摘要:

Real world ecosystems (as opposed to their mathematical counterparts) are often enormously complex associations of species which interact in diverse ways. As a matter of practical necessity, field ecologists can rarely specify, much less quantify, all of the interactions. Consequently, empirically derived equations purporting to describe the dynamics of such systems generally consider fewer than the total number of interacting species. The present paper calls attention to this reduction in dimensionality and explores some of its consequences. In particular, attention is called to what are termed the Abstracted Growth Equations, those of reduced dimensionality, and to the way that these expressions derive from the underlying n—variable equations. The degree to which the Abstracted Equations accurately describe the dynamics of the species of interest is shown to depend on the time scale of these species relative to that of the species which are omitted. A general result relating the product of the eigenvalues of the Abstracted Equations to the corresponding product for the n—variable equations is proved. It is further pointed out that the distinction between Abstracted and n—variable equations suggests experiments which at least in principle should enable the empiricist to estimate the importance of species and interactions which are omitted. The relationship between Abstracted and n—variable equations is also discussed with regard to measuring competition coefficients and related parameters, and also to the problem of determining whether or not higher order interactions are present in laboratory microcosms. The analysis concludes by comparing the stability properties of several simplified models of community interactions with those of the corresponding one—species Abstracted Equations. It is shown, for the case of difference equations, in particular, that analysis of the one—species models may often lead one to conclude that the system is stable, whereas in fact it is unstable due to overdamping. The final Discussion relates the results of the present paper to previous studies that anticipate the view presented here, and comments on the quarrel that has developed between those ecologists who believe in the existence of community—wide patterns of body size and the like and those who reject this view. It is suggested that the resolution of this dispute may depend on our ability to classify subsystems of species (i.e., guilds) with regard to the extent to which their internal organization is influenced by variation in the larger communities in which they are embedded. Finally, it is shown that Roughgarden's principal results for community coevolution can be deduced from the Abstracted Growth Equations of a particular subset of the entire community.

ISSN:0012-9615    

DOI:10.2307/2937321

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

To investigate how inputs of water and nutrients influence the structural and functional characteristics of cypress wetlands, primary productivity, respiration, transpiration, tree growth, biomass, basal area, stem density, standing stocks of phosphorus, and litterfall were measured in several kinds of Florida cypress ecosystems subjected to varying inflows of water and nutrients. A scrub cypress forest, natural and sewage—enriched cypress domes, and a cypress floodplain forest were studied. Floodwaters in these ecosystems ranged from stagnant and phosphorus poor (0.01—0.08 mg P/L) to flowing and phosphorus rich (0.66—5.86 mg P/L). Inputs of total phosphorus transported by water to these forests varied from 0.11 g P°m—2°yr—1in the scrub cypress forest to 1625 g P°m—2°yr—1in the floodplain forest. Phosphorus content of aboveground biomass (ranging from 0.25 to 4.78 g P/m2) appeared to be positively related to phosphorus inputs. Biomass and other structural characteristics of the study sites appeared to be related to stand history rather than to phosphorus inputs. For example, there was little difference in aboveground biomass between natural cypress domes (20.6—26.6 kg/m2), a sewage—enriched dome (21.7 kg/m2), and a floodplain forest (28.4 kg/m2). Aboveground biomass for the scrub cypress forest was low (3.6 kg/m2). Net daytime photosynthesis (2.1—13.7 g C°m—2ground surface°d—1), plant respiration (0.9—10.9 g C°m—2) ground surface°d—1) and an estimate of gross primary productivity increased with increasing inputs of total phosphorus. Aboveground biomass production (sum of wood production, ranging from 44 to 1080 g°m—2°yr—1, and litterfall, ranging from 224 to 941 g°m—2°yr—1) increased with increasing total phosphorus inputs in the low range, but biomass production leveled in the high input range. Total water loss from cypress forests increased with increasing P inputs. Mean daily transpiration rates from the scrub cypress forest (1.0 mm) and cypress domes (3.1—3.8 mm) were lower while those from the floodplain forest (5.6 mm) were similar to evaporation rates from open water bodies. Ratios of transpiration to net daytime photosynthesis were also low (156—221 g H2O/g organic matter), showing that cypress is efficient in its water use. In cypress domes and scrub cypress forest, where water may be limiting, trees appear to adjust t potential water stress through leaf morphology adaptations and minimum canopy development (leaf area index, LAI = 0.53—3.4 m2/m2). When water is not potentially limiting, as in the floodplain forest, adaptations to conserve water were lacking (e.g., LAI = 8.5 m2/m2). A dome with added sewage effluent had higher leaf biomass, leaf area index, and chlorophyll ° content, and more phosphorus stored in leaves than untreated domes. Net primary productivity, litter production, and wood production increased approximately twofold over pre—effluent rates or untreated cypress domes.

ISSN:0012-9615    

DOI:10.2307/2937322

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

The structure of a tropical rocky intertidal community on Taboguilla Island on the Pacific coast of Panama is characterized by extremely low abundances of noncrustose algae and sessile animals, indistinct vertical zonation patterns (a result of the low abundances), and the occurrence of most invertebrates (except barnacles) and upright algae in holes and crevices (as opposed to open, smooth surfaces). This contrasts strikingly with two temperate rocky intertidal communities, which have high covers of plants and animals, more obvious zones, and invertebrates and upright algae occurring both on relatively homogeneous surfaces and in holes and crevices. Field experiments were done and observations were made in the Panama community to test the effects of different types of consumers (both predators and herbivores) on their prey and on the types of escapes utilized by the prey. Consumer exclusion experiments suggest that (1) predation and herbivory are severe at all times of the year, (2) consumer pressure is a cumulative function of many types and species of predators and herbivores, (3) the primary effect of larger fishes and crabs is to restrict most prey to three—dimensional refuges (holes and crevices), and (4) the primary effect of smaller consumers, mostly invertebrates, is to keep abundances of the prey low. Thus, in the Panama system, three—dimensional space (holes and crevices) appears to be particularly important as a refuge from consumers, while escapes from consumers in body size, time, or two—dimensional space (e.g., in a higher zone) documented so frequently in temperate areas, assume secondary importance for many prey. This restriction of the types of escapes utilized by prey species appears to be a consequence of two main factors: the presence of fast—moving consumers (i.e., herbivorous and predaceous fishes and herbivorous crabs which are absent or rare in the two temperate communities), and the year—round foraging of all consumers.

ISSN:0012-9615    

DOI:10.2307/2937323

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

Regional and local environmental changes spanning the last 30 000 yr have been reconstructed based on paleoecological studies of the sediments in Rockyhock Bay, a peat—filled Carolina Bay in Chowan County, northeastern North Carolina. During the Plum Point Interstadial (30 000—21 000 BP), temperate forests with both deciduous and coniferous taxa occupied the area. Some boreal elements and constituents of northern hardwoods forests were also present. Climate was somewhat cooler than at present, and Rockyhock was a shallow—water body. During the full—glacial and most of the late—glacial (21 000—10 000 BP), the regional forests were boreal in character and were dominated by northern pines and spruce. Many other boreal taxa were present. The climate was colder, more continental, and drier than at present. Water table in the basin was higher, and there was a dense growth of Isoetes on the bottom sediments. There is indication of slight lowering of water table about 14 400 yr ago. Deciduous forests containing constituents of modern white pine—hemlock—northern hardwoods forests replaced the boreal forests about 10 000 yr ago. By 7200 yr ago, most of the "northern hardwoods" taxa had been replaced by other deciduous species including sweetgum. Between 7200 and 5000 yr ago, the water table dropped, peat began to form throughout the basin, and swamp forests with blackgum, cedar, magnolia, and may swamp shrubs began to develop. The swamp forests were modern in aspect by 4000 yr ago. During the last 4000 yr, there is indication of at least one secondary successional change within the bay, perhaps induced by a disturbance such as fire.

ISSN:0012-9615    

DOI:10.2307/2937324

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

This paper presents some concepts and methodology essential for the analysis of population dynamics of univoltine species. Simple stochastic difference equations, comprised of endogenous and exogenous components, are introduced to provide a basic structure for density—dependent population processes. The endogenous component of a population process is modelled as a function of density in the past p generations, including the most recent. The exogenous component of the process consists of all density—independent components of the ecological factors involved, including enhance variations. The model is called a pthorder density—dependent process. For a successful analysis of a population process by the above model, it is important that the process be in a state of statistical equilibrium, or stationarity. The simplest notion of stationarity is introduced, and the average behavior of the process, under this assumption, discussed. The order of density dependence in the population process of a given species depends on its interaction with other species involved in the food web. Considering certain attributes of the food web, in particular the limited number of trophic levels, the pyrmaid of numbers, the linear linkages between closely interacting species, and niche separation among competing species, it is argued that the order of density dependence is probably not much higher than three. A second—order model is perhaps adequate in many practical cases. The dynamics of some lower order density—dependent processes are compared by simulations, with a view to showing the effect of density—dependent and density—independent components at different orders. Several types of density dependence are discussed. If a given factor influencing the temporal variation in density is by itself influenced by density, it is called "causally density—dependent," which may reveal itself by some degree of correlation with density. A density—independent factor, however, may also show some sort of correlation with density in the recent past. This is called "statistically density—dependent." Such statistical density dependence may be due to: (1) spurious correlation, (2) bias in an estimator of the correlation coefficient, (3) autocorrelations in the density—independent factor, and (4) an intriguing mathematical property of the stochastic process. Particularly because of the last two reasons, it is often difficult to distinguish, by correlation method, between causal and statistical density dependence. Distinction also exists between temporal and spatial density dependence, the latter not necessarily implying the involvement of the former. The importance of the distinction between these types of density dependence is discussed in relation to the data analysis and model building. A Statistical analysis of the effect of ecological factors on population dynamics is attempted. Since it is often difficult to determine, by correlation, the causally density—dependent structure of a population process under the influence of some unknown density—independent factors, it is suggested that we reverse the procedure to determine the effect of the density—independent factors first. To confirm the involvement of some suspected density—independent factors in the species dynamics, I propose several methods of correlation between annual fluctuations in some population parameters, such as density, rate of change in density, and their transforms, and those in suitable indices of the suspected factors. Merits, demerits, and limitations of these methods are also discussed. To simplify the arguments, the correlation models are set up first without stage division, and then are elaborated to those in which the whole generation span is divided into several life—cycle stages, so that life table information can be used effectively for the identification of the density—independent factors involved in each stage. A set of life tables of the spruce budworm, Choristoneura fumiferana (Clem.), is analyzed to provide an example of the application of the above concepts and methods. Concluding remarks include some notes on designing life table studies.

ISSN:0012-9615    

DOI:10.2307/2937325

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

An individual animal may suffer simultaneously from several different maladies, which lead to its eventual death. Such "overlaps" between contemporaneous mortality factors create some problems in partitioning, in a consistent manner, the animals killed among several specific causes. The aim of this paper is to develop the notion of "killing power" of a mortality factor, so that we can properly evaluate and incorporate in a model the effect of any given factor on the dynamics of the animal population concerned. I first introduce the basic notion of killing power under the assumption that the mortality factors involved are operating independently of each other. The killing power of a given factor is then shown to be equivalent to the marginal probability of an animal being killed by that factor. The assumption of independence of the mortality factors is relaxed, and the more general notion of conditional killing power is introduced. This is equivalent to the conditional probability of an animal being killed by a given mortality factor, when that individual has not been simultaneously affected by other contemporaneous factors. The concept of mutually exclusive mortality factors is discussed to clarify an apparent confusion in some published literature. Typical examples of this type of mortality factor are an extended drought and excessively wet weather, both of which may kill some animals, but which will not occur simultaneously. Therefore, unless the stage division in the life tables so broad that these types of factors can occur in the same stage, we do not need to consider mutually exclusive mortality factors. How to evaluate the killing power of a mortality factor is illustrated with some examples taken from the literature. Also discussed is a more complicated case, in which the annual fluctuation in the proportion of animals killed by a given factor is correlated with those killed by other contemporaneous mortality factors. Concluding remarks include some notes on the division of stages in a life table so as to facilitate the evaluation of the killing powers of as many mortality factors as possible.

ISSN:0012-9615    

DOI:10.2307/2937326

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY