bluuebunny/crossref_metadata_2025 huggingface dataset:
Fire Affects Ecophysiology and Community Dynamics of Central Wisconsin Oak Forest Regeneration
Authors: Reich Peter B. , Abrams Marc D. , Ellsworth David S. , Kruger Eric L. , Tabone Tom J.
Published: May 2006

In order to understand better the ecophysiological differences among competing species that might influence competitive interactions after, or in the absence of, fire, we examined the response to fire of four sympatric woody species found in intermediate—sized gaps in a 3—yr—old mixed oak forest in central Wisconsin. Selected blocks in the forest were burned in April 1987 by a low—intensity controlled surface fire. The fire had significant effects during the following growing season on community structure, foliar nutrient concentrations, and photosynthesis. Acer rubrum seedling density declined by 70% following the fire, while percent cover increased several—fold in Rubus allegheniensis. In general, leaf concentrations of N, P, and K were increased by the fire in all species, although the relative enhancement decreased as the growing season progressed. Daily maximum photosynthetic rates were 30—50% higher in burned than unburned sites for Prunus serotina, Quercus ellipsoidalis, and R. allegheniensis, but did not differ between treatments for A. rubrum. Mean sunlit photosynthetic rates and leaf conductances were stimulated by the burn for all species, with the greatest enhancement in photosynthesis measured in Q. ellipsoidalis. Leaf gas exchange in R. allegheniensis was most sensitive to declining leaf water potential and elevated vapor pressure gradient, with Q. ellipsoidalis the least sensitive. Fire had no discernable effect on water status of these plants during a year of relatively high rainfall. In comparison with other species, A. rubrum seedlings responded negatively after fire–both in terms of survival/reproduction (decline in the number of individuals) and relative leaf physiological performance. Fire enhanced the abundance of R. allegheniensis and the potential photosynthetic performance of R. allegheniensis, P. serotina, and particularly Q. ellipsoidalis. We conclude that post—fire stimulation of net photosynthesis and conductance was largely the result of enhanced leaf N concentrations in these species.
Killer Factor as a Mechanism of Interference Competition in Yeasts Associated with Cacti
Authors: Ganter Philip F. , Starmer William T.
Published: May 2006

Many yeasts in the genus Pichia are killers (yeasts that secrete proteins toxic to other yeasts). The genus is subdivided into complexes comprised of species with similar morphologies, physiological abilities, and percentage of (guanine + cytosine) comprising their nuclear DNA. Here, we test the hypothesis that ecological factors influence the types of toxins produced by members of two Pichia complexes (the Pichia kluyveri and Pichia opuntiae complexes). Members of each complex are separated by geography or host range, but all live in decaying cactus tissue. Similarities among killer toxins were investigated by testing strains from each complex in the laboratory for their ability to kill a standard set of 70 yeasts. Principal components analysis demonstrated that the killer phenotype was constant within a species, but not within the complexes. The probability that a toxin would kill a particular yeast depended on some ecological characteristic of the yeasts tested as sensitives (i.e., the region, host plant, or habitat from which the yeasts were collected). We further tested the hypothesis that ecological factors influence killer phenotype by analyzing within—community patterns of killing. Killer strains from two different cacti were tested for their ability to kill a subset of the yeasts actually collected from the same pockets of rotting cactus tissue from which the killer strains were collected or from rot pockets in neighboring cacti. When the yeasts tested for sensitivity were classified a priori by their probability of interacting with each killer strain, it was shown that the sensitivity to killer toxin was correlated with the probability of interaction. Further, the yeast community composition of a rot pocket depended on the presence or absence of a killer strain in that pocket. We demonstrated that killer toxins could change community composition by growing strains of two sensitive yeasts in the presence of a killer or a non—killer strain of P. kluyveri. The killer strain reached a higher cell density than the non—killer strain in the presence of sensitive yeasts. The density of sensitive yeasts grown with a killer strain was lower than that of a strain grown with a non—killer strain of P. kluyveri.
Nutritional Ecology of an Insect Host‐Parasitoid Association: The Pea Aphid‐Aphidius Ervi System
Authors: Sequeira Richard , Mackauer Manfred
Published: May 2006

Evolutionary host—size models assume that host quality for parasitoid growth and development is a linear function of host size. To test this assumption, we compared the growth patterns of the hymenopteran parasitoid Aphidius ervi when it developed in different nymphal instars of apterous pea aphids, Acyrthosiphon pisum. At daily intervals, unparasitized aphids and aphids that were parasitized at age 24, 48, 72, and 120 h (corresponding to nymphal instars L1—L4) were dissected. We weighed parasitoid larvae and host remains to determine changes in their relative growth rates, based on wet mass (WM) and dry mass (DM). The growth in DM of unparasitized aphids followed a sigmoid curve. Aphids that were parasitized by A. ervi continued to grow for 5—7 d before their DM started to decline. Trajectories of the parasitoid's growth during its larval and pupal stages could be described by nonlinear equations. The maximum larval DM and the time for oviposition to adult eclosion of parasitoids varied with host age at parasitization, and thus suggested differences in host quality. We proposed that, for parasitoids developing in growing and feeding stages of the host, host quality is not a linear function of host size at parasitization but reflects the degree of integration of the two systems.
Resource Partitioning in Bumblebees: Experimental Studies of Foraging Behavior
Authors: Inouye David W.
Published: May 2006

A system comprising 2 species of bumblebess (Bombus appositus and Bombus flavifrons) and 2 species of flowers (Delphinium barbeyi and Aconitum columbianum) in Gothic, Colorado, USA, was manipulated to determine whether resource utilization by each bumblebee species was influenced by the presence of the other species of bumblebee. Each bumblebee species concentrated its foraging efforts on a different flower species, apparently choosing the species whose corolla tube length matched its proboscis length most closely. When each bumblebee species was temporarily removed from its preferred flower species, the remaining bumblebee species increased visitation to the other, previously less—utilized, flower species. The remaining bumblebees visited more flowers per stay in the patch, suggesting that they were finding greater amounts of nectar in the absence of other bumblebee species. These removal experiments demonstrated that the bumblebees were sampling flowers frequently enough and were flexible enough in their foraging behavior to respond rapidly to short—term changes in nectar availability. In another area, where its preferred flower species and the other bumblebee species were absent, B. flavifrons foraged actively on the flower species it rarely used in Gothic. This observation and the experiments demonstrate that resource utilization by a bumblebee species is influenced by the presence of other species and suggest that the phenomenon of competitive release can be observed in bumblebees. In this system,interspecific exploitation competition appears to be the primary mechanism involved in resource partitioning.
Macrofossil and Pollen Representation of Coniferous Trees in Modern Sediments From Washington
Authors: Dunwiddie Peter W.
Published: May 2006

Pollen and conifer needle macrofossils were collected from surface sediments in 30 ponds near Mount Rainier to study representation of forest trees in data from modern sediments. Percentages of major taxa calculated from these data were compared with basal area measures of forest composition within 30 m of each pond. Macrofossil percentages of Abies amabilis, Abies lasiocarpa, Pseudotsuga menziesii, Tsuga heterophylla, and Tsuga mertensiana were significantly correlated with their basal areas in nearby forests; none was greatly over— or underrepresented. Macrofossil assemblages were similar in replicate samples from surface sediments, as well as from parallel cores spanning 6000 yr. Fossil needle assemblages can therefore provide estimates of past species composition in coniferous forests in the Pacific Northwest. Also, widespread distribution of pollen was demonstrated by high pollen percentages of taxa that were absent from the flora at the samples sites, such as Alnus rubra. Pollen of Pinus and Tsuga heterophylla was overrepresented, whereas that of Abies and Tsuga mertensiana was underrepresented. Pollen percentages of most conifer taxa were twice the values reported in another study from Mount Rainier using moss polsters. Comparisons between the macrofossil and the pollen data indicate that conifer macroffossil assemblages more accurately reflect forest composition near sample sites, and also provide greater taxonomic resolution.
The Relative Importance or Snow Avalanche Disturbance and Thinning on Canopy Plant Populations
Authors: Johnson E. A.
Published: May 2006

Snow avalanche return intervals on two avalanche paths in the southern Canadian Rockies were estimated from scarred trees and shrubs. The interval between avalanches increased exponentially down each path. The tree and shrub diameters at which avalanches could cause breakage were predicted using both the mechanics of large delections in tapered beams and the resulting stem bending stress, and these predictions were confirmed by comparison to actual diameters at breakage on avalanche paths. Lodgepole pine (Pinus contorta) and Engelmann spruce (Picea engelmannii) bend when small, but were broken by avalanches when larger than °6 cm in diameter at the base. Glandular birch (Betula glandulosa) and willow (Salix glauca) never grew large enough to break. Breakage was influenced by size rather than wood elasticity of strength. Information on thinning mortality was reconstructed from live and dead stems in two Engelmann spruce populations and one lodgepole pine population. Dead trees wer cross—dated, using ring widths, to determine time of death. Avalanche mortality of trees was more important than thinning mortality when the average interval between avalanches was <150 yr. The shift from shrub— to tree—dominated growth habit down the avalanche path occurred when the average interval between avalanches was less than 15 to 20 yr.
Predator Avoidance in Seagrass Meadows: Prey Behavior, Microhabitat Selection, and Cryptic Coloration
Authors: Main Kevan L.
Published: May 2006

Behavioral responses to predators may influence distribution and abundance patterns of prey. Earlier predator—preference experiments and analyses of natural diets revealed that the caridean shrimp Tozeuma carolinense is underrepresented in the diet of pinfish, Lagodon rhomboides, the dominant predatory fish in marine seagrass meadows in the southeastern USA. I examined the influence of prey behavioral responses, microhabitat shifts, and cryptic coloration on prey accessibility to Lagodon with a combination of field observations and laboratory experiments. Tozeuma's behavior and microhabitat choice were extremely similar in the field and the laboratory and were tightly coupled to the seagrasses Tozeuma inhabits. In the presence of predatory fishes, Tozeuma increased the time spent in some otherwise rare behaviors. In particular, laboratory experiments revealed that the behavioral response of moving around the grass blade, to which a shrimp was clinging, resulted in a significant increase in shrimp survival, and thus is strongly adaptive. In addition, Tozeuma shifted microhabitats in response to predators; individuals that did not respond were often consumed. For the shrimp, predator—avoidance behaviors were far more important than cryptic coloration in eluding predatory fish. The presence of a physical structure alone did not necessarily provide protection for shrimp from these visual predators; the combination of active predator—avoidance behaviors and the presence of a structurally complex, opaque substrates resulted in a visual barrier between predator and prey. Such prey behavior—microhabitat links can be more important than physical interference of the habitat as factors affecting foraging success of visual predators.
Competition and the Role of Habitat Selection in Gerbillus Allenbyi and Meriones Tristrami: A Removal Experiment
Authors: Abramsky Z. , Sellah C.
Published: May 2006

In northern Israel, two seed—eating gerbiline rodents exhibit exclusive distributions. Gerbillus allenbyi is the only species inhabiting coastal sand dunes between Tel Aviv and Mt. Carmel. Meriones tristrami is also found in the same coastal region, but there it is restricted to nonsandy habitats. However, in the coastal area north of Mt. Carmel where G. allenbyi does not occur, M. tristrami inhabits dunes. This type of distribution is usually interpreted to result from competition. A perturbation experiment conducted in the narrow transition strip between soil and sand in an area where the above species are sympatric revealed no ongoing competition between the two species. It is suggested that strong interspecific competition occurring in the past resulted in genetically determined habitat selection in the present.
Seasonal Changes in Tissue Water Relations of Three Woody Species of the Quercus‐Carya Forest Type
Authors: Parker William C. , Pallardy Stephen G. , Hinckley Thomas M. , Teskey Robert O.
Published: May 2006

Tissue water relations of white and northern red oaks and mockernut hickory were studied during the growing season of 1979. In all species, osmotic potentials at full saturation and turgor loss point decreased during the period of leaf maturation; subsequently, osmotic potential responded to soil moisture availability. Estimates of the bulk modulus of elasticity and the relative water content at the turgor loss point revealed that white oak possessed less elastic leaf tissue than did either northern red oak or hickory. The bulk leaf pressure potential associated with the initiation of stomatal closure was lower in white oak and hickory (0.2 MPa) than in northern red oak (0.4 MPa) and remained seasonally constant, while total leaf water potential associated with stomatal closure was lower during periods of drought as osmotic potential decreased. The drought—tolerating behavior of white oak is consistent with its frequent occurrence and success in xeric habitats. Northern red oak and mockernut hickory exhibited responses more typical of drought—avoiding species, which would result in sustained turgor—mediated processes essential for growth and high competitive ability at moderate moisture stresses characteristic of more mesic habitats.
Analysis of Interferences Between C(3) and C(4) Grasses in Relation to Temperature and Soil Nitrogen Supply
Authors: Christie Edward K. , Delting James K.
Published: May 2006

The replacement series experimental design and mathematical procedures of de Wit were used to study the influence of soil nitrogen supply and temperature on the growth, in monoculture and in mixture, of two North American grassland species, Agropyran smithii (a C3 pathway species) and Bouteloua curtipendula (C4). At 20/12°C day/night temperatures, direct competition for soil nitrogen resulted in lowered growth of Bouteloua when growth in mixtures with Agropyran. However, at 30/15° day/night temperature the situation reversed, in that the growth of Agropyron was restricted when grown in the presence of Bouteolua, through competition for nitrogen. Based on the relative crowding coefficient, competitive ability is more closely related to temperature and the photosynthetic pathway type than soil nitrogen supply. The relative crowding coefficient for Agropyron was much greater at 20/12° than 30/15°, whereas values for Bouteloua were greatly depressed at 20/12° compared with 30/15°. Analysis of the competitive and noncompetitive aspects of plant interference occurring in this study is also made. Although differences in nitrogen uptake and use occurred between these two species, the growth differential at two temperatures would result in uneven utilization of the soil nitrogen resource on a seasonal basis. There was little evidence to support the postulate that differences in nitrogen—use efficiency between C3 and C4 pathway plants may be advantageous for C4 plants only.
The Effects of Reduction in Trout Density on the Invertebrate Community of a Mountain Stream
Authors: Allan J. David
Published: May 2006

An experimental reduction in trout density was carried out for 4 yr to determine whether the numbers or species composition of aquatic invertebrates would be affected. In a Colorado stream, the standing crop of trout (mainly Salvelinus fontinalis) prior to this study was 4.86 g/m2, typical of infertile trout streams. Repeated electroshocking of a 1220 m long experimental section kept trout stocks at 10—25% of this initial value during the summers of 1975—1978. Density of invertebrates in the benthos (number of invertebrates per square metre) showed no consistent differences between the trout removal section and upstream and downstream control sections for the majority to taxa examined. Drift density (number of invertebrates per 1000 cubic metres) also failed to show an effect of the trout removal. Some taxa were significantly more abundant at one site than another, but this was attributable to changes along the stream gradient or differences in phenology among the sites rather than experimental reduction of trout. As trout stomach content analysis revealed very intensive grazing of a few taxa aquatic insects, their failure to increase in the trout removal section was unexpected. Statistical analysis showed that because of the variability of the system, a doubling or halving of numbers must occur to be detectable. Within these limits, it is concluded that removal of trout had no significant effect on the prey community, and two explanations are offered. Trout may actually consume only a small fraction of total prey, although this conclusion is limited by inability of current techniques to sample the benthos fully. It is also likely that because streams do not provide areas where fish are predictably absent, the invertebrate community is highly adapted to fish predation and so is not sensitive to manipulations in fish density.
Benthic Invertebrate Responses to Size and Density Manipulations of the Common Mummichog, Fundulus Heteroclitus, in an Intertidal Salt Marsh
Authors: Kneib R. T. , Stiven A. E.
Published: May 2006

Most studies of the effects of predation on the organization and structure of prey communities have ignored predator population size structure and density. In the present investigation, we designed a series of field experiments to test the effects of these predator population properties on the intertidal benthic invertebrates of a salt marsh. The common mummichog, Fudulus heteroclitus, was the primary predator in our experiments. Three size—classes of mummichogs (small, 40—50) mm Total Length [TL]: medium, 50—69 mm TL; large >70 mm TL) were maintained at each of three densities (1, 2 and 4 fish/m2) in 10—m2 enclosures on the salt marsh. Water—filled pits were included in each cage as low—tide refuges for the fish. Each fish treatment combination was replicated three times. In addition, there were three uncaged 10—m2 plots (open controls) and three caged plots from which Fundulus were excluded (closed controls). Three cylindrical core samples (0.005 m2) were extracted from the sediment to a depth of 10 cm in each of the 33 experimental plots 1, 4, and 10 mo after initiation of the experiment in June 1978. Densities of the small, surface—dwelling invertebrates which make up most of the benthic community (Manayunkia aestuarina, Streblospio benedicti, Leptochelia rapax, ostracods, hydrobiid snails, and Gemma gemma) were highest in enclosures containing large fish, and lowest in closed controls and in enclosures containing small fish. Intermediate invertebrate densities occurred in enclosures containing medium—sized mumichogs. The effects of fish size were enhanced at high fish densities (2—4 fish/m2), although fish density alone had little effect on invertebrate abundance. We considered several hypotheses that could explain the findings. Small epibenthic predators—disturbers like the grass shrimp, Palemonetes pugio, were commonly observed in the experimental enclosures. The gut contents of fish taken from enclosures indicated that the shrimp were ingested only by medium and large size—classes of F. heteroclitus. We suggest that infaunal densities increased in response to the control of small epibenthic predator—disturbers (i.e., P. pugio) by the larger F. heteroclitus.
Temporal Variation in Birds and Fruits Along an Elevational Gradient in Costa Rica
Authors: Loiselle Bette A. , Blake John G.
Published: May 2006

We studied temporal fluctuations in fruit production by plants and in populations of understory fruit—eating birds at three elevations (50, 500, and 1000 m) in Costa Rican wet forests over a 12—16 mo period to investigate effects of resource variation on bird movements and community structure. We used mist nets to monitor changes in frugivore abundance, migration patterns, and breeding and molting cycles. We sampled understory fruits of each forest concurrent with studies of frugivores. Both frugivores and fruit exhibited considerable seasonal variation in abundance. Peak frugivore capture rates occurred during peak periods of ripe fruit abundance. Altitudinal migrants left lower montane (1000 m) forest during periods of fruit scarcity and wee present in lowland (50 m) and foothill (500 m) forest when ripe fruit was abundant. Migrants, both altitudinal and temperate, accumulated fat before migration, and perhaps (for altitudinal migrants) in anticipation of breeding. Some residents also put on fat before breeding. Breeding was seasonal at all forests and occurred when ripe fruit abundance was low. Results of this study indicate that birds may track changes in resource abundance. Thus, variation in resource abundance influences dynamics of bird communities, both in terms of species composition and abundance. Further, results illustrate the importance of viewing communities from different scales; dynamics at a local scale (e.g., one elevation) can be influenced by changes in conditions (e.g., fruit abundance) elsewhere. That some species regularly moved along elevational gradients implies that preservation of many species and of the biotic integrity of entire systems may require conservation of large, connected blocks of suitable habitat.
Reversion of Forest to Tundra in the Central Yukon
Authors: Cwynar Les C. , Spear Ray W.
Published: May 2006

Pollen and plant macrofossil analyses of sediments from three sites in the central Yukon that are presently in shrub tundra provided a record of former forest establishment. Shrub tundra with groves and gallery forest of balsam poplar occupied the region between 10 000 and 8000 BP. At 9400 BP white spruce (Picea glauca) populations expanded, and open white spruce woodlands persisted until 6500 BP when black spruce (Picea mariana) and green alder (Alnus viridis) populations increased, resulting in open spruce woodlands with a distribution of species probably similar to that commonly found today in the northern boreal forest: white spruce on drier south—facing slopes and on alluvial sites with balsam poplar, and black spruce on colder, wetter sites on north—facing slopes and valley bottoms. At 5000 BP forest began to revert to shrub tundra, abruptly at first but then more gradually. The modern groves of spruce in the region, which are mostly white spruce, therefore are probably relict populations surviving in favorable microsites. These results support the conclusion from other recent studies that Alaska and northwest Canada experienced warmer summers than today from as early as 10 000 BP to °6000 BP in response to increased summer insolation arising from changes in the earth's orbital parameters as predicted by the Astronomic Theory of climate change. The decline of forest after 5000 BP is not associated with any significant changes in the abundance of shrub or herb pollen types, suggesting that the ground vegetation has behaved independently of the tree populations.
Effects of Temperature and Substrate Quality on Element Mineralization in Six Arctic Soils
Authors: Nadelhoffer K. J. , Giblin A. E. , Shaver G. R. , Laundre J. A.
Published: May 2006

We compared the effects of temperature on rates of microbial respiration, N mineralization, nitrification, and P mineralization in soils from six arctic ecosystems located along a toposequence on Alaska's North Slope. Soils from these ecosystems were incubated aerobically in the laboratory for 13 wk and at temperatures representative of field values during a typical growing season. Rates of C and N mineralization were insensitive to temperature between 3° and 9°C but increased by factors of 2 or more between 9° and 15°. For both C and N, differences in mineralization rates among soils were greater than differences due to incubation temperature within single soils. This suggests that the quality of soil organic matter varies widely among these ecosystems and is more important than soil temperature differences in controlling rates of these processes in the field. Nitrification occurred in all soils, even at 3°, but there were large differences among soils in nitrification potentials. Overall differences in P mineralization between soils were small. Rates of P mineralization, however, decreased with increasing temperature in soils from some sites and increased with temperature in others. Carbon respired during the 13—wk incubations ranged between 1.5 and 8% of total soil organic C across soil types incubation temperatures. In contrast to the relatively high C mineralization rates in these soils, net N and P mineralization rates were very low and were likely due to high microbial demand for these nutrients. High microbial demand for mineral nutrients can severely limit plant N and P availability in arctic soils.
Leaf Litter and Larval Mosquito Dynamics in Tree‐Hole Ecosystems
Authors: Fish Durland , Carpenter Stephen R.
Published: May 2006

Larvae of Aedes triseriatus mosquitoes feed on microbes that decompose leaf litter in tree—hole ecosystems. Scanning electron micrographs indicate that browsing by mosquitoes substantially reduces microbial abundance on decaying leaves. Experiments using laboratory microcosms demonstrate that increased larval density decreases larval survivorship, pupation rates, pupal biomass, and total yield. Rapidly decomposing leaf litter (sugar maple) supports more mosquito growth than slowly decomposing litter (beech and black oak). In our experiments, mosquito yield was apparently regulated by larval density and detrital dynamics.
Resource Overlap and Competition in Pond Snails: An Experimental Analysis
Authors: Brown Kenneth M.
Published: May 2006

Several ponds were quantitatively sampled in northeastern Indiana, and the relative abundances of pulmonate snails were determined. Feeding preferences, on several natural food types, were estimated in feeding trials in the laboratory. The two main objectives of the study were to determine (1) which species pair had the highest resource overlap along the habitat and food dimensions of the niche, and (2) whether these species actually suffered decreased growth or fecundity due to competition. Sampling data and feeding trials indicate that Aplexa hypnorum is a specialist in both habitat and food use, and is a detritivore in heavily wooded, temporary ponds. Lymnaea elodes is more common in partially wooded, temporary ponds, and specializes on periphyton and carrion. Physa gyrina has a broad utilization of both the habitat and food dimensions of the niche. Helisoma trivolvis is a generalist in food preference, but was common in this study only in a permanent pond. These data, and niche overlap estimates based on them, predict that L. elodes and P. gyrina should have the greatest overlap on resources. Manipulative field experiments, in three different ponds, suggest that high overlap along the habitat and dietary dimensions of the niche results in competition in these freshwater snails. Interspecific competition decreased fecundity in P. gyrina, but not in L. elodes, in two separate experiments. For both species, interspecific competition had less of an effect than increased conspecific densities on growth rates. Physa generally did better in the less productive, temporary ponds, while L. elodes had higher fecundities and growth rates in a permanent, more productive pond. Lymnaea elodes was considered the better competitor because interspecific competition had smaller effects on growth and reproduction than intraspecific competition. Data on size—specific reproductive rates suggest that the earlier reproducing P. gyrina may be able to exploit more ephemeral habitats as refuges.
Local Geographic Distributions of Bumblebees Near Crested Butte, Colorado: Competition and Community Structure
Authors: Pyke Graham H.
Published: May 2006

It was hypothesized that the local geographic distributions of bumblebees near Crested Butte, Colorado and the community patterns exhibited by these bumblebees are the products of competition for plants. To evaluate this hypothesis several transects were established and at regular intervals throughout a summer growing season, data were collected along each transect on how many bumblebees of each species and caste were visiting the various plant species. The distributions and abundances of the plant species involved were recorded qualitatively. Seven species of bumblebees accounted for 97% of all bumblebees observed and in this paper attention is restricted to these species. Five other species were observed in very low numbers. Each bumblebee species had a different distributional pattern. The seven bumblebee species form four groups in terms of both their proboscis lengths and the corolla lengths of the plants they preferentially visit. Long—, medium—, and short—tongued groups were most often observed foraging at flowers with long, medium, and short corollas, respectively. Furthermore proboscis lengths of bumblebees tended to be very similar within each group but quite dissimilar between groups. The fourth group consisted of a single short—tongued species which has well—developed mandibles which enable it to rob nectar from many plants with long corollas. It also feeds legitimately on short—corolla flowers. Some anomalies in the above groupings are mentioned and discussed. When the data on bumblebees and plant distributions are combined with data on flower preferences, a pattern consistent with the competition hypothesis emerges. Within each proboscis—length group, bumblebee species tend to replace one another altitudinally in a manner consistent with the hypothesis. The nectar—robbing species is most abundant in areas where a plant that is usually visited by hummingbirds is most common. Other species of bumblebees are unable to gain access to the nectar of this plant. In any uniform well—isolated area, similar floristically to the present study area, only three or four species of bumblebees appear to be able to coexist. Furthermore, a bumblebee community in such an area will apparently consist of a short—, a medium—, and a long—tongued species and in some cases a short—tongued nectar—robber. These patterns are also consistent with the competition hypothesis, as similarity in proboscis length reflects similarity in diet and the intensity of competition should, for these bumblebees, be closely related to diet similarity. It is possible that the observed distributional patterns could also be explained on the basis of different distributions of suitable nest sites for each bumblebee species or different responses to local variations in climatic conditions. Neither alternative seems able, however, to explain the observed patterns.
An Experimental Analysis of the Food Location Behavior of Frugivorous Bats
Authors: Fleming Theodore H. , Heithaus E. Raymond , Sawyer William B.
Published: May 2006

Models of optimal foraging often separate the time and energy expended in getting to a feeding area (commuting costs) from the time and energy expended in searching for food (search costs) in animals that are "pure searchers." We predict that under certain circumstances (e.g., a relatively uniform resource distribution) searching and commuting behavior will be synchronous, whereas under other circumstances (e.g., a relatively patchy resource distribution) these two behaviors will be temporally separated. We have tested these predictions by studying the foraging and food location behavior of several species of Costa Rican frugivorous bats using mist—netting programs, radiotelemetry, and fruit relocation experiments. Concurrent observations were made on the phenology and distribution patterns of the six fruit species used in the relocation experiments. The mist—netting program allowed us to determine the food habits of the common frugivorous bats in our study area and to identify the major chiropteran dispersal agents of the six fruit species. The radio—tracking program provided detailed information on the foraging behavior of one of the most important seed dispersal agents, Carollia perspicillata, whose individuals commute up to 2.7 km from a central day roost before beginning to feed. Results of the fruit relocation experiments, in which ripe fruits of the six species were individually placed on "fruits poles" located either close to or at a considerable distance (>0.5 km) away from conspecific plants in areas known to be used by bats, indicated the following: (1) fruits of two species of Piper, which are highly preferred by Carollia and which occur in low nightly densities for extended periods of time, has as high (°.90) a probability of being found away from conspecific plants as when they were near conspecifics; (2) fruits of two species of Ficus and Muntingia calabura, which are very patchily distributed in time and/or space, has a significantly higher probability (.30—.50 vs. .02—.12) of being found near rather than away from conspecifics; and (3) when placed on poles with Piper fruits, fruits of Ficus ovalis, Chlorophora tinctoria, and Muntingia calbura had a significantly higher probability (.40—.60 vs. .08—.13) of being found than when placed in similar areas without Piper fruits. These results indicate that certain bats (e.g., Carollia perspicillata and Glossophaga soricina) are constantly "on the alert" for ripe Piper fruits while commuting, whereas bats that eat Ficus, Chlorophora, or Muntingia fruits (e.g., G. soricina or Arbibeus jamaicensis) are not constantly "on the alert" for these fruits while commuting. Bats feeding on Piper probably search and commute simultaneously, whereas those feeding on Ficus or Muntingia separate commuting and searching behavior. The food location behavior of frugivorous bats appears to be highly responsive to differences in the spatiotemporal distribution patterns of their food resources.
Water Relations and Water use Patterns for Artemisia Tridentata Nutt. In Wet and Dry Years
Authors: Campbell Gaylon S. , Harris Grant A.
Published: May 2006

Plant H2O relations and soil moisture depletion and recharge were followed in a stand of Artemisia tridentata near Washtucna, Washington during 1973 and 1974. Precipitation during the 1972—73 recharge season was 14.5 cm, 11 cm below normal. The 1973—74 precipitation was 35.7 cm, or 10 cm above normal. The 2 yr were therefore ideal for comparing plant behavior on wet vs. dry years. Soil moisture was depleted to around —70 bars in 1973 and —60 bars in 1974 to depths of 2.5 m. Leaf H2O potentials were —10 bars in the spring and decreased to —50 to —60 bars in the summer of 1973. In 1974, summer leaf H2O potential was —30 bars. Osmotic potentials were around —20 bars in the spring of 1973 and throughout the spring and summer of 1974. In the summer of 1973 osmotic potentials dropped to —60 bars. Stomatal diffusive conductances of 0.5 cm/s (resistance of 2 s/cm) were common during the spring and summer of 1974 and the spring of 1973. Summer conductances in 1973 were .05 cm/s (resistance of 20 s/cm). A simple model is proposed which predicts evaporation and transpiration from daily precipitation and potential evapotranspiration. Model parameters included soil hydraulic properties and maximum transpiration rate as a function of available soil moisture. The agreement between model prediction and measured values is within the uncertainties imposed by the input data. The model predicts 6.5 and 8.2 cm of evaporation for 1973 and 1974 respectively. In 1973 this was half of the total precipitation received. In 1974 it was only one fourth of the total.
Competition Between the Great Tit and the Blue Tit Outside the Breeding Season in Field Experiments
Authors: Dhondt Andre A. , Eyckerman Roman
Published: May 2006

Great and Blue Tits coexist in the same preferred habitat. To test the hypothesis that during winter these two titmice compete we excluded Great Tits from nestboxes in two areas. In both experimental areas Blue Tit breeding numbers increased significantly, whereas in a control area no increase was observed. We provide evidence that in winter, competition between Great and Blue Tits is for roosting sites, the Great Tit excluding Blue Tits. The two species may also compete for food. We conclude that Blue Tit breeding population size is limited by interspecific competition with the Great Tit in winter.
Competition for Bumblebee Pollinators in Rocky Mountain Plant Communities
Authors: Pleasants John M.
Published: May 2006

Meadoew plant communities at four sites in the Colorado Rocky Mountains were studied to evaluate the influence of competition for bumblebee pollinators. The niche relationships among plant species were characterized by overlap with respect to species of bumblebee visitors and times of flowering. Species in each community could be divided into two to three guilds based upon the identity of the species' major bumblebee visitor. Only the members of a guild would potentially be in competition with one another for pollinators. The competition hypothesis is that guild members have segregated blooming periods which would minimize competition. The null hypothesis is that blooming periods are dispersed randomly through time. The competition hypothesis was tested by comparing the actual temporal spacing pattern for a guild with random patterns generated by computer simulation. For 10 out of 11 guilds the sequence of blooming periods was more regular than expected (either at the statistically significant level [five cases] or near it [five cases]). I also examined the relationships among guild members using another measure of the competitive effect of one species on another which is more accurate than temporal overlap. This variable, called competitive load, includes information on the overlap, abundance, and floral attractiveness of competitors. It is an indicator of the number of visits a species loses to competitors. In general species were found to lose approximately half the number of visits they could potentially receive during the time they were in flower. All species received approximately the same number of visits despite large differences in floral abundance. Thus the pool of available pollinator visits appears to be rather evenly divided among the species in a community. This regularity in resource partitioning is further inferential evidence of competition for pollinators. Rare species avoid the potentially heavy loss of visitors to more abundant competitors by having flowers which are more attractive to pollinators. Attractiveness appears to be a function of the nectar production rate of a species' flowers. There are some exceptions to these generalizations. Species which can also reproduce vegetatively are less attractive, and lose more visits that expected. The competitive effect that on species experiences because of the presence of another may be the result of a loss of pollinator visits (exploitation) and/or a disruption of conspecific pollen flow (interference). Species may reduce the interference component of competition by spatial isolation. For the one guild that had a random flowering sequence with broad overlap among species, spatial isolation and competition between guild members were positively correlated. Temporal divergence and differential attractiveness are seen to be primarily a means of avoiding exploitation competition.
The Distribution and Origin of Mima Mound Grasslands in San Diego County, California
Authors: Cox George W.
Published: May 2006

In North America, Mima—type earth mounds are found west of the Mississippi River from southern Canada to northern Mexico. They occur on poorly drained soils with a shallow basement layer or permanent water table. In San Diego County, California, Mima mounds are found at the edge of coastal salt marshes above a shallow water table, on coastal marine terraces having a cemented, clay—capped hardpan, on foothill slopes and stream—cut terraces over shallow bedrock, and in cismontane valleys, mountain meadows, and the margins of desert—edge marshes where a dense subsurface clay horizon creates poor drainage. Major hypothesis of mound orgin involve (1) frost—sorting, (2) erosion, (3) wind disposition, and (4) fossorial rodent actitivy. At Miramar Mounds National Landmark in San Diego, soil samples (1980 cm3) were collected from the tops, edges, and neighboring basin centers of 10 mounds for analysis of small rock content. Soil plugs containing metal markets were inserted in pocket gopher tunnels at mound edges, and the movement of these markers traced with a metal detector. Mounds on a 0.58—ha plot were mapped and their dimensions measured. Contray to all but the fossorial rodent hypothesis, gravel and small pebbles that such rodents are able to move were concentrated in mound soils. Clearing of the experimental soil plugs by pocket gophers was accompanied by a significant moundward translocation of mined soil. The spacing of mounds tended towards uniformity, but intermound distance increased with increased significantly with increase in mound size. The distribution of mound fields in San Diego County corresponds closely to the distribution of original valley grassland inferred for a 10—yr fire cycle. It is hypothesized that Mima mounds are an indicator of original grassland environments.
Size‐Selective Predation on Gammarus Pseudolimnaeus by Trout and Sculpins
Authors: Newman Raymond M. , Waters Thomas F.
Published: May 2006

Length distributions of the stream amphipod Gammarus pseudolimnaeus were obtained from analysis of the stomach contents of age I and older brook (Salvelinus fontinalis), brown (Salmo trutta), and rainbow trout (S. gairdneri) collected from July 1979 to May 1980, and of sculpins (Cottus cognatus) and young—of—year trout. Length distributions of Gammarus in benthic and drift samples were also analyzed for comparison with stomach—content data. For all sample dates, length of Gammarus in the stomachs was significantly greater than in the benthic samples. Mean and median sizes in the drift were usually smaller than in the benthos, indicating that length distributions in the drift did not directly account for the length distributions of Gammarus in the stomachs. Age I and older trout generally consumed more Gammarus in the day than at night, and those consumed at night tended to be larger. The occurrence of larger Gammarus in the night drift than in the day drift and an observed lower exploitation rate at night support Allan's drift/predation risk hypothesis. Size selective predation by sculpins and young—of—year trout was significant but less pronounced than in age I and older trout. Although there was seasonal differences in the lengths of Gamarus in fish stomachs as well as in the drift and benthos, size—selective predation occurred throughout the year. Size—selective predation may have its greatest impact on Gammarus populations in the winter, when density and growth rates are low.
Effects of Bud Demography and Elongation Patterns on Betula Cordifolia Near the Tree Line
Authors: Maillette Lucie
Published: May 2006

Because plants acquire their structure through the activity of their meristems, whether meristems die, remain dormant, multiply, or develop inflorescences has distinct morphological implications. Studies of bud demography have the potential to explain the form of many temperate—zone trees. In this paper, I present field data collected on tree—line mountain white birches (Betula cordifolia Regel) whose buds were censused over a period of 1—3 yr. Birches ranged in shape from fully erect trees (7—8 m tall) in sheltered forest sites, to low polycormic shrubs (< 1 m tall) in nearby exposed tundra sites. Three questions were asked. (1) Is size merely an expression of age? (2) Is shoot elongation limited by (a) environmental factors, (b) low levels of meristematic activity, or (c) excessive allocation to reproduction? (3) Do bud and branch mortality vary from site to site? If so, is that variation a result of (a) herbivory or (b) climatic factors? Reproduction and browsing were not significant factors contributing to the reduced size of tundra birches. These birches however, did produce smaller long shoots and suffered heavier bud mortality during winter than birches at the other sites, with important year—to—year fluctuations, and thus parts of questions 2a and 3b were answered. After exposure to an extreme winter, most birches can resprout from dormant buds in the root collar area and form polycormic individuals. Both the frequency and severity of winter injury were related to the degree of exposure to the elements, and appear to be key factors in the morphogenesis of tree—line birches.
Defensive Regurgitation of Allelochemicals Derived From Host Cyanogenesis By Eastern Tent Caterpillars
Authors: Peterson Steven C. , Johnson Nelson D. , LeGuyader John L.
Published: May 2006

Eastern tent caterpillars regurgitate a droplet of enteric fluid when attacked by predatory ants. In laboratory bioassays, the regurgitated fluid of caterpillars fed cyanogenic young foliage of black cherry was more repellent to ants than was regurgitated fluid of caterpillars fed foliage that was less cyanogenic. Chemical analysis showed that hydrogen cyanide (HCN) and benzaldehyde, produced during host cyanogenesis, were present also in the regurgitated fluid. In bioassays with pure solutions, ants tolerated the odor of HCN but were repelled by benzaldehyde. This evidence indicates that incorporation of host—derived benzaldehyde into regurgitated fluid protects tent caterpillars from ant predation. The use or labile, plant—derived compounds in enteric discharges may be an important, but generally overlooked, insect defense.
Regulation of Bald Eagle Reproductive Rates in Southeast Alaska
Authors: Hansen Andrew J.
Published: May 2006

I examined the influence of food abundance and habitat quality on reproduction of Bald Eagles (Haliaeetus leucocephalus) in the Chilkat Valley, Alaska. The proportion of active nests was greatest and the timing of laying earliest in portions of the study area where food was most abundant. Clutch size was not related to food availability. Offspring survival was higher in breeding areas where food was experimentally provided than in control areas. Whether nests were active was also related to 2 of 11 habitat features measured at breeding areas. One habitat variable was associated with nest success. The findings suggest that variable productivity and a surplus of non breeders are the norm in the region as a result of high variability of annual food supplies and strong intraspecific competition. The results further suggest that the decline of this species in other parts of North America was at least partially due to alteration of eagle food sources and habitats by humans.