Disease in Marine Systems

Parasites and pathogens use many marine organisms as hosts; the resulting mortalities can lead to a range of changes, not only in the host population but in the habitat, resultantly altering community structure. Human impact escalates the number of stressors affecting marine ecosystems, making it increasingly important to understand these diseases and the timing of their outbreaks. Anthropogenic changes, such as overfishing and the introduction of terrestrial diseases into the marine environment alter community structures. Furthermore global issues, such as climate change and pollution are thought to increase disease prevalence, in both terrestrial and marine ecosystems. Previous reviews have suggested that over the past three decades, the increase in disease outbreaks has been coupled with climate change; however, lack of data on marine communities is preventing direct testing of this hypothesis.

The authors of this paper developed a proxy method to test a prediction of the increasing disease hypothesis: that reports of disease in scientific literature have increased since 1970. All online literature from ISI web of science, and quantified reports of disease in natural populations of marine organisms were searched from 1970-2001. The investigation looked at nine different taxonomic groups: turtles, corals, mammals, urchins, molluscs, sea grasses, decapods, sharks/rays and fishes, and detected important trends of disease. This method was the first, unbiased, quantitative use of normalised trends in literature, to investigate an ecological hypothesis.

The method showed an increase in total disease reports for all groups, however when the data was normalised an increase was only shown in turtles, corals, mammals, urchins and molluscs. No significant trend was shown in sea grasses, decapods, sharks/rays and counter to the hypothesis a decrease in disease was shown in fish. The results were tested for author bias, and no significant changes occurred in the data reviewed. The proxy method works on the assumption that: actual change in disease overtime will be accompanied by a corresponding change in publishing frequency by scientists. The method has been shown to work before; however it is limited by the inability to distinguish between an event that did not occur and an event which was not reported.

Overall two implications can be taken from the data collected. Firstly, the reported increases of disease were not due to increased study by marine biologists; and secondly, factors such as global change can have complex effects on disease. Temperature change has been linked to an increase in turtle and mollusc disease; along with coral bleaching, while bioaccumulation of toxins in mammals, due to pollution, has been shown to increase susceptibility to disease. Finally the decline in fish diseases has been found to correspond with the reduction of the fish population, due to overfishing. This is thought to have reduced the abundance of disease transmission and lead to the documented and observed decline in fish parasites. Finer scale investigations into disease and its impacts on each taxonomic group are required to fully understand the impacts and causes of marine diseases.

Review of: Ward, J.R. Lafferty, K.D. The Elusive baseline of marine disease: Are disease in ocean ecosystems increasing? PLOS Biology, 2 (4) pp. 542-547.