Rarity in the coral genus Acropora: implications for biodiversity conservation

Richards, Zoe Trisha (2009) Rarity in the coral genus Acropora: implications for biodiversity conservation. PhD thesis, James Cook University.

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Abstract

The processes that determine why some species are rare and others common are of major ecological and evolutionary significance. While theories concerning the causes and consequences of rarity have been developed for terrestrial systems, it is not clear to what extent they apply to marine invertebrates such as corals. In this study, data for the genera Acropora were used to test key ecological, phylogenetic and population genetic hypotheses about rarity and the results are interpreted in the context of biodiversity conservation. Overall, my goal is to improve information about rare coral species to enable better decisions about their threatened status, and to provide quality data to pilot fresh evaluations of marine invertebrate conservation action.

A new multi-scale model of coral rarity was developed, which shows that 54% of Acropora species examined in the NW Pacific Ocean are characterised by one of five different types of rarity. I found that thirteen species of Acropora are vulnerable to global extinction because they are rare and restricted across all scales of distribution and abundance. Twenty-two species were found to have a widespread global distribution but to occur in low abundance at a small number of sites across that range – these species are vulnerable to local extinction. I demonstrate that signals of rare species can be captured in multivariate analysis with inverse transformation. I show the inclusion of rare species in ecological data reveals more about community structure than examining common species alone.

Bayesian analyses of nuclear and mitochondrial sequence data imply that, although all of the rare species examined evolved relatively recently, phylogeny alone cannot account for the rarity of these corals because many common species have the same relative age. Members of the horrida and echinata groups occur in basal positions in both mitochondrial and nuclear topologies suggesting these lineages are far older than currently understood. My results show complex patterns of allele sharing along with mitochondrial monophyly coupled with nuclear polyphyly. I infer that in rare species, these patterns provide the first unequivocal evidence of hybridization in Indo- Pacific corals. A case study of a new population of elkhorn coral discovered in the Pacific Ocean is used to highlight the challenges corals pose to the conservation and management of endangered species.

Nine highly polymorphic microsatellite loci were used to examine the amount of genetic diversity and level of inbreeding in rare corals and closely related congeners. Overall, rare species of Acropora did not have significantly lower levels of genetic diversity or higher levels of inbreeding than common congeners. However, species specific microsatellite data suggests that at least 3 rare corals are genetically depleted. Another has 100% observed heterozygosity and I infer this species is a F1 hybrid. My findings of non-depletion in some rare species and depletion in some populations of common species, in conjunction with the lack of significantly greater levels of inbreeding in common species, means general conservation genetic hypotheses about the genetic resilience of rare/common species are not always relevant for species in the coral genus Acropora.

Alarmingly, my population genetic results suggest that over 90% of the Acropora species examined here have lower mean allelic diversity at individual loci than a ‘conservative mean’ published in a recent review of scleractinian coral genetic diversity. Estimates presented here of numbers of alleles per locus are likely to be underestimates of genetic diversity due to the relatively small sizes of populations sampled. Despite this, my startling results suggest that even though Indo-Pacific Acropora are the most species rich group of corals, as a group, they may be far less genetically diverse than first thought.

Results of this research provide new insights into ecological and genetic theories about the relationship between rarity, phylogeny and diversity for marine, modular organisms. I show how an examination of occupancy types can help prioritise conservation action by informing management which species have the highest and lowest extinction risk in their region. I suggest that given the poor knowledge of the responses of coral reef species to environmental change and management actions, further targeted species-specific monitoring and research is needed to detect change and gauge the success of conservation action. Overall, the results of this research show that rare Acropora corals do not always conform to the expections of conservation genetic theory. Further the ecological, and likely evolutionary, significance of hybrid corals poses a major challenge to conservation legislation, and I suggest that an urgent re-evaluation of conservation policies for marine invertebrates is warranted.

Item ID: 11408
Item Type: Thesis (PhD)
Keywords: corals, coral reefs, Acropora, Indo-Pacific, rarity, conservation genetics, genetic diversity, biodiversity conservation, phylogenetics, inbreeding, hybridisation, population structure, threatened species
Date Deposited: 27 May 2010 04:55
FoR Codes: 06 BIOLOGICAL SCIENCES > 0604 Genetics > 060411 Population, Ecological and Evolutionary Genetics @ 100%
SEO Codes: 96 ENVIRONMENT > 9605 Ecosystem Assessment and Management > 960506 Ecosystem Assessment and Management of Fresh, Ground and Surface Water Environments @ 50%
96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 50%
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