Fluvial geomorphology as a predictor of ecological variation: downstream trends in bedload grain size and the effect upon invertebrate egg mass density along an upland mountain stream, Victoria, Australia

This research project examines the influence of downstream shifts in the size and density of emergent rocks upon the density and aggregation of invertebrate egg masses selectively laid upon emergent substrate along the Acheron River, Victoria, Australia. Hydrobiosid caddisflies are common insects in streams throughout Australasia. In the Acheron River, adult females of three hydrobiosid species (Apsilochorema sp., Taschorema sp., Ulmerochorema sp.) have been shown to exhibit strong egg laying (oviposition) patterns limited by the following physical parameters; females of all three species preferentially lay their eggs upon large, in-stream rocks emerging from the water surface and velocity of flow also greatly influences oviposition choice. It may be assumed that sediment in rivers typically fine in a downstream direction and because of this, the size, density and abundance of large, emergent rocks typically decrease along a downstream continuum. This results in a corresponding decrease in the number of potential oviposition sites as one moves along the longitudinal profile, which raises interesting questions regarding the density and population dynamics of these invertebrate species. For example, as one moves downstream, are greater numbers of female invertebrates forced to lay larger aggregations of egg masses upon the increasingly rare emergent substrate until substrate is so scarce that invertebrate egg masses can no longer be laid? Assuming that oviposition substrata is a limiting resource, may a positive relation between egg mass density and emergent rock density be expected to exist? Changes in median grain size and emergent rock density were investigated across 11 study sites spanning a 14km stretch of the Acheron River. Simultaneous egg counts for three hydrobiosid species were also undertaken at each site. As expected, median grain size and emergent rock density decreased along the river's longitudinal profile. Interestingly, correlational analyses revealed a positive correlation between emergent rock density (no./m2) and egg mass density (no./m2) for one hydrobiosid species (Apsilochorema sp.), while no significant correlations were seen between egg mass aggregation and rock density for all three hydrobiosid species. It appears that, at least during the autumn / winter study period of this project, the abundance of oviposition sites may possibly determine the abundance of hydrobiosid egg masses laid in each site. However, the lack of a discernible relation between the availability of oviposition sites and crowding of egg masses on individual rocks also suggests that egg mass saturation does not occur during autumn and winter. Upstream dispersal of adult females to sites with a greater number of emergent rocks is a possible explanation of the above trends. This speculation concurs with past research in the ecological literature, which suggests that local processes affecting the larval and adult stages of stream insects may act independently and at different scales, thus decoupling local population dynamics of successive generations. Further research, however, is required to test this hypothesis