Madhav Gadgil

Life historical consequences of natural selection

The tremendous variation in the life-history patterns of organisms is best explained as adaptive.any organism has a limited amount of resources at its disposal, and these have to be partitioned between reproductive and nonreproductive activities. A larger share of resources to reproductive activities, that is, a higher reproductive effort at any age, leads to a better reproductive performance at that age; this may be considered a as profit function. This reproductive effort also leads to a reduction in survival and growth and consequent diminution of the reproductive contribution of the succeeding stages in the life history; this may be considered as a cost function. Natural selection would tend to an adjustment of the reproductive effort at every age such that the overall fitness of the life history would be maximized. A model of life history processes has been developed on the basis of these considerations. It leads to the following predictions: 1. If the form of the profit function is convex, or that of the cost function concave, the optimal strategy may be to breed repeatedly. Otherwise, the optimal strategy is to breed only once in a suicidal effort like a salmon (big-bang reproduction). 2. The value of reproductive effort continuously increases with age in the case of repeated reproducers. 3. If all the stages in the life history following a certain age are adversely affected, the age of reproduction will tend to be lowered in the case of big-bang reproducers, and the reproductive effort at all ages preceding that stage will tend to increase in the case of repeated reproducers. 4. As the reproductive potential increases with size at a slower rate, reproductive effort will be lower at maturity, reproductive effort will increase at a higher rate with age, and growth will continue beyond maturity. 5. A uniform change in the probability of survival from one age to the next at all ages would have no effect by itself, on the age of reproduction in big-bang breeders or on the distribution of reproductive effort with age in the repeated reproducers. 6. Such a change in survivorship would lead to a change in the equilibrium density of a population. If the population is resource limited, this would affect the availability of resources to the members, of the population in such a way that an increase in mortality would increase the availability of the resources. 7. For a resource-limited organism a greater availability of resources would lead to a lowering of the age of reproduction in the case of the big-bang breeders, and to a greater reproductive effort at ail ages for the repeated breeders.

THE CONCEPT OF r- AND K-SELECTION: EVIDENCE FROM WILD FLOWERS AND SOME THEORETICAL CONSIDERATIONS

The relationship of the demographic parameters of a population to its ecological niche constitutes one of the central problems of population biology. A most interesting theoretical notion pertinent to this problem is r- and K-selection (MacArthur 1962; Cody 1966; MacArthur and Wilson 1967; Hairston, Tinkle, and Wilbur 1970; Roughgarden 1971). The central idea of r- and K-selection is that populations living in environments imposing high density-independent (D.I.) mortality (r-strategists) will be selectively favored to allocate a greater proportion of resources to reproductive activities at the cost of their capabilities to propagate under crowded conditions, and conversely, populations living in environments imposing high density-dependent (D.D.) regulation (K-strategists) will be selectively favored to allocate a greater proportion of resources to nonreproductive activities, at the cost of their capabilities to propagate under conditions of high D.I. mortality. From the argument just stated, it may be deduced that the birth rate of an r-strategist will be greater than that of a related K-strategist. However, increased birth rate under conditions of high D.I. mortality is not sufficient evidence for an r-strategy, because, as demonstrated later in this paper, any increase in D.I. mortality must by itself produce a new equilibrium of birth and death rates at higher values of both. The crucial evidence needed for r- and K-selection is whether an organism is allocating a greater proportion of its resources to reproductive activities (r-strategists) than another related one (K-strategist) under any and all D.D. and D.I. mortality conditions.

Dispersal: Population Consequences and Evolution

Most animal and plant populations are divided into a number of local populations with some dispersal of individuals from one site to another. A theoretical investigation of the phenomenon of dispersal suggests the following consequences: Isolated and poorly accessible sites will tend to become less crowded than an average site as a result of dispersal. An episode of dispersal will result in uneven crowding at the various sites. Variation in the degree of crowding resulting from dispersal will depress the total population size of a species over its entire range. Variation in the carrying capacity with time will lead to an analogous depression of the mean population size. Spatial variation in the carrying capacities of the sites will favor a sensitive response leading to a rapid increase in the emigration rate with crowding, while variation with time will disfavor a response very sensitive to crowding. Variation in space will favor the emigration of a small fraction of the population, while variation in time will favor the emigration of a larger fraction.

Male Dimorphism as a Consequence of Sexual Selection

A number of insect species and red deer possess two forms of males differing from each other in the extent of development of devices involved in competition for females. Such forms may represent genetically based alternative strategies, one form being inferior in combat but wasting little energy in developing expensive weaponry, the other form being superior in combat but burdened with great energy expenditure to achieve this superiority. The return on investment in weaponry for the latter form depends not on the absolute value of investment, but on the extent of investment relative to the other forms present in the population. Such coevolution leads to an escalation of investment in devices of male competition. This costly arms race comes to an end when those investing in weaponry are just as well off as those which have totally opted out of such investment. Such a mechanism could precisely equalize the selective advantages of the two alternatives. Such coevolution is therefore a possible mechanism for the maintenance of a genetic polymorphism.

Growth Form and Reproductive Effort in Goldenrods (Solidago, Compositae)

Based on Gadgil and Solbrigs (1972) ideas concerning the distribution of resources between the reproductive and vegetative tissues, it is predicted that the ratio of reproductive biomass/total biomass (reproductive effort) will decline with the increasing successional maturity of the community. This prediction was confirmed in field studies utilizing four species of goldenrods. The distribution of resources among the various vegetative tissues should depend on the nature of the limiting factor and the growth form of the dominant plants competing for this limiting factor. Thus, it is predicted that the ratio of stem biomass/total biomass will decline and that of leaf biomass/total biomass will increase for light-limited plant populations as the growth form of competitors changes from being one of the same stature (herbs with herbs) to being one of greater stature (herbs with trees). These two predictions are likewise confirmed in goldenrods. The problem of how these differences in the distribution of resources among tissues are maintained in closely growing populations of a single species was examined. It was found that populations of a given species bloomed at different times, thus reducing the gene flow between these populations. This indicates some degree of isolation which could allow for the maintenance of ecotypic variation.

Traditional Ecological Knowledge, Biodiversity, Resilience and Sustainability

Much of the world’s biodiversity has been in the hands of traditional peoples, societies of hunters and gatherers, herders, fishers, agriculturists, for great many generations. Most living resources of the earth have been utilised for a historically long time; exceptions are few (e.g., open-ocean and deep-sea species). As Gomez-Pompa and Kaus [1990] observed, even tropical forests of the ‘Amazon were not untouched environments but the result of the last cycle of abandonment’ by traditional users. The fact is that pre-scientific, traditional systems of management have been the main means by which societies have managed natural resources over millennia [Berkes and Farvar, 1989; Gadgil et al., 1993]. Biological diversity has persisted despite, and in some cases because of, these systems of management so that we have any biodiversity today to speak about.

Male-female differences in foraging on crops by Asian elephants

Males and females may differ in morphology or behaviour because of contrasting factors affecting their reproductive success. In polygynous mammals with a marked sexual dimorphism, males are more likely to exhibit risky behaviour promoting reproductive success (Trivers 1985). In this study the authors present evidence that pubertal and adult male Asian elephants, Elephas maximus (above 15 years) incur greater risks than female-led family herds by foraging on cultivated crops which have more nutritive value than wild food plants.

NEW MEANINGS FOR OLD KNOWLEDGE: THE PEOPLE'S BIODIVERSITY REGISTERS PROGRAM

The program of Peoples Biodiversity Registers (PBR) is an attempt to pro- mote folk ecological knowledge and wisdom in two ways: by devising more formal means for their maintenance, and by creating new contexts for their continued practice. PBRs document folk ecological knowledge and practices involving the use of natural resources, with the help of local educational institutions and NGOs working in collaboration with local, decentralized institutions of governance. During 1996-1998, 52 such documents were prepared from village clusters distributed in eight states and union territories representing a wide spectrum of ecological and social regimes of India. They reveal a picture of generally declining productivity and diversity of living resources outside of intensively managed ecosystems. There are, however, notable exceptions; two of our case studies provide ex- amples of self-organized systems of management that have successfully protected, and indeed promoted, restoration of forest and wildlife resources. The PBRs also indicate a widespread erosion of practical ecological knowledge and of traditions of sustainable use and conservation. This is linked to the fact that those most intimately dependent on and knowledgeable about biodiversity belong to the economically and politically most disad- vantaged segments of the society. In consequence, conservation and sustainable use of biodiversity are not a high priority among the development aspirations held by the people. Nevertheless, people are concerned about degradation of the base of living resources and offer a number of concrete suggestions on their management. In fact, in a few cases, the PBR exercises have encouraged people to put such measures for more prudent use of local biodiversity resources into practice. The process of preparation of PBRs, as well as the resultant documents, could serve a significant role in promoting more sustainable, flexible, participatory systems of management and in ensuring a better flow of benefits from economic use of the living resources to the local communities.

The sacred groves of Western Ghats in India

Then ignoring the pleadings of the king, she wandered into the sacred grove of Kumara. Her mind bewildered by the curse of her Guru, she failed to notice this transgression into an area forbidden to women. No sooner did she enter, than she was transformed into a vine clinging to a tree at the boundary of the grove. Kalidasa in Vikramorvasiyam (c. 300 A.D.)

Carbon flow in Indian forests

The present study estimates the net emission of carbon from the forest sector in India. For the reference year (1986), the gross emission from deforestation in that year, plus committed emissions from deforestation in the preceding years, is estimated to be 64 × 106 t of C. The carbon sequestration (or net woody biomass accumulation in trees for long-term storage) from the area brought under tree plantations and the existing forest area under forest succession is estimated to offset the gross carbon emission in India, leading to no net emissions of carbon from the forest sector. Medium-term projections for India (for the year 2011) show that under a ‘business as usual’ scenario at current rates of afforestation, projected carbon emissions would continue to be balanced by sequestration.