Herbivore regulation in urban agroecosystems: Direct and indirect effects

Urban agroecosystems can provide habitat for biodiversity and can benefit human communities through urban food provisioning. Moreover, urban agroecosystems could be managed so as to optimize ecosystem services like natural pest control provided by trophic interactions between natural enemies and herbivores. As in other ecosystems, predation and parasitism regulate herbivores in urban settings, but less is known about the relative importance of direct and indirect effects at local and landscape scales in highly managed urban agroecosystems. We collected data on herbivore (cabbage aphid) density and parasitism ratios (proportion of parasitized aphid “mummies”) in 25 community gardens in three counties in the California central coast, USA. We used structural equation modeling to examine the effects of direct factors (host plant characteristics and parasitism) and indirect factors (soil, garden, and landscape characteristics) on herbivore density changes at two time points in the growing season (June and August). Aphid density, but not parasitism, varied across counties over the season, and there was a strong negative relationship between aphid density and parasitism. Direct effects were strong drivers of aphid density but not parasitism. In June, aphid density increased with host plant volume but decreased with greater floral density, while parasitism was only influenced by aphid density. In August, host plant volume similarly positively affected aphid density, and soil water holding capacity increased host plant volume. In addition, host plant density had a strong negative effect on parasitism. Urban gardeners may be able to reduce aphid pest densities by increasing floral resource density and strategically spatially distributing host plants throughout garden beds, though these processes depend on the season. The indirect effects of soil water holding capacity on aphid densities further suggest a critical role of human management on pest populations and pest control services through soil amendments and irrigation.     

Direct and indirect effects of n-species competition

Summary A measure of net interspecific competition is proposed which incorporates both direct and indirect effects between each species pair in a community. Indirect effects of one species on another may be mediated through other species in the community such that changes in the first species induce changes in other species which in turn directly affect the second species. Even in a competitive community these indirect effects are not necessarily always of a competitive nature and may sometimes actually act to oppose the effects of direct competition. Species pairs which are strong competitors in isolation may exhibit little net competition or even appear mutualistic depending on the other species present in the community.An expression for the net competitive effects between species pairs is derived in terms of the elements of the -matrix representation of the direct competitive effects. Examples calculated from the literature suggest that indirect competitive effects may often act to counterbalance direct competition between otherwise strong competitors in natural communities.     

Differential regulation of multiple overlapping promoters in flagellar class II operons in Escherichia coli

The cer –Xer dimer resolution system of plasmid ColE1 is highly selective, acting only at sites on the same molecule and in direct repeat. Recombination requires the XerCD recombinase and accessory proteins ArgR and PepA. The Escherichia coli chromosome dimer resolution site dif and the type II hybrid site use the same recombinase but are independent of ArgR and PepA and show no site selectivity. This has led to the proposal that ArgR and PepA are responsible for the imposition of constraint. We describe here the characterization of a novel class of 'conditionally constrained' multimer resolution sites whose properties support this hypothesis. In the presence of ArgR and PepA, plasmids containing conditionally constrained sites are monomeric, but in their absence, extensive multimerisation is seen. A mutant ArgR derivative (ArgR110), which is defective in cer -mediated dimer resolution, remains able to prevent plasmid multimerisation by a conditionally constrained site. This implies that the accessory factors block recombination in trans rather than ensuring rapid multimer resolution. When the distance between the ArgR and XerCD binding sites in a conditionally constrained site was altered by a non-integral number of helical turns, the site became unconstrained. Constraint was restored by the insertion of a full helical turn.     

Direct and indirect effects in microcosm communities of protists

Increased complexity in biological communities can increase the variety of interactions among species, but the relative strengths and long-term consequences of various direct and indirect interactions require further investigation. I studied interactions among four species of protists by monitoring their population dynamics when they were cultured either together or in seven different subset communities. Two protists were bacterivores (Chilomonas and Tetrahymena) and two were predators (Actinosphaerium and Euplotes). Actinosphaerium was omnivorous, and could eat both predatory Euplotes and bacterivores. Three indirect effects occurred among the four species of protists, including indirect facilitation of one predator by the other, apparent competition between bacterivores, and indirect facilitation of one bacterivore by the omnivorous predator. Community structure and invasibility depended on both direct and indirect effects; thus both can be mechanisms for assembly rules.     

Direct and indirect regulation of gonadotropin-releasing hormone neurons by estradiol

Estrogen signaling to GnRH neurons is critical for coordinating the preovulatory surge release of LH with follicular maturation. Until recently it was thought that estrogen signaled GnRH neurons only indirectly through numerous afferent systems. This minireview presents new evidence indicating that GnRH neurons are directly regulated by estradiol (E2), primarily through estrogen receptor (ER)-beta, and indirectly through E2-sensitive neurons in the anteroventral periventricular (AVPV) region. The data described suggest that E2 generally represses GnRH gene expression but that this repression is transiently overcome by indirect E2-dependent signals relayed by AVPV neurons. We also present evidence that the AVPV neurons responsible for relaying E2 signals to GnRH neurons are multifunctional gamma aminobutyric acid-ergic/glutamatergic/neuropeptidergic neurons.     

Direct and indirect transgenerational effects alter plant-herbivore interactions

Theory suggests that environmental effects with transgenerational consequences, including rapid evolution and maternal effects, may affect the outcome of ecological interactions. However, indirect effects occur when interactions between two species are altered by the presence of a third species, and can make the consequences of transgenerational effects difficult to predict. We manipulated the presence of insect herbivores and the competitor Medicago polymorpha in replicated Lotus wrangelianus populations. After one generation, we used seeds from the surviving Lotus to initiate a reciprocal transplant experiment to measure how transgenerational effects altered ecological interactions between Lotus, Medicago, and insect herbivores. Herbivore leaf damage and Lotus fecundity were dependent on both parental and offspring environmental conditions. The presence of insect herbivores and Medicago in the parental environment resulted in transgenerational changes in herbivore resistance, but these effects were non-additive, likely as a result of indirect effects in the parental environment. Indirect transgenerational effects interacted with more immediate ecological indirect effects to affect Lotus fecundity. These results suggest that explanations of ecological patterns require an understanding of transgenerational effects and that these effects may be difficult to predict in species-rich, natural communities where indirect effects are prevalent.     

Direct and indirect food web regulation of microbial decomposers in headwater streams

The direct and indirect regulation of primary productivity has been well established in autotrophic‐based ecosystems; however, less is known about the processes affecting decomposers in detrital‐based ecosystems. Because, small headwater, woodland streams are a dominate feature in most ecosystems and are tightly linked to terrestrial detritus, understanding decomposer‐mediated functions in these systems is critical for understanding carbon processes across the landscape. In this light, we conducted a microcosm and mesocosm experiment to test the direct and indirect food web effects on decomposers in small stream ecosystems. The results from the microcosm experiment supported an existing literature, demonstrating that nutrients directly stimulate decomposers and that microbivores directly reduce decomposers. Based on well‐founded food web theory in autotrophic systems, we predicted that fishes from different trophic‐functional guilds would indirectly stimulate decomposers by enhancing dissolved nutrients and by reducing microbivore densities. Our mesocosm experiment partially supported these predictions. Specifically, we found that fishes that consumed mostly terrestrial foods increased decomposers from the bottom–up by enhancing allochthonous nutrient loading into the stream ecosystems. Contrary to our predictions, however, predatory fishes that consume microbivores did not increase decomposers from the top–down. Rather, in streams with the predatory fish species, microbivores increased (rather than decreased) on leaf litter. This may have resulted from an experimental artifact associated with refuge provided by leaf packs. In conclusion, our data demonstrate that decomposers are regulated by similar direct and indirect processes important in autotrophic‐based ecosystems. This provides further evidence that food web processes can regulate leaf decomposition and flux of detrital carbon through ecosystems.     

Direct and indirect effects of obestatin peptides on food intake and the regulation of glucose homeostasis and insulin secretion in mice

Obestatin is a recently discovered peptide hormone that appears to be involved in reducing food intake, gut motility and body weight. Obestatin is a product of the preproghrelin gene and appears to oppose several physiological actions of ghrelin. This study investigated the acute effects of obestatin (1-23) and the truncated form, obestatin (11-23), on feeding activity, glucose homeostasis or insulin secretion. Mice received either intraperitoneal obestatin (1-23) or (11-23) (1 micromol/kg) 4h prior to an allowed 15 min period of feeding. Glucose excursions and insulin responses were lowered by 64-77% and 39-41%, respectively, compared with saline controls. However this was accompanied by 43% and 53% reductions in food intake, respectively. The effects of obestatin peptides were examined under either basal or glucose (18 mmol/kg) challenge conditions to establish whether effects were independent of changes in feeding. No alterations in plasma glucose or insulin responses were observed. In addition, obestatin peptides had no effect on insulin sensitivity as revealed by hypoglycaemic response when co-administered with insulin. Our observations support a role for obestatin in regulating metabolism through changes of appetite, but indicate no direct actions on glucose homeostasis or insulin secretion.     

Direct and indirect effects of leptin on adipocyte metabolism

Leptin is hypothesized to function as a negative feedback signal in the regulation of energy balance. It is produced primarily by adipose tissue and circulating concentrations correlate with the size of body fat stores. Administration of exogenous leptin to normal weight, leptin responsive animals inhibits food intake and reduces the size of body fat stores whereas mice that are deficient in either leptin or functional leptin receptors are hyperphagic and obese, consistent with a role for leptin in the control of body weight. This review discusses the effect of leptin on adipocyte metabolism. Because adipocytes express leptin receptors there is the potential for leptin to influence adipocyte metabolism directly. Adipocytes also are insulin responsive and receive sympathetic innervation, therefore leptin can also modify adipocyte metabolism indirectly. Studies published to date suggest that direct activation of adipocyte leptin receptors has little effect on cell metabolism in vivo, but that leptin modifies adipocyte sensitivity to insulin to inhibit lipid accumulation. In vivo administration of leptin leads to a suppression of lipogenesis, an increase in triglyceride hydrolysis and an increase in fatty acid and glucose oxidation. Activation of central leptin receptors also contributes to the development of a catabolic state in adipocytes, but this may vary between different fat depots. Leptin reduces the size of white fat depots by inhibiting cell proliferation both through induction of inhibitory circulating factors and by contributing to sympathetic tone which suppresses adipocyte proliferation. This article is part of a Special Issue entitled: Modulation of Adipose Tissue in Health and Disease.     

Direct and indirect effects of the pRb tumor suppressor on autophagy

Autophagy, an intracellular degradation pathway involved in cell survival or demise, is tightly controlled by complex regulatory mechanisms. A link between the Rb tumor suppressor and autophagy is now emerging. pRb plays a critical role in cell cycle progression and survival as well as the differentiation of certain cell types. Recently, we have reported that during skeletal myogenesis, Rb-deficient myoblasts fuse to form short myotubes that quickly degenerate. Myotube degeneration was associated with increased autophagic flux and inhibition of autophagy rescued the defect leading to long, twitching myotubes. We propose that Rb-loss sensitizes cells to autophagy via direct and indirect mechanisms and we discuss how these might affect cancer progression and response to chemotherapy.     

Direct and indirect effects of the pRb tumor suppressor on autophagy

Autophagy, an intracellular degradation pathway involved in cell survival or demise, is tightly controlled by complex regulatory mechanisms. A link between the Rb tumor suppressor and autophagy is now emerging. pRb plays a critical role in cell cycle progression and survival as well as the differentiation of certain cell types. Recently, we have reported that during skeletal myogenesis, Rb-deficient myoblasts fuse to form short myotubes that quickly degenerate. Myotube degeneration was associated with increased autophagic flux and inhibition of autophagy rescued the defect leading to long, twitching myotubes. We propose that Rb-loss sensitizes cells to autophagy via direct and indirect mechanisms and we discuss how these might affect cancer progression and response to chemotherapy.     

Direct and indirect effects of weather variability in a specialist butterfly

1. Climate change poses serious threats to the long‐term persistence of many animal and plant populations. Species having specific niche requirements, or characterised by highly co‐evolved interactions, will face the greatest challenges. An example is represented by Maculinea alcon (Denis & Schiffermüller), a monophagous and univoltine butterfly species, which lays eggs only on larval host plants which occur inparticular phenological conditions. 2. The present 2‐year study focused on two M. alcon populations, both located at the southern boundaries of the species, but facing different climatic conditions (360 m, low altitude versus 860 m, high altitude). Population vulnerability with respect to direct and indirect effects of climate change was analysed, focusing on two important aspects of butterfly biology, i.e. the flight activity of adults and the degree of synchrony in the larval plant–insect interactions. 3. It was observed that, when positive temperature anomalies are reached, the temperature can exert detrimental effects on adults' activity. At a low altitude, in a hotter than usual year, a temperature threshold was recorded (around 32 °C), above which the activity of butterflies is inhibited. In contrast, at a high altitude, temperature increases maintain the opportunity to enhance butterfly activity. Altitudinal differences were also observed in the phenology of the two interacting species, which generate stronger asynchrony at low altitudes. 4. High‐ and low‐altitude populations represent different conservation units: a global increase in temperature would pose a serious threat to the lowland populations, whereas high‐altitude populations would gain a greater role in assuring the persistence of M. alcon at its southern boundaries.     

Direct and indirect effects of termites on savanna tree-seedling growth

Termites are considered to be ecosystem engineers because they modify their biophysical environments. We tested the effects of soil-nutrient alteration in termite-enriched soils compared with termite-free open savannas. We also tested whether non-nutrient alterations (soil disturbance) induced by termites led to changes in tree-seedling growth. Soil samples taken from termite-enriched soils and adjacent open savanna sites in KwaZulu-Natal, South Africa were analyzed for nitrogen, pH, organic carbon and water-holding capacity. Seeds from three dominant tree species, Acacia sieberiana, Celtis africana and Ziziphus mucronata, were grown in soils taken from termite-enriched soils and adjacent savannas. Overall, organic carbon and nitrogen content were higher in termite-enriched soils than in adjacent savannas. We found that these differences in nutrients did not directly affect seedling growth rates or final height. However, C. africana had increased growth rates in compacted termite-enriched soils, while A. sieberiana seedlings were taller in non-compacted soils. We conclude that the indirect effects of disturbance by termites may be as important as the direct effects of increased nutrients for growth of savanna trees.     

Direct and indirect effects of body weight on adult wages

Previous estimates of the association between body weight and wages in the literature have been conditional on education and occupation. In addition to the effect of current body weight status (body mass index (BMI) or obesity) on wages, this paper examines the indirect effect of body weight status in the late-teenage years on wages operating through education and occupation choice. Using the National Longitudinal Survey of Youth 1979 data, for women, we find that a one-unit increase in BMI is directly associated with 1.83% lower hourly wages whereas the indirect BMI wage penalty is not statistically significant. Neither a direct nor an indirect BMI wage penalty is found for men. However, results based on clinical weight classification reveal that the indirect wage penalty occurs to a larger extent at the upper tail of the BMI distribution for both men and women via the pathways of education and occupation outcomes. Late-teen obesity is indirectly associated with 3.5% lower hourly wages for both women and men. These results are important because they imply that the total effect of obesity on wages is significantly larger than has been estimated in previous cross-sectional studies.     

Direct and indirect climate change effects on photosynthesis and transpiration

Climate change affects plants in many different ways. Increasing CO(2) concentration can increase photosynthetic rates. This is especially pronounced for C(3) plants, at high temperatures and under water-limited conditions. Increasing temperature also affects photosynthesis, but plants have a considerable ability to adapt to their growth conditions and can function even at extremely high temperatures, provided adequate water is available. Temperature optima differ between species and growth conditions, and are higher in elevated atmospheric CO(2). With increasing temperature, vapour pressure deficits of the air may increase, with a concomitant increase in the transpiration rate from plant canopies. However, if stomata close in response to increasing CO(2) concentration, or if there is a reduction in the diurnal temperature range, then transpiration rates may even decrease. Soil organic matter decomposition rates are likely to be stimulated by higher temperatures, so that nutrients can be more readily mineralised and made available to plants. This is likely to increase photosynthetic carbon gain in nutrient-limited systems. All the factors listed above interact strongly so that, for different combinations of increases in temperature and CO(2) concentration, and for systems in different climatic regions and primarily affected by water or nutrient limitations, photosynthesis must be expected to respond differently to the same climatic changes.