摘要:

A 403—cm core was recovered from Hanging Lake in unglaciated northern Yukon. Twenty—one radiocarbon dates indicate that the section is at least 25 000 and possibly 33 000 yr old; they permit the calculation of pollen influxes for the full—glacial in eastern Beringia. Numerical methods were used to divide the pollen stratigraphy into five zones. From prior to 33 000 to 18 450 BP, a herb zone was dominant (zone HL 1) with high percentages of Gramineae, Artemisia, and Cruciferae. However, the low pollen influx, ranging from 5—100 grains°cm—2°yr—1, the low organic content of the sediment, and the occurrence of open—ground taxa all indicate that the vegetation cover was sparser than it is today. The arctic—alpine affinities of the herb pollen show that generically the vegetation was akin to modern arctic plant communities. Modern fellfield communities in the northern Yukon and Siberia have a rich and endemic Artemisia flora and they can produce pollen spectra comparable to that of the herb zone. Percent and influx values for spruce, alder, and birch increased slightly during subzone H 1B (21 680—18 450 BP); this subzone probably represents an interstadial. From 18 450 to 14 600 BP, a Salix—Cyperaceae zone (HL 2) occurred, suggesting the development of snowbed and willow scrub communities in sheltered areas. Between 14 600 and 11 100 BP Betula pollen dominated (zone HL 3) indicating the spread of dwarf birches, but the influx data show that this initial increase was modest compared with the subsequent zone and thus dwarf birches were probably restricted at this time to more favorable habitats. The spread of birch together with the increased total pollen influx, the higher organic content of the sediment, and the increased richness of herb pollen indicates that the local flora was more diverse and that the vegetative cover increased. The climate must have warmed. Zone HL 4 (Ericales zone) spanned the period from 11 100 to 8900 BP. Wet heath communities became locally abundant, poplar was more abundant at the beginning of this zone than at any subsequent time, the ranges of Typha latifolia and Myrica gale were greater than today, and pollen influx and sedimentation rate both increased greatly. Spruce became regionally abundant. These varied changes are probably in response to a warmer and wetter climate; they provide evidence for a late Pleistocene to early Holocene warm interval initially recognized by McCulloch and Hopkins (1966). Zone HL 5 (Alnus crispa zone) has been dated from 8900 BP to the present; it reflects the regional expansion of Alnus crispa on organic soils.

ISSN:0012-9615    

DOI:10.2307/2937342

出版商:Ecological Society of America    

年代:1982

数据来源: WILEY

摘要:

Although predispersal seed predation by insects is common, no test exists of its effect on plant recruitment. This study examines seed predation in the population dynamics of a native, temperate shrub, Haplopappus squarrosus H. and A. (Asteraceae), over an elevational gradient in the coastal sage scrub vegetation of San Diego County, California USA. Frequency and abundance of H. squarrosus increase from coast to mountains. Expected abundance, based on flowers initiated, was highest at the coast and lowest in the interior, the opposite of the observed adult plant distribution. Overall flower and seed predation by insects was high (44—73%) and was greatest at the coast. Insect exclusion experiments at sites along the gradient had three main results. (1) Seedling recruitment was proportional to the number of viable seeds after seed predation. The exclusion plots had significantly higher numbers of seedlings established at all sites than did control plots; the greatest increase was at the coast. (2) Seedling survivorship was independent of both seedling and adult densities. (3) Juvenile recruitment was proportional to seedling establishment. Predation by insects prior to release of seeds played a critical role in the population recruitment of H. squarrosus within and among sites along the gradient. Survivorship of established plants, in addition, was as high at the coast as it was in the interior. These results suggest that indigenous insect seed predators can be a major force controlling the dynamics of a native plant species over its natural distributional range.

ISSN:0012-9615    

DOI:10.2307/2937343

出版商:Ecological Society of America    

年代:1982

数据来源: WILEY

摘要:

The colony defense strategies of the three honeybee species in Thailand were studied to examine the influence of predation on tropical honeybee societies. Each species focuses its defenses upon different stages in the predation sequence of detection—approach—consumption. This radiation in defense strategies apparently reflects each species' preadaptation by worker size (small, medium—sized, or large) and nest site (cavity or tree branch) to a different pattern of colony defense. Wasps, birds, and primates probably have difficulty finding the small, dispersed colonies of Apis florea, whose nests are built low on the branches of dense, shrubby vegetation. Once found, however, they are easily approached and overpowered because their low, exposed nests are accessible and their small workers inflict relatively painless stings. When overwhelmed, the bees quickly abandon their nest; later, they return to salvage wax. Ants find A. florea nests easily and at least one species (Oecophylla smaragdina) easily kills these small bees. However, sticky bands of resin encircling the nests' slender substrate branches prevent ants from invading A. florea nests. Cavity—nesting colonies of Apis cerana are conspicuous with their medium—sized bees streaming in and out of low, clearly visible entrance holes in caves and hollow tress. However, gaining access to A. cerana nests is difficult. Large predators cannot pass through the small entrance opening and small predators are overpowered by entrance guards. But if a large predator can breach a nest cavity's walls, it faces an only moderately powerful stinging defense. Apis cerana colonies are relatively small and their workers are not fiercely aggressive. Predators easily find the large, sometimes aggregated colonies of Apis dorsata, whose nests hang in the crowns of the tallest forest trees. But only skilled fliers and climbers can reach these lofty nests. Those which do face massive stinging attacks from the large colonies of these relatively giant, ferocious bees. Nests of both open—nesting species, Apis florea and A. dorsata, are protected by a three— to six—layer curtain of bees over the comb. Apis cerana colonies lack these curtains but are protected by their nest cavity walls. A curtain of inactive guards requires a large labor force. The high worker: brood ratio in A. florea relative to A. cerana colonies suggests that the age polyethism schedules of the open— and cavity—nesting species are tuned differently to generate the appropriate proportions of guard bees. Each species' colony defense system consists of numerous interwoven lines of adaptation, including nest site, nest architecture, colony population, labor allocation to defense, age polyethism schedule, colony mobility, and worker morphology, physiology, and behavior. Predation has been a pervasive and powerful force in the evolution of these tropical bee societies.

ISSN:0012-9615    

DOI:10.2307/2937344

出版商:Ecological Society of America    

年代:1982

数据来源: WILEY

摘要:

The life histories of four species of intertidal limpets, Cellana tramoserica, Notoacmea petterdi, Patella peroni, and Patelloida alticostata, which occur on the same shore but in different tidal zones, are compared. The population dynamics (recruitment, growth, and mortality) and the causes of mortality of each species are investigated. C. tramoserica and P. peroni have similar rates of growth and mortality, but P. peroni has a higher reproductive effort (the proportion of assimilated energy devoted to reproduction). Pd. alticostata has lower rates of growth and mortality than the above species, but its reproductive effort is similar to that of C. tramoserica. N. petterdi has the lowest rates of growth and mortality, but its reproductive effort is similar to, or perhaps slightly lower than that of C. tramoserica and Pd. alticostata. The causes of mortality vary among species. Predation by wrasses is the main cause of mortality of P. peroni and Pd. alticostata, which occur in the sublittoral and lower littoral zones, respectively. The principal cause of mortality of adult C. tramoserica, a midlittoral species, is starvation. N. petterdi, which occurs in the supralittoral zone, is a very long—lived species and the causes of its mortality could not be determined. Semiquantitative estimates of extrinsic adult mortality rates are obtained for the four species of limpet, and these are shown to be correlated with interspecific differences in reproductive effort. Adults of each species assume different levels of risk during reproduction. The adults of species which occur in seasonal environments appear to take greater risks when breeding than those of species which occupy more constant environments.

ISSN:0012-9615    

DOI:10.2307/2937345

出版商:Ecological Society of America    

年代:1982

数据来源: WILEY

摘要:

Recovery of a desert stream after an intense flash flooding event is described as a model of temporal succession in lotic ecosystems. A late summer flood in Sycamore Creek, Arizona, virtually eliminated algae and reduced invertebrate standing crop by 98%. Physical and morphometric conditions typical of the preflood period were restored in 2 d and the biota recovered in 2—3 wk. Algal communities responded rapidly and achieved a standing crop of nearly 100 g/m2in 2 wk. Community composition was dominated by diatoms early in succession and by filamentous greens and blue—greens later. Macroinvertebrates also recolonized denuded substrates rapidly, largely by immigration of aerial adults and subsequent oviposition. Growth and development were rapid and several generations of the dominant mayfly and dipteran taxa were completed during the 1st mo of recovery. Invertebrate dry biomass reached 7.3 g/m2in 1 mo. Gross primary production (Pg) measured as O2increased in a similar asymptotic fashion and reached 6.6 g°m—2°d—1in 30 d. Pgexceeded community respiration (R) after day 5 and Pg/R averaged 1.46 for the remainder of the 2—mo sequence. This ecosystem is thus autotrophic and exports organic matter downstream and by drying, laterally. Uptake of nitrate and phosphorus were proportional to net primary production and exhibited a marked downstream decline in concentration during both light and dark periods. Temporal trajectories of various community and ecosystem attributes are compared with those suggested by Odum (1969) to be diagnostic of successional status. Agreement was poor in attributes which are especially modified in open, frequently disturbed ecosystems such as streams.

ISSN:0012-9615    

DOI:10.2307/2937346

出版商:Ecological Society of America    

年代:1982

数据来源: WILEY