Site effects outweigh riparian influences on ground-dwelling beetles adjacent to first order streams in wet eucalypt forest

Abstract

In wet eucalypt forest with a rainforest understorey the vegetation adjacent to first order streams does not form a distinct riparian strip. This study investigated the riparian response of terrestrial ground-dwelling beetles adjacent to four such streams in Tasmania, Australia. Beetle assemblages varied more between the four sites than they did with distance from stream within sites, where they exhibited a measurable but subtle riparian response. The extent of the riparian zone varied between the four study sites, with a 1-5 m riparian zone at three sites and a gradually changing community up to 50-100 m upslope at one site. There was a trend for greater between plot variability immediately adjacent to the streams, possibly because this is a more highly disturbed environment. None of the habitat variables measured were consistently associated with riparian or upslope assemblages of beetles, probably explaining the subtlety of the beetles' riparian response. Forest conservation efforts for terrestrial species should not necessarily be focused on the riparian zone in preference to upslope areas.

Are streamside buffers edge-affected habitat for ground-dwelling forest beetle assemblages?

“…Two beetle species, Baeocera TFIC sp 02 and C. ater, were indicators for continuous upslope forest in the comparison with riparian habitats, while no species was found to be a significant indicator in the comparison between streamside reserves and continuous riparian habitats. Greater similarity of streamside reserve assemblages to continuous riparian than to upslope assemblages is not surprising, because we sampled the interior, more 'riparian' parts of the streamside reserves, and our previous research in the same forest type showed that beetle assemblage composition in the immediate riparian zone differs from that of upslope areas (Baker et al 2007b). In our study, the beetle species commonly collected in streamside reserves were those that were commonly collected in the other unlogged habitats.…”

“…In this study, Indicator Species Analysis also found a number of species characteristic of unlogged and regeneration habitats, although some different indicator species were identified compared to those identified in Baker (2006). The present study involved intensive trapping within a smaller study region than our previous studies (Baker 2006;Baker et al 2007b), thus it is possible that certain species were collected from a higher proportion of sites, resulting in significance in the Indicator Species Analysis. …”

“…Other Tasmanian studies of beetles in 100-m-wide non-riparian wildlife habitat strips also found ground-dwelling beetle assemblages to be edge-affected (Grove 2004;Grove and Yaxley 2005). Since narrow corridors in riparian are likely to support different assemblages of beetles (this study, Baker et al 2007b) and birds (e.g. MacDonald et al 2002), it is recommended that the network of wildlife habitat strips be augmented to more often include upslope as well as riparian habitat.…”

“…the zone least likely to be influenced by edge effects. Riparian conditions would be likely to influence beetles in at least a portion of a 10 m wide zone immediately adjacent to streams Baker et al 2007b). Despite the relatively close proximity of pitfall traps within transects, neither autocorrelation nor pitfall trap depletion were found to significantly influence catches at this spacing in this forest type .…”

“…Edge effects into streamside reserves in these forests have been previously estimated to extend as far as the streams in most cases (possibly up to *65 m) (Baker et al 2009). Grounddwelling beetles have also been demonstrated to be sensitive to riparian influences in this forest type Baker et al 2007b), with lower abundance and possibly reduced species richness in some riparian areas, although common species do not appear to be specialised to either riparian or upslope habitat. Therefore the combined impacts of riparian influences and edge effects in streamside reserves are likely to result in reduced habitat value relative to reserve corridors located in the upslope areas subject to timber harvesting .…”