Hindawi Publishing Corporation Journal of Marine Biology Volume 2011, Article ID 490198, 8 pages doi:10.1155/2011/490198 Research Article 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) G. S. Aeby,1 D. G. Bourne,2 B. Wilson,2 and T. M. Work3 1 Hawaii Institute of Marine Biology, Kaneohe, HI 96744, USA 2 Australian Institute of Marine Science, PMB 3, Townsville, QLD 4810, Australia 3 US Geological Survey, National Wildlife Health Center, Honolulu Field Station, Honolulu, HI 96850, USA Correspondence should be addressed to G. S. Aeby, [email protected] Received 16 July 2010; Accepted 27 November 2010 Academic Editor: Judith D. Lemus Copyright © 2011 G. S. Aeby et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 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 species that perform a similar function, irrespective of their taxonomic affinities [6, 7]. For example, herbivores such Ecosystem resilience can be defined as the capacity of a as reef fish and sea urchins, are an important functional system to absorb disturbance and reorganize so as to retain group, which help to maintain the balance between corals the basic ecosystem services [1, 2]. Regime shifts in ecosys- and algae. Coral reefs that have high species diversity would tems are increasingly common as a consequence of human have a higher capacity to absorb a disturbance since the activities that erode resilience. This is especially apparent for loss of any one species could potentially be compensated coral reefs worldwide, which are in decline primarily due to for by the actions of others (functional redundancy). In the overharvesting, pollution, disease, and climate change [3–6]. Caribbean, overfishing reduced the abundance of important Active and adaptive coral reef management is critical if we fish herbivores, but this was initially compensated for by are to maintain these ecosystems, which requires an under- a corresponding increase in sea urchin populations [8–10]; standing of those processes that support coral reef resilience. when sea urchin populations subsequently collapsed from A critical component underlying ecosystem resilience is adiseaseoutbreak[11], this led to massive losses of corals the diversity of functional groups and their response to [12]. Resilience also requires variability in the response of disturbance [1, 2]. A functional group is a collection of the species within functional groups to the perturbation 2 Journal of Marine Biology (response diversity) [1, 2]. If all species respond in a similar 2005. AWS was documented on Tutuila in American Samoa manner then functional redundancy is lost. Coral reefs with in 2004 [22] and in 2008 we conducted a parallel study a high diversity of species should therefore be more resilient on AWS dynamics. Our objectives were to (1) compare to change, with a greater probability that some component how differences in biological variables between the two of the species pool will be able withstand a specific stressor regions influence the impact of disease events (e.g., the allowing for functional redundancy and response diversity. number of susceptible host species (species-richness), host, The capacity of coral reefs to withstand the impacts abundance (colony density and percent cover), and size class of coral disease is of increasing concern. Coral disease has structure of the host coral; (2) document the initial AWS severely altered coral reefs in the Caribbean [13–17]andis outbreak levels (prevalence) and subsequent disease spread increasing on reefs across the Indo-Pacific [18–25]. Models (incidence) in each region; (3) compare the virulence (degree of global climate change predict that disease outbreaks, of colony mortality) of AWS on individual colonies between worldwide, will continue to increase through time [3, 4, 26– regions and among different host species; (4) determine 28], and it is therefore important that managers are given whether AWS is transmissible and investigate any variation sufficient information to predict how coral reefs may respond in transmissibility among host species. to future disease events in terms of their resistance and resilience. A basic premise of epidemiology is that increased host abundance enhances disease transmission, both initially 2. Methods into a population and subsequently within it, resulting in 2.1. Description of Sites. The Northwestern Hawaiian Islands increased disease severity [29–31]. As such, the diversity- (NWHI) are a chain of small rocky islands, atolls, coral disease hypothesis predicts that low host species diversity islands, and reefs that span 1,800 km over more than five canresultinanincreaseindiseaseseverityofspecialist degrees of latitude in the northwestern portion of the pathogens, since low host richness can, through relaxed Hawaiian Archipelago. The research site was located at interspecific competition, increase relative abundances of ◦ ◦ French Frigate Shoals (23 50 N, 166 10 W), which is a one or more susceptible hosts [32, 33]. The diversity-disease crescent-shaped atoll approximately 27 km in length with hypothesis is well supported in plant-pathogen systems [34– a well-formed barrier reef and lagoon [52]. The NWHI 39] as well as some animal disease systems [40–43]. This are now part of the Papahanaumoku¯ akea¯ Marine National hypothesis has not yet been adequately tested within the coral Monument. The territory of American Samoa has a total land reef environment (but see [44]); however given examples area of 76.1 square miles and includes five volcanic islands from terrestrial ecosystems, coral reefs in species-rich regions (Tutuila, Aunu’u, Ofu, Olosega, Ta’u) [53]. The research should be more resistant and/or resilient to coral disease than ◦ site was located in Vatia on the north side of Tutuila (14 reefs in species-poor regions. If so, then species diversity ◦ 14 S, 170 40 W). These two regions were opportunistically might be a good indicator for managers in predicting the chosen for comparison as each had reported outbreaks of outcome of disease events on reefs and an impetus for man- AWS and were accessible for study. aging for species diversity. Within the Indo-Pacific, marine organisms exhibit a longitudinal biodiversity gradient with species-richness declining with increasing distance from the 2.2. Disease Prevalence and Incidence. Within each region, Indo-Australian Archipelago [45–47]. For example, over 500 preliminary visual surveys were conducted to identify sites coral species are found in the Indonesian-Philippines centre which were dominated by Acropora sp. and that had an of diversity compared to less than 80 species within Hawaii ongoing AWS outbreak. When multiple outbreak sites were [48, 49]. Presumably, impacts of disease on coral reefs could found, those sites having the highest AWS levels were chosen be more severe in low diversity regions, especially for those for the study. The Northwestern Hawaiian Islands was diseases, which are more host specific such as Montipora surveyed in May 2005 and resurveyed in May 2006. American white syndrome [50]orAcropora white syndrome [20], Samoa was surveyed in June 2008 and was resurveyed in which have only been observed in the field affecting corals of September 2009. aspecificgenus(e.g.,Montipora and Acropora, respectively). At each outbreak site, two 25 m transect lines were laid However, little is known regarding the relationship between end-to-end and separated by approximately 3 m. The sub- coral disease severity and coral species richness. strate at the start of each transect (and every 5 meters there- We present the first study to examine the role that after) was marked with either a numbered cow tag or a steel biodiversity may play in influencing the impact of coral pin ensuring that future surveys could be conducted over the disease outbreaks, by comparing the dynamics of Acropora same area on the reef. Coral colony density and size class white syndrome (AWS) on a reef in a species-poor region structure were documented by recording Acropora colonies (Northwestern Hawaiian Islands) (reef coral species = 80) by size class along the transect lines. All Acropora sp.,witha [49] with a reef in a species-rich region (American Samoa) colony center within one meter on either side of the transect (reef coral species = 276) [51]. AWS is a coral disease line (25 m × 2 m), were enumerated and placed into one of that results in progressive tissue loss and which has caused seven size classes: <5 cm, 5–10, 10–20, 20–40, 40–80, 80–160, extensive coral mortality in many regions throughout the and >160 cm.