Effects Of Disturbance Frequency Research Articles

Numerical and Experimental Investigation of a Semi-Active Vibration Control System by Means of Vibration Energy Conversion

A vibration control concept based on vibration energy conversion and storage with respect to a serial-stiffness-switch system (4S) has previously been proposed. Here, we first present a rotational electromagnetic serial-stiffness-switch system as a novel practical vibration control system for experimental validation of the concept and, furthermore, an improved control strategy for higher vibration suppression performance is also proposed. The system consists of two spring-switch elements in series, where a parallel switch can block a spring. As an alternating mechanical switch, the experimental system uses two electromagnets with a shared armature. By connecting the armature to the rotating load or the base, the electromagnets decide which of the two spiral springs is blocked, while the other is active. A switching law based on the rotation velocity of the payload is used. Modelling and building of the experimental system were carried out. The corresponding experiment and simulation were executed and they matched well. These results prove that our serial-stiffness-switch system is capable of converting vibration energy and realizing vibration reduction under a forced harmonic disturbance. The effects of disturbance frequency, disturbance amplitude and sampling frequency on the system performance are shown as well. A position feedback control-based switching law is further put forward and experimentally verified to improve the repositioning accuracy of the disturbed system.

Effects of disturbance frequency and severity on plant traits: An assessment across a temperate flora

Abstract Recent analyses of plant traits across large sets of species have revolutionized our understanding of plant functional differentiation. However, understanding of ecological relevance of this differentiation is contingent upon knowledge of environmental preferences of species, namely along gradients of disturbance and productivity for which no quantitative data were available until recently. We examined the relationships of key functional traits (life‐history categories, leaf‐height‐seed (LHS) traits, clonal growth and bud bank traits) in the herb‐dominated flora of Central Europe to species niche positions along the gradients of disturbance frequency, disturbance severity and productivity. Life‐history categories and bud bank size showed the strongest response to disturbance and productivity, whereas relationship of LHS traits was much weaker. A number of traits, including clonal growth form and bud bank size, showed a significantly unimodal response to disturbance frequency. Responses of many traits to disturbance frequency were different from their responses to disturbance severity. Our findings support the notions that disturbance and productivity are key gradients of species functional differentiation and that disturbance severity and frequency select for different trait suites. Furthermore, the data indicate that in a predominantly herbaceous flora, the traits of life span, clonal growth and resprouting show stronger relationship with the environment than the LHS traits, which are more important in floras with high proportions of woody species. Since most previous trait analyses are based on woody‐plant‐dominated floras, patterns revealed in a herb‐dominated flora deepen our understanding of the full range of variation within the plant kingdom. A plain language summary is available for this article.

Coexistence of species with different dispersal across landscapes: a critical role of spatial correlation in disturbance.

Disturbance is key to maintaining species diversity in plant communities. Although the effects of disturbance frequency and extent on species diversity have been studied, we do not yet have a mechanistic understanding of how these aspects of disturbance interact with spatial structure of disturbance to influence species diversity. Here we derive a novel pair approximation model to explore competitive outcomes in a two-species system subject to spatially correlated disturbance. Generally, spatial correlation in disturbance favoured long-range dispersers, while distance-limited dispersers were greatly suppressed. Interestingly, high levels of spatial aggregation of disturbance promoted long-term species coexistence that is not possible in the absence of disturbance, but only when the local disperser was intrinsically competitively superior. However, spatial correlation in disturbance led to different competitive outcomes, depending on the disturbed area. Concerning ecological conservation and management, we theoretically demonstrate that introducing a spatially correlated disturbance to the system or altering an existing disturbance regime can be a useful strategy either to control species invasion or to promote species coexistence. Disturbance pattern analysis may therefore provide new insights into biodiversity conservation.

Numerical Simulations of Hydrodynamics and Heat Transfer in Wavy Falling Liquid Films on Vertical and Inclined Walls

The flow of thin falling liquid films is unstable to long-wave disturbances. The flow instability leads to development of waves at the liquid–gas interface. The effect of the waves on heat and mass transfer in falling liquid films is a subject of ongoing scientific discussion. In this work, numerical investigation of the wave dynamics has been performed using a modified volume-of-fluid (VOF) method for tracking the free surface. The surface tension is described using the continuum surface force (CSF) model. With low disturbance frequency, solitary waves of large amplitude are developed, which are preceded by low-amplitude capillary waves. With high disturbance frequency, low amplitude sinusoidal waves are developed. The waveforms dependent on the Reynolds number and disturbance frequency are summarized in a form of a regime map. A correlation describing the separation curve between the sinusoidal waves regime and solitary waves regime is proposed. The wave parameters (peak height, length, and propagation speed) are computed from the simulation results and compared with available experimental correlations in a wide range of parameters. The effects of the disturbance frequency and the plane inclination angle on the wave dynamics have been studied. The interaction of waves initiated by simultaneous disturbances of two different frequencies has been investigated. The heat transfer in the wavy film has been simulated for the constant wall temperature boundary condition. The effects of Prandtl number and disturbance frequency on local and global heat transfer parameters have been investigated. It has been shown that the influence of waves on heat transfer is significant for large Prandtl numbers in a specific range of disturbance frequencies.

Interactions between frequency and size of disturbance affect competitive outcomes

AbstractDisturbance has many effects on ecological communities, and it is often suggested that disturbance can affect species diversity by altering competitive outcomes. However, disturbance regimes have many distinct aspects that may act, and interact, to influence species diversity. While there are many theoretical models of disturbance‐prone communities, few have specifically documented how interactions between different aspects of a disturbance regime change competitive outcomes. Here, we present a model of two plant species subject to disturbance which we then use to examine species coexistence over varying levels of two aspects of disturbance: frequency, and spatial extent (i.e., area disturbed). We show that the competitive outcome is affected differently by changes in each aspect and that the effect of disturbance frequency on species coexistence depends strongly on the spatial extent of the disturbance, and vice versa. We classify the nature of these interactions between disturbance frequency and extent on the basis of the shape of the resulting coexistence regions in a frequency–extent parameter plane. Our results illustrate that different types of interaction can result from differences in life‐history traits that control species‐specific sensitivity to frequency and extent of disturbance. Thus, our analysis shows that the various aspects of disturbance must be carefully considered in concert with the life‐history traits of the community members in order to assess the consequences of disturbance.

Wave disturbance overwhelms top-down and bottom-up control of primary production in California kelp forests.

We took advantage of regional differences in environmental forcing and consumer abundance to examine the relative importance of nutrient availability (bottom-up), grazing pressure (top-down), and storm waves (disturbance) in determining the standing biomass and net primary production (NPP) of the giant kelp Macrocystis pyrifera in central and southern California. Using a nine-year data set collected from 17 sites we show that, despite high densities of sea urchin grazers and prolonged periods of low nutrient availability in southern California, NPP by giant kelp was twice that of central California where nutrient concentrations were consistently high and sea urchins were nearly absent due to predation by sea otters. Waves associated with winter storms were consistently higher in central California, and the loss of kelp biomass to winter wave disturbance was on average twice that of southern California. These observations suggest that the more intense wave disturbance in central California limited NPP by giant kelp under otherwise favorable conditions. Regional patterns of interannual variation in NPP were similar to those of wave disturbance in that year-to-year variation in disturbance and NPP were both greater in southern California. Our findings provide strong evidence that regional differences in wave disturbance overwhelmed those of nutrient supply and grazing intensity to determine NPP by giant kelp. The important role of disturbance in controlling NPP revealed by our study is likely not unique to giant kelp forests, as vegetation dynamics in many systems are dominated by post-disturbance succession with climax communities being relatively uncommon. The effects of disturbance frequency may be easier to detect in giant kelp because it is fast growing and relatively short lived, with cycles of disturbance and recovery occurring on time scales of years. Much longer data sets (decades to centuries) will likely be needed to properly evaluate the role of disturbance relative to other processes in determining patterns of NPP in other systems.

Effects of disturbance frequency, species traits and resprouting on directional succession in an individual‐based model of forest dynamics

Summary 1. Succession theory focuses on the position of species along the shade tolerance gradient and their ability to colonize recently disturbed patches and has for decades overlooked resprouting as a key trait in community patterns. 2. We study how different species traits interact with disturbance frequency to change species dominance in the canopy, focusing on the effects of resprouting ability. 3. We develop an individual‐based model that simulates the dynamics of three species paper birch Betula papyrifera Marsh, white pine Pinus strobus L. and sugar maple Acer saccharum Marsh., characterized by different successional strategies, as observed in northern Minnesota forests. We tested (i) how different disturbance frequencies, (ii) sugar maple resprouting and (iii) paper birch resprouting change successional patterns. 4. We show that three disturbance frequency classes produce three different outcomes, each dominated by a different species, with lower disturbance frequency favouring later successional species. 5. The importance of resprouting ability for success in the canopy depends on the species’ other life‐history traits. Sugar maple is able to dominate the community at all disturbance frequency classes with resprouting, whereas the inclusion of resprouting for paper birch does not change the successional patterns. 6. White pine is indirectly favoured by sugar maple’s resprouting ability, excluding paper birch from the community at disturbance frequency classes that would see paper birch dominance in the absence of resprouting. 7. Synthesis. We model tree life histories in a disturbed environment and test how the inclusion of resprouting changes succession patterns. Our results confirm the recent interest of taking into account resprouting, as it can bend succession directionality, and thus change community composition in response to disturbance. This has wide consequences for predicting forest diversity patterns as well as invasion phenomena in a changing world.

The effect of frequency and nature of pedestrian approaches on the behaviour of wandering albatrosses at sub-Antarctic Marion Island

The effects of disturbance frequency, pedestrian group size, history and approach distance were tested through standardised approaches to 148 brooding/guarding wandering albatrosses (Diomedea exulans) at Marion Island in 2006. Seldom-visited birds were approached at different frequencies over a 3-day period and chick survival was monitored 2 weeks later. Two-person approaches were made to some seldom-visited birds and to birds that have recently been bled. Birds close to the station and in a long-term study colony were also approached. non-vocal response (NVR), vocal response (VR) and overall response (OR) of individuals were analysed. Frequency of approach did not influence short-term behavioural responses, but affected chick survival significantly—nests approached most often had the highest proportion of failures. Over the long-term, there was sensitisation to disturbance. Low OR ranks were found for 71.7% of seldom-disturbed birds, 41.7% of study colony birds and 28.6% of birds close to the station. Management regulations at the island are likely to be effective in protecting this species from human disturbance.

Effects of Disturbance Frequency on Canopy Age Structure and Species Composition in a Managed Midwestern Oak Savanna

In the Midwest, the oak savanna is a unique plant community located between the western prairies and northern forests, but, once the most common plant community in Wisconsin, it is now one of the region's rarest plant communities. To assess the success of varying intensities of management practices, standard dendrochronology and vegetation surveying methods were used to determine the age structure and species composition in oak savanna sites that varied from intensely managed to unmanaged. Size and age distributions revealed that the number of distinct cohorts of oaks increased in less managed sites. Seedlings and saplings were present in high frequencies in all sites; however, management practices such as fire applications that were too frequent apparently decreased oak survivorship, as did the competition in the closed canopies of the unmanaged sites. A fire frequency of 2-3 years eliminates competition, but a frequency of 4-6 years, typical of fire-adapted forest communities, allows for slower-growing species and younger oaks to survive, while still maintaining a low density. In these conditions, where growth rates vary in time, the use of dendrochronology is necessary to obtain an accurate canopy structure for fire-adapted forest communities. Quercus macrocarpa was dominant in the more managed sites, but shade-tolerant species were dominant in the unmanaged site. At present, the canopy composition in the managed sites represented an oak savanna assemblage; however, the unmanaged composition was shifting to a woodland assemblage. Thus, remnant oak savanna can be over- or under-managed, and unmanaged oak savanna can be overlooked as woodlands.

The response of benthic rhizopods to sediment disturbance does not support the intermediate disturbance hypothesis

Disturbance is one of the mechanisms which counteract competitive exclusion of populations in resource‐limited communities, thereby facilitating coexistence and maintaining community species diversity. The intermediate disturbance hypothesis predicts maximum diversity at intermediate disturbance intensities and frequencies. This paper reports results of an experimental test of this hypothesis using a coastal benthic community of rhizopods (Protozoa: Rhizopoda), and experimental sediment resuspension as a simulated natural disturbance. We carried out two experiments of 5 d duration which focussed on the effects of resuspension intensity and frequency, respectively, on the abundance, species richness and on the Shannon‐Weaver diversity index of rhizopod communities in surface sediments of natural sediment cores from the coastal southern Baltic. Care was taken to adjust the experimental treatments to the natural disturbance regime in this area.Twenty‐four and 28 rhizopod species were present during the intensity and frequency experiment, respectively. Small bacterivorous rhizopods of the Vannellidae, Cochliopodidae, Paramoebidae and Rhizopoda incertae sedis dominated the communities during both experiments. Rhizopod abundance, species richness and diversity increased towards the end of the intensity experiment, but they did not show effects of disturbance intensity. Similarly, no effects of disturbance frequency were found during the frequency experiment. Our results indicate that coexistence and community diversity maintenance in benthic rhizopod communities, and probably in benthic heterotrophic protistan communities in general, may rely on different mechanisms than intermediate disturbance, such as trophic niche separation and high rates of dispersal and colonisation.

Effects of disturbance frequency, intensity, and area on assemblages of stream macroinvertebrates

Disturbance frequency, intensity, and areal extent may influence the effects of disturbance on biological communities. Furthermore, these three factors may have interacting effects on biological diversity. We manipulated the frequency, intensity, and area of disturbance in a full-factorial design on artificial substrates and measured responses of benthic macroinvertebrates in a northern Vermont stream. Macroinvertebrate abundance was lower in all disturbance treatments than in the undisturbed control. As in most other studies in streams, species density (number of species/sample) was lower in disturbed treatments than in undisturbed controls. However, species density is very sensitive to total abundance of a sample, which is usually reduced by disturbance. We used a rarefaction method to compare species richness based on an equivalent number of individuals. In rarefied samples, species richness was higher in all eight disturbed treatments than in the undisturbed control, with significant increases in species richness for larger areas and greater intensities of disturbance. Increases in species richness in response to disturbance were consistent within patches, among patches with similar disturbance histories, and among patches with differing disturbance histories. These results provide some support for Huston's dynamic-equilibrium model but do not support the intermediate-disturbance hypothesis. Our analyses demonstrate that species richness and species density can generate opposite patterns of community response to disturbance. The interplay of abundance, species richness, and species density has been neglected in previous tests of disturbance models.

How Species with Different Regeneration Niches Coexist in Patchy Habitats with Local Disturbances

We used a two-species simulation model to study mechanisms of coexistence of annual plants in patchy habitats with local disturbances. In habitats with nested scales of patchiness, short dispersal is advantageous because favorable habitat tends to be aggregated. The invasion of a resident population with short dispersal distance by a species with longer-range dispersal was simulated for combinations of habitat pattern, disturbance frequency and germination strategies. A germination strategy was defined by the type of response to disturbance (disturbance-broken when disturbances trigger germination, risk-spreading when germination is insensitive to disturbance) and the dormancy fraction at dispersal. Simulations estimated the long-term low-density growth rate of the invader, the mean local crowding (number of competing seeds per invader seed at each site) and the effective fecundity of each species (the mean number of seeds successfully dispersed per adult plant). Crowding increased with habitat suitability and decreased with increasing dormancy fractions for the resident. Effective fecundity in a landscape can be taken as a measure of competitive ability. The short-dispersing resident invariably had higher effective fecundity, but this difference decreased with increasing suitability, i.e. competitive differences decreased. Coexistence depended on both habitat suitability and disturbance frequency. Maximum coexistence was obtained for habitats of intermediate suitability with moderately frequent disturbances. General linear modelling of the long-term low-density growth rate showed that coexistence results from a reduction in local crowding. This growth rate also increased for increasing habitat suitability and connectivity, and for a higher dormancy fraction of the resident species. The effects of disturbance frequency and of invader's dormancy fraction depended on the type of dormancy of the resident species. The analysis showed that 2 different mechanisms are involved in the coexistence of species with different niches. Differences in regeneration niches permit coexistence through competitive equivalency with trade-offs between dispersal and germination traits, but for a limited range of habitat pattern and disturbance conditions. On the other hand, coexistence through density fluctuations of a disturbance-broken species and storage effects can be achieved for a broad range of environmental conditions and species germination strategies. Species coexistence thus results from the combination of two mechanisms. Evidence from natural communities is discussed. Our results also demonstrate the importance of detailed attention to spatial patterns and dispersal because of the complexity of spatial effects. Further, spatial pattern and disturbance frequencies need to be considered jointly to understand the dynamics of diversity.

Experimental disturbance and the maintenance of species diversity in a stream community.

In order to test the role of disturbance and the effects of disturbance frequency on stream communities, an experiment was conducted in New Hope Creek, North Carolina, USA. Patches of cobbles were tumbled 0, 1 or 2 times in a 6 week span. These tumbling disturbances lasted only 30 seconds. The recovery of the macroinvertebrates was monitored.Most taxa showed major reductions in population density immediately following the disturbance. The percent reduction of a given taxon in disturbed vs. control patches ranged from 21.4-95%. Recovery to near normal population levels was achieved in about four weeks. A second disturbance caused similar population reductions as the first one, and delayed the recovery.The macroinvertebrate community in cobbles was demonstrated to be resilient in that populations quickly regained their predisturbance densities. Rare taxa did not selectively colonize disturbed patches. The implications of these findings for the intermediate disturbance hypothesis and the structure of stream communities is discussed. Disturbance is a major determinant of lotic community structure and species diversity.