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Comparison of the in-stream fauna and resources of Tasmanian river reaches lined with willows or with other riparian types


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Read, M 1999 , 'Comparison of the in-stream fauna and resources of Tasmanian river reaches lined with willows or with other riparian types', PhD thesis, University of Tasmania.

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The widespread distribution of willow trees (Salixfragilis) has been thought to impact
deleteriously on in-stream faunas in south-eastern Australian rivers. This thesis aimed to
address some of the speculation in the literature regarding the impacts of willows through
three main research areas. Firstly, a survey was used to compare riparian function of willow
vegetation to native riparian vegetation and associated impacts on macroinvertebrate
populations. Secondly, the same approach was used to examine differences in
macroinvertebrate and fish populations between willowed vegetation and reaches where
willows has been removed. Finally, the role of willow large woody debris (LWD) in
Tasmanian rivers was investigated. This involved a census of large woody debris standing
stocks in 142 reaches on Tasmanian rivers. The ecological role of willow LWD was
investigated via a comparison of in-stream native wood to willow wood and the associated
effects on macroinvertebrate and fish populations. In this thesis, large woody debris (LWD)
refers to large organic woody material defined conventionally as greater than 1.0m in length
and 0.1m in diameter (Gippel, 1995).
The principal effects of willow vegetation on the biota occurred in summer and were due to a
combination of shading effects and decreased water quality and alterations to channel
morphology in willowed reaches. While reaches in native riparian zones supported higher
densities and numbers of taxa, these were significantly lower in willowed reaches. A slight
effect was observed in autumn as macroinvertebrate diversity in willowed reaches was lower
than native reaches. I concluded that willows act as a poor surrogate for native riparian
vegetation. Comparisons between willowed reaches and reaches where willows had been removed
revealed major differences in resources derived from riparian vegetation. Willowed reaches
had high organic matter standing stocks and usually low epilithic growth on the substrate. In
contrast, removal reaches had lower organic matter standing stocks and higher epilithic
biomass. The macroinvertebrate populations reflected these differences. Although no differences were observed in summary variables such as density or taxon number, differences
were found between functional feeding groups. Groupings generally reflected the food sources
available in either a vegetated reach with a high organic input and a dense canopy or a nonvegetated
reach with no canopy, higher incidental sunlight and therefore a denser epilithic
cover. A separate study revealed that in extreme situations willowed reaches are severely
impacted with a large decline in water quality and high organic standing stocks eliminating
most intolerant taxa. Fish populations at these sites were also depauperate, while at remaining
sites fish species showed a strong relationship with their preferred habitat. Census estimates of woody debris revealed that rainforest vegetation has the highest standing
stock of LWD across a spectrum of ripariap. types. Usually removal of woody native
vegetation often in concert with active removal of in-stream L WD accounts for lower wood
loadings in the Tasmanian rivers surveyed. Willow LWD is not common in rivers in
Tasmania and is a poor ecological substitute for the more complex native debris, which
supported higher densities and richness of macro invertebrate taxa than willow wood;
however, both wood types supported similar community composition. LWD provided
important habitat for the fish populations surveyed and reduced or negligible standing stocks
of LWD corresponded to a reduction in the number and size of particular fish species.
The findings confirm some of the speculations regarding the impact of willows on rivers in
south-eastern Australia. Wiflows were found to be a poor surrogate for native vegetation
although they provided important riparian resources in the absence of any vegetation at all.
The restoration of riparian zones and selective and strategic removal of will owed vegetation
over the long term and replacement with endemic vegetation should minimise the ecological
impacts of riparian vegetation removal on macro invertebrates and fish.

Item Type: Thesis - PhD
Authors/Creators:Read, M
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