Reef fishes on isolated islands: community structure, endemism and extinction

Hobbs, Jean-Paul A. (2011) Reef fishes on isolated islands: community structure, endemism and extinction. PhD thesis, James Cook University.

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Ecology strives to identify the processes determining species diversity, species composition and population abundance. Island communities have served as the natural laboratories for the development and testing of ecological theories. Islands also provide the opportunity to determine whether there are differences in the ecological processes that structure mainland and isolated communities. To date, most of the theory and empirical studies of island communities have focused on terrestrial organisms. However, islands can be equally instructive about the mechanisms favouring the presence, absence and abundance of marine species. Therefore, the aim of this thesis is to determine whether the processes structuring terrestrial island communities apply in the marine environment. Ecological hypotheses spanning the fields of biogeography through to conservation biology are tested using reef fish communities at the remote Christmas Island and the Cocos (Keeling) Islands in the tropical eastern Indian Ocean.

Like small isolated terrestrial communities, the reef fish communities at Christmas and Cocos Islands were found to be species poor and contained a distinct taxonomic composition with an over-representation of species with high dispersal potential. Despite low species richness, there was no evidence of density compensation, with population densities on the islands similar to species-rich neighbouring mainland assemblages. In contrast to terrestrial communities, species at the edge of their range did not have lower abundance than species at the centre of their range, and endemic species had substantially higher abundance than widespread species. Overall, the observed patterns conform to predictions from terrestrial ecological hypotheses, indicating that similar processes are important in determining species richness and community composition in marine and terrestrial communities on isolated islands. However, observed patterns in abundance did not conform to expectations from terrestrial theory, and this appears to be due to the different life histories of marine and terrestrial species.

Local environmental factors can also be important in structuring reef fish communities; however, few studies have examined their role on oceanic reefs. Regression tree analysis of angelfish and butterflyfish communities revealed that large physical gradients (island location, exposure, depth, habitat complexity) are more important than small-scale biotic factors (live coral cover, algal cover and habitat diversity) in determining the community structure of reef fishes at these oceanic islands.

Christmas and Cocos Islands are also situated on the Indo-Pacific biogeographic border, and in the terrestrial environment, biogeographic borders represent important areas for hybridisation. Eleven hybrid coral-reef fishes (across six families) were identified at the islands: the most recorded hybrids of any marine location. In most cases, at least one of the parent species is rare (< 3 individuals per 3000 m2), suggesting that hybridisation has occurred due to a scarcity of conspecific partners. The Islands also represent a marine suture zone because many of the hybrids have arisen through interbreeding between Indian and Pacific Ocean species. For these species, it appears that past climate changes allowed species to diverge in allopatry, while recent conditions have facilitated contact and subsequent hybridisation at this biogeographic border.

Isolated islands often contain a high proportion of endemic species, which suffer high rates of extinction because of an association among three traits that threaten species persistence: small geographic range size, low abundance and ecological specialisation. This study found that endemic angelfishes at Christmas and Cocos Islands did not conform to these interrelationships. Endemic angelfishes were 50-80 times more abundant than widespread species and were not more specialised than widespread congeners. High abundance and lack of specialisation by endemic reef fishes may compensate for the extinction risk posed by having an extremely small geographic range.

Endemic species, and isolated populations of widespread species, are also at risk of extinction because they tend to have low genetic diversity. Examination of angelfish mtDNA revealed that the endemic C. joculator exhibit high haplotype (h > 0.98) and nucleotide diversity (Christmas π% = 3.63, Cocos π% = 9.99). The isolated populations of widespread angelfishes (C. bispinosa and C. flavicauda) present at Christmas Island also had high haplotype (h > 0.99) and nucleotide diversity (π% = 2.81 and π% = 5.78%, respectively). The genetic diversity of all three study species are among the highest reported for marine fishes and may have been caused by high abundance, relict populations, multiple clades and rapid mutation rate. High genetic diversity should reduce extinction risk in these species because it increases their evolutionary potential to adapt to the changing environmental conditions that are forecasted for coral reefs.

In summary, this study tested the generality of terrestrially-derived ecological relationships related to island communities. Some of these ecological hypotheses were found to apply to marine communities, whereas others did not. New hypotheses have been proposed to explain why marine communities do not always conform to these ecological generalisations. By combining field, laboratory and molecular studies with datasets constructed from the literature, this study has provided a thorough examination of the ecology of reef fishes on isolated islands and advances our understanding of marine ecology.

Item ID: 17932
Item Type: Thesis (PhD)
Keywords: biogeography; hybridisation; community ecology; endemism; extinction risk; genetic diversity; reef fishes; islands; assemblage structures; Indo-Pacific; angelfishes; genetic specialisation; abundance; coral reef fishes; hybridization; genetic specialization; Christmas Island; Cocos (Keeling) Islands
Date Deposited: 28 Nov 2011 23:16
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 34%
05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050202 Conservation and Biodiversity @ 33%
06 BIOLOGICAL SCIENCES > 0603 Evolutionary Biology > 060302 Biogeography and Phylogeography @ 33%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 100%
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