bluuebunny/crossref_metadata_2025 huggingface dataset:
Phenotypic Plasticity of the Size at First Reproduction in Daphnia: The Importance of Maternal Size
Authors: Lampert Winfried
Published: May 2006

I propose a mechanism of rapid phenotypic response of size at first reproduction (SFR) that may allow relatively large species of Daphnia to survive under variable fish predation. A single clone of D. magna isolated from a shallow lake with planktivorous fish is able to express the full range of SFR observed in the field (1.8—3.0 mm) when raised under controlled temperature and food conditions in the laboratory. Control of SFR is predominantly based on size differences in the eggs of the first and the following broods of Daphnia. First—brood eggs are smaller. They hatch into smaller neonates that mature at a smaller size. In the lake, fish may selectively remove large females, and this will increase the share of first—brood females in the population. Offspring of these females will mature at a much smaller size, so that the size response to predation is intensified. When predation ceases, some females can grow to larger sizes. They will contribute many large offspring to the population because they have larger clutch sizes and larger eggs. This leads to a rapid shift toward large SFR. A single clone of D. magna was hatches from isolated eggs to measure the components of maternal size dependency, which is basic to this hypothesis. Strong positive relationships were found between female size and egg size, egg size and neonate size, neonate size and SFR, size of a female and its offspring's SFR, and female size and clutch size. The predicted pattern of SFR in relation to average body size is found in the lake, but the pattern of egg size is not clear, as egg size is strongly affected by changes in food abundance. This mechanism of phenotypic response may be a fortuitous effect of selection for maximum egg numbers in the first brood, but the increase of offspring size with maternal size may also be adaptive itself.
Testing Models of Optimal Diet Assembly by the Generalist Herbivorous Lizard Cnemidophorus Murinus
Authors: Dearing M. Denise , Schall Jos. J.
Published: May 2006

The diet of the predominantly herbivorous Bonaire Island whiptail lizard (Teiidae: Cnemidophorus murinus) was examined to assess three models of diet selection by generalist herbivores. These models were: single—nutrient maximizing, toxin avoiding, and nutritional wisdom. At three sites were gathered data on the diet of the lizard over a full year (both wet and dry seasons) on the relative abundance of all plant food types available to the lizards during that period, and on the nutritional composition of each plant type. Thirteen nutritional variables were measured, including content of energy, protein, minerals, water, and potentially toxic plant secondary compounds, and digestibility of protein. The lizards were generalist feeders, consuming a wide variety of flowers, fruits, leaves, nectar, and some animal material. Most stomachs contained more than one food type, suggesting C. murinus typically samples several kinds of plant materials each day. The lizards were selective with regard to foods eaten; most foods were not taken in proportion to their availability in the environment. Potential plant foods varied in nutritional quality, but no one nutrient was correlated with dietary preferences. Multivariate analysis revealed that preference or avoidance of a potential food type could be predicted by a combination of nutritional properties, but these differed among sites. Despite differing plant assemblages at each site, annual intake of nutrients by the lizards was similar among sites. The results best support the nutritional wisdom hypothesis; the Bonaire whiptail lizard may assemble its diet to obtain the proper balance of required nutrients, while avoiding dangerous levels of plant secondary compounds.
Size‐Related Effects of Predation on Habitat Use and Behavior of Porcupines (Erethizon Dorsatum)
Authors: Sweitzer Richard Alan , Berger Joel
Published: May 2006

Ecological and behavioral strategies that reduce predation were examined in North American porcupines (Erethizon dorsatum) by testing two hypotheses: (1) habitat use varies with activity level and age class of animals, and (2) with increasing nutritional stress, individuals accept increased predation risk to avoid starvation. Observational and experimental data were gathered over a 2—yr period for a Great Basin Desert porcupine population during winter. Field experiments included manipulations of perceived risk using predator urines and simulated predator encounters with a dog (Canis familiaris). Compared to foraging animals, sleeping porcupines of all age classes avoided high risk (i.e., open) areas. Mass loss varied by habitat with individuals foraging in open areas (high risk) experiencing less mass loss than those foraging in groves (low risk). Age class also affected habitat use by foraging porcupines. Juveniles invariably remained in low—risk areas, even when more risky habitats contained potentially better quality forage. Two—year—old porcupines shifted to high risk areas as body condition deteriorated, but condition exerted little influence on habitat use by adults, as this age class utilized habitat similarly throughout winter. These data reveal how a species with formidable antipredator morphology uses habitats in risk—sensitive ways.
A Postfire Shift From Lichen‐Spruce to Lichen‐Tundra Vegetation at Tree Line
Authors: Arseneault Dominique , Payette Serge
Published: May 2006

The environmental changes associated with a fire—induced shift from old—growth lichen—spruce krummholz to lichen—tundra vegetation have been evaluated at a tree line site in northern Quebec. Tree ring and growth form patterns of black spruce (Picea mariana) remains lying on the ground in a lichen—tundra community were used to reconstruct, within a 4600—m2 quadrat, the structure of a conifer stand at the time of the burn (°AD 1750). The prefire spruces were the last members of a long regenerative sequence leading to maintenance of a lichen—spruce stand formed after a burn °1700 yr BP. At this time postfire recruitment was most likely facilitated by a favorable climatic conditions. Before the 1750 fire event the krummholz was predominantly maintained by layering, due to severe climatic conditions at least since the beginning of the Little Ice Age (°AD 1580). The site was deforested by the 1750 fire, because of the limited regenerative potential of stunted spruce. Postfire spruce exclusion has been responsible for major environmental changes associated with a thinner snow cover. Along the border of the deforested site snow cover depth was controlled by living spruces, whereas postfire shrubs (mostly dwarf birch, Betula glandulosa) of the lichen—tundra stand were unable to trap drifting snow. A significant decrease in thickness of the snow cover following deforestation was deduced from a comparative analysis of stem morphology of prefire spruces and present snow conditions. Inception of gelifluction lobes and mudboils was also caused by a change in the soil thermal regime associated with a reduced snow cover. Our results confirm the hypothesis that several lichen stands of the forest—tundra are postfire communities succeeding from degraded conifer stands during cold periods of the late Holocene.
The Demography of Fine Roots in a Northern Hardwood Forest
Authors: Hendrick Ronald L. , Pregitzer Kurt S.
Published: May 2006

The production, development, and mortality of fine roots in a northern hardwood forest was monitored for 1 yr using minirhizotrons. Roots were divided into two strata based upon their depth in the soil, <30 cm and >30 cm. Cohort analyses of roots produced in the spring of 1989 revealed that while almost 50% of fine roots at both depths survived after 346 d, the number of white roots in each cohort declined very rapidly. Virtually all roots had turned brown after 346 d. The probability of a surviving white root turning brown was much greater than the probability that it would die at all times of the year, and the bulk of root mortality was accounted for by brown roots. Analysis of root length production and mortality showed that total annual length mortality at the <30 cm depth was 76% of the initial standing crop and 110% at >30 cm depth. Fine root production and mortality occurred simultaneously throughout the year, and production was slightly greater than mortality at both depth. Total root length peaked in the summer at both depths, and overwinter production and mortality as rather low. Production of white and brown root length indicated that roots near the soil surface were undergoing much more rapid rates of browning than deep roots. Loss of root length between sampling dates was largely due to roots that died and rapidly decayed or otherwise disappeared.
Recruitment Limitation, Mortality, and Population Regulation in Open Systems: A Case Study
Authors: Hughes Terence P.
Published: May 2006

In most populations of mammals and birds offspring live with (and initally may depend upon) their parents. Consequently, population size is strongly influenced by local fecundity. In contrast, most species of marine invertebrates have a dispersive larval stage, so that local fecundity has little or no effect on the establishment of new individuals. Similarly, many insects, amphibians, and fish have multiple life stages that live apart. Population regulation in such open systems is fundamentally different from more closed populations. I investigated the role of recruits in determining and regulating local population size of a sessile colonial invertebrate, Cellepora pumicosa (Bryozoa, Ascophora). Recruitment into a field population of C. pumicosa over an 18—mo period was less than post—recruitment mortality, causing a decline from 230 to 64 colonies (6.1 to 2.3% cover). The mortality rate was density independent. Colony growth rate declined among larger individuals, which usually regressed in size (shrank) before they died. The mechanics of recruitment limitation was further investigated using a simple demographic model. The model was designed to mimic an open population in which recruits were added to the local adult population at a rate independent of local fecundity or density. After they recruited, colonies could grow, shrink, or die at rates determined in the field. There were no density—dependent terms in the model, yet the population attained an equilibrium size and structure that was determined by a balance between rates of recruitment and size—specific mortality (i.e., storage) of recruited individuals. The equilibrium occurred because the per capita number of recruits declined as more and more individuals became established. Modelling based on the measured demography of C. pumicosa demonstrates that the observed levels of recruitment and mortality will limit the population size to very low levels, although relatively small increases in numbers of recruits would yield a substantially large population. In comparison, post—recruitment mortality would have to decrease greatly and remain density independent to achieve the same result. In contrast to the result for short—lived animals such as C. pumicosa, the population size of longer lived organisms is more sensitive to changes in mortality than recruitment, even when mortality is density independent and populations are recruitment regulated.
The Exclusion of Limnetic Cladocera from Turbid Glacier‐Meltwater Lakes
Authors: Koenings J. P. , Burkett Robert D. , Edmundson John M.
Published: May 2006

Glacial lakes, turbid (@>5 nephleometric turbidity units) with suspended particles (1—30 @mm), have both lower May—November levels of chlorophyll a and temperatures compared to nonglacial systems. Macro—zooplankton densities are also lower and dominated by Cyclops and Diaptomus. Extensive surveys showed that, regardless of the presence or absence of planktivorous fish, filter—feeding caldocerans (e.g., Bosmina, Daphnia, and Holopedium) were only absent from the limnetic zooplankton community of the glacial lakes. Both laboratory and in situ biochamber experiments demonstrated that turbidity reduced Daphnia survival and recruitment. Nondiscriminating filter feeders ingest glacial silt (average diameter of °11 @mm) because the size—range overlaps that of the phytoplankton. We speculate that such an inefficient foraging strategy, especially when silt levels are high and algal numbers low, lowers energy extractable from ingested food below maintenance levels. Thus, the limnetic macro—zooplankton community of most Alaskan glacial lakes is restricted to either the selective herbivore Diaptomus and the raptorial feeding Cyclops, or to just Cyclops.
Role of Enemy‐Free Space and Plant Quality in Host‐Plant Selection by Willow Beetles
Authors: Denno Robert F. , Larsson Stig , Olmstead Karen L.
Published: May 2006

Phratora vitellinae and Galerucella lineola are two leaf beetles that feed on willows (Salix) in central Sweden. When disturbed, larvae of P. vitellinae exude droplets of a defensive secretion from dorsal glands. The secretion contains salicyl aldehyde, the precursors for which are plant—derived salicylates like salicin and other chemically similar phenolic glycosides. In contrast, larvae of G. lineola lack dorsal glands, cannot convert salicylates to salicyl aldehyde, and no not produce such a defensive secretion. We evaluate the adult oviposition pattern, larval performances, and suceptibility to predators of these two beetles on three species of willows, two rich in salicylates (Salix fragilis and S. dasyclados) and one poor in salicylates (S. viminalis). Females of G. lineola oviposited preferentially on S. viminalis and S. fragilis where larvae developed rapidly and survival was high, and avoided S. dasyclados where larval performance was poor. Variation in leaf pubescence, toughness, and nitrogen content across willow species not explain observed patterns of larval performance. However, larval performance was inversely related to the concentration of salicylates in the three willows. By contrast, the oviposition preferences of P. vitellinae did not correspond well to patterns of larval performance. Salix fragilis was by far preferred choice of oviposition, and females rarely placed eggs on either S. viminalis or S. dasyclados. The performance of P. vitellinae larvae differed markedly on the two salicylate—rich willows; larvae survived best and developed most rapidly on S. fragilis, but mortality was high and development slow on S. dasyclados. Poor larval performance on S. dasyclados was associated with high concentrations of simple phenolic compounds other than salicylates. Despite differences in performance, larvae fed these two salicylate—rich willows produced a defensive secretion which effectively repelled coccinellid predators. Larvae fed the salicylate—poor S. viminalis survived and developed well in the absence of enemies, but lacked the salicylate—based defensive secretion and were readily eaten by coccinellids. Females of P. vitellinae apparently avoided S. viminalis as an oviposition host, not because larvae performed poorly, but because larvae were defenseless against some predators when raised on this salicylate—poor willow. We argued that herbivores such as P. vitellinae obtain enemy—free space on hosts from which they sequesters plant—derived allelochemicals for defense. The result appears to be dietary specialization on host—plant species that provide these precursors for defense. Other related herbivores like G. lineola that do not employ plant—derived defensive secretions, are not so constrained and feed on a wider range of host willows.
Balsam Fir on Isle Royale: Effects of Moose Herbivory and Population Density
Authors: Brandner Thomas A. , Peterson Rolf O. , Risenhoover Ken L.
Published: May 2006

Balsam fir (Abies balsamea) population parameters were analyzed in Isle Royale National Park, Michigan, to asses moose (Alces alces andersoni) herbivory in relation to the population density of both fir and moose. Fir population parameter were determined at nine study sites, each representing a different combination of relative fir and moose density (high, medium, and low levels, respectively). In general, moose suppression of fir height growth and recruitment to the canopy increased with increasing moose density and decreasing fir density. Where fir densities were low, sapling heights were restricted to <1 m regardless of moose density. In such sites, sapling growth suppression has been continuous since a peak in the moose population in the 1920s. Canopy fir trees at most sites established prior to arrival of moose on Isle Royale around 1910. At high fir density release from growth inhibition was common following a period of low moose numbers from the mid—1970s to early 1980s. Here this release should facilitate recruitment of fir and effect replacement of canopy losses due to senescene. In other areas, however, continuing canopy looses and lack of recruitment due to intensive herbivory by moose will greatly reduce the fir component of the canopy, effectively preventing any return to the fir—dominated forests of the past.
Carbon Relations of Flowering in a Semelparous Clonal Desert Perennial
Authors: Tissue David T. , Nobel Park S.
Published: May 2006

Agave deserti is a long—lived, semelparous perennial of the northwestern Sonoran Desert that flowers after °50—55 yr. Measurements of CO2 exchange over 24—h periods indicated that leaves of flowering rosettes had 24% more net CO2 uptake than leaves of adjacent nonflowering rosettes during the first month of inflorescence production. Net CO2 uptake for leaves of flowering rosettes was 32% less thereafter than for leaves of nonflowering rosettes, as senescing leaves of flowering rosettes exhibited dramatic reductions in nitrogen and chlorophyll contents. During the course of flowering, levels of total nonstructural carbohydrate (TNC) in the leaves of flowering rosettes dropped from 38 to 6% of the leaf dry mass, indicating substantial translocation of stored carbon to the inflorescence. TNC reserves of the rosette provided 70% of the carbon required to produce the 1.53 kg inflorescence, and CO2 uptake by the leaves and the inflorescence provided the remaining 30%. Rosettes must attain a minimum size (> 1000 g dry mass) to initiate flowering, unless they are connected to a large flowering rosette. Small rosettes did not produce inflorescences when their rhizome connection to a large rosette was severed °4 wk before inflorescences emerged, suggesting that a chemical signal is transmitted through the rhizome that induces the small rosette to flower precociously. Small flowering rosettes could not complete formation of the inflorescence unless partially supported by carbon supplied by the connected large rosette. The contribution of the large rosette declined from 74% for a 0—30 g dry mass connected rosette to 35% for a 200—600 g rosette. For both small and large flowering rosettes, the translocation of substantial carbohydrate reserves from the leaves is essential for production of the inflorescence.
Pollen Production and Plant Density Affect Pollination and Seed Production in Taxus Canadensis
Authors: Allison Taber D.
Published: May 2006

Mean pollen production and mean nearest neighbor distance were recorded for several populations of Taxus canadensis and correlated with the proportion of ovules pollinated and seed set. Distance and pollen production together explained 86% of the variation in pollination success, each variable significantly adding to the regression when adjusting for the other. Seed set was correlated significantly with pollen production and nearest neighbor distance separately, but the multiple regression including the latter two variables was not significant. Seed set was correlated most strongly with pollination success and mean ovule production (R2 = 0.7), suggesting that variation in seed set among Taxus populations was a combination of differences in pollen and resource availability.
Plasticity in Plant Size and Architecture in Rhizome‐Derived vs. Seed‐Derived Solidago and Aster
Authors: Schmid B. , Bazzaz F. A.
Published: May 2006

Old—field perennials of the genera Aster and Solidago, known to have long and short rhizomes, respectively, are expected to represent two types of genet architectures. We collected these plants at different field sites and grew their seeds and rhizomes in loose and compact soils to investigate the following questions about size and architectural plasticity in these clonal perennials: (1) to what extent do plant size and architecture vary among environments and within taxa, (2) does such variation obscure the differences in rhizome length and architecture between taxa, and (3) because these plants regenerate by clonal growth and sexual reproduction, how does mode of regeneration affect this variation? In all fields sites as well as in the contrasting experimental environments Aster and Solidago had similar size (e.g., shoot height and plant mass) but different architecture. Whereas Aster always produced long rhizomes and much—branched shoots with few main—stem leaves, Solidago produced short rhizomes and little—branched shoots with many main—stem leaves. Size variation among environments was large in both genera. Architecture was influenced by environment in Aster but not in Solidago. Phenotypic variation for size and architectural characters among populations and families within genera was small. In both genera, plants grown from rhizomes and from seeds of the same (maternal) parents had similar mean performance, but phenotypic variation among environments was greater for rhizome—derived than for seed—derived plants. We conclude that in these plants the environment influences variation in architecture less strongly than it affects variation in size. Because differences among populations and families were also small, the two taxa Aster and Solidago are well characterized by their respective plant architecture. We suggest that the influence of mode of reproduction on phenotypic flexibility may reflect a stronger architectural “blueprint” in seed—derived plants (less flexible) as compared with rhizome—derived plants (more flexible).
Post‐Defaunation Recovery of Fish Assemblages in Southeastern Blackwater Streams
Authors: Meffe Gary K. , Sheldon Andrew L.
Published: May 2006

We analyzed fish assemblage structure at 37 sites in South Carolina streams before and nearly one year after experimental defaunation to test assemblage resiliency. Decreases in stream depth and width in the second year reflected an intervening drought, but habitat structure remained highly correlated between years. Fish assemblages recovered well over four scales of analysis. Total fishes sampled, collective assemblage properties (species richness, density, biomass, and mean mass of fish), local assemblage structure, and single—species attributes generally did not significantly differ after defaunation, as determined by species— and individual—abundance correlations, detrended correspondence analysis, and a proportional similarity index. These assemblages were not randomly structured units, but were largely deterministic systems highly predictable from local habitat structure.
Avian Community Organization and Habitat Structure
Authors: Willson Mary F.
Published: May 2006

Bird species diversity was linearly correlated with foliage height diversity and curvilinearly with total percent vegetation cover. The addition of trees in a vegetational series has a disproportionate effect on the addition of species, primarily by the addition rather than the expansion of guilds. No basic relationship of species—packing within guilds is associated with bill or body size except frequently within two—member guilds. Estimated abundance and biomass of birds does not appear to be related to productivity of the habitats, in contrast to reports form the literature. Similarity of bird species composition is not related to similarity of foilage distribution, when like study areas are compared. Avifaunas of grasslands generally differed more among themselves than did those of forests. Bird species overlaps were correlated with foilage height overlaps only for part of the variational range for two— and three—layered habitats, and little if at all for grasslands. Width of "habitat—niche" is not related to numerical dominance, taxonomic or ecological categories. Because many of these results do not coincide with previous attempts at ecological generalization, great care in such attempts seems strongly indicated. Studies should be made of meticulously delineated subcommunities, resource measurement, and reproductive success, as well as events in the nonbreeding season.
Comparative Feeding Ecology of Wintering Abult and Juvenile Royal Terns (Aves: Laridae, Sterninae)
Authors: Buckley F. G. , Buckley P. A.
Published: May 2006

Comparison of the feeding behavior of adult and juvenile Royal Terns, Sterna maxima, showed the following: adults spent less time foraging over a given stretch of beach, making almost twice as many dives per minute as juveniles; while initial prey capture rates were the same on both age—classes, adult dived more frequently, and therefore caught more prey per unit time; both rarely dropped their prey without recovering it, but juveniles dropped their prey, subsequently recovering it, almost 14 times as often as adults, thereby expending considerably more energy per prey capture than adults. Analyses of counts of sitting and feeding terns confirmed predicted nonrandom associations: feeding adults avoided all other birds except their offspring, but, when resting, avoided even juveniles more often than expected; feeding juveniles occurred almost equally frequently alone, with adults, and in small flocks (adults never so occurred). An attempted time—budget analysis, limited by unmarked birds, suggests that adults feed more in the early morning hours, resting or foraging for their young later in the day; juveniles apparently do most of their solo fishing between 0900 and 1200 h, and in general spend less day time roosting than do adults. Analysis of data on recovered, banded terns indicate that significantly more juveniles than adults are caught on fishhooks and that adults' ability to recognize capturable prey is a prime factor in their fishing success. These results confirm Lack's and Ashmole's basic hypothesis that extended parental care and reduced feeding ability of young have played a significant role in the evolution of seabird clutch size. Considered in terms of the Holling—Griffiths—Watt "components of predation" analysis, our data suggest that these foraging differences must be incorporated in future predatory—prey models. The component analysis approach ought to be extended to other terms, especially over longer periods of time post—fledging. Salt and Willard's conclusions from their component analysis of the foraging behavior of Forster's Tern, reexamined in the light of our findings, lead us to believe that insufficient data are available to decide whether Forster's and Royal Terns better fit Holling—Griffiths—Watt or Lotka—Volterra predator—prey models, because these models fail to consider important aspects of the terns' biology and because certain data needed for their use were not obtained in either study.
Fungal Succession on Loblolly Pine and Upland Hardwood Foliage and Litter in North Mississippi
Authors: Watson E. S. , McClurkin D. C. , Huneycutt M. B.
Published: May 2006

Over 70 species of fungi were found in litter by direct observation and cultural methods including washing and surface sterilization. Thirteen were found only in pine litter, 10 in hardwood, and 49 in both litters. Basidiomycete mycelia were also found in certain layers, but no species identification was attempted. Thirteen species plus the unidentified Basidiomycetes were considered important in the fungal succession. Of the important species, Deuteromycetes predominated and Ascomycetes were second in importance on both living foliage and in the L layer. Deuteromycetes, Phycomycetes, Ascomycetes, and Basidiomycetes were all prominent on F litter. Basidiomycetes seemed to be dominant in the H layer.
Nitrogen and Carbon Isotope Ratios in Seabird Rookeries and their Ecological Implications
Authors: Mizutani Hiroshi , Wada Eitaro
Published: May 2006

Various samples around gull and penguin rookeries were examined for nitrogen and carbon isotope ratios. The results showed that input of organic nitrogen from avain breeding activity, followed by its aerobic decomposition in soil and by volatilization of the resultant ammonia, strongly dominated nitrogen flow in both rookeries. Plants, animals, and soils in the rookeries appeared dependent on the nitrogen thus provided. In the penguin rookery, soil organic carbon was also derived from breeding activity. A large input of organic carbon by gulls, however, appeared to have quickly escape from the gull rookery through respiration. It had little effect on the soil organic carbon; the main source of organic carbon there seemed to have come from higher plants. Major pathways of material flow in the seabird rookeries were deduced from the isotopic data. Study of stable isotopes should enable us to draw similar pictures for other ecosystems; it is particularly useful in places where frequent visits are impractical.
Does Foliage Damage Influence Predation on the Insect Herbivores of Birch?
Authors: Bergelson Joy M. , Lawton John H.
Published: May 2006

Damage caused by insect herbivores to their host plants may influence the interaction between herbivores and their natural enemies in several ways. Two important possibilities are by providing cues that attract predators and by forcing herbivores to move, which in turn increases their chances of encountering a predator. To explore these possibilities we experimentally investigated the relationship between the presence of damage and the rates of predation on two insect herbivores of birch, Betula pendula. Although both Coleophora serratella (Lepidoptera: Coleophoridae) and Apocheima pilosaria (Lepidoptera: Geometidae) suffered significant effects of predation, there was no interaction between the presence of foilage damage and predation rates on either of these birch herbivores. To understand these negative results better, we examined the movement behavior of the herbivores and foraging behavior of the predators. The presence of foliage damage did not markedly increase movement by either of the herbivores; Apocheima showed no response to foliage quality, while Coleophora moved only from the immediate vicinity of damage. When experimentally forced to move more, Apocheima were indeed more vulnerable to predation by wood ants; but surprisingly, Great Tits were not sensitive to increase movement of Apocheima prey. A simple population model indicates that the effect of foliage damage on herbivore movement is unlikely to be substantial enough to markedly change predation intensity. Overall, we find little support for the hypothesis that foilage damage increases herbivore movement sufficiently to alter predator—imposed mortality of birch herbivores.
Rates of Weed Spread in Spatially Heterogeneous Environments
Authors: Bergelson Joy , Newman Jonathan A. , Floresroux Ernesto M.
Published: May 2006

Communities subject to disturbance are apparently vulnerable to invasion in that they contain a large fraction of the introduced plant species persisting in any given area. Still, ecologists have a poor understanding of how details of the disturbance regime influence the success with which invaders spread through a community. In this paper, we report results from an experiment that investigates how the spatial distribution of bare ground influences the rate at which offspring of an introduced invader spread through a perennial ryegrass community. Artificially created gaps of three sizes were positioned along linear transects according to one of two spatial distributions. These size transects (one for each treatment combination) were positioned as spokes on a wheel and Senecio vulgaris was introduced into the center. After controlling for the total amount of gap area, we monitored the number and position of Senecio vulgaris establishing for two subsequent generations. Our results showed that the rate of spread of Senecio vulgaris was sensitive to both the gap size and the gap distribution; plants moved a greater distance when the gaps were large and underdispersed. Simulation models confirm that this pattern follows from the way in which gaps sample from seed shadows. We derive an analytical expression to explain why the mean seed dispersal distance does not predict the spread of Senecio vulgaris adults, and show how rates of spatial spread will change under different spatial patterns of disturbance. Finally, a model of spatial point processes confirms that the distribution of gaps mediates the rate of weed spread in two dimensions as well.
Short‐Term Responses of Alpine Buttercups to Experimental Manipulations of Growing Season Length
Authors: Galen Candace , Stanton Maureen L.
Published: May 2006

We examined the role of growing season length in regulating percentage cover and seed mass of the alpine snow buttercup, Ranunculus adoneus, in a snowbed habitat. Season length varies from °50 d in early—melting portions of the snowbed to 35 d in late—melting locations. Percentage cover and seed mass of snow buttercups decrease from early— to late—melting locations. Time available for growth was manipulated experimentally by altering the snowmelt schedule. Advancing snowmelt by 14 d in typically late—melting portions of the snowbed led to a threefold increase in percentage cover of snow buttercups (P < .025). However, delaying snowmelt to the same extent in typically early melting locations had little influence on snow buttercups. These results indicate that time available for growth limits cover of snow buttercups in late—emerging vegetation, whereas other factors regulate cover of snow buttercups in early—emerging vegetation . Perturbation of snowmelt scheduled also had significant effects on seed mass (P < .0055). These effects are probably mediated through changes in time available for seed maturation. Snow manipulation treatment accounted for 71% of the variation in seed maturation interval. Maturation interval, in turn, was correlated with seed mass (P < .01). Reducing or increasing the seed maturation interval by 12 d caused a 33% change in seed mass. These findings show strong direct effects of growing season length on plant performance at diverse life history stages.
A Comparison of the Impacts of Leaf‐ And Sap‐Feeding Insects on Growth and Allocation of Goldenrod
Authors: Meyer Gretchen A.
Published: May 2006

Many plants support both leaf— and sap—feeding insects, yet comparative studies of plant responses to different types of damage are rare. I compared the impacts of a leaf—chewing beetle (Trirhabda sp.), a xylem sap—feeding spittlebug (Philaenus spumarius), and a phloem sap—feeding aphid (Uroleucon caligatum) on growth of goldenrod (Solidago altissima). My goals were (1) to measure insect impacts on plant size as a function of herbivore loads, and (2) to determine the mechanisms by which insect feeding affects plant growth, using the techniques of plant growth analysis. Plants were grown outdoors in large pots, and insects were placed on them at a range of densities. A series of four whole—plant harvests was performed, with the final harvest occurring after 3 wk of insect feeding. Insect mass gain per plant was used as the basis for the comparison, since the insects differed greatly in size. Spittlebugs were the most damaging insect, beetles were intermediate, and no effects of aphid feeding were detected. After 3 wk of feeding, both spittlebugs and beetles reduced total leaf mass, total leaf area, and root mass, and the impact of the spittlebug was 5—6 times greater than that of the beetle. Spittlebug feeding also reduced the mass of the apical buds, stem mass, and the number of lateral stems, while beetle and aphid damage had no effect on these variables. Dry mass allocation to leaves, stems, and roots was generally not affected by insect feeding, resulting in smaller but proportionally similar plants by the end of the feeding period. Plant relative growth rates (dry mass increase per unit dry mass per day) were reduced by both spittlebugs and beetles but not aphids, and the impact of the spittlebug was more severe than that of the beetle. None of the insects affected net assimilation rates (dry mass increase per unit leaf area per day), indicating that the insects did not reduce plant growth by altering plant physiology. Instead, reductions in relative growth rates were explained by changes in plant morphology; feeding by both spittlebugs and beetles strongly reduced specific leaf area (leaf area/leaf mass). This reduction of leaf area relative to leaf mass was the mechanism by which both insects decreased plant relative growth rates. This result suggests an explanation for why simulated damage frequently fails to mimic damage by actual herbivores. When leaves are artificially removed by clipping there will be an immediate change in dry mass allocation patterns, but specific leaf area will not necessarily be affected.
Avian Hatching Asynchrony: Brood Classification Based on Discriminant Function Analysis of Nestling Masses
Authors: Harper R. Given , Juliano Steven A. , Thompson Charles F.
Published: May 2006

Studies of hatching asynchrony in altricial birds have used several methods to determine if a brood hatches synchronously or asynchronously. The most commonly used methods include making frequent nest visits during the time of hatching and using nestling mass or other measures of size on a given day to estimate nestling age or to describe the extent of the size hierarchy within broods. We made daily nests visits to determine if House Wren (Troglodytes aedon) broods hatched synchronously or asynchronously, and then compared classification of these broods based on discriminant function analysis with that based on mass differences of nestlings. The best misclassifications rate based on discriminant function analysis was 17.5%, whereas the rates based on differences (23.1%) and estimates of nestling age (27.5%) were higher, but not significantly higher. The most frequent error ocurring in discriminant function analysis was misclassifying asynchronous broods as synchronous broods, whereas the most frequent error based on mass differences of nestlings was misclassifying synchronous broods as asynchronous ones. Discriminant function analysis based on coefficient of variation of nestling mass and number of nestlings was about as effective (error rate = 17.5%) as was classification using individual masses. We recommend that investigators who wish to categorize broods objectively as synchronous or asychronous based on nestling masses or other measures of size should use a discriminant function analysis.
Gap‐Phase Regeneration in a Tropical Forest
Authors: Brokaw Nicholas V. L.
Published: May 2006

Gap—phase regeneration of trees was described for the first 5—6 yr of regrowth in 30 treefall gaps (20—705 m2) in tropical moist forest on Barro Colorado Island, Panama. Trees were classified as pioneers (saplings found only in gaps) or primary species (saplings found in gaps and in the understory of mature forest). In most of the gaps studied, stem densities rose rapidly after gap formation, then levelled off or declined by years 3—6. This pattern was particularly marked in some large gaps (>150 m2), where pioneers attained high densities, then experienced heavy mortality. Stem density of primary species did not vary with gap size. In large gaps the mean rate of growth in height was greater for pioneers than for primary species, size—class distribution broadened more for pioneers than for primary species, and early recruits of both regeneration types grew faster than later ones. Gap formation fosters regeneration of pioneer and primary species and, in this forest, produces patches that differ markedly in tree population dynamics, species composition, and growth rate.
Regulation of Seed‐Set in Polemonium viscosum: Floral Scents, Pollination, and Resources
Authors: Galen Candace
Published: May 2006

The effects of pollination and resource levels on seed—set in Polemonium viscosum were evaluated at three elevations: 3528, 3640, and 4025 m in the Rocky Mountains of Colorado. Resource levels were enhanced by weeding out heterospecific neighbors of the plants studied. Flowers are either sweet— or skunk—scented, and the morphs are ecotypically adapted to differences in resource and pollination availability. At the low site, where bumble bees are uncommon and flies common, sweet flowers set fewer seeds than skunky ones; at the highest elevation were bumble bees are common and flies uncommon, sweet flowers set more seeds than skunky ones. Hand pollination increased seedset, indicating that seed—set was pollen limited at all sites. The effects of weeding on seed—set were less consistent than those of hand pollination. Nonetheless, weeding enhanced seed production in sweet—scented flower at the low elevation, and in concert with pollen supplementation also enhanced seed—set in skunky—scented flower at the medium elevation. This pattern is consistent with the hypothesis of ecotypical differentiation between the floral morphs.
Nutritional Values of Wild Fruits and Consumption by Migrant Frugivorous Birds
Authors: Johnson Robert A. , Willson Mary F. , Thompson John N. , Bertin Robert I.
Published: May 2006

To learn some of the bases for consumption of temperate fleshy fruits by birds, we examined nutritional and morphological traits of temperate fleshy fruits and made laboratory observations on consumption of individual fruit species using 18 fruit species and 11 migrant frugivorous bird species in Illinois. The only seasonal trends in fruit traits were interspecific increases in absolute quantity of potassium and protein per fruit. Fruit energy content did not differ among species having bicolored vs. monochrome or small vs. large fruit displays. The fruit mass consumed was correlated best with dry pulp mass per fruit, providing significant positive correlations in 6 of 11 frugivorous species. Large fruit size relative to bill size did not appear to affect fruit consumption over the range of fruit sizes and bird species used. Because retained energy was correlated with mass consumed, the fruit pulp mass consumed was in most cases a good index of the energy obtained. Some significant differences occurred in digestive efficiency of a bird species eating different fruit species, and among different bird species eating a single fruit species, but no trends were apparent. Regurgitated seeds generally spent less time in a bird than did defecated seeds, facilitating more rapid disposal of seed ballast. Smaller birds defecated only small seeds and regurgitated some small seeds as wells as large ones, whereas larger birds defecated all smaller seeds may larger ones. Consequently, resultant seed shadows may depend upon both bird and seed size.
Seasonal Food Limitation of Detritivores in a Montane Stream: An Experimental Test
Authors: Richardson John S.
Published: May 2006

Many species of detritivorous invertebrates in small streams depend almost entirely on inputs of leaf litter for their nutritional requirements, however the concentration of this resource varies considerably seasonally. An experiment designed to test the hypothesis that productivity of coarse particle detritus feeders (shredders) is seasonally food limited was performed using replicate streamside channels that received one of three input rates of whole leaf detritus (rates equal to those naturally falling into montane streams, and two levels of increased inputs). Seven of the nine common shredder species attained significantly higher adult mass, higher densities, or both when food was supplemented. Larval densities of Malenka spp. and Brillia retifinis were significantly higher when additional food was added. Brillia retifinis densities were more than 10 x greater in high food additions than in the "natural" treatment; B. retifinis apparently fills a role as a fugitive in this system and is better able to track shifts in resource abundance by virtue of its short generation time. Zapada cinctipes and Z. haysi were more dense in high and intermediate food input treatments during the last half of the summer. Other taxa had nonsignificant trends to higher densities when extra food was provided. Most of the change in benthic densities can be attributed to decreased rates of emigration with increased food supply. Six of the eight taxa for which adult mass at emergence was measured were significantly more massive (4—46%) when food was added. Both sexes exhibited this increase in mass, but females gained proportionally more in most species. There were no detectable changes in the timing of adult emergence due to food manipulations. The biomass of most taxa increased in proportion of the overall increase in biomass of the common coarse—detritus consumers. The main exceptions to that pattern were a disproportionate increase in the percentage of the biomass represented by Brillia retifinis and a decrease in the representation of Zapada cinctipes. The responses of this community to food supplementation demonstrate food limitation of detritivores. The exponential increase in benthic biomass under food addition shows the scope for productivity if food were not limiting. These coexisting species benefitted from enhanced food supply on a spatial scale that is relevant for population level processes. The large seasonal variation in resource abundance under natural conditions creates bottlenecks during periods of low food supply, which constrain subsequent production even during periods when food is abundant.
Fish Community Structure in Dry‐Season Stream Pools
Authors: Capone Teresa A. , Kushlan James A.
Published: May 2006

Fish assemblage patterns were examined in 40 dry—season pools in a hydrologically variable river drainage in northeast Texas, to test the hypothesis that fish assemblage structure is correlated with environmental gradients of habitat characteristics. The study focused on two questions: 1. Do discrete, indentifiable fish assemblage patterns exist? 2. If so, how are they related to environmental conditions? The null hypothesis that fishes were randomly distributed among pools was rejected, indicating that assemblage patterns existed. Cluster analyses of the relative abundance of fish species detected three fish assemblages dominated by: mosquitofish; black bullhead; and sunfish—shiner—mosquitofish. Factor analysis reduced environmental variables to six factors representing pool depth, pool persistence, channel size, canopy cover, pool substrate, and pH, by which assemblages could be discriminated. These results demonstrated a relation between fish assemblage structure and environmental conditions in the pools. The discriminating variables varied along a gradient, and fish assemblages varied similarly. The study supported model predictions for gradient trends in species richness, species composition, fish size, and fish density, but not for species density.