Many invasive species proliferate uncontrollably because their original herbivores/ predators are not present in the new habitats (“Enemy Release Hypothesis”). The generalist marine herbivores of western Mediterranean: Sarpa salpa, Paracentrotus lividus and Arbacia lixula have a diet that includes a variety of native and invasive species (such as Caulerpa cylindracea), and can play a key role in the control of invasive species.

  

Main herbivores of the Western Mediterranean ( Sarpa salpa, Paracentrotus lividus and Arbacia lixula)

Herbivory can have a direct effect limiting or favoring invasive species, depending on whether the herbivory is on invasive or on native species. It can also have an indirect effect, modifying the composition and structure of the algal communities, and therefore increase or decrease the interspecific competition between invasive and native species. In cases of presence of invasive algae, discerning the importance of these biotic factors (predation and competition) in invasive processes is of vital importance to predict the evolution of the different invasive species, as well as the most vulnerable or resilient habitats.

In this project we study the relative importance of generalist herbivores, the characteristics of native communities, and  abiotic parameters (temperature) as resistance agents against algae invasions. We perform this study in the Cabrera National Park, together with an ecological and genetic monitoring of benthic communities affected by the invasions of foreign algae. In this Park we have a time series of temperature and abundance of invasive species in a long-term time frame, 2005-2021 for the ecological monitoring of invasive algae and 2007-2021 for continuous temperature measurements.

METHODS

Diver doing a transect to
quantify the abundance of sea urchins.

In Cabrera Archipelago National Park we carry out an estimation of the main herbivores, both fish ( Sarpa salpa) and sea urchins ( Paracentrotus lividus and Arbacia lixula). The preferences and consumption capacity of invasive seaweeds by these herbivores are known from previous studies. So the estimation of the herbivores’ abundance will allow an approximation of the impact and control on invasive algae.

In the sampling campaign we did  25 m long and 5 m wide counting transects of Sarpa salpa in three locations. The abundance and size class of the observed specimens were determined in each transect. Urchin densities were evaluated at 5 sampling stations using 25*1 m transects. In these transects we quantified the abundance and size of the urchins. The recruitment of the two sea urchin species was also quantified by lifting large specimens, below which the recruits are usually found and then measured and counted.


Arbacia lixula recruit