Prevalence of pelagic dependence among coral reef predators across an atoll seascape.

Coral reef food webs are complex, vary spatially and remain poorly understood. Certain large predators, notably sharks, are subsidized by pelagic production on outer reef slopes, but how widespread this dependence is across all teleost fishery target species and within atolls is unclear. North Malé Atoll (Maldives) includes oceanic barrier as well as lagoonal reefs. Nine fishery target predators constituting ca. 55% of the local fishery target species biomass at assumed trophic levels 3-5 were selected for analysis. Data were derived from carbon (δ13 C), nitrogen (δ15 N) and sulphur (δ34 S) stable isotopes from predator white dorsal muscle samples, and primary consumer species representing production source end-members. Three-source Bayesian stable isotope mixing models showed that uptake of pelagic production extends throughout the atoll, with predatory fishes showing equal planktonic reliance between inner and outer edge reefs. Median plankton contribution was 65%-80% for all groupers and 68%-88% for an emperor, a jack and snappers. Lagoonal and atoll edge predators are equally at risk from anthropogenic and climate-induced changes, which may impact the linkages they construct, highlighting the need for management plans that transcend the boundaries of this threatened ecosystem.

Chronic spearfishing may indirectly affect reef health through reductions in parrotfish bite rates.

The grazing behaviour of two Caribbean parrotfish, a fished species, the stoplight parrotfish Sparisoma viride and a non-fished species, the striped parrotfish Scarus iseri, were studied in the presence (fished site) and absence (marine reserve) of chronic spearfishing activity. Diurnal feeding periodicity did not differ between the sites in either species: roving individuals had significantly higher bite rates in the afternoon, while territorial individuals foraged consistently throughout the day. Mean bite rate varied between sites in both species. Abundance, biomass and bite rates of S. viride were all significantly higher within the reserve, except for roving S. viride which had a higher mean bite rate in the afternoon outside the reserve compared with within it, attributable to maximisation of feeding in the afternoon when fishing risk was lower. Scarus iseri mean abundance and bite rate were greater outside the reserve, potentially because reduction in large territorial herbivores allowed S. iseri to feed more rapidly. By reducing the grazing potential of the remaining S. viride individuals the effect of fishing is greater than would be predicted from biomass changes alone. Less grazing by S. viride would not be compensated for by the increase in grazing by S. iseri because the latter feeds on different algae. Spearfishing of key parrotfish species reduces grazing potential directly by extraction and indirectly by changing behaviour.

Novel tri-isotope ellipsoid approach reveals dietary variation in sympatric predators.

Sympatric species may partition resources to reduce competition and facilitate co-existence. While spatial variation and specialization in feeding strategies may be prevalent among large marine predators, studies have focussed on sharks, birds, and marine mammals. We consider for the first time the isotopic niche partitioning of co-occurring, teleost reef predators spanning multiple families. Using a novel tri-isotope ellipsoid approach, we investigate the feeding strategies of seven of these species across an atoll seascape in the Maldives. We demonstrate substantial spatial variation in resource use of all predator populations. Furthermore, within each area, there was evidence of intraspecific variation in feeding behaviors that could not wholly be attributed to individual body size. Assessing species at the population level will mask these intraspecific differences in resource use. Knowledge of resource use is important for predicting how species will respond to environmental change and spatial variation should be considered when investigating trophic diversity.

High refuge availability on coral reefs increases the vulnerability of reef-associated predators to overexploitation.

Refuge availability and fishing alter predator-prey interactions on coral reefs, but our understanding of how they interact to drive food web dynamics, community structure and vulnerability of different trophic groups is unclear. Here, we apply a size-based ecosystem model of coral reefs, parameterized with empirical measures of structural complexity, to predict fish biomass, productivity and community structure in reef ecosystems under a broad range of refuge availability and fishing regimes. In unfished ecosystems, the expected positive correlation between reef structural complexity and biomass emerges, but a non-linear effect of predation refuges is observed for the productivity of predatory fish. Reefs with intermediate complexity have the highest predator productivity, but when refuge availability is high and prey are less available, predator growth rates decrease, with significant implications for fisheries. Specifically, as fishing intensity increases, predators in habitats with high refuge availability exhibit vulnerability to over-exploitation, resulting in communities dominated by herbivores. Our study reveals mechanisms for threshold dynamics in predators living in complex habitats and elucidates how predators can be food-limited when most of their prey are able to hide. We also highlight the importance of nutrient recycling via the detrital pathway, to support high predator biomasses on coral reefs.

Reef flattening effects on total richness and species responses in the Caribbean.

There has been ongoing flattening of Caribbean coral reefs with the loss of habitat having severe implications for these systems. Complexity and its structural components are important to fish species richness and community composition, but little is known about its role for other taxa or species-specific responses. This study reveals the importance of reef habitat complexity and structural components to different taxa of macrofauna, total species richness, and individual coral and fish species in the Caribbean. Species presence and richness of different taxa were visually quantified in one hundred 25-m(2) plots in three marine reserves in the Caribbean. Sampling was evenly distributed across five levels of visually estimated reef complexity, with five structural components also recorded: the number of corals, number of large corals, slope angle, maximum sponge and maximum octocoral height. Taking advantage of natural heterogeneity in structural complexity within a particular coral reef habitat (Orbicella reefs) and discrete environmental envelope, thus minimizing other sources of variability, the relative importance of reef complexity and structural components was quantified for different taxa and individual fish and coral species on Caribbean coral reefs using boosted regression trees (BRTs). Boosted regression tree models performed very well when explaining variability in total (82·3%), coral (80·6%) and fish species richness (77·3%), for which the greatest declines in richness occurred below intermediate reef complexity levels. Complexity accounted for very little of the variability in octocorals, sponges, arthropods, annelids or anemones. BRTs revealed species-specific variability and importance for reef complexity and structural components. Coral and fish species occupancy generally declined at low complexity levels, with the exception of two coral species (Pseudodiploria strigosa and Porites divaricata) and four fish species (Halichoeres bivittatus, H. maculipinna, Malacoctenus triangulatus and Stegastes partitus) more common at lower reef complexity levels. A significant interaction between country and reef complexity revealed a non-additive decline in species richness in areas of low complexity and the reserve in Puerto Rico. Flattening of Caribbean coral reefs will result in substantial species losses, with few winners. Individual structural components have considerable value to different species, and their loss may have profound impacts on population responses of coral and fish due to identity effects of key species, which underpin population richness and resilience and may affect essential ecosystem processes and services.

Negative maternal or paternal effects on tactic inheritance under a conditional strategy.

Alternative behavioral and life-history tactics are common in animal populations. The conditional strategy model provides a powerful explanation for the evolution and persistence of such tactics, as it allows alternative tactics to be perpetuated even if there is tactic inheritance and tactics yield unequal mean fitness. In many biological systems negative maternal or paternal effects complicate the inheritance of condition and, hence, the inheritance of alternative tactics. Indeed, the inheritance of condition may result in the alternation of tactics across generations. In this paper, we show that the conditional strategy is robust to these effects on progeny condition. There is a unique and stable proportion of tactics under standard inheritance and unequal tactic fitness, and these two important properties of the conditional strategy hold even if negative maternal or paternal effects on progeny condition cause tactics to alternate across generations. However, the dynamics of tactic proportions pursuant to a perturbation of the equilibrium tactic proportions depend on the form of tactic inheritance. An application of our theoretical results to data from a population of smallmouth bass (Micropterus dolomieu) in which negative paternal effects dictate progeny condition reveals that age at first reproduction in males alternates regularly across generations. Furthermore, the model indicates that the population would return rapidly to equilibrium if the proportions of males that mature early or late in life were perturbed from the equilibrium within the system. This example shows how the model of the conditional strategy can be used to gain insight into tactic dynamics in situations where some of the model parameters are difficult or impossible to measure empirically.