Substrate and shade: mechanisms of willow tree influence on the macroinvertebrate community of Heeney Creek, South Island, New Zealand

Abstract

Experiments were conducted in an effort to isolate the relative contributions of substrate size distribution and shading to the reduction in macroinvertebrate abundance associated with riparian willows in Heeney Creek, New Zealand. The effect was apparently not related to the availability of food. Periphyton primary production was not significantly different among open, artificially shaded, and willow shaded sites, in spite of the intense shading, and previous studies have indicated that allochthonous food sources are more important to invertebrates at the willow-shaded sites than at the open sites. An experiment in which the substrate size distributions associated with willow-shaded and open sites were reconstituted in containers, and incubated in the stream for colonisation, indicated that there were significant effects of substrate type. These were, however, relatively minor. The major effect was from another, undetermined factor associated with willows, as the containers at artificially shaded sites had higher abundances than at willow shaded sites, regardless of substrate type. Willows are known to produce a variety of compounds that inhibit the feeding of terrestrial animals, and we postulate that exudates from their roots may similarly affect the invertebrates in small, closely overgrown streams such as Heeney Creek.