bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of Mineral Nutrition on Delayed Inducible Resistance in Alaska Paper Birch
Authors: Bryant John P. , Reichardt P. B. , Clausen T. P. , Werner R. A.
Published: May 2006

In subarctic forests, birch (Betula) trees respond to severe (50—100%) manual defoliation by delayed inducible resistance (DIR). This plant response to defoliation is characterized by a decline in the nutritional quality of leaves for immature insects for several years after defoliation events, and concomitant changes in leaf chemistry that may be detrimental to insect nutrition, that is, a decline in leaf nitrogen and an increase in leaf phenols. Two explanations of delayed inducible resistance have been proposed. (1) The active defense response hypothesis claims that delayed inducible resistance is an active response to defoliation per se rather than merely a passive consequence of recovery from the stress of severe defoliation. (2) In contrast, the carbon—nutrient balance (CNB) hypothesis claims that delayed inducible resistance is caused by nutritional stress resulting from severe defoliation. We used two experiments to test these hypotheses. (1) In a three—way factorial field experiment we reared spear—marked black moth (Rheumaptera hastata) larvae on Alaska paper birch (B. resinifera) saplings that had experienced combinations of 100% manual defoliation and fertilization with N and P in previous years, and measured larval survival and pupal mass. In association with these measurements of larval performance, we assayed leaf condensed tannin levels and the concentrations of N and P in leaves, and correlated the results of these assays with larval performance. (2) In a laboratory experiment we tested the biological activity of condensed tannin and linalool, the major secondary metabolites of Alaska paper birch leaves, by treating leaves collected from previously undefoliated Alaska paper birch saplings with combinations of condensed tannin and linalool, and measuring the performance of spear—marked black moth larvae reared on these leaves. Our results supported predictions of the carbon—nutrient balance hypothesis. We found that fertilization with nitrogen, the nutrient limiting growth of Alaska paper birch in our study site, mitigated delayed inducible resistance, and that condensed tannin is likely to be the major chemical cause of delayed inducible resistance in Alaska paper birch. In our field experiment we also found that fertilization of Alaska paper birch with phosphorus, a nutrient that does not limit the growth of Alaska paper birch in our study site, affected levels of condensed tannin in leaves and the sapling's defoliation history influenced this effect. Thus, future studies of the effects of mineral nutrition on secondary metabolite production by woody plants and their responses to herbivory should consider nutrients that do not limit growth in addition to those that do limit growth.
Variation in Foliar 15N Abundance and the Availability of Soil Nitrogen on Walker Branch Watershed
Authors: Garten Charles T.
Published: May 2006

Spatial patterns in natural 15N abundance (°15) in soil, soil solutions, and non—N2—fixing plants were studied in the deciduous forest on Walker Branch Watershed near Oak Ridge, Tennessee. This study was undertaken to test the hypothesis that foliar °15N values are related to the availability of inorganic nitrogen in mineral soil. Soils collected in or near valley bottoms on the watershed had higher levels of net nitrogen mineralization and net nitrification potential than those sampled from ridges and slopes. More positive foliar °15N values occurred in valley bottoms, which, relative to other positions on the watershed, were characterized by greater availability of soil nitrogen and lower C—to—N ratios in the Oi—horizon, in the surface mineral soil, and in autumn leaf fall. Although leaf nitrogen concentrations changed significantly over the course of the growing season, there was little seasonal variation in foliar °15N values. A hypothesis about the relative importance of different sources of nitrogen to the forest and how nitrogen cycling varies with topography in this nitrogen—deficient ecosystem was derived, in part, from spatial patterns in natural 15N abundance. There appear to be two processes affecting the topographic patterns in foliar 15N abundance on this watershed: (1) greater uptake from isotopically heavy pools of inorganic soil nitrogen by plants in valley bottoms, and (2) uptake of isotopically light ammonium—N in atmospheric deposition by plants on ridges and slopes (where the availability of inorganic soil nitrogen to plant roots is more limited). Results from this study indicate that foliar °15N values are positively correlated with net nitrification potential in surface soil.
The Effect of Inflorescence Bud Removal on Tuber Production in Helianthus Tuberosus L. (Asteraceae)
Authors: Westley Lynn C.
Published: May 2006

I investigated allocation to sexual and asexual reproduction in the Jerusalem artichoke, Helianthus tuberosus L. (Asteraceae), a clonal pseudoannual that is an herbaceous ramet for one season, but propagates as a perennial through tubers that quickly dissociate from the parent plant. H. tuberosus ramets grown from tubers allocated 24% of their end—of—season biomass to tubers and 9% to sexual organs, with substantial variation attributable to four genetic lines cultured from eastern Iowa. Experimentally induced failure of sexual reproduction increased allocation to asexual reproduction under common garden conditions. Plants with inflorescence buds removed produced more (82 vs. 69) and larger (4.4 g vs. 3.8 g) tubers than those with unlimited sexual reproduction. Total biomass allocation to reproduction in plants receiving the bud—removal treatment was equivalent to combined allocation to sexual and asexual reproduction in plants that produced flower heads and achenes. This suggests a complete diversion of resources from sexual to asexual reproduction when sexual reproduction is thwarted. Aboveground plant size did not change with treatment. These results suggest a flexible life history that can respond to sexual failure resulting from low pollinator availability or damage to inflorescences from herbivory or weather, with enhanced facultative production of asexual tubers capable of sexual reproduction the following year. Hereditary variation in allocation leaves open the possibility of local adaptation.
Male Reproductive Success in Sessile Invertebrates: Competition for Fertilizations
Authors: Yund Philip O. , McCartney Michael A.
Published: May 2006

Recent in situ fertilization studies of free spawning and brooding marine organisms have focused almost exclusively on the yield of fertilized ova (female reproductive success). As a consequence, we known little about the factors that determine male reproductive success. If marine organisms compete for fertilizations (as do many terrestrial organisms), then a male's reproductive success should be reduced by the presence of other males. We tested this hypothesis via in situ experiments employing allozyme markers for both a colonial ascidian (Botryllus schlosseri) and a bryozoan (Celleporella hyalina). Under moderate density conditions, the presence of closer male—functioning colonies reduced the fertilization success of more distant males in both species. In C. hyalina, male fertilization success also increased with allocation to sperm production. In addition, selfing rates in this species were negatively correlated with the abundance of outcross sperm. These results suggest that male reproductive success in sessile marine invertebrates must be assessed as a function of the gamete output and spatial distribution of other males in a population, and that the performance of isolated males may yield overestimates of male fertilization success in natural populations.
Growth and Survival of Postmetamorphic Toads: Interactions among Larval History, Density, and Parasitism
Authors: Goater Cameron P.
Published: May 2006

I examined growth and survival of the European toad, Bufo bufo, from hatching to the approximate time of first hibernation. I varied tadpole density in experimental ponds such that individuals from low—density ponds emerged 48.5% larger than those from high—density ponds. In the laboratory, metamorphs from both pond densities were maintained in containers at densities of one or six. Nine weeks after metamorphosis they were exposed to 0 or 80 larvae of the lungworm, Rhabdias bufonis. A factorial experiment aimed to determine (1) the extent to which conditions experienced by larvae carried over to the terrestrial stage (2) the effects of resource limitation and past history on a host's response to a potential pathogen. The density of metamorphs had the strongest effect on growth and survival: 18 wk after metamorphosis, those raised alone were °80% heavier than those in groups and they had 31% higher survival. However, larval history also affected growth and survival and affected how metamorphs responded to density. First, single toads emerging from low—density ponds were 14.5% larger at the time of hibernation than those from high—density ponds. The mechanism for this growth advantage probably lies in the consistently higher growth rates of single toads from low—density ponds, especially during the first 3 wk after metamorphosis. Second, survival in group containers was higher for toads from low—density ponds, especially during the first few weeks after metamorphosis when most deaths involved toads from high—density ponds. These results support a carry—over effect between larval history and subsequent performance in an organism with a complex life cycle. Infection with lungworms had no detectable effect on metamorph growth or survival. This result contrasts earlier studies on this system, possibly due to the relatively low worm burdens or low statistical power. Yet, the results may also indicate that the predicted impact of infection on hosts, especially those limited by resources, is not as straightforward as theoretical studies suggest.
Effects of Invasion of an Aspen Forest (Canada) by Dendrobaena Octaedra (Lumbricidae) on Plant Growth
Authors: Scheu Stefan , Parkinson Dennis
Published: May 2006

Effects of invasion of an aspen forest in the Canadian Rocky Mountains by the earthworm Dendrobaena octaedra (Savigny) on nutrient mineralization, soil microflora, and plant growth were investigated during the growth period of 1992. Experimental chambers with reconstructed forest floor were placed in the field and destructively sampled after 7 and 14 wk. D. octaedra enhanced the shoot biomass of the grass Agropyron trachycaulum (Link) Malte (Poaceae) and increased the shoot—to—root ratio during early plant growth. Microbial biomass, basal respiration and respiratory quotient qCO2 in L/F layer material were reduced by D. octaedra but increased in the H layer. The nutrient (NH4+, NO3—, PO43—) content in soil was also affected by D. octaedra but the effects were small. Effects of the earthworms on soil nutrient content were masked by the great variation in the data and by leaching of nutrients from experimental chambers.
Searching for Cryptic Prey: A Dynamic Model
Authors: Dukas Reuven , Clark Colin W.
Published: May 2006

We present a dynamic model to examine sequential foraging decisions of predators searching for cryptic prey. We identify key elements of information processing and ecological factors determining the costs and benefits of two foraging alternatives. On the one had, specializing on a single prey type while ignoring other types increases the distance a forager must move; this involves increases the distance a forager must move; this involves increased energy and time expenditures. On the other hand, switching between searching for several prey types has the obvious cost of an initial reduction in the probability of detecting prey immediately after switching. Switching also has the less apparent cost of a forager's reduced ability to assess the probability of prey presence. We show that if the cost of switching is sufficiently high, and a forager estimates that one prey type is more common than others, then the forager should search for this prey only, unless conditions change.
The Role of Growth in Maintaining Spatial Dominance by Mussels (Mytilus Edulis)
Authors: Petraitis Peter S.
Published: May 2006

Spatial dominance is maintained by the balance of three processes: mortality, recruitment, and growth. Patches of mussels facing high mortality may show no change in percent cover (i.e., dominance) as long as growth by the remaining survivors can fill in the vacant space. A graphical model is developed to examine the possibility that growth may ameliorate the effects of predation and physical disturbance on spatial dominance. In the model, growth, mortality, and recruitment are depicted as vectors on a set of isoclines for log percent cover. The 100% isocline and analogous to the self—thinning line. Self—thinning data for the mussel Mytilus edulis are used to establish the log—percent isoclines. A review of published growth and mortality data suggests mussels can use growth to offset the effects of mortality. Small mussels can easily double in length in 1 yr, and this sufficient to withstand 60% mortality per year without a loss in percent cover. Since both growth and mortality tend to be size specific, the point at which growth can no longer offset mortality depends on the environmental conditions that affect rates of growth and mortality. The model and analysis can be easily extended to other species and suggest that species with rapid growth and large variation in body size need not reply on recruitment to maintain spatial dominance, even when faced with high rates of predation.
Toward A Method For Measuring Instantaneous Fungal Growth Rates In Field Samples
Authors: Newell S. Y. , Fallon R. D.
Published: May 2006

Methods are available in the literature for the measurement of instantaneous growth rates in field samples of photosynthetic microbes and procaryotic saprotrophic microbes, but there has been no such method for eucaryotic saprotrophic microbes (members of the kingdom Fungi). We have devised a technique for estimating instantaneous growth rates for ergosterol—containing fungi in field material, and preliminarily applied the technique to estimation of fungal productivities and throughputs in two types of standing—dead grass (one saltwater, one freshwater), obtaining plausible values (e.g., conversion efficiencies, total fungal production ° leaf mass loss, were calculated to be 36—64%). This method is based on measuring rates of radiolabeled acetate incorporation into ergosterol. We examined several potential problem areas for use of radiolabeled precursors in measurement of microbial molecular syntheses, and its extrapolation to microbial productivity. Key findings were: (a)5 mmol/L added acetate (required for maximization of detection of change in rates) had positive or neutral impact on 48—h fungal growth, and no shift—up (sudden upward change in rate) or lag in incorporation of acetate into ergosterol occurred between 0.25 and 1.25 h of incubation of naturally decaying leaf (Spartina alteriflora) samples with 5 mmol/L acetate; (b) bacterial assemblages on leaves did not contribute radioactivity to the ergosterol fraction after incubation with radioacetate (as opposed to > 30 Bq per centimetre length of leaf containing fungal mycelium); (c) the average empirical factor for conversion from nanomoles 14C—acetate incorporated to micrograms organic fungal mass produced was 8.2 mg/nmol (coefficient of variation, 27%). We tentatively conclude that the acetate—to—ergosterol method is tenable for measurement of natural fungal productivity.
Nitrogen Content of Food Plants and Vulnerability of Pieris Rapae to Natural Enemies
Authors: Loader Colleen , Damman Hans
Published: May 2006

We tested the commonly made assumption that reduction in leaf quality reduces the survivorship of herbivores indirectly by increasing their exposure to natural enemies. Caterpillars of the butterfly Pieris rapae were placed on collard plants that had been experimentally manipulated to have leaves either rich or poor in nitrogen. On low—nitrogen plants caterpillars both developed more slowly and devoted more of their time to feeding than on high—nitrogen plants. When exposed to large (> 2 mm) predators and parasites in a natural setting, the survivorship of caterpillars on low—nitrogen collards was reduced greatly compared to caterpillars protected from predators and parasites. Survivorship of caterpillars on high—nitrogen plants was reduced significantly less dramatically when exposed to predators and parasites. Small (< 1—2 mm) pupal parasites that were able to reach the caterpillars through the mesh of the predator exclusion cages emerged more frequently from individuals reared on high—nitrogen plants.
Seasonal Drought and the Phenology of Understory Shrubs in a Tropical Moist Forest
Authors: Wright S. Joseph
Published: May 2006

The vegetative and reproductive growth of tropical forest plants often occurs in short bursts separated by longer quiescent periods. Seasonal changes in moisture availability may be a cause and, if so, may cue growth in a proximate sense or limit growth in an ultimate sense. Two 2.25—ha plots of mature forest were irrigated for three consecutive dry seasons on Barro Colorado Island, Panama, to discriminate among the various possibilities. The responses of three Piper species and four Psychotria species are reported. Irrigation disrupted the timing of leaf flush for all seven species. Irrigation increased annual leaf production for just one species, however, and had no effect on annual inflorescence production for the Psychotria nor on stem radial growth. These responses are consistent with the hypothesis that water stress constrains the timing of growth while only rarely limiting total annual growth. However, leaf turgor potentials observed among control plants may not have been sufficiently low to constrain leaf and inflorescence production, and other effects of irrigation cannot be discounted. Possible phenological cues that were affected by irrigation include soil moisture content, air temperature, relative humidity, and the timing of nutrient inputs from decaying litter. There was no evidence that a "water stress requirement" must be met during the dry season before wet season budbreak. The gradual decay in synchrony for leaf production for all four Psychotria species suggests that endogenous rhythms contribute to growth periodicities in the genus.
Nonlethal Effects of a Predator on Competitive Interactions Between Two Anuran Larvae
Authors: Werner Earl E.
Published: May 2006

I examine the nonlethal effects of a predator, an odonate larva (Anax junius), on competitive interactions between two species of anuran larvae (bullfrog, Rana catesbeina, and green frog, R. clamitans). The anurans coexist in permanent ponds, which sometimes contain high densities of A. junius. Laboratory experiments indicated that in the absence of the predator, the two species were similar in competitive abilities and grew at virtually identical rates. In the presence of caged A. junius, however, the bullfrog grew to larger and the green frog much smaller than when the predator was absent. Both species reacted to the presence of A. junius by reducing activity and altering space use. I argue that the reductions in activity in the presence of the predator affected the two species differently, which in turn changed competitive interactions. I discuss the relation between activity level and competitive ability, and the potential for predators to alter competitive relations by affecting activity levels of syntopic species (a higher order interaction). Vulnerability to predators also increases with activity, and I discuss the implications of an activity—mediated trade—off between growth rate and predation risk to interactions among anuran larvae. Finally, I present a method for quantitatively evaluating the net balance of (negative) direct mortality effects and (sometimes positive) indirect effects that alter competitive ability.
Clutch Size Determination in the Marsh Tit (Parus Palustris)
Authors: Nilsson Jan-Ake
Published: May 2006

By providing supplemental food to Marsh Tits (Parus palustris) from the day the first egg was laid, effects on clutch size could be directly attributed to food without the confounding effects on laying date. Females provisioned with additional food responded by laying, on average, on egg more than control females. Differences in territory quality did not influence this result because, on territories occupied in both years, one more egg was laid in the year of provisioning. Although the rate and amount of extra food provided were equal during the two experimental years, the resulting clutch sizes of provisioned females differed between years by more than one egg in absolute terms. Further, the mean clutch size of provisioned females declined seasonally in parallel to that of unprovisioned females. I conclude that clutch size in Marsh Tits is determined by a tactical decision based on declining values of offspring with the progress of the season. However, within the limits sets by this tactical decision, the availability of food during egg—laying also plays a role.
Applications of Nucleic Acid Hybridization in Microbial Ecology
Authors: Holben William E. , Tiedje James M.
Published: May 2006

Nucleic acid hybridization techniques offer a new approach to answer old, intractable questions in microbial ecology as well as new questions. These include characterization of the predominant, yet unculturable populations in nature, the role of the environment in gene expression, and the extent of gene exchange among communities in nature. The essence of this methodology is the denaturing and annealing of complementary strands of nucleic acid molecules. The specificity of this hybridization reaction can be controlled such that only identical, or nearly identical, sequences in a complex mixture of nucleic acids extracted from a population or community can anneal. Labeled DNA or RNA sequences (probes) are introduced into hybridization reactions to identify and quantitate a particular organism containing the complementary target sequence. Methods for the recovery and purification of DNA from soils and sediments are given, as well as important considerations for the selection of probe and target sequences, and for the methods of detection and quantitation. Some advantages of this methodology include the abilities to: (1) detect populations without prior culturing of organisms, (2) detect specific organisms without the need for selectable markers, (3) detect multiple populations in the same analysis, and (4) detect genetic rearrangements or gene transfer in natural communities.
Body Size, Prey Size, and Community Structure in Insectivorous Mammals
Authors: Dickman C. R.
Published: May 2006

This study investigates the relationship between body size and prey size in three two—species communities of insectivorous mammals in Australia and England. In capacity all species maximized their net energy intake by feeding preferentially on large prey, but the field in larger species of insectivore in each community ate prey that were 25—29% longer than those eaten by smaller species. Direct field observations showed that the larger species of insectivores dug extensively in the topsoil and subsurface litter to obtain prey, whereas the smaller species foraged on the litter surface or in trees. The larger species also exploited denser and structurally more complex microhabitats than their smaller relatives. These differences in foraging behavior allowed their larger insectivores in each community to exploit prey that were 7—19% longer than those encountered by the smaller species. Controlled removal of the larger species of insectivore from two communities produced marked changes in the microhabitat use and prey size selection of the remaining species. In the first community, a small eutherian shrew (Sorex minutus) ate more long prey and tended to move into tall grass when a larger congener, S. araneus, was removed. In the second community a small dasyurid marsupial (Antechinus stuartii) also ate more long prey and decreased its use of trees when the larger A. swainsonii was removed. These results provide no evidence that body size directly influences prey size selection in insectivorous mammals. However, body size differences may influence prey size selection indirectly by conferring an asymmetric competitive advantage on larger insectivores, thus allowing them to actively exclude smaller species from microhabitats containing larger prey. Community diversity of insectivorous mammals should therefore depend on the availability of different foraging microhabitats, and not on the distribution of prey sizes.
Impacts of Supplemental Feeding on Survival Rates of Black‐Capped Chickadees
Authors: Brittingham Margaret Clark , Temple Stanley A.
Published: May 2006

The availability of winter food may strongly influence overwinter survival and, hence, limit certain bird populations in northern regions. Using the Jolly—Seber method of estimating survival rates from recapture and reobservation data, we compared the survival rates of 418 individually marked Black—capped Chickadees, Parus atricapillus, having access to supplemental food with those of 158 chickadees without access to supplemental food. During three winters (October through April, 1982—1985) chickadees with access to supplemental food had higher average monthly survival rates (95 vs. 87%), higher overwinter survival rates (69 vs. 37%), and higher standardized body masses (an additional 0.13 g) than birds on control sites. Differential survival occurred primarily during months with severe weather (> 5 d below —18°C). During these months, high energy demands probably made it difficult for birds without access to supplemental food to obtain sufficient energy from dispersed natural sources. In addition, during periods of extreme weather when foraging may be difficult, the extra fat carried by individuals that are supplementally fed may increase the probability of survival.
Local Differences in Host Use by Two Populations of the Colorado Potato Beetle
Authors: Horton David R. , Capinera John L. , Chapman Phillip L.
Published: May 2006

Local differences in performance on three host species (Solanum sarrachoides, S. rostratum, potato [S. tuberosum]) were demonstrated for two nonagricultural populations of Colorado potato beetle. Populations from north—central and northeastern Colorado showed differential larval growth rate, survival, size, and tendency to diapause. Statistically significant host x population interactions indicated that these differences were host—species dependent. Further analysis suggested that the two populations were most successful on the locally abundant and annually predictable host species of their respective community. Population differences in performance were pronounced on one wild host species (S. sarrachoides); performance was similar on the other host species. Extent of host specificity was related to composition of the flora. The population that was exposed to all three host species in the field showed high rates of survival on all hosts; there was no indication that beetles from this population showed major trade—offs in performance among hosts. The second population showed highly reduced survival and growth on the wild host species that is absent from its immediate range. Implications of these results for addressing questions about the beetle's original host shift to potato are discussed. Our results, in particular the success of these nonpest populations on potato, suggest that no major physiological changes in performance were necessary in the beetle for colonization of potato.
Interactive Effects of Dragonfly Predation in Expermental Pond Communities
Authors: Van Buskirk Josh
Published: May 2006

One goal of ecology is to generate dynamic models that predict the species composition of complex natural communities. The general applicability of such models will depend on how often unpredictable higher order interactions occur between competitors and predators. I experimentally tested for interactions in a two predator—multiple prey community by manipulating the presence or absence of two species of predatory dragonfly larvae (Anax junius and Tramea carolina) in replicated outdoor artificial ponds containing identical number of four species of anuran tadpoles and a diverse zooplankton assemblage. The impact of predation on prey communities was measured by the relative abundances and larval performances (size at metamorphosis, length of larval period, and growth rate) of the anurans, and by the densities of zooplankton sampled at 10—d intervals. Zooplankton reached high densities in the experimental ponds (up to 2700 animals/L). Dragonfly predation had a marginally significant impact on zooplankton species composition, and the effects on zooplankton size structure were not significant. Predation by Anax and Tramea profoundly modified the species composition of anurans: the relative abundance of toads (Bufo americanus) increased in the presence of predators, as two hylid species (Hyla crucifer and Pseudacris triseriata) decreased. The interaction between Anax and Tramea on anuran species composition was additive. The performance of surviving larvae of two anurans (Bufo and Pseudacris) was significantly improved by predation (growth rate increased and larval period shortened), whereas the performance of two others (Hyla and Rana utricularia) showed no response to dragonflies. The absence of strong interactions between the predatory odonates implies that in some cases it may be valid to build models of complex ecological communities by additively combining results from more simple subsets of species.
Energetics of Vertical Migration in Chaoborus Trivittatus Larvae
Authors: Swift Michael C.
Published: May 2006

One of the recent theories for the adaptive value of vertical migration states that migrants gain an energetic advantage over nonmigrants because by alternating between areas of high and low temperatures they are able to partition energy into growth more efficiently than nonmigrants. The energetics of the vertical migration of fourth—instar Chaoborus trivittatus larva in Eunice Lake, British Columbia was studied to identify and quantify this hypothesized energetic advantage. Fourth—instar C. trivittatus larvae undergo a regular, synchronous, diel vertical migration which exposes them to a wide range of temperature and prey density. Feeding occurs primarily at night and near the surface. Although all zooplankton in Eunice Lake are potential prey, Diaptomus kenai constitutes the majority of the biomass in the diet of fourth—instar larvae; Carbon assimilation efficiency of both copepods and cladocerans by C. trivittaus is ° 68%. Respiration rate increases linearly with temperature over the range 5—25 degrees C, although there is a suggestion of a plateau in O_2 consumption over the temperature range to which the larvae are exposed during migration. Temperature and ration size interacted to determine larval growth rate; fluctuating temperatures limited growth regardless of prey density while at constant high temperature prey density limited the growth rate. Analysis of several possible migration strategies showed that on an energetics basis alone growth will be maximized by either staying near the surface where there is food, or by vertically migrating with a physiologically determined periodicity based on individual feeding history. Laboratory growth experiments and the results of computer simulations of vertical migration show that the energetic hypothesis for the adaptive value of vertical migration does not hold for C. trivittatus larvae in Eunice Lake. These larvae do not follow either of the migration strategies that is energetically most efficient. No alternative hypothesis to explain their migration pattern is attractive. Possibly it is maintained by immigration from populations exposed to strong selection for this behavior pattern.
Dragonfly Production and Prey Turnover
Authors: Benke Arthur C.
Published: May 2006

Annual production was calculated for three dominant larval odonate populations (Ladona deplanata, Epitheca spp., and Celithemis fasciata) coexisting in the littoral zone of an abandoned farm pond. Dragonfly populations and their prey were collected simultaneously with an Ekman grab at 2—wk to 1—mo intervals. Production for each dragonfly species was calculated using both the Allen curve method and the removal—summation method. Annual production for the three dominant species was ° 6 g/m2 (dry wt), and total odonate production was estimated as 8 g/m2. Weekly odonate production was consistently > 0.3 g/m2 from July through September. With a mean annual standing stock of ° 2 g/m2, the odonate annual turnover ratio (TR) is ° 4. Weekly TRs were highest during summer months, generally declining as individual size increased. From back calculations of the amount of food required to feed the odonates, it as estimated that a minimal value of annual prey production was 18 g/m2 and a minimal estimate of weekly prey production in the summer months was almost 1 g/m2. With an observed prey standing stock of 0.6 g/m2, this would necessitate an annual prey TR of 30 and a weekly summer TR > 1. Since such an indirect method of calculating prey production does not account for other sources of prey mortality such as fish predation and emergence, the TRs are considered to be minimum values, perhaps only half of actual values. These relatively high TR values agree quite well with estimates based upon the amount of food required to sustain fish predators. The significance of these estimates made using indirect methods is that they are considerably higher than estimates made using more traditional direct methods. An attempt is made to resolve this discrepancy by examining sources of error associated with both approaches. Furthermore, since odonates have a standing stock frequently two to three times that of their prey and a large daily consumption capacity, I suggest that prey are saved from annihilation because they can find sufficient refuges.
An Experimental Test of Mate Defense in an Iguanid Lizard (Sceloporus Graciosus)
Authors: Deslippe Richard J. , M'Closkey Robert T.
Published: May 2006

Males of many iguanid lizard species defend territories against conspecific males. We investigated whether territoriality in male sagebrush lizards (Sceloporus graciosus) was associated with defense of mates. Studies on the sagebrush lizard in western Colorado, USA, included mark—recapture census and removal of adult females from the territories of resident males. Adults and juveniles were clearly distinguished by body size. Resident males did not abandon their territories following the removal of adult females. Resident males did not expand the size of their territories following the loss of adult females. Males were not defending juvenile females because most male territories did not include juvenile females. Sagebrush lizards are long lived, and territory defense probably ensures several mating opportunities. We postulate that loss of a mate in one season may be less important than the long—term benefits of territory maintenance and repeated reproduction.
Specialized Host Plant Performance of Pea Aphid Clones is not Altered by Experience
Authors: Via Sara
Published: May 2006

Within species of polyphagous herbivores, individuals may prefer or have greater fitness on one host than on others within the species range. Such intraspecific specialization can be caused either by genetic differences or by the effects of experience (conditioning) on a given host plant. Separating the causes of intraspecific variation in host use in an important issue in understanding the potential for host shifts in herbivorous species. In the pea aphid, a polyphagous pest of forage crops, various clones collected from a local area have been shown to be specialized such that they have higher fitness on either alfalfa or red clover. The experiment reported here was designed to test whether this observed specialization could be significantly altered by prolonged experienced on the alternate plant species. In a factorial design, replicate lineages of two specialized clones were maintained on both alfalfa and red clover for three generations; each line was then tested on both crops. Experience on the alternate crop had no significant effect on the relative performance of these two specialized clones. These results support the suggestion that differences in host plant performance among pea aphid clones are genetically based. They also suggest that individuals that move between crops are likely to be selected against, since they appear to be unable to significantly improve in performance through time as a result of experience. Such selective elimination of intercrop migrants will promote genetic divergence of nearby subpopulations on different crops.
Coarse Woody Debris in Douglas‐Fir Forests of Western Oregon and Washington
Authors: Spies Thomas A. , Franklin Jerry F. , Thomas Ted B.
Published: May 2006

Amounts and structural characteristics of coarse woody debris (CWD) were examined in relation to stand age and site moisture condition in 196 Pseudotsuga menziesii stands in western Oregon and Washington. Stands ranged from 40 to 900 yr old, and most if not all, originated after fire. In a chronosequence from the Cascade range, the amount of CWD followed a U—shaped pattern for stands <500 yr old, with moderate levels (92 Mg/ha) in stands <80 yr old, lowest levels (<50 Mg/ha) in stands 80—120 yr old, and highest levels (173 Mg/ha) in stands 400—500 yr old. After 500 yr the amounts of CWD declined to intermediate levels. In the southern Coast Range, lowest levels (32 Mg/ha) of CWD were in the youngest stands (60—80 yr), primarily because they inherited little CWD from the preceding (prefire) stands. In the Cascade Range, levels of CWD inherited from preceding stands were highest in young stands and declined to near zero by 250 yr. The overall decay rate constant (k) for snags and logs in the Cascade Range, calculated indirectly from the chronosequence, was 0.029 yr—1. Volume and biomass of CWD differed significantly in old—growth stands (>200 yr old) among site moisture classes. Dry sites averaged 72 Mg/ha moderate sites 137 Mg/ha, and moist sites 174 Mg/ha. The dynamics of CWD were modeled for three fire histories, each beginning with an initial fire in an old—growth stand but differing in number and severity of subsequent fires. All three models exhibited low values of CWD between 80 and 200 yr. The lowest and most prolonged minimum in CWD between 80 and 200 yr. The lowest and most prolonged minimum in CWD during succession occurred when additional fires burned early in succession, which probably happened preceding many stands in the southern Coast Range. The results of the study indicate that a steady—state condition in CWD may not be reached for >1000 yr, and that the nature and timing of disturbance play a key role in the dynamics of CWD in the dynamics of CWD in the region.
Early Successional Pathways and the Resistance and Resilience of Forest Communities
Authors: Halpern Charles B.
Published: May 2006

Vegetation changes were studied for 21 yr in two clearcut logged and slash—burned Pseudotsuga forests in the western Cascade Range of Oregon. Detrended correspondence analysis (DCA) was used to examine the successional relationships among six understory communities exposed to a gradient of disturbance intensity. Euclidean disturbances between pre— and postdisturbance samples in ordination space were used to compare community resistance to disturbance and long—term recovery, or resilience. Ordination through time for plant communities revealed a common pattern of rapid floristic change away from predisturbance composition, followed by gradual, unidirectional return. Early, but transient, convergence of successional pathways was common among mesic— and dry—site communities, reflecting the broad distribution of colonizers and the floristic similarity of predisturbance understories. Distinct sequences were observed on moist sites, reflecting more unique residual and colonizing floras. Ordinations also revealed increasing compositional change with disturbance intensity. Successional sequences were dominated by residual species on relatively undisturbed sites and by alternate suites of invading species on moderately disturbed and burned sites. Variation in the response gradient between watersheds reflected the modifying influence of local environment, stand history, and change in succession. Resistance and resilience varied little among plant communities but were generally lowest for the depauperate Coptis community and greatest for the compositionally and structurally diverse Polystichum and Rhododendrom—Gaultheria types. Both measures were strongly influenced by disturbance intensity. The stability of Pseudotsuga understories derives from the moderate tolerance of initial understory dominants to burning and in their ability to subsequently perennate from subterranean structures. Variation in the long—term response of communities reflects complex interactions between species life history, disturbance intensity, and chance, suggesting that both deterministic and stochastic factors must be considered in evaluating community stability and response to disturbance.
Demographic Cost of Reproduction in a Neotropical Herb: An Experimental Field Study
Authors: Horvitz Carol C. , Schemske Douglas W.
Published: May 2006

There are few empirical data for plants demonstrating unambiguously that an increase in current reproductive effort results in a demographic cost, i.e., a significant decrease in future growth, survival, or reproduction. In this study we experimentally created low— and high—reproductive—effort plants in a natural field population of a neotropical perennial herb and measured subsequent growth, survival, and reproduction. During the season subsequent to the experimental treatment, there were no significant differences in survival, growth, or reproduction between the two treatments. Thus, for long—lived perennial plants, an increase in reproduction may not result in a demographic cost. More experimental studies are needed to determine the generality of this result.
Fruits Rhinoceros Eat: Dispersal of Trewia Nudiflora (Euphorbiaceae) in Lowland Nepal
Authors: Dinerstein Eric , Wemmer Chris M.
Published: May 2006

To document the dispersal of the fruits of the tropical tree Trewia nudiflora by a large herbivore, we examined several aspects of the Trewia nudiflora—Rhinoceros unicornis interaction. The fruits of Trewia nudiflora (Linn.) (Euphorbiaceae), a common riverine forest tree of southern Nepal, are large, hard, and dull—colored upon ripening. Although unattractive to arboreal and volant frugivores (monkeys, bats, and birds), Trewia fruits are eaten by greater one—horned Asian rhinoceros (Rhinoceros unicornis). During the rainy season (June—October), Trewia fruits become an important food source for rhinoceros. Trewia seeds remained from 3 to 7 d inside the gut of captive rhinoceros, but gut treatment itself had no significant effect on germination success. Manuring the seeds in dung significantly increased seedling growth. Trewia is shade—intolerant; despite abundant fruit fall and reasonable seed germination, seedling recruitment was poor under the forest canopy. Uningested fruits often remained hard throughout the monsoon, preventing seeds from germinating. Seeds defecated into grassland latrines created by rhinoceros developed into robust seedlings, whereas seedlings on forest latrines generally died soon after germination or after the monsoon. Heavy seed loads in dung, low germination from uningested fruits, and rapid growth of Trewia seedlings on grassland latrines demonstrate the role of rhinoceros in dispersal and recruitment of woody species in riverine grasslands and the significance of megafaunal dispersal.
Role of Macroinvertebrates in Nitrogen Dynamics of a Desert Stream
Authors: Grimm Nancy B.
Published: May 2006

Organismal nitrogen budgets (nitrogen ingested, egested, excreted, and utilized in production) were constructed for collector—gatherer macroinvertebrates and grazing snails of a Sonoran Desert stream. Twenty—seven percent of ingested nitrogen was utilized in insect tissue production, 9—31% was excreted as ammonia, and the remainder (42—64%) was egested. Of nitrogen utilized in production, only 26% resulted in increased standing stock during a 20—d successional period. The remainder was lost to predation and nonpredatory mortality (70%) or as emergent adult insects (4%). Snail excretion was 9—13%, and egestion was 26—39% of ingestion. Of nitrogen ingested by snails 50—68% was used in tissue production. As a percentage of nitrogen retained by the stream ecosystem, increased storage of N in insect biomass was 10%, insect emergence was 1%, and excretion recycled up to 70% of that amount back to the dissolved nitrogen compartment. Collector—gatherer macroinvertebrate influence on nitrogen dynamics, especially via recycling of excreted ammonia, increased over successional time.
Trapline Foraging by Hermit Hummingbirds: Competition for an Undefended, Renewable Resource
Authors: Gill Frank B.
Published: May 2006

Traplining hummingbirds face a conflict between waiting for undefended floral nectar to accumulate to higher levels of profit and losing accumulated nectar to competitors. The goal of this research was to learn the rules and temporal patterns of flower visitation by Long—tailed Hermit Hummingbirds (Phaethornis superciliosus). Monitors of visits by color—marked individuals, particularly lek males, to natural flowers established patterns of feeding in space and time. Monitors of visits to nectar feeders that were refilled at fixed intervals established patterns of adjustment to nectar availability and competitive losses. Competing hummingbirds tended to segregate among undefended feeding sites, with one individual becoming the primary user of each site. Competition caused hummingbirds to visit a feeder frequently, often before a scheduled refill. In this way one individual could harvest most of the provided nectar. Under conditions of nearly exclusive use of a feeder with artificial nectar, the hummingbirds adjusted their visits to operant (fixed—interval) schedules of food presentation. The scheduled harvesting of floral nectar by traplining hermit hummingbirds thus entails the adaptive use of short, elapsed time intervals.
Diet Selection by a Heteromyid Rodent: Role of Net Metabolic Water Production
Authors: Frank Craig L.
Published: May 2006

Kangaroo rats (family Heteromyidae) are primarily granivores. These desert rodents do not drink free water, but depend on performed water in their diet and water produced by oxidation of food (metabolic water). Oxidation of different nutrients produces different net amounts of metabolic water. At low humidities, oxidation of carbohydrates produces a net metabolic water gain, lipid metabolism results in a net water loss, and protein metabolism produces a severe water loss, mainly through urination. Under humid conditions, carbohydrate metabolism again produces a net water gain, lipid oxidation produces a large net metabolic water gain, and although this is slightly reduced, protein metabolism still results in a large water loss. Diet composition therefore influences heteromyid water balance. Preference experiments involving semisynthetic diets that were energetically equivalent tested the influence of net metabolic water production on diet preferences of Dipodomys spectabilis. At low humidities, kangaroo rats in a positive water balance preferred: (1) intermediate—protein, (2) high—lipid, and (3) high—carbohydrate diets. When water stressed, the rodents preferred low—protein diets, and the preference for high—lipid diets decreased. At high humidities, the rodents always preferred high—lipid diets. These results indicate that when energy contents were equivalent, kangaroo rats usually preferred diets that produce the greatest net metabolic water gain.