Baseline Caribbean Reefs

Caribbean coral reefs started to deteriorate before systematic monitoring began and so questions remain about how reefs have changed since human impact and if they have transitioned into functionally ‘novel’ states. To explore these questions, we mapped and bulk-sampled several hectares of mid-Holocene reefs in Caribbean Panama and the Dominican Republic and compared the composition and ecological function of these pre-human impact reefs to nearby modern reefs. We quantified the remains of all major reef groups, but focus here on molluscs, corals, and fishes. Filter feeding molluscs are twice as abundant relative to other feeding modes on modern reefs, commensurate with eutrophication from land use changes. At the same time, large herbivorous gastropods declined significantly in size due to millennia of human selective harvesting. We observed the well-documented loss of Acroporid corals and a functional shift in coral communities towards weedier, slower growing, and brooding species. Some modern coral communities appear to retain some historical functions, and isolated Acropora refugia do persist, but the corals in them are less robust than those in the mid-Holocene, questioning their functional resilience to future change. Reef fish otolith assemblages suggest an 80% decline in non-harvested fish and a relative increase in planktotrophy—patterns best explained by the loss of coral structure and eutrophication. Counterintuitively, otolith sizes suggest that non-harvested fish are larger than they were in the past, a result that suggests lower mortality rates from reduced predation due to a loss of predators. This conclusion is supported by the estimated 71% decline in shark abundances and 400% increase in evidence of damselfish algal-gardening on modern reefs. These examples illustrate how both bottom-up and top-down processes have reshaped the structure, trophic interactions and ecosystem functions of Caribbean reefscapes.