Impacts of Forest Management on Land Snails and Slugs
The Xerces Society has just released a literature review summarizing the effects of logging, road building and burning on snails and slugs. This document, funded by the Interagency Special Status and Sensitive Species Program, was designed to help BLM and Forest Service biologists better understand and evaluate the effects of forest land management actions on terrestrial mollusks, and implement effective conservation measures to meet agency Sensitive species policy goals. We are hopeful that this review will be a useful tool not only for Forest Service and BLM personnel, but also for other federal, state, and private land managers faced with gastropod conservation issues.
Snails and slugs are essential components of forest ecosystems. They decompose forest litter, recycle nutrients, build soils, and provide food and calcium for birds, amphibians, reptiles, small mammals, and invertebrates. Although mollusks have been a crucial part of the ecology of temperate forests for millennia, recent loss and fragmentation of natural habitats due to clearcut logging, road-building, and altered fire regime have resulted in both extinction and extinction risk for many mollusk species (e.g., Curry et al. 2008). Mollusks (including aquatic species) represent 20% of all threatened animals, and 37% of known animal extinctions since 1600 A.D. (Seddon 1998 in Dunk et al. 2004). In an era where the extinction rate is an estimated 400 times the natural rate (reviewed in Werner & Raffa 2000), it is important for land managers to take mollusks into consideration when developing or re-evaluating strategies for managing forests ecosystems to achieve forest health and biodiversity conservation goals.
Key findings of the review include:
- While some level of exposure in the physical environment is tolerated by certain mollusks, most species are extremely sensitive to temperature and moisture extremes.
- Research suggests that the majority of snails and slugs are dependent on litter from deciduous trees and have higher abundances in multispecies forests with strong broadleaf components. Additionally, mollusks in deciduous forests appear to rebound from disturbance more quickly than in coniferous forests.
- Forests with old-growth characteristics supply microhabitat and microclimate conditions capable of supporting a diversity of mollusks, and forest age is often positively correlated with mollusk richness and abundance.
- Numerous studies stress the importance of refugia in gastropod recolonization potential and community resilience following forest disturbance. Since land mollusks are small animals with limited mobility and dispersal capabilities, the maintenance of refugia in disturbed habitat is particularly important for this group. Refugia should include logs, snags, fallen branches, and other forms of coarse woody debris, as well as areas with thick leaf-litter. Woody debris and litter provide islands of habitat, food, and protection from microclimatic extremes, increasing species’ tolerance of temporarily inhospitable environments.
- Research suggests that in order to reduce microclimate extremes and protect gastropods, partial cuts should be favored over clearcuts, aggregated (group) retention over dispersed retention or thinning, and larger group retention over smaller group retention. In particular, harvesting with large group retention helps to maintain pre-harvest boreal gastropod assemblages and will likely conserve boreal gastropod species if used as a tool for biodiversity management.
- Fragmented habitat limits the dispersal and post-disturbance recolonization potential of gastropods. Tracts of intact forest and connected groups of old trees help provide dispersal corridors for gastropods and can lead to significant increases in the survival of disturbance-sensitive species.
- Research suggests that techniques that minimize soil compaction and damage to (or removal of) the organic layer favor survival of gastropods. For example, Timberjacks have been found to cause less damage to the organic mat and resident invertebrate populations than feller bunchers, single-grip harvesters, and grapple skidders.
- Due to the tendency of mollusks to avoid non-vegetated and/or dry environments, even narrow, unpaved roads with low traffic densities are barriers to the dispersal of mollusks.
- Numerous studies have found negative and long-lasting responses of gastropods to fire, including population extirpation and reductions in abundance and species richness. Small burns surrounded by unburned plots have been most successful at maintaining gastropod community structure. Although there is little information comparing gastropod responses to differences in burn severity and frequency, it is presumed that a fire regime involving low-intensity burns at infrequent fire-return intervals (>5 years) would best maintain gastropod communities.