CRAMP Rapid Assessment. Bio-Indicator Results
By examining all 61 explanatory variables, we found that the structuring of coral reefs involves complex interactions and each factor alone is a weak predictor of any of the response variables, explaining only a small portion of its variability.
Yet a combination of factors can explain a significant portion of the variability. Many factors combine to influence coral reef communities. Both natural factors (rugosity, depth and wave energy) and anthropogenic factors (organics, human population, management protection and distance from a stream) influence biotic assemblage characteristics. Although these factors are the most influential in explaining the observed variability in coral community structure, many other factors combine to varying degrees to influence biological populations. The figure below shows which factors are most influential to coral and fish populations. Factors colored in red have a negative relationship with the response factor. For example, human population and high wave energy have an inverse relationship with coral cover. Coral decreases as population and wave heights increase.
Factors that significantly influence biological variables
factors affect the habitat classes in a complex manner. Environmental
factors that are important in differentiating all habitat classes include
benthic community composition and sediment grain-size parameters. Impaired
sites show a strong relationship to some factors but not to others. For
example, some measures of disturbance (organics and human population) are
correlated with all sites, while the level of marine protection influences
reference sites alone.
Indicators of anthropogenic impact
Specific factors or assemblages of organisms can provide early warning signs of coral reef decline. Rapid assessments can assist in identification of heavily impacted sites. These sites can be identified by departure from reference values established for a select number of factors with high discriminatory power. However, sites in early stages of decline are more difficult to quantify due to the extreme variability found in coral reef communities. In order to quantify declining conditions, temporal variation must be defined and subsequent surveys with suitable statistical precision must be conducted. Monitoring of sites, whose rank is near the endpoints of the entire suite of sites for particular discriminating variables, can be used to quantify the progression of declining conditions.
Among the variables with high discriminatory power are rugosity, human population, sediment associated organics, and silt/clay. These environmental variables encompass both natural (rugosity) and anthropogenic (population and sediments) variability. Values for coral cover, numerical fish abundances and silt/clay that fall outside reference site ranges can also be used to detect impairment. These combined variables can be used to identify degraded sites, or stations within a site where impacts are strongest.
For habitat-scale physical parameters, bulk sediment deposit composition, wave regime, depth, substrate, and rugosity can be used to define Hawaiian coral reef condition.
Rugosity measurements are heavily influenced by coral cover and diversity, which are also highly correlated with fish populations. Measurements of spatial complexity may prove to be a rapid way to conduct preliminary assessments of both coral and fish communities. Although the use of a rugosity index cannot substitute for fish abundance data, it can serve as a relative indicator of differences between sites over large spatial scales where abbreviated surveys are necessary. There are clear advantages to this indirect measure of abundance since high variability exists among fish populations.
Corals have been reported to stratify by depth and waves, with wave energy documented as the most dominant forcing function structuring coral communities in Hawai‘i.
The significance of depth in explaining coral cover is analogous to stratification of vegetation by elevation, the most apparent environmental gradient in terrestrial ecology. The phenomenon of coral cover increasing with increasing depth is partially a function of decreasing wave energy. This study supports Peter Glynn’s research conducted in the eastern Pacific suggesting that physical factors like depth and wave regimes control shallow environments, while biological factors are the forcing function in deeper waters. Yet our research also found that waves are not the controlling factor in sheltered, shallow waters where anthropogenic factors can dominate.
Rugosity is also heavily influenced by waves. Species with lobate or encrusting forms tend to inhabit regions with high wave energy. Species with highly branched morphology, low skeletal strength and high fracture rates, reside in regions with little wave exposure, such as in bays and near sheltered shorelines. This distribution may have evolved as an adaptive response to disturbance by waves.
Fine particles of silt and clay, correlated with coral species abundance are also partially structured by wave energy. Winnowing of fine grain particles in high energy regimes select for larger, coarser grain sizes while smaller organic particles can remain in areas with little wave disturbance. Yet, where fine sediment overwhelms the system, sedimentation rather than wave energy, becomes the dominant forcing function on community structure.
Along with wave heights, wave direction is also important in structuring coral communities. Distinct and consistent directional wave patterns prevail throughout the year. A storm surf gradient exists along Hawaiian shorelines increasing in a clockwise direction. Larger winter swells arrive on the north shores of the islands originating from the North Pacific Swell, while less exposed south shores receive lower energy South and Trade Wind Swells. These long period swells are influential in biostratification of species, spatial heterogeneity and structuring of coral reef communities. Anomalous changes in wave direction can significantly impact coral communities.
Wave patterns affecting the main Hawaiian Islands.
At a local scale, factors such as proximity to perennial streams and proximity of human population can help define the condition of Hawaiian coral reefs. These two measures of disturbance were found to be highly influential in defining reef condition.
Anthropogenic factors can also be important in the structuring of coral reef communities. Water quality reflects the resident population and adjacent watershed uses. Streams in Hawai’i have a long history of alteration and diversion, with few intact streams remaining today. Statistical analyses indicate that sites in close proximity to high human population and perennial streams had lower coral cover and species richness.
Although these factors play an important role in structuring coral communities, coral reefs are complex, interrelated systems that are influenced by numerous physical, biological and chemical factors that continually interact to manipulate biotic populations.
Last Update: 04/21/2008
By: Lea Hollingsworth
Hawai‘i Coral Reef Assessment & Monitoring Program
Hawai‘i Institute of Marine Biology
P.O. Box 1346
Kāne‘ohe, HI 96744