Boris Worm

Rapid worldwide depletion of predatory fish communities.

Serious concerns have been raised about the ecological effects of industrialized fishing, spurring a United Nations resolution on restoring fisheries and marine ecosystems to healthy levels. However, a prerequisite for restoration is a general understanding of the composition and abundance of unexploited fish communities, relative to contemporary ones. We constructed trajectories of community biomass and composition of large predatory fishes in four continental shelf and nine oceanic systems, using all available data from the beginning of exploitation. Industrialized fisheries typically reduced community biomass by 80% within 15 years of exploitation. Compensatory increases in fast-growing species were observed, but often reversed within a decade. Using a meta-analytic approach, we estimate that large predatory fish biomass today is only about 10% of pre-industrial levels. We conclude that declines of large predators in coastal regions have extended throughout the global ocean, with potentially serious consequences for ecosystems. Our analysis suggests that management based on recent data alone may be misleading, and provides minimum estimates for unexploited communities, which could serve as the ‘missing baseline’ needed for future restoration efforts.

Rebuilding Global Fisheries

Fighting for Fisheries In the debate concerning the future of the worlds fisheries, some have forecasted complete collapse but others have challenged this view. The protagonists in this debate have now joined forces to present a thorough quantitative review of current trends in world fisheries. Worm et al. (p. 578) evaluate the evidence for a global rebuilding of marine capture fisheries and their supporting ecosystems. Contrasting regions that have been managed for rebuilding with those that have not, reveals trajectories of decline and recovery from individual stocks to species, communities, and large marine ecosystems. The management solutions that have been most successful for rebuilding fisheries and ecosystems, include both large- and small-scale fisheries around the world. Catch restrictions, gear modification, and closed areas are helping to rebuild overexploited marine ecosystems. After a long history of overexploitation, increasing efforts to restore marine ecosystems and rebuild fisheries are under way. Here, we analyze current trends from a fisheries and conservation perspective. In 5 of 10 well-studied ecosystems, the average exploitation rate has recently declined and is now at or below the rate predicted to achieve maximum sustainable yield for seven systems. Yet 63% of assessed fish stocks worldwide still require rebuilding, and even lower exploitation rates are needed to reverse the collapse of vulnerable species. Combined fisheries and conservation objectives can be achieved by merging diverse management actions, including catch restrictions, gear modification, and closed areas, depending on local context. Impacts of international fleets and the lack of alternatives to fishing complicate prospects for rebuilding fisheries in many poorer regions, highlighting the need for a global perspective on rebuilding marine resources.

How many species are there on Earth and in the ocean

The diversity of life is one of the most striking aspects of our planet; hence knowing how many species inhabit Earth is among the most fundamental questions in science. Yet the answer to this question remains enigmatic, as efforts to sample the worlds biodiversity to date have been limited and thus have precluded direct quantification of global species richness, and because indirect estimates rely on assumptions that have proven highly controversial. Here we show that the higher taxonomic classification of species (i.e., the assignment of species to phylum, class, order, family, and genus) follows a consistent and predictable pattern from which the total number of species in a taxonomic group can be estimated. This approach was validated against well-known taxa, and when applied to all domains of life, it predicts ∼8.7 million (±1.3 million SE) eukaryotic species globally, of which ∼2.2 million (±0.18 million SE) are marine. In spite of 250 years of taxonomic classification and over 1.2 million species already catalogued in a central database, our results suggest that some 86% of existing species on Earth and 91% of species in the ocean still await description. Renewed interest in further exploration and taxonomy is required if this significant gap in our knowledge of life on Earth is to be closed.

Predicting ecological consequences of marine top predator declines

Recent studies document unprecedented declines in marine top predators that can initiate trophic cascades. Predicting the wider ecological consequences of these declines requires understanding how predators influence communities by inflicting mortality on prey and inducing behavioral modifications (risk effects). Both mechanisms are important in marine communities, and a sole focus on the effects of predator-inflicted mortality might severely underestimate the importance of predators. We outline direct and indirect consequences of marine predator declines and propose an integrated predictive framework that includes risk effects, which appear to be strongest for long-lived prey species and when resources are abundant. We conclude that marine predators should be managed for the maintenance of both density- and risk-driven ecological processes, and not demographic persistence alone.

Global phytoplankton decline over the past century

In the oceans, ubiquitous microscopic phototrophs (phytoplankton) account for approximately half the production of organic matter on Earth. Analyses of satellite-derived phytoplankton concentration (available since 1979) have suggested decadal-scale fluctuations linked to climate forcing, but the length of this record is insufficient to resolve longer-term trends. Here we combine available ocean transparency measurements and in situ chlorophyll observations to estimate the time dependence of phytoplankton biomass at local, regional and global scales since 1899. We observe declines in eight out of ten ocean regions, and estimate a global rate of decline of ∼1% of the global median per year. Our analyses further reveal interannual to decadal phytoplankton fluctuations superimposed on long-term trends. These fluctuations are strongly correlated with basin-scale climate indices, whereas long-term declining trends are related to increasing sea surface temperatures. We conclude that global phytoplankton concentration has declined over the past century; this decline will need to be considered in future studies of marine ecosystems, geochemical cycling, ocean circulation and fisheries.

Global patterns and predictors of marine biodiversity across taxa

Global patterns of species richness and their structuring forces have fascinated biologists since Darwin and provide critical context for contemporary studies in ecology, evolution and conservation. Anthropogenic impacts and the need for systematic conservation planning have further motivated the analysis of diversity patterns and processes at regional to global scales. Whereas land diversity patterns and their predictors are known for numerous taxa, our understanding of global marine diversity has been more limited, with recent findings revealing some striking contrasts to widely held terrestrial paradigms. Here we examine global patterns and predictors of species richness across 13 major species groups ranging from zooplankton to marine mammals. Two major patterns emerged: coastal species showed maximum diversity in the Western Pacific, whereas oceanic groups consistently peaked across broad mid-latitudinal bands in all oceans. Spatial regression analyses revealed sea surface temperature as the only environmental predictor highly related to diversity across all 13 taxa. Habitat availability and historical factors were also important for coastal species, whereas other predictors had less significance. Areas of high species richness were disproportionately concentrated in regions with medium or higher human impacts. Our findings indicate a fundamental role of temperature or kinetic energy in structuring cross-taxon marine biodiversity, and indicate that changes in ocean temperature, in conjunction with other human impacts, may ultimately rearrange the global distribution of life in the ocean.

Patterns and ecosystem consequences of shark declines in the ocean.

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.

Consumer versus resource control of species diversity and ecosystem functioning

A key question in ecology is which factors control species diversity in a community. Two largely separate groups of ecologists have emphasized the importance of productivity or resource supply, and consumers or physical disturbance, respectively. These variables show unimodal relationships with diversity when manipulated in isolation. Recent multivariate models, however, predict that these factors interact, such that the disturbance–diversity relationship depends on productivity, and vice versa. We tested these models in marine food webs, using field manipulations of nutrient resources and consumer pressure on rocky shores of contrasting productivity. Here we show that the effects of consumers and nutrients on diversity consistently depend on each other, and that the direction of their effects and peak diversity shift between sites of low and high productivity. Factorial meta-analysis of published experiments confirms these results across widely varying aquatic communities. Furthermore, our experiments demonstrate that these patterns extend to important ecosystem functions such as carbon storage and nitrogen retention. This suggests that human impacts on nutrient supply and food-web structure have strong and interdependent effects on species diversity and ecosystem functioning, and must therefore be managed together.

Extinction, survival or recovery of large predatory fishes.

Large predatory fishes have long played an important role in marine ecosystems and fisheries. Overexploitation, however, is gradually diminishing this role. Recent estimates indicate that exploitation has depleted large predatory fish communities worldwide by at least 90% over the past 50–100 years. We demonstrate that these declines are general, independent of methodology, and even higher for sensitive species such as sharks. We also attempt to predict the future prospects of large predatory fishes. (i) An analysis of maximum reproductive rates predicts the collapse and extinction of sensitive species under current levels of fishing mortality. Sensitive species occur in marine habitats worldwide and have to be considered in most management situations. (ii) We show that to ensure the survival of sensitive species in the northwest Atlantic fishing mortality has to be reduced by 40–80%. (iii) We show that rapid recovery of community biomass and diversity usually occurs when fishing mortality is reduced. However, recovery is more variable for single species, often because of the influence of species interactions. We conclude that management of multi–species fisheries needs to be tailored to the most sensitive, rather than the more robust species. This requires reductions in fishing effort, reduction in bycatch mortality and protection of key areas to initiate recovery of severely depleted communities.

Management effectiveness of the world's marine fisheries.

A global analysis shows that fishery management worldwide is lagging far behind international standards, and that the conversion of scientific advice into policy, through a participatory and transparent process, holds promise for achieving sustainable fisheries.