Case Studies II: Reef Diversity & Resistance

Having established that coral diversity is declining due to diseases, lets now look to answer these questions:
----->  What are the ramifications of coral disease on the rest of the reef systems organisms?
----->  Is there room for adaptation/recovery/resistance?


What are the ramifications of coral disease on the rest of the reef systems organisms?
Ostrander et al 1999: Rapid transition in the structure of a coral reef community

Fig.30: San Salvador (Lynchburg, 2012)

Ostrander and others did a 40 month study around the island of San Salvador. There the randomly chose 10 permanent locations offshore and use steel intersections for observations. Then they observed how factors including bleaching, disease, increased macroalgae, and physical disturbance affects Carribean reefs organisms. 


Though disease was one of many factors being observed, this study is a great example of how all of the factors mentioned above work to acutely affect coral reef community and structure. 
Within their observation period, they saw a massive bleaching event which significantly reduced coral abundance. 


In addition, Hurricane Lili in October 1996 was a physical disturbance that impacted the reef system. This disturbance did not greatly affect coral abundance but it heralded an increase macroalgae abundance. Simultaneously, we see a decline in the grazer urchin, Diadema antillarum, due to overfishing and nutrient-loading. Thus, the corals lose their competitive edge to macroalgae with more bleaching, less mutualistic relations, more disease, etc. 

Fig.31: Shift towards macroalgae abundance

Figure 31 shows a graph that charts the increase in macroalgae seemingly at the expense of coral decline. We see that the major contributor to the shift was a physical disturbance, Hurricane Lili. Hurricane Lili occurred in October, and dramatically changed macroalgal abundance. 
We see that before it, macroalgae experienced fluctuations but increased mostly when bleaching happened in 1995 but was sharply declining before the hurricane. But after it, the macroalgae increased sharply by possibly taking advantage of the hurricane's aftereffects with rapid turnover. 
On the other hand, coral experiences a decline during bleaching but the hurricane had no great affect on its abudance. But such lack of prosperity in conjunction with macroalgal increases, shows a shift in coral reef structure that has the decreases in sea urchins show is most likely to decrease biodiversity there. 


 Is there room for adaptation/recovery/resistance?
Just relying on the previously mentioned data can leave one dejected about the future of coral reefs. But what if there is room for hope? What if there are cases of some recovery, resistance, or adaptation? Lets look at some below!


Mydlarz et al 2009: What are the physiological and immunological responses of coral to climate warming and disease?


In this research paper, Mydlarz and others noted that there is some recovery against disease and that there are possible mechanisms and relationships that can make resistance possible in the future.


The coral itself, the holobiont, can fight pathogens with innate immune responses that can slow or even stop pathogen and tissue damage from progressing. Corals have amobeocytes, antioxidants, and some other thermal resistance variability, showing that corals have some plasticity to climate change. 


It's not just the coral itself but also the algal symbionts and surface microbial community that can respond to pathogens. In some diseases, it's not the coral but the algal symbiont that the pathogen targets. Thus, a change in algal composition to one that is unsusceptible to disease could be an adaptation. In addition, the microbial communities can protect the coral host by acting as barrier to pathogens and also releasing antibacterial compounds expelling pathogen.


The paper also mentions numerous instances where other studies have shown disease to disappear on certain coral species on which that specific disease used to be prevalent. This leads one to hypothesize that not only can coral evolve resistance but that it is already happening. 

Fig.32: Mydlarz et  al 2009
 

Figure 32 shows a diagram which the authors use to help pinpoint what sort of balance they think is capable of reducing and resisting disease in corals.  From their perspective, coral host, algal symbiont, and surface microbial community can combine in a such a way to survive stressors and reduce mortality


Do Reef Organisms Help Coral Resist Coral Disease?
Raymundo et al 2009: Functionally diverse reef-fish communities ameliorate coral disease

Fig.33: Philippines Coral Reef (Gwapo, 2012)

Raymundo and others help show that coral reef ecosystem's resilience is related to conserving its functional diversity by top-down effects. They hypothesized that trophically diverse reef fish coral environments will have less coral disease than overfished reef. 
They had seven location in the Phillippines. At each location, there were two sites, one that was a marine protected area (MPA) and the other that was an unprotected area. 


Their results found that disease prevalence was much lower in MPA areas due to increased fish diversity than in unprotected areas which had greater disease prevalence due to decreased fish diversity from overfishing. In addition, they explores how fish diversity's trophic components might be linked to coral disease. 


Figure 34 shows that there is a marked difference in amount of disease prevalence between locations. The MPA areas generally saw far less disease incidences than the unprotected fished areas.


They found that the increased presence of coral-eating chaetodontid butterflyfish, which are rarely fished, are linked to increased disease prevalence. Thus, these butterflyfish might possibly be vectors for coral disease.

Fig.34: Disease disparity between locations(Raymundo et al 2009)

Fig.35: Butterflyfish, possible disease vector? (Veelen, 2007)

Summary of Studies: 
The above mentioned studies have shown that there might be a shift in reef organism composition, from coral dominant to macroalgal dominant. Consequently, they have also seen a reduction in grazers like sea urchins which further enhance algal populations. 
However, there is some room for resistance to pathogens if coral holobiont, algal symbionts, and microbial community can find a balance that negates pathogen progress. 
Furthermore, other studies have shown that there already has been evolution of resistance on field with some corals no longer being susceptible to certain diseases. Finally, coral reefs with greater fish diversity (thus less overfishing) help ameliorate against disease.


So what can we do about all of this? Let's look at the next page

Text information from respective studies