Molting, Growth and Reproductive Strategies in the Terrestrial Isopod, Armadillidium Vulgare

E-ISSN： 1939-9170|57|6|1179-1194

ISSN： 0012-9658

Source： Ecology, Vol.57, Iss.6, 1976-11, pp. : 1179-1194

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Abstract

The relationship between molting, growth and reproduction was studied in the terrestrial isopod, Armadillidium vulgare. Reproduction and growth are tied together in three major ways: (1) Among reproductive ♀ ♀ the growth rate is less than that of comparable nonreproductive ♀ ♀. This is because both the wt gain at ecdysis (initially H_O) uptake, but ultimately replaced by biomass) and the frequency of molting are less for reproductive ♀ ♀ than for nonreproductive ♀ ♀. This reduced growth associated with reproduction is most pronounced among the smaller ♀ ♀; (2) The estimated total energy devoted to both growth and reproduction among reproductive ♀ ♀ is about the same as the energy devoted only to growth among nonreproductive ♀ ♀ (3) Among individual reproductive ♀ ♀ there was an unexpected positive correlation between growth and reproduction: ♀ ♀ producing larger (size—specific) broods also gain more weight. This contrasts markedly with the clear reduction in growth in the comparison between reproductive and nonreproductive ♀ ♀. It is suggested that this apparently anomalous result may be explained by differential mortality. In natural field populations, large ♀ ♀ produce 2 broods/season and small ♀ ♀ produce only 1 brood/season. Using a simple model relating ♀ size to the number of offspring produced in either 1 or 2 broods, this difference in reproductive output between large and small ♀ ♀ can be explained as an adaptive response to the constraint imposed by the trade off between growth and reproduction. The predicted size at which individuals shift from a one—brooded to a two—brooded strategy is nearly identical to the observed size at which this change in reproductive output occurs in a field population. Using a simple model, the analysis of reproductive strategies is extended to variable environments. It is suggested that reproductive strategies optimizing fitness in a variable environment may destabilize the population's resistance to environmentally imposed disturbances and result in large numerical fluctuations.