Ecological variables, including physiognomic-structural attributes, and classification of Indonesian coral reefs

Abstract

Communities are distinguished by biological and physical features, such as size and shape of organisms and dead substrata, which are characteristic expressions of the organizing forces in the community. We measured 87 of such features in 39 transects on seaward-facing reef slopes in the eastern Indonesian archipelago, but did not identify coral species. We aimed to identify the basic variables that are indispensable to classify coral reef communities. This would give ecological information on variation in reef communities and show exactly which data must be recorded in the field. Principal Component Analysis (PCA) of the data matrix showed the following variables to be important in the ordination of transects along the axes: coral colony shape, loose fragments, bare bottom, coral tissue wounds, rubble, sediment/rubble, crustose coralline algae, excavating sponge, miscellaneous organisms, coral overgrowth, interaction coral/non-coral, Acanthaster, maximum size coral colonies, tabular Acropora, massive Porites, fungiids, angle slope, and crevices. We used the transect data to define four groups of environmental conditions: ‘sheltered’, ‘exposed’ (to water movement), ‘biologically disturbed’ and ‘physically disturbed’. Discriminant Analysis was employed to classify additional transects. It appeared that a minimum of 9 variables has to be measured in the field (rubble, thick branching corals, fungiids, sediment/rubble, two largest-colonies diameters, massive Porites, angle slope, Acanthaster) to assign transects to one of those groups (P < 0.10). With just 14 variables the classification of transects was 100% correct. Two additional groups of environmental conditions are recognizable with, respectively, prominence of competitive interactions and Acanthaster predation. There were too few transect data to characterize these groups satisfactorily for Discriminant Analysis.