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Demographic studies on selected species of the genus Calamus L. (Arecaceae)

Demographic studies on selected species of the genus Calamus L. (Arecaceae)
P. V. Anto

2005

Department of Botany, University of Calicut, Kozhikode 673635, Kerala, INDIA.

ABSTRACT

Demography is the study of population changes and their causes throughout the life cycle. During the past three decades, the exploitation of rattans from the Western Ghats was very extensive resulting in the disappearance of rattans from all the accessible areas. Large scale destruction and fragmentation of the forest have further aggrevated the situation. In Kerala part of Western Ghats, rattans are restricted mostly to remote areas in the forests and some species are on the verge of extinction. Hence conservation measures have to be initiated urgently to protect this valuable natural resource.

Demographic and population ecological studies will help to analyse the changes that is taking place with in a population and to understand their regulating mechanisms. This understanding will lead to formulation of the management criteria for the species under study. Four commercially important species were selected for the present study, viz, C. thwaitesii, C. hookerianus, C. vattayila and C. delessetianus.

The main objectives were:
  1. To study the population structure and the changes taking place in it over a period of time and
  2. To develop conservation strategies for the species selected.
The study plots were selected in the natural forest areas in the Western Ghats at Athirapilly under Vazhachal Forest Division in Central Circle (Thrissur district) and at Nelliyampathy under Nemmara Forest Division in Olavakkode Circle (Palakkad district). Two rattan species, C. thwaitesii and C. delessertianus were common at Athirappilly forest areas with moist deciduous forests. At Nelliampathy, C. vattayila and C. delessertianus were common. The forest type was evergreen.

The number of individuals coming under different life stages namely seedling, juveniles, sub-adults and adults were taken periodically at four months interval for three years. The height, girth, number of individuals in each clump, production of new leaves, suckers etc were noted. Average internodal lengths of each species also were measured.

For each of the life stage, yearly transition probabilities Gi, yearly probabilities of surviving in the same stage, Pi, average yearly fecundities, Fij were calculated. These parameters were used to construct transition matrices and from these matrices, the finite rate of population increase λ. were calculated as the dominant eigen values and the stage distributions 'w', were given by the corresponding right eigen vectors. The elasticity values for the matrix coefficients were also calculated.

In order to study the population dynamics, recruitment and death rates were found out and population flux was calculated. Of the two species selected from a single forest type, even though the environmental conditions were the same, one species registered faster growth. C. thwaitesii from moist deciduous forest and C. delessertianus from evergreen forest were growing faster when compared to C. hookerianus and C. vattayila respectively. C. thwaitesii and C. delessertianus are large diameter rattans, and actually they need more time for elongation than the medium diameter rattans. But the shorter stage durations for C. thwaitesii and C. delessertianus shows that these two species are well adapted to the environment in which they grow. C. vattayila show short internodes at the basal region and this suggests a deficiency of light. In this species, a reversal of juvenile to seedling is very high which has no effect on transition but more influence on stage duration. The sub adult stage duration is very long here. C. delessertianus shows very high value of sexual fecundity when compared to other species.

C. vattayila is having the longest life duration followed by C. hookerianus and C. delessertianus. C. thwaitesii is having the shortest life duration.

In the matrix models, λ is the dominant eigen value, which indicates population growth rate. All the selected species except C. vattayila show a tendency towards increase in growth rate. The λ value of C. vattayila is less than one (0.94) which indicates a decline in population. In population studies, a decrease in adult survival rate is detrimental to population growth. In C. vattayila, there is a decrease in adult population. Hence this species can be considered as threatened.

The λ value is 1.21 for C. delessertianus which shows that it is the most successful species in its habitat.

Seedling mortality rate is high among the species seen in the moist deciduous forests when compared to the species in the evergreen forests. This shows that the seedling is shade demanders. The rate of adult recruitment is very high among the species seen in moist deciduous forests which shows that adult plants are light demanders.

When yearly probabilities of surviving and remaining in the same state (P values) were analysed for the four species studied, it was seen that the adult and the sub-adult stages were commonly critical in changing the finite rate of the population increase in the three species except C. vattayila. In C. vattayila, juvenile and adult stages influence the value.

Although sexual fecundity values of all species are higher, they have very little influence on finite rate of population increase. But the high elasticity of vegetative fecundity values of C. hookerianus has much influence on its population growth.

C. thwaitesii shows a normal life cycle pattern with regular transition from seedlings to adults. In C. hookerianus juveniles are the major population. Other than seedlings, suckers also contribute to juvenile population. But increased shoot density in a clump leads to competition which results in the death of a number of shoots reducing the number of sub-adults.

In case of C. vattayila and C. delessertianus, there were only limited number of adult plants which resulted in low recruitment rates. In C. vattayila, immature fruits are eaten by small mammals and birds. Single stemmed nature of these two species also is detrimental to the population.

The balancing of birth and death rate determines the size of the population. C. thwaitesii and C. delessertianus show maximum birth rate, but in C. vattayila the death rate is greater. There is no obvious seasonal variation in the recruitment and death of the species studied.

The regression reveals that in C. thwaitesii juvenile and sub-adult stages are more fitted for lineartrend among life stages in the population where as it is the juveniles and adult stages in C. hookerianus. In C. vattayila and C. delessertianus number of juveniles shows a linear trend.

In C. thwaitesii, the rate of increase in population is more than one which is an indication of the stable nature of the population. This shows that the population is very much adapted to its habitat.

In C. hookerianus, also the rate of increase in population is slightly greater than one indicating a stable population.

In C. vattayila, the rate of increase in population was less than one towards the end of the study period. This condition, if continued without any management programme, will lead to the population collapse and the species will become extinct in the near future.

C. delessertianus also shows a tendency to unstable population. But the population growth rate λ greater than one (1.2) in this species which shows that the population is actually increasing.

Both biotic and abiotic factors influence the population growth. The major abiotic factors are man, animals including insects and micro organisms.

In the present study, C. vattayila shows a declining population rate and very low survival probability. Hence drastic conservation measures should be taken for this species. In in situ conservation the juvenile stage of this species should be protected. Raising plantations is the best ex situ conservation method for this species. Since availability of mature seeds is a problem here, tissue culture techniques can be made use of to produce sufficient number of seedlings.

C. delessertianus also requires conservation strategies. Here the adult stage is to be protected. Since this is large diameter rattan this is extracted heavily. Here also ex situ conservation is a must.