bluuebunny/crossref_metadata_2025 huggingface dataset:
Population Consequences of Predation‐Sensitive Foraging: The Serengeti Wildebeest
Authors: Sinclair A. R. E. , Arcese P.
Published: May 2006

The "predation—sensitive food" (PSF) hypothesis proposes that both food and predation necessarily limit populations, because as food becomes limiting animals take greater risks to obtain more food, and some of these are killed. Alternative hypotheses are "predator regulation" where predators hold the prey population well below starvation levels; and "surplus" predation where predators kill only those prey that are excluded from optimal habitat and are dying from starvation. The predictions from these hypotheses were tested by examining body condition of Serengeti Wildebeest (Connochaetes taurinus) over 24 yr (1968—1991). Two phases of population growth were examined: 1968—1973 when the population was increased with superabundant food; and 1977—1991 when the population was stationary and regulated by intraspecific competition for food. Three categories of data were compared: live animals, predation kills, and nonpredation deaths. Body condition was measured from bone marrow, the last reserves of fat in ungulates. The predator regulation hypothesis predicts that the marrow condition should be similar to the predation and live samples. The surplus hypothesis predicts the predation and nonpredation samples should be similar. The PSF hypothesis predicts that marrow condition of the predation sample should be (1) poorer than that of the live sample, (2) better than that of the nonpredation sample, and (3) better when food is limiting than when it is abundant. Analyses of the frequency distribution of marrow categories showed that both the predation and nonpredation samples were significantly poorer than that of the live population. In both increase and stationary phases of population growth, the predation sample was in better condition than the nonpredation sample. The predation sample was not quite significantly better (P = 0.052) when food was limiting. These results are consistent with the PSF hypothesis and inconsistent with both of the alternative hypotheses. Female and male condition was similar in the predation sample, but females were killed at a younger age. Lions and hyenas killed animals in similar condition, but lions took animals at a younger age. The results suggest that (1) body condition affects the vulnerability of individual wildebeest to predation, and (2) predation jointly limits the population with intraspecific competition by removing animals from the population that are in better condition than those that are starving.
Selective Herbivory and Plant Community Structure in a Mid‐Atlantic Salt Marsh
Authors: Furbish C. E. , Albano Marianita
Published: May 2006

Factors affecting the distribution of the grasses Spartina alterniflora and Distichlis spicata in a mid—Atlantic salt marsh were examined. A series of eight shallow wells, four in patches of each grass type, was used to describe physicochemical conditions known to limit the distribution of both grasses. Tidal amplitude, surface of subsurface salinity, and subsurface oxidation—reduction potential were found to be within range, but suboptimal, for both grasses. Evidence of selective grazing upon S. alterniflora was found through examination of grazing sign within the grass patches, observations of feral horse feeding behavior, and examination of feral horse feces for grass epidermal fragments. An exclosure experiment simulated preferential grazing and showed that S. alternifora responded negatively while D. spicata responded positively to simulated preferential grazing of S. alterniflora. These results point to a competitive relationship between the grasses under suboptimal conditions for dominance of either species. Selective herbivory (analogous to predation) upon S. alterniflora was shown to be a plausible factor impacting the competition relationship to favor D. spicata. Location along physical gradients, interspecies competition, and herbivory are discussed in relation to salt marsh plant communities.
The Regulation of Maternal Investment in an Indeterminate Flowering Plant (Lotus Corniculatus)
Authors: Stephenson Andrew G.
Published: May 2006

Lotus corniculatus is an indeterminate flowering herbaceous perennial that regularly produces many more flowers than mature fruits. In addition, the number of inflorescences, the number of flowers per inflorescence, the proportion of flowers that initiate, abort, and develop fruits to maturity, the number of seeds per fruit, and seed mass vary within and among individuals of this species. In this study, these components of maternal investment were measured under control, resource enriched (N—P—K fertilizer), and resource deprived (partial defoliation) conditions on a garden population of even—aged and similar—sized individuals. A controlled pollination experiment showed that resources rather than pollination limit the reproductive output of the plants in this study. The data also show that flower number and seed yield are regulated during the reproductive episode in response to resource availability. An analysis of the various components of flower number and seed yield reveals that L. corniculatus regulates flower number and seed yield under the different resource conditions primarily by altering the number of successive inflorescences/infructescences. Plants subjected to the three experimental treatments did not, however, differ in the proportion of flowers that produced a mature fruit or in the proportion of initiated fruits that matured (or aborted). On each treatment only one of every three flowers produced a mature fruit, and three of every five initiated fruits aborted. These findings suggest that the ratio of flowers to fruits has been selected for reasons other than uncertainties in resource availability. The flowers that fail to produce a mature fruit could be functioning only as males, or they could be providing plants with a "choice" of fruits to mature.
Downy Woodpecker Foraging Behavior: Efficient Sampling in Simple Stochastic Environments
Authors: Lima Steven L.
Published: May 2006

I describe a study in which free—roaming downy Woodpeckers (Picoides pubescens) were allowed to forage in three different, patchy stochastic environments where patches (small, thin logs with holes drilled into them to hold food items) contained either zero or a fixed number of food items. All the patches in an environment were, however, identical in appearance. Downy Woodpeckers systematically searched the holes of a patch for food items, and thus the foraging task for an energy—maximizing woodpecker was to determine to what extent to sample a patch without success (without finding a food item) before giving the patch up as being empty and moving on. A model is presented to determine the optimal sampling solution. Although this foraging task is relatively simple, optimal solutions to many stochastic foraging problems may be quite complex. Thus, the observed foraging behavior is compared to the model's predictions as well as to some suggestions form the literature of simple behavioral mechanisms (e.g., a fixed giving—up—time strategy) that animals may use to approximate optimal solutions. The results show that the woodpeckers clearly recognized the fact that some patches contained food items while others did not, and use this information in following a sampling strategy more sophisticated than any of the suggested simple behavioral mechanisms. The predicted sampling behavior on empty patches and that observed were in qualitative but not quantitative agreement. The model predicted that a single number of holes in a patch should be sampled without success while, in fact, a distribution of number of holes sampled was observed. The mode of the distributions, however, corresponded to the predicted value in two of the three environments. It is shown that the woodpeckers did not know with complete accuracy at least some of the model's parameters, and that this may be related to a weak counting ability. A weak counting ability, the need to sample for environmental changes, a stochastic element in a bird's behavior itself, or some combination of these factor may contribute to the observed variability in sampling behavior. Nevertheless, perhaps through some ability to estimate time, the woodpeckers exhibited a sampling strategy very similar to that predicted by a simple energy maximization model, and such sampling abilities are crucial to more complex stochastic foraging models.
Egg Development of a Chydorid Cladoceran, Chydorus Sphaericus, Exposed to Constant and Alternating Tempeatures: Significance to Secondary Productivity in Fresh Waters
Authors: Meyers Dewey G.
Published: May 2006

Eggs of the chydorid cladoceran, chydorus sphaericus, were exposed to five constant temperatures of 10°, 15°, 20°, 25°, and 30°, and three 12.12 h diel thermal cycles of 10°:20° (daily mean [x] of 15°), 15°:25°, (° = 20°), and 20°:30° (x = 25°). Diurnal temperature fluctuations decreased the duration of development and, consequently, accelerated the developmental rate an average of 10% over the constant thermal regimes that were equal to the daily means of the alternating schedules. Both the Edmondson (1960) indirect method of calculating duration and the direct method were equally accurate. Both methods revealed durations that varied inversely with temperature and rates that varied directly, and disclosed a curvilinear relationship of temperature to duration and rate of development. The reduced duration was shown not to be caused by ineffective acclimation procedure, erroneous mathematical analysis and the assumed linearity of the rate—temperature function (Kaufmann effect), or pulsed egg laying. This supports the conclusion that hatching time actually was reduced and points to the occurrence of an unexpected physiological alteration of the developing embryo. Duration reductions, caused by the alternating thermal conditions, appear to provide an adaptive energetic advantage of enhanced metabolic egg development to littoral Cladocera. To test the effect of reduced duration of egg development associated with alternating temperatures on the calculation of durations under fluctuating field conditions, four methods of determination were compared. The "egg—specific weighted mean rate," although the most accurate technique with about a 3% overestimate of actual developmental time, requires precise knowledge of the developmental starting times of each egg, which has, until now, prevented its application in field studies. Because of relative ease of field application and negligible predictive error of less than 5%, the "weighted mean rate" of calculating duration from averaged rates of selected time intervals from continuous field thermal recordings is the method of choice in field studies. Thus, even though a real physiological alteration accompanies dynamic thermal regimes as experienced in the field, the rate—summation method of estimating duration accommodates this durational reduction to well within acceptable levels of current field accuracy. For greater accuracy in predicting the duration of development and, consequently, the birth rate of field populations, it is suggested that water temperature be continuously monitored, diurnal position of the study population be determined, acclimation of test animals be conducted prior to experimentation, a rate—summation method be applied, and mean field temperature calculations be made over the smallest practical time intervals.
Intertidal Community Structure: Space‐Time Interactions in the Northern Gulf of California
Authors: Lively Curtis M. , Raimondi Peter T. , Delph Lynda F.
Published: May 2006

Long—term studies are required for an understanding of how temporal variation and space—time interactions affect the structure of communities. Here we report on a long—term study of the independent effects of, and the interactions among, two sources of temporal variation (seasonal and annual) and two sources of spatial variation for a rocky intertidal community in the northern Gulf of California. The sources of spatial variation were: (1) microspatial effects due to the foraging patterns of a common predatory snail (Acanthina angelica) and (2) macrospatial effects due to differences among sites. The results from semiannual samples of 100—cm2 quadrats showed highly significant temporal and spatial effects for all members of the sessile community (barnacles, mussles, algae) and for limpets over the 8—yr study period. There were also highly significant season X space interactions for all sessile members of the community, which probably resulted from seasonal settlement by the sessile members of the community, and aestivation by the predator. Finally, we observed highly significant year X space effects as well year X season X space effects for most species. These latter interactions can be understood as an amplification of seasonal and spatial effects due to the largely unpredictable differences among years. An Analysis of the variance components showed that most of the variation in percentage cover of barnacles and a brown encrusting alga was due to microspatial effects, while most of the variation in mussels, limpets, and a green alga was due to year and season effects. This combination of results suggests that competition and predation by Acanthina are relatively more important in controlling the distributions and local abundances of barnacles and encrusting algae, and that unpredictable differences among years in settlement are more important in controlling the local population densities of mussels and limpets. The importance of these differences are discussed in relation to interpretation of short—term experimental studies in population and community studies.
Parasitoid Aggregation and the Stabilization of a Salt Marsh Host‐Parasitoid System
Authors: Reeve John D. , Cronin James T. , Strong Donald R.
Published: May 2006

We examine a salt marsh host—parasitoid system, consisting of the planthopper Prokelisia marginata and its egg parasitoid Anagrus delicatus, for evidence of stabilizing parasitoid behavior. We first determine if there is sufficient parasitoid aggregation to potentially stabilize the Prokelisia—Anagrus interaction, using methods that infer parasitoid behavior from the spatial pattern of parasitism. We then test a basic assumption implicit in theory, that the degree of parasitoid aggregation is invariant with respect to changes in overall host and parasitoid density, and through time. Our results suggest that Anagrus behavior should, in theory, exert a consistent stabilizing effect on Prokelisia—Anagrus dynamics. Using field experiments, and direct sampling of the spatial distribution of parasitoids, we then try to determine if parasitoid aggregation is in fact responsible for the heterogeneous pattern of parasitism observed in the field. The results of these studies indicate that there is insufficient parasitoid aggregation to account for this heterogeneity. Instead, factors such as the degree of tidal inundation, or the age of host eggs, may cause host patches to differ in their vulnerability to parasitoid attack.
Polyunsaturate Content and Diet Selection by Ground Squirrels (Spermophilus Lateralis)
Authors: Frank Craig L.
Published: May 2006

Golden—mantled ground squirrels (Spermophilus lateralis) are small herbivores that hibernate during winter. Previous experiments have demonstrated that a high polyunsaturate diet enhances hibernation by ground squirrels. Polyunsaturated fatty acids cannot be synthesized by mammals, but they are commonly produced by plants. It was therefore predicted that ground squirrels, during the 2 mo prior to hibernation, select diet items of a relatively high polyunsaturate content. This hypothesis was tested in a series of laboratory diet selection experiments involving S. lateralis and semisynthetic diets of different polyunsaturate contents. The stomach contents of free—ranging S. lateralis were also analyzed for polyunsaturate content. As predicted, the squirrels preferred semisynthetic diets of relatively higher polyunsaturate contents. Stomach content analyses suggest that the dietary lipids of free—ranging ground squirrels have high polyunsaturate contents. These results indicate that polyunsaturate content may influence the foraging behavior of ground squirrels.
Competition in Two Species of Desert Annuals Along a Topographic Gradient
Authors: Pantastico-Caldas Marissa , Venable D. Lawrence
Published: May 2006

Details on the nature of density—dependent processes in desert annuals and their interaction with density—independent factors are critical to an understanding of adaptation to variable environments and the role of environmental variation in the maintenance of species coexistence. In this study we examined intra— and interspecific competitive interactions along a topographic gradient for two co—occurring species of desert winter annuals, Plantago patagonica and Pectocarya recurvata. This investigation was carried out in a creosote flat in the Sonoran Desert during 1987—1988, a growing season with high plant densities and low rainfall. Three stations were established along the topographic gradient: one in the sandy rivulets of a tertiary wash or arroyo, one in the sandy clay soil at the base of a small hill, and the third on the southeast side of the gently sloping (9°) hill. The wash habitat had the greatest moisture availability (soil water potential) shortly after rainfall. Yet it dried out quickly, presumably due to greater evaporation and percolation, leaving the base of the hill with the greater average moisture availability. The slope had the lowest moisture availability. Competition experiments were set up at each station using a neighborhood design with varying density in either mono— or mixed cultures. Target plants were scored for survival, fecundity, and size. Plantago and Pectocarya were found to have equivalent competitive effects in all three habitats using either species as a target: the species identity of neighbors had no measurable effects so that competition coefficients appear to be equal. However, density—response curves had different slopes in the different habitats along the gradient: both species had flat density—response curves on the slope, while Plantago had its steepest curve in the wash and Pectocarya had its steepest curve at the base of the hill. While density—dependent processes were important determinants of success for these desert annuals in this low—rainfall year, the strength of the density response varied spatially. We argue that the habitat—density interactions that differ among species may promote their coexistence.
Facilitative and Inhibitory Interactions Among Estuarine Meiobenthic Harpacticoid Copepods
Authors: Chandler G. Thomas , Fleeger John W.
Published: May 2006

In the meiofauna communities of Louisiana estuaries (USA), the harpacticoid copepods Scottolana canadensis and Pseudostenhelia wellsi are predominant species. Scottolana canadensis is a semisessile burrow—dweller capable of subsurface suspension and deposit feeding. Pseudostenhelia wellsi is also semisessile but builds extensive networks of mucus tubes within the top 1 cm of muddy sediments, and appears to graze on its inner tubewalls. Tube building by P. wellsi generates meiofauna—sized structure and adds cohesiveness to surface sediments, as well as providing potential food and grazing substrates for other meiofuna. Monospecific patches of P. wellsi and S. canadensis (250 individuals/5 cm2) were artificially generated in laboratory microcosms to determine if the unique lifestyle and sedimentary effects of either species facilitate or inhibit colonization by two other errant, burrowing harpacticoids common in the community, Nitrocra lacustris and Cletocamptus deitersi. These two species share similar foraging and burrowing behaviors and similar effects on sediment structure, which sharply contrast with those of P. wellsi and S. canadensis. Pseudostenhelia wellsi tube patches facilitated colonization by both S. canadensis and N. lacustris, but strongly inhibited colonization by C. deitersi. Scottolana canadensis patches were unattractive to N. lacustris. As P. wellsi showed the strongest effects on colonization by other harpacticoids, its mechanisms of facilitation/inhibition were also studied. In laboratory microcosms, cultured S. canadensis and N. lacustris were offered patches of azoic sediments, mucin—enriched sediments without structure, azoic sediments with agar tube mimics (structure), and sediments with natural P. wellsi tubes (mucus and structure). Both mucus enrichment and inert tube structure acted as strong facilitants to N. lacustris copepodites and adults overall. However, neither effect alone facilitated patch colonization by N. lacustris adult females and S. canadensis copepodites and adults. Their colonization was facilitated specifically by natural P. wellsi tubes. These experiments demonstrate that species interactions in harpacticoid communities can quickly influence spatial patterns, and those patterns may be mediated by species—specific effects on the sedimentary environment (e.g., mucus tube, burrows, increased flushing, erodability, etc.). However, spatial patterns cannot be predicted easily by contrasting the compatibility of one species' biogenic effects with those of another.
Salivary Water Loss to Seeds In Yellow Pine Chipmunks and Merriam's Kangaroo Rats
Authors: Vander Wall Stephen B.
Published: May 2006

Seeds are known to serve as sources of preformed and metabolic water for desert rodents, but water loss to hygroscopic seeds has received little attention. Salivary water loss by yellow pine chipmunks (Tamias amoenus) and Merriam's kangaroo rats (Dipodomys merriami) to Jeffrey pine (Pinus jeffreyi) and antelope bitterbrush (Purshia tridentata) seeds was measured to determine its magnitude and potential importance. Chipmunks rapidly lost salivary water to individual seeds inserted into their cheek pouches. Jeffrey pine seeds (129 mg) absorbed 9.8 mg of water and bitterbrush seeds (36 mg) absorbed 4.6 mg of water when held in the cheek pouches for 15 min. Only 3—5% of this water can be reclaimed if the chipmunk eats the seed because most of the absorbed water resides in the inedible seed coat, which the animal discards. The kangaroo rats, which possess external, fur—lined cheek pouches, lost no water to seeds held in their cheek pouches for 15 min. Chipmunk salivary water loss to Jeffrey pine and bitterbrush seeds was equivalent to 19 and 59%, respectively, of the water (performed plus metabolic water) contained in those seeds. Salivary water loss is a major and previously overlooked component of the water budgets of granivorous rodents with internal cheek pouches. The presence of fur—lined cheek pouches in heteromyid rodents (Kangaroo rats, pocket mice, and kangaroo mice) may have contributed to the great success of these rodents in the deserts of North America by solving the problem of salivary water loss to the seeds they handle and store.
Treefall in a Mixed Oak‐Pine Coastal Plain Forest: Immediate and Historical Causation
Authors: Matlack Glenn R. , Gleeson Scott K. , Good Ralph E.
Published: May 2006

In a local convective storm, 85 trees were damaged in an 18.3 ha stand in the Pine Barrens region of southern New Jersey, USA. To determine what factors place individual trees at risk, damaged trees were measured and graded for degree of fire scarring and fungal rotting. Similar measurements were made on 280 undamaged individuals. Ages were determined in a subsample by coring. Only two of the seven species present suffered wind damage (Quercus prinus and Q. velutina). Damaged trees were moved more severely rotted than undamaged individual both within and among species. Presence of fungal rot was strongly linked to scarring of the trunk by fire, which in turn was determined by date of recruitment relative to historical episodes of fire. Risk was not influenced by either tree size, proximity to existing gaps, taper of the stem, or interspecific differences in wood strength. Vulnerability to wind damage in this sand was determined by a summation of disturbance effects influencing tree recruitment and health over the previous 90 yr.
Indirect Effects, Prey Susceptibility, and Habitat Selection: Impacts of Birds on Limpets and Algae
Authors: Wootton J. Timothy
Published: May 2006

I experimentally manipulated avian predation pressure in an intertidal community to determine its direct and indirect influence on the abundance and distribution of three limpets and their algal food source. Birds reduced the overall abundance of Lottia digitalis only; L. strigatella abundance increased and the abundance of L. pelta, the species most frequently consumed, did not change. Through crypsis, the goose barnacle Pollicipes polymerus and the mussel Mytilus californianus indirectly affected the abundance of L. digitalis and L. pelta in opposite ways by changing the risk of succumbing the bird predation. These limpets also exhibited strong habitat selection for their cryptic substrates. By altering the amount of preferred habitat, birds indirectly influenced limpet abundance: gull predation reduced the area covered by P. polymerus, releasing M. califonianus from space competition. By consuming limpet grazers and reducing space competition, birds indirectly enhanced the abundance of algae. L. strigatella sizes generally fell below the range of limpet sizes consumed by birds. Consequently, birds indirectly increased L. strigatella density by reducing the intensity of exploitative competition with other limpet species; L. strigatella biomass declined significantly with increasing biomass of the other two limpets, and with decreasing algal cover. These results demonstrate that indirect effects and apparently adaptive behaviors can counteract (or reinforce) direct interactions between species pairs, suggesting that conclusions from short—term experiments emphasizing species pairs can be tenuous.
Intraspecific Response to Variation in Food Resources by Attwater's Pocket Gopher
Authors: Williams Lawrence R. , Cameron Guy N.
Published: May 2006

We examined the effect of variation in availability of dietary resources on demographic variation among seven populations of Attwater's pocket gopher. Pocket gophers were live—trapped to assess demographic parameters, and additional individuals were used to determine detailed reproductive information and dietary habits. Attwater's pocket gopher exhibited variation in population density, ovulation rate, pre—implantation mortality, proportional loss of young, and total recruitment, but not in litter size, sex ratio, or age structure among the seven populations. Dietary analyses revealed that annual dicots were avoided in a habitat where their proportional occurrence was high, but the preferred dietary item (perennial dicots) was consumed in proportion to its occurrence. Population density (carrying capacity) was limited by availability of resources, primarily availability and dispersion of preferred perennial dicots. Predictability of density of pocket gophers by dietary resources was low, °12%, compared to that of terrestrial herbivores. Low population density was associated with low total recruitment, low survival of young, and longer movement by individuals through the habitat. We propose an hypothesis for regultion of population density. As availability of perennial dicots decreases, pocket gophers move greater distances to obtain this resource. Increased movement increases the probability of aggressive, intraspecific encounters. These encounters increase aggression levels in the populations, result in greater loss of young through dispersal or mortality, and generally decrease recruitment. Loss of individuals decreases population density to a level set by availability of resources and level of aggression decreases.
The Relationship between Reproduction and Survival in Known‐Aged California Gulls
Authors: Pugesek Bruce H. , Diem Kenneth L.
Published: May 2006

Survival from 1984 to 1985 was compared among known—aged California Gulls 4—23 yr old. Data supported a hypothesis of increasing reproductive effort with age. A flood during 1984 terminated breeding during the incubation phase of part of the colony, thus permitting an estimate of reproductive costs. Yearly survival of gulls whose nest flooded was 92% compared to 72% among gulls that carried out the reproductive cycle. Adult survival was inversely related to level of fledging success in 1984. Survival declined and fledging success increased with increasing age. However, gulls of differing ages within the same fledging success category had the same probability of survival. No evidence supported the hypothesis that fledging success increased with age because disproportionately larger numbers of individuals with "good phenotypes" survived to old age. Cross—sectional data showed that fledging success peaked at ages 16—19 and declined among gulls 20—23, suggesting senescence in extremely old gulls. However, longitudinal data indicated that fledging success did not decline among old gulls. The few gulls surviving to extreme old age did so by consistently exerting low reproductive effort and fledging few offspring.
Carbon Balance Limits the Microdistribution of Grimmia laevigata, a Desiccation‐Tolerant Plant
Authors: Alpert Peter , Oechel Walter C.
Published: May 2006

Despite their ability to tolerate long and severe drought and extreme temperature, desiccation—tolerant plants are typically excluded from the most xeric microhabitats. It was hypothesized that this is due to an inability of the plants to maintain a positive cumulative carbon balance during repeated cycles of wetting and drying under highly xeric conditions. To test this hypothesis, the response of net CO2 flux to light, temperature, plant water content, and previous desiccation were measured in the desiccation—tolerant moss Grimmia laevigata, the dominant green plant on exposed granitic boulders in the inland chaparral of San Diego County California. Patterns of net CO2 flux and results from a simple model of carbon balance were consistent with the hypothesis.
Slugs as Generalist Herbivores: Tests of Three Hypotheses on Plant Choices
Authors: Rathcke Beverly
Published: May 2006

Three hypotheses explaining plant choices by generalist herbivores were tested by measuring leaf acceptabilities to three slug species, each of which lives in a different successional stage of vegetation. The acceptabilities of 61 plant species were determined in laboratory feeding trials by offering individual slugs leaf discs of a test and a control plant (lettuce) and recording consumption. The plant—defense hypothesis, that plants predictable to herbivores should have greater defense commitments and be less acceptable than unpredictable plants, was contradicted when predictability was judged by successional status. The acceptabilities of climax forest plants and early—successional plants were not significantly different for any of the three slug species. This was also true within woody species and within herbs. However, the plant—defense hypothesis was supported when predictability was measured as seasonal leaf persistence. As seasonal leaf persistence of forest herbs increased, acceptability declined. As alternative explanation based on growth rate is discussed. Alternatively, feeding choices were hypothesized to be determined by adaptations of these generalist herbivores to plants available in their local habitats (the herbivore—adaptation hypothesis). Because slugs are generally restricted to feeding on herbs, woody species were predicted to be less acceptable than herbs and this was strongly supported. Tree leaves were seldom eaten by any of the three slug species. In addition, each slug preferred a few herbaceous species unique to its specific habitat, suggesting local adaptation, but the actual mechanism remains to be determined. If both plants and slugs are coevolving, plant choices would depend upon where we examine the coevolutionary spiral, but they should be unique for each slug species as it coevolves with local plants (the coevolution hypothesis). In these tests, feeding responses of the three slug species were significantly positively correlated and unique responses were not evident. These hypotheses depend upon the probabilities of plant—herbivore encounters, which can be difficult to determine. For these slug species stem heights of only 10 cm acted as effective feeding barriers. Plants were vulnerable during emergence, but they can escape slugs if they emerge early in the spring. Because such subtle factors may determine encounter rates, predictions of feeding choices and plant defenses may be difficult to make. Specific adaptations may not be evident from these tests of general patterns. Field observations suggest that these introduced slug species are exerting significant selective pressure on the morphologies, phenologies, and defenses of native plants.
Life History Evolution in Stochastic Environments: A Graphical Mean‐Variance Approach
Authors: Real Leslie A. , Ellner Stephen
Published: May 2006

A small—variance approximation technique for examining the evolution of reproductive effort is developed, using a decomposition of fitness into arithmetic mean and variance components. Trait values often translate nonlinearly into fitness components such as offspring number or growth rate. A nonlinear relationship between phenotypic traits and fitness implies that the expected value of the fitness component will be a function of the arithmetic mean and variance in the traits influencing a given fitness component. The maximum expression of traits is often, however, a function of reproductive effort. Reproductive effort then influences the mean and variance in traits that affect fitness. Consequently, we can use these techniques to determine the optimal allocation of resources to current reproduction via their effects on the distribution of trait values. The optimal pattern of allocation is influenced by the quality and variability of the habitat and by the curvature of the trait—fitness relationship. The full range of adaptive responses, from semelparity to different degree of iteroparity, can be predicted from the model. We extend the model to cases where reproductive effort can be partitioned into different offspring types, for example, offspring produced through chasmogamous and cleistogamous flowers. When more than one type of offspring is produced, then reproductive risks can be spread across offspring types and the conditions favoring semelparity are less restrictive. Semelparity is especially favored if the offspring types exhibit negative covariance in their contributions to growth rate. This new technique appears generally consistent with traditional life history theory, but can lead to some new insights that result from the explicit partitioning of the effects of central tendency and variability in trait—fitness relationships.
Independence of Savanna Grasses from Soil Organic Matter for Their Nitrogen Supply
Authors: Abbadie Luc , Mariotti Andre , Menaut Jean-Claude
Published: May 2006

In Lamto savanna (Cote d'Ivoire), grass δ15N (≈‐1.3%) is much lower (has a smaller absolute value) than soil organic matter δ15N (≈‐1.3%). In order to understand such a discrepancy, we have analyzed 15N natural concentrations in the four major sources of mineral nitrogen that can meet the annual requirements of plants: bulk precipitation, mineralization of humified soil organic matter, atmospheric dinitrogren fixation, and decomposition of plant litter. The first source (negative δ15N) only contributes ≈7% of nitrogen requirements, as does the second (δ15N≈+7%) due to a very low humus mineralization rate. The third source (δ15N = 0) contributes up to 17%, due to nonsymbiotic N2 fixation by microorganisms associated with grasses, legumes being almost absent from the savanna. All these processes cannot account for the low δ15N of grasses, suggesting that most of the assimilated nitrogen originates from the decay of root material (δ15N≈‐1.1%).
Modification of Antipredator Behavior of Caecidotea Intermedius by Its Parasite Acanthocephalus Dirus
Authors: Hechtel Laura J. , Johnson Cheryl L. , Juliano Steven A.
Published: May 2006

The isopod Caecidotea intermedius serves as the intermediate host for the acanthocephalan Acanthocephalus dirus. C. intermedius is preyed upon by the northern creek chub, Semotilus atromaculatus, which also serves as the definitive host for A. dirus. The effects of the parasite on the antipredator behavior of C. intermedius were examined. We tested the hypothesis that behavioral changes induced in C. intermedius by the parasite are by—products of increased energy demands induced by the parasite (i.e., increased hunger). Infected and uninfected C. intermedius were placed in a divided aquarium with 0, 1, or 2 creek chubs. Leaf discs provided both a food source and a refuge from the predator. In the presence of the creek chubs, uninfected C. intermedius avoided the predator, and were found more frequently in refugia as the number of creek chubs increased. However, infected C.intermedius were associated with the side of aquarium containing the predator, and spent significantly more time out in the open away from the refuge regardless of the number of creek chubs. These data show that the antipredator behavior exhibited by C. intermedius is altered by A. dirus, and that such alterations are unlikely to be simply products of increased energy demands. Parasite—induced behavioral changes appear to increase C. intermedius' vulnerability to predation, thereby increasing the likelihood of A. dirus completing its life cycle. We find little evidence for greater foraging need as the mechanism indicating changes in antipredator behavior.
Effects of Age and Experience on Reproductive Performance of Wood Ducks
Authors: Hepp Gary R. , Kennamer Robert A.
Published: May 2006

Data from a long—term study of Wood Ducks (Aix sponsa) breeding in South Carolina were used to test whether reproductive performance was age specific and to evaluate several hypotheses proposed for age—specific variation. We used known—aged females from 1 through 5 yr of age. Yearling females initiated nests 11—19 d later than older females; heavier females; independent of age, nested earlier than lighter females. One—way analyses of covariance using female body mass and nesting date as covariates indicated that clutch size, mean egg mass, number of ducklings per nest, and the percentage of eggs hatching (hatching success) were independent of female age. Probability of nests producing at least one duckling (nest success) also was not related to female age. We separately tested effects of breeding experience and female age class (yearling vs. adult) on reproductive performance. In the context of this study, females were considered as experienced if they previously were captured using nests boxes and inexperienced if there was no record of nest box use. Adult females with previous breeding experience initiated nests an average of 26 d earlier than adults without previous experience; body mass of experienced adults was greater than that of inexperienced adults. Adult females designated as "inexperienced" may have nested previously in natural cavities and were simply changing to nest boxes. Nest—site fidelity is known to affect nesting date, so we also compared nesting dates of inexperienced adults with a subset of experienced females that changed nest sites. Inexperienced adults nested 12 d later than experienced adults that switched nest sites and were nesting on different wetlands from the previous year. Next, we controlled nesting experience and tested specifically for age effects by comparing inexperienced adults with yearlings. Nesting date of inexperienced adults and yearling females did not differ, but body mass of these adults was greater than that of yearlings. There was no evidence that differential survival of heavy, early—nesting yearling females explained age—related patterns in nesting date and female body mass. In fact, late—nesting yearlings survived better than yearling females that initiated nests early. Furthermore, Jolly—Seber estimates of annual survival did not decline with female age, suggesting that older females were able to compensate for any increase in reproductive effort associated with early initiation of nests. Binary regression analyses showed that later nesting females were less likely to initiate second nests and more likely to have nests destroyed by predators. Female Wood Ducks nesting early produced larger clutches, hatched more young from successful nests, were at less risk from predators, and improved their chances of initiating second nests. Adult females at southern latitudes, therefore, should begin nesting as early as possible. Yearling females survive better if they begin nesting later in their 1st yr.
Plant Production and Soil Microorganisms in Late‐Successional Ecosystems: A Continental‐Scale Study
Authors: Zak Donald R. , Tilman David , Parmenter Robert R. , Rice Charles W. , Fisher Frederick M. , Vose James , Milchunas Daniel , Martin C. Wayne
Published: May 2006

Annual C inputs from plant production in terrestrial ecosystems only meet the maintenance energy requirements of soil microorganisms, allowing for little or no net annual increase in their biomass. Because microbial growth within soil is limited by C availability, we reasoned that plant production should, in part, control the biomass of soil microorganisms. We also reasoned that soil texture should further modify the influence of plant production on soil C availability because fine—textured soils typically support more microbial biomass than coarse—textured soils. To test these ideas, we quantified the relationship between aboveground net primary production (ANPP) and soil microbial biomass in late—successional ecosystems distributed along a continent—wide gradient in North America. We also measured labile pools of C and N within the soil because they represent potential substrate for microbial activity. Ecosystems ranged from a Douglas—fir forest in the western United States to the grasslands of the mid—continent to the hardwood forest in the eastern U.S. Estimates of ANPP obtained from the literature ranged from 82 to 1460 g°m—2°yr—1. Microbial biomass C and N were estimated by the fumigation—incubation technique. Labile soil pools of C and N and first—order rate constants for microbial respiration and net N mineralization were estimated using a long—term (32 wk) laboratory incubation. Regression analyses were used to relate ANPP and soil texture with microbial biomass and labile soil C and N pools. Microbial biomass carbon ranged from 2 g/m2 in the desert grassland to 134 g/m2 in the tallgrass prairie; microbial N displayed a similar trend among ecosystems. Labile C pools, derived from a first—order rate equation, ranged from 115 g/m2 in the desert grassland to 491 g/m2 in the southern hardwood forest. First—order rate constants for microbial respiration (k) fell within a narrow range of values (0.180 to 0.357 wk—1), suggesting that labile C pools were chemically similar among this diverse set of ecosystems. Potential net N mineralization rates over the 32—wk incubation were linear in most ecosystems with first—order responses only in the alpine tundra, tallgrass prairie, and forests. Microbial biomass C displayed a positive, linear relationship with ANPP (r2 = 0.51), but was not significantly related to soil texture. Labile C also was linearly related to ANPP (r2 = 0.32) and to soil texture (r2 = 0.33). Results indicate that microbial biomass and labile organic matter pools change predictably across broad gradients of ANPP, supporting the idea that microbial growth in soil is constrained by C availability.
Nitrogen Use Efficiency of Carex Species in Relation to Nitrogen Supply
Authors: Aerts Rien , de Caluwe Hannie
Published: May 2006

We studied the effect of nitrogen supply (3.3 and 20.0 g°m—2°yr—1) on the nitrogen use efficiency (NUE) of three Carex species that differ in maximum growth rate. The rank order of maximum growth rate of these species is: Carex diandra @< Carex rostrata < Carex acutiformis. We distinguished two components of NUE: the mean residence time of nitrogen (MRT) in the plant and the nitrogen productivity (A). In this concept, NUE equals the product of MRT and A, and thus equals total productivity divided by the total N loss in litter. The N productivity (A) of C. diandra was significantly higher than that of C. rostrata and C. acutiformis. In each species A decreased strongly at enhanced N supply. The MRT of N differed substantially between species, but was not significantly affected by N supply. Contrary to what is generally found, the species with the lowest maximum growth rate (C. diandra) had the highest N turnover rate (lowest MRT), while the species with the highest maximum growth rate (C. acutiformis) had the lowest N turnover rate (highest MRT). In all species, the NUE decreased with increasing N supply. As a result of the observed patterns in N productivity and mean residence time, the species with the lowest maximum growth rate (C. diandra) had the lowest nitrogen use efficiency, while the species with the highest maximum growth rate (C. acutiformis) had the highest NUE. These results are not in agreement with the generally observed patterns in NUE along gradients of primary productivity. Comparisons of the results of this study with those of other studies show that generalizations of patterns of NUE along gradients of primary productivity should explicitly take into account which growth forms are compared (evergreen species vs. deciduous species, or [congeneric] species with the same growth form). Moreover, this study shows that factoring NUE into its components is a useful tool in the analysis of the adaptive significance of the N economy of plants.
Fertilization Mitigates Chemical Induction and Herbivore Responses Within Damaged Oak Trees
Authors: Hunter Mark D. , Schultz Jack C.
Published: May 2006

Previous work has shown that fertilization can mitigate across—year induced resistance among individual Alaska paper birch, supporting the hypothesis that induction results from nutrient deficiency rather than active defense. The present study suggests that fertilization can prevent induction responses in oak within one season, and among leaves within individual trees. Saplings of two oak species, divided among fertilizer treatments within a plantation, were defoliated to varying degrees by a gypsy moth outbreak during spring. We monitored foliage chemistry and insect herbivore distributions during summer and fall of the same year. Damaged leaves within unfertilized Quercus prinus saplings showed increases in foliar astringency and proanthocyanidins. Within fertilized trees, damage did not induce increases in either astringency or proanthocyanidins. Defoliation also induced increases in proanthocyanidins and astringency within Q. rubra saplings, but induction was unaffected by fertilization. On both tree species, the distributions of certain insect guilds were skewed away from damaged leaves on unfertilized trees, but independent of damage on fertilized trees. We suggest that the effects of fertilization on induction can vary among tree species, and can occur at more than one spatial and temporal scale. Our data support the view that nutrient availability may affect phytochemical induction and herbivore responses.