RESEARCH NEWS & VIEWS to the quark-model picture of protons and neutrons. According to that picture, the neutron is made from one u (up) quark and two d (down) quarks, and the proton from two u quarks and one d quark. According to QCD, the u and d quarks have exactly the same interaction with gluons, the mediators of the strong force between quarks. But their masses need not be equal, and in fact they are not: the d quark is heavier than the u quark. Because the neutron differs from the proton in containing a d quark in place of a u quark, this contribution tends to make the neutron heavier. Nowadays, we understand that protons and neutrons are complex objects, of which the quark model provides only a crude caricature. For accurate work we must solve the equations of QCD and QED directly, as Borsanyi et al. have done, by putting powerful computers to work at clever algorithms. But u and d quarks still enter as elements of fundamental theory, and they still have different masses and different electric charges, but identical strong interactions. Thus, the two sources of neutron–proton mass difference that figured in our heuristic discussion also underpin the rigorous theory. If the only place where the mass difference between the u and d quarks occurred were in the proton–neutron mass difference, we would simply be trading one number for another. But the u and d (and strange) quarks occur inside dozens of different quark-containing sub atomic particles, all of whose masses must be obtained using the same quark-mass values in the QCD and QED equations. Therefore, the quark masses have been determined in a vast number of cases, and the theory is stringent. Borsanyi and colleagues actually calculated the masses of a large number of particles (among them the neutron and proton) and obtain a consistent fit to the observed values. At this point, an important subtlety deserves mention. Part of the mass of a u quark is associated with the energy of its electric field (and likewise for the d quark). In fact, that ‘selfenergy’ is, formally, a divergent (non-finite) quantity. Finite, physically meaningful quantities are extracted by calculating finite changes in the self-energy as the quark occurs in different environments, for example when it is bound inside different hadrons. Some physicists find this renormalization theory disconcerting. They would prefer to have a theory in which all fundamental quantities, whether measurable or not, are finite. Disconcerting it may be, but the success of Borsanyi and colleagues’ calculations, being firmly based on renormalization theory, profoundly attest to nature’s assent to it. The authors’ work is a major technical achievement that pushes the envelope of available computer power. Reading it the other way: with increased computer power, it will be possible to improve on the precision of
their result, which is still limited. They have demonstrated convincingly, for the first time, that fundamental theory gives the right sign for the proton–neutron mass difference (not a trivial thing, because, as we have discussed, crude electrostatics gives the opposite result), and have obtained the correct magnitude within a few tens of per cent uncertainty. From a broader perspective, it is a milestone achievement to include both QCD and QED accurately in the same calculation, because the techniques usually used in those fields — respectively, direct numerical solution (lattice gauge theory) and perturbation theory (Feynman graphs) — are so different. This progress encourages us to predict a future in which nuclear physics reaches the level of precision and versatility that atomic physics has already achieved, with vast
implications for astrophysics, and conceivably for technology. We can look forward to much more accurate modelling of supernovae and neutron stars than has so far been possible, and entertain dreams of refined nuclear chemistry, enabling, for example, dense energy storage and ultrahigh-energy lasers. ■ Frank Wilczek is at the Center for Theoretical Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA. e-mail:
[email protected] 1. Borsanyi, Sz. et al. Science 347, 1452–1455 (2015). 2. Particle Data Group. http://pdg.lbl.gov 3. Kinoshita, T. (ed.) Quantum Electrodynamics (World Scientific, 1990). 4. Kronfeld, A. S. & Quigg, C. Am. J. Phys. 78, 1081–1116 (2010). This article was published online on 8 April 2015.
EC O LO GY
Recovering the potential of coral reefs An analysis of fish declines in coral reefs shows that simple fishing limits and implementation of marine protected areas can be enough to support recovery of coral ecosystem resilience. See Letter p.341 N I C H O L A S K . D U LV Y & H O L LY K . K I N D S VAT E R
F
ishing has transformed today’s coral reefs, but its effects can be insidious and hard to detect1,2. Defining conservation strategies without an appropriate frame of reference is therefore a serious challenge for coral-reef conservation. In this issue, MacNeil et al.3 (page 341) use a combination of data from protected, near-pristine and fished reefs to document the extent of fish biomass declines that have arisen as a result of fishing. Furthermore, they provide comprehensive evidence that simple forms of fisheries management can successfully restore fish-community biomass. Conservation biologists typically rely on monitoring over time to track the decline and recovery of biodiversity and ecosystem states4. But ecosystem-scale underwater research on coral reefs has been possible only in the past 20 years, so that few suitable time series exist for this purpose. To overcome this challenge, MacNeil and colleagues use a space-for-time substitution5 to estimate fish biomass from underwater surveys of 832 reefs across the world’s tropical oceans. The authors combine data on fish biomass in marine protected areas (MPAs), in which fishing is prohibited, with data from 22 unfished sites that are more than
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200 kilometres from the nearest human settlements — the most pristine reefs in the world. This provides an estimate of historical biomass in coral-reef ecosystems on an unprecedented global scale. The authors find that, on average, there is no more than one tonne of fish biomass in each hectare of protected or near-pristine coral reef, although local ecological conditions can lead to considerable variation (Fig. 1). By comparison, 83% of the fished reefs — both managed and unmanaged — have less than half of this biomass. The range of depletion varies widely, from the most severely degraded reefs in the Caribbean and western Pacific, to almost undetectable depletion in the most remote, least-inhabited islands, such as Pitcairn and Easter islands. The reefs in Guam and Papua New Guinea are near collapse, with only 10% of the historical estimate of fish biomass present. Although these declines seem dire, an equally important finding is that fisheries management works. This is a message of hope to those working in conservation. Over the past decade, many have given up on fisheries management because it is perceived as being too difficult, expensive or beyond the capacity of academics and non-governmental organizations6. Many instead turned to MPAs as a blanket solution to marine-conservation challenges. But to be effective, MPAs need to be
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NEWS & VIEWS RESEARCH
Figure 1 | How overfished is this reef? MacNeil et al.3 show how conservation targets and the recovery rates of key fish groups can be estimated from large-scale comparisons of fish biomass at protected and remote sites.
protected and enforced, which requires them to be large, old and isolated7. Effective MPAs can halt declines, but the build-up of biomass to historical levels takes time. MacNeil and colleagues show that recovery takes at least 35 years, twice as long as previous estimates8. Patience, persistence and continued financial investment will be essential to the success of the ocean’s increasing number of MPAs. As MacNeil and colleagues recognize, MPAs are simply not an option in areas where people depend on fish from reefs. Coral reefs lie in the waters of more than 100 developing countries, many of which have dense, rapidly growing coastal populations. Enforced MPAs might not be viable because of the burden of displacing fishers, the unknown effects of redistributing fishing and the time it takes for biomass to recover. But the authors show that those reefs
that had some form of management, such as restrictions on fishing equipment, species or access, had 27% more fish biomass than reefs open to fishing. Even in depleted reef communities, regulations protecting key species can promote ecosystem resilience and recovery. For example, prohibiting specific equipment can allow herbivorous fishes to recover, promoting coral resilience9. MacNeil et al. take this analysis one step further, comparing MPAs of different ages to predict the recovery speed and sequence of different fish groups following implementation of management measures. Their models predict that species at the base of the food web, including herbivores, will recover rapidly. Some of these low-trophic-level species, such as parrotfishes, recover in a nonlinear manner, reaching the greatest biomass soon after
management is implemented. The researchers predict that these species will be most abundant — and therefore at their most effective for grazing, excavating or scraping away algal overgrowth that limits coral growth — at the time when the reefs recover half of their historic fish biomass. Piscine predators have historically been the first group to be overfished, and this study shows that they are the last to recover. Because they are almost absent from present-day reefs, their relevance to healthy coral ecosystems is sometimes overlooked. But piscine predators have two essential roles in reef communities. First, they suppress mesopredators such as starfish, preventing trophic cascades that change the dominant reef substrate from hard coral to algal overgrowth. Second, they integrate oceanic and reef food webs, feasting on the planktivorous fishes that vacuum up oceanic zooplankton10. Without predatory fishes, reefs are potentially condemned to a state of lowered biomass. Prevention of this negative outcome requires effective fisheries governance, including improved monitoring, equipment restrictions to reduce unintentional catch, and increased transparency in the supply and trade of high-value seafood products11. There has been much discussion about coral-reef conservation, but little analysis of the efficacy of alternative management options. Currently, most of the world’s coral reefs have little or no management — in part because of the persistent lack of recognition by international development agencies and local governments of the social and economic benefits that small-scale fisheries have for the poorest coastal peoples of the world11. MacNeil et al. provide definitive confirmation that simple fisheries governance tools, including protected areas and equipment, access and species restrictions, can be effective. If adopted seriously, these measures can secure a sustainable future for coral reefs and the people who depend on them. ■ Nicholas K. Dulvy and Holly K. Kindsvater are in the Department of Biological Sciences, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada. e-mail:
[email protected] 1. Wing, S. & Wing, E. Coral Reefs 20, 1–8 (2001). 2. Polunin, N. V. C. & Roberts, C. M. (eds) Reef Fisheries (Chapman & Hall, 1996). 3. MacNeil, M. A. et al. Nature 520, 341–344 (2015). 4. Collen, B. et al. Conserv. Biol. 23, 317–327 (2009). 5. Pickett, S. T. A. in Long-term Studies in Ecology (ed. Likens, G. E.) 110–135 (Springer, 1989). 6. De Santo, E. M. J. Environ. Manage. 124, 137–146 (2013). 7. Edgar, G. J. et al. Nature 506, 216–220 (2014). 8. Molloy, P. P., McLean, I. B. & Côté, I. M. J. Appl. Ecol. 46, 743–751 (2009). 9. Hughes, T. P. et al. Curr. Biol. 17, 360–365 (2007). 10.Hamner, W. M., Jones, M. S., Carleton, J. H., Hauri, I. R. & Williams, D. M. Bull. Mar. Sci. 42, 459–479 (1988). 11. Sadovy, Y. Fish Fisheries 6, 167–185 (2005). This article was published online on 8 April 2015. 1 6 A P R I L 2 0 1 5 | VO L 5 2 0 | NAT U R E | 3 0 5
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