摘要:

It is argued that the problem of pattern and scale is the central problem in ecology, unifying population biology and ecosystems science, and marrying basic and applied ecology. Applied challenges, such as the prediction of the ecological causes and consequences of global climate change, require the interfacing of phenomena that occur on very different scales of space, time, and ecological organization. Furthermore, there is no single natural scale at which ecological phenomena should be studied; systems generally show characteristic variability on a range of spatial, temporal, and organizational scales. The observer imposes a perceptual bias, a filter through which the system is viewed. This has fundamental evolutionary significance, since every organism is an "observer" of the environment, and life history adaptations such as dispersal and dormancy alter the perceptual scales of the species, and the observed variability. It likewise has fundamental significance for our own study of ecological systems, since the patterns that are unique to any range of scales will have unique causes and biological consequences. The key to prediction and understanding lies in the elucidation of mechanisms underlying observed patterns. Typically, these mechanisms operate at different scales than those on which the patterns are observed; in some cases, the patterns must be understood as emerging form the collective behaviors of large ensembles of smaller scale units. In other cases, the pattern is imposed by larger scale constraints. Examination of such phenomena requires the study of how pattern and variability change with the scale of description, and the development of laws for simplification, aggregation, and scaling. Examples are given from the marine and terrestrial literatures.

ISSN:0012-9658    

DOI:10.2307/1941447

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

Organismal movement is frequently treated as a diffusion process arising from a simple random walk in a spatially uniform environment. However, anomalous diffusion may arise due to: (1) intrinsic departures from random movements of individuals; or (2) the effects of barriers that impede, or corridors that facilitate, movement. We examine anomalous diffusion in field studies of Eleodes beetles (Coleoptera: Tenebrionidae) and in simulations of correlated random walk on maps of real and artificial landscapes. We show how diffusion alternates between ordinary and anomalous diffusion depending on movement rules, landscape pattern, and the spatial and temporal scales of observation. Recent theories of diffusion in spatially complex media predict power law relations for anomalous diffusion. Over time scales of 5—500 s, Eleodes exhibited power laws for: (1) mean squared displacement with time; and (2) the mean time to travel from the center to the perimeter of circles of various radii. In grasslands, diffusion exponents changed significantly at a radius of 42 cm, which characterized the size of grass and bare soil patches. A second change in diffusion at scales of 24—600 h characterized home range activity. Marked discrepancies between the dynamics of beetles in the field and in simulations suggest a need for more comprehensive models of individual movement that use different rules for various domains of space and time. Studies of anomalous diffusion identify the relative effects of environment vs. innate behavior and reveal a range of scales over which the effects pertain.

ISSN:0012-9658    

DOI:10.2307/1941448

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

Two common features of biological communities are (a) complex interactions among species, which make community dynamics sensitive to initial conditions, and (b) spatial heterogeneity, which fragments large—scale ecological systems into a mosaic of patches, hereafter termed a "metacommunity." This computer simulation study examines the effect of complex interaction on the global and local dynamics of metacommunities. Patches are physically identical and differ only in the initial proportion of species that colonize the patches. The random variation is then magnified by deterministic interactions that cause patches to follow different trajectories based on initial conditions. After a period of interaction, individuals from all patches join in global pool of dispersers that colonize a new "generation" of patches. Complex interactions can have at least two important effects on metacommunity dynamics. First the number of species coexisting in the metacommunity can greatly exceed the number of species coexisting in any single patch, despite the fact that the patches are physically identical, the species do not differ in colonization ability, and stochastic effects are absent after the colonization stage. Second, complex interactions provide a new source of variation upon which natural selection can operate at the patch level, providing a mechanism for the evolution of functionally organized communities.

ISSN:0012-9658    

DOI:10.2307/1941449

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

Few studies have directly addressed the effects of disturbance on spatial and temporal heterogeneity. Spatial heterogeneity. Spatial heterogeneity is the degree of dissimilarity in species composition from one point to another into a community, whereas temporal heterogeneity is compositional change within a site over time. The purposes of this study were to determine (1) if a quadratic relationship exists between within—site heterogeneity and disturbance frequency as predicted by the intermediate disturbance hypothesis (IDH), (2) if disturbed and undisturbed sites have similar heterogeneity as implied by the disturbance heterogeneity hypothesis (DHM), and whether or not these results differed with scale, and (3) if there is a relationship between spatial and temporal heterogeneity as implied by the DHM. Analyses were based on plant species composition data collected over 9 yr in quadrats permanently located in experimental management units subjected to differ burning frequencies at Konza Prairie Research Natural Area, Kansas, USA. The relationship between disturbance frequency and within—site heterogeneity was opposite that predicted by the IDH. Heterogeneity was lowest at intermediate disturbance frequencies. Heterogeneity in annually burned prairie was lower than in unburned prairie and prairies burned once every 4 yr in contrast to predictions of the DHM. However, this relationship did not hold at larger spatial scales. There was a positive relationship between within—site spatial and temporal heterogeneity on annual burned sites, sites burned once every 4 yr, and nearly so on sites burned every other year. Within—site heterogeneity was negatively correlated with cover of Andropogon gerardii, and positively correlated with total richness and species diversity. Studies of variation, in addition to averages, will increase our ability to predict patterns of species distribution and abundance within and between communities in response to disturbance.

ISSN:0012-9658    

DOI:10.2307/1941450

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

The effects of rainfall amount, rainfall distribution, and grazing intensity on net primary production and herbivory were examined for a dry tropical savanna. Aboveground net primary production in permanently protected savanna ranged from 329 to 841 g.m—2.yr—1and was positively related to annual rainfall. It was not related to early rainy season rainfall, but was related to rainfall of the later part of the rainy season. Grazing simulated 4—45% greater net primary production, the simulation being higher at lightly and moderately grazed sites. Consumption, as estimated by comparing net primary production inside temporarily fenced and outside the fenced plots, was 476— 34 g.m—2.yr—1. Consumption in the present savanna was positively related to net primary production, and to annual rainfall. Green biomass, being in function of soil moisture, was greater in the high rainfall year. Belowground net primary production in permanently protected savanna (BnpP) ranged from 404 to 838 g.m—2.yr—1and was significantly related to annual rainfall. BnpP was not related to early rainy season rainfall but was related to late rainy season rainfall. Total net primary production ranged from 836 to 1579 g.m—2.yr—1and was also related to annual rainfall than early rainy season rainfall. Grazing reduced belowground net primary production by 25—65% in grazed savanna (BnpG). Maximum reduction occurred in heavily grazed savanna. BnpG was inversely related with rainfall. Total net primary production (herbaceous + woody vegetation) of the savanna ranged from 11.9 to 19.1 Mg.ha—1.yr—1. The total net primary production of the savanna was similar to that of the native dry deciduous forest; however, the production of herbaceous species was 61% lower in the forest. The total net primary production of savanna was higher than that of the dry tropical forest when the belowground net primary production of the 0—50 cm layer was considered.

ISSN:0012-9658    

DOI:10.2307/1941451

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

To determine the long—term effect of alder on soil fertility, biogeochemical fluxes were measured and calculated for two pairs of adjacent, 55—yr—old stands dominated by conifers, primarily Douglas—fir (Pseudotsuga menziesii), and by conifers and nitrogen—fixing red alder (Alnus rubra). At a low—fertility site in the Wind River Experimental Forest in southwestern Washington, biomass of the alder—conifer stand (289 Mg/ha) exceeded that of the conifer stand (171 Mg/ha), and the aboveground net primary production (ANPP) of the alder—conifer stand (10.3 Mg.ha—1.yr—1) was more than twice that of the conifer stand (4.8 Mg.ha—1.yr—1). At a more fertile site in the Cascade Head Experimental Forest in western Oregon, both biomass and ANPP were higher than at Wind River, and biomass and ANPP were higher in the conifer stand (584 Mg/ha and 19.2 Mg.ha—1.yr—1) than in the alder—conifer stand (342 Mg/ha and 10.7 Mg.ha—1.yr—1). Nitrogen accretion in the alder—conifer stand at Wind River averaged 54 kg.ha—1.yr—1for the 52 yr since stand establishment, with a current rate of N fixation of °75 kg.ha—1.yr—1. For the alder—conifer stand at Cascade Head, N accretion averaged 73 kg.ha—1.yr—1for 55 yr, with a current N—fixation rate of 85 kg.ha—1.yr—1. The cycling of all nutrient appeared very malleable under the influence of alder. At Wind River, return of nutrients in fine litterfall in the alder—conifer stand ranged from 1.5 (P) to 7.9 (N) times those in the conifer stand; whereas at Cascade Head, these ratios ranged from 1.7 (S) to 4.2 (N). Nutrient—use efficiencies (kilograms of ANPP per kilogram of nutrient uptake) were generally lower for the alder—conifer stands at both sites. Denitrification appeared negligible (<0.3 kg.ha—1.yr—1) in all stands. Leaching of organic plus inorganic N ranged from °5 kg.ha—1.yr—1for the conifer stand at Wind River, to 50 Kg.ha—1.yr—1for the alder—conifer stand at Cascade Head.

ISSN:0012-9658    

DOI:10.2307/1941452

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

A field experiment was conducted to examine the relative importance of soil factors and plant species on communities of vesicular—arbuscular mycorrhizal (VAM) fungi. Populations of VAM fungal spores were studies in 4—yr—old monocultures of five successional grass species grown in gradient of soil mixtures ranging from pure subsurface sand to pure sandy loam topsoil. A total of 19 species of VAM fungi were found across all treatments. Of the 12 most abundant VAM fungal species, 6 species had a significant dependence on both soil mixture and host species, while 2 were dependent only on soil and 2 only on host. To our knowledge, these are the first results indicating that even closely related hosts (five grasses) may cause divergence in VAM fungal communities on initially identical soils. Cluster analysis of the similarity of fungal communities by host plant species showed that fungal communities in the two late successional grasses to be most similar to one another and least similar to the fungal communities in the early successional grass species. Cluster analysis of the similarity of fungal communities by soil mixture showed the fungal communities in the sandy end of the soil gradient diverged predictably from the fungal communities in the black soil end of the gradient. These results support the hypothesis that soil factors and plant species may be of equal importance in regulating the species composition of VAM fungal communities.

ISSN:0012-9658    

DOI:10.2307/1941453

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

The purpose of this study was to examine the effect of abundant native large herbivores on ecosystem function of a spatially and temporally heterogeneous temperate grassland. Net aboveground primary production (ANPP), large herbivore consumption (C), and dung deposit (D), an index of nutrient flow from herbivores to the soil, were measured in grassland ad shrub—grassland habitat on winter, transitional, and summer range used by herbs of elk (Cervus elaphus) and bison (Bison bison) in northern Yellowstone National Park. Temporary exclosures (5—7 per site) were moved ever 4 wk during the snow—free season to determine ANPP and C. Data were collected during 1988, a year of drought and unusually high elk and bison population levels, and 1989, a climatically near—average year, with dramatically fewer elk and bison. All three processes, ANPP, C, and D, varied widely among sites: ANPP range: 16—589 g/m2, C range: 0—306 g/m2, and D ranged: 0—68 g.m2. An average of 45% of ANPP was consumed by herbivores. Production and consumption, and consumption and dung deposition were positively correlated across all sites. In addition, sites were grazed when plants were growing. There was a 19% reduction in ANPP from 1988 to 1989, likely caused by death or injury to plants during the 1988 drought. Drought also appeared to be partially responsible for reductions in elk and bison from 1988 to 1989, which were coincident with declines in C and D. Results indicate direct effects and suggest indirect effects of a single—season drought on grassland function that will persist for several years after the event.

ISSN:0012-9658    

DOI:10.2307/1941454

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

Large mammalian herbivores can influence the dynamics and structure of ecosystems by selectively removing tissues of specific plant species. The plant community composition can be altered as animals feed on some species but not others, changing the biomass, production, and nutrient cycling of an entire ecosystem. We used four paired moose (Alces alces) exclosures and browsed plots (built between 1948 and 1950) on Isle Royale, Michigan, to examine the influence of moose on aboveground biomass, production, and annual litterfall of boreal vegetation in 1987. Tree biomass was significantly greater (X = 230 vs. 150 Mg/ha, df = 3, P<.05), shrub biomass was significantly less (X = 1.9 vs. 3.1 Mg/ha, P<.05), and herb biomass was significantly less (X = 0.2 vs. 0.8 Mg/ha, P<.05) in exclosures than in browsed plots. Tree production was greater in exclosures than in browsed plots (X = 7.9 vs. 5.0 Mg.ha—1.yr, P = .05), but there was no difference in the production per unit biomass between exclosures and browsed plots. Shrub production in exclosures was similar to that of browsed plots (X = 3.5 vs. 2.3 Mg.ha—1.yr—1, P<.05), despite total vegetation biomass differences between paired plots. There was significantly greater herb litter produced in the browsed plots than in the exclosures (X = 0.7 vs. 0.1 Mg.ha—1.yr—1, P<.05). Moose browsing prevented saplings of preferred species from growing into the tree canopy, resulting in a forest with fewer canopy trees and a well—developed understory of shrubs and herbs. In addition, browsing may have altered the eventual balance of white spruce (Picea glauca) was balsam fir (Abies balsamea), causing an increase in the former and a decrease in the latter. Thus, browsing by moose influences in long—term structure and dynamics of the boreal forest ecosystem, which has important implications for forest ecosystem management, especially where the population dynamics of moose are regulated.

ISSN:0012-9658    

DOI:10.2307/1941455

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

Studies of natural and simulated herbivory were conducted to assess the effects of secondary herbivory and plant association on the reproductive success of Ipomopsis aggregata. Over the 5—yr period of this study 77% of all plants were browsed by ungulate herbivores at some time during the flowering season. Of these, 33% were subsequently browsed. Removal of the single inflorescence stimulated the production of, on average, five new flowering stalks from dormant lateral buds along the remaining portion of the plant's stem. Although regrowth shoots were initially avoided by ungulates following the removal of scarlets gilia's single inflorenscence, plant types were secondarily browsed following stem elongation and flower bud formation. Secondary herbivory had no effect on the compensatory outcome. Plants that were naturally browsed produced significantly higher numbers of flowers and fruits than plants that were not eaten, even when plants were secondarily browsed. Because there were no significant differences in numbers of seeds produced per fruit or in seed mass, an increase in total fruits produced by browsed plants resulted in an increase in fitness through seed production. Observational and experimental results indicate that I. aggregata switches from a "mutualistic" to an "antagonistic" interaction with its ungulate herbivores in order to achieve its greatest fitness. Results of experimental clipping showed that high levels of secondary herbivory on I. aggregata would be detrimental, decreased fitness by °70%. An apparent change in plant quality following the initial bout of herbivory, however, deters high levels of subsequent herbivory, restricting tissue removal to the tips of the plant. When plants were found in close association with either pine or grasses (to add in the potential negative effects of competition), browsed plants still outperformed control plants, producing significantly more flowers and fruits than uneaten control plants. As in a previous study, these results support the contention that mammalian herbivores can benefit plants enhancing plant fitness.

ISSN:0012-9658    

DOI:10.2307/1941456

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY