bluuebunny/crossref_metadata_2025 huggingface dataset:
Harvester Ant Foraging and Shrub‐Steppe Seeds: Interactions of Seed Resources and Seed Use
Authors: Crist Thomas O. , MacMahon James A.
Published: May 2006

Granivore—seed interactions involve a feedback between granivore seed selectivity and seed availability. We examined this feedback to determine how seed preferences by the western harvester ant, Pogonomyrmex occidentalis, related to seed availability and, in turn, affected the soil seed pool. Preferences were estimated from natural diets as well as from experiments that controlled seed size, relative availability, and distance from ant nests. Seed availability to ants varied with season and over 2 yr. Colony activity and seed intake rates were correlated with seed availability. Seed preference by ants was correlated with the seasonal availability of preferred species, but not with unpreferred seeds. From the soil seed pool, ants preferentially harvested small, sound seeds. They removed 9—26% of the potentially viable seed pool each year, and as much as 100% of available preferred species. Seed densities were lower 2—7 m from nests, where foraging activity was concentrated, than 7—12 m from nests. In controlled preference experiments. P. occidentalis was unselective near nests, but preferred large seeds with higher assimilable energy content in trials 10 m from nests. A relatively low foraging activity > 7 m, however, suggests that this distance—dependent preference is rarely manifested in natural conditions and does not measurably affect soil seed dynamics. Our results point to the importance of studying diet choice in a natural context; preferences measured under unexperimental conditions may not correspond to natural diets. Such discrepancies in food preference measurements will affect predictions about how consumers influence the population dynamics of resource organisms.
Proximate Limits to Seed Production in a Herbaceous Perennial Legume, Lathyrus Vernus
Authors: Ehrlen Johan
Published: May 2006

Resources, pollen, and predation limit fruit and seed set in Lathyrus vernus. Removal of flowers increased fruit and seed set among remaining ovaries and ovules, but did not affect seed mass. The distributions of the increased fruit and seed set among remaining ovaries indicated specific and variable patterns of translocation. The importance of pollen relative to resource deficiency varied between two experimental seasons, among individuals, and among flowers within shoots. Pollen supplementation increased fruit and seed set in one of two experimental seasons. When shoots were supplied with extra pollen a greater proportion produced fruits, but the number of fruits per fruit—producing shoot did not increase. Differences in fruit set among flowers with different phenology and different position within inflorescences persisted also among pollen—supplemented flowers. Furthermore, while pollen availability sometimes may limit fruit initiation, maturation seems to be limited primarily by resources. Genets that received supplementary pollen in 1988 produced fewer flowers in 1989, indicating that long—term fecundity is not necessarily pollen limited. Lastly, exclusion of the main ovary predators increased fruit set.
Prediction of Rarities from Habitat Variables: Coastal Plain Plants on Nova Scotian Lakeshores
Authors: Hill N. M. , Keddy P. A.
Published: May 2006

Predictive models relating species richness of rare plants to measured habitat variables were developed using data from the shoreline vegetation of lakes in southwestern Nova Scotia. These lakes contain large disjunct populations of many Atlantic Coastal Plain plant species whose main ranges are along the eastern seaboard of the United States. The richness of rare coastal plain herbs was easier to predict than richness of the "background flora" of wide—ranging species from noncoastal plain elements. Multiple—regression models using habitat variables accounted for 83% of the variability in species richness of rare coastal plain species but only 45% of that for the background flora. Richness was best correlated with the two inter—related variables, watershed area and shoreline width. The mechanism underlying this pattern appears to be that flooding kills woody plants, thereby reducing competition from shrubs and creating open expanses of shoreline.
Measuring Pattern Diversity
Authors: Scheiner Samuel M.
Published: May 2006

In this paper I develop a new viewpoint with regard to species diversity by defining a new type, pattern diversity, to add to the previous categories of inventory and differentiation diversity. By defining and naming this entity, we expand our notions of the properties of ecological units, can integrate it with other ecological concepts, and investigate it. Pattern diversity is a measure of the relative arrangement of subunits within an ecological unit, such as communities in a landscape. In turn, pattern diversity has three types: spatial, temporal, and compositional. Compositional pattern diversity is measured as mosaic diversity using a method called affinity analysis. Mosaic diversity measures landscape complexity and is a function of two properties of species patterns: the variation in species richness among communities and the variation in commonness or rarity among species (evenness). A low value of mosaic diversity is indicative of a simple landscape with one or a few underlying environmental gradients and dominanted by a few species; a high value is indicative of a complex landscape with many environmental gradients and no ubiquitous species. Statistical inference can be used for determining standard errors and for comparing with null models. Mosaic diversity is relatively insensitive to the number of sites sampled, the number of species sampled, and the intensity of sampling of rare species. Thus, pattern diversity and its measure–mosaic diversity–hold out promise to provide a new perspective on species diversity and the rules governing the assemblage of communities.
Effects of Settlement and Fire Suppression on Landscape Structure
Authors: Baker William L.
Published: May 2006

Natural landscapes subject to disturbances have a patchy structure that is important to many species living in these landscapes. This structure may be modified when the disturbance regime is altered by either climatic change or human influences (e.g., fire suppression), yet little is known about how this structure will change. I used a GIS (geographic information system)—based spatial model and data on historical changes in fire sizes and intervals to simulate the effects of settlement and fire suppression on the structure of the landscape in the Boundary Waters Canoe Area, Minnesota. I used seven measures to assess change in landscape structure. Settlement and fire suppression altered some but not all components of landscape structure. Settlement produced an immediate significant effect on some measures (age, shape, Shannon diversity, richness, and angular second moment), but no effect on other measures (size, fractal dimension). In contrast, suppression produced an immediate response in fewer measures (shape, Shannon diversity, richness), a delay for several decades in the case of some measures (age, fractal dimension), and a delay for hundreds of years in the case of other measures (size, angular second moment). Landscapes that have been altered by settlement and fire suppression cannot be restored using traditional methods of prescribed burning, which will simply produce further alteration. Causes of landscape change cannot be separated without control landscapes that lack prescribed burning, fire suppression, or other alterations of the natural fire regime.
Diet Choice and the Funcional Response of Beavers
Authors: Fryxell J. M. , Doucet C. M.
Published: May 2006

We investigated the effects of changing sapling availability on foraging selectivity and cutting rates by beavers in large experimental enclosures. As predicted by an energy—maximizing contingency model, the mean size of saplings cut by beavers increased with distance from the lodge and was positively correlated with sapling density. Species selectivity was also positively correlated with sapling density. Trembling aspen (Populus tremuloides) was preferred to speckled alder (Alnus rugosa), and red maple (Acer rubrum) was the least preferred species. The functional responses of beavers presented with saplings of a single species and of similar size differed from those recorded in trials with a single species of saplings of variable sizes and from trials with three species of saplings of variable sizes. Size—selective foraging by beavers reduced the maximum rate of sapling cutting at high sapling densities relative to the single—size trials, but both treatments showed similar cutting rates at low sapling densities. Species—selective foraging by beavers reduced cutting rates at low sapling densities relative to the single—species trials, but both treatments showed similar cutting rates at high sapling densities. Species—selective foraging introduced a slight inflection in the functional response curve for preferred species, which could have a stabilizing effect on trophic interactions.
Information Processing and Prey Detection
Authors: Dukas Reuven , Ellner Stephen
Published: May 2006

We present a model to examine how foragers should divide attention among different potential prey types in order to maximize their net rate of energy intake. We define attention as the brain's capacity to process information simultaneously. Our model is based on neuropsychological studies suggesting first, that predators who divide attention among an increasing number of different prey types decrease their ability to detect any given type, and second, that this decrease is larger when prey items are more difficult to detect. Our model also incorporates the effects of search rate on forager's probability to detect prey. The model predicts that foragers encountering cryptic prey should devote all attention to a single prey type. When encountering conspicuous prey, foragers should divide attention among the different prey types. For prey types that differ in conspicuousness but are equal in energy content, handling time, and density, foragers should give more attention to the more conspicuous prey if the conspicuousness values of all prey types are relatively small. However, when all prey are more conspicuous, foragers should devote more attention to the less conspicuous prey type. We suggest that our model may serve to explain and predict some of the foraging decisions of animals searching for cryptic or conspicuous prey, and especially studies on "search image" formation.
Absence of Trade‐Offs Between Sexual Size Dimorphism and Early Male Emergence in a Butterfly
Authors: Nylin Soren , Wiklund Christer , Wickman Per-Olof , Garcia-Barros Enrique
Published: May 2006

Protandry, here defined as the earlier emergence of males, is a common feature in life histories and could be the result of sexual selection on males to maximize matings, or alternatively and incidental by—product of other selection pressures on the sexes. If protandry is selected for per se, theory predicts that it should be associated with seasonal environments where there is little overlap between generations. The degree of protandry should be insensitive to environmental conditions. Moreover, on the assumption that males and females grow at the same rate as larvae, a trade—off between development time and size is expected to result in a strong association between protandry and female—biased sexual size dimorphism. These predictions were tested by a combination of comparative and experimental studies on five populations of the speckled wood butterfly, Pararge aegeria, from central and south Sweden, England, Spain, and the island of Madeira. Protandry was associated with seasonal environments, as it was only exhibited in the three northernmost populations. Protandry in these populations remained largely constant in a variety of temperatures, both under direct development, when protandry results from a sex difference in development time through the egg, larval, and pupal stages, and under diapause development, when it results from a sex difference in pupal development time only. These results indicate that protandry is selected for per se through sexual selection in seasonal environments. Similar female—biased size dimorphism occurred in protandrous and non—protandrous populations alike, and hence sexual size dimorphism in P. aegeria is not a result of selection for protandry, nor the causal factor behind protandry. Protandry and sexual size dimorphism appear to be largely decoupled traits in the life history evolution of P. aegeria. This is achieved by means of variation in pupal developmental time and variation in the relative growth rates of the sexes. Variation in growth rates is likely to be a general phenomenon and may make possible independent optimization of size and development time (age at sexual maturity), and accordingly influence expected patterns of size—related trade—offs.
The Effects of Complex Trophic Interactions on a Marine Microbenthic Community
Authors: Walters Keith , Moriarty D. J. W.
Published: May 2006

The consequences of complex trophic interactions for the dynamics of a natural microbenthic community from an intertidal Australian seagrass bed were examined during experiments in January, March, April, and June 1989. Effects of meiofauna and protist density manipulations on bacterial numbers and division rates and microalgal biomass were determined over 6 d each months. Results varied among experiments, microbial groups, and vertical positions within the sediment. Meiofaunal effects increased bacterial division rates but not densities in January subsurface and March surface sediments. In June, enhanced meiofaunal densities decreased subsurface but increased surface bacterial densities. A complex, tri—trophic—level interaction among meiofauna, protists, and bacteria probably caused the increased bacterial densities in June surface sediments. Meiofauna had no discernible effect on bacterial densities and division rates in January surface and March subsurface sediments even though relatively small (<50%) differences between treatments could be detected with reasonable statistical power. Changes in microalgal biomass could not be attributed to experimental treatments, and meiofauna likely have a limited effect on microalgae in this intertidal seagrass habitat. Results indicate that the combined effects of meiofauna can alter microbial densities and growth rates and significantly affect microbenthic community structure. Spatial and temporal variability in the nature of trophic interactions will determine the cumulative effects of meiofauna on microbial communities.
Vegetation Dynamics, Fire, and the Physical Environment in Coastal Central California
Authors: Callaway Ragan M. , Davis Frank W.
Published: May 2006

Current concepts of vegetation dynamics include that of the shifting landscape mosaic, but evidence for shifting mosaics in disturbed and undisturbed systems is primarily based on negative spatial relationships among adults and recruits, and not on measurements of actual shifts over time. We used aerial photographs to measure transition rates as evidence for mosaic shifts among grassland, coastal sage scrub, chaparral, and oak woodland communities in central coastal California between 1947 and 1989. In unburned plots without livestock, transition from grassland to coastal sage scrub was 0.69% per year, coastal sage scrub to oak woodland was 0.30% per year, and oak woodland to grassland was 0.08% per year. These transition rates, considered together, indicate that vegetation patterns may be dynamic on landscapes dominated by these communities. In burned plots without livestock, and in unburned plots where livestock were not excluded, transition rates were lower, except for the conversion of oak woodland to grassland. In burned plots, a high rate of transition of coastal sage scrub to grassland was measured. Markov chain models predicted much less directional change in community proportions in either grazed or burned conditions than in ungrazed, unburned conditions. Some transition rates varied with substrate and topographical position, indicating that fire, grazing, and the physical environment interacted to determine direction and rate of vegetation change. Variation in transition on different substrates suggests that only portions of the vegetation of these landscapes may be dynamic, with some patches in certain combinations of environment and disturbance that change rapidly, and other patches that remain static as edaphic or topographic climax communities.
Corvid Density and Nest Predation in Relation to Forest Fragmentation: A Landscape Perspective
Authors: Andren Henrik
Published: May 2006

The abundance and distribution of corvids (Common Raven, Corvus corax; Hooded Crow, C. corone; Jackdaw, C. monedula; Black—billed Magpie, Pica pica; and European Jay, Garrulus glandarius) were studied across a gradient from a landscape dominated by agricultural land to a landscape dominated by forest in south—central Sweden. The total density and the predation rate on dummy nests increased as the proportion of agricultural land increased. This supports the suggestion that the density of corvids increases as forest becomes fragmented and intermixed with agricultural land, causing an increase in nest predation in small forest fragments. However, the different species responded differently to the proportion of forest in the landscape. The two forest—living corvids, the Jay and Raven, were absent from small forest fragments in a matrix of agricultural land and mainly preyed upon dummy nests inside the large forest fragments. Thus, both of these species were habitat specialists restricted to forests. Three corvids basically inhabit agricultural land, the Hooded Crow, Jackdaw, and Magpie. The Jackdaw and the Magpie showed a strong preference for agricultural land and they mainly preyed upon dummy nests in agricultural land. On the other hand, the density of Hooded Crows was higher in landscapes with a mixture of agricultural land and forest than in landscapes dominated by either agricultural land or forest, indicating a use of both agricultural land and forest. Further, Hooded Crows regularly preyed upon nests in both forest habitat and agricultural land; it is a habitat generalist. Therefore, the Hooded Crow was the most important species in the corvid family, causing increased predation pressure close to forest—farmland edges and in small forest fragments surrounded by agricultural land.
Reproductive Energetics and Prey Harvest of Leach's Storm‐Petrels in the Northwest Atlantic
Authors: Montevecchi W. A. , Birt-Friesen V. L. , Cairns D. K.
Published: May 2006

Leach's Storm—Petrels (Oceanodroma leucorhoa), the smallest and most abundant seabirds that breed in the northwest Atlantic, are not usually included in avian energetics models. To round out an energy analysis of seabirds spanning the full range of body sizes in the northwest Atlantic, the field metabolic rates (FMRs) of breeding Leach's Storm—Petrels were measured using doubly labeled water techniques (DLW). Metabolic, dietary and, demographic data were used as inputs in a populations energetics model to estimate the energy requirements and prey harvests of the Leach's Storm—Petrel population of eastern North America. FMRs of storm—petrels that remained in burrows averaged 83.3 ± 19.4 KJ/d. FMRs increased significantly with time (i.e., x in hours per day) at sea (y = [85.8 ± 6.5] + [3.13 ± 0.48]x), and metabolism at sea was estimated to be 161 ± 18 KJ/d. FMRs integrated over activity on land and activity at sea averaged 124 KJ/d during incubation and 142 KJ/d during chick rearing. Incubating adults weighed significantly more than adults rearing chicks, although adult mass did not vary with age of chick. Basal metabolic rate (BMR, measured as O2 flux) averaged 2.02 ± 1.01 cm3°g—1°h—1, equivalent to 45.4 ± 30.4 KJ/d, very similar to values obtained for breeding conspecifics in the Bay of Fundy and in the Bering Sea. Thermal conductance averaged 0.124 ± 0.065 cm3°g—1°h—1°°C—1. FMR/BMR ratios averaged 2.73 for incubating birds and 3.13 for birds rearing young. Population energetics modelling indicated that during a 7—mo breeding period Leach's Storm—Petrels at colonies in the northwest Atlantic Ocean consume >74 000 Mg of prey, mostly myctophids and amphipods (mostly Hyperia galba), as wall as euphausiids (mostly Meganyctiphanes norvegica) and other prey (decapods, copepods, isopods). Leach's Storm—Petrels make up °80% of the breeding seabird population in eastern Canada, but owing to small body size and metabolic efficiency, they account for comparatively little of the energy that flows through the avian assemblage of the northwest Atlantic. An intra—specific, inter—colony difference in FMR is suggested: FMRs measured in a Newfoundland colony were significantly higher than those determined with the same DLW procedure in the Bay of Fundy near the southern limit of the species' breeding range in the northwest Atlantic. Differences in ambient temperatures and wind speed associated with different oceanographic regimes may generate energetic differences. Like inter—colony differences in feeding ecology, inter—colony differences in FMR need to be considered in population and community energetics models and in meso— and mega—scale extrapolations.
Fish Trophic Guilds and the Stucture of a Tropical Stream: Weak Direct vs. Strong Indirect Effects
Authors: Flecker Alexander S.
Published: May 2006

Freshwater fishes exhibit spectacular biodiversity in the tropics. In contrast to temperate streams, where insectivorous fishes generally predominate, detrital— and algalfeeding fishes are widespread components of tropical ichthyofaunas. I compared the direct and indirect effects of a variety of fishes in structuring a neotropical stream insect assemblage. An exclosure/enclosure experiment was preformed to examine the role of different fish trophic guilds and densities on patterns of patch colonization by invertebrates. Insectivorous characid fishes had relatively weak impacts on overall invertebrate abundance, although significant reductions were observed for selected taxa (the mayflies Baetis sp. 2 and Leptohyphes, and the caddisflies Smicridea and Alisotrichia). This contrasted sharply with the strong effects of the grazing armored catfish Chaetostoma milesi and the detritivore Prochilodus mariae, which caused significant reductions in abundance of common taxa throughout the invertebrate assemblage. In addition, grazers and detritivores substantially reduced the amount of sediments and associated detritus accumulated on stream bottom substrata. A second experiment was preformed to evaluate the mechanism by which grazing and detritivorous fishes influence insect abundance. Insect abundance was compared between a treatment where resource depression and bioturbation was simulated by removing sediments manually to fish exclosure/enclosure treatments. As in the previous experiment, grazers and detritivores significantly reduced the abundance of many common invertebrate taxa, as well as detrital and algal resources. However, taxon—specific differences in colonization of the sediment—removal treatment suggest that several factors are important in explaining responses to fishes by insects. Many common taxa displayed decreases in abundance both in sediment—removal and fish treatments, indicating that sediment—processing activities by epibenthic fishes are important in explaining patch colonization for many members of the invertebrate assemblage. However, sediment depletion is not sufficient to explain the response of the abundant invertebrate, Baetis sp. 1, which showed substantial reductions in cages containing fishes (open and grazer enclosures), but high densities in both the fish—exclusion and the sediment—removal treatments. These data suggest that intimidation by fish, as measured by the presence of grazers, results in low rates of patch use by this numerically dominant mayfly. These findings contrast with studies reporting relatively weak effects of fishes in temperate zone streams. Fishes may be strong interactors in some neotropical steams, not through direct lethal consumption of prey, but rather by modifying the distribution and abundance of resources important to stream insects. In some instances the simple intimidation associate with the presence of a diverse and abundant fauna of bottom—feeding fishes also may influence insect abundance and distribution.
Absence of Interspecific Competition Among Tallgrass Prairie Grasshoppers During a Drought
Authors: Evans Edward W.
Published: May 2006

The grasshoppers Phoetaliotes nebrascensis and Orphulella speciosa overlap extensively in both diet and habitat in native tallgrass prairie at Konza Prairie, Kansas. I tested for interspecific competition between these two species during a drought year when primary productivity in the prairie was unusually low. In late June 1989, I transferred young nymphs of P. nebrascensis among enclosed plots (each 550—900 m2) to produce three pairs of plots with grasshoppers removed or added. Pretransfer densities in the six plots were estimated at 6 nymphs/m2. Experimental transfers resulted in densities of P. nebrascensis in addition plots 2—3 times as large as in paired removal plots; significant differences in density between addition and removal plots persisted throughout the summer. Densities of O. speciosa in plots with P. nebrascensis removed or added did not differ significantly throughout the growing season of 1989, nor in early summer of 1990. Furthermore, growth and development of O. speciosa in these plots (as measured by rate of maturation, and adult femur length and mass in 1989) were not reduced in the presence of high densities of P. nebrascensis, nor was reproductive potential (measured as the ratio of young nymphs present in early summer 1990 to adults in late summer 1989). These results are similar to those of a previous experiment at the same site during 1987 when primary productivity was moderate. Even under the extreme conditions of 1989, with particularly high ratios of grasshoppers to available food resources, variation in P. nebrascensis density did not adversely affect populations of O. speciosa. These results contrast with recent results for sedentary phytophagous insects, but are consistent with the hypothesis that free—ranging insect herbivores may rarely experience interspecific competition for food resources.
Simulated Effects of Grazing on Soil Nitrogen and Mineralization in Contrasting Serengeti Grasslands
Authors: Seagle Steven W. , McNaughton S. J. , Ruess Roger W.
Published: May 2006

The roles of edaphic characteristics and grazing in regulating nitrogen cycling and belowground processes were simulated for the shortgrass and tallgrass regions of the Serengeti National Park, Tanzania, by coupling a model of grassland productivity with a model of belowground decomposition processes. Grasslands of each region were simulated under ungrazed and observed grazing intensities to establish reference points for sensitivity analyses using a wide range of grazing intensities and different microbial biomasses. The consequences of patchy carbon and nitrogen deposition were also simulated. Results indicate that nitrogen cycling processes are affected differentially by grazing in the shortgrass and tallgrass regions. Net nitrogen mineralized by the microbial biomass in the shortgrass simulations declined from the ungrazed condition at low grazing intensities, and proceeded to increase from moderate to high grazing intensities. Plant nitrogen uptake was significantly correlated with net mineralization, and soil mineral nitrogen levels were greatest at moderate grazing intensities. In tallgrass simulations, soil mineral nitrogen increased with grazing intensity; net nitrogen immobilization occurred over much of the grazing intensity range because of low decomposition substrate quality. Tallgrass plant nitrogen uptake responded to urine nitrogen input to the soil and was uncorrelated with net mineralization. The potential for nitrogen competition between grasses and microbes was greater in the tallgrass than in the shortgrass because of lower total system nitrogen and lower grazing intensity in the tallgrass. The ecosystem—wide soil nitrogen gradient of the Serengeti governs most properties of nitrogen cycling, while within—region edaphic conditions and grazing create significant variation in local ecosystem processes.
Avian Predation on Anolis Lizards in the Northeastern Caribbean: Inter‐Island Contrast
Authors: McLaughlin John F. , Roughgarden Jonathan
Published: May 2006

This paper presents results of a field study of avian predation on Anolis lizards on islands in the northeastern Caribbean, addressing the question of what causes annual fluctuation in lizard abundance. The two main potential avian predators, the Pearly—eyed Thrasher (Margarps fuscatus) and the American Kestrel (Falco sparverius), were studied on three adjacent islands in the northern Lesser Antilles: Anguilla, St. Martin, and St. Eustatius. Anoles on Anguilla were found to be quantitatively unaffected by avian predation; thrashers there did not eat vertebrates, and the kestrel density was too low to have significant impact on the lizard population. In contrast, thrashers in moist habitat on St. Martin ate vertebrates including anoles, and thrasher predation potentially accounted for the anole decline to the dry—season abundance low. Additionally, an unanticipated thrasher niche shift was discovered. Thrasher body size and diet appear to be related to the abundance of a congeneric species, M. fuscus. On Anguilla, where M. fuscus was absent, M. fuscatus was smaller and had a non—vertebrate diet. On St. Eustatius, M. fuscus was abundant while M. fuscatus was larger and preyed on anoles. M. Fuscus was rate on St. Martin, where M. fuscatus had an intermediate body size and a diet comparable to that on St. Eustatius.
Sex, Dominance, and Microhabitat use in Wintering Black‐Capped Chickadees: A Field Experiment
Authors: Desrochers Andre
Published: May 2006

I studied foraging patterns of Black—capped Chickadees (Parus articapillus) wintering in flocks in poplar forests of central Alberta, Canada. Females exploited highest and outermost tree parts, while males (which socially dominate females) foraged low and close to trunks. Females spent more time probing and found fewer prey per probe than males, but both sexes had similar feeding rates. I temporarily removed all eight males from three flocks, females foraged lower and closer to trunks than in three control flocks. Experimental females spent less time probing than controls, but had similar feeding rates. When captive males were released, they returned to their flock and experimental females shifted back to their initial foraging behavior. The results show that sex—specific foraging patterns in this population were affected by avoidance by females of microhabitats used mostly by dominant males. I suggest that predator avoidance makes the tree parts exploited by males more profitable.
Secondary Succession, Gap Dynamics, and Community Structure in a Southern Appalachian Cove Forest
Authors: Clebsch Edward E. C. , Busing Richard T.
Published: May 2006

Forest succesion was studied in a species—rich cove forest of the Great Smoky Mountains, Tennessee. Long—term compositional and structural changes in secondary stands, 15—63 yr after agricultural abandonment circa 1920, were compared to an adjacent old—growth stand. Later successional trends were projected by computer simulation. The colonizing stand (15 yr) was dominated by Liriodendron tulipifera and Robinia pseudoacacia. Common old—growth species absent at this early stage included Aesculus octandra, Fagus grandifolia, and Tsuga Canadensis. By year 40, Liriodendron dominated the overstory and all old—growth dominants were represented in the understory. After 60 yr, Acer saccharum was clearly the most abundant understory species; gap model projections indicated that after 200 yr it will dominate the forest. The actual old—growth stand was dominated by Acer saccharum. Species richness peaked at mid—succession (°50 yr), while diversity (H') and evenness (J') of species biomass were highest at the old—growth stage. Richness peaked with the coexistence of shade—intolerant colonizers in the overstory and shade—tolerants in the understory. Canopy—gap processes of the secondary and old—growth stands differed sharply. The old—growth canopy was composed of large, spreading, and overlapping crowns, while the second—growth stand was composed of narrow, nonoverlapping crowns. Gaps in the secondary stand tended to be small (<100 m2) but numerous. The regeneration of intolerants was low and individual gap microsites were not markedly different from the understory as a whole. Old—growth gaps were often larger, higher light microsites which contrasted sharply with the shaded forest floor. The wider range of gap sizes in old forests accommodates a variety of species, including intolerants, contributing to the high canopy species diversity of old—growth cove forests.
Role of Debris Dams in the Structure and Functioning of Low‐Gradient Headwater Streams
Authors: Smock Leonard A. , Metzler Glenn M. , Gladden James E.
Published: May 2006

The importance of debris dams to organic matter dynamics and the macroinvertebrate community of two low—gradient, headwater streams on the Coastal Plain of Virginia was examined through sampling of natural dams and by experimental manipulation of dam abundance. Buzzards Branch had permanent flow and a sand substrate; Colliers Creek had intermittent flow and an organic substrate. Dam abundance varied from 8 to 13 dams/100m of stream length and covered only 1—3% of channel surface. Annual mean storage of organic matter (ash—free dry mass; particles >0.15 mm) in debris dams was 922 g/m2 and 3356 g/m2 in Colliers Creek and Buzzards Branch, respectively. Wood >16 mm constituted 73—80% and fine particulate organic matter (FPOM; particles 0.15—1 mm) <1% of total organic matter in the streams. Dams stored 21% and 85 of the coarse particulate organic matter (CPOM) on the channel surface in Colliers Creek and Buzzards Branch, respectively. Storage was highly seasonal, with highest concentrations following autumnal leaf fall and only 5—17% of December storage present in February. Dams were net exporters of large amounts of small particles of CPOM and FPOM. Macroinvertebrate density and biomass in dams was correlated with changes in organic matter storage both over seasons and between streams. Annual mean densities were 8915 individuals/m2 and 22 302 individuals/m2, and biomass was 0.3 g/m2 and 3.2 g/m2, in Colliers Creek and Buzzards Branch, respectively. Densities were at least 10 times, and biomass 5 times, as great in dams as on sediment. Increasing the abundance of dams increased organic matter storage, macroinvertebrate abundance, and the relative contribution of shredders to biomass, both in dams and on sediment. Increasing dam abundance also increased retention of leaves during base and especially storm flow. Widespread removal of riparian vegetation throughout the southeastern Coastal Plain, resulting in a decrease in the number of dams in streams, no doubt has altered the ecological characteristics of these streams relative to historical, pristine conditions. In particular, the transport of both energy and nutrients to downstream riverine areas likely is greater, and production of higher consumer levels probably is lower under present conditions.
Latitudinal Clines: A Trade‐Off between Egg Number and Size in Pacific Salmon
Authors: Fleming Ian A. , Gross Mart R.
Published: May 2006

The latitudinal variation in clutch size found in many animal species, including Pacific salmon, has been an enigmatic problem in ecology. We analyze egg number and egg size of 17 populations of coho salmon (Oncorhynchus kisutch) distributed over a latitudinal gradient in North America. These populations have a significant latitudinal increase in their egg number. But this increase is accompanied by a significant latitudinal decrease in their egg size. The total biomass of eggs produced also declines with latitude. Thus, the positive latitudinal trend in egg number cannot be explained by latitudinal variation in the total investment in eggs. This suggests that local optima in egg size may result in latitudinal clines in egg number. This reasoning, that egg number evolves around selection for egg size, is in fact predicted by life history theory and may explain the clutch size patterns observed in many organisms.
Estimation of Recruitment from Immigration Versus In Situ Reproduction Using Pollock's Robust Design
Authors: Nichols James D. , Pollock Kenneth H.
Published: May 2006

Recruitment to animal populations can occur through both immigration and in situ reproduction. These two components of recruitment are conceptually distinct and lead to different mechanistic models of population dynamics. We describe a capture—recapture design that can be used to obtain separate estimates of two recruitment components. We then illustrate the use of our method and estimators with capture—recapture data from a population of Microtus pennsylvanicus at the Patuxent Wildlife Research Center in Maryland.
Juvenile Competitive Bottlenecks: The Perch (Perca Fluviatilis)‐Roach (Rutilus Rutilus) Interaction
Authors: Persson Lennart , Greenberg Larry A.
Published: May 2006

In size—structured populations interspecific competitors often have been suggested to have their greatest impact on the juvenile life stage. We hypothesized that roach (Rutilus rutilus), by virtue of their superior foraging ability on zooplankton, would cause juvenile perch (Perca fluviatilis) to shift from feeding on zooplankton to macroinvertebrate prey, the prey consumed by adult perch. This should potentially increase size—class competition in perch, producing a competitive bottleneck at the macroinvertebrate feeding stage. To test this, we experimentally examined how roach density affected both the diet and growth of young—of—the—year (0+) and 1—yr—old (1+) perch as well as how it affected the zooplankton and benthic macroinvertebrate resource levels in 10 enclosures in a pond over a 3—mo period. The densities of 0+ and 1+ perch were held constant between enclosures, where the density of roach was varied over five density levels. A significant effect of roach density on zooplankton (cladoceran) biomass was observed over the experimental period. By the end of the experiment, the biomass of zooplankton was low in all enclosures except those without roach, and the mean size of two zooplankters, Bosmina and cyclopoid copepods, decreased with increasing roach density. In contrast, the total biomass of macroinvertebrates in the enclosures did not change with increasing roach density. There was, however, evidence that chironomids were released from predation by Sialis due to an increased consumption of Sialis by 1+ perch. As predicted from our hypothesis, 0+ perch shifted from pelagic zooplankton to benthic macroinvertebrates as roach density increased. The roach density at which 0+ perch shifted to feed on benthic prey decreased over the experimental period, but occurred at the same density of pelagic prey. The proportion of zooplankton in the diet of 1+ perch and roach also decreased with increasing roach density, with 1+ perch totally dropping zooplankton from their diet and roach shifting to plant to detrital material. Over time we observed that roach increased its utilization of plant and detrital material. We found density dependence in growth rates of roach and 0+ perch as well as 1+ perch. The growth data suggested that the main competitive effect of roach occurred during the first half of the experiment. This was related to roach's high consumption of plant material at all roach densities at the end of the experiment and to the low water temperature during the second half of the experiment.
Removal Rates of Seeds Adapted for Dispersal by Ants
Authors: Hughes Lesley , Westoby Mark
Published: May 2006

To learn what scope there may be for a mother plant to modify whether her seeds are removed by ants, and how quickly, we investigated (1) how initial seed removal by ants may vary in times (years, season, time of day) and space (within and between sites, and (2) how large these sources of variation are. We focused on seed of two common ant—dispersed species in a field experiment on 20 x 20 m plots at each o three sites in Kuringai Chase National Park in southeast Australia. The most important influence on removal rate over the first 12 h of exposure was the precise location of seeds on the ground (54% of total variation). This variation did not persist over longer periods of time and, except during winter, all seeds had a high probability of being removed within 2—3 d. Time of year also had a strong influence had a strong influence on removal rate (21% of total variation) with the highest rates occurring in summer and the lowest in winter. These changes were mainly attributable to changes in ambient temperature. Differences in removal rates between the two seed species used was the third most important source of variation (4%). There was significant variation in removal rate between the same months in different years, in two of the three between—year comparisons. The effect of year, and of interactions between year and other factors, accounted for between 7 and 18% of total variation. Complementary experiments showed that ants were the only important removers of seed at the study sites and that seed age up to 1 yr did not influence removal rate. Experiments comparing removal rates of clumped seeds to those of single seeds showed no consistence difference. The high probability of removal of elaiosome—bearing seeds within 2—3 d of seedfall means it is unlikely that ant—dispersed plants in this vegetation ever face the problem of their seeds not being encountered and transported by an ant. Instead, the problem may be how to be removed by those ant species whose behavior will give the seed the best chance of survival and establishment.
Grassland Patch Dynamics and Herbivore Grazing Preference Following Urine Deposition
Authors: Day T. A. , Detling J. K.
Published: May 2006

Field experiments were performed over two growing seasons to investigate the response of Schizachyrium scoparium (C4 photosynthetic pathway) and Poa partensis (C3) to natural and simulated bison urine deposition in a northern, mixed prairie in South Dakota. We also assessed potential feedbacks of urine deposition on herbivore grazing by monitoring grass response to defoliation and herbivore grazing preference for vegetation occupying urine patches. Total aboveground biomass and root mass were higher and root: shoot ratios were lower on urine patches than in the surrounding plant community. Higher total aboveground biomass on urine patches resulted primarily from increased aboveground P. pratensis production. Urine deposition in May had little effect on aboveground production of S. scoparium except during July when S. scoparium was most active. Urine disposition date and plant penology appear important in determining changes in species composition. Following urine deposition, aboveground N concentrations of P. pratensis and S. scoparium were higher on patches relative to conspecifics off patches. This increase in N concentration following urine deposition was greater in P. pratensis. We suggest the largest increase in P. pratensis biomass following urine deposition is related to its relatively large response to increased soil N availability and its rhizomatous habit. Root N concentrations were higher on urine patches. Poa pratensis on urine patches initiated growth earlier in the season and postponed senescene relative to plants off patches. Aboveground production following clipping was greater on urine patches and N concentrations in regrowth of both species were higher than concentrations in plants not previously clipped. Above ground herbivore utilization was greater on urine patches than on adjacent vegetation. Although urine patches covered only 2% of the study site, they provided 7% of the biomass and 14% of the N consumed by aboveground herbivores from June through August. Urine patches probably provided an even greater source of forage and N for herbivores earlier and later in the growing season when surrounding vegetation was most quiescent.
Role of Resources and Disturbance in the Organization of an Old‐Field Plant Community
Authors: Carson Walter P. , Pickett S. T. A.
Published: May 2006

The roles of disturbance and resource availability in influencing species richness and plant abundance were examined by conducting a factorial experiment for 2 yr in an old field in New Jersey, USA, dominated by goldenrods (Solidago spp.). Replicate plots were treated by (1) adding macronutrients, (2) adding water, (3) tying back tall herbs to increase light to ground layer species, and (4) creating gaps in the vegetation by disturbing the soil with hand trowel early in the growing season. Nutrient additions increased the cover of the herbaceous canopy and enhanced the dominance of Solidago spp. Water additions also increased canopy during the second growing season, a year with low seasonal rainfall. Light enrichment as a consequence of canopy manipulation resulted in the competitive release of subcanopy species, primarily Fragaria virginiana. Other subcanopy dominants were limited by different resources; Hieracium pratense responded most strongly to water and Rumex acetosella to nutrients. These differences in limiting resources may be responsible for the continued coexistence of these three subcanopy species. Additionally, we observed marked seasonal and yearly fluctuations in the severity of resource limitation which might serve to favor different species in this community at different times. Species richness was decreased by light enrichment alone, whereas light with irrigation interacted to increase species richness. Nutrient additions had little direct effect on species richness A single small disturbance at the growing season had little impact on species richness or abundance. Relative to the influence of resources, small—scale disturbance plays only a minor role in this community. Our results suggest that nonequilibrium organization of old—field communities can occur via a shifting resource base in the absence of natural disturbances.