magnetic fields and energetic particles. At the helio pause, they will take in situ measurements of the interstellar plasma for the first time.

The first crossing of the termination shock, by Voyager 1, occurred when it was 94 au from the Sun, during a several-month period of rapid (~100 km s–1) inward motion of the shock. A gap in data transmission during this time meant that its detailed structure could not be determined. This first crossing nonetheless established a number of the shock’s proper-ties, including its distance, and the facts that it fluctuates considerably, moving in and out for months at a time, and that its overall shape is blunt9 rather than bullet-shaped or rounded.

The crossings of the shock (at 84 au) by Voyager 2 add to our understanding in several ways, as detailed in the five new papers1–5, not least because the working plasma detector per-mitted a much richer set of observations. The fact that the Voyager 2 crossings occurred some 10 au closer to the Sun than those of Voyager 1 confirms earlier suggestions10 that the shock is laterally asymmetric (pushed in on one side), possibly because of the presence of an inclined local interstellar magnetic field11–13. The multi-ple crossings by Voyager 2 were presumably the result of dynamic changes in the shock — quick inward and outward motions, ripples propagat-ing along the shock face or perhaps re-forming of the shock, phenomena much like those seen in the water analogue depicted in Figure 1.

The plasma data returned by Voyager 2 have also established the importance of interstellar ‘pickup ions’. These are interstellar neutral atoms that have entered the solar wind and become ionized; they have a higher energy than the thermal particles, and strongly affect the processes at the shock. For a ‘standard’ shock, the downstream flow would be sub-sonic, but Voyager 2 found it to be cooler than expected and supersonic. Consistent with this, the higher-energy accelerated particles have an energy density comparable to that of the plasma and magnetic field. These facts establish that the shock at the Voyager 2 crossing is quite dif-ferent from previously observed shocks, pos-sibly because of the pickup ions.

The observations of the two Voyagers show that the termination shock is very complex, and many ambiguities remain. It will be a long time before we receive more in situ data, but — fortunately — remote observations from the inner heliosphere should fill some of the gaps. Electromagnetic waves from the solar-wind plasma in the heliosheath are too weak to be observable, but its interactions with ambi-ent neutral atoms in the heliosheath give rise to energetic neutral atoms (ENAs) that can be observed at Earth (much as light or other radiation can). The IBEX spacecraft, to be launched later in 2008, will exploit this princi-ple to provide a global view of the heliosheath. The recently launched STEREO spacecraft can also observe ENAs.

A paper by Wang et al.14, also in this issue (page 81), reports on initial observations of

ECOLOGY

Return of the nicheMathew A. Leibold

Two ideas vie for prominence in community ecology — ‘niche partitioning’ and ‘neutral theory’. A survey of patterns of tree abundance in tropical forest prompts fresh thinking on their respective effects.

New data and novel analyses invigorate old debates. A provocative example, published in the journal Ecology, comes in the form of a paper by Kelly et al.1 that will spark fresh argument over the question of the factors that determine patterns of biodiversity.

Traditional explanations for the local co- existence of species hold that the balance of nature is delicately related to differences in how species interact with their local environments (their ‘niches’), with populations of each species being primarily regulated by distinct environ-mental factors2. Such niche partitioning results in stable frequency dependence, in which each species increases relative to others when it is rare, and decreases when it is common3. This venerable view has been confronted with the contention4,5, arising from recent modelling work, that stochastic demography and disper-sal are more important, and that they allow the widespread coexistence of species with identical niches. This ‘neutral theory’ has provided pos-sible explanations for the occurrence of highly diverse communities that challenge the tradi-tional view, and has indicated ways to account for them with simple models.

The resulting clash of ideas has led to a possible synthesis that finds a place for both niche and neutrality6,7. The key to this synthesis is the thought that niche partitioning is increas-ingly less likely, and neutral dynamics more likely, as species come to resemble each other

more closely. This synthesis would retain niche partitioning as a component of community structure, but would use neutral theory to resolve the matter of why niche partition-ing cannot by itself explain highly diverse communities.

This is the context in which Kelly and col-leagues’ work1 is set. They examined abundance patterns of trees in a highly diverse tropical forest in Mexico, and focused especially on pairwise patterns of relative abundance between closely related (congeneric) spe-cies (Fig. 1, overleaf) and less closely related species. They found that coexisting congeners had relative abundances that indicated stable frequency dependence, whereas less closely related pairs of species showed patterns consist-ent with neutral theory. These results are sur-prising — not only is it the most closely related pairs of species that show frequency depend-ence, rather than less closely related pairs, but also the less closely related pairs show relative abundances similar to those expected with neu-tral dynamics. Both of these aspects contradict the proposed synthesis described above. The authors also show that the type of frequency dependence manifested by closely related pairs of species is consistent with, and indicative of, mechanisms associated with temporally fluc-tuating environments. This suggests that such species-pairs coexist because one does better than another at different times, for instance

these ENAs from the STEREO spacecraft, which complement the in situ Voyager observa-tions from the heliosheath. The energy spectra are consistent with the Voyager observations, but the observed variation of the intensity with direction is unexpected. Particularly interest-ing and puzzling to this writer are the observed double maxima of the fluxes, as a function of longitude, near the direction from which the interstellar medium is flowing.

Over the past few years, the stream of in situ and remote data from the outer reaches of the heliosphere has revolutionized our view of how the Sun interacts with the Galaxy. More is to come as the Voyagers continue their jour-ney, and as remote data, such as those from STEREO and the forthcoming IBEX mission, become available. ■ J. R. Jokipii is in the Department of Planetary Sciences, University of Arizona, Tucson, Arizona 85721, USA.

e-mail: jokipii@lpl.arizona.edu

1. Richardson, J. D., Kasper, J. C., Wang, C., Belcher, J. W. & Lazarus, A. J. Nature 454, 63–66 (2008).

2. Decker, R. B. et al. Nature 454, 67–70 (2008). 3. Stone, E. C. et al. Nature 454, 71–74 (2008). 4. Burlaga, L. F. et al. Nature 454, 75–77 (2008).5. Gurnett, D. A. & Kurth, W. S. Nature 454, 78–80 (2008).6. Burlaga, L. F. et al. Science 309, 2027–2029 (2005).7. Decker, R. B. et al. Science 309, 2020–2024 (2005).8. Stone, E. C. et al. Science 309, 2017–2020 (2005).9. Jokipii, J. R., Giacalone, J. & Kota, J. Astrophys. J. 611,

L141–L144 (2004).10. Lallement, R. et al. Science 307, 1447–1449 (2005).11. Ratkiewicz, R. et al. Astron. Astrophys. 335, 363–369

(1998).12. Pogorelov, N. & Matsuda, T. J. Geophys. Res. 10, 237–245

(1998).13. Opher, M., Stone, E. C. & Liewer, P. C. Astrophys. J. 640,

L71–L74 (2006).14. Wang, L., Lin, R. P., Larson, D. E. & Luhmann, J. G. Nature

454, 81–83 (2008).

See also pages xi and 24, and video (www.nature.com/nature/videoarchive/voyager).

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during periods of drought, rather than because they specialize on different resources.

At first glance, these results imply that closely related species show niche partition-ing, whereas interactions between less closely related species are determined by neutral dynamics. However, this raises two puzzling questions.

First, how similar do species have to be to result in neutrality between them? Kelly and colleagues’ findings indicate that, even in a highly diverse community, closely related con-geners are not similar enough to show neutral dynamics. One possibility is that close related-ness is not reflected in ecological similarity. But this seems unlikely, because such relatedness is highly correlated with similarity of growth form, demography and general ecology.

Second, to what degree does finding relative abundance patterns consistent with neutrality really indicate that stochastic demography and dispersal regulate interactions between spe-cies-pairs? A notable point here is that Kelly et al. looked at pairwise relative abundance distributions, whereas most previous studies examined such distributions in entire, and often highly diverse, assemblages. Kelly and colleagues’ findings tend to confirm that the fit of relative abundance distributions to neutral theory is a poor diagnostic for the importance of stochastic demography and dispersal. It may instead be that the structure of environmental variation among factors in the community itself may have random components. Some previ-ous models of niche partitioning8,9 have such features, and to some extent these models match the data of Kelly and colleagues.

What is surprising is that this component of niche partitioning does not also affect closely related species. The authors argue that the interactions between such species differ in type from those involving less related species because they show the signature of temporally fluctuating environments, whereas less closely related species seem less likely to be affected by this kind of niche partitioning. The mecha-nisms that generate temporal fluctuations in niche relations are not known, but such fluctu-ations may account for some aspects of highly diverse communities. However, it still seems unlikely that they can fully explain the appar-ent coexistence of hundreds of species in some communities, including this one.

Thus Kelly and colleagues’ results provide answers to some aspects of the debate between neutralists and niche partitioners. But they leave open other issues that the tentative synthesis seemed to have resolved.

One issue is whether there is a role for neu-tral dynamics at all. The new findings provide surprisingly little support for such dynamics but do not rule them out. Perhaps ecological neutrality does occur among some subsets of species in these communities, but, if so, it is not closely associated with phylogenetic relatedness. The other issue is the need for an explanation for the high diversity in these

communities. The appealing component of the synthetic view was that stochastic demog-raphy and dispersal could enhance diversity in a community with limited niches to an arbitrary degree that would depend on exter-nal factors (speciation and biotic exchanges, for example). If stochastic demography and

dispersal are not important, even in a diverse community, explanations for diversity must be found elsewhere.

Kelly and colleagues’ findings1 indicate that much of the unresolved component of diver-sity involves the co-occurrence of compara-tively unrelated species. This is a clue that the pro cesses involved may not fall in the normal realm of conventional niche theory. This the-ory has focused on local dynamics, but regional factors may affect local patterns in unpredict-able ways. For example, many species may be present as ‘sink populations’ that are main-tained by dispersal from other locations where they have higher fitness as a result of different local conditions10. This is just one possibility, but it does suggest that regional factors may be a key to understanding highly diverse com-munities such as these tropical forests.

Where does all this leave us? My view is that Kelly and colleagues’ paper will turn out to have a considerable impact — but as much in stimulating fresh thinking as in directly clarifying the relative roles of neutral and niche-partitioning processes. ■

Mathew A. Leibold is in the Section of Integrative Biology, University of Texas at Austin, Austin, Texas 78712, USA.e-mail: mleibold@mail.utexas.edu

1. Kelly, C. K., Bowler, M. G., Pybus, O. & Harvey, P. H. Ecology 89, 962–970 (2008).

2. Hutchinson, G. E. Cold Spring Harb. Symp. Quant. Biol. 22, 415–427 (1957).

3. Chesson, P. Annu. Rev. Ecol. Syst. 31, 343–366 (2000).4. Bell, G. Am. Nat. 155, 606–617 (2000).5. Hubbell, S. P. The Unified Neutral Theory of Biodiversity and

Biogeography (Princeton Univ. Press, 2001).6. Leibold, M. A. & McPeek, M. A. Ecology 87, 1399–1410

(2006).7. Adler, P. B., HilleRisLambers, J. & Levine, J. M. Ecol. Lett. 10,

95–104 (2007).8. Nee, S., Harvey, P. H. & May, R. M. Proc. R. Soc. B 243,

161–163 (1991).9. Sugihara, G. Am. Nat. 116, 770–787 (1980).10. Mouquet, N. & Loreau, M. Am. Nat. 162, 544–557 (2003).

Figure 1 | Common ground. Two adult congeneric trees — Bursera instabilis and B. heteresthes — in intimate coexistence at Chamela Biological Station in Mexico, the site studied by Kelly et al.1. Although the two species are distinct in bark coloration, they are otherwise similar in most respects.

ATMOSPHERIC CHEMISTRY

Her dark materials Kevin Zahnle

A glitch in the history of sulphur isotopes could imply that methane emitted by the ancient biosphere created a high-altitude photochemical smog, which governed the climate in a distinctly Gaian way.

Free oxygen became abundant in Earth’s atmosphere around 2.3 billion years ago, a change that was preceded or accompanied by at least three major ice ages. These ice ages themselves occurred immediately after an abrupt change in the relative abundance of isotopes of sulphur in sedimentary rocks around 2.46 billion years ago1–3. The sulphur isotopes indicate that the photochemistry of the atmosphere altered dramatically, and the cold climate suggests that a major greenhouse

gas, probably methane, had been removed. A somewhat similar pattern in the sulphur isotope record half a billion years earlier was reported recently, also associated with an ice age, and it has been suggested that this might indicate that there had been a transient period of high atmospheric oxygen4.

Writing in Earth and Planetary Science Letters, Domagal-Goldman et al.5 offer a counter-proposal: the sulphur isotopes and the ice age could have been caused by increased

C. K

. KEL

LY

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© 2008 Macmillan Publishers Limited. All rights reserved© 2008 Macmillan Publishers Limited. All rights reserved