1,720,963 research outputs found
Herbaceous biomass species diversity relationships in nutrient hotspots of a semi-arid African riparian ecosystem
No full textThe hump-back relationship between species diversity and productivity predicts highest species richness at
intermediate levels of biomass, and low species numbers in least and most productive habitats. Sodic patches of
semi-arid savannas are considered ‘nutrient hotspots’ by producing high-quality forage. The impact of biomass
reduction (due to overgrazing) and accumulation (in the absence of herbivores) on herbaceous species richness
and diversity is still unknown for these hotspots. We tested the relationship between biomass and herbaceous
species richness/diversity in savanna sodic sites across varying biomass levels obtained through different
herbivore exclosures. LOWESS and quadratic regression analyses revealed a unimodal species richness/
diversity–biomass relationship for biomass <2 500 kg ha−1. Species richness and diversity in the sodic zone peaked
at 1 300 kg ha−1, followed by a steady decline. At biomass levels exceeding 2 500 kg ha−1, i.e. where herbivores
have been excluded for 10 years, the decline in species richness and diversity stabilised. Despite many debates
surrounding unimodal relationships and the intermediate disturbance hypothesis, this study confirmed a peak
in species richness and diversity at intermediate biomass levels in a herbivory-adapted ecosystem. Herbivores
drive ecosystem heterogeneity and enhance herbaceous species richness and diversity by keeping biomass at
intermediate levels (i.e. <2 500 kg ha−1 for this sodic site
The impact of herbivore exclusion on forb diversity: comparing species and functional responses during a drought
No full textRainfall and herbivory shape savannah herbaceous communities, but these disturbances are being altered globally. To assess potential consequences of such alterations, we evaluated herbivore effects on species and functional diversity during an episodic drought in a sodic savannah using data collected from long‐term herbivore exclosures in the Kruger National Park, South Africa. Herbaceous life forms are rarely acknowledged as distinct functional entities. Moreover, the functional ecology of forbs remains elusive. Here, we present disturbances–responses by forbs separately from grasses. We hypothesised that combinations of intense utilisation and drought would be associated with low diversity and high dominance at species and functional levels for both life forms. Contrary to our hypothesis, low forb and grass diversity was associated with long‐term herbivore exclusion, which exceeded expected undesirable effects of intense utilisation and drought. Grasses responded less sensitively, suggesting that forbs respond dynamically to changes in herbivore assemblage when these alterations are combined with drought. Consistent with patterns in savannah systems, forbs contributed significantly to species and functional trait diversity. High forb diversity is suggested to enhance resilience of this nutrient‐rich ecosystem against declines in its functioning when subjected to drought and alterations in herbivor
Drought responses of forb and grass communities in communal and protected rangelands
Full text linkMSc (Environmental Sciences), North-West University, Potchefstroom Campus, 2019The structure, growth, dynamics and productivity of herbaceous vegetation in semi-arid savannas are strongly limited by rainfall variability and nutrient availability. These factors interact with other drivers of savanna vegetation structure and function, of which herbivory remains among the strongest disturbance agents. Herbivore pressure is applied through grazing and/or browsing by multiple herbivore guilds of which their effects vary among rangeland types and herbivore densities. The spread of African pastoralism along with the predicted increase in frequency and severity of drought events highlights the need to improve our scientific understanding of herbaceous community responses to changes in rainfall variability and rangeland type. While grasses are generally the principal component in rangeland productivity and herbaceous dynamics studies, forbs are disproportionately underrepresented, whilst hosting the most diverse components of rangeland plant communities. Since African savanna vegetation structure and function have co-evolved with native herbivores, the interactive effects of land-use change towards single-species pastoralism and frequent or extensive drought events are expected to prompt significant changes in the herbaceous layer of semi-arid savannas. Grass community responses to changes in rangeland type and rainfall variability are well documented, although forb community dynamics have largely been neglected in the past. Forbs comprise an important part of the herbaceous layer as they significantly contribute to savanna ecosystem diversity and function. Plant functional traits adapted to tolerate disturbances are considered central to the functioning of herbaceous vegetation. Global climate and land use change necessitate the identification of drought- and herbivore tolerance traits to understand the functioning of the complete herbaceous component. The drought of 2014 – 2016 provided a unique opportunity to investigate the effects of a severe drought event on herbaceous community responses to distinct rangeland land-use practices in a nutrient-poor semi-arid Lowveld savanna of South Africa. The study aimed to evaluate and compare the drought responses of forbs and grasses on community- and functional level within two contrasting rangelands (wildlife and livestock). Sampling of herbaceous vegetation data were conducted towards the end of the drought (October 2016), and repeated several months after significant rainfall (January 2017). Field surveys took place in two different rangelands in the Gazankulu area, South Africa. The two rangeland types included a protected area, hosting a diverse community of large indigenous wildlife, and a communal rangeland with a long history of cattle grazing. Sampling of floristic data was conducted per herbaceous life form, permitting comparisons of forb and grass community responses to rainfall variability and rangeland type. Species abundances, basal cover per species and plant functional traits were recorded in a total of 120 fixed 1m2 plots across rangeland types. Results revealed life-form specific responses to rainfall and rangeland type. Post-drought herbaceous communities were significantly different from drought-communities, although grass assemblages in the communal rangeland were not affected by drought release. Forbs responded significantly to rainfall variability and rangeland type on community and functional level. For all the species richness and diversity indices forbs responded significantly within the protected area, except for species evenness. The communal rangeland did not respond significantly for either life form in any of the richness and diversity indices. Grasses responded significantly in species abundance and Shannon-wiener diversity index, however forbs had the greatest interaction with rainfall variability and rangeland type. Plant functional types did not respond as expected. In the drought year perennial forb and grass plant functional types dominated the herbaceous layer. Post-drought conditions were characterised by unpalatable perennial forbs dominated and palatable perennial grass plant functional types. Forbs had higher functional diversity with the most plant functional types within both rainfall years. The communal rangeland functional and compositional characteristics remained constant across rainfall years which could indicate drought-tolerance for this community. However, the protected area did show resilience through high plant functional type abundanceNational Research Foundation (NRF)Unit for Environmental Sciences and Management (UESM)Master
Herbaceous responses to herbivory, fire and rainfall variability differ between grasses and forbs
No full textHerbivory and fire are known to shape plant community structure and function in savanna ecosystems worldwide, yet these drivers are increasingly being altered in their behaviour, or completely excluded. Furthermore, herbaceous responses to rainfall in semi-arid and arid savannas may outweigh the effects of herbivory and fire, especially in nutrient-rich ecosystems. Despite considerable recognition of herbaceous responses to drivers in savanna systems, few studies consider grasses and forbs as distinct herbaceous functional entities. To address this shortcoming, we used long-term herbaceous vegetation data collected from herbivore and fire exclusion treatments in the Kruger National Park (KNP) in South Africa. We investigated response patterns in herbaceous abundances per functional group for three sampling years, which represented different rainfall conditions (i.e., average for 2010, above average for 2001, and below average for 2015. Dynamic shifts in forb versus grass dominance were revealed across treatments. Surprisingly, palatable annual forb communities seemed more resilient to herbivores, or their removal, dominating over palatable annual grass communities. Furthermore, unpalatable perennial forbs were revealed to be more resilient to herbivore presence than their absence. Palatable perennial grass abundances were consistently enhanced by above-average rainfall conditions, whereas equal dominance of palatable grasses and forbs was evident during the drought year. This highlights that palatable perennial functional groups (not grasses alone) provide important ecosystem functions, such as forage stability, and hence functional redundancy to absorb disturbances such as droughts. Moreover, rainfall variability and herbivory are considered the main drivers of palatable perennial functional groups in this nutrient-rich ecosystem. Palatable perennial grass abundances differed significantly from other alleged unfavourable herbaceous functional groups (e.g., unpalatable perennial grasses, annual grasses, and annual and perennial forbs) and, although they presented contrasting patterns for each sampling year, fire and herbivory treatments, palatable perennial grasses remained the dominant functional group in this ecosystem type. Therefore, this study did not support previous findings that conditions such as drought, fire and herbivory favour unfavourable functional groups at the expense of palatable perennial grasses. Observed patterns provide evidence of a dynamic and less predictable coexistence between grasses and forb
The community ecology of herbaceous vegetation in a semi-arid sodic savanna
Full text linkPhD (Environmental Sciences), North-West University, Potchefstroom Campus, 2018.Ecological conservation of savanna ecosystems is dependent upon interactive mechanisms involving bottom-up drivers such as nutrient availability, and top-down controls relating to fire, herbivory and water availability at various spatial scales. Alterations in diverse savanna herbivore communities, suppression of natural fire regimes and increased rainfall variability alter the functioning of these mechanisms. Protected areas, such as the Kruger National Park (KNP), South Africa, provide valuable natural experimental settings where vegetation structure and function are being maintained by similar drivers under which they have evolved. Since plant communities form the structural and functional basis for most terrestrial ecosystems, functional understanding of species is becoming progressively important. Despite increased awareness of understanding resilience in complex systems, there is limited information available on the underlying functions of herbaceous life forms. This is particularly true for the forb component within the herbaceous layer, which is generally overlooked in ecological studies. Research presented in this thesis primarily aimed to evaluate how the species–and functional composition of the herbaceous layer of a semi-arid sodic savanna responds to changes in herbivory, fire and rainfall. Since the observed patterns in savanna community ecology is driven by underlying effects of herbivory, fire and rainfall variability, this study furthermore aimed to evaluate how interactions within the herbaceous component relate to the drivers they are exposed to, or released from. Specific objectives were therefore to test interactive effects of long-term exposure and/or exclusion of: (1) herbivory and fire on forb and grass diversity and abundance patterns of various functional groups, and how these effects interact with rainfall variability in a nutrient-rich semi-arid savanna ecosystem without elephants, (2) elephants (partial herbivore loss) versus all large mammalian herbivores (LMH) (total herbivore loss) on forb and grass diversity patterns and differences in forb and grass abundances of various functional groups, and how these effects interact with rainfall variability in a fire-excluded nutrient-rich semi-arid savanna system, (3) herbivory and fire during an episodic drought on system function by evaluating patterns in herbaceous species composition, trait diversity and functional group assemblages in a system without elephants, and (4) herbivory during an episodic drought on system function by evaluating patterns in herbaceous species composition, trait diversity and functional group assemblages in a system without fire. Results obtained from this study suggested that semi-arid savanna herbaceous community dynamics are largely dependent on variable life-form (i.e., grass and forb) responses to common savanna drivers at both species and functional
level. These findings add to current understanding of the community ecology of savanna herbaceous layers by acknowledging the important ecological role of a previously neglected herbaceous life-form, the herbaceous forb component. However, further research on forbs within sites with different soil conditions and geographical aspects is necessary to improve the understanding of savanna herbaceous communities and hence the management of herbivore forage security when considering complex environmental changes.National Research Foundation (NRF)Doctora
Herbaceous plant diversity responses to various treatments of fire and herbivory in sodic patches of a semiarid riparian ecosystem
Full text linkMSc (Environmental Sciences), North-West University, Potchefstroom Campus, 2014Understanding relationships between large herbivores and plant species diversity in dynamic riparian
zones, and more specifically sodic zones, is critical to biodiversity conservation. Sodic patches form
an integral part of savanna ecosystems because of the ecosystem services and functions they provide,
i.e. accumulation of nutrients, provision of open spaces for predator vigilance and formation of wet
season wallowing points. Furthermore, these key resource areas sustain body condition for dry season
survival and support reproduction through nutritional benefits, making them „nutrient hotspots.‟
The Nkuhlu research exclosures in the Kruger National Park (KNP) provide a unique opportunity to
investigate spatial and temporal heterogeneity patterns within riparian zones, and how these patterns
are affected by fire and herbivory. A monitoring project was initiated to answer questions pertaining
to the dynamics of the herbaceous layer and was aimed at determining (a) whether there exists
meaningful variance in herbaceous plant species richness and diversity across different treatments of
fire and herbivory in the ecologically sensitive sodic zone, (b) if temporal shifts in plant species
composition and diversity occurs, (c) whether an increase in herbaceous biomass, an artifact of
herbivory and fire exclusion, suppresses herbaceous plant species diversity and richness, and (d)
whether there exists a significant relationship between herbaceous biomass and species
richness/diversity. The Nkuhlu exclosures consist of three herbivory treatments, each divided into a
fire and no-fire treatment, hence six treatment combinations overall. Herbivory treatments consisted
of, (1) a partially fenced area designed to specifically exclude elephants (giraffes are also excluded
due to body size), (2) an open, unfenced area and (3) a fully fenced area, designed to exclude all
herbivores larger than a hare. Herbaceous vegetation was sampled in two 1 m2 circular sub-plots in
the eastern and western corners of each of the 82 fixed plots. Biomass of each plot was estimated with
a Disc Pasture Meter (DPM) by sampling ten points diagonally within each plot. DPM-readings were
converted to kg/ha according to latest conversions for the Lowveld Savanna.
Species richness and biomass showed significant variance across treatments for the 2010 dataset,
whereas no significant variation in herbaceous species diversity was perceived. Combined treatment
of fire absence and herbivore presence contributed to higher forb species richness in the sodic zone.
Biomass was significantly higher in fully fenced areas where herbivores were excluded, opposed to
the open and partially fenced areas. Although no significant variation was recorded for diversity
across treatments, lowest diversity was recorded in the absence of all herbivores, especially in
combination with fire treatment. After nine years of herbivory exclusion, diversity of herbaceous
species varied significantly. Herbaceous species composition changed over time in areas exposed to herbivory, while composition of fully fenced treatments did not reveal change. A hump-shaped
relationship exists between herbaceous species richness/diversity and field biomass, at least for areas
with biomass levels not exceeding 2500 kg/ha. Herbivores are therefore considered essential in
sustaining herbaceous plant species richness and system heterogeneity in the sodic zone, since
herbaceous species richness/diversity was higher in herbivore presence and herbaceous species
composition changed over time in areas exposed to herbivory. Although statistically non-significant,
fire seems to suppress species richness.
Conservation implications: This study could be used as framework to advance and develop sciencebased
management strategies for, at least, the sodic zones of the KNP. Research in these exclosures
contributes to our understanding of these landscapes and benefit ecosystem conservation planning. It
also provides valuable long-term data for key ecological processes.Master
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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