Action F5. Modelling of the expansion of Caulerpa taxifolia.
Partner: Direcció general de Biodiversitat.

To develop a predictive model of the expansion of Caulerpa taxifolia and Caulerpa prolifera through the comparative study of the regulation of horizontal growth occurring in different substrates and environmental conditions.

To develop the action a company reputed for its expertise in this matter was hired.
The work consisted in the elaboration of a survey to study the growth norms of Caulerpa taxifolia and Caulerpa prolifera, in order to model its expansion in Posidonia oceanica fields round the Balearics.

Monthly at location measurements, applying marking techniques, of the elongation and ramification of the stolons, rate of frond formation and ramification angle of the stolons of Caulerpa taxifolia and Caulerpa prolifera growing in fields of Posidonia. Design of a numeric model based on the empiric norms of clone growth , in order to simulate the clone expansion of Caulerpa taxifolia and C.prolifera.

Monthly empiric estimates have been collected for one and a half years in order to quantify the rules of annual clone growth that regulate the growth of patches of C.taxifolia and C. prolifera (autochthonal species) : distance between neighbouring fronds, elongation rate of the stolon , ramification rate of the stolon , angle of ramification, and average life span of the stolons and fronds.

The results obtained indicate that in accordance with growth rules, C.taxifolia would occupy space at a much slower rate than C.prolifera. For example, C.taxifolia extends its stolons at a slower rate than C.prolifera (25.4 cms. year- 1 in C.taxifolia, 122 cms. year- 1 in C.prolifera). Based on the obtained estimates of clone growth rules for both species, a numeric model was then designed to simulate the growth of patches. The model predicts that the development of a C.taxifolia patch is not linear where young patches (3-5 years) increase their shape exponentially and grow in ramifications, whereas the older patches increase their size more slowly and grow centrifugally. Nevertheless, patches older than 8 years expand at a constant and maximum rate (aprox.15 cm year-1).
This model assumes that occupation of space may be limited by competition between fronds in the patch, but does not contemplate that there may be competition between Caulerpa and other species. It is, however, possible that the expansion of patches of C.taxifolia in Posidonia oceánica fields which are in good state (i.e. not in decline) is limited by the presence of P.oceanica. Whereas the model predicted that patches around 10 years old would have a diameter of 1-2 meters, in Cala d’Or, where the invasion of C.taxifolia began in 1992, the diameter of the larges patches was only of 0.6 m.
The model has been adjusted to simulate the development of C.prolifera patches in order to be able to determine comparatively the capacity of C.taxifolia to expand.

Due to the present low incidence of C.taxifolia and its naturally occurring recession in Balearic waters, the modelling will be useful in the hypothetical case that Caulerpa should begin to spread again, as the model would make it possible to predict its expansion. This model will make it easier to take measures to avoid any harm to Posidonia fields, and can also be applied to model the growth of other species that may affect Posidonia, for with the adequate parameters a simulation for any species can be developed.
Thus, the aims of this action have been fulfilled as the goal was to model the growth and spreading of C.taxifolia, although, for the moment, fortunately, it is not necessary to apply the model as C.taxifolia in natural recession.

Graph of the model of expansion