bluuebunny/crossref_metadata_2025 huggingface dataset:
Demographic Studies of the Chaparral Shrub, Ceanothus Megacarpus, in the Santa Ynez Mountains, California
Authors: Schlesinger William H. , Gill David S.
Published: May 2006

Survivorship patterns and the development of plant community structure were studied in 11 stands of chaparral dominated by Ceanothus megacarpus in the Santa Ynez Mountains, Santa Barbara County, California. Mature plants of C.megacarpus are not fire—resistant and do not resprout following fires; buried, dormant seeds germinate in response to fires, often forming dense, nearly pure, even—aged stands. Self—thinning through intraspecific competition begins about 5 years after fire, presumably when the shrub canopies begin to overlap. Thinning proceeds during the next 10 to 15 years depending on the density of shrubs. Study sites of varying density and age show that the size—frequency distributions of the living shrubs change through the thinning process as a result of mortality which is concentrated among the smaller individuals. Thinning often reduces stand densities by 50% or more; the mortality is greater in stands of higher initial density. Actively thinning stands show a slope of —1.23 in a double—logarithmic plot of stand density versus mean weight per plant, determined by dimension analysis. During thinning, the horizontal pattern of shrubs becomes less clumped. Eventually self—thinning ceases at a point when the stand has reached a high, relatively constant level of biomass. The stand is then dominated by relatively few, large individuals and net primary production is balanced by the death of older plant parts.
Root Weights and Carbohydrate Reserves of Big Sagebrush
Authors: Sturges D. L. , Trlica M. J.
Published: May 2006

Mature mountain big sagebrush plants near the bottom and midway up a hillside had maximum rooting depths between 183 and 213 cm, °60 cm deeper than roots of Wyoming big sagebrush plants just below the ridge. Maximum lateral root spread was between 122 and 152 cm. Approximately 60% of total root system weight was located in the surface 30 cm of soil. Total nonstructural carbohydrate concentrations in root decreased from 6.8% in plants at the bottom site to 3.8% in plants at the ridge site. This response could have resulted from an environmental gradient, by a release from competition, or from subspecies differences. Total carbohydrate reserves of individual plant root systems were °7.7 g.
Host Selection by Mycophagous Drosophila
Authors: Jaenike John
Published: May 2006

Host—selection behavior was studied in the mycophagous species, Drosophila falleni, D. putrida, and D. testacea. By marking flies with fluorescent dusts and recapturing them, it was determined that individuals of each of the 3 Drosophila species are attracted to >1 species of mushroom. There appeared to be little variation among individuals in their preferences for the different species of mushrooms. The polyphagy of individuals is probably selected favored by the unpredictability of fructification of individual mushroom species.
Resource Use by Bill Morphs of An African Finch: Evidence for Intraspecific Competition
Authors: Smith Thomas Bates
Published: May 2006

The African estrildid finch Pyrenestes ostrinus exhibits a polymorphism in bill size that is not related to sex. This paper examines the feeding ecology, sessonal changes in abundance, and diet overlap of the two bill morphs on a study are in south—central Cameroon. The morphs fed on two major sedge species, hard and soft. The diets of the small— and large—billed morphs were influenced by their feeding efficiencies on the sedges and seed abundance. When both major sedges were abundant, diet overlap between morphs was high, but overlap declined during the major dry season when overall food abundance was low. During the dry season, large—billed morphs specialized on the hard—seeded sedge species while small—billed morphs broadened their diet to include other foods. This pattern of resource use is consistent with increased intraspecific compeition for food and similar to patterns of resource use between distinct species during food—lean times. A model for the evolution of the polymorphism is presented.
Seasonal Decline in Reproductive Performance of Song Sparrows
Authors: Hochachka Wesley
Published: May 2006

Seasonal variation in reproductive success and nestling size were studied in a population of Song Sparrows (Melospiza melodia). I tested whether seasonal variation in reproductive success was due to differences among parents or territories, or whether the variation was an effect of time felt by all individuals in the population. Clutch size declined in the population through the breeding season, and the decline was due to some classes of females both nesting later and laying smaller clutches. Late lying and smaller clutches of yearling females were a major contribution to seasonal decline in clutch size, although seasonal decline in clutch size was also observed within age classes of females. Hatching success was roughly 10% lower for second broods than first broods, but did not vary with date that the nest was intiated. Survival of offspring did not vary seasonally during the period that offspring were in the care of their parents. Although nestling size was greater later in the breeding season, the probability of independent offspring recruiting into the breding population was lower for young born later in the breeding season. The seasonal decline in recruitment of offspring was not due to differences in the quality of territories or parents.
Seed Size and Plant Growth Form as Factors in Dispersal Spectra
Authors: Westoby Mark , Rice Barbara , Howell Jocelyn
Published: May 2006

All vascular plant species were listed on eight sites near Sydney, Australia. Four sites were on infertile and four on fertile soil. Each of the 355 species was classified according to seed mass (measured as fresh mass of embryo plus endosperm), morphological adaptations for dispersal by different vectors, and growth form. The infertile—soil sites had more species and more cover adapted for dispersal by ants, and the fertile—soil sites had more species and more cover with fleshy fruits adapted for dispersal by vertebrates. We tested the hypothesis that this difference could be interpreted as a secondary correlate of seed size or of growth form. Species with smaller seed mass of growing to <2 m tall were significantly more likely to be adapted for dispersal by ants relative to vertebrates. An indirect association via growth form was capable of accounting for up to 84% of the relationship between soil type and dispersal mode, and indirect association via seed mass for up to 23% of the relationship; the two together could account for a maximum of 85%. The indirect association association via seed mass relatively weaker because seed masses proved not to be very different between these soil types. There remained a significant residual tendency for species in any given seed size class and growth form to be more likely to be dispersed by vertebrates relative to ants on fertile soils than on infertile soils.
Patterns of the Distribution and Abundance of Competing Species when Resources are Heterogeneous
Authors: Naeem Shahid
Published: May 2006

Detecting interspecific competition from census data may be difficult and often impossible when resources are spatially and temporally heterogeneous. This difficulty comes about because the effects of interspecific competition vary with the levels of resources. This phenomenon is clearly shown in the assemblage of arthropods found living in Heliconia wagneriana bracts, microaquatic habitats that are readily manipulated and censused. I manipulated the number of flowers that naturally fall into the bracts and serve as food for many of the arthropods. By the addition or the removal of flowers, the effects of interspecific competition for these resources became evident in changes of the distribution and abundance of the arthropod species. Analyses of these changes revealed that competition was readily detected for two species when resources were diminished, but not detectable when resources were augmented. Thus, census studies of communities that use heterogeneous resources are likely to include assemblages in patches where competition is not occurring, along with other patches where competition is occurring, but knowledge of the resource heterogeneity can itself be used to discern the effects of competition.
Variation in Life History Parameters of California Red Scale on Different Citrus Cultivars
Authors: Hare J. Daniel , Yu Dicky S. , Luck Robert F.
Published: May 2006

We measured survival, growth rate, and initial egg complement of California red scale, Aonidiella aurantii, when reared on foliage and bark of four citrus cultivars in order to detrmine the extent that variation among citrus cultivars may introduce variation in the number and quality of scale for utilization by the introduced parasitoid Aphytis melinus. Among bark substrates, lemon (Citrus limon) and grapefruit (Citrus paradisi) were most suitable for scale survival and growth. Bark of mandarin (Citrus unshiu) was nearly unsuitable for California red scale survival, and bark of sweet orange (Citrus sinensis) was intermediate in suitability. Foliage was a more uniformly suitable substrate for California red scale in that variation in survival and adult sizes was generally insignificant when scale were reared on different cultivars. Substantial differences in concentrations of free animo acids and soluble protein occurred in foliage and bark among citrus cultivars, but these differences were not correlated with differences in scale performance. The more uniform densities and sizes of scale on leaves across cultivars may have contributed to the development of A. melinus' propensity to search the leaves on the tree's exterior more frequently and regularly than the bark on the tree's interior for individual scales suitable for the reproduction of female progeny.
The Window of Vulnerability of a Shoot‐Galling Sawfly to Attack by a Parasitoid
Authors: Craig Timothy P. , Itami Joanne K. , Price Peter W.
Published: May 2006

We studied the susceptibility of a shoot—galling sawfly, Euura lasiolepis, to attack by the parasitoid Lathrostizus euurae. Each Euura had a window of vulnerability: a limited time period when the parasitoid attacked it. Lathrostizus attacked a limited range of gall sizes and ages during each sample period. Gall age was related to its position on a shoot, and Lathrostizus shifted the gall position it attacked as it tracked vulnerable—aged galls. The parasitoid emergence period matched the window of vulnerability of its host. Lathrostizus attacked and successfully parasitized a low proportion of hosts encountered. A low interval—parasitism rate uniformly distributed among Euura galls formed at different times is consistent with the pattern expected if the window of vulnerability influenced attack. The window of vulnerability was formed by the interaction of gall and sawfly development rates. Two hypothesis that defined the window were supported: (1) the host development hypothesis, that Lathrostizus is a larval parasite and does not attack galls containing eggs and (2) the gall toughness hypothesis, that large, old galls that are tough are not attacked by Lathrostizus. Our data did not support (3) the ovipositor limitation hypothesis, that some galls were too large for the larva inside to be reached by the Lathrostizus ovipositor. We observed that if these galls were soft, they were attacked. Lathrostizus parasitism rates on Euura varied widely among willow clones. Willows differed in the rate of gall toughening, which produced different—sized windows and thus contributed to variation in parasitism rates among Euura populations on different willow clones.
An Experimental Separation of Interference and Exploitative Competition in Larval Damselfly
Authors: Anholt Bradley R.
Published: May 2006

Density—dependent reduction in survival, growth, and development rates of larval damselflies can be the result of depletion of food resources or increased interference costs (energy or time) associated with behavioral interactions. While interference has been implicated in several studies, no direct test of this hypothesis by manipulating interaction frequency or intensity has been attempted. To separate these mechanisms, I simultaneously manipulated habitat complexity (number of perches) to alter the frequency of behavioral interactions, larval density, and food supply. Damselflies became more evenly distributed among available perches as the density per perch increased, demonstrating that there were behavioral responses to the manipulation of habitat complexity. Food supply and damselfly density strongly affected survival, timing of emergence, and mass at emergence. However, the proportion of the variation in these performance variables attributable to the habitat complexity manipulation was tiny. In spite of the overt nature of the interactions among individuals, the costs appear to be very low. Future work that implicates behavioral mechanisms in population processes will have to attempt more direct manipulations of the behavior itself to test the hypothesis before concluding that behavior is the cause of an observed pattern.
Resource Limitation in a Stream Community: Phosphorus Enrichment Effects on Periphyton and Grazers
Authors: Hart David D. , Robinson Christopher T.
Published: May 2006

A field experiment was used to test whether stream periphyton was nutrient limited, and if so, whether the grazing insects that consumed this resource were food limited. Eight replicate wooden flumes were placed in an unshaded riffle in Augusta Creek, a third—order, hardwater trout stream in southwestern Michigan (USA). Phosphorus, in the form of Ca(H2PO4)2, was added to four of these flumes for 105 d. The average concentration of soluble reactive phosphorus (SRP) maintained in P—enriched flumes (85 mg/L) was >5 times as great as in control flumes (15 mg/L). After 77 d of nutrient enrichment, periphyton biomass per unit area on substrates with reduced grazer densities was 1.8 x as great in P—enriched flumes as in control flumes. The larvae of two species of grazing caddisflies (Leucotrichia pictipes and Psychomyia flavida) in P—enriched flumes also had higher individuals mass, developmental rates, and population densities. These responses resulted in standing crops of final—instar larvae in P—enriched flumes that were 2.1x and 1.5x as great as those in control flumes for Psychomyia and Leucotrichia, respectively. This demonstration of food limitation supplements previous behavioral studies showing that both grazer species aggressively defend feeding territories, and contributes to a growing body of evidence suggesting that resource limitation is more important in some stream food webs than previously recognized. Resource limitation of these grazer populations may be due in part to the rarity of catastrophic flooding and drying in Augusta Creek. Such stable flows could permit consumer populations to reach higher densities than in frequently disturbed streams.
Ecology and Breeding Phenology of Larval Hyla Andersonii: The Disadvantages of Breeding Late
Authors: Morin Peter Jay , Lawler Sharon P. , Johnson Elizabeth A.
Published: May 2006

We exploited natural variation in the breeding phenology of Hyla andersonii, the Pine Barrens Treefrog, to test whether offspring hatching at different times during the breeding season differed in growth and survival. We studied the growth and survival of two successive cohorts of Hyla tadpoles in 12 artificial ponds, where we also manipulated abundances of two kinds of organisms that might interact seasonally with Hyla: aquatic insects and Bufo tadpoles. We also measured whether the abundance of periphyton changed seasonally in response to manipulations of aquatic insects and tadpoles, to described temporal patterns of resource availability that might affect tadpole growth and survival. Additions of hatchlings occurred 34 d apart, and the first cohort completed development before the addition of the second cohort. Additions of aquatic insects and Bufo woodhousii tadpoles, separately or together, did not significantly affect survival in either cohort. Although the initial larval density of the fish cohort was 1.5 times that of the second, the first cohort survived better, grew and developed more rapidly, and metamorphosed at larger size that the second cohort. Competition from Bufo tadpoles and insects, as inferred from reduced growth rates relative to controls without these interspecific competitors, was more pronounced in the first cohort than in the second, and coincided with reduced standing crops of periphyton, and important food for tadpoles. Although breeding late in the season reduced the growth and survival of offspring, potential reductions in the fitness of late—breeding adults are problematic because of uncertainties about the number of breeding efforts made by each female each year. Breeding late would be disadvantageous if different subpopulations breed only early or only late in the season, and if the patterns seen in our study hold for natural ponds. However, if late breeding efforts represent additional clutches deposited by adults that have already reproduced earlier that year, production of additional offspring later in the season could increase fitness.
On the Relative Abundance of Species: Postfire Changes in a Coastal Sage Scrub Rodent Community
Authors: Price Mary V. , Waser Nickolas M.
Published: May 2006

Mac Arthur's Q—Minimization theory of competition—mediated community structure suggests a protocol for predicting species abundances from knowledge of the availabilities and species utilizations of critical resources. Using this protocol, we compared predicted changes in rodent abundances with those observed over the two and one—half years following a fire in a topographically uniform area of California coastal sage scrub vegetation. Approximately half of our permanent census grid covered an area burned in 1979, and half covered an adjacent unburned area. A separate trapping survey at a nearby unburned site showed that the five rodent species common to the permanent grid had distinct capture frequencies in four structural microhabitats. We used these microhabitat—specific capture frequencies as resource—utilization functions for each species, because structural microhabitats appear to be related to underlying resources that limit rodent populations. Resource—availability functions were derived from censuses of vegetation and other structural features on burned and unburned sides of the permanent grid. Predictions from Mac Arthur's theory were upheld in that (1) abundances of each species on one side of the grid relative to the other correlated significantly with relative K values on the two sides of the grid (K values were calculated as the overlap between microhabitat utilization and availability functions); (2) relative abundances of the five rodent species on the burned side of the grid were correlated with those predicted from relative K values; and (3) the total microhabitat utilization of the rodent community on each side of the grid matched overall microhabitat availability on that side well and matched availability on the other side poorly. These results provide support for a potentially powerful protocol for predicting identities and relative abundances of coexisting species. Although this protocol should provide results similar to those from a more inductive one relying on empirical foreknowledge of spatial correlations between resource and consumer abundances, it differs in important respects.
Local and Latitudinal Variation in Predation on an Herbivorous Marine Snail
Authors: Fawcett Michael H.
Published: May 2006

Intertidal grazing molluscs show striking local and geographic variation in distribution and body size. Their density is generally greatest in the upper and mid—intertidal zones on rocky shores of the Pacific coastal of North America, and least in the low intertidal zone. However, the elevation of the lower limits of their shore—level distribution varies considerably among sites. In addition, the lower limits are generally higher south of central California than in the north, and the average body size is usually smaller in the south than farther north. This is exemplified by the commonest molluscan grazer in the mid—intertidal region of many rocky shores on this coast, the trochid snail Tegula funebralis. A hypothesis to explain these patterns, that they are due to variation in predation, was tested. At sites where predator density was high, regardless of latitude, the lower limit of Tegula's intertidal range was higher on the shore, compared with areas containing few predators. Shell width was usually smaller, and increased with elevation wherever predators were abundant. Predators were more abundant at southern than at northern sites. The primary predators of Tegula are octopuses (Octopus bimaculoides and O. bimaculatus), the starfish Pisaster ochraceus, and crabs of the genus Cancer. The octopuses occur only at the southern sites, whereas Pisaster and Cancer are found throughout the geographic range of the study. Reciprocal transplants of marked Tegula, done between several southern and northern sites, showed that the lower limit of distribution of adult Tegula on the shore at a given site is influenced by the snails' defensive behavior. Where predators were abundant, both southern and northern Tegula placed low on the shore rapidly climbed to higher levels than in locations where predators were scarce. The probable cause of this upward migration is an avoidance response to chemical exudates from predators. Southern Tegula also have an inherently different response than northern Tegula; southern snails always climbed upwards sooner and reached higher levels than northern snails. This resulted in predators killing more of the northern snails in these experiments. The behavioral difference between southern and northern Tegula may be the result of long—term differences in the intensity of predation by octopuses, which are much rarer in the intertidal north of central California than south of there. Young—of—the—year Tegula were found mainly in the mid—intertidal region. The shift in the direction of shore—level size gradients of Tegula populations between high and low predation sites may be caused by movements of adult snails to levels above or below the less mobile juveniles. The relative scarcity of Tegula and other grazing molluscs in the low intertidal zone is probably due both to upward migration and high mortality rates, with predation being a major cause of both.
The Spotted Owl Fracas: Mixing Academic, Applied, and Political Ecology
Authors: Simberloff Daniel
Published: May 2006

Recent intensive field study and modelling of the Spotted Owl foster optimism that scientists can help solve problems in managing threatened species and can cooperate with government agencies to stem the decline of biotic diversity. The effort shows that field biologists can quickly conduct coordinated research on a species whose habits make it difficult to study, and that modellers can quickly use field data to refine questions and to predict effects of potential management strategies. However, the key empirical finding–that the owl's home range typically encompasses huge amounts of old—growth forest–has generated enormous unease among forest industry interests. They do not relish the prospect that the United States Department of Agriculture Forest Service will sequester thousands of acres of forest with timber worth thousands of dollars per acre. The Forest Service, in its tentative plan for managing the Spotted Owl, seems to have taken a middle course. Their preferred alternative includes substantially fewer and smaller sites than are recommended by an advisory panel formed by the major ornithological societies. However, this plan includes much more old growth than the previous one does. Should this plan be enacted, the small chance for long—term survival of the spotted Owl will require increased adherence by individual forests to the spirit as well as the letter of the plan.
Can Leaf Aspect Affect Herbivory? A Case Study wih Tea
Authors: Banerjee Barundeb
Published: May 2006

Leaves of tea cultivars are markedly different in leaf geometry, with leaf angle varying from erect through semi—erect to horizontal; a particular cultivar has only one type of leaf. Defoliators and leaf chewers feed mostly on horizontal leaves. Leaf rollers and miners colonize only semi—erect leaves. Sap feeders inhabit all three types of leaves, with the largest species accumulation on erect leaves.
Predator Avoidance by Cricotopus: Cyclomorphosis and the Importance of Being Big and Hairy
Authors: Hershey Anne E. , Dobson Stanley I.
Published: May 2006

In laboratory experiments, we tested in importance of body size and long abdominal hairs in determining susceptibility of two aquatic midges, Cricotopus sylvestris and C. bicinctus, (Diptera: Chironomidae), to predation by Hydra and the damselfly Ischnura verticalis. The chironomids co—occur on dense macrophytes in the Yahara River, Dane County, Wisconsin, USA, and differ structurally in that C. sylvestris has long hairs and C. bicinctus has short hairs. We also monitored Hydra density and C. sylvestris hair length for 2 yr. Large C. bicinctus were less susceptible to predation by both Hydra and small Ischnura, but prey size had no effect on susceptibility to large Ischnura. Long hairs offered no protection from Ischnura, but did protect C. sylvestris from Hydra, relative to short—haired C. bicinctus or C. sylvestris with experimentally shortened hairs. Hair length is negatively allometric relative to body size in ontogeny; hair absolute length remains constant as the larvae increase in size. Hair length was also cyclomorphic; successive generations of C. sylvestris showed a seasonal pattern of hair length. Hydra density also varied seasonally. When Hydra density was high, hairs on C. sylvestris were longest; when Hydra density was low, hairs were shortest. The observed cyclomorphic and allometric patterns of hair length were both beneficial characteristics in the presence of Hydra, and the benefits were additive.
Influences of Flow Regime on Development and Desiccation Response of Lotic Diatom Communities
Authors: Peterson Christopher G.
Published: May 2006

Diatom communities developed in either sheltered or direct—current habitats in the Colorado River below Lake Mead were subjected to short—term desiccation to test the hypothesis that communities inhabiting placid (sheltered) environments are less resistant to unpredictable disturbance than are those from more rigorous (direct—current) habitats. Direct—current communities exhibited lower cell accumulation rates and supported lower biomass and diatom densities than sheltered communities, suggesting that current shear and flow variability limited accumulation in direct current. Diatom communities that developed in sheltered habitats were less resistant to desiccation stress than communities from more rigorous flow conditions. Both biomass and cell densities were reduced by desiccation on sheltered substrates, whereas the same desiccation frequency in direct current had either no effect or caused increases in abundance parameters, depending on community age. Desiccation caused greater taxonomic change in sheltered than in direct—current environments. Low resistance to desiccation in sheltered environments was probably due to lack of nutrient renewal by current and to competition for resources in high—density communities. Temporal variability in ash content was greater in undisturbed than in desiccated communities in both flow regimes, indicating greater binding capacity of desiccated communities and suggesting that mucilage production increased with desiccation. In direct—current communities, where diatom accrual was suppressed by shear forces associated with fast flow, mucilage production after desiccation may have enhanced diatom immigration success, resulting in both higher densities and diversity in desiccated communities.
Paedomorphosis in Ambystoma Talpoideum: Effects of Density, Food, and Pond Drying
Authors: Semlitsch Raymond D.
Published: May 2006

The expression of metamorphosis and paedomorphosis in the salamander Ambystoma talpoideum was examined in a three—factor experiment using artificial ponds. This experiment was designed to measure simultaneously the effects of initial larval density, food level, pond drying regime, and their interactions on the percentage of individuals metamorphosing or becoming paedomorphic. A higher percentage of individuals metamorphosed from drying low—density, and high—food ponds than from constant water level, high—density, and low—food ponds. A higher percentage of individuals became paedomorphic in constant water—level and low—density ponds than in drying and high—density ponds. Food level had no effect on the expression of paedomorphosis. Initial density, food level, and drying regime all had significant effects on the mean snout—vent length of larvae in ponds. The percentage of paedomorphic individuals in a pond was positively correlated with mean snout—vent length in the population. Analysis of covariance using snout—vent length as the covariate removed density and food level effects on the percentage of metamorphic individuals but did not remove the effects of drying regime. Use of snout—vent length as covariate in an analysis of covariance for the percentage of paedomorphic individuals failed to remove significant effects of density or drying regime. Only initial density affected the survival of larvae. The factors manipulated in this experiment affected the expression of paedomorphosis both directly through the physical environmental conditions that they created and indirectly through the growth and body size of larvae. These results suggest that differences in environmental factors among populations can affect the life—history pathways of individuals and lead to a predominance of paedomorphosis in permanent aquatic habitats with favorable growth environments.
An Approach to the Perturbation Analysis of Optimal Life Histories
Authors: Caswell Hal , Real Leslie A.
Published: May 2006

It has been suggested that, while natural selection may not produce optimal phenotypes, it should at least produce populations whose phenotypes differ in the directions of their optima. Optimization models are thus most readily tested in terms of the response of an optimal strategy to perturbations. This paper presents a method for calculating the perturbation response of a constrained optimum when the constraint function is at least approximately linear. We consider examples from life—history theory, but the method is widely applicable. Even for simple two—variable optimization problems subject to linear constraints, the response to perturbations is surprisingly indeterminate and sometimes counter—intuitive. This indeterminacy is resolved by the values of the second derivatives of fitness with respect to its components.
Clutch Size in Birds: A Predation Perspective
Authors: Lima Steven L.
Published: May 2006

A simple model of clutch size in nidicolous birds is developed in which the cost of reproduction is the risk of predation to both the parent and its dependent young. An analysis of the model shows that (1) conventional ideas of food limitation, though sufficient, are not necessary for existence of an optimal clutch size; (2) predation as the sole cost of reproduction is adequate for the existence of an optimal clutch size; and (3) one need not expect a clutch—size—dependent physiological cost to a parent (leading to increased mortality) to be a major determinant of clutch size. In addition, several experimental and observational results that purport to demonstrate food limitation in breeding birds are also consistent with the idea of predation risk as the major cost of reproduction. Current ideas of clutch size determination and the costs of reproduction are considered in light of the above results, and possible syntheses are suggested. A predation—risk perspective may offer considerable insight into the evolution of clutch size in birds.
Documenting Cmpleteness, Species‐Area Relations, and the Species‐Abundance Distribution of a Regional Flora
Authors: Miller Ronald I. , Wiegert Richard G.
Published: May 2006

Four data sets containing randomly distributed species observations were computer generated. Each data set was characterized by a different species—abundance distribution: (1) the distribution of rare vascular plants in the southern Appalachians, (2) a uniform distribution, (3) a Poisson distribution, and (4) a truncated lognormal distribution. A computerized sampling methodology (miller 1986) recorded the statistically significant and unique species—area relations distinguishing each data set. The most robust species–are relations (i.e., highest r2 adjusted values) typified the rare southern Applachian plant distribution and the truncated canonical lognormal distribution. A method is presented for obtaining a reliable estimate of the total number of species characterizing a taxonomic group within a region. This method is based on analyses of the accumulation of new species vs. The accumulation of observations recorded from a sampling of each of the data sets. A statistically robust negative exponential model was fitted to each of these accmulation distributions. From this model, 195 rare vascular plant species were predicted to inhabit the central southern Appalachians region. This estimate was in close agreement with the actual total of 188 species determined from 150 yr of field observations. Thus, this method of fitting the data from a set of species observations to the negative exponential model provided an estimate of the total number of species for a taxonomic category (rare vascular plants) and a region (southern Appalachians).
Animal Density Estiamtion Using a Trapping Web Design: Field Validation Experiments
Authors: Parmenter Robert R. , MacMahon James A.
Published: May 2006

A recent developed trapping web sampling design (Anderson et al. 1983) and a series of four density estimation models were evaluated using known field populations of ground—dwelling darkling beetles (Tenebrionidae: Eleodes spp.) in a shrub—steppe ecosystem in southwestern Wyoming. Known densities (range 794—11 793 beetles/ha) of marked beetles were established annually in two circular enclosures (415.5 m2 each) from 1984 to 1987. After releasing the beetles, we sampled the marked populations in each enclosure, using 141 pitfall traps arranged in a web design. The resulting data were analyzed with four density estimation models (Fourier series, exponential power series, negative exponential, and half normal) using the program TRANSECT. We then evaluated each model's ability to estimate the true beetle density (D). We found that the negative exponential model was most reliable, estimating D "correctly" (i.e., the 95% confidence interval of D included D) in 100% of the trials (n = 11), and having a mean coefficient of variation (CV) of 20% and no detectable bias. The exponential power series and Fourier series also performed well, providing confidence interval coverage of D in 91 and 73% of the trials, respectively. However, the precision of the exponential power series model was extremely low, resulting in very wide confidence intervals. The half normal model consistently underestimated the true densities, and provided only 18% confidence interval coverage. Tests of the sampling design's assumptions (regarding animal capture probabilities and movements) using actual field data showed that, in most cases, all assumptions were satisfied; even when assumptions were violated, the there successful models continued to function well. An improved procedure for setting confidence intervals is also provided. Based on these analytical results, and considering the logistical and theoretical advantages of trapping webs over trapping grids, we recommend the use of the web design and distance measure analyses for estimating population densities of mobile small animals.
The Life History Consequences of Resource Depression in Daphnia Pulex
Authors: Lynch Michael
Published: May 2006

The life history response of a clone of Daphnia pulex to nine concentrations of a defined food supply is described and combined with an energy analysis. Three features of this clone were essentially invariant with respect to food level: the length–mass regression, the durations of instars, and the size—specific fractions of net energy intake invested in reproduction. The main response to food limitation was a reduction in size—specific net energy intake and in egg size. Simple mathematical functions are fit to the data on these relationships and combined to produce a model that reconstructs the age—specific patterns of growth and reproduction quite accurately. The energy cost of molting is shown to be a major constraint on the evolution of body size in Daphnia.
Dynamics of Mojave Desert Shrub Assemblages in the Panamint Mountains, California
Authors: Webb Robert H. , Steiger John W. , Turner Raymond M.
Published: May 2006

We studied shrub communities in the Panamint Mountains of the Mojave Desert to determine whether vegetational changes after disturbance can be characterized as succession according to modern successional theory. We found, on a variety of disturbed and undisturbed sites, that the rate of change was a function of the type and age of disturbance. Recent debris—flow deposits were colonized by shrub assemblages of different species composition than those on the surrounding, older debris—flow deposits and other geomorphically stable surfaces. Colonization of human—disturbed sites was highly variable, but species compositions were different from the predisturbance species composition. In Grayia—Lycium assemblages, Grayia spinosa reasserted its dominance over colonizers relatively quickly. In Coleogyne assemblages, typically found on older geomorphic surfaces, species composition differences persisted considerably longer, depending on the severity of the initial disturbance. Extremely stable assemblages, dominated by Coleogyne ramosissima, occurred on the oldest, least disturbed surfaces. The variability of species composition among disturbed sites was greater than the variability among undisturbed and geomorphically stable sites, in accord with convergent succession. Models of desert succession should consider several factors: (1) colonization is dependent largely on the severity of disturbances and residual biotic components; (2) the time span for recovery may be longer than past periods of climatic and geomorphic stability; and (3) colonizing species may have considerable range in their life—history strategies.
Furanocoumarins in Wild Parsnip: Effects of Photosynthetically Active Radiation, Ultraviolet Light, and Nutrients
Authors: Zangerl A. R. , Berenbaum M. R.
Published: May 2006

Previous studies of wild parsnip, Pastinaca sativa L., revealed a significant degree of genetically controlled variation in furanocoumarin production. In this study, the influence of environmental factors, namely, soil nutrients, photosynthetically active radiation, and ultraviolet (UV) radiation, on the production of furanocoumarins in the wild parsnip was examined experimentally. Aboveground tissues of plants grown under all combinations of three light intensities and three nutrient levels (N, P, K) were analyzed for the absolute and relative amounts of furanocoumarins. In a second experiment the effect of ultraviolet radiation on furanocoumarin production was evaluated by growing plants under full sunlight with and without an ultraviolet screen. Light and nutrient availability jointly affected the concentration of four of the six furanocoumarins present (sphondin and angelicin being the exceptions). Both nutrients and light were limiting factors in furanocoumarin production insofar as low availability of either resource limited the effect of variation in other resource. Relative amounts of the furanocoumarins were independently influenced by light and nutrient availability. UV radiation increased the concentration of all but two of the furanocoumarins (imperatorin and sphondin) and also influenced that relative amounts of furanocoumarins. Since furanocoumarins are UV—phototoxic to many organisms, including insects, the response of parsnip plants to UV radiation may affect their resistance to herbivores.
Foraging Behavior of Aphytis Melinus: Effects of Patch Density and Host Size
Authors: Reeve John D.
Published: May 2006

The influence of size and density of the California red scale (Aonidiella aurantii) on the behavior of the parasitoid Aphytis melinus was examined. Aphytis appears to employ a simple strategy of searching the same amount of time in patches of different host density, and as a result the same fraction of hosts was encountered at all host densities. Only the first few of the encountered hosts were actually parasitized, however, probably due to egg depletion. Different instars of the California red scale yielded Aphytis progeny of different size and sex ratios. Small hosts, such as the second instar, produced small Aphytis with a male—biased sex ratio. The third instar, a larger host, produced much larger Aphytis and a more balanced sex ratio. Perhaps because of its low quality, the second instar was parasitized at a lower rate than the third instar, in both the field and laboratory. Parasitism on second instars in the field was lower when third instars were abundant. Host size distribution may have significant effects on Aphytis size and sex ratio, with implications for biological control of the red scale.
Translocation Experiments: Bosmina Responses to Copepod Predation
Authors: Kerfoot W. Charles
Published: May 2006

Regional surveys of the small cladoceran Bosmina longirostris reveal a complex morphological pattern. Body size and shape vary greatly, with the most pronounced differences found in the posterior mucrones and anterior antennules. Some lakes have short—featured clones, most have intermediates, while a few have very long—featured clones. Neighboring lakes of similar morphometry can have short— or long—featured clones. Possible explanations for the geographic patterns include: (1) initial colonization events, (2) competitive release following colonization, (3) hybridization between formerly isolated breeding stocks, and (4) natural selection from resident predators, particularly omnivorous copepods. Field translocation experiments demonstrated the importance of Epischura predation in determining the success of long—featured clones. Prey risk was both size— and shape—dependent, with fitness determined largely through the differential survivorship of early instars. Increased fitness associated with longer features was evident at several levels: (1) during clonal recruitment from resting eggs, (2) in the success of translocated stocks, and (3) as developmental responses within exposed prey populations. In long—term exposures to Epischura, feature lengths of short—featured stocks increased by several standard deviations. Mucro length showed the greatest response, although antennule lengths and egg lengths also showed significant increases. During the Epischura lacustris—Bosmina longirostris interaction, responses were clearly the joint effect of two separate phenomena: selective responses (due to differential mortality) and chemical induction (developmental plasticity). Mucro and antennule responses were accompanied by significant declies in coefficients of variation.
Competition and the Local Distribution of the Grass Stipa Neomexicana
Authors: Gurevitch Jessica
Published: May 2006

Within a grassland in southern Arizona, Stipa neomexicana occurs only on dry ridge crests with low total grass cover, while total grass cover is greater below the ridge crests in moister, low—lying areas. I hypothesized that Stipa neomexicana was limited to these dry ridges by competitive exclusion. This hypothesis was tested by removal experiments conducted at three positions along the topographic gradient. The responses of Stipa were compared with those of Aristida glauca and other neighboring species. The predictions made under the hypothesis were confirmed; competitors limited seedling establishment, seeding survival, flower production, and the growth of mature plants. The response to the removal of competitors increased from the ridge crest to the downslope positions. Competition depressed estimated finite rates of population increase for Stipa neomexicana, particularly on the lower slope. Thus, increasing competition from neighboring grasses along the topographic gradient was responsible for restricting Stipa neomexicana to the unfavorable ridge crest sites.
Effects of the Herbicide Atrazine on Aquatic Insect Community Structure and Emergence
Authors: Dewey Sharon L.
Published: May 2006

The herbicide atrazine was applied to 0.045—ha experimental ponds in two replicate concentrations of 0, 20, 100, and 500 mg/L each. Physical, chemical, and biological variables were measured and aquatic insect community structure was monitored in the eight ponds using partially submerged funnel emergence traps. Atrazine had no effect on water temperature or oxygen concentration. Mean turbidity did increase significantly with atrazine concentration, but turbidity could not be correlated with abundance or species richness of emerging insects. Macrophyte production decreased with increasing herbicide concentration; however, the macroalga Chara sp. showed resistance up to 100 mg/L. The benthic insect community was also negatively affected by atrazine. Abundance of emerging individuals of the chironomid Labrundinia pilosella was significantly reduced at atrazine concentrations as low as 20 mg/L, while other, less abundant species showed similar declines. Benthic insect species richness (S), species equitability (J'), and total emergence all declined significantly with atrazine addition. In general, abundances of nonpredatory insects were greatly reduced with the addition of atrazine (20 mg/L), while predatory insects showed no response to the herbicide. In addition, emergence periods of several herbivorous insect species shifted to an earlier time in atrazine—treated ponds; the shift was statistically significant for Oxyethira pallida. The lowest concentration at which atrazine affected aquatic insects in the pond system (20 mg/L) is one order of magnitude lower than the lowest concentration previously shown to have a direct toxic effect on the midge Chironomus tentans in single—species laboratory experiments. Furthermore, because of the differential response of predatory and nonpredatory insect species to atrazine in this study, it is suggested that the effect of the herbicide on the insect community in the experimental ponds may be primarily indirect, presumably through reduction of the food of nonpredators (periphyton, macrophytes) and, to some extent, their habitat (macrophytes).