The current interest in risks associated with classical biological control led us to review the literature to determine whether the introduction of multiple biological control agents has been more effective than the introduction of a single control agent. We analyzed 59 projects against weeds and 108 projects against insect pests. Establishment of control agents was significantly higher in single-agent projects than in multiple-agent projects against insect pests, but not in projects against weeds. The success of biological control against weeds increased with the number of agents released. However, no relationship was found between the number of agents released and biological control success for insect pests. These results suggest that negative interactions may play a significant role amongst biological control agents of insects. Thus, multiple releases may be effective against weeds only. In over 50% of the successful multiple-agent projects against weeds and insect pests, a single agent was shown to be responsible for success. This result indicates that, in a majority of biological control projects, multiple agents are not released for a cumulative control effect but to increase the likelihood that the right control species is released (lottery model). Considering the possibility of negative interactions among biological control agents and taking into account the risks associated with natural enemy introductions, we recommend restraint in the introduction of multiple agents for biological control.

Summary1. A wide range of spatially explicit simulation models have been developed to forecast landscape dynamics, including models for projecting changes in both vegetation and land use. While these models have generally been developed as separate applications, each with a separate purpose and audience, they share many common features.2. We present a general framework, called a state-and-transition simulation model (STSM), which captures a number of these common features, accompanied by a software product, called ST-Sim, to build and run such models. The STSM method divides a landscape into a set of discrete spatial units and simulates the discrete state of each cell forward as a discrete-time-inhomogeneous stochastic process. The method differs from a spatially interacting Markov chain in several important ways, including the ability to add discrete counters such as age and time-since-transition as state variables, to specify one-step transition rates as either probabilities or target areas, and to represent multiple types of transitions between pairs of states. 3. We demonstrate the STSM method using a model of land-use/land-cover (LULC) change for the state of Hawai'i, USA. Processes represented in this example include expansion/contraction of agricultural lands, urbanization, wildfire, shrub encroachment into grassland and harvest of tree plantations; the model also projects shifts in moisture zones due to climate change. Key model output includes projections of the future spatial and temporal distribution of LULC classes and moisture zones across the landscape over the next 50 years. 4. State-and-transition simulation models can be applied to a wide range of landscapes, including questions of both land-use change and vegetation dynamics. Because the method is inherently stochastic, it is well suited for characterizing uncertainty in model projections. When combined with the ST-Sim software, STSMs offer a simple yet powerful means for developing a wide range of models of landscape dynamics.

Pacific giant salamanders (Dicamptodon tenebrosus Good) in the Chilliwack River valley of southwestern British Columbia are at the periphery of their range, and therefore of conservation concern. Although logging is a potential threat to the species, no studies have examined how clear-cutting affects its terrestrial stage. We used radio telemetry to compare the movements of 35 terrestrial Pacific giant salamanders at sites with three different logging histories: forested, clearcut to the stream margin, and clearcut with riparian buffer strips. The results demonstrate that logging affected movements of the salamanders. Salamanders in clearcuts remained significantly closer to the stream, spent more time in subterranean refuges, and had smaller home ranges than those at forested sites. During a dry year, salamanders in clearcuts were significantly more dependent on precipitation for their movement than salamanders in forested habitats. Salamander movement behavior in riparian buffer strips was not significantly different from that at forested sites but was significantly different from that at clearcut sites. Riparian buffer strips appear to mitigate some of the negative effects of clearcuts on salamander movement.

Abstract. 1. Western tent caterpillars hatch in the early spring when temperatures are cool and variable. They compensate for sub-optimal air temperatures by basking in the sun.2. Tent caterpillars have cyclic population dynamics and infection by nucleopolyhedrovirus (NPV) often occurs in populations at high density.3. To determine whether climatic variation might influence viral infection, the environmental determinants of larval body temperature and the effects of temperature on growth and development rates and larval susceptibility to NPV were examined.4. In the field, larval body temperature was determined by ambient temperature, irradiance, and larval stage. The relationship between larval body temperature and ambient temperature was curvilinear, a property consistent with, but not necessarily limited to, behaviourally thermoregulating organisms.5. Larvae were reared at seven temperatures between 18 and 36 C. Larval growth and development increased linearly with temperature to 30 C, increased at a lower rate to 33 C, then decreased to 36 C. Pupal weights were highest for larvae reared between 27 and 30 C. 6. The pathogenicity (LD 50 ) of NPV was not influenced by temperature, but the time to death of infected larvae declined asymptotically as temperature increased.7. Taking into account larval growth, the theoretical yield of the virus increased significantly between 18 and 21 C then decreased slightly as temperatures increased to 36 C. 8. Control and infected larvae showed no difference in temperature preference on a thermal gradient. The modes of temperature preference were similar to those for optimal growth and asymptotic body temperatures measured in the field on sunny days.9. Warmer temperatures attained by basking may increase the number of infection cycles in sunny springs but do not protect larvae from viral infection.

This report describes a land management modeling effort that analyzed potential impacts of proposed actions under an updated Bureau of Land Management Resource Management Plan that will guide management for 20 years on 4.6 million hectares in the Great Basin ecoregion of the United States. State-and-transition models that included vegetation data, fire histories, and many parameters (i.e., rates of succession, fire return intervals, outcomes of management actions, and invasion rates of native and nonnative invasive species) were developed through workshops with scientific experts and range management specialists. Alternative restoration scenarios included continuation of current management, full fire suppression, wildfire use in designated fire use zones, wildfire use in resilient vegetation types only, restoration with a tenfold budget increase, no restoration treatments, and no livestock grazing. Under all the scenarios, cover of vegetation states with native perennial understory declined and was replaced by tree-invaded and weed-dominated states. The greatest differences among alternative management scenarios resulted from the use of fire as a tool to maintain native understory. Among restoration scenarios, only the scenario assuming a tenfold budget increase had a more desirable outcome than the current management scenario. Removal of livestock alone had little effect on vegetation resilience. Rather, active restoration was required. The predictive power of the model was limited by current understanding of Great Basin vegetation dynamics and data needs including statistically valid monitoring of restoration treatments, invasiveness and invasibility, and fire histories. The authors suggest that such computer models can be useful tools for systematic analysis of potential impacts in land use planning. However, for a modeling effort to be productive, the management situation must be conducive to open communication among land management agencies and partner entities, including nonprofit organizations.