Critical assessment and ramifications of a purported marine trophic cascade (original) (raw)
Related papers
2007
Large shark species in the northwest Atlantic were considered for inclusion in this category based on their size and occurrence of elasmobranchs in their diet. Eleven species met these criteria (Table S1). These sharks are among the largest (notable exclusions being basking and whale sharks, which feed at much lower trophic levels), reaching maximum lengths ranging from ~2.0m in blacktip and sandbar sharks up to 5-6m in great hammerhead and great white sharks (S1-S3). Bull, blacktip, sandbar, and scalloped hammerhead reach sexual maturity below or close to 2m, but all others mature at a greater length (S1-S3). These large fishes are all tertiary consumers (trophic level ≥4) with catholic diets. Five species (bull, great hammerhead, tiger, sand tiger, and great white sharks) are true apex predators, while the remaining six species feed at and near the top of the food web. Smaller elasmobranchs form a key component of the diet of large sharks (S1, S2, S4, S5), and conversely, sharks are the most common predators of other elasmobranchs (S6, S7). Among the large sharks, however, there is considerable variation in the proportion of elasmobranchs consumed in their diet. Bull, great hammerhead, sand tiger, and great white sharks are each considered to be important predators on other elasmobranchs, with about 30-40% of their diet comprised of these fishes (S5). For the other species, the proportion of elasmobranchs in their diet has ranged in different studies between approximately 1 and 15% (see Table S1 references; S5). We compiled data on elasmobranch consumption by each of the large sharks, with particular consideration of the species included in the elasmobranch mesopredator category (see below). At the species level there is evidence that large sharks are predators of seven of the elasmobranch
PLOS ONE
Top predators can exert strong influences on community structure and function, both via direct, consumptive effects, as well as through non-consumptive, fear-based effects (i.e. predation risk). However, these effects are challenging to quantify, particularly for mobile predators in marine ecosystems. To advance this field of research, here we used baited remote underwater video stations (BRUVs) to assess how the behavior of mobile fish species off Cape Cod, Massachusetts, was affected by exposure to large sharks. We categorized sites into three levels of differential shark predation exposure (white sharks, Carcharodon carcharias) and quantified the relative abundance and arrival times (elapsed time before appearing on screen) for six mobile fish prey groups to the BRUV stations. Increased large shark exposure was associated with a decrease in overall prey abundance, but the overall response was prey group-specific. Foraging of smooth dogfish, a likely important prey item for large sharks in the system, was significantly reduced in areas frequented by white sharks. Specifically, the predicted probabilities of smooth dogfish bait contacts or bite attempts occurring were reduced by factors of 5.7 and 8.4, respectively, in areas of high exposure as compared to low exposure. These modifications were underscored by a decrease in smooth dogfish abundance in areas of high exposure as well. Our results suggest that populations of large, roving sharks may induce food-related costs in prey. We discuss the implications of this work within the context of the control of risk (COR) hypothesis, for the purposes of advancing our understanding of the ecological role and effects of large sharks on coastal marine ecosystems.
Elasmobranchs are among the oldest and most successful predators in the ocean, yet one of the most vulnerable to the direct and indirect effects of fishing. Many populations are rapidly declining around the world, and an increasing number is listed as threatened or endangered. The broader ecosystem consequences of these declines, and whether other marine predators can replace sharks, are open questions. In this thesis, I used a diverse set of data and modeling techniques to analyze long-term changes in elasmobranch populations in the Mediterranean Sea, and the consequences of shark declines on marine ecosystems. Because of its long history of fishing, the Mediterranean offers a unique perspective on the response of marine communities to exploitation over long time scales. Here, I reconstructed the history of elasmobranch exploitation over the past 200 years in pelagic, coastal and demersal communities. Results were combined meta-analytically to derive a general pattern of change for...
Patterns and ecosystem consequences of shark declines in the ocean
Ecology Letters, 2000
Whereas many land predators disappeared before their ecological roles were studied, the decline of marine apex predators is still unfolding. Large sharks in particular have experienced rapid declines over the last decades. In this study, we review the documented changes in exploited elasmobranch communities in coastal, demersal, and pelagic habitats, and synthesize the effects of sharks on their prey and wider communities. We show that the high natural diversity and abundance of sharks is vulnerable to even light fishing pressure. The decline of large predatory sharks reduces natural mortality in a range of prey, contributing to changes in abundance, distribution, and behaviour of small elasmobranchs, marine mammals, and sea turtles that have few other predators. Through direct predation and behavioural modifications, top-down effects of sharks have led to cascading changes in some coastal ecosystems. In demersal and pelagic communities, there is increasing evidence of mesopredator release, but cascading effects are more hypothetical. Here, fishing pressure on mesopredators may mask or even reverse some ecosystem effects. In conclusion, large sharks can exert strong top-down forces with the potential to shape marine communities over large spatial and temporal scales. Yet more empirical evidence is needed to test the generality of these effects throughout the ocean.
Expanded trophic complexity among large sharks
To understand the effects of predator removal in marine ecosystems requires accurate estimates of trophic position and trophic structure that have been difficult to obtain to date. For example, most sharks are classified as diet generalists that feed around trophic position 4, but this classification contradicts observations of diverse feeding behaviour among large species, suggesting that trophic structure has been oversimplified among upper trophic level species. To test this assumption, bulk δ 15 N and δ 13 C values of 13 shark species constituting the large shark assemblage off southern Africa were integrated into (i) a hierarchical Bayesian model, accounting for body size and variable sample sizes among species, and (ii) a dietary δ 15 N-dependent enrichment model to quantify individual and assemblage-wide trophic position and structure. Compound specific isotopic analysis of amino acids (CSIA-AAs) for a subset of species was used to verify results. Although discrepancies occurred between methods, overall these data confirm that large sharks, including several globally threatened species, feed at markedly higher trophic positions and across a broader trophic range than is currently assumed. This identifies a lower degree of functional equivalence among the assemblage. Such complex trophic structure among large sharks suggests that cascading effects from species-specific removals in food webs may be weaker but more pervasive than currently assumed. Reassignment of the trophic structure of large marine predators has important consequences for any potential regulatory and stabilizing roles in marine food webs.
Loss of Large Predatory Sharks from the Mediterranean Sea
Conservation Biology, 2008
Evidence for severe declines in large predatory fishes is increasing around the world. Because of its long history of intense fishing, the Mediterranean Sea offers a unique perspective on fish population declines over historical timescales. We used a diverse set of records dating back to the early 19th and mid 20th century to reconstruct long-term population trends of large predatory sharks in the northwestern Mediterranean Sea. We compiled 9 time series of abundance indices from commercial and recreational fishery landings, scientific surveys, and sighting records. Generalized linear models were used to extract instantaneous rates of change from each data set, and a meta-analysis was conducted to compare population trends. Only 5 of the 20 species we considered had sufficient records for analysis. Hammerhead (Sphyrna spp.), blue (Prionace glauca), mackerel (Isurus oxyrinchus and Lamna nasus), and thresher sharks (Alopias vulpinus) declined between 96 and 99.99% relative to their former abundance. According to World Conservation Union (IUCN) criteria, these species would be considered critically endangered. So far, the lack of quantitative population assessments has impeded shark conservation in the Mediterranean Sea. Our study fills this critical information gap, suggesting that current levels of exploitation put large sharks at risk of extinction in the Mediterranean Sea. Possible ecosystem effects of these losses involve a disruption of top-down control and a release of midlevel consumers.
Marine Ecology Progress Series, 2012
This study shows that a sharp ecosystem transition in dominant communities during the early 1990s, from finfish to crustaceans, was common to the 2 northernmost Northwest Atlantic ecosystems, the Newfoundland−Labrador shelf (NL) and the northern Gulf of St. Lawrence (nGSL). Fishery and survey data show that populations of Atlantic cod Gadus morhua, typical of most finfish species, collapsed during the late 1980s to early 1990s in both systems, while Greenland halibut Reinhardtius hippoglossoides populations changed little. Biomass of northern shrimp Pandalus borealis increased following the collapse of cod in both systems, likely due, at least in part, to release of predation pressure. Predation appeared to have relatively little effect on biomass of snow crab Chionoecetes opilio. Shrimp replaced capelin as the principal prey in the diet of NL cod and nGSL Greenland halibut in the mid-1990s. The contribution of shrimp to the predator diets was generally highest when neither capelin nor other suitable prey (fish or squid) were available. We conclude that the NL and nGSL ecosystems are similar in form and function, differing from Canadian Atlantic ecosystems further south. The implications of a change in predominant forage species from capelin to shrimp in these systems are unknown, but could conveivably include changes in the pathway and efficiency of energy flow.
Key Features and Context-Dependence of Fishery-Induced Trophic Cascades
Conservation Biology, 2010
Abstract: Trophic cascades triggered by fishing have profound implications for marine ecosystems and the socioeconomic systems that depend on them. With the number of reported cases quickly growing, key features and commonalities have emerged. Fishery-induced trophic cascades often display differential response times and nonlinear trajectories among trophic levels and can be accompanied by shifts in alternative states. Furthermore, their magnitude appears to be context dependent, varying as a function of species diversity, regional oceanography, local physical disturbance, habitat complexity, and the nature of the fishery itself. To conserve and manage exploited marine ecosystems, there is a pressing need for an improved understanding of the conditions that promote or inhibit the cascading consequences of fishing. Future research should investigate how the trophic effects of fishing interact with other human disturbances, identify strongly interacting species and ecosystem features that confer resilience to exploitation, determine ranges of predator depletion that elicit trophic cascades, pinpoint antecedents that signal ecosystem state shifts, and quantify variation in trophic rates across oceanographic conditions. This information will advance predictive models designed to forecast the trophic effects of fishing and will allow managers to better anticipate and avoid fishery-induced trophic cascades.Resumen: Las cascadas tróficas disparadas por la pesca tienen implicaciones profundas para los ecosistemas marinos y los sistemas socioeconómicos que dependen de ellos. Con el número de casos reportados incrementando rápidamente, han emergido atributos clave y similitudes. Las cascadas tróficas inducidas por pesquerías a menudo presentan tiempos de respuesta diferenciales y trayectorias no lineales entre los niveles tróficos y pueden ser acompañados por cambios en el estado de los ecosistemas. Más aun, su magnitud parece ser dependiente del contexto, variando como una función de la diversidad de especies, la oceanografía regional, la perturbación física local, la complejidad del hábitat y la naturaleza de la pesquería misma. Para conservar y manejar ecosistemas marinos explotados, existe una necesidad imperiosa por mejorar el entendimiento de las condiciones que promueven o inhiben las consecuencias en cascada de la pesca. Más aun, la investigación futura debe examinar la interacción de los efectos tróficos de la pesca con otras perturbaciones humanas, identificar especies estrechamente interactuantes y los atributos del ecosistema que confieren resiliencia a la explotación, determinar los rangos de disminución de depredadores que provoca las cascadas tróficas, identificar antecedentes que indiquen cambios de estado de los ecosistemas y cuantificar la variación en las tasas tróficas en condiciones oceanográficas diferentes. Esta información mejorará los modelos predictivos diseñados para estimar los efectos tróficos de la pesca y permitirá que los manejadores anticipen y eviten las cascadas tróficas inducidas por pesquerías.Resumen: Las cascadas tróficas disparadas por la pesca tienen implicaciones profundas para los ecosistemas marinos y los sistemas socioeconómicos que dependen de ellos. Con el número de casos reportados incrementando rápidamente, han emergido atributos clave y similitudes. Las cascadas tróficas inducidas por pesquerías a menudo presentan tiempos de respuesta diferenciales y trayectorias no lineales entre los niveles tróficos y pueden ser acompañados por cambios en el estado de los ecosistemas. Más aun, su magnitud parece ser dependiente del contexto, variando como una función de la diversidad de especies, la oceanografía regional, la perturbación física local, la complejidad del hábitat y la naturaleza de la pesquería misma. Para conservar y manejar ecosistemas marinos explotados, existe una necesidad imperiosa por mejorar el entendimiento de las condiciones que promueven o inhiben las consecuencias en cascada de la pesca. Más aun, la investigación futura debe examinar la interacción de los efectos tróficos de la pesca con otras perturbaciones humanas, identificar especies estrechamente interactuantes y los atributos del ecosistema que confieren resiliencia a la explotación, determinar los rangos de disminución de depredadores que provoca las cascadas tróficas, identificar antecedentes que indiquen cambios de estado de los ecosistemas y cuantificar la variación en las tasas tróficas en condiciones oceanográficas diferentes. Esta información mejorará los modelos predictivos diseñados para estimar los efectos tróficos de la pesca y permitirá que los manejadores anticipen y eviten las cascadas tróficas inducidas por pesquerías.
Trophic ecology of an abundant predator and its relationship with fisheries
Marine Ecology Progress Series, 2013
Trophic studies are key components in animal ecology and fisheries research. Although stomach samples are often obtained from fisheries, diet studies that consider the influence of fisheries on dietary results are still lacking. Here, the diet of the draughtboard shark Cephaloscyllium laticeps, an abundant mesopredator in Tasmanian waters, was investigated. Stomach samples were obtained from gillnet and craypot fisheries sourced from 4 regions: central (100% gillnet), east coast (63% gillnet, 37% craypot), northwest (100% gillnet), and southwest Tasmania (100% craypot). Overall, C. laticeps consumed the same prey types in all regions, but the importance of some prey varied significantly between regions. Generalized linear models showed that region was the main factor affecting prey abundance in the diet. Fishing method had some influence on the abundance of some prey (crabs, octopus, and other molluscs (gastropods and bivalves)), but the effect of fishing method on pot-related species such as Jasus edwardsii (lobster) and octopus was not as prevalent as expected. The common occurrence of C. laticeps as a bycatch species and its high consumption of targeted fishery species (lobsters and octopus) indicates that C. laticeps has a strong interaction with the fisheries. Therefore, the relationship between these fishery species and C. laticeps should be considered in food web studies in Tasmanian waters.