Effects of fishing and trophic interactions on tropical reef builders: Coralline algae and corals

While coral reef systems have been extensively studied, little research has focused on one of the major tropical reef builders, crustose coralline algae (CCA). In addition to reef building, some CCA provide essential cues inducing settlement of coral larvae. Herbivorous grazers may modify the distribution of CCA with cascading effects on coral recruitment, leading to ecosystem-wide effects. A major consequence of fishing is the removal of predatory fishes, leading to changes in the dominant herbivores present. Kenya has a more complete grazing guild than the much studied Caribbean because it still has abundant sea urchin populations. The dominant grazers in protected reefs are fishes, while the dominant grazers in fished reefs are sea urchins. I investigated the differential effects of fish and sea urchin grazing (mediated by fisheries management and climate change) on abundance and species composition of CCA using long-term data, field experimentation, and field surveys. In contrast with studies in temperate systems and in the Caribbean, here, both fish and sea urchin grazing reduced the cover of CCA. However, sea urchin grazing intensity was stronger than that of fishes and the effects of an ocean warming event, and depleted CCA in fished reefs. Fishing-induced changes in grazer type also modified species composition of CCA, with twice as much inductive CCA in protected than on fished reefs. Coral recruitment was strongly correlated with CCA, and especially inductive CCA, across multiple coral families. Thus, fishing changes the distribution and composition of CCA with potentially strong effects on coral settlement. To understand the relative contribution of the major processes that contribute to coral recruitment (settlement and post-settlement mortality), I investigated the spatial scales of variation in recruitment within a protected reef. When fish grazing is precluded, settlement patterns reflect delivery processes, with variation concentrated at the largest measured spatial scale (1000 m). However, settlement patterns were modified by fish grazing, resulting in variation concentrated at the smallest spatial scale (1 m). By using an ecosystem-level approach, I was able to determine how grazers directly and indirectly influence tropical reef builders, and understand how humans may change ecosystem dynamics via unexpected pathways.