A high level of coral cover doesn’t always mean a high level of species diversity
By: Zoe Richards
The health and productivity of coral reefs is rapidly declining. Hard corals are the principal builders of coral reef ecosystems; however they are struggling to survive due to pollution, catchment clearing and climate change. The task of curtailing these declines is immense.
To protect coral reef ecosystems, a portfolio of management approaches are needed. This includes Commonwealth and State leadership and community participation. Most importantly, management actions must be based upon pragmatic science.
Currently the health of coral reefs is reported in terms of the level of coral cover. It is assumed that a reef with high cover is healthier than one with low cover. This information ultimately influences the selection of marine reserves.
However, new research shows that reefscape metrics such as coral cover are not linearly related to richness of a coral species.
This means that if a reef is designated as healthy and consequently given an elevated level of protection (for example, made a no-take zone) because it has a high level of coral cover, the biodiversity benefit of protecting this reef may not be as high as first expected.
Moreover, results of this study show that the relationship between coral cover and coral diversity is far from simple. Coral biodiversity actually peaks at intermediate levels of coral cover.
This finding is not unusual, considering coral communities can be dominated by a single or small number of species that can reach an incredibly high level of cover. This can occur when a community is in an early phase of recovery after a disturbance or when there have been no disturbances for a long time.
Alternatively, it is quite common for coral communities to have a large number of species that occur as small, sparse colonies. Therefore even if coral cover is not high, there is a high level of diversity. If coral biodiversity is high, this benefits all of the fish, crustaceans and other associated marine life.
Knowing about the lack of a positive linear relationship between coral cover and coral biodiversity is important information for coral reef managers. This is because biodiversity lies at the core of ecosystem health, productivity and functioning.
Especially on disturbed reefs, a large pool of species is required to sustain ecosystem structure and function. Evidence is mounting that individual species are important to ecosystem resilience and that even small changes in diversity can have significant impacts on ecosystem function.
One of the most ominous consequences of reef degradation is the loss of biodiversity. Despite this, in most reef regions there is little information about species occurrences, or about species' responses to management and environmental change.
Moreover, financial and logistical constraints mean that proxy measures are not necessarily used in coral reef monitoring.
The best proxies are those that are reliable, easily quantified and documented as a simple linear function. Results presented in this study indicate that while coral cover performs poorly as a measure of species richness; genus diversity performs well because it is strongly and linearly associated with species richness. This suggests that collecting data on coral genus diversity would be a valuable addition to coral reef monitoring programs.
Ultimately the loss of biodiversity is irreversible and the ecosystem effects are unpredictable. The ability to detect and monitor biodiversity is vital if we are to protect it.