bluuebunny/crossref_metadata_2025 huggingface dataset:
Relationships between Surface Sediment Diatom Assemblages and Lakewater Characteristics in Adirondack Lakes
Authors: Charles Donald F.
Published: May 2006

Relationships between surface sediment diatom assemblages and lakewater characteristics were studied in 38 lakes in the Adirondack Mountains of northern New York. Most of the lakes are dilute, poorly buffered, and oligotrophic to mesotrophic. The diatom flora (240 taxa identified) is typical for circumneutral to acidic lakes. The purposes of this study were to identify the environmental factors most strongly related to the distributions of diatom taxa and the overall composition of diatom assemblages, and to derive equations to infer lakewater pH from diatom assemblage data. Relationships between diatom assemblages and environmental gradients were analyzed using reciprocal averaging ordination (RA). Correlations between RA axis 1 and pH—related factors were strong (pH, r2 = 0.90; alkalinity, r2 = 0.81; p(Ca positive charges), r2 = 0.67; pMg, r2 = 0.78; pNa, r2 = 0.48; p—total Al, r2 = 0.58; conductivity, r2 = 0.47). Correlations were weaker (but still statistically significant) with elevation, epilimnion temperature, and concentrations of SO4, Cl, and Si. Total P, chlorophyll a, water color, and mean depth were not important in explaining differences among assemblages. Cluster analysis of lakes and diatom taxa, multiple regression of taxon cluster groups with lakewater pH, and the occurrence patterns of individual taxa also indicated that pH—related factors are closely associated with diatom distributions. Euplanktonic taxa were present only rarely in surface sediments of lakes having a pH <5.8, alkalinity <20 μmol/L, and concentrations of Ca positive charges <100 μmol/L and total Al >60 mg/L. Predictive equations were derived for inferring lakewater pH from diatom assemblage data. Most were based on approaches of Hustedt (1939), Nygaard (1956), Merilainen (1967), and Renberg and Hellberg (1982). Agreement between predicted and measured pH was very good; r2 values ranged from 0.61 to 0.94 and se from 0.28 to 0.60 pH units. These predictive relationships can be used to interpret stratigraphic diatom assemblages to reconstruct lake pH histories. No significant relationships existed between Araphidinae: Centrales ratios and measures of trophic status.
Survival and Reproductive Effort in Banner‐Tailed Kangaroo Rats
Authors: Waser Peter M. , Jones W. Thomas
Published: May 2006

Nine years of demographic data for two unmanipulated populations of banner—tailed kangaroo rats (Dipodomys spectabilis) in Arizona provide measures of survival, fecundity, age structure, and density. We use these data to search for costs of reproduction in two important components of fitness: survival and future fecundity. We also examine the interaction effects among reproductive effort, age, sex, and density on survival and future fecundity. None of the analyses reveals a significant negative effect of reproductive effort on survival or future fecundity. This result may be seen as evidence that reproduction has no measurable costs in fitness for bannertails. However, another explanation is that bannertails adjust their reproductive effort to individual circumstances so as to minimize costs.
Fine Root Production Estimates and Belowground Carbon Allocation in Forest Ecosystems
Authors: Nadelhoffer Knute J. , Raich James W.
Published: May 2006

We compared published estimates of the net fine root production (FRP) in forest sites to litterfall and aboveground net primary production (ANPP) to test whether annual rates of fine root and aboveground production vary together at global scales. We also compared FRP estimates to theoretical upper limits as defined by our previously published relationship between total root allocation (TRA, carbon allocated to FRP plus live—root respiration) and litterfall. Estimates of the carbon content of FRP in the total data set ranged from 25 to 820 g°m—2°yr—1 and were not correlated with annual litterfall of ANPP. Different methods used for estimating fine root production, however, showed contrasting results. Estimates derived using either sums of seasonal changes in fine root biomass ("Sequential Core" method) or differences between annual maximum and minimum fine root biomass ("Maximum — Minimum" method) were not correlated with either litterfall of ANPP. Sequential Core estimates were often high relative to predicted TRA values, whereas Maximum — Minimum estimates were generally <50% of TRA. The small number of FRP estimates derived from root—free cores ("Ingrowth Core" method) were not correlated with measures of aboveground production but were all well below predicted TRA values. In contrast to results of other methods, FRP estimates derived using ecosystem N budgets ("N Budget" method) were positively correlated with both litterfall and ANPP. Comparing FRP estimates based on N budgets with previous results of forest soil C budgets suggested that annual fine root production increases with aboveground production and that °1/3 of TRA is used for production of fine roots.
Distribution‐Free and Robust Statistical Methods: Viable Alternatives to Parametric Statistics?
Authors: Smith Stephen M.
Published: May 2006

In much ecological hypothesis testing, Type II errors may be more serious than type I errors; ecologists are encouraged to make an a priori assessment of the relative costs of both types of error. Distribution—free statistics such as the Mann—Whitney procedure are not necessarily more powerful nor more robust than their parametric analogues.
Response of a Predator to Variation in Prey Density at Three Hierarchical Scales Lady Beetles Feeding on Aphids
Authors: Ives Anthony R. , Kareiva Peter , Perry Renee
Published: May 2006

We investigated the response of two species of lady beetles, Coccinella 7—punctata and Hippodamia variegata, to spatial variation in the density of aphids on fireweed, Epilobium angustifolium. Our approach involved experimental manipulations of aphid densities and field observations of lady beetle foraging at three different scales: (1) Individual lady beetles on individual fireweed stems (2) populations of lady beetles on individual fireweed stems, and (3) populations of lady beetles in populations of fireweed stems. At the finest scale, we found that individual lady beetles remained on single fireweed stems longer in the presence of larger aphid colonies. However, this behavioral response to aphid density was weak, explaining only 11 and 4% of the total variance in the length of time C. 7—punctata and H. variegata remained on the stems. To examine directly the population—level effects of this individual lady beetle behavior, we conducted a second series of experiments that focused on the response of populations of lady beetles; we released large numbers of lady beetles into field plots in which we had manipulated the number and size of aphid colonies. By sampling the distribution of lady beetles, we showed that the number of lady beetles found on fireweed stems depended significantly on the presence and size of aphid colonies. The average number of both C. 7—punctata and H. variegata found on a stem was 10—20 times greater when the stem contained a large aphid colony that when the stem contained no aphids. Finally, we examined the consequence of these patterns at the scale of populations of lady beetles and populations of fireweed in 25—m2 field plots. We found that the number and size of aphid colonies within the plots explained 50 and 90% of the variation among plots in the average length of time that C. 7—punctata and H. variegata remained. Taken together, these results demonstrate that although individual lady beetle response to aphid density is extremely weak, the cumulative effect of many individuals can produce strong population—level aggregation of lady beetles in areas of high aphid density.
Body Size‐Related Coexistence: An Approach Through Allometric Constraints on Home‐Range Use
Authors: Basset Alberto
Published: May 2006

This paper investigates whether body size—related constraints on home—range resource harvesting could lead to coexistence between interspecific competitors of different body size under conditions of complete niche overlap. With this objective, I analyzed the influence of body size on induced and sustainable resource limitation and its implications on the interaction between an individual and a fixed biomass of larger competitors in a four—dimensional space, consisting of two spatial dimensions describing competitor home range, resource availability, and time. It was shown that body size—related spatiotemporal constraints on home—range resource harvesting, trophic optimization, and relativity of resource availability determine absolute and relative amounts of unused home—range resources, restricting the influence of resource limitation induced by an individual to a definite size distance around its size. It was therefore concluded that: (1) when competition occurs asymmetrically with a superiority of large animals, size differences alone could allow coexistence, independently of any kind of resource partitioning; and (2) superiority of large animals should result from the resource density control that a larger competitor imposes on the smaller one whenever size differences for stable coexistence occur. Existing evidence of an inefficiency in home—range resource exploitation seems large enough to suggest a view of guilds based on a hierarchy of inclusive home ranges.
Effects of Urine Deposition on Small‐Scale Patch Structure in Prairie Vegetation
Authors: Steinauer E. M. , Collins S. L.
Published: May 2006

Large grazing mammals contribute to the species diversity of grasslands via direct and indirect effects of defoliation and urine deposition. We examined the influence of one and two applications of simulated bovine urine on vegetation structure on (1) tallgrass prairie burned either every 2 or 4 yr at Konza Prairie Research Natural Area (KPRNA), Kansas, and (2) unburned sandhills prairie at the Niobrara Valley Preserve, Nebraska. We also examined the influence of urine and the clipping of graminoids on an annually burned site at KPRNA. Plant abundance in general increased on urine patches but the response appeared dependent on litter accumulation. C4 grasses increased at the annual burn and Niobrara sites where litter levels were low. C3 forbs increased at the 2— and 4—yr burn sites where litter levels were high. Urine treatment significantly affected community composition at all but the 2—yr burn site. Alpha—diversity decreased on urine patches at the annual and 4—yr burn sites but increased on urine patches at the Niobrara site. Beta—diversity increased on urine patches at the annual burn and Niobrara site but decreased on urine patches at the 4—yr burn site. The clipping of graminoids at the annual burn site reduced both α— and β—diversity and graminoid abundance while forb abundance was not affected.
Protandry in Aedes Sierrensis: The Significance of Temporal Variation in Female Fecundity
Authors: Kleckner Cheryl A. , Hawley William A. , Bradshaw William E. , Holzapfel Christina M. , Fisher Ian J.
Published: May 2006

Protandry, the early emergence of males into a seasonally breeding population, has been described mathematically as a mating strategy resulting from sexual selection on both sexes. A key implicit assumption in protandry models is that all matings contribute equally to a male's reproductive success. We build a simulation model of protandry based on field censuses and investigate the consequences of size—specific temporal variation in female fecundity for the optimal timing of protandry in the western treehole mosquito, Aedes sierrensis. We show that theoretical predictions of protandry are robust when differential female fecundity is incorporated into a model of protandry in A. sierrensis. In addition, we utilize field data and laboratory experiments to elucidate the selective forces acting on both sexes of this mosquito. Under conditions of reduced per capita resources, males minimized development time by pupating at lower mass; females maximized mass by delaying pupation. These gender—specific, homeostatic adjustment to food and density, which result in density—dependent protandry, indicate that protandry is the result of selection on independent fitness criteria in each sex.
Relative Impact of Interactions within and between Trophic Levels During an Insect Outbreak
Authors: Auerbach Michael
Published: May 2006

The aspen blotch miner, Phyllonorycter salicifoliella, has occurred at extremely high, "epidemic" densities on its quaking aspen host trees for at least 10 yr in Itasca Park, Minnesota, USA. During the 1986 through 1988 growing seasons, I censused density, survival, and sources of mortality for eggs, larvae, and pupae on branches at two heights on four host trees. I constructed cohort life tables to determine: (1) dominant sources of mortality, and (2) which sources of mortality were spatially density dependent. The impact of parasitism on leaf miner success was also measured by excluding parasitoids with cages from four trees in 1986. Most mortality occurred during larval development and was caused by interference, parasitism, and unknown causes. Interference means that one larva kills another when mines coalesce during the first three instars. Egg mortality, predation, and death from premature leaf abscission were generally low, except in 1988 when a severe drought caused significant early leaf fall. I tested if dominant sources of mortality were density dependent at four spatial scales: per leaf, twig, branch and tree. Larval interference was the only consistently density—dependent cause of death. No between—trophic—level interactions imposed density—dependent mortality. Although interference was spatially density dependent, it appears incapable of regulating P. salicifoliella population size. Instead, P. salicifoliella densities approached population "ceilings" delimited each year by availability of young leaves during oviposition. Cessation of the outbreak is unlikely until stochastic meteorological events increase overwintering mortality and/or severely disrupt synchrony between ovisposition flights and availability of young foliage.
Models of Multiple Dispersers from the Nest: Predictions and Inference
Authors: Tonkyn David W. , Plissner Jonathan H.
Published: May 2006

Many vertebrate populations exhibit large variation in natal dispersal distances. Murray (1967) showed by simulation that this variation need not have a genetic basis, but could result from two factors, chance variation in proximity to suitable sites, and competition with sibs and neighbors for those sites. Waser (1985) developed an analytic model of the first of these effects, which predicts the probability that an individual will locate an available site within any given distance, under two limiting search strategies. We extend the model to consider the distances traveled by multiple dispersers from a site, under both search strategies, and show that competitive exclusion by early dispersers can further increase the variation in dispersal distances. The incorporates the effects of competition with siblings, but not with dispersers from other natal sites, and brings us part way toward an analytic solution to Murray's original model. In addition, we discuss both the difficulties inherent in testing models of dispersal, and the importance of doing so for the understanding of various behavioral, population, and evolutionary processes.
Worldwide Limitation of Finch Densities by Food and Other Factors
Authors: Schluter Dolph , Repasky Richard R.
Published: May 2006

We compared nonbreeding finch densities (biomass) with seed abundance in arid and semiarid sites in Kenya, the United States, Brazil, and Argentina. Our objectives were to examine whether densities are limited by food, and whether a given seed abundance results in similar finch densities on different continents. We also wished to estimate the relative roles of food and other biotic factors in determining absolute density, by comparing the observed densities to those that might potentially by achieved on a given food supply. We used data from the finches of the Galapagos Islands as an approximate measure of the food—imposed upper limit to finch density because food there is short, emigration is restricted, and predation is reduced. Half the variation in finch densities across sites around the world could be explained by standing seed abundance, strongly suggesting that finches on mainlands are food limited. Densities on different continents were roughly similar when seed supplies were the same, despite the different phylogenetic origins of finches present. Nevertheless, average densities on mainlands were only about one—quarter the densities found on Galapagos islands. Low mainland densities possibly result from competition with other granivores (e.g., rodents and ants). However, mainland finches restrict their foraging to areas of relative safety, where seed densities may be lower than elsewhere, hinting that predation may also reduce densities. We suggest that predation and possibly other biotic factors limit mainland populations jointly with food, and do not simply reduce densities below a point where food is limiting.
Seasonal Phytoplankton Demographic Processes and Experiments on Interspecific Competition
Authors: Carney Heath J. , Richerson Peter J. , Goldman Charles R. , Richards Robert C.
Published: May 2006

Population dynamics of Lake Tahoe phytoplankton species were studied during 1985. Four dominant diatoms were examined: Cyclotella glomerata, Cyclotella ocellata, Synedra amphicephala, and Synedra radians. The role of interspecific resource—based competition during spring and summer was tested in three ways: (1) species distribution patterns along the vertical gradient (upper 100 m of the water column), (2) measurements of in situ growth and loss (sinking, grazing, natural death, diffusion) processes, and (3) transplants of assemblages between depths. All three tests indicated that resource—based competition was important. Species clearly segregated with thermal stratification and nutrient depletion in the epilimnion. C. glomerata peaked within the epilimnion (0—20 m), S. radians became dominant in the deep chlorophyll maximum region (60—90 m), and the other species peaked at intermediate depths. Resource—limited growth was a dominant process during this time, and sinking and natural death were significant losses. Experimental transplants of assemblages between two depths (15 and 75 m) demonstrated competitive interactions in the phosphate—limited region near the surface, but there was no evidence of competition in the deeper light—limited region. This suggested asymmetrical competition along the vertical gradient. Primary productivity nutrient—enrichment bioassays and growth kinetic assays (phosphate, nitrate, light) also indicated that phosphate and light limitation were important bases of species interactions.
Vegetation Dynamics in Amazonian Treefall Gaps
Authors: Uhl Christopher , Clark Kathleen , Dezzeo Nelda , Maquirino Pedro
Published: May 2006

In a 1—ha plot of old—growth tierra firme forest near San Carlos de Rio Negro, Venezuela (northwest Amazon Basin), 88 trees ≥ 10 cm dbh out of 786 die over a 10—yr period. Most deaths resulted in the formation of small (5—100 m2) canopy openings (gaps). Occasionally, large gaps are formed in this region when strong winds topple many trees together. In five small (single—treefall) gaps and one large (multiple—treefall) gap we studied changes in soil fertility and nutrient leaching, and also plant establishment, mortality, and growth during the first 4 yr following gap formation. Gaps were divided into four zones, or microhabitats: a trunk zone, an open zone (between bole and forest edge), a crown zone, and a root—pitzone. Sampling was conducted in each microhabitat. Soil nutrient levels in single—treefall gaps did not differ in a predictable fashion in response to microhabitat within gap of gap age. Moreover, except for a small, short—term increase in NO3—N, leaching losses from single—treefall gaps did not differ from forest levels. Advance regeneration plays a dominant role n treefall gap succession at San Carlos. Under closed forest, advance regeneration has a mean annual survivorship of °80%; height growth is only a few centimetres a year, and leaf retention times frequently exceed 4 yr. Four years after gap formation, advance regeneration accounted for 97% of all trees ≥ 1 m tall in the single—treefall gaps and 83% of all trees in the multiple—treefall gap. Almost all trees in both gap types were of primary forest species; pioneer trees comprised only a small fraction of the regrowth. In general, microhabitat within gaps did not influence plant density, or plant establishment and mortality patterns. However, mortality was much higher for individuals that germinated after gap formation than for individuals present as advance regeneration. Plant growth within treefall gaps was influenced by gap size, gap microhabitat type, gap age, and plant size. Seedlings and saplings of forest trees in the multiple—treefall gap grew three times as fast as those in the single—treefall gaps. Within single—treefall gaps, height growth was greater in the trunk and open zones than in the crown zone, and trees generally grew more slowly as gaps aged. Furthermore, tree growth was positively correlated with tree size, causing the size differential between trees of different heights to expand as gaps aged. As San Carlos, tree seedlings do not grow to the canopy during a single gap event; rather, canopy closure occurs by growth of larger pole—sized trees that survived treefalls or by lateral expansion of bordering subcanopy trees. Our results indicate that neither gap size, microhabitat with gaps, nor gap age have measurable effects on nutrient loss, nor do they appear to affect plant density, plant establishment, or plant mortality. Size, microhabitat, and temporal affects are minimized, in large part, because of the great importance of advance regeneration in gap succession.
Productivity of Epipelic Algae in Tundra Ponds and a Lake near Barrow, Alaska
Authors: Stanley Donald W.
Published: May 2006

The biomass and productivity ((14)C) of epipelic benthic algae were measured in several tundra ponds and in a lake near Point Barrow, Alaska. In the ponds (50 m diam X 20 cm deep) the epipelic algae (mostly Chlorophyta and Cyanophyta) are most abundant in the upper 2 cm of sediment but their total biomass is low (0.5—1.5 g C°m(—2)) and no surface mat of cells forms, largely because of the continuous mixing of the flocculent surface sediments by a variety of benthic invertebrates. Epipelic photosynthesis is limited to a 3—mo thaw period (June through August) and has a single broad peak in mid—July of ° 200 mg C°m(—2)° day(—1). Epipelic production ranged from 10—4 g C m—2 yr—1 (1971—1973), compared to 1 g C m—2 yr—1 for the pond phytoplankton. In contrast, epipelic production in nearby Ikroavik Lake (12 km2 X 2 m deep) was only 2.3 g C°m(—2)°yr(—1) because of lower light intensities at the sediment surface, lower water temperatures, and a shorter ice—free period than in the ponds.
Life History and the Relationship Between Food Availability and Foraging Effort
Authors: Abrams Peter A.
Published: May 2006

Simple theoretical models are used to explore the effect of changed food density on optimal foraging effort when a forager faces a trade—off between energy (nutrient) gain and risk of mortality. Models of simple iteroparous and semelparous life histories show that the optimal effort or amount of risk may increase or decrease with increasing food density. Increases are more likely for iteroparous than for semelparous life histories, and are more likely when the increase in food availability is of short duration. The possibility of starvation makes it more likely that foraging effort will decrease with increasing food density. Organisms whose foraging goal is to minimize mortality are especially unlikely to increase effort in response to increased food availability. If there is a cost associated with the amount of food ingested, effort is more likely to decrease with increasing food. Some limitations of previous theoretical treatments of the foraging effort question are discussed. Previous experimental results illustrate the full range of responses to food density predicted here.
Relative Abundance of Littoral Zone Fishes: Biotic Interactions, Abiotic Factors, and Postglacial Colonization
Authors: Hinch Scott G. , Collins Nicholas C. , Harvey Harold H.
Published: May 2006

We used multivariate ordination techniques to establish relationships among the abundances of the most frequently occurring littoral zone fishes (fishes that occurred in ≥60% of the lakes) in 25 central Ontario lakes. Previously, most large—scale comparative studies of freshwater fish communities only measured species presence/absence. Abundance estimates provide an alternative, and perhaps more sensitive, means of assessing the relative importance of biotic processes, geological/chemical factors, and historical determinants of present—day community structure. Seventy percent of the variation in abundances was described by the first two axes from a correspondence analysis. Abundances of white sucker (Catostomus commersoni) and rock bass (Ambloplites rupestris), and abundances of brown bullhead (Ictalurus nebulosus) and smallmouth bass (Micropterus dolomieui) were negatively related. However, abundances of these species pairs were not related where the species occurred in sympatry, suggesting that species presence/absence strongly influenced these patterns of abundance. Most of the variation in species abundance and distribution was attributed to physical factors associated with species colonization processes (i.e., differences in postglacial colonization rates may have caused negative association between white sucker and rock bass). An independent 45—lake data set was used to confirm these zoogeographic patterns. Predation by northern pike (Esox lucius) may be partly responsible for variation in abundance and distribution of white sucker. Where pike and sucker co—exist, sucker abundance is lower than where sucker exist without pike. Habitat differences between lakes provided the best explanation for the variation in abundance of smallmouth bass and brown bullhead. Biogeographic processes, physical environmental factors, and predatory processes have been very influential in shaping abundances and distributions of ubiquitous littoral zone fishes in Ontario.
Fire and Grazing in the Tallgrass Prairie: Contingent Effects on Nitrogen Budgets
Authors: Hobbs N. Thompson , Schimel David S. , Owensby Clenton E. , Ojima Dennis S.
Published: May 2006

Fire and grazing occur together in many of the world's grasslands, but their effects on nutrient cycling have usually been studied as if they acted separately. We hypothesized that grazing by large herbivores results in conservation of nitrogen that would otherwise be lost from burned grasslands. We tested this hypothesis in a series of experiments on burned and unburned tallgrass prairie grazed by cattle. We manipulated grazing using exclosures and mowing. Combustion losses of N from ungrazed plots (1.8 g°m—2°yr—1) burned in the spring were double those from similarly burned, grazed plots (0.9 g°m—2°yr—1). These losses represented about half of the preburn, aboveground stocks of N. The magnitude of N loss was proportional to the standing crop biomass available for combustion. Fire temperatures and energy release were reduced by grazing. We used mowing to simulate locally heavy grazing in patches. In the absence of burning, mowing patches increased the likelihood that a patch would be regrazed and caused persistent reductions in the residual biomass remaining in a patch at the end of the growing season. Mowing did not influence patch utilization or residual biomass when pastures were burned. Thus, the effects of fire on grassland N budgets were modified by grazing, and the effects of grazing on the patch structure of grasslands were modified by fire. We conclude that accurately predicting volatile losses of nutrients from grassland ecosystems resulting from biomass burning may depend on understanding effects of grazing.
The Consequences of Herbivory for Dwarf Fireweed: Different Time Scales Different Morphologial Scales
Authors: Doak Daniel F.
Published: May 2006

Despite widespread interest in the impact of herbivores on plants, very few studies have addressed the morphological mechanisms by which plants compensate for losses due to herbivore attack, and fewer still have examined compensation in long—lived, herbaceous species. I conducted a 3—yr experimental study of dwarf fireweed's (Epilobium latifolium) responses to real and simulated attack by a momphid caterpiller. I found that the type and extent of plant responses change with morphological and temporal scale. For instance, damage shoots showed effective compensation, producing between 89 and 583% more branch tissue than undamaged shoots. However, clumps of shoots are joined by perennial rhizomes; a single year of herbivore damage to a clump's shoots reduced net growth by > 75% over a 3—yr period. Clump form also changed following attack; in the year after damage shoot number was reduced by 51%, while mean shoot size either increased or was unaffected. Further complicating the caterpiller's impact on its host was its inhibition of mammalian herbivory: simulated caterpiller damage reduced the probability of mammalian attack from 20 to 6%. These complicated and very different changes following herbivory illustrate the need to study compensation mechanistically and in the context of a plant's biotic environment in order to understand its importance and consequences.
Prey Selection by the Tropical Snail Thais Melones: A Study of Interindividual Variation
Authors: West Lani
Published: May 2006

I studied the feeding behavior of marked individuals of the carnivorous marine snail Thais melones in a rocky shore habitat along the Pacific coast of Panama. These snails consumed a variety of invertebrate species such as bivalves, limpets, and polychaetes. The population of Thais melones was generalized in overall diet, but individuals varied in degree of specialization. Feeding sequences of 282 marked snails were divided into size classes for the 100 snails observed through five or more sequential feeding attacks. These sequences revealed two interesting patterns. First, diets varied from specialized to generalized even among individuals of similar size foraging adjacent to one another in the same habitat. Second, among individuals that exhibited more specialized diets, the prey species of choice was not always the same. Individual diets of snails were monitored twice daily in the field for time periods ≤3.5 mo. Two of five snails that were observed over longer periods showed consistency of prey choice with more specialized diets for 11 and 13 mo until they were lost. Neither of these patterns appeared to result from biased movements of individual snails among different habitat patches since mapped foraging tacks of individual snails intersected and overlapped extensively. Both of these results do, however, reveal that no single "optimal" pattern of prey selection was exhibited by these size—matched, co—occurring snails. In addition, for medium—sized snails (20—30 mm shell length), rates of spiral shell growth measured over the same time period revealed a statistically significant association with diet diversity: individuals with a less diverse (i.e., more specialized) diet tended to grow more rapidly, yet the relationship was not significant for small (<20 mm) or large (>30 mm) snails. These studies indicate that pattern of prey choice (i.e., specialized or generalized) may be as important as the composition of the diet on correlates of fitness. Superiority of a specialized diet (regardless of specific prey identity) could occur through increased efficiency in prey handling.
The Timing of Sexual Maturity in Clonal Animals
Authors: Harvell C. Drew , Grosberg Richard K.
Published: May 2006

The indeterminate growth potential of many clonal organisms sets them apart from solitary organisms on two demographic counts: (1) fecundity may increase indeterminately and (2) the age—specific probability of genet mortality may decline exponentially with size. In contrast to most solitary organisms, the fecundity benefits of postponing sexual reproduction are not necessarily offset by a rapid increasing risk of mortality among clonal organisms with indeterminate growth. We examined patterns and causes of variation in the timing of sexual maturity in clonal organisms through experimental manipulations of two phyletically distinct, sessile, colonial marine invertebrates. In the bryozoan Membranipora membranacea, both crowding by conspecifics and simulated grazing triggered the onset of sexual maturity. Within a population of the colonial ascidian Botryllus schlosseri, there are two genetically determined life history morphs. The semelparous morph grows determinately and reproduces at a fixed size. The iteroparous morph can grow indeterminately and reproduces when extrinsic factors (e.g., substratum limitation) intervene. Analysis of the literature on other sessile, clonal taxa suggests that reproduction is timed differently for annual, indeterminately growing, and perennial, determinately growing, taxa. Many annual clonal species show extreme flexibility in size of first reproduction and reproduce over a wide range of sizes depending on extrinsic factors. Many long—lived clonal species delay reproduction throughout unfavorable conditions until attaining some minimum size. Beyond this size, a combination of age and extrinsic factors appears to modify the actual size at first reproduction.
Distribution of Energy Use and Biomass Among Species of North American Terrestrial Birds
Authors: Maurer Brian A. , Brown James H.
Published: May 2006

The distribution of biomass and energy use among species with different body sizes provides an empirical basis for studying ecological processes that determine species diversity. Biomass and energy use distributions were determined for North American terrestrial birds from data on population density and body mass of 380 species and data on energy use obtained from the literature. Using these data, several hypotheses regarding the specific form of biomass (summed for all species in a body size category) and energy use distributions were evaluated. Biomass continued to increase in successive log body mass intervals, but this was not due simply to increasing species numbers. Energy use initially increased in these same intervals but leveled off above a body mass of °80 g. Energy used by average populations of individuals species was uniformly distributed between the lower and upper bounds of each log body mass interval. In addition, the upper boundaries on biomass and energy use for individual species paralleled closely the biomass and energy use distributions. Qualitatively similar patterns were obtained for plant— and animal—eating birds considered separately, and for birds in 14 arbitrarily defined subregions of the North American continent. There were important quantitative differences among energy use distributions for the 14 subregions. Subregions at lower latitudes had energy use distributions that were nearly an order of magnitude higher than those of regions at higher latitudes. These results imply that previous hypotheses to explain biomass and energy use distributions were not of sufficient generality to account for both similarities among distributions of very different systems (e.g., birds and aquatic plankton) and spatial variation among systems composed of similar species. A more general hypothesis should consider the importance of inherent physiological constraints on energy use and environmental limitations on energy availability. The process that influence resource allocation in a large assemblage of many species may result in statistical patterns of energy use and biomass that tend to maximize ecological quantities analogous to entropy in statistical physical systems.
Relationship Between Crab Spider Misumena Vatia Nesting Success and Earlier Patch‐Choice Decisions
Authors: Morse Douglass H.
Published: May 2006

To test the relationship between foraging—theory predictions and the survival of offspring, I studied the crap spider Misumena vatia. These spiders decrease egg predation by guarding their nests. This advantage occurs largely during the first half of the nesting period. Nests of females removed after laying were less successful than those from which the female was not removed, but success of nests abandoned during the second part of the nesting period did not differ from that of females remaining until the young had left. Nests guarded by large females were more successful in some years than those guarded by small females; however, no size—related difference in success occurred in nests from which females were removed. Since large egg masses result directly from accurate choice of hunting sites and subsequent high prey capture (Fritz and Marse 1985), the nest—guarding period often enhances earlier reproductive success and augments selection for accurate choice of hunting sites.
Survival, Development, and Size of Larval Tiger Beetles: Effects of Food and Water
Authors: Knisley C. Barry , Juliano Steven A.
Published: May 2006

We manipulated the amount of food and water (soil moisture) available to natural populations of larval tiger beetles in Arizona. Supplemental food increased survivorship of first instars to the second instar in all species in at least one of two years. Supplemental soil moisture usually increased, but sometimes decreased, survival of first instars. Interaction of food and water treatments was significant in a few cases, but the nature of the interaction varied greatly among species and years. Significant mortality of first instars from natural enemies occurred, and may explain the anomalous cases of first—instar survival with food exclusion (since enemies were also excluded by this treatment). Food supplementation increased first—instar—to—adult survivorship for all species and third—instar—to—adult survivorship for most species; in contrast, water supplementation had no effect on most species. Food manipulation, but not water manipulation, also affected development time or size of adults (usually both) for all seven species tested. Supplemental food produced larger adults and resulted in more rapid maturation. Reduced food resulted in delayed development, and this usually resulted in death. The results of this study provide one of the few cases in which simultaneous effects of two major ecological factors have been analyzed for a large set of similar species in an array of habitats. The prevalence of food limitation and the frequent lack of a detectable consistent interaction between food availability and water availability in this system indicate that food limitation and resource competition may be the dominant factors controlling these organisms.
An Experimental Study of Habitat Use in an Assemblage of Ozark Minnows
Authors: Gorman Owen T.
Published: May 2006

The underlying causes of habitat use in an assemblage of six Ozark minnow species were investigated in an experimental laboratory study. An earlier field study had shown that the assemblage was segregated in pools primarily by a single habitat variable, vertical position in the water column. Although the field study indicated that interspecific interactions affected species' use of vertical position, a lack of control over species' combinations and densities prevented determination of the relative contribution of habitat selection vs. interspecific interactions in affecting habitat—use patterns. In the laboratory experiments in the present study, 75—L aquaria were used to simulate pool habitat. A series of selection experiments determined the preferred vertical distribution of each species; only three species preferred distributions distinct from other species. A series of multispecies experiments determined the effect of interspecific interactions on vertical distributions. A response was considered an association if a species' distribution shifted significantly from its preferred distribution toward that of the heterospecific, and was considered a dissociation if it shifted away from the heterospecific. Of 45 possible interactions, 34 were significant and were composed of 18 associations and 16 dissociations. Species—pairs interactions appeared to be species specific and could be observed across multispecies experiments. Mean pairwise overlaps for preferred distributions and multispecies experiments were very similar, but mean overlaps for associative interactions were larger than those in dissociative interactions. The near—balance of the two types of interactions explains the lack of reduction in mean overlap in the multispecies combinations. In the three— and four—species experiments, interactions were complex; a strong interaction between one species pair buffered interactions between other pairs and determined the overall segregational pattern. The similarity of segregrational patterns in the four—species experiments and the field study suggests that multispecies interactions contribute to habitat—use patterns in the assemblage. The ability of each species to modify its (preferred) vertical distribution in response to heterospecifics indicates that assemblage members are relatively flexible in their use of stream habitats and also explains the variability of habitat use observed in the field.
On Recovery, Markov Chains, and Canonical Analysis
Authors: Orloci L. , Orloci M.
Published: May 2006

Canonical analysis of time—series data identified the stationary Markov chain as a plausible mathematics for the recovery process in burned heathland and exploited mixed forests. The process appeared robust and highly deterministic. The results indicated that a steady state may be attained in less than a decade in heathland with continued grazing, but may require more than a century in the mixed forest after exploitation ceased. These contradict, in part, findings by others who could not justify, based on field estimates of transition probabilities and axiomatic tests, the stationary Markov chain as a valid model.
Water Relations of Jamaican Upper Montane Rain Forest Trees
Authors: Kapos Valerie , Tanner Edmund V. J.
Published: May 2006

We studied the water relations of an upper montane rain forest at an elevation of °1550 m in the Blue Mountains of Jamaica to investigate whether the growth of the forest is limited by water shortage and/or whether the characteristically hard, leathery leaves of tropical montane rain forest trees are an adaptation to water shortage. Daily courses of leaf water potential, leaf relative water content, leaf resistance, and leaf temperature were followed for nine tree species at the driest time of year. The lowest leaf water potential measured was —1.7 MPa. Leaf resistance was usually low and rarely showed increases that might be attributed to water shortage; it was significantly correlated with light levels. Leaf relative water content was always >86% and was not correlated with leaf water potential. Leaf sublethal water deficits (SLD) averaged 27% (12 species). There was no relationship between SLD and leaf structural characteristics. We conclude that water supply rarely, if ever, seriously limits the growth of these montane forests and that their xeromorphic leaves are not an adaptation of water limitation.
Within‐ and Between‐Species Variation in Vegetative Phenology in Two Forest Herbs
Authors: Brown Robert L. , Ashmun Jeffrey W. , Pitelka Louis F.
Published: May 2006

We analyzed within‐ and between‐species variation in vegetative phenology for two
coexisting forest herbs, Aster acuminatus and Clintonia borealis. For each species, phenological variation
was analyzed at three levels: pooled across all study areas, among study areas, and among patches
at each study area. Aster acuminatus and Clintonia borealis differed in their mean phenological patterns,
especially with respect to mean dates of emergence, maturation, and senescence. There was greater
synchrony among ramets of C. borealis as compared with A. acuminatus for most phenological traits.

Significant correlations between ramet (plant) size and a number of phenological traits were observed,
particularly for the dates of emergence and death and total ramet lifespan. The same general
patterns of correlation were evident within individual patches, within study areas, and for the pooled
sample of all ramets for each species. Correlations between size and phenology were generally stronger
for ramets of A. acuminatus than for ramets of C. borealis.

There were few significant differences in phenology among study areas, especially after variation
due to ramet size was removed by covariance analysis. In contrast, there was considerable variation
among patches at each study area for both species. For A. acuminatus, differences among patches in
mean ramet size explained much of the phenological variation. For C. borealis, patches differed in
their phenological patterns independent of variation in mean ramet size. Some of the phenological
differences between C. borealis and A. acuminatus may be causally related to differences in the extent
to which ramets of one clone remain physiologically integrated.
An Empirical Bayes Approach to Analyzing Recurring Animal Surveys
Authors: Johnson Douglas H.
Published: May 2006

Recurring estimates of the size of animal populations are often required by biologists of wildlife managers. Because of cost or other constraints, estimates frequently lack the accuracy desired but cannot readily be improved by additional sampling. This report proposes a statistical method employing empirical Bayes (EB) estimators as alternatives to those customarily used to estimate population size, and evaluates them by a subsampling experiment on waterfowl surveys. EB estimates, especially a simple limited—translation version, were more accurate and provided shorter confidence intervals with greater coverage probabilities than customary estimates.
A Neighborhood Analysis of Herbivory in Bouteloua Gracilis
Authors: Holmes Robert D. , Jepson-Innes Karen
Published: May 2006

This study examined whether an individual plant's risk of herbivory was affected by its nearest neighbor. Individually potted tillers of Bouteloua gracilis were placed in a natural habitat with one of two neighbors (B. gracilis conspecifics or Aristida spp.), and in one of two cover treatments (high: in center of neighbor canopy, and low: at periphery of canopy). After one month, the experimental tillers were collected and scored for herbivory. We found that the less palatable neighbor, Aristida spp., reduced the probability that herbivores would feed upon experimental tillers in one of the two years of our study, but in both years increased the amount of feeding on those tillers that were attacked. The amount of cover did not alter the proportion of plants fed upon, but did affect the number of feeding marks for plants that were fed upon. Our results demonstrate that small—scale spatial pattern within a community is an important determinant of herbivory. A tiller's neighbor exerted a stronger effect on the amount of herbivory it experienced than did either history of previous herbivory or larger scale spatial effects. Our study also reveals that the ways in which neighbors affect herbivory are complex and may vary from year to year.