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quirement for the doctorate the prep- aration of a short paper on ‘the social significance of my dissertation’. All research in population biology can be justified in terms understandable to nonprofessionals and students should learn early how to make such explanations.

Above all, population biologists should try to cool down the technical differences of opinion that are so often interpreted by outsiders as in- dicating that our discipline has no agreed-upon body of results that are important for society to understand. For example, the neutrality con- troversy and the debate over punctu- ated equilibrium have both given ammunition to creationists. Need- less to say, we should not suppress legitimate disagreement to avoid arming fools. But both sides could be more careful to respect the views of the other and to reiterate in publica- tions the basic agreement on both

evolution as a historical process and the undisputed role of natural selec- tion in that process.

Finally, population biologists should make greater efforts to build bridges to the social sciences. Scien- tists in both areas often face very similar problems in dealing with enormously complex systems of immediate importance to Homo sapiens. Both often find it logistically or ethically impossible to carry out critical experiments. And the efforts of the two groups to solve human problems will be enhanced much more by cooperation than confronta- tion. Economists might not under- stand ecology, but ecological solu- tions to environmental dilemmas in- evitably have an economic dimen- sion.

Enough of the sermon; back to The Club of Earth. Its formation was an attempt to attract the attention of the media, and thus of the public. The

Club will speak out occasionally on matters of public policy; frequent statements would not be ‘news’. The group is small enough that members can confer on statements and re- solve differences expeditiously. Its members are well enough known within science as a whole and suf- ficiently senior and diverse that their opinions cannot be disregarded as easily as those of a single spokes- man. In short, The Club of Earth is one small step of the great many that should be taken by our community to improve the clout of population biology.

References 1 Beardsley, T. (1986) Nafure 323, 193 2 Mooney, H.A. and Drake, J.A.. eds (1986) Ecology of Biological invasions of North America andHawai/, Springer Verlag

Monarch Butterfly Migration in North America: Controversy and

Conservation Stephen 6. Malcolm

The monarch butterfly is the most spectacular example of insect migra- tion known. Monarchs are threat- ened by the destruction of their over- wintering sites in Mexico, California and elsewhere, and many efforts are being made to conserve these sites. However, a controversial recent sug- gestion, that some monarch popula- tions may not migrate at all, has jeopardized some of these efforts. This article assesses the evidence for and against the new suggestion.

At a recent meeting* to discuss the biology and conservation of monarch butterflies (Danaus plexip- pus), Adrian Wenner and Ann Harris from the University of California, Santa Barbara, asked the question ‘Do California monarchs really mi- grate?‘. The question is significant for two reasons. First, the monarch’s annual migration in North America is the only example, worldwide, of a ‘threatened phenomenon’, as desig- nated by the International Union for the Conservation of Nature. Second- ly, the question has important implications for understanding the nature of animal migration.

The paper presented by Wenner and Harris was prefaced by a widely circulated press release that went a

*Los Angeles County Natural History Museum, Callfor- nla, September 1986, to be pubhshed by the Museum.

Stephen Malcolm is at the Dept of Zoology, University of Florida, Gainesville, FL 32611, USA.

little further to question whether monarchs migrate at all. The ques- tion was designed to be provocative since it queries a remarkably unified body of opinion that monarchs con- stitute the clearest and most dra- matic example of insect migration knownl,z. As such, the question has sparked an avalanche of newspaper and popular scientific articles on the monarch (e.g. Ref. 3).

Migration in western populations What are the established facts that

could decide whether monarchs do or do not migrate? The evidence of Wenner and Harris, very simply, sug- gests that monarchs are able to breed all year round in Santa Bar- bara county, southern California, be- cause some cultivated and coastal milkweeds are continuously present as larval food and winter tempera- tures are insufficiently cool to main- tain reproductive diapause. Thus the monarchs, which are milkweed spe- cialists as larvae, may not require periods of reproductive diapause during the winter around Santa Bar- bara. From their larval census, Wen- ner and Harris consider that Califor-

nian monarch distribution in space and time is the result of range expan- sion and contraction through ran- dom movements, rather than directed migration between breed- ing sites rich in milkweeds and over- wintering sites devoid of milkweeds. They also found that monarchs aggregated in overwintering clusters are heavy with fat reserves and show very little wing wear-two character- istics which may be inconsistent with long distance migration. Similarly, Tuskes and Brower4 found little wing wear and high lipid content of monarchs in overwintering aggrega- tions along the central California coast.

The evidence for migration by monarchs has been collected pri- marily by Fred Urquhart and his several thousand collaborators who, since 1937, have regularly tagged monarchs and published evidence of recaptures1+7. Of the thousands of tagged monarchs released in west- ern North America (British Columbia, Oregon, Idaho, Nevada and Califor- nia), 26 recaptures were made in the western states of Washington, Ore- gon and California. These monarchs

i - , Ii,‘, .,‘I, 135

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Fig. 1. The approximate breeding areas (stippled) and overwintering sites in California (C) and Mexico (M) of monarch butterflies east and west of the Rocky Mountains in the USA, Canada and Mexico. The lines are drawn between mark-release (open circles) and recapture points (solid circles), with solid lines representing southerly autumn movement and dashed lines as northerly spring records. Data from Refs 5, 6,8 and 9.

had flown distances ranging from 29 to 1360 km, with four individuals flying more than 1000 km to California6f7 (Fig. 1). These autumn recaptures were all of monarchs that flew south to their point of recapture. The 14 monarchs that had flown further than 300 km south were caught in California, or southern Ore- gon.

Earlier records of 14 Californian monarchs tagged in January, Febru- ary and March5 indicate from capture-recapture directions in spring that at this time of year monarchs fly north, north-east, or east (Fig. 1). Thus, although the evi- dence for monarch migration in Cali- fornia is scanty, there is objective evidence for northerly spring migra- tion and southerly autumn migra- tion, much as monarchs east of the Rocky Mountains have been found to migrate.

Evidence from other populations Wenner and Harris challenge the

accepted dogma that the western population of monarchs can be readily compared with the eastern population, from which they appear to be separated by the Rocky Moun- tains (Fig. 1). There is no scarcity of evidence for migration of the eastern population and because monarchs in California overwinter in dense aggregations on coastal trees - much like the huge aggregations of

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millions of monarchs that overwinter in Mexico - both populations are thought to migrate annually to over- wintering sites separated in time and space from milkweed-rich breeding areas*.

Urquhart has documented south- westerly autumn movements of eastern monarchs of up to 3000 km from the northern USA to high alti- tude overwintering sites in fir forests on the transvolcanic mountains of central Mexico* (Fig. 1). Similar long distance flights in spring show that monarchs remigrate north-east- wards for up to 3500 km from over- wintering sites to breeding sitesg, suggesting that some monarchs may be capable of migrating over a round trip of up to 6500 km.

A new chemical ‘fingerprinting’ technique has shown clearly that 85% of monarchs overwintering in Mexico at 19”N contain particular cardenolides (cardiac-active ster- oids) that they can only have taken from their larval host plant Asclepias syriacal (the commonest North American milkweed that does not grow further south than 35”N). Therefore the monarchs overwinter- ing in Mexico must have migrated south in autumn from the Great Lakes area (where A. syriaca is most abundant) to Mexico. These same monarchs then remigrate north in the spring to recolonize early spring, southern US milkweeds*.

Monarchs overwintering in Mex- ico, like those in California, are also in excellent condition and carry enor- mous fat reserves of up to 50% body weightzsll. Autumn migrants fuel their migration, and up to five months of overwintering, by exten- sive feeding on superabundant floral nectar sources along the coast of the Gulf of Mexico, en route for Mexico*.

Thus it seems that the western and eastern populations of monarchs are remarkably similar in their seasonal long range movements and the nature of overwintering. Wenner and Harris are right to highlight the paucity of information on western monarch movement, but the extra- polation from results gathered in one southern Californian county, to sug- gest that Californian monarchs do not migrate, lacks parsimony. Range expansion and contraction is not a viable alternative to migration be- cause random movements from coastal overwintering sites to track the seasonal appearance and dis- appearance of widely distributed lar- val milkweed resources would be strongly selected against. Such un- directed movements would make milkweed species with particular habitat and temporal distributions very hard to locate. Since monarchs are not freeze-tolerant, no mechan- ism would exist for monarchs any- where in the expanded range to locate predictably sites in which they could survive temporal gaps in larval resources. Monarchs would then either evolve freeze tolerance or be restricted to coastal milkweeds, near suitable overwintering sites. But monarchs are not restricted to the west coast, they are not freeze- tolerant, and they do move long dis- tances. Thus - using any of the cur- rent ecological or behavioural defini- tions of migration1*,13 - monarchs both west and east of the Rocky Mountains migrate in space between areas of larval milkweed resources and overwintering sites.

Rather than denying monarch migration, the observations of Wen- ner and Harris are best appreciated in the light of other similar observa- tions of monarch winter breeding in southern California14, Arizona’s, southern Floridal6, and south- eastern Australia”,l8. In Australia, James’* has shown that warm temperatures inhibit long-distance flight of monarchs but stimulate ovarian development, and cool temperatures inhibit ovarian de- velopment while facilitating long- distance flight. In these benign cli- mates, without freezing winter ex- tremes, monarchs are most likely to

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display the mixed life history tactic that Wenner and Harris have observed, with plastic physiological and behavioural responses to weak abiotic cues.

Day length is also likely to be associated with monarch migration since the butterflies migrate north in March and south in September, at the two times when rates of day length change are maximal. Such timing suggests that day length changes may offer directional cues to monarch migration. Monarchs in the northern hemisphere could re- spond to day length by migrating north when it is increasing and south when it decreases.

Research currently in progress at the Los Angeles County Natural His- tory Museum on monarch move- ment and at the University of Califor- nia, Santa Cruz, on Californian monarch breeding and overwinter- ing biology (both presented at the Los Angeles meeting) will help to understand the latitudinal variation in interactions between monarchs and their breeding and non-breeding habitats. Wenner and Harris are ideally situated to contribute to this effort by examining the plasticity of the monarch’s life history in south- ern California.

Migration and conservation This controversy has had most im-

pact on the conservation efforts of two organizations: The Monarch Project (of the Xerces Society) in California and Monarca AC in Mex- ico. Not surprisingly, the overwinter- ing sites are the main focus of con- servation efforts in Mexico and Cali- fornia since these small areas of fra- gile high montane and coastal forest are increasingly threatened by man’s activities. In Mexico the threat comes from the depredations of a local sub- sistence enonomy; in California, property speculators and developers call the tune of overpriced land use.

Although aggregations of thou- sands of overwintering monarchs have been known and admired for more than a century in California, it is the Mexicans who have achieved the most notable success in conserv- ing overwintering sites by gov- ernmental decree3,1g; despite the fact that the enormous Mexican overwintering colonies of hundreds of millions of butterflies were only ‘discovered’ 12 years ago by Brugger and Brugger working with Urquhartzo.

In contrast, the Californian con- servation effort is fraught with dif- ficulties since most monarch roosts are on private land. Mexican

monarch roosts are on common land, or ejidos, in a subsistence enonomy that includes grazing rights and the use of forest wood for fuel. Thus the development of a cash enonomy is a comparatively effec- tive and inexpensive way to compen- sate the ejidatarios for the loss of their land rights.

Of course in California, land with a view of the sea commands such high prices that no conservation, or even federal, organization could ever afford to buy large areas for protec- tion. Noble efforts are being made by The Monarch Project to persuade property owners to sign ‘easements’ in which they give up the rights to manipulate trees in which monarchs overwinter. These efforts may be threatened by suggestions that monarchs do not migrate. The past three years have witnessed the des- truction of 7 of the 45 known perma- nent coastal Californian overwinter- ing sites (C. Nagano, pers. com- mun.), and an eighth site is at pres- ent being transformed from mon- arch trees to avocado pear trees. The new suggestions will be seized upon by developers as an excuse for further destruction.

Biologists working on monarchs in Canada, USA, Mexico and Australia should perhaps consider carefully the implications of their research in- terpretations for the conservation of this international insect traveller.

Right: autumn mtgrant in Texas, en route for Mexico. Below: sun-basking monarchs overwintering in Mexico. Photographs by Tonya van Hook.

References 1 Johnson, C.G. (1969) Migration and Dispersal of lnsecfs by Night, Methuen 2 Brower, L.P. (1985) Contrib Mar. Sci Suppl. 27,748-785 3 Norman, C. (1986) Science 233, 1252-1253 4 Tuskes, P.M. and Brower, L.P (1978) Ecol. ErVomol. 3,141-153 5 Urquhart, F.A. (1960) The Monarci~ Butterfly, University of Toronto Press 6 Urquhart, F.A. and Urquhart, N.R. (1977) Can. Entomol. 109,1583-1589 7 Baker, R.R. (1978) TheEvolutIonary Ecology ofAnimal Migration. Holmes & Meier 8 Urquhart, F.A. and Urquhart, N.R. 11978) Can. J. Zoo/. 56,1759-1764 9 Urquhart, F.A. and Urquhart, N.R. (1979) Can. Entomol. Ill, 15-l 8 10 Seiber, J.N., Brower, L.P., Lee, SM.. McChesney, M.M., Cheung, H.T.A., Nelson, C.J. and Watson, T R 11986)

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J. Chem. Ecol. 12,1157-l 170 11 Brower, L.P., Calve& W.H., Hedrick, L.H. and Christian, J. (1977) J. Lepid. Sot. 31,232-242 12 Taylor, L.R. and Taylor, R.A.J. (1983) in The Ecology ofAnimal Movement (Swingland, I.R. and Greenwood, P.J., eds), pp. 181-214, Clarendon Press

13 Kennedy, J.S. (1985) Contrib. Mar. Sci. Suppi. 27,5-26 14 Urquhart, F.A., Urquhart, N.R. and Munger, F. (1970) J. Res. Lepid. 7, 169-181 15 Funk, R.S. (1968) .I. Lepid. Sot. 22, 63-64 16 Brower, L.P. (1961) Ecoiogy42,76-83

17 James, D.G. (1981) Gen. Appi. Entomoi. 13,47-53 18 James, D.G. (1983) Aust. J. Zooi. 31, 491-498 19 New York Times (editorial), 1 September, 1986 20 Urquhart, F.A. (1976) NatiGeogr. 150, 161-173

Population Biology and the Management of Fisheries D.H. Cushing

The successful management and ex- ploitation of fisheries requires de- tailed knowledge of the population dynamics and life histories of com- mercial species, and of the factors affecting recruitment and mortality. Much progress has been made in recent years, exemplified by the re- port of the 1984 Dahlem Workshop on the exploitation of marine com- munities, and by subsequent re- search discussed in this article. In some fisheries at least, management decisions can now have a firm scien- tific base.

In April 1984, Robert May invited to a Dahlem Workshop’ ‘some peo- ple knowledgeable about fisheries’ and ‘some unversed in fishery lore (but including population geneti- cists, community ecologists and re- source economists) to think about problems in the exploitation of marine resources’. There were four themes: the dynamics of single populations; the dynamics of sys- tems with many species; manage- ment under uncertainty; and multi- species management. In this article I examine some of the discussions of the Workshop, and comment on some important subsequent developments.

Recruitment, mortality and density dependence

The central problem of fisheries research is the generation of recruit- ment and to determine the effect of the parent stock upon this process - the traditional stock-recruitment re- lationship. The Workshop believed that the density-dependent proces-

D.H. Cushing was formerly at the Fisheries and Food Fisheries Laboratory, Ministry of Agricul- ture, Lowestoft, UK. Present address: 198 Yar- mouth Road, Lowestoft, Suffolk NR32 4AB, UK.

ses take place after metamorphosis or on the nursery ground. There were two reasons for this belief. First, metamorphosed plaice settle from a volume to a surface and so the opportunity for predation is greater. Bevertonl, showed that the mortality of nursery ground plaice increased with increasing density to a plateau (based on observations from three or four stocks; the incre- ment in mortality with density in each stock was much less than the total for all). The second reason was that recruitment was not predictable from bongo net samples of fish lar- vae (a bongo net comprises two long plankton nets which, however, do not sample the later larvae very well).

More recently, Ware and Lambert* have shown that the mortality of mackerel larvae in a small bay in Nova Scotia depends on their initial density, so density-dependent mor- tality can occur during larval life. Cushing3 showed that because had- dock larvae have a narrow range of food size preference they must grow with their food cohorts, and that they are not too dilute to affect the density of their food; hence, in the late larval stages the larval mortality could well be density-dependent. Van der Veer4 found that an index of recruitment in southern North Sea plaice can be linked to the numbers of stage 4b larvae (the last larval stage before complete metamorphosis) flooding in the plankton into the Waddensee, the mudflats on the northern coast of Holland. These papers suggest that the essential processes may start somewhat earlier in the life history than was thought at the Workshop.

Shepherd and Cushing5 noted that in many exploited stocks recruitment was not reduced despite the fact that fishing mortality (F) exceeded natu- ral mortality (M) by a factor of three or more. In a stock-recruitment curve, the relationship between the

number of recruits and the stock biomass is a measure of mortality at stages from eggs to recruitment. If the recruitment is not reduced at the point where the line (F=3M+) in- tersects the stock-recruitment curve then the coefficient of density- dependent mortality is high (>l.O) in the unexploited stock. Van der Veer4 shows that at the time of settlement metamorphosed plaice are eaten by shrimps, and that the post- settlement mortality is not density- dependent. The mortality observed in Beverton’s relationship in each of the three or four stocks is either that at settlement or after it (which would then conflict with van der Veer’s observation). On average, the cumu- lative density-dependent mortality at the time of settlement in van der Veer’s results amounted to 0.56 - probably enough to satisfy the cri- terion quoted above when the ex- ploited stock is very much less than the unexploited one. Hence it is possible that the essential processes are completed a little earlier in the life history than had been thought.

The question arises whether the generation of recruitment is a pro- cess separate from that of popula- tion regulation or whether there is a single process that controls each. One might imagine that environmen- tal factors operate during the larval stages (which would account for the rather rough links between recruit- ment and climatic events) because the effect of climate is upon the production cycle6; then density- dependent mortality would operate at a later stage in the life history. For example, Roughgarden’ showed that the settlement of barnacles on an area of rocky shore near San Francisco was governed by the amount of free space available. Pre- sumably if larval dispersion away from the rocks was high, the oppor- tunity for density-dependent mortal- ity would be low and vice versa, as if there was indeed a two-stage pro- cess (shown by Elliott’ for the sea trout).

Another thesis states that when the larvae grow at less than their maximal rate due to food shortage (because of environmental factors) their cumulative mortality, by preda-

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