bluuebunny/crossref_metadata_2025 huggingface dataset:
Predation Risk Influences the Use of Foraging Sites by Tits
Authors: Suhonen Jukka
Published: May 2006

In coniferous forests of Central Finland, tits (Paridae) and the Goldcrest (Regulus regulus) exploit nonrenewable resources in their group territories during the winter. Results of many studies have indicated that interspecific competition restricts the use of foraging sites in mixed—species winter flocks. However predation is a significant mortality factor in these species, and predation risk might also restrict the selection of foraging sites. The Pygmy Owl (Glaucidium passerinum) is the main diurnal predator of these small passerines during winter. Predation risk from the Pygmy Owl varies greatly with the availability of voles as alternative prey. The purpose of this study is to examine the effects of predation risk on the use of foraging sites in coniferous forest tits during winter. I collected foraging data from Willow (Parus montanus) and Crested Tits (P. cristatus) during one winter when vole populations had crashed and another winter when vole density was high to see if predation risk affects the selection of foraging sites by these species. Both Willow and Crested Tits changed to foraging in the inner parts of coniferous trees after the vole populations had crashed and when the Pygmy Owls started to hunt the tits. Additionally, in both pine and birch Willow Tits moved towards the treetops under high predation risk. Since all the foraging shifts were towards sites that are regarded to be safest from predation risk, it seems that predation risk in addition to interspecific competition influences the use of foraging sites by tits in coniferous forest.
Bottomland Vegetation Distribution along Passage Creek, Virginia, in Relation to Fluvial Landforms
Authors: Hupp Cliff R. , Osterkamp W. R.
Published: May 2006

Persistent distribution patterns of woody vegetation within the bottomland forest of Passage Creek, Virginia, were related to fluvial landforms, channel geometry, streamflow characteristics, and sediment—size characteristics. Vegetation patterns were determined from species presence as observed in transects and traverses on landforms developed along the stream. Distinct species distributional patterns were found on four common fluvial geomorphic landforms: depositional bar, active—channel shelf, floodplain, and terrace. Independent hydrologic characteristics (flow duration and flood frequency) were determined for each of the landforms. Vegetation data were analyzed by binary discriminant analysis, principal components analysis, and detrended correspondence analysis. Results and related field observations suggest that certain species are significantly associated with specific fluvial landforms. Vegetation patterns appear to develop more as a result of hydrologic processes associated with each fluvial landform rather than from sediment—size characteristics. Flood disturbance may be an important factor in maintaining the vegetation patterns, which may therefore be used as indicators for particular hydrogeomorphic site conditions.
Competition and Food Selection: Field Tests of a Theory
Authors: Pimm Stuart L. , Rosenzweig Michael L. , Mitchell William
Published: May 2006

A graphical model describing the optimal choices of two species competing for resources in two types of habitats is tested with hummingbirds in the field. In this model, both species prefer taking resources from one of the habitat types. However, one of the species (the dominant), by virtue of interference competition, can gain access to the better patch more readily than the other (the subordinate). The model begins with the result of single—species optimal foraging models: at low densities of birds, only the better patch type should be selected, but as density increases, both should be used. Interspecific competition should not lead to qualitatively different behaviors for the dominant species because the effects of the subordinate are weak. For the subordinate, however, there is a third class of behaviors; under the pressure of high densities of the dominant, the subordinate may totally avoid the better patch and use only the poorer. We tested the validity of the model's predictions using three species of hummingbirds: Blue—throated (Lampornis clemenciae), Rivoli's (Eugenes fulgens), and Black—chinned (Archilochus alexandri) coming to feeders containing 1.2 mol/L or 0.35 mol/L sucrose solution. There were no detectable effects of Rivoli's on Black—chinned or vice—versa. This allowed us to test the model's predictions about the interactions between the other pairs of species. Blue—throateds were dominant in both cases. The amount of time birds spent feeding (or in the case of Blue—throateds, feeding and defending) was analyzed, rather than actual densities. Wide ranges of feeding and defending times were obtained using species removals and natural seasonal changes in densities. These two pairs of species exhibited all the model's features. In particular, the dominant's aggressive behavior forced the subordinate to restrict itself to a patch type known to be inferior.
Foraging Decisions, Patch Use, and Seasonality in Egrets (Aves: Ciconiiformes)
Authors: Erwin R. Michael
Published: May 2006

Feeding Snowy (Egretta thula) and Great (Casmerodius albus) egrets were observed during two breeding seasons in coastal New Jersey and two brief winter periods in northeast Florida. A number of tests based on assumptions of foraging models, predictions from foraging theory, and earlier empirical tests concerning time allocation and movement in foraging patches was made. Few of the expectations based on foraging theory and/or assumptions were supported by the empirical evidence. Snowy Egrets fed with greater intensity and efficiency during the breeding season (when young were being fed) than during winter. They also showed some tendency to leave patches when their capture rate declined, and they spent more time foraging in patches when other birds were present nearby. Great Egrets, in Contrast, showed few of these tendencies, although they did leave patches when their intercapture intervals increased. Satiation differences had some influence on feeding rates in Snowy Egrets, but only at the end of feeding bouts. Some individuals of both species revisited areas in patches that had recently been exploited, and success rates were usually higher after the second visit. Apparently, for predators of active prey, short—term changes in resource availability (resource depression) may be more important than resource depletion, a common assumption in most OFT models.
Random Search by Herbivorous Insects: A Simulation Model
Authors: Cain Michael L.
Published: May 2006

A stochastic simulation model that relates the success of various herbivore searching behaviors to the density and arrangement of food plants was developed. It was designed to be field—testable with all model parameters directly measurable in field experiments. The model uses probability distributions and parameter ranges that are in accordance with published herbivore movement data. In the model, herbivore insect searching success depends in a complex manner upon parameters such as insect move lengths, turning angle concentration, reactive distance, risk of mortality while searching, plant density, plant dispersion, and plant stand size. Simulation results preclude simple generalizations and indicate that the outcome of interactions between plant density, plant dispersion, and herbivore searching behavior depends heavily upon parameter values. I argue that species—to—species shifts in the details of the simple searching process that I simulate could explain the (experimentally observed) high variation in herbivore response to vegetation texture. I discuss plant and insect strategies that are optimal" within biological constraints. For example, results from the model suggest that under many conditions clumping by plants can be an effective escape strategy from searching herbivores. This contradicts the claim that plant spatial distribution does not affect insect searching success, a claim that is justified only if it is assumed that organisms search (infinitely long) without biological cost. The importance of considering the cost of ineffective search is emphasized.
Modifying Lodgepole Pine Stands to Change Susceptibility to Mountain Pine Beetle Attack
Authors: Waring R. H. , Pitman G. B.
Published: May 2006

The premise that mature lodgepole pine forests are susceptible to mountain pine beetle attack when physiologically stressed was supported experimentally by manipulating the canopy density and availability of nitrogen in a 120—yr—old forest exposed to a high population of beetles. Where canopy density was reduced, ether by us or by the insects, surviving trees significantly increased their resistance to attack over a 3—yr period. Increased resistance was reflected by changes in wood production per unit of leaf area (tree growth efficiency). Improved nitrogen nutrition hastened tree recovery but did not prevent attacks by beetles until growth efficiencies exceeded 100 g of wood production per square metre of foliage. Growth efficiency, as here defined, is an index of vigor that may reflect the relative ability of susceptible trees to produce defensive compounds following attack.
Phosphorus Spiralling in a Woodland Stream: Seasonal Variations
Authors: Mulholland Patrick J. , Newbold J. Denis , Elwood Jerry W. , Ferren Leigh Ann , Webster Jackson R.
Published: May 2006

Four radiotracer releases were performed over an annual period in 1981—1982 to determine seasonal variation in indices and pathways of phosphorus spiralling in Walker Branch, a small woodland stream in eastern Tennessee, USA. Each release consisted of an addition of °370 MBq each of carrier—free 32PO4 and 3H2O over a 1—h period during baseflow. Concentrations of 32P and 3H dissolved in stream water were measured intensitively at several stations downstream for the radiotracer input during and immediately following each release. Activity of 32P in coarse particulate organic matter (CPOM), fine particulate organic matter (FPOM), and aufwuchs was measured 2—3 h after each release and at various intervals for 7 wk. Total biomass of CPOM, FPOm, and aufwuchs at the time of each release was also measured. Uptake of 32PO4 from the water was greatest in November and lowest in August, Uptake length (Sw) of phosphorus, defined as the average distance travelled by a PO4 ion dissolved in water, varied from 22 m in November to 97 m in August. Uptake of 32PO4 by CPOM was generally greatest, with °50% of total uptake, while that by aufwuchs was lowest, with <15% of the total. CPOM abundance was the major determinant of whole—steam PO4 uptake rate and Sw. Turnover length (Sp) of phosphorus, defined as the average distance traveled by an atom of P taken up by particulate material, was short compared to Sw, varying from 1 m in November to 3 m in January. Consequently total spiralling length (S) of P varied from 23 in November, just after peak autumn leaf fall, to 99 m in August, and reflected primarily the travel of P in the dissolved form. Our results indicate that the greatest increase in Sw, (and consequently in S) in Walker Branch occurs in late autumn or winter after storms reduce the abundance of CPOM in the lower portions of the stream bed. Although we calculate that Sp may increase by one of two orders of magnitude for short periods during storms, the greatest effect of storms on P spiralling over the long term is their impact on the quality of CPOM and FPOM in the stream bed after the return to baseflow. For most of the year, detrital organic carbon probably influences phosphorus spiralling more than phosphorus spiralling influences the processing of organic carbon in Walker Branch. Only during the fall and early winter periods, when CPOM abundance is high and Sw is short, does phosphorus spiralling exert strong control over biotic processes downstream.
Fiddler Crab Regulation of Spartina alterniflora Production on a New England Salt Marsh
Authors: Bertness Mark D.
Published: May 2006

While a great deal of research has been directed at understanding the high productivity of marsh grasses, little attention has been paid to the potential importance of marsh fauna in influencing marsh grass production. The mud fiddler crab, Uca pugnax, excavates burrows in Spartina alterniflora—dominated salt marshes along the East Coast of North America. Here, I examined experimentally the effect of fiddler crab burrowing on S. alterniflora production. At low intertidal heights, S. alterniflora production was robust and characterized by tall—form plants with roots and rhizomes that penetrate deeply into the substrate. At higher tidal heights, low productivity, short—form S. alterniflora with a dense superficial root mat prevailed. Fiddler crabs differentially affected grass production at different tidal heights. In tall—form S. alterniflora stands at intermediate tidal heights, experimental reduction of fiddler crab density for a single growing season decreased aboveground production by 47% and increased root mat density by 35%. In contrast, on the lower intertidal soft edge of the marsh and in the high intertidal, dense root mat, short—form S. alterniflora zone, experimental reduction of crab density did not have as marked an effect on cordgrass production. Crab burrows were shown to increase soil drainage, soil oxidation—reduction potential, and in situ decompostion of belowground plant debris. The relationship between fiddler crabs and S. alterniflora appears to represent a faculatative mutualism. On the sewart edge of marshes in soft sediment that would not otherwise support burrow structures, the roots, rhizomes, and debris of S. alterniflora provide structural support and consequently facilitate burrowing. At intermediate tidal heights, intensive burrowing activity increases aboveground grass production and prevents the establishment of a dense root mat, while crabs are largely precluded from burrowing in the dense root mat, short—form S. alterniflora stands at higher tidal heights. Therefore, while initially dependent on S. alternifora in soft sediments, U. pugnax burrowing activity appears to maintain a habitat suitable for continued burrowing, and as a byproduct, increases cordgrass production and maintains tall—form S. alterniflora stands.
Deep‐Sea Hydrothermal Vent Mussels: Nutritional State and Distribution at the Galapagos Rift
Authors: Smith K. L.
Published: May 2006

Rich faunal communities have been discovered around active deep—sea hydrothermal vent sites. The mussel Bathymodiolus thermophilus (Bivalvia, Mytilidae) is one dominant member of this hard—substrate community along the Galapagos Rift, east of the Galapagos Islands in the equatorial eastern Pacific. These mussels are most dense around the vent effluent and decrease in number with increased distance away from the discharge site. At the Mussel Bed vent, at 2490 m depth, the nutritional state of mussels (oxygen consumption and chemical composition) from a densely populated site °1 m from a warm—water fissure was compared to that of mussels from a sparsely populated peripheral site to examine the importance of food supply on mussel distribution. The submersible Alvin was used to collect bottom—water samples for chemical analysis, to manipulate in situ respirometers, and to collect specimens for chemical composition analysis. The quantity of particulate organic carbon immediately above the bottom (<10 cm) was twice as high (207.0 mg/L) at the dense site at the peripheral site (106.4 @m/L). Other parameters measured showed no significant variation (P > .05) between sites, including water temperature, current speed, and concentrations of dissolved oxygen, nitrate, and dissolved organic carbon. Concentrations of ammonium, nitrite, and hydrogen sulfide were below the detection limits of the assay used. Oxygen comsumption rates (R) as a function of mussel dry mass (M) were higher at the dense site (R = 0.23M038) than at the peripheral site (R = 0.098M0.48). Dense—site mussels had robust soft—tissue structure whereas peripheral mussels had an emaciated appearance. Water content was lower but condition index values and glycogen content were higher in the dense mussels. Less distinct differences were noted in total lipid and protein content between populations. Mussels examined 312 d after reciprocal site—to—site transplanting conformed in water and glycogen content to their adopted populations. I conclude that the nutritional state of the dense—site mussels is considerably higher than that of the peripheral mussels and is probably related to increased nutritive supply at the dense site. Possible causative factors are differences in the nutritive quality of organic matter and influences of symbiotic chemoautotrophs in the gills of these mussels.
Indirect Facilitation of Granivorous Birds by Desert Rodents: Experimental Evidence from Foraging Patterns
Authors: Thompson Daniel B. , Brown James H. , Spencer Wayne D.
Published: May 2006

In order to quantify the response of birds to experimental manipulations of seed availability and densities of granivorous rodents and ants, we counted granivorous birds and measured diurnal and nocturnal seed removal on 24 plots during winter months in the Chihuahuan Desert. Removal of single, widely spaced millet seeds provided a reliable measure of bird and rodent foraging activity. Avian foraging activity increased in response to supplemental seeds, but decreased in response to long—term removal of all rodents and all ants. Although birds potentially compete for seeds with rodents and ants, these results suggest that, in the long term, indirect facilitation dominates the interactions among all three taxa. The positive effect of the other granivores on birds may be mediated through changes in habitat structure (e.g., reduction in the density of annual plants). The increase of avian foraging in response to seed addition and the high spatial and temporal variance in the patterns of seed removal by birds indicate that in desert habitats granivorous birds use their mobility to find and exploit high concentrations of seeds. Thus, birds may reduce the spatial variation in seed abundance and reduce the densities of seed available to other granivores. The foraging behavior of birds and rodents revealed by these experiments clarifies the response of granivores to experimental manipulations reported earlier, such as the lack of biomass compensation by rodents in response to seed addition.
Chaos in a Three‐Species Food Chain
Authors: Hastings Alan , Powell Thomas
Published: May 2006

A continuous time model of a food chain incorporating nonlinear functional (and numerical) responses exhibits chaotic dynamics in long—term behavior when biologically reasonable parameter values are chosen. The appearance of chaos in this model suggests that chaotic dynamics may be common in natural food webs.
Regulation of Planktonic Microbial Communities by Nutrients and Herbivores
Authors: Pace Michael L. , Funke Elizabeth
Published: May 2006

We conducted a series of single—factor and factorial experiments in two north—temperate lakes (Peter and Paul lakes, Gogebic County, Michigan, USA) to study regulation of heterotrophic microbial communities. In single—factor experiments large Daphnia were added to enclosures, and these were compared to enclosures without Daphnia. These treatments contrast the major food—web configurations that occur in these lakes as a consequence of cascading trophic interactions. Bacterial abundance and the incorporation of methyl [3H]—thymidine into DNA by bacteria were similar in treatments with and without Daphnia. Ciliates were significantly reduced while heterotrophic flagellates were only marginally reduced by Daphnia additions in the Peter Lake experiments. Protozoans were not affected by Daphnia additions in the Paul Lake experiment. Factorial experiments compared the relative significance of large daphnids and nutrients as regulators of bacteria and protozoa. Treatments included controls, Daphnia additions, nutrient (N plus P) additions, and a combined Daphnia—nutrient treatment. Bacteria responded to nutrient additions but not to Daphnia. Protozoa, on the other hand, were strongly affected by Daphnia and to a lesser extent by nutrients. These experiments suggest that the trophic cascades is truncated at the level of protozoans. Protozoa appear to be partially regulated by predators, whereas bacteria show no response to changes in either Daphnia or protozoa.
Component of Plant Competition Along an Experimental Gradient of Nitrogen Availability
Authors: Wilson Scott D. , Tilman David
Published: May 2006

We tested for variation in the intensity of above— and belowground competition along a 5—yr—old experimental gradient of nitrogen availability in a Minnesota old field. Standing crop increased, species richness decreased, and species composition varied significantly as nitrogen availability increased. Transplants of the three grasses that were dominants at the three levels of the nitrogen gradient (low; Schizachyrium scoparium, intermediate: Poa pratensis, high: Agropyron repens) were grown along the gradient with no neighbors present, with only the roots of neighbors present, or with both the roots and shoots of neighbors present. Resource measurements indicated that the treatment with only neighbor roots present provided at light regime similar to that in which all neighbors had been removed but a nitrogen regime similar to that in which all neighbors were present. At low nitrogen availability, transplants grown with only neighbor roots generally did not differ significantly in biomass or growth rate from those grown with both neighbor roots and shoots, suggesting that the bulk of neighbor effects at low nitrogen were belowground. At high nitrogen availability, plants grown with only neighbor roots generally grew significantly larger than those grown with both roots and shoots of neighbors, but not as large as plants grown with no neighbors, suggesting that both above— and belowground competition occurred. At low nitrogen availability, plants grown with neighbors weighed 3—12% as much as those grown without; at the highest rate of nitrogen addition, plants grown with neighbors weighed 12—58% as much as those grown without, indicating that competition occurred on both poor and rich soils. The intensity of competition, measured as the suppression of transplants by neighbors, did not vary significantly with nitrogen availability. Further, the per—gram effect of neighbors, measured as the slope of the relationship between transplant performance and neighbor biomass, did not vary significantly with nitrogen supply rate. In total, competition was an important force at all points along the experimental productivity gradient, but competition shifted from being mainly belowground in the least productive vegetation to both above— and belowground in fertilized plots.
Effects of Food Web Compensation After Manipulation of Rainbow Trout in an Oligotrophic Lake
Authors: Elser James J. , Luecke Chris , Brett Michael T. , Goldman Charles R.
Published: May 2006

Stocking of the dominant planktivore of Castle Lake (rainbow trout) was discontinued to examine the impact of food web interactions on zooplankton communities and inter— and intra—annual dynamics of ecosystem properties (light penetration, primary productivity). Dynamics of zooplankton and ecosystem processes were examined for 3 yr following the manipulation and compared to 2—3 yr of premanipulation data. Sampling of vertebrate and invertebrate planktivores documented shifts in other members of the zooplanktivore guild as rainbow trout declined. Reduction of rainbow trout densities led to compensatory responses in other components of the Castle Lake fish assemblage as brook trout and golden shiners increased in abundance. This compensation resulted in increased rates of vertebrate planktivory on daphnids within 2 yr after trout stocking was discontinued. Zooplankton shifts in response to discontinuance of trout stocking were more rapid, particularly an immediate increase in a previously rare invertebrate predator (Diacyclops thomasi). Other limnological parameters also responded rapidly following the manipulation: water transparency declined and primary productivity (PPr) increased. In addition, intra—annual patterns (i.e., seasonal development) and the vertical distribution (shallow vs. deep) of PPr appeared to be affected by the food web manipulation. Our results indicate that complexities of real food webs complicate the prediction of the outcome of food web perturbations. Reduction of the previously dominant planktivore (rainbow trout) led to increases in other zooplanktivores (Diacyclops, golden shiners, brook trout) that resulted in enhanced predation pressure on zooplankton herbivores. Our results also indicated that alterations in water quality parameters (transparency, PPr) in response to food web alterations need not necessarily be mediated through changes in the abundance of Daphnia, as strong limnological responses preceded reductions in Daphnia by a year. We hypothesize an alternative mechanism for food—web—induced changes in lake ecosystem dynamics: changes in water clarity and productivity can result when cyclopoid predation strongly affects micrograzers.
Adaptive Radiation in Sticklebacks: Trade‐Offs in Feeding Performance and Growth
Authors: Schluter Dolph
Published: May 2006

Divergent Natural selection for efficient exploitation of alternative resources is thought to be a primary cause of adaptive radiation. I transplanted two recently diverged stickleback species (Gasterosteus sp.) and their Hybrids between habitats to test three predictions of this hypothesis: (1) the form of natural selection on feeding traits should differ between resource environments; (2) fitness trade—offs should occur between morphs specialized for different resources; (3) feeding efficiency should be the basis of fitness differences between morphs. Growth rate was used as a surrogate component of fitness. All three predictions were upheld. The larger of the two species (the Benthic), which has a larger mouth, deeper body, and fewer, shorter gill rakers than the second species (the Limnetic), had a twofold growth advantage in the littoral zone of the lake. This pattern was reversed in the open—water habitat, where the Limnetic species grew at double the rate of the Benthic. Hybrids, which are morphologically intermediate, tended to be intermediate in growth in both habitats. Growth rates were closely predicted by earlier laboratory measurements of feeding efficiency of the three forms in the same habitats, thus linking growth (and probably fitness) with feeding performance and morphology. I compare the strengths of the present approach with other methods that have been used to test the role of resources in adaptive radiation. I also argue that steep trade—offs in fitness along the littoral—open—water habitat gradient have two further consequences. First, they may cause Hybrids to have low fitness, allowing Benthics and Limnetics to coexist in the absence of perfect premating isolation between them. Second, they may promote ecological character displacement. Steep trade—offs may thus help explain why divergence along the littoral—open—water gradient is so common in fish that have colonized low—diversity lakes.
Density‐Related Dispersal in Planthoppers: Effects of Interspecific Crowding
Authors: Denno Robert F. , Roderick George K.
Published: May 2006

In dense populations, most planthoppers (Homoptera:Delphacidae) produce fully winged migratory forms that can escape to new habitats. Under less crowded conditions, flightless morphs with reduced wings result. For two sympatric species, Prokelisia marginata and Prokelisia dolus, interspecific crowding was found to be as strong a stimulus for the production of migrants as intraspecific crowding. The effects were reciprocal for both species and were demonstrated both in the laboratory and field. However, because migratory forms were triggered at a much lower density in P. marginata than P. dolus, interspecific interactions were asymmetric, with P. dolus having a far greater influence on the wing form of P. marginata. Interspecific interactions did not directly influence survival, development time, or body size, but reproductive capability was indirectly affected because migratory females are less fecund than their flightless counterparts. Intraspecific crowding adversely affected the survivorship, development time, and body size of P. marginata, but had no influence on the fitness of P. dolus. P. marginata is a much more migratory species than P. dolus. Consequently, mobility is associated with the extent that each species' fitness is reduced by intraspecific crowding. Our results demonstrate that for wing dimorphic insects, the effects of interspecific competition can be extended to include altered wing form, a character that directly influences dispersal capability as well as reproductive potential. We suggest that when wing—dimorphic species share a common habitat, interspecific interactions can influence population dynamics by increasing emigration and decreasing population growth.
Temperature Regulation in Puerto Rican Anolis Lizards: A Field Test Using Null Hypotheses
Authors: Hertz Paul E.
Published: May 2006

I used null hypotheses about basking behavior and body temperatures (Tb) to evaluate the extent and effectiveness of behavioral temperature regulation in Anolis cristatellus and A. gundlachi. I sampled these species in January and August near the extremes of their altitudinal distributions in Puerto Rico. Hollow electroformed copper lizard models, randomly positioned within habitats occupied by the lizards, served as operative temperature (Te) thermometers, providing baseline predictions about basking rates and Tb of nonthermoregulating lizards. Comparison of the basking behavior and Tb of lizards to the basking rates and temperatures of the models provided unambiguous evidence of nonrandom thermoregulatory activity in some of the lizard populations. Although basking opportunities were similar in the habitats occupied by both species, basking rates differed markedly: A. cristatellus basked more frequently than did randomly positioned models, and its basking rates were higher at high elevation and in January; A gundlachi used basking sites at random at both elevations in both months. In addition, A. cristatellus limited its activity, particularly at high elevation, to times when operative temperatures were relatively high or basking sites were available (i.e., sunny weather). As a result of its opportunistic basking activity, mean Tb of A. cristatellus was often elevated above mean Te, and A. cristatellus populations experienced only limited variation of mean Tb with elevation and season. In contrast, A. gundlachi mean Tb approximated mean Te, and the Tb of A. grundlachi varied more with elevation and season. Mean Tb in A. cristatellus was always higher than the mean Tb of A. gundlachi at every elevation and month combination. The data also suggest that the low Tb experienced by A. gundlachi at high elevation might dramatically reduce sprinting ability, whereas the Tb experienced by A. cristatellus allows a majority of lizards at high elevation to sprint quickly.
A Demographic Study of Two Populations of the Seaweed Ascophyllum Nodosum
Authors: Aberg Per
Published: May 2006

Very little is known about the demography in size—structured seaweed populations, and this is especially true for populations in variable environments. Thus, the demography of the brown alga Ascophyllum nodosum was analyzed with a matrix population model. This was built on a 3—yr study in two populations on the Swedish west coast, where >1100 individuals were marked and followed twice each year. The environmental variability is due to the presence of ice during some of the winters. The frequency of ice years in the study area is known, and is higher at one site compared to the other. During the study there were 2 yr with ice and one without at both sites, and these temporal changes in the environment resulted in large variations in the vital rates of A. nodosum. The individuals were divided into five size classes and the population dynamics at years without ice were characterized by low mortality rates and high transition probabilities for growth to larger sizes, while years with ice had high mortality rates and high transition probabilities for breakage to smaller sizes. Of the 25 possible transitions in the life cycle graph all except 2 had nonzero entries, which means that the adult life of A. nodosum individuals can be described as plastic growth between all five size classes. A crude estimate of the recruitment showed that both populations will increase in numbers in ice—free years, but will decrease in years with ice. The size—dependent fertility rate is probably subject to errors, and thus the survival matrices were scaled with a fertility function built on total reproductive biomass per individual, allowing analyses of the demography at different levels of the asymptotic growth rate @l1. One of the main differences between the populations was that the stable size distribution for survival matrices in ice—free years was dominated by size class 4 at one population but class 5 at the other. Thus, to achieve the same value of @l1 higher fertility rates were needed at the population dominated by class 4. Elasticity analysis of the transition matrices showed the same tend, although the main result from that analysis was that growth to larger sizes of remaining in the same class contributed more to @l1 than reproduction. This was valid for all levels of @l1 investigated.
Age and Survivorship of Diapausing Eggs in a Sediment Egg Bank
Authors: Hairston Nelson G. , Van Brunt Robert A. , Kearns Colleen M. , Engstrom Daniel R.
Published: May 2006

We determined the densities of diapausing eggs of the copepod Diaptomus Sanguieneus in sediments from two small freshwater lakes in Rhode Island. Sediment cores, sliced at 1—cm intervals, showed that egg densities ranged between 4 x 104 and 8 x 104 eggs/m2 near the sediment surface and declined to very low values at depths of 10—15 cm in both lakes, although eggs were found as deep as 30 cm in the sediment of one lake. Between 10 and 50% of these eggs hatched in short—term laboratory experiments, and actual egg viability is probably higher. 210Pb—dating revealed relatively constant sedimentation rates in both lakes, and we use this information to estimate egg ages. In one lake, the mean diapausing—egg age is 70.4 yr (median age = 45.9 yr) and the maximum age of eggs we hatched was 332 yr. In the other lake, the mean egg age is 48.9 yr (median age = 35.9 yr) and the maximum age of eggs we hatched was 112 yr. We calculated egg mortality rates by regressing 1n(egg density) on the age of the sediment from which the eggs were taken to obtain estimates of 1.1 and 1.5% mortality/yr for the two lakes. Diapausing eggs of zooplankton represent a long—lived life history stage of an otherwise short—lived organism. They provide generation overlap that can have substantial significance for both ecological and evolutionary dynamics.
Growth of Birch and Pine Seedlings in Relation to Grazing by Soil Fauna on Ectomycorrhizal Fungi
Authors: Setala Heikki
Published: May 2006

It is hypothesized that mutualistic relationships can be persistent due to a third “external” species constraining the otherwise unlimited growth of the mutualists. By consuming mycorrhizal fungi, soil fauna may exert such a control over the development of the plant—fungus symbiosis. In this study I investigated the influence of soil fauna on the mutualistic association of ectomycorrhizal (EM) fungi and seedlings of birch and pine in transparent experimental microcosms simulating coniferous forest soil. The soil was defaunated, reinoculated with soil microbes, and seedlings of silver birch (Betula pendula) or Scots pine (Pinus sylvestris) infected with 3—4 taxa of EM fungi were separately planted in the microcosms. Half of the microcosms were thereafter reinoculated with diverse soil fauna. Microcosms were incubated in a climate chamber with varying illumination and temperature regimes for two annual cycle of the plant. After 57 wk the biomass of EM fungi was significantly reduced in the refaunated microcosms, the amount of fungi on birch and pine roots being 50 and 20%, respectively, of that in the non—refaunated controls. Similarly, the total microbial biomass in the bulk soil was significantly lower in the refaunated systems. The shoot production of both tree species was significantly enhanced in the presence of fauna: by the end of the experiment 1.5 (birch) and 1.7 (pine) times more aboveground biomass was produced in the refaunated microcosms than in the controls. Concentrations of N and P in the foliage of both tree species were also higher in the refaunated microcosms. These data indicate that the obvious reduction in EM fungi due to soil fauna is not harmful for the plant—fungus symbiosis provided that the community composition of the belowground food web is complex enough to ensure efficient mobilization of nutrients in the mycorrhizosphere.
Growth‐Related Constraints on Diet Selection by Sunfish
Authors: Walton William E. , Hairston Nelson G. , Wetterer James K.
Published: May 2006

Diet selectivity by the planktivorous bluegill sunfish (Lepomis macrochirus) is correlated with the fishes' visual resolution. Poor visual capabilities are thought to impose a constraint on the ability of small sunfish to chose prey. Li et al. (1985) suggested that sunfish subjected to conditions of poor visual resolution select prey in accordance with a model in which prey choice is determined by apparent prey size. Further, enhanced visual resolution permitted large sunfish to assess absolute prey size more accurately than could smaller conspecifics and, consequently, large—sized sunfish consumed relatively more large prey than was predicted by the apparent size model. Here, we examine the visual capabilities and prey selection of very small sunfish, extend the analyses of Li et al. by determining the optimal diet sets for the fish used in their study, and compare prey selection by all sunfish size categories to that predicted by the apparent size and optimal foraging models. Contrary to the conclusions of Li et al., small sunfish (<20 mm standard length [SL]) were more selective than was predicted by the apparent size model and preferentially attacked small—sized Daphnia. The optimal foraging "prey" model predicted correctly the size—related changes in mean size of prey attacked by sunfish, whereas the apparent size model predicted a preference for large Daphnia for all sizes of fish. Yet, the observed diet breadths were always larger than those predicted by the optimal foraging model. The visual system any energy considerations constrain prey selection in small sunfish but operate in opposite fashions.
Predicting Digestible Protein and Digestible Dry Matter in Tannin‐Containing Forages Consumed by Ruminants
Authors: Hanley Thomas A. , Robbins Charles T. , Hagerman Ann E. , McArthur Clare
Published: May 2006

Equations for predicting digestible protein and digestible dry matter on the basis of laboratory analytical techniques (Robbins et al. 1987a, b) were tested with in vivo digestion trials using black—tailed deer (Odocoileus hemionus sitkensis). Leaves of seven natural, tannin—containing forages and twigs from one browse species were fed fresh in total collection digestion trials. The equation for estimating digestible protein was based on the plant's crude protein content and a measure of the tannin's protein—precipitating capacity. The equation for estimating digestible dry matter was based on a measure of tannin astringency and the concentrations of neutral detergent fiber, lignin, cutin, and biogenic silica. Both equations predicted in vivo results accurately, although a slight modification of the laboratory technique was recommended for determining neutral detergent fiber. Sodium sulfite should be used when analyzing tannin—containing forb and shrub leaves and omitted for relatively tannin—free forages. The equations provide useful measures of both the ecological effectiveness of tannins as digestion—reducing agents and the nutritional value of the ingested plant matter for the animals.
Sex Ratio and Neighbor Effects in Monospecific Stands of Juniperus Virginiana
Authors: Vasiliauskas S. A. , Aarssen L. W.
Published: May 2006

Sex ratio, sexual dimorphism, and neighbor effects were investigated in 20 monospecific stands involving 3500 trees of Juniperus virginiana, a dioecious gymnosperm, near its northern limit in eastern Ontario. We tested the hypothesis that the intensity of competition for a given tree will depend on the sex of its nearest neighbor because of differential costs of reproduction in male and female plants. Population sex ratio was significantly male biased in only one stand but was male biased when data from all stands were combined. Stand sex ratio was not related to age structure, stand density, or local competition intensity (based on separation distance/size regression). Male trees were significantly taller and had greater diameters in only three stands. In the combined data from all stands, however, there was a greater representation of male trees among the larger diameter trees and among the taller trees. Male trees were also significantly taller in the pooled data set. These results suggest that female trees pay a slightly but significantly greater cost for reproduction in terms of reduced vegetative growth. However, the effects of close neighbor proximity did not depend on a neighbor's sex, and male and female trees were randomly distributed relative to each other. Hence, males and females appear to compete with similar intensities against each other and are not spatially segregated into separate "niches." The overall greater height for males may be interpreted as a consequence of sexual selection, as greater height in male trees would confer greater success as a pollen donor, especially in dense monospecific stands characteristic of this species.
Autotrophic Carbon Sources for Fish of the Central Amazon
Authors: Forsberg B. R. , Araujo-Lima C. A. R. M. , Martinelli L. A. , Victoria R. L. , Bonassi J. A.
Published: May 2006

Effective management of the Amazon's commercial fish populations requires an understanding of the factors controlling their production. A fundamental step in the investigation of fish production is to identify the plant groups that contribute energy to fish foodwebs. Stable isotope data for plants and 35 fish species were used to identify autotrophic carbon sources for the central Amazon fish community. Adult fish, aquatic macrophytes, tree parts, periphyton, and phytoplankton were collected in lakes and other flooded environments along the central Amazon floodplain and analyzed for carbon stable isotope composition (°13C) by mass spetroscopy. °13C values for plants ranged from —39.4 to —11.9% with averages of —33.3, —28.8, —27.6, —26.2, and —12.8% for phytoplankton, flooded forests trees, C3 aquatic macrophytes, periphyton, and C4 macrophytes, respectively. The average for all C3 plants (phytoplankton, trees, C3 macrophytes, and Periphyton) was —29.1%, while the average for C4 plants (mainly C4 macrophytes) was —12.8%. Mean °13C values for adult fish ranged from —37.0 to —19.8% with an average of —28.8%. Fish and plant data were used in an isotope mixing model to estimate the relative contribution of different plant groups to fish carbon. C4 macrophytes, which contributed over half of the primary production on the floodplain, accounted on average for only 2.5—17.6% (minimum to maximum) of the carbon in fish. The C3 plants, as a group, were the primary carbon source for 34 fish species, and accounted for an average of 82.4—97.5% of the carbon in all species. Phytoplankton, a minor C3 producer, accounted for a minimum of 36.6% of fish carbon on average, and was the principal carbon source for the commercially important characiform detritivores. Several alternative hypothese are proposed to explain the apparent selective transfer of C3 carbon through Amazon fish foodchains.
Why Predation Rate Should Not be Proportional to Predator Density
Authors: Abrams Peter A.
Published: May 2006

In most mathemetical models of population dynamics in ecological communities, the death rate of prey species is a linearly increasing function of the density of their predator(s). If antipredator behavior is incorporated into such models it becomes very unlikely that prey death rate is a linear function of predator density. In addition, it is possible for the prey death rate to decrease, rather than increase, as a function of the population density of its predator. Decreasing predation rates may be common when the prey organism is attempting to accumulate resources for growth or reproduction. Decreasing predation rates are less likely when the prey organism is attempting to minimize mortality. These results are earlier ones on indirect effects generated by optimal foraging (Abrams 1984, 1991a, b) suggest that higher order ineractions shoyld be extremely common in natural communities.
Functional Response of Herbivores in Food‐Concentrated Patches: Tests of a Mechanistic Model
Authors: Gross John E. , Shipley Lisa A. , Hobbs N. Thompson , Spalinger Donald E. , Wunder Bruce A.
Published: May 2006

Type II functional responses are frequently observed in herbivores feeding in patches where plants are concentrated in space. We tested a mechanistic model of regulation of intake rate of herbivores foraging in food—concentration patches (Laca and Demment 1992, Spalinger and Hobbs 1992) that accounts for asymptotic, Type II responses. The model is based on the hypothesis that competition between cropping and chewing regulates instataneous intake rate in response to changes in the size of bites obtained by the forager. We tested this hypothesis and examined the ability of our model to account for observations of intake rate of 12 species of mammalian herbivores ranging in body mass over 4 orders of magnitude. We measured short—term intake rates of mammalian herbivores feeding in hand—assembled patches of plants. We varied bite size by changing plant height and density in patches offered to herbivores, and observed dry matter intake rates in response to this variation. Averaged across species, our model accounted for 77% of the variance in food intake rate (P <.001 for all species). Predictions of maximum intake rate closely resembled observations of processing capacity, demonstrating that processing rather than cropping sets an upper limit on short—term intake. Tests of model mechanisms provided strong support for the hypothesis that competition between cropping and chewing is responsible for the Type II functional response seen in herbivores feeding in food—concentration patches. The model was able to consistently predict intake rates observed in 16 previous studies. These results indicate that plant characteristics regulating bite size (e.g., leaf size and geometry, spinescen) frequently control instantaneous rates of food intake by mammalian herbivores.