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Experimental exclusion of insectivorous predators results in no responses across multiple trophic levels in a water-limited, sagebrush-steppe ecosystem

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  • Idaho Department of Fish and Game
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Many of the existing parametric and nonparametric tests for homogeneity of variances, and some variations of these tests, are examined in this paper. Comparisons are made under the null hypothesis (for robustness) and under the alternative (for power). Monte Carlo simulations of various symmetric and asymmetric distributions, for various sample sizes, reveal a few tests that are robust and have good power. These tests are further compared using data from outer continental shelf bidding on oil and gas leases.
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We used bird-centered vegetation measurements to assess selection of shrub-defined patches by foraging Sage Sparrows (Amphispiza belli) and Brewer's Sparrows (Spizella breweri) over a period of 3 yr. Principal components of habitat variation based on a random sample of 900 patches were patch size, species composition (big sagebrush [Artemisia tridentata] vs. green rabbitbrush [Chrysothamnus viscidiflora] vs. gray rabbitbrush [C. nauseosus]), and plant vigor. Both bird species differed from random (P < 0.001) in their selection of patches in each year, but did not differ from each other. Patch structure varied significantly among years (P < 0.001), as did each species' use of patches. Between-year changes in the two bird species' selection patterns were highly correlated with changes in the habitat, and with each other. These correlations suggest that shrubsteppe sparrows 'track' interannual variation in patch composition and structure (which is presumably induced by precipitation variation) but maintain nonrandom use patterns within years.
Article
Urban and suburban forests provide important recreational values to the public and can also contain high levels of biodiversity even though management regimes of such forests often include the clearing of bushes, shrubs and small trees to make them more attractive to people. In a field experiment, we studied the importance of bush canopies as foraging sites for insectivorous birds. In suburban forests, the canopies of rowan (Sorbus aucuparia) and alder buckthorn (Frangula alnus) were net-enclosed for two months at three study sites in SW Sweden. We compared abundance, biomass and body size distributions of arthropods between experimental and control canopies of rowan and alder buckthorn and found the effects to be strong. Biomass of arthropods was generally an order of magnitude higher on canopies protected from bird predation than on controls. Dermaptera contributed most to the biomass estimated in net-enclosed bushes. In certain taxa, however, the biomass was not significantly affected by the experimental treatment, possibly in part due to on-going reproduction during the experimental period. Furthermore, median body size in net-enclosed bushes was 1.3–3.0 times the control, excluding Araneae. The relative difference in arthropod biomass due to the experimental treatment was related to the density of breeding insectivorous birds at each site. We concluded that bush canopies are important foraging sites for birds in suburban forests, although we cannot rule out trait-mediated effects. Management regimes that include clearing of the ground may affect the bird community negatively.
Article
Bell System Technical Journal, also pp. 623-656 (October)
The respiratory activity of plants in the light, measured as CO:! release from the TCA cycle or O-2 consumption by the respiratory chain, varies between 25 and 100% of the dark respiratory activity. This has been interpreted as evidence for an inhibition of respiration during photosynthesis. However, studies with specific respiratory inhibitiors have shown that oxidative phosphorylation occurs in the Light and provides the cytosol with ATP, which is required for sucrose synthesis. Respiratory activity in the light might also be required to sustain a high photosynthetic capacity and might even prevent photoinhibition. Sources of redox equivalents for oxidative phosphorylation can be photosynthetically generated, externally oxidized NADPH; photorespiratory oxidation of glycine to serine; or a partial activity of the TCA cycle. Fifty to seventy-five percent of the redox equivalents produced in the mitochondria remain in the matrix and can function in ATP syntheses. The other 25-50% are exported via the mitochondrial malate-oxaloacetate shuttle and can function in nitrate reduction in the cytosol or hydroxypyruvate reduction in the peroxisomes.
Article
The enemies hypothesis holds that predatory insects and parasitoids are more effective at controlling populations of herbivores in diverse systems of vegetation than in simple ones. Eighteen studies that tested the enemies hypothesis are reviewed. Of those studies reporting mortality from predation or parasitism, nine found higher mortality rates in diverse systems; two found a lower mortality rate; and two found no difference. The mechanisms that are thought to underlie the enemies hypothesis and directions for future research are discussed. Evidence suggests that the enemies hypothesis and the resource concentration hypothesis (which predicts that herbivores more easily find, stay in, and reproduce in monocultures of host plants than in polycultures) are complementary mechanisms in reducing numbers of herbivores in diverse agricultural systems.
Article
The sagebrush biotype is the largest in the western United States. This vast sagebrush community is thought to harbor equally vast and diverse arthropod communities, but these remain little explored. Our objective was to examine the diversity, abundance, and seasonal phenology of arthropod taxa found on the dominant shrub of the sagebrush ecosystem, big sagebrush (Artemisia tridentata). We wanted to improve understanding of this little-studied arthropod assemblage that may play significant roles in the dynamics of sagebrush populations and the sagebrush ecosystem. We sampled free-living and gall-forming arthropods from a stratified random sample of sagebrush plants at the Barton Road Ecological Research Area, Idaho, resulting in a sample of over 8000 individuals and 232 morphospecies. Species richness and abundance declined from May to August, and abundance of most taxa similarly declined over the summer. A few taxa, including Acari (mites), were notably more abundant in August. Fluid feeders were the most diverse and abundant free-living feeding guild during all months and comprised up to 79% of morphospecies. The gall formers included 4713 individuals of 12 species of gall flies (Rhopalomyia spp.), primarily (97%) R. ampullaria. Abundance of galls increased from small to large (presumably young to old) plants. Overall, A. tridentata was host to a high diversity of arthropods, some of which have potential to cause or mitigate significant damage to their host plant. Arthropods seem likely to have the greatest impact on sagebrush early in the growing season, when they are most diverse and abundant. Documentation of the full diversity of arthropods associated with sagebrush required samples taken throughout the growing season, but a single sample early in the growing season captured a high proportion of taxa.
Article
Many factors can influence the top-down and bottom-up dynamics of phytophagous insects. Although interactions between herbivore species have been frequently shown to be ecologically important, the effects of such horizontal trophic interactions on the relative roles of top-down and bottom-up forces have gone largely unstudied. In this paper we report on the results of a factorial field experiment in which we examined the effects of within-trophic-level interactions on the top-down and bottom-up dynamics of a salt marsh planthopper. We manipulated the bottom-up effects of plant quality by increasing soil salinity, and manipulated top-down effects by decreasing the intensity of parasitoid attack with yellow sticky traps that removed hymenopteran parasitoids. We applied these treatments to plots in two patches of the host plant, one with low densities of lepidopteran stem borer larvae, and one with high densities of stem borers. We maintained the treatments and monitored planthopper density for ten months, from March through December 1999. Increased salinity significantly increased planthopper density within one month of the first application of salt. The rapid response of the planthopper to salt treatments suggested a chemical mechanism, perhaps mobilization of bound nitrogen. Yellow sticky traps, although significantly reducing parasitism of planthopper eggs, had little impact on hopper density. The density of lepidopteran stem borers, however, had an even greater impact on planthopper density than did salt treatments, with high stem borer plots supporting much lower densities of hoppers. Stem borer density also reduced the response of the planthopper to other treatments, especially salt supplementation. The results of this study show that the impact of within-trophic-level interactions can significantly change herbivore trophic dynamics and can be even more important than either top-down or bottom-up effects in determining herbivore density.
Article
Elevated atmospheric CO2 may alter decomposition rates through changes in plant material quality and through its impact on soil microbial activity. This study examines whether plant material produced under elevated CO2 decomposes differently from plant material produced under ambient CO2. Moreover, a long-term experiment offered a unique opportunity to evaluate assumptions about C cycling under elevated CO2 made in coupled climate–soil organic matter (SOM) models. Trifolium repens and Lolium perenne plant materials, produced under elevated (60 Pa) and ambient CO2 at two levels of N fertilizer (140 vs. 560 kg ha−1 yr−1), were incubated in soil for 90 days. Soils and plant materials used for the incubation had been exposed to ambient and elevated CO2 under free air carbon dioxide enrichment conditions and had received the N fertilizer for 9 years. The rate of decomposition of L. perenne and T. repens plant materials was unaffected by elevated atmospheric CO2 and rate of N fertilization. Increases in L. perenne plant material C : N ratio under elevated CO2 did not affect decomposition rates of the plant material. If under prolonged elevated CO2 changes in soil microbial dynamics had occurred, they were not reflected in the rate of decomposition of the plant material. Only soil respiration under L. perenne, with or without incorporation of plant material, from the low-N fertilization treatment was enhanced after exposure to elevated CO2. This increase in soil respiration was not reflected in an increase in the microbial biomass of the L. perenne soil. The contribution of old and newly sequestered C to soil respiration, as revealed by the 13C-CO2 signature, reflected the turnover times of SOM–C pools as described by multipool SOM models. The results do not confirm the assumption of a negative feedback induced in the C cycle following an increase in CO2, as used in coupled climate–SOM models. Moreover, this study showed no evidence for a positive feedback in the C cycle following additional N fertilization.
Article
Insect herbivores can reduce growth, seed production, and population dynamics of host plants, but do not always do so. Big sagebrush (Artemisia tridentata) has one of the largest ranges of any shrub in North America, and is the dominant and characteristic shrub of the extensive sagebrush steppe ecosystem of the western United States. Nevertheless, the impact of insect herbivores on big sagebrush, its dominant and characteristic shrub, is largely unknown. Occasional large effects of insect herbivore outbreaks are documented, but there is little knowledge of the impact of the more typical, nominal herbivory that is produced by the diverse community of insects associated with big sagebrush in natural communities. In 2008, we removed insects from big sagebrush plants with insecticide to evaluate whether insect herbivores reduced growth and seed production of big sagebrush. Removal of herbivores led to significant and substantial increases in inflorescence growth (22%), flower production (325%), and seed production (1053%) of big sagebrush. Our results showed the impact of insect herbivory in the current growing season on the growth and reproduction of big sagebrush and revealed an unrecognized, significant role of non-outbreak herbivores on big sagebrush. KeywordsArthropod-Defoliator-Feeding guild-Trophic dynamics-Sagebrush steppe
Article
Insectivorous birds can increase plant growth by consuming herbivorous insects and reducing insect damage. However, plant traits such as the level of chemical defense may affect the quantity and quality of insects, and alter the foraging behavior of birds. Therefore, I predicted that plant traits can also modify the effect of birds on leaf damage and plant growth. This study compared the effect of insectivorous birds on the herbivory and growth of two chemically different willow species, weakly defended Salix phylicifolia and strongly defended S. myrsinifolia under two fertilization levels. Half of the willows were protected from birds using a translucent gill-net, which did not limit access by insects. The effect of birds on the densities of leaf-chewing insects and leaf damage was considerable on unfertilized S. phylicifolia but less obvious on fertilized ones. The effect of bird predation was negligible on S. myrsinifolia, which had very low insect densities in all treatments. Birds increased the growth of the experimental willows, but the effect was clear only in unfertilized S. phylicifolia. I suggest that birds avoided foraging on willows with low populations of insects and little visible damage. The study shows that bird predation can alter the patterns of insect densities we see on willows, emphasizing the importance of considering multitrophic effects when studying plant-insect interactions.
Article
Differences of collembolan communities within the organo-mineral A layer were studied in relation to physico-chemical changes in humus at nine sites of beech forests (Fagus sylvatica L.) and first generation spruce stands (Picea abies (L.) Karst.), planted on former beech forest 30 years ago (Central Pyrenees, France). Changes in humus form were caused by the spruce plantation and occurred mainly within the fermentation horizon where acidifying litter accumulation increased the horizon depth. The recent replacement of beech by spruce induced a shift from mull towards moder humus forms, which is explained by the decreasing organic matter turnover rate. A significant decrease in the three exchangeable cations and pH under spruce was also observed. Collembolan species diversity within the A horizon was significantly lower under spruce at this early stage of the silvogenetic cycle. Differences between species composition of Collembola between the two forest stands is related to changes in environmental conditions (e.g. nutrient availability, soil porosity, soil moisture). This study shows how forest management practices are susceptible to modify biological activity within the A horizon under exotic conifer plantations.
Article
The biota of the food web of the sand community in the Coachella Valley desert include 174 species of vascular plants, 138 species of vertebrates, >55 species of arachnids, and an unknown (but great) number of microorganisms, insects (2000-3000 estimated species), Acari and nematodes. Trophic relations are presented in a series of nested subwebs and delineations of the community. Complexity arises from the large number of interactive species, the frequency of omnivory, age structure, looping, the lack of compartmentalization, and the complexity of the arthropod and soil faunas. Patterns from the Coachella web are compared with theoretical predictions and "empirical generalizations' derived from catalogs of published webs. The Coachella web differs greatly: chains are longer, omnivory and loops are not rare, connectivity is greater, top predators are rare or nonexistent, and prey-to-predator ratios are >1.0. The evidence argues that actual community food webs are extraordinarily more complex than those webs cataloged by theorists. -from Author
Article
Low precipitation and high temperatures in deserts limit primary productivity, and reduce rates of herbivory and microbial decomposition. As a result, the small amount of plant tissue produced in deserts dies unconsumed and turns to litter that is consumed by macrodetritivorous arthropods, which are preyed upon by small ectothermic arthropods and reptiles. This ectotherm-based food chain is energetically efficient and, despite the low productivity of the desert habitat, is able to support a fourth trophic level of endothermic predators, i.e. mammals and birds, that forage over large areas and locate their prey visually from a distance. This four-link chain results in trophic interactions that run from the large endothermic predators through the ectothermic ones to the macrodetritivores. Thus, macrodetritivores are released from predation and become food-limited. However, the full expression of these interactions occurs only in low productivity habitats with low plant cover. In productive habitats, plant cover blocks the vision of endothermic predators and provides refuge to small ectothermic ones. This results in ectothermic predators becoming abundant in habitats with high plant cover and controlling their prey, the macrodetritivores. Thus, the increase in productivity in deserts decreases the amount of energy that reaches top predators as it has a mainly structural effect: to increase plant cover and mediate predation interactions.
Article
Traditionally, herbivorous insects are thought to exhibit enhanced performance and outbreak dynamics on water-stressed host plants due to induced changes in plant physiology. Recent experimental studies, however, provide mixed support for this historical view. To test the plant-stress hypothesis (PSH), we employed two methods (the traditional vote-counting approach and meta-analysis) to assess published studies that investigated insect responses to experimentally induced water-deficit in plants. For insects, we examined how water deficit affects survivorship, fecundity, density, relative growth rate, and oviposition preference. Responses were analyzed by major feeding guild (sap-feeding insects and chewing insects) and for the subguilds of sap-feeders (phloem, mesophyll, and xylem feeders) and chewing insects (free-living chewers, borers, leaf miners, and gall-formers). Both vote counting and meta-analysis found strong negative effects of water stress on the performance of sap-feeding insects at large and on members of the phloem- and mesophyll-feeding subguilds in particular. Both analytical techniques demonstrated a nonsignificant response for chewing insects at large due to the offsetting effects of water stress on the different subguilds. For example, our analyses found consistent positive responses for borers, negative responses for gall-formers, and in consistent responses for free-living species and leaf miners. Overall, our analyses strongly challenge the historical view that herbivorous insects exhibit. elevated performance and outbreak dynamics on water-stressed plants. Rather, there is widespread evidence that many phytophagous insects, especially sap-feeders, are adversely affected by continuous water stress. Despite enhanced foliar nitrogen during times of plant stress, concurrent reductions in turgor and water content interfere with an herbivore's ability to access or utilize nitrogen. To explain the discrepancy between the observed outbreaks of phytophagous insects on water-stressed plants in nature and the negative effects detected in many experimental studies where plants are continuously stressed, we propose a "pulsed stress hypothesis" whereby bouts of stress and the recovery of turgor allow sap-feeders to benefit from stress-induced increases in plant nitrogen. Our finding that phloem-feeding insects respond positively on intermittently stressed plants but exhibit poor performance on continuously stressed ones is consistent with this hypothesis and suggests that the phenology of water stress as it mediates nitrogen availability may hold the key to understanding how water stress affects the population dynamics of insect herbivores.
Article
Growth of vegetative structures occurred in the spring when water was not limiting, and shrubs in both watering treatments exerted little stomatal control over water loss. Reproductive growth occurred through the summer when water was limiting, and supplemental watering increased reproductive growth. Shrubs conserved water during summer by abscising leaves and lowering stomatal conductance of both vegetative and reproductive modules in response to decreases in xylem pressure potential and increases in evaporative demand. Leaf abscission can occur without decreasing the amount of carbon available for reproductive growth because inflorescences are capable of positive photosynthetic rates comparable to vegetative leaves. Resource limitations were, however, reflected in the efficiency of water use during tissue construction; floral leaves and floral heads of shrubs not receiving supplemental water were produced with higher water-use efficiency. Conservative use of water during production of vegetative modules would offer no advantage to A. tridentata because neighbouring species are also most active at this time. Reproductive growth in A. tridentata occurs during summer when neighbouring species are largely dormant, and so efficient use of water may allow development of reproductive structures to continue throughout the summer even with limited supplies of water. -from Authors
Article
Arid environments are characterized by limited and variable rainfall that supplies resources in pulses. Resource pulsing is a special form of environmental variation, and the general theory of coexistence in variable environments suggests specific mechanisms by which rainfall variability might contribute to the maintenance of high species diversity in arid ecosystems. In this review, we discuss physiological, morphological, and life-history traits that facilitate plant survival and growth in strongly water-limited variable environments, outlining how species differences in these traits may promote diversity. Our analysis emphasizes that the variability of pulsed environments does not reduce the importance of species interactions in structuring communities, but instead provides axes of ecological differentiation between species that facilitate their coexistence. Pulses of rainfall also influence higher trophic levels and entire food webs. Better understanding of how rainfall affects the diversity, species composition, and dynamics of arid environments can contribute to solving environmental problems stemming from land use and global climate change.
Article
Airborne communication between individuals, called "eavesdropping" in this paper, can cause plants to become more resistant to herbivores when a neighbor has been experimentally clipped. The ecological relevance of this result has been in question, since individuals may be too far apart for this interaction to affect many plants in natural populations. We investigated induced resistance to herbivory in sagebrush, Artemisia tridentata, caused by experimental clipping of the focal plant and its neighbors. We found no evidence for systemic induced resistance when one branch was clipped and another branch on the same plant was assayed for naturally occurring damage. In this experiment, air contact and plant age were not controlled. Previous work indicated that sagebrush received less damage when a neighboring upwind plant within 15 cm had been experimentally clipped. Here we found that pairs of sagebrush plants that were up to 60 cm apart were influenced by experimental clipping of a neighbor. Furthermore, we observed that most individuals had conspecific neighbors that were much closer than 60 cm. Air contact was essential for communication; treatments that reduced airflow between neighboring individuals, either because of wind direction or bagging, prevented induced resistance. Airflow was also necessary for systemic induced resistance among branches within an individual. Reports from the literature indicated that sagebrush is highly sectorial, as are many desert shrubs. Branches within a sagebrush plant do not freely exchange material via vascular connections and apparently cannot rely on an internal signaling pathway for coordinating induction of resistance to herbivores. Instead, they may use external, volatile cues. This hypothesis provides a proximal explanation for why sagebrush does not demonstrate systemic induced resistance without directed airflow, and why airborne communication between branches induces resistance.