Functional biogeography: evaluating community assemblage patterns and ecosystem functioning in intertidal systems using trait-based approaches
- Authors: Gusha, Molline Natanah C
- Date: 2022-10-14
- Subjects: Biogeography , Marine algae , Benthic ecology , Invertebrates , Functional redundancy , Ocean temperature , Biology Classification
- Language: English
- Type: Academic theses , Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/365792 , vital:65790 , DOI https://doi.org/10.21504/10962/365792
- Description: Analyses of taxonomic diversity patterns within coastal systems has been critical in the development of the theory of biogeography. Increasing evidence, however, shows that the variety of functions that species perform in ecosystems (rather than their taxonomic identity) is a better predictor of the influence of the environment on the species. This information has been useful in predictive ecology leading to the development of trait-based approaches (TBA). Until the late 1970s, however, limited effort (particularly in marine systems) was channeled towards patterns in functional species traits and how they may be affected by changes in environmental gradients. Here, I mapped the functional biogeography of the South African coastline based on a suite of species' reproduction and development traits. Because species composition is one of the key tools used by zoogeographers to map species distribution patterns, I expected lower variability in trait composition within main biogeographic regions than in intervening transition zones based on the habitat templet theory and following the biomass ratio and limiting similarity hypotheses. In brief, the habitat templet theory proposes that “the habitat provides a templet upon which evolution forges species characteristics”, while the biomass ratio hypothesis assumes that the most abundant species traits determine ecosystem functioning. The limiting similarity hypothesis also sometimes referred to as the niche complementarity hypothesis, however, predicts that species can coexist if their niches complement one another. In light of the habitat being an evolutionary templet, abiotic and biotic habitat patterns were measured as nearshore SST and chlorophyll-a gradients, respectively. I expected the SST gradient to act as the stronger key filter of trait diversification because temperature is often considered the most influential environmental factor affecting species survival with seasonality of SST affecting the timing of spawning and along with food availability, possibly influencing fecundity. Functional trait data were thus compiled for macroinvertebrate species collected from fifty-two rocky shore sites from three main bioregions (east, south, and west) and two transition zones (south-west and south-east). Biological trait analysis and functional diversity indices were used to evaluate how traits related to species development and reproduction respond to temperature and chlorophyll -a (used as a proxy for food availability) gradients along the coastline. GLMM and hierarchical cluster analyses showed distinct patterns/shifts in SST and chlorophyll-a gradients across bioregions, with two main breaks in SST separating the east and south-east overlap (SEO) bioregions from the south, south-west overlap (SWO) and west bioregions. In contrast, chlorophyll-a exhibited three major breaks with the east, SEO–south–SWO, and west clustering independently of each other. The RLQ analysis (a type of co-inertia analysis) which simultaneously ordinates 3-matrix datasets [i.e., (environment × site[R]), (species × site[L]) and (species × traits [Q])] showed that the higher SST gradient on the east and SEO promoted higher abundance and biomass of simultaneous hermaphrodites while higher chlorophyll-a gradients on the SWO and west coasts strongly promoted reproductive maturity at larger-sizes. The combined fourth-corner analyses showed that the modalities within the development trait domain responding to chlorophyll-a gradients primarily included filter feeders, sessile and swimming species and also species living on the infratidal zone. In addition, the reproduction trait domain showed higher sensitivity and association to differences in chlorophyll-a and SST gradients than development traits. Overall, SST and chlorophyll-a gradients influenced the distribution of the most dominant traits as indicated by shifts in community-weighted mean trait values across bioregions. This suggests the importance of habitat filtering on coastal species reproduction. A separate study evaluating the influence of large-scale biogeographic effects vs the micro-scale biogenic habitat structure offered by coralline seaweeds across 24 sites revealed some notable effects of both factors on the diversity and abundance of macroalgal epifauna. There was a notable biogeographic influence on epifauna, with the SEO recording the highest epifaunal species richness and abundance, followed by the south coast, then the SWO and lastly the west coast. In addition, the total biomass gradient of the corallines followed a similar trend. The epifauna however, showed no host-specificity, illustrating that epifauna may not be species–centric as commonly assumed, and the higher diversity of epifaunal diversity may well be simply because those corallines are the available habitat within the sampled part of the coastline. Lastly, macroinvertebrate trait distribution on the South African coastline confirms that the habitat, particularly the biotic filter (in this case chl-a) provides a templet upon which evolution forges species traits. However, since temperature is a proxy for nutrient availability (cold upwelling brings nutrients), then temperature drives chlorophyll-a. Subsequently this means the abiotic component indirectly drives trait distribution by influencing the biotic environment (chl-a). For epifauna species, also, the coralline diversity and composition can also be regarded as a biotic filter influencing the epifaunal abundances and composition across different bioregions. Moreover, since temperature is regarded as a conservative trait in seaweeds, temperature tolerance defines the biogeographical boundaries of seaweeds, therefore temperature may be indirectly affecting epifauna abundances through coralline species diversity and biomass. In summary, considering the deterministic processes governing ecosystem functioning and community assemblage, the mass ratio and limiting similarity hypotheses showed complementary effects. Different bioregions provided variable support for these two hypotheses, but overall, the mass ratio hypothesis (weighted by species biomass) received stronger support and may be more meaningful to the interpretation of ecosystem functioning and persistence within rocky shore systems. Lastly, although, the SWO showed some of the characteristics of a subtraction zone based on the relatively low abundance, diversity, and biomass measures. Nonetheless, there was evidence of high functional redundancy across all other four bioregions. This suggests that in the context of development and reproduction traits, the rocky shore ecosystem along the SA coastline may be functionally stable at this stage. , Thesis (PhD) -- Faculty of Science, Zoology and Entomology, 2022
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- Date Issued: 2022-10-14
Stock structure of Patagonian toothfish Dissostichus eleginoides (Smitt 1898, family Nototheniidae) in the Southwest Atlantic
- Authors: Lee, Brendon
- Date: 2022-10-14
- Subjects: Otoliths , Patagonian toothfish Geographical distribution , Fish tagging , Biogeography , Microstructure , Microchemistry
- Language: English
- Type: Academic theses , Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/365599 , vital:65763 , DOI https://doi.org/10.21504/10962/365599
- Description: The identification of discrete self-sustaining productive units in marine populations is essential for achieving sustainable fisheries objectives. Marine fish populations frequently exhibit dynamic characteristics across their life-histories, displaying variability in spatial structure and mixing patterns, both within and among populations. The incoherent application of management boundaries on biological populations can bias stock assessment results and have important implications on sustainable fisheries management. Patagonian toothfish (Dissostichus eleginoides) is a long-lived, slow-growing, late-maturing, deep-sea, benthopelagic species. It forms the basis of important and highly lucrative industrial and artisanal fisheries across its distribution. Patagonian toothfish have complex life-histories characterised by high dispersal potential during the egg and larval phase, a wide depth range because of their ontogenetic migratory behaviour, and large adult size that is capable of undertaking long-distance active movements (>200 km). These characteristics provide opportunities for high levels of connectivity, and as such, the stock structure is not well understood. We applied an integrated, multidisciplinary approach to provide an improved understanding of the complex stock structure dynamics for Patagonian toothfish on the Patagonian Shelf, specifically in relation to the shelf, slope, and deep-sea plains around the Falkland Islands. Research results were focused on aspects pertaining to (1) geographic variation in phenotypic characters (otolith shape); (2) a description of the spatial-temporal distribution patterns; (3) the active movements of deep-sea adults (tag-recapture study); and (4) the identification of early life-history dispersal through otolith microstructure and microchemical chronologies. Results from the study indicate high regional connectivity during the early life-history stages derived from at least two spawning contingents into spatially discrete nursery areas (cohort groups) on the Falklands Shelf. Fish followed distinct ontogenetic pathways into deeper waters adjacent to the areas wherein juvenile settlement into a demersal habitat occurred. There is little to no evidence of mixing among cohort groups during their ontogenetic migration into deep-sea adult habitats, reflecting a mixed population based on oceanographically defined egg and larval dispersal. The majority of the adult component of the population continue to display high site fidelity. However, between 9 and 25% of the population, consisting predominantly of larger reproductively capable adults undertake long-distance dispersal behaviour, identified as home-range relocations from the adult deep-sea habitats towards three of the known southern spawning grounds in the region. Results are suggestive of a requirement for improved collaborative efforts for regionally-based management approaches with careful consideration of local stock contingents. Future monitoring and research priorities should focus on the identification of reproductive potential, dispersal pathways and settlement patterns of stock contingents to inform the dynamics of mixed stock origins across the Patagonian region. While many aspects regarding the stock structure remain unresolved, results derived from the current studies can be used to inform the development of management measures to ensure the continued recovery and sustainable management of Patagonian toothfish within the region. , Thesis (PhD) -- Faculty of Science, Ichthyology and Fisheries Science, 2022
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- Date Issued: 2022-10-14