Showing papers on "Disturbance (geology) published in 1989"

The Maintenance of Species Diversity by Disturbance

Abstract: Hypotheses that relate disturbance to the production and maintenance of species diversity are reviewed. The hypotheses have been classified traditionally by the effect of the disturbance on the community: Those that involve selective mortality, which maintains the species diversity of a community in equilibrium, and those that invoke causing random, localized, mass mortality, which prevent the community from reaching and equilibrium. Regardless of this difference, most hypotheses predict that the greatest number of species will occur at intermediate leves of disturbance. We develop graphical models for equilibrium and nonequilibrium situations, which we show to differ only in respect to temporal and spatial scale. We maintain that the usual outcome-the greatest number of species at intermediate levels of disturbance-can be explained by assuming that there are trade-offs in species-specific abilities that place constraints on immigration to, and extinction in, patches. Changes in immigration and extinction...

The ecological concept of disturbance and its expression at various hierarchical levels

Abstract: Current definitions of disturbance are intuitive, narrow, and only implicitly based on system structure. This is because the concepts are based on experience at particular levels of organization or on systems whose structure is well known. The definitions are thus inadequate for the development of a general theory of ecological disturbance. A universally applicable definition would 1) identify the object disturbed; 2) distinguish between change in the object that is disturbance versus change that is not; and 3) distinguish between direct and indirect consequences of disturbance. To meet these requirements, we formally link the hierarchical organization of ecological objects and the concept of disturbance. Any persistent ecological object will have a minimal structure, or system of lower level entities that permit its persistence. Disturbance is a change in the minimal structure of an object caused by a factor external to the level of interest. Using these definitions, disturbance can be unequivocally identified and associated with various specific ecological levels of organization. Because of the dependence of the concept of disturbance on recognizing the minimal structure of ecological systems, application of the concept will advance as refined models of the hierarchical structure of ecological systems are elaborated.

A methodology for estimating canopy disturbance frequency and intensity in dense temperate forests

Abstract: Analysis of the frequency of past moderate and high-intensity disturbances has been hindered in forests of complex age structure by methodological problems. A methodology is proposed for developing a disturbance chronology in such stands by identifying the probable date of canopy accession for each sample tree. Canopy accession dates are based on an evaluation of radial growth pattern and early growth rates of existing canopy trees. Canopy disturbance intensity is defined as the percentage of sample trees with canopy accession events in each decade. Rotation periods for disturbances of various intensities are calculated from the chronology. The method was evaluated using 893 increment cores from 70 plots in northern hardwood stands of western Upper Michigan. The estimated average disturbance rate for all plots and decades was 5.7–6.9% of land area per decade, with an implied average canopy tree residence time of 145–175 years. These estimates are similar to those obtained by on-site estimates of canopy tr...

Landscape heterogeneity and disturbance

Abstract: Section 1 Introduction.- 1. Landscape Ecology: State of the Art.- 2. Scale Effects in Landscape Studies.- Section 2 Disturbances in the Landscape.- 3. Landscape Ecology and Air Pollution.- 4. Parasites, Lightning, and the Vegetation Mosaic in Wilderness Landscapes.- 5. Fire, Grazing, and the Landscape Heterogeneity of a Georgia Barrier Island.- 6. Disturbance by Beaver (Castor canadensis Kuhl) and Increased Landscape Heterogeneity.- 7. Suppression of Natural Disturbance: Long-Term Ecological Change on the Outer Banks of North Carolina.- 8. Vegetation Dynamics in a Southern Wisconsin Agricultural Landscape.- Section 3 Implications for Landscape Management.- 9. Landscape Restoration in Response to Previous Disturbance.- 10. Patch-Within-Patch Restoration of Man-Modified Landscapes Within Texas State Parks.- 11. Progressiveness Among Farmers as a Factor in Heterogeneity of Farmed Landscapes.- Section 4 Conclusion.- 12. The Ethics of Isolation, the Spread of Disturbance, and Landscape Ecology.

Impairment of the inferior alveolar nerve after sagittal split osteotomy.

Abstract: The incidence and degree of neurosensory disturbance of the inferior alveolar nerve, as well as its recovery course, were studied on 46 sides in 23 patients who had undergone bilateral sagittal split osteotomies, by means of subjective symptoms, light touch, anaesthesiometer and two-point discrimination. The degree of disturbance was classified into mild, moderate and severe grades by the threshold pressure shown in tests with the anaesthesiometer. The disturbance, which was almost exclusively limited to mild (37%) and severe (28%) grades, was observed in 67% of the sample at one week. The disturbance disappeared completely within one to three months postoperatively in most sites with mild disturbance, and within three months to one year in half of the severely affected sites. Although the recovery was delayed in the other half of the severely disturbed sites, the disturbance was of mild grade at one and a half years. The overall incidence of disturbance at one year was 15%. Computed tomographic examination of the ascending ramus showed that the narrowest width between the mandibular canal and the buccal cortical plate ranged from 0 mm to 3.2 mm with a mean of 1.6 +/- 0.9 mm (SD) and it was less than 1.2 mm in 91% of sites with a severe grade disturbance, whereas it was distributed in a range of 0.9 mm to 3.2 mm in sites with no disturbance or with mild or moderate disturbance. The importance of preoperative computed tomography to indicate the location of the mandibular canal and the use of a thin cement spatula for the osteotomy was stressed, to avoid or reduce postoperative development of neurosensory impairment.

Evidence for episodic sand-blow activity during the 1811-1812 New Madrid (Missouri) earthquake series

Abstract: Four major earthquakes of the 1811-1812 New Madrid series caused widespread ground disturbance, mostly in the form of sand blows, in the alluvial valley of the Mississippi River. Although a recurrence interval of 600-700 yr has been estimated for major shocks, recent excavations at an archeological site in northeastern Arkansas revealed no evidence of ground disturbance for at least 1.3 ka prior to 1811. Sand blows attributed to the New Madrid series show that such features can have multiple episodes of sand extrusion through a single dike and vent.

A dynamic games approach to controller design: disturbance rejection in discrete time

Abstract: It is shown that the discrete-time disturbance-rejection problem, formulated in finite and infinite horizons, can be solved by making direct use of the available results on linear-quadratic zero-sum dynamic games. Under perfect state measurements the approach leads to a minimax controller which achieves the best performance bound, and also to a characterization of all linear controllers under which disturbance attenuation does not exceed a prescribed bound. For the former, the worst-case disturbance turns out to be a correlated random sequence with a discrete distribution, which means that the problem (viewed as a dynamic game between the controller and the disturbance) does not admit a pure-strategy saddle point. Also formulated is a stochastic version of the problem, where the disturbance is a partially stochastic process with fixed higher order moments (other than the mean). Here the minimix controller depends on the energy bound of the disturbance, provided that it is below a certain threshold. Several numerical studies are included to illustrate the main results. >

Catchment disturbance inferred from paleolimnological studies of three contrasted sub-humid environments in Morocco

Abstract: Paleolimnological techniques for assessing recent drainage basin disturbance are evaluated in three Moroccan lakes with catchments contrasted in terms of land-use and vegetation. Rates of sediment accumulation in the two lakes with agricultural catchments were relatively high (>1.6 cm yr-1) in the most recent past. Dilution effects prevented core dating by the 210Pb method alone and post-1953 chronologies were constructed by combining 210Pb and 137Cs data. The recent sediment accumulation rate at the currently least disturbed site, where natural Cedrus forest is still abundant, was relatively low (<0.4 cm yr-1) but has increased since the mid-19th century. Magnetic, geochemical, pollen, and diatom studies of all three lake sediment cores linked with modern field survey data show that soil erosion in the most vegetationally disturbed catchment (Dayat-er-Roumi) has been high throughout the recent past and that intensity peaks are probably associated with wetland drainage operations beginning in the 1940's. At the partially forested site (Dayat Affougah), pre-1950's woodland clearance and other land-use changes are the likely cause of past major soil erosion episodes. The site currently dominated by natural Cedrus forest (Lac Azigza) shows only minor disturbance during the past c. 150 years although a major soil erosion episode occrred in the 17th century. Paleolimnological analysis has clearly demonstrated that major landscape change has occurred at all three sites. However, only at the two sites with catchment cultivation do previously accelerated soil erosion and lake sediment accumulation rates persist to the present. Information essential for formulation of appropriate management plans is presented and the importance of paleolimnology in assessing man-induced lake-catchment disturbance is stressed.

Researcher Disturbance in Colonies of Wading Birds: Effects of Frequency of Visit and Egg-Marking on Reproductive Parameters

Abstract: -In two closely matched colonies of Tricolored Herons (Egretta tricolor) we found no differences in reproductive parameters of the one visited frequently (16 times) and the other visited infrequently (7 times). Visits to colonies were standardized and were initiated only after most of the herons had partially completed their clutches. These results suggest that limited colony visitations beginning after courtship and early egg-lalying do not result in large disturbance effects. We caution, however, that these findings should not be applied without further testing, especially since the colonies we studied were relatively free of aerial and ground predators. We also found that letters marked on the sides of eggs were turned down by parental birds significantly more often than expected due to chance. We suggest researchers label eggs inconspicuously on the ends or not at all. Received 13 December 1988, accepted 29 June 1989.

T1he effects of winier seismic trails on tundra vegetation in northeastern alaska, u.s.a.

Abstract: The effects of winter seismic trails on tundra vegetation were studied on the Coastal Plain of the Arctic National Wildlife Refuge. Seismic exploration occurred during the winters of 1984 and 1985. Thirty-four permanent study plots were established the following summers, representing the range of disturbance which occurred in each of seven major vegetation types. Plots were visited two of the first three summers after disturbance. At each plot plant cover was measured using a point frame, disturbance characteristics were recorded, and ground surface elevations were surveyed. Plant cover was lower on most disturbed plots than on their adjacent controls, with decreases as high as 87%o the first summer following disturbance. The species most sensitive to disturbance were evergreen shrubs, followed by willows, tussock sedges, and lichens. Willow height in riparian shrubland plots was significantly reduced by 5 to 11 cm (from an average of 16 cm, p<0.05). Little recovery of plants occurred in the second or third summers after disturbance; only four plots in river floodplain habitats (Dryas terrace and riparian shrubland) showed improvements in cover of a few species. Track depressions ranging from 5 to 15 cm occurred at three plots in moist sedge-shrub tundra and increased significantly (p<0.05) at one plot between the first and third summers following disturbance. Continuation of this study will provide information on the long-term impacts and recovery rates of winter seismic trails.

Natural and disturbance-induced demographic variation in an infaunal polychaete Nephtys incise

Abstract: Demography of the infaunal polychaete Nephtys incisa was investigated for periods of 1 yr prior to and following disturbance (dredge material disposal) at a site in central Long Island Sound, USA. Infaunal grab samples were taken at 5 stations 200 m to 3 km apart. The demography of populations at each station was based on age-classes spanning juveniles to adults 4 + yr of age. Agespecific survivorship and fecundity were based on changes in mean density of each cohort and a positive correlation between female size and egg production, respectively. Analyses of population matrix models indicated pre-disposal populations had positive population growth rates, despite differences in vital rates among stations. During the second year, population growth rates were reduced by 50r!X, below population maintenance levels, at each station, primarily due to recruitment failure across the entire study site. Population growth rates were reduced an additional 25 'Yo by disturbance at the dump site and next closest stahon (200 m away) due to decreased worm size and survivorship relative to other stations, and the absence of recoloniz~tion by worms > 2 yr of age. Populations of N. incisa appear to experience several demographic 'states', related to periods of potential population growth, decline and recovery from disturbance. Based on analyses of related demographic parameters, there is a conconlitant change in the contribution different age-classes make to population growth. During periods of growth 2-yr-old worms make the greatest contribution, older age-classes during declines, while younger age classes become important dunng recovery from disturbance. These differences result from temporal and spatial fluctuations in recruitment, ind~vidual growth and reproductive activity. Responses of long-lived marine infauna to disturbance likely depend on their current demographic state at the time of disturbance (reflecting demographic conditions such as size/age structure) and factors external to the population (e.g. environmental influences on settlement and recruitment or the type of disturbance]. In this case, the disposal of contaminated dredge material had a negative impact on vital rates and potential population growth of N. incjsa at and 200m away from the disposal site, but little or no effect on populations 400 ni to 3 km away.

Introduced versus native plants in austral forests

Abstract: Despite the concern of some nineteenth- and early twentieth-century biologists, relatively few alien species have become established in undisturbed forests of Australia, Madagascar or New Zealand. All three countries have large 'naturalized' floras adapted to frequent disturbance. The short-term advantages of these alien plants - short life cycles, fast growth, abundant seed and plasticity of habit - have allowed them to cover large areas of disturbed ground and to coexist with native plants in early successional vegetation. Very few persist in mature forest. The threat to the austral forests is human disturbance, and there is need for research on the native plants which perform the specialized role of repairing canopy after disturbance.

Comparison of two LQG-based methods for disturbance rejection

Abstract: When the disturbances acting on a system reflect the dynamics of the process generating them (i.e. disturbances other than white noise), this information can be exploited for the design of disturbance rejection controllers. Two modern control approaches for designing linear disturbance rejection controllers are discussed: disturbance modeling and frequency shaping of cost functionals. Both methods are extensions of linear-quadratic-Gaussian theory. It is shown that although the philosophies used to develop the two control schemes are very different, there is a distinct relationship between them. This is formalized by showing that the two methods are equivalent for the single-input-single-output case. The duality of the two methods is discussed for the multi-input-multi-output case. >

Effects of wave exposure on the community structure of a plant-dominated, fringing-reef platform: intermediate disturbance and disturbance-mediated competition

Abstract: Effects of wave-exposure on a fore-reef community were investigated by quantitative biomass sampling and field manipulations at Galeta Point, Caribbean Panama. Three predictions of Connell's (1978) intermediate-disturbance hypothesis are examined: (1) community diversity should be low in highly disturbed habitats, as few species can tolerate extreme biotic and abiotic condihons; (2) community diversity should be low in benign habitats, as most species are excluded by the competitive dominants; and (3) maximal community diversity should occur at intermediate conditions, as neither competition nor disturbance exert a dominating influence. In general, data support the intermediatedisturbance model. The predictions apply best to guilds of species that occupy primary substratum and are directly affected by wave-induced disturbance and space-light competition. Species adapted to environmental extremes or associated with the competitive dominants may be equally important in determining community richness. Tolerance to disturbances defines which species become dominant. Habitat complexity, competition, and mutualism secondarily influence the composition of this community. Selection for competitive ability among seaweeds becomes increasingly important in benign environments. Throughout most of the fore-reef zone, Laurencia papillosa is the competitive dominant, as defined by its ability to sequester and dominate space. As wave exposure subsides, Acanthophora spicifera competitively reduces the abundance of L. papillosa in some microhabitats. Spatial-temporal conditions determine the superior competitor. A balance among competitive abilities, mutualistic interactions, and tolerances to distrubances promotes the coexistence of these species.

The Role of Snow Cover in Limiting Surface Disturbance Caused by Winter Seismic Exploration

Abstract: The relationship between snow cover and the degree of surface disturbance caused by winter seismic vehicles was investigated on the Arctic Coastal Plain of the Arctic National Wildlife Refuge in northeastern Alaska. Ninety study plots were established on seismic lines and camp moves in tussock tundra and moist sedge-shrub tundra. Total snow depth and its components, slab layer and depth hoar, were measured during the winter. Plant cover changes, tussock disturbance, visibility and disturbance levels were determined at the study plots in the summer. Disturbance was found to be generally lower when snow depths were greater. In tussock tundra, plots with snow depths over 25 cm had significantly less disturbance than those with under 25 cm (p <0.05). The relationship between snow cover and disturbance was less clear in moist sedge-shrub tundra, where disturbance appeared to be less at snow depths above 25 cm, but these differences were not statistically significant (p <0.05). Slab depth, which does not include the loose layer of depth hoar, provided a better measure of protective snow cover in most sedge-shrub tundra, as slab depths over 20 cm resulted in significantly less disturbance (p <0.05). Moderate-level disturbance (25-50% decrease in plant cover) did not occur on trails where snow depths were at least 25 cm in tussock tundra and 35 cm in moist sedge-shrub tundra. Low-level disturbances (less than 25% decrease in plant cover) occurred on trails with snow depths as high as 45 cm in tussock tundra and 72 cm in moist sedge-shrub tundra. Key words: surface disturbance, winter seismic exploration, seismic trails, tundra, snow depth, Alaska, Arctic National Wildlife Refuge, Arctic Coastal Plain

Airphoto Analysis of Winter Seismic Disturbance in Northeastern Alaska

Abstract: Airphoto interpretation was used to quantify the extent of disturbance caused by seismic exploration on the 60,000 ha coastal plain of the Arctic National Wildlife Refuge during the winters of 1984 and 1985. The relationships of vegetation type, trail location and traffic pattern to the amount of disturbance were investigated. Approximately 20% of the seismic trails were photographed at 1:6000 scale, using color infrared film. Ground data collected at 194 sites were used to develop a photo interpretation key describing the photo signatures of seven vegetation types and four disturbance levels. Vegetation types and disturbance levels were determined for 4914 circles of 3 mm diameter on the aerial photos (18 m ground distance). Fourteen percent of the points were interpreted as having no disturbance (level 0), 57% had level 1 disturbance (low), 27% had level 2 (medium) and 2% had level 3 (high). Wet or partially vegetated areas were the least susceptible to disturbance. Vegetation types with mounds, tussocks, hummocks or high-centered polygons and dryas terraces were more heavily disturbed. Camp move trails and overlapping seismic and camp move trails created in 1984 caused more disturbance than other trail types due to multiple passes of vehicles over narrow trails. U.S. Fish and Wildlife Service monitors were more successful at minimizing disturbance the second year by requesting that vehicle operators avoid multiple passes on the same trail, sensitive vegetation types and areas of low snow cover. Key words: airphoto analysis, winter seismic exploration, seismic trails, vegetation disturbance, traffic patterns, Alaska, Arctic National Wildlife Refuge, arctic coastal plain