• Energy Research

We show how theoretical developments in macroecology, life-history theory and food-web ecology can be combined to formulate a simple model for predicting the potential biomass, production, size and trophic structure of consumer communities. The strength of our approach is that it uses remote sensing data to predict properties of consumer communities in environments that are challenging and expensive to sample directly. An application of the model to the marine environment on a global scale, using primary production and temperature estimates from satellite remote sensing as inputs, suggests that the global biomass of marine animals more than 10 −5 g wet weight is 2.62×10 9 t (=8.16 g m −2 ocean) and production is 1.00×10 10 t yr −1 (31.15 g m −2 yr −1 ). Based on the life-history theory, we propose and apply an approximation for distinguishing the relative contributions of different animal groups. Fish biomass and production, for example, are estimated as 8.99×10 8 t (2.80 g m −2 ) and 7.91×10 8 t yr −1 (2.46 g m −2 yr −1 ), respectively, and 50% of fish biomass is shown to occur in 17% of the total ocean area (8.22 g m −2 ). The analyses show that emerging ecological theory can be synthesized to set baselines for assessing human and climate impacts on global scales.

Fisheries and aquaculture make a crucial contribution to global food security, nutrition and livelihoods. However, the UN Sustainable Development Goals separate marine and terrestrial food production sectors and ecosystems. To sustainably meet increasing global demands for fish, the interlinkages among goals within and across fisheries, aquaculture and agriculture sectors must be recognized and addressed along with their changing nature. Here, we assess and highlight development challenges for fisheries-dependent countries based on analyses of interactions and trade-offs between goals focusing on food, biodiversity and climate change. We demonstrate that some countries are likely to face double jeopardies in both fisheries and agriculture sectors under climate change. The strategies to mitigate these risks will be context-dependent, and will need to directly address the trade-offs among Sustainable Development Goals, such as halting biodiversity loss and reducing poverty. Countries with low adaptive capacity but increasing demand for food require greater support and capacity building to transition towards reconciling trade-offs. Necessary actions are context-dependent and include effective governance, improved management and conservation, maximizing societal and environmental benefits from trade, increased equitability of distribution and innovation in food production, including continued development of low input and low impact aquaculture.

A general rule in ecology is that the abundance of species or individuals in communities sharing a common energy source decreases with increasing body size. However, external energy inputs in the form of resource subsidies can modify this size spectrum relationship. Here, we provide the first test of how a marine resource subsidy can affect size spectra of terrestrial communities, based on energy derived from Pacific salmon carcasses affecting a forest soil community beside streams in western Canada. Using both species-based and individual approaches, we found size structuring in this forest soil community, and transient community-wide doubling of standing biomass in response to energy pulses from Pacific salmon carcasses. One group of species were clear outliers in the middle of the size spectrum relationship: larval calliphorid and dryomyzid flies, which specialize on salmon carcasses, and which showed a tenfold increase in biomass in their size class when salmon were available. Thus, salmon subsidize their escape from the size spectrum. These results suggest that using a size-based perspective of resource subsidies can provide new insights into the structure and functioning of food webs.

Overfishing and habitat degradation through climate change pose the greatest threats to sustainability of marine resources on coral reefs. We examined how changes in fishing pressure and benthic habitat composition influenced the size spectra of island‐scale reef fish communities in Lau, Fiji. Between 2000 and 2006 fishing pressure declined in the Lau Islands due to declining human populations and reduced demand for fresh fish. At the same time, coral cover declined and fine‐scale architectural complexity eroded due to coral bleaching and outbreaks of crown‐of‐thorns starfish, Acanthaster planci. We examined the size distribution of reef fish communities using size spectra analysis, the linearized relationship between abundance and body size class. Spatial variation in fishing pressure accounted for 31% of the variation in the slope of the size spectra in 2000, higher fishing pressure being associated with a steeper slope, which is indicative of fewer large‐bodied fish and/or more small‐bodied fish. Conversely, in 2006 spatial variation in habitat explained 53% of the variation in the size spectra slopes, and the relationship with fishing pressure was much weaker (∼12% of variation) than in 2000. Reduced cover of corals and lower structural complexity was associated with less steep size spectra slopes, primarily due to reduced abundance of fish <20 cm. Habitat degradation will compound effects of fishing on coral reefs as increased fishing reduces large‐bodied target species, while habitat loss results in fewer small‐bodied juveniles and prey that replenish stocks and provide dietary resources for predatory target species. Effective management of reef resources therefore depends on both reducing fishing pressure and maintaining processes that encourage rapid recovery of coral habitat.

Assessing Biodiversity Declines Understanding human impact on biodiversity depends on sound quantitative projection. Pereira et al. (p. 1496 , published online 26 October) review quantitative scenarios that have been developed for four main areas of concern: species extinctions, species abundances and community structure, habitat loss and degradation, and shifts in the distribution of species and biomes. Declines in biodiversity are projected for the whole of the 21st century in all scenarios, but with a wide range of variation. Hoffmann et al. (p. 1503 , published online 26 October) draw on the results of five decades' worth of data collection, managed by the International Union for Conservation of Nature Species Survival Commission. A comprehensive synthesis of the conservation status of the world's vertebrates, based on an analysis of 25,780 species (approximately half of total vertebrate diversity), is presented: Approximately 20% of all vertebrate species are at risk of extinction in the wild, and 11% of threatened birds and 17% of threatened mammals have moved closer to extinction over time. Despite these trends, overall declines would have been significantly worse in the absence of conservation actions.

Theory predicts that bottom-heavy biomass pyramids or ‘stacks’ should predominate in real-world communities if trophic-level increases with body size (mean predator-to-prey mass ratio (PPMR) more than 1). However, recent research suggests that inverted biomass pyramids (IBPs) characterize relatively pristine reef fish communities. Here, we estimated the slope of a kelp forest fish community biomass spectrum from underwater visual surveys. The observed biomass spectrum slope is strongly positive, reflecting an IBP. This is incongruous with theory because this steep positive slope would only be expected if trophic position decreased with increasing body size (consumer-to-resource mass ratio, less than 1). We then used δ15N signatures of fish muscle tissue to quantify the relationship between trophic position and body size and instead detected strong evidence for the opposite, with PPMR ≈ 1650 (50% credible interval 280–12 000). The natural history of kelp forest reef fishes suggests that this paradox could arise from energetic subsidies in the form of movement of mobile consumers across habitats, and from seasonally pulsed production inputs at small body sizes. There were four to five times more biomass at large body sizes (1–2 kg) than would be expected in a closed steady-state community providing a measure of the magnitude of subsidies.

AbstractThe oceans face a biodiversity crisis, but the degree and scale of extinction risk remains poorly characterized. Charismatic species are most likely to garner greatest support for conservation and thus provide a best‐case scenario of the status of marine biodiversity. We summarize extinction risk and diagnose impediments to successful conservation for 1,568 species in 16 families of marine animals in the movie Finding Nemo. Sixteen percent (12–34%) of those that have been evaluated are threatened, ranging from 9% (7–28%) of bony fishes to 100% (83–100%) of marine turtles. A lack of scientific knowledge impedes analysis of threat status for invertebrates, which have 1,000 times fewer conservation papers than do turtles. Legal protection is severely deficient for sharks and rays; only 8% of threatened species in our analysis are protected. Extinction risk among wide‐ranging taxa is higher than most terrestrial groups, suggesting a different conservation focus is required in the sea.