The ups and downs of life: population expansion and bottlenecks of helminth parasites through their complex life cycle.

The fundamental assumption underpinning the evolution of numerous adaptations shown by parasites with complex life cycles is that huge losses are incurred by infective stages during certain transmission steps. However, the magnitude of transmission losses or changes in the standing crop of parasites passing from upstream (source) to downstream (target) hosts have never been quantified in nature. Here, using data from 100 pairs of successive upstream-downstream life stages, from distinct populations representing 10 parasite species, we calculated the total density per m2 of successive life stages. We show that clonal amplification of trematodes in their first intermediate host leads to an average 4-fold expansion of numbers of individuals at the next life stage, when differences in the longevity of successive life stages are taken into account. In contrast, trophic transmission to the definitive host results in almost no numerical change for trematodes, but possibly in large decreases for acanthocephalans and nematodes, though a correction for longevity was not possible for the latter groups. Also, we only found a positive association between upstream and downstream stage densities for transmission involving free-swimming cercariae in trematodes, suggesting a simple output-recruitment process. For trophic transmission, there was no coupling between downstream and upstream parasite densities. These first quantitative estimates of ontogenetic rises and falls in numbers under natural conditions provide new insights into the selective pressures acting on parasites with complex cycles.