Case Studies I: Coral Diversity

Unfortunately, there are not that many research studies done regarding only coral diseases. However, this trend might change as more people become aware of the implications of climate change on coral reefs. Using different case studies, we can attempt to analyze the impacts of climate change and disease.

Specifically, lets try to see if we can answer these questions:
-----> Are coral diseases reducing coral diversity?
-----> What are the ramifications of coral disease on the rest of the reef system's organisms?
-----> Is there room for adaptation/recovery/resistance?


Are Coral Diseases Reducing Coral Diversity?
Overall the majority of the studies showed that coral disease is reducing abundance of coral reef species, with some experiencing harsher survival chances than others.

Bruckner et al 1997: Spread of black-band disease epizootic through the coral reef system in St. Ann's Bay, Jamaica

Fig.26: St.  Ann's Bay (Morris, 2010)

Over a period of a year and a half, Bruckner and others chronicled distribution, abundance, and dispersion patterns of black band disease in the Caribbean.

Within the 20 cites, 5.2% of massive coral species became infected with the disease. After 19 months, disease incidence increased dramatically, spreading over 3 kilometers in direction of water current.

The species expected to have the most disease prevalence was actually infected at a frequency of 2.2%. Instead, another species was infected at a frequency of 8.6% due to excessive runoff in that region. Thus, not only increased temperatures make corals vulnerable but so do anthropogenic causes like nutrient loading.

Fig.27: Bruckner et al 1997

Figure 27 is a bar graph showing the progressive increase of disease with time. Disease incidence increases in the summer, most likely due to increased temperatures.
The averaged infections of the most infected species, S. sidereal, were compared to all other corals to show how deeply excessive run-off increased its infection rates. Thus, it's not just temperature and ocean acidification that can increase pathogenesis but also anthropgenic causes like excessive run-off..

Bruckner et al 2009: Ten years of change to coral communities off Mona and Desecheo Islands, Puerto Rico, from disease and bleaching

Fig.28: Desecheo Island (Gwenn, 2011)

For a period of around 10 years, researchers followed disease prevalence in reef species nearby Mona and Desecheo Islands.

Reefs off of Puerto Rico saw 30 to 80% loss in coral cover due to decreased populations of certain species over time due to disease and bleaching. Two species composed the majority of the corals, being larger, older, and most abundant.

Outbreaks of diseases, including white plague, from 1998 to 2001, affected 30 to 60% of M.annularis colonies. Though, disease prevalence fell in years afterwards, the year 2005 again experienced mass bleaching that again made the species susceptible to disease.

Abundance has reduced from 24 to 32% and the colonies that remain have less than half their living tissue left. Furthermore, the species are finding it difficult to recruit, resheet, and fend off predators and competitors like macroalgae and sponges

Fig.29: Bruckner et all 2009

The graph in Figure 29 shows the prevalence of white plague disease over the given timespan in the different species. The trend shows that in hot years 1999-2001 and 2005, when bleaching events increased so did white plague disease incidence.

However, different species were affected differently, with the two largest species (squares) being affected most significantly. In comparison to the last hot event of 1999-2001, the 2005 event the proportion of populations infected were dramatically more.

Such changes have huge effects on coral species composition as the more abundant and bigger ones show the most decline.

Summary of studies:

The underlying theme of these studies is that coral composition sees a significant and negative change as disease prevalence increases. Just like bleaching, disease prevalence is associated with rising temperatures.

We see that the most abundant, dominant coral species which are usually larger are more severely affected than the less dominant and smaller coral species.

Thus, one hypothetical future is seeing an increased presence and eventual domination of small coral species.

In addition, it is not just rising temperatures that can affect coral composition but also anthropgenic activities like excessive nutrient run-off which can make certain coral more susceptible to infections.

Next, we will see if we can answer the other two questions regarding other organisms' composition and possibility for adapting.

Text information compiled from respective studies