摘要:

The aims of this paper are to compare the thermal ecology of four species of varanid lizards that occupy a range of habitats and climatic regions, and to assess the efficacy of methods for evaluating the extent to which ectothermic animals exploit their thermal environments. Hertz et al. (1993) have proposed several indices of thermoregulation, and these are evaluated with respect to our data from varanid lizards. The thermoregulatory characteristics of three tropical monitor lizards (Varanus panoptes, V. gouldii, and the semiaquatic V. mertensi), and the temperate—zone V. rosenbergi were studied throughout the year. Radiotelemetry was used to measure the body temperatures (Tbs) of free—ranging animals, and microclimatic data were collected to determine the range of possible Tbs that an animal could achieve. Operative temperatures (Tb's) were estimated by biophysical models for each set of animal characteristics and microclimatic conditions. The Tb's selected by animals in a laboratory thermal gradient were used to determine the set—point range of Tb's that the animals voluntarily select. Plots that superimpose Tb's, Tb's, and the set—point range across the day are extremely useful for describing the thermoregulatory characteristics of ectotherms. These plots can be used to determine the extent to which the animals exploit their thermal environment: we define an index of thermal exploitation (Ex) as the time in which Tb's are within the set—point range, divided by the time available for the animal to have its Tb's within the set—point range. Only V. mertensi was active throughout the year. In general, during seasons of inactivity, the Tb's of inactive species fell outside the set—point range, but during periods of activity all species selected Tb's within their set—point range. The temperate—zone species (V. rosenbergi) thermoregulates very carefully during periods when environmental conditions allow the animals to attain the set—point range, and V. gouldii also thermoregulates carefully in the wet season. V. mertensi selects Tb's that are significantly lower than the other species both in the field and in the laboratory, and thermoregulatory indices of this species were intermediate relative to the other species. The amount of time spent in locomotion each day was not correlated with the indices of thermoregulation: the most active species, V. panoptes, was, with respect to several indices, the least careful thermoregulator. The type of question that is being addressed, with respect to the interactions between an animal's thermal environment and its thermoregulatory behavior, determines the appropriateness of the various indices of thermoregulation. The Ex index describes the thermoregulatory characteristics of ecotherms in a heterogeneous thermal environment, and in such an environment a large amount of information can easily be interpreted graphically. This index is less useful in a thermally homogeneous environment.

ISSN:0012-9615    

DOI:10.2307/2963472

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

We examined the importance of the epiphytic lichen, Ramalina menziesii, in a blue oak (Quercus douglasii) woodland of California, using a factorial experiment in which lichens were removed from tree canopies and the soil surface for 3—yr period. Each blue oak contained an average of 3.8 kg of lichen biomass, totaling 590 kg/ha for the entire woodland; oak leaf biomass was 958 kg/ha. Removal of lichens affected the interception of rainfall by the forest canopy and the deposition of water and nutrients in throughfall collected beneath the canopy. Trees with lichens had higher deposition of total N, organic N, Ca, Mg, Na, and Cl, and a lower deposition of SO4under their canopy; deposition of NO3, NH4, K, and total P was not affected. Accounting for the uptake of nutrients by lichens, we calculate that canopy lichens enhance the receipt of N and P from the atmosphere by 2.85 and 0.15 kg·ha—1· yr—1, respectively. The bulk deposition (wetfall + sedimentation) of N and P for the same period was 0.88 and 0.06 kg·ha—1·yr—1, respectively. Because retranslocation of N occurred prior to leaf abscission, the percentage of N in leaf litter was approximately half that in lichen litter. Lichen litter significantly slowed the decomposition of oak leaves on the forest floor. Oak leaf and lichen litter decomposition was slow, with low N release; whereas P release was much more rapid. The removal of lichens had no effect on leaf biomass or nutrient content, acorn productivity, or tree—ring increment of the oak trees, probably because soil pools of N and P were large relative to the magnitude of the flux changes caused by the manipulations. In fact, no differences in soil mineralization rates among the treatments were detectable. Overall, the manipulations significantly affected annual intersystem capture of nutrients, but the magnitude of these changes was insufficient to affect intrasystem fluxes or forest productivity.

ISSN:0012-9615    

DOI:10.2307/2963473

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

With a view toward understanding variation in species composition among different forest communities, I examined species—specific growth and mortality of juvenile trees (2.3—78 mm diameter at 10 cm above the ground) at three contrasting sites. Two sites differing in soil mineralogy and elevation (schist/gneiss uplands vs. calcareous bedrock valley) were situated in northwestern Connecticut, USA. To examine variation over a more extensive geographic scale, I located the third site in central—western Michigan, USA. Among the three sites, the deciduous species (American beech, white ash, and sugar maple) showed little intraspecific variation in models of relative radial growth rate as a function of light availability. Strikingly, a substantial component of the variation in radial growth at the Michigan site could be explained by sapling growth models originally calibrated for the Connecticut sites. In contrast to the deciduous species, the evergreen species (white pine and eastern hemlock) exhibited significant intraspecific differences in growth models between the two Connecticut sites. Intraspecific variation in growth models among sites was characterized by significantly different estimates of growth at low light. This result suggests that light and other resources can be simultaneously limiting, and challenges the application of Liebig's law of the minimum to tree sapling growth. For the deciduous species, mortality processes exhibited more variation among the sites than did growth. I found significant site differences in mortality as a function of recent growth for both sugar maple and white ash on the calcareous soils in comparison to the schist/gneiss soils in Connecticut. In general, the mortality functions at the Michigan and the upland Connecticut site were similar, and both differed from the Connecticut calcareous site. Community dynamics at each of the three sites were simulated with a model of forest dynamics called SORTIE, which incorporated observed differences in sapling mortality and growth functions among the sites. Using SORTIE, among—site differences in only juvenile survivorship and growth were sufficient to predict dominant species in the adult canopy, demonstrating the critical importance of sapling stages to community dynamics. If a species performs differently at a particular site, its interactions with other species at the site will also change, potentially contributing to further differences in community species composition. Changes in species' performance in response to different site conditions and the species' subsequent interactions are critical to understanding differences among the dynamics and compositions of these forest communities.

ISSN:0012-9615    

DOI:10.2307/2963474

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

This paper presents the biotic, sedimentary, geomorphic, and climatic history of the upper part of the Roberts Creek Basin, northeastern Iowa for the late—glacial and Holocene, and compares these records with a C—O isotopic sequence from Coldwater Cave, 60 km northwest of Roberts Creek. The biotic record (pollen, vascular plant and bryophyte macrofossils, and insects) is preserved in floodplain alluvium that underlies three constructional surfaces separated by low scarps. Each surface is underlain by a lithologically and temporally distinct alluvial fill. The highest surface is underlain by the Gunder Member of the Deforest Formation, dating from 11 000 to 4000 yr BP; beneath the intermediate level is the Roberts Creek Member, dating from 4000 to 400 yr BP; and the lowest level is underlain by the Camp Creek Member, deposited during the last 380 yr. Pollen and plant macrofossils in the alluvial fill show that a typical late—glacial spruce forest was replaced by Quercus and Ulmus in the early Holocene. This early—to—middle Holocene forest became dominated by mesic elements such as Acer saccharum, Tilia americana, Ostrya virginiana, and Carpinus caroliniana as late as 5500 yr BP; in contrast, the closest sites to the west and north were at their warmest and driest and were covered by prairie vegetation between 6500 and 5500 yr BP. After 5500 yr BP, the forest in the Roberts Creek area was replaced by prairie, as indicated by a rich assemblage of plant macrofossils, although only Ambrosia and Poaceae became abundant in the pollen record. The return of Quercus ≈ 3000 BP (while nonarboreal pollen percentages remained relatively high) indicates that oak savanna prevailed with little change until settlement time. The bryophyte assemblages strongly support the vascular plant record. Rich fen species characteristic of boreal habitats occur only in the late—glacial. They are replaced by a number of deciduous—forest elements when early—to—middle Holocene forests were present, but mosses of forest habitats completely disappear when prairie became dominant. A few deciduous—forest taxa return during the late—Holocene, when oak savanna prevailed. The C—O isotopic record from stalagmitesin Coldwater Cave indicates a relatively stable environment from ≈ 8000 to 5100 yr BP, when the δ13C values indicate a change in vegetation dominated by C3(predominantly forest) to C4(predominantly prairie) plants. About 4900 yr BP, the rise in18O values indicates a temperature increase of ≈ 1.5°C. The fact that the vegetational change suggested by the δ13C values preceded the temperature increase suggests that fire may have been an important factor in converting forest to prairie. Abundant charred seeds and other plant material at Roberts Creek 4830 yr BP support this hypothesis. The18O values remain constant from ≈ 5100 to ≈ 3000 yr BP, but the δ13C values gradually rise, indicating that soil formed under forest takes at least 2000 yr for its carbon to reach equilibrium after replacement by prairie vegetation. The return of oak to form savanna is reflected in the gradual decline of δ13C values in the last 3000 yr BP; O isotopic values drop sharply by ≈ 1°C ≈ 2800 yr BP and then were relatively stable. In contrast to the vegetational and isotopic records, the insect assemblages suggest little change in the local environments throughout most of the Holocene. All of the beetle taxa presently occur in eastern Iowa. The relative stability through the Holocene indicates that both open grassland and riparian woodland elements were present throughout. Settlement, land clearing, and land cultivation by EuroAmericans in the region caused rapid erosion of the upland landscape, the deposition of 1—2 m of sediment across the floodplain, a replacement of the native vegetation with ruderal species, a decimation of the native insect fauna, and a degradation of water quality in the stream. These changes in the landscape, vegetation, and insect faunas are as striking as those associated with glacial—interglacial transitions. The timing and direction of changes in the vegetation at Roberts Creek generally correlate well with the carbon and oxygen isotopic record in speleothems at nearby Coldwater Cave and indicate that climate was the main forcing function. However, the contrast between the vegetational change and the stability of the beetle population suggests that climatic changes were subtle. We hypothesize that the factors involved in the Holocene changes were seasonal changes in temperature and precipitation that may not have resulted in much mean annual change. Such changes may have affected the vegetation more than the insect fauna.

ISSN:0012-9615    

DOI:10.2307/2963475

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

To determine the extent of host—plant specialization, the pollen sources of the 72 anthidiine species (family Megachilidae, subfamily Megachilinae, tribe Anthidiini) of Europe, North Africa, and Asia Minor were investigated by microscopic analysis of ≈ 1800 pollen loads of females. By this measure, 31 species (43%) were oligolectic (relatively specialized to pollen source) at the level of plant tribe, plant subfamily, or plant family. Exclusive pollen sources of these bees throughout their geographic ranges are flowers of the Cardueae (Compositae), the Asteroideae (Compositae), the Papilionoideae (Leguminosae), the Lamioideae (Labiatae), the Nepetoideae (Labiatae), the Dipsacaceae, or the Campanulaceae. Thirteen species (18%) were found to exhibit a strong, but not exclusive, preference for the Papilionoideae (Leguminosae), the Labiatae, the Cardueae (Compositae), and Zygophyllum (Zygophyllaceae), respectively, while 25 species (35%) proved to be more markedly polylectic, visiting the flowers of up to 17 different plant families for pollen. The plants exploited by three species (4%) are insufficiently known. By far the most important pollen sources of the anthidiine bees as a whole are the Compositae (41.7%) followed by the Leguminosae (23.1%) and the Labiatae (13.0%). The phylogenetic relationships of the anthidiine bees were estimated by a cladistic analysis based on 115 characters to trace possible evolutionary patterns of diet composition. Based on the estimated phylogeny, at least eight shifts of oligoleges between different plant taxa and six transitions between oligolecty and polylecty appear to have occurred. Four transitions were from oligolecty to polylecty whereas two transitions are of unknown direction, both directions being equally parsimonious. Assuming that the ancestral state in the anthidiine bees was oligolectic, the present distribution of oligolectic and polylectic species can be explained solely by switches from the oligolectic to the polylectic habit and by shifts of oligoleges between different plant taxa. Three of four transitions from oligolecty to polylecty are accompanied by a reduction in bee body size. The significance of this size reduction with respect to the polylectic habit is discussed. The oligolectic anthidiine species visit significantly fewer flower species for pollen during a single foraging bout than the polylectic species. On average, 1.4 plant species were recorded in the loads of specialists compared to 2.2 for generalists. Two monophyletic groups of bees belonging to the genus Anthidium are equipped with a pollen—collecting apparatus consisting of specialized hairs localized either on the face or on the underside of the thorax. It is used to remove pollen from the raised anthers of flowers of the Labiatae and the Scrophulariaceae and to brush pollen from the flat inflorescences of some Compositae, respectively. The observation of flower—visiting females of several anthidiine species revealed that pollen uptake is far from an accidental process. Basic pollen—harvesting techniques have reached a high degree of efficiency: females of a given bee species worked flowers of a certain plant species in a fixed manner, the structures used for pollen uptake from the same flower type were found to be largely the same among anthidiine bees of different taxonomic groups, and no distinct differences with respect to the basic pattern of pollen removal from flowers of the same architecture were obvious when comparing oligolectic and polylectic species. On the other hand, polyleges showed a high intraspecific flexibility regarding the organs used for pollen uptake from flowers of a different architecture.

ISSN:0012-9615    

DOI:10.2307/2963476

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY