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Impacts of Willow Control on Flora and Fauna It is believed that the impacts of strategic willow control on native wildlife are minimal, except in the case of certain bird species. Willows provide less suitable habitat for wildlife than native vegetation but some local disruption to individual species would be expected. Over the longer term, the replacement of problem willows with native vegetation is believed to be beneficial to local wildlife. Prior to any willow control activities an assessment would need to be undertaken that investigates localized impacts on wildlife. For practical reasons this would need to be relative to the size of the on-ground works.
The main impact on wildlife is the potential to disrupt bird breeding events, in particular for the Darter and Cormorant (pers. comm. McGuiness 2005). Significantly, Darter numbers have been recorded in slightly higher numbers through the autumn months (although they have been recorded during all seasons) (Allan 2003) and this is usually the optimum time to undertake willow control. Darters are considered to be an ‘uncommon breeding visitor’, and tend to be present in higher numbers during times of inland drought. They are most frequently found in the eastern end of Lake Burley Griffin, Jerrabomberra Wetlands and the Molonglo River opposite Duntroon (Allan 2003). Over the 25 years of records, Darters have been recorded in the ACT every year, but only 12 of these years were reported as breeding events. In a study undertaken in the 1980’s (Nix, cited by Allan 2003), nests and fledglings were found in Weeping Willows along Molonglo Reach. One particular tree hosted eight Cormorant nests in addition to Darter nests. These breeding events have since been repeated at the same site sporadically until 1997-98 when nests were observed in the same willows (Taws, cited by Allan 2003). Another breeding event occurred in 2002-03 where several (25+) nests were observed in willows between the Lake and the first bridge up the Molonglo River. It was estimated that there were between 50 and 100 Darters on that stretch of river (Holland 2003, cited by Allan 2003). Other recent sightings of Darter nests and fledglings have been on the eastern side of the National Museum of Australia and Black Mountain Peninsula.
Many other species of birds also use willows for roosting. Of note is the Nankeen Night Heron, which is known to use willows as shelter during the day (pers. comm. Lintermans 2006). The only way to properly determine the extent to which willows are used by birds would be through on-ground surveys. However, it may be possible to extrapolate from existing information, knowledge of habitat needs and surveys from the surrounding area. Representatives from Canberra Ornithologists Group and other bird experts could be called upon to advise on this, for any specific willow control works.
The only aquatic plant specifically noted as part of the willow survey was Cumbungi or Bulrush (Typha sp). This was due to the likelihood of this plant colonising areas after willow control activities. When the willows are present it is suppressed by shading. When the willows are removed it may prove very useful as an almost immediate (and natural) replacement for willow roots. Cumbungi was noted in most areas of the survey, and would is expected to establish quite quickly on bare banks once the canopy layer is removed. Cumbungi holds soil together, filters water and provides habitat. It is particularly useful habitat for fish and aquatic birds.
In some areas, however, Cumbungi may not be desirable as it impedes access and interferes with recreational activities. An assessment would need to be made as to whether it needs to be addressed in terms of impacts. In some areas where revegetation is planned it may also pose a problem reducing species diversity. Cumbungi distribution is largely controlled by depth so in the case of the Lake its spread would be limited. It tends to be more of a problem in shallow streams and waterbodies where all open water is colonized due to a lack of natural controls. Part of the maintenance of the lake involves monitoring the growth of submerged and emergent aquatic plants. The purpose is to minimise nuisance and potential safety risks to lake users. Under this program, excessive plant growth is controlled by harvesting from selected recreational areas (such as Yarramundi rowing course, swimming areas, and boat launching and mooring areas). This is normally carried out in early summer months with the harvested weed removed to avoid impacting on water quality.
Willows provide shade and shelter from predators for aquatic life. Unfortunately, in many circumstances (such as along constricted rivers) they provide an overabundance of shade that actually restricts diversity of aquatic creatures. This is certainly the case along sections of the Molonglo River. In the Lake area it is of mixed impact given the vast expanse of open water and alternative habitat. Potential impact of willow control on fish and macroinvertebrates is likely to be localized and minimal, although it should be recognised that there is likely to be some impact (pers. comm. Lintermans 2006), particularly where large trees are removed. The main impacts would be an increase in light and reduction of woody habitat. Fish are likely to benefit where there is colonization by Cumbungi and other aquatic plants stimulated by the extra light. It is unclear whether animals such as Platypus and water rats use willows to any great degree. There have been conflicting reports on this matter (pers. comm. Lintermans. 2006).
Impacts of Willow Control on Flooding As a general rule, removal of willows (or any vegetation) in large numbers from river systems, improves the conveyance of flow. Therefore in the immediate area of control works floods are shorter in duration and lower in height than downstream, where floods are larger and of longer duration. This is because water that was previously slowed by the vegetation is transferred downstream faster. In the case of Lake Burley Griffin the flood impact of willow removal is not likely to be a problem as an enormous number of willows would need to be removed from the tributaries before a notable effect on the height of the Lake was achieved. In addition to this, water levels are controlled by the release of water from Scrivener Dam. In the Molonglo River, elevated flood levels may be experienced along downstream reaches if a willow control program was put in place that removed a large proportion of all the upstream willows. Comparing flood heights before and after willow control activities and correlating them with rainfall events could monitor this. However, it is debatable whether the flood effects would be significant enough to warrant the additional time and expense of a monitoring program specifically for that purpose. If willow control is undertaken using best management practices, i.e. a staged approach with appropriate revegetation, significant changes in flood height should not be of major concern. It is something to be aware of when planning works. | ||||||||||||||||||
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