Competition for space in benthic environments: the allelopathic and overgrowth responses of native sponges and a non-native ascidian on a coral reef in Bonaire, NA
This study explores the competitive and allelopathic interactions of native sponges and the non-native ascidian, Trididemnum solidum, in the space-limited coral reef environments of Bonaire, NA. The study had two main goals: (1) to identify native sponge species and provide estimates of sponge and T. solidum percent cover, and (2) investigate the allelopathic and overgrowth responses of native sponges and T. solidum when they are engaged in spatial competition with each other. Belt transects and modified nearest-neighbor methods were used to quantify abundance, species diversity, and interactions between native sponges and T. solidum at the Karpata dive site on Bonaire. Overall, it was found that percent cover of sponges significantly increased with depth while percent cover of T. solidum varied among depths, reaching a maximum at 11 – 15 m. Twenty-two species of sponges were recorded with species composition and abundance varying among depths while diversity among depths was not significantly different (p = 0.880). It was found that percent cover of T. solidum had a significant effect on the number of contact interactions. A closer look at contact interactions revealed that T. solidum frequently (87.5 % - 100.0%) overgrew sponges and caused tissue necrosis but was itself never observed to be overgrown. Staged interactions between two abundant encrusting sponges (Ulosa ruetzleri and Halisarca sp.) and T. solidum showed that native sponge growth is impaired by the ascidian and that T. solidum uses allelopathy when expanding its colonies. As described by this study, the success of T. solidum in its expanded range may provide support for two additional hypotheses: the evolution of increased competitive ability hypothesis, which attributes the success of non-native species to their increased ability to overgrow native organisms, and the novel weapons hypothesis which explains that non-native species are successful because they harbor allelopathic chemicals that native organisms have not evolved defenses for.