bluuebunny/crossref_metadata_2025 huggingface dataset:
Coordination of Photosynthetic and Alkaloidal Responses to Damage in Uninducible and Inducible Nicotiana Sylvestris
Authors: Baldwin Ian T. , Ohnmeiss Thomas E.
Published: May 2006

Defense and regrowth after herbivore attack are not mutually exclusive alternatives for most plants, yet few studies have examined the coordination of the processes responsible for these two plant functions. To this end, we studied the coordination of alkaloidal and photosynthetic responses to simulated herbivory in the context of changes in leaf nitrogen in plants grown under a range of nitrate supply rates in two experiments. In the first experiment, damage—induced changes in leaf nicotine, total nitrogen, nitrate, and photosynthetic rate (PR) were monitored in same—aged undamaged leaves of young Nicotiana sylvestris plants grown in pots. In the second, the alkaloidal response to damage was uncoupled from damage by growing plants in pots for >150 d, causing them not to respond to leaf damage with increased nicotine concentrations (uninducible). We propose that the changes in PR and nicotine content induced by damage reflect the allocation of resources to regrowth and defense, respectively, and examine the predictions of the optimal defense (OD) theory regarding these responses. We have previously established that neither constitutive nor induced nicotine production is a passive consequence of a nitrogen imbalance in excess of growth requirements as is predicted by the carbon/nutrient (C/N) theory. If PR reflects the fitness value of the leaf and damage reflects a high probability of future damage, we interpret the OD theory to predict that PR and nicotine content should be correlated, and that damage should increase the amount of nicotine allocated for a given PR. Nicotine, nitrogen, and PR increased in a coordinated fashion in response to leaf damage in the inducible plants. In both experiments, PR and nitrogen were highly correlated, but damage did not affect the PR—nitrogen relationship. In the first experiment with inducible plants, nicotine and nitrogen were also highly correlated. However, damage significantly increased the slope of the nicotine—nitrogen relationship 1.6—fold. Similarly, nicotine and PR were significantly correlated and the slope of the nicotine—PR relationship increased significantly (1.9—fold) in response to damage. These results are consistent with the predictions of the OD theory. Despite this coordination, alkaloidal and photosynthetic responses can be uncoupled. Regardless of nitrate supply rate, damaged uninducible plants exhibited no significant increase in nicotine content, but significantly increased their PR in response to damage with a correlated increase in leaf nitrogen content. Nicotine and PR were not significantly correlated in undamaged plants, but were significantly correlated in damaged plants. Unless other defenses are activated in uninducible plants, these results may reflect a priority of growth over defense in uninducible plants with slow growth rates or reduced rooting volume.
Effects of Changing Density and Food Level on Metamorphosis of a Desert Amphibian, Scaphiopus Couchii
Authors: Newman Robert A.
Published: May 2006

Amphibians that breed in temporary ponds provide a good opportunity to study the ecological and evolutionary consequences of environmental variability. Ephemeral aquatic habitats provide larval amphibians a transient and highly variable opportunity for growth. In the desert ponds used by Couch's spadefoot toad (Scaphiopus couchii), tadpole density varies considerably among pods and often increases within a pond as it dries. Models of optimal size and timing of metamorphosis predict that, relative to constant high resource environments, metamorphosis should occur at a smaller size in constant low resource environments, and smaller and earlier in declining resource environments, assuming all else is equal. Considerable evidence supports the first prediction, but few studies have examined the effect of decreasing resource availability. I conducted an experiment to examine the effects of increasing density and decreasing food level on metamorphosis in Couch's spadefoot toads. Tadpoles at constant high per capita food metamorphosed at the largest sizes. At low or decreasing per capita food, tadpoles metamorphosed at a uniformly small size, but varied in the time required to reach that size. Tadpoles from both increasing density and decreasing food treatments metamorphosed earlier than tadpoles from constant density, constant food treatments. These results support the idea that tadpoles can respond adaptively to resource variation.
Anolis Sagrei and Anolis Cristatellus in Southern Florida: A Case Study in Interspecific Competition
Authors: Salzburg Mark A.
Published: May 2006

This study examined the possibility of interspecific competition between Anolis sagrei and A. cristatellus, two introduced species of lizards that co—occur in the vicinity of Miami, Dade County, Florida. In an area of sympatry, structural habitat use differed significantly between the two species. Comparison of habitat use by sympatric A. sagrei and allopatric sagrei in immediately adjacent control areas revealed no significant differences. Upon removal of A. cristatellus from an area of sympatry, habitat use by A. sagrei shifted in a manner consistent with an hypothesis of interspecific competition; no such shift occurred in adjacent control populations of allopatric A. sagrei. Studies of competition that are based solely upon comparisons of resource use in sympatry and allopatry often yield spurious conclusions. On the assumption that the resource use measured in this study bears on the fitness of the two species, results of this study provide strong experimental evidence that A. sagrei and A. cristatellus are presently in competition in southern Florida.
The Effects of Host‐Finding and Colonization Rates on Abundances of Parasitoids of a Gall Midge
Authors: Hopper Keith R.
Published: May 2006

Five wasp species parasitize Rhopalomyia californica, a midge that forms galls on Baccharis pilularis. The relations between wasp abundances and colonization and host—finding rates were addressed with two types of experiments. In the first experiment, galls were stripped from a large area of host plants in the field, and recolonization by the midge and wasps was monitored. Colonization rate, which includes dispersal, search, and successful parasitization, correlated with field abundance of the wasp species in undisturbed habitat, with one exception. Torymus koebelei was the slowest colonizer but was the most abundant wasp. In a second set of experiments, the host—finding rates of the wasp species were measured in the greenhouse, by exposing hosts at field density to single female wasps. The wasp species differed in the number of hosts parasitized per day. Furthermore, this measure of host—finding rate correlated with abundance in the field, again with one exception. T. koebelei was third in host—finding ability, although first in abundance. Colonization rate measured in the field correlates strongly with host—finding rate measured in the laboratory. This suggests that most of the differences among the wasps in colonization rate resulted from differences in ability to find hosts on a small spatial scale (e.g., on a single host plant) and not from differences in dispersal over longer distances. Host—finding rate and fecundity were found not to correlate. These experiments and others on the same system suggest that host—finding rate and larval competitive ability determine the abundances of these wasps.
Resource Limitation in Populations of the California Vole
Authors: Ford R. Glenn , Pitelka Frank A.
Published: May 2006

The roles of food and water supply and of dispersal in the summer population declines of Microtus californicus were studied in two sets of manipulation experiments in a predator—free environment. The first set of experiments was carried out during a year of high vegetative productivity. Enclosed populations were either provided with ad libitum food and water during the period of summer population decline, or maintained as controls. Populations that were supplemented increased steadily during the summer, while control populations declined at moderate rates. Differences between control— and supplemented—population trends were generally attributable to increased reproductive activity and lowered age at sexual maturity in the supplemented populations. The second set of experiments was carried out during a year of low vegetative productivity. Both enclosed and unenclosed populations were supplemented or treated as controls. During the period of winter—spring population increase, enclosed populations showed better survivorship than did unenclosed populations. During the summer, both enclosed and unenclosed control populations declined rapidly to extinction, while enclosed and unenclosed supplemented populations declined at moderate rates and began to increase again in the fall. Enclosure appeared to have no measurable impact on decline phase population performance, but food and water supplementation improved survivorship and increased the proportion of females breeding. Analysis of variance indicated that both year of study and the supplemental treatment had a significant effect, among enclosed populations, on the rate of population change during the period of summer decline. During the 2nd yr of study, rate of population change and survivorship were significantly altered by supplement, but enclosure had no effect on declining populations. Populations from both years of study showed a significant negative correlation between initial densities and rate of population change during the winter—spring period. among both enclosed and unenclosed populations. The mechanism underlying this density dependence, however, was not apparent. During the decline phase, initial densities had little effect on rates of population change, but vegetative production was significantly correlated with survivorship, breeding rates, and rate of population change.
Plant Chemistry, Insect Adaptations to Plant Chemistry, and Host Plant Utilization Patterns
Authors: Zangerl Arthur R. , Berenbaum May R.
Published: May 2006

The role of plant chemistry in determining the suitability of herbivore—free individuals within a plant population has rarely been investigated. In this study, we examined the importance of furanocoumarin chemistry in determining distribution patterns of the parsnip webworm (Depressaria pastinacella) in a population of its host plant, wild parsnip (Pastinaca sativa). Parsnip webworms feed almost exclusively on wild parsnip, a species that produces furanocoumarins, compounds that are toxic or deterrent to many insects. Samples of plants hosting actively feeding parsnip webworms (attacked) and plants exhibiting no damage (unattacked) were collected in the field and brought to the laboratory, where they were examined for content of three furanocoumarins, xanthotoxin, bergapten, and sphondin. In addition, they were bioassayed with ultimate—instar parsnip webworms; growth, consumption rate, and rate of in vitro metabolism of furanocoumarins were compared between larvae fed undamaged tissue from attacked plants and larvae fed tissue from unattacked plants. Attacked plants contained less than half the amount of xanthotoxin and sphondin, two of the three furanocoumarins assayed, than did unattacked plants. Growth and consumption rates of larvae fed tissue of unattacked plants also were reduced significantly (40 and 24%, respectively) compared to larvae fed attacked plants. Moreover, in vitro metabolism of xanthotoxin and bergapten was reduced by 50% in larvae fed tissue from unattacked plants compared to larvae fed tissues of attacked plants. Cluster analysis placed plants into three groups based on furanocoumarin chemistry. One group, characterized by high concentrations of all three furanocoumarins, contained only unattacked individuals. A second group consisting of plants intermediate in furanocoumarin concentration, contained predominantly unattacked individuals, and last group, characterized by low concentrations of all three furanocoumarins, was dominated by attacked individuals. When large larvae are fed plant tissues with below—average total furanocoumarin concentrations, consumption rate is the most important larval characteristic determining growth rate. However, when larvae are fed plant tissues with above—average concentrations of furanocoumarins, the capacity to metabolize furanocoumarins is also important. These results indicate that plant chemistry, relative to larval ability to tolerate plant chemicals, can to a large extent explain the distribution of this insect within a population of its host plant.
Patch‐Size Effects on Plant Phenolics in Successional Openings of the Southern Appalachians
Authors: Shure Donald J. , Wilson Lawrence A.
Published: May 2006

We examined the seasonal changes in plant chemical defenses of key tree species (Robinia pseudoacacia, Liriodendron tulipifera, Acer rubrum, and Cornus florida) regenerating during early succession in different patch sizes within Southern Appalachian forests. Trees of each species were sampled from five sizes of forest openings (0.016, 0.08, 0.4, 2.0, and 10 ha; two replicates each) and from the understory of surrounding forests. Leaves of rapidly growing, pioneer species (Robinia and Liriodendron) were slightly less tough and contained lower levels of hydrolyzable tannins and total phenolics, but higher condensed tannin concentrations than the slower growing, more shade—tolerant species (Acer and Cornus). These results generally support earlier findings indicating the evolutionary development of greater levels of constitutive phenolics in slow—growing than fast—growing species. The differential use of condensed and hydrolyzable tannins by slow— and fast—growing species may reflect adaptive trade—offs in the allocation of different types of phenolics. Regenerating trees often had tougher leaves and contained higher levels of phenolics in large than small openings by midsummer. The differences in phenolics were more pronounced for condensed tannins and when comparing the smallest canopy openings with larger patch sizes. Plant phenolics were also generally lower in understory trees than in conspecific sprouts in openings. These findings may reflect phenotypic carbon/nutrient adjustments as light availability changes across the patch—size gradient. The greater sunlight in large than small openings apparently promotes excess carbon production and a metabolic buildup of phenolic compounds. Independent measures of phenolic compounds peaked at different times during the growing season. Hydrolyzable tannins reached peak levels quite early whereas condensed tannins increased later in the growing season. Total phenolics peaked in midsummer. Herbivore damage was consistently high (5—9%) on Robinia sprouts in all openings. Insect herbivory on the remaining three species (1—5%) was significantly lower in large than small patch sizes. Tree species regenerating in the high sunlight of larger openings in Southern Appalachian forests thus tend to have tougher leaves, contain greater phenolics, and experience less herbivore damage than conspecifics in small openings.
Gas Exchange Dynamics in C^3 and C^4 Grasses: Consequence of Differences in Stomatal Conductance
Authors: Knapp Alan K.
Published: May 2006

In full sunlight (photosynthetic photon flux density, 0.4—0.7. μm, PPFD), stomatal conductance to water vapor (g) and transpiration rates (E) are usually higher, and water use efficiency lower, in C3 than C4 species. In the C3 grass Bromus inermis, the difference in conductance between full sun (photosynthetic photon flux [PPFD] > 1500 μmol°m—2°S—1) and shade (PPFD ° 400 μmol°m—2°s—1) was twice as large as in the C4 species Sorghastrum nutans (226.7 vs. 97.8 mmol.m—2.S—1, respectively). Similar differences were measured in four additional C3 and C4 grasses. Variability in PPFD incident on leaves is very common in grasslands, and stomata are known to respond relatively rapidly to fluctuations in this resource. This study evaluated the consequences of different responses in stomatal conductance between C3 and C4 grasses during periods of sunlight variability. Photosynthesis (A, measured as CO2 uptake) and g varied substantially in both B. inermis and S. nutans as PPFD fluctuated between sun and shade levels (4—10 min intervals). Time constants derived form first—order equations describing responses in g after abrupt changes in PPFD indicated that g approached steady—state levels more rapidly in S. nutans than in B. inermis. However, because of the greater magnitude of change in g between sun and shade PPFD in B. inermis, the absolute rate of change in g in this C3 grass was twice as rapid compared to S. nutans (25.7 vs. 12.4 mmol°m—2°s—1°min—1 for reductions in g, respectively). New steady—state levels in g were achieved in 12—13 min after a change in PPFD in S. nutans and 15—19 min in B. inermis. A statistically based two—state (sun/shade) simulation model was used to quantify costs in A and E of the nonsteady—state responses measured for both species as PPFD varied. Costs were calculated as the difference between model output of A or E based on field responses and model output based on hypothetical gas exchange responses in which A and g changed instantaneously. Results suggested that even though the absolute rate of change in g changed was greater in the C3 grass, costs in E during periods of short—term (2—6 min) variations in PPFD were as much as sixfold higher than in the C4 species. Lower cost in E in C4 species during periods of variable PPFD (due to inherently smaller alterations in g) is a previously unrecognized consequence of the C4 photosynthetic pathway. C4 species, and grasses in particular, often are most successful in high—PPFD, water—limited environments in which leaf—level variability in PPFD occurs daily from cloud cover and within—canopy shading. In these environments, stomatal responses that conserve water under nonsteady—state conditions may contribute significantly to the success of C4 species.
Effects of Fragmentation on the Richness of Vertebrates in the Florida Scrub Habitat
Authors: McCoy Earl D. , Mushinsky Henry R.
Published: May 2006

Structures of habitat fragments interact with autecologies of resident species to produce patterns in taxonomic richness. Understanding this interaction is crucial to habitat conservation. We studied the vertebrates inhabiting fragments of the severely threatened sand pine scrub habitat of interior peninsular Florida. We surveyed 16 scrubs in south—central Florida that were categorized as either “large,” “medium,” or “small.” For each scrub, we determined (1) vegetational structure, (2) vegetational composition, (3) distance to the nearest scrub, (4) distance to the nearest larger scrub, (5) presence/absence of potential “corridor“ habitats between scrubs, (6) types of habitats between scrubs, (7) distance to the nearest permanent water, (8) percent coverage of surrounding habitats, and (9) area reduction over time. The principal methods used to census nonavian taxa were pitfall and double—ended funnel traps, while the principal method for avian taxa was transect surveys. Many of the nine scrub attributes were intercorrelated. Scrub size was the only attribute that correlated strongly with vertebrate richness. Distance, both to permanent water and to the nearest scrub, correlated strongly with nonavian richness after the influence of size was removed, however. We detected significant nestedness, but when the taxon—area relationship was taken into consideration, we could not demonstrate that any taxa were excluded from small scrubs. We constructed joint lists of taxa from various hypothetical “archipelagos” of the 16 individual scrubs, and found that the archipelagos often supported more taxa than large individual scrubs. We found no tendency for small scrubs to be depauperate, in general, and no matter how rare taxa were identified, they were more common than expected in medium and small scrubs, and less common than expected in large scrubs. The mere observation of either a taxon—area relationship or a nested taxonomic composition reveals little about underlying ecological processes. Our detailed analysis leads us to conclude that no evidence indicates the need for single large scrub reserves for resident vertebrates: an archipelago of individually smaller reserves could support at least as many taxa. This conclusion is based solely on the simple presence—absence of taxa in scrubs that were treated as replicates within size categories, however, and consideration of the abundances and distributions of individual taxa, and of the variation among scrubs of similar size, may lead to a different conclusion.
Invasions and Competitive Displacement among House Geckos in the Tropical Pacific
Authors: Case Ted J. , Bolger Douglas T. , Petren Ken
Published: May 2006

Since the 1930s the common house gecko, Hemidactylus frenatus, a sexual species, has been inadvertently introduced to many tropical Pacific islands. Using mark—recapture censuses and visual gecko searches we found that Lepidodactylus lugubris, an asexual gecko previously common on these islands, is nearly 800% more abundant on buildings in the urban/suburban environment on islands that lack H. frenatus than it is on islands were H. frenatus is present. On buildings in Hawaii and Fiji that have been surveyed in different years, the proportion of H. frenatus relative to L. lugubris has significantly increased over time. The degree of numerical dominance is also related to climate and habitat: L. lugubris is relatively more common on the more mesic, cooler, windward sides of large islands compared to the more arid sides of islands and in general H. frenatus does not penetrate forest habitats, where L. lugubris remains one of the most common geckos. Overall, buildings with external electric light have more geckos than unlit buildings. Where the two species occur on the same buildings H. frenatus is closer, on average, to the prime feeding sites near lights that attract insects. In the absence of H. frenatus, L. lugubris is found closer to the lights. We hypothesize that the previously demonstrated agonistic dominance of H. frenatus over L. lugubris (Bolger and Case 1992) is likely to lead to competitive superiority when insect prey are concentrated into patches that are structurally simple (like flat building walls with lights), allowing easy detection of prey and intruders.
Light‐Limited Growth and Competition for Light in Well‐Mixed Aquatic Environments: An Elementary Model
Authors: Huisman Jef , Weissing Franz J.
Published: May 2006

Light is never distributed homogeneously since it forms a gradient over biomass. As a consequence, the common theories on nutrient competition are not applicable to competition for light. In this paper, we investigate a model for light—limited growth and competition among phytoplankton species in a mixed water column. The model is based on standard assumptions such as Lambert—Beer's law of light absorption, a Monod equation for carbon uptake, and constant specific carbon losses. By introducing the concept of quantum return, we show that the dynamics of growth and competition can be quantified not only in terms of depth but also directly in terms of light availability. We argued that the crucial measure for phytoplankton growth is not a "critical depth" but a "critical light intensity," I*out. For each species, I*out corresponds to the equilibrium light intensity at the bottom of a water column when the species is grown in monoculture. I*out plays a role similar to the "critical nutrient concentration" R* used in models of nutrient—limited growth. For a constant light supply, the species with the lowest I*out will competitively exclude all other species. There are, however, some important differences between R* and I*out. Whereas R* reflects both the local and the total balance between nutrient uptake and nutrient losses, I*out only reflects the total carbon balance. Moreover, I*out decreases with increasing light supply, whereas R* is independent of the nutrient supply. As a consequence, (1) the outcome of competition for light may depend on the light supply, (2) the compensation point is not a good predictor for the outcome of competition, (3) the resource ratio hypothesis does not apply when species compete for both nutrients and light. The outcome of competition for nutrients and light may depend on the nutrient and light supply, on the mixing depth, and on the background turbidity due to inanimate substances.
Phosophorus Nutrition in White‐Tailed Deer: Nutrient Balance, Physiological Responses, and Antler Growth
Authors: Grasman Brian T. , Hellgren Eric C.
Published: May 2006

Phosphorus (P) is considered, except for sodium, to be the most limiting, mineral for growth and reproduction in large mammalian herbivores worldwide and a predictor of ungulate distribution and abundance. Knowledge of phosphorus nutrition and seasonal adaptation to dynamics in mineral needs and availability in wild herbivores is incomplete. This paper reports data on experiments designed to evaluate physiological responses to low dietary P and to determine phosphorus requirements in male white—tailed deer (Odocoileus virginianus), a browsing cervid with presumed elevated mineral requirements during antlerogenesis. Adult (>2 yr) male deer were housed individually and placed on three levels of dietary P (0.14—0.19%, 0.24—0.27%, and 0.34—0.36% of diet dry mass) from June 1989 to September 1991. We conducted balance trials to measure seasonal P retention and to model annual and seasonal dietary P requirements. Data were collected on serum chemistry, endocrine function, antler growth, antler composition, and tail bone composition in two trials designed to test the hypothesis that P is limiting to deer antler growth in southwestern habitats. Phosphorus balance varied by diet and tended to fluctuate seasonally. Retention was highest for the 0.19% P diet and greatest during the fall. The variation in P balance resulted from increased efficiency in P metabolism in animals on the 0.14—0.19% P diet. Dietary phosphorus requirements were estimated at 72.2 mg°kg—0.75°d—1 (0.14% P at study intake rates) on an annual basis, changing seasonally from 94.4 mg°kg—0.75°d—1 (0.16%) during spring to 57.4 mg°kg—0.75°d—1 (0.11%), and 59.6 mg°kg—0.75°d—1 (0.12%) during summer and fall, respectively. Feed consumption, body mass, antler size and composition, and bone composition did not vary by diet. Diet x month interactions were observed in 1990 and 1991 for serum activity of alkaline phosphatase (AP), manifested in higher activity on the 0.14—0.19% P diet during the peak period of antler growth . changes in AP were an indicator of physiological compensation to low dietary P. Other key serum chemistries and hormonal concentrations did not vary by diet. These results suggest that P limitation for antler growth is unlikely for adult browsing cervids. We predict sensitivity of the distribution and abundance of male cervids to P limitation to be a function of feeding strategy and body size.
Mechanism of Southward Migration of a Noctuid Moth [Agrotis Ipsilon (Hufnagel)]: A Complete Migrant
Authors: Showers William B. , Keaster Armon J. , Raulston Jimmy R. , Hendrix William H. , Derrick M. Ellison , McCorcle Michael D. , Robinson John F. , Way Michael O. , Wallendorf Michael J. , Goodenough John L.
Published: May 2006

To determine whether an economically important moth species emigrates from the western Corn Belt (Iowa and Missouri) to the gulf coastal areas of Louisiana and Texas each autumn, adult populations of Agrotis ipsilon (Hufnagel) males were monitored with traps baited with female sex pheromone, 16 July to 31 October 1986—1987. Concurrently, to estimate the potential for southward displacement of A. ipsilon, moths marked with a vital dye, Calco Red N—1700, were released at dusk 28 September and 2 October 1986, and 10 and 17 September 1987, from Ankeny, Iowa. Increased nightly moth—capture in Texas was positively related to numbers of nights with near—surface (100—300 m) northerly airflow (southward displacement of air) and decreased moth—capture in the western Corn Belt. Capture in Louisiana was also negatively correlated with capture of moths in the western Corn Belt. The recapture in autumn 1986 of two moths in Missouri, 397 km and 460 km south of the release site, respectively, was possible because of nightly near—surface northerly airflow. Near—surface northerly airflow the night of and subsequent to 17 September 1987 was responsible for a marked moth being recaptured near Neota, Illinois, and another being recaptured near Brownsville, Texas, 276 km southeast and 1900 km southwest of the release site, respectively. We conclude that during late summer and autumn, A. ipsilon, and probably other insect species, respond to deteriorating environmental conditions by using certain weather systems with northerly (southward displacement) near—surface airflow to complete midcontinental migrations to more favorable areas.
Hatching Asynchrony in American Goldfinches: An Experimental Study
Authors: Skagen Susan Knight
Published: May 2006

I examined Lack's (1954, 1968) hypothesis that asynchronous hatching is an adaptive response to food shortage during the breeding season by comparing growth and survival of nestlings in asynchronous and artificially created synchronous broods of American Goldfinches (Carduelis tristis). I also examined the effects of seasonal and environmental factors on nestling growth and survival and on hatching asynchrony. Nestlings in asynchronous broods were more likely to diverge in mass and did so at a faster rate than those in synchronous broods. The lightest nestlings of asynchronous broods grew more slowly than their heavier nestmates and than all nestlings in synchronous broods. Brood reduction occurred more frequently in asynchronous broods. Survival and growth rates decreased throughout the breeding season and during inclement weather. Hatching intervals increased during the season. When brood reduction or differential growth among nestmates occurred in asynchronous broods, suggesting that the energy available for growth was limiting, heavy nestlings in asynchronous broods grew nonsignificantly faster than heavy nestlings in concurrent synchronous broods. This trend implies that when insufficient food is delivered to nestlings, asynchronous hatching may provide a slight advantage for older nestlings. Growth rates of all nestlings, however, were greatest in highly synchronous broods. Explanations other than growth of young must be invoked to explain why extreme synchrony is not more common in goldfinches. I discuss constraints on the normal pattern of hatching asynchrony characteristic of this species. The proximate mechanism for differential feeding and brood reduction is discussed.
Body Size, Interspecific Interactions, and Use of Foraging Sites in Tits (Paridae)
Authors: Alatalo Rauno V. , Moreno Juan
Published: May 2006

In mixed—species flocks, the heavier tit species forage in the inner tree parts, while the smaller species exploit food items of the outer canopy. Using aviary experiments we studied the relative importance of two possible explanations for body size—foraging site correlation. The first hypothesis is that larger bodied tits are more efficient than smaller ones in exploiting the inner tree parts, while the reverse would be true in the outer canopy. However, the heavier species, the Willow Tit Parus montanus, was not more efficient in food searching in the inner tree parts than the smaller species, the Coal Tit Parus ater. The second hypothesis is based on interference competition with the socially dominant (heavier) species selecting the most profitable foraging sites, and forcing the smaller species to less rewarding places. In fact, in confrontation experiments only individuals of the smaller species changed their foraging sites in the presence of an individual of the other species in the aviary. In sum, body size influences social dominance relations and thereby relegates the smaller species to poorer foraging sites, but presumably gives them an advantage in exploitation competition via reduced energy demands for basal metabolism.
Demography of a Habitat Generalist, The White‐Footed Mouse, in a Heterogeneous Environment
Authors: Adler Gregory H. , Wilson Mark L.
Published: May 2006

A population of white—footed mice (Peromyscus leucopus) in southeastern Massachusetts was lived—trapped monthly in five habitat types for 5 yr in order to study the demography of a habitat generalist. We identified three demographic groups (two of low density and one of high density), which differed primarily in density, adult survival, proportion of males breeding, and variability in the proportion of males. The low—density segments of this population were at times able to achieve demographic performance equivalent to the high—density segments. Several demographic variables were related linearly to microhabitat gradients derived from a principal components analysis of 24 habitat variables, but similar demographic structure was found in different habitat types. A simple model is presented that relates demography to environmental suitability in a habitat generalist. According to this model, population density, productivity, and survival increase, and numerical and demographic variability decrease, along a gradient of increasing environmental suitability. Intensity of intrinsic regulation also may increase with environmental suitability, which would contribute to population stability. We hypothesize that in habitat generalists such as Peromyscus leucopus, population numbers are regulated along a stable trajectory in areas of high suitability, but may show little or no intrinsic regulation in poorer habitats. Furthermore, demographic structure may shift as environmental conditions improve or decline in quality. This highly flexible demographic structure may enable populations of generalists to persist in poor habitats.
Experimental Analysis of the Grazing Interaction Between a Mayfly and Stream Algae
Authors: Hill Walter R. , Knight Allen W.
Published: May 2006

The interaction between the grazing mayfly Ameletus validus and periphyton in a small, northern California stream was examined by manipulating the density of the mayfly in flow—through plexiglass channels. Containing natural cobble substrate and located in situ, the channels established an initial gradient of A. validus at 0, 0.5, 1, and 4 times the average density of the mayfly in Barnwell Creek. After 23 d, A. validus significantly depressed periphyton standing crop: ash—free dry mass (AFDM) at the 0, 0.5, 1, and 4 N grazer densities was 5.067 ± 1.389 (se), 1.829 ± 0.173, 1.741 ± 0.325, and 1.009 ± 0.199 g/m2 (ANOVA: P < .01). The mayfly also influenced two structural attributes of the periphyton, increasing the amount of chlorophyll a per unit biomass and decreasing the relative contribution of the loose, upper layer to total periphyton biomass. Principal component analysis of algal relative abundances contrasted the effect of grazing on two groups of diatoms. A group of species found primarily in the loose layer of periphyton (Nitzschia spp., Surirella spiralis, Cymatopleura elliptica, and Navicula cryptocephala) was disproportionately reduced in abundance, while an adnate group (Gomphonema clevei, Achnanthes minutissima, Synedra ulna, Rhoicosphenia curvata, and an undescribed species of Epithemia) increased its relative abundance with increasing grazing pressure. The decline in relative abundance of the loose layer diatoms did not appear to result from selective consumption by A. validus, but may have been mediated by a reduction of inorganic sediment in the periphyton by A. validus. Inorganic sediment was highly correlated with the relative abundances of the loose layer group of diatoms, a group of species that are adapted for locomotion on sediment substrates. A. validus growth in the experimental channels was strongly density dependent. Growth in length over 23 d for the 0.5, 1, and 4 N treatments was 2.24 ± 0.17, 1.80 ± 0.23, and 1.15 ± 0.25 mm (ANOVA: P < .01). The significantly greater growth of A. validus at subnormal densities in the experimental channels suggested that the A. validus population in Barnwell Creek was food—limited.
Beech Range Extension and Vegetation History: Pollen Stratigraphy of Two Wisconsin Lakes
Authors: Webb Sara L.
Published: May 2006

The pollen stratigraphy of two small lakes in eastern Wisconsin (Radtke Lake, Washington county, and Gass Lake, Manitowoc County) records the Holocene (past 10 000 yr) spread of beech (Fagus grandifolia: Fagaceae). Radiocarbon dates were obtained for the oldest stratigraphic levels at which beech pollen appeared consistently in amounts > 0.5% of terrestrial pollen. A spatially continuous pattern of beech expansion from the north was ruled out, because beech trees grew in Wisconsin by 6000 BP, 2000 yr before adjacent populations were established to the north. Alternative geographic patterns of speed (from the south or east) were spatially discontinuous, requiring seed dispersal distances of perhaps 25—130 km. That beechnuts could be dispersed across such distances suggests (1) the involvement of Blue Jays, Passenger Pigeons, or other vertebrates, and (2) a capacity for reaching climatically controlled range limits, given sufficient time despite such discontinuities in habitat. A lag 1000—2000 yr between the establishment of source populations in Michigan and Indiana and the appearance of beech in Wisconsin suggests that low—probability dispersal events were involved and that dispersal constraints limited the range of beech during this time, although climatic and edaphic explanations for the lag cannot be ruled out. Pollen data from the two sites reveal other features of vegetation history in eastern Wisconsin: an open Picea—Fraxinus woodland prior to 11 000 BP; sequence of Picea, Abies, Betula, and then Pinus forests between 11 000 and 7500 BP; the establishment of a coniferous/deciduous forest ecotone ("tension zone") ° 7000 BP in this region; and the presence of Quercus—dominanted deciduous forests from 7000 BP until 110 BP (time of Euro—American settlement), a period punctuated by a gradual decrease in Ulmus populations (° 4500 and 5700 BP at the two sites) and by an increase in mesophytic tree abundance at the expense of Quercus after 3500 BP.
Modeling Markovian Dependence in Populations of Aralia Nudicaulis
Authors: Kenkel N. C.
Published: May 2006

I examined spatial patterns of two populations of the clonal herb Aralia nudicaulis for evidence of spatial inhibition among neighboring ramets. Second—order spatial analysis revealed that ramet patterns of both populations were regular at local spatial scales, a result consistent with the proposition that localized, inter—ramet interactions are important in reducing spatial overlap. Localized ramet interactions suggest Markovian dependence, which is defined when an event (e.g., occurrence of a ramet) at X is dependent solely on the existence or otherwise of an event within a distance ° of X. Evidence of Markovian dependence in the populations was tested by fitting Markov point—process models to the observed ramet patterns. The populations conformed well to the Markov model, the results indicating that both ramet spatial patterns were Markov of range ° = 18 cm. This inhibition distance corresponds closely to the mean horizontal radius of an A. nudicaulis ramet, indicating that interactions occur at the spatial scale of the individual. I suggest that a likely mechanism for the development of locally regular spatial patterns in these populations in inter—ramet competition for a limiting resource, probably light.
On Measuring Fitness Gain Curves in Plants
Authors: Emms S. K.
Published: May 2006

The great variety of sexual systems possessed by flowering plants can be understood in terms of sex allocation theory, which assumes that individuals allocate resources to male vs. female reproductive structures so as to maximize total fitness. The relative shapes of the fitness gain curves for each sex can be used to predict and explain such allocation patterns. Several recent studies have attempted to measure the shapes of these gain curves. Typically, these studies have regressed estimates of male or female fitness against flower number for a set of plants of different size. Here I argue that this method is inappropriate, and that it can lead to incorrect conclusions about the true shapes of the gain curves if some third, causal factor, such as genotype, size, or environmental conditions, affects both allocation and the fitness returns from that allocation. I suggest that experimental manipulation of flower number is likely to be the most useful and easily interpretable technique for estimating the shapes of gain curves in plants.
Effects of Meadow Vole Population Density on Tree Seedling Survival in Oil Fields
Authors: Ostfeld Richard S. , Canham Charles D.
Published: May 2006

Voles (Mammalia: Arvicolidae) are reported to be key consumers of tree seedlings in some habitats, but their importance in old—field succession is not known. Natural populations of voles fluctuate widely in density, and we sought to determine the effects of vole density on seedling predation. We used a randomized block design, with meadow voles maintained at three density levels (low, medium, high; three replicates of each) in field enclosures of 17 mo. We introduced young seedlings of five tree species into the enclosures. Three cohorts of seedlings were planted under intact vegetation, and one was planted in small areas from which we had cleared all vegetation. Overall predation rates were high for all cohorts planted under intact vegetation. There was a significant effect of vole density on seedling predation rate for two of these three cohorts; nearly all seedlings were destroyed in enclosures with high vole density. Predation rates by voles were highest for white ash and red maple, intermediate for white pine, and lowest for eastern hemlock and red oak. For the cohort planted in small clearings, mean predation rates were low and there was no density effect. Spatial variation in seedling predation rates was higher in enclosures with low and medium densities of voles than in high—density enclosures. In three of the cohorts there was a significant block effect, with the same block consistently having the highest predation rates. Evidence suggests that the block effect was caused by genetic differences among voles. Seedling predation by voles can: (1) delay old—field succession by reducing the number of tree invaders; (2) create patchiness by concentrating surviving seedling in certain microsites; and (3) alter relative abundances of invading species via differences in palatability or apparency. We suggest that the population dynamics of voles be viewed as a keystone process in old—field succession.
Body Size‐Prey Relationships in Insectivorous Marsupials: Tests of Three Hypotheses
Authors: Fisher D. O. , Dickman C. R.
Published: May 2006

There is a strong positive correlation between the body sizes of dasyurid marsupials and the mean sizes of the invertebrate prey. This study tests three hypotheses proposed to explain this relationship, using 21 species of dasyurids (body mass range: 5–200 g) throughout continental Australia: (1) the maximum prey size that can be physically handled increases with dasyurid size due to the restricted gapes of forces of biting of smaller dasyurids; (2) the prey size that maximizes rate of energy intake varies with dasyurid size, with animals preferentially consuming the most profitable prey; and (3) prey sizes encountered during foraging vary with dasyurid size. Field observations of dasyurids were made at 20 study areas between January 1980 and February 1991. Most dasyurids were live—trapped, weighed, and sexed before being released at the trap site. We then tracked each animal and sampled potential prey in their foraging microhabitats. Dasyurids diets were examined using scats analysis. We measured rate of energy intake for captive dasyurids by offering different—sized cockroaches, measuring processing times, and calculating the energy content of cockroaches used. The first hypothesis was rejected. Both large and small dasyurids could grasp and pierce the largest prey offered in the laboratory, which were similar in size to the largest prey potentially available in the field. The second and third hypotheses were supported. Individuals of the smaller species maximized rates of energy gain in captivity by feeding preferentially on small prey, and their foraging trails in the field traversed microhabitats where mean potential prey size encountered was small (2–4 mm long). In contrast, larger dasyurids preferred large prey, obtained a greater rate of energy gain from them, and foraged in microhabitats where prey lengths averaged @>6 mm. There was a tendency for larger dasyurids to include progressively larger prey in their diets than were available, on average, along their foraging trails. This may reflect increasing selectivity for larger prey. However, it may also reflect a reduced susceptibility to predation on increased competitive ability for large dasyurids in productive microhabitats, and hence reflect a size—based advantage in encountering large prey. These results indicate that net energy yield and the use of microhabitats with different prey sizes are the most important determinants of the body size—prey size relationship in dasyurid marsupials.
The Life History Consequences of Resource Depression in Ceriodaphnia Quadrangula and Daphnia Ambigua
Authors: Lynch Michael
Published: May 2006

An analysis of the life history and energetics responses of two small planktonic cladocerans, Daphnia ambigua and Ceriodaphnia quadrangula, to food limitation is presented. The results are consistent with an earlier study of D. pulex in that the size—specific instar durations and proportional investments in reproduction are independent of the food level. Compared to D. pulex, these smaller species spend more time in early instars but less time in adult instars. Of the energy available for (growth + reproduction), a much greater fraction is allocated to reproduction than in the case of similar—sized D. pulex, and it quickly reaches an asymptote of 94—97%. As in the case of D. pulex, the net rate of energy intake reaches a plateau shortly after maturity. This, combined with the increased cost of molting as the animal grows, causes age—specific reproduction to level off and eventually decline. The maximum rate of net energy intake is much lower and the cost of molting much higher for D. ambigua and C. quadrangula than for the larger D. pulex. This puts an upper limit on body size for the two smaller species that is much lower than that attainable in D. pulex. The data suggest the possibility that the size of planktonic cladocerans may be less a direct consequence of size—selective mortality than a correlated response to selection operating on energy—related traits.