Variation In Strength Of Interactions Research Articles

Superior moderate-strength Pd–Pt/Al-based composite tailored for dehydrogenation of perhydroacenaphthene

Pd–Pt-based catalysts are promising for the dehydrogenation of perhydroacenaphthene (PHAN) molecules, yet their effectiveness is hampered by strong adsorption of the product C12H8 and limitations under extreme conditions. Herein, utilizing a relational model involving d-band center, sub-surface coverage, and H atom and C12H8 adsorption energies, the most dehydrogenation-prone crystal facet was identified. Following this, employing Mulliken charge and Sabatier principle, we screened positively charged Pd–Pt/111_O2(Al) surface with moderate interaction strength through the calculation of interaction energy between the chosen Pd–Pt surface and thermally stable MgAl2O4(111) terminal surfaces. Crucially, an extensive examination into reaction mechanisms, microkinetics, and the variation in interaction strength between support and active components at 873.15 K and 3 MPa was conducted. It was observed that the rate-determining step for PHAN molecules on the Pd–Pt/111_O2(Al) surface was C12H19* + * → C12H18* + H*, with a rate of 2.57 × 10-23 s−1. The dehydrogenation of C12H10 to C12H8 proved more favorable than from C12H20 to C12H10, with the latter strengthening support and active components, unlike the weakening effect of the former. This work offers essential foundational support for the development of excellent performance catalysts and the dehydrogenation process of superior heat sink fuels.

Evaluating top-down, bottom-up, and environmental drivers of pelagic food web dynamics along an estuarine gradient.

Identification of the key biotic and abiotic drivers within food webs is important for understanding species abundance changes in ecosystems, particularly across ecotones where there may be strong variation in interaction strengths. Using structural equation models (SEMs) and four decades of integrated data from the San Francisco Estuary, we investigated the relative effects of top-down, bottom-up, and environmental drivers on multiple trophic levels of the pelagic food web along an estuarine salinity gradient and at both annual and monthly temporal resolutions. We found that interactions varied across the estuarine gradient and that the detectability of different interactions depended on timescale. For example, for zooplankton and estuarine fishes, bottom-up effects appeared to be stronger in the freshwater upstream regions, while top-down effects were stronger in the brackish downstream regions. Some relationships (e.g., bottom-up effects of phytoplankton on zooplankton) were seen primarily at annual timescales, whereas others (e.g., temperature effects) were only observed at monthly timescales. We also found that the net effect of environmental drivers was similar to or greater than bottom-up and top-down effects for all food web components. These findings can help identify which trophic levels or environmental factors could be targeted by management actions to have the greatest impact on estuarine forage fishes and the spatial and temporal scale at which responses might be observed. More broadly, this study highlights how environmental gradients can structure community interactions and how long-term data sets can be leveraged to generate insights across multiple scales.

The microstructure characteristics of the 1-ethyl-3-methylimidazolium thiocyanate–methyl acetate−methanol systems

The ionic liquid (IL) has demonstrated promising potential as an entrainer in efficiently separating the methyl acetate−methanol azeotropic mixture using extractive distillation. However, comprehending the fundamental mechanism responsible for azeotropism vanished by an IL entrainer is crucial for its large-scale applications. This study aims to explore the microstructure characteristics of the 1-ethyl-3-metfighylimidazolium thiocyanate ([EMIM][SCN])–methyl acetate−methanol systems using FTIR and DFT calculations. The focus is on understanding the fundamental reasons behind azeotrope breaking facilitated by the IL entrainer. Initially, the v(aromatic C–H) region of the imidazolium cation was analyzed to evaluate the interaction strength between the entrainer and methyl acetate/methanol. The results indicated stronger interactions between [EMIM][SCN] and methanol. As a consequence of this variation in interaction strength, the relative vapor pressure of the two constituents was impacted, ultimately resulting in the disturbance of the azeotrope. Subsequently, the v(C=O) region of methyl acetate was utilized to detect interaction complexes in various systems, assisted by excess spectra. Various interaction species, including methyl acetate–methanol/2methanol and methyl acetate–ion cluster/ion pair/cation were found. Notably, when the mole fraction of IL exceeded 0.087, the IL effectively disrupted the interaction complex between methyl acetate and methanol, thereby eliminating azeotropy in the methyl acetate–methanol system.

Variation in body size drives spatial and temporal variation in lobster-urchin interaction strength.

How strongly predators and prey interact is both notoriously context dependent and difficult to measure. Yet across taxa, interaction strength is strongly related to predator size, prey size and prey density, suggesting that general cross-taxonomic relationships could be used to predict how strongly individual species interact. Here, we ask how accurately do general size-scaling relationships predict variation in interaction strength between specific species that vary in size and density across space and time? To address this question, we quantified the size and density dependence of the functional response of the California spiny lobster Panulirus interruptus, foraging on a key ecosystem engineer, the purple sea urchin Strongylocentrotus purpuratus, in experimental mesocosms. Based on these results, we then estimated variation in lobster-urchin interaction strength across five sites and 9 years of observational data. Finally, we compared our experimental estimates to predictions based on general size-scaling relationships from the literature. Our results reveal that predator and prey body size has the greatest effect on interaction strength when prey abundance is high. Due to consistently high urchin densities in the field, our simulations suggest that body size-relative to density-accounted for up to 87% of the spatio-temporal variation in interaction strength. However, general size-scaling relationships failed to predict the magnitude of interactions between lobster and urchin; even the best prediction from the literature was, on average, an order of magnitude (+18.7×) different than our experimental predictions. Harvest and climate change are driving reductions in the average body size of many marine species. Anticipating how reductions in body size will alter species interactions is critical to managing marine systems in an ecosystem context. Our results highlight the extent to which differences in size-frequency distributions can drive dramatic variation in the strength of interactions across narrow spatial and temporal scales. Furthermore, our work suggests that species-specific estimates for the scaling of interaction strength with body size, rather than general size-scaling relationships, are necessary to quantitatively predict how reductions in body size will alter interaction strengths.

Interpreting the Evolutionary Echoes of a Protein Complex Essential for Inner-Ear Mechanosensation.

The sensory epithelium of the inner ear, found in all extant lineages of vertebrates, has been subjected to over 500 million years of evolution, resulting in the complex inner ear of modern vertebrates. Inner-ear adaptations are as diverse as the species in which they are found, and such unique anatomical variations have been well studied. However, the evolutionary details of the molecular machinery that is required for hearing are less well known. Two molecules that are essential for hearing in vertebrates are cadherin-23 and protocadherin-15, proteins whose interaction with one another acts as the focal point of force transmission when converting sound waves into electrical signals that the brain can interpret. This "tip-link" interaction exists in every lineage of vertebrates, but little is known about the structure or mechanical properties of these proteins in most non-mammalian lineages. Here, we use various techniques to characterize the evolution of this protein interaction. Results show how evolutionary sequence changes in this complex affect its biophysical properties both in simulations and experiments, with variations in interaction strength and dynamics among extant vertebrate lineages. Evolutionary simulations also characterize how the biophysical properties of the complex in turn constrain its evolution and provide a possible explanation for the increase in deafness-causing mutants observed in cadherin-23 relative to protocadherin-15. Together, these results suggest a general picture of tip-link evolution in which selection acted to modify the tip-link interface, although subsequent neutral evolution combined with varying degrees of purifying selection drove additional diversification in modern tetrapods.

Species abundance correlations carry limited information about microbial network interactions.

Unraveling the network of interactions in ecological communities is a daunting task. Common methods to infer interspecific interactions from cross-sectional data are based on co-occurrence measures. For instance, interactions in the human microbiome are often inferred from correlations between the abundances of bacterial phylogenetic groups across subjects. We tested whether such correlation-based methods are indeed reliable for inferring interaction networks. For this purpose, we simulated bacterial communities by means of the generalized Lotka-Volterra model, with variation in model parameters representing variability among hosts. Our results show that correlations can be indicative for presence of bacterial interactions, but only when measurement noise is low relative to the variation in interaction strengths between hosts. Indication of interaction was affected by type of interaction network, process noise and sampling under non-equilibrium conditions. The sign of a correlation mostly coincided with the nature of the strongest pairwise interaction, but this is not necessarily the case. For instance, under rare conditions of identical interaction strength, we found that competitive and exploitative interactions can result in positive as well as negative correlations. Thus, cross-sectional abundance data carry limited information on specific interaction types. Correlations in abundance may hint at interactions but require independent validation.

Environmental context dependency in species interactions

Ecological interactions are not uniform across time and can vary with environmental conditions. Yet, interactions among species are often measured with short-term controlled experiments whose outcomes can depend greatly on the particular environmental conditions under which they are performed. As an alternative, we use empirical dynamic modeling to estimate species interactions across a wide range of environmental conditions directly from existing long-term monitoring data. In our case study from a southern California kelp forest, we test whether interactions between multiple kelp and sea urchin species can be reliably reconstructed from time-series data and whether those interactions vary predictably in strength and direction across observed fluctuations in temperature, disturbance, and low-frequency oceanographic regimes. We show that environmental context greatly alters the strength and direction of species interactions. In particular, the state of the North Pacific Gyre Oscillation seems to drive the competitive balance between kelp species, asserting bottom-up control on kelp ecosystem dynamics. We show the importance of specifically studying variation in interaction strength, rather than mean interaction outcomes, when trying to understand the dynamics of complex ecosystems. The significant context dependency in species interactions found in this study argues for a greater utilization of long-term data and empirical dynamic modeling in studies of the dynamics of other ecosystems.

The Importance of Electrostatics and Polarization for Noncovalent Interactions: Ionic Hydrogen Bonds vs Ionic Halogen Bonds

A series of 26 hydrogen-bonded complexes between Br− and halogen, oxygen and sulfur hydrogen-bond (HB) donors is investigated at the M06-2X/6–311 + G(2df,2p) level of theory. Analysis using a model in which Br− is replaced by a point charge shows that the interaction energy ({Delta E}_{Int}) of the complexes is accurately reproduced by the scaled interaction energy with the point charge ({Delta E}_{Int}^{PC}).This is demonstrated by {Delta E}_{Int}=0.86{Delta E}_{Int}^{PC} with a correlation coefficient, R2 =0.999. The only outlier is (Br-H-Br)−, which generally is classified as a strong charge-transfer complex with covalent character rather than a HB complex. {Delta E}_{Int}^{PC} can be divided rigorously into an electrostatic contribution ({Delta E}_{ES}^{PC}) and a polarization contribution ({Delta E}_{Pol}^{PC}).Within the set of HB complexes investigated, the former varies between -7.2 and -32.7 kcal mol−1, whereas the latter varies between -1.6 and -11.5 kcal mol−1. Compared to our previous study of halogen-bonded (XB) complexes between Br− and C–Br XB donors, the electrostatic contribution is generally stronger and the polarization contribution is generally weaker in the HB complexes. However, for both types of bonding, the variation in interaction strength can be reproduced accurately without invoking a charge-transfer term. For the Br−···HF complex, the importance of charge penetration on the variation of the interaction energy with intermolecular distance is investigated. It is shown that the repulsive character of {Delta E}_{Int} at short distances in this complex to a large extent can be attributed to charge penetration.

High Redundancy as well as Complementary Prey Choice Characterize Generalist Predator Food Webs in Agroecosystems

Food web structure influences ecosystem functioning and the strength and stability of associated ecosystem services. With their broad diet, generalist predators represent key nodes in the structure of many food webs and they contribute substantially to ecosystem services such as biological pest control. However, until recently it has been difficult to empirically assess food web structure with generalist predators. We utilized DNA-based molecular gut-content analyses to assess the prey use of a set of generalist invertebrate predator species common in temperate agricultural fields. We investigated the degree of specialization of predator-prey food webs at two key stages of the cropping season and analysed the link temperature of different trophic links, to identify non-random predation. We found a low level of specialization in our food webs, and identified warm and cool links which may result from active prey choice or avoidance. We also found a within-season variation in interaction strength between predators and aphid pests which differed among predator species. Our results show a high time-specific functional redundancy of the predator community, but also suggest temporally complementary prey choice due to within-season succession of some predator species.

To Everything There Is a Season: Summer-to-Winter Food Webs and the Functional Traits of Keystone Species.

From a trophic perspective, a seasonal increase in air temperature and photoperiod propagates as bottom-up pulse of primary production by plants, secondary production by herbivores, and tertiary production by carnivores. However, food web seasonality reflects not only abiotic variation in temperature and photoperiod, but also the composition of the biotic community and their functional responses to this variation. Some plants and animals-here referred to as seasonal specialists-decouple from food webs in winter through migration or various forms of metabolic arrest (e.g., senescence, diapause, and hibernation), whereas some plants and resident animals-here referred to as seasonal generalists-remain present and trophically coupled in winter. The co-occurrence of species with divergent responses to winter introduces seasonal variation in interaction strengths, resulting in summer-to-winter differences in trophic organization. Autumn cooling and shortening day length arrests primary productivity and cues seasonal herbivores to decouple, leaving generalist carnivores to concentrate their predation on the few generalist herbivores that remain resident, active, and vulnerable to predation in winter, which themselves feed on the few generalist plant structures available in winter. Thus, what was a bottom-up pulse, spread among many species in summer, including highly productive seasonal specialists, reverses into strong top-down regulation in winter that is top-heavy, and concentrated among a small number of generalist herbivores and their winter foods. Intermediate-sized, generalist herbivores that remain active and vulnerable to predation in winter are likely to be keystone species in seasonal food webs because they provide the essential ecosystem service of turning summer primary productivity into winter food for carnivores. Empirical examination of terrestrial mammals and their seasonal trophic status in the boreal forest and across an arctic-to-tropics seasonality gradient indicates seasonal specialization is more common among herbivores, small body sizes, and in regions with intermediate seasonality, than among carnivores, large body size, and regions where summers are very short or very long. Better understanding of food webs in seasonal environments, including their vulnerability and resilience to climate change, requires a multi-season perspective.

Plant survival and keystone pollinator species in stochastic coextinction models: role of intrinsic dependence on animal-pollination

Coextinction models are useful to understand community robustness to species loss and resilience to disturbances. We simulated pollinator extinctions in pollination networks by using a hybrid model that combined a recently developed stochastic coextinction model (SCM) for plant extinctions and a topological model (TCM) for animal extinctions. Our model accounted for variation in interaction strengths and included empirical estimates of plant dependence on pollinators to set seeds. The stochastic nature of such model allowed us determining plant survival to single (and multiple) extinction events, and identifying which pollinators (keystone species) were more likely to trigger secondary extinctions. Consistently across three different pollinator removal sequences, plant robustness was lower than in a pure TCM, and plant survival was more determined by dependence on the mutualism than by interaction strength. As expected, highly connected and dependent plants were the most sensitive to pollinator loss and collapsed faster in extinction cascades. We predict that the relationship between dependence and plant connectivity is crucial to determine network robustness to interaction loss. Finally, we showed that honeybees and several beetles were keystone species in our communities. This information is of great value to foresee consequences of pollinator losses facing current global change and to identify target species for effective conservation.

Global patterns in the effects of predator declines on sea urchins

Latitudinal gradients in the strength of biotic interactions have long been proposed, but empirical evidence for the expectation of more intense predation, herbivory and competition at low latitudes has been mixed. Here, we use a meta‐analysis to test the prediction that predation pressure on sea urchins, a group of consumers with a particularly strong influence on community structure in the world's oceans, is strongest in the tropics. We then examine which biotic and abiotic factors best correlate with biogeographic and within habitat patterns in sea urchin responses to predation. Consistent with expectations, predator impacts on sea urchins were highest in tropical coral reefs and decreased towards the poles in rocky reef habitats (> 25° absolute latitude). However, latitude and temperature were weakly correlated with effect sizes, and the strongest predictor of predator impacts was sea urchin species. This suggests an important role of prey identity (i.e. traits including behaviour, physical, and chemical defences) rather than large scale abiotic factors in determining variation in interaction strengths. Ecosystem‐shaping sea urchins such as Tripneustes gratilla, Diadema savignyi and Centrostephanus rodgersii were strongly impacted by consumers, indicating a tight coupling between predators of these species and their boom and bust prey. Anthropogenic activities such as over‐fishing, climate change and habitat destruction are causing rapid environmental change, and understanding how predation pressure varies with temperature, across habitats and among prey species, will aid in predicting the likelihood of ecosystem wide effects (via trophic cascades).

Beyond body mass: how prey traits improve predictions of functional response parameters.

Understanding the factors that determine the strength of predator-prey interactions is essential to understanding community structure and stability. Variation in the strength of predator-prey interactions often can be attributed to predator mass and prey mass, or abiotic factors like temperature. However, even when accounting for these factors, there remains a considerable amount of unexplained variation that may be attributed to other traits. We compiled functional response data from the literature to investigate how predator mass, prey mass, prey type (taxonomic identity), temperature, and prey defenses (hard vs soft integument) contributed to the variation found in the predator-prey interactions between freshwater cyclopoid copepods and their prey. Surprisingly, our results indicate that prey identity (taxonomic group) and defenses (hard vs soft integument) are more important for generating variation in interaction strengths than body mass and temperature. This suggests that allometric functions can only take us so far when attempting to better understand variation in individual predator prey interactions, and that we must evaluate how other traits influence interaction strengths. Identifying additional factors such as prey defenses may enable us to better predict potential changes in the structure and function of planktonic and other food webs by better accounting for the variation in the interactions between generalists and their many prey types.

Consumer–plant interaction strength: importance of body size, density and metabolic biomass

Explaining variability in the strength and sign of trophic interactions between primary consumers and plants is a long‐standing research challenge. Consumer density and body size vary widely in space and time and are predicted to have interactive effects on consumer–plant interactions. In a southern US salt marsh, we used replicate field enclosures to orthogonally manipulate the body size (mass) and density of a dominant consumer (a snail). We investigated impacts (leaf damage and biomass) on monocultures of cordgrass, the foundation species, over three months. Increasing consumer density and body size increased leaf damage additively and, as predicted, multiplicatively reduced plant biomass. Notably, size and density determined the sign of consumer impact on plants: low to medium densities of small consumers enhanced, while high densities of large consumers strongly suppressed, plant biomass. Finally, total consumer metabolic biomass (mass0.75) within an enclosure parsimoniously explained plant biomass response, supporting theoretical predictions and suggesting that multiplicative effects of density and body size resulted from their effects on total metabolic biomass. The consequences of changes in consumer density and body size resulting from anthropogenic perturbations may therefore be predicted based on metabolic biomass.SynthesisConsumer size, density and biomass can all strongly affect consumer–plant interactions. Though density and body size have been extensively studied as drivers of variation in interaction strength, the role of biomass as the ultimate driver has been less appreciated. We manipulated body size and density of a single consumer species and, based on metabolic theory, integrated these into a single variable: total metabolic biomass. Our results suggest that changes in interaction strength attributed to size or density may in fact be due to changes in metabolic biomass. This metric could thus serve as a useful tool in further understanding species interactions.

CAUSES OF VARIATION IN BIOTIC INTERACTION STRENGTH AND PHENOTYPIC SELECTION ALONG AN ALTITUDINAL GRADIENT

Understanding the causes of variation in biotic interaction strength and phenotypic selection remains one of the outstanding goals of evolutionary ecology. Here we examine the variation in strength of interactions between two seed predators, common crossbills (Loxia curvirostra) and European red squirrels (Sciurus vulgaris), and mountain pine (Pinus uncinata) at and below tree limit in the Pyrenees, and how this translates into phenotypic selection. Seed predation by crossbills increased whereas seed predation by squirrels decreased with increasing elevation and as the canopy became more open. Overall, seed predation by crossbills averaged about twice that by squirrels, and the intensity of selection exerted by crossbills averaged between 2.6 and 7.5 times greater than by squirrels. The higher levels of seed predation by crossbills than squirrels were related to the relatively open nature of most of the forests, and the higher intensity of selection exerted by crossbills resulted from their higher levels of seed predation. However, most of the differences in selection intensity between crossbills and squirrels were the result of habitat features having a greater effect on the foraging behavior of squirrels than of crossbills, causing selection to be much lower for squirrels than for crossbills.

The periodic modulation of a Bose-Fermi mixture in double-well trap

In this paper, we study the self-trapping of a Bose-Fermi mixture in a periodic modulation field by adjusting fermionic number and interaction parameter in a double-well potential. We find that the self-trapping of bosons can be affected by interatomic self-interaction and interspecific interaction parameter. Moreover, we notice that the self-trapping of bosons gives rise to a critical phenomenon with the variation of interaction strength and fermionic number.

Positive plant interactions and community dynamics

Positive plant interactions and community dynamics

Distinct Peptide Binding Specificities of Src Homology 3 (SH3) Protein Domains Can Be Determined by Modulation of Local Energetics across the Binding Interface

The yeast Nbp2p SH3 and Bem1p SH3b domains bind certain target peptides with similar high affinities, yet display vastly different affinities for other targets. To investigate this unusual behavior, we have solved the structure of the Nbp2p SH3-Ste20 peptide complex and compared it with the previously determined structure of the Bem1p SH3b bound to the same peptide. Although the Ste20 peptide interacts with both domains in a structurally similar manner, extensive in vitro studies with domain and peptide mutants revealed large variations in interaction strength across the binding interface of the two complexes. Whereas the Nbp2p SH3 made stronger contacts with the peptide core RXXPXXP motif, the Bem1p SH3b domain made stronger contacts with residues flanking the core motif. Remarkably, this modulation of local binding energetics can explain the distinct and highly nuanced binding specificities of these two domains.

Mechanical control of magnetic states of finite carbon chains encapsulated in single wall carbon nanotubes

Effects of a lateral strain on finite carbon chains encapsulated in armchair carbon nanotubes are investigated using spin-polarized density functional theory. A strain induced transition between magnetic and nonmagnetic states of the encapsulated carbon chain is predicted at certain value of the applied strain which can be easily achieved experimentally, due to variation in interaction strength between the single wall carbon nanotube and the encapsulated carbon chain. Furthermore, this transition is reversible within a certain range of the applied strain. The proposed mechanical control of magnetic state of the hybrid system opens possibility for nanodevices such as mechanomagnetic switch or piezomagnetic sensors.

Charge and Spin Pairing Instabilities in Hubbard Nanoclusters

Electron coherent and incoherent pairings and other types of correlations are studied exactly for the ensemble of small clusters with different geometries under variation of interaction strength, electron doping and temperature. Charge and spin collective excitations yield intriguing insights into level crossing degeneracies, phase separation and condensation. Criteria for spin-charge separation and recombination driven by interaction strength and temperature are found. Phase diagrams resemble a number of inhomogeneous, nanoscale phases and pseudogap regions seen recently in high TC cuprates, manganites, multiferroics and CMR nanomaterials.