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I ' NATURE . VOL 341 . 14 SEPTEMBER 1989 community structure ignores evidencc that herbivorcs not only dcpend on, but sti'ongly influence, the plants they eat. In The Origin of Species, Darwin rcported that, on a small cleared plot, he "marked all the seedlings of our native weeds as they came up, and out of 357 no less than 295 were destroyed, chiefly by slugs and insects." More recent studies show clearly that the fcedback effects of herbivores on plant populations are both negative"' and positive". In the light of these studies, it is far from an expected finding that the complex interplays between plants and herbivores sllould yield a simple proportionality between NAP and NSP. Moreover, the constant of proportionality, onethousandth, is far lcss than might have been guessed from widely quoted ecological efficiencies of the order of 10-20 per cent. Nor would the trickle-down theory explain why consumption by herbivores rises as the square of net foliage production, as McNaughton ctal. find. If studies of food-web structure demonstrate a rough proportionality between the numbers of basal and intermediate species (trophic species, really, since biological species with identical predator species and identical prey species are counted as a single unit), and if future studies of foodweb function confirm the finding of McNaughton et al. that NSP and NAP are proportional, the next problem, and IVEWSANDVIEWS p - I ECOLOGY Just proportions in food webs Joel E. Cohen I N FOOD webs of natural communities with vcrying numbcrs of species, the number of green plants and other species that consume no prey in the wcb (basal species) is ncarly proportional, on ilvcrage. to the number of species that have both predators and prey in the wcb (intermediate species), accordi~lg to studies rcported over the past dozen yc;lrsl'. Empirical rules"' of that typc describe structural or i~natomic;~lproperties of food wcbs. An import;int qucstion in community ecology is wllcthcr parallel regularities exist in the way in which food webs function"", On page 142 of this issuev, McNaughton el ul. report remarkable regularities in measures of food web function. Their findings indicate thc existence of a physiology of food webs that gcncralizcs ocross natural cornn~unities. Their report opens up tllc distant. but highly attractive, prospect of linking the previously known structural regularities with new functional patterns. McNaughton cl 111. show that thc biomass, consutnption and nct secondary productivity (NSP) of herbivores ;Ire all rel;~tcdto the net abovc-ground primary productivity (NAP)of a natural habitat by simplc powcr laws. Particularly striking is that the exponent of the power law that relalcs NSP to NAP is 1101 sig~~ificantly dilfcrcnt from 1: to a first al~proximation. NSP and NAP arc dircctly proportional. In cl'fcct, the cncrgy convcrlcd inlo herbivorcs pcr square mctrc of sulfi~ceurea pcr year is, on average, iibout onethousandth of the e n e r g th:it is converted into plants per square metre of surface area pcr year. The ecological energy pyran~id has a much largcr base than second storey. This proportionality would b e a triviality if scco~ldaryproductivity wcrc passively controlled and limitcd by primary productivity. But this tricklc-down view of NATURE . VOL 3 4 1 . 14 SEPTEMBER 1989 opportunity, is to relate the two proportionalities. In the 35 studies where NSP and NAP were both measured directly, are the numbers of basal trophic species proportional to the numbers of herbivore trophic species? What is the relation between primary or secondary productivity and the diversity of biological and trophic species in these studies? Stay tuned! But as Mark Twain advised a young reporter: first get your facts straight, then you can distort 'em. McNaughton et al. I very young science. A backward glance to acknowledge that their findings would be the early stages of other, now maturer, considerably strengthened by more data sciences shows the tremendous power of from tropical forests and other structur- empirical proportionalities. Joseph Louis ally complex systems. At present, their Proust proved that elements combine in results rely heavily on structurally simple definite proportions by weight when they form chemical compounds (the law of vegetation such as grassland and tundra. Moreover, the wide scatter of the assem- definite composition), laying a corner-, bled data with respect to the log-log linear stone of the empirical support for the regression proposed to describe them atomic theory". Gregor Mendel found brings to mind the statistical maxim that. fixed proportions of different phenotypes on log-log coordinates, even an elephant in, offspring in his experiments on plant looks like a straight line. The report of breeding; his explanation in terms of McNaughton et al. does not have all the genes was phenomenological. Erwin answers, but it hassome and, betterstill, it 1. Cohen. J. NBlure 270.165-166 (1977). Briand. F. 8Cohen;J. Nature307.264-267 (1984). raises important new questions. , 2.3. Jeffrles. MJ. & Lawton. J.H. Freshwater Biolo~v ~. - 15. . Both the structural and the functional 105-112 (1985). proportionalities found in food webs are 4. Suglhars. G.. Schoenly. K. 1Trombla. A. Science 245. 48-52 (1989). purely descriptive. Though a model exists 5. Cokn. J.E.. Briand. F. 4 N m s n . C.M. Communily Food that explains much of food-web strucWebs: Data and meory Blornslhernslicr Vol. 20 (Sprlnger, Heldelberg. 1989). t u r ~ " ,it too is phenomenological; and 6. Lawton. J.H. 8 Warren. P.H. Trends Ecol. Ewl. 3. 242there is no hint of a mechanistic explana245 (1888). 7. Plmm. S. Foodwebs (ChapmanandHall. London. 1982). tion for the observed patterns in the report 8. Cole. JJ.. Flndlay. S.E.G. PLovell. G.M. (eds) Cornparaof McNaughton et a/.. But ecology is a NEWS AND VIEWS Chargaffs discovery that .the concentration of adenine equals that of thymine and that the concentration of guanine equals that o f cytosine in DNA from various sources was one of the chief clues to Watson and Crick's solution of DNA structure. Time will reveal the scientific consequences of the new proportionalities in food-web structure and function. The possibilities are exciting. 0 Joel E. Cohen is Professor of Populations at The Rockefeller University, New York, New York, 10021-6399,USA. 9. lo. 11. 12. 13. tlw Analyses OlEcosystems: Patterns. ~echanismsand Theories (Springer. New York. 1990). McNaughton. SJ.. Oeslerheld. M.. Frank. D.A. 8 Willisrns. K.J. Nature 341. 142-144 (1989). Dirzo. R. In Perspectives on Plant Populalion Ecolo.~ (eda Dirzo. R. 4 Ssrukhln. J.) 141-165 (Sinsuer. Sunderland. Maasachusetu. 1984). Blanchl. 1,s..Jones. C.G. & Shschok. M. Trends Ecol. EWl. 4.234-238 11989). Cohen. J.E. (L C.M. ~ r m R. . soc. 0224.421448 (1885). Jafk. 0. Crucibles: me ~ t o ~ o l ~ h e m r s r r y ( ~ a w New cett. York. 1957). c man.