Day: April 10, 2012

The entire report, Large Predators Limit Herbivore Densities in Northern Forest Ecosystems, is available here.

ScienceDaily (Apr. 9, 2012) — A survey on the loss in the Northern Hemisphere of large predators, particularly wolves, concludes that current populations of moose, deer, and other large herbivores far exceed their historic levels and are contributing to disrupted ecosystems. The research, published recently by scientists from Oregon State University, examined 42 studies done over the past 50 years.

It found that the loss of major predators in forest ecosystems has allowed game animal populations to greatly increase, crippling the growth of young trees and reducing biodiversity. This also contributes to deforestation and results in less carbon sequestration, a potential concern with climate change.

“These issues do not just affect the United States and a few national parks,” said William Ripple, an OSU professor of forestry and lead author of the study. “The data from Canada, Alaska, the Yukon, Northern Europe and Asia are all showing similar results. There’s consistent evidence that large predators help keep populations of large herbivores in check, with positive effects on ecosystem health.”

Densities of large mammalian herbivores were six times greater in areas without wolves, compared to those in which wolves were present, the researchers concluded. They also found that combinations of predators, such as wolves and bears, can create an important synergy for moderating the size of large herbivore populations.

“Wolves can provide food that bears scavenge, helping to maintain a healthy bear population,” said Robert Beschta, a professor emeritus at OSU and co-author of the study. “The bears then often prey on young moose, deer or elk — in Yellowstone more young elk calves are killed by bears than by wolves, coyotes and cougars combined.” In Europe, the coexistence of wolves with lynx also resulted in lower deer densities than when wolves existed alone.”

In recent years, OSU researchers have helped lead efforts to understand how major predators help to reduce herbivore population levels, improve ecosystem function and even change how herbivores behave when they feel threatened by predation — an important aspect they call the “ecology of fear.”

“In systems where large predators remain, they appear to have a major role in sustaining the diversity and productivity of native plant communities, thus maintaining healthy ecosystems,” said Beschta. “When the role of major predators is more fully appreciated, it may allow managers to reconsider some of their assumptions about the management of wildlife.”

In Idaho and Montana, hundreds of wolves are now being killed in an attempt to reduce ranching conflicts and increase game herd levels. The new analysis makes clear that the potential beneficial ecosystem effects of large predators is far more pervasive, over much larger areas, than has often been appreciated.

It points out how large predators can help maintain native plant communities by keeping large herbivore densities in check, allow small trees to survive and grow, reduce stream bank erosion, and contribute to the health of forests, streams, fisheries and other wildlife.

It also concludes that human hunting, due to its limited duration and impact, is not effective in preventing hyper-abundant densities of large herbivores. This is partly “because hunting by humans is often not functionally equivalent to predation by large, wide-ranging carnivores such as wolves,” the researchers wrote in their report.

“More studies are necessary to understand how many wolves are needed in managed ecosystems,” Ripple said. “It is likely that wolves need to be maintained at sufficient densities before we see their resulting effects on ecosystems.”

“The preservation or recovery of large predators may represent an important conservation need for helping to maintain the resiliency of northern forest ecosystems,” the researchers concluded, “especially in the face of a rapidly changing climate.”

The Xerces Society for Invertebrate Conservation is an international nonprofit organization that protects wildlife through the conservation of invertebrates and their habitat.  They have just released this new literature review summarizing the effects of logging, road building and burning on snails and slugs, which was funded by the Forest Service/BLM Interagency Special Status and Sensitive Species Program.

Summary:
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.

Download the entire literature review here.