Reproductive ecology and population dynamics in a scleractinian coral community

Harriott, Vicki (1983) Reproductive ecology and population dynamics in a scleractinian coral community. PhD thesis, James Cook University.

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Abstract

Patterns of scleractinian distribution and abundance, reproductive ecology, recruitment and mortality were studied over a 2 year period on a patch reef site at Lizard Island, Great Barrier Reef. The purpose of the study was to examine aspects of the population ecology of a range of coral species, and to determine the relationships between community structure and the ecology of the component species in that community. In particular, an attempt was made to examine dynamic aspects of community structure.

In a series of five 10m x 1m transects, 117 scleractinian species were recorded. Coral cover was significantly higher on the shallow reef top (2m deep) than at the reef base (< 9m deep), but there were no significant differences in number of colonies, number of species, Shannon's diversity index, Pielou's evenness, or mean size class of colonies between the two sites. Coral species showed distribution patterns that could be related to a depth gradient.

Reproductive ecology of five scleractinian species present in this community, Lobophyllia corymbosa (Forskal), Favia favus (Forskal), Porites lutea (Edwards and Haime), Porites australiensis (Vaughn) and Pocillopora damicornis (Linnaeus) was studied for two years. All but P. damicornis released gametes that were probably fertilized externally. Two major reproductive patterns were found amongst the 4 non-viviparous species: L. corymbosa and F. favus were simultaneous hermaphrodites and released gametes over several days in summer; P. lutea and P. australiensis were dioecious and released gametes over several weeks to several months respectively, also in summer. The predominance of non-viviparous species amongst those sampled supports the generalization that a brief annual spawning period with larvae developing externally may prove to be the dominant form of sexual reproduction in hermatypic corals.

The fifth species, Pocillopora damicornis showed indications of seasonality in gametogenesis and planula release at Lizard Island, in contrast with several previous reports on the species at other locations. Gametogenesis occurred predominantly in winter, and planulae were released with lunar periodicity that was dependent on the season. P. damicornis planulae settled preferentially on biologically conditioned, algal-covered substrata, rather than unconditioned, bare coral substrata, but showed subsequent mortality inversely related to this settlement preference.

Coral mortality rate varied amongst these five species. P. damtcornts had the highest mortality, followed by L. corymbosa, the massive Porites species, and F. favus in order of decreasing mortality rate. Mortality was very high during a summer period when extensive bleaching (=loss of zooxanthellae) was noted. For most species, mortality rate declined with increasing colony size.

There were no significant differences in recruitment (at ›1cm diameter) or mortality rates between colonies in shallow and deep transects over an 18 month period. Between 32% and 48% of all colonies recorded in transects died during the 18 month study period. Despite the potential for large changes in generic abundances as a result of the high rate of turnover in coral colonies, there was usually less difference in the same transect over time than there was between neighbouring transects, i.e. the composition of the coral fauna tended to be maintained over time. Recruitment and mortality rates varied greatly among genera, but were similar in genera from the same families.

Coral spat recruited exclusively onto the lower and vertical surfaces of coral blocks used as settlement plates. Fish grazing on plates in shallow water and sediment deposition in deeper water were concluded to be primary causes of spat death. Spat of three families were abundant. Acroporids were most abundant and recruited mainly in summer, but at low densities at other times. Pocilloporids were second most abundant and recruited evenly throughout the year. Poritids were least abundant of these families and recruited almost exclusively in summer. These temporal patterns could be related to known spawning times of some coral species. Spat were most abundant in shallow sites, but the familial composition of spat was similar at deep and shallow sites. The deep/ shallow variation in abundances of spat on settlement plates, and the fact that these differences were not reflected in recruitment at 1cm diameter in transects, raises questions as to whether either settlement space or availability of planulae limit recruitment.

Overall, both coral species diversity and colony mortality rate (which may be assumed to reflect the degree of disturbance at a site) did not vary significantly between the deep and shallow sites. This is consistent with the intermediate disturbance model of diversity maintenance. However, the fact that coral composition at the site did not change over 18 months is consistent with equilibrium models of diversity maintenance. A mechanism for succession in the absence of major interspecific competition is proposed, and a number of processes, both equilibrium and non-equilibrium, are concluded to play a role in the maintenance of high diversity in this community.

Item ID: 24108
Item Type: Thesis (PhD)
Keywords: coral communities; corals; Favia; life histories; life history; Lizard Island; Lobophyllia; Pocillopora; population dynamics; Porites; reproductive ecology; Scleractinia; stony corals
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Date Deposited: 13 Dec 2012 23:29
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060207 Population Ecology @ 50%
06 BIOLOGICAL SCIENCES > 0603 Evolutionary Biology > 060308 Life Histories @ 50%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 100%
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