bluuebunny/crossref_metadata_2025 huggingface dataset:
Comparative Population Ecology of Eleven Species of Rodents in the Chihuahuan Desert
Authors: Brown James H. , Zeng Zongyong
Published: May 2006

Comparisons of mark—recapture data on life histories and population dynamics of the 11 commonest species of nocturnal desert rodents inhabiting our experimental study site in the Chihuahuan Desert of extreme southeastern Arizona permitted assessment of the role of evolutionary relationships and ecological factors in the coexistence of these species. The species varied greatly in population density, extent of interannual variation in abundance, timing of reproduction, extent to which reproduction was seasonal, rate of disappearance of marked individuals, frequency and distance of lifetime dispersal movements, but perhaps less so in death rate and maximum longevity. Most of the species showed positively correlated year—to—year fluctuations in population density, suggesting that they responded similarly to interannual variation in precipitation, primary production, and availability of food resources. In contrast, there were both positive and negative correlations in seasonal patterns of reproductive activity and population density. Lifetime dispersal movements were inversely related to body size, suggesting that energy constraints cause the smallest species to move among rich patches in a coarse—grained manner. Patterns of similarities and differences among closely related (congeneric and confamilial) species suggested that evolutionary constraints sometimes, but not always, limited variation in life history and demography. The relationship between population ecology and competition among these species was not clear. We interpret the diversity of life histories and population dynamics in these coexisting species to be a consequence of: (a) a productive and spatially and temporally variable environment that provides a variety of resources that may be used in different ways, (b) historical biogeographic events that have made available a large regional pool of species from which potential colonists can be drawn, and (c) differences in population ecologies among the species that evolved primarily in other environmental contexts, but that permit coexistence by enabling the species to use different resources or to use the same resources in different ways.
Life History and Population Energetics of the Dobson Fly, Corydalus Cornutus
Authors: Brown Arthur V. , Fitzpatrick Lloyd C.
Published: May 2006

Rates and energies of consumption (C), egestion (F), assimilation (A), respiration (R), growth (Pg), production of exuviae (Pev), and production of egg masses (Pr) with associated efficiencies, and effects of seasonal temperature, weight and life—history stage upon them were examined for a typical individual and cohort of Corydalus cornutus (L.) from a stream in north—central Texas. Dobson flies are univoltine with 11 larval instars. Emergence, oviposition, and hatching occurred from late May into August. The typical dobson fly hatched in mid—June, grew rapidly until November, and again in March, reaching adult size prior to pupation in early June. A0dult females fed to provide energy to yolk eggs, produce cases and for maintenance during their week of reproductive activities. Metabolic compensation enabled larval dobson flies to maintain fairly constant rates of R during winter (6.07 J g—1 h—1, 5—15°C) and summer (16.15 J g—1 h—1, 20—30°C); with a seasonal acclimatization threshold occurring between 15—20°C. Metabolic constancy is adaptive and probably contritubed to high ratios of P/R (1.94) and P/A (66%) for individual larvae. Rates of C, F, A, and R, but not assimilation efficiency (AE = A/C X 100), were influenced by temperature and size. The energy budget for a typical dobson fly during the 47 wk as a larva was: C = 17,435, A = 14,401, F = 3,033, Pg = 8,682, Pev = 828 and R = 4,892 J. Embryos respired 0.45 J/wk, prepupae 1,494 J/wk, male pupae 2,130 J/wk, female pupae 1,900 J/wk, male adults 2,615 J/wk and female adults 3,075 J/wk. Prepupal and pupal exuviae were 602 J and 502 H respectively. The average adult female's minimal reproductive effort was 2,791 J for eggs and 774 J for egg cases, 54 and 100% of her A and P respectively, 17% of her life—history A. Annual energetics of the larval population were; C = 165,343, A = 136,574, F = 28769, Pg = 54,610, Pev = 15,096, Pr = 1,502—1,711 and R = 66,869 J m—2.
Comparative Foraging Behavior of Six Stingless Bee Species Exploiting a Standardized Resource
Authors: Hubbell Stephen P. , Johnson Leslie K.
Published: May 2006

We report on differences in foraging and recruitment patterns among 6 stingless bee species simultaneously exploiting a large grid of baits in a tract of tropical dry forest in Costa Rica. Comparative data for the species were obtained on the following parameters of foraging: time to initial bait discovery, rate of discovery of additional baits, rate of worker recruitment, time till attainment of approximate steady—state number of workers and visited baits, frequency of number of workers per bait, degree of site constancy of foraging within and between days, and responses to presence of intraspecific and interspecific rivals at baits. All species had unique aspects to their foraging, which reflected whether the species mark food sources and lay trails with pheromones, the degree to which workers forage solitarily, and their use of aggression. The behavior of Trigona fuscipennis, the pheromone—using group forager, was particularly distinctive. It found baits the most slowly, was the most site constant, and excluded all other bees from its baits. We hypothesize that in group foragers, extreme localization of workers and resultant slowness of discovery of new food sources represent a trade—off for the advantages of group defense of good sources. Other responses to rival bees included bullying tactics by lone, marauding workers of the large, facultative recruiter, Trigona silverstriana, and persistent, opportunistic insinuation by small, solitary foraging species. Spatial segregation of foraging location between the species increased over time on each day of the experiment, especially among interspecifically aggressive species. The observed diversity of tempo and mode of foraging reinforces the hypothesis that coexistence among these apparently food—limited social bees does not result from the simple partitioning of food resources by size or taxon, but from rather subtle partitioning based upon diversity in the timing, persistence, renewal rate, and spatial dispersion of their limited food resourves.
Sensing Vegetational Patterns with Pollen Data: Choosing the Data
Authors: Webb T. , Laseski R. A. , Bernabo J. C.
Published: May 2006

Comparisons between modern pollen and tree—inventory data from northeastern North America show how the clear portrayal of vegetational patterns in pollen data depends upon several choices made in selecting and preparing these data. These choices concern the size and location of the area studied, the types of pollen examined, the quantitative measure of pollen used, the types of sediments sampled, the number of analysts contributing samples to the data set, and the degree of smoothing applied to the data either by adding samples together or by combining pollen types into groups. Our results show that these choices can control not only what scale of vegetational pattern will appear on maps of pollen data but also whether these maps portray vegetational patterns at all. Of the choices examined, the most important were those concerning the size of the area studied, the types of pollen examined, and the amount of smoothing used. Before presenting the results, we introduce an expression for a signal—to—noise ratio that contrasts the degree of vegetational variation affecting the pollen data with the size of the uncertainties and nonvegetational variation (e.g., changes in pollen dispersal) that also influence these data. The effects of the different choices in data selection are discussed in light of their effect on this ratio.
The Statistical Analysis of Ecophysiological Response Curves Obtained from Experiments Involving Repeated Measures
Authors: Potvin Catherine , Lechowicz Martin J. , Tardif Serge
Published: May 2006

Physiological ecologists often analyze the responses of physiological or biochemical traits to environmental factors such as temperature, irradiance, water potential, or the concentrations of CO2, O2, and inorganic nutrients. The data for such a response curve typically are gathered by sequential sampling of the same plant or animal, and their analysis should explicitly allow for this repeated—measures design. Unfortunately, the statistical analysis of response curves in ecology generally has either been ignored or incorrectly done. In an effort to encourage rigorous analysis of response data, we address statistical treatment of response curves and illustrate the correct alternatives that are available. Four different statistical methods for analyzing response curves are considered: analysis of variance with repeated measures (ANOVAR), multivariate analysis of variance with repeated measures (MANOVAR), a nonparametric split—plot analysis (NP split—plot) and parametric comparison of models fitted to the data by nonlinear regression. Analyses of the CO2 dependence of photosynthesis in the C4 grass Echinochloa crus—galli following chilling are used as examples of these different methods. ANOVAR, potentially the most powerful analysis, makes stringent assumptions about the variance—covariance structure of the data. Within limits these assumptions can be relaxed and a corrected significance level used. When the variance—covariance structure badly violates the ANOVAR assumptions, MANOVAR or NP split—plot are viable alternatives. In physiological ecology, however, the use of MANOVAR frequently is limited by small sample sizes and the tendency for the number of levels of the treatment factor to exceed the sample size. Greater attention to experimental design can avoid this problem. The NP split—plot is based on simple assumptions and could be widely used. The comparison of curves fitted by nonlinear regression is also distribution free and provides an interesting alternative when the responses are amenable to fitting. For any of these analyses to be viable the thoughtful choice of experimental protocols and design is essential.
Lobster Predation on Mussels: Shore‐Level Differences in Prey Vulnerability and Predator Preference
Authors: Robles C. , Sweetnam D. , Eminike J.
Published: May 2006

On a Rocky islet off the coast of Southern California, intertidal beds of the mussel Mytilus californianus are subject to predation by spiny lobsters, Panulirus interruptus, during evening high tides. Mussel shell thickness and degree of inflation (width—to—height ratio) increased with length, but the increases were significantly less for mussels growing on relatively low shore levels. Vertical differences in mussel density, size frequency distribution, and shell morphology were described with transect surveys at low tide. Lobster distribution and abundance were recorded by divers during nocturnal high tides. Laboratory experiments and field observations were done to determine (1) the maximum vulnerable, "critical" lengths above which the mussels cannot be crushed, (2) preferred sizes below the critical lengths, and (3) whether critical lengths or predator preferences varied with the shore—level differences in mussel morphology. Critical lengths, preferences, and other measures of predator performance were determined from predator electivity experiments in laboratory aquaria. The lobsters displayed size preferences that fell far below the critical lengths. Both critical and preferred sizes increased with lobster carapace length, and, consequently, the largest lobsters were able to kill the largest mussels in the bed. Prey handling times ranged from 5 s to 1.6 h, and increased with prey/predator size ratios. For a given carapace length, the critical length was greater for lower shore mussels. For a given mussel length, lower shore mussels required shorter handling times and were preferred over upper shore mussels. Intraspecific variation in prey defenses can affect predator performance. In the intertidal mussel bed, such variation occurs over small spatial scales. In contrast to studies of intertidal prey refugia, few of the prey were invulnerable. Therefore, predator preferences and the relative availability of preferred types may determine aspects of prey population structure.
Residual Effects of Grazing: Inhibition of Competitor Recruitment by Encrusting Coralline Algae
Authors: Breitburg Denise L.
Published: May 2006

Adult—adult and adult—recruit competitive interactions between members of the same pair of species can have differing outcomes; thus, chronology and history may be important determinants of existing community structure. Examples of different outcomes of adult—adult and adult—recruit interactions can be found among species that are potential competitors for space on rocky subtidal substrate. As found in the present study, crustose coralline algae significantly decrease recruitment by sessile epibenthic invertebrate species from several taxa. Nonetheless, sessile invertebrates appear to be superior competitors for space as adults, since established individuals or colonies can overgrown coralline algal crusts. It is suggested that (1) by decreasing total invertebrate recruitment, even in the temporary absence of grazing on portions of the substrate, extensive encrusting coralline algal cover may contribute to the persistence of "coralline algae—sea urchin communities" (composed mostly of crustose coralline algae, sea urchins, and sea stars), and (2) by altering the relative abundance of other recruiting species, grazing—resistant organisms such as crustose coralline algae may decrease the probability that, after grazing abates, a community will return to its pregrazed species composition.
An Experimental Analysis of a Life History Theory
Authors: Luckinbill Leo S.
Published: May 2006

r— and K—selection were applied to bacterial populations for up to 400 generations in the form of density—independent or density—dependent population controls in both batch and semi—continuous culture. Shifting ratios of competitively neutral genetic markers signaled the adaptation of bacterial strains to selection. Pairwise competition of r— with K—adapted strains showed adaptation under density—dependent and independent and independent controls to be nonspecific; that is, the adaptive trade—off predicted in theory did not occur here. R— and K—selection only adapted populations to the same thing; growth. Predicted adaptations in growth rate, saturation density, cell size, productivity, and efficiency also did not occur. These bacteria, however, were found to have the ability both to grow rapidly and exploitatively, or alternatively to survive up to 7 wk starvation at high density. The latter mechanism, allowing survival through prolonged periods of intense resource deprivation, is clearly a K type of adaptation. E. Coli appears to have hedged its environmental bets by the capacity for either rapid growth and exploitation, or persistence in the face of depleted resources, as the situation demands.
The Frequency of Density Dependence in Insect Host‐Parasitoid Systems
Authors: Stiling Peter D.
Published: May 2006

I examined case studies of density—related parasitism in insect host—parasitoid systems reported in the literature. For 171 examples, the frequency of density dependence was 25% (n = 43), of inverse density dependence 23% (n = 39), and of independence 52% (n = 89). This is a considerably lower frequency of density dependence than previously reported in other reviews (Lawton and McNeill 1979, Lessells 1985, but see Dempster 1983). Biological features of case histories gave few clues as to the expected frequency of density dependence. Such features included host order, life stage attacked, parasite group, native vs. introduced status of hosts or parasites, number of attacking parasitoid species, mobile vs. sedentary hosts, and monophagous vs. polyphagous hosts or parasitoids. Biotic and abiotic factors may, however, provide good reasons for a low overall frequency of density dependence.
Influence of Moisture, Temperature, and Substrate on Snapping Turtle Eggs and Embryos
Authors: Packard Gary C. , Packard Mary J. , Miller Kirk , Boardman Thomas J.
Published: May 2006

Flexible—shelled eggs of common snapping turtles (Chelydra serpentina) were incubated on two substrates (sand and vermiculite) at each of three temperatures (26.0°, 28.5°, 31.0°C) and three moisture regimes (water potentials initially —150 kPa, —550 kPa, —950 kPa) in a factorial experiment assessing the influence of these variables on the water relations of eggs and the development of embryos. Hatching success was high on wet substrates at 26.0° and 28.5°, but declined at the highest temperature and on drier media. Net absorption of water by viable eggs, duration of incubation by embryos, and size of hatchlings, were positively correlated with wetness of substrates and negatively correlated with temperature. Turtles hatching from eggs at 26.0° were males regardless of the wetness of the medium, whereas those emerging form eggs at 28.5°and 31.0° were females. These patterns of response characterized eggs incubated on sand as well as those on vermiculite. Findings from this study indicate that temporal and spatial variations in moisture and temperature within and among natural nests probably elicit ecologically important variation in size and sex of hatchling snapping turtles.
Patterns of Plant Species Richness Along Riverbanks
Authors: Nilsson Christer , Grelsson Gunnel , Johansson Mats , Sperens Ulf
Published: May 2006

We studied bank vegetation between the spring high—water level and the summer low—water level along two rivers in northern Sweden. The hypothesis that natural and ruderal species would show different downstream patterns of species richness was tested by sampling species composition and environmental variables along 200 m long stretches of riverbank 10 km apart. Natural species richness was highest in the midreaches of both rivers, whereas ruderal species showed a significant, monotonic increase downstream. There are no obvious mechanisms producing the quadratic pattern of natural species richness. The downstream increase in ruderal species suggests a founder effect depending on larger artificial disturbances near the coast, but alternative explanations are also given. Total species richness did not exhibit any interpretable downstream patterns. The only factors significantly correlated with total species richness along both rivers were substrate heterogeneity and substrate fineness. Total species richness increased with substrate heterogeneity and was at maximum at intermediate levels of substrate fineness. This suggests that co—occurrence of natural and ruderal species is most likely where substrate types are numerous and dominant substrate particle size intermediate.
Kernel Methods for Estimating the Utilization Distribution in Home‐Range Studies
Authors: Worton B. J.
Published: May 2006

In this paper kernel methods for the nonparametric estimation of the utilization distribution from a random sample of locational observations made on an animal in its home range are described. They are of flexible form, thus can be used where simple parametric models are found to be inappropriate or difficult to specify. Two examples are given to illustrate the fixed and adaptive kernel approaches in data analysis and to compare the methods. Various choices for the smoothing parameter used in kernel methods are discussed. Since kernel methods give alternative approaches to the Anderson (1982) Fourier transform methods, some comparisons are made.
Testing Theories of Clutch Overlap with the Reproductive Ecology of Heterandria Formosa
Authors: Travis Joseph , Farr James A. , Henrich Sabine , Cheong Ron T.
Published: May 2006

We used laboratory and field studies of the reproductive ecology of the poeciliid fish Heterandria formosa to examine the possible mechanisms that have driven the evolution of superfetation (simultaneous provision of maternal care to sets of dependent offspring in diverse stages of development). Increased food levels in the laboratory increased the number of broods that a female produced by increasing the level of superfetation. Brood sizes were determined by female body size and were unaffected by food level. Offspring size was unaffected by food level but varied with female age. Different females had different age—specific offspring sizes. Brood size and offspring size were not correlated, even when female size was held apart. In the field the level of superfetation increased with the total number of embryos, even when female size was held apart. Larger, older females displayed higher levels of superfetation than smaller, younger females. Superfetation appeared to increase offspring production rate without compromising brood size or individual offspring size. The correlations among reproductive traits that were displayed by H. formosa were quite different from those found in poeciliid fish without superfetation. Heterandria exhibited a demographic regime characterized by overlapping generations but high adult mortality rates relative to juvenile mortality. We used these results to test three general hypotheses for the evolution of clutch overlap. The results support Burley's (1980) hypothesis that clutch overlap is an adaptation to maximize offspring production rate in the face of limited maternal effort. We modified this hypothesis to specify production of large offspring. In poeciliids, the critical tests of this modification will involve comparing the reproductive and demographic patterns observed in species such as Heterandria formosa, which occupies an extreme position with regard to clutch overlap, with species such as Gambusia affinis, which does not display clutch overlap, yet is often syntopic with Heterandria. Similar comparative studies in demography could address the question of why clutch overlap is not more common.
Effects of Nutrients and Zooplankton Size on the Structure of a Phytoplankton Community
Authors: Vanni Michael J.
Published: May 2006

In situ enclosure experiments were conducted over two summers (1980—1981) to assess the effects of nutrient (N and P) enrichment and zooplankton size structure on the phytoplankton community of an oligo—mesotrophic lake containing planktivorous fish and small zooplankton species. Exclusion of fish from the community resulted in an increase in mean individual zooplankton size, primarily because the cladocerans Ceriodaphnia and Diaphanosoma attained larger sizes. In 1980, total cladoceran biomass was also greater in the absence of fish. In 1980, total phytoplankton density was significantly lower in the fishless enclosures (where zooplankton were larger and grazing rates presumably higher), at a given nutrient level. The proportion of phytoplankton density comprising species with gelatinous sheaths or other protective coverings was greater in the fishless enclosures, under both enriched and unenriched conditions, presumably because these species are relatively resistant to zooplankton grazing. In 1981, when lower fish densities were used (than in 1980), total phytoplankton density was lower in the fishless enclosures only toward the end of the experiment and only in unenriched enclosures. In enriched enclosures the presence of fish had no effect on total phytoplankton density. However, the proportion of phytoplankton density made up of small, edible (to zooplankton) phytoplankton species was lower in the absence of fish throughout the experiment in both unenriched and enriched treatments. In both years, nutrient enrichment caused phytoplankton density to increase greatly, in the presence and abscence of fish. Nutrient enrichment increased phytoplankton density a much greater amount than did reduction in zooplankton size. The results of these experiments demonstrate that even relatively small changes in zooplankton size (i.e., shifts in size within only small species) can result in significant alterations of the phytoplankton community. However, the increase in zooplankton body size resulting from fish exclusion could not buffer all effects of nutrient enrichment on total phytoplankton density.
Territory Size Variation in the Ovenbird: The Role of Habitat Structure
Authors: Smith T. M. , Shugart H. H.
Published: May 2006

The hypothesis that structural habitat cues are the proximate factor determining territory size was tested by examining the relationships among habitat structure, prey abundance, and intrapopulation variation in territory size in Ovenbirds (Seiurus aurocapillus). Territory size was negatively correlated with prey abundance, with territory size decreasing as prey abundance per unit area increased. In addition, there was a significant difference in prey abundance per unit area between territory sites and areas of the study site not occupied by ovenbirds. A regression of prey abundance with variables describing the habitat structure of territory sites was significant, with habitat structure accounting for 73% of the variation in prey abundance among territories. This regression analysis, in combination with an additional discriminant function analysis of habitat occupancy, suggested a gradient of habitat quality as a function of vegetation structure that is related to both habitat selection and variation in territory size. To determine the possible mechanisms responsible for the inverse relationship between prey abundance and territory size, several hypotheses were considered. Both sampled prey abundance and predicted prey abundance as a function of habitat structure provided adequate models to explain the observed variation in territory size. Although an index of intraspecific competition was correlated with territory size, it was not significantly correlated with either sampled prey abundance or predicted prey abundance. Therefore, the index is not sufficient to account for the observed inverse relationship between prey abundance and territory size. A partial correlation analysis of territory size with prey abundance and predicted prey abundance supported the structural cues hypothesis, with variation in territory size being related to structural features of the habitat rather than prey abundance per se.
Wind Effects on Needles of Timberline Conifers: Seasonal Influence on Mortality
Authors: Hadley Julian L. , Smith William K.
Published: May 2006

In the Rocky Mountains, USA, year—round wind exposure has been correlated with widespread needle dehydration and death in winter for alpine timberline conifers. Needle death may be due solely to the drying effects of winter wind or may also result from predisposition to winter injury associated with suboptimal summer growth conditions. Our experiments showed that in the lower timerbline ecotone in southwest Wyoming, needle mortality was primarily due to winter wind and cuticle abrasion; death was frequent only in wind—exposed needles of flagged trees and surface needles of krummholz mats. At higher elevations where only krummholz mats and a few flagged trees exist, mortality averaged ≥75% for needles unprotected by snow, regardless of wind exposure. Snow covered needles had low mortality throughout the timberline ecotone. Winter death of naturally wind—exposed needles of Picea engelmannii occurred at ≤—4.5 MPa water potential (≤60% relative water content). Cuticular resistance of wind—exposed needles declined from 100—250 ks/m in autumn to ≤30 ks/m by midwinter. Experimentally reversing the windward—leeward orientation of small, flagged trees in early winter resulted in lower xylem pressure potentials and needle viabilities for newly wind—exposed (originally leeward) compared to newly sheltered (originally windward) shoots. Also, sheltering exposed branches from winter wind on flagged trees in the lower timberline ecotone (3200 m) increased mean overwinter needle survival from near 0 to ≥50% in both Picea engelmannii and Abies lasiocarpa. Scanning electron micrographs of dehydrate wind—exposed needles collected in March at this site showed little cuticular surface wax, probably because of windborne ice crystal abrasion. However, winter dehydration and death in both experimentally and naturally wind—sheltered needles at higher elevation may have been due to inadequate needle maturation during summer, which could act to exclude flagged trees from the upper timberline ecotone.
Avian Nest Dispersion and Fledging Success in Field‐Forest Ecotones
Authors: Gates J. Edward , Gysel Leslie W.
Published: May 2006

Observations of 21 species of open—nesting passerines breeding in contiguous field and forest habitats at Rose Lake Wildlife Research Area, Michigan, were made during 1974 and 1975. Data were collected on nest dispersion, clutch—size, and fledging success in relation to the field—forest edge. Losses of eggs or nestlings were attributed to predation, inclement weather, Brown—headed Cowbird (Molothrus ater) parasitism, nest desertion, hatching failure, and adult death. Each bird species seemed to have a preferred distance from the habitat discontinuity that was used as a nest site. Furthermore, nests were not uniformly distributed on the area. Over one—half of the nests were found within ±15 m of the habitat discontinuity. Seventy—five percent of the nests belonged to birds characteristic of mixed breeding habitats, i.e., birds requiring an open overstory canopy with elevated singing and observation perches and dense cover near the ground for nesting and feeding. These mixed—habitat species also accounted for the increase in avian species nesting near edges. Based on Kendall rank and partial rank correlation tests, increasing numbers of nests and the percentage of total clutches, <3 eggs were found to be negatively correlated with increasing distance from the habitat discontinuity. Correlation between fledging success and increasing distance from the edge was positive and highly significant. Of the several mortality factors investigated, predation and cowbird parasitism were found to be the most important. The increased predation rate with decreased distance from the edge was attributed primarily to a functional response to higher numbers of nests and a greater activity of potential nest predators in the vicinity of the habitat discontinuities. Our results indicate that habitat suitability decreases with increasing numbers of nests toward the narrow field—forest edges. Although such abrupt habitat discontinuities did attract a variety and abundance of birds characteristic of habitats with mixed life—form, these discontinuities seemed to function as "ecological traps" by concentration nests and thereby increasing density—dependent mortality. Ironically, the cowbird was also a victim of the increased predation rate. As these man—made forest edges are of recent origin, they are perhaps unrepresentative of the ecological niche in which these species evolved, and thus they may be poorly adapted to cope with the increased nest predation.
Experimental Evidence for Intraspecific Competition in a Lepidopteran Leaf Miner
Authors: Bultman Thomas L. , Faeth Stanley H.
Published: May 2006

We manipulated field densities of the leaf—mining insect Cameraria sp. nov. (Lepidoptera: Gracillariidae) by caging them on branches of Quercus emoryi in central Arizona to test for intraspecific competition. High density cages (n = 7 replicates) had mean larval densities about 3 times those of low—density cages (n = 3 replicates). Our results demonstrate intraspecific competition in the form of density—dependent larval mortality. While mortality for leaf miners co—occurring on leaves was higher than for those occurring singly on leaves, larval mortality on leaves with multiple mines was just as high in low— as in high—density cages. That is, high mortality in high—density treatments was not related to interactions of larvae co—occurring on leaves. Rather, greater mortality in high density treatments resulted from increased mortality of miners on singly mined leaves in high—density treatments compared to those on singly mined leaves in low—density treatments. We suggest that ovipositing Cameraria compete with one another via interference when searching for leaves of superior quality. Natural field densities of Cameraria were highly variable. We conclude that natural high—density aggregations of Cameraria may result in some intraspecific competition, but, for the most part, Cameraria remains at low levels such that the competitive effect we found would not usually operate. However, intraspecific competition may act as a ceiling that prevents Cameraria aggregation on branches of Emory oak from exceeding the levels we observed in the field. Phytophagous insects that are commonly aggregated may compete to some degree even at relatively low densities, such as those of many leaf miners, if leaf quality (chemical/physical) is variable and leaves superior for insect growth and survival are rare.
Habitat‐Specific Solute Retention in Two Small Streams: An Intersite Comparison
Authors: Munn Nancy L. , Meyer Judy L.
Published: May 2006

We measured uptake rates of phosphorus, nitrate, calcium, and dissolved organic carbon within two headwater streams, one in the Appalachian mountains of North Carolina and the other in the Cascade range of Oregon. The major physical differences between these two streams are parent geology and local geomorphic structure. Uptake rates were measured following low—level nutrient releases during summer 1987 in 20—m reaches with different geomorphology. The relative importance of biotic vs. abiotic sorption of nitrogen and phosphorus by sediments from the two streams was assessed by laboratory experiments. Nitrate—N uptake rates were high for the western stream (11.9 mg°m—2°min—1) and low for the eastern stream (3.9 mg°m—2°min—1) during the summer. The debris dam reaches in Oregon were the most retentive of N (shortest uptake length of 17 m). Conversely, soluble reactive phosphorus (SRP) uptake rates were higher for the eastern stream (18.6 mg°m—2°min—1), primarily through biotic processes. SRP uptake lengths were short for the cobble (32 m), debris dam (35 m), and rock outcrop (40 m) reaches in the eastern stream. Uptake of SRP in either stream was not related to sediment size fraction but rather to a combination of sediment infiltration rates and quality of organic material. Calcium (Ca) uptake lengths were long in the Oregon stream (1278 m) but short in the Appalachian stream (106 m). Surprisingly, the eastern stream was more retentive of Ca than nitrate (shorter uptake lengths for Ca) during this time period. Debris dams greatly enhanced retention of dissolved organic carbon in both streams (60% of all uptake in the eastern stream and 81% in the western stream), although retention was greater in the eastern stream. The lower uptake rate of phosphorus and higher uptake rate of nitrate in the Oregon stream were expected based on geographic location and parent geology; streams in this area drain catchments of volcanic origin and tend to have low N:P (atomic; 1.8 for the western stream) ratios, indicating potential N limitation. Streams flowing over granitic bedrock, such as the eastern stream, tend to have lower P availability (N:P = 15.5). The combined results of laboratory and field measurements indicate that in the eastern stream, strong biotic control of P uptake coupled with high P demand result in relatively short P uptake lengths and a strong impact of P spiraling on ecosystem dynamics. In the western stream, strong biotic control of N uptake combined with strong N demand result in short N uptake lengths. This is especially true at sites of downed timber that retain both FPOM and CPOM, creating a high N demand (shortening N spirals).
Exploitation of Seed Aggregates by Merriam's Kangaroo Rat: Harvesting Rates and Predatory Risk
Authors: Bowers Michael A.
Published: May 2006

The simple cost—benefit model of foraging and patch usage of Gilliam and Fraser (1987) was tested for Dipodomys merriami, using data from the laboratory and field. Costs are modeled as the risk of being preyed upon while foraging from a particular patch and benefits as the amount of energy harvested per unit time. The model predicts that foraging rodents should choose patches where the risk: gain ratio is minimal. Field tests of the model using three types of 1—g seed samples (i.e., pure milo, pure millet, and a mixed sample of the two) showed that the extent to which seed aggregates were exploited (an estimate of patch residency time) increased with the rate at which seeds are harvested (determined through laboratory trials) and decreased with increasing predatory risk as apparently affected by microhabitat and nocturnal illumination. The patch encounter rate for single seeds and 1—g seed aggregates did not differ, but rodents consistently removed a greater proportion of seeds from the latter. These results suggest that foraging decisions involve a balancing of energy gain with the perceived risk of being eaten, that these are evaluated at the scale of individual resource patches, and that D. merriami accepts proportionately more risk at higher resource levels. While many studies have inferred the importance of both predation and resources to the population and community ecology of desert rodents, this study is one of the first to examine the interaction of responses to predation and patch quality. It is noted that predators may concentrate their prey in refugia, thereby increasing competition for resources among prey species. Hence, awareness of interactions both within and between trophic levels may be required to understand how communities are organized.
Soil Organic Matter Assimilation by a Geophagous Tropical Earthworm Based on (delta)^(13)C Measurements
Authors: Martin A. , Mariotti A. , Balesdent J. , Lavelle P.
Published: May 2006

Assimilation of soil organic matter and fresh plant debris by Millsonia anomala, a tropical geophagous earthworm, was investigated by measuring changes in the 13C/12C ratio of their tissues when fed on organic matter naturally labelled by 13C. Individuals collected from a soil colonized by C4 plants (C4 soil) that were fed on C3 soil had >50% of tissue C derived from the C3 soil after 33 d. Assimilation of organic matter associated with different particle size fractions was investigated with individuals fed for 25 d on C4 soil with C4 particle size fractions substituted in turn for the equivalent C3 soil particle fraction. A significant labelling of earthworms was observed with both the 250—2000 μm and the 0—20 μm particle size fractions. Addition of fresh C3 plant debris to the C4 soil also resulted in a significant change in earthworm C isotope ratio. Assimilation of fresh plant material was greater than that of soil organic matter. This shows that young M. anomala are able to assimilate young organic matter (fresh plant debris, coarse soil organic matter) as well as fine soil organic matter, both of which classically have been regarded as strongly resistant to decomposition in models of soil organic matter dynamics.
Importance of Mixing, Thermal Stratification and Light Adaptation for Phytoplankton Productivity in Lake Tahoe (California‐Nevada)
Authors: Tilzer Max M. , Goldman Charles R.
Published: May 2006

Lake Tahoe is highly transparent (Secchi readings 20—42 metres) and never freezes over. Deep mixing throughout its 500—metre water column occurred in March 1974 and 1975. Between June and October 1974, a distinct epilimnion of 15— to 30—metre thickness existed. Phytoplankton, which is dominated by diatoms, showed no significant vertical shifts in species composition. Chlorophyl α concentrations were highest at the bottom of the trophogenic zone (75—100m) during thermal stratification. Maximum photosynthetic rates were observed at greater depths during winter mixing of the lake than during thermal stratification. In situ experiments, in which incubations flasks containing phytoplankton from 3 discrete depths were distributed over the trophogenic zone, were conducted to study light adaptation. Results indicated that seasonal variation in the vertical distribution of photosynthesis was controlled to a greater extent by algal adaptation to prevailing subaquatic light conditions than by variations in solar irradiance. During deep mixing of the lake, algae adapted to average prevailing low light intensities. Thermal stratification, however, caused populations to remain at certain depths sufficiently long to adapt to ambient light intensities. Thus, phytoplankton at shallow depths was light adapted, whereas deep populations remained dark adapted. During thermal stratification, light inhibition was minimized and dim light in deep water was utilized efficiently. Thus, at a defined light—saturated photosynthetic rate, integral photosynthesis in general was enhanced during thermal stratification relative to levels attained during lake mixing.
Natural Selection on Behavioral Phenotypes of the Lizard Uta Stansburiana
Authors: Fox Stanley F.
Published: May 2006

Behavioral differences of resource utilization between juvenile survivors and nonsurvivors of the desert side—blotched lizard Uta stansburiana stejnegeri were measured. Marked individuals were followed through time in enclosed field plots so disappearance indicated death. In the 1st yr of the study, one characterized by light selection pressure, survival was independent of most of the behavioral variables. In the 2nd yr of the study, one characterized by heavy selection pressure, oversummer survivors were active under conditions of clearer skies, warmer temperatures, later morning hours, earlier evening hours, and were associated with vegetation furnishing more shade and shelter than nonsurvivors. Two—way analysis of variance demonstrated behavioral differences between survivors and nonsurvivors, independent of age. Survivors were seen less frequently during the summer of heavy selection pressure, and were more specialized in their activity (smaller phenotypic breadth). The home ranges of survivors contained significantly greater densities of yucca and shelter holes, and lesser densities of Croton. The diversity of plants and other elements on the home ranges of survivors was significantly greater than on those of nonsurvivors. Older, larger juvenile lizards moved to the unmodified sector of an experimental plot in which vegetation was cut away. Those older juveniles which showed some activity on the modified sector experienced heavy mortality. No genetic basis for the behavioral differences between survivors and nonsurvivors was obtained. The behavioral variation in the population, if directly heritable, may be maintained by opposing, fluctuating selection pressures; or, if a consequence of microhabitat differences, by the limiting amount of superior habitat used somewhat exclusively by surviving lizards. The quality of the home range an individual inhabits probably influences his behavior and ultimately his survival. Aggressive behavior may influence the quality of home range in individual obtains.
Can Competition Be Detected Using Species Co‐Occurrence Data?
Authors: Hastings Alan
Published: May 2006

I determined the equilibrium structure of a model of competition with an arbitrary number of species. The species were assumed to live on a large number of identical islands (patches), and all species were assumed to have similar characteristics. In model system, competition was included by letting the per species extinction rate on patches increase with the number of species. I showed that with reasonable sample sizes and strong competition, it is not possible to show statistically that the resulting species distributions differ from random assortment. In more realistic situations where species and islands differ, competition would be even more different to detect, so using species co—occurrence data to look for competition is likely not to detect even strong competition.
The Responses of Plankton Communities in Experimental Ponds to Atrazine, the Most Heavily Used Pesticide in the United States
Authors: DeNoyelles Frank , Kettle W. Dean , Sinn Donald E.
Published: May 2006

Experimental ponds received single additions of the herbicide atrazine in concentrations of 20 and 500 mg/L, and were compared to control ponds for 136 d. Atrazine is an inhibitor of photosynthesis, and both concentrations depressed phytoplankton growth in the ponds within a few days. This was followed by successional changes leading to the establishment of species of phytoplankton more resistant to inhibition by atrazine. Laboratory studies verified this resistance and verified effects on other species at concentrations of atrazine as low as 1—5 mg/L. When and to what extent resistant species appeared in the phytoplankton communities differed with treatment. At the atrazine concentration of 500 mg/L, there was a delayed appearance but eventually a greater biomass and persistent of these species. The grazing zooplankton influenced these differences and were in turn affected by them. Natural interactions such as competition and predation among the species of the communities greatly affected their responses to the toxic chemical. The importance of atrazine as an environmental pollutant is suggested by these responses to concentrations of 1—5 mg/L, which are common downstream in many agricultural watersheds, 20mg/L, which is the high level found in these waters, and 500 mg/L, which is the high level found in waters directly adjacent to treated fields.
Ecosystem Analysis of the Tallgrass Prairie: Nitrogen Cycle
Authors: Risser P. G. , Parton W. J.
Published: May 2006

Nitrogen cycling in tallgrass prairie was studied by using nitrogen content and concentration data for various ecosystem components on grazed and ungrazed tallgrass prairie in northeast Oklahoma. The seasonal dynamics of N in various plant parts was described, and the annual flow of N among different compartments was calculated. A simulation model was used to study the effect of grazing, fertilization, irrigation, and fire on nitrogen cycling. The results show that grazing, fertilization, and irrigation increase the nitrogen cycling rates. Annual spring fires reduce plant uptake and mineralization of N from plant parts, while mineralization of N from organic matter is increased.
Atmospheric CO_2 Concentrations Within a Mixed Forest: Implications for Seedling Growth
Authors: Bazzaz F. A. , Williams William E.
Published: May 2006

We studied carbon dioxide concentrations in a mixed deciduous forest in New England, USA by making continuous measurements at 0.05, 0.2, 3, and 12 m above the soil surface. The measurements began in early March and continued until the end of November 1985; therefore, they spanned the growing season and parts of the dormant seasons both before and afterwards. The data were compared with those from Mauna Loa, Hawaii, which represent global atmospheric CO2 levels in the Northern Hemisphere. The results show strong seasonal and daily variations in CO2 concentrations at all heights in the forest. On average, during the growing season, CO2 levels were generally higher in the forest than in bulk air at Mauna Loa. The highest level of CO2 was found near the forest floor and the lowest at the 12—m level. Daily levels of CO2 were constant throughout the day in the dormant season and were the same for all heights in the forest. However, during the growing season, the CO2 concentrations were lowest during the middle of the day, especially at the 12 m height. Thus, this study shows that the CO2 concentrations in the forest may be quite different than those in bulk air and that seedlings, saplings, and mature trees may experience different CO2 environments for which they may show different responses in photosynthesis, growth, and water use. Moreover, a tree may experience different CO2 environments as it grows towards the canopy, and different modules of an individual may also be growing in different CO2 atmospheres.