bluuebunny/crossref_metadata_2025 huggingface dataset:
Complex Ant‐Plant Interactions: Rain‐Forest Ants as Secondary Dispersers and Post‐Dispersal Seed Predators
Authors: Levey Douglas J. , Byrne Margaret M.
Published: May 2006

We describe the role of leaf—litter ants (Pheidole spp.) in the seed bank dynamics of several small—seeded shrubs in a Costa Rican lowland rain forest. Three ants harvest seeds from frugivore feces and cache them in their nests in partially decomposed twigs. From 24 to 38% of the colonies (depending on ant species) contain cached seeds and 25—32% have seeds on refuse piles. Experiments with captive colonies of Pheidole nebulosa and P. nigricula demonstrated that °6% of harvested Miconia nervosa and M. centrodesma seeds and deposited on refuse piles. Because seeds generally retain viability longer than the nest twig remains intact, harvested seeds are not trapped inside twigs. Experimental plantings of 4—d—old Miconia nervosa seedling on two types of substrate (ant refuse pile vs. topsoil) under two light levels (equivalent to small and large clearings) demonstrated that seedlings grew faster and survived better on refuse piles under light levels typical of small clearings. Light levels typical of large gaps are not necessarily advantageous for establishment of understory rain forest plants. These results illustrate the dynamic nature of a tropical soil seed band and the complexity of plant—animal interactions that occur there. Ants are simultaneously antagonistic and mutualistic towards seeds, killing most but significantly benefiting some. This interaction is extremely common (Pheidole density >300 individuals/m2) and likely influences plant recruitment patterns. Our results challenge the generalizations that small seeds are largely protected from predation because of their size, that post—dispersal seed harvesting is equivalent to seed predation, that competition among seedings from a frugivore defecation is common, and that small seeds accumulated over long periods of time in the seed bank. We hypothesize that differential harvesting treatment of seeds by ant species may be a mechanism underlying community—level patterns of regeneration in small—seeded plants. Also, the incidental benefits to uneaten seeds deposited on refuse piles provide additional evidence that myrmecochory evolved in plants that strengthened rare but consistent benefits of being harvested by "granivorous" ants.
Efficiency‐Maximizing Flight Speeds in Parent Black Terns
Authors: Welham Clive V. J. , Ydenberg Ronald C.
Published: May 2006

Optimal foraging theory assumes that individuals maximizing a given energy currency maximize their fitness. Though models with different currencies (and assumptions) have been successful at describing forager behavior, discriminating among currencies has proven difficult because models optimizing different currencies often make similar predictions. A field experiment was designed to use flight speeds of Black Terns (Chlidonias nigra) feeding their young to test predictions from optimal foraging models that maximize (1) the net energy gained per unit of energy expended (efficiency: EFF), (2) the net rate of energy intake (NREI), and (3) the daily delivery rate (DR), respectively. Manipulating the distance an adult had to fly between a feeding enclosure and its nest enabled us to discriminate among the three currencies. Predicted speeds of the EFF model best matched the observed flight speeds. Maximizing efficiency resulted in the lowest delivery rate to the nest, but it was also the only currency that predicted adult daily energy expenditures within the theoretical limit (DEEmax) estimated for Black Terns (206 kJ/d; Kirkwood 1983). The feeding enclosure represented an unusually profitable food source for the terns, and this may have resulted in artificially high predictions for flight speed, particularly for the NREI and DR currencies. We therefore made another set of predictions from the three models using estimates of foraging success from parents who had never foraged from the feeding enclosure, and compared these predictions with the measured airspeeds. The EFF model was again the best predictor of flight speeds even though none of the three models predicted energy expenditures in excess of DEEmax. Though the DR model resulted in the highest total daily energy delivered to the nest, this represented only a small gain relative to the NREI and EFF models. Black Terns parents may maximize EFF because this currency is least likely to result in daily energy expenditures that exceed their metabolic limit.
Consequences of Ratio‐Dependent Predation for Steady‐State Properties of Ecosystems
Authors: Ginzburg Lev R. , Akcakaya H. Resit
Published: May 2006

Ratio—dependent and prey—dependent models of trophic interactions make very different predictions about the steady—state (equilibrial) properties of ecosystems, such as the response of equilibrium abundance and biomass of organisms in each trophic level to increased primary productivity or nutrient input. Prey—dependent theory predicts alternating positive, negative, and zero responses of trophic levels to increased productivity, whereas ratio—dependent theory predicts proportional increase in all trophic level biomasses. We analyzed data on the nutrient input and the biomasses of phytoplankton, zooplankton, and fish across lakes to distinguish between ratio—dependent and prey—dependent models. The results show parallel increases in all trophic levels as a result of increased nutrient input, demonstrating that natural systems are closer to ratio dependence than to prey dependence.
Interspecific Variation in Fruit Shape: Allometry, Phylogeny, and Adaptation to Dispersal Agents
Authors: Herrera Carlos M.
Published: May 2006

Investigations on fruit and fruiting characteristics of animal—dispersed, fleshy—fruited plants have been generally interpreted in terms of adaptations by plants to dispersal agents. Most often, these studies did not formulate non—adaptive, alternative null hypotheses, and the potential influence of phylogenetic effects on observed patterns was not assessed either. This paper presents an analysis of interspecific variation in fruit shape (as assessed by length and width) among vertebrate—dispersed plants of the Iberian Peninsula. Tests of predictions from both adaptive (to dispersal agents) and null (based on allometry) hypotheses are presented and the influence of phylogenetic effects is also accounted for. Interspecific variation in fruit shape was unrelated to seed dispersal mode ("bird dispersed" vs. "bird plus mammal dispersed"), and predictions from adaptive hypotheses were not supported. Variation in fruit shape did not depart significantly from that predicted by the allometry—based, null hypothesis. Deviations of individual species from the predicted allometric relationship were unrelated to dispersal mode, and originated from genus— and species—specific variation in fruit shape. There exists a considerable influence of phylogeny on fruit shape variation, and nearly half of total variance is attributable to variation among genera within families. After accounting for phylogenetic effects, the null hypothesis still held within taxonomic categories above the species level. Observed constancy in the relative variation of fruit length and width, despite variation in dispersal mode and morphological type, is interpreted in terms of shared morphogenetic and physical constraints independent of dispersal. The implications of substantial phylogenetic effects on interspecific patterns of variation in fruit and fruiting traits are discussed.
Mechanisms and Consequences of Intraspecific Competition in a Carabid Cave Beetle
Authors: Griffith David M. , Poulson Thomas L.
Published: May 2006

We have been able to clearly study the mechanisms and consequences of intraspecific competition, in contrast to most studies, because of the cave beetle (Neaphaenops tellkampfi, Carabidae) has no competitors and only a single available prey item, cricket eggs (Hadenoecus subterraneus, Rhaphidophoridae), in our study area in Mammoth Cave National Park. The sexes do not differ in size or external morphology and we have never seen interference with copulating pairs and so the likelihood of competition for mates or female choice seems low or nonexistent compared to competition for food. Both in the field and laboratory we assessed: (1) foraging effort by numbers of holes dug: (2) potential foraging success by whether the holes were deep enough to reach a buried cricket egg; and (3) time since foraging success by abdominal distension and body mass. We assessed both exploitation competition, the indirect effect of beetles on each other via cricket egg prey, and interference competition, the direct effect of beetles on each other. To demonstrate exploitation competition we showed that (1) beetles in the field reduced the density of egg prey, (2) lowered egg density in the laboratory resulted in decreased per capita harvest rate, and (3) in the field reduced harvest rates resulted in reduced fecundity at times of year when cricket numbers, fecundity, and egg—laying were lowest and beetle densities were highest. To demonstrate interference competition we showed that beetles reduced each other's foraging effectiveness. In laboratory arenas per capita foraging effectiveness decreased when egg density was held at 10 and beetle density increased from 1, to 2, to 4 per arena. With increasing beetle density there were significant declines in holes dug per beetle, depths of holes, and eggs eaten per beetle. In addition beetles spent less time foraging. In the field interference competition, as assayed by decreasing hole depths, was significantly greater in areas of high beetle density than in areas with low beetle density. Fights for holes occurred only in areas of high beetle density. Preempting holes or stealing eggs could be advantageous since bioenergetic calculations showed that the cost of digging for eggs is as high as two—thirds of a beetle's energy budget. In the field in areas of high beetle density there was no defense of holes <3 mm deep and defense of all holes >7 mm deep. Beetles that had recently eaten eggs were found significantly more often under rocks and on the ceiling where we infer that chances of eggs being stolen are low.
Canopy Gap Characteristics and Drought Influences in Oak Forests of the Coweeta Basin
Authors: Clinton B. D. , Boring L. R. , Swank W. T.
Published: May 2006

Canopy gaps in southern Appalachian mixed—Quercus forest were characterized to assess the impact of the 1985—1988 record drought on patterns of tree mortality in relation to topographic variables and changes in overstory composition. Using permanent transects, we samples 68 canopy gaps within the Coweeta Basin. Among 1—5 yr old gaps, the most common gap type was the 1—yr—old single—tree snag, accounting for 49% of all gaps sampled; 65% of all gaps occurred within 2 yr of the drought peak. Gap area ranged from 40 to 850 m2, averaged 239 m2, with a median of 152 m2. Analysis of 1988 color infrared (IR) aerial imagery yielded a gap formation rate of 0.8 gaps°ha—1°yr—1 in mixed—Quercus stands affecting 2.0% of the area in this forest type in 1988. The most frequent gap—forming species were Quercus coccinea, Q. rubra, and Q. velutina, respectively. Evidence suggests that moisture stress brought on by severe drought increases the susceptibility of Quercus species of the shoe—string fungus Armillaria mellea, which may result in significant shifts in canopy composition in these forests.
Sand‐Colored Nighthawks Parasitize the Antipredator Behavior of Three Nesting Bird Species
Authors: Groom Martha J.
Published: May 2006

On beaches along the Manu River of southeastern Peru, four bird species (Black Skimmer, Rynchops niger; Large—billed Tern, Phaetusa simplex; Yellow—billed Tern, Sterna superciliaris; Sand—colored Nighthawk, Chordeiles rupestris) nest in mixed—species associations. Costs and benefits of the association were distributed asymmetrically among the species. Nighthawks never actively defended their nests against predators. Terns and skimmers defended their nests by aggressively mobbing predators, and indirectly defended nearby or adjacent nests of nighthawks. As a result, nighthawks had greater hatching success when nesting near terns and skimmers. Terns and skimmers spent more time engaged in antipredator behaviors and being vigilant, but less time in parental care, and experienced lowered hatching success when among large nighthawk groups. In addition, both tern species fledged significantly fewer young per successful nest when nesting with many nighthawks. Results of this study demonstrate the first known parasitic relationship among members of a mixed—species nesting group.
Aquatic Predators and Tadpole Prey in Central Amazonia: Field Data and Experimental Manipulations
Authors: Gascon Claude
Published: May 2006

Experimental manipulations of aquatic predators (odonate naiads and fishes) and prey (three tadpole species: Osteocephalus taurinus, Phyllomedusa tomopterna, and Epipedobates femoralis) investigated the role of predators in the organization of a tropical larval anuran assemblage. Eighteen months of surveys of 31 primary forest lentic sites examined phenological patterns of tadpole and predator species. I tested for a relation between predator and metamorph presence using the Mantel—Haenszel chi—square. I tested for differences in survival and development for each tadpole species as a function of predator identity. I examined correlations between growth of surviving tadpoles and both the number of conspecific and the total number of surviving tadpoles. Two—way ANOVAs tested for differences in predation rate among predators and experiments. Finally, I tested for differences in selectivity for each prey species. Odonate naiads were the most common predator in upland isolated pools and peccary wallows, while fish were most common in lowland, stream—associated habitats. No relation was found between the presence of metamorphs and of predators in natural sites, except for metamorph presence (a positive relation for all tadpole species pooled) and presence of aeshnid larvae in natural sites. Predators reduced tadpole survival in three of four different prey—species combinations. Aeshnid naiads were the most effective predators, followed by libellulid naiads. Fish were never the most effective predators and their effect was variable. Predation rate on each of the three prey species varied among predators and sometimes depended upon which species were present, indicating that predation rate in the system was a function of the particular combination of prey species. No significant differences were found in selectivity for any prey species among individual predator types. All predators, however, preferred P. tomopterna, the less abundant species, over O. taurinus whenever the two species were together. Growth of tadpoles was inversely related to the number of surviving conspecific tadpoles. Predators, therefore, have the potential to modify the composition of tadpole species assemblages through differential predation.
A Field Tests of Interspecific Competition on Oviposition of Gall‐Forming Sawflies on Willow
Authors: Fritz Robert S. , Price Peter W.
Published: May 2006

We performed a manipulative field experiment to test for competitive effects of the stem galling sawfly (Euura lasiolepis) on three other gall—forming sawflies on arroyo willow (Salix lasiolepis). Within a willow clone, three branches were left untreated, three branches had E. lasiolepis females placed on them in nylon screen bags during that species' oviposition period, and three branches were bagged during that same period but had no sawflies introduced. When oviposition by E. lasiolepis was completed, bags were removed, and three other sawflies were permitted to oviposit on the branches. We found no significant interspecific competitive effects on the densities of three species in response to 9— and 17—fold increase in densities of galls of the stem—galling sawfly on branches of field plants in 1985 and 1986, respectively. The densities of stem gallers on the experimental branches were high, but were near densities observed on some unmanipulated field plants. The density of the petiole galler in 1985 was higher on branches with the stem galler than was the density of the leaf galler. This suggested a facilitation effect, which was not repeated in 1986. The leaf folder had significantly greater densities than the leaf galler or petiole galler. However, bagging of branches resulted in greatly reduced densities of the leaf folder compared to unbagged control branches in both years. The petiole galler and leaf galler were not affected by bagging in either year, probably due to their later oviposition phenology. Within treatment branches, where stem gall densities were high, these three sawfly species did not significantly avoid oviposition on shoots with stem galls, and in 1986 there were generally positive co—occurrence patterns of each of these species with stem galls on shoots. We conclude that interspecific competitive effects on gall density was not important under these field conditions for the leaf galler and the petiole galler, but bagging effects on the leaf folder density could have reduced the likelihood of detecting competitive effects by the stem galler on this species. Differences in the results of this study with an earlier experiment may be related to compensatory ability of the larger field plants and differences in stem gall densities.
Modelling Distributional Patterns of Woodland Birds along a Continental Gradient
Authors: MacNally Ralph C.
Published: May 2006

There are several ways to approach the question of how distributions of species are determined, including field experimentation and computer or mathematical modelling. If large numbers of species are to be considered simultaneously, modelling is the most practicable method. The current paper examines the relative capacities of a set of models to account for patterns of distributions of 92 species of forest and woodland birds along a 250—km transect in southeastern Australia. Multivariate analyses of habitat ordinations (nonmetric, multidimensional scaling: NMDS) were used to characterize the habitat breadth and habitat position of each of the avian species. Breadth is the range of structural types of habitat that a species occupies. Position is a measure of the “representativeness” of the typical habitat type occupied by a species relative to the entire sample of habitat types. There was a significant inverse correlation between breadth and position (r = —0.70). The models were used to try to account for this inverse relationship and related statistics, including the amount of co—occurrence between species. Three models based on the geographic proximity of sites provided reasonable representations of the observed patterns, except for co—occurrence, which all models consistently underestimated. Several more complicated techniques were used to try to increase the explanatory power of the models, but only one of these methods provided a justifiable improvement. The modelling demonstrated that many of the distributional patterns observed for this avifauna could be accounted for by using simple site—selection algorithms, and by assuming that species behaved independently. The major source of variation unaccounted for by the models was a consistent underestimation of levels of co—occurrence between avian species, which may mean that further modelling may require interdependence rather than independence of species.
Sex Ratios of Red Maple (Acer Rubrum) Populations in Northern Lower Michigan
Authors: Sakai Ann K.
Published: May 2006

Red maple (Acer rubrum) on nine different sites in northern lower Michigan showed generally male—biased sex ratios across a wide variety of habitats and through time. These ratios were not correlated with moisture or nutrient conditions of the sites and for more mesic sites, ratios did not appear to be affected by age of the stands. The more xeric sites showed greater male bias in sex ratios with age of the stand. Within the xeric sites, the sex ratio of regularly flowering trees was more male biased than the ratio of trees at first reproduction. Although data appear to support resource allocation theory predicting more male—biased ratios in older or poorer habitats, the male—biased sex ratios did not result from measurable differences in sex expression of trees originating from sprouts vs. seeds, sex—related differences in size at first reproduction, or sex—related differential mortality, or because of patterns of sex lability. Differences in sex ratio were of minor importance in reproductive success of individuals in the xeric plots given the lack of reproduction from seed. Reproduction from seed was higher in mesic areas, but sex—related differences in life history characteristics were not evident in these areas. The similarity of ratios in a diversity of habitats and through time suggests that the ratios may reflect the underlying genetic patterns of sex expression, rather than subsequent responses to the environment. These data suggest that sex ratios alone are not sufficient to infer proximate mechanisms for biased sex ratios or sex—related differences in resource allocation.
Sex and Drugs and Herbivores: Sex‐Biased Herbivory in Arroyo Willow (Salix Lasiolepis)
Authors: Boecklen William J. , Price Peter W. , Mopper Susan
Published: May 2006

We document through correlative and experimental analyses sex—biased herbivory in the arroyo willow, Salix lasiolepis. Male willows typically support higher densities of five species of tenthredinid sawflies than do female willows. Significant intersexual variation sawfly densities exists in naturally occurring willow clones and in clones grown in a common garden. Arroyo willow is sexually dimorphic in leafing phenology and in aspects of leaf chemistry. Male willows have significantly longer vegetative shoots than do females at the time of sawfly oviposition (P = .01). Males also have significantly higher foliar concentrations of nitrogen, phosphorus, and potassium than do females. Female willows have a marginally significant (P = .093) higher concentration of total phenols. Of the dimorphic traits, only variation in leaf phenology is a consistent predictor of variation in sawfly densities among willow clones.
Plant Successional Stage and Insect Herbivory: Flea Beetles on Sand‐Dune Willow
Authors: Bach Catherine E.
Published: May 2006

The effects of early plant successional stage on the abundance and amounts of damage caused by a specialist herbivore were investigated for sand—dune willow, Salix cordata (Salicaceae), growing on sand dunes of varying ages. Both adults and larvae of Altica subplicata (Coleoptera: Chrysomelidae) were very abundant on host plants growing on the youngest and intermediate—aged dunes, but were absent from most plants growing on the oldest dunes. Amounts of beetle damage were greatest on the youngest dune, with nearly half of the plants having no leaves at the end of the season, and damage decreased with increasing successional stage. To explain these patterns of herbivory, laboratory and field studies we used to examine beetle survivorship, feeding preferences, and movement patterns. There was no difference in larval survivorship or growth rate as a function of the age of the community in which host plants were growing. Plant successional stage also did not influence larval feeding preferences, however, adults preferred leaves from the oldest dunes. Mark—recapture experiments revealed significantly longer residence times for beetles released in younger successional areas than in older successional areas, indicating that beetle movement behavior was consistent with patterns of abundance on dunes of varying ages. Several hypotheses were tested to determine whether beetle abundances were influenced by other factors that vary with successional stage, including host plant attributes (host plant height and nutritional quality) and attributes of the plant community surrounding host plants (abundance of surrounding nonhost neighbors and proximity to neighboring host plants). The increase in host plant height with succession was not responsible for the absence of beetles on older dunes, since taller plants on the younger dunes supported even higher beetle abundance than did shorter plants. Leaf nitrogen content decreased with succession and was consistent with patterns of beetle abundance. Although plant successional stage influenced numbers of neighboring host plants as well as herbaceous dicots and woody plants, differences between numbers of beetles on dunes varying in successional stage were not caused by the composition of the surrounding plant community. These results suggest that patterns of herbivore abundance were most strongly affected by movement patterns and host plant quality, and that effects of successional stage on herbivory are an important influence on herbivore—plant dynamics in successional systems.
Sex‐Specific Life History Tactics and Risk‐Taking in Coho Salmon
Authors: Holtby L. Blair , Healey Michael C.
Published: May 2006

Differences in size and abundance between males and female at sexual maturity are common in vertebrates. Evolutionary explanations for these differences generally assume a consistent pattern within a species. Among Pacific salmon (Oncorhynchus spp.) and coho salmon (O. kisutch) in particular, however, sexual size dimorphism and sex ratio are not consistent among or within populations, making extant explanations unsatisfactory. In this paper we describe the variation size dimorphism and sex ratio within and among coho salmon populations and propose a model to account for this variation. Our data consist of a 16—yr study of coho salmon in carnation Creek, British Columbia, and shorter term studies of 30 other coho populations ranging from California to Alaska. Among mature coho salmon in Carnation Creek males outnumbered females but females were larger, and the male/female sex ratio varied directly with the female/male size ratio. Mean female size was constant over 16 yr, while male size varied inversely with the male/female sex ratio. Females apparently had higher mortality rates than males, and the difference in mortality rates appeared to increase as the animals neared maturity. Sex— and size—ratio data from other coho salmon populations indicate that there are two general population types. In one type males outnumber females, and females are larger at maturity. In the other type the sexes are equally abundant and equal in size. We propose that the range of size for successful reproduction in coho salmon is set by conditions in the breeding environment, and that the range differs for males and females because of the different roles that the sexes play in reproduction. The sex—specific constraints on reproductive size lead to sex—specific foraging strategies and risk—taking. Interpopulation differences in size and sex ratio are determined by differences in physical characteristics of the breeding environment, spawning population density, and the relative riskiness of ocean nursery areas. Within populations, variation is determined by the interaction of sex—specific foraging strategies with interannual variation in ocean feeding conditions and predator abundance.
The Physiological Ecology of Daphnia: Development of a Model of Growth and Reproduction
Authors: McCauley Edward , Murdoch William W. , Nisbet Roger M. , Gurney William S. C.
Published: May 2006

Patterns of growth, development, and reproduction have been observed in many Daphnia species, and there have been some attempts to explain them using models that take into account rates of intake, assimilation, maintenance, and energy allocation rules. We show, however, that existing models cannot capture some essential features of individual growth, especially under conditions of low food supply that are typical of field conditions. These features include: (1) a sigmoid growth curve, and (2) the time to starvation or the performance of individuals during periods of low food availability. We propose and test a new hypothesis based on the idea that allometric relationship for physiological rates are stage dependent. We show that ingestion rates increase much faster with juvenile body size than with adult body size for several Daphnia species. Existing data suggest that allometric relationships for respiration are not stage dependent, and we derive a maintenance function that takes into account overheads associated with growth and basal metabolic rates. The new allometric relationships for ingestion and maintenance, along with an accurate description of the onset of maturity and partitioning of energy between growth and reproduction, can account for the sigmoid growth pattern displayed by Daphnia. Existing models cannot explain Daphnia's performance when food availability is low, and this led us to examine how Daphnia stores energy and uses reserves. Our review synthesizes disparate observations on the structure and dynamics of reserves, and forms the basis for a new model of Daphnia pulex.
Optimal Foraging and the Size Selection of Prey by the Bluegill Sunfish (Lepomis Macrochirus)
Authors: Werner Earl E. , Hall Donald J.
Published: May 2006

The bluegill sunfish, Lepomis macrochirus, is known to select prey on the basis of size. We present evidence that this size selection is related to the optimal allocation of time spent searching for, and handling prey. A model relating search and handling time to energy return is constructed to determine the optimal breadth of diet. Prey are permitted to differ in size and relative abundance. All elements of the model are estimated from experiments with the bluegill feeding on populations constructed from size classes of Daphnia magna. Relative visibility of the different prey sizes markedly affects relative encounter rates or "effective" proportions. Effective proportions are determined empirically from feeding experiments and theoretically from reaction distance in order to correct for this bias. Search time is then manipulated by varying absolute abundance of prey. At low absolute abundance, prey of different size are eaten as encountered. As prey abundance is increased, size classes are dropped sequentially from the diet in accordance with the theory. Search and handling times are estimated from these experiments and quantitative comparisons with the model indicate these changes in diet maximize return with respect to time spent foraging.
Fog Precipitation in Coastal California Forests
Authors: Azevedo J. , Morgan D. L.
Published: May 2006

Summer fog precipitation beneath coastal forests was examined at two forested sites on ridges bordering the Eel River Valley in northern California. Each site was outfitted with bottle—funnel gauges and standard mechanical gauges placed on the forest floor and screened bottle—funnel gauges to trap fog in open areas. As much as 42.5 cm of fog water were collected beneath the forest crown during the summer fog season although differences in crown exposure and dimension caused high variability in seasonal totals caught beneath individual trees. A typical fog precipitation event begins during late evening and ends just after sunrise. It sometimes continues for several days and produces fog drip amounts of up to 8 cm. The nutrients and water captured form drifting fog by intercepting trees, shrubs, and grasses affect water balances and nutrient cycling within such coastal ecosystems. Fog may influence species composition, and the character of the soils and vegetation of such coastal forests must certainly reflect to some degree the prevalence of summer fog.
The Effects of Space and Enrichment on a Predator‐Prey System
Authors: Luckinbill Leo S.
Published: May 2006

The prolonged coexistence of the predator Didinium nasutum and its prey Paramecium aurelia was obtained in these experiments. In a large experimental volume, predators reduced the prey population to a low density but were unable to capture them all. In small experimental volumes, all the P. aurelia were captured. In large—volume experimental systems enriched with abundant bacterial food, Didinium also captured all the prey. Thus, a large arena and the limitation of the prey population by its food supply are necessary for stable interaction and assure the survival of both predator and prey. Damped population oscillations of predator and prey occurred in the absence of refuges for prey or of physical heterogeneity.
Interpreting Breeding Patterns from Census Data: A Solution to the Husting Dilemma
Authors: Gill Douglas E.
Published: May 2006

Semelparous, iteroparous, periodic, and erratic breeding patterns were observed in montane populations of the red—spotted newt, Notophthalmus viridescens, in western Virginia from 1974 to 1983. Most of the observed absences and gaps in the records of both males and females are explained by a null model that combines the small errors of sampling with independently estimated rates of annual mortality. Most newts naturally breed every year until their deaths, but a significant number of females and males did skip breeding opportunities. Because the variation in breeding histories could have been environmentally induced, no adaptive significance is ascribed to it.
Physiological Plasticity in Electromorphs of Blood Proteins in Free‐ranging Microtus ochrogaster
Authors: McGovern Mike , Tracy C. Richard
Published: May 2006

On two 1—ha plots, one fenced in and one not, populations of the prairie vole, Microtus ochrogaster, exhibited annual cycles in population density in 1977 and 1978, as they had since 1971. Blood was analyzed by electrophoresis to determine the frequencies of occurrence of several allozymes of polymorphic blood proteins. There was great seasonal and annual variability in how well population density correlated with the frequency of each of two electromorphs of transferrin (Tf) and one electromorph of leucine aminopeptidase (LAP). Also, during some seasons males with certain electromorphs had significantly greater body mass than those with other morphs. Furthermore, of the 137 individuals captured more than once, 40 (29.2%) showed changes in electromorphs of Tf, and 24 (17.5%) showed changes in LAP electromorphs. These data support earlier studies showing these proteins to be physiologically plastic. This study and others suggest that conclusions about the genetic composition of populations based on electrophoretic analyses must be supported by evidence that the protein systems studied do not show lability within individuals under abiotic or biotic stress. This study also suggests that electrophoresis may be a useful tool in assessing the physiological state of individuals exposed to different natural environments.
Effects of Reduced Perch‐Height Separation on Competition between Two Anolis Lizards
Authors: Rummel John D. , Roughgarden Jonathan
Published: May 2006

Some effects of the presence of Anolis bimaculatus (snout—vent length [SVL] 85 mm) on A. wattsi (SVL 49 mm) were assessed in a field experiment on the island of St. Eustatius, Netherlands Antilles. A. wattsi normally perches below 0.5 m. A. bimaculatus above 1.5 m. The effects of reducing this perch—height separation on the interaction of these species was studied in enclosures experimentally modified to contain A. wattsi alone, A. wattsi with A. bimaculatus and a normal range of available perch positions, or A. wattsi with A. bimaculatus but no available perch positions above 1.5 m in height. The presence of A. bimaculatus at normal perch heights caused A. wattsi to use perch positions with hotter microclimates and to be active at different times of day than when A. bimaculatus was absent, but A. bimaculatus did not affect the growth, egg production, or prey selection of A. wattsi during this study. Effects of perch—lowered A. bimaculatus on A. wattsi were identical, but the A. bimaculatus themselves had lower growth rates, egg production, and food intake when compared to those with access to the normal range of perches. The absence of a strong effect on A. wattsi is consistent with the results of a previous reciprocal experiment; this further establishes that there is an inverse relationship between interspecific competition and resource partitioning. The effect of A. bimaculatus on A. wattsi was, however, greater than that of A. wattsi on A. bimaculatus, which supports the hypothesis that the competitive relationship between these two species is asymmetric. Our finding that the perch lowering did not increase the effect of A. bimaculatus on A. wattsi suggests that perch height is not an axis of niche separation important to the competition between these two species.
Planktivore Prey Selection: The Reactive Field Volume Model vs. the Apparent Size Model
Authors: Wetterer James K. , Bishop Christopher J.
Published: May 2006

Two models proposed to explain prey selection by visually foraging planktivorous fish, the reactive field volume model (RFVM) and the apparent size model (ASM), have been found to yield similar dietary predictions in many situations. However, we found earlier formulations of the ASM to be incorrect. The correct predictions of these two models were compared. Published empirical data from an earlier fish foraging study were reanalyzed and found to fit both models quite well. Interpretation of data from an unpublished study, performed under conditions that should have allowed discrimination between the two models, was somewhat ambiguous. When we performed a similar study the results were consistent with the predictions of the ASM, but differed very significantly from those of the RFVM.
The Analysis of Class Dispersion Patterns Using Matrix Comparisons
Authors: Harvey L. Edward , Davis Frank W. , Gale Nathan
Published: May 2006

A matrix comparison method is demonstrated for measuring the relative dispersion of different classes of individuals (e.g., species, age classes, life forms) in a sample of plants whose distributions has been mapped. The method employs combinatorial data analysis procedures and a Monte Carlo approach to significance testing; it is particularly suited to the study of subsets of interplant distances in relatively small data sets. As an example, within—site tree dispersion patterns in a Jamaican wet limestone forest are analyzed. Dispersion patterns vary among species, between size classes of the same species, and for the same species in different stands.
An Experimental Study of Competition among Fugitive Prairie Plants
Authors: Platt William J. , Weis I. Michael
Published: May 2006

We assessed competition among five wind—dispersed perennial fugitive plant species. The species constitute a guild on small open sites formed by foraging badgers on the Cayler Prairie Preserve in northwestern Iowa, USA. The outcome of interactions among individuals at a site depended on the intervals between immigrations. When immigrations were separated by more than one growing season, preemptive competition occurred: initial colonists were not affected, but most later arrivals died during the first growing season, and none survived to maturity. When multiple immigrations occurred within a single year, exploitative competition depressed growth and propagule production of co—occurring species in inverse proportion to their propagule size. By manipulating the density of colonization sites and the production of propagules, we demonstrated a direct nonlinear relationship between immigration and intra— and interspecific competition. With infrequent immigration, neither preemptive nor exploitative competition was likely among offspring; reproductive success of parents was proportional to the likelihood of immigration. As immigration increased, preemptive and exploitative competition among offspring was accelerated, and per—propagule gains in parental reproductive success declined. Losses of reproductive success were inversely related to species—specific exploitative capabilities. For individuals of each species, the competition threshold was that immigration rate above which reproductive success was lowered due to the combined effects of preemptive and exploitative competition are directly related (as with competition for sites or resources within sites), competition increases with the density of the limiting resource and will have greatest effect on plants at high densities of the limiting resource. Our results further indicate that intra— and interspecific competition are likely to influence reproductive success within local populations of these fugitive species. Mean fecundities among adjacent populations of fugitive plants were more similar than expected for noninteracting populations distributed along a gradient of site densities, suggesting that interspecific competition has influenced life history attributes in this guild. Differences in life history attributes influencing immigration and exploitative capabilities may result in there being regions along the gradient within which each species is competitively superior. Persistence in the guild may be maintained by: (1) an inverse relationship between immigration and exploitative ability, (2) variation in the density of sites, and (3) the occurrence of both preemptive and exploitative competition on sites.
Size Hierarchies in Experimental Populations of Annual Plants
Authors: Weiner Jacob
Published: May 2006

The effects of inter— and intraspecific interference on size hierarchies (size inequalities) were investigated in populations of the annual plants Trifolium incarnatum and Lolium multiflorum. Variables experimentally manipulated included plant density, species proportions, soil fertility, and spatial pattern of plantings. Densities were below those for extensive density—dependent mortality. Size inequality always increased with increasing density. Plants grown individually showed very low inequality, while plants grown at the highest density had the most developed hierarchies. Size inequality usually increased with an increase in productivity when interference was occurring. When dominant in mixtures, Lolium showed less size inequality than in monoculture, while the suppressed species, Trifolium, usually displayed an increase in inequality. Spatial pattern appeared to be less important than other factors in causing size inequalities; plants sown in a uniform spatial pattern showed significantly lower size inequality than plants sown in a random pattern in only one out of four cases. Inequality in reproductive output of Trifolium, as estimated by dry mass of flower heads, was always greater than inequality in plant in dry mass. The results support a model of plant interference in which large plants are able to usurp resources and suppress the growth of smaller individuals more than they themselves are suppressed. While interference decreases mean plant mass, it increases both relative variation in plant mass and the concentration of mass within a small fraction of the population.
Heliconius Caterpillar Mortality during Establishment on Plants With and Without Attending Ants
Authors: Smiley John T.
Published: May 2006

Rates of disappearance (mortality) in Heliconius caterpillars of various sizes were investigated by experimental placement on Passiflora host plants growing in their native rain forest habitat. Survivorship of experimental larvae was similar to that of Heliconius larvae in natural populations: 60% mortality during the initial 2 d, followed by 30% mortality during each 2—d period thereafter. Survivorship was not dependent on larval size or instar, nor was it related to larval growth rate or host plant isolation from surrounding foliage. However, survivorship was appreciably lower on host plants on which ants were attending extrafloral nectaries (70% vs. 45% mortality in first 2 d), indicating that ants may be a primary source of high mortality. I conclude that for these Heliconius, high first—instar mortality is not due to the small size of the caterpillars but rather is a consequence of ant attendance to some host plants; most caterpillars on plants with ants are killed in the first instar, while caterpillars on ant—free plants have much lower mortality and make up the majority of the cohort in later instars.
Energy Allocation during Lactation in Cotton Rats (Sigmodon hispidus) on a Restricted Diet
Authors: Mattingly D. Kevin , McClure Polley A.
Published: May 2006

The effects of restricted food availability on lactating Sigmodon hispidus from Kansas (KSh) and Tennessee (TSh) populations were studied in the laboratory. Mothers received 75% of food intake required by nonreproductive females of similar body mass. KSh control and restricted females produced significantly larger litter sizes at birth (6.9 and 7.3) than TSh females (4.7 and 5.0). At weaning (12d), mean litter size of restricted KSh females (4.3) was similar to that of control (4.4) and restricted TSh females (4.5). Mean litter size for control KSh females (6.6) was significantly larger than for the other three groups. KSh females of smaller body mass suffered higher offspring mortality during lactation than did larger KSh females. Mean body mass of control KSh young (20.1 g) was significantly larger than of control TSh young (16.5 g) at age 12 d. Mean body masses of restricted KSh and TSh young (12.5 and 11.2 g) were not significantly different at 12 d of age, and young of both groups were smaller than controls. Smaller restricted KSh females produced young that grew more slowly than those of larger restricted KSh females. Variation in the growth of restricted TSh young, however, was dependent upon litter size, as was growth for both TSh and ksh controls. During lactation, restricted TSh and KSh females lost an average of 31% of parturition body mass; comparable values for control TSh and KSh were 13% (16 g) and 9% (17 g), respectively. In the nest, body temperature of young of restricted KSh and TSh females averaged °1°—2°C lower than body temperatures of control young. Development of the ability of pups to respond metabolically to cold, however, did not differ significantly between control and restriction treatments or between populations. Thus, under severe food restriction, females with larger litter size (KSh) lost young until litter sizes were similar to those for females with smaller original litters (TSh). Decreased growth and body temperatures of nestlings in food—restricted groups reduced the energy demands of lactation so that smaller litters could survive to weaning. Growth and survivorship of KSh young were largely determined by the body mass of the dam, indicating the potentially significant role of body size for small mammals that experienced periodic food shortages during lactation.
Nonlinearities in Competitive Interactions: Component Variables or System Response
Authors: Kerfoot W. Charles , DeMott William R. , Levitan Charles
Published: May 2006

Nonlinear responses for component demographic variables do not necessarily imply nonlinear responses for net population variables. Two examples, one drawn from the laboratory and one from the field, contrast the responses of Daphnia brood size and per capita growth rate during density manipulations. While the response of brood size is markedly nonlinear, the response of the per capita growth rate is approximately linear. These examples caution against inferences about population change based solely on knowledge of single component variables. We also emphasize the importance of checking physiological adjustments over the entire density range.