Wildlife Conservation in Bangladesh David J. Chivers

TREE vol. 1, no. 2, August 1986

In this overpopulated country dominated by rivers and coasts, eco- nomic needs channel zoological re- search into fisheries (and disease control) and invertebrates that affect crop production, as was reflected in the deliberations of the conference. Yet, there is increasing realisation of the importance of forests (as a sus- tained source of timber) and the birds and mammals that they har- bour, whose status reflects the ‘health’ of the forests.

confronted by serious obstacles.

Bangladesh is one of the most densely populated countries in the world; it is almost surrounded by India, with a short frontier with Burma in the south-east, and the Bay of Bengal in the south. It is situated in the world’s largest deltaic plain, with fertile alluvial soils between the rivers Padma (Ganges), Jamuna (Brahmaputra) and Meghna. There are forested hill tracts to the east and south-east. The monsoonal climate results in alternating seasons of flooding and drought, the severity of which is increasing with the clear- ance of forests (and the effects of the dams on the Ganges and Brahma- putra in India). Rice and fish are the most important crops, with jute, tea,

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BAY OF BENGAL

P 90 92

Fig. 1. Forested area; of Bangladesh. Shaded areas: more or less continuous forest cover. Stippled areas: scattered forest patches.

rubber and cottage industries pro- viding the main exports.

I attended the Fifth National Zoo- logical Conference, organised by the Zoological Society of Bangladesh at the University of Chittagong in March 1986. There were 108 papers accepted for presentation, reflecting the balance of research interests among Bangladeshi zoologists: 35 on fish, 22 on arthropods, 20 on crop pests, 10 on snails, worms, snakes, turtles and crocodiles, 11 on disease and only eight on birds and mam- mals.

David J. Chivers is at the Sub-department of Veterinary Anatomy, University of Cambridge, Tennis Court Road, Cambridge CBZ lQS, UK.

The forested area of Bangladesh (Fig. 1) has decreased from 13% to less than 7% in’the last few years’ because of the size and growth rate of the human population and re- quirements for fuel wood, timber and agricultural land! Since so little forest remains, the arguments preva- lent in other parts of South-east Asia for conservation in terms of climatic, soil and water balance and economic potential in terms of food, medicines and timber are rather weak. Never- theless, Bangladesh encompasses a rich variety of dryland (dipterocarp semi-evergreen forest in the Chitta- gong Hill Tracts; sal forest at Mad- hupur) and wetland (Sundarban) ecosystems and, being at the junct- ion of the Indian and Chinese sub- continents, it has an especially rich and unique fauna’.

There are four National Parks, ten Wildlife Sanctuaries and one Game Reserve in Bangladesh, but despite valiant efforts of the Forest Depart- ment staff, even these are not safe from encroachment and damage. There is an urgent need for a system of wildlife officers within the Forest Department, providing career oppor- tunities for the able, energetic and well-trained wildlife ecologists now emerging from the universities, as well as a focus on the needs of wild- life. There is little funding even for basic commodities of teaching and research; travel on wildlife matters depends on public transport, and equipment is limited and primitive.

It is important to conserve this diversity for economic as well as aesthetic reasons, and in particular to expand the reafforestation pro- grammefor sustained yields of fuel wood, thereby reduc- ing pressure on the remain- ing forests. Bangladeshi scientists are well aware of this need, and are striving to protect ecosystems and to identify the animals and plants that are valuable either as food or in the con- trol of pests of growing and stored crops. Funds are short, however, and nation- al priorities are not geared to effective forest conserva- tion.

The Wildlife Society of Bangladesh nevertheless provides a focus for the dedicated wildlife biologists from the universities, many of whom have filtered into other walks of life be- cause of the lack of career opportuni- ties. The Society is campaigning vigorously for more effective habitat protection and management, calling for a system of wildlife officers in the Forest Department and challenging government decisions. It is develop- ing projects on capped langurs (Fig. 2) and the Madhupur National Park, turtles and marine reserves, gharials and river conservation, and ele- phants and forest conservation. The Society is particularly concerned with education, especially in rural areas, as an aid to improved habitat

While it is right that the main scientific effort should centre on improving crop yields - of fish as well as plants - the role of the re- maining forests in environ- mental stability and as reservoirs of irreplaceable genetic material should not be neglected. There are Bangladeshi scientists work- ing in this direction, under the inspiration and guidance of the ornithologist Profes- sor K.Z. Husain, but they are in the minority and are

Fig. 2. The capped langur, Presbytis pileata, in sal forest in the Madhupur National Park.

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TREE vol. 7, no. 2, August 1986 reviews

protection and management. Those concerned with wildlife re-

search and conservation, howev.er, urgently need exposure to appropri- ate methods employed elsewhere in the world, funds from international agencies for personnel and equip- ment (including books, vehicles

and projectors), and the support of cannot afford to neglect. overseas bodies (preferably interna- tional) in representations to the Gov- ernment of Bangladesh about key References conservation issues. Bangladesh offers an intriguing array of ecosys-

1 Timm, R.W. (1981) Proc. ThirdNat/

terns and ecological problems, with Zoo/. Conf. Dacca, Bangladesh, 23-24 2 Prater, S.H. (1971) The Bookoflndian

prospects for resolution, which we Animals, Bombay Natural History Society

Sex Ratios in Geographically Structured Populations

Michael Bulmer

In his book on sexual selection’, Darwin documented evidence that the primary sex ratio (the proportion of males at concep- tion) is about % in a wide variety of species. Otherwise. he explained, a newly conceived member of the rare sex will, on average, have more offspring than one of the common sex, since each offspring has one mother and one father; thus there is frequency-dependent selection in favour of parents producing the rare sex. Darwin formulated this explanation in the first edition ( I82 I ) for monogamous species, but he failed to extend it to polygamous species, and in the second edition ( 1874) he retracted it completely. It was left to Fishefi to develop the theory in the more general form that there should be equal parental expenditure on the two sexes, allowing for the possibility that one sex may cost more to produce than the other. Despite the wide applicability of Fisher’s principle, recent work on sex ratio evolution has focused on situations where it breaks down3. Hamilton4 first pointed out that Fisher’s argument assumes population- wide competition for mates, whereas most natural populations have a geographical population structure in which limited dis- persal imposes constraints on mating pat- terns. What are the consequences for the sex ratio?

Local mate cornpetItIon Hamilton4 considered the follow-

ing model to investigate the latter question. The world consists of a large number of patches, each of which is colonised by n mated females; their offspring mate at random with each other within the patch, and the newly mated females disperse in population- wide competition for the n breed- ing sites available in each patch in the next generation. The essential

Michael Bulmer is at the Department of Biomathematics, 5 South Parks Road, Oxford OXI 3UB, UK.

feature of this model is that mating occurs before dispersal. The optimal sex ratio can be found by an argu- ment (Box I) based on inclusive fitness theory5.

Two forces can be identified leading to a female bias? ( I ) dimin- ishing returns associated with pro- ducing sons because of competi- tion between males for mates; (2) producing more daughters in- creases the mating opportunities of one’s sons if sib-mating occurs at random. The effect of the first force on its own can be studied in a model in which females lay their male eggs and their female eggs in different patches7 or in a model in which mating occurs after dispersal but only females disperse8m9. In both models the optimal sex ratio is (n- 1)/(2n- I), which is slightly less female-biased than equation (4) in Box I.

A more rigorous but more laborious analysis of an exact population genetics model leads to the same results as the above more heuristic inclusive fitness argumentlo. The two methods are in fact equivalent under weak selec- tion and additive gene action5.

Empldcal evidence Hamilton4 compiled a list of 26

haplo-diploid insects and mites which usually mate with their sibs and have an extreme female- biased sex ratio. The most bizarre is a mite which has one male and about I4 females in a batch of progeny; the male mates his sisters inside his mother and dies before he is born.

Box 1. Hamilton’s model for local mate competition. Suppose that most females produce M male

and F female offspring, while a rare mutant type produces m males and f females (with the same total offspring number). Since the mutant female will be in a patch with n- I normal females, her inclusive fitness, representing her genetic con- tribution to future generations, can be written as

W = vtrDMf + v,rs, [mjm. (11

The two terms represent fitness through produc- ing females and males, respectively; V+(V,) is the value of a female (male) in projecting genes into future generations; rDM(rsM) is the relatedness of a daughter (son) td her mother; and the term in square brackets is the average number of matings each male expects to get.

At equilibrium the fitness of any mutant type must be less than that of the normal type, so that W is at a maximum when m=M and f=F; hence SW/am = 6W/6f, evaluated at this point. Solving this equation we obtain as the optimal sex ratio

where

r. _ In-11 P 121 2n

P = 2v, rsMGvmrsM + vtrDMl 131

For diploids, Y, = v, and rDM = rs,,, by symmet- ry, so that p = I and the optimal sex ratio is

r* = (n - 1)/2n. (4)

Thus the sex ratio should be female-biased, the bias increasing as n decreases.

Most animals to which the model applies are haplo-diploid. sex being determined by whether or not the mother fertilises the egg, which makes it easy for the sex ratio to respond adaptively to selection. For haplo-diploidy vr = 2v,. while the ratio rsM/rDM varies with the level of inbreeding between 2 (no inbreeding) and I (complete in- breeding). In Hamilton’s model it can be shown that

p = (4n-21/(4n- I). (5)

Thus a slightly greater female bias is expected in a haplo-diploid than in a diploid species.

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