Decadal variability in sponge abundance and biodiversity on an Indo-Pacific coral reef

Natural communities can undergo large temporal changes in abundance and species composition that may be difficult to detect without long-term ecological monitoring. Characterizing temporal variability in coral reef fauna is critical for predicting how reef ecosystems will be impacted by environmental change. Sponges are an ecologically important component of coral reefs, yet descriptions of temporal dynamics of multi-species sponge assemblages are scarce. We studied temporal changes in abundance and biodiversity of an Indonesian coral reef sponge assemblage over 13 yr (2005−2017). Mean (±SE) sponge abundance (sponge patches m) initially increased from 124.06 ± 8.46 to 183.73 ± 12.12 in 2005−2007 (p < 0.001), declined to 66.53 ± 10.62 in 2007−2014 (p < 0.001), and increased to 105.87 ± 15.42 in 2014−2017 (p < 0.001). These patterns in sponge abundance did not depend on water temperature. Overall, we recorded 141 sponge taxa. However, species composition showed strong temporal patterns driven by a few abundant taxa (e.g. Protosuberites sp., Sycon sp., and Pericharax sp., respectively accounting on average for ~25, 20, and 5% of total sponge abundance). Species richness increased with sponge abundance (p < 0.001), whereas evenness decreased due to dominance of some taxa in years of high sponge abundance (p = 0.002). Our study highlights that the abundance and biodiversity of Indo-Pacific sponge assemblages undergo dramatic temporal changes driven by species-specific population variability. This variability has important implications for designing monitoring programs, for interpreting experimental studies, and for understanding long-term responses of coral reefs to perturbations.