THE VARIED ROLES OF SNAILS - National Universities Commission

d. LIFE CYCLE & LIFE BUDGET SUDIES:
Other important things involved in the ecology of snails
include growth rates, key factor analyses, capacity for
increase, net reproductive rate that constitute Life Budget
studies. Iheagwam and Okafor (1984) collected long term
data on the numerical and production changes in Bulinus
globosus and Lymnaea natalensis (gastropods). They
followed the population dynamics and production of many
consecutive cohorts of the snails and found roles for
density and habitat quality in controlling reproductive rates
in the gastropods. They concluded that horizontal (cohort)
life tables were statistically acceptable in such studies and
encouraged its use in the study of other species of
gaqstropods.
To calculate cohort production, the life cycles are often
divided into time intervals while for production in the
snails it will be subdivided into age intervals. Such life
tables are prepared by following the survival of consecutive
cohorts of snails over their life span. Egg production
estimates are used to calculate the net reproductive rates
(RO). From such life tables information on age specific
survivorship rates (LX), age specific fecundity rates (MX)
and mortality rates(qx) are also obtained for each species.
Analysis of the brood years performances usually show a
positive linear correlation. The degree of linearity of the
data collected often suggests how the carrying capacity of
the habitat is declining.
Survivorship data suggest the type of regulation of the
brood size which in itself narrows the number of survivors
at maturity. The survival pattern when integrated with the
reproductive rates often assesses the success or failure of
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