Population Demographics in Species With Biphasic Life Cycles

Abstract

Abstract. We develop and test models for the population dynamics of species that undergo regular alternations of generations between independent, free-living, haploid and diploid phases. The models are patterned after the dioecious, haploid-diploid lifecycle of many marine algae. If the two phases have equal demographic rates, all models (with or without density dependence) predict a stable distribution with a ratio of :1 (ϳ60% ͙2 haploids and 40% diploids). We find that observable deviations from this distribution can occur when the demographic rates (mortality, fecundity) between phases vary widely. Field surveys of three macroalgal species with independent, free-living haploids and diploids, Mazzaella flaccida, M. laminarioides, and M. splendens, reveal a consistent pattern of haploid dominance that significantly exceeds this null model expectation (72%, 76%, and 66%, respectively). These species also exhibit significant variation in haploid frequency among sites. We estimated the per capita fecundity and mortality rates for each phase of both M. flaccida and M. laminarioides and used this information to address two basic questions: (1) Which parameter better explains the overall high relative haploid abundance for each species? and (2) Which parameter better explains the variation in relative haploid abundance among sites?Differences in haploid and diploid mortality rates did not explain the high relative abundance of haploids for either M. flaccida or M. laminarioides. However, differences in fecundity rates between phases were consistent with the high relative abundance of haploids for M. flaccida; diploids usually had significantly higher per capita fecundity rates than haploids. For M. laminarioides, there were no significant differences in fecundity rates between phases.For M. flaccida, among-site variation in relative abundance of haploids can be explained by differences in mortality rates. The relative abundance of haploids was positively correlated with the ratio of diploid:haploid mortality rates; the sites with the highest ratio of diploid:haploid mortality had the highest percentage of haploids. Differences in mortality rates for M. laminarioides, and fecundity rates for M. flaccida and M. laminarioides, could not explain the observed variation in haploid abundance among sites.

Responses of the haploid-to-diploid ratio of isomorphic biphasic life cycles to time instability

“…Dissimilarities between ploidy phases were found over the fecundity rates by Santos and Duarte [20], Scrosati et al [24], Thornber and Gaines [29] and Thornber et al [30]. These could be seasonal, herbivory dependent and could possibly be due to their different cytological processes of gamete/spore production.…”

“…Then, the ramets in Vandenberg should survive more, attain bigger sizes or get sexually mature later relative to those in Piedras Blancas. Thornber and Gaines [29] showed that a higher diploid fecundity was driving the average haploid dominance in Mazzaella flaccida, while their H:D spatial variability could only be explained by differences in mortality rates. Accordingly, Vieira and Santos [34] suggested that only through ploidy dissimilarities in survival may conditional differentiation drive an effective spatial niche partition.…”

Ecophysiological studies of the non-indigenous speciesGracilaria vermiculophylla(Rhodophyta) and its abundance patterns in Ria de Aveiro lagoon, Portugal

“…For G. vermiculophylla, Rueness (2005) described a higher abundance of vegetative individuals in European waters. The different latitudinal, biological and physical processes might explain these differences (Thornber & Gaines, 2004). The patches we investigated in the Ria de Aveiro exhibited high variation in reproductive status, with some being almost always vegetative and others having a predominance of carposporophytic individuals.…”

Susceptibility of spores of different ploidy levels from AntarcticGigartina skottsbergii(Gigartinales, Rhodophyta) to ultraviolet radiation

“…Analysis of variance (ANOVA) is presented in Table 2. phase dominance in most Gigartinales species and that of G. skottsbergii population in South America (Piriz 1996;Avila et al 1999;Faugeron et al 2004). Phase dominance of specific macroalgal life stage may result from differences in spore production between generations (Scrosati & De Wreede 1999;Thornber & Gaines 2004) or fertilization rate (Fierst et al 2005). In the triphasic life history of red macroalgae, the carposporophyte phase represents an adaptation to minimize the effect of both low and variable fertilization rates, where the cloning of the zygote amplifies the production ratio of carpospores relative to tetraspores (Searles 1980).…”