The role of microhabitats within mangroves: an invertebrate and fish larval perspective
- Authors: Vorsatz, Lyle Dennis
- Date: 2020
- Subjects: Mangrove ecology -- South Africa , Mangrove forests -- South Africa , Niche (Ecology) , Rhizophora mucronata , Acanthaceae , Rhizophoraceae , Fishes -- Larvae -- South Africa , Aquatic ecology -- South Africa
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/167644 , vital:41499
- Description: Microhabitats provided through structural complexity are central for the diversity, productivity, connectivity and niche differentiation within and among ecosystems. Mangrove forests afford juvenile fish and invertebrates with nursery and recruitment habitats, facilitated by the fine scale configuration of their specialised root systems. Although the importance of mangroves for resident and transient juveniles is well recognised, the roles that mangrove microhabitats play for larvae is not yet comprehensively understood. This study aimed to determine how microhabitats with varying degrees of complexity influence the composition, abundance and distribution of larval communities that inhabit mangrove forests and the physiological responses of larvae to acute temperature variations in relation to ontogenetic stage and microenvironment exposure. Two relatively pristine study sites were selected to represent a warm temperate and subtropical mangrove system in the Eastern Cape and KwaZulu-Natal on the east coast of South Africa, respectively. The differences in complexity among the root systems of Rhizophora mucronata, Avicennia marina and Bruguiera gymnorhiza were assessed using 3D scanning and the computed 3D models were then analysed using four complexity metrics. Results indicated that A. marina is the most complex in terms of surface-volume ratio, R. mucronata has the most interstitial space among its roots and B. gymnorhiza and R. mucronata differ in their fractal dimensions. Larvae collected in each microhabitat at each site using light traps showed that, despite temperature and salinity homogeneity across microenvironments, spatio-temporal differences occurred in both fish and invertebrate assemblages. This trend suggests that microhabitat structural complexity exerts an influence on larval community composition by acting as a microscape of available habitat, which ensures ecological linkages within and among the mangrove forest and adjacent ecosystems. In addition, the oxygen consumption rates of mangrove-associated brachyuran larvae varied according to mangrove microhabitat, whereby larvae collected at less complex environments had the highest metabolic rates at increased temperatures. Moreover, ontogenetic shifts in physiology were prevalent as older brachyuran larvae were more eurythermal than earlier stages, suggesting that thermally stressful events will have a greater impact on recently spawned larvae. Overall, the interstitial spaces within individual root systems are the most important complexity measure, as utilisation of these mangrove microhabitats is scale-dependent, and larvae will most likely occupy spaces inaccessible to large predators. Likewise, microscale variation in the environmental conditions and ontogenetic stage of brachyuran larvae within the mangrove microscape, can amplify the physiological responses to rapid temperature variations. Results suggest that early stage larvae are the most vulnerable to mass-mortality, and if thermally stressful events increase in frequency, duration and magnitude, the larval supply for the successful recruitment into adult populations could be under threat. Through linking how mangrove microhabitat complexity influences larvae in terms of community metrics and physiology, this study paves the way for further advancement of our understanding of how microscale processes emerge into meso- and macroscale patterns and influence the stability and functioning of highly productive ecosystems.
- Full Text:
- Date Issued: 2020
- Authors: Vorsatz, Lyle Dennis
- Date: 2020
- Subjects: Mangrove ecology -- South Africa , Mangrove forests -- South Africa , Niche (Ecology) , Rhizophora mucronata , Acanthaceae , Rhizophoraceae , Fishes -- Larvae -- South Africa , Aquatic ecology -- South Africa
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/167644 , vital:41499
- Description: Microhabitats provided through structural complexity are central for the diversity, productivity, connectivity and niche differentiation within and among ecosystems. Mangrove forests afford juvenile fish and invertebrates with nursery and recruitment habitats, facilitated by the fine scale configuration of their specialised root systems. Although the importance of mangroves for resident and transient juveniles is well recognised, the roles that mangrove microhabitats play for larvae is not yet comprehensively understood. This study aimed to determine how microhabitats with varying degrees of complexity influence the composition, abundance and distribution of larval communities that inhabit mangrove forests and the physiological responses of larvae to acute temperature variations in relation to ontogenetic stage and microenvironment exposure. Two relatively pristine study sites were selected to represent a warm temperate and subtropical mangrove system in the Eastern Cape and KwaZulu-Natal on the east coast of South Africa, respectively. The differences in complexity among the root systems of Rhizophora mucronata, Avicennia marina and Bruguiera gymnorhiza were assessed using 3D scanning and the computed 3D models were then analysed using four complexity metrics. Results indicated that A. marina is the most complex in terms of surface-volume ratio, R. mucronata has the most interstitial space among its roots and B. gymnorhiza and R. mucronata differ in their fractal dimensions. Larvae collected in each microhabitat at each site using light traps showed that, despite temperature and salinity homogeneity across microenvironments, spatio-temporal differences occurred in both fish and invertebrate assemblages. This trend suggests that microhabitat structural complexity exerts an influence on larval community composition by acting as a microscape of available habitat, which ensures ecological linkages within and among the mangrove forest and adjacent ecosystems. In addition, the oxygen consumption rates of mangrove-associated brachyuran larvae varied according to mangrove microhabitat, whereby larvae collected at less complex environments had the highest metabolic rates at increased temperatures. Moreover, ontogenetic shifts in physiology were prevalent as older brachyuran larvae were more eurythermal than earlier stages, suggesting that thermally stressful events will have a greater impact on recently spawned larvae. Overall, the interstitial spaces within individual root systems are the most important complexity measure, as utilisation of these mangrove microhabitats is scale-dependent, and larvae will most likely occupy spaces inaccessible to large predators. Likewise, microscale variation in the environmental conditions and ontogenetic stage of brachyuran larvae within the mangrove microscape, can amplify the physiological responses to rapid temperature variations. Results suggest that early stage larvae are the most vulnerable to mass-mortality, and if thermally stressful events increase in frequency, duration and magnitude, the larval supply for the successful recruitment into adult populations could be under threat. Through linking how mangrove microhabitat complexity influences larvae in terms of community metrics and physiology, this study paves the way for further advancement of our understanding of how microscale processes emerge into meso- and macroscale patterns and influence the stability and functioning of highly productive ecosystems.
- Full Text:
- Date Issued: 2020
Interspecific interactions of the sea urchin Parechinus angulosus and the effect of variations in microhabitat availability
- Authors: Farquhar, Michael Robert
- Date: 1995
- Subjects: Sea urchins , Niche (Ecology) , Sea urchins -- Habitat
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5720 , http://hdl.handle.net/10962/d1005406 , Sea urchins , Niche (Ecology) , Sea urchins -- Habitat
- Description: Intertidal populations of the sea urchin Parechinus angulosus were examined at seven sites along the south and east coasts of South Africa. At the five southerly sites, P. angulosus occurred in allopatry, while, at the two northerly sites, it occurred in sympatry with several species of Indopacific sea urchins. At the five southerly sites, there was a significant correlation of number of sea urchins per pool with the surface area of the overhang in that pool. This relationship broke down for P. angulosus at the two northerly sites, where there was a significant correlation between these two factors for Stomopneustes variolaris. The density of populations of P. angulosus at the two northerly sites, although not significantly different from all five southerly sites, were considerably lower. There was a significant difference between the mean size of populations at the seven sites. Although no direct evidence is available, a competitive interaction between these two species is proposed to explain the observed patterns of microhabitat utilization and densities. Two series of manipulative experiments were conducted at Kowie Point, where P. angulosus occurs in sympatry, The first, involved urchin removals from three experimental sites. The percentage cover of four functional algal groups was monitored in three experimental and three unaltered control sites over a three month period in spring (Sept. - Dec.) 1993. Two 100 point 0,25m² random point quadrats were thrown at each of the six sites at approximately monthly intervals over the experimental period. Since initial cover of algae varied at the six sites, ANOVA's of the change in percentage cover of four functional algal groups were used to determine treatment effects. No treatment effects were identified for any of the four functional algal groups. There was a strong negative correlation between the percentage cover of foliose algae and encrusting coralline algae, suggesting the possibility of a competitive relationship between them. The second, involved the experimental manipulation of microhabitat availability. The blocking of overhangs, by means of cement filled bags, resulted in an approximate 50% decrease in the total number of urchins in the two experimental pools, and the two control pools without overhangs. However, in the control pool with overhangs there was a slight increase in the number of urchins over the same period. Clearly, the presence of suitable shelters, is a prerequisite for the maintenance of dense intertidal population of P. angulosus. It is proposed that, due to the exposed nature of the South African coast, intertidal populations of P.angulosus are restricted to inhabiting suitable shelters from which they emerge to feed on passing drift algae. The implications of these findings are discussed in terms of current ecological literature.
- Full Text:
- Date Issued: 1995
- Authors: Farquhar, Michael Robert
- Date: 1995
- Subjects: Sea urchins , Niche (Ecology) , Sea urchins -- Habitat
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5720 , http://hdl.handle.net/10962/d1005406 , Sea urchins , Niche (Ecology) , Sea urchins -- Habitat
- Description: Intertidal populations of the sea urchin Parechinus angulosus were examined at seven sites along the south and east coasts of South Africa. At the five southerly sites, P. angulosus occurred in allopatry, while, at the two northerly sites, it occurred in sympatry with several species of Indopacific sea urchins. At the five southerly sites, there was a significant correlation of number of sea urchins per pool with the surface area of the overhang in that pool. This relationship broke down for P. angulosus at the two northerly sites, where there was a significant correlation between these two factors for Stomopneustes variolaris. The density of populations of P. angulosus at the two northerly sites, although not significantly different from all five southerly sites, were considerably lower. There was a significant difference between the mean size of populations at the seven sites. Although no direct evidence is available, a competitive interaction between these two species is proposed to explain the observed patterns of microhabitat utilization and densities. Two series of manipulative experiments were conducted at Kowie Point, where P. angulosus occurs in sympatry, The first, involved urchin removals from three experimental sites. The percentage cover of four functional algal groups was monitored in three experimental and three unaltered control sites over a three month period in spring (Sept. - Dec.) 1993. Two 100 point 0,25m² random point quadrats were thrown at each of the six sites at approximately monthly intervals over the experimental period. Since initial cover of algae varied at the six sites, ANOVA's of the change in percentage cover of four functional algal groups were used to determine treatment effects. No treatment effects were identified for any of the four functional algal groups. There was a strong negative correlation between the percentage cover of foliose algae and encrusting coralline algae, suggesting the possibility of a competitive relationship between them. The second, involved the experimental manipulation of microhabitat availability. The blocking of overhangs, by means of cement filled bags, resulted in an approximate 50% decrease in the total number of urchins in the two experimental pools, and the two control pools without overhangs. However, in the control pool with overhangs there was a slight increase in the number of urchins over the same period. Clearly, the presence of suitable shelters, is a prerequisite for the maintenance of dense intertidal population of P. angulosus. It is proposed that, due to the exposed nature of the South African coast, intertidal populations of P.angulosus are restricted to inhabiting suitable shelters from which they emerge to feed on passing drift algae. The implications of these findings are discussed in terms of current ecological literature.
- Full Text:
- Date Issued: 1995
- «
- ‹
- 1
- ›
- »