摘要:

The existing warm (Larrea) deserts of the Southwest are Holocene expansions replacing late—Pleistocene, evergreen woodlands of low—statured junipers, pinyon pines, and live oaks; these woodlands have been isolated by complementary contraction to the slopes of higher mountains that rise like islands from the modern desert sea. Because pinyon—juniper woodland is now so widespread on the similar fault—block mountains of the Great Basin, even as far north as southern Idaho, it would seem reasonable to suppose that the modern "cold" (Artemisia, Atriplex) deserts were similarly wooded during the last glacial. However, conclusive Neotoma macrofossil evidence (4514C—dated assemblages are reported here) documents major latitudinal displacement of vegetation that precludes pinyon—juniper woodland in the northern and central Great Basin at that time. On the other hand, the entire Mohave Desert sector (south of °37°N) served as an extensive Pleistocene refugium for pinyon—juniper woodland, as documented by an additional 48 dated Neotoma deposits. During the Wisconsinan glacial in the southeastern corner of Oregon, a 42°27'N, there was a subarctic landscape of hyperboreal, prostrate shrublet—junipers (Juniperus horizontalis and J. communis) and widespread patterned ground, even at the near—basal elevation of 1460 m. The pleniglacial vegetation of the central Great Basin at 39°N in eastern Nevada and western Utah, was dominated by a regional subalpine forest of bristlecone pine (Pinus longaeva), associated with minor but consistent boreal juniper (J. communis) down to 1660 m, close to the base level imposed by pluvial Lake Bonneville. Spruce has not been recorded below 1900 m during the last glacial. At a lower range of elevation (1350—1525 m), available south of the southeastern rim of the Bonneville basin at 37°30'N. Pinus longaeva was replaced by limber pine (P. flexilis), Douglas—fir (Pseudotsuga), and montane red cedar (J. scopulorum); existing woodland juniper (J. osteosperma) was lacking, but the subalpine J. communis was present at this local base level. Theory of island biogeography, as applied to ecological islands atop the high mountains of the Great Basin, is reexamined in the light of the drastic vegetational displacements documented in the detailed Quaternary macrofossil record. Species/area plots of montane—subalpine conifers presently distributed on 54 Great Basin mountaintops show an overall insular pattern that is especially well developed on the subset of 38 ecological islands east of 116°W; the slope of z = 0.26 is close to the theoretical value for islands in equilibrium. All 11 taxa of montane—subalpine conifers that penetrate the Great Basin deeply have their main distributions in the Rocky Mountains; only three wide—ranging species occur also in the Sierra Nevada. A long sundering trough in the western Great Basin parallels and isolates the Cascade—Sierran uplift with low—elevation barriers that impede migration, but in the eastern Great Basin there are high connecting divides to the western Rockies, especially via an axial route southeast of the Bonneville basin. There is an east—west pattern of declining species richness of11 montane conifers in the Great Basin that correlates with distance from the rocky Mountain pool of 12 coniferous species. The pleniglacial subalpine forests in the lowlands of the central Great Basin had only one to three species of conifers (e.g., Pinus longaeva, Picea engelmannii, Juniperus communis). During the great late—glacial/Holocene (12 000—8000 yr BP) warming of climate, these shifted upward in elevation and were augmented in the east (but not in the west) by as many as five additional species of montane conifers. Macrofossil evidence indicted that the later Holocene arrivals dispersed across barriers of woodland and desert that by then isolated the shrunken montane islands. Moderately long—range transport of seeds by birds is deduced as follows: a northward latitudinal shift of 500—640 km during the Holocene is documented for several species of relatively thermophilous conifers, including the heavy—seeded, late—maturing pinyon pine. A 640—km migration in 8000 yr (80 m/yr) is indicated for pinyon, but the most generous estimate of its dispersal rate via the wind/gravity mode is a plodding 0.4 m/yr (3.2 km/8000 yr), orders of magnitude too slow. Seed dispersal by Clark's Nutcrackers and Pinyon Jays, however, is both the prevalent mode and amply swift enough to fit the known migrational history. Hence, the islands—in—equilibrium pattern indicated by the typically insular slope (z = 0.26) for montane conifers on the eastern set of mountaintops in the Great Basin is a reflection of Holocene immigration via sweepstakes dispersal being offset by extinction on the smaller islands. Both extinction and immigration of conifers are documented in the late—glacial/early—Holocene Neotoma record from the small Confusion Range in the east—central Great Basin of western Utah.

ISSN:0012-9615    

DOI:10.2307/1942644

出版商:Ecological Society of America    

年代:1983

数据来源: WILEY

摘要:

Detritus dynamics in Big Hurricane Branch, a second—order stream at Coweeta Hydrologic Laboratory in the southern Appalachian Mountains, were simulated with a computer model, using data from a variety of Coweeta stream studies. The model was used to evaluate the role of macroinvertebrates in the stream. Macroinvertebrates accounted for only a small portion of the respiration of detritus; their major role was conversion of benthic detritus into transported detritus. Macroinvertebrates were responsible for 27% of annual particulate organic matter (POM) transport, though when they were removed there was only a 10% reduction in POM transport because of a compensatory increase in storm transport. The contribution of macroinvertebrates to POM transport during nonstorm periods was much more significant, as high as 83% in late summer. Based on an annual budget, macroinvertebrates decrease the efficiency of detritus processing in low—order streams, because they increase transport loss. On a longer time scale, however, macroinvertebrates prevent accumulation of large amounts of detritus in the stream and major losses during infrequent large storms. By stabilizing long—term detritus export dynamics, they provide an important link between low—order and higher—order streams.

ISSN:0012-9615    

DOI:10.2307/1942645

出版商:Ecological Society of America    

年代:1983

数据来源: WILEY

摘要:

Population dynamics of the intertidal sea anemones Anthopleura elegantissima (Brandt) and A. xanthogrammica (Brandt) were investigated by long—term monitoring and experimental manipulation of populations in Washington State. Sizes of individual A. xanthogrammica and A. elegantissima fluctuated seasonally, increasing during the spring and early summer and shrinking or reaching a pleateau the rest of the year. Data from marked individual A. xanthogrammica showed that adults (>6.5 cm) moved very little and may persist for at least several decades. Only 3 of 160 mapped adult anemones disappeared over 2 yr. Juveniles moved more often and thus appeared and disappeared at much higher rates. Growth of A. xanthogrammica in control and experimental removal areas was compared by a new technique. One—year growth increments were used to compare growth in each size class by analysis of variance and were also used to plot long—term growth trajectories and expected reproductive success (habitat suitability, Fretwell 1972). Growth was much more rapid in areas with experimentally reduced density than in control areas. The control area with the lowest density had the greatest individual growth rates. Differences in growth trajectories and expected reproductive success between control areas and the experimental areas indicate that adult density affects growth of most size classes. The mechanism is probably both noninterference competition for prey and direct interference by adult tentacle crowns blocking capture by smaller individuals. Clonal aggregations are formed by longitudinal fission of A. elegantissima. (A. xanthogrammica never reproduce asexually.) Monitored clonal aggregations of A. elegantissima showed that fission rate is greatest during the fall and winter, averaging 0.17 divisions per clonal individual per year in all areas. Only the larger individuals in each clone divided and a single individual never divided more than once in a given year. Rate of fission was independent of mean individual size within clones. Therefore, clones in more suitable habitats produced larger individuals and thus had greater gonad (offspring) production. Clones in the high intertidal were composed of very small individuals, had the lowest rates of fission, and were clearly occupying a marginal habitat.

ISSN:0012-9615    

DOI:10.2307/1942646

出版商:Ecological Society of America    

年代:1983

数据来源: WILEY

摘要:

I examined patterns of species distributions in ground ants on Barro Colorado Island (BCI), Republic of Panama, using baited transect samples and Berlese extraction of litter arthropods. I sampled during wet and dry seasons in 1976 and 1977. All sites sampled were under closed—canopy rain forest at least 75 yr old. More species and more individuals were collected in wet than dry season using either method. Evenness of species abundance at baits also increased during the wet season. Ant activity was higher in 1977 than 1976 at all periods; this was correlated with increased rainfall during the dry season. Measures of species abundance and activity were positively correlated with protection from physical stresses. Over all seasons, fewer species and fewer individuals were collected at drier, sunnier sites. Patterns of seasonal and annual change were similar at bait transects and in Berlese samples. Species that were both active at baits and collected in Berlese samples varied in abundance the same way in both sampling methods. Several lines of evidence suggest that moisture availability is an important contributor to these patterns of among—site and among—season variation. First, ant activity, measured in several different ways, increased rapidly when moisture availability was increased, either through rainfall or through experimental watering during the dry season. Second, moisture availability may affect the distribution of suitable nest sites. This hypothesis was supported by differences in the number of colonies collected in Berlese samples from sites which differ in moisture availability. Third, the availability of food among sites may be correlated positively with soil moisture content, but there was not enough evidence to test this possibility. Fourth, there is some evidence that reduction of army ant raids in drier parts of the island during the dry season may concentrate the effects of these ant predators in moist, sheltered areas of BCI. Establishment and persistence of a given species in an area is a complex function of the availability of suitable food and nest sites and the strength of competitors and predators. Patchy species distributions were more the result of changes in the frequency of a species' occurrence among areas, than due to the existence of unique groups of species at separate sites, at least during the periods I sampled. Thus physical conditions were probably at least marginally acceptable to a large fraction of the fauna at all sites. However, differences in abundance and activity of ant species were correlated with changes in moisture availability. I have considered four hypotheses to account for these patterns, but they are by no means exhaustive. Moisture availability, acting through many intermediate steps, has major effects on the small—scale composition of ant faunas, even in a relatively sheltered environment like a tropical forest floor.

ISSN:0012-9615    

DOI:10.2307/1942647

出版商:Ecological Society of America    

年代:1983

数据来源: WILEY