Effects of riparian willow trees(Salix fragilis)on macroinvertebrate densities in two small Central Otago, New Zealand, streams

Abstract

The effects of an introduced willow tree species (Salix fragilis) on the densities of macroinvertebrates were examined in two Central Otago, New Zealand, streams during 1991. Significantly lower invertebrate densities and biomass were observed in willow-lined sections of the streams than in nearby open sections in summer, autumn, and winter. This result was observed in riffles and pools, for most dominant species and nearly all functional feeding groups. The effect was not associated with differences in the amount of fine paniculate organic matter (< 1 mm), stone surface organic layer biomass, or chlorophyll a concentration, which were similar it open and willow-shaded sites. Amounts of coarse paniculate organic matter (> 5 mm) were significantly higher in willow-shaded riffles, but this did not result in increased abundance or biomass of shredders. Willow trees reduced incident stream illumination by as much as 80%, but did not appear to influence water chemistry between open and willow-shaded sites. The decreased invertebrate densities probably result from a decrease in average substrate size and/or a lowering of food production through shading effects. M93063

The influence of water quality and macroinvertebrate colonization on the breakdown process of native and exotic leaf types in sub-alpine stream

“…Some studies suggested that invasive species can cause alterations at different levels in the structure and functioning of ecosystems (Ehrenfeld 2010). Moreover, the establishment of exotic riparian plants can alter the exchange of organic material between terrestrial and lotic ecosystems, both changing the quantity (Mineau et al 2011(Mineau et al , 2012 or the quality (B€ arlocher & Graça 2002) of terrestrial litter inputs, leading to alterations in the composition (Serra et al 2013) and density (Lester et al 1994) of benthic assemblages. Many studies have shown that, for instance, the introduction and spread of Eucalyptus globulus plantations in Spain had a strong effect on in-stream terrestrial detritus colonization and degradation (Chauvet et al 1997;Pozo et al 1998;Ferreira et al 2006), while other studies have shown weak influence of exotic species on streams (Braatne et al 2007).…”

Prey availability and diet of juvenile brown trout (Salmo trutta) in relation to riparian willows (Salix‐spp.) in three New Zealand streams

“…This may be brought about by shade decreasing secondary production and invertebrate densities (e.g., Allen 1951;Hopkins 1976;Behmer & Hawkins 1986). However, Lester et al (1994) found that the effect of shading by willows on macroinvertebrate abundance and biomass was not associated with differences in the amount of fine particulate organic layer matter, stone surface organic layer biomass, or chlorophyll a concentration. Likewise, large seasonal increases in coarse particulate organic matter (CPOM) resulting from leaf fall, did not result in an increase in the abundance or biomass of shredders in the macroinvertebrate communities of two streams studied by Lester et al (1994).…”

The effects of riparian forest management on the freshwater environment: a literature review of best management practice

“…For example, OGrady (1993) reported that the cover of aquatic vegetation was reduced by 60% in stream sections flowing under a complete conifer canopy compared to those measured in adjacent open streams. Lester et al (1994) showed that where the riparian willow canopy in a New Zealand stream reduced the incident illumination by up to 80%, the macro invertebrate abundance and biomass was just half that recorded in open streams. However, Glova and Sagar (1994) studied the effects of different levels of shade on three New Zealand streams and showed that the biomass and diversity of benthic invertebrates, as well as the abundance and biomass of brown trout, tended to be higher in moderately shaded stream sections compared with those with dense willow shading or open areas.…”