Indirect interactions among predators: role of positive and antagonistic feeding behaviors in marine systems
Author(s)
Galasso, Nicola Maria
Date Issued
June 12, 2013
Type
Doctoral Thesis
Abstract
With this Phd I focused on few aspects of ecological interactions that arise from that research of food sources (i.e. predator-prey interactions and predator-predator interactions). In particular I focused my attention on facilitation and interactions among predators, working on few models that are based on predator-prey interactions of the Western Mediterranean upper subtidal. The first step was the investigation of a facilitative interaction that takes place in the upper subtidal of the “Ustica Island” Marine Protected Area (MPA) between the starfish Marthasterias glacialis (Mg), the wrasse Thalassoma pavo (Tp) and the sea urchin Paracentrotus lividus (PL). During the attacks of the starfish to the sea urchin, the latter uses its tube feet for the escape. Tp then is able to consume the tube feet that are now available. In order to verify the facilitative effect of Tp behavior on Mg predation I measured and confronted the attack and escape speed of starfish and sea urchin. I then proceeded with the amputation of sea urchin tube feet proving how the escape speed would indeed diminish increasing the percentage of tube feet removed. In this way it was possible to verify the existence of a facilitative interaction among the species studied. This interaction lays within a scenario of habitat shifting also due to human presence (both direct and indirect). I thus had the chance to bring and investigate an example of the importance of indirect interaction within the ecological studies. The next step was aimed to deepen the knowledge of the feeding behavior of Tp previously described. In particular I investigated which were the stimuli that mostly attract Tp (vision of the interaction, chemical signals, noise…). I then arranged a manipulative experiment that would take advantage of another well-known feeding behavior of Tp in Ustica MPA. It is indeed common practice of scuba diver to break open sea urchin in order to attract fish and entertain the tourists. The species that mostly and firstly react to the breaking of the sea urchin is Tp. The breaking of a sea urchin by a scuba diver was then the experimental model used to evaluate the reaction of Tp to external stimuli. This complex stimulus was divided into its components (chemical stimuli characterized by a mixture of all the guts; auditive as the noise of the breaking of the test; visual…etc.) and these were presented on the field to the Tp in order to evaluate the response, measured as the number of individuals that approached. It was then possible to find out that the visual stimulus was the one that more than the others regulates the reaction of Tp to external stimuli. This result is easily extendible to the Mg/Pl interaction giving a wider and more complete comprehension of the scenario of the interaction. Finally I focused on the interaction among conspecific predators that prey upon the same prey. Precisely I set up lab experiments to study eventual between sub-adult individuals of the lobster Palinurus elephas (Pe) during the predation on the sea urchin Paracentrotus lividus (Pl). The predation by sub-adult individuals is not known so firstly I had to arrange an experiment with the aim of understanding the predatory abilities of single Pe individuals. More precisely I wanted to discover which was the largest sea urchin size that a single lobster was able to consume. Once that size was found I gave to a couple of lobsters sea urchins of the first not consumed size in the previous experiment. The goal was to verify if two lobsters were able, cooperating, to consume sea urchins larger than what single lobsters could do. I also used cameras to record the interactions and, in this way, recognize, describe and quantify both predatory and interactive behaviors. I then was able to notice how basically competitive behavior would establish
among lobsters. Sea urchins were actually consumed by a single lobster with no facilitative nor cooperative mechanisms involved.
Additional information
Dottorato di ricerca in Ecologia e gestione delle risorse biologiche
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