bluuebunny/crossref_metadata_2025 huggingface dataset:
Seed and Seedling Ecology of a Monocarpic Tropical Tree, Tachigalia Versicolor
Authors: Kitajima Kaoru , Augspurger Carol K.
Published: May 2006

Tachigalia versicolor is a monocarpic canopy tree species on Barro Colorado Island, Panama, This 2—yr demographic and physiological study examined whether seed and seedling survival of this monocarpic species is high compared to nonmonocarpic species previously studied, as life history theory predicts. For the weighted average of two trees, the percentage of offspring estimated to survive from dispersed seed to germination, 1 yr, and 2 yr was 30, 7.2, and 4.3%, respectively. The large seeds (500—600 mg) suffered greater mortality in absolute numbers than did the seedlings. Major mortality agents of seeds were bruchid beetles (Amblycerus tachygaliae) and terrestrial vertebrates, while vertebrate herbivores and damping—off fungi killed seedlings. Seedlings survival and growth were enhanced under canopies of dying T. versicolor adults, relative to under canopies of living conspecific and nonconspecific adults. The majority of 2—yr—old seedlings occurred within 0.40 m from the parent trees, reflecting the original peak of the distribution of wind—dispersed seeds. Few seedlings persisted beyond 40 m. Seedling survival in the first 2 mo was higher in shaded understory than in light—gaps, due to higher vertebrate herbivory in sun than shade. Seedling growth and survival beyond 2 mo was higher in light—gaps than in shaded understory. T. versicolor seedlings exhibited a wide range in maximum net photosynthetic rate when grown in contrasting light conditions, and were able to acclimate to a sudden increase in light level by means of morphological and physiological adjustments of newly developed leaves. These results indicate that T. versicolor can utilize a wide range of light environments in the forest during seedling establishment. Relative to nomonocarpic wind—dispersed tree species previously studied in the same community. T. versicolor ranks near the top in its seed mass, dispersal capability, photosynthetic flexibility to contrasting light conditions, and probability of seedling survival in the shaded understory through 1 yr. These characteristics enable T. versicolor to establish large numbers of seedlings. The mean total number of offspring of two trees estimated to survive through 2 yr was 4395 out of 103 055 dispersed seeds and 30 529 germinated seeds. In comparison to nonmonocarpic species, juvenile survival of T. versicolor is high as expected for this monocarpic tree species.
Stand Density Effects in Young Red Alder Plantations: Productivity, Photosynthate Partitioning, and Nitrogen Fixation
Authors: Bormann B. T. , Gordon J. C.
Published: May 2006

We evaluated the effects of stand density on nitrogen (N2) fixation, net primary production (NPP) photosynthate partitioning, and canopy characteristics in 5—yr—old red alder (Alnus rubra Bong.) plantations. Our study used acetylene reduction and dimension analysis techniques. Trees in low—density stands (initially spaced 2.74 x 2.74 m) had the highest mass, and surface—area components as well as N2 fixation. Mid—density stands (initially spaced 1.22 x 1.82 m) had the highest per—unit—area values for leaf mass, canopy volume, branch mass and surface area, root and stump mass, net branch production, aboveground net production, and N2 fixation. The highest density stands (initially spaced 0.61 x 1.22 m) had the highest values per unit area of the variables: wood volume, bole and total aboveground dry mass, and net bole production. Nodule dry mass per unit area was approximately equal in the mid— and high—density stands, averaging 146 kg/ha. A high correlation between N2 fixation and leaf mass per tree (r = 0.892) supports an earlier hypothesis (Gordon and Wheeler 1978) about field—grown alder. The lack correlation between leaf mass and N2 fixation per unit area (averaging 2.15 Mg/ha and 70 kg°ha—1°yr—1, respectively) suggests that high—density stands allocated less photosynthate to nodules of N2 fixation.
Temporal Variability in the Community Structure of Fish on Coral Patch Reefs and the Relation of Community Structure to Reef Structure
Authors: Sale Peter F. , Douglas William A.
Published: May 2006

Fish assembled on 20 lagoonal patch reefs were censused eight times over 33 mo. Reefs, which ranged in size from 2.71 m2 to 28.35 m2 surface area, supported an average of 128 fish of 21 species at any one time. These were drawn from a total pool of 143 species, and, because of successive recruitments and losses of individual fish, each reef supported many more species during the study than were present at any one census. Structure of the assemblages on each reef, in terms of species number, of fish, and species composition, varied through time. Mean proportional similarity of assemblages on the same reef was 0.568, °15% greater than that between assemblages on different reefs (0.422). Structural attributes of reefs, other than size, were of little value in predicting the structure of the fish assemblages formed. The results are compatible with an essentially nonequilibrial view of reef fish communities. This view holds that species recruit to reef sites at varying rates, and independently of each other, while individuals are lost from sites (through death or emigration) also in a way unstructured with respect to the species composition of the resident fauna.
Fire Frequency and Community Heterogeneity in Tallgrass Prairie Vegetation
Authors: Collins Scott L.
Published: May 2006

Few studies have directly addressed the effects of disturbance on spatial and temporal heterogeneity. Spatial heterogeneity. Spatial heterogeneity is the degree of dissimilarity in species composition from one point to another into a community, whereas temporal heterogeneity is compositional change within a site over time. The purposes of this study were to determine (1) if a quadratic relationship exists between within—site heterogeneity and disturbance frequency as predicted by the intermediate disturbance hypothesis (IDH), (2) if disturbed and undisturbed sites have similar heterogeneity as implied by the disturbance heterogeneity hypothesis (DHM), and whether or not these results differed with scale, and (3) if there is a relationship between spatial and temporal heterogeneity as implied by the DHM. Analyses were based on plant species composition data collected over 9 yr in quadrats permanently located in experimental management units subjected to differ burning frequencies at Konza Prairie Research Natural Area, Kansas, USA. The relationship between disturbance frequency and within—site heterogeneity was opposite that predicted by the IDH. Heterogeneity was lowest at intermediate disturbance frequencies. Heterogeneity in annually burned prairie was lower than in unburned prairie and prairies burned once every 4 yr in contrast to predictions of the DHM. However, this relationship did not hold at larger spatial scales. There was a positive relationship between within—site spatial and temporal heterogeneity on annual burned sites, sites burned once every 4 yr, and nearly so on sites burned every other year. Within—site heterogeneity was negatively correlated with cover of Andropogon gerardii, and positively correlated with total richness and species diversity. Studies of variation, in addition to averages, will increase our ability to predict patterns of species distribution and abundance within and between communities in response to disturbance.
Maintenance Respiration and Stand Development in a Subalpine Lodgepole Pine Forest
Authors: Ryan Michael G. , Waring Richard H.
Published: May 2006

We examined a chronosequence of subalpine lodgepole pine stands to test the hypothesis that low net primary production in older forest stands is caused by higher maintenance respiration costs of woody tissue. We predicted that respiration of woody tissue (particularly stem sapwood) would be greater in older stands and that the higher maintenance costs would account for observed low wood production. For a unit of ground surface, the carbon flux involved in wood production and association constructed respiration was 210 g.m—2.yr—1 in a 4—yr—old stand, but declined to 46 g.m—2.yr—1 in a 245—yr—old stand. However, maintenance respiration of woody tissue in stems and branches consumed only 61 g.m—2.yr—1 in the 40—yr—old stand and 79 g.m—2.yr—1 in the 245—yr—old stand. The slight, nonsignificant increase in maintenance respiration of woody tissues could not explain the dramatic decline in aboveground wood production in the old—growth stand.
Flower Selection and the Dynamics of Lipid Reserve in Two Nectarivorous Butterflies
Authors: May Peter G.
Published: May 2006

This study examines flower choice by two syntopic butterfly species, Agraulis vanillae (Nymphalidae) and Phoebis sennae (Pieridae), and the energy consequences of foraging selectivity. Foraging selectivity is influenced by proboscis of P. sennae is °~1 cm longer than that of A. vanillae, it is able to forage at long—corolla flowers (three Ipomoea spp.) inaccessible to A. vanillae. Relative to other nectar sources exploited by the two species, these Impomoea species are highly rewarding energetically. Phoebis sennae increases its energy intake relative to A. vanillae by exercising greater discrimination between high— and low—reward nectar sources, avoiding several species with small nectar volumes that are avidly visited by A. vanillae. Consequently, the average foraging profit of P. sennae is nearly 4 times that by A. vanillae. These differences in foraging selectivity energy intake are closely related to energy storage and depletion rates, measured as age—related changes in gross lipid contents of adult butterflies. Adult activities in the less selective species, A. vanillae, are fueled to a greater degree with lipid reserves set aside at metamorphosis; A. vanillae emerge from pupae with 13—16% of dry mass allocated to lipids, and these lipid stores decline at a variable rate throughout adult life. By contrast, in P. sennae lipid accumulated adult feeding provides the largest proportion of energy stores. P. sennae emerge from the pupa with only 6% of the dry mass allocated to lipids, and accumulate lipid stores to levels as high as 40% of dry mass through most of adult life, depleting these lipid reserves only in late adulthood. Further, A. vanillae show significant sexual dimorphism in lipid content throughout life, with females containing greater quantities, while P. sennae show no such dimorphism. These results are discussed with respect to Boggs' model concerning the allocation of larval—derived resources to reproduction.
Stability of an Aquatic Macroinvertebrate Community in a Multiyear Hydrologic Disturbance Regime
Authors: Boulton Andrew J. , Peterson Christopher G. , Grimm Nancy B. , Fisher Stuart G.
Published: May 2006

We compared rates and directions of benthic aquatic macroinvertebrate succession following eight spates of varying magnitude that occurred in different seasons over 3 yr in Sycamore Creek, a Sonoran Desert stream. A consistent cycle of seasonal change in assemblage composition occurred each year, little altered by spates. Changes reflected variations in presence or absence rather than relative abundance of taxa. Seasonal patterns were confirmed by plotting temporal changes in densities of common taxa. Invertebrate abundance (mostly oligochaetes and mayflies) peaked in spring. "Summer" dominants included gastropod Physella virgata and the caddisfly larva Cheumatopsyche arizonensis. Assemblages composition remained relatively consistent during spring over 3 yr when high discharge was prolonged, whereas there was a major change in autumn community structure between 1984 and 1986, probably, reflecting low discharge during a drought in 1986. Drying apparently influenced assemblage composition more than spates, possibly by altering habitat availability and the intensity of biotic interactions as surface stream volume shrank. Assemblage resistance to disturbance by spates was variable. Similarly, resistance of individual common taxa varied within and among taxa, and like assemblage resistance, was not simply a function of spate magnitude or timing (season). Resilience was generally high. Succession rate (degree of change in assemblage composition) decline during succession in all but spring sequences, which displayed no consistent trend. The two summer sequences had highest initial succession rates (in first 30 d postspate), possibly reflecting higher water temperatures, as also exhibited late—successional increases in succession rate. Spatial variation in assemblage composition was uncorrelated with any physical variable measured. Factors known to influence ecosystem—level processes such as primary productivity (e.g., inorganic nitrogen flux, days since spate) also affected community—level aspects such as aquatic invertebrate assemblage composition in Sycamore Creek. Discharge and water temperature had lesser but detectable effects, and probably contributed to the marked seasonality in assemblage composition. Further comparisons of collective properties of ecosystems and communities within other biomes may identify "common denominators" that characterize responses to disturbance and environmental change. This will remove the different perceptions about stability we gain by using response variables that are assessed only at a community or ecosystem level.
Does the Spatial Scale of Experimentation Matter? A Test with Tadpoles and Dragonflies
Authors: Gascon Claude , Travis Joseph
Published: May 2006

We report the results of a study designed to examine whether the spatial scale of experimentation in artificial ponds (cattle watering tanks) influences the results of manipulative studies of competitive and predatory interactions. The experiment consisted of two initial densities of Rana utricularia (Ranidae) tadpoles (0.035 and 0.14 animals/L) and the absence of presence (0.005 animals/L) of predatory Tramea lacerata (Odonata, Libellulidae) naiads in each of two water depths in cattle tanks of equal dimensions. Depth of tank did not significantly affect survival rate of tadpoles either as a main effect or as part of any interactive effect. At low tadpole densities they increased them significantly. Length of larval period increased significantly with increases in the density of surviving tadpoles at similar rates at both water depths, but deeper tanks produced significantly longer larval periods for the same tadpole density. Size at metamorphosis decreased significantly with increases in the density of surviving tadpoles, but it decreased at a significantly faster rate in the shallower tanks. The results indicate that experiments at smaller spatial scales do no necessarily distort the numerical dynamics in the system but that they can overestimate the importance of density variation in producing variation in some phenotypic characters.
Nest Predation and Differential Insular Extinction among Selected Forest Birds of Central Panama
Authors: Sieving Kathryn E.
Published: May 2006

Barro Colorado Island (BCI) is a 75—yr—old tropical forest isolate in Gatun Lake in the Republic of Panama, where depredation of understory bird nests is hypothesized to be more intense than in nearby mainland forest. Eight of 12 bird species in the terrestrial insectivore guild, with historical breeding records from BCI, are now extinct on the island. I tested the hypothesis that interspecific variation in nest design and placement underlies differential avian extinction from BCI. Selecting five species from the terrestrial insectivore guild, two that are extinct and three that persist on BCI, I constructed 200 hand—made mimics of their nests (40 each of five types) using species—specific nest materials collected in local forest. Artificial nests, containing Coturnix quail eggs as bait, were placed on two island and two mainland plots for representative incubation periods (15 d) and checked at 3—d intervals for predation. Experiments were repeated in February, late April, and June of 1988 to assess seasonal shifts in predation pressure in relation to timing of breeding in understory birds, and temporal constancy of interspecific differences in nest lost rates. BCI nest losses were higher than on the mainland in two of the three experiments and predation intensity varied seasonally. Peak losses occurred in April—May (late in the dry season), immediately before the onset of breeding in understory insectivorous birds, suggesting nest predation may influence the timing of nesting activities. Two nest types designed to mimic the nests of species now extinct from BCI suffered lower predation on BCI and on the mainland than three nest types of species persisting on BCI. However, ratios of island to mainland predation rates were disproportionately higher for BCI—extinct nest types than for BCI—persistent nests in two of the three samples. Results suggest that over evolutionary time species—specific rates of nest loss may determine clutch replacement capability, thereby causing variation in tolerance of inflated insular nest predation. In this way, interspecific variation in nest design and placement among terrestrial insectivorous birds may underlie the pattern of differential extinction from Barro Colorado Island.
An Experimental Study of the Structure of Herbivorous Insect Communities in a Salt Marsh
Authors: Vince Susan W. , Valiela Ivan , Teal John M.
Published: May 2006

Factors regulating the numbers and species of herbivores insects in a salt marsh were experimentally investigated by chronic fertilization of plots. All biweekly nutrient enrichments (urea or three dosages of a mixed fertilizer) resulted in increased grass standing crop in low and high marsh habitats. Plant nitrogen content increased only in those plots receiving the two highest levels of mixed fertilizer. Grass responses were most rapid where the highest nutrient load was applied. In all plots, fertilization resulted in increased standing crop of low and high marsh insect herbivores. However, the greatest herbivore loads occurred where grass nitrogen content as well as standing crop had increased. Life history data and laboratory experiments indicated that the herbivore increases were largely due to in situ changes in survivorship and fecundity in response to higher plant nitrogen content. The herbivore response to fertilization was rapid, tracking in a single summer the nutrient and biomass changes of the grasses. The rate of increase, however, differed among herbivore species, and was positively related to the annual number of generations. Nutrient additions did not alter the diversity of the high marsh herbivore community, but led to increased diversity in low marsh. This was achieved primarily through greater equitability of the species abundances. All of the herbivore species collected in the fertilized plots occurred in the undisturbed salt marsh. The low marsh herbivore community responded to fertilization mainly with greater numbers of mirids, cicadellids, and grasshoppers, although normally dominated by the delphacid Prokelisia marginata. Potential increases in number of P. marginata in response to fertilization were apparently checked by the increased abundance of spiders. The delphacids were more susceptible to spider predation than other salt marsh herbivores, due to their small size and low position in the grass canopy. Plant nutritional quality is of major importance in governing the abundance of salt marsh herbivores, with life history characteristics and predation interacting to structure the community.
The Influence of Island and Mainland Lakeshore Landscapes on Boreal Forest Fire Regimes
Authors: Bergeron Yves
Published: May 2006

In order to characterize the fire regime of the southern boreal forest and to understand the way in which landscape and fire regime interact, a detailed study of fire history was undertaken in two adjacent contrasting landscapes in northwestern Quebec. The fire history for the islands of Lake Duparquet was reconstructed and compared to that of the surrounding lakeshore. Fire occurrence was determined by archival search and by collection of information from the fire scars. Dendroecological techniques were used to determine years of stand initiation on the lakeshore and for a subsample of the islands. Stand initiation data were used to estimate fire frequency, fire cycle, and fire intensity (lethal and non—lethal fires). Tree composition and surficial deposits were sampled in order to assess the possible effect of topography and forest cover on fire frequency. Dated fire scars (n = 273) revealed 82 fire years, with the earliest dated to 1593. Islands experienced more fire years (56) than the lakeshore (37), and fire years were uncorrelated. Most 72%) of the lakeshore stands were initiated by a few large fires (eight), whereas the fire size on islands was variable, ranging from <10% to 100% of the total surface of the island. Despite these differences in fire regime, fire occurrence and fire frequency decreased concurrently on the islands and the lakeshore starting °120 yr ago. The lakeshore fire cycle was 63 yr before 1870 and 99 yr since 1870, whereas the fire cycle on the islands was slightly longer (74 yr and 112 yr) during the same intervals. The long—term decrease in fire frequency of both the lakeshore and the islands was possibly driven by climatic change. Fire frequency was generally similar for all surficial deposits and fuel types on the lakeshore, whereas fires were more abundant in pine woodland growing on bedrock on the islands. Lakeshore fires were generally of lethal intensity, whereas both a long cycle of lethal fires and a shorter cycle of non—lethal fires were observed on 50% of the islands. The morphology and location of islands with both types of fires suggest that a higher frequency of lightning strikes resulted in a regime characterized by more frequent fires and fires of variable intensity. The specific fire regime observed in the island landscape may be the cause of the abundance of pines, whose presence may contribute, in turn, to the continuation of this regime of less intense and more frequent fires. We suggest that fire regime in the boreal forest is controlled by long—term climate change at the regional scale, and by a strong interaction with landscape at the local scale, both components having a great impact on the distribution and the dynamics of boreal vegetation.
Soil, Nitrogen, Phosphorus, and Organic Matter Processing by Earthworms in Tallgrass Prairie
Authors: James Samuel W.
Published: May 2006

Earthworms represent the dominant fraction of soil macroinvertebrate biomass in Kansas tallgrass prairie and thus may have a large impact on nutrient budgets. To evaluate this impact I estimated cast production and rates of nutrient processing by the native genus Diplocardia and by exotic Lumbricidae in permanent plots on two soil types at Konza Prairie Research Natural Area. Total cast production was estimated from field population estimates, soil climate data, and cast production—temperature relationships obtained in the laboratory. The organic matter and mineral N and P contents of surface casts by month were used to estimate total processing of these substances in the field. Earthworms present in the study sites were Diplocardia longiseta, D. kansensis, D. rugosa, D. singularis, D. smithii, D. verrucosa, and the European species Aporrectodea caliginosa and Octolasion cyaneum. Estimated total cast production and nutrient processing by Diplocardia spp. exceeded that of exotics in both soil types, in spite of higher populations of Lumbricidae in one soil. Total annual soil consumption by all earthworms was 4—10% of the A—horizon, depending on soil type. Organic matter equivalent to 10% of total soil organic matter in the top 15 cm or to 100—300% of plant annual belowground production appears to pass through the earthworms each year. Mineral N processed was °10—12% of annual plant N uptake, comparable to half of the input from precipitation, while the P processed was equivalent to 50% of annual uptake. In contrast to other grassland systems studied, the introduction of Lumbricidae had a negative effect on soil turnover and nutrient mineralization due to the lower throughput of Lumbricidae and their relative intolerance of summer soil temperatures.
Habitat Use and Community Structure in an Assemblage of Cottid Fishes
Authors: Norton Stephen F.
Published: May 2006

I applied an observational approach, using strip transects, to document the abundance and the spatial distributions of members of an assemblage of subtidal fishes (Scorpaeniformes: Cottidae) in the San Juan Islands, Washington, USA. I then assessed the extent to which species—specific distributions were correlated with habitat availability and the potential for interspecific competition to modify patterns of abundance and resource use. The six common cottid species were Artedius fenestralis, A. harringtoni, Asemichthys taylori, Chitonotus pugetensis, Icelinus borealis, and Jordania zonope. A comparison of their observed distributions with the distributions of microhabitats along the transects revealed that all species demonstrated significant affinities for or avoidance of many of the predominant substrates: clean rock, silt rock, shell sand, gravel, mud, and algae. Using a stepwise regression technique, I found that the densities of individual cottid species were significantly correlated with the percentage cover of these substrate types along the transects and with transect depth. Several lines of evidence suggest that interspecific competition does not play a strong role in this assemblage. Species pairs that co—occurred on at least some transects did not show disjunct distributions. Microhabitat overlap of these species pairs was higher in sympatry (comparison of transects with both species present) than in allopatry (comparisons of transects with only one species present), counter to the expectation of niche divergence. Correlations of species densities after correcting for habitat heterogeneity with the Schoener—Crowell—Pimm technique revealed only one significant negative density correlation out of thirty potential correlations. Cottid diversity was not correlated with structural diversity along transects in spite of strong associations of each species with habitat structure. This suggests that the transect areas were undersaturated in the number of individuals and/or species of cottid fishes. The apparent lack of competition among subtidal cottids contrasts with studies on intertidal fish communities dominated by cottids for which there is observational evidence of interspecific competition.
Direct Observations of Owls and Heteromyid Rodents: Can Predation Risk Explain Microhabitat Use?
Authors: Longland William S. , Price Mary V.
Published: May 2006

Coexisting heteromyid rodent species of North American deserts differ in habitat use and in locomotory morphology. Quadrupedal species forage primarily in structurally complex microhabitats, such as under bush canopies, while bipedal species forage in open spaces. A common explanation for this morphology—microhabitat association is that species differing in morphology also differ in vulnerability to predators, that microhabitat structure affects predation risk, and that animals preferentially forage in the safest microhabitats. We tested this for two bipedal and two quadrupedal heteromyid species (matched by body size), and one cricetid species, by quantifying effects of habitat and illumination on activity and on risk of predation by Great Horned Owls. Capture frequencies were lower for all heteromyid species than for the cricetid species, Peromyscus maniculatus. Heteromyid activity was lower in open habitat and under bright illumination. Illumination had no significant effect on risk, perhaps because rodents changed activity patterns under full moon to compensate for a potential increment in risk. Habitat, however, did affect risk: all species were attacked and captured more frequently in the open. Bipedal species were attacked relatively more in the open than were quadrupeds. If these results apply to all predators, they indicate that predation alone cannot account for the divergent microhabitat associations of bipedal and quadrupedal species. Bipedal heteromyids, however, escaped owl attacks more frequently than did quadrupeds of equivalent size. It is therefore conceivable that they experience lower overall risk in nature, where owls may preferentially attack more easily captured prey species when given a choice. Under these circumstances, owl predation could reinforce divergent microhabitat specializations based on some other factor, such as foraging economics, by restricting quadrupeds more strongly than bipeds to the safety of bushes.
Food, Predation Risk, and Microhabitat Selection in a Marsh Fish Assemblage
Authors: McIvor Carole C. , Odum William E.
Published: May 2006

Fishes moving onto the surface of a tidal freshwater marsh from an adjacent stream were sampled with flume nets in spring through autumn for 2 yr. Significantly higher numbers were found at sites adjacent to shallow—sloped depositional banks than at sites adjacent to deeper, steeper sloped erosional banks of the stream. Marsh surface features appeared similar, so explanations for this apparent habitat selection were sought in the subtidal portion of the stream. To determine whether the relative abundance of benthic invertebrate prey differed in the two environments, litter bags were placed in the subtidal for 4 wk. Despite higher numbers of invertebrates in erosional zones, both the wet biomass per sample and the mean wet biomass per organism were significantly greater in depositional sites. To test the hypothesis that invertebrate food is more available to fishes in the depositional subtidal, banded killifish (Fundulus diaphanus) were placed in feeding enclosures in both depositional and erosional subtidal habitats in two streams. These fish obtained significantly more food in the shallow depositional subtidal. To test the hypothesis that predation pressure differs with subtidal geomorphology, mummichogs (Fundulus heteroclitus) were tethered on the first half of the rising tide in both depositional and erosional environments in three creeks. Significantly more mummichogs disappeared from tethers and were presumed taken on erosional banks. We conclude that two mechanisms operate in the stream subtidal to give rise to the greater fish abundance at marsh surface sites adjacent to depositional areas. At low tide, when small fishes are confined to creek channels, they select shallow depositional habitats where (1) the availability of benthic invertebrate prey is greatest and (2) predator pressure is less. As the tide rises and inundates the marsh surface, these small fishes seek shelter on the marsh surface adjacent to their preferred low—tide refuge.
Philopatry, Dispersal, and Habitat Saturation in the Banner‐Tailed Kanagaroo Rat, Dipodomys Spectabilis
Authors: Jones W. Thomas , Waser Peter M. , Elliott Lee F. , Link Nancy E. , Bush Betsy B.
Published: May 2006

We report natal dispersal distances for 331 banner—tailed kangaroo rats (Dipodomys spectabilis) at high and low population density. The data were collected in 29 censuses of marked individuals in two populations over 8 yr. One population underwent four—fold variation in density. The other was consistently at high density. Because the number of burrow systems (i.e., breeding sites) remained fairly constant, we interpret changes in density as changes in degree of habitat saturation. Dispersal distances were shorter and the proportion of philopatric young (moving less than a home range diameter) was greater at high density than at low density. This result does not conform to models of dispersal based on studies of cycling small mammals or to a simple competition model of dispersal. All data are consistent with a habitat saturation model, which holds that high—density saturated conditions favor philopatric tendencies in offspring.
Plant Influences on Parasitism of Two Leafminers: A Test of Enemy‐Free Space
Authors: Gross Paul , Price Peter W.
Published: May 2006

We evaluated the importance of host plant traits as causes for a much greater parasitoid species richness on Tildenia inconspicuella (Lepidoptera: Gelechiidae), a leafminer on horsenettle, Solanum carolinense, than on tildenia georgei, a leafminer on groundcherry, Physalis heterophyllaa var. ambigua. In three field experiments, we found no evidence for short—term effects of host plants on parasitoids. None of the parasitoid species normally associated with the horsenettle miner successfully parasitized groundcherry miners transferred to horsenettle. Rates of parasitism and composition of parasitoid species did not differ for horsenettle miners on horsenettle vs. on eggplant, nor for groundcherry miners in tied vs. untied leaves. Instead, long—term effects of host plant morphology on leafminer behavior explained differences in parasitism between the two leafminer species. Whereas the mine structure of the groundcherry miner allows the larva to leave its mine and move freely over the leaf surface, the horsenettle miner stays within its mine, even when disturbed. We found that this difference has made the horsenettle miner vulnerable to more parasitoid species; it can be exploited, not only by parasitoids that can attack vagile hosts, but also by parasitoids restricted to concealed, sedentary hosts. Evidence further indicated that horsenettle trichome morphology has favored the evolution of this obligate endophytism in the horsenettle miner by selecting against mine architectures that would allow miners easy access to the leaf surface. When experimentally placed on leaf surfaces, miner larvae could not feed on horsenettle because they could not penetrate its meshwork of stellate trichomes. On groundcherry, where trichomes are thin and flexible, larvae easily initiated mines. We argue that obligate endophytism, which is uncommon among related gelechiids, is a specific adaption to horsenettle morphology that has made the horsenettle miner available to more parasitoid species than its vagile relatives.
Nutrition Mediates Reproductive Trade‐Offs with Age‐Specific Mortality in the Beetle Callosobruchus Maculatus
Authors: Tatar Marc , Carey James R.
Published: May 2006

Trade—offs between mortality and reproduction play a central role in theories of life history and of senescence. Yet the fundamental question of how reproduction affects mortality is unresolved. We simultaneously manipulated both egg production and adult food availability in the beetle Callosobruchus maculatus and measured their effects on age—specific mortality. We show that there are two distinct mortality trade—offs of egg production. An early trade—off is observed at ages < 18 d and is conditional on current adult nutrition; it is potentially related to energy exhaustion. The late trade—off is observed after 20 d of age, is progressive, and occurs despite the availability of current adult diet; it is potentially related to senescence. The expression of both mortality trade—offs depends on the interaction of early reproduction and nutritional state at the time of reproduction. Food availability at the time of high egg production completely mitigates the early mortality trade—off and lowers the late mortality trade—off by °12%. Most mechanistic explanations for the trade—off between egg production and survival have assumed that eggs compete for the allocation of nutrients that are necessary for survival. However, experiments typically alter only egg production or nutrition, and cannot distinguish between nutrient allocation and alternative mechanisms where some direct, permanent somatic insult results from reproduction. By manipulating both egg production and nutrient availability, we distinguish between these alternatives and unambiguously demonstrate that nutrient allocation can be a mechanism for mortality trade—offs.
Aphid Performance in An Alder (Alnus) Hybrid Zone
Authors: Gange Alan C.
Published: May 2006

Population sizes of insect herbivores on natural plant hybrids may be higher, lower, or intermediate to those on the parent species. I studied the biology and population dynamics of a species of free—living aphid (Pterocallis alni) in a hybrid zone of alder (Alnus sp.). The parent trees were Alnus glutinosa and A. incana and their hybrid was A. pubescens. Hybrid trees differed from parents in phenological and morphological characters. Leaf trichome density on hybrid trees was intermediate to trichome densities on A. glutinosa (high density) and A. incana (low density), and thus appeared to be inherited in an additive manner. High trichome density on A. incana resulted in very high mortality and eventual elimination of aphids colonizing this tree. Aphid populations on the hybrid A. pubescens were higher than on A. incana, but lower than on A. glutinosa. However, aphid susceptibility did not appear to be inherited in a strictly additive fashion, as there was a dominance deviation in favor of the susceptible parent. This lack of additivity in insect resistance was most likely due to threshold effect if trichome density, with the density on the hybrid species below the level for the tree to be aphid resistant. Population differences on A. glutinosa and A. pubescens were not caused by differences in growth or reproduction of aphids between hosts. Cage—reared aphids on these two hosts had identical performance in five successive generations. The effect of caging probably reduced wind movement of trichomes and thereby reduced the probability of aphids being dislodged from their feeding site. Predator and parasitoid species responded in a similar manner to the aphids; their numbers were intermediate on the hybrid hosts.
Species Interactions and Substrate Specificity among Log‐Inhabiting Bryophyte Species
Authors: McAlister Suzanne
Published: May 2006

Fallen logs in temperate deciduous forests are a transient substrate on which a dozen or more bryophyte species can co—occur. This study aimed to assess whether establishment capabilities and species interactions determine the habitat specificity of these species. I performed a modified Dewit replacement series experiment on three moss species (Anomodon rostratus, typical on bark of hardwood logs; Leucobryum albidum, characteristic of Pinus Logs; and Platgyrium repens, a generalist species) in order to evaluate their competitive interactions. The species were grown from leaf fragments sown on four substrates: Pinus bark and wood, and Quercus bark and wood. Substrate had a significant effect on establishment for all three species. However, only on Pinus bark was the frequency dependent on the species sown. Randomization test analogs of analysis of variance indicated that the biomass of Anomodon and Platygyrium grown in monoculture different significantly across substrates, while that of Leucobryum did not. There was considerable variability in performance within bryophyte species and there was no evidence for significant interactions between pairs of species.
Do Female Red‐Winged Blackbirds Limit Their Own Breeding Densities?
Authors: Searcy William A.
Published: May 2006

In territorial, polygynous species such as the Red—winged Blackbird (Agelaius phoeniceus), either of two processes might limit densities of breeding females: (1) females resident on a territory might repel new settlers (aggressive density limitation), or (2) prospective settlers might in their own interests avoid already—settled territories (passive density limitation). Two removal experiments were performed to test for either type of limitation in a population of Red—winged Blackbirds in northwestern Pennsylvania. First, randomly selected primary females were removed before nesting began. The number of newly settling females was no greater on removal territories than on controls, while remaining harems were significantly smaller on removal territories. Nesting success was no greater on removal territories, despite the reduction in breeding density. Second, one to three females were removed from randomly chosen territories later in the breeding season. Again, the number of new settlers was no greater on removal than on control territories, while remaining harems were significantly smaller on removal territories. Nesting success was no different on removal and control territories, but nest predation was significantly higher on removal territories. Two further experiments tested specifically for aggressive limitation. First, levels of aggressiveness shown towards taxidermic mounts by primary females were related to settlement patterns on their territories. Aggression score was found not to correlate with either harem size or with the length of the delay between the nest starts of the primary and secondary females. Second, primary females were treated with testosterone to increase their aggressiveness. Treated females gave significantly higher levels of aggressive displays than controls. However, harem sizes were no smaller on territories of treated females, nor did secondary females nest later on such territories than on control territories. Thus, there is no evidence that female Red—winged Blackbirds in this population are limiting their own breeding densities either passively or aggressively. The absence of passive limitation and of any negative density dependence in reproductive success weighs against the acceptance of the polygyny threshold model for this species.
Biological and Cultural Success of Song Memes in Indigo Buntings
Authors: Payne Robert B. , Payne Laura L. , Doehlert Susan M.
Published: May 2006

Song behavior, breeding success, and survival of individuals and the survival of their local song traditions or memes were compared in two populations of Indigo Buntings (Passerina cyanea) in southern Michigan. First—year males were less successful than older males. First—year males with the song of a neighbouring adult tended to be more successful in mating and in fledging young than were 1st—yr males that retained an individualistic song. Adults that were members of a local song neighourhood tended to be more successful than adults that were not. Reproductive and survival success of individual birds did not vary with the number of males in the meme. The different song traditions (memes) are equivalent in biological terms to mating success, breeding success, and survival of their bearers. Memes are successful in a cultural sense if they survive from year to year. Meme survival is determined largely by the number of individual birds in the song group; chances are high that at least one male with the song will return in the larger groups. Songs are learned outside the family context, kin usually do not have the same song, and a female selects a mate independently of whether his song is the same as her father's. The change over time in the local meme pool is independent of any obviously associated genetic traits, and cultural extinction of song memes is described in probabilistic terms of meme size.
Thermally Imposed Time Constraints on the Activity of the Desert Lizard Sceloporus Merriami
Authors: Grant Bruce W. , Dunham Arthur E.
Published: May 2006

We investigated the thermal biology of the lizard Sceloporus merriami in a hot, arid locality in west Texas. The effects of the thermal environment on lizard activity were hypothesized to be extensive because S. merriami body temperatures average lower than the body temperatures of any other North American desert iguanid (32.2°C). To test this hypothesis, we compared lizard thermal microclimate use and rates of lizard movements, feeding, and social displays with estimates of the relative availability of thermal microclimates near 32.2°. These comparisons were made throughout the lizards' morning activity period. The thermal environment had a major impact on S. merriami and restricted individual activities (movement rates, feeding strikes, and social displays) to a 2—h period beginning around local sunrise (1000) and to a brief period in late afternoon. In the morning, rates of activities were maximal within 1 h after local sunrise. This activity maximum coincided with the time that the average available environmental temperature approximated the mean body temperature for S. merriami. Also at this time, lizards occupied thermal microclimates randomly with respect to microclimate availability on their home ranges, whereas when activity rates were lower, lizard microclimate use was a nonrandom. Thus, when thermally favorable microclimates were most available, lizard activity was least constrained. S. merriami timed the initiation, peak, and cessation of overall and microclimate—specific activity to coincide with the overall and microclimate—specific availability of favorable operative temperatures.
Size‐Related Effects on Search Times in North American Grassland Female Ungulates
Authors: Berger Joel , Cunningham Carol
Published: May 2006

Feeding and searching (= vigilance) rates arise as a result of many interrelated factors including trophic level, diet, reproductive condition, sex, habitat, body mass, and potential predation pressure. Because of unique ecological conditions in which the confounding influences of all but two of these variables could be minimized, we examined the hypothesis that body mass alone accounts for interspecific differences in search times, and tested it with females of four sympatric native North American ungulates (Bison bison, Antilocapra americana, Ovis canadensis, and Odocoileus hemionus). When the effects of group size were controlled, smaller bodied species were more vigilant (per unit body mass) than larger ones. However, search times (ST) also scaled to body mass, and between 81 and 97% of the ST variance was explained by either exponential or power functions. To remove the potential bias that predators exert different influences on species of varying size, search times of bison in areas with and without their major predator, wolves (Canis lupus), were contrasted; search times did not differ between sites. Our results highlight the importance of designing field research that controls for confounding variables prior to attempting to scale behavioral processes to ecological events.
Chlorophyll Variability, Nutrient Input, and Grazing: Evidence from Whole‐ Lake Experiments
Authors: Carpenter Stephen R. , Kitchell James F. , Cottingham Kathryn L. , Schindler Daniel E. , Christense David L. , Post Daivd M. , Voichick Nicholas
Published: May 2006

A Planktivore Lake with small herbivorous zooplankton and a Piscivore Lake with large grazers were monitored for 2 yr and then enriched for 2 yr with inorganic phosphorus and nitrogen. An unenriched lake with an undisturbed fish community served as a reference ecosystem. Added nutrients increased biomass of both phytoplankton and zooplankton. In the Planktivore Lake, phytoplankton were stimulated more than zooplankton. This situation was reversed in the Piscivore Lake. Time series models predicting chlorophyll from P input rate and crustacean length fit well for total chlorophyll and the edible fraction (<35 μm), but were less successful for the large fraction (<35 μm). A 1—mm change in mean crustacean length had about the same effect on chlorophyll as a decrease in P input rate of 1 mg.m‐3—.d‐1—. There was no evidence of interaction between grazer and P input effects. Although effects of grazers and P input are equal and additive, the range of mean crustacean lengths among lakes is ≈1 mm, while the range of P input rates is substantially >1 mg.m‐3d‐1. Therefore, the potential for increasing eutrophication by P input exceeds the potential for controlling eutrophication by food web manipulation. Nevertheless, biomanipulation can reduce chlorophyll concentration at P input rates of 1.5 mg.m‐3.d‐1 or more; these rates characterize a large fraction of the world's stratified lakes.