Showing papers on "Disturbance (geology) published in 1986"

Optimal rejection of persistent bounded disturbances

Abstract: In this paper, we formulate the problem of optimal disturbance rejection in the case where the disturbance is generated as the output of a stable system in response to an input which is assumed to be of unit amplitude, but is otherwise arbitrary. The objective is to choose a controller that minimizes the maximum amplitude of the plant output in response to such a disturbance. Mathematically, this corresponds to requiring uniformly good disturbance rejection over all time. Since the problem of optimal tracking is equivalent to that of optimal disturbance rejection if a feedback controller is used (see [7, sect. 5.6]), the theory presented here can also be used to design optimal controllers that achieve uniformly good tracking over all time rather than a tracking error whose L 2 -norm is small, as is the case with the currently popular H_{\infty} theory. The present theory is a natural counterpart to the existing theory of optimal disturbance rejection (the so-called H_{\infty} theory) which is based on the assumption that the disturbance to be rejected is generated by a stable system whose input is square-integrable and has unit energy. It is shown that the problem studied here has quite different features from its predecessor. Complete solutions to the problem are given in several important cases, including those where the plant is minimum phase or when it has only a single unstable zero. In other cases, procedures are given for obtaining bounds on the solution and for obtaining suboptimal controllers.

Effects of disturbance on diversity in mixed-grass prairie

Abstract: Seven grassland treatments representing different disturbance regimes were sampled within a large area of mixed-grass prairie in southwestern Oklahoma, USA. Species diversity was low on the undisturbed and most severely disturbed grasslands. The results also indicate, however, that grassland diversity was not a simple function of disturbance rate, size or intensity. Instead, species diversity was maximized under a combination of natural disturbances. To some extent, the increase in species diversity was the result of increased habitat diversity associated with a type of disturbance. That is, disturbances such as grazing and wallowing have a cumulative effect one grassland diversity. Overall, the structure one grassland communities can not be accurately determined without considering the structure of the natural disturbance regime.

Waves caused by a moving disturbance in a shallow channel of finite width

Abstract: The flow created by an impulsively started pressure distribution travelling at a constant velocity in a shallow channel is investigated. The restricted Green-Naghdi theory of fluid sheets is used to perform the three-dimensional calculations. The results show remarkable similarity to model tests. In particular, these calculations predict the periodic generation of two-dimensional solitons in front of and travelling faster than the disturbance if the disturbance is large enough. Behind the disturbance a complicated, doubly corrugated set of waves is formed. The computations also predict that periodic creation of solitons is accompanied by a correspondingly periodic oscillation of the wave drag, as well as a dramatic increase in the mean wave drag.

Man as an Intertidal Predator in Transkei: Disturbance, Community Convergence and Management of a Natural Food Resource

Abstract: The indigenous peoples of Transkei, South Africa, remove shellfish from the shore to supplement their diet. Paired exploited and protected rocky shores were compared in terms of community structure and of the size structures of component species. Relative abundance of algae and sessile, inedible invertebrates is greater, and modal and mean sizes of exploited species are smaller at exploited sites than at protected sites. Disturbance through selective predation increases species richness, in line with the \"intermediate disturbance hypothesis\", and also leads to community convergence towards a common state. The recovery potential of these systems is high and the resource could best be managed on a \"rotational cropping\" basis.

Effects of disturbance frequency on stream benthic community structure in relation to canopy cover and season

Abstract: Field experiments were conducted to examine the effects of disturbance frequency on invertebrates and periphyton colonizing bricks in a third order Rocky Mountain (USA) stream. After an initial colonization period (30 days), sets of bricks were turned over at intervals of 0, 3, 9, 27, or 54 days. Invertebrate species richness and density were reduced as disturbance frequency increased. These trends were evident for both seasons (summer and fall) and sites (open vs. closed canopy). Invertebrate species diversity (H′) displayed no effect during the fall experiment; however, H′ was reduced at high frequencies of disturbance during the summer experiment. Baetis tricaudatus was the most abundant invertebrate on the substrata at both sites and seasons. Alloperla, Baetis, Cinygmula, Chironomidae, Drunella grandis, Hydropsyche, and Seratella tibialis increased in absolute abundances as disturbance frequency decreased. Four other abundant taxa (Capnia, Cleptelmis, Glossosoma, and Isoperla) displayed no clear respo...

Rodent Ecology and Burrowing Behavior: Predicted Effects on Archaeological Site Formation

Abstract: Studies of burrowing rodent ecology are reviewed to identify the dynamics of rodent disturbance and to predict its impact on archaeological deposits. Data from a central California site are used to evaluate the proposed effects of rodent behavior. Burrowing activity appears to segregate soil contents by size, causing artificial concentrations of small materials near the surface, and larger materials at depths between 30 and 60 cm. Despite extensive stratigraphic disturbance, rodent burrowing seems to have minimal effects on horizontal material distributions.

History of late- and post-glacial vegetation and disturbance around Upper South Branch Pond, northern Maine

Abstract: The changing character of vegetation and the effects of disturbance on vegetation are inferred from pollen, plant macrofossils, charcoal, and microlepidopteran larvel head capsules in sediment core...

Measures of resilience: the response of coastal sage scrub to fire

Abstract: Measures of four components of resilience are developed and used to quantify the response of coastal sage scrub to fire in southern California: (1) elasticity (rate of recovery following disturbance), (2) amplitude (threshold of disturbance beyond which recovery to the original state no longer occurs), (3) malleability (extent of alteration of the new stable-state from the original) and (4) damping (extent and duration of oscillation in an ecosystem parameter following disturbance). Vegetation and soil properties measured before fire, and for the first 5–6 yr after fire on four coastal (Venturan association) and four inland (Riversidian association) sites of coastal sage were used to follow changes. In addition, results from a simulation model of post-fire succession in Venturan coastal sage scrub (the FINICS model of Malanson) were used to examine resilience behavior over a 200 yr period. Resilience behavior of coastal sage scrub is critically influenced by the presence of a competitive mix of inherently strongly and weakly resprouting species. Sites dominated by weak resprouters exhibit lower elasticity and less damping of year-to-year fluctuations in composition in the early post-fire years. Sites with a mixture of weak and strong resprouters have a lower threshold of disturbance (amplitude) before species extirpation occurs, a result intensified by a higher frequency of disturbance. Malleability is also greater in these systems under higher disturbance frequency.

Response of Permafrost Terrain to Disturbance: A Synthesis of Observations from Northern Alaska, U.S.A.

Abstract: Former exploratory drilling sites in the National Petroleum Reserve-Alaska, are examples of the long-term physical modifications resulting from disturbance of perennially frozen terrain. Camp const...

Resilience at the level of the plant community

Abstract: 1. Resilience is considered in relation to six major mechanisms of disturbance. Many kinds of potential disturbance (but not all) can be met by resistance rather than resilience. 2. Each species may respond to a potential disturbance, and recover afterwards, in a unique way. 3. Two types of resilience are recognized: in situ and by migration. 4. Different key characters are involved in plants being suited to the four basic dimensions of one type of disturbance: extent, frequency, intensity and timing. 5. Four types of positive inter-plant relationship can be found during succession where resilience is by migration: enablement, facilitation, exclusion and inhibition. 6. Measurement of resilience is complex, especially in relation to patchy distributions of plants; for species conservation the species-area relationship may be a very useful criterion by which to judge elasticity. 7. A community containing a moderate number of species is likely to be more resilient in terms of ground cover, soil conservation, run-off and productivity than one with few species provided the various species differ appreciably in growth rate, life-form, longevity and regeneration niche, but it is likely to be less elastic in conservation of species content. Adding many sparse species is unlikely to increase elasticity in respect of ground cover etc, and is very likely to reduce elasticity of species content. 8. High resilience in situ does not prepare a community for a disturbance involving resilience by migration. In particular, long exposure of mediterranean systems to disturbance by drought and natural fire has not prepared them for human disturbance (agriculture, road-building, quarrying etc). 9. Different kinds of pioneers are needed in different types of derelict man-made communities, particularly in neglected pastures compared with grass-free areas such as quarries. 10. Some communities subject to human disturbance have lost many species and, considered in their own right, they may be more resilient than the original communities. 11. Patterns of human disturbance have changed markedly through time, and more attention should be paid to the responses of plants to changes in disturbance regime.

Effects of disturbance on epiphytic community architecture

Abstract: Microdistributional patterns of attached algal communities on Cladophora and glass slides were compared under varying disturbance regimes in the Upper Mississippi River, using light and scanning electron microscopy. Localized physical disturbance, induced by boat traffic and wind-generated wave action in the main channel, inhibited development of complex attached algal communities and maintained an adnate flora with two-dimensional architecture. In contrast, communities protected from disturbance developed a more complex, three-dimensional architecture. A high degree of architectural similarity was observed between communities on Cladophora and glass substrates exposed to similar levels of disturbance. Communities exposed to severe disturbance resembled ones in early stages of colonization and development, whereas less disturbed communities were similar to ones in advanced stages of development. These results demonstrated that turbulence strongly influences the community structure of periphyton on both micro and macro scale levels.

Returns and Winter-Site Fidelity of North American Migrants Banded in Belize, Central America

Abstract: The rapid destruction of Central and South American forests is causing concern over the potential impact of deforestation on North American migrant birds that winter in these areas (see Rappole and Morton 1985,and Terborgh 1980, for discussion and references). A t least 49 of these migrant species, which spend a majority of the year in the tropics,demonstrate winter-site fidelity (McNeil 1982, see Loftin 1977 for summary of species), and thus may be particularly sensitive to habitat destruction. Studies of winter-site fidelity in the Neotropics to date have been widely dispersed geographically and most have not compared results between sites which have been subjected to differing degrees of human disturbance. We report returns from wintering North American migrants banded at three study sites in Belize, Central America, each of which has been subjected to different degrees of human disturbance

Research in Behavioral Disorders/Emotional Disturbance: A Survey of Subject Identification Criteria.

Abstract: A survey of 323 research studies in behavioral disorders/emotional disturbance was conducted to determine the nature and prevalence of the behavioral disorders/emotional disturbance identification ...

Man as an intertidal predator in Transkei: disturbance, community convergence and management of a natural food

Abstract: The indigenous peoples of Transkei, South Africa, remove shellfish from the shore to supplement their diet. Paired exploited and protected rocky shores were compared in terms of community structure and of the size structures of component species. Relative abundance of algae and sessile, inedible invertebrates is greater, and modal and mean sizes of exploited species are smaller at exploited sites than at protected sites. Disturbance through selective predation increases species richness, in line with the "intermediate disturbance hypothesis", and also leads to community convergence towards a common state. The recovery potential of these systems is high and the resource could best be managed on a "rotational cropping" basis.

Recent advances in the treatment of cerebellar ataxias.

Abstract: The term ataxia was originally applied to a specific disturbance of motor function that results in loss of coordination and voluntary movements Subsequently, ataxia was defined as a disturbance which, quite independent of any motor weakness, alters direction and extent of voluntary movement and imp

Personality dysfunction and behavioral disturbance in early adolescence.

Abstract: This study examines the prevalence of personality disturbance in a nonclinical sample of young adolescents and the relationship between personality functioning and behavior problems. Sixty-three adolescents, initially screened at age 10 using teacher and parent behavior rating scales, were assessed at age 13 by means of a semistructured psychiatric interview. In addition, their parents and teachers again completed behavior rating scales. The findings indicate a prevalence in the population of 54% who are clear of personality disturbance, 19% who display marked disturbance, and 27% who display some disturbance. Disturbance in personality functions was found to be associated with increased anxiety and depression, school difficulties and interpersonal withdrawal. Behavior indicative of conduct problems and hyperactivity did not differentiate the clear, moderately disturbed, and markedly disturbed adolescents.

Seismicity and tectonics in Wales

Abstract: The distribution of instrumental and historical seismicity in Wales is described, including two events in 1984 for which good hypocentral locations were determined. Most of central Wales is aseismic throughout the historic record, suggesting that this is not a feature of reporting bias or population distribution alone. There appears to be a relation between the epicentres and some of the major fault structures of Wales, including the Bala Fault and the Swansea Valley Disturbance. The relationship between faults and seismicity is examined in detail for the two recent events, and more generally for the rest of the data, which have much lower locational accuracy. Stress measurements for NW Europe are shown to give a consistent NW-SE direction for the maximum horizontal stress. This is appropriate for sinistral strike-slip on N-S vertical faults. First motion data for both 1984 events also give a nodal plane in this orientation. In the first case, the event can be clearly attributed to mapped N-S vertical faults SE of Newtown, and in the second, a deep N-S normal fault under the Lleyn Peninsula is inferred as the most likely fault plane orientation from regional geological considerations. The proposed activity of the Bala Fault and the Swansea Valley Disturbance can also be explained within this stress field by their thrust reactivation as low angle structures. The general difficulty of relating the remaining seismicity to geological structures is demonstrated by the fact that the second 1984 event could not be directly connected with any surface structure, or even with known structures at depth.