The Maintenance of Diversity on Coral Reefs: Empirical Tests of the Role of Aggregation in Promoting Coral Species Richness

EPA Grant Number: F5E20892
Title: The Maintenance of Diversity on Coral Reefs: Empirical Tests of the Role of Aggregation in Promoting Coral Species Richness
Investigators: Idjadi, Joshua A.
Institution: University of Delaware
EPA Project Officer: Lee, Sonja
Project Period: September 1, 2005 through August 1, 2007
Project Amount: $94,232
RFA: STAR Graduate Fellowships (2005) RFA Text |  Recipients Lists
Research Category: Academic Fellowships


This project will use manipulative experiments and mensurative surveys to test the role of aggregation in promoting coral species coexistence. Specifically, the investigation will focus on the influence of the spatial dispersion of strong and weak competitors on the ability of the less competitive species to grow and survive (coexist) in artificial competitive arenas.

There are worldwide reports of coral reef degradation involving reduced coral abundance and diversity. This project will provide information about what processes maintain diversity on coral reefs and about how diversity might be restored. There is no shortage of theories and models regarding processes, which increase diversity and species coexistence. This project will focus on empirical tests of a spatial heterogeneity mechanism specifically, the aggregation/segregation hypothesis. The aggregation/segregation hypothesis simply predicts that common corals are more aggregated in speciose communities. The grouping of aggressive, competitive species leaves suitable, vacant habitat and thus helps promote the presence of a greater diversity of less competitive organisms. In addition, when neighboring corals are more likely to be conspecifics than heterospecifics, the spatial separation of different species prevents exclusion by the competitive dominant. This project will address the hypothesis by using both mensurative and manipulative studies at two study sites on two Pacific coral reefs across a range of coral species richness.


Preliminary experiments indicated that an aggregated pattern of strong competitors could positively influence the growth of less competitive species. Weakly competitive coral branches were placed on plastic tiles in competition with conspecifics and heterospecifics with treatments representing different levels of aggregation and the plates were attached to the substratum in lagoonal habitats in Moorea, French Polynesia and Heron Island, GBR, Australia. Growth and survivorship were recorded after one year. Corals competing with the same number and type of competitors grew almost twice as much in aggregated patterns. Further planned experiments will test the roles of interspecific competition among strong competitors and the importance of colony position in aggregated and regulary dispersed patches on the growth and survivorship of weak competitors. The project will also test the influence of competitive release (via open space in which to grow) on the aggregation effect observed in earlier experiments. Mensurative surveys are being conducted to confirm the occurrence of natural aggregation and to establish the frequency of these competitive interactions on the natural reef.

Expected Results:

If further experiments confirm the importance of aggregation in promoting coral species coexistence, this study will have implicated one of the putative mechanisms for generating diversity on coral reefs. This information could be used to assess the condition of healthy reefs and to guide reef restoration efforts.

Supplemental Keywords:

fellowship, coral reef community ecology, coral species diversity, coral species richness, species coexistence, spatial heterogeneity, aggregation, segregation, competition, competitive exclusion, coral reef restoration, ecology, coral decline,, Scientific Discipline, Ecosystem Protection/Environmental Exposure & Risk, Habitat, Environmental Monitoring, habitat dynamics, community composition, aggregation, diversity of species, habitat patch dynamics, ecological consequences, spatial species separation

Progress and Final Reports:

  • 2006
  • Final