摘要:

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.

ISSN:1051-0761    

DOI:10.2307/1941946

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

The impacts of four different management systems on diversity of ground—foraging ants were examined to assess potential for conservation of biological diversity in human—influenced agricultural or agroforestry systems. Using a variety of indices, diversity was measured along a gradient of anthropogenically disturbed habitats. The gradient, in terms of increasing levels of disturbance, ranged from primary rain forest, abandoned cacao plantations, productive cacao plantations, to banana plantations. The primary forest and abandoned cacao plantations were situated in a reserve and the productive cacao plantations, and banana plantations were located on privately owned land within 35 km of the reserve in the Sarapiqui Valley of Costa Rica. To sample ground—foraging ants, we placed tuna baits along the ground in each habitat. Although tuna baits attract a generalist assemblage of ants that may be better able to adapt to more disturbed sites, diversity of this community was significantly reduced with increased disturbance. Ant diversity was not different for forest and abandoned cacao sites except in terms of evenness, with the forest having a more even distribution of species. Forest and abandoned cacao sites were significantly more diverse than both productive cacao and banana plantations. The latter two did not differ significantly from each other in terms of diversity, but there was considerable variability within each of these habitat types. The results of this study suggest that conservation of diversity may be possible in a "mosaic" of different land uses, but further studies, such as those addressing the importance of scale and nearby source populations, are necessary before management practices can be implemented.

ISSN:1051-0761    

DOI:10.2307/1941947

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

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.

ISSN:1051-0761    

DOI:10.2307/1941948

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

The desert tortoise is a threatened species living in the deserts of the American Southwest. Using size—structured demographic models, we analyzed the status of the tortoise in the Western Mojave desert and evaluated the effectiveness of possible management 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 respiratory 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.

ISSN:1051-0761    

DOI:10.2307/1941949

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

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.

ISSN:1051-0761    

DOI:10.2307/1941950

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

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, °1 November through the end of April. Simulated elk survival for three winters (1987—1988; 1988—1989; 1990—1991) agreed with observed data. A factorial simulation experiment was conducted to explore the effects on ungulate survival of fire size, fire pattern, and winter severity during an initial postfire winter (when no forage is available in burned areas) and a later postfire winter (when forage is enhanced in burned areas). Initial ungulate population sizes were held constant at 18 000 elk and 600 bison. Winter severity played a dominant role in ungulate survival. When winter conditions were extremely mild, even fires that affected 60% of the landscape had no effect on ungulate survival during the initial postfire winter. The effects of fire on ungulate survival become important when winter conditions were average to severe, and effects were apparent in both the initial and later postfire winters. The spatial patterning of fire influenced ungulate survival if fires covered small to moderate proportions of the landscape (e.g., 15% or 30%) and if winter snow conditions were moderate to severe. Ungulate survival was higher with a clumped than with a fragmented fire pattern,suggesting that a single, large fire is not equivalent to a group of smaller disconnected fires. The interaction between fire scale and spatial pattern suggests that knowledge of fire size alone is not always sufficient to predict ungulate survival.

ISSN:1051-0761    

DOI:10.2307/1941951

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

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. Consideration 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 differences 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.

ISSN:1051-0761    

DOI:10.2307/1941952

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Two greenhouse experiments were conducted to evaluate the performance of individual oat (Avena sativa) target plants as influenced by neighbor oat plants with different spatial arrangements. Crop spatial arrangement was separated into three components: (1) population density, (2) distance between plants (distance dispersion, DD), and (3) angular arrangement of plants about one another (angular dispersion, AD). Distance dispersion was quantified using a weighted mean distance from a target individual to its neighbors. Angular dispersion of neighbors around the target was quantified using a measure of circular variance. DD and AD were combined to create a dispersion index (DI). At constant density, distance dispersion treatments were combined with angular dispersion treatments in a randomized—block factorial design. Target plant biomass production was predicted using a rectangular hyperbola equation including population density, DD, AD, and DI as independent variables. Fifty—five and 44% of the residual variation (the variation not accounted for by density alone) in individual plant yield was accounted for when spatial arrangement (DI) was added to the regression. The approach can be used to compare the intensity of competition among different crop planting patterns.

ISSN:1051-0761    

DOI:10.2307/1941953

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Three different approaches for assessing the impact of a permanent increase in water level on wetland vegetation were studied using a long—term, controlled, and replicated experiment. These three approaches were: (1) digitized vegetation maps derived from aerial photographs; (2) vegetation data (species abundance, species diversity) from 10 permanent quadrats in each cell; and (3) Bray—Curtis similarity indices comparing the composition of the vegetation in permanent quadrats within a cell and among cells. This study was conducted in a 10—celled wetland complex in the Delta Marsh, Manitoba, Canada. There were three water level treatments: (1) the normal or mean regulated water level in the surrounding Delta Marsh, (2) the medium (30 cm above normal), and (3) the high (60 cm above normal). There were four, three, and three cells (ranging in total area from 6 to 8 ha), respectively, in each treatment. The vegetation in all cells had been reestablished with a drawdown just prior to this study. To reduce cell—to—cell variation, we adjusted the indicators derived from vegetation maps from 1985 through 1989 by subtracting the mean values of the same indicators in 1979 and 1980, after 15—16 yr of normal water level conditions. The adjusted percentage of a cell covered with open water increased significantly and two other adjusted indicators, the number of vegetation types and the number of multispecies vegetation types, decreased significantly in the flooded treatments. The percentage of a cell covered with sparse emergent vegetation and percentage covered with standing litter did not differ significantly among treatments. In permanent quadrats, species richness, total shoot density of the emergent species, and the Shannon diversity index showed significant treatment effects: all three declined in the flooded treatments. The Simpson's index, however, did not show a treatment effect. When Bray—Curtis similarity indices comparing the vegetation either among permanent quadrats within a cell or for the same permanent quadrat in a cell among years were used, either within—cell vegetation heterogeneity or ongoing successional changes in the vegetation made it impossible to detect treatment effects.

ISSN:1051-0761    

DOI:10.2307/1941954

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Watershed liming has been suggested as a long—term mitigation strategy for lake acidity, particularly in areas subject to high levels of acidic deposition.However, virtually no information has been available on the impacts of liming on wetland vegetation. In 1989, 1100 Mg of limestone (83.5% CaCO3) were aerially applied to 48% (100 ha) of the Woods Lake watershed in the west—central Adirondack region of New York as part of the first comprehensive watershed liming study in North America. We inventoried wetland vegetation in 1.0—m2plots before liming and during the subsequent 2 yr. Within this period liming influenced the cover, frequency, or importance values of only 6 of 64 wetland taxa. The cover of Sphagnum spp. and of the cespitose sedge Carex interior decreased in control relative to limed plots, and cover of the rhizomatous sedge Cladium mariscoides increased nearly threefold in limed areas. These two sedges, which are relatively tall, are characteristic of more calcareous habitats. Cover of the grass Muhlenbergia uniflora, cover and importance value of the sundew Drosera intermedia, and frequency of the forb Hypericum canadense were adversely affected or inhibited by lime. It is unclear whether liming directly inhibited the growth of these three small—statured species, or whether the adverse effects of lime were mediated through shifts in competitive interactions with other species. The limited responses that we observed to liming, along with changes that occurred in control plots over the study period, may indicate that in the short term watershed liming was no more of a perturbation than the environmental factors responsible for natural annual variation in wetland communities.

ISSN:1051-0761    

DOI:10.2307/1941955

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY