unshaded reaches (Latta, 1 974; Lester, 1 992; Glova &Sagar, 1994) are inconsistent,
and further research is required in order to determine the possible impacts of the wholesale , .removal of introduced willows on river ecosystems.
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The most obvious effects of removal of will owed vegetation from river banks is the loss of
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shade and a reduction of local allochthonous organic inputs - two key functions of riparianL vegetation for rivers (Cummins, 1993). Furthermore, many ecologists predict that, with
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.. -1 reduction in riparian shading, the river should shift from autotrophy to heterotrophy (Minshall
· et al., 1983; Shields & Nunally, 1984) and, to varying degrees, the composition of the
invertebrate fauna should reflect this (Cummins, 1 974; Minshall et al., 1983; Gregory et al., 1991). This is supported by many studies that have compared the faunal communities of shaded forested reaches to unshaded reaches (Tuchman & King, 1993; Lena t &Crawford,
1994; Reed et al., 1994; Tait et al., 1994). The removal of riparian vegetation may also be expected to affect the dynamics and structure of tish communities. Many fish species require the cover and habitat functions provided by riparian vegetation, and this has been documented
in Australia (Koehn & 0' Connor, 1990; Davies, 1995) and elsewhere (Dolloff: 1986;
Penczak, 1995; Collares-Pereira e t al., 1997).
Despite these obvious differences in riparian vegetation function, little documentation exists specifically for differences between reaches lined by willows vegetation and those where willows have been cleared. Some studies of the in-stream fauna have involved comparisons
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between willowed reaches and cleared banks (Latta, 1974; Lester et al., 1 994a) or differing
densities of willows along river banks (Glova &Sagar, 1 994) in New Zealand. Both Latta (1 974) and Glova & Sagar (1994) concluded that dense plantings of willows along stream banks are likely to be detrimental to aquatic invertebrates and fish, but that moderate densities
. are likely to improve conditions compared to those experienced in nearby pasture sites. Lester
et al. (1 994a) found that willows had adverse effects on macroinvertebrate populations and they suggested that shading, and changes in water chemistry and substrate composition from willow root infiltration of the benthos may have been contributing factors.
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som,e of the hypothesised differences between willow lined reaches and reaches had been removed, I conducted a survey of seven medium-sized rivers in
; As in Chapter 2, I used a survey-based approach to document differences in the
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� fish, macroinvertebrate fauna, and key habitat variables between willow-lined reaches and
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.removal reaches in order to determine the possible impacts of willow removal on in-streami�
fauna. By examining these differences across a nwnber of rivers, I sought to find which'.'
patterns were clear and consistent. As in Chapter 2, while a survey does not establish.
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causation between vegetation types and in-stream fauna by itself, it acts as an important step in generating plausible hypotheses for later testing. Secondly, a survey approach was the only avenue possible as BACI approaches as outlined by Underwood (199la) were difficult to conduct around the often haphazard removal of vegetation by community groups whose objectives lay outside the aims of my study. The only alternative was to remove vegetation as part of the survey as carried out previously by Latta (1974). This was not practical due to
financial and time constraints of the study, or possible given a moratorium placed on removal of willows during the course of the survey; moreover, the likely time-lag between willow removal and an observable response by in-stream fauna was probably outside the timeframe imposed by constraints imposed on Ph.D. research.
Specifically, I hypothesised that in shallow environments, differences between
macroinvertebrate communities would be most apparent in all seasons due to consistent differences in litter inputs and shading between willowed reaches and reaches where willows had been removed. For fish communities, differences in important cover variables resulting from willow removal should be reflected by differences in fish community composition and abundance between the two types of reaches. As an adjunct to the survey, I suspected that differences would be more profound when comparing removal reaches to reaches severely choked by willows, which resemble "willow swamps" defined by Collier (1993). These reaches possess little flow due to willow blockage, high organic standing stocks, and low dissolved oxygen levels; these reaches are so different, they warranted separate investigation from the shallow rivers in the rest of the survey. I hypothesised that differences in
macroinvertebrate and fish communities would be greatest in comparisons between degraded willow reaches and unshaded reaches.
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objective of the survey was to provide ecological information for riparian
and restoration activities. The removal of willows along river banks is
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in Australia at present and the lack of relevant ecological information
surrounding willow removal activities and associated impacts is a key issue confronting ·ecologists> water managers and community groups. Findings from this research will clarify
some of the ecological misconceptions regarding removal of willows along Australian rivers and provide management prescriptions for many community groups that carry out riparian management activities.