Impact of exotic willow roots (Salix spp.) as habitat for aquatic invertebrate communities in South Australian stream

Abstract

Abstract: The principal objective of this study was to compare differences in species richness and abundance of aquatic invertebrate communities between natural willow roots and artificial aluminium wire mesh substrates. A field experiment was conducted at Deep Creek in the Mount Lofty Ranges (South Australia) to investigate colonisation of willow roots by aquatic invertebrates. Two different types of substrate with differing degrees of complexity were used. Willow roots with only primary roots left intact; lateral and primary roots left intact; and no roots were cut down and aluminium wire mesh of three different mesh sizes: coarse (15 mm), medium (9 mm) and fine (1.5 mm), were allowed to colonise for 30 and 90 days. Aquatic invertebrates were significantly more abundant on willow roots than on aluminium wire mesh substrates. However, there was no significant difference in species richness. Habitat complexity and period of colonisation are important determinants of invertebrate community structure on each substrate type. Scrapers (primarily gastropods) often constituted the greatest abundance on willow roots. The introduced hydrobiid snail, Potamopyrgus antipodarum, was significantly more abundant especially on the most complex of willow root structures and after a longer period of colonisation (90 days). Findings of this study suggested that willow roots have habitat value for aquatic invertebrates, especially the hydrobiid snail and decrease in structure associated with willow roots resulted in a lower abundance of invertebrates. Carefully management of willow removal should take into account the sudden removal of willow roots which may disrupt the invertebrate communities which utilise the roots as habitat. It is also evident that a long period of colonisation is needed to obtain a stable invertebrate community, especially during revegetation efforts.