University of Tasmania

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Habitat complexity, spatial scale and grazing interactions in a temperate river

posted on 2023-05-27, 15:58 authored by Robson, Belinda
Complex habitat architecture is associated with high species richness and is thought to weaken trophic interactions between species. This thesis examines the effect of different levels of architectural complexity on the strength of the trophic interaction that exists between algae and invertebrate grazers in Mountain River, Tasmania. This river contains two major riffle types, with contrasting habitat architecture: bedrock platforms, which have a very simple architecture, and boulder-cobble riffles, which are very complex. Two spatial scales of habitat architecture were studied: microhabitats and riffles. The research strategy began with one year of stratified sampling of the riffle types and their microhabitats for invertebrates. Two manipulative experiments were then carried out to determine the effect of contrasting riffle scale architectural complexity on trophic interaction strength between epilithic algae and invertebrate grazers. Grazers were removed from some experimental plots and not others and the response of the algae was measured. This was followed by a large scale experiment that simultaneously compared microhabitat scale and riffle scale architecture using artificial substrata. Grazers were removed from half of the substrates and the response in algal growth was measured. To observe the role of habitat architecture in the response by invertebrates to spates, invertebrate densities were censused regularly before and after a small spate. The differences in habitat architecture between the riffle types were quantified using fractal geometry. Invertebrate body size spectra were constructed from the seasonal samples and related to changes in fractal dimension with scale in order to determine whether habitat architecture had controlled animal body sizes. The dominant invertebrate species were the same in the two habitats; however, species richness was higher in the more complex riffle type, and animal densities were greater in the simpler riffle type. More complex riffle scale habitat architecture reduced the effect of the grazer population on the algal population. Manipulation of habitat architecture at two spatial scales indicated that microhabitat scale architecture had a greater influence on species richness and density than riffle scale architecture, and also determined the outcome of competition between grazing invertebrates. Rock tops in the more complex riffle type were a flow refuge from low magnitude flood events. The boulder-cobble riffles had a greater average fractal dimension than bedrock and showed changes in fractal dimension with scale that were absent in bedrock platforms. These differences in habitat shape could be directly related to differences in the body size spectra of the two habitats. Habitat architecture at both spatial scales therefore had a significant effect on all the aspects of invertebrate community structure and function that were studied.


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Copyright 1995 the author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s). Includes bibliographical references (leaves 147-159). Thesis (Ph.D.)--University of Tasmania, 1997

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