摘要:

Three species of massive reef—building coral, Goniastrea aspera, G. favulus, and Platygyra sinensis, were studied on two fringing reef flats in the central Great Barrier Reef from 1982 to 1984. Total population sizes ranged from 25 to 292 colonies, and remained relatively constant. Population structures and dynamics based on both age and size were described. Differences between these two classifications were primarily due to tissue fission or shrinkage of colonies. In populations dominated by small size classes, young corals were not necessarily the most abundant. Similarly, populations dominated by the largest size classes were not always dominated by the oldest corals. In several populations, mean colony size decreased slightly with increasing age, though variability in size also increased. Mortality patterns were similar in all three species, with the youngest genets and smallest ramets suffering the highest death rates. While mortality in older or larger corals was low, the incidence of partial mortality and fission was higher in these groups. Age—specific fecundity increased rapidly once reproductive age was reached at ≈5 yr, but fecundity decreased slightly in the older age classes due to the decrease in mean colony size. Estimated mean generation times ranged from 33 to 37 yr.

ISSN:0012-9615    

DOI:10.2307/2937107

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

Zooplankton species composition was manipulated in an 8—mo long series of experiments in outdoor 450—L tanks. The first experimental stage evaluated the impact of cyclopoids (Mesocyclops, Microcyclops, Cyclops) and calanoids (Diaptomus) on each other. The second and third stages evaluated the effects of Daphnia and mosquitofish (Gambusia affinis) on the calanoid—cyclopoid interaction. Cyclopoid predation initially caused a 50—70% reduction in calanoid abundance. After a few months, however, cyclopoids caused>300% increases in calanoid abundance, apparently by preventing calanoid overexploitation of the phytoplankton. The presence of calanoids depressed cyclopoid abundance by up to 70—90%, presumably by diminishing the availability of edible phytoplankton to cyclopoid nauplii and of prey, such as rotifers, to cyclopoid adults. The presence of Daphnia depressed calanoid abundance, probably by reducing the phytoplankton abundance, but did not affect cyclopoid abundance, perhaps because cyclopoid adults could feed on items other than phytoplankters. Coequality (i.e., similarity in abundance) of closely related taxa was in 4 of 5 cases favored by the addition of a more distantly related taxon. Thus the addition of cyclopoids favored the coequality of two Diaptomus species (D. clavipes, D. siciloides), the addition of these Diaptomus species favored the coequality of two cyclopoid genera (Mesocyclops, Microcyclops), and the addition of Daphnia favored coequality of cyclopoids and calanoids and coequality of the two Diaptomus species, but the dominance of Microcyclops over Mesocyclops. Effects of Gambusia included a drastic reduction of Daphnia populations, moderate reduction of calanoid populations, and negligible effects on cyclopoid populations.

ISSN:0012-9615    

DOI:10.2307/2937108

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

Analysis of food web structure and temporal dynamics is essential to understanding energy flow and population dynamics of species, and may contribute to conservation, wildlife management, and disease and pest control. This report synthesizes all the observational studies of food web dynamics to which we have access. Most published food webs are static and cumulative: they depict information gathered over many occasions. A web observed over a single, relatively short time period is time specific. Here we analyze the relation between cumulative and time—specific versions of webs in 16 published cases. Fourteen of the 16 webs are from detritus—based habitats that harbor large fractions of arthropod species: carcasses, tree fluxes, felled logs, treeholes, dung pads, and an acidic pond. The other two webs describe soybean fields and the arctic tundra. These webs are presented here in a consistent format and are analyzed in four ways. First, we quantified temporal trends and levels of variation in nine web properties: the percentages of species in the web that are top predators (%T), intermediate species (%I), and basal species (%B); the ratio of number of prey species to number of predator species (P); the mean chain length (µ); the product of species richness and connectance (S × C); and the numbers of total species, newly arriving species, and local extinctions. In most webs %I and %T fluctuated widely; the latter generally increased in time or remained constant, while the former correspondingly decreased or remained constant. Since the number of basal species usually varied little, changes in %B were inversely associated with changes in species richness over successional and seasonal time scales. Predictable changes in P, µ and S x C accompanied the changes in %B, %I, and %T. The numbers of total species, new arrivals, and local extinctions displayed no consistent increasing or decreasing trends. Second, we compared cumulative and time—specific webs from the same habitat to determine which properties, if any, of time—specific webs might be predicted from cumulative webs. In cumulative webs, P, µ, and %T came closest to the median of the values from time—specific webs, followed by %I, S x C, and %B Cumulative webs, which usually appear in general ecology textbooks, overestimate S x C and underestimate %B relative to time—specific versions. In five studies cumulative webs were completed when the last or next—to—last samples were taken; additional sampling in these cases would probably have uncovered more species. Third, opportunistic species were removed from four time—specific webs to determine how these species influenced web structure. Removing one top—feeding opportunistic species from each web caused a dramatic rise in %T, small reductions in %I, S × C, µ, and P, and a negligible rise in %B. A single opportunistic species, even though it makes only rare and brief visits to a habitat, can dramatically reshape web structure. Fourth, properties of cumulative and time—specific versions of the 16 food webs were compared to properties of cumulative webs in two published web catalogs. The cumulative versions of the 16 webs grossly resemble the cumulative webs in both prior catalogs, but the median S × C is greater and the median %B is lower in the 16 cumulative webs than in either prior catalog. Even for these two statistics, the median value for the 16 cumulative webs falls well within the range of variation of both prior catalogs. The time—specific webs in the 16 cases differ from those of the two prior catalogs somewhat more than do the cumulative webs. Comparisons between time—specific and cumulative versions of a web, one system at a time, are more sensitive than rough comparisons between collections of webs because the methods used to define species and links are (usually) consistent within a study.

ISSN:0012-9615    

DOI:10.2307/2937109

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

We conducted a field experiment to assess interrelationships between leaf—tissue secondary chemistry, avian predation, and the abundance and diversity of arthropods occurring on sagebrush (Artemisia tridentata) in central Oregon. Arthropods were removed from individual shrubs, some of which were then caged to exclude birds. Secondary chemistry and arthropods were sampled at intervals up to 56 wk following the defaunation/caging treatment. Recovery rates differed among arthropod taxa and functional groups. Several sap—sucking homopterans and hemipterans reached control levels within 2—4 wk of the treatment, whereas abundances of parasitoids and predators recovered to match control numbers only 6 wk after defaunation. Abundances of several herbivorous leaf—chewing taxa (primarily lepidopteran larvae) remained significantly depressed even 56 wk after the treatment. Fungivores (oribatid mites) reached greater abundances on defaunated than on control shrubs by the end of the experiment. There were also significant changes in the concentrations or frequency of occurrence of several chemical compounds following the defaunation treatment. Several hydrocarbons, sesquiterpenes, and monoterpenes that were present in most shrubs exhibited sharply reduced concentrations in leaf tissues within 4 wk of the treatment, whereas some alcohols and ketones (linalool, borneol, thujone), which occurred at relatively low frequencies among control plants, increased dramatically in their frequencies of occurrence following arthropod removal. Both changes persisted for the duration of the experiment. We found several significant associations between the abundance or occurrence of arthropod taxa or groups and concentrations or frequencies of secondary compounds, but these were most prevalent among the leaf—chewing lepidopterans. We suggest that the shrubs responded to the removal of herbivorous, leaf—chewing arthropods by altering chemical allocation patterns; the changes in chemistry persisted for over a year because recolonization of the defaunated plants by these herbivores was slow. Effects of the caging treatment were much less obvious. The recovery of the diversity of arthropods known to constitute prey for birds in this system was slower on shrubs protected from avian predation than on exposed shrubs, but a year after defaunation diversity had increased to higher levels on the protected shrubs. Few arthropod taxa or functional groups differed in abundance or frequency of occurrence between caged and uncaged shrubs, although fungivores (which are not eaten by birds) increased to levels on protected shrubs that were nearly twice those on the uncaged controls by the end of the experiment. Between—shrub variance in the abundance and diversity of bird—prey taxa was greater among exposed than protected shrubs, possibly reflecting the effects of area—restricted searching by the birds. There were no differences in leaf—tissue secondary chemistry between caged and uncaged shrubs.

ISSN:0012-9615    

DOI:10.2307/2937110

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

Steppe communities of interior Alaska are restricted to steep, generally southfacing bluffs. Two competing hypotheses explain this distribution: (1) steppe taxa are restricted to bluffs by their requirements for the abiotic conditions of these sites, and (2) steppe taxa have broader physiological ranges than expressed in nature, but are restricted to bluff sites by competition with other plants. We addressed these hypotheses using correlative and experimental methods to identify controls over the distribution of steppe and forest understory species on Eagle Bluff, Alaska. We developed a causal model and used path analysis, a correlative method, to estimate strengths of hypothesized causal relationships. Based on the results of path analysis we hypothesized that light intensity and soil moisture were important controls over the composition of the vegetation and species distributions. We tested this hypothesis by examining the responses of two species native to steppe (Linum lewisii and Potentilla hookeriana) and two species native to forest (Moehringia lateriflora and Pyrola secunda) to manipulation of light intensity, soil moisture, and soil type. The forest species had reduced survival and biomass when grown in the high light and dry soil moisture characteristic of steppe environments. Conversely, survival of steppe taxa was independent of treatments, and the response of survivors to treatments differed between species. Linum had higher seed production with higher soil moisture than it normally experiences and grew less under lower light intensity. Potentilla responded positively to higher moisture and had weakly negative (but not significant) response to reduced light levels. Our experiments confirmed the importance of light and soil moisture to vegetation on Eagle Bluff, but revealed the individualistic nature of species response to environment.

ISSN:0012-9615    

DOI:10.2307/2937111

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY