Microbial disease in coral reefs: An ecosystem in transition

Abstract

Infectious disease outbreaks are considered an important factor for the degradation of coral reefs. Reef-building coral species are susceptible to the influences of black band disease (BBD), characterized by cyanobacteriadominated microbial mat that migrates rapidly across infected corals, leaving empty coral skeletons behind. We investigate coral-macroalgal phase shift in presence of BBD infection by means of an eco-epidemiological model under the assumption that the transmission of BBD occurs through both contagious and non-contagious pathways. It is observed that in presence of low coralrecruitment rate on algal turf, reduced herbivory and high macroalgal immigration, the system exhibits hysteresis through a saddle-node bifurcation and a transcritical bifurcation. Also, the system undergoes a supercritical Hopf bifurcation followed by a saddle-node bifurcation if BBD-transmission rate crosses certain critical value. We examine the effects of incubation time lag of infectious agents develop in susceptible corals after coming in contact with infected corals and a time lag in the recovery of algal turf in response to grazing of herbivores by performing equilibrium and stability analyses of delay-differential forms of the ODE model. Computer simulations have been carried out to illustrate different analytical results.2010 Mathematics Subject Classification. Primary: 92B05, 92D25; Secondary: 92D40.

Macroalgal allelopathy in the emergence of coral diseases

“…The system with stability at the interior equilibrium exhibits a sudden change of transition associated with saddle-node bifurcation when the WPD-II transmission rate becomes low. This supports our previous claim [5] that the system will be coral-dominated and stable with low rate of infection. However, in our previous study [5], with low contagious infection rate we found no noticeable change in the dynamics of the system even with high rate of non-contagious infection.…”

“…This supports our previous claim [5] that the system will be coral-dominated and stable with low rate of infection. However, in our previous study [5], with low contagious infection rate we found no noticeable change in the dynamics of the system even with high rate of non-contagious infection. The current paper demonstrates a Hopf bifurcation followed by coral-depleted steady state with high rate of infection.…”

“…Our model originates from the models used in [8,23] and is a direct extension of the models studied in [4,5] by assuming that the disease spreads among corals according to a Susceptible→Infected (SI) type compartmental eco-epidemiological model [2]. For free-moving terrestrial organisms, transmission rates are mainly dependent on the contact among susceptible and infectious hosts, whereas in the marine environment disease agents move more widely and rapidly within the water column [16].…”

“…An eco-epidemiological model is considered to study the dynamics of coral reef benthic system in presence of a microbial infection, WPD-II on corals. We have modified our previous models [4,5] by introducing macroalgal toxicity on corals and by considering the dynamics of pathogens. Our model assumes the transmission of WPD-II through free living pathogens shed by infected corals at a rate proportional to the product of susceptible coral cover and pathogen concentration in the environment.…”