Demographic processes and spatial heterogeneity in community structure and dynamics of corals on the reef crest

Nelson, Victoria Mary (1994) Demographic processes and spatial heterogeneity in community structure and dynamics of corals on the reef crest. PhD thesis, James Cook University of North Queensland.

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This study investigates some of the demographic processes (recruitment, mortality, fragmentation and fission) which cause spatial heterogeneity in coral reefs. Permanent quadrats at four sites (separated by 100s to 1000s of metres) on the reef crest at Lizard Island were photographed approximately every three months for 33 months. The total number of colonies, percent cover and composition of corals showed no substantial changes over 33 months but differed markedly among sites. The total abundance and percent cover of corals varied two- to threefold among sites. Species composition differed among sites, but Acroporids dominated the composition of assemblages at all four sites. Three of the sites had similar species composition, dominated by A cropora hyacinthus, but the fourth was characterised by a suite of species that were rare at other sites.

Recruitment was an important process influencing differences among sites in total abundance and composition of corals. More than 3000 sexual recruits were detected in 80 m² over two years. Rates of recruitment differed by twofold among sites, and species composition also differed. The abundance and composition of recruits were closely correlated with the abundance and composition of established assemblages. Three sites had similar composition of recruits, while the fourth site was distinguished by species that were rare elsewhere.

Post-recruitment mortality also played an important role in determining relative abundances of species at a site. Fewer than 25% of recruits survived over two years (< 25%). Rates of mortality differed by twofold among sites and exaggerated differences among sites established by recruitment. There was no evidence of density-dependent mortality during the two years of the study; survival of recruits was not correlated with density or cover of established colonies, nor with density of recruits. Survival of recruits differed fourfold among species, ranging from 13% for Montipora spp. to 52% for Pocillopora damicornis. The three most abundant species on the reef crest, A. hyacinthus, A. gemmifera and P. verrucosa, all had high rates of recruit survival (45% over 2 years).

Mortality of colonies already established at the start of the study also differed among sites. Mortality was significantly greater at one site (71%) than at the other three (53 - 56%). Survival of established colonies differed fivefold among species. Robust and/or large species (A. gemmifera, P. eydouxi, A. monticulosa, A. cuneata and A. hyacinthus) had > 60% survival, while smaller, more delicate species had lower (< 30%) survival (e.g. Montipora spp, A. nobilis).

Colonisation of bare substratum on the reef crest by fragments is apparently uncommon, but can occur for some species. Over 423 fragments were detected in the quadrats over 33 months, but 90% died or moved out of the quadrats at all sites. Ninety-one percent of fragments were Acroporids. Fragments of A. hyacinthus had the highest survival, with 19% of fragments still alive after 12 months. Fragments of A. nana and A. nobilis had greater rates of mortality, with 5% and 8%, respectively, still alive after 12 months.

Local abundances of some species (e.g. A. monticulosa, A. robusta) may have been enhanced by fission, but daughter colonies of the majority of common species on the reef crest did not survive. Fission of colonies occurred frequently (7.2 daughter colonies.m⁻².yr⁻¹) and in all taxonomic groups. There was a tenfold difference among sites in the number of daughter colonies produced by fission which was related to the species composition of assemblages at each site. Survival of daughter colonies differed by more than 25-fold among species, with daughter colonies of A. monticulosa, A. gemmifera, Ponies sp and A. robusta all with survival greater than 20% over 2 years.

In summary, differences among sites in numerical abundance and composition were largely determined by recruitment, but differential mortality among sites and species was also important. The study clearly illustrated that coral communities differed greatly in both structure and dynamics among sites on the reef crest. For this reason, a general understanding of the processes important in shaping assemblages of corals is dependent on quantifying both spatial and temporal variation in these processes.

Item ID: 33789
Item Type: Thesis (PhD)
Keywords: community structure; coral assemblages; coral reef ecology; corals; GBR; Great Barrier Reef; recruitment; reef crests; reef ecology
Date Deposited: 21 Jul 2015 05:09
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 100%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 100%
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