bluuebunny/crossref_metadata_2025 huggingface dataset:
Longitudinal Patterns of Ecosystem Processes and Community Structure in a Subarctic River Continuum
Authors: Naiman Robert J. , Melillo Jerry M. , Lock Maurice A. , Ford Tim E. , Reice Seth R.
Published: May 2006

Ecosystem processes and community structure in running waters of the boreal forests of Quebec, Canada, are strongly influenced by climate and channel geomorphology. Here we present an overview of a project examining longitudinal trends as small streams gradually coalesce into large rivers, summarizing our results in a series of budgets and predictive equations describing changes in organic carbon dynamics and community structure. There were significant trends with stream order for 70% of the 73 components, processes, and ratios examined. Of 46 independent components examined, 63% showed a significant trend with stream order. As stream size increased from 1st to 9th order there was a decrease in total carbon inputs (i.e., precipitation, throughfall, primary production, and allochthonous materials) followed by a gradual increase due to greater primary production in streams >6th order. The standing stock of carbon decreased exponentially downstream, and total carbon outputs (i.e., respiration, leaching, methane evasion, and insect emergence) increased slightly downstream. Nevertheless, some ecosystem—level processes, as well as community structure, showed equivocal trends, which were apparently due to the hierarchical scale of examination and the relative degree of physicochemical vs. biological control, of the processes and communities. The data, when placed in a watershed perspective, showed that total carbon inputs were evenly distributed by stream order throughout the 19 871—km2 Moisie River drainage network. Most carbon was stored in the small 1st to 3rd order streams, whereas the majority of organic carbon was metabolized in the 7th to 9th order rivers. Fluvial transport of organic carbon to the Gulf of St. Lawrence was nearly three times that of the measured total annual input, suggesting that inputs of dissolved organic carbon in groundwater were more important than previously expected. Ecosystem—level measurements of carbon retention and utilization also showed significant trends with stream order. The spiraling length for carbon increased exponentially from 8—15 km in small streams to 426 km in the 9th order river. There was a concomitant decrease in reach retention with stream order, while the rate coefficient of respiration and rate of downstream movement increased with order. The stream metabolism index, a measure of ecosystem efficiency, increased from 1st to 7th order, thereafter decreasing as streams became larger. These trends with stream order were related to physical gradients in channel dimensions, hydrology, riparian influences, and sunlight. We conclude that these subarctic lotic ecosystems have numerous strong relationships with stream order and that the dynamics can be described by a relatively small set of predictive equations.
Regulation of Structure in Complex Systems: Experimental Temporary Pond Communities
Authors: Wilbur Henry M.
Published: May 2006

Interactions among competition, predation, and disturbance in determining the abundances of four species of anurans were studied in a factoral experiment using 36 replicated experimental ponds. Hatchlings of the four species (Rana utricularia, Scaphiopus holbrooki, Bufo americanus, and Hyla chrysoscelis) were introduced at the same relative abundances at a low or high initial density. Competition determined survival, body size at metamorphosis, and length of larval period in high—density communities, which were food—limited. The predatory salamander Notophthalmus viridescens did not alter either the total number of metamorphs or their combined biomass in the high—density communities, but the relative abundance of metamorphs was shifted as Scaphiopus holbrooki, the competitive dominant, was selectively eaten. In low—density communities, predation decreased survival and biomass production of tadpoles, often to zero, in all species. For each of the four combinations of tadpole density and presence or absence of predators, ponds were drained at three rates to simulate different drying regimes. Competition slowed growth and thus increased the risk of desiccation in high—density populations in drying ponds. Predation ameliorated the effects of competition, allowing survivors to grow rapidly enough to metamorphose before ponds dried. Survival of tadpoles to metamorphosis, body size at metamorphosis, and the timing of metamorphosis were determined in a complex way by interactions among all of the treatment effects and the life history characteristics of the four species studied. Scaphiopus was the species least sensitive to tadpole density and was the competitive dominant in ponds without newts. It has a rapid growth rate and metamorphosed soon enough to escape desiccation. It suffered the greatest risk of predation and was eliminated from even some of the high—density communities. Rana was most successful in low—density communities without predators. No Rana survived in rapidly drying (50—d) ponds because of insufficient time to obtain a minimum size for metamorphosis. Rana were also eliminated from most populations exposed to predation. The effect of competition on Bufo in high—density populations, few or no survivors, was reversed by predation as newts selectively fed on Scaphiopus and Rana. This result was most striking in the tanks that dried most rapidly. Hyla did very poorly in all slowly drying (100—d) tanks compared with controls because of intense competition. It had moderate success in high—density communities where newts had removed most competitors. These results show that biological and environmental factors interact to determine the structure of anuran communities. Neither competition no predation is the single unifying force, but rather they interact to determine the different consequences of the date of drying of a pond to the success of each species.
An Experimental Analysis of Natural Selection on Body Size of Hatchling Turtles
Authors: Janzen Fredric J.
Published: May 2006

A complete understanding of life history evolution requires an appreciation for the influence of natural selection and genetic variation on intrapopulation phenotypic variation in key life history traits. Experimental manipulation provides particularly useful insight into the biological importance of this variation. Consequently, I undertook a field experiment in which I manipulated body size and evaluated selective predation in order to investigate the ecological and evolutionary significance of phenotypic variation in hatchling turtles. I incubated eggs from 17 clutches of common snapping turtles (Chelydra serpentina) on wet and dry substrates, producing "large" and "small" hatchlings, respectively. Hatchlings were individually marked, measured, and then released simultaneously at a typical nest site in a National Wildlife Refuge area, Whiteside County, Illinois from which the eggs collected originally. Subsequently, hatchlings were recaptured in drift fences erected along the edge of the Mississippi River. Sixty—six of 112 hatchling C. serpentina were recaptured. Survivorship was not related to clutch, incubation conditions, or locomotor performance, but was significantly size dependent. Natural selection favored larger hatchlings. The strength of selection acting directly on three size traits, as measured by standardized selection gradients (°), was 0.327 (PCI [principal component 1] °overall size and mass), 0.282 (PC3 = plastron length), and 0.162 (PC4 = trade—off between hatchling mass and width). The opportunity for selection was moderate (I =0.703), but did not constrain selection. Full—sib heritabilities of body size of hatchling turtles were variable (h2wet = 0.72 ° 0.34 and h2dry = 0.17 @+ 0.28), but were not significantly different between the two moisture treatments. These results provide support for the "bigger is better" hypothesis. Larger hatchling Chelydra serpentina exhibited significantly greater survivorship than smaller individuals during movement from the nest site to water. However, larger body size of hatchling turtles may not evolve rapidly because the strength of selection was moderate in magnitude and the heritability was relatively low. These results emphasize the need for more detailed ecological investigations of the early life history of these organisms. Furthermore, this study highlights the utility of experimental manipulations of phenotypic variation for evaluating the potential impact of natural selection on life history evolution.
Ecology of Mast‐Fruiting in Three Species of North American Deciduous Oaks
Authors: Sork Victoria L. , Bramble Judy , Sexton Owen
Published: May 2006

We conducted an 8—yr study of flower and acorn production in three species of oak in east—central Missouri: white oak (Quercus alba), red oak (Q. rubra), and Black oak (Q. velutina). The goal was to evaluate whether mast—fruiting, the synchronous production of large seed crops followed by small seed crops, is simply a response to weather conditions or is actually an evolved reproductive strategy. In this paper, we address four specific questions: (1) Are annual fluctuations in acorn crop size the result of synchronous production of acorns by individuals? (2) Are large acorn crops the result of large flower crops and/or high survival of those flowers to mature fruit? (3) To what extent do weather variables account for variation in acorn production among years? (4) Does acorn crop size correlate negatively with prior acorn production? Red oak and white oak showed a greater degree of mast—fruiting than did black oak. Within a species, individuals tended to produce large acorn crops in the same years, but each species differed in which years they produced large crops. The size of a given acorn crop was determined by both flower abundance and survival of flowers to fruit. Principal components and multiple—regression analyses were used to describe the relationship between weather variables and acorn production. The first principal component explained the largest amount of the variation in black oak (R2 = 0.55) and the red oak (R2 = 0.89). In white oak, two principal components combined to explain 77% of the variation in acorn production. The weather variables that were associated with these principal components included spring temperature (positive effect) and summer drought (negative effect). Past acorn production had a major impact on the size of the current acorn crop, with each species showing a different pattern. In black oak, the current acorn crop was negatively correlated with the crop 3 yr prior but positively correlated with the crop 2 yr prior. In red oak, acorn crop size 1, 2, and 3 yr prior had negative correlations with current acorn crop, while acorn crop size 4 yr prior was positively correlated. In white oak, there were negative correlations between acorn crop size and crop size 1, 2, and 4 yr prior but a positive correlation with the acorn crop size 3 yr prior. These data are consistent with the hypothesis that mast—fruiting species must store resources during some years in order to produce a mast crop. We discuss the possibility that these three species may have inherent cycles of reproduction that are modified by the impact of weather conditions, black oak with 2—yr cycles, white oak with 3—yr cycles, and red oak with 4—yr cycles. We conclude that the patterns of acorn production for black oak, red oak, and white oak are not simply responses to weather events but are also a function of prior reproductive events. This suggests that masting is an evolved reproductive strategy.
Host Range of Generalist Caterpillars: Trenching Permits Feeding on Plants with Secretory Canals
Authors: Dussourd David E. , Denno Robert F.
Published: May 2006

To test if secretory canals limit the host ranges of polyphagous moth larvae, we measured the growth of Trichoplusia ni (cabbage looper) and Spodoptera ornithogalli (yellow—striped armyworm) on plants in nine families, including species with and without secretory canals. Larvae were reared to the final instar on excised leaves of the test plant, weighed, then enclosed in the field on either detached leaves with depressurized canals or intact leaves with undamaged canals. After 24 h, the larvae were reweighed and the leaves were examined for evidence of trenching. For comparison, nine additional species of generalist caterpillars were tested on Lactuca serriola (with latex canals) using the same procedure; five of these species were also tested with Petroselinum crispum, which has resin canals. Trichoplusia larvae gained mass on detached and intact leaves of all plant species tested. Larvae cut trenches in plants with canal—borne secretions (Apiaceae–resin, Asteraceae: Cichorieae–latex, Cucurbitaceae–phloem sap), and thereby deactivated the canal systems. Plant species lacking exudates were not trenched. Spodoptera larva failed to cut trenches and performed poorly (with one exception, Daucus carota) on intact leaves of plants with exudates, the same species that T. ni trenched. Detached leaves supported rapid growth by S. ornithogalli larvae, documenting the suitability of these plants in the absence of functional canals. On the latex—bearing Lactuca serriola. S. ornithogalli also grew rapidly on leaves distal to trenches cut by T. ni. This laboratory experiment confirmed that armyworms could feed on plants bearing secretory canals if the larvae were capable of trenching. Performance of the nine other generalist species was likewise correlated with behavior; only species adept at trenching grew rapidly on intact L. serriola and P. crispum. Our results, corroborated across both plant and insect taxa, indicate that the effectiveness of canal—borne exudates as a defense is dependent upon the behavioral capabilities of the herbivore. Generalists lacking trenching behavior are unable to compensate through biochemical or physiological mechanisms. Their host ranges are indeed limited by canalicular defenses due to their inability to deactivate the canals by trenching.
The Allometry of Nitrogen to Growth and an Inducible Defense under Nitrogen‐Limited Growth
Authors: Ohnmeiss Thomas E. , Baldwin Ian T.
Published: May 2006

As plants become nitrogen—limited in their growth, carbon/nutrient (C/N) theory predicts a decreasing allocation of nitrogen to nitrogen—intensive defense compounds, while optimal defense (OD) theory predicts an increasing allocation to defense; CN ratio of the plant does not change, while OD theory does. We compare C/N and OD theory predictions for the inducible nicotine production of a native tobacco, Nicotiana sylvestris. We define allocation as the slope of the ln—ln allometric relationship between whole—plant nicotine and nicotine—free biomass in undamaged and damaged plants, harvested every other day in eight consecutive harvests. Relative growth, nitrogen, and nitrogen in nicotine accumulation rates (RGRs, RNARs, and RNicARs) were calculated to estimate the instantaneous investment of nitrogen in nicotine and the growth of plant tissues. Nicotine accumulation continues during nitrogen stress, resulting in a 3.29—fold increase in the nicotine pools of undamaged plants, and is accelerated in response to leaf damage, resulting in 4.06 fold increase in the nicotine pools of damaged plants. Nitrogen pool size did not change after day 2, and both undamaged and damaged plants allocated an increasing proportion of their nitrogen pool to nicotine throughout the experiment. Damage increased the allocation of nitrogen to nicotine; the slope of the ln nicotine vs ln nicotine—free biomass regression for damaged plants was significantly greater (16.9%; P < .041) than the slope for undamaged plants as measured during the period of damage—induced nicotine accumulation (days 2—10 damage). Allocation of nitrogen to nicotine accumulating is clearly not a passive function of the nitrogen in excess of growth requirements, and damage increases this allocation. Trade—offs between biomass and nicotine production over the course of the experiment between damaged and undamaged plants were not apparent. However, the short—term estimates indicate otherwise. On day 2, there were a significant decrease in RGR, with a correlated increase in RNicAR in response to damage, suggesting that there was a short—term trade—off between allocation to growth and nicotine accumulation or other induced processes. By the end of the experiment, each damaged plant had accumulated 0.595 mg N in nicotine above that accumulated by undamaged plants, and additional investment representing 1.94% of the total nitrogen pool. However 2 d after damage, damaged plants allocated 0.131 mg more N to nicotine than undamaged plants, representing a short—term investment of 17.7% of the nitrogen potentially made available by the decrease in RGR (.0740 mg). Plants invest substantial portions of their nitrogen in an induced defense, but our estimates do not include the nitrogen costs of production, storage, transport, detoxification, or the indirect cost of this induced defense.
Diversity in Body Size and Feeding Morphology within Past and Present Vulture Assemblages
Authors: Hertel Fritz
Published: May 2006

Vultures comprise two geographically isolated and taxonomically distinct groups, Old World accipitrids and New World vulturids, and provide a classic case of convergent evolution. In both regions, several species of vultures often feed together in large numbers on carcasses. Behavioral studies of East African and Amazonian vultures have documented parallels in apparent ecological separation within this guild of specialized scavengers. Here morphological differences in skull, beak, and manibular dimensions are compared among sympatric vultures in East Africa, South Africa, the Indian subcontinent, Amazonia, and the Pleistocene Rancho La Brea deposits in California. A discriminant function analysis based on morphological indices separates three basic feeding types: rippers, gulpers, and scrappers. Vultures of the three feeding types are present in both Pleistocene and Recent assemblages and show a similar distribution of body sizes into three size classes, suggesting that competition has favored similar pathways of ecological separation. This is true even when phylogenetic differences among some of the assemblages are partially accounted for. Comparisons between the fossil and extant New World vultures indicate that more specialized species were prone to extinction and that there has been a reduction in body size since the Pleistocene.
The Effects of Enrichment of Three‐Species Food Chains with Nonlinear Functional Responses
Authors: Abrams Peter A. , Roth James D.
Published: May 2006

This article investigates the dynamic behavior of a model of an unstable food chain consisting of three species or trophic levels. The model assumes that each of the consumer species has a Holling type 2 functional response. The study examines how dynamic behaviors and top—level population densities change with the carrying capacity of the bottom level. In all cases examined, the mean population density of the top species initially increases, but eventually decreases as carrying is increased. In some cases, a sufficient increase in the carrying capacity of the bottom species leads to extinction of the top species. If the maximum rate of change of the top species is sufficiently slow relative to the middle species, the period of the population cycles in the top species can change rapidly with a small change in the carrying capacity of the bottom species. Low carrying capacities produce simple cycles with a long period, while high carrying capacities produce simple cycles with a short period. For intermediate carrying capacities, more complex dynamical behaviors, including chaos, may occur. Systems that have only one equilibrium point with all species present may have two alternative attractors with very different densities of the top—level species. The responses of the bottom two species to an increased carrying capacity are also discussed. The results imply that nutrient enrichment may adversely affect high—level consumers and that reductions in the density of top—level consumers may make it difficult or impossible to restore their populations.
Effect of pH Variation of Interspecific Competition Between Two Species of Hylid Tadpoles
Authors: Warner Susan C. , Travis Joseph , Dunson William A.
Published: May 2006

When species differ in their ability to cope with varying abiotic conditions, a change in an abiotic factor can potentially alter the outcome of biotic interactions. The purpose of this study was to examine this phenomenon. We tested the effects of an abiotic factor (pH) and two biotic factors (intraspecific and interspecific competition) on survival and growth to metamorphosis by larvae of two anurans (Hyla gratiosa and Hyla femoralis) in outdoor tanks. Two levels of pH (4.5 and 6.0) and three levels of density (0, 30, or 60 embryos per 580 L tank) of each species were arranged in a fractional factorial design and replicated threes times. Larvae of H. gratiosa exhibited typical patterns of intraspecific density—dependent responses: as density increased, survival and size at metamorphosis decreased and larval period increased. The interactions of the abiotic factor and the biotic factors were complex in this experiment. The addition of H. femoralis at high pH caused decreased survival and increased larval period of H. gratiosa, but at low pH these negative effects were not detected. Variation in pH had no effect on the size at metamorphosis of H. gratiosa. Interspecific competition from H. femoralis caused decreased size at metamorphosis of H. gratiosa only at the high initial densities. Some of the responses of H. femoralis were different than those of H. gratiosa. In this species, intraspecific competition affected only the size at metamorphosis. Low pH increased the survival of H. femoralis, but had no effect by itself on any other trait. Increased conspecific density and pH interacted only in their effect on the size at metamorphosis of H. femoralis. Interspecific competition resulted in decreased size at metamorphosis but only increased larval period at the high initial densities. Interspecific competition at the high density interacted with pH to increase survival and size at metamorphosis at low pH in contrast to those results at high pH. Overall, the larval period was the response most strongly affected in H. gratiosa while for H. femoralis the size at metamorphosis was the most strongly affected. Hyla gratiosa had more negative impact on H. femoralis than vice versa. This may be due to the larger body size of H. gratiosa. Low pH enabled H. femoralis to compete more effectively, because the negative impact of H. gratiosa was lessened. Even in a relatively simple simulation, complex interactions occurred between biotic and abiotic factors. This finding demonstrates the importance of multi—factor analysis in furthering our understanding of the structure of assemblages.
Acclimation of Photosynthetic Phenotype to Environmental Heterogeneity
Authors: Spencer William E. , Teeri James , Wetzel Robert G.
Published: May 2006

Inducible C4—like photosynthetic metabolism in Hydrilla verticillata leaf tissue elicits variability in photosynthetic phenotype, expressed as CO2 compensation point (Τ). We conducted a field and laboratory study to investigate the ecological and adaptive significance of this physiological phenomenon. Spatial horizontal environmental heterogeneity was observed within clonal populations of H. verticillata in Florida, USA. Measured at midday, the edge habitat at the expanding periphery of the clone exhibited a dissolved inorganic carbon (DIC) concentration of 0.7 mmol/L, pH 7.1, a dissolved oxygen (DO) level of 0.13 mmol/L, and biomass of 0.2 kg/m2. The mat habitat, located 200 cm towards the interior of the surface mat, exhibited DIC 0.1 mmol/L, pH 10.2, DO 0.48 mmol/L, and biomass 0.8 kg/m2. DIC depletion and DO supersaturation characterized the mat habitat for most of the day and much of the growing season. Furthermore, net photosynthesis, daily carbon gain, and relative growth rate (RGR) of H. verticillata were reduced 80% by mat conditions compared to edge conditions. Τs of H. verticillata were positively correlated with CO2 and bicarbonate concentration, and negatively correlated with pH, DO, and biomass. Low and high Τ photosynthetic phenotypes were associated with the mat and edge habitats, respectively. Photosynthetic phenotype of H. verticillata appears to acclimate to environmental heterogeneity within a clone in the field. Net photosynthesis and daily carbon gain of low Τphenotype H. verticillata was 128% and 40% greater than the high Τ phenotype when measured in the mat habitat, but was 21% lower than the high Τphotosynthetic phenotype when measured in the edge habitat under low quantum flux. Laboratory experiments showed a negative curvilinear relationship between the Τ of H. verticillata and plant density. The data demonstrate that plasticity in photosynthetic phenotype of H. verticillata is a density—dependent, physiological response that optimizes carbon gain within a stressful heterogeneous environment. Evolution of facultative C4—like photosynthetic metabolism in H. verticillata may have been an adaptation to the contraints imposed upon carbon gain by DIC and quantum flux limitation in the mat habitat.
Impact of Insect Herbivores on Competition Between Birds and Mammals for Pinyon Pine Seeds
Authors: Christensen Kerry M. , Whitham Thomas G.
Published: May 2006

Few studies have considered impacts of insect cone herbivores on bird and mammal seed consumers. Harvests of pinyon pine (Pinus edulis) cones by insects, birds, and mammals were negatively correlated both within and across sites. Birds and mammals significantly increased their cone harvest when the other group was experimentally prevented access to trees, and mammals harvested more cones when insects were removed from trees with an insecticide. These results suggest that stem— and cone—boring insects (primarily Dioryctria albovittella), birds (Clark's Nutcrackers, Pinyon Jays, and Scrub Jays), and mammals (cliff chipmunks and rock squirrels) compete for pinyon pine seeds. Additionally, insects affected the interaction between birds and mammals because the ratio of mammal to bird harvest increased up to 350—fold as insect cone consumption increased. Because insect—infected and uninfested pinyon pines are genetically different, these observations and experiments suggest that genetic interactions between plants and herbivores can affect the structure of a seed—consumer community.
Stage‐Structured Interactions in Bluegill: Consequences of Abult Resource Variation
Authors: Mittelbach Gary G. , Osenberg Craig W.
Published: May 2006

Ontogenetic shifts in diet and habitat by the bluegill sunfish (Lepomis macrochirus) separate its life history into two functionally distinct stages: juveniles, which are restricted by predators to feeding in vegetated littoral habitats, and adults, which feed on zooplankton (Daphnia) in the open water. Through comparative and experimental studies in natural lakes, we show how variation in Daphnia size and abundance has a strong impact on adult bluegill foraging gain and growth, and how the bluegill in turn influences Daphnia abundance and size. A two—life—stage model is used to predict how natural variation in Daphnia productivity among lakes should affect both adult and juvenile bluegill densities and growth rates. The model predicts that adult bluegill growth and density should be positively correlated among lakes, while juvenile growth and density should be negatively correlated in these same lakes. These predictions arise despite density dependence acting in each life—stage. A long—term data set on bluegill growth and abundance in a series of natural lakes supports these predictions. Finally, we consider what factors might drive variation in Daphnia abundance and size structure among lakes, and suggest that the volume (and/or productivity) of the hypolimnetic refuge available to Daphnia is a key factor. The strong interaction between adult bluegill and Daphnia, coupled with the bluegill's two—stage life history, provides an important mechanism by which the dynamics of littoral species (fish and their invertebrate resources) are coupled to the dynamics of open—water zooplankton.
Larval Dispersion and Survivorship in a Leaf‐Mining Moth
Authors: Simberloff Daniel , Stiling Peter
Published: May 2006

We observed and quantified leaf miner distribution on Quercus geminata in order to determine its relation to leaf abscission and the effects of leaf abscission on larvae. A leaf—mining moth, Stilbosis quadricustatella, exhibited a clumped distribution of mines among leaves of the evergreen oak Quercus geminata. Mines tended to be on large, peripheral, and undamaged leaves so that leaves were often multiply minded. Mined leaves, especially those with multiple mines, tended to abscise early. Unless miners in an abscised leaf were very near to pupation, abscission killed them. Premature leaf fall was by far the largest source of larval mortality for this moth. A preliminary analysis suggested that the clumping of miners was no more favorable to the tree (in terms of number of leaves abscised early) than to the insect.
The Influences of Conspecific and Heterospecific Residents on Colonization
Authors: Danielson Brent J. , Gaines Michael S.
Published: May 2006

To determine if dispersing prairie voles, Microtus ochrogaster, are prevented from establishing home ranges in habitat already occupied by conspecifics or potentially competitive species, voles were introduced into enclosed populations of: the same species; southern bog lemmings, Synaptomys cooperi; cotton rats, Sigmodon hispidus; or an empty enclosure. The results indicated that colonization by dispersing voles was negatively affected by resident conspecifics. Introduced females were more strongly affected than males during the vegetative growing season but not during the nongrowing season when reproductive activity is typically low. Resident bog lemmings also negatively affected colonization by dispersing voles. However, both sexes of introduced voles were similarly affected in both seasons. There was no evidence of postcolonization competitive effects, suggesting that interspecific competition does not occur between established resident individuals. Cotton rats, which have only recently become part of the small mammal community in Kansas, did not adversely affect colonization by dispersing voles or have adverse post—colonization effects on their survival and reproduction. The ability of residents to inhibit colonization by another species may facilitate the coexistence of M. ochrogaster and Synaptomys cooperi by retarding the competitive exclusion of either species until annual fluctuations in reproduction and density create an abundance of suitable but unoccupied space. This type of coexistence is similar to the storage effect in lottery models of competitive coexistence.
Distribution of Seeds in Sonoran Desert Soils: Implications for Heteromyid Rodent Foraging
Authors: Price M. V. , Reichman O. J.
Published: May 2006

We used a hierarchical sampling design to investigate temporal and spatial patterns of variation in the topsoil seedbank at a site on the Santa Rita Experimental Range, 50 km south of Tucson, Arizona. Replicate sets of contiguous 4 ° 4 cm samples to 2 cm depth ("transects") were collected three times per year, from 1980 to 1983, from four microhabitats that are differentiated by desert heteromyid rodents: large and small open spaces, under canopies of large shrubs, and under canopies of small trees. We also analyzed texture and organic content of soils from the four microhabitats. Seeds from 54 plant taxa were encountered, most of them rare. Average seed mass across species was 1.28 mg/seed, but because tiny seeds were most numerous, the abundance—weighted average was 0.16 mg/seed. Total seed abundance was highly variable, ranging from 0 to 256 seeds and from 0 to 146 mg per sample. The fourfold temporal variation in mean mass or number of seeds per sample was largely due to between—year rather than between—season effects. Seed standing crops were positively correlated with rainfall during the previous 6—mo rainy period, and were lowest in fall and winter of 1982 after 2 yr of poor winter rains. Mean seed number per sample, which reflects primarily abundance of tiny seeds, did not vary among microhabitats. Mean per—sample seed mass, which reflects abundance of the large seeds eaten by rodents, did vary among microhabitats, however, with values being higher under shrubs and trees than in open spaces. The low mean abundance in open spaces was due more to a high frequency of seedless samples than to low maximum per—sample abundance. Hence, profitable seed patches do occur in open spaces, but are rarer than under shrubs or trees. Within microhabitats there was significant variation among replicate transects and among samples within transects. On a local scale, seeds occurred in patches 12 cm in diameter in all microhabitats and sampling periods. Seed species composition varied seasonally and among microhabitats, and there was an overall correlation between seed and adult microhabitat distribution for spring ephemerals. Soil from open spaces was finer and had lower organic content than that under shrub or tree canopies. Our results suggest that the resource upon which desert granivores depend is highly variable in time and space. Microhabitat is one of the most conspicuous sources of variation, and heteromyid rodent species differ in use of those microhabitats (open spaces vs. areas under canopies of perennial plants) that differ most noticeably in seed abundances, species composition, and soil characteristics.
Demographic Consequences of Larval Development Mode: Planktotrophy vs. Lecithotrophy in Streblospio Benedicti
Authors: Levin Lisa A. , Caswell Hal , DePatra Kathy D. , Creed Elizabeth L.
Published: May 2006

This paper examines the demographic consequences of planktotrophic and lecithotrophic development in an estuarine polychaete. Two strains of Streblospio benedicti (Spionidae) were reared in the laboratory from birth through death at 20°C and salinity 34%. Survivorship and reproductive data were collected weekly and were used to construct life tables and population projection matrices for each development mode. Planktotrophic females reproduced earlier, and had higher fecundity and a shorter generation time than lecithotrophic females, but also exhibited higher mortality early in life. Despite the apparently opportunistic nature of the planktotrophic life history traits, the finite rate of increase (l) in the lecithotrophic strain (1.319 wk—1) exceeded that of the planktotrophic strain (1.205 wk—1). Net reproductive rate (R0) was also higher for the lecithotrophs (93.4) than for the planktotrophs (17.6) Peak reproductive values were attained earlier in planktotrophs than in lecithotrophs. Sensitivity analyses indicate that l is most sensitive to changes in larval and juvenile survivorship, and that the differences in l were almost completely determined by life table differences during the first 15 wk of life. The potential population growth rates obtained in this study agree well with those estimated for other opportunistic polychaete species such as Capitella sp. I and Polydora ligni. Under uniform conditions the two strains of S. benedicti achieved similar growth rates with very different life history traits. We hypothesize that each combination of traits may be adaptive under different circumstances in the field.
Effects of an Algivorous Minnow on Temperate Stream Ecosystem Properties
Authors: Gelwick Frances P. , Matthews William J.
Published: May 2006

We used a field experiment to test for effects of the algae—grazing minnow Campostoma anomalum on stream ecosystem dynamics. Eight whole pools in Brier Creek, in south—central Oklahoma (USA), served as replicates. Following a stream—resetting flood, four pools with natural densities of Campostoma and four pools with no Campostoma were maintained by fences across riffles for 61 d. Subsequently, a changeover was conducted. Fish were removed from two of the previously grazed pools and added to two of the previously nongrazed pools; the other four pools remained as grazed and nongrazed controls. The changeover was maintained for 33 d. A repeated—measures analysis of variance showed significant grazing or grazing ° time effects for variables related to organic matter processing, algal primary productivity and community composition, and invertebrate density. Principal components analysis showed complete separation of grazing treatments on the combined first two axes (accounting for 34.6% of the variance). Correlations of variables with these axes corresponded to results of the univariate ANOVAs; pools with Campostoma had lower algal standing crops, higher biomass—specific net primary productivity, greater percent blue—green algae, lower areal coverage by filamentous green algae, lower bacterial density, smaller benthic particulate organic matter fractions, and higher invertebrate density than pools without Campostoma. The changeover indicated that effects of grazing by these fish can be rapid when they enter nongrazed pools, but that effects can linger in formerly grazed pools despite markedly reduced densities of these algivorous fish.
The Nature of Population Stability in Eurosta Solidaginis, A Nonoutbreaking Herbivore of Goldenrod
Authors: Cappuccino Naomi
Published: May 2006

Eurosta solidaginis (Diptera: Tepritidae) is a common, nonoutbreaking herbivore of goldenrod (Solidago altissima) whose populations fluctuate little through time. A large body of theory is based upon the assumption that nonoutbreaking phytophagous insects are regulated at "endemic equilibria" by the temporally density—dependent mortality caused by their natural enemies. This equilibrium viewpoint has been challenged recently by proponents of "nonequilibrium theory," even though the evidence against population regulation is largely observational and likely to be biased by the almost exclusive study of abundant, pest species. To understand the nature of population stability in Eurosta solidaginis, I altered the density of 20 subpopulations, and looked for compensatory changes in mortality that could allow the populations to return to their pre—experimental density. Mortality diminished in those subpopulations whose density was decreased. Moreover, the change in mortality depended significantly on the change in density; subpopulations in which density was most severely diminished enjoyed a greater decrease in mortality. Of four individual mortality agents, only early larval death, which may be partly caused by a defensive reaction by the host plant, showed significant density dependence. No within—generation (spatial) density dependence was detected in either the first or the second (experimental) generation. At the end of the density—manipulation experiment, all Eurosta were removed from the patches. Most patches were recolonized within one generation.
Competitive Ability and Adaptation to Fertile and Infertile Soils in Two Eriophorum Species
Authors: McGraw James B. , Chapin F. Stuart
Published: May 2006

We investigated the role of competition in adaptation to varying nitrogen (N) levels for two Alaskan species of Eriophorum found naturally in low— vs. high—nutrient soils. In a growth—chamber experiment, seedlings were grown at a range of proportions (0, 0.5, 1.0) of the two species and at a range of densities (n = 1,2, and 4 plants per pot) under low— and high—N treatments. In a field experiment, plants were reciprocally transplanted between low— and high—nutrient sites into both unvegetated (noncompetitive) and vegetated (competitive) plots. The two species responded similarly to density and nitrogen in pure cultures, but in mixtures the species from the high—nutrient site (E. scheuchzeri (showed a greater growth response to N than did the species from the low—nutrient site (E. vaginatum). Analyzed as a replacement series, the experiment showed a reversal in relative competitive ability (as measured by the relative crowding coefficient) of the two species between low— and high—N treatments at high density. Tissue N concentrations were higher and more responsive to N addition for E, scheuchzeri. Nitrogen use efficiency was higher for E. vaginatum in all treatments, while N uptake efficiency was higher for E. scheuchzeri in all treatments. In the field, survival, growth, and flowering responses to reciprocal transplanting suggested local adaptation of the two species to their respective home sites. In noncompetitive plots, the growth response mirrored that seen in the growth chamber. However, after 2 yr there was a clear reversal between sites in relative aboveground masses of the two species. The local species acquired more nitrogen than the alien species at both sites. We conclude that Eriophorum vaginatum and E. scheuchzeri possess traits typical of species adapted to infertile and fertile soils, respectively. The contrasting ecological responses of the two species are due, at least in part, to differences in their nutritional physiology, including differences in nutrient use efficiency, nutrient uptake efficiency, and root: shoot ratio. In the field, these differences result in differential growth, reproduction, and survival, while in the growth chamber, the differences result in differential ability to accumulate biomass and reversal in competitive ability as a function of nutrient availability. This and other studies suggest that adaptations to low, as well as high, resource levels may confer improved competitive ability.
Marsh Seed Banks of the Okefenokee Swamp: Effects of Hydrologic Regime and Nutrients
Authors: Gerritsen Jeroen , Greening Holly S.
Published: May 2006

Seedling germination from the seed banks of two marshes in the Okenfenokee Swamp, a deep macrophyte marsh and a shallow sedge marsh, was examined in the greenhouse under three hydrologic regimes and four nutrient conditions. Experimental hydrologic conditions of 3—mo duration, were constant exposure (no standing water), 1—mo exposure followed by reinundation, and constant inundation, repeated four times in 1 yr; nutrient conditions were amendments of N, P, N and P, and a control. The macrophyte marsh, whose water level fluctuates irregularly, has three guilds of species adapted to germination under different hydrologic conditions: drought—adapted, such as Rhyschospora inundata, which germinates in dry (exposed) conditions; inundation—adapted, such as Nymphaea odorata, which germinates only under water; and generalists, such as Eriocaulon compressum, which germinates in both exposed and inundated conditions. The relative abundances of these guilds fluctuate with the frequency and duration of drawdown in the habitat. Rhychospora inundata is a drought—adapted fugitive species in macrophyte marshes of the Okefenokee Swamp. It has a large population of seeds in the sediment, which germinate during a drought. Reinundation stimulates flowering and fruiting of adult plants. The sedge marsh, whose water level fluctuates regularly, has a single guild of species, dominated by Carex wateriana, which will not germinate in standing water. Nutrient content seedlings was parallel to the nutrients available. Nitrogen appears to limit marsh plant growth during drawdown, while phosphorus appears to limit a few species during inundation. As a result, most plants grow more rapidly in standing water, especially those that require drawdown for germination. The result is that this ombrotrophic, nutrient—poor wetland becomes more "lake—like," or P limited, during inundation, and more "land—like," or N limited, during low water levels.
Population Ecology of Uinta Ground Squirrels
Authors: Slade Norman A. , Balph David F.
Published: May 2006

The behavior and ecology of Uinta ground squirrels (Spermophilus armatus) at the Utah State University Forestry Field Station northeast of Logan, Utah, were studies 1964—71 to determine the role of behavior in population regulation. In 1968 the population was reduced experimentally to about one—half the previous density. The study area consisted of a lawn area where resident squirrels more than replaced themselves, and an area of mixed shrubs and grasses where they did not. Surplus squirrels from the lawn raised the population density off the lawn. Also, habitat suitabilities varied with population densities. Hence, densities in the two areas were not proportional to habitat preferences. Before the reduction the population density fluctuated widely but the mean change for 5 yr was approximately zero. A potentially high rate of increase was curtailed by disappearance of juveniles and yearlings; juveniles tended to disperse from the natal burrow. The principal changes following the reduction were increases in percentage of yearling squirrels breeding and of juveniles remaining on the study area. Litter size and proportion of adult females breeding did not change significantly, but losses during hibernation, due in part to badger predation, decreased. Dispersal of squirrels from the study area played a key role in population regulation. Dispersers probably suffered higher mortality than sedentary squirrels, but dispersal was not necessarily maladaptive for the individual. Dispersers may have been unable to produce young at high densities; thus emigration offered the only opportunity for reproduction.
Winter Weight Dynamics in Microtus Pennsylvanicus
Authors: Iverson Stuart L. , Turner Brian N.
Published: May 2006

A population of meadow voles (Microtus pennsylvanicus) 75 mi northeast of Winnipeg, Manitoba, Canada was live—trapped biweekly for two winters. From August to February of each year the mean population weight decreased by 30% to 40%. This decrease was not due to preferential death of the heavier animals because their survival was better than would have been expected if survival and weight were independent. part of the weight loss was due to the immigration of young animals that failed to gain weight. However, much of the decrease in mean weight was due to that lost by individuals, because many samples of animals released and recaptured 2 wk later had lost significant amounts of weight. Four marked ♂ ♂ trapped from August to February lost an average of 28.6% of their weights while 14 marked ♀ ♀ lost 45.5% in the same time period. In February the mean weights of individuals and the population began to increase. It is suggested that weight loss is not simply a result of lack of food, but is an adaptive response cued by some factor such as day length.
The Effect of Decreasing Water Potential on Net CO2 Exchange of Intact Desert Shrubs
Authors: Odening Walter R. , Strain B. R. , Oechel W. C.
Published: May 2006

This study compares the effects of seasonal (in situ) and experimentally—induced drought on three desert shrubs of different growth types. The effect of decreasing water potential on CO2 uptake was used as the response indicating differences in desert adaptation. Larrea divaricata Cav., Encelia farinosa Gray, and Chilopsis linearis Cav. were selected as representing evergreen, drought—deciduous, and winter—deciduous species respectively. Field measurements of water potential and photosynthesis were made simultaneously on plants of all three species. Data were collected in the field at monthly or bimonthly intervals for a year. Additional experiments were conducted in a transplant garden and under controlled environments in a phytotron. Larrea has a high protoplasmic tolerance to drought stress and maintains net photosynthesis throughout extended periods of low water potential. The photosynthetically active period of Encelia is prolonged, and water loss reduced, by seasonal leaf variability and the drought deciduous habit. These phenomena permit a greater photosynthetic efficiency over a wider range of water potentials than would be possible with a single leaf type. Decreasing water potentials affect Chilopsis the most, but the presence of xeromorphic leaf tissue augments the phreatophytic habit in maintaining relatively high water potentials. A transpiration—retarding mechanism in Chilopsis is also hypothesized.