Healthy Reefs
Indicators of Structural Integrity
Structural indicators include the key structural components of an ecosystem, including abiotic (e.g., salinity, temperature) and biotic (e.g., abundance/distribution of organisms) attributes. These abiotic and biotic components contribute to maintaining biodiversity. Reef structure plays an important role in determining the way reef ecosystems function.
Biodiversity
Coral reefs are among the most biologically diverse ecosystems in the world accounting for one third of all marine fish species. Despite their ecological and economic significance to humans, loss of marine biological diversity is occurring at alarming rates due to overexploitation, pollution, habitat alteration, and global climate change. Marine systems are inherently different from their terrestrial counterparts in that they are open systems with larval dispersal. Activities that act as barriers to this open system (e.g., disruption of larval flow, habitat fragmentation) can reduce genetic diversity and overall ecosystem functioning. Our knowledge of loss of marine biodiversity stems primarily from commercially exploited species (e.g., whales, fishes, etc), while the knowledge of less conspicuous species is scant. High biological diversity is often equated to a healthy ecosystem. By the same token, high loss of biodiversity has been used as a proxy for identifying ecosystem degradation. Reefs with higher biodiversity, such as those in the Indo Pacific, have generally shown greater resilience to disturbances than those reefs with fewer species, such as those in the Caribbean, although there may also be additional contributing factors unrelated to biodiversity.
Abiotic
Reefs depend on highly specific environmental conditions and corals, in particular, have adapted to tolerate relatively narrow ranges of these conditions. “Natural” levels of these abiotic conditions (e.g., sediment, nutrients) have been altered or disrupted by human activities resulting in increased stress to the biota. Several key factors are important for controlling modern reef types and locations, and these vary over spatial and temporal scales: small-scale controls are those influencing organisms at a reef-wide level (e.g., light, nutrients, sediment, salinity and temperature); intermediate-scale controls function within ocean basins and include physical oceanographic factors (e.g., temperature, wave, salinity, nutrients); and larger-scale controls (e.g. 1,000 km/over long time periods) act on the global scale and include tectonics, carbon dioxide levels and sea level rise.
Habitat Extent
Coral reefs are closely tied to surrounding ecosystems, particularly seagrasses and mangroves. Coral reefs dissipate wave energy, which provide environments suitable for seagrass and mangrove colonization and protects them from erosion during storms and strong wave action. In turn, mangroves and seagrasses improve water quality by stabilizing and reducing concentrations of sediments and nutrients. They also provide a critical habitat for juvenile life stages of many important commercial fish and invertebrate species. These ecosystems interact to form a complex and dynamic mosaic of habitats that serve as foraging areas, nurseries, and physical and chemical buffers. The presence of sea grasses, mangrove and coral reef habitats in close association to one another is considered especially valuable since proximity facilitates a natural energy flow between systems, creates corridors for transient species, and provides critical habitat for many reef species at various developmental stages.
Community Structure
The community structure, or benthic composition, of coral reef ecosystems is a major force shaping reef communities. Competition for the limited supply of prime reef real estate is very high, particularly the ongoing battle between macroalgae and corals. The species-specific responses of both the corals and the algae are very important and complex, with both physical (bottom-up) and biological (top-down) factors playing a critical role in competition and overall community composition. In addition to the composition of the substrate, the rugosity or complexity of the reef surface is also important, as reefs with higher topographic complexity provide more habitats for fish and invertebrate inhabitants.
- Key Structural Indicators
- Table of Structural Indicators
- Ranking Results —Structural Indicators (pdf version - 61KB)
