Modeling the recovery process after mass bleaching

　

Soyoka Muko

Department of Biology, Kyushu University, Japan

　

In coral communities, the composition of morphological types is very different in each habitat. For instance, in Okinawa, Japan, “Branching Acropora spp.” dominated the protected site, whilst “Table-like Acropora sp.” were abundant at the exposed site. In 1998, mass bleaching occurred and most of corals has been dead in this area. The study of the recovery process after mass bleaching provides us an opportunity to understand the demographic process, larval settlement, growth, and death, which form and maintain the observed patterns.

We formulate a simple model for the dynamics of coverage of the two morphotypes. The model incorporates the space-limited recruitment and the space-limited growth. The larval settlement rate is proportional to the amount of vacant space in the local habitat and to the abundance of larvae in the water column. The growth rate of colony size (coverage area) increases with the size and with the fraction of vacant space. The coverage X_i of type i are

where x_i⁰ is the size of newly settled adult; L_i, the abundance of larvae; s_i, the efficiency of larval settlement; g_i⁰, the maximum growth rate; and u_i, the mortality. F, is the amount of free space:

F=A-X₁-X₂

The result of the model show that recovery process has three phases. [1] In the beginning, the relative abundance of the two types is controlled by the ratio of the larval settlement. [2] When the vacant space becomes occupied, both the settlement of larvae and the growth of settled colonies affect the dynamics of coverage. [3] After the free space is depleted, both larval settlement and growth become very small. Now the slow process of colony death comes to have an influence and causes the final convergence to the equilibrium composition.

The dominance of table-like corals at the exposed site is often regarded as an adaptation for the severe wave action. However the same pattern can be explained by larger recruitment rate of table-like corals, if the total amount of recruitment is large. In order to distinguish these hypothesis, we are investigating the demographic processes of the two morphotypes at three different sites in Sesoko Island, Okinawa.

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