemblages, including invasive species. Marine overwash of the berm and a short mouth breaching aided the return of estuarine species for only very brief periods (weeks). We conclude that although estuarine species are quick to re-colonise the estuary after a breach, prolonged mouth closure along extended stretches of coast might severely reduce estuarine invertebrate biodiversity. Management options are preferably the provision of an adequate freshwater inflow to allow natural breaching, or else artificial breaching, to facilitate several openings per year as well as for a long enough tidal phase to permit the establishment of an estuarine salinity gradient.
South African Journal of Science, 2019
In their article 'Science in the service of society: Is marine and coastal science addressing Sou... more In their article 'Science in the service of society: Is marine and coastal science addressing South Africa's needs', Cochrane et al. 1 express concern, based on an analysis of abstracts from a single South African Marine Science Symposium (SAMSS 2017) that too little research is either interdisciplinary or 'actionable'-defined as science whose results translate easily to policies, management actions or industry. They argue that science is disconnected from the needs of society, may not adequately benefit society and, therefore, that the science risks losing support and credibility. To remedy such drawbacks, they propose funding actionable science as a priority, including in the tertiary education arena, and emphasising interdisciplinary research. In conclusion, they offer eight recommendations for future funding of marine science. Because their article may substantially influence the policies of funding agencies and thus the trajectory of marine science in South Africa, it is important that their claims are examined in the wider context of how science benefits society. Cochrane et al. 1 's arguments are built on four premises which do not stand up to scrutiny. Firstly, they assume a clear distinction between pure, basic and applied (or actionable) research. Secondly, they underestimate, to the point of discounting, the value of 'pure' science in advancing the goals and imperatives that, in their opinion, should be urgently addressed. Thirdly, they assume an unrealistic linearity in the way that science translates to policy and management. Finally, they do not account for the manner in which the funding system already addresses many of their concerns.
Ecological Indicators, 2019
Ecological Network Analysis (ENA) has provided insights into the structure, function, and transfo... more Ecological Network Analysis (ENA) has provided insights into the structure, function, and transformation of ecosystems for more than forty years. Key insights from ENA focus on how the patterns of directed weighted transactions among system components (e.g., species, functional groups, economic sectors) create emergent and often unexpected relationships in ecosystems that affect system function and sustainability. Flow analysis, also called throughflow analysis, is one of several core techniques in ENA. Generally, it traces the flux of energy or matter through the network from inputs to outputs. During the forty-years of development, flow analysis has accreted multiple extensions and modifications. In this concept and synthesis paper, we review four flow analyses and show how they are conceptually linked by partitioning flows across subsets of pathways within networks. These flow analyses include: (1) the definition of throughflow, a measure of the total processing power of a network; (2) Leontief's decomposition based on walk length, indicating the direction and distance of energy or matter flow; (3) Finn's measure of recycling of matter in networks; and (4) five mode analysis, characterizing flows according to their origin and destination. Presenting these techniques side-by-side with a common conceptual framework reveals overlaps and distinctive elements among the analytic products. This synthesis clarifies the flow analyses tools and their applications to ecological and socioeconomic networks and provides example applications. Further, new insights are presented by combining existing flow analyses to calculate novel indices that further characterize the flow structure of networks. For example, both indirect flows in networks and cycling are highly important features in networks. In order to determine the proportion of indirect flows generated through cycling, we can use the ratio of Cycled Flow identified from Finn's analysis and the indirect flows identified in the Leontief analysis. As ENA matures through additional analysis development and applications, it will continue to provide insights into ecosystems and contribute to the broader area of network science.
Ecological Indicators, 2017
Various indicators rooted in the concepts of information and entropy have been proposed to be use... more Various indicators rooted in the concepts of information and entropy have been proposed to be used for ecological network analysis. They are theoretically well grounded and widely used in the literature, but have always been difficult to interpret due to an apparent lack of strict relations with node and link weight. We generated several sets of 10,000 networks in order to explore such relations and work towards a sounder interpretation. The indices we explored are based on network composition (i.e., type and importance of network compartments), or network flows (i.e., type and importance of flows among compartments), including Structural Information (SI), Total System Throughput (TST), Average Mutual Information (AMI), Flow Diversity (H), and Ascendency (ASC). A correlation analysis revealed a lack of strict relationships among the responses of the investigated indicators within the simulated space of variability of the networks. However, fairly coherent patterns of response were revealed when networks were sorted by following a "bottom-up" criterion, i.e. by increasing the dominance of the large-sized top predator in the network. This ranking is reminiscent of ecosystem succession, along which the prominence of higher trophic level organisms progressively increases. In particular, the results show that a simple increase in organisms having large size and low consumption rates is potentially able to simultaneously lead to an increase of different types of information (as SI, H and AMI), thus also emphasizing the importance of bionomic traits related to body size in affecting information-related properties in a trophically connected community. The observed trends suffer from a certain dispersion of data, which was diminished by imposing specific and ecologically meaningful constraints, such as mass balancing and restriction to certain range of the ratio A/C, an index related to the viability of ecological networks. These results suggest that the identification of a set of effective constraints may help to identify improved conditions for applicability of the investigated flow-based indicators, and also provide indication on how to normalise them with respect to meaningful network properties or reference states. Thus, in order to increase confidence in the derived network metrics describing a particular ecosystem state, and thus increase their applicability, it is advisable to construct replicate networks by taking the variability of input data into account, and by applying uncertainty and sensitivity analyses.
Systems Ecology: Ecological Network Analysis, Ascendency
Whole Food-Web Studies
Treatise on Estuarine and Coastal Science, 2011
Whole food-web studies of mangrove ecosystems are rare. However, several components of the flora ... more Whole food-web studies of mangrove ecosystems are rare. However, several components of the flora and fauna and selected linkages from primary food sources (primary producers and detritus) and between heterotrophs have been studied in detail. Considerable attention has been given to carbon imports and exports from mangrove ecosystems in the form of Dissolved Organic Carbon (DOC), Particulate Organic Carbon (POC), and CO2. The comparatively few whole food-web studies on mangroves from different continents thus far produced fairly wide ranges (and thus few generalizations) of ecosystem level indices, including trophic efficiencies, extent of recycling, or total energy throughput.
Ecological Network Analysis, Ascendency
Encyclopedia of Ecology, 2008
The theory of ascendency was developed to express a nonmechanical view of ecosystem function and ... more The theory of ascendency was developed to express a nonmechanical view of ecosystem function and development. Ascendency (A) captures the growth and development of ecosystems. Growth is denoted by the total system throughput. Development is calculated by means of information theory as average mutual information (AMI). AMI is the information gained by reducing the uncertainty about where a quantum of material flows in a trophic flow network. Initial uncertainty conditions are calculated by Shannon’s diversity index, and a posteriori uncertainties by conditional probabilities of the occurrence of a transfer. As such, ascendency describes the organization (degree of constraints) of ecosystems. Its upper end is the initial uncertainty, or diversity of flows. The difference between the initial uncertainty and A is called the overhead, which constitutes the remaining uncertainty and indeterminacy in the network. Overheads are apparent due to imports, exports, respiration, and parallel pathways or redundancies. Ecosystems live with a tradeoff between organized performance (A) and sundry repertoires to insure reliability (overhead). Ecosystems are thought to show a propensity towards increasing A. The concept of ascendency has been extended to describe nutrient limitations sensu Liebig’s law of the minimum and thus provides a theoretical basis for Liebig’s law.
Estuarine, Coastal and Shelf Science, 2015
In South African estuaries, there is no knowledge on the resilience and variability in phytoplank... more In South African estuaries, there is no knowledge on the resilience and variability in phytoplankton communities under conditions of hypersalinity, extended droughts and reverse salinity gradients. Phytoplankton composition, abundance and biomass vary with changes in environmental variables and taxa richness declines specifically under hypersaline conditions. This research thus investigated the phytoplankton community composition, its resilience and variability under highly variable and extreme environmental conditions in an estuarine lake system (Lake St. Lucia, South Africa) over one year. The lake system was characterised by a reverse salinity gradient with hypersalinity furthest from the estuarine inlet during the study period. During this study, 78 taxa were recorded: 56 diatoms, eight green algae, one cryptophyte, seven cyanobacteria and six dinoflagellates. Taxon variability and resilience depended on their ability to tolerate high salinities. Consequently, the phytoplankton communities as well as total abundance and biomass differed along the salinity gradient and over time with salinity as the main determinant. Cyanobacteria were dominant in hypersaline conditions, dinoflagellates in marine-brackish salinities, green algae and cryptophytes in lower salinities (brackish) and diatoms were abundant in marine-brackish salinities but survived in hypersaline conditions. Total abundance and biomass ranged from 3.66 Â 10 3 to 1.11 Â 10 9 Cells/L and 1.21 Â 10 6 to 1.46 Â 10 10 pgC/L respectively, with the highest values observed under hypersaline conditions. Therefore, even under highly variable, extreme environmental conditions and hypersalinity the phytoplankton community as a whole was resilient enough to maintain a relatively high biomass throughout the study period. The resilience of few dominant taxa, such as Cyanothece, Spirulina, Protoperidinium and Nitzschia and the dominance of other common genera such as Chlamydomonas, Chroomonas, Navicula, Gyrosigma, Oxyrrhis, and Prorocentrum, provided the carbon at the base of the food web in the system and showed that even during the extended period of drought, a foundation for productivity can be provided for once conditions improve.
Salinity tolerance of the South African endemic amphipodGrandidierella lignorum(Amphipoda: Aoridae)
African Journal of Aquatic Science, 2014
ABSTRACT The endemic amphipod Grandidierella lignorum is an organism potentially suited to the to... more ABSTRACT The endemic amphipod Grandidierella lignorum is an organism potentially suited to the toxicity testing of coastal water and sediment in South Africa. The ranges of salinity and temperature at which G. lignorum can be used for toxicity testing were investigated under laboratory conditions, in order to avoid potential confounding effects induced by these non-contaminant factors on test data interpretation. The data are discussed in the context of the known ecology of this amphipod. Amphipods were exposed to salinities of 0–56 for 96 h. Salinities were prepared using natural seawater and synthetic sea salt. Grandidierella lignorum tolerated all salinities, but showed highest survival at salinities of 7–42. Salinity tolerance was modified by temperature, with highest survival occurring between 10 and 25 °C. These represent the range of conditions at which toxicity testing can be performed. Salinity tolerances to natural and synthetic seawater did not differ significantly. Synthetic sea salt can thus be used to manipulate the salinity of media for toxicity testing without it acting as a confounding variable. The tests performed also highlight the importance of resting laboratory-cultured G. lignorum for up to six weeks between the harvesting of individuals for use in experiments.
Quantifying the impact of stress on soil ecosystem development using Information Indices (Ecosystem Network Analysis)
Marine Ecology Progress Series, 2014
Central issues for aquatic food webs
Dynamic Food Webs, 2005
Aquatic ecosystems worldwide provide important resources for human populations. Estuarine and mar... more Aquatic ecosystems worldwide provide important resources for human populations. Estuarine and marine habitats provide many essential ecosystem services, including climate regulation, yet currently these systems are impacted over large areas through over-exploitation and degradation. Moreover, aquatic ecosystems represent an integrative picture of environmental impacts and management practices operating in the terrestrial hinterland. Scientists have as yet not been able to develop predictive whole ecosystem simulation models, and no single method of food web analysis has been subjected to extensive validation of its predictive capabilities. This chapter describes factors that act at the level of individuals and populations, but whose influence on the food web dynamics and ecosystem functioning is as yet poorly understood. It is suggested that the neglect of these factors, among others to be discovered and explored, contributes to the uncertainty of predictive ecosystem simulation models. The use of larger-scale network approaches and the need for quantitative descriptors of food webs is emphasized, and concluded with challenges in application of food web theory to management issues in aquatic systems.
Aquatic Food Webs
This book provides a synthesis of theoretical and empirical food web research. Whether they are b... more This book provides a synthesis of theoretical and empirical food web research. Whether they are binary systems or weighted networks, food webs are of particular interest to ecologists by providing a macroscopic view of ecosystems. They describe interactions between species and their environment, and subsequent advances in the understanding of their structure, function, and dynamics are of vital importance to ecosystem management and conservation. This book covers issues of structure, function, scaling, complexity, and stability in the contexts of conservation, fisheries, and climate. Although the focus of this volume is upon aquatic food webs (where many of the recent advances have been made), many other issues are addressed.
Organic Geochemistry, 2003
The response of mangrove ecosystems to environmental change can be examined with stable isotopic ... more The response of mangrove ecosystems to environmental change can be examined with stable isotopic tracers of C and N. The 13 C and 15 N of a taphonomic series of Rhizophora mangle L. (Red mangrove) leaves were analyzed from Twin Cays, Belize, to facilitate reconstruction of past mangrove ecosystems. On Twin Cays, fresh leaves of dwarf R. mangle trees ($ 0.5 m high) were found to have more negative d 15 N values (mean=À10%) and more positive 13 C values (mean=À25.3%) compared to tall R. mangle trees (mean d 15 N=0%, d 13 C=À28.3%). These isotopic differences can be related to nitrogen and phosphorus availability [Ecology 83 (2002) 1065]. We investigated three taphonomic stages in the fossilization of R. mangle leaves into peat with the following: (1) senescent leaves; (2) fallen leaves on the surface of the peat; and (3) sub-fossil leaves found within mangrove peat. In addition, by examining natural leaf assemblages we established that 13 C and 15 N of R. mangle leaves were not altered during senescence, despite a significant (50%) decrease in the N%. Modern dwarf and tall trees could still be identified from 13 C and 15 N analyses of the leaf assemblages found directly below a tree. Dwarf and tall trees could also be identified from 13 C analyses of leaves that had decomposed for four months. Although dwarf and tall trees could not be statistically separated after four months according to d 15 N analyses, leaves with very negative 15 N (À7%) were only collected below dwarf trees. Leaf fragments were present in $ 50 cm long cores of peat from four sites on the island, and their isotopic compositions were determined. The ranges of 13 C (À29 to À22%) and 15 N (À11 to +2%) values from sub-fossil leaves were similar to the ranges from modern leaves (d 13 C=À29 to À23%, d 15 N=À11 to +1%). The sub-fossil leaf isotopic compositions were independent values, in comparison to the uniform values of the surrounding peat. Because of the stability and persistence of the stable isotopic signals, they could contribute significantly to a multi-proxy approach to mangrove palaeoenvironmental reconstruction.
Journal of Marine Systems, 2005
Carbon flow networks of the Kromme, Swartkops and Sundays estuaries, situated on the south coast ... more Carbon flow networks of the Kromme, Swartkops and Sundays estuaries, situated on the south coast of South Africa, were analysed using ecological network analysis. A major difference between the three estuaries is the freshwater inflow regimes: The Kromme Estuary receives very little freshwater (annual mean 0.07 m À3 s À1), the Swartkops Estuary receives annually about 0.82 m À3 s À1 while the Sundays Estuary receives on average about 2.74 m À3 s À1 annually. Ecological network analysis revealed differences between most ecosystem attributes such as the cycling of carbon, trophic structure, the organisational and developmental status in terms of ascendency and redundancy, and regarding the contribution to and dependencies of compartments to and on one another in the network. Due to the lack of frequent freshwater inflow and consequently the renewal of the nutrient pool, the Kromme Estuary recycles most of its material and showed the highest detritivory/herbivory ratio (57:1), whereas the Sundays recycles the least proportion and had the lowest detritivory/herbivory ratio at 10:1. The Sundays Estuary, hitherto believed to be dominated by pelagic production, was found to rely more on the benthic biota in terms of carbon throughput as inferred from the contribution and dependency coefficients, than on pelagic communities. Due to the low rate of fresh water inflow into the Kromme Estuary, the absence of an axial salinity gradient, a comparatively high Finn Cycling Index of 40%, and the long water exchange time, this system appears to have developed into an barmQ of the sea. System level properties such as the A/C ratio, the Average Mutual Information index, and the food web connectance index, increase from the lowest values calculated for the Kromme Estuary, intermediate for the Swartkops Estuary, and highest for the Sundays Estuary, while the FCI followed an inverse trend between the three systems.
Journal of Marine Systems, 2011
Data-poor environments Ecosystem modelling allows for an understanding of the structure and funct... more Data-poor environments Ecosystem modelling allows for an understanding of the structure and functioning of data-limited ecosystems provided that models undergo extensive sensitivity analyses to explore the levels of uncertainty. We explored one such data-limited system, the KwaZulu-Natal (KZN) Bight, a river-influenced bight on the east coast of South Africa. Potential system states of the KZN Bight were created by constructing multiple models in Ecopath with Ecosim, carrying out sensitivity analyses and comparing outputs. Sensitivity analyses showed that models were most sensitive to apex predator parameters and a comparison of outputs showed the important influence of riverine detritus on system functioning. To demonstrate the KZN Bight models could reproduce known differences to other ecosystems a comparison of the nutrient-poor KZN Bight to the nutrient-rich Southern Benguela was carried out. This confirmed that the KZN Bight was considerably smaller in biomass, productivity and fishery landings than the Southern Benguela with the systems being detritusdriven and phytoplankton-driven respectively. The KZN Bight relied on large detritus imports from rivers and had higher cycling through the system. The reliance on detritus import from rivers has riverine and fishery management implications as a decrease in riverine detritus caused a decrease in biomass of biotic groups, including targeted fishery species.
Uploads
Papers by Ursula Scharler