The population biology of Crenobia alpina (Dana)
An investigation was carried out into the relationship between environment and reproduction in the freshwater triclad species. By studying two populations occupying adjacent habitats, under varying conditions of temperature, flow rate and resource availability, the effects of habitat variability on the di~ribution~ density, size structure and the levels of se>:ual and ase>:ual reproduction within each population were assessed. The population occupying a habitat which was characterised by its eurythermic temperature regime, high flow rates and complex macroinvertebrate community, occurred at low densities. Within this population, individuals were larger, on average, than in the adjacent population, and sexual reproduction occurred at high levels throughout the year, with asexual reproduction (by binary and multiple fission) also occurring throughout the yea~, but at lower levels. The seasonal nature of this habitat was reflected within the popUlation of , which exhibited spring peaks in density, followed by summer peaks in the level of sexual reproduction. In contrast, the adjacent population occupied a habitat which was characterised by its stenothermic temperature regime, low flow rates and a less complete macroinvertebrate community, and occurred at much higher densities. Within this population, individuals were smaller, on average, than in the adjacent population, and sexual reproduction was virtually absent, wi th asexual reproduction (by binary and multiple fission) occurring throughout the year at appreciable levels. The lack of seasonality within this habitat was similarly reflected in the lack of any seasonal fluctuations in density, or level of (in this case asexual) reproduction within the population. The level of food availability varied seasonally in both habitats, however, and was generally similar, in terms of biomass, in bath areas. A hypothesis was presented which of triclads within each habitat related the density to the total food availability, measured as stream drift. It was suggested that at low population densities, the relatively higher levels of net resource availability per i ndi vi dual favoured the occurrence of sexual reproduction, in contrast with situations of high population density, where the relatively lower levels of net resource availability per individual inhibited the occurrence of sexual reproduction, thus favouring asexual reproduction. This hypothesis was supported by the results of laboratory investigations in other studies, together with the evidence gained from field observations from this study, particularly the observation that in the low dem::.i ty population, seasonal cycles in the level of sexual reproduction were out of phase with seasonal cycles of a similar nature in the level of popUlation denSity. Further evidence, from the results of .a field manipulation experiment, was presented which supported the hypothesis. In an area of high popUlation denSity, density was reduced artificially, resulting in a significant increase in the level of sexual reproduction within the population. population, It was concluded that in the high density intense intraspecific competition for food resulted in a low net level of food availability per individual, which in turn inhibited the process of sexualisation in triclads from that area. The results from these two populations of ~~~!e~~~, indicating that net food availability controls the occurrence of sexual reproduction in this species, are in marked contrast to the findings of previous studies, in which habitat temperature is implicated as the dominant environmental influence on this process.