bluuebunny/crossref_metadata_2025 huggingface dataset:
Amino Acid Concentrations in Extrafloral Nectar of Impatiens Sultani Increase after Simulated Herbivory
Authors: Smith Lisa L. , Lanza Janet , Smith Garon C.
Published: May 2006

In many plants, extrafloral nectars attract ants that protect the plant from herbivory. Previous work indicates that both sugars and amino acids contribute to ant attraction and that herbivory may affect nectar production or composition. This study was designed to measure the effect of defoliation on sugar and amino acid concentrations in the extrafloral nectar of Impatiens sultani. Twenty—nine fall—raised plants and 45 spring—raised plants were subjected to varying degrees of defoliation in which 25, 50 or 75% of each leaf was removed. Extrafloral nectar was collected immediately before and 24 h after defoliation for the fall—raised plants; additionally, nectar was collected 24h before and 48 and 72 hr after defoliation in the spring—raised plants. Nectars were analyzed for sugar and amino acid content by means of high—pressure liquid chromatography (HPLC). Twenty four hours after defoliation, amino acid content increased dramatically in plants subjected to all three levels of defoliation' amino acid concentrations of nectars 72 h after defoliation were indistinguishable from predefoliation levels. No change in sugar concentrations or nectar volumes was detected. Additional work is needed to determine if the increase in amino acid is adaptive and serves to attract more plant protectors.
Patch‐Size Effects on Early Succession in Southern Appalachian Forests
Authors: Phillips Donald L. , Shure Donald J.
Published: May 2006

Four sizes of forest opening (0.016, 0.08, 0.4, 0.4, and 2.0 ha; two replicates each) were established in a Southern Appalachian forest to examine the effects of disturbance size on earl successional community structure and function. Solar radiation, soil temperature, and air temperature were all higher in large openings than small openings and increased from edge to center of disturbance patches. Aboveground net primary productivity (NPP) was 3—4 times as highe in larger (2.0 ha) as small (0.016 ha) openings, presumably in response to greater light availability in large patches. Stump and root sprouts of tree species accounted for the largest fraction of NPP in all patch sizes. Herbs, vines, shrubs, advance regeneration trees, and tree seedlings had progressively smaller NPP, respectively. Vegetation biomass reached 0.7—2.6% of undistributed forest levels and aboveground NPP reached 17—58% of forest levels by the 2nd yr after cutting. Plant species richness was generally higher in large than small patches. Tree species composition shifted considerably followed disturbance. Liriodendron tulipifera was important before and after logging. Large canopy dominants such as oaks and hickories were relatively unimportant sources of sprouts during early revegetation. Instead, minor canopy and understory species such as Robinia pseudoacacia, Halesia carolina, Acer rubrum, Cornus florida, and Magnolia fraseri were the major sprouters in all patch sizes. The N—fixing black locust (Robinia) was much more important in large than small openings. Disturbance size within the Southern Appalachians thus affects microenvironment, species composition, and NPP during early revegetation.
The Reproductive Consequences Associated with Inducible Alkaloidal Responses in Wild Tobacco
Authors: Baldwin Ian T. , Sims Cynthia L. , Kean Sybil E.
Published: May 2006

We experimentally examined the phenotypic cost (as measured by the effect on seed set) associated with an inducible alkaloidal response in Nicotiana sylvestris (Solanaceae). Alkaloid concentrations were partially uncoupled from leaf damage in two experiments with the use of two techniques: (1) applying auxins to damaged leaf edges, and (2) minimizing the damage to veins of the leaf. Both of these techniques inhibited the induced response. In both experiments with flowering—stage plants, damaged plants with elevated alkaloids contents had lower reproductive output than similarly damaged plants with low alkaloid contents. Plants were able to compensate for damage inflicted during the rosette stage; reproductive output of damage plants did not differ from that of control plants. In addition, we found a significant negative correlation between seed number and leaf alkaloid content in undamaged full—sib plants. While these results are consistent with the hypothesis that the alkaloidal response to damage is associated with diminished reproductive output, and decrement in reproductive output cannot be directly attributed to alkaloid production because other unmeasured responses to damage may accompany the alkaloidal response to damage and thereby confound these measures of phenotypic cost. Last, we examined the consequences of altering leaf alkaloid concentrations, within the range found during the induced response, on rates of carbon gain and found no effect in tobacco.
Interference Competition in Postmetamorphic Salamanders: Interspecific Differences in Aggression by Coexisting Species
Authors: Walls Susan C.
Published: May 2006

I examined whether interference competition occurred among the terrestrial juveniles of two pond—breeding salamanders, Ambystoma maculatum and A. talpoideum. In a laboratory experiment, I observed intra— and interspecific agonistic behavior between pairs of recently metamorphosed individuals by staging encounters between resident salamanders (i.e., possessors of a refugium) and (1) conspecific, (2) heterospecific, and (3) surrogate (control) intruders. Resident A. maculatum exhibited significantly more threat and submissive behavior to live intruders than to surrogates, with more agonistic behavior direct toward conspecifics than to heterospecifics. Intruders likewise exhibited significantly more threat and submissive behavior to conspecific residents than to heterospecific residents. In contrast, neither resident nor intruding A. talpoideum altered their behavior in response to differing types of opponents. These results indicate that interference competition may operate in the terrestrial environment. Further, the agonistic behavior of terrestrial Ambystoma is the antithesis of that of the larvae of these two species, which suggests an ontogenetic switch in behavior at metamorphosis. These data are the first indication of such a shift in competitive abilities in amphibians. This behavioral switch may sustain coexistence between these two species by counterbalancing the competitive advantage of one species in the aquatic stage with that of the other species in the terrestrial stage.
Insect Attraction to Ultraviolet‐Reflecting Spider Webs and Web Decorations
Authors: Craig Catherine L. , Bernard Gary D.
Published: May 2006

The foraging performance of any predator is dependent on its ability to locate prey. All spiders produce silks and many locate insects by producing silk traps. We measured the reflective properties of silk produced by primitive, non—web—weaving spiders and derived aerial web spinners. We found that primitive spiders produce silks that reflect ultraviolet (UV) light and primitive aerial web weavers spin UV—reflecting catching silks that attract Drosophila. Derived, web—spinning spiders in the genus Argiope, however, produce catching silks that exhibit low reflectivity in the UV and, in fact, reflect little light at all. Nevertheless, Argiope decorate their webs with bright, UV—reflecting bars and crosses that attract prey. We found that more insects were intercepted per hour by decorated webs with spiders than by undercorated webs from which the spider had been removed. In addition, within—web analyses showed that when only half of a web was decorated, more insects were intercepted by the decorated halves than the undecorated web halves. We propose that UV—reflecting decorative silks, together with the UV—reflecting body surfaces of A. argentata, act as a visual display that attracts prey.
An Experimental Examination of Nest‐Site Segregation by Two Peromyscus Species
Authors: Dooley James Lawrence , Dueser Raymond D.
Published: May 2006

The role of interspecific competition for nest sites in the coexistence of the white—footed mouse (Peromyscus leucopus noveboracensis) and the cloudland deermouse (Peromyscus maniculatus nubiterrae) was experimentally investigated in the montane forests of southwestern Virginia. Radiotelemetry was used on control grids, with both species present, and reciprocal removal grids to contrast nest—site selection with and without contraspecifics present. On control grids, Peromyscus maniculatus used only arboreal nests while P. leucopus used both ground and arboreal nests. In addition, relative to P. maniculatus, those P. leucopus which did select arboreal nests, used lower cavities and trees of smaller diameter at breast height (dbh). These results provide strong evidence for vertical nest—site segregation, and confirmed observations from previous studies. Though both species demonstrated shifts in nest selection between treatments in the removal experiment, the analysis of the P. maniculatus response was statistically suspect and both responses appeared to be ecologically insignificant (that is, shifts were not towards the type of nest site used by the removed species). Additionally, home range analysis and examination of nest availability suggest that large portions of suitable habitat on each grid may not have been used and that nest sites were not a limiting resource. We suggest, therefore, that interspecific competition for nest sites was probably not important to co—existence during the course of the study.
Forest Succession in Relation to River Terrace Development in Olympic National Park, Washington
Authors: Fonda R. W.
Published: May 2006

The floodway zone of the Hoh River exhibits four terrace levels of different ages, formed by erosional activity of the river on valley fills. The vegetation in this valley is in a long—term seral sequence as shown by the zonal pattern in relation to aging and development of these land surfaces. Succession starts on gravel bars, which are dominated by Alnus rubra and Salix scouleriana. The following sequential forest communities, and associated ages of land surfaces, are found: Alnus rubra on alder flats (80—100 yr); Picea sitchensis—Acer macrophyllum—Populus trichocarpa on first terraces (400 yr); Picea sitchensis—Tsuga heterophylla on second terraces (750 yr); and Tsuga heterophylla on third terraces. The latter represents the climax community for the river terrace sere, and it occurs on surfaces exposed by retreating Pleistocene alpine glaciers. The first three terraces are derived from Neoglacial alluvial fills. There is a strong correlation among zonation patterns, forest succession, age of terraces, soil moisture, and soil profile development. Available soil moisture is an important factor governing the zonal sequence. The younger land surfaces are significantly drier than the older terraces. Plants on alder flats and first terraces must withstand greater moisture stress than those of second and third terraces. As the land surface ages, the soil profile develops; deeper, more mature soils are found away from the river. The term "Olympic rain forest" is inappropriately applied to this vegetation; "temperate moist coniferous forest" is more appropriate not only for forests in the Hoh Valley, but also for the rest of the Olympic Mountains and vegetation along the northern Pacific coast.
Intraspecific Competition between Drosophila Larvae in Serial Transfer Systems
Authors: Gilpin Michael E.
Published: May 2006

Experiments showed the overriding importance for population regulation of larval competition as compared to oviposition. A dominance hierarchy exists based on larval age (or size). The mechanism of the larval competition is not simple exploitation, but must also involve some component of interference; these may be labeled, respectively, K and α strategies.
Heavy Metal Tolerance in Typha Latifolia without the Evolution of Tolerant Races
Authors: McNaughton S. J. , Folsom T. C. , Lee T. , Park F. , Price C. , Roeder D. , Schmitz J. , Stockwell C.
Published: May 2006

Clones of the broad—leaved cattail (Typha latifolia L.) and soil samples were obtained from near a zinc smelter and from a control location. In the smelter location, soil zinc concentration was 385 times higher, cadmium content 37 times higher, and lead content 16 times higher. No evidence for the evolution of heavy metal tolerance could be detected in 2 x 2 experiments in which genotypes from both locations were grown on both soils. Growth of genotypes from both locations was inhibited on the heavy metal soil, but not to the extent that would be expected from previous studies of heavy metal effects. This is the first case described in which a species has been able to colonize heavy metal soils in the absence of the evolution of tolerant races.
Character Displacement and Biogeography of the Pygmy Shrew in Northern Europe
Authors: Malmquist Mats G.
Published: May 2006

Two ecologically similar shrew species, Sorex minutus and S. araneus, were studied in sympatry on the Swedish mainland and in allopatry on Irelend, the Outer Hebrides, and two islands in the Baltic Sea, in order to examine competitive character displacement in skull morphology. Eight skull measures were compared on a total of 228 specimens. S. minutus had significantly smaller jaws in sympatry. It was concluded that this difference is a result of interspecific competition; alternative explanations were tested and rejected. S. araneus did not exhibit morphological differences between sympatric and allopatric populations. The difference in response of the two species, it was concluded, is due to their different relative abundance in sympatry, which implies an asymmetry in their competitive effect on one another. It is suggested that late—Quaternary zoogeographic patterns during periods of low sea level can explain the present distribution of allopatric pygmy shrew populations.
Silica as a Defense against Herbivory and a Growth Promotor in African Grasses
Authors: McNaughton S. J. , Tarrants J. L. , McNaughton M. M. , Davis R. D.
Published: May 2006

Grasses and dung were collected in the Serengeti National Park and analyzed for silica content by wet ashing. Grasses from grasslands differing in the grazing intensities experienced were grown in the laboratory in a factorial experiment to determine factors controlling tissue silicification. Concentrations of silica in tissues of plants collected in the field were higher than have been reported for any other plants abundant in grazing ecosystems. Silica contents in the field were higher in more heavily grazed grasslands and in tissue produced earlier in the growing season. Animal dung contained substantial quantities of silica. Laboratory experiments indicated that tissue silicification was increased by defoliation, was higher in plants from more heavily grazed grasslands, varied in different organs and species in patterns confirming current hypotheses about plant defense, and was affected by the availability of soluble silica in the nutrient medium. Silica in the nutrient medium promoted the yield of unclipped plants substantially. Total yield was 18% higher than that of control plants, although hydroponic experiments with solutions prepared and handled in plastic indicated that silica was not a growth requirement, except, perhaps, at the ecologically unrealistic concentrations that might result from reagent contamination. Yield stimulation by silica was differentially distributed among organs, tending to promote photosynthetically active tissues and crowns. Flowering of one species was promoted by silica. Leaves of silica—fed plants were larger. Leaf blade chlorophyll concentrations were 15% higher in silica—fed plants from the more heavily grazed grasslands. The results suggest that complex patterns of grass silicification had a role in the radiation of grazing animals and grasses and may contribute to maintaining the biotic diversity of contemporary grassland—savanna ecosystems by influencing the partitioning of forage species and organs among grazers. Growth promotion by silica may be due to the substitution of mineral support for carbon—based support associated with the deposition of silica in the intercellular spaces of aerial tissues. Since soils of the Serengeti region commonly have pH levels above neutrality, where the availability of silica is low, silica supply could influence primary productivity and resultant energy and nutrient flow through the trophic web in the native environments of the plants.
Individualistic Growth Response of Tundra Plant Species to Environmental Manipulations in the Field
Authors: Chapin F. Stuart , Shaver Gaius R.
Published: May 2006

In undisturbed arctic tussock and wet meadow tundras we increased air temperature with a plastic greenhouse, increased nutrient availability by NPK fertilization, and decreased light intensity with shade cloth to determine the factors limiting growth of tundra plants. After 2 yr of these manipulations we measured growth of each major vascular species and one moss species. Each species showed a different pattern of growth response to alteration of light, air temperature, and nutrient regimes, indicating that no single factor limits growth of all species in these communities. Growth of canopy species (Betula nana, Ledum palustre, Carex bigelowii, and Eriophorum vaginatum) was reduced by experimental shading more than was growth of understory species (e.g., Vaccinium vitisidaea and Rubus chamaemorus). Species typical of nutrient—rich sites (Betula nana, Rubus chamaemorus, and Polygonum bistorta) generally responded more to nutrient addition than did species typical of nutrient—poor sites (e.g., Empetrum nigrum), although there were species characteristic of fertile sites (Salix pulchra) and infertile sites (Ledum palustre) which did not show this pattern of nutrient response. Species that grow in warm hollows between tussocks showed less growth in response to increased air temperature than did canopy species. We suggest that lack of a single common factor limiting growth of all species in tussock and wet meadow tundras implies that (1) each species is individualistically distributed, as described by the continuum model of community organization, (2) as a result of competition and/or distinct evolutionary histories, the growth of each species is limited by a different combination of environmental factors, and (3) production by individual species varies greatly from year to year, but production by the whole vegetation is more stable, because years that are favorable for growth of some species cause a compensatory decrease in growth of other species.
Water Relations and the Maintenance of Sierran Conifers on Hydrothermally Altered Rock
Authors: DeLucia Evan H. , Schlesinger William H. , Billings W. D.
Published: May 2006

Unusual, nutrient—poor soils derived from hydrothermally altered bedrock support open forests of Sierran conifers (Pinus ponderosa, P. jeffreyi, and others) amongst the sagebrush and pinyon—juniper vegetation on typical desert soils of the Great Basin in western Nevada. As an index of soil moisture availability, we compared predawn xylem pressure potential (XPP) and diurnal patterns of stomatal conductance for various species at three sites, with paired, adjacent soils derived from unaltered and hydrothermally altered parent material. At high soil moisture availability, Great Basin shurbs maintained higher maximum leaf conductance (2—3 mm/s) than Sierran conifers (<1.2 mm/s). During the growing season XPP declined for all species at all sites, but during extended drought values for Great Basin shrubs (—2.70–4.47 MPa) growing on unaltered soil were consistently lower than for Sierran conifers (—0.97_—1.12 MPa) growing onaltered soil. While few species were found on both soil types, Amelanchier alnifolia showed lower predawn XPP on soils derived from unaltered (—3.22 MPa) than from altered (—2.05 MPa) parent paterials. These data suggest that shrubs rapidly deplete soil moisture in the Great Basin desert, restricting the establishment of Sierran conifers to soils derived from altered rocks, which otherwise support minimal vegetation cover. The @°13C of foilage from conifers were higher than from Great Basin shrubs, suggesting that conifers maintain higher seasonally integrated water—use efficiencies.
Biphasic Mass Gain in Migrant Hummingbirds: Body Composition Changes, Torpor, and Ecological Significance
Authors: Carpenter F. Lynn , Hixon Mark A. , Beuchat Carol A. , Russell Robert W. , Paton David C.
Published: May 2006

Body mass of migrant Rufous Hummingbirds (Selasphorus rufus) on refueling stopovers increased on average from 3.2 to 4.6 g over a period ranging from several days to 3 wk. In birds arriving with body masses below °3.5 g, the initial period of mass gain was very slow. This slow gain was not explained by energy costs associated with territory establishment or learning to secure food, since it occurred even in years when nectar resources were superabundant and territoriality was nearly nonexistent. Data on body composition indicate that mass gain up to °3.5 g was due to deposition of nonlipid body components, which we hypothesize to be proteins involved in rebuilding muscle catabolized during the last stage of the recent migratory flight. Following the initial phase of slow mass gain, an accelerating rise in body mass consisted entirely of lipid gain. On average, overnight mass loss decreased prior to migration, suggesting that nocturnal torpor facilitated lipid deposition. The slow phase of mass gain is potentially important constraint on migrating hummingbirds, because if they deplete their fat stores and allow their body mass to fall 3.5 g, they incur a substantial cost in terms of greatly increased time spent on the subsequent stopover.
Relationships between Contemporary Pollen and Vegetation Data from Wisconsin and Michigan, USA
Authors: Bradshaw R. H. W. , Webb Thompson
Published: May 2006

Scatter diagrams and regression analysis of paired pollen and tree—inventory data show how pollen percentages represent the percent basal area for the major arboreal genera in Michigan and Wisconsin. We show that the relationship between pollen and tree percentages for each taxon is generally similar for two states of comparable size and similar vegetation (Wisconsin and Michigan), but that the relationship is influenced by the size of the pollen—collecting site and the size of the area surveyed for trees around each site. These results provide information concerning the relative size of the pollen—source area for seven arboreal pollen types: Pinus, Quercus, Betula, Tsuga, Ulmus, Fagus, and Acer, listed in descending order of pollen—source area. Moderate—sized lakes (30—150 ha) accumulate significant quantities of Pinus and Quercus pollen produced farther than 30 km away, but accumulate relatively few Fagus grains from >4.5 km, and even fewer Acer grains from >2.3 km. The source areas for Betula, Tsuga, and Ulmus pollen lie within 30 km of each lake, and significant quantities of these grains travel farther than 4.5 km. Regression analysis of data from basins of different size supports the hypothesis that small basins collect their pollen from a smaller area of the surrounding vegetation than do large basins.
Plasticity in Yield Components in Natural Populations of Three Species of Sesbania
Authors: Marshall D. L. , Fowler N. L. , Levin D. A.
Published: May 2006

To discover whether plasticity in resource allocation differs among related species, we examined variability in seed mass, seeds per fruit, ovules per flower, fruit survivorship, and fruits per unit plant size in Sesbania macrocarpa, S. drummondii, and S. vesicaria. The species differed in the amounts of variability of the different components of yield, and seed mass was not always the least variable component of yield. The components of yield differed significantly among years, sites, and plants. Differences among plants in size, density of surrounding plants, distance to the nearest neighbor, and density of surrounding fruits had little measurable effect on the components of yield. Differences in the position of fruits on single plants affected seed mass in S. drummondii and S. vesicaria and seeds per fruit in S. vesicaria. The components of yield showed a negative relationship among species, but were only weakly correlated within species, probably because environmental and genetic effects on the yield components acted in different directions. The differences in plasticity of yield components among species probably reflected differences among the species in response to stresses such as herbivory and drought, but may also have reflected their different life histories (annual vs. perennial). Given that each response pattern carries a set of consequences, natural selection may have acted to produce a different, least costly response to stress in each species.
Density‐Dependent Growth, Body Condition, and Demography in Feral Donkeys: Testing the Food Hypothesis
Authors: Choquenot David
Published: May 2006

The food hypothesis of population regulation was tested using two predation—free populations of feral donkeys in tropical northern Australia. Growth, body condition, and age—specific rates of fecundity and mortality were compared between the two populations. One of the populations was roughly one—half the density of the other, following an experimental reduction 3—4 yr previously. Fecundity was high and independent of density, with >70% of mature females pregnant in both populations. Age at maturity and juvenile mortality displayed some density dependence. Populations conformed to predictions of the food hypothesis, abundance being ultimately regulated by food—related juvenile mortality. At high density, juvenile mortality was three times as great as at low density, and was associated with poorer juvenile body condition and slower growth. Differences in rates of juvenile mortality are associated with nutritional status of lactating females, which is in turn determined by density—dependent competition for quality forage. The effects of competition for forage upon juvenile mortality synchronize lactation with an annual flush of quality forage at the onset of the monsoonal wet season.
Daytime Energy Management by Tropical Hummingbirds: Responses to Foraging Constraint
Authors: Tiebout Harry M.
Published: May 2006

Because hummingbirds are extremely sensitive to energy stress, yet often face conditions when energy intake is reduced or energy expenditures must be increased, they should have flexible energy budgets. I experimentally measured the ability of two syntopic tropical hummingbird species to manage daytime energy budgets under conditions simulating natural foraging constraint, defined here as low nectar secretion rate, low flower density, or both. I tested a territorial species, Amazilia saucerottei, and a traplining species, Chlorostilbon canivetii, individually in a large flight cage at two rates of food delivery (HIGH = ad libitum, LOW = 64% of ad libitum) and two perch—to—feeder distances (NEAR = 4 m, FAR = 20 m). Both species increased flight time when distance was increased (FAR > NEAR by 44%) and when rate of food delivery was reduced (LOW > HIGH by 23%). However, birds on the HIGH food delivery rate did not increase food intake to compensate for increased flight expenditures to the FAR feeder. Birds on LOW food maintained total rates of energy expenditure that were less than birds on HIGH food, and LOW food birds experienced no effect of feeder distance on expenditure. These results appear to be due primarily to short—term reductions in perching metabolic rates (PMRs) by birds on LOW food or with the FAR feeder. Reduced daytime PMRs helped to mitigate the potentially negative impacts of foraging constraint: rates of mass loss of the LOW food birds were an estimated 48% less than if birds had maintained normal PMRs. Reduced PMRs also enabled birds visiting the HIGH FAR feeder to maintain rates of mass gain equal to unconstrained birds (HIGH NEAR feeder). Relative differences between species in energy management reflected their foraging models. The low—reward trapliner paid the greatest energy costs at LOW food, regardless of distance, due to its overall tendency for high flight time. In contrast, the territorialist did poorly in the HIGH FAR treatment, due to flight expenditures associated with defending a rich but dispersed resource. Although individual birds exhibited considerable flexibility in energy management, the foraging mode represented by each species appeared to be energetically specialized for particular levels of food availability and dispersion.
Inorganic Particles Alter Competition in Grazing Plankton: The Role of Selective Feeding
Authors: Kirk Kevin L.
Published: May 2006

Suspended sediments (clay and silt particles) differentially inhibit cladoceran populations, but not rotifer populations, and can change the outcome of competition between rotifers and cladocerans in favor of rotifers (Kirk and Gilbert 1990). This paper provides a mechanistic explanation for the population and community effects of suspended clay. Feeding experiments with radioactively labeled phytoplankton cells (Cryptomonas) showed that the presence of suspended clay (<2—@mm particle size) significantly decreased the phytoplankton ingestion rates of five cladoceran species (Ceriodaphnia dubia, Daphnia ambigua, D. galeata mendotae, D. magna, and D. pulex) by 13—83% but not those of three rotifer species (Keratella cochlearis, K. crassa, and Synchaeta pectinata). When radioactively labeled phytoplankton cells and clay particles were offered simultaneously, cladocerans were less selective for phytoplankton over clay than were rotifers. Thus, cladocerans ingested more suspended clay particles than did rotifers, and this caused the reductions in phytoplankton ingestion rates. Smaller cladocerans were less selective for phytoplankton over clay than were larger cladocerans, and their phytoplankton ingestion rates were thus reduced more by clay. Rotifers fed more selectively than either large or small cladocerans, were able to avoid ingesting clay particles, and therefore were not inhibited by suspended clay. Thus, differences in the feeding modes and selectivities of rotifers and cladocerans form the mechanistic basis for the change in competitive outcome in favor of rotifers observed in the presence of suspended clay. This mechanism may form the basis for changes in the relative abundance of rotifers and cladocerans in turbid lakes and reservoirs.
Role of Nutrient Cycling and Herbivory in Regulating Periphyton Communities in Laboratory Streams
Authors: Mulholland Patrick J. , Steinman Alan D. , Palumbo Anthony V. , Elwood Jerry W. , Kirschtel David B.
Published: May 2006

In this study we examined the role of nutrient cycling and herbivory in regulating stream periphyton communities. Population, community, and ecosystem—level properties were studied in laboratory stream channels that had nutrient inputs reduced compared to channels where ambient nutrient levels were maintained. We reduced nutrient inputs in four of eight channels by recirculating 90% of the flow, whereas the other four channels received once—through flow of spring water. We examined the interaction between herbivory and nutrients by varying the number of snails (Elimia clavaeformis) among streams with different nutrient input (circulation) regimes. Reduction in nutrient input viar recirculation resulted in lower concentrations of nutrients in the water but did not result in significant differences in biomass, carbon fixation, or algal taxonomic composition. However, herbivory had large effects on these characteristics by reducing biomass and areal rates of carbon fixation and simplifying periphyton taxonomy and physiognomy. Lower rates of nutrient input significantly affected characteristics associated with nutrient cycling. Streams with reduced nutrient inputs had lower periphyton nutrient contents, higher ratios of total : net uptake of P from water, and higher rates of phosphatase activity than streams with ambient nutrient inputs. However, the effects of reduced nutrient input on cycling characteristics were reduced or eliminated by intense herbivory. Our results indicate that nutrient cycling can increase in response to reductions in nutrient input once sufficient biomass or detritus accumulates to facilitate cycling, thereby allowing biomass accrual and carbon fixation rates to be maintained at levels achieved at higher nutrient inputs. In streams with low nutrient input, cycling can meet a large fraction of the nutrient demand of the periphyton. Because our experimental design involved a reduction from ambient nutrient levels rather than the usual enrichment techniques (e.g., additions to water, nutrient—diffusing substrates) in aquatic ecosystems, we have been able to observe how biological structure and processes are influenced by external nutrient inputs at the low natural levels of most undisturbed streams.
Experiments on Mechanisms of Tree Establishment in Old‐Field Succession: Seedling Survival and Growth
Authors: De Steven Diane
Published: May 2006

Mechanisms influencing the relative establishment success of early successional tree species in North Carolina Piedmont old fields were investigated in multi—factor field experiments. Six trees species (Acer rubrum, Fraxinus americana, Liquidambar styraciflua, Liriodendron tulipifera, Pinus taeda, and Ulmus alata) were sown into old fields at equivalent densities. In this paper I evaluate relative seedling performance (survival and growth) with respect to two principal factors: (1) competition from old—field vegetation (weeded vs. vegetated plots) and (2) browsing by vertebrate herbivores (exclosures vs. open plots). Manipulation of the first factor tests Connell and Slatyer's (1977) alternative "models" of succession. Seedlings were monitored over a 3—yr period in each of two experiments, one initiated in a wet spring (1984) and the other in a dry spring (1985). Most seedling mortality occurred in the first growing season. In the 1984 experiment, old—field vegetation reduced survival of all species; however, competitive reductions were not large except in Liriodendron, which appeared unable to survive in herbaceous cover, and in Acer, which survived poorly in the absence of competition as well. In the other four species, survival in old—field vegetation was similar; after high mortality in the first year, seedlings exhibited continued slow mortality with time. The dry spring of 1985 resulted in greater overall mortality in all species and greater variation in species responses to vegetation cover. In contrast to survival, height growth was strongly reduced by herbaceous vegetation in all hardwood species, but not in Pinus, which grew fairly well in competition with the herbaceous community. Characterization of these responses as tolerance vs. inhibition is complex and requires consideration of long—term demographic behavior. Three species (Fraxinus, Liquidambar, and Ulmus) were damaged by vertebrate browsers, but browsing intensity varied according to site and was most severe in Ulmus, with the result that its survival and growth were selectively reduced. The expected relative prevalence of these hardwood species, based on their emergence success and seedling performance in the experiments, does not fully correspond to their apparent relative occurrence in young Piedmont hardwood communities. A significant factor that may account for the discrepancies is differential seed rain, which can numerically offset low rates of seedling survival. Patterns of seed production by the hardwood species appear consistent with the hypothesis.
Internal Cycling of Nitrate in Soils of a Mature Coniferous Forest
Authors: Davidson Eric A. , Hart Stephen C. , Firestone Mary K.
Published: May 2006

Gross rates of N mineralization, immobilization, and nitrification were measured by 15N isotope dilution in a 10—yr—old conifer plantation and in a mature conifer forest. Gross rates revealed nutrient cycling characteristics that differ from expectations based on more common measures of net rates. Although net mineralization rates were somewhat higher in the young forest than in the old forest, gross mineralization rates in the old forest were 2—3 times as high as gross mineralization rates in the young forest, indicating more rapid turnover of inorganic—N pools in the old forest. Net mineralization rates were <14% of gross mineralization rates. Smaller NO3— pool size and lower net nitrification rates in the old forest than the young forest might lead to the conclusion that the old forest is a non—nitrifying ecosystem and that nitrate is important only in the N cycle of the young forest. However, gross nitrification rates were similar in both young and old forests. Microbial assimilation of NO3— was also significant in both forests, indicating a rapid turnover of a small but important NO3— pool. Microbial assimilation may be an important pathway for NO3— retention in forest ecosystems.
Ecomorphology of Insectivorous Bats: A Test of Predictions Using Two Morphologically Similar Species
Authors: Saunders Matthew B. , Barclay Robert M. R.
Published: May 2006

Wing morphology and echolocation call design have been used to make predictions regarding habitat—use and resource partitioning in insectivorous bats. Species with very different morphologies and/or calls forage in different habitats or in different ways. However, bat communities contain many morphologically similar species, which would be predicted to overlap ecologically. We tested the reliability of such predictions and assumptions by investigating the habitat use and diet of two similar species, the little brown bat (Myotis lucifugus) and the long—legged bat (Myotis volans), in southern Alberta, Canada. We recorded the echolocation calls of free—flying individuals and measured wing morphology. The echolocation calls of the two species overlapped in design, although on average the calls of M. volans were shorter and had higher maximum frequency than those of M. lucifugus. Both species had low wing loading and aspect ratio although M. lucifugus had significantly higher values than did M. volans and was thus expected to forage in areas with less environmental clutter. Field observations did not substantiate this, however. We assessed the foraging ecology of the bats by: (1) monitoring species—specific echolocation calls of free—flying individuals, (2) capturing bats in mist—nets set in various habitats, (3) observing the foraging behavior of light—tagged individuals, and (4) assessing the diet from fecal samples. There was no evidence of temporal partitioning of resources by the two species, but some spatial partitioning occurred. Although individuals of both species generally fed in open areas, M. lucifugus foraged primarily low over water while M. volans fed high above the ground and along cliff edges. Neither fed consistently in cluttered areas despite wing and echolocation call designs that should permit such behavior. Despite the overlap in foraging areas, the diets of the two species differed significantly. Myotis volans ate primarily lepidopterans while M. lucifugus ate a wider variety of prey, including dipterans, lepidopterans, and coleopterans. Our results suggest that there are several levels at which morphology influences the ecology of insectivorous bats. Large differences may influence foraging—habitat choice by restricting some species to certain habitats. At another level, however, we suggest that small differences influence prey availability rather than habitat availability. Bats may select habitats based on the distribution of suitable prey rather than due to direct limitations imposed by morphology or echolocation call design.
Effects of Piscivore‐Mediated Habitat Use on Resources, Diet, and Growth of Perch
Authors: Diehl Sebastian , Eklov Peter
Published: May 2006

We investigated experimentally how presence or absence of different piscivores influenced habitat use, diet, and individual growth of two size classes of juvenile perch (Perca fluviatilis) and abundances of juvenile perch resources in different habitats. Pond enclosures encompassed 3 x 6 m of dense vegetation and 9 x 6 m of open habitat, and were stocked with 75 young—of—year and 30 1—yr—old perch and with either three piscivorous perch, three northern pike (Esox lucius), or no piscivores. Northern Pike were both larger and possessed a larger gape than piscivorous perch. To isolate behavioral responses of juvenile perch to piscivores, we replaced consumed young—of—year perch. Prey fish mortality was lowest without piscivores and highest with northern pike. Young—of—year perch increased their use of vegetation in the presence of both piscivores, whereas 1—yr—old perch increased their use of vegetation only with northern pike. For both age classes of prey fish, increased use of the vegetation led to reduced individual growth, owing to two complementary mechanisms. First, the physical complexity of submerged macrophytes likely interfered with the benthic feeding of perch. Second, increased use of the (relatively small) vegetated habitat increased the mean density experienced by prey fish. Piscivore—induced changes of prey fish densities in the two habitats had substantial effects on the biomass of prey fish resources in the open habitat, but only minor effects in the vegetation. Sialis lutaria, the major predatory macroinvertebrate (°50% of total macroinvertebrate biomass in the open habitat), and total predatory macroinvertebrates were positively affected by piscivores in the open habitat, but not in the vegetation. Chironomids (<3% of total macroinvertebrate biomass in the vegetation) and the sizes of nonpredatory macroinvertebrates were negatively affected by piscivores in the vegetation, but not in the open habitat. Biomass of nonpredatory macroinvertebrates, Cladocera, and Copepoda did not differ among treatments in either habitat. From our review of field experiments, vulnerable prey fish always change their habitat use in the presence of piscivores. Behaviorally mediated indirect effects of piscivores on individual growth rates and prey fish resources were just as frequently observed as direct effects of piscivores on prey fish survival.
Cold Hardiness and Overwintering Strategies of Hatchlings in an Assemblage of Northern Turtles
Authors: Costanzo Jon P. , Iverson John B. , Wright Michael F. , Lee Richard E.
Published: May 2006

Field and laboratory studies were conducted during 1989—1994 to investigate the overwintering strategies of hatchling turtles representing four families native to western Nebraska. Whereas hatchling snapping turtles (Chelydra serpentina) and spiny soft—shelled turtles (Apalone spinifera) overwinter in aquatic habitats, yellow mud turtles (Kinosternon flavescens) and ornate box turtles (Terrapene ornata) burrow below the natal nest and hibernate in sandy soil. Painted turtles (Chrysemys picta) overwinter within their shallow natal nests, but this species, and T. ornata, tolerate extensive tissue freezing. Overwintering behaviors of these species are consistent with indices of physiological cold hardiness and patterns of geographic distribution. Frost commonly penetrated and persisted below 10 cm, the soil depth at the which hatchling C. picta routinely hibernate. Field and laboratory data suggested that hatchling C. picta survive either by remaining supercooled (unfrozen) or by tolerating tissue freezing, the strategy employed depending on prevailing physiological and microenvironmental conditions. Whereas relatively lower temperatures can be survived in the supercooled state, supercooling capacity may be limited via the inoculation of body fluids by environmental ice. Alternatively, whereas freeze tolerance fortuitously is promoted by ice inoculation, this strategy may be viable only at relatively high subzero temperatures. A cold—hardness strategy based on both survival mechanisms may promote winter survival in hatchling C. picta by conferring protection under dynamic physiological and microenvironmental conditions. Physiological cold hardiness and behavior are integrated determinants of the northern distributions of temperate region turtles.
Effects of Needle Architecture on Warbler Habitat Selection in a Coastal Spruce Forest
Authors: Parrish Jeffrey D.
Published: May 2006

Habitat associations of mobile foragers are well documented, but causes behind such patterns are seldom as well defined. Black—throated Green Warblers (Dendroica virens) attain greater densities in red (Picea rubens) than in white spruce (P. glauca) forests in coastal Maine and use red spruce foliage significantly more than predicted on the basis of availability. These habitat associations may be explained best by small—scale differences in vegetation structure, which can influence foraging efficiency. Red spruce have shorter needles held at more acute angles to woody stems than do white spruce. I predicted that D. virens prefers red spruce because of ease of perching and foraging on needles with this structure. Moreover, red spruce preferences should be exaggerated in moist vegetation, since white spruce needle structure and arrangement may wet and reduce plumage insulation capabilities more quickly. Three aviary experiments were performed with D. virens to test the role of structural differences in vegetation on foraging—site and perching—site selection: wetting of vegetation, decreasing white spruce needle angle to resemble red spruce, and reduction of white spruce needle length to approximate red spruce. Moistening of red spruce branches did not affect the foraging or microhabitat use of experimental warblers. Needle angle reduction did not change perching—site preferences, but foraging significantly shifted in favor of white spruce. Needle length manipulations resulted in significantly greater use of white spruce than in controls, for both perching and foraging substrates. The relationships found in this study between individual habitat selection and fine—scale features of habitat structure are consistent with the larger—scale patterns of habitat occupancy seen in this system, and have important implications in understanding habitat requirements for declining forest passerine populations. _kw Dendroica virens