Abstract
Life history theory states that selection acts upon individuals to produce optimal reproductive success, through maximising number of recruited offspring. Selection pressures vary across different environments and subsequently produce a wide variation in optimal life history, resulting in variation of life history traits. Previous research has identified that life history variation can occur across large scale latitudinal and elevational gradients and between small scale allopatric populations. Research concerning the diets and life history variation at early life stages, particularly in fish, is severely limited. This thesis investigates the importance of diets and resource availability on the observed life history variation between four species of the ‘Galaxias vulgaris complex’; G. anomalus, G. eldoni, G. depressiceps and G. pullus across an altitude gradient and between life history categories at the larval life stage.
Diet analyses displayed that Chironomidae were the most prevalent food item, which accounted for 62% of the observed diet contents, followed by Copepoda (20%) and Algae (8%). Chironomidae commonly dominate small freshwater fish diets, and as predicted, larval galaxiid populations displayed a correlation between larval galaxiid length and prey item size, as expected in gape-size limited fish. Additionally, species was observed to be a much poorer predictor of prey item length than either larval length or site. Galaxiids also displayed higher selectivity when exposed to extreme resource abundance. However, there were no significant trends observed between galaxiid diets and elevation or life history categories. Conversely, invertebrate communities displayed significantly greater diversity at fast life history populations than intermediate life history populations. Additionally, diets consistently displayed significantly lower diversity than their respective invertebrate community.
This research highlights the importance of investigating multiple selection pressures when attempting to discern causal effects of life history variation. It is likely that, while resource abundance is important in defining life history, other selection pressures, such as disturbance and predation may be acting as stronger selection pressures upon populations of larval galaxiids.