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Entrepreneurial insects Stuart L. Pimm
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NATURE | VOL 411 | 31 MAY 2001 | www.nature.com
from sun-loving Helianthemum chamaecistus to Geranium species. These live in habitats that were previously too cool for this butterfly. Indeed, the authors find that whereas females of stationary populations prefer to lay eggs on the same species of plant that they ate in their youth, expanding populations — irrespective of their upbringing — prefer the widely distributed geraniums. Spreading into patches of geraniums has allowed the butterfly to cross the previously impossibly large gaps in the distribution of Helianthemum. Thomas et al. also consider two species of bush cricket that have also increased their ranges over the past 20 years. One of these, the long-winged conehead bush cricket, comes in long-winged as well as extra-longwinged forms; Roesel’s bush cricket has flightless and flying forms. On the margins of these species’ geographical ranges, the incidence of the more mobile forms is greatly increased. That leads, the authors assume, to a greater potential for dispersal. Can one ever predict what will happen as climate warms? Will species march towards the warming poles or up mountainsides in an orderly fashion? Both answers are surely ‘no’. Even during the post-glacial warming of the past ten millennia, species moved at very different rates4. The present rate of warming is much faster, so orderly changes will be even less likely. What Thomas et al.2 have described are several synergistic mechanisms — changes in the ability of insects to move through habitat patches, changes in their food supply, and in their ability to disperse — that lead to geographical ranges expanding faster than expected. These synergies are unexpected even for species as well documented as British butterflies. For the tropics, far to Britain’s south, where most butterfly species live (and live largely unknown lives), concerns about the effects of global climate change should be heightened, not allayed. The mechanisms documented by Thomas et al. are not only unexpected but both idiosyncratic and capricious.
Figure 2 Brown argus — shift in its choice of host plant to Geranium species.
Before your colleagues snatch this copy of Nature, ask them whether they think a warming climate will increase or decrease the density of a species’ preferred habitat patches; offer more or fewer food plants; and favour more or less population dispersal. In each case, the correct answer is ‘it depends’. The same answer will probably apply if one is investigating the north, middle or south of a species’ geographical range. However much information is available, one might as well toss a coin as to try to predict individual range changes in detail. They depend on too many interacting factors and will be obvious only after the event. One can make more general predictions, however. The outcomes of individual coin tosses are uncertain, but their aggregate behaviour is not. Don’t bet a million pounds on the result of one toss, but go ahead and predict at least one tail from 20 tosses of a fair coin and you will be on to a winner. Metaphorically, a species with a large geographical range, typical for species ROBERT WILSON
ritain sits off the unfashionable fringe of a continent near the bottom of the biodiversity league1. The size of Britain’s butterfly fauna elicits expressions of sympathy from lepidopterists elsewhere. There are also occasional murmurs of encouragement — global warming might favour those species that once lived precariously, hoping for summers with enough sunshine. Will these butterflies rise to the challenge or remain provincial, hidebound by their local adaptations and so restricted at the margins of their geographical ranges to isolated subpopulations? Writing on page 577 of this issue2, Thomas and colleagues show that two butterfly species, as well as two species of bush cricket, are unusually entrepreneurial at the edges of their ranges. With so many Britons expressing so much passion for so few butterflies, Thomas et al. can consider butterfly ranges that have been mapped out in exquisite detail over the past two decades. The silver-spotted skipper (Fig. 1) was once found only on warm, south-facing chalk hillsides in southeast England. Now it lives even on north-facing slopes there. There is more to this result than an addition to Parmesan’s catalogue3 of northward-moving butterflies. Like other butterflies, the skipper has many subpopulations that are more or less isolated from each other, reflecting the patchy distribution of suitable breeding habitats. Some patches of habitat will be too small or too isolated to support persistent butterfly populations. A warming climate has created more and larger patches. In turn, these provide steppingstones to yet other suitable patches that were previously too remote or too small to have been occupied permanently. Because of climate warming, the total area of habitat available to the silver-spotted skipper has doubled since 1982, but the butterfly’s geographical range has expanded threefold. The distribution of the brown argus butterfly (Fig. 2) also expanded from the 1970s to the 1990s, according to two atlases that compile records of butterfly distribution across 10-kilometre-square grids. In the south of England, the range has filled in and more of these squares are now occupied than in the 1970s. In the north, the range has expanded considerably from what was previously only sporadic occurrence. As Thomas et al. reveal, there has also been a shift in the butterfly’s choice of host plant,
ROBERT WILSON
Four British insect species have increased their geographical range as a result of climate warming. The underlying mechanisms vary from a change in food to an increase in habitat. Other species may not be quite so lucky.
Figure 1 Silver-spotted skipper — its range has expanded threefold since 1982. © 2001 Macmillan Magazines Ltd
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100 YEARS AGO As a general rule, the pitch of a musical note does not in any way depend upon its intensity, but solely upon the wave-length. It appears probable, however, that any wave motion of very great intensity produces distorted effects. Thus we find that a very loud sound may so affect the ear of the observer as to appear flatter than it really is. This is a purely subjective effect… If a C tuning-fork (middle C, 256 vibrations per second) be strongly bowed, and then be quickly brought near the ear, before its loud note has had time to die away, the sound will appear flattened to about Bb1, or even A1, the amount of the effect being different to different ears… The amount of the subjective effect differs with different individuals, both in pitch and in intensity. What to one person appears a flattening of a minor 3rd , to another auditor appears a flattening of only a major 2nd, but in every case it appears to be a flattening and not a sharpening. Also the loudness of the subjective note appears different, even to the different ears of the same person… Much of my own musical work has been done amongst male voices, and I have frequently noticed that a singer of good concert-room power may, if practising in a small room, seemingly sing with flat intonation. From Nature 30 May 1901. 50 YEARS AGO The leading article in Nature of April 28 on “Biologists in the Modern State” draws attention once more to the need for biologists with sound basic training in one of the physical sciences. The Agricultural Research Council is seriously concerned about the shortage of plant and animal physiologists with an adequate knowledge of chemistry and physics and of biochemists with biological experience. Posts in these subjects have remained unfilled for long periods owing to lack of suitable applicants. It is recognized by the Council that it is rarely possible in the course of study for a first degree to include adequate training in both biological and physical sciences; and hence applications for the Council’s studentships from candidates wishing to study biology after graduating in a physical science, or to study a physical science more thoroughly after training in biology with a physical science as a subsidiary subject, are given very sympathetic consideration. From Nature 2 June 1951. 532
in temperate regions, is tossing many global-warming coins. Each population might live on different plants (or perhaps suites of them), across landscapes with different structures of habitat patches, and in range centres or at range margins. And each population will make different tradeoffs between its ability to disperse and other essential biological functions. In aggregate, among widespread species, there will be winners and losers, but there will always be survivors. In the tropics, each of the many more participants in the global-warming gamble plays a far riskier game. Each outcome will be as idiosyncratic and biologically fascinating as those uncovered by Thomas and co-workers. But, because tropical species typically have smaller geographical ranges5,
each species bets its existence on far fewer tosses. Human actions have forced small ranges on many species, while others are born this way. The broad lesson from Thomas and colleagues’ results should not be awe at how quickly a few species benefit from global change, but concern with how rapidly many may be harmed by it. ■ Stuart L. Pimm is at the Center for Environmental Research and Conservation, MC5556, Columbia University, 1200 Amsterdam Avenue, New York, New York 10027, USA. e-mail:
[email protected] 1. Myers, N., Mittermeier, R. A., Mittermeier, C. G., da Fonseca, G. A. B. & Kent, J. Nature 403, 853–858 (2000). 2. Thomas, C. D. et al. Nature 411, 577–581 (2001). 3. Parmesan, C. et al. Nature 399, 579–583 (1999). 4. Overpeck, J. T., Webb, R. S. & Webb, T. III Geology 20, 1071–1074 (1992). 5. Stevens, G. C. Am. Nat. 133, 240–246 (1989).
Superconductivity
Rehearsals for prime time Paul Grant Superconductivity seems to have been forever waiting in the wings. Although superconducting power cables are about to go live, will the newest material, magnesium diboride, become the class act of the future? ractical superconductors made their debut in the early 1960s. This followed the discovery of intermetallic compounds that promised high-performance electromagnets cooled by liquid helium to temperatures of about 4 K. But it took 20 years for one of the players, niobium– titanium (NbTi), to become the material of choice for superconducting wires in real applications, such as magnetic resonance imaging and high-energy physics. In 1986, a new family of high-temperature superconductors burst on the scene, based on complex copper-oxide materials. This time the potential lay in electric power applications using refrigeration technologies spanning 20 to 100 K. Despite the brittleness of the copper oxides, a viable, albeit expensive, wire technology was in place by 1992 and ‘street performances’ in the form of power cables are currently in production. Then, this January, in what perhaps was the briefest billing given a modern physics discovery, Nature published a paper1 reporting superconductivity at 39 K in magnesium diboride (MgB2), an unexpected new talent for such a simple compound, which has been around since the 1950s. Now, barely five months later, some 35 physicists report in three acts2–4 (starting on page 558 of this issue) substantial progress towards improving the properties of MgB2 that are vital to applications requiring high electric current and magnetic field. Superconductivity, like other players in the theatre of science, moves faster these days.
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Whether or not a material is superconducting depends not only on its temperature but also on the strength of the ambient magnetic field, arising either from current flow or an applied field. In addition to having zero electrical resistance, an ordinary ‘typeI’ superconductor has the ability to shield itself completely from magnetic fields. But above a critical field, HC, the superconductivity is destroyed. However, in the 1930s it was noticed that an applied magnetic field could partially penetrate some materials, eventually called ‘type-II’ superconductors, and the sample would remain superconducting. The magnetic flux flows through magnetic vortices, whose cores behave as simple metallic conductors, surrounded by superconducting regions. When the field is high enough the number of vortices occupies the total volume of the sample, and all superconductivity is lost. All practical superconductors are type II. These magnetic vortices are the source of another operating limit on superconductors — the maximum superconducting current they can support. When a current flows in a type-II superconductor it produces a force on the magnetic vortices, causing them to move and creating electrical resistance through friction with the atomic lattice. But, thankfully, defects in the material can fix or ‘pin’ enough vortices such that they remain motionless, at least until the current and field become sufficiently strong that the vortices are ‘unpinned’ and so generate resistance. The behaviour of vortices therefore NATURE | VOL 411 | 31 MAY 2001 | www.nature.com
