The phenomenon of biological invasions represents one of the major causes of the global crisis of biodiversity. Understanding the elements which control the process of invasion is therefore of high interest for the preservation of natural ecosystems. For this aim, it is necessary to determine specifically which are the processes that regulate the spatial and temporal distribution of the introduced species. These processes depend on the biological and physical processes in habitats being threatened. For instance, the presence of other introduced species and anthropic disturbances as well as their interaction is known to contribute to regulating invasion.
Descriptive studies suggest that the structure of resident populations in areas colonized by invasive seaweeds differs, in terms of species diversity and relative abundances, from populations in non-invaded areas. In the invaded areas, frequently, a minor diversity is observed. This kind of variations could be assigned directly to the presence of allochthonous seaweed, which could be a disturbing agent. In this case, by experimental removal or reduction of that particular invader, diversity should increase. Alternatively, environmental conditions, variable at small scale, could be directly responsible for the alteration of the resident assemblages and facilitating the settlement of invasive seaweed. In this instance, removing the invading seaweed would not change in a significant way the structure of local assemblages, but restoring environmental conditions would be necessary to generate a positive response of resident assemblages.
The aim of the present study is to estimate the spatial distribution, the temporal dynamics and the effects on native populations of two species of macroalgae introduced on the shallow rocky reef along the coast of Livorno: Rhodophyta Asparagopsis taxiformis (Delile) Trevisan de Saint-Leon of uncertain origin and the native Australian Chlorophyta species Caulerpa racemosa (Forsskal) J.Agardh.
The study took place on shallow rocky reefs (4 to 6 meters of depth). The percentage cover of the two seaweeds has been measured by visual sampling every three months at three different sites that were selected randomly, at a distance of about 2 km one from another. In addition, an experimental study has been performed at Antignano.
For this aim, 12 units ( 20 cm x 20 cm squares) have been assigned to each of these 4 treatments: 1) total removal of the whole population; 2) a 100% removal of the invasive seaweed’s covering; 3) a 50% removal of the invasive seaweed’s covering; 4) examination (no manipulation on the population).
It seems that A. taxiformis has basically a punctiform distribution with a trend for decreasing percentage cover in fall. The maximum percentage cover of C. racemosa corresponded, instead, to the period in which the cover of A. taxiformis was reduced to a minimum. This discrepancy in the period of maximum development could suggest that competition mechanisms might regulate the distribution of these two introduced seaweeds.
The spatial distribution of C.racemosa also appears to be more homogeneous compared to A. taxiformis. This could depend on variations of the physical properties of the environment at a small spatial scale. By using plaster balls it was possible to determine that the distribution of C. racemosa is less influenced by hydrodynamics than A.taxiformis. The punctiform distribution of the Rhodophyta could therefore be justified by its weak tolerance to hydrodynamic forces.
Contrary to the extensively documented effect caused by C.racemosa on the native benthic assemblages, A. taxiformis did not show any effect on the assemblages it was associated to. In this context, it is important to stress that a comparison of the data collected in march 2010 and the data from march 2011, highlights a 64% decrease of the cover by A.taxiformis in control quadrats. Thus, it is possible to hypothesize that A. taxiformis does not persist long enough in given area to generate significant effects on native benthic assemblages.