Coral Diversity and the Severity of Disease Outbreaks: A Cross-Regional Comparison of Acropora White Syndrome in a Species-Rich Region (American Samoa) with a Species-Poor Region (Northwestern Hawaiian Islands)
The dynamics of the coral disease, Acropora white syndrome (AWS), was directly compared on reefs in the species-poor region of the Northwestern Hawaiian Islands (NWHI) and the species-rich region of American Samoa (AS) with results suggesting that biodiversity, which can affect the abundance of susceptible hosts, is important in influencing the impacts of coral disease outbreaks. The diversity-disease hypothesis predicts that decreased host species diversity should result in increased disease severity of specialist pathogens. We found that AWS was more prevalent and had a higher incidence within the NWHI as compared to AS. Individual Acropora colonies affected by AWS showed high mortality in both regions, but case fatality rate and disease severity was higher in the NWHI. The site within the NWHI had a monospecific stand of A. cytherea; a species that is highly susceptible to AWS. Once AWS entered the site, it spread easily amongst the abundant susceptible hosts. The site within AS contained numerous Acropora species, which differed in their apparent susceptibility to infection and disease severity, which in turn reduced disease spread. Manipulative studies showed AWS was transmissible through direct contact in three Acropora species. These results will help managers predict and respond to disease outbreaks. 1. Introduction Ecosystem resilience can be defined as the capacity of a system to absorb disturbance and reorganize so as to retain the basic ecosystem services [1, 2]. Regime shifts in ecosystems are increasingly common as a consequence of human activities that erode resilience. This is especially apparent for coral reefs worldwide, which are in decline primarily due to overharvesting, pollution, disease, and climate change [3–6]. Active and adaptive coral reef management is critical if we are to maintain these ecosystems, which requires an understanding of those processes that support coral reef resilience. A critical component underlying ecosystem resilience is the diversity of functional groups and their response to disturbance [1, 2]. A functional group is a collection of species that perform a similar function, irrespective of their taxonomic affinities [6, 7]. For example, herbivores such as reef fish and sea urchins, are an important functional group, which help to maintain the balance between corals and algae. Coral reefs that have high species diversity would have a higher capacity to absorb a disturbance since the loss of any one species could potentially be compensated for by the actions of others (functional redundancy). In the
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