Sandra J. DeBano

Investigating temporal patterns of a native bee community in a remnant North American bunchgrass prairie using blue vane traps.

Abstract Native bees are important ecologically and economically because their role as pollinators fulfills a vital ecosystem service. Pollinators are declining due to various factors, including habitat degradation and destruction. Grasslands, an important habitat for native bees, are particularly vulnerable. One highly imperiled and understudied grassland type in the United States is the Pacific Northwest Bunchgrass Prairie. No studies have examined native bee communities in this prairie type. To fill this gap, the bee fauna of the Zumwalt Prairie, a large, relatively intact remnant of the Pacific Northwest Bunchgrass Prairie, was examined. Native bees were sampled during the summers of 2007 and 2008 in sixteen 40-ha study pastures on a plateau in northeastern Oregon, using a sampling method not previously used in grassland studies—blue vane traps. This grassland habitat contained an abundant and diverse community of native bees that experienced marked seasonal and inter-annual variation, which appears to be related to weather and plant phenology. Temporal variability evident over the entire study area was also reflected at the individual trap level, indicating a consistent response across the spatial scale of the study. These results demonstrate that temporal variability in bee communities can have important implications for long-term monitoring protocols. In addition, the blue vane trap method appears to be well-suited for studies of native bees in large expanses of grasslands or other open habitats, and may be a useful tool for monitoring native bee communities in these systems.

Soil Morphologic Properties and Cattle Stocking Rate Affect Dynamic Soil Properties

Abstract Soil properties that influence the capacity for infiltration and moisture retention are important determinants of rangeland productivity. Monitoring effects of grazing on dynamic soil properties can assist managers with stocking rate decisions, particularly if monitoring takes into account environmental variability associated with inherent soil morphological properties. On a Pacific Northwest Bunchgrass Prairie in northeast Oregon, we applied three cattle stocking rates (0.52, 1.04, and 1.56 animal unit months · ha−1) and an ungrazed control in a randomized complete block design for two 42-d grazing seasons and measured the change in four dynamic soil properties: soil penetration resistance, soil aggregate stability, bare ground, and herbaceous litter cover. To address apparent environmental heterogeneity within experimental units, we also utilized a categorical soil factor (termed Edaphic Habitat Types or EHT), determined by characterizing soil depth, texture, and rock fragment content at sample sites. Stocking rate did not affect extent of bare ground or soil aggregate stability. Stocking rate had a significant effect on penetration resistance, which was greatest at the high stocking rate (1.6 J · cm−1 ± 0.1 SE) and lowest in the control (1.1 J · cm−1 ± 0.1 SE). For litter cover, the effects of stocking rate and EHT interacted. In two rocky EHTs, litter cover was highest in the controls (60% ± 6 SE; 50% ± 3 SE) and ranged from 27% ± 3 SE to 33% ± 6 SE in the stocking rate treatments. Measures of penetration resistance, aggregate stability, and bare ground were different across EHTs regardless of stocking rate, but did not interact with stocking rate. Our study demonstrates that response of dynamic soil properties to stocking rates should be considered as a useful and accessible approach for monitoring effects of livestock management decisions on rangeland conditions.

Short-term responses of native bees to livestock and implications for managing ecosystem services in grasslands

Rangelands are significant providers of ecosystem services in agroecosystems world-wide. Yet few studies have investigated how different intensities of livestock grazing impact one important provider of these ecosystem services—native bees. We conducted the first large-scale manipulative study on the effect of a gradient of livestock grazing intensities on native bees in 16 40-ha pastures in the Pacific Northwest Bunchgrass Prairie. Each pasture was exposed to one of four cattle stocking rates for two years and grazing intensity was quantified by measuring utilization. We measured soil and vegetation characteristics related to floral and nesting resources as well as several metrics of the bee community. Increased grazing intensity significantly reduced vegetation structure, soil stability, and herbaceous litter and significantly increased soil compaction and bare ground. Native bees responded with changes in abundance, richness, diversity, and community composition. Responses varied with taxa and time of season. Bumble bees were sensitive to grazing intensity early in the season, showing reduced abundance, diversity, and/or richness with increased intensity, potentially because of altered foraging behavior. In contrast, sweat bees appeared unaffected by grazing. These results show that native bee taxa vary in their sensitivity to livestock grazing practices and suggest that grazing may potentially be a useful tool for managing pollination services in mosaic agroecosystems that include rangelands.

Impact of season-long water abstraction on invertebrate drift composition and concentration

Surface water abstraction from rivers for irrigated agriculture is one of the largest uses of freshwater resources in the world. Water abstraction has important impacts on the structure of riverine assemblages. However, little work has examined the chronic, season-long impacts on ecosystem functions. Invertebrate drift is an important ecosystem function of river systems influencing nutrient cycling, food webs, and invertebrate population dynamics. We examined the season-long impact of reduced discharge resulting from multiple points of abstraction on drift assemblage composition, concentration, and total drift load. Early in the season, water abstraction had little impact on drift assemblage composition. However, later in the irrigation season, the drift assemblage at sites impacted by water abstraction diverged from upstream, control sites. The degree of change in assemblage composition at impacted sites was related to the amount of water abstracted such that sites with the lowest discharge also had assemblages that differed most strongly from control sites. Drift assemblages at impacted sites became dominated by tolerant microcrustaceans. In addition, water abstraction resulted in an increase in drift concentration (ind./m3). However, despite this increase in concentration at impacted sites, total drift load (# of invertebrates drifting in the river) decreased with decreasing discharge.

The effect of soil depth and exposure to winter conditions on survival of the potato tuberworm, Phthorimaea operculella

The potato tuberworm, Phthorimaea operculella Zeller (Lepidoptera: Gelechiidae), is an important pest of solanaceous crops including potato [Solanum tuberosum L. (Solanaceae)]. Recent evidence of survival of the pest in the Columbia Basin of Oregon and Washington, USA, is surprising as potato tuberworm is most commonly a pest in tropical and subtropical regions. Potato tuberworm was studied in a manipulative experiment to determine how stage (egg, larva, and pupa) and soil depth affected the potential for winter survival. In early January 2007, eggs, tubers infested with larvae, and pupae were placed in the soil at 6, 10, and 20 cm depths, or left on the soil surface. Each potato tuberworm life stage was sampled at 7–17‐day intervals for 90 days and survival was estimated at each time period. Eggs survived up to 54 days with the greatest survival observed at 6‐cm soil depth and the lowest survival on the soil surface. However, egg survival was significantly reduced after 1 month of exposure to winter conditions. Larvae were able to survive up to 30 days with the greatest survival observed at 20‐cm soil depth. Tubers at the surface and buried at 6 cm were frozen; thus, no larval survival was recorded. The pupal stage showed a greater tolerance to winter conditions than the egg or larval stages, surviving up to 91 days of exposure. These results suggest that the pupal stage is probably the stage most likely to survive winter conditions in the northwestern USA.

The Status of the ‘Red-Listed’ Bombus occidentalis (Hymenoptera: Apiformes) in Northeastern Oregon

Abstract The western bumble bee, Bombus occidentalis, is included on the red list of bees by The Xerces Society. It was once a common bumble bee west of the Cascades but in the late 1990s it experienced a dramatic decline along coastal regions. The cause was speculated to be due to the introduction of pathogens from captive-bred bumble bees used for pollination of greenhouse crops. In extensive surveys conducted in western and southern Oregon, 10 individuals have been recorded since 2000. In this note, we report the collection of 49 individual B. occidentalis over two years in the Zumwalt Prairie Preserve of northeastern Oregon. This finding shows that B. occidentalis persists in northeastern regions of the Pacific Northwest, either because of geographic isolation from or potential resistance to the pathogens that decimated populations in the western part of the region. Further research is needed to determine its occurrence in other regions of its historical range to assess the extent of its decline. In addition, conservation efforts are critical for protection of this species in both agricultural ecosystems and in native habitats.

Insect Visitors and Pollination Ecology of Spalding's Catchfly (Silene spaldingii) in the Zumwalt Prairie of Northeastern Oregon

ABSTRACT: Silene spaldingii S. Watson (Spaldings catchfly) is a threatened wildflower that relies on insect-mediated pollination. However, its pollination ecology is not well understood, particularly in the Zumwalt Prairie of northeastern Oregon, which contains the largest known S. spaldingii population. Our objectives were to: (1) describe the principal insect visitors to S. spaldingii in the Zumwalt Prairie, (2) quantify the available pool of pollinators in the area, and (3) determine whether the visitation rate to individual plants is associated with the density of S. spaldingii at the patch scale, as predicted by the resource concentration hypothesis, and/or by the density and composition of non-S. spaldingii blooming plants, as predicted by the facilitation and competition hypotheses. We recorded insect visits to S. spaldingii during peak bloom at 30 patches and characterized the local bee community using blue vane traps. We quantified the patch-scale density of S. spaldingii and the composition and abundance of other blooming species at each patch. Two bumble bee species comprised all observed visits, although they constituted only 20% of the total bees sampled on the prairie. Bumble bees showed a high degree of host fidelity even when other blooming plants were present. Per capita visitation rates increased with catchfly density and blooming plant abundance at the patch scale, supporting the resource concentration and facilitation hypotheses. Silene spaldingii in the Zumwalt Prairie appears to rely on a narrow pool of pollinators that may preferentially visit it over other blooming plants, and more dense patches of S. spaldingii may increase pollination efficiency.

A method for subsampling terrestrial invertebrate samples in the laboratory: estimating abundance and taxa richness.

Abstract Significant progress has been made in developing subsampling techniques to process large samples of aquatic invertebrates. However, limited information is available regarding subsampling techniques for terrestrial invertebrate samples. Therefore a novel subsampling procedure was evaluated for processing samples of terrestrial invertebrates collected using two common field techniques: pitfall and pan traps. A three-phase sorting protocol was developed for estimating abundance and taxa richness of invertebrates. First, large invertebrates and plant material were removed from the sample using a sieve with a 4 mm mesh size. Second, the sample was poured into a specially designed, gridded sampling tray, and 16 cells, comprising 25% of the sampling tray, were randomly subsampled and processed. Third, the remainder of the sample was scanned for 4–7 min to record rare taxa missed in the second phase. To compare estimated abundance and taxa richness with the true values of these variables for the samples, the remainder of each sample was processed completely. The results were analyzed relative to three sample size categories: samples with less than 250 invertebrates (low abundance samples), samples with 250–500 invertebrates (moderate abundance samples), and samples with more than 500 invertebrates (high abundance samples). The number of invertebrates estimated after subsampling eight or more cells was highly precise for all sizes and types of samples. High accuracy for moderate and high abundance samples was achieved after even as few as six subsamples. However, estimates of the number of invertebrates for low abundance samples were less reliable. The subsampling technique also adequately estimated taxa richness; on average, subsampling detected 89% of taxa found in samples. Thus, the subsampling technique provided accurate data on both the abundance and taxa richness of terrestrial invertebrate samples. Importantly, subsampling greatly decreased the time required to process samples, cutting the time per sample by up to 80%. Based on these data, this subsampling technique is recommended to minimize the time and cost of processing moderate to large samples without compromising the integrity of the data and to maximize the information extracted from large terrestrial invertebrate samples. For samples with a relatively low number of invertebrates, complete counting is preferred.

Native Bees Associated with Isolated Aspen Stands in Pacific Northwest Bunchgrass Prairie

ABSTRACT: In the western United States, up to 90% of quaking aspen (Populus tremuloides Michx.) is estimated to have disappeared since European settlement due to human activities. This loss poses a significant threat to many forms of biodiversity associated with aspen, including plants, birds, and mammals. However, no work has investigated native bee diversity associated with aspen. Our objectives were to: (1) describe the diversity, taxonomic composition, and sex ratios of bees in aspen stands in native bunchgrass prairie of northeastern Oregon, (2) compare bee communities in aspen with those in an adjacent grassland, and (3) document differences in floral resources associated with each habitat. We sampled native bees and quantified blooming non-anemophilous plant richness in four aspen stands and four locations in bunchgrass prairie three times during the summer. Bee abundance increased in both habitats as the summer progressed. Although taxa richness and overall abundance of bees did not differ significantly between habitats, community composition did. Bumble bees (Bombus) were more abundant in aspen stands, while sweat bees (Lasioglossum and Halictus) were more abundant in grassland locations. Four species of bumble bees were significantly associated with aspen stands and sex ratios of bumble bees were significantly more male-biased in aspen stands compared to grassland locations. Floral resources are one potential driver of observed differences between habitats. In late summer, aspen stands had higher blooming plant richness, as well as a distinctive floral community. This study suggests that conserving aspen benefits not only many plants and vertebrates, but also ecologically significant invertebrates, such as native bees.

Morphometric condition as a measure of energetic content and reproductive potential in Dactylotum variegatum

Abstract Morphometric condition (i.e., condition estimated from some aspect of body size) is frequently calculated from mass-size residuals, and is often used to infer something about an animals energetic state. However, controversy exists about the validity of using mass-size residuals for this purpose. Recent work suggests the approach is appropriate in mammals, but it is unclear if morphometric condition is useful in the context of understanding the energetic state of grasshoppers and whether differences in morphometric condition are associated with variation in fecundity-related characteristics in females. To address this question, I examined male and female Dactylotum variegatum, sampled from a mosaic of grazed and ungrazed grasslands in southeastern Arizona, to determine whether morphometric condition corresponded with lipid and glycogen levels in males and a variety of reproductive characteristics in females. D. variegatum was found to be strongly sexually dimorphic relative to mass and femur length. The relationship between mass and femur length was linear for both males and females, an important assumption underlying the use of mass-size residuals as a measure of condition. Structurally larger males had more lipid and glycogen content. After controlling for the effect of size on lipid and glycogen content, a positive relationship was found between morphometric condition and lipid and glycogen levels, although the relationship was relatively weak for glycogen. In females, femur length, mass, and morphometric condition were not significantly associated with the number of ovarioles or the mean number of eggs or corpora lutea per ovariole. Only the mean number of resorption bodies per ovariole was significantly related to mass and morphometric condition, with larger and better conditioned females having fewer resorption bodies. The results of this study suggest that mass-size residuals in this species of grasshopper are related to energetic condition, but that other factors, such as variation in water and protein content, may weaken the relationship. Morphometric condition probably has limited value in estimating fitness of wild-caught D. variegatum females.