letters to nature attention to those events which give a variance reduction greater 15. Jeanloz, R. & Wenk, H.-R. Convection and anisotropy of the inner core. Geophys. Res. Lett. 15, 72±75 than 70%. Most core-sensitive modes are also sensitive to mantle (1988). 16. Karato, S. Inner core anisotropy due to magnetic ®eld-induced preferred orientation of iron. Science structure. Some events yield signi®cantly different rotation angles 262, 1708±1711 (1993). for different mantle corrections29,30. We regard this as an indication 17. Yoshida, S., Sumita, I. & Kumazagawa, M. Growth model of the inner core coupled with the outer core of serious noise contamination, and discard these events for a dynamics and the resulting elastic anisotropy. J. Geophys. Res. 101, 28085±28103 (1997). 18. Bergman, M. I. Measurements of elastic anisotropy due to solidi®cation texturing and the implica- particular mode. This conservative choice sometimes results in very tions for the Earth's inner core. Nature 389, 60±63 (1997). few events to constrain the rotation rate for individual modes; for 19. Su, W-J. & Dziewonski, A. M. Inner core anisotropy in three dimensions. J. Geophys. Res. 100, 9831± example, only nine events constrain the rate for mode 3S2, and the 9852 (1995). rotation rate determined for this mode is less certain. We plot the 20. Shearer, P.M. & Toy, K. M. PKP(BC) versus PKP(DF) differential travel times and aspherical structure in Earth's inner core. J. Geophys. Res. 96, 2233±2247 (1991). apparent rotation angles for several modes as a function of event date 21. McSweeney, T. J., Creager, K. C. & Merrill, R. T. Depth extent of inner core seismic anisotropy and (Fig. 3). The error bars represent the range of angles over which the implications for geomagnetism. Phys. Earth. Planet. Inter. 101, 131±156 (1997). ®t to the receiver strips is adequate. The best-®tting straight line is 22. Song, X. Anisotropy of the Earth's inner core. Rev. Geophys. 35, 297±313 (1997). also indicated, along with the corresponding apparent rotation rate. 23. Woodhouse, J. H. The coupling and attenuation of nearly resonant multiplets in the earth's free oscillation spectrum. Geophys. J. R. Astron. Soc. 61, 261±283 (1980). For the hypothesis to be acceptable, all the modal splitting 24. Woodhouse, J. H. & Dahlen, F. A. The effect of a general aspherical perturbation on the free functions should appear to be rotating at the same rate. Figure 4 oscillations of the earth. Geophys. J. R. Astron. Soc. 53, 335±354 (1978). summarizes our best estimate of each mode's rotation rate. Also 25. Woodhouse, J. H. & Giardini, D. Inversion for the splitting function of isolated low order normal 8 mode multiplets. Eos 66, 300 (1985). shown are the results of another recent mode study and body-wave 26. Ritzwoller, M., Masters, G. & Gilbert, F. Observations of anomalous splitting and their interpretation 1,3 studies . Even though some of the modes shown in Fig. 4 indicate in terms of aspherical structure. J. Geophys. Res. 91, 10203±10228 (1986). signi®cantly non-zero rotation rates (some indicate a slight west- 27. Giardini, D., Li, X.-D. & Woodhouse, J. H. Splitting functions of long period normal modes of the ward inner-core rotation), these rates do not agree with each other. earth. J. Geophys. Res. 93, 13716±13742 (1988). 28. Li, X.-D., Giardini, D. & Woodhouse, J. H. Large-scale three-dimensional even-degree structure of the The mean rotation rate for all modes in our study is 0:01 6 0:218 per Earth from splitting of long-period normal modes. J. Geophys. Res. 96, 551±577 (1991). year, which is obviously insigni®cantly different from zero. The 29. Masters, G., Johnson, S., Laske, G. & Bolton, H. A shear velocity model of the mantle. Phil. Trans. R. results shown are obtained using model SB10L1830 to perform the Soc. Lond. A 354 1385±1411 (1996). mantle corrections, but other models give rather similar results 30. Masters, G., Bolton, H. & Laske, G. Joint seismic tomography for P and S velocities: how pervasive are chemical anomalies in the mantle? Eos 80, S14 (1999). (0:03 6 0:218 per year for S16B3029. Recent body-wave analyses suggest that the rotation rate may be as small as 0.2±0.38 per year in Acknowledgements an eastward sense. Such a rotation rate (also indicated in Fig. 4) is The data used in this study were collected at a variety of global seismic networks and marginally consistent with most modes. We can certainly rule out a obtained from the IRIS-DMC, GEOSCOPE and BFO. This research was supported by the rate of 18 eastward, as well as a signi®cant westward rotation. US NSF. Perhaps complexity near the ray turning points or systematic Correspondence and requests for materials should be addressed to G.L. changes in earthquake location procedures could allow smaller (e-mail: [email protected]). rotation rates to be compatible with the body-wave data. We believe that a model analysis is the best way to determine inner-core rotation. First, we are dealing with large-scale vibrations ................................................................. which are insensitive to errors in event locations and to local structure in the inner core. Second, the method is independent of Environmental warming alters the earthquake source mechanism. Third, we do not need to worry about how much of the splitting functions we observe are caused by food-web structure heterogeneity or anisotropyÐall we care about is whether they change with time. Our results are marginally consistent with the and ecosystem function small rotation rates reported in most recent body-wave analyses, though our best value indicates that the inner core is not rotating Owen L. Petchey, P. Timon McPhearson, Timothy M. Casey at a signi®cant rate relative to the mantle. This would be in accord & Peter J. Morin with the notion that the inner core is gravitationally locked to the mantle10. M Department of Ecology, Evolution and Natural Resources, 14 College Farm Road, Cook College, Rutgers University, New Brunswick, New Jersey 08901-8511, USA Received 11 May; accepted 6 August 1999. .............................................................................................................................................. 1. Song, X. & Richards, P. G. Seismological evidence for differential rotation of the Earth's inner core. Nature 382, 221±224 (1996). We know little about how ecosystems of different complexity will 2. Su, W., Dziewonski, A. M. & Jeanloz, R. Planet within a planet: rotation of the inner core of the Earth. respond to global warming1±5. Microcosms permit experimental 274, 1883±1887 (1996). control over species composition and rates of environmental 3. Creager, K. C. Inner core rotation rate from small-scale heterogeneity and time-varying travel times. Science 278, 1248±1288 (1997). change. Here we show using microcosm experiments that extinc- 4. Souriau, A. Earth's inner coreÐis the rotation real? Science 281, 55±56 (1998). tion risk in warming environments depends on trophic position 5. Souriau, A. New seismological constraints on differential rotation rates of the inner core from Novaya but remains unaffected by biodiversity. Warmed communities Zemlya events recorded at DRV, Antarctica. Geophys. J. Int. 134, F1±F5 (1998). disproportionately lose top predators and herbivores, and become 6. Souriau, A., Roudil, P. & Moynot, B. Inner core differential rotation: facts and artefacts. Geophys. Res. Lett. 24, 2103±2106 (1997). increasingly dominated by autotrophs and bacterivores. Changes 7. Creager, K. C. Large-scale variations in inner core anisotropy. J. Geophys. Res. (in the press). in the relative distribution of organisms among trophically 8. Sharrock, D. S. & Woodhouse, J. H. Investigation of time dependent inner core structure by the de®ned functional groups lead to differences in ecosystem func- analysis of free oscillation spectra. Earth Planets Space 50, 1013±1018 (1998). tion beyond those expected from temperature-dependent physio- 9. Masters, G., Laske, G. & Gilbert, F. Autoregressive estimation of the splitting matrix of free-oscillation multiplets. J. Geophys. Res. (submitted). logical rates. Diverse communities retain more species than 10. Buffet, B. A. A mechanism for decade ¯uctuations in the length of day. Geophys. Res. Lett. 23, 3803± depauperate ones, as predicted by the insurance hypothesis, 3806 (1996). which suggests that high biodiversity buffers against the effects 11. Masters, G. & Gilbert, F. Structure of the inner core inferred from observations of its spheroidal shear modes. Geophys. Res. Lett. 8, 569±571 (1981). of environmental variation because tolerant species are more 6,7 12. Woodhouse, J. H., Giardini, D. & Li, X.-D. Evidence for inner core anisotropy from free oscillations. likely to be found . Studies of single trophic levels clearly show Geophys. Res. Lett. 13, 1549±1552 (1986). that warming can affect the distribution and abundance of 13. Morelli, A., Dziewonski, A. M. & Woodhouse, J. H. Anisotropy of the inner core inferred from PKIKP species2,4,5, but complex responses generated in entire food webs travel times. Geophys. Res. Lett. 13, 1545±1548 (1986). 14. Poupinet, G., Pillet, R. & Souriau, A. Possible heterogeneity of the earth's core deduced from PKIKP greatly complicate inferences based on single functional groups. travel times. Nature 305, 204±206 (1983). We used microcosms containing aquatic microbes to investigate NATURE | VOL 402 | 4 NOVEMBER 1999 | www.nature.com © 1999 Macmillan Magazines Ltd 69 letters to nature Low A Predators Low B Actinosphaerium (3) Herbivores Bacterivores Herbivores Bacterivores Hypostome sp. P. bursaria (2) Euplotes (5) Colpidium Producers Producers Chlamydomonas Diatom sp. A Bacterial assemblage (Chlorella) Bacterial assemblage Diatom sp. B Predators High A High B Actinosphaerium Predators Stentor B (3) Stentor B (5) Spirostomum (4) Herbivores Bacterivores Herbivores Bacterivores Hypostome sp. (5) P. bursaria (3) Euplotes (5) Paramecium Frontonia A (2) Halteria (5) Frontonia B (5) Colpidium Stentor A Monostyla Stentor A (4) Rotaria (5) Producers Chlamydomonas (4) Producers Scenedesmus Desmid sp.