Sponges contribute to large number of functions in coral reef ecosystems. Among these, bioerosion is perhaps one of the most widely studied, largely due to the important contribution of excavating sponges to the carbonate budget on coral reefs (up to 95 % of the total internal bioerosion). Despite our current knowledge, much of the literature is centred on individual-based observations, and little is known about their ecological role and interactions with other reef taxa in complex coral reef systems. The aim of this thesis was to quantify the ecological interactions of bioeroding sponges with major reef taxa by scaling up individual observations to population and ecosystem-based approaches. A cosmopolitan, abundant and highly competitive bioeroding sponge from Glover’s Atoll, Belize (Cliona tenuis) was used as model species. Monitoring of C. tenuis populations throughout 2009 indicated a trade-off between reproduction and growth, with the highest growth rates (31.4 ± 5.6 mm.y–1) occurring in summer, and a peak in reproductive output during winter. Populations typically show strong left-skewed size frequency distributions, mostly represented by juvenile-size individuals (46%), suggesting that regulating mechanisms (e.g. competition and predation) may be acting in constraining the transition of juveniles to adult sizes. Long-term in situ manipulations showed no effect of predation, yet competition with macroalgae significantly reduced the size of the sponge by 38% ± 11% (SE). While C. tenuis exhibit high growth and recruitment rates that could theoretically result in rapid population growth, the likelihood of sponges forming an alternative stable state as reefs sustain greater levels of disturbance is unclear. An analytical modelling approach of the interplay between macroalgae, coral and sponge was used to explore the likelihood of alternate stable states. The results show that irrespective of successful sponge invasion, inclusion of this third antagonist (in the interplay between coral and macroalgae) can qualitatively affect the likelihood of alternative stable state. The model exhibits emergent properties suggestive of intransitivity between the three competing taxa. Despite the potential of C. tenuis to benefit from disturbance, there are few cases in the literature reporting increases in bioeroding sponge abundance followed disturbance. Therefore, regulating mechanisms such as competition with other taxa, recruitment limitation or mortality are expected to exert demographic control on the populations of bioeroding sponges when space limitation is relaxed due to coral mortality. To determine processes regulating sponge populations, an individual-based spatial modelling approach was used to simulate the population dynamics of C. tenuis in a dynamic ecosystem environment. Using an orthogonal hypothesis testing approach, it was found that competition, and to a lesser extend partial mortality of the sponge tissue, largely regulate the population structure of C. tenuis. While reductions in coral cover may temporarily favour the rapid colonization by sponges, the competitive superiority of macroalgae may steal the opportunity from the opportunists.Fondo Nacional de Ciencia, Tecnología e Innovación (FONACIT
