How Rats Wreak Havoc on Coral Reefs

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RESEARCH NEWS & VIEWS superconductor. Therefore, Majorana fermi- called phonons, which provide an unwanted the observation of such an effect. More work, ons should exist on the edge of α-RuCl3. contribution to the thermal conductance. both theoretical and experimental, is required The direct detection of Majorana fermions Banerjee et al. went to great lengths to to fully understand the implications of these in condensed-matter systems was never going minimize this phonon contribution. They experiments. Nevertheless, it is undoubtedly to be easy. Such particles must be electrically carried out their experiments at temperatures exciting that the quest for Majorana fermions neutral and therefore cannot participate in of about 20 mK and used a sophisticated design is heating up in this manner. ■ electrical transport (although they can medi- of a source and drains to avoid the coupling ate such transport in superconductors8,9). of edge modes to phonons. By comparison, Kirill Shtengel is in the Department of However, although Majorana fermions Kasahara and colleagues’ experiment was much Physics and Astronomy, University of are unable to conduct current, they can less intricate and required temperatures of only California, Riverside, California 92521, USA. conduct heat. about 5 K. These authors could not detect a e-mail: [email protected] Electrons can conduct both electricity and signal of half-integer quantization at lower 1. Majorana, E. Nuovo Cimento 14, 171–184 (1937). heat. As a result, metals — which contain temperatures, which probably suggests that the 2. Wilczek, F. Nature Phys. 5, 614–618 (2009). many free electrons — are typically good heat system transitioned to a different phase. Their 3. Banerjee, M. et al. Nature 559, 205–210 (2018). conductors. This idea is formalized by the results also indicate that a substantial amount 4. Kasahara, Y. et al. Nature 559, 227–231 (2018). 5. Read, N. & Green, D. Phys. Rev. B 61, 10267–10297 Wiedemann–Franz law, which states that elec- of heat was carried by phonons. (2000). trical conductivity is directly proportional to Under these circumstances, it should 6. Kitaev, A. Ann. Phys. 321, 2–111 (2006). thermal conductivity divided by temperature. be surprising that the authors saw signs of 7. Jackeli, G. & Khaliullin, G. Phys. Rev. Lett. 102, 017205 (2009). Although the identification of this relationship quantized Hall heat transport — the heat con- 8. Mourik, V. et al. Science 336, 1003–1007 (2012). is often lauded as one of the early successes of duction in the direction perpendicular to that of 9. He, Q. L. et al. Science 357, 294–299 (2017). solid-state theory, the proportionality constant the thermal gradient — by Majorana fermions. 10. Simon S. H. Phys. Rev. B 97, 121406 (2018). 11,12 11. Ye, M., Halász, G. B., Savary, L. & Balents, L. Preprint is not universal for ordinary metals: scatter- However, two recent studies have argued at https://arxiv.org/abs/1805.10532 (2018). ing processes, which limit both electrical and that phonon coupling not only is not detri- 12. Vinkler-Aviv, Y. & Rosch, A. Preprint at https://arxiv. thermal conductivity, affect these properties mental, but also can actually be necessary for org/abs/1805.11587 (2018). differently in different metals. However, if the motion of particles in a material is ballistic (if there is effectively no ECOLOGY scattering), both electrical and thermal conductivity are quantized and proportional to the number of propagating modes (conduction channels). Each electron mode contributes a How rats wreak havoc unit of thermal conductance, and, crucially, each Majorana mode contributes only half a unit. Both Banerjee et al. and Kasahara et al. on coral reefs observed this fraction of thermal conductance on the edges of their condensed-matter The introduction of non-native rats can devastate island ecosystems. It now systems. emerges that these rats also harm a complex web of interactions linking seabirds The existence of Majorana edge modes in a with the algae and fishes of nearby coral reefs. See Letter p250 condensed-matter system is a strong indica- tor that the topological order of the system is non-Abelian — which means, for example, NANCY KNOWLTON latter islands, the larger populations of birds that a collection of the system’s excitations produced larger deposits of guano — nitrogen- has a huge number of quantum states with on-native rats that invade tropical rich bird excrement. This nitrogen is mostly the same energy. The non-Abelian nature of islands can cause problems for the obtained from food that the birds consume the quantum Hall state studied by Banerjee ecosystems they invade1. These during long-distance foraging trips to parts of et al. has long been expected (albeit not con- Nintruders can decimate the native popula- the ocean that, thanks to their higher levels of firmed beyond reasonable doubt). However, tions on which they feed, such as plants and nutrients, are 100 to 100,000 times more pro- Kasahara and colleagues’ findings provide the terrestrial invertebrates. Bird populations can ductive than the waters in the immediate vicin- first experimental evidence of a non-Abelian plummet, too, when rats eat eggs and nestlings. ity of an island. The nitrogen deposition rates spin liquid. Although more work is needed The complex, indirect effects of rodent pres- on the rat-free islands were 251 times greater to confirm the exact nature of this state, the ence can spread deeply and widely through per hectare than were those on the rat-infested discovery of such an unconventional phase of island food webs2. However, little attention islands. Using a technique to identify different matter is truly exciting. has been paid to the indirect impacts of such isotopic forms of nitrogen, the authors could Banerjee and colleagues used their invasive species on adjacent coral-reef com- distinguish this ‘imported’ seabird nitrogen measurements to try to discriminate between munities. On page 250, Graham et al.3 address from locally derived nitrogen. This enabled different candidate non-Abelian states. This this for sites on the Chagos Archipelago in Graham and colleagues to track where the task is harder than obtaining evidence for the Indian Ocean (Fig. 1), comparing coral seabird-deposited nitrogen ended up. non-Abelian topological order. It relies on reefs surrounding six rat-infested islands with Some nitrogen was absorbed by plants counting both fractional and integer contri- those adjacent to six islands that lacked rats. on the islands, and some entered the ocean butions to the system’s thermal conductance, The authors find that reefs near rat-infested through rain or breaking waves. For example, which, in turn, requires certain assumptions islands have fewer nutrients, fewer fishes and 100 metres from the shore of rat-free islands, to be made about the process by which dif- reduced numbers of fishes grazing on the algae both a type of sponge and a type of mac- ferent propagating modes reach thermal that compete with corals. roalga had elevated levels of nitrogen derived equilibrium10. The issue of equilibration is One of the most marked effects of rats was from seabird foraging, compared with the further complicated by the fact that the edge a 760-fold decline in the number of nesting levels recorded near rat-infested islands. At modes can reach equilibrium not only with seabirds per hectare on rat-infested islands 230 metres from the shore of rat-free islands, each other, but also with lattice vibrations compared with rat-free islands. On the the concentration of seabird-derived nitrogen

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in turf algae and in the muscle tissue of a species of alga-eating damselfish was higher than such measurements on rat-infested

NICK GRAHAM islands. By measuring growth rings in damselfish ear bones, Graham and colleagues showed that the damselfish in the waters around rat-free islands grew faster and at any given age were larger than those living beside rat-infested islands, presumably because their food was richer in nitrogen. Looking at all types of reef fish, the authors found that the total biomass of the fish population was 48% higher around rat-free islands than around rat-infested islands. Moreover, of all the types of reef fish, the abundance of herbivorous (alga-eating) fishes was the most negatively affected by the presence of rats. Herbivorous fish are particularly important for coral reefs, because their grazing prevents the algae from overgrowing and killing corals. Around rat-free islands, parrotfishes, a group of herbivores, grazed the entire surface of the reef 9 times per year, whereas around rat-infested islands the equivalent figure was only 2.8 times per year. Because parrotfishes feed with powerful beaks, there was also more bioerosion and greater production of sand on coral reefs surrounding rat-free islands; however, the amount of living coral was not lower than that on rat-infested islands. The dramatic effects that Graham and colleagues document provide a comprehen- sive picture of how reefs are interconnected with the surrounding land and seascapes. The Figure 1 | Booby chick on a nest above a coral-reef lagoon in the Chagos Archipelago. Graham et al.3 movement of organisms around such habi- report their studies of how non-native rats affect the ecosystems of islands and adjacent coral reefs in tats generates genetic connections between the Chagos Archipelago in the Indian Ocean. They find that rat-free islands have substantially more regions that have long been appreciated by seabirds than do rat-infested islands. Moreover, nitrogen deposits from seabird excrement has a positive biogeographers and geneticists. In marine effect on nearby coral reefs through nutrient cycling, which generates nitrogen-rich algae that boost the fish population. conservation, the generation of networks of marine-protected areas that take into account this genetic connectivity between coral reefs This work has immediate practical be on the lookout for how these rat-infested is increasingly a part of the planning process. implications, particularly because reefs are versus rat-free islands recover from the surely John Donne’s poem ‘No Man is an Island’ under grave threat around the world. Many inevitable next coral-bleaching event. ■ explores the nature of human connections, of the early losses of coral reefs were due to and, as others have noted4 in a similar vein, overfishing8, and a scarcity of herbivorous Nancy Knowlton is in the Department of “no reef is an island”, either. fishes continues to make reefs less resilient9,10. Invertebrate Zoology, National Museum of However, the nutritional (trophic) connec- Now, however, one of the major causes for con- Natural History, Smithsonian Institution, tions linking coral reefs with other marine and cern over reef survival is the impact of climate Washington DC 20013, USA. terrestrial ecosystems haven’t been studied as change and the ability of reefs to recover from e-mail: [email protected] extensively as have the genetic connections. disturbances due to oceanic warming, which is 1. Harper, G. A. & Bunbury, N. Glob. Ecol. Conserv. 3, This is surprising, given the long-standing puz- causing mass coral bleaching and death affect- 607–627 (2015). zle of how coral reefs thrive in nutrient-poor ing even remote and protected reefs11. 2. Nigro, K. M. et al. Restor. Ecol. 25, 1015–1025 waters — a phenomenon commonly called Adding rats to the list of dangers to reefs (2017). 3. Graham, N. A. J. et al. Nature 559, 250–253 (2018). Darwin’s paradox because Charles Darwin might seem discouraging. Yet the discovery 4. Schill, S. R. et al. PLoS ONE 10, e0144199 (2015). highlighted this enigma. Tight recycling of of the negative impacts of rats on reefs does 5. de Goeij, J. M. et al. Science 342, 108–110 (2013). energy and nutrients certainly helps5, as do point directly to a specific strategy that could 6. Hamner, W. M., Colin, P. L. & Hamner, P. P. Mar. Ecol. Prog. Ser. 334, 83–92 (2007). the oceanic plankton that are captured by the slow the pace of reef degradation. Rats and 7. Williams, J. J., Papastamatiou, Y. P., Caselle, J. E., proverbial “wall of mouths” that the fishes on other invasive mammals have been success- Bradley, D. & Jacoby, D. M. P. Proc. R. Soc. B 285, a reef present6. fully eradicated from hundreds of islands12, 20172456 (2018). 8. Pandolfi, J. M. et al. Science 301, 955–958 (2003). Graham and colleagues’ study adds to our with beneficial effects on many terrestrial 9. Jackson, J. B. C., Donovan, M. K., Cramer, K. L. & growing appreciation of the importance of ecosystems. Graham and colleagues suggest Lam, V. (eds) Status and Trends of Caribbean Coral long-distance nutritional subsidies for reefs, that the same strategy, and others more gener- Reefs: 1970–2012 (Glob. Coral Reef Monit. Netw., IUCN, 2014); go.nature.com/2jdpbvp generated not only by seabirds as documented ally aimed at protecting seabirds, should be a 10. Adam, T. C., Burkepile, D. E., Ruttenberg, B. I. & here, but also by wide-ranging underwater priority for islands associated with coral reefs, Paddack, M. J. Mar. Ecol. Prog. Ser. 520, 1–20 predators such as sharks7. Notably, human helping to buy time while society comes to (2015). 11. Hughes, T. P. et al. Nature 556, 492–496 (2018). impacts can disrupt the subsidies in both of grips with and, one must hope, slows climate 12. Russell, J. C. & Holmes, N. D. Biol. Conserv. 185, 1–7 these cases. change. In the meantime, scientists will now (2015).