|dc.description.abstract||Among the drivers of the patchy distribution of organisms in space and time are abiotic factors, including physical disturbance, and biotic interactions such as predation and competition. Our understanding of the interplay of these factors is far from complete, especially in frequently disturbed ecosystems.
The first of three main experiments investigated the individual and combined effects of flood disturbance and fish predation on benthic invertebrates, algae and leaf decay rates in a reach of the flood-prone Kauru River in Otago, New Zealand. Bed movements during a mid-sized flood were simulated by removing substratum from 16 scour patches, depositing substratum in 16 fill patches, and leaving 16 stable patches unchanged (patch size 0.49m2). Fish were excluded from half the patches using electric exclosures. (My first data chapter describes how this method was adapted for use in the Kauru.) The community on ceramic tiles was studied for 58 days. Local bed disturbance had many short-term and long-term effects on invertebrate and algal biomass distributions. Fish presence had far fewer effects but four of six significant interactions between disturbance and predation indicated a stronger impact of predation in stable patches.
I included competition among invertebrate grazers as a third factor in my second 43- day experiment in the Kauru River. The competition treatment consisted of twice-weekly removals of Potamopyrgus antipodarum from half the patches. Local bed disturbance affected the benthic community far more often than fish predation or snail grazing. The frequency of disturbance effects was highest shortly after the disturbance and decreased with time, whereas the few effects of biotic factors occurred on the last sampling date.
In my final experiment, I used streamside channels to investigate two topics not addressed in the reach-scale experiments. First, predatory fish are known to influence stream invertebrates not only by direct consumption, but also by altering their behaviour. Second, different fish predators can affect prey communities in different ways. I investigated the individual and combined effects of flood-related bed disturbance and predation by two dominant fish species. Bed movements caused by floods were simulated by tumbling the substratum in half the channels at the start of the experiment. Six channels each were stocked with trout or upland bullies or had fish excluded. Biological response parameters were determined 0, 14 and 28 days after the disturbance, and invertebrates in the entire channel substrata on day 28. Disturbance frequently affected a range of response parameters. Presence or absence of predatory fish, by contrast, had no significant effects on overall invertebrate standing stocks but affected invertebrate densities on surface stones in 40% of cases and invertebrate activity on surface stones in all cases. Native bullies featured more often than exotic trout in causing density changes and equally often in causing changes to grazer behaviour.
My combined experimental findings from a flood-prone river imply that in the presence of physical disturbance, biotic interactions may play a weaker role in determining the distribution of stream organisms than under stable conditions.||en_NZ