Tropical deforestation and species extinction

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2

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Ln N(t) 1. Second difference A2100.N = IOR, /V,T,-Zlog. N,+log. A( as a function Oi log. N, Ri = 0.91. Data from the spruce budworm Fia.

(Choristoneura

fumiferana)“.

explained. Turchin’ used nine parameters in a Taylor expansion of Eqn 12 and explained less variation in this data set. Our experience with other data is that with the three parameters of g=a+pln

N,+yln(N,/N~-,)

we can achieve better fits than are possible using Eqn 12 with three times as many parameters. This suggests that the proposed redefinition has at least a pragmatic advantage. In opposition to Eqns 12 and 13, another class of lagged models has been based on concentrated delays with lags other than one. May4 used these models as a basis for what he termed ‘detail-free’ explanations of population cycles. In recent years Berryman has been a major contributor to the use of such fixed lag models with F = F(Nk_T),T Z 1. Taneyhill’s letter above points out

the counterintuitive property of these models: increasing the rate of reproduction causes an increase in the period of oscillations. In our experience this is not only counter to intuition but also falsified by the evidence. Oscillatory insect species with higher reproduction rates generally show shorter rather than longer periods of oscillation. This, of course, does not prove that the dimension should necessarily be two but that it should include more than one lagged density with two being minimally sufficient to obtain the effect in the right direction. Consequently, I believe that Eqn 13 is an equation of minimal complexity that satisfies our current modeling needs better than the ones commonly in use.

Lev Ginzburg Dept of Ecology and Evolution, State University of New York, Stony Brook, NY 11794, USA

References 1 Turchin, P. (1992) Nature344, 660-663 2 Turchin, P. and Taylor, A.D. (1992) Ecology 73,289-305 3 Royama, T. (1984) Ecol. Monogr. 54, 429-462 4 May, R.M. (1981) in Theoretical Ecology (May, KM., ed.), pp. 4-25, Blackwell Scientific Publications 5 Berryman, A.A. (1992) Trends Ecol. Evol. 7,316

This correspondence

is now closed - ed.

WhyCooperate? Game Theory and Kin Selection Dugatkin et al.’ recently provided a thoughtful perspective on the evolution of cooperation, suggesting extensions of current theory. Categorizing cooperative behavior into evolutionary classes’, they show that different evolutionary mechanisms are likely to favor cooperation in different species (perhaps even different behaviors within single species, e.g. cooperative hunting3 versus communal nursing4 in lions). This important insight is easily overlooked in a drive for generality. Dugatkin et al. usefully suggest extending game theoretic models to explain cooperation arising through by-product mutualism5 or group selection6, rather than focusing solely on reciprocity’. However, they then state (p. 204) that ‘because our

concern is with cooperation among unrelated individuals we do not discuss [kin-selected cooperation]‘. I suggest that by isolating kin-selection from direct mechanisms, they risk oversimplifying cooperation in the real world. For instance, Dugatkin et a/. use cooperative hunting in female lions to illustrate by-product mutualism, but this is not an example of cooperation among nonrelatives: pride-mate lionesses are invariably close relativesE. In several cases, Dugatkin et al. focus their concern on non-kin-selected evolutionary mechanisms, but do not restrict themselves to analysis of cooperation among nonrelatives - a dangerous recipe. Direct fitness payoffs may be intertwined with indirect (kin-selected)

payoffs when players are related. Because most cases of cooperation (particularly elaborate, costly or long-term cooperation) are found where interactants are related4Gg, this concern is practical. Consider cooperative hunting in female lions, which Dugatkin et al. suggest arises through by-product mutualism, because decisions to cooperate are not apparently conditional on relatedness. The core of a lion pride arises from stable matrilines, so that the mean value of r among females is high, with low variance’. Hence, a rule of thumb that transferred the conditionality of hunting decisions on r to a simple conditionality on pride membership would usually be reliable. Stealthy hunting probably favors easilyevaluated conditionalities. The observation that lionesses’ cooperation wasn’t conditional on r, given the high and invariant expected value of r, cannot be used to argue that cooperation is not favored by (possibly dependent on) the high degree of relatedness that invariably exists. If relatedness is entirely distributed above a threshold for cooperation, then cooperation will not covary with r, even if cooperation is in fact conditional on r. Even below the threshold, limited variation in r will yield limited variation in cooperative behavior. The contrast with cooperation in male lions is illuminating. Unlike females, male lions include relatives and nonrelatives in their coalitions’. nonrelative males are Tellingly, admitted only to small coalitions’. Larger coalitions, whose members’ reproductive success suffers with each additional member, are closed to nonrelatives. Dugatkin et al. conclude that ‘rather than kinship, a “common enemy” - size of prey - is the mechanism underlying cooperative hunting’ (emphasis mine). I suggest that a more complicated mechanism may exist, dependent both on the existence of difficult prey (providing a direct payoff to cooperation through a better catch rate), but also dependent on close genetic ties within a pride (ameliorating the necessary evil of sharing cooperatively hunted food). The test case for by-product mutualism among females - prides of unrelated lionesses - does not exist*. Controversy over the causes of cooperation has been common’, perhaps in part because field studies have not always measured direct and indirect payoffs to all strategies. dwarf For instance, among

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mongooses, the decision to remain a cooperating subordinate is favored entirely by indirect selection in some age-sex classes, and almost entirely by direct selection in others (S.R. Creel and P.M. Waser, unpublished). Dugatkin and colleagues point out important holes in current models of cooperation. By suggesting routes of development, they enhance the likelihood of producing broad, realistic models. One further direction for development is the integration of models of kin-selected benefits with game-theoretic models of cooperation based on direct fitness. We

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References 1 Dugatkin, L.A., Mesterton-Gibbons, M. and Houston, A.I. (1992) Trends Ecol. Evol. 7,202-205 2 Mesterton-Gibbons, M. and Dugatkin,

L.A. 0. Rev. Biol. (in press) 3 Scheel, D. and Packer, C. (1991) Anim. Behav. 41,697-710 4 Packer, C., Lewis, S. and Pusey, A.E. (1992) Anim. Behav. 43,265-281 5 Brown, J.L. (1983) in Advances in Behavior (Rosenblatt, J.S., ed.), pp. l-37, Academic Press 6 Wilson, D.S. and Sober, E. (1988) J. Theor. Biol. 136,337-356 7 Axelrod, Ft. and Hamilton, W.D. (1981) Science211, 1390-1396 8 Packer, C., Gilbert, D.A., Pusey,A.E. and O’Brien, S.J. (1991) Nature 351, 562-565 9 Brown, J.L. (1987) Helping and Communal Breeding in Birds, Princeton University Press

Alternatives to Deforestation tops the ranking because it identifies the causes of extinctions, focuses on rehabilitation of damaged habitat, then suggests sustainable and profitable alternatives for the people affected by or implicated in current destruction. The emphasis throughout the 17 chapters is prescriptive and pragmatic. While empirically grounded in reliable science and sociology, it does not allocate any space to theory. Fearnside’s chapter compellingly deflates the fallacy of ‘sustainable’ cattle in pasture Amazonia, notwithstanding Serrao and Toledo’s attempted defense. Deforestation Tropical and Species Extinction is the only one to include chapters on two (Amazonia and Papua New Guinea) of the three main tropical forest areas. (Not one chapter of all three books is devoted to African forests.) Of the seven chapters, many focus on predicting extinction rates, and one (Heywood and Stuart) concludes that ‘...there is little evidence of extinctions at the rates predicted by some theoretical models’. The Browns’ chapter on Brazilian forests focuses on current irrational and anti-economic land abuse and clearly provides optimal conservation strategies. Marshall and Swaine’s Tropical Rain Forest: Disturbance and Recovery focuses on deforestation and extinctions where they are among the most severe in the world - in the SE Asian rainforests. This book provides a better understanding of the processes causing extinctions, especially selective logging, rather than

how to reduce destruction. We know there are enormous riches trapped in the dwindling rainforests; we know little about how to exploit such forests sustainably, and we don’t know how to use the promise of such wealth as a tool for conservation. The work outlined in this book may have far-reaching consequences because of the highly commendable attention paid to training: because ‘PhDsIha’ are more effective than ‘kms of barbed wire’ in conservation, it is encouraging to see that 12 PhDs and four Masters, mostly Asians, have recently been trained. The sad and well known findings in Marshall and Swaine’s book, borne out by those of the other books, bear repeating. Current tropical forest logging extinguishes species directly; practically no tropical forestry (~0.1%) is now sustainable, and sustainable logging is decreasingly likely in today’s increasingly overpopulated world. The indirect impacts of logging are similarly severe: increasing fire risks, edgeeffects, drought, improved human access along logging trails, and conversion to non-economic (ab)uses (e.g. cattle ranching). Peninsular Malaysian timber exports are now ceasing; Sabah is predicted to be logged out by 1995, Sarawak by 2002, and Kalimantan by 2010. Instead of outrage at the unnecessary economic, human and biodiversity losses, this volume shows that the need for conservation units is scarcely reduced even by improved logging. The price of host tolerance

will have better, more broadly encompassing models of evolution if direct and indirect fitness are not separated: a gene-copy is a genecopy is a gene-copy.

Scott Creel Wildlife Conservation Research Unit, Dept of Zoology, University of Oxford, South Parks Road, Oxford, UK OX1 3PS

Book Reviews Prioritiesfor the Tropics Alternatives

to Deforestation

edited by A.B. Anderson, Columbia University Press, 1992. $24.00 pbk (xiv + 281 pages) ISBN 0 23106893 X Tropical Deforestation and Species Extinction edited by T.C. Whitmore and J.A. Sayer, Chapman & Hall, 1992. f 74.95 pbk (xix + 153 pages) ISBN 0 412 45520 X Tropical Rain Forest: Disturbance and Recovery edited by A.G. Marshall and Swaine, The Royal Society, f 19.50 (UK) f27.00 (ROW) pages) ISBN 0 85403 458 7

M.D. 1992. (135

The major dilemma confronting all concerned with tropical deforestation and species extinctions is the balance between yet more intellectualizing and research on the one implementation of hand, and already widely agreed conservation priorities on the other. ‘The loss of biodiversity by the destruction of natural habitats...is the folly our descendants are least likely to forgive us’, as Ed Wilson best put it. When a species is threatened with extinction, is it more important to study it or to save it? The problem is urgent; extinction rates are the highest they have ever been and are accelerating fast. This is the critical lens through which I rank these three books. 72