Showing papers in "Ecological Applications in 1994"

On Beyond Baci: Sampling Designs That Might Reliably Detect Environmental Disturbances

Abstract: Much sampling to detect and quantify human environmental disturbances is flawed by a lack of appropriate replication. BACI (Before—After—Control—Impact) designs have only a single control location, and any conclusions from them are illogical. Asymmetrical designs using one putatively impacted and several control locations can reliably detect a variety of environmental impacts, including those that do not affect long—run mean abundances, but do alter temporal variance. When abundances of populations in different locations show temporal interaction, the asymmetrical designs allow tests for impact that are not possible in BACI designs. Asymmetrical designs are also extendable to sample at hierarchical spatial and temporal scales. The power of tests using asymmetrical designs is great for non—interactive sets of abundances, but greatest for pulse (short—term) responses to disturbances, large alterations of temporal variance, or combinations of temporal variance, or combinations of sustained, press responses in mean abundance coupled with altered temporal heterogeneity. Power in temporally interactive sets of data is generally poor. Alternatives to pre—disturbance sampling, including generalized assessment of spatial and temporal variances and experimental impacts, may provide better guidance for detection of human disturbances.

A Benthic Index of Biotic Integrity (B-IBI) for Rivers of the Tennessee Valley

Abstract: Invertebrate data from rivers in the Tennessee Valley were used to: (1) evaluate the utility of 18 characteristics of invertebrate assemblages (attributes) to assess the biological condition of streams and (2) develop a comprehensive benthic invertebrate index that reflects important aspects stream biology and responds to the effects of human society in detectable ways. We used data from the Tennessee Valley Authority's (TVA) fixed—station monitoring program established in 1986 to evaluate water resources in the Tennessee Valley. The streams covered sites with various types and levels of human disturbance. TVA biologists sampled riffles and pools using quantitative methods. To evaluate attributes we determined (1) the ability of each attribute to distinguish among sites (analyses of variance), (2) the associations among attributes (correlation coefficients), (3) the concordance of assessments of sites using each attribute with two independent site assessments (water quality and analysis of fish assemblages based on the index of biotic integrity), and (4) the variability of each attribute across habitats (riffles and pools) and ecoregions. Thirteen attributes (total taxa richness and taxa richnesses of intolerant snails and mussels, mayflies, caddisflies, and stoneflies; relative abundances of Corbicula, oligochaetes, omnivores, filterers, grazers, and predators; dominance; total abundance) were valuable in discriminating sites, exhibited concordance with other methods of assessment, and were relatively uncorrelated among themselves. Five attributes (sediment—surface taxa richness; relative abundances of shredders, gatherers, detritivores, and chironomids) were not acceptable for inclusion in the index because they either were not in concordance with other methods of assessment or were strongly correlated with other attributes. Based on the results of this evaluation, 13 attributes were included in a benthic index of biotic integrity (B—IBI). The B—IBI was evaluated using a more extensive fixed—station data set and was tested using an independent data set where samples were taken above and below the release of an industrial effluent to the North Fork Holston River. The fixed—station sites were correctly ranked, although rankings were not always consistent for pool and riffle habitats. B—IBI also distinguished the industrial impact. The B—IBI has potential as an assessment tool for streams, although more testing and evaluation of both the attributes and the index are warranted.

The Effects of Habitat Fragmentation on Midwestern Grassland Bird Communities

Abstract: The influence of area and vegetation structure on breeding bird communities associated with 24 Illinois grassland fragments (0.5—600 ha) was studied between 1987 and 1989 to document the effects of habitat fragmentation in a severely fragmented midwestern landscape. Fragment area strongly influenced bird communities within grasslands and accounted for a high percentage of the variation in mean breeding bird species richness among fragments (R2 = 0.84). Breeding bird species richness patterns within 4.5—ha subsections of these grasslands also significantly increased with fragment size. Eight of the 15 (53%) most common bird species had distributions among fragments that were significantly influenced by habitat area, whereas six species (40%) had distributions within fragments that were significantly influenced by vegetation structure only. The Dickcissel (Spiza americana) was the only species with a distribution within fragments that was not significantly associated with either habitat area or vegetation structure. Four groups of birds were identified by an analysis of habitat area and vegetation structure preferences of individual species: area—sensitive species (5 species), edge species (3), vegetation—restricted species (6), and the Dickcissel. Estimates of minimal area requirements for the five area—sensitive species ranged from 5 to 55 ha. Discriminant analyses of habitat suitability within fragments suggests that the absence of area—sensitive grassland bird species form some small fragments may result, in part, from limited habitat availability. All five area—sensitive species, however, also regularly avoided structurally suitable habitat on small grassland fragments. As a result of the considerable extent to which native and, more recently, agricultural grasslands have declined in the Midwest, habitat fragmentation is likely to have caused midwestern grassland bird declines, especially for area—sensitive species.

Habitat fragmentation, native insect pollinators, and feral honey bees in argentine, "chaco serrano"'

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Atmospheric Deposition of Nutrients and Pollutants in North America: An Ecological Perspective

Abstract: Research on air pollution and acidic deposition during the last 15 yr has greatly increased our knowledge of the rates and the processes of atmospheric deposition. The invigoration of the field has been a direct result of interchange and cooperation among ecosystem ecologists, micrometeorologists, and plant physiological ecologists who each approach the study of atmospheric deposition from different perspectives. This has led to the widespread realization among ecologists of the importance of dry and cloud deposition and the introduction of new methods to estimate these fluxes. In this paper I summarize the current understanding of atmospheric deposition processes, measurement methods, and patterns of deposition in North America. Dry deposition measurements are still highly uncertain in most cases, and methodology is still an active area of research and debate, but it seems clear that ecologists will require a suite of different methods to evaluate dry deposition fluxes of the various elements important to ecosystems. Standard model formulations have been developed for estimating dry and cloud deposition, and these models are finding wide use in flux estimation at sites where direct measurements are unavailable. National monitoring networks for wet and dry deposition have been established and are providing information on continental— and regional—scale patterns. Research has demonstrated that deposition rates are increased substantially at high—elevation sites by enhancement of wet, dry, and especially, cloud deposition. Patterns of O3 exposure are also different at high—elevation sites. The deposition of O3, while mechanistically similar to that of other gases, has been treated differently in the ecological and botanical literature because of its direct phytotoxicity at ambient concentrations in industrialized areas. Current efforts in ozone exposure research involve determining appropriate exposure indices for interpreting biological responses and coupling models of atmospheric transport with models of ozone disposition within the plant.

Predicting the impact of Turtle Excluder Devices on loggerhead sea turtle populations

Abstract: In this paper, we modified and updated a stage—based population model for loggerhead sea turtles (Caretta caretta) and used the model to project potential population—level effects of the use of turtle excluder devices (TEDs) in trawl fisheries of the southeastern US. We reduced the seven—stage model of Crouse et al. (1987) to a five—stage model and performed sensitivity analyses on the matrix. The most sensitive matrix parameters were those dealing with survival while remaining in a stage, rather than growth from one stage to the next or reproductive output. Population growth rate was most sensitive to survival in the large juvenile stage, followed by small juvenile survival. Large juveniles are the most common size class among stranded dead turtles found on beaches; 70—80% of strandings are thought to be related to trawl fisheries. Simulations of our loggerhead model based on estimated effects of TED regulations on stage—specific survivorship suggested that southeastern US loggerhead populations should increase, but rather slowly. If TEDs were required during the shrimping season in offshore areas only (as they were from 1990 to 1992), 70 yr or more would be required for the simulated population to increase by an order of magnitude. Recent estimates of TED effects from South Carolina strandings data suggest a similar recovery rate. Good compliance with regulations requiring TEDs year—round in all waters could allow the population to increase nearly twice as fast as that expected under the "seasonal offshore" regulations. We also used a Leslie matrix version of the model to illustrate the expected transient response in the numbers of females expected on nesting beaches (due to shifting age—size structures with TED use). Rather than a monotonic increase, we expect an initial increase in the number of nesting females, followed by a leveling off or slight decline (perhaps 10—15 yr from now), followed by another increase. The magnitude of the projected population increase will depend upon the actual increases in stage—specific survivorship due to TED regulations. New, or compensatory, sources of mortality could slow or reverse this projected recovery.

Responses of Trace Gas Fluxes and N Availability to Experimentally Elevated Soil Temperatures

Abstract: We are conducting a field study to determine the long-term response of belowground processes to elevated soil temperatures in a mixed deciduous forest. We established 18 experimental plots and randomly assigned them to one of three treatments in six blocks. The treatments are: (1) heated plots in which the soil temperature is raised 5?C above ambient using buried heating cables; (2) disturbance control plots (cables but no heat); and (3) undisturbed control plots (no cables and no heat). In each plot we measured indexes of N availability, the concentration of N in soil solutions leaching below the rooting zone, and trace gas emissions (CO, N20, and CH4). In this paper we present results from the first 6 mo of this study. The daily average efflux of CO2 increased exponentially with increasing soil temperature and decreased linearly with increasing soil moisture. A linear regression of temperature and the natural logarithm of C02 flux explained 92% of the variability. A linear regression of soil moisture and C02 flux could explain only 44% of the variability. The relationship between soil temperature and C02 flux is in good agreement with the Arrhenius equation. For these CO2 flux data, the activation energy was 63 kJ/mol and the Q,0 was 2.5. The daily average uptake of CH4 increased linearly with increasing soil temperatures and decreased linearly with increasing soil moisture. Linear regression could explain 46% of the variability in the relationship between temperature and CH4 uptake and 49% of the variability in the relationship between soil moisture and CH4 uptake. We predicted the annual C02 flux from our study site in 1991 using two empirical relationships: the relationship between air temperature and soil temperature, and the re- lationship between soil temperature and C02 flux. We estimate that the annual C02-C flux in 1991 was 712 g/m2 from unheated soil and 1250 g/m2 from heated soil. By elevating the soil temperature 5?C above ambient, we estimate that an additional carbon flux of 538 g.m-2 yr-' was released from the soil as CO2.

Modeling Population Viability for the Desert Tortoise in the Western Mojave Desert

Abstract: The desert tortoise is a threatened species living in the deserts of the Amer- ican Southwest. Using size-structured demographic models, we analyzed the status of the tortoise in the Western Mojave desert and evaluated the effectiveness of possible manage- ment measures. Our demographic analyses agree with the trends reported by field censuses in showing rapid population decline. Importantly, simulations that include variation and correlation in vital rates yield highly variable forecasts of population growth, indicating the uncertainties inherent in even short-term projections of tortoise population sizes. To determine where conservation efforts and data collection should be focused, we performed a series of sensitivity analyses in which the effects of changing different vital rates were quantified. We found that the rate of population growth is most sensitive to the survival of large adult females and that improving survival of this size class to reputably "pristine" rates could reverse population declines; in contrast, large improvements in other vital rates will not, alone, reverse population decline. Thus, shooting, off-road vehicles, upper respi- ratory tract disease (URTD), and other major sources of adult mortality should be the primary focus of management strategies. Finally, we discuss the impact of the proposed expansion of the U.S. Army's Fort Irwin, which would reduce the Western Mojave tortoise population by < 13%. We argue that the expansion could have a grave impact on the long- term population viability of the tortoise, but that this impact will be a function of the management of remaining tortoise habitat. We conclude that, although many anthropogenic impacts threaten the tortoise, the species' future could be bright, provided that research and land-use planning focus on biologically important aspects of its life history.

Tropical rain forest conversion to pasture: Changes in vegetation and soil properties

Abstract: The effect of converting lowland tropical rainforest to pasture, and of subsequent succession of pasture lands to secondary forest, were examined in the Atlantic Zone of Costa Rica. Three replicate sites of each of four land-use types representing this disturbance-recovery sequence were sampled for changes in vegetation, pedological properties, and potential nitrogen mineralization and nitrification. The four land-use types included primary forest, actively grazed pasture (10-36 yr old), abandoned pasture (abandoned 4-10 yr) and secondary forest (abandoned 10-20 yr). Conversion and succession had obvious and significant effects on canopy cover, canopy height, species composition, and species richness; it appeared that succession of secondary forests was proceeding toward a floristic composition like that of the primary forests. Significant changes in soil properties associated with conversion of forest to pasture included: (1) a decrease in acidity and increase in some base exchange properties, (2) and increase in bulk density and a concomitant decrease in porosity, (3) higher concentrations of NH[sub 4][sup +], (4) lower concentrations of NO[sub 3][sup [minus]], (5) lower rates of N-mineralization, and (6) in some cases, lower rates of nitrification. Chemical changes involving cations associated with conversion from forest to pasture indicated increases in soil fertility under the pasturemore » regimes, while changes associated with nitrogen indicated decreases in fertility. Physical changes in density and porosity were deleterious with respect to infiltration, percolation, aeration, and, ultimately, erodability. Beyond the practical aspects of land management, many of these changes are very important to carbon and nitrogen cycling and to the emission and consumption of biogenic trace gases.« less

Environmental Limits on Net Primary Production and Light-Use Efficiency Across the Oregon Transect

Abstract: Due to climate differences, an extreme range in productivity occurs along a 250-km, west-east transect at 144? north latitude in western Oregon, USA, where coniferous evergreen forests dominate. As part of the Oregon Transect Ecosystem Research (OTTER) project, our objective was to evaluate how climate constrains net primary production (NPP) by limiting the utilization of intercepted photosynthetically active radiation (IPAR). The forests measured along the transect intercepted from 22% to 99.5% of the incident PAR. With data collected from recording meteorological stations installed near each site, we defined the hourly conditions when photosynthesis was partly or completely limited by drought, extreme humidity deficits, or frost. From this analysis we calculated that the fraction of incident PAR that could be utilized throughout the year ranged from 92% in the coastal rainforests to <25% in the juniper woodland. NPP varied from 3 to 26 Mg hat l yr-I with the fraction of belowground NPP, estimated from litterfall, increasing from 20% to 60% of the total as the environment becomes harsher. Light-use efficiency (e), calculated under conditions when the environment did not constrain photosynthesis, averaged 0.8 g/MJ for aboveground NPP and 1.3 g/MJ for total NPP.

Relating Riparian Vegetation to Present and Future Streamflows

Abstract: The intense demand for river water in arid regions is resulting in widespread changes in riparian vegetation. We present a direct gradient method to predict the vegetation change resulting from a proposed upstream dam or diversion. Our method begins with the definition of vegetative cover types, based on a census of the existing vegetation in a set of 1 ° 2 m plots. A hydraulic model determines the discharge necessary to inundate each plot. We use the hydrologic record, as defined by a flow duration curve, to determine the inundation duration for each plot. This allows us to position cover types along a gradient of inundation duration. A change in river management results in a new flow duration curve, which is used to redistribute the cover types among the plots. Changes in vegetation are expressed in terms of the area occupied by each cover type. We applied this approach to riparian vegetation of the Black Canyon of the Gunnison National Monument along the Gunnison River in Colorado. We used TWINSPAN to cluster plots according to species occurrence. This analysis defined three vegetative cover types that were distinct in terms of inundation duration. Quantitative changes in the extent of cover types were estimated for three hypothetical flow regimes: two diversion alternatives with different minimum flows and a moving average modification of historical flows. Our results suggest that (1) it is possible to cause substantial changes in riparian vegetation without changing mean annual flow, and (2) riparian vegetation is especially sensitive to changes in minimum and maximum flows. Principal advantages of this method are simplicity and reliance on relatively standard elements of plant community ecology and hydrologic engineering. Limitations include use of a single environmental gradient, restrictive assumptions about changes in channel geometry, representation of vegetation as quasi—equilibrium cover types, and the need for model validation.

Dynamics and Pattern of a Managed Coniferous Forest Landscape in Oregon

Abstract: We examined the process of fragmentation in a managed forest landscape by comparing rates and patterns of disturbance (primarily clear-cutting) and regrowth between 1972 and 1988 using Landsat imagery. A 2589-km(exp 2) managed forest landscape in western Oregon was classified into two forest types, closed-canopy conifer forest (CF) (typically, greater than 60% conifer cover) and other forest and nonforest types (OT) (typically, less than 40 yr old or deciduous forest). The percentage of CF declined from 71 to 58% between 1972 and 1988. Declines were greatest on private land, least in wilderness, and intermediate in public nonwilderness. High elevations (greater than 914 m) maintained a greater percentage of CF than lower elevations (less than 914 m). The percentage of the area at the edge of the two cover types increased on all ownerships and in both elevational zones, whereas the amount of interior habitat (defined as CF at least 100 m from OT) decreased on all ownerships and elevational zones. By 1988 public lands contained approximately 45% interior habitat while private lands had 12% interior habitat. Mean interior patch area declined from 160 to 62 ha. The annual rate of disturbance (primarily clear-cutting) for the entire area including the wilderness was 1.19%, which corresponds to a cutting rotation of 84 yr. The forest landscape was not in a steady state or regulated condition which is not projected to occur for at least 40 yr under current forest plans. Variability in cutting rates within ownerships was higher on private land than on nonreserve public land. However, despite the use of dispersed cutting patterns on public land, spatial patterns of cutting and remnant forest patches were nonuniform across the entire public ownership. Large remaining patches (less than 5000 ha) of contiguous interior forest were restricted to public lands designated for uses other than timber production such as wilderness areas and research natural areas.

Detection of Environmental Impacts: Natural Variability, Effect Size, and Power Analysis

Abstract: The power of any test of an environmental impact is simultaneously constrained by (1) the variability of the data, (2) the magnitude of the putative impact, and (3) the number of independent sampling events. In the context of the Before–After—Control–Impact design with Paired sampling (BACIP), the variability of interest is the temporal variation in the estimated differences in a parameter (e.g., population density) between two unperturbed sites. The challenges in designing a BACIP study are to choose appropriate parameters to measure and to determine the adequate number and timing of sampling events. Two types of studies that are commonly conducted can provide useful information in designing a BACIP study. These are (1) long—term studies that provide estimates of the natural temporal and spatial variability of environmental parameters and (2) spatial surveys around already—perturbed areas ("After—only" studies) that can suggest the magnitude of impacts. Here we use data from a long—term study and an After—only study to illustrate their potential contributions to the design of BACIP studies. The long—term study of parameters sampled at two undisturbed sites yielded estimates of natural temporal variability. Between site differences in chemical—physical parameters (e.g., elemental concentration) and in individual—based biological parameters (e.g., body size) were quite consistent through time, while differences in population—based parameters (e.g., density) were more variable. Serial correlation in the time series of differences was relatively small and did not appear to vary among the parameter groups. The After—only study yielded estimates of the magnitude of impacts through comparison of sites near and distant from a point—source discharge. The estimated magnitude of effects was greatest for population—based parameters and least for chemical—physical parameters, which tended to balance the statistical power associated with these two parameter groups. Individual—based parameters were intermediate in estimates of effect size. Thus, the ration of effect size to variability was greatest for individual—based parameters and least for population and chemical—physical parameters. The results suggest that relatively few of the population and chemical—physical parameters could provide adequate power given the time constraints of most studies. This indicates that greater emphasis on individual—based parameters is needed in field assessments of environmental impacts. It will be critical to develop and test predictive models that link these impacts with effects on populations.

Community‐Wide Consequences of Trout Introduction in New Zealand Streams

Abstract: Twenty-five years ago not one of these rivers had the least interest for the angler … the rod of the fisherman never cast a shadow on their waters; every one of these mighty rivers, every one of the thousand creeks and streams that flow into them … were tenantless and profitless to the sportsman (Spackman 1892, as cited in McDowall 1990a) … brown trout have since been stocked in almost every conceivable lake, river, or stream, such that the present naturalized population encompasses every suitable ecological niche within the confines of New Zealand (MacCrimmon and Marshall 1968).

Ecological Remote Sensing at OTTER: Satellite Macroscale Observations

Abstract: Coarse, global-scale, satellite remotely sensed observations are compared with ground measurements collected during the OTTER study. The objective was deri- vation of ecological and environmental variables from the satellite data needed to define primary production in western Oregon. Observations from the TOMS sensor and the AVHRR sensor provide estimates of incident PAR radiation, intercepted PAR, atmo- spheric humidity, air temperature, vapor pressure deficit, and drought. The satellite ob- servations compared favorably with the coincident ground measurements, but the strength of the relation was modest in some cases. Atmospheric attenuation of the remotely sensed measurements and ground measurement quality both limit stronger relations. This study demonstrates that satellite remote sensing is capable of providing information needed for macroscale ecological monitoring. Currently, it appears possible, from AVHRR and TOMS observations, to derive periodic approximations of ecological conditions sufficient to drive a simple-production-efficiency-type model. More refined remote-sensing methods will be needed to provide the type of measurement precision required by more refined ecosystem models.

Biological Inventory Using Target Taxa: A Case Study of the Butterflies of Madagascar

Abstract: Biotic inventories provide critical data for conservation planning, but frequently, conservation decisions are made without surveys, due to lack of time, funds, or appropriate methodology. A method, target taxon analysis, is therefore proposed for streamlining regional biotic inventories, while simultaneously increasing their taxonomic coverage and spatial resolution. In this method, regional inventories focus on a number of narrowly defined target taxa, chosen to represent collectively an array of higher taxa. Such target taxa should be information rich; in other words, the pattern of species distributions in these taxa should correlate either with patterns of environmental heterogeneity or with distributional patterns of species in unrelated taxonomic groups. It is suggested that clades that experienced an evolutionary radiation within the region are likely to be information rich for conservation planning at or within this regional scale. Such clades will be identifiable as low—ranking, species—rich taxa with high endemism. The information richness of these potential target clades can then be evaluated by direct gradient methods of analysis that relate community compositional change to environmental factors, or by correlating distributional patterns of species among separate target clades. To assess this approach to biological inventory, a species—rich genus and subgenus of endemic butterflies from the island of Madagascar were chosen as target taxa and were evaluated for information richness in comparison to the entire butterfly fauna of Madagascar. Using canonical correspondence analysis and other analytical techniques, the subgenus of Malagasy Henotesia species (Satyrinae) proved to be as good or better than the entire butterfly fauna at delineating a variety of environmental gradients at both local and landscape scales. The endemic genus Strabena (Satyrinae) was only able to delineate such patterns under a restricted set of conditions. However, this genus, while species rich in Madagascar, was not exceptionally diverse nor were its species members abundant within the study area. It is concluded that target taxon analysis is a potentially useful tool for providing high—quality data while expanding coverage of taxonomic diversity for conservation planning.

Testing forest-bgc ecosystem process simulations across a climatic gradient in oregon'

Abstract: Field measurements from the Oregon Transect Ecological Research project (OTTER) were used to validate selected process simulations in the FOREST-BGC eco- system model. Certain hydrologic, carbon, and nitrogen cycle process simulations were tested in this validation, either comparatively across sites, or seasonally at single sites. The range of simulated ecosystem-process rates across the OTTER sites was large; annual evapotranspiration (ET) ranged from 15 to 82 cm, net photosynthesis (as carbon) from 2.2 to 22.8 Mg/ha, and decomposition (as carbon) from 1.0 to 7.2 Mg ha-' Iyr-'. High cor- relations between predicted and measured data were found for: aboveground net primary production, R2 = 0.82; 100-yr stem biomass, R2 = 0.79; and average leaf nitrogen con- centration, R2 = 0.88. However, correlations for pre-dawn leaf water potential and equi- librium leaf area index (LAI) were much lower, because successful simulation of these variables requires accurate data for soil water-holding capacity. Defining the water-holding capacity of the rooting zone and the maximum surface conductance for photosynthesis and transpiration rates proved to be critical system variables that defied routine field mea- surement. Although many other processes are simulated in FOREST-BGC, no other pro- cesses had repeated field data sets for validations. Problems in parameterizing the model from disparate data sets are also evaluated, with suggestions for using ecosystem modeling in future integrated research programs.

The Effects of Management Systems on Ground-Foraging Ant Diversity in Costa Rica

Abstract: The loss of biological diversity is an ecological crisis of profound and universal impact (Wilson 1988). Most of this loss is occurring in tropical regions as a result of conversion of forest to agriculture and pasture (Myers 1984). The “traditional” solution to reduce loss of species has been to establish national parks and other protected areas and attempt to exclude local populations of people from them. While these pristine parks are critical to protecting many species that cannot survive habitat modification, small islands of pristine forest may not be optimal for the protection of many species on a long-term basis, particularly if they are surrounded by huge expanses of disturbed habitats that restrict migration (Lovejoy et al. 1986, Bierregaard et al. 1992, but see Wu et al. 1993).

Height of White‐Flowered Trillium (Trillium Grandiflorum) as an Index of Deer Browsing Intensity

Abstract: The height of white-flowered trillium (Trillium grandiflorum) is a useful indicator of deer browsing intensity. In their foraging activities deer select larger plants over smaller plants. Because flowering plants are larger than nonflowering plants, the number of plants in flower decreases with increasing browsing intensity. As browsing intensity increases, the height of the Trillium becomes shorter in successive growing seasons, presumably due to the loss of photosynthetic capacity and reduction in belowground re- sources. Trillium stem height was positively correlated with reproductive output by pe- rennial herbaceous plants and negatively correlated with the percent of the herbaceous understory that is browsed. This indicates change in stem height is an indication of the general status of the herbaceous flora as influenced by deer browsing. Based on deer pop- ulation densities associated with study sites supporting Trillium populations with stable stem heights and flowering plants, maintenance of deer densities of 4-6 individuals/km2 is recommended for deciduous forests in northeastern Illinois. In eastern United States, research workers who assume they are studying relatively undisturbed sites should be aware that intense deer browsing may have imposed an alteration on their study sites.

Predator-Prey Theory and Management of Multispecies Fisheries

Abstract: In population modelling the functional forms of models are at least as important as are parameter values in expressing the underlying biology and in determining the outcome. Here the influence of differing biological assumptions as to predator interference on the forms of predator—prey models is discussed, and it is shown how this aspect of the underlying biology can influence the conclusions to be drawn from multispecies population models as to how "competition from natural predators" affects human harvesting of natural populations. The importance of these issues for the current controversy over the effects of marine mammals on fisheries is stressed. See full-text article at JSTOR

Remote-Sensing-Based Condition Assessment for Nonequilibrium Rangelands Under Large-Scale Commercial Grazing

Abstract: Range condition assessment procedures that rely on field-collected botanical data face major problems in nonequilibrium rangelands, which are spatially variable and extensively grazed. These problems include the difficulty of interpreting changes in plant species composition and the logistics of obtaining representative data for large areas. Con- sideration of ecosystem behavior through time and in space shows that certain spatial and temporal patterns exist that may be used to isolate the impact of grazing from other processes. The patterns also make it possible to distinguish between temporary changes and those that are more long term. All relevant patterns may be expressed in terms of total plant cover and may be monitored from remote sensing satellites. It is therefore possible to derive a set of range condition indicators that may be measured and monitored from space. These indicators use trends in average vegetation cover with distance from water at the end of very wet periods, trends in cover variance with distance from water, and the magnitude of observed vegetation response at individual points in the landscape compared with that which is expected when vegetation recovers fully from grazing. It is also possible to use spatial variability in the rate of cover depletion after rainfall to infer relative dif- ferences in the amount of forage present. When used in combination, the methods offer a realistic alternative to field-based assessment and are capable of detecting many types of rangeland degradation. They are also considerably cheaper to use.

Landscape pattern response to changes in pattern generation rules: land-use legacies in forestry

Abstract: The Pacific Northwest of the United States is currently embroiled in an acrimonious debate over the management of federal forest lands. Constructive resolution of this debate will require better information on a broad range of forest management issues. This study focuses on one such issue: the development of landscape pattern in response to alternative forest cutting plans and the degree to which established landscape patterns can be changed. Dispersed cutting has been conducted on federal lands in the western United States for > 40 yr, but alternative cutting plans are now being considered. To assess the effects of different disturbance processes on the development of landscape pattern, we compare dispersed- and aggregated-cutting plans using a simple, rule-based simulation model that incorporates realistic regulatory and logistic constraints. Our results indicate that, once established, the landscape pattern created by dispersed disturbances is difficult to erase without a substantial reduction in the disturbance rate or a reduction in the minimum stand age eligible for disturbance. Change in landscape pattern can lag substan- tially behind change in the rules governing pattern generation.

Response of Early‐ and Late‐Flowering Plants to Fire Season in Experimental Prairies

Abstract: Fire maintains tallgrass prairies, but fire season is virtually unexplored as a formative influence of prairie floristics. Replicated mid—July burns, simulating the peak of lightning—caused fire in natural grasslands, strongly influenced species composition and cover in experimental restorations in southern Wisconsin. Perennials that flowered before mid—July showed only 17% cover in unburned plots and 6% in plots burned in March, but 46% cover in plots burned in mid—July. Late—flowering species dominating the remnants managed with dormant—season burns accounted for 80% cover in unburned plots and 92% cover in plots burned in March, but only 47% cover in plots burned in July. Management practices that employ dormant—season burns may produce quite different communities than those maintained by midsummer burns that simulate the timing of lightning fires. See full-text article at JSTOR

Simulating Winter Interactions Among Ungulates, Vegetation, and Fire in Northern Yellowstone Park

Abstract: The interaction of large-scale fire, vegetation, and ungulates is an important management issue in Yellowstone National Park. A spatially explicit individual-based simulation model was developed to explore the effects of fire scale and pattern on the winter foraging dynamics and survival of free-ranging elk (Cervus elaphus) and bison (Bison bison) in northern Yellowstone National Park. The Northern Yellowstone Park (NOYELP) model simulates the search, movement, and foraging activities of individuals or small groups of elk and bison. The 77 020-ha landscape is represented as a gridded irregular polygon with a spatial resolution of 1 ha. Forage intake is a function of an animal's initial body mass, the absolute amount of forage available on a site, and the depth and density of snow. When the energy expenditures of an animal exceed the energy gained during a day, the animal's endogenous reserves are reduced to offset the deficits. Simulations are conducted with a 1-d time step for a duration of 180 d, [approximately]1 November through the end of April. Simulated elk survival for three winters (1987-1988; 1988-1989; 1990-1991) agreed with observed data.

Seasonal Patterns and Remote Spectral Estimation of Canopy Chemistry Across the Oregon Transect

Abstract: We examined seasonal changes in canopy chemical concentrations and content in conifer forests growing along a climate gradient in western Oregon, as part of the Oregon Transect Ecosystem Research (OTTER) study. The chemical variables were related to seasonal patterns of growth and production. Statistical comparisons of chemical variables with data collected from two different airborne remote-sensing platforms were also carried out. Total nitrogen (N) concentrations in foliage varied significantly both seasonally and among sites; when expressed as content in the forest canopy, nitrogen varied to a much greater extent and was significantly related to aboveground net primary production (r = 0.99). Chlorophyll and free amino acid concentrations varied more strongly than did total N and may have reflected changes in physiological demands for N. Large variations in starch concentrations were measured from pre- to post-budbreak in all conifer sites. Examination of remote-sensing data from two different airborne instruments suggests the potential for remote measurement of some canopy chemicals. Multivariate analysis of high-resolution spectral data in the near infrared region indicated significant correlations between spectral signals and N concentration and canopy N content; the correlation with canopy N content was stronger and was probably associated in part with water absorption features of the forest canopy. The spectral bands that were significantly correlated with lignin concentration and content were similar to bands selected in the other laboratory and airborne studies; starch concentrations were not significantly related to spectral reflectance data. Strong relationships between the spectral position of specific reflectance features in the visible region and chlorophyll were also found.

The Importance of Soil Water in the Recruitment of Bouteloua Gracilis in the Shortgrass Steppe

Abstract: In the shortgrass steppe region of North America there is a controversy about the ability of the dominant species to recruit from seedlings. The prevailing view is that Bouteloua gracilis is incapable of recruitment from seedlings in areas receiving <380 mm of annual precipitation. A common explanation for this situation is that environmental conditions permitting seedling establishment are infrequent. To assess the frequency of environmental conditions appropriate for the recruitment of B. gracilis we used a soil water simulation model and long—term climatic data in conjunction with detailed information about the ecophysiological requirements for seed germination and growth of seminal and adventitious roots. We found that recruitment events occur as frequently as every 30—50 yr on silty clay, silty clay loam, and silty loam soils, but less than once in 5000 yr on sandy soils. Simulated frequencies of recruitment were sufficient to account for the observed abundance of B. gracilis in 7 of 11 soil textures...

Human Disturbance and Long-Term Changes on a Rocky Intertidal Community

Abstract: The effects of human recreational activities on a rocky intertidal habitat on the coast of San Diego, California, USA were investigated. Organisms susceptible to collection for food, bait, or aquaria were identified and served as key species for the study of biota disturbance. This study examined three major aspects: (1) Distribution and activities of people along the shoreline were documented. (2) Distribution and density of echinoderms and molluscs inhabiting the cryptic underrock surface were sampled along an identified gradient of disturbance. (3) Densities of conspicuous organisms inhabiting the underrock surface at the most disturbed location detected in the spring of 1971 were compared with those detected in the spring of 1991. Surveys of human activity made during weekends with low tides exposing most of the intertidal zone (below O MLLW) during the daylight hours between December 1990 and August 1991 showed a definite pattern. People concentrated in an area with a 200 m radius centered on the primary accesses; areas farther from these points were less visited. The study site stood along a gradient of human disturbance, and the sampling of organisms along this indicated a gradient of biota disturbance as well. The density of all species was reduced in the more heavily visited intertidal area. The underrock community at the most heavily visited location changed substantially from spring 1971 to spring 1991. The density of most of the macroorganisms decreased between the two dates, with the exception of the density of small gastropods, which increased. Even if more long—term studies are needed for determining the actual status of communities under the influence of human disturbance, the combination of spatial and long—term studies shows the importance of setting better policy and more establishing more effective reserved in order to enhance and maintain species diversity and density.

Sea‐Level Rise and the Reduction in Pine Forests in the Florida Keys

Abstract: Forests dominated by Pinus elliottii var densa have undergone a reduction in area in the Florida Keys (USA). A previous investigation interpreted the presence of halophytic species in a former pine forest in Key Largo as evidence of sea-level rise. We therefore examined aerial photos and field evidence to learn how the 15-cm rise in local sea level over the last 70 yr had affected the distribution of pines on a second island, where intact pine forests still remained in 1991. The distribution of in situ dead pine stems showed that the area occupied by pines on Sugarloaf Key was 88 ha at some time prior to the earliest available aerial photographs, in 1935. The area of pine forest was reduced to 46 ha by 1935, and continued to decrease through 1991, when it covered 30 ha. The pattern of pine mortality was related to topographic position, with the areas where pines died earliest occupying the lowest elevations. Our analysis of current vegetation patterns showed that the areas of earliest pine mortality are now populated by a higher proportion of halophytic plant assemblages than areas of more recent pine mortality. We also compared the physiological responses of pines in two portions of the island: one where pine forest reduction had been most pronounced, and a second where the extent of the forest had changed little over the past 50 yr. Both groundwater and soil water salinity were higher in the area of rapid pine forest reduction, and the pines sampled there exhibited higher physiological stress, as indicated by pre-dawn water potential and stemwood carbon isotope ratios. These results suggest that the salinization of ground- and soil water that occurs as sea level rises is a major factor in the reduction of pine forests of Sugarloaf Key. If sea level continues to increase, the Florida Keys will experience a decline in both landscape and species diversity, as species-rich upland communities are replaced by simpler mangrove communities. This pattern may also occur in other low-lying island ecosystems with limited freshwater resources.

Remote Sensing of Seasonal Leaf Area Index Across the Oregon Transect

Abstract: Remotely sensed data acquired from four remote-sensing instruments on three different aircraft platforms over a transect of coniferous forest stands in Oregon were analyzed with respect to seasonal leaf area index (LAI). Data from the four instruments were corrected for the varying seasonal and geographic atmospheric conditions present along the transect. Strong logarithmic relationships were observed between seasonal maximum and minimum LAI and the simple ratio (SR) (near infrared/red reflectance) calculated from the broad-spectral-band Thematic Mapper Simulator (TMS), as well as from the narrow-spectral-band Airborne Visible/Infrared Imaging Spectrometer (AVIRIS), the Compact Airborne Spectrographic Imager (CASI), and a Spectron SE590 spectro-radiometer (R(exp 2) = 0.82-0.97). The TMS SR reached an asymptote at an LAI of approx. 7-8. However, the SE590 and the CASI SR continued to increase up to the maximum LAI of 10.6. The variability of the relationship between the AVIRIS SR and LAI increased at stands with LAIs greater than 7, making a trend in the AVIRIS SR-LAI relationship at LAIs greater than 7 difficult to discern. The SRs of the coniferous forest stands measured by the narrow-spectral-band instruments were higher than they were from the broad-spectral-band TMS. This is attributed partially to the integration of the TMS over a broad wavelength region in the red and more strongly to calibration differences between the sensors. Seasonal TMS SR trends for four time periods for some of the stands deviated from the expected seasonal LAI trends, possibly because of smoke and very low sun angles during some of the acquisition periods. However, the expected SR differences for the seasonal minimum and maximum LAI were observed for all of the sensors for nearly all of the forest stands. This study, demonstrates that remotely, sensed data from both broad- and narrow spectral band instruments can provide estimates of LAI for use in forest ecosystems simulation models to estimate evapotranspiration, photosynthesis, canopy turnover, and net primary production over large areas.

Root Gap Dynamics in Lodgepole Pine Forest: Nitrogen Transformations in Gaps of Different Size

Abstract: Belowground responses to aboveground disturbance were studied in experimental gaps created in a 95—yr—old stand of Pinus contorta in southeastern Wyoming. One—, 5—, 15—, and 30—tree clusters were felled to create a series of gaps in the root mat, and solution—phase N was monitored over two consecutive snow—melt periods via tension—tube water collectors. We hypothesized that dissolved and extractable nitrogen concentrations would not exceed predisturbance levels until a threshold canopy gap size had been achieved. As predicted, NOx—N attained significantly higher solution N concentrations (2—5 mg/L) only with the death of 15 trees or more. However, dissolved organic nitrogen decreased gradually with increasing gap size. Net mineralization and nitrification were studied using 30—d in situ incubation assays in each gap. Extractable nitrate routinely was negligible until the 30—tree gaps had been attained. Predicting the effects of disturbance on nutrient cycling, including timber—harvesting practices, requir...