Topographic controls on the invasion of Pteronia incana (Blue bush) onto hillslopes in Ngqushwa (formerly Peddie) district, Eastern Cape, South Africa
- Kakembo, V, Rowntree, Kate M, Palmer, Anthony R
- Authors: Kakembo, V , Rowntree, Kate M , Palmer, Anthony R
- Date: 2007
- Language: English
- Type: Article
- Identifier: vital:6689 , http://hdl.handle.net/10962/d1006708
- Description: The role of topographic factors in the invasion of hillslopes by Pteronia incana, an unpalatable shrub, was investigated. The study combined field observations with image analysis based on high-resolution infrared imagery. A Digital Elevation Model (DEM) of 20 m spatial resolution was used to derive terrain parameters. The Topographic Wetness Index (WI), a component of the TOPMODEL, was derived from the DEM and its relationship with the spatial distribution of P. incana was explored. The absence/presence of P. incana was noted to be strongly influenced by slope angle and aspect. The probability for P. incana occurrence increased with slope steepness and southerly slope orientation. Abandoned and grazing lands were identified as the main invasion hotspots on hillslopes. The combined influence of slope gradient and aspect, and land use was noted to have promoted the invasion. This is borne out by the concentration of the invasion on abandoned steep slopes with a southerly orientation. The WI confirmed the bearing local topographic variations have on P. incana spatial distribution such that, P. incana was associated with the low WI values of convexities. The coupling between local topography and soil surface crusting underpins soil moisture variability. This in turn determines the competition between the patchy P. incana and grass species and the eventual replacement of the latter by the former. Restoration efforts of the invaded lands should focus on trapping of sediment and litter, and moisture retention on the inter-patch bare areas.
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- Authors: Kakembo, V , Rowntree, Kate M , Palmer, Anthony R
- Date: 2007
- Language: English
- Type: Article
- Identifier: vital:6689 , http://hdl.handle.net/10962/d1006708
- Description: The role of topographic factors in the invasion of hillslopes by Pteronia incana, an unpalatable shrub, was investigated. The study combined field observations with image analysis based on high-resolution infrared imagery. A Digital Elevation Model (DEM) of 20 m spatial resolution was used to derive terrain parameters. The Topographic Wetness Index (WI), a component of the TOPMODEL, was derived from the DEM and its relationship with the spatial distribution of P. incana was explored. The absence/presence of P. incana was noted to be strongly influenced by slope angle and aspect. The probability for P. incana occurrence increased with slope steepness and southerly slope orientation. Abandoned and grazing lands were identified as the main invasion hotspots on hillslopes. The combined influence of slope gradient and aspect, and land use was noted to have promoted the invasion. This is borne out by the concentration of the invasion on abandoned steep slopes with a southerly orientation. The WI confirmed the bearing local topographic variations have on P. incana spatial distribution such that, P. incana was associated with the low WI values of convexities. The coupling between local topography and soil surface crusting underpins soil moisture variability. This in turn determines the competition between the patchy P. incana and grass species and the eventual replacement of the latter by the former. Restoration efforts of the invaded lands should focus on trapping of sediment and litter, and moisture retention on the inter-patch bare areas.
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Towards a definition of SUBJECT in binding domains and subject-oriented anaphora
- Authors: de Vos, Mark
- Date: 2007
- Language: English
- Type: Article
- Identifier: vital:6141 , http://hdl.handle.net/10962/d1011593
- Description: The question of subjecthood has dogged linguistic science since ancient times. However, in current versions of Minimalism, subjects do not have primitive status and can only be defined in derived terms. However, subjects and the broader theoretical notion of SUBJECT remain important in linguistic description. This paper develops a definition of subjecthood in terms of set-theoretic notions of functional dependency: when a feature, say phi, determines the value of some other feature, say u-phi. This notion is used to describe various phenomena where subjecthood has been invoked: binding domains and subject-oriented anaphors.
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- Authors: de Vos, Mark
- Date: 2007
- Language: English
- Type: Article
- Identifier: vital:6141 , http://hdl.handle.net/10962/d1011593
- Description: The question of subjecthood has dogged linguistic science since ancient times. However, in current versions of Minimalism, subjects do not have primitive status and can only be defined in derived terms. However, subjects and the broader theoretical notion of SUBJECT remain important in linguistic description. This paper develops a definition of subjecthood in terms of set-theoretic notions of functional dependency: when a feature, say phi, determines the value of some other feature, say u-phi. This notion is used to describe various phenomena where subjecthood has been invoked: binding domains and subject-oriented anaphors.
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Unconscious nobility: the animal poetry of Harold Farmer
- Authors: Wylie, Dan
- Date: 2007
- Language: English
- Type: Article
- Identifier: vital:6122 , http://hdl.handle.net/10962/d1004705
- Description: I want to suggest that Harold Farmer's poetry works repeatedly in this area of ambiguity, a zone of tension triangulated, as it were, between three impulses. First : a notion (or even the fact) that a sense of community depends on 'knowing' what the 'other' is thinking or feeling, and on being able to articulate that knowledge. Second : suspecting, or even knowing, that certain reaches of the mind of the 'other' are fundamentally, and fascinatingly, unknowable - of the realm of the unconscious. And third : knowing (or just fearing or hoping) that any secure distinction between ourselves-as-humans and ourselves-as-sharing-animal-traits is artificial, or at least permeable. Hence, while Farmer's wild animals are perpetually on the brink of disappearing from sight and understanding, it is precisely that mysteriousness which attracts us, can sometimes envelop us, and even speak to us. In having spoken and been spoken to, we are somehow ennobled.
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- Authors: Wylie, Dan
- Date: 2007
- Language: English
- Type: Article
- Identifier: vital:6122 , http://hdl.handle.net/10962/d1004705
- Description: I want to suggest that Harold Farmer's poetry works repeatedly in this area of ambiguity, a zone of tension triangulated, as it were, between three impulses. First : a notion (or even the fact) that a sense of community depends on 'knowing' what the 'other' is thinking or feeling, and on being able to articulate that knowledge. Second : suspecting, or even knowing, that certain reaches of the mind of the 'other' are fundamentally, and fascinatingly, unknowable - of the realm of the unconscious. And third : knowing (or just fearing or hoping) that any secure distinction between ourselves-as-humans and ourselves-as-sharing-animal-traits is artificial, or at least permeable. Hence, while Farmer's wild animals are perpetually on the brink of disappearing from sight and understanding, it is precisely that mysteriousness which attracts us, can sometimes envelop us, and even speak to us. In having spoken and been spoken to, we are somehow ennobled.
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Wave exposure effects on population structure and recruitment in the mussel Perna perna suggest regulation primarily through availability of recruits and food, not space
- McQuaid, Christopher D, Lindsay, Tracy L
- Authors: McQuaid, Christopher D , Lindsay, Tracy L
- Date: 2007
- Language: English
- Type: Article
- Identifier: vital:6968 , http://hdl.handle.net/10962/d1012033
- Description: Recruitment and population structure of Perna perna in low shore mussel beds were investigated over 15 months at six sites along the south coast of South Africa. Initial, subjective classification of sites as wave exposed or wave sheltered (three of each) was confirmed using the dissolution of cement blocks to measure average water flux and dynamometers for maximum wave force. Recruitment occurred throughout the year, but recruit (1–5 mm) densities were significantly higher from January to April 1996 on both shore types. Recruit densities were positively correlated with adult (>15 mm) densities for both shore types (P < 0.05) but the correlations were extremely weak (r 2 < 0.06 in each case). In areas with 100% cover, adult size (mean and maximum lengths) was greater on exposed sites, but density showed the reverse and was negatively correlated with maximum wave strength (r = −0.84). Despite differences in adult densities and sizes, biomass, which is a product of the two, showed no significant difference between the two shore types (ANOVA P > 0.05). Thus wave exposure dramatically affects density, recruitment and mussel size, but not recruitment timing or biomass where there is 100% cover, and mediates a three-way interaction among food supply, larval supply and intraspecific competition for space. In contrast to shores with saturation recruitment, mussel biomass here appears to be limited by recruit supply and constraints of food, especially on sheltered shores, while density is regulated through intraspecific competition for space primarily on exposed shores and at small spatial scales.
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- Authors: McQuaid, Christopher D , Lindsay, Tracy L
- Date: 2007
- Language: English
- Type: Article
- Identifier: vital:6968 , http://hdl.handle.net/10962/d1012033
- Description: Recruitment and population structure of Perna perna in low shore mussel beds were investigated over 15 months at six sites along the south coast of South Africa. Initial, subjective classification of sites as wave exposed or wave sheltered (three of each) was confirmed using the dissolution of cement blocks to measure average water flux and dynamometers for maximum wave force. Recruitment occurred throughout the year, but recruit (1–5 mm) densities were significantly higher from January to April 1996 on both shore types. Recruit densities were positively correlated with adult (>15 mm) densities for both shore types (P < 0.05) but the correlations were extremely weak (r 2 < 0.06 in each case). In areas with 100% cover, adult size (mean and maximum lengths) was greater on exposed sites, but density showed the reverse and was negatively correlated with maximum wave strength (r = −0.84). Despite differences in adult densities and sizes, biomass, which is a product of the two, showed no significant difference between the two shore types (ANOVA P > 0.05). Thus wave exposure dramatically affects density, recruitment and mussel size, but not recruitment timing or biomass where there is 100% cover, and mediates a three-way interaction among food supply, larval supply and intraspecific competition for space. In contrast to shores with saturation recruitment, mussel biomass here appears to be limited by recruit supply and constraints of food, especially on sheltered shores, while density is regulated through intraspecific competition for space primarily on exposed shores and at small spatial scales.
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Xylem as well as phloem sustains severe damage due to feeding by the Russian wheat aphid
- Saheed, S A, Liu, Lin, Jonsson, L M V, Botha, Christiaan E J
- Authors: Saheed, S A , Liu, Lin , Jonsson, L M V , Botha, Christiaan E J
- Date: 2007
- Language: English
- Type: Article
- Identifier: vital:6541 , http://hdl.handle.net/10962/d1005983
- Description: Investigation of comparative effects of feeding damage by the Russian wheat aphid (RWA, biotype SA1, Diuraphis noxia Mordvilko) on leaf blades of susceptible and resistant wheat cultivars (Triticum aestivum L. var Betta and Betta-Dn1 respectively) were carried out to establish the level of ultrastructural damage caused by this aphid and the possible limitation of damage induced which could be ascribed to the resistance gene Dn1 over the susceptible cultivar. Ultrastructurally, Betta-Dn1 sustained less damage to the vascular tissue as well as to the mesophyll during the experimental period. Both inter- and intracellular probes resulted in considerable saliva deposition as the aphids probed for suitable feeding sites. Salivary tracks were observed between and within mesophyll, bundle sheath cells as well as the vascular tissue, including the xylem. Disruption of organelles and cytoplasm resulted from cell probing and sheath deposition. Cell and organelle damage was more evident in the non-resistant Betta cultivar. The aphids probed for and fed from thin-walled sieve tubes preferentially. Few thick-walled sieve tubes showed evidence of either aphid probing or feeding-related damage. Saliva was deposited when the aphids probed inter- and intracellularly for feeding sites. The aphids appeared preferentially to probe for and feed from thin-walled sieve tubes, as few thick-walled sieve tubes showed evidence of damage. Vessels, apparently probed for water, contained watery saliva that encased the secondary walls and sealed pit membranes between probed vessels and xylem parenchyma. The xylem probed by the RWA was rendered non-functional, probably contributing to symptoms of leaf roll, chlorosis and necrosis, which were observed within two weeks of infestation in the susceptible Betta cultivar. This damage was limited in the resistant Betta-Dn1 cultivar during the same time frame.
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- Authors: Saheed, S A , Liu, Lin , Jonsson, L M V , Botha, Christiaan E J
- Date: 2007
- Language: English
- Type: Article
- Identifier: vital:6541 , http://hdl.handle.net/10962/d1005983
- Description: Investigation of comparative effects of feeding damage by the Russian wheat aphid (RWA, biotype SA1, Diuraphis noxia Mordvilko) on leaf blades of susceptible and resistant wheat cultivars (Triticum aestivum L. var Betta and Betta-Dn1 respectively) were carried out to establish the level of ultrastructural damage caused by this aphid and the possible limitation of damage induced which could be ascribed to the resistance gene Dn1 over the susceptible cultivar. Ultrastructurally, Betta-Dn1 sustained less damage to the vascular tissue as well as to the mesophyll during the experimental period. Both inter- and intracellular probes resulted in considerable saliva deposition as the aphids probed for suitable feeding sites. Salivary tracks were observed between and within mesophyll, bundle sheath cells as well as the vascular tissue, including the xylem. Disruption of organelles and cytoplasm resulted from cell probing and sheath deposition. Cell and organelle damage was more evident in the non-resistant Betta cultivar. The aphids probed for and fed from thin-walled sieve tubes preferentially. Few thick-walled sieve tubes showed evidence of either aphid probing or feeding-related damage. Saliva was deposited when the aphids probed inter- and intracellularly for feeding sites. The aphids appeared preferentially to probe for and feed from thin-walled sieve tubes, as few thick-walled sieve tubes showed evidence of damage. Vessels, apparently probed for water, contained watery saliva that encased the secondary walls and sealed pit membranes between probed vessels and xylem parenchyma. The xylem probed by the RWA was rendered non-functional, probably contributing to symptoms of leaf roll, chlorosis and necrosis, which were observed within two weeks of infestation in the susceptible Betta cultivar. This damage was limited in the resistant Betta-Dn1 cultivar during the same time frame.
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