Forest Investment Account

Abstract of FIA Project 6080009

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Terrestrial gastropods as indicators for monitoring ecological effects of variable-retention logging practices: pre-disturbance surveys at experimental sites, May - October 2002, annual progress report

Author(s): Ovaska, Kristiina; Sopuck, Lennart G.; Biolinx Environmental Research Ltd.
Imprint: Sidney, BC: Biolinx Environmental Research Ltd., 2003
Subject: Forest Investment Account (FIA), Gastropoda, British Columbia, Snails, Ecological Research
Series: Forest Investment Account (FIA) - Land Base Investment Program


Forest-dwelling slugs and land snails are sensitive to changes in moisture and temperature conditions and the forest floor structure associated with logging. In the autumn of 1999, we initiated a pilot study in coastal forests of British Columbia to examine the potential of these animals as focal species within Weyerhaeuser’s Ecological Monitoring Program. Initially, our emphasis was to characterize gastropod faunas of different forest types, examine sensitivities of different species, and develop efficient sampling methods. In 2001, we began collecting pre-disturbance data on species diversity and relative abundance of gastropods at two experimental sites: Tsitika (R917) on north-eastern Vancouver Island and Stillwater (R949) near Powell River on the coastal mainland. In 2002, we collected pre-disturbance data at two additional experimental sites: Port McNeill (R817) on Vancouver Island and East Yakoun (R1000) on the Queen Charlotte Islands. A further objective in 2002 was to refine sampling methodologies useful for designing an efficient monitoring program for this group. This report presents the results of the 2002 field season at these two experimental sites. The designated treatments at both the Port McNeill and East Yakoun sites were clearcut, three variable retention (VR) treatments, and an uncut control. At each site, we set up 20 sampling plots (4 in each treatment area). Each plot consisted of 10 sampling stations where we surveyed gastropods with artificial cover-objects (constructed of corrugated cardboard sheets) and litter sampling (extraction of small snails from dried samples of litter). At both sites, we inspected the cover-objects four times for gastropods sheltering underneath them or adhering to the various layers. We collected litter samples at each station (5 liters/plot for a total of 100 liters) twice at the Port McNeill site and once at the East Yakoun site. The number of species of terrestrial gastropods found at the Port McNeill and East Yakoun sites was similar (14 and 16 species, respectively). This diversity appears to be typical of lower elevation coniferous forests in coastal British Columbia. At the Port McNeill site, we found 12 species of snails and two species of slugs. The most common group was Tightcoil snails (Pristiloma species) that represented 34% of 2041 gastropods caught with artificial cover-objects and 22% of 1499 small snails extracted from litter samples. One snail, Broadwhorl Tightcoil (P. johnsonii), was detected only with litter sampling. Slugs and carnivorous snails were relative poorly represented and comprised only 7% and 5% of all artificial cover-object captures, respectively. Small litter snails (9 species) dominated the gastropod fauna at this site. The average density of small snails was 4.1 snails/liter of litter in June and 4.0 snails/liter of litter in October. At the East Yakoun site, we found 12 species of snails and four species of slugs. As at the Port McNeill site, the most common group was Tightcoil snails, which represented 32% of 1595 detections with artificial cover-objects and 21% of 577 small snails extracted from litter samples. In contrast to the Port McNeill site, slugs and carnivorous snails were well represented and comprised 22% and 12% of the detections with iii artificial cover-objects, respectively. Small litter snails were represented by seven species, and their average density was 3.1 snails/liter of litter. Two species of snails (Amber Glass, Nesovitrea electrina, and Denticulate Tightcoil, Pristiloma lansingii) were detected only with litter samples. At this site, we encountered two unusual gastropods that did not match the descriptions for any known species: a small semi-slug (8 specimens), which probably represents an undescribed jumping-slug (Hemphillia species), and a clear-shelled Tightcoil (15 specimens), which is either another undescribed species or an albino form of the Striate Tightcoil (Pristiloma stearnsii). Our previous studies suggest that Tightcoil snails may be sensitive to logging and are probably good focal species for investigating ecological effects of different logging practices. Other candidates for focal species are the large carnivorous snail Robust Lancetooth (Haplotrema vancouverense), which was common at the East Yakoun site, and the small litter snail Western Flat Whorl (Planigyra clappi), which was common – although unevenly distributed – at the Port McNeill site. Small Vertigo snails (Vertigo species) were also identified as potential focal species and were common at both sites. With a few exceptions, artificial cover-object data indicated that the abundance (average number per plot) of most gastropod species was similar among the designated logging treatments and control areas during the pre-disturbance surveys. However, many species, particularly small snails, exhibited aggregated dispersion patterns among sampling plots in uncut forest, most likely in response to microhabitat conditions. For example, at the Port McNeill site, one plot in the designated clearcut treatment and one plot in the designated control area supported very high densities of small litter snails, probably because the plots were located in moist depressions with abundant herbaceous vegetation. In contrast, large snails and slugs were more evenly distributed among plots. Characterizing variation in patterns of abundance before logging takes place is extremely important to prevent erroneous conclusions about the effects of logging treatments, when in fact observed differences are due to pre-existing habitat conditions. A priority for gastropod studies in 2003 is to initiate pre-disturbance surveys at additional experimental sites to provide replication. The designated VR-treatments at the four experimental sites surveyed so far for gastropods are somewhat different and include comparisons of different levels of retention in groups (10%, 20%, and 30% retention; Tsitika and East Yakoun sites), different group sizes (Port McNeill site), and different levels of retention with dispersed, scattered trees (5%, 10%, and 30% retention; Stillwater site). Ideally, at least three sites would have a replicate set of treatments. Two study areas are planned for 2003: White River/Memekay area on Vancouver Island and Goat Island/Lois Lake area near Powell River on the mainland. To complete the experiments, post-logging surveys are to be conducted at the established sampling plots at all experimental sites to examine responses of gastropods to the logging treatments. Because some of the experimental sites are in second growth forest, surveys of benchmark, old growth stands would also be beneficial to obtain information on gastropods within undisturbed habitats.

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Updated August 02, 2006 

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