摘要:

The salt marsh snail Cerithidea californica is first intermediate host to a diverse guild of larval trematode parasites. In Bolinas Lagoon, in central California, the site of this study, at least 15 species of trematodes infect snail populations. This study investigated patterns of interspecific association and interaction among members of this parasite guild. Seven to 19 host subpopulations were sampled annually at each of two sites in the lagoon from 1981 to 1988. Mixed—species infections constituted only 2.5% of the 5025 infections examined in the study. A Monte Carlo simulation procedure demonstrated that the numbers of such infections were often less than would be expected by chance, especially when the overall prevalence of infection was high. Patterns of association between particular pairs of species depended on whether the species' life histories include redial or only sporocyst larvae. Species that develop as rediae were predominantly negatively associated with other redial species and with most species that develop only as sporocysts. There was weak evidence of positive interspecific association between a few redial and sporocyst—only species, while members of other such pairs were distributed independently. Associations between sporocyst—only species were either weakly positive or neutral. Snails carrying known infections were marked, released, and recaptured at both study sites. During their exposure in the field, some initial infections were invaded by another parasite species that often excluded the first parasite. The vulnerability of a parasite species to invasion and replacement by another differed among the tested species. Infections of the largest redial species, Parorchis acanthus, were especially resistant to replacement, while those of the smallest redial species, Euhaplorchis californiensis, were the most frequently excluded. Four other species were invaded or replaced at intermediate rates. The two largest redial species, P. acanthus and Himasthla rhigedana, were responsible for>90% of the invasions or exclusions. Direct observations showed that the rediae of these species prey on the larval stages of other species, as do the rediae of Echinoparyphium sp. This direct form of interspecific antagonism is probably the primary mechanism by which such species exclude others from host snails, as has been widely demonstrated in similar freshwater snail—trematode systems. While hierarchical, negative interactions prevent the coexistence of species at the level of the individual host, the mark—recapture study showed that rates of exclusion are low for most subordinate species, with the exception of Euhaplorchis californiensis. At the level of the host subpopulation, the assemblage of larval trematodes is diverse, and its composition is temporally and spatially variable. There is no trend toward dominance of the assemblage by large redial species as the level of infection rises within aging cohorts or subpopulations of hosts. These patterns of guild structure within host cohorts and subpopulations are consistent with the hypothesis that recruitment processes rather than interspecific interactions primarily determine the composition and relative abundance of species at this regional level. Several characteristics of snail—trematode systems that may promote regional coexistence of such a large number of potentially interacting parasite species are the isolated and subdivided nature of the host resource, the aggregated distributions of larval stages, and the differential exploitation of different—sized hosts. Many features of this system are consistent with Price's (1980) non—equilibrial view of parasite communities.

ISSN:0012-9615    

DOI:10.2307/2937176

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

Herbivory can potentially affect the speed and direction of plant succession by favoring the development of a community dominated by grazing—resistant species. This idea was tested experimentally by examining the effects of the planktonic herbivore, Daphnia, on phytoplankton succession in a naturally eutrophic lake. Phytoplankton succession was characterized by two major transitions in community structure. Algal dominance shifted from small diatoms and chlorophytes during the spring bloom to cryptophyte flagellates during the clear—water phase. After the clear—water phase, dominance shifted to filamentous blue—greens (cyanobacteria). Algal species positions in the successional sequence were repeatable from year to year, despite interannual shifts in the timing of species' peak abundances associated with a drastic change in the food web. In addition, evidence is presented to suggest that Daphnia—induced changes in water clarity may have fostered increases in epilimnetic mixing, which, in turn, may have stimulated brief blooms of large—celled algae after clear—water periods. Daphnia manipulation in large enclosures, and whole—lake observations before and after a fish kill, showed that intense grazing promoted the transition from edible, spring—bloom species to similarly edible, cryptophyte flagellates. In contrast, Daphnia grazing retarded further succession to grazing—resistant, filamentous blue—greens. Thus, the effects of herbivory on algal succession were not predictable from the relative susceptibilities of these algal species to grazing mortality. These results underscore the importance of indirect effects in the herbivore—plant interactions of planktonic communities. The observation that a single species of herbivore had opposite effects during two successional transitions implies that caution should be exercised when extrapolating grazer effects beyond the time scale of an experiment.

ISSN:0012-9615    

DOI:10.2307/2937177

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

Variation in the timing of seed dispersal among individuals of Cornus drummondii was marked at two sites in east—central Illinois. Individuals whose fruit crops were removed most quickly shared few characteristics. Fruit removal rates, dispersal efficiency (percent of total crop removed), and dispersal success (estimated percent of dispersed seeds attributable to each individual) were not closely related to characteristics of the fruits or the immediate environment of the parent plant. Although small fruit crops were often depleted proportionately faster than large crops, dispersal success relative to others in the population was positively related to crop size; i.e., plants with larger crops contributed more seeds to the total pool of dispersed seeds than plants with smaller crops. Over the 4—yr study, individual plants often exhibited between—year differences in ripening time, crop size, and pulp characteristics, but between—year correlations of traits were only sporadically significant. Crop size showed the most consistent between—year correlation (i.e., plants with large crops tended to make large crops in all years), but even this trait was not always correlated between years. Dispersal success was usually correlated between years, but dispersal efficiency was not. Annual (and sometimes site) differences in most dispersal—related plant traits and in dispersal efficiency and dispersal success were common. No plant traits except crop size had consistent effects on dispersal success, although we found some evidence of selective foraging. We suggest that fruit removal may be more a function of avian biology (flocking and social behavior, frequency of foraging interruptions, sun—seeking in cold weather, etc.) than of most characteristics of the fruiting plant. The ecological consequences of differences in dispersal phenology were variable between years. Postdispersal seed predation and germination success showed no consistent seasonal trends. The most important consequence of failure of early fruit dispersal may be a high probability of nondispersal (falling, rotting, being eaten by a predispersal seed predator or poor dispersal agent). Great variation in space and time in the outcome of bird/fruit interactions means that studies directed at the evolution of the interactions need to have both larger samples sizes and longer duration than has been feasible so far.

ISSN:0012-9615    

DOI:10.2307/2937178

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

Pollen records from northern Grand Teton National Park, the Pinyon Peak Highlands, and southern Yellowstone National Park were examined to study the pattern of reforestation and climatic change following late—Pinedale Glaciation. The vegetational reconstruction was aided by analyses of associated plant macrofossils and the modern pollen rain of the region. Radiocarbon—age determinations and tephrochronology provided a chronologic framework to help correlate pollen records among sites. The fossil records indicate that alpine meadow communities, with Betula and Juniperus, were present between ≈14 000 and ≈11 500 yr BP. This early assemblage implies a lowering of modern upper treeline by at least 600 m and a climate was ≈5—6° colder than present. Between 11 500 and 10 500 yr BP, increased temperature and winter precipitation allowed first Picea, and then Abies and Pinus cf. albicaulis to spread into areas underlain by andesite and nonvolcanic bedrock. By 10 500 yr BP, the fossil record in these areas resembled modern spectra from subalpine forest. In contrast, the Central Plateau of Yellowstone, which is underlain by infertile rhyolitic soils, was treeless prior to ≈10 000 yr BP. The absence of late—glacial subalpine parkland in this area is attributed to the same edaphic factors that limit Picea, Abies, and Pinus albicaulis from the rhyolite plateau today. Between 10 000 and 9500 yr BP, Pinus contorta forest developed throughout the region in response to further warming. Pseudotsuga and Populus were present between 9500 and 5000 yr BP, suggesting a warmer, drier climate than today and more frequent fires. In the last 5000 yr BP mixed forests of Picea, Pinus, and Abies have developed on andesitic and nonvolcanic terrain, and closed forests of Pinus contorta have persisted on rhyolitic substrates. The vegetational patterns are attributed to a combination of climatic and nonclimatic controls operating on different spatial and temporal scales. Climatic changes brought about by the retreat of Laurentide ice and the amplification of the seasonal cycle of radiation explain the broad patterns of vegetational change on millennial time scales. On shorter time scales and smaller spatial scales, substrate differences and migration history shaped the vegetational variability.

ISSN:0012-9615    

DOI:10.2307/2937179

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

We grew four perennial grass species (Poa pratensis, Agropyron repens, Agrostis scabra, and Schizachyrium scoparium) for 5 yr in monocultures and in pairwise competition plots on an experimental nitrogen gradient. The gradient contained plots ranging from 100% sand to 100% black soil, plus plots that received additional N fertilizer. To examine the impact of initial conditions on the long—term outcome of interspecific competition, three competitive situations were created: seed vs. seed competition (both species planned simultaneously), seed invasions (each species added as seed to year—old monocultures of the other), and vegetative invasions (dividers separating adjacent monocultures of two species removed after 1 yr). Extractable soil NO3—and NH4+were measured to test if species differences in the concentration of available soil N in monoculture (i.e., R* for N, Tilman 1982) could predict the long—term outcome of competition. By year 5, Schizachyrium displaced or greatly reduced the biomass of both Poa and Agropyron on the soil mixture gradient (the mixed soils but not the added—N plots) independent of the wide range of starting conditions. On these soils, Schizachyrium monocultures had significantly lower soil concentrations of both NO3—and NH4+than either Poa or Agropyron monocultures. Similarly, Agropyron displaced or greatly reduced the biomass of Agrostis by year 5. Agropyron monocultures had significantly lower concentrations of NO3—and NO3—+ NH4+, but not NH4+, than Agrostis monocultures. In contrast, no competitive displacement occurred in competition between Poa and Agropyron, and initial differences persisted over 5 yr. Monocultures of these two species did not differ in NO3—concentration, but did differ for NH4+and NO3—+ NH4+. Thus, species differences in ability to deplete soil NO3—successfully predicted the outcome of competition for all four species pairs on the soil mixture gradient. If resource preemption or asymmetric competition had been the mechanism of competition, initial conditions would have affected the long—term outcome of competition. Rather, these results support the R* (i.e., resource reduction) model for competition for soil N. In the added—N fertilizer plots, Schizachyrium had decreased biomass in competition with both Poa and Agropyron. However, neither Agropyron nor Poa appeared to have an advantage when they competed with each other in the added—N plots. For these three species pairs, the 5—yr results of competition in the added—N plots, which had greatly reduced light availability because of increased production and litter accumulation, depended on initial conditions. In the fourth pair, Agrostis was displaced by Agropyron in all competition treatments in the added—N plots. Thus, we cannot reject the hypothesis that resource preemption (i.e., asymmetric competition) is important in light competition.

ISSN:0012-9615    

DOI:10.2307/2937180

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY