bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of Plant Toughness, Calcification, and Chemistry on Herbivory by Dolabella Auricularia
Authors: Pennings Steven C. , Paul Valerie J.
Published: May 2006

Most studies of feeding preferences of herbivores have focussed on the role of plant secondary metabolites; very few studies have experimentally tested the importance of plant toughness or calcification as feeding deterrents, although low preference has repeatedly been shown to correlate with these factors. Feeding preferences of the unusual generalist herbivore Dolabella auricularia, an opisthobranch gastropod which, like many other herbivores of that group, sequesters secondary metabolites from some algae, were consistent with our expectation that they might not feed on algae and seagrasses that were tough, calcified, or chemically rich. We experimentally examined the importance of each of these three features in deterring feeding by Dolabella. Consumption of plants by Dolabella was negatively correlated with plant toughness, as determined by penetrometer measurements. Given a choice between two species of plants, Dolabella always preferred the softer of the pair. However, when both species were ground up and incorporated into soft carrageenan cakes, Dolabella were significantly less selective in 3 of 4 cases where the tougher plant had no other known line of defense, and in 1 of 3 cases where the tougher plant also had chemical or CaCO3 defenses. Frondose calcified algae on Guam are ≥50% CaCO3 by dry mass. CaCO3 deterred Dolabella feeding at concentrations of 28 and 50% of dry mass, but not at 16%. CaCO3 occurs in algae in two crystal structures, calcite and aragonite; both deterred Dolabella feeding. CaCO3 and chemical defenses of Halimeda macroloba did not interact synergistically to enhance effectiveness. Dolabella feeding was not deterred by organic extracts from most plants, even when offered at very high concentrations. Extracts from many of these plants had previously been shown to deter feeding by fishes at lower concentrations. Of the 12 plants tested, Dolabella feeding was deterred only by extracts from Caulerpa racemosa, Hormothamnion enteromorphoides, and Halimeda macroloba. We isolated and tested the two major metabolites of Caulerpa, caulerpenyne and caulerpin, and identified caulerpenyne as the deterrent agent. These results indicate that Dolabella are relatively unaffected by chemical defenses and yet are deterred by other plant defenses such as calcification and toughness. The fact that Dolabella can sequester algal secondary metabolities may explain Dolabella's relative indifference to chemically rich plant extracts.
Ectoparasitism as a Cause of Natal Dispersal in Cliff Swallows
Authors: Brown Charles R. , Brown Mary Bomberger
Published: May 2006

Nestling Cliff Swallows (Hirundo pyrrhonota) in southwestern Nebraska that were relatively heavily parasitized by hematophagous fleas (Ceratophyllus celsus) and swallow bugs (Oeciacus vicarius) dispersed to nonnatal colonies to breed the subsequent year, whereas nestlings that were relatively lightly parasitized returned to their natal colony to breed. There were no significant differences between dispersers and nondispersers in natal clutch size, natal brood size, relative hatching date, natal body mass, natal nest's distance from the colony's center, and natal nest's age. There were no sex differences in dispersal tendencies. Dispersing birds tended to move to smaller colonies to breed, and in some cases settled in breeding colonies later than nondispersers. Ectoparasitism may be a major cause of natal dispersal in Cliff Swallows and perhaps should be considered another potential cause of dispersal in general, especially in group—living species that may be associated with large numbers of highly co—evolved ectoparasites.
Predation on Dispersing Misumena Vatia Spiderlings and Its Relationship to Maternal Foraging Decisions
Authors: Morse Douglass H.
Published: May 2006

Misumena vatia (Thomisidae) spiderlings leaving nests guarded by their mothers experienced lower predatory pressure than those leaving unguarded nests. However, this relationship was independent of the size of either their mother or brood, traits affected by differences in maternal foraging success. The principal spiderling predator at this stage, the small salticid spider Metaphidippus insignis, hunted about unguarded, but not guarded, nests. Even the smallest Misumena mothers, being much larger than Metaphidippus, effectively guarded against the salticid, quickly driving it from their nests. This nonselective predation on the spiderlings, combined with the superior success of large broods exposed to parasitism by ichneumonid wasps, demonstrates that highly successful female Misumena foragers enjoy marked fitness advantages, at least through their offspring's dispersal from the nest. It supports the basic, though seldom tested, assumption of optimal foraging theory that success in foraging translates into enhanced lifetime fitness.
Complex Interactions Between Dispersal and Dynamics: Lessons From Coupled Logistic Equations
Authors: Hastings Alan
Published: May 2006

A study of one of the simplest systems incorporating both dispersal and local dynamics, coupling two discrete time logistic equations, demonstrates several surprising features. Passive dispersal can cause chaotic dynamics to be replaced by simple periodic dynamics. Thus passive movement can be stabilizing, even in a deterministic model without underlying spatial variation in the dynamics. The boundary between initial conditions leading to qualitatively different dynamics can be a fractal, so it is essentially impossible to specify the asymptotic behavior in terms of the initial conditions. In accord with several recent studies of arthropods and earlier theoretical work, density dependence may only be detectable at a small enough spatial scale, so efforts to uncover density dependence must include investigations of movement.
Effectiveness of Tortoise Beetle Larval Shields Against Different Predator Species
Authors: Olmstead Karen L. , Denno Robert F.
Published: May 2006

Larvae of the tortoise beetles Charidotella bicolor and Deloyala guttata carry shields formed from exuviae and feces over their bodies that are though to provide protection from natural enemies. We investigated the effectiveness of shields as a defense against three groups of invertebrate predators (mandibulate, piercing/sucking and chelicerate taxa) common in tortoise beetle habitats. Choice experiments, functional response studies, measurements of predator body size and mouthpart length, and a literature review indicated that predators with long piercing/sucking mouthparts (i.e., Heteroptera) were the most effective predators of tortoise beetles. For example, relatively large heteropterans with long, piercing/sucking mouthparts (e.g., Nabis americoferus and Podisus maculiventris) were not deterred by shields whereas small predators with short beaks (e.g., Geocoris punctipes) were. Likewise, coccinellid predators with short mandibles were deterred by the shield, while a carabid predator with long mandibles (Lebia fuscata) was effective at killing tortoise beetle larvae. No clear patterns of the effectiveness of shields against chelicerate predators were evident. Choice experiments also indicated that small tortoise beetle larvae were more susceptible to predator attack than were larger instars. Additionally, the effectiveness of the shields against different predators was quite similar between the two tortoise beetles. Field experiments indicated that predators are an important source of mortality for tortoise beetle larvae in natural settings, and high larval mortality is associated with elevated densities of predators with piercing/sucking mouthparts, such as heteropterans. Thus, shields provide some but not universal protection against the variety of predators present in tortoise beetle communities.
The Role of Regeneration Stages in the Distribution of Edaphically Restricted Fynbos Proteaceae
Authors: Mustart Penelope J. , Cowling Richard M.
Published: May 2006

Regeneration stage requirements can be determinants of adult plant distributions. We investigated this in two species—pairs: Protea abtusifolia and Leucadendron meridianum occurring on shallow soils overlying limestone, and P. susannae and L. coniferum on the adjacent, deep, weakly acidic, colluvial sands. We tested the hypothesis that abiotic regeneration stage requirements at the seed germination and/or seedling establishment stages determine adult distribution patterns. We excluded competitive effects in our experiments. We preformed reciprocal seed transplants in the field, and monitored seedling emergence, establishment, and mortality over two summers. We determined iron, aluminum, calcium, and magnesium contents of 1—yr—old shoots, as well as soil temperature and moisture and midsummer water stress measurements of seedlings. There was higher seedling emergence of seed of all species on the limestone soils, negating the idea that seeds had germination requirements specific to their natural soil types. We suggest that this higher emergence is related to the higher moisture contents of the limestone soils. Seedlings of the limestone species performed better on their own soil type, with lower seedling mortality and overall increased survival (taking into account seed germination and seedling establishment) on their own soil type. Seedlings of the colluvial sands species, on the other hand, had slightly increased seedling mortality, and overall less survivorship, on their own soil type. Despite this, seedlings of these two species growing on the colluvial sands were larger than their counterparts on the limestone soils. There were both more and larger seedlings than those of P. obtusifolia and L. meridianum on the colluvial sands. Thus competitive exclusion is invoked to explain the sole persistence of the colluvial sands species on their native substrate.
Modeling of Wave Regeneration in Subalpine Abies Forests: Population Dynamics with Spatial Structure
Authors: Sato Kazunori , Iwasa Yoh
Published: May 2006

The wave regeneration of forest trees is studied theoretically in model populations with a lattice structure, in which each lattice point corresponds to stand containing a cohort of trees. It is assumed that the tree height at each site increases at a constant rate and that trees die in unit time if they are taller than their windward neighbors, with the height difference greater than a critical value. Starting from a random initial distribution, the spatial pattern becomes a saw—toothed shape, and moves at a constant rate downwind without changing its shape. In addition to this basic model, four models are also examined in which both the absolute height (or age) and the height difference between neighbors affect the tree mortality. Two rules (AND and OR rules) produce very irregular final patterns that are not saw toothed, but the other two (SUM and PRODUCT rules) often generate patterns that are close to saw toothed and more regular than those produced by the basic model. Both periodic boundary conditions and fixed height boundary conditions are examined. Two—dimensional models tend to produce more regular wave regenerating patterns than one—dimensional models.
Early Spring Nitrogen Dynamics in a Temperate Forest Landscape
Authors: Groffmann Peter M. , Zak Donald R. , Christensen Soren , Mosier Arvin , Tiedje James M.
Published: May 2006

Early spring nitrogen cycling in temperate forests is dynamic and important to site fertility and retention of N in these ecosystems. In this study, we combined short—term (2 d) 15N—based measurements of plant uptake, microbial nitrification, denitrification, and immobilization with more conventional measurements of these processes over an 8—wk period from early March to early May within a temperature forest landscape unit in Michigan, USA. Measurements were made in two landscape positions, a summit position with a well—drained soil and a toe—slope position with a poorly drained soil. While soil mineral N levels showed little spatial and temporal variation over the 8—wk period, nitrification rates, microbial biomass (chloroform—labile) N, and denitrification were highly variable. The poorly drained soil consistently had high levels of nitrification and denitrification relative to the well—drained soil. Pools of microbial biomass N increased by a factor of 10 over the 8—wk period in both soils, but were relatively stable during April, when the 15N experiment was conducted. Microbial biomass appeared to be the key regulator of the fate of added 15N. In the well—drained soil, the largest movement of N was into microbial biomass and total soil N. As a result of this strong immobilization, there was relatively little nitrification and denitrification of 15N in the well—drained soil. In the poorly drained soil, there was no apparent movement of 15N into microbial biomass. As a consequence, availability of 15NH4+ to nitrifiers was high, and rates of nitrification were very high. Accumulation of NO3— in the poorly drained soil, along with high soil moisture, forested high denitrification N losses. The results suggest that N retention processes and the fate of either atmospherically or agriculturally derived N inputs will likely show strong temporal and spatial variation during the early spring period in temperate forest landscapes.
Stability, Regulation, and the Determination of Abundance in an Insular Song Sparrow Population
Authors: Arcese Peter , Smith James N. M. , Hochachka Wesley M. , Rogers Christopher M. , Ludwig Donald
Published: May 2006

The population dynamics of the Song Sparrow, Melospiza melodia, were studied for 15 yr on Mandarte Island, in southwestern British Columbia, Canada. This population exhibited a high median density (7.8 females/ha) and fluctuated strongly (10—fold or more) from year to year. The population received few successful immigrants, even at low densities, and its dynamics were thus driven by local events. Two strong density—dependent regulating mechanisms were detected. First, reproductive output was strongly depressed at high densities because of an increased rate of nest failure and a decline in mean clutch size. Greater nest failure at high density was due to increased predation on eggs and nestlings. Nest failure increased with the rate of nest parasitism by Brown—headed Cowbirds (Molothrus ater) and was lowest when cowbirds were absent from Mandarte, suggesting that cowbirds either cause or facilitate nest failure. Second, the rate of juvenile recruitment was inversely related to the density of adults, because of the interference competition for breeding territories between adults and yearlings. The survival rate of adults was independent of population density. Survival rates of adults and juveniles were negatively affected by weather in February 1989, when an annually cold spell coincided with the loss of over 90% of the population. A second severe population crash occurred during the winter of 1979—1980, but the cause of this crash is not known. On two other occasions, periods of severe winter weather occurred at the same time as a substantial fraction of the population disappeared. However, not all periods of poor weather were accompanied by marked disappearances of birds. Adult females survived better than adult males during the breeding season but equally well outside it. We found no correlations between the survival rates of juveniles and adult males and females in years without a population crash suggesting that there is no common cause underlying the observed variation in survival among age and sex classes in most years. Simple models of the population indicate that either density—dependent reproductive success or density—dependent recruitment of locally hatched juveniles can regulate population size. These two regulatory factors operate in a sequential and independent manner. Despite the presence of strong regulating factors, population crashes caused marked instability in population size, nearly leading to extinction in 1989. The rate of juvenile recruitment was the only factor closely related to the net rate of population growth in noncrash years and in all years combined, indicating that juvenile recruitment is the central factor affecting the abundance of Song Sparrows on Mandarte Island.
Nonlethal Impacts of Predator Invasion: Facultative Suppression of Growth and Reproduction
Authors: Fraser Douglas F. , Gilliam James F.
Published: May 2006

We asked whether invasions by a predator in a patchy environment altered only the death rate of the prey, or whether there were also nonlethal effects, i.e., alternations in three other vital rates: net emigration, reproduction, and individual growth rates. Field studies documented the patch use of the guppy Poecilia reticulata and the killifish Rivulus hartii in pools of a second—order forest stream in Trinidad, before and after invasion by the piscivorous fish Hoplias malabaricus. Experiments revealed that the predator altered the within—pool use of space by Poecilia and Rivulus, and caused significant emigration of the prey from pools in which it was present. Further, intimidation by the predator in an experimental stream suppressed total egg production in Rivulus by °50%, and created spatial patchiness (more eggs laid in safer pools) and temporal patchiness (pulses of eggs) in egg production. The presence of the predator also induced shifts to shallow riffle areas and significantly reduced the growth rate of adult but not juvenile Rivulus. In contrast to the familiar paradigm that increased predation rates results in compensatory increases in per capita reproductive rates and/or growth rates as the population is thinned, we found that the threat of predation suppressed rates of reproduction and growth in predator—occupied patches.
Partialling out the Spatial Component of Ecological Variation
Authors: Borcard Daniel , Legendre Pierre , Drapeau Pierre
Published: May 2006

A method is proposed to partition the variation of species abundance data into independent components: pure spatial, pure environmental, spatial component of environmental influence, and undetermined. The new method uses pre—existing techniques and computer programs of canonical ordination. The intrinsic spatial component of community structure is partialled out of the species—environment relationship in order to see if the environmental control model still holds. The method is illustrated using oribatid mites in a peat blanket, forest vegetation data, and aquatic heterotrophic bacteria. In this latter example, the new method is shown to be complementary to another approach based on partial Mantel tests.
Demography of an Age‐Structured Annual: Resampled Projection Matrices, Elasticity Analyses, and Seed Bank Effects
Authors: Kalisz Susan , McPeek Mark A.
Published: May 2006

The role of age—structured seed banks in influencing population dynamic parameters was investigated in a natural population of the winter annual Collinsia verna. Seed persistence was quantified by creating experimental seed banks in the field with seeds of known age. Survival and fecundities of adult plant stages were determined in quadrats of naturally occurring individuals in the field population. Bootstrapping was applied to the resulting data set to estimate means and 95% confidence intervals for population growth rate, stable age distributions, reproductive values, and elasticities. Analyses were performed to determine the extent of variation in these demographic parameters between two consecutive years and between three transects within each year. The results indicate that the presence of a seed bank, even of short duration, was critical to the demography of this population. The population was expanding rapidly in the 1st year of the study (growth rate 1.80), but was declining during the 2nd year (0.41). While the seed bank was demographically important in some transects during the good year (Year 1) the demographic effects of the seed bank were seen most significantly in the poor year (Year 2). The results of this study indicate the need for including age—structured seed banks in demographic analyses and the potential importance of seeds in age—structured seed banks in the shaping of plant life histories.
Nutritional Value of Hypogeous Fungus for a Forest‐Dwelling Ground Squirrel
Authors: Cork S. J. , Kenagy G. J.
Published: May 2006

Hypogeous fungi often contain high concentrations of nitrogenous compounds, vitamins, and minerals and are major dietary items for many rodents and other small mammals in North American forests. For these reasons ecologists have presumed hypogeous fungi to be of high nutritional value for small mammals. In a series of feeding experiments we investigated the nutritional quality of sporocarps (fruiting bodies) of the hypogeous fungus Elaphomeyces granulatus in the diet of the golden—mantled ground squirrel Spermophilus saturates, which is highly mycophagogus throughout its season of activity. We compared the digestive utilization of the fungus by this small mammal with digestibility of a range of plant foods available in nature. The sporocarps of E. granulatus had a high concentration of nitrogen, but 80% of this nitrogen was in the totally indigestible spores and the relatively indigestible cell walls of the peridium, and only half of the remaining 20% was protein nitrogen. Even though the squirrels ate the peridium in preference to the spore—laden core, the digestibility of the ingested nitrogen was only °50%, which is much lower than the digestibility of most plants. The digestible energy content of the fungus was low compared with that of conifer seeds and similar to that of grasses and legumes that are naturally eaten by the ground squirrels in small amounts. The overall (dry—matter) digestibility of E. granulatus sporocarps (60%) appears to be near the minimum digestibililty on which golden—mantled ground squirrels are able to maintain themselves. Therefore, sporocarps of E. granulatus cannot be regarded as a high—quality dietary items in terms of the availability of nutrients. We suggest that this is true for hypogeous fungi in general and that their value as a food resource derives mainly from their great abundance during the active season of small mammals such as ground squirrels and from their emission of strong odors, which makes mature sporocarps highly detectable. These features should maximize the yield of energy and nutrients to small mammalian mycophagists in relation to foraging effort.
Metabolism and Food Availability as Regulators of Production in Juvenile Cotton Rats
Authors: Derting Terry L.
Published: May 2006

Recent studies have shown positive relationships between mass—specific rates of basal metabolism (BMR) and production rates for interspecific comparisons of mammals. This study investigated associations between BMR and production rates by determining whether simultaneous increases in BMR and production occur at the individual level. Growth, reproductive development, and food—processing rates of juvenile Sigmodon hispidus with naturally occurring or experimentally elevated levels of BMR and with ad libitum or restricted food availability were compared. When food was readily available, elevated BMR was associated with higher ingestion rates, faster and more efficient growth, and earlier sexual development. When food was restrived, elevated BMR was associated with severely reduced mass gains. These results suggest that basal metabolism, either directly or indirectly, contributes to mammalian rates of production.
Paleoecology of Range Limits: Beech in the Upper Peninsula of Michigan
Authors: Woods Kerry D. , Davis Margaret B.
Published: May 2006

Fossil pollen was used to map American beech (Fagus grandifolia) populations in Wisconsin and Michigan during the last 8000 yr. Among questions addressed were: (1) What were routes and rates of range expansion? (2) Did range expansion follow a "wave—front" model or occur by coalescene of outliers? (3) Has the range of beech been determined by environmental tolerance or by dispersal rate? Presettlement distributions were mapped from survey records. Range expansion was reconstructed using pollen diagrams from 34 sites, most in a 30—50 km grid. We show beech moving through southern Michigan into Wisconsin and Michigan's Upper Peninsula. During much of the last 8000 yr the range of beech was relatively stable and presumably environmentally controlled. Brief delays may have been occasioned by dispersal barriers. Range expansions occurred during the last millennium, as well as 3000—2500 yr ago and 8000—5000 yr ago. Several separate colonizations across Lake Michigan formed temporary outliers. No other past outliers were detected, although modern outliers are known; however, even with closely spaced sites, outliers may have been undetected.
Nectar Standing Crops in Delphinium Nelsonii Flowers: Spatial Autocorrelation among Plants?
Authors: Waser Nickolas M. , Mitchell Randall J.
Published: May 2006

Several aspects of nectarivore foraging behavior have been interpreted as responses to spatial reward patchiness of the kind documented for Delphinium nelsonii floral nectar by Pleasants and Zimmerman (1979). Working with this same species over 3 yr, however, we were unable to detect substantial pattern in nectar standing volumes, either through contingency analyses or spatial autocorrelation. Although spatial outocorrelations between rewards of neighboring plants were positive in 5 or 6 samples examined, only one value was statistically significant. Spatial autocorrelations over longer distances were erratic. We used computer simulations of nectarivores foraging in a large plant population to explore factors that promote reward patchiness. Simulations suggest that moderate patchiness will develop at all but extremely low or high flower visitation rates. Rates were intermediate at our sites and those of Pleasants and Zimmerman, however, so visitation intensity does not seem to explain the discrepancy between our results and theirs. On the other hand, reward patchiness in simulations declined substantially as nectarivores exhibited less area—restricted foraging. Hummingbirds were important visitors at our sites, and fly farther between plants than the bumble bees that predominated at Pleasant's and Zimmerman's sites. Finally, simulations suggest that spatial patchiness is lower when interplant coefficient of variation in nectar production rate is large, as in our populations.
Regrowth and Palatability of Acacia Shoots Following Pruning by African Savanna Browsers
Authors: DuToit Johan T. , Bryant John P. , Frisby Kathleen
Published: May 2006

Stands of two species of African Acacia were monitored for 1 yr in a natural savanna ecosystem, to compare shoot regrowth and leaf chemistry in lightly browsed and heavily browsed trees. Where ungulates concentrated at a seasonal waterhole, A. nigrescens was more severely browsed than the thornier and finer leafed A. tortilis. Shoot regrowth in heavily browsed A. nigrescens more than compensated for herbivory, as net annual shoot extension was not significantly different from that in light browsed trees. Foliage of heavily browsed A. nigrescens was higher in nutrients and lower in condensed tannins than foliage of light browsed trees. We propose that severe pruning by browsing ungulates reduces intershoot competition for nutrients, promoting rapid shoot regrowth. Carbohydrate demands of rapid regrowth reduce carbon—based secondary metabolite synthesis. This results in patches of highly palatable browse that attract further browsing, generating a browsing regrowth feedback loop. Such patches may be considered analogous to grazing lawns.
Foliar Leaching, Translocation, and Biogenic Emission of 35S in Radiolabelled Loblolly Pines
Authors: Garten Charles T.
Published: May 2006

Foliar leaching, basipetal (downard) translocation, and biogenic emission of sulfur (S), as traced by 35S, were examined in a field study of loblolly pines. Four trees were radiolabeled by injection with amounts of 35S in the MBq range, and concentrations in needle fall, stemflow, throughfall, and aboveground biomass were measured over a period of 15—20 wk after injection. The contribution of dry deposition to sulfate—sulfur (SO42——S) concentrations in net throughfall (throughfall SO42——S concentration minus that in incident precipitation) beneath all four trees was >90%. Calculations indicated that about half of the summertime SO2 dry deposition flux to the loblolly pines was fixes in the canopy and not subsequently leached by rainfall. Based on mass balance calculations, 35S losses through biogenic emissions from girdled trees were inferred to be 25—28% of the amount injected. Estimates based on chamber methods and mass balance calculations indicated a range in daily biogenic S emission of 0.1—10 mg/g dry needles. Translocation of 35 to roots in nongirdled trees was estimated to be between 14 and 25% of the injection. It is hypothesized that biogenic emission and basipetal translocation of S (and not foliar leaching) are important mechanisms by which forest trees physiologically adapt to excess S in the environment.
Comparison of Detritus Dynamics in Two Tidal Freshwater Wetlands
Authors: Findlay Stuart , Howe Karin , Austin H. Kay
Published: May 2006

We have examined the generation and persistence of detritus in two contrasting tidal freshwater wetlands on the Hudson River. These wetlands offer a difference in vegetation, with Tivoli South Bay dominated by a floating—leafed macrophyte (water—chestnut, Trapa natans) and North Bay a typical Typha marsh. In South Bay, there was a large amount of water—chestnut dry biomass (400 g/m2) available to enter the detritus pool, but there was no increase in the standing stock of benthic organic matter following senescence of water—chestnut. Our estimates show that mineralization plus leaching of dissolved material are sufficient to remove much of this detritus. In the Typha marsh, there is a large amount of detritus generated (°25% of annual primary production) and this material persists as a thick layer. Decomposition of this litter is very slow (0.3/yr). A portion of the litter may be exported because decomposition alone cannot account for the observed rate of disappearance from the marsh surface. Microbial abundance was used to estimate the amount of heterotrophic biomass supported by these different types of detritus. Bacterial growth on water—chestnut detritus is relatively slow (106 cells°mg—1°d—1), resulting in a turnover of bacterial biomass in 10—36 d. Bacterial and fungal biomass associated with Typha were low, and could not account for the observed increase in nitrogen content.
Effects of Larval Density in Ambystoma Opacum: An Experiment Large‐Scale Field Enclosures
Authors: Scott David E.
Published: May 2006

This experiment was designed to measure the effects of larval density on larval traits in the salamander Ambystoma opacum, and to ascertain whether previous studies conducted at smaller spatial scales or higher densities produced artifactual results. Density effects on larval growth, body size at metamorphosis, length of larval period, and survival to metamorphosis were studied in A. opacum in large—scale (41 m2 and 23 m2) field enclosures in two temporary ponds. Each enclosure contained indigenous populations of prey (zooplankton and insects) and predators, as well as the range of microhabitats present in these natural ponds. Initial larval densities were chosen to represent high and low levels of naturally occurring mean densities. Larvae at high density grew more slowly, exhibited longer larval periods, were smaller at metamorphosis, and had reduced survival compared with those raised at the low density. At one site where experiments were conducted during three years, larval quality and quantity were enhanced in the year of longest hydroperiod. When ponds dried early, larvae metamorphosed sooner, were smaller, and exhibited lower survival. Slower growth at high densities probably increased mortality through an increased probability of desiccation in the early—drying years, and increased susceptibility to predation in all years. Larval survival in the enclosures was not elevated compared to natural ponds. as may occur in predator—free small enclosures and artificial ponds. These results suggest that, in natural ponds, the importance of intraspecific competition is dependent upon hydroperiod, and the intensity of competition influences predation risk. Thus, both density—dependent and density—independent factors affect body size and recruitment of larval A. opacum into the adult population. The use of large—scale field enclosures has advantages and disadvantages: it allows the examination of density—dependent process under natural conditions and provides high statistical power because of low variability in larval traits; however, experimental designs must be simple and underlying mechanisms are difficult to identify.
A Field Test of Risk‐Sensitive Foraging in Black‐Capped Chickadees (Parus Atricapillus)
Authors: Barkan Christopher P. L.
Published: May 2006

The combination of a nonlinear functional relationship between energy intake and fitness, along with stochasticity in foraging success, has led to the development of risk—sensitive foraging models. These models are significant because they can lead to predictions different from those of the traditional deterministic models based upon a premise of simple energy intake rate maximization. The risk—sensitive foraging models suggest that the energy state of the forager could affect its sensitivity to the variance in foraging success associated with a particular strategy. Empirical tests conducted on several species of small, captive animals have generally supported the risk—sensitivity models; however, no experiments on free—living animals had been done. I conducted the first field test of a risk—sensitive foraging model. The test was conducted on Black—capped Chickadees (Parus atricapillus), using an experimental feeder that mimicked their natural foraging substrate and allowed me to manipulate the expected value and the variances of prey. I estimated chickadee daily energy expenditure and their rate of food intake and determined them to be on a positive energy budget while using the feeder. The birds were given a choice of two prey types with equal expected values but different variances. Consistent with a risk—sensitive foraging model, the chickadees showed a clear, significant preference for the lower variance prey. Analysis of the foraging rates of the birds revealed no significant difference related to prey type other than the difference in variance. The results suggest the importance of the incorporation of environmental stochasticity into models of animal foraging.
Long‐Term Vegetation Change at a Fully Protected Sonoran Desert Site
Authors: Turner Raymond M.
Published: May 2006

To investigate desert vegetation dynamics, I undertook an open—ended study of a site that offers a combination of multiple observations through time with continuous protection from domestic livestock and other human impacts. The site is MacDougal Crater in the Sierra del Pinacate Reserve, Sonora, Mexico. Three sources of data have been used: a series of exactly matched photographs, begun in 1907; detailed permanent—plot maps, dating from 1959—1960; and an age—distribution analysis of a 170—yr—old population of Carnegiea gigantea. The crater vegetation is dominated by the woody perennials Cercidium microphyllum, Encelia farinosa, Prosopis sp., and Larrea tridentata, and the columnar cactus Carnegiea gigantea. Various populations of Larrea tridentata declined 50—90%, and Cercidium declined 60%, during the first half of this century with little or no recruitment since. Carnegiea numbers increased fourfold over the same period. A 200—fold increase in Prosopis in the playa—like crater center occurred in the early 1970s. Elsewhere on the crater floor, Encelia density increased markedly during the same period from insignificant levels in the early 1960s. Age distribution analysis for the Carnegiea population reveals three major establishment peaks during the 1790 period. Recruitment and morality records from the three sources of data are compared with regional climate records. The high mortality for some of the species was probably the result of the prolonged drought during 1936—1964. Establishment surges for some appear related to periods of unusually heavy precipitation during certain seasons. Clearly, desert communities are highly responsive to changes in the climate regime under which they grow.
Influences of Floral Variation on Pollen Removal and Seed Production in Wild Radish
Authors: Young Helen J. , Stanton Maureen L.
Published: May 2006

The effects of corolla size, pollen production, and pollinator visitation frequency on pollen removal (an estimate of male reproductive success) and pollen deposition and seed production (female success) were studied in wild radish (Raphanus sativus, Brassicaceae) in 1987. Wild radish exhibits variation in corolla size, petal coloration, and anther size. A previous study (Stanton and Preston 1988) found no correlation between corolla size and estimates of female success, suggesting that flower size may be selected upon through variation in male reproductive success. For flowers receiving a single honey bee visit, corolla size was positively correlated with pollen removal, but not with pollen deposition on stigmas or seed production. Flowers receiving an average of 3.7 honey bee visits (in the 1987 study) also showed a correlation between corolla size and pollen removal (seed production was not monitored in this study). In contrast, when flowers were visited an average of 5.7 times by honey bees (the 1988 study), there was no significant relationship between corolla size and either pollen removal or estimates of female reproductive success (pollen deposition on stigmas and seed production). Honey bees demonstrated a preference for large flowers, either by significantly overvisiting large flowers or by visiting the large flowers, before visiting smaller flowers. Under natural conditions, most radish flowers receive numerous visits, but even with frequent visitation, at least 20% of the pollen a flower produced was not removed. At the same time, flowers with greater pollen production had a larger proportion of pollen removed. Pollen removal was a positive decelerating function of honey bee visitation frequency. The number of pollen grains removed per visit declined with increasing honey bee visits. The male fitness function (pollen removal plotted against pollen production) was positive and decelerating in two experiments; positive and linear in one study. Large flowers may have greater male success than small flowers by virtue of their greater pollen production, their greater pollinator visitation frequency, and their earlier pollinator visitation. If selection is acting on floral traits in wild radish through variation in male success, increases in both corolla size and pollen production are likely to be selected for.
Comparative Demography of Three Graminoids Infected by Systemic, Clavicipitaceous Fungi
Authors: Clay Keith
Published: May 2006

Few experimental studies have addressed the demographic consequences of plant—microbe symbiosis in natural communities. In this study the survival, growth, and reproduction of tall fescue grass (Festuca arundinacea), smut grass (Sporobolus poiretii), and green sedge (Cyperus virens) infected by the imperfect fungus Acremonium coenophialum, the ascomycetous fungus Balansia epichloe, and B. cyperi, respectively, were compared with uninfected conspecifics in field experiments conducted in natural plant communities in southern Louisiana. Equal numbers of infected and uninfected ramets of each species were propagated in the greenhouse from infected and uninfected seed lots (F. arundinacea) or from field—collected infected and uninfected plants (S. poiretii and C. virens). Ramets of the two grasses were planted into a grassland community where the two species were dominant members of the community. Ramets of the sedge were planted into a marshy area where C. virens occurred naturally. Numbers of tillers were counted for each ramet upon planting for subsequent use as a covariate, and each ramet was censused from one to three times a year for three consecutive years. All survivors were then harvested and aboveground dry biomass was determined. For F. arundinacea, infected ramets exhibited higher survival, flowering frequency, vegetative tiller production, and biomass over the 3—yr period. Relative fitness, calculated from survival and inflorescence production, was nearly twice as high for infected ramets, indicating a mutualistic relationship between F. arundinacea and its endophytic fungus. For S. poiretii, growth and survival of infected and uninfected ramets were similar, but flowering was greatly suppressed in infected ramets. Relative fitness of infected ramets was only °1/10 that of uninfected ramets, indicating a pathogenic relationship. There was evidence of substantial contagious spread of the fungus in the experimental population. Infected ramets of C. virens had significantly higher survival over 3 yr and tended to be bigger than uninfected ramets. Inflorescences were produced by both types of ramets, but the inflorescences of infected ramets were always aborted. The results indicate that, within a closely related group of fungi, both pathogenic and mutualistic relationships with host plants exist. Maternally inherited, seed—borne endophytes similar to the one in F. arundinacea are found in any grasses, suggesting that analogous mutualisms are common. The greater survival and vegetative vigor of infected C. virens call into question the traditional notion of fitness as applied to plants capable of extensive asexual reproduction. The biological mechanisms responsible for the greater survival and growth of infected plants probably include induced alterations in the host plant's hormonal balance, greater tolerance to abiotic stresses, and increased resistance to herbivores and pathogens.
Herbivore Diets, Herbivore Colonization, and the Escape Hypothesis
Authors: Thomas C. D.
Published: May 2006

The escape hypothesis predicts that isolated individuals of a given plant species will escape their specialized herbivores and so survive better than individuals growing in clumps. This phenomenon could promote plant coexistence. I examined assumptions underlying the hypothesis by considering insect herbivores associated with neotropical Passiflora vines. I found that the hypothesis was probably of only limited value because plants were attacked by a range of herbivore species, most of which were not really specialized. Furthermore, some oligophagous herbivore species colonized isolated P. pittieri more readily than they colonized plants that were near to other P. pittieri plants: isolated plants did not escape herbivory. Because of the activities of herbivores that feed on >1 plant species, @>2 plant species that share herbivores may not show independent patterns of herbivory or independent local distributions.
An Ecological Analysis of Chaparral and Pine Forest Bird Communities on Santa Cruz Island and Mainland California
Authors: Yeaton Richard Ivan
Published: May 2006

A comparative study of chaparral and pine forest bird communities on Santa Cruz Island and adjacent mainland areas was made during 1969—72. Five fewer species occurred in the island chaparral bird community and four fewer species in the island pine forest bird communities. Changes in bird species composition occurred most notably in the island pine forest, which is utilized by species commonly found in the chaparral. No significant changes in niche breadths were observed for the communities in general; however, a slight tendency for increased foraging height diversities on the island was observed. Average niche overlap values between species present on both the mainland and the island are greater on the island, mainly because of an increased vertical habitat overlap. By use of the observed mainland densities and competition coefficients composed of mainland overlap values, island densities were predicted for four bird species that show greater densities in the island chaparral community. Hence, competitive release occurs in the chaparral bird community. All communities studied are in a state of equilibrium, and the island communities are at least approaching a state of saturation equivalent to that of the mainland communities.
Vegetation Patterns on a Southern Appalachian Watershed
Authors: Day Frank P. , Monk Carl D.
Published: May 2006

The vegetation on a relatively undisturbed hardwood forest watershed at Coweeta Hydrologic Laboratory, Franklin, North Carolina was sampled, and estimates of density, basal area, and above—ground biomass were computed. These vegetational parameters and five topographic variables (elevation, aspect, slope angle, distance from stream channel, and distance from water divide) were used to analyze site—species relationships on the watershed. The primary analytical techniques used were correlation analysis and principal components ordination. Major changes in the vegetation since the introduction of chestnut blight were also examined. The vegetation on the watershed was found to be dominated by oaks, though considerable change had occurred in the vegetation composition since the appearance of chestnut blight. Total basal area on the watershed was 25.6 m2/ha and the total above—ground biomass was 139,900 kg/ha. Significant correlations were found between 13 major species and one or more of the topographic variables. The ordination results revealed species groupings related to the correlation results. Distance from the stream, distance from the water divide and elevation, which produce a soil moisture gradient, were the important topographic factors determining species distribution at Coweeta.
Plant Species Diversity in Old‐Field Succession on the Georgia Piedmont
Authors: Nicholson Stuart A. , Monk Carl D.
Published: May 2006

Species diversity of vascular plants was determined in 51 seral communities ranging form 0.2—200? yr old. Species richness (S = number of species), information content (H'pop = oSsi = 1 pi loge pi), redundancy R = (H'pop o H'max)/(H'max o H'min), and evenness (J'pop = H'pop /logeS) were calculated for numbers of individuals in four strata (ground layer, shrub, understory, and canopy. Richness (S) in the strata and major growth forms (herbs, woody vines, shrubs, and trees) increased rapidly following their establishment, then at a decreasing rate throughout the remainder of succession. Equitability, (1 o R, J'pop), on the other hand, increased to near maximum levels immediately after establishment but changed little thereafter. Contrasts in seral richness and equitability trends suggest differing regulatory mechanisms–equitability being regulated primarily by short term factors (resource levels) and richness by long term factors (community stability and evolutionary time).
Coevolution of Some Seed Beetles (Coleoptera: Bruchidae) and Their Hosts
Authors: Center Ted D. , Johnson Clarence D.
Published: May 2006

Original observations and a review of the literature revealed many adaptations evolved in seed beetles (Bruchidae) to circumvent the defenses of leguminous plants against seed—feeding insects. Analysis was based on 38 bruchid species and 44 species of host plant. In response to seed toxicity the beetles either avoid the toxins or develop a resistance to them. If the plant produces a greater number of seeds a beetle is apt to feed on several seeds during its life. Other defenses such as rapid dehiscence of the pod, special characteristics of the husk, synchronization of seed set, and special characteristics of the seeds are also circumvented by the Bruchidae. This pattern of coevolution probably rarely allows the host plant to entirely exclude the seed predators.