ecology of phasmids - KLUEDO - Universität Kaiserslautern

Life history & potential population growth 36
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Equation 3-3 F x = F d L f a f
where F d = Mean number of fertilized eggs produced per day and female
L f = Mean adult lifetime of females
af = Number of females
For the calculation of ΣFx knowledge about the sex ratio of a population is inevitable. Female-biased
populations will result in higher values for the net per capita rate of increase R and hence in faster
growth. Replacing ΣFx in Equation 3-2 by F d L f a f shows that the proportion of females in the
M. diocles lab population (estimated by the sex ratio) is included by the term af/a0.
The outlined model in Equation 3-1 describes the infinite exponential population growth in the absence
of limiting factors. Such hypothetical populations do not reflect biological facts. Factors affecting
population growth can act on every life stage of an organism. Here, I assessed the effect of limiting
factors in the first life stage. More precisely, I estimated egg mortality due to fungal infestation and
failed hatching. A reduction in hatching will result in a lower net per capita rate of increase R (or basic
reproductive rate R0 respectively) because a0 > ax and fewer females enter the reproductive adult stage.
The population will still grow exponentially but the proportion of eggs that fail to hatch decelerates the
increase. This model assumes that failed hatching is constant.
3.3 Results
3.3.1 Demographic population parameters and life history traits
In the M. diocles lab population sex ratio was balanced (females : males = 1.01; Table 3-1). Females and
males showed a pronounced sexual dimorphism, differing significantly in size and weight. In average,
females weighed more than three times more and were ca. 20 % longer than males (Table 3-1).
Consistently, it took females approximately 8 days longer to develop from nymph to adult, resulting in a
total developmental period of 108 days (Table 3-1).
Mean adult lifetime differed significantly between sexes, with females living approximately 13 days
longer than males (i.e., 60 vs. 47 days; cf. Table 3-1). Generally adult lifetimes varied enormously
among individuals. At maximum an individual female tripled average adult lifetime and lived for 177
days. Likewise maximum lifetime for males was more than twice as long as male mean adult lifetime.
Females produced in average 0.66 eggs per day (Table 3-1). But individual females posed up to ten eggs
in one day. Mean egg production was modestly related to female weight (RS = 0.43; cf. Figure 3-1).
In average nymphs hatched after 71 days. Duration of egg development ranged considerably from 35 to
172 days but clearly peaked with 50 % of all nymphs emerging between days 68 and 74 after
oviposition (Figure 3-2).