bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of an Introduced Bacterium on Bacterial Communities on Roots
Authors: Gilbert Gregory S. , Parke Jennifer L. , Clayton Murray K. , Handelsman Jo
Published: May 2006

The objective of this work was to determine whether introduction of a bacterium altered microbial communities associated with roots. We conducted experiments in the field and in a growth chamber to determine whether coating soybean seeds with marked strains derived from the biological control agent Bacillus cereus UW85 affected the bacterial community in the rhizosphere of soybeans. We characterized 2651 individual isolates of bacteria from root—free soil and from soybean seedlings based on 43—50 physiological attributes. Discriminant analysis of the bacterial communities according to these attributes showed that the communities of rhizosphere bacteria that developed on non—treated plants and on plants grown from seeds coated with a single strain of bacteria were sometimes dramatically different. This occurred even when the introduced strain did not persist as a common member of the community. In two of four experiments we could as easily differentiate between bacterial communities on roots of UW85—treated and non—treated seedlings as between the communities in the rhizosphere and root—free soil. In the other two experiments we could differentiate only between communities in root—free soil and on roots. In the comparison of bacteria from root—free soil and from the soybean rhizosphere, we found that bacteria from root—free were more likely to degrade complex carbon sources than were rhizosphere bacteria, whereas bacteria from roots were resistant to more antimicrobial substances and were able to grow on a wider range of simple carbon sources than were bacteria isolated from root—free soil.
Niche Breadth and Life History Variation in a Hybrid Daphnia Complex
Authors: Weider Lawrence J.
Published: May 2006

Laboratory life table experiments using multiple clones of Daphnia galeata, D. hyalina, and D. galeata x hyalina hybrids quantified differences in niche breadth among these taxa along a gradient of food carbon levels (0.2—2.0 mg/L). Two—way analysis of variance revealed significant main effects (i.e., species, food level) for all life history features examined, as well as significant species x food level interactions for several life history characters (e.g., age at first reproduction and intrinsic rate of increase, r). Nested analysis of variance revealed significant species level effects for all life history characters examined, as well as significant clone level effects for all characters except r. Overall, species level effects explained approximately twice the proportion of the total variance for individual traits when compared with clone level effects (means of 32.8 and 15.8%, respectively). Significant interspecific differences in breadth of response (i.e., niche breadth) were observed for estimates of r. D. galeata exhibited a significantly greater niche breadth across food treatments than D. hyalina; D. galeata x hyalina showed an intermediate response breadth, which was not significantly different from either parental species. The existence of considerable intraspecific (i.e., clonal) variation in several life history traits, implies that any interpretation of life history variation among members of hybrid species complexes may depend critically on the genetic (clonal) make—up of the populations examined. These results are discussed in reference to the coexistence of these taxa in nature.
Mate‐ and Oviposition‐Influenced Host Preferences in the Coral‐Feeding Snail Cyphoma Gibbosum
Authors: Nowlis Joshua P.
Published: May 2006

Mate—searching has been hypothesized to influence the host preferences of small grazers, yet no previous field data have examined this idea. I examined the host preferences of Cyphoma gibbosum, a generalist gastropod that feeds on gorgonian corals, and tested if mate searching or oviposition preferences contributed to variation in host preferences. I constructed experimental reefs containing three species of gorgonian corals at uniform densities and placed snails on these reefs either alone or in group of four. Half of the groups of snails were single—sexed while the other half contained two snails of each sex. I observed snail movements by conducting daily censuses of three reefs and constructed transition matrices describing the movements of snails in each treatment among the three species of coral. Snails showed preferences for some hosts over others, and these preferences varied due to both mate searching and oviposition. Snails preferred to occupy Briareum asbestinum over Plexaurella dichotoma or Pseudopterogorgia acerosa, but these preferences were strongly influenced by previous experience. Moreover, females tended to move from B. asbestinum to Plexaurella dichotoma to oviposit, and back to B. asbestinum afterwards. Finally, both males and females moved frequently towards members of the opposite sex, in order to copulate, which influence their distribution among hosts. These data reveal that behaviors associated with mating can influence host preferences and need to be considered in understanding host utilization patterns.
Elevated Herbivory in Plant Hybrid Zones: Chrysomela Confluens, Populus and Phenological Sinks
Authors: Floate Kevin D. , Kearsley Michael J. C. , Whitham Thomas G.
Published: May 2006

Using the free—feeding beetle Chrysomela confluens (Coleoptera: Chrysomelidae), we examined the hypothesis that naturally occurring hybrid zones between two cottonwood species are "sinks" or centers of insect abundance. Over a 3—yr period, we found that 94% of the C. confluens population was restricted to a 13—km hybrid zone, which represents <3% of the cottonwood population. Of several potential mechanisms tested, expanded host phenology in the hybrid zone, relative to pure zones, best explained the distribution of C. confluens which is dependent upon newly flushed leaves for optimal growth and reproduction. The hybrid zone is a superior beetle habitat because: (1) early leaf flush in the hybrid zone provides the first source of food for beetles in spring and (2) staggered leaf phenologies in the hybrid zone allow beetles to shift only newly flushed Fremont trees as foliage of sympatric hybrid and narrowleaf trees declines in quality. This shift by ovipositing females can result in a 600% increase in fecundity relative to nonshifting females. Additionally, 10—yr common garden experiment shows that there is a strong genetic component to the timing of leaf flush, which suggests a long—term stability of resources in the hybrid zone that beetles can rely on. For C. confluens, the hybrid zone is a "phenological sink" that increases beetle fecundity and leads to chronically high herbivory year after year. Because movement from the hybrid zone reduces fecundity, emigration into adjacent pure zones is likely hindered. This hypothesis requires staggered phenologies between overlapping host species; it probably does not require hybridization between host species. Further, this hypothesis may in part explain frequent outbreaks of insects in nurseries and plantations. Thus, two distinct mechanisms have been identified that result in increased insect abundance in plant hybrid zones; "phenological sinks" resulting from staggered plant phenologies and "hybrid sinks" resulting from increased susceptibility of hybrids to insect attack. Such studies suggest that hybrid zones are important natural laboratories for the study of plant—herbivore interactions.
Sex and the Single Mustard: Population Density and Pollinator Behavior Effects on Seed‐Set
Authors: Kunin William E.
Published: May 2006

The local density of a plant population can influence both the number of pollinators attracted and the behavior (and thus pollination efficiency) of those that are. I present the results of a field experiment performed on Brassica kaber, a self—incompatible annual crucifer, designed to test for the existence of density effects in pollination and to separate the two major component effects: reductions in the quantity and in the quality of pollinator services. B. kaber plants were grown in fan—shaped density arrays in six floral backgrounds chosen to manipulate pollinator foraging behavior. Pollinators visiting focal plants behaved as generalists in backgrounds of the similar—flowered Brassica hirta, behaved as floral specialists where no alternative floral resources were provided (three treatments), and displayed a mixture of behaviors in two background plantings where dissimilar flowers were available. Pollinator visitation rates to focal B. kaber plants declined sharply at low density in all backgrounds except the similar flower (B. hirta) treatment. Widely spaced plants displayed significantly reduced seed—set in all backgrounds studied. The most dramatic density effects occurred where pollinators were induced to behave as generalists, suggesting that density—related declines in pollinator quality are more important than parallel declines in the quantity of visits.
Influence of Plant Allelochemicals on the Tobacco Hornworm and its Parasitoid, Cotesia Congregata
Authors: Barbosa Pedro , Gross Paul , Kemper John
Published: May 2006

Experiments were conducted to determine if three plant defenses affected the parasitoid Cotesia congregata in a fashion that would influence herbivore—parasitoid interactions. We evaluated the developmental rate, size, and survival of the parasitoid Cotesia congregata, when reared from Manduca sexta fed on diets containing either nicotine, rutin, or hordenine. These results were compared to the effects of the same three chemicals on unparasitized M. sexta. The influences of the three plant defenses differed. In general, however, the effects of these allelochemicals on the parasitoid paralleled those on the unparasitized hornworn. The one major exception was that concentrations of nicotine that had little or no effect on the hornworm caused significant mortality of parasitoids. Neither rutin nor hordenine had large or consistent negative effects, although rutin had a pronounced effect on both hornworm and parasitoid developmental times.
Seed Entrapment in Alpine Ecosystems: Effects of Soil Particle Size and Diaspore Morphology
Authors: Chambers Jeanne C. , MacMahon James A. , Haefner James H.
Published: May 2006

The spatial distribution of diaspores in seed banks can be significantly affected by physical processes that act on diaspores after they reach the soil surface. We examined how diaspore morphology and soil particle size affect diaspore incorporation into soil in a disturbed alpine ecosystem on the Beartooth Plateau, Montana, USA. Diaspores of alpine species with varying morphology were sown over soils of five different particle sizes and later collected from three depths. Regardless of diaspore morphology, the total number of diaspores trapped increased with increasing particle size until a threshold soil particle size was reached above which no more diaspores were trapped. At small particle sizes (0.5—1.0 and 1.0—2.0 mm) small diaspores and diaspores with adhesive seed coats were trapped, but most large diaspores moved horizontally across the surface and were not trapped. The majority of those diaspores trapped were at the 0—1 cm depth at small particle sizes. At large particles sizes (2.0—4.0, 4.0—8.0, and 8.0—16.0 mm) high numbers of large diaspores were trapped, and more diaspores moved vertically through the soil column. In small particle size soils small diaspores reached greater depths than large diaspores. Diaspores with adhesive seed coats responded more like large diaspores in terms of vertical movement. Diaspore length and eccentricity (length/width) were highly correlated with entrapment at small particle sizes and appeared to have the greatest effect on horizontal movement. Mass and width were significantly correlated with numbers of diaspores trapped in large particle sizes and were influencing vertical movement. Models based on the Weibull probability distribution were used to describe diaspore "survival" on the soil surface and to describe vertical movement in the soil column. This study indicates that on exposed soils in windy environments diaspore morphology and soil particle size greatly affect the spatial distribution of diaspores in seed banks. For diaspores of a given species, the optimal soil type traps a high number of diaspores but precludes significant downward movement.
Effects of Avian Predation on Grasshopper Populations in North Dakota Grasslands
Authors: Fowler Ada C. , Knight Richard L. , George T. Luke , McEwen Lowell C.
Published: May 2006

We experimentally studied the effects of avian predation on grasshopper abundance in western North Dakota during the summers of 1988 and 1989. Grasshopper densities in 15 ° 15 m plots from which birds were excluded (NO BIRDS) were compared with similar—sized plots where birds were allowed to forage (BIRDS). Plots were established in early June at 16 sites (8 per year), and grasshopper densities were estimated from hoop counts in NO BIRDS and BIRDS plots at 2—wk intervals until the end of July. There were significantly more grasshoppers in NO BIRDS plots than in BIRDS plots in 1989 (P < .0001), but not in 1988 (P = .137). The difference between treatments in 1989 was first detected 6 wk after the exclosures were erected. Between years there were no differences in initial grasshopper abundance in all treatments (P > .388). After the final hoop count each year, sweep—net sampling was also used to estimate grasshopper densities. By this method, average grasshopper density in late July was 26% and 37% lower in BIRDS plots than in NO BIRDS plots in 1988 and 1989, respectively. Average length, total biomass of grasshoppers, species richness, and species diversity, however, did not differ between the treatments.In 1988, 2 of 15 grasshopper species were significantly more abundant in the NO BIRDS plots. There was no difference between the treatments among 16 species identified in the 1989 samples, but the power of our tests to detect differences for individual species was low. Our results support the hypothesis that avian predation reduces insect populations at low and moderate densities.
Enhanced Foraging Efficiency Through Information Centers: A Benefit of Coloniality in Cliff Swallows
Authors: Brown Charles R.
Published: May 2006

Cliff Swallows (Passeriformes: Hirundo pyrrhonota) in southwestern Nebraska, USA, nest in colonies that serve as information centers in which unsuccessful individuals locate and then follow successful individuals to aerial insect food resources. I investigated to what degree colony size affected the efficiencies at which Cliff Swallows foraged for food, and to what degree information transfer among colony residents might represent a benefit of living in colonies. The number of Cliff Swallows departing from a colony each hour to forage increased significantly with colony size, meaning that individuals did not wait as long to locate appropriate foraging associates in large colonies as in small colonies. Waiting intervals between an individual's arrival at its nest and its departure on its next foraging trip increased as the number of birds nesting in a colony declined. Parental Cliff Swallows in large colonies returned with food for their nestlings more often and brought more food per trip, than did parental Cliff Swallows in small colonies. As a result, nestling body mass at 10 d of age increased significantly with colony size, when the confounding negative effects of ectoparasites on nestling body mass were removed by fumigating nests. Adult Cliff Swallow body mass late in the nesting season during the period of feeding of nestlings increased significantly with colony size even in the presence of blood—sucking ectoparasites (in nonfumigated colonies). Experimental reduction of large Cliff Swallow colonies to the approximate size of small colonies suggested that nestling and adult body mass in these colonies did not vary with colony location, meaning that enhanced foraging efficiency of birds in larger colonies was probably attributable to more efficient transfer of information among the larger number of colony residents and was probably not attributable to differences in local resource characteristics near colonies of different sizes. Enhanced foraging efficiency through information centers is likely a major benefit of coloniality for Cliff Swallows.
Herbivore Grazing Increases Polyphenolic Defenses in the Intertidal Brown Alga Fucus Distichus
Authors: Van Alstyne Kathryn L.
Published: May 2006

Although predator—induced defenses have been reported for several species of terrestrial vascular plants, they have not been previously described in aquatic or nonvascular plants. In this study, field manipulations were used to demonstrate the presence of inducible chemical defense production in the intertidal brown alga Fucus distichus. When experimentally damaged, Fucus increased its concentrations of polyphenolic compounds by °20% over uninjured control plants within 2 wk. These increases occurred in the area where the plant was injured and within adjacent undamaged branches. The increase in concentrations of polyphenolic compounds in clipped plants in these experiments corresponded well with differences in phenolic levels in naturally grazed and ungrazed algae. Herbivorous snails (Littorina sitkana) showed a preference for clipped plants immediately after they were wounded. However, over a 2—wk period the snails shifted their preference towards the uninjured control plants, corresponding with the increase in polyphenolic levels within the experimentally damaged plants. L. sitkana spent less time feeding on clipped plants, and these plants lost °50% less tissue (by surface area) to grazers than did uninjured algae. The presence of induced defenses in algae causes plant quality to vary spatially and temporally. This may result in variation in intra— and interspecific food preferences of herbivores, and ultimately may affect benthic algal community structure.
Flowering Phenologies: Bog, Woodland, and Disturbed Habitats
Authors: Heinrich Bernd
Published: May 2006

The following phenology, in Maine, of 48 species of native biotically pollinated angiosperms from bogs, and 44 native species from woodland is compared with the pattern of blooming among 92 species (31% exotic) from disturbed habitats. Apportionment of the temporal blooming niches of the component plant species varied significantly (p < .005) between the three habitats. Flowering in bogs was spread fairly evenly throughout the growing season. In woodland, flowering occurred primarily before the trees leafed out, while in disturbed habitats blooming peaked in midsummer. Wind—pollinated trees and shrubs of all three habitat types had overlapping blooms in early spring. The blooming patterns of 1972 are compared with those of a few indicator species for 1973, 1974. 1975, and early spring 1976. The three types of habitats, having different presumed selective pressures on blooming time, served as mutual controls in the analysis of blooming times from the community perspective.
Areal Distribution and Stratigraphy of Diatoms in the Sediments of Lake Sallie, Minnesota
Authors: Bradbury J. P. , Winter T. C.
Published: May 2006

The patterns of modern diatom sedimentation in presently eutrophic Lake Sallie, Minnesota, USA, have aided in elucidating the trophic changes in the lake caused by man's activities since the 1870s. The modern sedimentation patterns were determined from a transect of surface sediment samples taken through littoral and profundal zones of the lake and the trophic history is interpreted from the diatom content of two lake—sediment cores. The transect shows that, in this lake, diatom valves are mostly deposited in the area of their living habitats and are not homogeneously redistributed by lake currents. Planktonic diatoms Stephanodiscus sp. cf. S. hantzschii, Stephanodiscus niagarae, and Melosira granulata dominate in the profundal sediments, and sessile Fragilaria species characterize the sediments beneath the littoral habitat. Fragilaria construens var. binodis prefers nutrient—rich water entering the littoral zone near the inflow of the Pelican River, a stream carrying secondary sewage effluent from the town of Detroit Lakes. The diatom stratigraphy indicates that the benthic and planktonic diatom communities have changed in species composition and probably in productivity as a result of cultural eutrophication. Spring—blooming Stephanodiscus sp. cf. S. hantzschii dominates the post—settlement record. A core near the Pelican River inflow shows a marked increase in Fragilaria construens var. binodis that probably reflects increased nutrient levels during the most recent history of Lake Sallie
Importance of Vegetative Cover to Cycles of Microtus Populations
Authors: Birney Elmer C. , Grant W. E. , Baird Donna Day
Published: May 2006

A hypothesis is presented that a site—dependent threshold level of vegetative cover is necessary for a population of Microtus to increase in numbers sufficiently to undergo a multi—year cycle. The hypothesis is supported by results of studies in grassland habitats in the tallgrass prairie of Oklahoma and Minnesota, the mixed grass prairie of South Dakota, and the shortgrass prairie of Colorado. Levels of cover below the threshold sometimes support resident, breeding populations of voles. Levels above the threshold may influence amplitude, duration, and synchrony of Microtus cycles.
Optimal Foraging Strategy of the Sheet‐Web Spider Lepthyphantes Flavipes Under Perturbation
Authors: Leclerc Jacques
Published: May 2006

Field work concerning a sheet—web spider, Lepthyphantes flavipes (Linyphiidae), found on the upper surface of a deciduous forest litter indicates that a physical perturbation like falling leaves has a major effect on spider movement. The proposed optimal foraging model using stochastic dynamic programming includes perturbation movement as opposed to foraging movement between web sites, with variable prey availability in space and time. The two possible strategies (fixed—site strategy or mobile strategy) are constrained by different energy costs and predation risks. Decision is base on prey availability of an occupied web site and on internal fat reserves. It is found that a fixed—site strategy is optimal in good web sites (better than the habitat mean) and in poor web sites when spiders have high fat reserves. A mobile strategy is optimal in poor web sites at low reserves. An optimal mobile strategy is much less profitable than an optimal fixed site strategy, but the latter is interrupted by perturbation. The model may explain the prevalence of the fixed—site strategy in L. flavipes and in Linyphiidae in general.
Effects of Cannibalism and Alternative Prey on Population Dynamics of Saduria Entomon (Isopoda)
Authors: Leonardsson Kjell
Published: May 2006

Field sampling and laboratory experiments were performed to investigate the importance of cannnibalism in the isopod Saduria entomon from the Gulf of Bothnia. In the experiments the vulnerability to cannibalism was high for small individuals. The vulnerable phase lasted °9 mo in the deep zone and 3 mo in the shallow zone. Large S. entomon individuals caught small conspecifics more efficiently than they caught the alternative prey Pontoporeia affinis (Amphipoda), and Neomysis integer (Mysidacea). Both relative and absolute densities affected the number of small conspecifies eaten. The preference for conspecifics increased with decreased with decreasing density of P. affinis. In a behavioral experiment a small but significant proportion of the small S. entomon specimens actively sensed and avoided aggregations of large conspecifics. In contrast to the findings of the laboratory experiments, only a small amount of S. entomon were found in the guts of large conspecifics from the field samples. Pontoporeia affinis was the dominant prey. Due to differences in the life history of S. entomon, conditions for cannibalism differ between shallow and deep zones, favoring higher rates of cannibalism in the deep zone. In the deep zone there were lower densities of alternative prey per predator, the vulnerable phase was three times as long, and the cannibals reached larger sizes. Calculations using information from laboratory data showed that cannabalism has the potential to cause substantial mortality in S. entomon in the deep zone also when inflated mortality rates were accounted for. In the field, the density of small S. entomon was negatively correlated with the density of large conspecifics, but positively correlated with the density of P. affinis. The size distributions of the field samples indicated a high mortality in deep—living young S. entomon. These patterns were consistent with the results of the laboratory predation experiments, which are consistent with the hypothesis that cannibalism produced these patterns.
Food and Shelter as Determinants of Food Choice by an Herbivorous Marine Amphipod
Authors: Duffy J. Emmett , Hay Mark E.
Published: May 2006

Because food and habitat are closely linked for small herbivores that live on plants, food choice in the field may be constrained by the need to choose plants that provide safe living sites. We investigated the importance of food value and refuge value in determining the plant utilization patterns of the herbivorous marine amphipod Ampithoe longimana. When offered a choice of five common seaweeds, this amphipod fed most readily on Dictyota and Hypnea and less readily on Sargassum, Chondria, and Calonitophyllum. Rates of feeding on the different seaweeds were unrelated to seaweed gross morphology, toughness, nitrogen, or protein content. When cultured on each of these seaweeds in the laboratory, amphipod survivorship was high on Dictyota (82%), intermediate (35 and 18%, respectively) on Sargassum and Hypnea, and low (0%) on the other seaweeds. Survivorship on the different diets was strongly correlated (r = 0.930) with algal protein content; however, neither protein content nor amphipod performance of the different diets was significantly related to feeding rates on those diets. Additionally, amphipods from the three seaweed species that produced some survivors did not differ in growth rate, fecundity, egg size, or age at first ovulation. Variance in survivorship, and related measures, among sibling groups of amphipods suggested that this amphipod population possessed heritable variation for performance on the different seaweed species. In the field, abundance of A. longimana on the different species of algae was more clearly related to the preference of omnivorous fishes for these algae than to feeding rates of the amphipods when given those algae in the laboratory.A. longimana was more abundant on Dictyota and Sargassum (both unpalatable to omnivorous fishes), than on Hypnea, Chondria, and Calonitophyllum (all of which are palatable to fishes). During the season when omnivorous fishes were abundant, density of A. longimana increased on Dictyota, which is chemically defended from fishes, but decreased or remained unchanged on the seaweeds that are more palatable to fishes. Competition with other amphipods as a group did not appear to explain the distribution of A. longimana among seaweeds, since there were no negative correlations between A. longimana abundance and total amphipod abundance in any month. The lack of any consistent relationship between host—plant use in the field and either feeding preference or diet value, as measured by survivorship and reproduction, suggests that host—plant use by A. longimana may be strongly constrained by requirements for shelter from predation.
Facilitation and Interference of Quercus Douglasii on Understory Productivity in Central California
Authors: Callaway Ragan M. , Nadkarni Nalini M. , Mahall Bruce E.
Published: May 2006

Controversy over the effect of Quercus douglasii on the productivity of California grassland has been fueled by conflicting reports. In some studies, understory grassland productivity was < 25% of open grassland productivity, whereas in other studies understory productivity exceeded 200% of that of surrounding grassland. We examined light, temperature, soil nutrients, soil moisture, and fine tree root distributions under selected Q. douglasii trees (12 that appeared to have suppressive effects on understory productivity, and 12 that appeared to enhance understory productivity) in order to determine how variations in these factors were associated with the differences in understory grassland productivities. We found that grassland productivity is likely to be facilitated by nutrients inputs via litterfall and throughfall under all trees, but that trees with low understory grassland productivities has substantially higher amounts of fine roots in the upper 50 cm of soil and much lower predawn xylem pressure potentials than trees with high understory productivities and presumably deeper root systems. Root exclosures reduced the negative effects of these trees on the dominant understory grass species, and further experiments indicated that the negative effects of the tree roots may partially result from allelopathic oak root exudates. Thus, shallow fine tree roots may inhibit understory productivity, and variations in Q. douglasii root morphology may explain the intertree variations in facilitating/interfering effects on understory species.
Disturbance Regimes of Midsuccessional Old Fields
Authors: Goldberg Deborah E. , Gross Katherine L.
Published: May 2006

We monitored the dynamics of natural gaps in vegetation in permanent plots in three old—field communities for 3 yr. The vegetation in all three fields was dominated by long—lived clonal perennials. Total area in gaps with exposed soil was very low, with <0.2% of the area of each field in gaps ≥10 cm in diameter (our minimum size). Most gaps were small; only 5 to 21% of all gaps found were °20 cm in diameter. Most (95%) of the new gaps were created by animal activity, with moles being the most common disturbance agent. There was considerable variation in disturbance and closure rates over space and time that resulted in differences in the availability of gaps within fields, among fields, among seasons, and among years. Within each field, disturbance rates were higher but closure rates tended to be lower in more xeric areas of the field, resulting in higher availability of gaps in the more xeric areas at each census. Within each year, both disturbance and closure rates were higher during the growing season than during the winter. These two trends seemed to balance each other, resulting in little change in the availability of gaps over the year. Disturbance rates did not differ among years, but closure rates were higher in a year with higher precipitation, resulting in a lower availability of gaps in the wetter year. Overall these gaps appear to be too rare and too small to play an important role in the persistence of the dominant perennials in the old—field communities we studied, many of which require large gaps to establish from seed. However, they may be important for the persistence of annuals with seed banks in these communities. Such gaps may also influence the rate of succession in these fields by influencing the pattern of colonization by woody plants.
Direct and Indirect Effects of Prior Grazing of Goldenrod upon the Performance of a Leaf Beetle
Authors: Brown D. Gordon , Weis Arthur E.
Published: May 2006

A plant's grazing history can influence the nutritional quality of its leaves and directly affect herbivore performance. In addition, leaf nutritional quality can indirectly affect herbivore performance by influencing food consumption. We examined the direct and indirect influences of prior grazing of Solidago missouriensis upon leaf nutritional quality and the growth and fecundity of Trirhabda canadensis (Coleoptera: Chrysomelidae). We fed leaves from both previously grazed (during 1988 and 1989) and ungrazed goldenrod plots derived from a single S. missouriensis clone to larvae and adults of T. canadensis in laboratory performance trials. We computed standard insect performance indices and used structural equation modeling and path analysis to interpret the results of these trials. Prior grazing of goldenrod had a direct negative impact upon the growth of T. canadensis larvae in spring of 1990; a concomitant reduction in foliage consumption accentuated this effect. Larvae fed leaves from grazed plots accumulated biomass and N more slowly, used them less efficiently, and reached a lower final mass than did larvae fed leaves from plots without prior folivory. Goldenrods produced a new flush of leaves following larval grazing in the spring of 1990. Therefore, adults feeding in plots defoliated by larvae ate younger leaves than those feeding in undefoliated plots. Leaves from defoliated plots had direct positive impacts upon growth and egg production of adult T. canadensis. Adults converted these young leaves into biomass with greater efficiency and preferred them to older leaves from ungrazed plots in choice tests. However, negative indirect effects of prior grazing upon initial adult mass and total foliage consumption negated any positive influence that these more nutritious leaves had upon growth, and made the overall impact of prior grazing upon egg production negative. These results indicate that impacts of food availability and quality must be considered jointly to understand how prior grazing influences herbivore performance.
Nested Species Subsets and Geographic Isolation: A Case Study
Authors: Kadmon Ronen
Published: May 2006

Insular systems tend to exhibit nonrandom patterns of species composition in which smaller species assemblages contain successive subsets of those found on richer ones. The fact that such nested patterns of species composition have usually been observed in systems where species number is correlated with island size has led to the hypothesis that differences among species in the effect of island size on the probability of extinction represent the main mechanism producing nested patterns of species composition. This study tested the hypothesis that differences among species in dispersal ability may interact with geographic isolation to produce nested subsets of species composition. This hypothesis was tested in a systems of woody plants inhibiting artificial islands that were created by the filling of the Clarks Hill Reservoir, Georgia, USA. All of the studied islands had been logged prior to their separation from the mainland and their species number 34 yr later was found to be negatively correlated with distance to the closest mainland. The results of the present analysis indicated that (1) species occupancy of the whole island system was highly nested, (2) nestedness occurred with respect to geographic isolation but not with respect to island size, (3) only a small portion of the overall species included in the study contributed to the observed nestedness, and (4) the degree to which individual species contributed to the observed nestedness was correlated with their dispersal properties, with wind—dispersed species showing no evidence for nestedness and species lacking adaptations for long—range dispersal showing the strongest patterns of nestedness. These results indicate that isolation effects may interact with species—specific dispersal properties to produce nested subsets of species composition.
Plant Species Richness on Islands Over a Century of Primary Succession: Lake Hjalmaren
Authors: Rydin Hakan , Borgegard Sven-Olov
Published: May 2006

Between 1882 and 1886 the water level of Lake Hjalmaren, Sweden, was lowered °130 cm. As a consequence, a large number of new islands arose. Lists of vascular plant species have been obtained for such islands of size range 50 m2 to 2.5 ha in 1886, 1892, 1903—1904, 1927—1928, and 1984—1985. Species—area relationships were slightly better described by the exponential S = C + z log A than with the power model S = CAz (fitted with nonlinear regression) in all stages of succession. There were no clear trends in the shape of the species—area curve over time. Except for the 1886 survey all slopes of the power model included the theoretically suggested value of 0.26 in their confidence intervals, indicating that this value has no obvious significance in relation to equilibrium or succession. On large islands (>0.3 ha) species number had reached at equilibrium at °95 species with immigration of °30 species balancing extinction between the last two surveys. Indications of interactions that led to decreases in species richness at late successional stages are weak. Small islands still continue to accumulate species, mainly due to substrate stabilization, which leads to formation of habitats for plant establishment. A stepwise regression indicates over the whole successional sequence log area as more important for variation in species number than descriptors of island shape, habitat diversity, or location. However, for individual islands the residuals in the species—area regression could be explained by special habitat features.
Succession in Old‐Field Plant Communities: Effects of Contrasting Types of Nutrient Enrichment
Authors: Carson Walter P. , Barrett Gary W.
Published: May 2006

We investigated the effects of monthly nutrient applications on succession in two old—field plant communities. Succession was monitored for 3 yr in 1—yr (younger) and 4—yr (older) experimental plots. Three 0.1—ha plots in each old field were treated with sludge, three with fertilizer, and two were left as untreated controls. In the younger community, herbaceous perennials and winter annuals replaced summer annuals by the 3rd yr of succession in control plots. Summer annuals, however, dominated enriched plots throughout the study. Species richness was significantly higher in enriched plots than in control plots during the 1st yr, but was significantly lower than control plots thereafter. Annual aboveground net primary productivity (ANPP) was significantly greater in enriched plots than in controls in the 1st yr, but the converse occurred in the 3rd yr. The type of nutrient enrichment affected ANPP; sludge plots had significantly lower ANPP than fertilizer plots in the 1st and 3rd yr. In the older community, two summer annuals and a biennial displaced dominant perennial grasses in enriched plots. In contrast to the younger community, ANPP was consistently greater in nutrient—enriched plots than in controls, and nutrient enrichment did not alter species richness. The type of nutrient enrichment affected the older community in an opposite manner from the younger community; sludge plots had significantly greater ANPP than fertilizer plots in the 3rd yr. We concluded that the age and physiognomy of the old—field community, the type of nutrients applied, and the duration of enrichment, each influenced the course of succession; responses observed in the 1st yr of enrichment were not indicative of later trends.
Persistent Differences in Two Forms of Spartina Alterniflora: A Common Garden Experiment
Authors: Gallagher John L. , Somers G. Fred , Grant Donna M. , Seliskar Denise M.
Published: May 2006

Tall— and short—form Spartina alterniflora plants were transplanted from the Delaware marsh to common garden plots during the summer of 1978 and irrigated with saltwater three times a week during that and each subsequent growing season. Nine years later plant biomass, culm height, density, and diameter and flowering frequency remained distinct. Underground regrowth reserves and root profiles likewise remained distinct for the two forms. Since the two growth forms had been living in the same environment for 9 yr and had retained many ological and physiological differences, some genetic control of the morphology and physiology of the two growth forms is highly probable.
Nitrogen Limitation of Production and Decomposition in Prairie, Mountain Meadow, and Pine Forest
Authors: Hunt H. W. , Ingham E. R. , Coleman D. C. , Elliott E. T. , Reid C. P. P.
Published: May 2006

The responses of decomposition and primary production to nitrogen supply were investigated in a shortgrass prairie, a mountain meadow, and a lodgepole pine forest. Nitrogen (N) supply was increased by applying ammonium nitrate, or decreased by applying sucrose. The litterbag technique was used to follow decomposition of leaves of the dominant plants: blue grama (Bouteloua gracilis) from the prairie, western wheatgrass (Agropyron smithii) from the meadow, and lodgepole pine (Pinus contorta) from the forest. Soil from beneath the litterbags was sampled at the time of litterbag retrieval in order to detect interactions between decomposition and properties of the underlying soil. There was no consistent effect of soil properties on decomposition rate, but there was a significant effect of litter type on N mineralization in the underlying soil. Decomposition was fastest in the forest, intermediate in the prairie, and slowest in the meadow. Blue grama decomposed faster than the other litters. Each litter type decomposed faster than expected when placed in its ecosystem of origin. This interaction suggests that decomposers in an ecosystem are adapted to the most prevalent types of litter. Nitrogen supply had a small but significant effect on decomposition rate. Within an ecosystem, there was a positive association between decomposition and accumulation of N within the litter, but this relationship was reversed when comparing across ecosystems, possibly because of the overriding effects of differences among ecosystems in abiotic factors. Aboveground net primary production was estimated in the grasslands by a single harvest at the end of the growing season, and growth increment of boles was measured in the forest. These indices of primary production showed a greater relative response to N fertilization than did decomposition, suggesting that primary production is the more N—limited process.
The Use of Regression for Detecting Competition with Multicollinear Data
Authors: Carnes Bruce A. , Slade Norman A.
Published: May 2006

Monte Carlo simulations were used to demonstrate that regression methods could be successfully used to estimate competition coefficients with collinear data, but only under conditions that may be difficult to meet with ecological data. Ordinary least squares performs well when estimating coefficients associated with noncollinear predictor variables. Stepwise regression and maximum eigenvalue least squares reduce collinearity by deleting information that, even though not statistically significant, may be important in accurately estimating interaction. We propose that apparent competition (Holt 1977) and the failure of regression techniques to detect competition when it is known to exist experimentally may be due to the omission or lack of measurement of critical elements of the community matrix.
Niche Separation of Two Damselfish Species by Aggression and Differential Microhabitat Utilization
Authors: Ebersole John P.
Published: May 2006

Two herbivorous, interspecifically territorial West Indian damselfishes, the beaugregory (Eupomacentrus leucostictus) and the three—spot damselfish (E. planifrons), both inhabit the back—reef area of coral reefs in a narrow depth band. A survey of coral heads in this habitat revealed that beaugregories utilize all types of heads, while three—spots are found only on branching or creviced heads with relatively complex internal structure. Translocations of individual fish and heterospecific pairs were performed to determine whether this differential microhabitat utilization is the result of differences in microhabitat preference between the two species, or aggressive interactions between them, or both. As indicated by duration of residence of translocated fish on experimental heads, three—spot damselfishes showed significant preference for heads with complex internal structure, while beaugregories showed no preference. In translocations of heterospecific pairs, three—spots won aggressive contests more often on internally complex heads, and beaugregories won more often on heads without internal structure, then would be expected on a random basis. Since this difference fell just short of significance (P = .053), translocations of heterospecific pairs were also done in aquaria with artificial heads. In these more controlled conditions, the preponderance of wins by three—spots on complex heads and beaugregories on simple heads reappeared as a significant difference, with occasional co—occupation of the complex head by both species. The observed differences in microhabitat utilization are thus due both to different preferences and aggressive interaction. It is probable that the aggressive interactions, a form of interference competition, and responsible for the evolution of the different habitat preferences. However, for both the three—spot damselfish and the beaugregory, it has been shown that a principal function of aggressive interspecific territoriality is the protection of food resources from exploitation competition.
A Determination of the Energetic Equivalence of the Risk of Predation
Authors: Abrahams Mark V. , Dill Lawrence M.
Published: May 2006

The influence of predation risk on patch choice was measured by examining the spatial distribution of 10 guppies (Poecilia reticulata) between two feeders, at one of which there was a risk of predation. The distribution was assumed to be ideal free. Nine unique situations were examined using all possible combinations of three risk levels and three diet levels, for each sex of guppy separately. Both sex and diet level influenced the effect of predation risk on patch choice. For the females the effect of risk was highest at the intermediate diet level. However, the males exhibited the opposite response: the effect of risk of predation was lowest at the intermediate diet level. A simple equation was then used to predict how much extra food (representing the energetic equivalent of risk) must be added to the risky patch for the guppies to become indifferent to the risk differences between the two types of patches. This manipulation caused a similar number of guppies to use both the risky and safe feeders, reducing or offsetting the influence of risk of predation. However, the male guppies were less influenced by this manipulation than were the females. The different results for the two sexes are consistent with known differences in their life histories, indicating that a knowledge of an animal's life history will often be necessary to understand how it makes trade—offs when choosing were to forage.