bluuebunny/crossref_metadata_2025 huggingface dataset:
Clutch Size, Nest Predation, and Distribution of Avian Unequal Competitors in a Patchy Environment
Authors: Moller Andres Pape
Published: May 2006

I studied reproductive success, nest predation, and most productive clutch size of seven passerine bird species (Barn Swallow Hirundo rustica, Great Tit Parus major, European Blackbird Turdus merula, Yellowhammer Emberiza citrinella, House Sparrow Passer domesticus, Chaffinch Fringilla coelebs, Black billed Magpie Pica pica) breeding in a patchy environment of tree lots in a matrix of farmland. Habitat patches of 0.07—3.61 ha consistent primarily of elm Ulmus glabra and various other deciduous trees, a thick undergrowth of bushes, primarily elder Sambucus nigra, and herbs. Clutch size of all species was independent of habitat patch size even after controlling for time of breeding and age of breeding birds, but brood size at fledging increased with increasing patch size for the Great Tit, European Blackbird, Yellowhammer, Chaffinch, and Black—billed Magpie, but not far the Barn Swallow and House Sparrow. The frequency of nest predation decreased with increasing size of habitat patches in the open—nesting European Blackbird, Yellowhammer, Chaffinch, and Black—billed Magpie, but not in the hole— or the semi—hole nesters. Brood size of Great Tit, European Blackbird, Yellowhammer, Chaffinche, and Black—billed Magpie increased with increasing patch size even after excluding nests suffering from nest predation. The most productive clutch size measured as the second—order polynomial regression of brood size at fledging on clutch size differed in relation to habitat patch size for the Great Tit, European Blackbird, Yellowhammer, Chaffinch, and Black—billed Magpie, but not for the Barn Swallow and House Sparrow, suggesting that there is no unique optimal clutch size in the first five species. A decreasing fraction of yearling breeding birds was found with increasing patch size in the Great Tit, European Blackbird, Yellowhammer, and Chaffinch, but not in the Barn Swallow and House Sparrow, Habitat patches differed in quality with respect to absolute food abundance and nest predation risks, and territorial species relying nearly exclusively on food resources found within patches or suffering from intense nest predation followed an ideal despotic distribution, while colonial species relying on food resources outside patches followed an ideal free distribution for unequal competitors.
An Invader Alters Germination and Growth of Native Dominant Tree in Hawai'i
Authors: Walker Lawrence R. , Vitousek Peter M.
Published: May 2006

Myrica faya, an introduced nitrogen—fixing tree, is rapidly invading volcanically disturbed forests in Hawaii Volcanoes National Park on the island of Hawai'i. We investigated the direct effects of Myrica on the establishment and growth of the native tree, Metrosideros polymorpha, in a forest that is recovering from burial under a 10—200 cm deep layer of volcanic cinder. The number of Metrosideros seedlings found under Metrosideros trees, in the open, and under Myrica trees was 0.12/m2, and 0.01/m2, and zero, respectively. Myrica litter inhibited Metrosideros germination in field experiments; germination occurred only when Myrica litter was removed. In open sites, the addition of nitrogen—rich Myrica soils increased dry mass accumulation of transplanted Metrosideros seedlings. Shade from Myrica canopies (or shade cloth) increased survivorship and height growth but not dry mass accumulation of Metrosideros seedlings. Isolated Metrosideros trees increased diameter growth in response to nitrogen fertilizer but not in response to the presence of adjacent Myrica trees, despite previous findings that Myrica trees elevate available nitrogen in the soil. Myrica basal diameter and height growth were consistently greater than that of Metrosideros in all size classes. Although Myrica does not readily invade closed, late—successional Metrosideros forests, on young, volcanically disturbed soils it is rapidly establishing dense, monospecific stands under which Metrosideros does not regenerate.
Experimental Analysis of the Role of Emigration in Population Regulation of Desert Pupfish
Authors: McMahon Thomas E. , Tash Jerry C.
Published: May 2006

Pupfishes (genus Cyprinodon) have existed for long periods in small desert springs and streams that often lack competitors, major predators, and large environmental changes. How their populations are regulated in these systems is poorly known. We experimentally examined the role of emigration in regulating populations of pupfish by (1) comparing the dynamics of populations held in four pools open to emigration with those of populations held in four pools closed to emigration over a 22—mo period and (2) comparing the rates of emigration from open pools having different densities of pupfish in relation to resources. When pupfish populations were prevented from emigrating, they exhibited symptoms of overpopulation similar to those well documented for fenced populations of small mammals. Their numbers increased more than populations in open pools, mortality was high, and body condition and recruitment were low. Again, as in small—mammal populations, these responses were absent in pupfish populations in open pools, where 84% of the total numbers of fish produced emigrated. The timing and magnitude of pupfish emigration was similar in all four open pools and was related to seasonal temperature changes. More males emigrated than females, and emigrants had significantly lower condition factors than residents. When densities of pupfish in relation to resources were doubled in two open pools, the percentages of pupfish that emigrated (41.8 and 42.2%) were almost equal to the 50% drop in resources. In contrast, only 15.5 and 16.0% emigrated from two control pools, where pupfish densities in relation to resources were left unchanged. Our results, demonstrating that emigration is a potentially significant mechanism for regulating populations of desert pupfish, are consistent with data from a variety of other animals (small mammals, hydra, roe deer), suggesting that population regulation via a behavioral spacing mechanism may be common among mobile animals.
Ultimate Adjustment of Clutch Size to Parental Feeding Capacity in a Passerine Bird
Authors: Slagsvold Tore , Lifjeld Jan T.
Published: May 2006

We examined Lack's (1954, 1966) hypothesis that the clutch size of altricial birds is adjusted to the number of young that can be fed and reared successfully by the parents. A critical test would be to manipulate the future potential of parents to rear the brood so that the female is already aware of this potential at the time clutch size is determined. We made such an experiment with Pied Flycatcher (Ficedula hypoleuca) females, by removing certain feathers from their wings and tail when they were incubating, and then removing the initial clutches to induce renesting. The "handicapped" females reduced the size of the repeat clutch more than control females, and they also fed their young at a lower rate, confirming Lack's hypothesis.
Estimation of Small‐Mammal Population Size
Authors: Menkens George E. , Anderson Stanley H.
Published: May 2006

Two methods of estimating small—mammal population size from multiple capture—mark—recapture occasions, the Lincoln—Petersen estimator and program CAPTURE, were evaluated and compared using computer simulation. Comparisons were made for population sizes of 50, 75, and 100, trapping periods of 5, 7, and 10 d and numerous patterns of capture probabilities. Program CAPTURE often failed to provide useful estimates of population size due to the poor performance of its model selection procedure. The Lincoln—Petersen estimator provides estimates of population size with low biases and standard errors except when uniform behavioral response occurs or when there is high heterogeneity in individual capture probabilities. We suggest that researchers use CAPTURE as a screening technique to check for evidence of uniform trap responses and/or high individual heterogeneity in capture probabilities. If these influences are not present, the Lincoln—Petersen estimator may be the best method for estimation of small—mammal population size.
Comparing Predator‐Prey Models to Luckinbill's Experiment with Didinium and Paramecium
Authors: Harrison Gary W.
Published: May 2006

When Leo Luckinbill (1973) grew Paramecium aurelia together with its predator Didinium nasutum in 6 mL of standard cerophyl medium, the Didinium consumed all the prey in a few hours. When the medium was thickened with methyl cellulose, the populations when through two or three diverging oscillations lasting several days before becoming extinct. When he used a half—strength cerophyl medium thickened with methyl cellulose, the populations maintained sustained oscillations for 33 d before the experiment was terminated. The data from this experiment provide a rare opportunity to test current predator—prey models. A standard differential equation predator—prey model with a carrying capacity for the prey and a saturating (Type 2) functional response predicts the outcome of Luckinbill's experiment qualitatively, but does not give a good quantitative fit to the data. Several modifications of this model are tested against the data for the populations grown in the medium thickened with methyl cellulose, using the Marquardt—Levenberg method to obtain the least squares best fit. Neither Leslie type models nor models with a ratio—dependent functional response do well, but adding either predator mutual interference or a sigmoid (Type 3) functional response improves the fit dramatically. Modeling the predator growth rate to depend on energy or nutrient storage instead of directly on the rate of consumption of prey, thus creating a delayed numerical response, along with predator mutual interference or a sigmoid functional response, produced the best models and gave excellent fits to the data. These models are further validated by the fact that changing only one or two parameter values to reflect the unthickened medium or the half—strength medium also gives reasonably good fits to the other data sets. The last model requires a more sigmoid functional response to fit the data in the thickened than in the unthickened medium, suggesting that an increase in the cost—benefit ratio of energy spent searching to energy gained capturing prey inhibits the predator searching at low prey densities.
Canopy Structure and Photon Flux Partitioning Among Species in a Herbaceous Plant Community
Authors: Hirose Tadaki , Werger Marinus J. A.
Published: May 2006

This paper demonstrates a new analysis of photon flux partitioning among species and an evaluation of the efficiency of photon flux capturing in terms of biomass investment. For that purpose, distributions of aboveground biomass, leaf area, and photon flux density (PPFD) were determined with the stratified harvest method in a stand of a tall herbaceous community on a floating fen at the time of peak standing crop. The stand contained 11 species and the photon flux absorbed by each species in the stand was estimated. Three tall dominant species absorbed 75% of the incident PPFD, while eight short subordinate species absorbed 2.5%. Tall species in the canopy received higher PPFD averaged over leaf area (°area). However, the PPFD absorbed per unit aboveground biomass (°mass) of the tall species was not higher than that of the subordinate species. °mass is a product of the leaf area ratio (LAR, the ratio of leaf area to aboveground biomass) and °area. There was a trade—off relationship between LAR and °area. To have a high °area, plants place their leaves at higher positions in the canopy, which necessarily decreases LAR. Aboveground biomass was regarded as an investment (cost) to capture PPFD (benefit). °mass, as a ratio of benefit to cost, indicates an efficiency of biomass investment to capture photon flux. Tall species appeared to have an advantage over subordinate species in receiving a large fraction of incident PPFD, while subordinate species have an advantage in efficiently using their biomass to capture PPFD.
Resource Allocation in Relation to Leaf Retention Time of the Wintergreen Rhododendron Lapponicum
Authors: Jonasson Sven
Published: May 2006

Extended leaf longevity has been explained both as an adaptation to conserve nutrients and to increase seasonal or life—time carbon gain. The relative importance of these possible adaptations was investigated in the wintergreen dwarf shrub Rhododendron lapponicum by analyses of responses of growth and nutrient allocation to early and late spring defoliation and to natural reduction of leaf biomass following flowering. Removal of old leaves at the time of budbreak had no effect on biomass accumulation and nutrient pools of the new shoots 1 mo later, although the defoliation prevented the transport of resorbed nutrients from old leaves to the new shoots. This transport accounted for by far the largest part of the increase in shoot N and P in underfoliated controls. Extra nutrients were not transported from stems and roots to compensate for the lost import from old leaves. Hence, defoliated plants compensated for the losses solely by increasing nutrient uptake. In contrast, defoliation 3 wk earlier caused a 29% reduction of annual shoot growth and a proportional decline of nutrient pools in the shoots, while the nutrient concentrations, reflecting the shoot nutrient status, did not decline. Almost identical responses were observed in unmanipulated plants the year after heavy flowering. Reproductive branches do not form leaves, resulting in a decreasing photosynthetic capacity and a reduction of resource storage and allocation to next year's leaf cohort. Although early and late spring defoliation caused equal losses of mobile nutrients, only the early defoliation decreased growth, suggesting that it was the loss of carbon rather than nutrients that affected growth. This support earlier observations that the extended retention time of leaves in wintergreen and evergreen species plays a greater role in improving the carbon balance than in supplying the new leaves with nutrients.
Ideal Free Distributions of Stream Fish: A Model and Test with Minnows, Rhinicthys Atratulus
Authors: Tyler Jeffrey A. , Gilliam James F.
Published: May 2006

Ideal Free theory has furthered our understanding of the processes determining the distribution of mobile foragers in a spatially heterogeneous (patchy) habitat. The Input Matching rule derived from Ideal Free theory has been used to predict forager distributions, but does not account for unshared environmentally induced costs that individuals may incur. Drift—feeding stream fish typically contend with such costs in the form of (1) an energetic costs of maintaining position while foraging in moving water, and (2) a decrease in the proportion of drift food items they can capture as current speed increases. These costs often differ between patches, and the cost of position maintenance is unaffected by changes in the number of fish foraging in a patch. Here, we developed an Ideal Free Distribution (IFD) model (IFD With Costs) to describe forager distributions in a habitat composed of patches in which foragers compete for food and incur an energetic cost when foraging that is unaffected by the number of competitors in the patch. We tested the ability of two versions of the IFD With Costs as well as two alternative IFD models to predict the distribution of minnows (Rinichtys atratulus) foraging in a laboratory flow—tank with two side—by—side patches. In all trials the patches differed in their supply of drift food, and in some trials the patches had different water velocities. When the patches' water velocities differed, the fish distributions (1) changed as the total supply of food in the two patches increased while the ratio of food supply in the patches remained constant, (2) did not match relative food availabilities in the patches, and (3) favored the slow water velocity patch. One version of the IFD With Costs predicted each of these results while none of the alternative models could account for these findings. The results support the hypothesis that fish quantitatively integrate energetic gains and costs in a manner similar to that described by the IFD With Costs when making patch selections.
The Consequences of Short‐Distance Larval Dispersal in a Sessile Marine Invertebrate
Authors: Olson Richard Randolph
Published: May 2006

The colonial ascidian—algal symbiosis Didemnum molle Herdman occurs in large numbers (densities of >100 colonies/m2) on shallow patch reefs in the lagoon at Lizard Island, Australia. Shallow—water currents in the lagoon are driven unidirectionally by the southeast trade winds most of the year. The patch reefs are several hundred metres apart and are separated by sand flats. The largest populations of D. molle occur on the upstream ends of the patch reefs. D. molle larvae, followed underwater, dispersed invariably downstream in the direction of the water current at °1—2 m/min. Although larvae were capable of swimming for up to 2 h, they generally settled in 10—15 min when given a suitable substratum such as a dark surface. Larvae experimentally denied a suitable substratum for 40 min settled rapidly when provided with a dark surface, suggesting that when larvae encounter a reef after drifting over sand flats, they probably settle immediately. A qualitative model of larval dispersal was developed to predict the settlement pattern of D. molle in the lagoon at Lizard Island. Three predictions of the model were tested by measuring larval recruitment upstream and downstream of an experimental patch reef for 1 wk before, then 1 wk after denuding the reef of all reproductive D. molle colonies. The results agree with the model, indicating that larval recruitment of D. molle in the lizard Island lagoon is a highly predictable process. The settlement pattern that results from the dispersal model describes the upstream distribution of adult D. molle colonies on patch reefs in the Lizard Island lagoon.
Prealighting Search Behavior and Host Plant Selection by Ovipositing Euphydryas editha Butterflies
Authors: Mackay Duncan A.
Published: May 2006

The relative contributions of nonrandom movement and alighting to prealighting host discrimination by ovipositing Euphydryas editha butterflies were examined. A montane California population of butterflies was studied at two sites in adjacent habitats. In each study site, only one host plant species was used by the butterflies; in one site the host was the annual Collinsia torreyi, in the other it was the perennial Pedicularis semibarbata. In the Pedicularis study site, butterflies discriminated in favor of the host both by searching in areas where it was dense and by alighting preferentially on host plants. In the Collinsia study site, no evidence of a significant prealighting discrimination towards hosts was obtained. The reasons for this difference are discussed; it is suggested that the suitability of Collinsia as a host has increased recently, but that the butterflies have not yet evolved behavioral mechanisms of prealighting search that would enable them to locate these plants efficiently.
Fire and Herbivory in a Great Salt Lake Marsh
Authors: Smith Loren M. , Kadlec John A.
Published: May 2006

The effects of fire and of herbivory by vertebrates on the standing crops and aboveground net primary production of Typha latifolia, Scirpus lacustris, S. maritimus, and Distichlis spicata were investigated in a freshwater marsh adjacent to the Great Salt Lake. Production of T. latifolia, S. lacustris, and S. maritimus vegetation responding after fire was not different from control areas, but flooding following fire eliminated D. spicata. Grazing by wetland vertebrates reduced net primary production of marsh macrophytes. Significant reductions occurred in previously burned areas of T. latifolia (48%), S. lacustris (25%), and S. maritimus (9%), illustrating the importance of the herbivory food chain for inland marshes. Evidence of increased protein in vegetation responding after fire and preferential grazing was consistent with the hypothesis that wetland vertebrates select vegetation of higher nutritive quality. However, alternative hypotheses such as increased foraging efficiency in burned areas could not be dismissed.
Brittlestar Beds: Low‐Predation Anachronisms in the British Isles
Authors: Aronson Richard B.
Published: May 2006

Two epifaunal, suspension—feeding ophiuroids, Ophiothrix fragilis and Ophiocomina nigra, each form dense populations around the British Isles. The ophiuroids carpet the substratum at densities in the hundreds to thousands per square metre. This investigation compared predation on ophiuroids between dense brittlestar beds and subtidal rocky reef communities. Ophiuroids are far less abundant in rocky reefs, and they occupy cryptic microhabitats. The mortality of experimentally tethered ophiuroids was significantly lower in brittlestar beds, where fishes and a portunid crab, Liocarcinus puber, were rare. Starfish, Asterias rubens, ate small numbers of tethered ophiuroids in brittlestar beds and rocky reefs. The low incidence of sublethal damage (regenerating arms) in brittlestar beds reflects a low natural encounter rate with predators. Dense ophiuroid populations declined precipitously in shallow water at the end of the Mesozoic. The near absence of "modern" predators and the consequent low predation levels in living brittlestar bed support the theory of a large increase in consumer pressure beginning in the Cretaceous.
The Ontogeny of Diet Selection in Fledgling Ospreys
Authors: Edwards Thomas C.
Published: May 2006

I studied the ontogeny of diet selection in fledgling Ospreys (Pandion haliaetus) in the 1985 and 1986 postfledging periods to (1) determine if young that forage from the same available fish resource base develop similar diets; (2) examine how diet choice is affected by interactions with conspecifics; (3) examine how within— and between—year differences in fish abundance in two different lake habitats (littoral and limnetic zones) affect patterns of diet selection; and (4) compare diet choice of fledglings with adults who simultaneously were feeding on the same resources base. Eight young in 1985 and 14 young in 1986 were color banded and observed throughout the 170—d postfledging period. Dynamics of the available fish resource base, which consisted of sunfish (Lepomis spp.), shad (Dorosoma spp.), and bass (Micropterus salmoides, Morone saxatilis), were also monitored. Fish abundance differed considerably between years, both by habitat and by fish species. The abundance of all fish was decreasing when young fledged in 1985 and increasing in 1986. Individual differences in diet selection were present in both years, with preference for each fish species in each year exhibited by at least one young. In addition, some young of each year exhibited no preference for any fish species, but instead captured fish in relation to their availability in the lake. Preference patterns of individuals remained constant throughout postfledging in both years, although the mean diet of all young shifted in 1985. Individual preference patterns were strongly related to whichever fish species was captured initially. Siblings, who tended to forage together throughout postfledging, had statistically similar diets. Diets of all young in 1985 and 9 of 14 young in 1986 differed from the mean diet of adults. Although diets of adults shifted during postfledging, diets of young remained constant, suggesting that young were less able than adults to respond to fluctuations in fish availability. Adults also preferentially foraged in the lake's littoral zone whereas young did not exhibit any tendency to choose one habitat over the other. It may be that recognition and use of habitat—related differences in fish abundance represent a skill level not yet developed by recently fledged Ospreys.
Sublethal Parasites and Host Energy Budgets: Tapeworm Infection in White‐Footed Mice
Authors: Munger James C. , Karasov William H.
Published: May 2006

One way to study a parasite's effect on the individual and population ecology of its host is to examine effects on the host's energy budget. A relatively innocuous effect of a gut parasite, such as decreased digestive efficiency, can potentially translate into an effect costly to host fitness, such as decreased reproduction, if other compensations (such as increased rate of food intake) do not occur. We found that infection by the tapeworm Hymenolepis citelli caused a 2% drop in dry—matter digestibility in host white—footed mice (Peromyscus leucopus). A crowding experiment indicated that this result should be applicable to a wide range of intensities of infection. However, we detected no compensation for this decreased digestive efficiency either in amount of food consumed or in mass change (which would indicate use of fat stores or changes in growth). In field experiments we used doubly labeled water to measure effects of tapeworms on field metabolic rate and water influx (potentially a measure of food intake rate), and temperature—sensitive transmitters to measure body temperature. We detected no compensations via these routes either. Our failure to detect compensations indicates that in the nonreproductive mice studied the decrease in digestive efficiency is not of sufficient importance to engender substantial compensations and is therefore unlikely to lead to fitness—altering effects. It is in reproductive animals or in animals subjected to food shortage that such effects would be expected.
Estimates of Pre‐Settlement Insect Damage in Australian and North American Forests
Authors: Morrow P. A. , Fox L. R.
Published: May 2006

Herbivores insect damage to broadleaved evergreen trees prior to widespread European disturbance of the environment was estimated from herbarium specimens collected in western North America and Australia. These data were used to determine whether the current pattern of higher levels of insect damage on Australian Eucalyptus trees than on comparable North American trees of similar habitat and leaf type is an inherent ecological characteristic of these systems or a consequence of recent European disturbance. The historical data confirm present day patterns. Several features of Eucalyptus and of Australian forests and woodlands may predispose them to higher levels of damage.
Drought Stress and Competition Effects on the Local Distribution of Pogogyne Abramsii
Authors: Bauder Ellen T.
Published: May 2006

A field experiment was conducted to determine the effect of competition and drought on the performance of Pogogyne abramsii at the dry end of a steep moisture gradient. Pogogyne is a rare, annual mint restricted to the vernal pools of San Diego County, California. Circular plots in a 1—m band above the elevation of highest water were seeded with Pogogyne as a target species and Pogogyne, Hypochoeris glabra, or Vulpia myuros var, myuros as the intra— or interspecific competitor. Densities of 1—8 competitors per plot were established. Four additional replicates of the Pogogyne/Hypochoeris combination at all eight densities had water added at 3—d intervals throughout the growing season. Additional water had a significant, positive effect on Pogogyne survivorship, with 73% of target plants surviving in water—added plots compared to 34% in unwatered plots. Watered and unwatered plants did not differ in mean final biomass. Suvivorship and final biomass of target Pogogyne plants were not different in low and high density plots. Intra— and interspecific competitors had an equally negligible effect on Pogogyne performance. Competitive effects on target plant biomass were assessed using numbers of individual competitors (initial and final densities) and estimated competitor cover and biomass per plot. In general <15% of the variance in biomass was explained by the abundance of competitors. Results of this study do not support the hypothesis that distribution of Pogogyne on the dry end of the water duration is limited by competition with grassland herbs.
Resilience of Soil Biota in Various Food Webs to Freezing Perturbations
Authors: Allen-Morley Carole R. , Coleman D. C.
Published: May 2006

We tested three predictions of food web behavior: (1) inclusion of a third trophic level will reduce populations on the second trophic level, allowing species on the first trophic level to increase; (2) food chains with an odd number of trophic levels release nutrients to the environment at a slower rate than food chains with an even number of trophic levels;' (3) the longer the food chain, or more complex the food web, the longer the community will take to recover from disturbance. Bacteria (Pseudomonas fluorescens, P. maltophilia, P. paucimobilis, P. stutzeri, and Alcaligenes sp.), and fungi (Fusarium oxysporum) were used for the first trophic level, and nematodes were used for the second (Acrobeloides sp. and Aphelenchus avenae) and third (Mononchus tunbirdgensis) trophic levels. Microorganisms were assembled to form food chains and food webs of different structures in sterile—soil microcosms. The microcosms were designed to allow repeated sampling without losing sterility. Soil samples were removed every 2 wk for 3 mo, then either the experiment was terminated, or samples were taken less frequently. Carbon was added when each sample was taken and the soil removed for analyses was replenished in sterile soil. Population abundance, soil ammonium—nitrogen, and carbon dioxide were measured throughout the course of the experiments. Once constant population levels were established (days 30—40), the systems were frozen to —1°C for 7 d. Sampling continued after the freeze for up to 100 d. The populations on the first trophic level were lower with increased food web complexity or food chain length, refuting the first predict ion. Net nitrogen mineralization and carbon release were greater with increased food web complexity, refuting the second prediction. We speculate that these results are closely linked to the organisms' generation times and species—specific traits, such as rapid grown following a stress (Acrobeloides sp.). Recovery after freezing was highly variable and was more a function of the species than of food web structure. Nutrient cycling and recovery from stress were more dependent upon the species that make up the community than upon the community structure itself.
The Effects of Crowding on Terrestrial Isopods
Authors: Ganter Philip F.
Published: May 2006

Caged populations of two species of oniscoid isopod, Armadillidium nasatum and A. vulgare, were maintained at varying levels of density, food availability, species composition, and population size structure. Growth rate, of mortality, and the size at first reproduction of the young animals were measured in each cage. Increased density reduced the growth rate, survivorship, and the size at first reproduction of both species. Limited food reduced the growth and mortality rates of A. nasatum and the growth rate of A. vulgare. Including both species in the cage increased the growth rate, survivorship, and size at first reproduction of A. nasatum and increased the growth rate of A. vulgare. The presence of large animals increased the growth rate of A. nasatum and reduced it for A. vulgare. These results demonstrate that the density and species composition affect growth, survivorship, and reproduction when food is in excess, probably by interference competition between conspecifics.
Residual Ordination Analysis: A Method for Exploring Vegetation‐Environment Relationships
Authors: Carleton T. J.
Published: May 2006

A technique of analysis is proposed for investigating relationships between a set of vegetation data and environmental variables from the same survey sample sites. The vegetation data are first condensed and transformed by detrended correspondence analysis, an ordination technique. From this point residual ordination analysis proceeds on the assumption that all relationships are linear. Canonical—correlation analysis is used to examine the relationships between the transformed vegetation variables (stand scores on ordination axes) and additive combinations of environmental variables. That combination which most parsimoniously explains the variance (redundancy) on the ordination axes is taken as the basic set of predictor variables. On the assumption that these environmental variables show mutual covariation, they are split into a single natural group (e.g., soil variables) and a remainder. Using linear regression techniques, the unique variation in the predictors of the remainder subset is derived. Through a similar sequence of operations the unique variation of this reminder subset is partialled out of the vegetation ordination and a residual ordination derived. Relationships between this residual ordination and the single natural group of the predictor set are investigated through canonical correlation and visual assessment of scatter plots. The process is repeated for other natural groups of environmental variables in the basic predictor set. The technique is illustrated with a small set of jack pine understory survey data. Residual ordination analysis indicates that forest fire frequency, soil type, and tree canopy composition all play a role in determining understory composition. The assumptions of the method are discussed, and residual ordination analysis is identified as an exploratory, rather than a confirmatory, technique in ecology.
Growth and Reproductive Allocation of Adenocaulon Bicolor Following Experimental Removal of Sunflecks
Authors: Pfitsch William A. , Pearcy Robert W.
Published: May 2006

We used experimental and multiple—regression approaches to test the hypothesis that the availability of direct light (sunflecks) is important for the growth and reproduction of Adenocaulon bicolor, a perennial herb occurring in the understory of a coastal redwood forest. Plants were shaded from direct—beam light (PFD = photosynthetic photon flux density) with shadow bands that still allow receipt of most of the diffuse light. After two growing seasons the shaded plants receiving only diffuse light were significantly smaller than control plants that received both direct and diffuse light. In general, shaded plants decreased in size whereas those receiving sunflecks remaining at the same size. Reproduction was reduced more than vegetative size in the shaded plants. Despite the dramatic effect of removing sunflecks from the light energy available to individual plants, we found no significant relationship between plant performance and the amount of direct PFD available in a microsite, as predicted from hemispherical canopy photographs. Instead, the size and reproductive output of plants growing under natural light regimes was correlated with the average daily diffuse light availability. Although additional direct PFD could be expected to increase carbon gain, the lack of a correlation between the amount of PFD and plant performance may be due to other factors such as water or nutrient stress that increase in microsites with increased direct light.
Alternative Seed Defense Mechanisms in Congeneric Plants
Authors: Siemens David H. , Johnson Clarence D. , Ribardo Keve J.
Published: May 2006

Plants in the same genus often differ by certain putative defense compounds even though they are attacked by the same or similar herbivores. We compared two such congeners in their interactions with the same seed predators. In the Sonoran Desert, seeds of the palo verdes Cercidium floridum and C. microphyllum (Leguminosae) were fed upon by the bruchid beetles Mimosestes amicus, M. ulkei, and Stator limbatus, several species of imperfect soil fungi, and at least one species of termite. Seed suitability was compared in field and laboratory experiments. Seeds of C. floridum were significantly more resistant except to a putative subpopulation of S. limbatus that normally feeds on C. floridum. Cercidium floridum seed resistance was located in the seed coat and was chemically based, as determined by a seed boat removal experiment and experiments in which we transferred extracts from resistant seeds to susceptible seeds. The resistance chemical(s) was not identified, though tests for tannins were negative. Alternatively, C. microphyllum abscised pods early, which significantly reduced attack by the seed predators. Early pod abscission was more effective at reducing seed predation than were resistant seed coats. We suggest that very different plant defense mechanisms may evolve under similar selection pressures within the same lineage, contrary to conventional wisdom.
Experimental Analysis of Microhabitat Selection in Water‐Striders (Heteroptera: Gerridae)
Authors: Spence J. R.
Published: May 2006

Associations between water—strider species and particular types of vegetation structure result proximately from habitat fidelity at the time of spring colonization and from different preferences for habitats within lakes. Individual gerrids discriminate among lakes during prereproductive dispersal on a trial—and—error basis. Gerrid species differ with respect to preference for emergent cover. Wind and surface disturbance increase preference for sheltered habitats in all species studied but extent of response varies among species. Gerris pingreensis and G. comatus actively seek out emergent cover and open habitats, respectively. G. buenoi is a habitat generalist but distribution of this species is affected by a tendency to avoid other species. Habitat associations of these three common Gerris species are explained in terms of habitat preferences and species interactions. Foraging efficiency varies with habitat structure and species morphology and therefore surface habitats provide a variable selective background for the evolution of gerrid species.
Influences on the Distribution of Carex Exilis: An Experimental Approach
Authors: Santelmann Mary V.
Published: May 2006

Carex exilis occurs in bogs and fens along the Atlantic coast, but only in fens in the midcontinent. I experimentally tested five hypotheses explaining this pattern. Reciprocal transplants were made, and no differences were found among populations, implying that there are no bog—adapted ecotypes. Instead, C. exilis growth and reproduction depended most on conditions under which plants were grown. Plants from all populations studied could complete their life cycle in coastal bogs, coastal fens, and midcontinental fens, but not in midcondinental bogs. Greenhouse experiments showed that salinity and acidity had no significant effect on germination and growth; increased nutrient supply and water table level did. Seedlings of C. exilis grew vigorously on peat from a midcontinental bog when the water table was kept artificially near the moss surface, but most died when the water table was at its natural level, 20 cm or more below the moss surface. The water table of coastal bogs is normally high. Water level affected competition for light between Sphagnum moss and C. exilis seedlings. Under the drier conditions of midcontinental bogs, Sphagnum overgrew the seedlings; under wetter conditions typical of coastal bogs, seedlings grew faster than Sphagnum, and coexisted with the moss.
Does Spatial Scale Affect the Incidence of Density Dependence? A Field Test with Insect Parasitoids
Authors: Stiling Peter , Throckmorton Ann , Silvanima Jay , Strong Donald R.
Published: May 2006

We tested the hypothesis that spatial scale of investigation has an effect on the probability of detecting density dependence in the field. Specifically, we examined parasitism rates by two different parasites of one host (the salt—marsh planthopper Prokelisia) with different dispersal capabilities. One parasite, the wasp Anagrus, attacked host eggs and flew widely over the marsh, traveling on the order of kilometres. The other, the strepsipteran Elenchus, parasitized host nymphs and adults. Females of this parasite cannot fly, and dispersal is by triungulinid nymphs, which walk on host plants and can only disperse on the order of metres from their spent maternal corpse. We found no dispersing plant—hoppers to be carrying Elenchus. We expected no density dependence by Elenchus on a large scale, that is, over an area of 1—2 km2 but possibly some evidence of density dependence on a small scale, that is, within a few metres. By similar logic, we expected Anagrus perhaps to show density dependence at both scales because it disperses so well. Parasitism rate by both insect species varied widely both in space, even between adjacent sites, and through times. We found little evidence of density dependence for either type of parasite at any spatial or temporal scale. Failure to detect density dependence may be due less to the effect of looking at the wrong spatial scale than to the idiosyncratic behavior of individual parasite species.
Prospects for Aircraft‐Based Gas Exchange Measurements in Ecosystem Studies
Authors: Matson Pamela A. , Harriss Robert C.
Published: May 2006

Aircraft sampling of the planetary boundary layer can be used to measure trace gas distributions and fluxes directly on regional scales. Laser—based detection and in situ atmospheric sampling can be used for fast—response, accurate measurements of gas concentrations in a column of air over time. Lidar (light detection and ranging) techniques provide a unique method for determining the structure and dynamics of the boundary layer over time. Under stable atmospheric conditions, the combination of these techniques allows estimates of gas exchange between the biosphere and atmosphere.
Habitat Preferences of African Ungulates Derived by Uni‐ and Multivariate Analyses
Authors: Ben-Shahar R. , Skinner J. D.
Published: May 2006

The paper presents a selection of results of a study of habitat preferences of 10 ungulate species on a South African nature reserve. Habitat parameters influencing the distribution of the ungulates were evaluated by incorporating a series of multiple regression analyses and correspondence analysis. The two approaches showed some similarities among preferences of ungulates in relation to the habitat parameters examined. Use of the combination of the broad view provided by the components of the correspondence analysis and the power of discrimination of individual parameters given by the multiple regression is suggested. The combination of different analytical approaches aids in overcoming limitations within each analysis and may provide further insights into the composition of the investigated system.
Carbon Flow From Lignocellulose: A Simulation Analysis of a Detritus‐Based Ecosystem
Authors: Moran Mary Ann , Legovic Tarzan , Benner Ronald , Hodson Robert E.
Published: May 2006

A simulation model is used to predict the fate of carbon derived from lignocellulose in the food web of a marsh community in the Okefenokee Swamp, Georgia. The model, constructed from empirical work spanning a 6—yr period, predicts the role of lignocellulose—derived detritus in supporting secondary production in this ecosystem. Although Carex walteriana lignocellulose is the dominant product of primary production in this marsh and the dominant source of detritus, the model predicts that it may not be quantitatively important in the support of tropic groups other than bacteria and protozoa. The efficiency of transfer lignocellulosic carbon through the microbial loop is low, and overall only 8% of the carbon used by bacteria and fungi is predicted to be ingested subsequently by metazoans. An average of 32% of the bacterial biomass in the marsh water and sediments is predicted by the model to be produced at the expense of lignocellulosic carbon, whereas <10% of the biomass of larger detritivorous animals can be traced to carbon from lignocellulose. Thus in this marsh ecosystem, detritus food webs based on other, less abundant, sources of detrital carbon, or perhaps classical plant—herbivore food webs, are more likely to be the major source of carbon for metazoan production.
Competition and Abiotic Stress Among Trees and Shrubs in Northwest Colorado
Authors: Welden Charles W. , Slauson William L. , Ward Richard T.
Published: May 2006

We tested several hypotheses about the relationship of competition to abiotic stress, using the vegetation of the semiarid Piceance Basin of northwestern Colorado. We studied competition among the shrubs Amelanchier utahensis, Artemisia tridentata, and Symphoricarpos oreophilus, and between the trees Pinus edulis and Juniperus osteosperma, in 10 sites. We calculated several indices of biotic moisture stress, based on the slope, aspect, and evaluation of each site. Competition was measured by regression of the distance separating neighboring plants vs. the sum of their canopy areas. The slope of such a regression (if significant and positive) measures the intensity of competition, and its coefficient of determination (r2) measures the importance of competition, between the plants involved. We detected competition among these plants in all but one combination of species and in most sites. No significant differences in the intensity of competition were found within species combinations. Significant differences in the importance of competition were found in one of three interspecific combinations of shrub species, and in two of three combinations of tree species. Neither the intensity nor the importance of competition showed any consistent relationship with any index of abiotic moisture stress. Thus, no hypothesized relationship between abiotic stress and competition is supported. Our data also show no consistent relationship between the importance of competition and its intensity, supporting our hypothesis that the intensity and the importance of competition are independent.
The Effects of Disturbance Size and Frequency on a Shortgrass Plant Community
Authors: Coffin D. P. , Lauenroth W. K.
Published: May 2006

We examined the effects of small, patch—producing disturbances on a shortgrass plant community dominated by the perennial grass blue grama (Bouteloua gracilis [H.B.K.] Lag. ex Griffiths). Size distributions and rates of occurrence were determined from field data and from the literature for B. gracilis plants and for three disturbance types: cattle fecal pats, western harvester ant mounds, and small animal burrows. A Spatially explicit simulation model was then used to determine the effects of each disturbance. The average turnover rates (percent affected per year) of the basal cover and density of B. gracilis plants in the simulated plots were calculated for three topographic positions and three grazing intensities. Disturbance size relative to the size of B. gracilis plants distinguished two classes of disturbances. Ant mounds and animal burrows were significantly larger than B. gracilis plants, while fecal pats were comparable in size to B. gracilis plants. The frequency of occurrence of a disturbance was found to be a good predictor of the amount of basal cover and the number of pants killed each year by each disturbance type. The fastest turnover rates for basal cover were associated with the smallest and most frequent disturbance type (fecal pats). The effects of these disturbances were found to be dependent on topographic position and grazing intensity, especially the former. Slopes and lowlands had faster turnover rates than uplands. Our results suggest that the recovery of B. gracilis is dependent on the spatial scale of the disturbance.