摘要:

This paper tests the proposition that a small set of plant, animal, and abiotic processes structure ecosystems across scales in time and space. Earlier studies have suggested that these key structuring processes establish a small number of dominant temporal frequencies that entrain other processes. These frequencies often differ from each other by at least an order of magnitude. If true, ecosystems therefore will have a few dominant frequencies that are endogenously driven and that are discontinuously distributed. This paper additionally tests the proposition that these structuring processes should also generate a discontinuous distribution of spatial structures coupled with the discontinuous frequencies. If that is the case, animals living in specific landscapes should demonstrate the existence of this lumpy architecture by showing gaps in the distribution of their sizes. This proved to be the case for birds and mammals of the boreal region forest and the short—grass prairie. Alternative hypotheses to explain the body mass clumps include architectural, developmental, historical, and trophic causes. These were all tested by comparing body—mass clump distributions (1) in ecosystems having different spatial structures (forest, grassland, and marine pelagic) and (2) in different animal groups having different body plans (birds and mammals) or feeding habits (carnivore, omnivore, and herbivore). The only hypothesis that could not be rejected is that the body—mass clumps are entrained by discontinuous hierarchical structures and textures of the landscape. There is evidence for at least eight distinct habitat "quanta," each defined by a distinct texture at a specific range of scales. These eight quanta together cover tens of centimetres to hundreds of kilometres in space and at least months to millennia in time. There is a striking similarity, but not identity, between the clump structure of prairie and boreal animals. This indicates that many processes that form qualitative habitat structure are common to both landscapes or ecosystems, but a few are landscape specific, particularly over larger scales. That conclusion is extended to all terrestrial ecosystems by an analysis of the body—mass clump structure of all North American birds. In contrast, there are striking differences in clump structure between landscapes and "waterscapes," indicating that fundamentally different processes shape structure in terrestrial and open ocean systems. The discontinuous body—mass structure provides a bioassay of discontinuous ecosystem structure. Mammalian carnivores, omnivores, and herbivores all show the same number of body—mass clumps, and the gaps in these distributions occur at the same body masses. Mammals and birds show the same number of body—mass clumps, but the mass gaps for mammals occur at larger sizes than those for birds in such a way that the log—transformed body—mass gaps for mammals are correlated linearly with those for birds. Hence there is a simple cross—calibration between the mammal and bird bioassays. I compiled and analyzed published data on home ranges in order to convert body masses into an absolute linear measure of geometric structures in the landscape. A new and general equation was developed relating home—range size to body mass, and was tested by reanalyzing published data for mammalian carnivores, omnivores, and herbivores and for birds. I conclude: (1) Birds and mammals of all trophic levels utilize resources in their foraging areas in the same way by measuring the spatial grain of habitat patches with a resolution defined as a function of their size (i.e., the animal's step length or minimum unit of measurement). The step length is a morphological function of the size of animals and is not significantly affected by trophic status or taxonomy of the groups considered. That explains why all trophic levels and both birds and mammals show the same qualitative body—mass clump structure. (2) Home—range data can convert the body—mass data to a quantitative estimate of texture, i.e., of fractal dimension of the landscape. The landscape forms a hierarchy that contains breaks in object sizes, object proximities, and textures at particular scales. Animals also demonstrate a hierarchy of decisions whose target suddenly shifts at specific scales in space and time. The interaction between these two hierarchies produces the discontinuous body—mass clump structure. The breaks in geometry in the landscape occur because structuring processes exert their influence over defined ranges of scale. The temporal and architectural structure of habitat quanta are in general determined by three classes of processes, each dominating over three different ranges of scale. Vegetative processes that determine plant growth, plant form, and soil structure dominate the formation of texture at fine microscales of centimetres to tens of metres in space and days to decades in time. At the other, macroscale extreme, slow geomorphological processes dominate the formation of a topographic and edaphic structure at large scales of hundreds to thousands of kilometres and centuries to millennia. At the mesoscales in between, contagious disturbance processes such as fire, insect outbreak, plant disease, and water flow dominate the formation of patterns over spatial scales of hundreds of metres to hundreds of kilometres. In addition, the direct impacts of grazing by large herbivores and of human activities, and the indirect effects of large predators and animal disease, further transform spatial patterns over these meso—scales. These processes operate on time scales of years to decades, making them critically important in determining whether present local, regional, and global human influences will trigger a transition in vegetation types, and, if so, how rapidly. The paper provides a direction for the development of programs to evaluate, monitor, and predict ecosystem and community changes across scales. The necessary research elements include (1) models that incorporate a few scale—dependent structuring processes to allow cross—scale analysis; (2) comparative studies of different disturbed and undisturbed landscapes using the animal body—mass bioassay technique to identify critical scales of ecosystem geometry; (3) analysis of remote imagery to identify spatial discontinuities and regions of scale invariance; and (4) behavioral studies of the hierarchy of animal decisions to identify species groups vulnerable to predicted (using models) or observed (using remote imagery) changes in vegetation geometry.

ISSN:0012-9615    

DOI:10.2307/2937313

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

Watershed 2 of the Hubbard Brook Experimental Forest was experimentally deforested in the winter of 1965—1966, and subsequent plant regeneration was suppressed by herbicide application until the 1969 growing season. Changes in species structure, plant abundance, aboveground primary productivity and biomass, and aboveground nutrient pools were recorded on 70 permanent plots distributed across the watershed during 1, 2, 3, 5, 11, and 20 yr of subsequent succession. Species richness increased rapidly over time but equitability and the Shannon—Wiener diversity index declined with succession. Plant densities of herb, shrub, sapling, and tree strata increased, then decreased, in successive waves as the vegetation increased in height. Basal area (stems>5.1 cm dbh) initially increased exponentially, then linearly after the 5th yr to attain a total of 18.7 m2/ha by the 20th yr. Primary productivity also increased exponentially at first, but then increased linearly through the 20th yr at which time annual aboveground primary productivity was 20 Mg/ha. Aboveground biomass increased linearly after the 5th yr until by the 20th yr it was 52 Mg/ha, 38% of the reference forest biomass. Aboveground nutrient pools of some elements accumulated faster than biomass by the 20th yr when calculated as percentages of the 55—yr—old reference forest pools and biomass. These included: potassium–52%, phosphorus–44%, magnesium–42%. Other accumulated more slowly than biomass: sulfur–32%, and nitrogen and calcium each–29%. While recovery rate on Watershed 2 was delayed in the first decade of regrowth, and composition was somewhat altered because of the 3—yr suppression, this forest nevertheless displayed a vigorous capacity for regeneration after suppression ceased.

ISSN:0012-9615    

DOI:10.2307/2937314

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

Physical disturbance can be an important force at the individual, population, and community levels of organization. The effects of disturbance may differ for mobile and sessile organisms, however, because of differences in the potential for escape and postdisturbance recolonization by survivors. I used field sampling and laboratory experiments to examine how episodic movement of severely oxygen—depleted (hypoxic) bottom water into nearshore habitat in the Chesapeake Bay affects population density, recruitment, and reproduction of a mobile species–the naked goby (Gobiosoma bosc), a benthic oyster bed fish. Oxygen depletion is a common physical disturbance in freshwater, estuarine, and coastal aquatic systems. In this study, episodic hypoxia influenced mortality, size structure of the population, reproductive behavior, and spatial distribution. Intrusion of severely hypoxic water occurred in late July and early August during the 2—yr study. These intrusions coincided temporally with peak periods of recruitment, and caused the most severe drops in dissolved oxygen concentrations in deep and mid—depth areas of the oyster reef, where recruitment was highest. Laboratory experiments suggested that newly settled recruits require higher oxygen concentrations for survival than do older individuals. Field samples also indicated that these new recruits are less able to escape to more highly oxygenated shallow water refuges when an intrusion occurs. Thus, the spatial and temporal patterns of recruitment and disturbance, and physiological requirements, combine to result in extremely high mortality of new recruits during severe intrusions. In contrast to effects on new recruits, some large juveniles and adults successfully migrate inshore when oxygen levels decline. In both field samples and laboratory experiments, adult males continued to guard eggs and shelters until dissolved oxygen closely approached lethal levels. Calculations based on size—specific physiological tolerances and swimming speeds suggest that the occurrence of lethal conditions in the fluctuating environment may be more predictable to larger individuals than to new recruits. This predictability may increase the possibility of an appropriate response to low oxygen disturbances by large juveniles and adults. After the disturbance abates, surviving individuals recolonize abandoned areas. This ability of mobile animals to recolonize a disturbed area as adults or juveniles, rather than solely through reproduction, may lead to differences in postdisturbance ecological interactions and differences in selection for colonizing ability between mobile and sessile species.

ISSN:0012-9615    

DOI:10.2307/2937315

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

Previous investigations, focused primarily on vertebrates, have noted substantial losses of eggs and embryos to predators and questioned why selection has not more commonly resulted in the evolution of chemically defended eggs or embryos. Hypotheses regarding the apparent rarity of such defenses have emphasized the potential incompatibility of actively developing tissues and toxic metabolites. Alternatively, this apparent pattern could be an artifact of our greater knowledge of vertebrates, which in general show few tendencies for synthesizing defensive metabolites in either juvenile or adult stages. In this study, we investigated adult and larval chemical defenses of a group of benthic marine invertebrates, the ascidians, in which the adults are often chemically rich, and we contrast our findings with what is known about chemical defenses of eggs and embryos from terrestrial and aquatic organisms. Our findings suggest that there is no fundamental incompatibility of rapidly developing juvenile tissues and bioactive metabolites, and that chemically defended eggs and larval stages may be common among some taxonomic groups. Ascidians are benthic invertebrates that often lack apparent physical defenses against predation, yet are common on coral reefs where predation by fishes is intense. In contrast to most co—occurring invertebrates, many ascidians also release large, conspicuous larvae during daylight hours when exposure to fish predation would be highest. Thus selection by predators might favor the evolution of distasteful larvae. In situ observations indicate that many conspicuous ascidian larvae are distasteful to potential consumers. We investigated the ability of secondary metabolites produced by taxonomically diverse ascidians from geographically distant locales to deter predation on both adults and larvae. Larvae from the Caribbean ascidian Trididemnum solidum were distasteful to reef fishes, and when organic extracts of individual larvae were transferred onto eyes of freeze—dried krill (a good larval mimic in terms of size and color), these eyes were rejected by fishes while control eyes (solvent only) were readily eaten. Larvae of the Indo—Pacific ascidian Sigillina cf. signifera were also distasteful to coral—reef fishes and contained the unpalatable bipyrrole alkaloid tambjamine C. When added to artificial foods at or below their natural mean concentrations and offered to consumers in field and laboratory feeding assays, the secondary metabolites produced by Trididemnum solidum (Caribbean Sea), Sigillina cf. signifera (Indo—Pacific), and Polyandrocarpa sp. (Gulf of California) significantly deterred feeding by co—occurring fishes and invertebrates. Secondary metabolites produced by Trididemnum cf. cyanophorum from the Caribbean Sea, Lissoclinum patella from the Indo—Pacific, and Aplidium californicum from the temperate Pacific, and the small stellate spicules common to many tropical didemnid ascidians did not significantly affect fish feeding. High—pressure liquid chromatography (HPLC) analyses of six didemnin cyclic peptides in individual colonies of Trididemnum solidum from one patch reef at Little San Salvador, Bahamas found large inter—colony differences in their concentrations. The mean concentration of didemnin B was more than double the concentration needed to significantly deter fish feeding in our field assays, and feeding tests with nordidemnin B showed that it deterred fish feeding across the entire range of natural concentrations. HPLC analysis of the extract from a combined collection of T. solidum larvae found adequate concentrations of didemnin B and nordidemnin B to account for their rejection by foraging fishes. We demonstrate that taxonomically diverse ascidians from habitats characterized by intense predation pressure produce secondary metabolites that significantly reduce predation on both adults and larvae, and suggest that this defensive chemistry may be crucial in allowing the release of large, well—provisioned larvae during daylight periods when larvae have the greatest probability of using photic cues to select physically appropriate settlement sites. Production of defensive secondary metabolites appears widespread among certain groups of ascidians, some of which are also known to concentrate acid and heavy metals as additional defensive strategies.

ISSN:0012-9615    

DOI:10.2307/2937316

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

We conducted field experiments to test the general hypothesis that the composition of decomposer communities and their trophic interactions can influence patterns of plant litter decomposition and nitrogen dynamics in ecosystems. Conventional (CT) and no—tillage (NT) agroecosystems were used to test this idea because of their structural simplicity and known differences in their functional properties. Biocides were applied to experimentally exclude bacteria, saprophytic fungi, and microarthropods in field exclosures. Abundances of decomposer organisms (bacteria, fungi, protozoa, nematodes, microarthropods), decomposition rates, and nitrogen fluxes were quantified in surface and buried litterbags (Secale cereale litter) placed in both NT and CT systems. Measurements of in situ soil respiration rates were made concurrently. The abundance and biomass of all microbial and faunal groups were greater on buried than surface litter. The mesofauna contributed more to the total heterotrophic C in buried litter from CT (6—22%) than in surface litter from NT (0.4—11%). Buried litter decay rates (1.4—1.7%/d) were ≈2.5 times faster than rates for surface litter (0.5—0.7%/d). Ratios of fungal to bacterial biomass and fungivore to bacterivore biomass on NT surface litter generally increased over the study period resulting in ratios that were 2.7 and 2.2 times greater, respectively, than those of CT buried litter by the end of the summer. The exclusion experiments showed that fungi had a somewhat greater influence on the decomposition of surface litter from NT while bacteria were more important in the decomposition of buried litter from CT. The fungicide and bactericide reduced decomposition rates of NT surface litter by 36 and 25% of controls, respectively, while in CT buried litter they were reduced by 21 and 35% of controls, respectively. Microarthropods were more important in mobilizing surface litter nitrogen by grazing on fungi than in contributing to litter mass loss. Where fungivorous microarthropods were experimentally excluded, there was less than a 5% reduction in mass loss from litter of both NT and CT, but fungi–fungivore interactions were important in regulating litter N dynamics in NT surface litter. As fungal densities increased following the exclusion of microarthropods on NT surface litter, there was 25% greater N retention as compared to the control after 56 d of decay. Saprophytic fungi were responsible for as much as 86% of the net N immobilized (1.81 g/m2) in surface litter by the end of the study when densities of fungivorous microarthropods were low. Although bacteria were important in regulating buried litter decomposition rates and the population dynamics of bacterivorous fauna, their influence on buried litter N dynamics remains less clear. The larger microbial biomass and greater contribution of a bacterivorous fauna on buried litter is consistent with the greater carbon losses and lower carbon assimilation in CT than NT agroecosystems. In summary, our results suggest that litter placement can strongly influence the composition of decomposer communities and that the resulting trophic relationships are important to determining the rates and timing of plant litter decomposition and N dynamics. Furthermore, cross placement studies suggest that the decomposer communities within each tillage system, while not discrete, are adapted to the native litter placements in each.

ISSN:0012-9615    

DOI:10.2307/2937317

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY