Dean E. Pearson

WEAK VS. STRONG INVADERS OF NATURAL PLANT COMMUNITIES: ASSESSING INVASIBILITY AND IMPACT

In response to the profound threat of exotic species to natural systems, much attention has been focused on the biotic resistance hypothesis, which predicts that diverse communities should better resist invasions. While studies of natural communities generally refute this hypothesis, reporting positive relationships between native species diversity and invasibility, some local-scale studies have instead obtained negative relationships. Most treatments of the topic have failed to recognize that all exotic invaders do not behave alike: while “weak” invaders become minor components of communities, “strong” invaders become community dominants at the expense of native species. At the same time, the specific impacts of strong invaders on communities are poorly documented yet critical to understanding implications of diversity loss. With these shortfalls in mind, we examined local-scale relationships between native and exotic plant taxa in bunchgrass communities of western Montana, USA. We found that measures of...

Indirect effects of host-specific biological control agents

Biological control is a crucial tool in the battle against biological invasions, but biocontrol agents can have a deleterious impact on native species. Recognition of risks associated with host shifting has increased the emphasis on host specificity of biocontrol agents for invasive weeds. However, recent studies indicate host-specific biocontrol agents can also exhibit substantial nontarget effects through indirect interactions and food-web subsidies. Based on an evaluation of these studies, we conclude that the interaction strength between biocontrol agents and their hosts is at least as important as host specificity for determining the outcome of biocontrol introductions. Host-specific biocontrol agents that establish, but fail to reduce the densities of their hosts can facilitate bottom-up effects that link the target weed to other native organisms through food webs, thereby expanding the impacts of the invasive weed. We believe that indirect nontarget effects of host-specific biocontrol agents arising from food-web subsidies could prove more deleterious to native species than are the direct nontarget effects currently recognized from host shifting.

Non-target effects of an introduced biological control agent on deer mouse ecology

Abstract Release of exotic insects as biological control agents is a common approach to controlling exotic plants. Though controversy has ensued regarding the deleterious direct effects of biological control agents to non-target species, few have examined the indirect effects of a ”well-behaved” biological control agent on native fauna. We studied a grassland in west-central Montana infested with spotted knapweed (Centaurea maculosa) to examine the effects of knapweed invasion and two gall flybiological control agents (Urophora affinis and U. quadrifasciata) on the native deer mouse (Peromyscus maniculatus). Stomach-content analysis revealed that Urophora were the primary food item in Peromyscus diets for most of the year and made up 84–86% of the winter diet. Stomach contents indicated that wild-caught mice consumed on average up to 247 Urophora larvae mouse–1 day–1, while feeding trials revealed that deer mice could depredate nearly 5 times as many larvae under laboratory conditions. In feeding trials, deer mice selected knapweed seedheads with greater numbers of galls while avoiding uninfested seedheads. When Urophora larvae were present in knapweed seedheads, deer mice selected microhabitats with moderately high (31–45% cover) and high knapweed infestation (≥46% cover). After Urophora emerged and larvae were unavailable to Peromyscus, mice reversed habitat selection to favor sites dominated by native-prairie with low knapweed infestation (0–15%). Establishment of the biological control agent, Urophora spp., has altered deer mouse diets and habitat selection by effecting changes in foraging strategies. Deer mice and other predators may reduce Urophora populations below a threshold necessary to effectively control spotted knapweed.

Comparison of proposed survey procedures for detection of forest carnivores

American marten (Martes americana), fisher (M. pennanti), wolverine (Gulo gulo), and lynx (Lynx lynx) are forest carnivores believed threatened by disturbance of late-successional forests. To manage forested ecosystems for these species, effective methods for their detection must be available. Recently, the U.S. Forest Service proposed standardized survey procedures for the detection of forest carnivores; this report presents the first critical assessment of these protocols. We compared dual-sensor remote cameras and soot-coated open and covered track plates in the same study areas over an 8-month period. Of the 4 species targeted by these procedures, we detected 3 (American marten, fisher, wolverine). The remote camera method ranked highest with respect to ease of use, effectiveness, and accuracy of identifications. However, track plates performed well for 2 species and, under certain circumstances, may be the method of choice. We suggest improvements for each method and encourage that such standardized procedures be applied over wide regions.

Biotic resistance via granivory: establishment by invasive, naturalized, and native asters reflects generalist preference

Escape from specialist natural enemies is frequently invoked to explain exotic plant invasions, but little attention has been paid to how generalist consumers in the recipient range may influence invasion. We examined how seed preferences of the widespread generalist granivore Peromyscus maniculatus related to recruitment of the strongly invasive exotic Centaurea stoebe and several weakly invasive exotics and natives by conducting laboratory feeding trials and seed addition experiments in the field. Laboratory feeding trials showed that P. maniculatus avoided consuming seeds of C. stoebe relative to the 12 other species tested, even when seeds of alternative species were 53-94% smaller than those of C. stoebe. Seed addition experiments conducted in and out of rodent exclosures revealed that weakly invasive exotics experienced relatively greater release from seed predation than C. stoebe, although this was not the case for natives. Seed mass explained 81% of the variation in recruitment associated with rodent exclusion for natives and weak invaders, with larger-seeded species benefiting most from protection from granivores. However, recruitment of C. stoebe was unaffected by rodent exclusion, even though the regression model predicted seeds of correspondingly large mass should experience substantial predation. These combined laboratory and field results suggest that generalist granivores can be an important biological filter in plant communities and that species-specific seed attributes that determine seed predation may help to explain variation in native plant recruitment and the success of exotic species invasions.

Characteristics of American marten den sites in Wyoming

We examined characteristics of den structures and den sites used by female American marten (Martes americana) for natal and maternal dens in the Sierra Madre Range, Wyoming. During 1988-95, we located 18 natal dens (parturition sites) and 97 maternal dens (sites where kits were present exclusive of parturition) used by 10 female marten. Important den structures included rock crevices (28%), snags (25%), red squirrel (Tamiasciurus hudsonicus) middens (19%), and logs (16%). Resource selection function (RSF) analysis showed that an individual selection model provided a significantly better fit than a null model or pooled selection model, indicating that the sample of marten selected maternal den sites that differed from random sites, and that individual animals did not select maternal den sites in the same manner. Six marten individually exhibited significant selection for maternal den sites within home ranges. Overall selection coefficients for maternal dens indicated the number of squirrel middens was the most important variable, followed by number of snags 20-40 em diameter at breast height (dbh), number of snags 2:41 em dbh, and number of hard logs 2:41 em in diameter. Selection of natal den sites was also significant via comparison between selection and no-selection models, with number of middens, number of Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa) >20 em dbh, and number of hard logs 2:41 em in diameter the most important variables in descending order of importance. Large logs, large snags, and large, live spruce and fir trees are important characteristics for marten den sites in the central Rocky Mountains. The prominence of middens at den sites suggests red squirrels provide important denning structures as well as prey for marten.

WEED-BIOCONTROL INSECTS REDUCE NATIVE-PLANT RECRUITMENT THROUGH SECOND-ORDER APPARENT COMPETITION

Small-mammal seed predation is an important force structuring native-plant communities that may also influence exotic-plant invasions. In the intermountain West, deer mice (Peromyscus maniculatus) are prominent predators of native-plant seeds, but they avoid consuming seeds of certain widespread invasives like spotted knapweed (Centaurea maculosa). These mice also consume the biological-control insects Urophora spp. introduced to control C. maculosa, and this food resource substantially increases deer mouse populations. Thus, mice may play an important role in the invasion and management of C. maculosa through food-web interactions. We examined deer mouse seed predation and its effects on seedling emergence and establishment of a dominant native grass, Pseudoroegneria spicata, and forb, Balsamorhiza sagittata, in C. maculosa-invaded grasslands that were treated with herbicide to suppress C. maculosa or left untreated as controls. Deer mice readily took seeds of both native plants but removed 2-20 times more of the larger B. sagittata seeds than the smaller P. spicata seeds. Seed predation reduced emergence and establishment of both species but had greater impacts on B. sagittata. The intensity of seed predation corresponded with annual and seasonal changes in deer mouse abundance, suggesting that abundance largely determined mouse impacts on native-plant seeds. Accordingly, herbicide treatments that reduced mouse abundance by suppressing C. maculosa and its associated biocontrol food subsidies to mice also reduced seed predation and decreased the impact of deer mice on B. sagittata establishment. These results provide evidence that Urophora biocontrol agents may exacerbate the negative effects of C. maculosa on native plants through a form of second-order apparent competition-a biocontrol indirect effect that has not been previously documented. Herbicide suppressed C. maculosa and Urophora, reducing mouse populations and moderating seed predation on native plants, but the herbicides direct negative effects on native forb seedlings overwhelmed the indirect positive effect of reducing deer mouse seed predation. By manipulating this four-level food chain, we illustrate that host-specific biological control agents may impact nontarget plant species through food-web interactions, and herbicides may influence management outcomes through indirect trophic interactions in addition to their direct effects on plants.

Small-mammal seed predation limits the recruitment and abundance of two perennial grassland forbs

Although post-dispersal seed predators are common and often reduce seed density, their influence on plant population abundance remains unclear. On the one hand, increasing evidence suggests that many plant populations are seed limited, implying that seed predators could reduce plant abundance. On the other hand,.it is generally uncertain whether the magnitude of seed limitation imposed by granivores is strong enough to overcome density-dependent processes that could compensate for seed loss at later stages. We examined the impact of seed predation by small mammals, primarily deer mice (Peromyscus maniculatus), on seedling recruitment and subsequent plant establishment of two perennial grassland forbs in western Montana, USA: Lupinus sericeus (Fabaceae) and Lithospermum ruderale (Boraginaceae). The experiment combined graded densities of seed addition for each species with a small-mammal exclusion treatment. Seedling recruitment and plant establishment were monitored in the experimental plots for up to three years. For both species, small-mammal exclusion increased the total number of seedlings that emerged, and these effects were still significant three years after seed addition, resulting in greater numbers of established plants inside exclosures than in control plots. We also found evidence of seed limitation, with increasing density of seeds added leading to increased numbers of seedlings. Results from seed addition and small-mammal exclusion experiments in later years also revealed significant impacts of small mammals on seedling emergence. These results suggest that granivores can have potentially important impacts in limiting forb abundance in grasslands communities.

Fire and mice: seed predation moderates fire's influence on conifer recruitment.

In fire-adapted ecosystems, fire is presumed to be the dominant ecological force, and little is known about how consumer interactions influence forest regeneration. Here, we investigated seed predation by deer mice (Peromyscus maniculatus) and its effects on recruitment of ponderosa pine (Pinus ponderosa) and Douglas-fir (Pseudotsuga menziesii) seedlings in unburned and recently burned fire-adapted montane forests in west-central Montana, USA. Deer mice were almost twice as abundant in burned than unburned stands. Deer mouse removal of seeds from petri dishes was two times higher in burned than in unburned stands, and seed removal levels were 8% higher for ponderosa pine than for the smaller Douglas-fir seeds. In seed-addition experiments, emergence of seedlings in deer mouse-exclusion cages was almost six times higher in burned compared to unburned forest. In both burned and unburned forest, emergence was lower for ponderosa pine than for Douglas-fir. Seedling survival to establishment did not differ between conifer species but was considerably higher in burned than in unburned forest. However, effects of seed predation on recruitment prevailed over fire effects: in cages allowing access by deer mice, emergence and establishment were extremely rare for both conifer species in both burned and unburned forest. This research suggests that consumer interactions can substantially influence recruitment even in fire-adapted forest ecosystems.

Effects of biological control agents and exotic plant invasion on deer mouse populations

Exotic insects are commonly introduced as biological control agents to reduce densities of invasive exotic plants. Although current biocontrol programs for weeds take precautions to minimize ecological risks, little attention is paid to the potential nontarget effects of introduced food subsidies on native consumers. Previous research demonstrated that two gall flies (Urophora affinis and U. quadrifasciata) introduced for biological control of spotted knapweed (Centaurea maculosa) dramatically affect the foraging ecology of the deer mouse (Peromyscus maniculatus), a native generalist predator with important trophic linkages. In the current study, we found that relative abundance of deer mice was elevated twofold in grassland habitats with high densities of spotted knapweed and gall fly food sources, compared to those dominated by native vegetation, in two of three years. Availability of gall fly larvae during the critical overwinter period appeared to reduce overwinter population declines of mice in knapweed-invaded habitats. These positive effects on populations apparently overshadowed negative effects on breeding productivity associated with knapweed invasion and loss of the gall fly resource during the summer. Our results suggest that insect biocontrol agents can subsidize native consumer populations, setting the stage for various indirect effects on food webs. Comprehensive understandings of the conditions under which introduced biological control agents may exhibit nontarget effects on native food webs are needed to further develop criteria for screening potential biocontrol agents before they are released.