Ecology of terrestrial gastropods and their response to conifer release treatments in northwestern Ontario

Abstract

The primary objective of this study was to document any changes in terrestrial gastropod species richness and density associated with alternative methods of managing competing vegetation on regenerating conifer plantations. Pre-treatment data were collected in 1993 and post-treatment data in 1994, following application of four conifer release treatments including, two chemical herbicides (Vision® and Release®), removal of vegetation by mechanical means (Silvana Selective/Ford Versatile), and manual cutting with brushsaws. Gastropod densities were estimated using cardboard sheets. A total of 47,595 gastropods was collected over the two years, comprising 21 species. No change in gastropod richness or density was detected between the four conifer release areas and the control in 1994, despite changes in vegetation cover following treatments. This was attributed to rapid re-establishment of the herbaceous layer following treatments which probably continued to provide favourable conditions for snails and slugs. Species richness and density of gastropods in a 9-year-old regenerating conifer plantation and an adjacent 70-year-old mixedwood forest were compared. Gastropod density was higher on the regenerating plantation (15.5±1.3 m-2) than in the mature forest (9.4±0.6 m-2) and species richness was also slightly greater (20 spp. vs. 18 spp., respectively). These differences were attributed to the more abundant near-ground vegetation and the greater amount of deciduous litter characterizing the regenerating plantation. Near ground temperature was measured on the four conifer release areas and the control using thermocouples. Temperature was measured 2 cm above cardboard sheets, directly beneath the sheets, and 2 cm deep in the humus layer. Results suggest that temperatures were not affected by the release treatments. Temperatures above and beneath the sheets were similar and greater than those in the humus layer throughout the early part of the summer. In August, temperature above and beneath the sheets cooled to below that in the humus layer. Gastropod collections were greatest when the temperature beneath the cardboard sheets was approximately 15°C. Possible limitations associated with the cardboard sheet sampling method were investigated. Gastropod collections from sheets enclosed with a barrier were compared to unenclosed sheets to determine the extent of horizontal movement. The mean density recovered beneath enclosed sheets (2.1±0.2 m-2) was less than that from unenclosed sheets (3.1db0.4 m-2) suggesting horizontal movement to the sheets does occur. However, whether gastropods are actually attracted to the sheets remains unknown. Gastropod collections from weathered sheets were contrasted with those from new sheets. Overall, total mean densities of gastropods from weathered sheets (27.3±4.1 m-2) and new sheets (16.2±2.6 m-2) did not differ, but four species were collected in greater numbers on the weathered sheets suggesting differential use of cardboard sheets by particular species.