Title
Predation and the distribution and abundance of tropical sea urchin populations
Author(s)
McClanahan, T.R.
Published
1998
Publisher
Journal of Experimental Marine Biology and Ecology
Abstract
Nine species of herbivorous sea urchin commonly inhabit shallow-water coral reefs of the Indian Ocean. The abundance, distribution and diversity of these species were studied in seven reef lagoons over a 7-year period to determine the factors that control population variation in time and space and the role that predation plays in their ecology at a scale = 0.1 hectare. Field studies of predation included experimentally tethering sea urchins (Predation Index), visual transects of fish to estimate the predator biomass and line transects of reef substrate rugosity (bottom contour complexity) which allowed for five possible measures of the hypothesized limiting resource refuge from predators. The amount of refuge was used to predict the abundance of individual species of sea urchin, species richness and diversity. It was predicted from pervious manipulative species interaction studies that the top competitor in the guild (Echinometra mathaei (de Blainville)) would be directly and closely controlled by predation because competitive dominance may often reflect an evolutionary and energetic trade- off with predator susceptibility and, therefore, E. mathaei abundance would decrease either linearly or logarithmically with measures of predation. Competitively-subordinate species were predicted to have unimodal ('hump- shaped') abundance relationships with refuge because these species should make an evolutionary trade-off between increased predator resistance and decreased competitive ability. Spatial and temporal variation in wet-weight estimates was high for all species except the numerical and competitive dominant, E. mathaei, and for all species combined. Spatial variation in species abundance was highest under conditions of high predation and low species dominance and decreased as the total wet weight of urchins increased and the competitive dominant occupied resources. The most common subordinate species frequently exhibited the predicted unimodal relationships with the best measure of refuge from predation but variation was high and the best- fit second-order polynomials predicted less than 55% of the variation in sea urchin biomass. There was evidence for partitioning of the spatial refuge resource among species particularly for E. mathaei (de Blainville), Tripneustes gratilla Linn. and Diadema setosum Leske at the low to intermediate levels of predation. There was, however, a large overlap in the peak abundance of Diadema savignyi (Michelin), Echinothrix diadema Linn., Toxopneustes pileolus Lam. and Stomopneustes variolaris Lam. at the highest levels of predation. Nonetheless, the partitioning of refuge among many species occurs more at the smaller scale of reef crevices (< 10 m2) than at the larger scale of ~0.1 hectare sites or whole sections of reef. Diversity and number of species, however, were strongly associated with measures of refuge and exhibited unimodal curves.
Keywords
abundance; coral reef lagoon; interspecific competition; predation; refuge; resource partitioning; sea urchin; spatial distribution; article; biodiversity; marine environment; nonhuman; population dynamics; Indian Ocean; Diadema savignyi; Diadema setosum; Echinoidea; Echinometra mathaei; Echinothrix diadema; Stomopneustes variolaris; Stomopneutes variolaris; Toxopneustes pileolus; Tripneustes gratilla; ECHINOMETRA-MATHAEI DEBLAINVILLE; REEF FISH POPULATIONS; HIGH DIVERSITY; CORAL REEFS; COMPETITION; RECRUITMENT; COMMUNITIES; HERBIVORY; PATTERNS; LAGOONS

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PUB12881