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Sea urchin: the keystone herbivore in Mediterranean marine macrophyte communities by Simone Farina

Reading Time: 8 minutes

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parole chiave: ricci di mare, estinzione 



photo credit andrea mucedola

Predation is a process where predatory interactions, by definition, exert clear direct effects at the population level as interspecies relationships. Predation is a class of interaction in which one species obtains direct benefits (the predator) while the second species is harmed (the prey). From the ecological point of view prey are resources for predators, and when the number of prey increase, the abundance of predators also increases due to the availability of the resource.


Paracentrotus lividus, photo credit andrea mucedola, 2016

Predation process is considered among the strongest species interactions, however, more than at population level, the effects of predation are considered with major importance in complex networks of connectivity since they may ripple out through the ecological community, indirectly changing the abundances of other species with as consequence strong indirect effects observed starting at community scale (Fretwell, 1987). Effectively this process can produce a progression of indirect effects across the lower trophic levels. Because the predator eats prey, when predators decrease the number of prey, they are realising from predation (or grazing) the next lower trophic level establishing a trophic cascade effect (Schmitz, Hambäck, & Beckerman, 2000). In this way the lower trophic level could proliferate with uncontrolled determining other effects on the next lower level. Indirect relationships between predators, herbivores and producers represent then a generalized trophic cascade, the indirect effect of predation on community. Definitively predation regulates directly population abundance and distribution but more indirectly all the trophic chain, pointing out the basic ecological concepts of the top-down control, the trophic cascades and the idea of keystone species.


Marine ecosystems provide excellent examples of indirect effects of predation. One of the most common trophic cascade in Mediterranean benthic ecosystems include predatory fishes, sea urchins and macrophytes, usually macroalgas but sometimes also seagrasses. Fishes eat urchins and urchins graze on algae. These trophic cascades have been seriously altered by overfishing, that have extirpate predatory fishes in many coastal areas, determining uncontrolled proliferations of sea urchins that often overgraze algal cover creating barrens (Steneck et al., 2002; Steneck, Vavrinec, & Leland, 2004). Sea urchin is one of the most important consumers of macrophytes and in many cases its voracity makes that the increasing of population density has a dramatic effect on their communities (Sala et al. 1998). These effects of overfishing have been reversed in Marine Protected Areas (MPAs), where the top-down control asserted by fish on sea urchins shapes indirectly lush vegetation, with indirect positive consequences on biodiversity (Palacin, Giribet, Garner, Dantart, & Turon, 1998; Valentine & Heck, 1991).

p8100006Therefore macrophyte ecosystems can experience a remarkable change when sea urchin abundance increases or decreases even if this invertebrate represents a very little proportion of the ecosystem(Alcoverro & Mariani, 2004; Camp, Cobb, & Van Breedfield, 1973; K. L. Heck & Valentine, 1995; Rose et al., 1999), and this fact characterises sea urchins as keystone species (Pinnegar et al., 2000; Planes, Garcia-Charton, Marcos, & Perez-Ruzafa, 2006). Predation is considered the prevalent mechanism of sea urchin population control after settling and it is well known that it generates a bottle-neck for population growth between recruits and adults, and represents a significant pressure until individuals reach the refuge size of 5.5 cm TD (Guidetti, 2004; Hereu, Zabala, Linares, & Sala, 2005).  The main sea urchin predators in the Mediterranean are the fishes Coris julis, Diplodus spp. and Sparus aurata, visual hunters of small and medium size classes. In addition to these, there are also many bottom predators whose effect on sea urchin populations is still little known (Sala & Zabala, 1996).  For example, species from the Muricidae sp. and the carnivore sea star Marthasterias glacialis are well known consumers of sea urchins that mainly hunt through chemotactic mechanisms (Bonaviri et al., 2009).  While directly measuring rates of predation in real world ecosystems is generally unfeasible, researchers have developed assay techniques to obtain relative estimates that can integrate longer periods of time and avoid observer artifacts (Hairston, 1989). In marine systems, measures of predation have relied heavily on tethering techniques, often using sea urchins as a model prey (McClanahan and Muthiga, 1989).

p8170235In this context, estimating the ability of predators to control urchin numbers is critical to understand ecosystem functioning (Farina et al., 2014; K. Heck & Valentine, 1995; K. Heck & Wilson, 1987; Mcclanahan & Muthiga, 1989; McClanahan, 1999; Pederson & Johnson, 2006; Shears & Babcock, 2002). Tethering techniques have been extensively used in experimental ecology as a tagging and constraining technique to assess predation for different species in various ecosystems and conditions (e.g., Gorman, Gregory, & Schneider, 2009). The most effective and commonly used tethering methods involve piercing the target organism with a hypodermic needle. For instance, with sea urchins this involves piercing the test from the oral to the aboral region, and passing a monofilament line through the skeleton, which is then used as a tether (Boada et al., 2015; Ebert, 1965) . Sea urchin survival is checked daily and the type of attack is also classified depending on the type of mark found: fishing line loop without sea urchin or broken skeleton was classified as fish attack, while a drill hole found on prey skeleton indicates benthic predator attack (Peharda & Morton, 2006).

This technique can be used to estimate pressure exerted by predators in a Marine Protected Area or in a zone with high fishing pressure. In this sense the estimation of the strength of the mechanism of the top-down control on sea urchins permit to valuate the effects of local fishing activity or conservation planning on the whole community (Boada et al., 2015).



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