SCIENTIFIC CORRESPONDENCE
the light source through the gradient of Phototactic purple decreasing light intensity; on the other Extinction or hand, if their movement is scotophobic, bacteria the colony should move in the opposite miscalculation? direction. R. centenum swarm colonies SIR - In 1883 T. Engelmann! discovered irradiated laterally with infrared light in SIR - Heywood et al. I have discussed that purple photosynthetic bacteria re fact moved rapidly towards the light uncertainties in modelling rates. One way verse their swimming direction when the source, as for positive phototaxis. to check a predictive model's validity is to light intensity is suddenly reduced. He Anoxygenic photosynthetic bacteria see how well it predicts past events. In this described this photosensory effect as and the oxygenic cyanobacteria and algae case, the species-area relation model does "Schreckbewegung" ('movement of share several similar physiological re fairly well in predicting the number of fright'), but it is more properly designated quirements, so it is not surprising that bird and mammal extinctions during the as a scotophobic response, past 500 years in the United States and that is, a fear of darkness. a Canada .. Scotophobic behaviour is Time (min) Infrared Here I consider species extirpated fol typical of motile purple bac lowing habitat destruction initiated by 2 teria, is independent of the o Europeans in North America , combined direction of illumination, with currently endangered or threatened 3 and does not occur if the 30 species , to check if species loss is con light intensity is reduced ..'Q" gruent with habitat loss. Although exact only gradually. By contrast, 60 figures are difficult to come by, I take as oxygenic cyanobacteria and 20% the amount of land removed from algae exhibit phototactic re- 120 native North American mammal and bird 4 sponses, that is, oriented habitats . Applying the species-area rela movement towards or away tion, So=kA z, gives the number of species from a light source, b originally present, and Sl=k(0.8AY irrespective of its intensity. Time (min) Visible gives the number remaining today (for We show here that the non non-isolates 0.12 :::; z :::; 0.17; for true sulphur purple bacterium o island isolates z = 0.25; ref. 5). Thus Rhodospirillum centenum2 1-(S/So)=1-(0.8Y is the fraction of can also move with respect 30 original species lost. to the direction of illumina- ..'Q"- The values of z usually observed for tion and that this photo 60 non-isolates indicate species loss is in the tactic behaviour is observ range 2.65-3.72%. The number of terrest able macroscopically. rial bird species threatened, endangered 120 When grown photo or extinct in North America is 18. The trophically in a liquid question is, then, does 18 correspond to medium, R. centenum cells 2.65-3.72% of the original bird fauna? bear a single polar Phototactic behaviour of R. centenum swarm colonies by Working backwards to deduce T, the total flagellum, but when grown time-lapse photography. a. An R. centenum swarm colony number of bird species originally present, exposed to 575 flW of infrared light of wavelength above 650 aerobically in darkness on gives 679 :::; T:::; 484, which does bound nm; b, swarm colony exposed to 2.500 flW of visible light of 0.8% agar containing 0.3% wavelength below 650 nm. Scale bar, 10 mm. the roughly 600 known breeding species in peptone they produce many North America, plus extinctions. peritrichous flagella. Such cells contain representatives of these three groups I used the non-isolate z because birds the normal complement of photosynthetic of phototrophs are phototactic. Photo are good dispersers and 'habitat islands' pigments3 and form phototactic 'swarm taxis is believed to help motile photo are less likely to be truly isolated for birds colonies'; the figure shows that lateral synthetic organisms move towards zones than for mammals. For mammals, which illumination of such a colony with infrared where conditions are better for growth; do not disperse as readily as birds, I chose light causes positive phototaxis (a) or for example, it is well known that cyano an isolate value for z of 0.25 (ref. 5). negative phototaxis with visible light (b). bacteria and anoxygenic photosynthetic This indicates a species loss of 5.4% for Action spectra and details of the re bacteria accumulate in discrete bands mammals. Using 19, the number of sponses of swarm colonies to simul in stratified lakes and laminated micro terrestrial mammal species threatened, 5 taneous illumination with perpendicular bial mats . endangered or extinct in North America, beams of light will be published elsewhere Lisa Ragatz and again working backwards, the orig (manuscript in preparation). Ze-YuJiang inal number of mammal species is around Several ad hoc assumptions would be Carl Bauer 352. This is in reasonable agreement necessary to explain the movement of R. Howard Gest with the roughly 300 extant species plus centenum swarm colonies towards or away Indiana University, extinctions. from light sources on the basis of sco Department of Biology, This thumbnail analysis supports the tophobic responses of randomly moving Bloomington, use of species-area curves to predict spe cells. Nevertheless, we also tcsted for Indiana 47405, USA cies extinctions due to habitat loss, at least authentic phototaxis using a technique 4 previously applied to phototactic algae . 1. Engelmann. Th. W. Arch. gesam. Physiol. Mensch. u. Thiere30. 95-124 (1883). 1. Heywood, V. H. etal. Nature 368. 105 (1994) Infrared light was passed through a con 2. Favinger.J" Stadtwald. R. & Gest. H. Ant. 2. Williams. J. D. & Nowak, R. M. in The Last Extinction (eds verging lens to focus on a swarm colony. v. Leeuwenhoek. J. Microbiol. 55. 291-296 Kaufman. L. &Mallory, K.) 115-148 (MIT, Cambridge. At the edge of the colony, the light (1989). Massachusetts. 1993). 3. Yildiz. F. H .. Gest. H. & Bauer. C. E.J. 8act.173. 3. Endangered and Threatened Wildlife and Plants Code of intensity was relatively high and the in 5502-5506 (1991). Federal Regulations 50: 17.11 & 17 .12 (US Dept of tensity gradient decreased in the direction 4. BUder.J.Jb. wiss. 80t. 58.105-220 Interior, Washington DC, 1992). (1917) 4. Ray. D. L. & Guzzo. L. Environmental Overkill (Regnery of the light source. A positive phototactic 5. Madigan, M. T. in Biology ofAnaerobic Microorganisms Gateway. Washington DC. 1993). organism or colony should move towards (ed. Zehnder. A. J. B.) 39-111 (Wiley. New York, 1988). 5. Preston. F. W. Ecology 43. 185 (1962). 104 NATURE· VOL 370 . 14 JULY 1994