Little is known about long-term changes in coral reef fish communities. Here we present a new technique that leverages fish otoliths in reef sediments to reconstruct coral reef fish communities. We found over 5,400 otoliths in 169 modern and mid-Holocene bulk samples from Caribbean Panama and Dominican Republic mid-Holocene and modern reefs, demonstrating otoliths are abundant in reef sediments. With a specially-built reference collection, we were able to assign over 4,400 otoliths to one of 56 taxa (35 families) though mostly at genus and family level. Many otoliths were from juvenile fishes for which identification is challenging. Richness (by rarefaction) of otolith assemblages was slightly higher in modern than mid-Holocene reefs, but further analyses are required to elucidate the underlying causes. We compared the living fish communities, sampled using icthyocide, with the sediment otolith assemblages on four reefs finding the otolith assemblages faithfully capture the general composition of the living fish communities. Radiocarbon dating performed directly on the otoliths suggests that relatively little mixing of sediment layers particularly on actively accreting branching coral reefs. All otolith assemblages were strongly dominated by small, fast-turnover fish taxa and juvenile individuals, and our exploration on taxonomy, functional ecology and taphonomy lead us to the conclusion that intense predation is likely the most important process for otolith accumulation in reef sediments. We conclude that otolith assemblages in modern and fossil reef sediments can provide a powerful tool to explore ecological changes in reef fish communities over time and space.
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.
The breadth of habitat occupied by a species, and the rate at which a species can expand into new habitats has important ecological and evolutionary consequences. Here we explore when extant species of free-living cupuladriid bryozoans expanded into new benthic Caribbean habitats that emerged during the final stages of formation of the Isthmus of Panama. Habitat breadth was estimated using the abundances of over 90,000 colonies in ten cupuladriid species, along with the ecological and sedimentary characteristics of the samples in which they occurred. Data reveal that all species expanded their habitat breadths during the last 6 Myr, but did so at a different tempo. ‘Young’ species - those that originated after 5 Ma - expanded relatively quickly, whereas ‘old’ species - those that originated before 9 Ma - took a further 2 Myr to achieve a comparable level of expansion. We propose that, like invasive species, young species are less restrained when expanding their habitat breadths compared to older well-established species. Understanding the mechanism causing this restraint requires further research.
Shark populations have declined over the last half century, but the patterns of change vary across space. Long-term records of shark abundance are limited, making it challenging to determine how local environmental conditions influence pre-exploitation shark baselines and their susceptibility to human impacts. We use shark scales (dermal denticles) preserved in coral reef sediments to reconstruct shark communities during the mid-Holocene and today across the Isthmus of Panama. We interpret these data alongside records of primary productivity, habitat, and fish abundance to explore energy flow to higher trophic levels on each coast. The Tropical Eastern Pacific is a productive system driven by seasonal upwelling with a long history of shark exploitation. The Caribbean coast, on the other hand, is oligotrophic and environmentally stable, with lower rates of harvesting. We find that denticle accumulation rates, a proxy for shark abundance, are an order of magnitude greater in Pacific Panama (Gulf of Panama) than in Caribbean Panama (Bocas del Toro). Primary productivity and fish abundance are also higher in Pacific Panama, helping to support these large predator populations. Denticle accumulation rates declined by 71% since the mid-Holocene in Caribbean Panama, including a selective loss of pelagic sharks. In contrast, modern denticle accumulation rates in Pacific Panama are comparable to their range of variability during the mid-Holocene, and the composition of denticle assemblages remained similar through time—suggesting that sharks in the Gulf of Panama have persisted despite intensive fishing. We postulate that the region’s high productivity might underlie its high shark abundance and apparent resilience by increasing available resources for predators. Our findings shed light on the role of energy in shaping natural variability in shark baselines and recovery potential. They also highlight the importance of incorporating oceanographic context into shark management.
Along the coastal Tropical Eastern Pacific (TEP), regions of strong seasonal upwelling bring cold, nutrient-rich waters, controlling ecological conditions and sustaining millions of people through large-scale fisheries. The TEP is also important for the regulation of global climate and is affected by large-scale environmental processes such as ENSO. How the nutrient dynamics of this region will respond to climate change and what the implications will be for coastal ecology remains unknown. Environmental records are needed that capture intra and inter-decadal variation and extend over millennia where these biotic and abiotic processes interact. Here, we develop a new sampling approach and construct two coral skeleton records (n = >600) from reef matrix cores that extend six millennia, from the upwelling Gulf of Panamá and the non-upwelling Gulf of Chiriquí. We ask what effects millennial-scale climate patterns have on upwelling in the region, and how the magnitude of upwelled nutrients influences ecological productivity and even human habitation. We combined multiple proxies using climatic (carbonate δ18O), nutrient (skeletal-organic matrix δ15N), diagenetic (taphonomic scoring), ecological (benthic community composition), and temporal (U-Th dates) data. Using Generalised Additive Models to assess variability, we find strong divergences in the nutrient (δ15N; range >5 ‰) records between Gulfs, while δ18O (range ~2‰) is more stable. The greatest variation in δ15N values occurs during times of high reef accretion whereas δ18O is constant, suggesting that nutrients, not temperature, are driving reef productivity. Taphonomic, taxonomic, and age data reveal periodic shifts and collapses of coral communities that differ between Gulfs. We end by drawing connections between these ecological shifts to the episodic human habitation documented during the late-Holocene and hypothesize what this may mean for ecosystem resilience and environmental management under future climate.
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