bluuebunny/crossref_metadata_2025 huggingface dataset:
Seed and Seedling Ecology of Neotropical Melastomataceae
Authors: Ellison Aaron M. , Denslow Julie Sloan , Loiselle Bette A.
Published: May 2006

Models of gap—phase regeneration in tropical wet forests are based primarily on studies of canopy trees. These paradigms are derived from studies of a small number of species whose life history traits place them easily into "pioneer" or "climax" species Woody plants of the forest understory, subcanopy trees and shrubs, are known to affect significantly regeneration dynamics in treefall gaps, yet we lack fundamental information on life history characteristics of understory plants that could permit their incorporation into gap regeneration models. We investigated intra— and interspecific variability in early life history characteristics of understory Melastomataceae species of different growth forms and adult distributions from the lowland tropical rain forest at La Selva Biological Station, Costa Rica. We examined germination patterns in vitro (22 species), effects on germination of passage through birds (5 species), seedling establishment in the field (3 species), and growth in different light environments (8 species). Established seedlings were significantly more abundant in gabs than in nearby understory sites. This pattern was generally applicable to all melastomes studied, regardless of the habitat where adults were normally most abundant. Seedlings were more common in the root pits and on the root mounds of uprooted trees, even though there was less light available to the seedlings in these microsites than elsewhere in the gaps. In a series of germination trials in vitro, most seeds germinated readily within 1—2 wk. Comparisons of germination rates between seeds collected from bird droppings and seeds extracted from fruits by hand gave varied results. With one exception, seeds of shade—tolerant species germinated more slowly following passage through birds, while seeds from high—light—demanding species germinated more rapidly after passing through birds. Seeds of three common species were sown directly into different microsites within gaps and understory control sites. In contrast to the germination trials, emergence percentages in the field ranged from 4% in understory sites to 21% in gap centers. Survival after 1 yr was low (0.1%—0.4%). After 1 yr, these seedlings were small (<2 cm tall) and produced few leaves. In similar light environments in the shade—house, melastome seedling survival was between 80% and100%, and growth increased with increasing light levels from 2% (corresponding to understory sites) to 20% (small gap centers) of full sun. With the exception of two species common to abandoned pastures and river banks, the species used in the shade—house growth experiments did not show significant increases in growth with increases in light form 20% to 40%. Suites of early life history traits described for tropical tree species are not consistent with the variation in pattern seen within this one family of understory species.
Effects of Nutrient Availability and Herbivory on Polyphenolics in the Seaweed Fucus Versiculosus
Authors: Yates J. L. , Peckol P.
Published: May 2006

Research seeking to explain intraspecific variations in plant phenolics has focused on two general paradigms, the resource availability (carbon/nutrients balance) and induced—defense models. We experimentally tested both hypothesis to explain changes in phlorotannin concentrations in the brown alga Fucus vesiculosus was collected monthly from two estuarine sites (Cape Cod, Massachusetts, USA) differing in nitrogen availability, and analyzed for polyphenolic levels and tissue—nitrogen concentrations. In situ nutrient enrichment experiments were conducted to measure changes in polyphenolic levels related to N availability. Stimulated grazing experiments examined the possibility of induced increases in polyphenolic concentrations in Fucus vesiculosus. Paired choice experiments conducted in the laboratory examined feeding selectivity by the snail Littorina littorea for either Fucus population. Field measurements and experiments revealed temporal and site—related changes in tissue constituents. Polyphenolic concentrations were consistently, sometimes two times, higher in Fucus vesiculosus from the low—N site compared with algae from the high—N site; tissue—N content was higher in the population from the high—N site. For both populations, tissue N was inversely correlated with polyphenolic concentrations. Only Fucus from the low—N site showed a significant reduction in polyphenolic concentrations under experimental enrichment. Regression analysis revealed a significant relationship (R2 = 0.71) between phenolic concentrations and growth rates for Fucus at the high—N, but not the low—N site. This suggests a growth cost associated with phlorotannin production for this phenolic—poor population due, perhaps, to C—limited growth. Phlorotannin concentrations increased significantly in clipped Fucus at the low—N site in two of three simulated grazing experiments, indicating at inducible response. We did not find a significant inducible response for the high—N, phenolic—poor population, and our choice experiments revealed a clear preference for this population by Littorina littorea. Our results offer support for both the induced—defense and carbon/nutrient balance hypotheses as explanation for variations in phlorotannin concentrations in Fucus vesiculosus. We suggest that within—species variation in polyphenolics in due to a complex interaction of environmental (nutrient availability, irradiance levels) and defense—related (grazing activity) factors.
Sexual Selection's Effects on Male Life History and the Pattern of Male Mortality
Authors: Clinton Walter L. , Le Boeuf Burney J.
Published: May 2006

Modifications of male life history due to sexual selection should be apparent in polygynous species such as elephant seals, in which sexual selection has produced a high degree of sexual dimorphism. In theory, male traits that confer a mating advantage bear survival disadvantage. The aim of this study was to examine the relationship between sexual selection and life history patterns in male northern elephant seals, Mirounga angustirostris. A life table, with age—specific estimates of mortality and reproduction, was constructed for male elephant seals from resighting data of tagged seals. Increased agespecific mortality was associated with the period of first reproduction by males, which occurred from 6—10 yr of age. A negative relationship was found between mating success and future survival in males that were beginning to breed. Older males showed no phenotypic costs to reproduction, and a positive, but not significant, relationship was shown between current mating success and future survival and mating success.
Dispersal in Kelps: Factors Affecting Spore Swimming and Competency
Authors: Reed Daniel C. , Amsler Charles D. , Ebeling Alfred W.
Published: May 2006

The distance over which propagules can successfully colonize new sites depends on processes that increase the time they remain competent while being dispersed. As do feeding larvae, algal spores can contribute to their own nutrition (via photosynthesis) during dispersal. We explored the dispersal potential of the kelps Macrocystis pyrifera and Pterygophora californica in laboratory experiments by examining (1) how long their spores can swim, (2) the contribution of energy derived from photosynthesis to spore swimming duration, and (3) the ability of spores to germinate and attach after they stop swimming. Results indicate that under photosynthetically saturating irradiance no spores of either species can swim longer than 120 h; <10% of the spores were still swimming after 72 h. When placed in the dark, spores did not swim longer than 72 h; <10% remained swimming after 48 h. More importantly, spores did not die after they stopped swimming; most germinated in the water column and retained their capacity to produce viable sporophyte recruits. The ability of spores to attach after they stopped swimming differed between the two species; settlement density declined in Macrocystis and increased in Pterygophora. Thus, the viable planktonic stage of these algae is not necessarily restricted to the spore but may include later life history stages. These results provide biological evidence that the spores and germlings of these kelps can remain competent in the plankton for extended periods of time, which is consistent with our previous findings that their dispersal can occur over greater distances than previously thought possible.
Prey Specificity in Chrysopa: An Interspecific Comparison of Larval Feeding and Defensive Behavior
Authors: Milbrath Lindsey R. , Tauber Maurice J. , Tauber Catherine A.
Published: May 2006

To examine pathways in the evolution of prey specialization in insects, we conducted a comparative experimental analysis of larval behavior in two sympatric predacious sister—species (Chrysopa quadripunctata, a generalist, and C. slossonae, a specialist on the woolly alder aphid). Responses of the larvae to food (Myzus persicae) and camouflaging material (waxy secretions from woolly alder aphids or exuviae from M. persicae) were quantified during four discrete developmental periods. Our analysis revealed species specific, ontogenetic, and environmentally induced variation in larval defensive and feeding behavior. The pattern of interspecific variation indicates that larval behavior could have subserved two processes in the evolution of prey specialization in the sister—species' progenitor: establishment on a specific prey and adaptation to the prey. First, the presence of the woolly alder aphid's secretions results in generalist larvae camouflaging themselves to a degree that rivals that of the specialist. Such phenotypic plasticity could have promoted the progenitor's initial adoption of the ant—tended woolly alder aphid as prey and its wool as a source of camouflaging material. Second, quantitative differences between the generalist's and specialist's behavior indicate that adaptation to the woolly alder aphid did not require the acquisition of novel behavioral traits in the larvae. It involved modifications in the expression of defensive behavior that existed in the generalist ancestor and alterations in the ability to attack and feed on robust prey.
Growth, Photosynthesis, and Resource Investment for Vegetative and Reproductive Modules of Artemisia Tridentata
Authors: Evans R. D. , Black R. Alan
Published: May 2006

Growth of vegetative and reproductive structures in Artemisia tridentata is temporally separated during the growing season; vegetative growth occurs during spring and early summer when soil moisture is most abundant, while reproductive growth occur during summer and fall when soil moisture may be limiting. Vegetative and reproductive structures may therefore exhibit contrasting efficiencies of resource acquisition and investment resulting from temporal differences in resource availability during their development. We examined the effect of water stress on growth, photosynthesis, and resource investment for vegetative and reproductive modules of Artemisia tridentata by applying supplemental water. No differences were observed in vegetative biomass between shrubs in the two watering treatments. Growth of vegetative structures occurred in the spring when water was not limiting, and shrubs in both watering treatments exerted little stomatal control over water loss. Conversely, reproductive growth occurred through the summer when water was limiting, and supplemental watering increased reproductive growth. Shrubs conserved water during summer by abscising leaves and lowering stomatal conductance of both vegetative and reproductive modules in response to decreases in xylem pressure potential and increases in evaporative demand. Leaf abscission can occur without decreasing the amount of carbon available for reproductive growth because inflorescences are capable of positive photosynthetic rates comparable to vegetative leaves. Water stress did not alter tissue construction costs or carbon and nitrogen contents for either vegetative or reproductive modules. Resource limitations were, however, reflected in the efficiency of water use during tissue construction; floral leaves and floral heads of shrubs not receiving supplemental water were produced with higher water—use efficiency. Conservative use of water during production of vegetative modules would offer no advantage to A. tridentata because neighboring species are also most active at this time. Reproductive growth in A. tridentata occurs during summer when neighboring species are largely dormant, and so efficient use of water may allow development of reproductive structures to continue throughout the summer even with limited supplies of water.
Factors Influencing Community Structure along Stress Gradients: Zooplankton Responses to Acidification
Authors: Locke Andrea
Published: May 2006

Acidification effects on zooplankton community structure are examined within the context of a model proposed by Menge and Sutherland, which was originally developed from studies of the effects of physical disturbance on marine intertidal zone communities. The model proposes that with increasing environmental stress, the factors predation, competition, and physical or physiological disturbance each predominate in turn in structuring food webs. Analyses of zooplankton communities along pH gradients do not support the Menge and Sutherland model. Patterns of species richness relationships with pH follow the general predictions of Menge and Sutherland (highest diversity of intermediate stress) in data from only 3 of 10 different studies that I examined. Predation and competition links in zooplankton food webs in acidified lakes near Sudbury, Ontario, maintain a constant ratio with acidification to pH 5.0, but at higher hydrogen ion concentrations food webs become highly variable.
Tannic Acid, Protein, and Digestible Carbohydrate: Dietary Imbalance and Nutritional Compensation in Locusts
Authors: Raubenheimer David
Published: May 2006

The combined effects of dietary imbalance and the allelochemical tannic acid on fifth stadium Locusta migratoria were investigated. In a factorial—design experiment, insects were fed artificial diets containing digestible carbohydrates and proteins in equal proportions (14 or 28%), or in a 1:2 or 2:1 ratio, with or without 10% tannic acid. Growth, consumption, and utilization efficiencies were measured over the course of the fifth stadium, and histological analyses of the midguts of newly molted adults were undertaken. While the consumption of diet was higher on the 14%—protein diets, consumption of protein remained lower than on the 28%—protein diets, indicating that compensatory feeding for protein was incomplete. Similarly, compensation for low dietary carbohydrates occurred, but was incomplete. Low levels of either nutrient resulted in increased intake of the other, but the effect of protein on carbohydrate intake was stronger than vice versa. There was a statistically interactive effect of carbohydrate and protein on the amounts of carbohydrate consumed, resulting from the fact that particularly high levels of carbohydrate were consumed when the diets simultaneously contained 28% carbohydrate and 14% protein. The terms "incidental augmentation" and "incidental restriction" of intake are introduced to describe the ways that certain nutrient groups may affect the intake of others in foods that are nutritionally imbalance. Tannic acid resulted in reduced efficiency of conversion of ingested nitrogen, but the efficiency of conversion of total ingested nutrients was higher on the diets containing tannic acid. A higher frequency of lesions was observed in the midgut epithelia of tannin—fed insects than controls, and it is suggested that the sloughing of necrotic epithelial tissues into the lumen and their subsequent egestion with the feces may account for the reduced efficiency of nitrogen conversion in these insects. A statistical interaction between protein levels and tannic acid demonstrated that tannic acid reduced consumption, but only of the low protein diets. Therefore, the effect of tannic acid was to restrict compensatory feeding for protein. A consequence of this was reduced growth in insects fed low—protein diets containing tannic acid. Total growth (dry mass) was lower on the 28%—protein diets and higher on the 28%—carbohydrate diets, and therefore corresponded with levels of carbohydrate intake. Nitrogen accumulation increased with dietary protein levels, but remained constant across carbohydrate levels despite increased intake of nitrogen on low—carbohydrate diets. This was accounted for by lower efficiency of conversion of nitrogen on the 14%—carbohydrate diets. Approximate digestibility was higher for the 14%—protein diets, possibly reflecting increased intake of easily digested carbohydrates. There were no statistically significant interactive effects between the two nutrients, or between either nutrient and tannic acid, on utilization efficiencies.
Postfleding Survival of European Starlings
Authors: Krementz David G. , Nichols James D. , Hines James E.
Published: May 2006

We tested the hypotheses that mass at fledging and fledge date within the breeding season affect postfledging survival in European Starlings (Sturnus vulgaris). Nestlings were weighed on day 18 after hatch and tagged with individually identifiable patagial tags. Fledge date was recorded. Marked fledglings were resighted during weekly two—day intensive observation periods for >9 wk postfledging. Postfledging survival and sighting probabilities were estimated for each of four groups (early or late fledging by heavy or light fledging mass). Body mass was related to postfledging survival for birds that fledged early. Results were not clear—cut for relative fledge date, although there was weak evidence that this also influenced survival. Highest survival probability estimates occurred in the EARLY—HEAVY group while the lowest survival estimate occurred in the LATE—NIGHT group. Sighting probabilities differed significantly among groups, emphasizing the need to estimate and compare survival using models which explicitly incorporate sighting probabilities.
Early Successional Patterns of Forest Species: Interactions of Life History Traits and Disturbance
Authors: Halpern Charles B.
Published: May 2006

Patterns of abundance were examined for vascular plant species during 21 yr of succession in two clear—cur and burned Pseudotsuga forests in the western Cascade Range of Oregon. A majority of forest understory species persisted through disturbance. Most colonizing species established within 2 yr after burning. Individualistic species responses were described by a series of broadly overlapping, unimodal curves of constancy and canopy cover, differing in time of initiation, duration, and magnitude. Thus, early successional change was characterized by gradual shifts in the abundance of generally persistent species. Eleven population patterns (species groups) were identified. Interactions of life history traits and disturbance explain the temporal trends of the most common species. Within the groups of invading species, the timing of initial establishment, as well as the timing and magnitude of peak abundance were related to the origin of propagules, phenological traits, potential for vegetative expansion, and temporal and spatial variation in disturbance. Abundance patterns of invading species were also influenced by stochastic and historical factors. Contrasting responses of species between sites reflected differences in histories of logging and slash burning. Within the groups of residual species, temporal patterns of abundance reflected initial species distributions, resistance to logging and burning disturbance, mode of reproduction, morphological traits, and spatial variation in disturbance intensity. These observations suggest that early secondary succesion in Pseudotsuga forests has a deterministic component, founded in the life history traits of the available species, and a stochastic component reflecting site history and variation in disturbance.
Simulating Cumulative Fire Effects in Ponderosa Pine/Douglas‐Fir Forests
Authors: Keane Robert E. , Arno Stephen F. , Brown James K.
Published: May 2006

A successional process model has been adapted for use with species from ponderosa pine/Douglas—fir (Pinus ponderosa var. ponderosa)/(Pseudotsuga menziesii var. glauca) forests of the inland Northwest. Its design allows modification for application to other forest types. This model, FIRSUM, simulates tree establishment, growth, and mortality, along with live and dead fuel accumulation, fire behavior, and fuel reduction on a 400—m2 plot. The modeling contains algorithms for influences on tree establishment and growth including temperature, water stress, light tolerance, and site quality. The model was used to predict 200 yr of forest succession for five different disturbance regimes. This allowed comparison of patterns of basal area by species, of duff and fuel accumulation, and on fire intensities among the following scenarios: (1) no fires (fire suppression), (2) consistent fire intervals of 10, 20, and 50 yr, and (3) a natural fire regime of variable intervals reconstructed from fire scarred trees. Frequent fires (10— and 20—yr intervals) were simulated to be of low intensity, resulting in scorch heights of 0.5—3.0 m. These fires prevented Douglas—fir saplings from surviving and becoming part of the overstory. Simulation results for the 10— and 20—yr fire intervals were similar to those for the natural fire regime, based on fire occurrence between AD 1600 and 1900. However, the occasional long fire intervals within the natural fire regime allowed greater regeneration success for ponderosa pine. Fires at regular intervals of 50 yr were more severe and resulted in a decrease of western larch (Larix occidentalis) after 150 yr, with a corresponding increase in ponderosa pine. Douglas—fir slowly increased in basal area and became established in the overstory after 200simulations years. The no—fire scenario allowed Douglas—fir to achieve dominance in the understory, and eventually in the overstory, thereby limiting survival of ponderosa pine and western larch regeneration. A test of the model showed predictions to be within 19% of field observations, and a sensitive analysis of FIRESUM showed parameters associated with the growth algorithm to be most critical for predicting successional trends.
Effects of Different Resource Additions of Species Diversity in an Annual Plant Community
Authors: Goldberg Deborah E. , Miller Thomas E.
Published: May 2006

A commonly observed phenomenon in plant communities is that the addition of a limiting resource leads to an increase in productivity and a decrease in species diversity. We tested the hypothesis that the mechanism underlying this pattern is a disproportionate increase in mortality of smaller or shade—intolerant species in more productive sites caused by reduction of light levels. We added water and/or one of three nutrients (nitrogen, phosphorus, and potassium) to a 1st—old—field community dominated by weedy annuals and measured effect on productivity, species composition, diversity, and light levels after one growing season. Diversity was not clearly related to productivity in this experiment. Watering increased productivity, but, contrary to expectations, had no effect on density of surviving plants, species diversity, or abundance of low—growing species. Almost all the increase in biomass with watering was due to a positive response by Ambrosia artemisiifolia, an upright annual that was the most common species in the canopy in all treatments. The addition of nitrogen had only a small positive effect on productivity, but strongly decreased density of surviving plants, species diversity, and abundance of most low—growing species. Only Ambrosia increased in abundance with nitrogen addition. The phophorus and potassium additions had little effect on the community. We suggest that the high mortality and low diversity in the nitrogen addition plots, but not in the more productive watered plots, was due to limitation by nitrogen earlier than limitation by water during the growing season. The consequence was earlier canopy closure and greater mortality due to light limitation.
Short‐ and Long‐Term Limitations to Fruit Production in a Tropical Orchid
Authors: Ackerman James D. , Montalvo Arlee M.
Published: May 2006

Fruit production in interoparous flowering plants can be limited by a variety of conditions that need not be mutually exclusive nor immediately evident. We examined short— and long—term constraints to fruit production in a Puerto Rican population of the infrequently pollinated orchid Epidendrum ciliare. Average natural fruit maturation rate ranged from 5 to 15% over 4 yr. To evaluate limitations to fruit production, pollinations were augmented for a randomly chosen experimental portion of a population for two consecutive years. During this period, only 10 and 15% of flowers on control plants were naturally pollinated. Hand—pollinations of nearly all flowers produced by experimental plants increased fruit production to 33 and 49%, compared to 5 and 8% for controls. Thus, fruit production within seasons was partially limited by pollinations. Fruit predation also was heavy for both control (28 and 29% of fruits initiated) and pollinations—augmented (20 and 32%) groups. No clear relationship existed between treatment and predation frequency. Furthermore, undamaged fruits often aborted (controls 11 and 37%, experimentals 15 and 28%), but frequencies were independent of self vs. outcrossed pollination and treatment. Seed crop mass declined as fruit set increased, suggesting that resources were limited. Long—term effects of heavy fruit loads were evident. Over 2 yr, the number of inflorescences, flowers, and flowers per inflorescence declined for experimentals relative to the controls. Moreover, plant size and vegetative proliferation decreased for experimentals relative to controls. Thus, through elevated fruit production the plant incurs future cost and lifetime fitness of E. ciliare might be more affected by resource constraints than by the other factors studied.
The Physiological Ecology of Daphnia: A Dynamic Model of Growth and Reproduction
Authors: Gurney W. S. C. , McCauley E. , Nisbet R. M. , Murdoch W. W.
Published: May 2006

In the preceding paper (McCauley et al. 1990) we developed a new model for the growth and fecundity of Daphnia based on a quantitative review of short—term physiological rates and energy allocation for D. pulex. In this paper, we formulate a fully dynamic version of this model and test its predictions against experimentally observed growth and fecundity schedules obtained independently under a variety of different experimental protocols. Preliminary testing falsifies two simplifying hypotheses made in our original development and we propose modifications of these hypotheses. With the aid of these modifications our model yields predictions that are in good agreement with a large body of data on the growth and fecundity schedules of individual D. pulex. Although our model contains 18 parameters, the values of the great majority are independently determined from short—term physiological measurements, leaving only two as freely adjustable "fitting parameters." The target dataset, which contains four complete growth curves (°80 observations) and 32 growth or fecundity characterizations, thus provides a stringent test of the model, and our success in matching our predictions to it provides strong evidence that measurements of short—term physiological rates can be used successfully as predictors of long—term growth and fecundity.
Host‐Plant Specialization Decreases Predation on a Marine Amphipod: An Herbivore in Plant's Clothing
Authors: Hay Mark E. , Duffy J. Emmett , Fenical William
Published: May 2006

We investigated the factors selecting for host—plant specialization and the roles that plant defensive chemistry may play in this process. We studied marine organisms because marine communities contain fewer specialized herbivores than terrestrial communities and, therefore, provide a simplified system for investigating certain aspects of host—plant specialization. Our study focused on the unusual domicile—building and feeding behavior of the amphipod Pseudamphithoides incurvaria, which is the only herbivorous marine amphipod known to specialize on a few closely related seaweeds. P. incurvaria lives in a portable, bivalved domicile that it constructs from the chemically defended brown alga Dictyota bartayresii. Chemical assays indicate that natural populations of Pseudamphithoides construct their domiciles from D. bartayresii even when this alga is rare compared to other Dictyota species and to other related genera in the family Dictyotaceae. In both choice and no—choice tests in the laboratory, Pseudamphithoides built domiciles from and selectively consumed species of Dictyota that produced dictyol—class diterpenes that deter feeding by reef fishes. Other brown seaweeds in the family Dictyotaceae, including a Dictyota species, that did not produce these fish—feeding deterrents were avoided by the amphipod. Amphipods removed from their domiciles were rapidly eaten when presented to predatory fish; amphipods in their normal domiciles were consistently rejected by fish. The defensive value of the domicile appeared to result from specific characteristics of the Dictyota from which it was built, since amphipods forced to build domiciles from the palatable green seaweed Ulva were rapidly eaten when these amphipods, in their domiciles, were exposed to predatory fish. Algal defensive chemistry directly cued domicile building. When the green alga Ulva was treated with pachydictyol—A (the major secondary metabolite produced by Dictyota bartayresii), domicile building by Pseudamphithoides increased in proportion to the concentration of pachydictyol—A. All data collected during this study are consistent with the hypothesis that predator escape and deterrence are primary factors selecting for host specialization by Pseudamphithoides incurvaria. Similar conclusions can be drawn for the limited number of other marine herbivores that are relatively specialized.
Catastrophe, Extinction, and Species Diversity: A Rocky Intertidal Example
Authors: Wethey David S.
Published: May 2006

Geographically widespread episodes of catastrophic mortality affect local species diversity. Sea ice has been a frequent catastrophic mortality agent on rocky shores in New England during the historical past. Empirical demographic data and the historical record of winters with sea ice were used in a population projection model of intertidal barnacles. Projections indicate that animals that live exclusively in the zone affected by ice would have become extinct under the conditions of sea ice disturbance that have existed in New England during the past 300 yr. This may explain the absence from New England of abundant, large, long—lived species with delayed reproduction that are restricted to the mid to low intertidal zone. Such species are common elsewhere in the world on shores unaffected by ice. Given the cyclic nature of the postglacial climate, global patterns of species richness may be influenced by climatically induced extinction pressures of this kind.
Algal Periphyton Growth on Nutrient‐Diffusing Substrates: An in situ Bioassay
Authors: Fairchild G. Winfield , Lowe Rex L. , Richardson William B.
Published: May 2006

Differences in nutrient limitation for dominant species within an algal periphyton community were determined using additions of N and P supplied by nutrient—diffusing artificial substrates. Sealed clay flowerpots were filled with 2% agar and one of nine nutrient treatments (all combinations of K2HPO4 at 0.0, 0.05, and 0.5 mol/L with NaNO3 at 0.0, 0.05, and 0.5 mol/L). The pots were submerged at 0.5 m depth in Douglas Lake, Michigan, and diffused N and P to their outer surfaces in proportion to internal concentrations. After 51 d the pots were scraped and analyzed for attached algae. Total algal biomass as chlorophyll a on the pots ranged from 0.17 ± 0.02 (SE) mg/cm2 for pots without added nutrients to 15.7 ± 2.0 mg/cm2 for pots with K2HPO4 at 0.05 mol/L and NaNO3 at 0.5 mol/L. chlorophyll a on pots containing just P (0.05, 0.5 mol/L) increased 6— to 10—fold over controls. The diatoms Epithemia adnata and Rhopalodia gibba and the blue—green alga Anabaena increased significantly on the P—only pots; these species are suspected of N—fixing capability. Chlorophyll a on pots containing just N (0.05 mol/L) increased 1.5— to 2—fold, though this increase was nonsignificant; Achnanthes minutissima, Gomphonema tenellum, and Cocconeis placentula showed enhanced growth on these pots. Combinations of N and P caused heavy growth of the filamentous alga Stigeoclonium tenue. Naviculoid diatoms were also most abundant on the N + P pots. Average nutrient levels in Douglas Lake during the study were: NH3, 2.02 μmol/L; NO3, 0.44 μmol/L; and PO4, 0.06 μmol/L. The low ambient concentrations of both N and P, together with results of the periphyton bioassay, indicate that the two nutrients may jointly limit overall growth, and that the form of growth limitation differs by species within the periphyton community.
Seedfall Pattern of Several Species of Bird‐Dispersed Plants in an Illinois Woodland
Authors: Hoppes William G.
Published: May 2006

I investigated spatial patterns of seedfall for several species of bird—dispersed plants in an east—central Illinois maple—hackberry woodland. Seed density around naturally growing fruiting plants and artificial fruit displays in treefall gaps was monitored with seed traps and compared with seedfall in undisturbed forest. Around individual fruiting plants, seedfall decline with distance from the seed source. Seedfall was highest near the source, but if the tail of the seed shadow crossed the edge of a treefall gap, there was usually another, smaller seedfall peak, producing a bimodal pattern. Among fruiting species, small seeds were dispersed farther and were much more likely to be dispersed into adjacent treefall gaps than large seeds. Seeds dispersed out of a treefall gap had a higher probability of falling into an adjacent treefall gap than would be expected if seeds fell uniformly around a source. Natural seedfall around a treefall gap was highest at the edge of the gap, lower in the center of the gap, and lowest in the undisturbed forest. About 50% of the seeds of bird—dispersed plants in the study area were estimated to have fallen into treefall gaps (16.8% of area). If seedling survival is higher in treefall gaps than in undisturbed forest for the bird—dispersed plants in the study area, patterns of seedfall indicate that furgivorous birds benefit the plants by (1) dispersing most seeds into treefall gaps, (2) by increasing the diameter of the seed shadow over that produced by gravity, (3) increasing the probability that a plant's seeds will be located at the site of a future treefall gap.
Food Supplementation in a Tropical Mainland Anole, Norops Humilis: Effects on Individuals
Authors: Guyer Craig
Published: May 2006

In this study I investigated how individuals of Norops humilis at a tropical lowland wet—forest site in Costa Rica used energy made available by supplemental feeding. I manipulated prey availability on three experimental plots by attracting arthropods to bits of rotting meat that were replenished weekly. Each experimental plot was paired with an unmanipulated control plot in a block design. Mark—recaputre data were collected on all plots over a 7—mo test period during which I made 9910 observations on 1582 individuals. Home—range size was not affected by food supplementation. Instead, home—range overlap was increased for all combinations of age—sex groups (juveniles, males, and females) except for females relative to other females and juveniles relative to females. Growth of females was not altered by supplemental food. For males, both the shape of the growth curve and the estimate of asymptotic largest size were altered by food supplementation. No difference in body mass relative to length was observed for either males of females. Rate of egg production of females of experimental plots was significantly more rapid than on control plots. Males appeared to shunt extra food energy into growth and females into reproduction. The responses of this mainland anole are similar to responses described for island anoles in that increased juvenile overlap, increased clumping of juveniles, and unchanged home—range sizes were observed. In contrast, the observation of unchanged relative body mass, unchanged female growth rate, and increased egg—production rates in supplentally fed N. humilis differ from observations of island anoles.
Omnivory and Food Chain Length in Zooplankton Food Webs
Authors: Sprules W. G. , Bowerman J. E.
Published: May 2006

We constructed plankton food webs for 515 lakes in northeastern North America on the basis of zooplankton species presence and diet data. Analysis of these webs indicated that glacial lakes containing the predatory species Mysis have more trophic levels and a greater incidence of omnivory than is typical of other lakes and a variety of other aquatic and terrestrial communities. Furthermore, this degree of complexity is greater than predicted by theoretical analyses of model food web dynamics. We discuss methodological and theoretical limitations of food web studies and hypothesize that a relatively high incidence of cannibalism, reciprocal predation, life history omnivory, and close linkage between omnivory and high trophic position contribute to the structural stability of glacial lake plankton communities.
Island Biogeography: Effect of Geographical Isolation on Species Composition
Authors: Kadmon Ronen , Pulliam H. Ronald
Published: May 2006

Island biogeography theory attempts to explain and predict among—island variation in species richness. However, two islands with the same number of species may still differ from each other considerably in their species composition. In this study we test the hypothesis that among—island variation in species composition is predictable and can be related to the corresponding differences in distance to the mainland. We focus on woody plants inhabiting islands in the Clarks Hill Lake, a reservoir completed in 1954 on the Savannah River, between Georgia and South Carolina, USA. Two groups of islands were sampled: islands that wee logged prior to the filling of the reservoir and islands that were not logged. Each island was surveyed for the presence of all tree and shrub species, and its distance from the mainland was determined. In both groups of islands, the degree to which two islands are similar in their species composition was negatively and significantly correlated with their difference in distance to the mainland. Species richness, however, was correlated with distance to the mainland only on logged islands. We conclude that geographic isolation may affect species composition on islands, and that such an effect may occur even in the absence of a corresponding effect on species richness.
Effects of Herbivorous Insects and Soil Fertility on Reproduction of Goldenrod
Authors: Meyer Gretchen A. , Root Richard B.
Published: May 2006

Plant responses to herbivore damage vary enormously, but the causes of this variation are not well understood. We examined how both insect feeding style and soil fertility influence the severity of herbivore impacts, by measuring the effects of feeding by a leaf—chewing beetle (Trirhabda sp.), a xylem sap—feeding spittlebug (Philaenus spumarius), and a phloem sap—feeding aphid (Uroleucon caligatum) on growth and reproduction of goldenrod (Solidago altissima). Our goals were to determine (1) whether these three insects differentially affect plant fitness, (2) whether soil fertility affects a plant's ability to compensate for damage, and (3) whether soil fertility and insect feeding style interact in determining herbivory impacts. Plants were grown outdoors in large pots at two levels of soil fertility, and insects were placed on them at a range of densities. Insects fed on the plants for °3 wk in spring, and then the plants were grown insect free for the rest of the season. We used insect mass gain per plant as the basis for comparing the effects of three insects, since the insects differed greatly in size. Spittlebug feeding reduced plant growth rates 3 times more than beetle feeding; aphid feeding had no detectable effect. The plants grew faster at higher soil fertility, but there was no interaction between insect impact and fertilizer level. Stem heights at the end of the season were increased by fertilizer and not affected by herbivory. Spittlebug feeding, and to a lesser extent aphid feeding, decreased the number of lateral stems, and again there was no interaction with soil fertility. Soil fertility had strong effects on plant sexual and asexual reproduction. Plants at the high level of soil fertility showed a three—fold increase in total seed production, a 5% increase in mean achene mass, almost a two—fold increase in rhizome mass, and increased percent nitrogen in seeds and rhizomes. Total seed production was reduced by herbivory. The pattern was the same as for plant growth rates; the spittlebug caused the strongest decrease, beetle feeding was second, and aphid feeding caused slight reductions. There was a strong interaction between insect impact and soil fertility. Total seed production was reduced only at the high level of soil fertility for all three insects. Spittlebug and beetle damage also delayed flowering, but only at the high level soil fertility. Rhizome mass, mean achene mass, and percent nitrogen in seeds and rhizomes were not affected by herbivory. The common assumption that plants are better able to compensate for herbivore damage at higher levels of soil fertility must be reconsidered. This study also suggests that sexual reproduction may be more sensitive to damage than asexual reproduction in perennial plants.
Effects of Simulated Herbivory and Intraspecific Competition on the Compensatory Ability of Birches
Authors: Hjalten Joakim , Danell Kjell , Ericson Lars
Published: May 2006

To improve our knowledge regarding plant responses to herbivory we subjected juvenile birches (Betula pubescens) to different types of simulated herbivory, i.e., removal of the top (1 cm) of the main stem or defoliaiton (including removal of main stem top), and to different levels of intraspecific competition (i.e., three stand densities). Treatments were also repeated during the following year. Birch responses to these treatments were measured as changes in plant height and dry mass. Defoliated plants showed reduced growth irrespective of the level of intraspecific competition. Topped plants in the low—density plots showed enhanced growth, i.e., over—compensation, whereas topped plants growth in the high— and medium—density plots were unaffected by the treatments. Our results indicate that we should expect a continuum of plant responses to herbivory. Thus, there is little value in asking whether herbivory has a positive or negative effect on plant growth. Instead, emphasis should be placed on identifying conditions under which we would expect different responses. Further, we suggest that overcompensation should not generally be considered as an adaptive response that has evolved as a consequence of natural selection due to herbivory, but rather as an indirect consequence of selection for competitive ability. Because apical dominance (assumed to be an adaptation to competition) restricts growth, the growth rate will increase if apical dominance is removed.
Construction and Analysis of a Large Caribbean Food Web
Authors: Goldwasser Lloyd , Roughgarden Jonathan
Published: May 2006

We document the construction of a relatively large food web (44 species) from the island of St. Martin in the northern Lesser Antilles, and compare it with patterns observed in other, generally smaller food webs. In constructing this web, we integrate data from a variety of studies, many of which focussed on Anolis lizards and their vertebrate predators. In addition to determining the links between predators and prey, we estimate the frequencies of predation (the link strengths), and find an approximately bell—shaped distribution with a majority of links of intermediate frequencies. Some of the properties of this web contrast strongly with those of webs in the ECOWeB compilation. In particular, our analysis shows this web to possess an unusual richness of intermediate species (relative to top predators or basal species) and of links between those intermediate species. The number and lengths of chains are also unusually high, as is the degree of omnivory. Nor does this web match the predictions of the cascade model, which predicts even higher proportions of intermediate species and links between them, and even more numerous chains. It appears that these and other differences are not due simply to the large number of species involved here, but it is not yet clear whether they should be ascribed to the completeness with which some of the diets are known, to differences between the ways this and other webs were constructed, or to unique ecological conditions on the island of St. Martin.
Allometric Theory: Extrapolations from Individuals to Communities
Authors: Cyr Helene , Pace Michael L.
Published: May 2006

Physiological rates of individual organisms are well related to their body size. these allometric relationships suggest that ecological rates should also be related to the size structure of organisms in populations, communities, and ecosystems. We describe size distributions of zooplankton communities and explore the implications of such distributions on community grazing rates. Ninety zooplankton communities, varying in biomass and in size distributions, were sampled in 28 lakes in northeastern North America and their grazing rates were predicted with an allometric equation. Zooplankton size distributions vary in shape but, on average, can be described as bimodal. Predicted community mass—specific grazing rate decreases with increasing mean body size (r2 = 0.92)and is only slightly affected by the shape of community size distributions. Community biomass on the other hand increases with mean body size (r2 = 0.39). Total zooplankton grazing rate is expected to be higher in communities dominated by large zooplankton, but this relationship is obscured (r2 = 0.15) by temporal and spatial variability in zooplankton biomass. Although body size is a powerful predictor of individual physiological rates, its importance is expected to be largely masked at the level of communities.
The Relationship of Plant Size to Self‐Pollination in Mertensia Ciliata
Authors: Gerber Monica A.
Published: May 2006

This study tested the hypothesis that self—pollination and, as a result, inbreeding rate increased with plant size in Mertensia ciliata, a self—compatible, profusely flowering perennial. The hypothesis is based on two subsidiary hypotheses, which were examined: (1) the dispersal of pollen among flowers and among plants is generally limited in plant populations; and (2) individual pollinators are likely to make longer visits to large than to small plants, and to spread higher proportions of self pollen rather than foreign pollen among flowers of large plants. Because M. ciliata exhibits inbreeding depression in seed—set and seed mass, increased self—pollination and inbreeding would lower the reproductive output of flowers on large plants. However, in experiments with bumble bees confined to nylon mesh cages, I found that the carry—over of pollen between flowers by bees is extensive in M. ciliata. In addition, individual bees visit only a few more flowers per visit to large than to small plants, and large plants attract more bees per minutes than small plants. Using these data, I constructed a mathematical model to estimate the amount of foreign pollen a flower should receive as a function of plant size. The model predicts that flowers on all plants should receive approximately equal amounts of foreign pollen, and that self—pollination and hence inbreeding will not vary with plant size. These predictions were supported by direct measures of flowers in the field: no differences were found in foreign pollen receipt and in reproductive output (seed—set and seed mass) between flowers from large and small plants.