دانلود رایگان مقاله رقابت برای محل سرپناه و مکانیسمی جهت جابجایی گونه گامارید

Abstract

Invasions of alien gammarid species have led to reduced abundance of many native gammarid species and earlier gammarid invaders. Intra-guild predation (IGP) has been suggested as the main mechanism causing such species displacements. This study elucidates a mechanism for species displacement that is based on competition for shelter, viz. species excluding each other from a shelter place. Preferences of Dikerogammarus villosus and Gammarus roeselii for shelter space were studied in laboratory experiments. In contrast with night conditions both species showed a strong preference for shelter sites under daytime conditions so that all experiments were carried out under permanent light conditions. Single individuals of D. villosus sheltered more than those of G. roeselii. Intraspecific competition experiments with different size classes and sexes showed that in contrast with D. villosus, the size class had a significant effect on the mean sheltering proportion of individuals of G. roeselii. When both species were brought together in a basin with a shelter site to test interspecific competition, individuals of G. roeselii were actively pushed out of their shelters within 24 h, while D. villosus showed no change in shelter. This led to a significantly increased predation risk for G. roeselii, which was confirmed in an experiment in which a benthic fish was added. IGP only manifested itself after 48 h, indicating that competition for shelter preceded IGP. When shelter opportunities are in short supply, shelter exclusion may be one of the initial mechanisms for gammarid species displacements.

Discussion

The experimental shelter sites were often used by both gammarid species under continuous light conditions. D. villosus was the stronger competitor for shelter sites, which eventually resulted in an increased mortality of G. roeselii in the presence of a predatory fish. Single individuals of D. villosussheltered more than single individuals of G. roeselii. These results match those of the intraspecific experiment, where individuals of D. villosus also sheltered more than large males and large females of G. roeselii. It seems that D. villosus is not affected by intraspecific competition, in contrast to G. roeselii, where small individuals showed higher sheltering rates than large individuals in intraspecific competition. Since smaller individuals are more vulnerable to cannibalism and predation by (larger) predators, they hide more likely in shelters (Platvoet et al. 2009a). The small males and small females of G. roeselii sheltered significantly more than large males and large females in the separate as well as the mixed size classes. The sheltering individualsin oursingle individual experiments were notsubject to competition by other individuals or species. Probably because shelter sites are only efficiently used when there is an increased predation risk (Magalhaes, Janssen, Hanna, & Sabelis, 2002), fewer single individuals of G. roeselii sheltered in comparison with sheltering individuals of G. roeselii in the intraspecific experiment.