bluuebunny/crossref_metadata_2025 huggingface dataset:
Diversity Relations and Succession in Californian Coastal Sage Scrub
Authors: Westman Walter E.
Published: May 2006

The facultatively drought—deciduous shrublands of coastal California and Baja California are lowest in species richness of the four Mediterranean—climate regions where this physiognomic type occurs. Alpha richness in the North American coastal sage scrub varies primarily with the abundance of herbaceous annual species. Herb levels in turn vary with differences in levels of precipitation, favorableness of temperature during the winter and spring growing season, shading by shrubs, soil nitrogen, and air pollution. Levels of herbaceous annuals are highest in the growing season following fire, and show a second pulse of abundance in stands 15—25 yr old. Mature stands of coastal sage scrub are typically low in species equitability, due at least in part to the shade—intolerance of the herbaceous understory and to reduced levels of soil nitrogen. Symbiotic nitrogen—fixing organisms are virtually absent from stands which have not burned in 20 yr or more. The pattern of postfire succession varies markedly with fire intensity; dominant shrubs sprout abundantly from root crowns only following less intense fires. While there are °50 widespread sage scrub species, more than half of the 375 species encountered in the present study of the sage scrub flora are rare in occurrence within the habitat range. In view of the reduction of the area of coastal sage scrub in California to 10—15% of its former extent and the limited extent of preserves, measures to conserve the diversity of the flora are needed.
Monkey Dispersal and Waste of a Neotropical Fruit
Authors: Howe Henry F.
Published: May 2006

Foraging birds and mammals were observed at a Panamanian rain forest tree Tetragastris panamensis (Burseraceae) in order to determine (1) relative importance of different dispersal agents and (2) the relationship between fruit production and dispersal. The system is of interest because fruits provide little but sugar, fruits are not depleted by animals (i.e., are superabundant), and the tree is visited by a variety of vertebrates with different effects on seed dissemination. Three mammals (Alouatta palliata, Cebus capucinus, Nasua narica) account for 97% of all seeds removed from the trees; the howler monkey (Alouatta) is alone responsible for 74% of the seeds removed. Seeds from the feces of these mammals are fully viable. In addition, two diurnal mammals, three nocturnal mammals, and nine birds ingest and disperse small numbers of seeds, but these species are numerically insignificant. Two parrots (Amazona farinosa and A. autumnalis) are important fruit thieves and seed predators, while three mammals and three additional birds kill or drop small numbers of seeds near the parent trees. Assemblage richness increases markedly with crop size, but most of 25 visitors are of little significance to the plant. Use by three monkeys closely reflects their relative abundance; the abundance of the coatimundi (Nasua) is unknown. This system is characterized by enormous seed waste. Of >430,000 seeds produced by 19 study trees in 1978, <4% had even a remote chance of establishment. The remainder were killed by parrots (6%), were dropped or spontaneously fell under parent trees (66%), or were doomed to intense competition in fecal clumps (24%). The three primary dispersal agents indirectly caused most seed and seedling mortality of dropping seeds under the tree crown or leaving them in fecal clumps, precluding survival of any but a small fraction of seedlings. Crop size ranged from 165 to 100,000 arillate seeds per season, but fruits appeared superabundant on all trees. Numbers of seeds dispersed increases dramatically with crop size, but the proportion dispersed was highest at plants with crops of intermediate size. The proportion of seeds killed by parrots increased with crop size. Disproportionate mortality of seedlings in high densities near adult trees indicated density—dependent mortality among juveniles. Wide geographical ranges of this tree and members of the frugivore assemblage, use of other food resources by common foragers, richness of the assemblage, variation in species visitation at individual trees, irregular annual fruits production, and obvious "inefficiecy" of the dominant dispersal agents from the perspective of the plant suggest that Tetragastris has a generalized dispersal strategy and is not closely coevolved with particular frugivores. A generalized dispersal system is expected from a tree, such as this one, that produces sugary fruit incapable of providing a balanced diet.
The Nitrogen Uptake Kinetics of Spartina Alterniflora in Culture
Authors: Morris James T.
Published: May 2006

Rates of nitrogen uptake were measured in cultures of Spartina alterniflora growing outdoors in continuously flowing nutrient solutions. A Michaelis—Menten model with Vmax expressed as an exponential function of temperature was an accurate predictor of uptake rates during the growing season. Half—saturation constants were estimated to be 0.057 @+ 0.016 mg N/L for NH4 and 0.124 @+ 0.034 mg N/L for NO3. These half—saturation constants are too low in comparison to levels of inorganic NH4 in marsh pore water to account for nitrogen—limited growth in the field. It was suggested that an edaphic factors(s), possibly an oxygen deficiency, or a metabolic poison such as H2S, or competition from other ions for carriers, might inhibit nitrogen uptake in the marsh in such a way as to increase the half—saturation constant for uptake. A gradient of such an environmental factor could account for gradients in morphology and productivity in communities of Spartina alterniflora.
Similarity and Diversity Among Coral Reef Fish Communities: A Comparison between Tropical Western Atlantic (Virgin Islands) and Tropical Central Pacific (Marshall Islands) Patch Reefs
Authors: Gladfelter William B. , Ogden John C. , Gladfelter Elizabeth H.
Published: May 2006

The fish assemblages of groups of large natural coral patch reefs in the tropical western Atlantic and Pacific oceans were censused visually during the summers of 1976 and 1978. Thirty—one reefs were in the northeastern Caribbean Sea (25 at St. Croix and 6 at Anegada) and 15 reefs were located at the southern end of Enewetak Atoll, Marshall Islands. The degree of similarity among the faunas in each group of reefs (using a similarity index based on the log of individual abundances) was correlated with environmental parameters of the reefs in both regions (complexity of surface topography, reef height, reef area, and position with respect to the main reef, currents and grassbeds). The mean degree of similarity among all the fish faunas in each area was the same at the two major sites (0.61 at Enewetak and 0.62 at St. Croix) as was the similarity among the faunas of the most uniform subset of reefs in each area (0.68 at Enewetak and 0.73 at St. Croix). These comparable levels of similarity suggest similar levels of predictability in the composition of reef fish faunas in the two areas, in contrast to previous studies on very small natural and artificial structures, which suggested a low level of predictability among Pacific reefs (Sale 1978). The discrepancy between our results and those of previous workers in the Pacific is partly are result of differences in the sizes of the reefs studied, being several orders of magnitude larger in the present case. Reef fish diversity was greater at Enewetak (mean number of species per reef = 93; H' = 5.38) than at St. Croix (mean number of species per reef = 64; H' = 4.58) but equitability values were the same in both areas (0.82 and 0.81, respectively). At both sites fish species diversity was positively correlated with reef surface complexity, projected reef surface area and reef height, but the correlation was higher at St. Croix for all parameters. This was probably attributable to the greater structural similarity of reefs of Enewetak. At Enewetak there was also correlation between H' and proximity to the main barrier reef. Major differences in the trophic composition of these fish faunas based on available published dietary information indicate that differences between the two sites are accounted for primarily in the proportions of diurnal planktivores and nocturnal general invertebrate feeders. These differences were probably the result of major environmental differences between the two sites; a high oceanic influence at Enewetak, resulting in abundant zooplankton, and the presence of dense grassbed (harboring abundant invertebrates; Randall 1963) surrounding the reefs at St. Croix.
Age Structure and Mortality Within a Group of Closely Related Mosses
Authors: Watson M. A.
Published: May 2006

The age distribution and mortality curves of six species of moss in the family Polytrichaceae were determined for each species in pure stand and in mixture at nine locations along an altitudinal gradient on Mount Washington, New Hampshire, USA. The shapes of the age distributions of adult shoots were found to be remarkably uniform both for species within site and for single species at different sites. There were no consistent differences in patterns of survivorship or mortality between species when found in pure stand and in mixture, suggesting that the presence of closely related species does not predictably affect the form of the age distribution. Each species does however exhibit an individualized pattern of response to interactions with closely related species. Polytrichum commune and Poolytrichastrum formosum exhibited decreased mortality in mixed populations while Polytrichastrum pallidisetum exhibited increased mortality. No differences were found between pure stands and mixtures in Polystichum juniperinum, Polytrichm piliferum or Polytrichastrum pallidisetum. Mortality increased throughout the adult life of these species approaching an asymptote late in life. Density influenced the form and properties of the age distribution, but it was impossible to clearly separate real effects from artifacts introduced by large differences in sampling size. Mosses appear to attain greater ages in high—density populations.
A Latitudinal Gradient in Rates of Ant Predation
Authors: Jeanne Robert L.
Published: May 2006

Predation by ants on wasp larva baits was assessed at five latitudes from 43°N to 2°S. Rates of predation were significantly greater in the tropics; that is, a food item was exploited by ants in significantly less time in the tropics than in the temperate zone. It was also found that predation rates were higher in low second—growth vegetation than in forests and higher on the ground than on vegetation, and that these differences were more pronounced in the temperature zone than in the tropics. Rates of predation occuring on buildings were lower than in natural situations at every latitude. The number of ant species taken at the baits increased from 22 at the northernmost locality to 74 at the southernmost. In all localities but the northernmost the forest habitat produced a greater diversity of species than the field habitat. There was greater degree of microhabitat specificity among ants in the tropics than in the temperate zone. The proportion of forest canopy specialists also increased toward the tropics. Overall predation rate and ant species richness were found to be positively correlated on a latitudinal scale, but this relationship broke down at the level of habitats and microhabitats.
Prey Preferences in the Desert Horned Lizard: Influence of Prey Foraging Method and Aggressive Behavior
Authors: Rissing Steven W.
Published: May 2006

Desert horned lizards Phrynosoma platyrhinos consumed the solitary—foraging seed—harvester ant, Pogonomyrmex califonicus, 10 to 100 times more often than expected based upon its relative abundance in the environment. The group—foraging seed—harvester ants, Pogonomyrmex rugosus and Veromessor pergandei, were correspondingly underrepresented in the horned lizard's natural diet. In a series of laboratory prey choice tests, where prey availabilities were held constant and equal, these preferences were confirmed: P.californicus was eaten more often than P. rugosus which was eaten more often than V. pergandei. These preferences correspond to differences in ant aggressive behavior which, in turn, correspond to differences in ant foraging—group size and foraging method. Colonies of solitary—foraging ants have small foraging groups and are generally incapable of mobbing a lizard, but they are also difficult to find. Colonies of group—foraging ants have large to massive foraging groups and vigorously mob lizards. Such aggressiveness is probably effective by increasing the risk of exposure of a lizard to predators of its own; the indirect costs of preying upon these ant species, normally assumed constant over all prey types for most predators, are variable here. Differences in experience with ant aggressive behavior probably account for differences in prey preferences found among adult lizards captured from the same area. Ant—species—specific predation by Phrynosoma may be an important ecological force structuring the Mohave Desert seed—harvesting ant community.
Woody Litter Decomposition Following Clear‐Cutting
Authors: Abbott D. T. , Crossley D. A.
Published: May 2006

Unconfined Quercus prinus woody litter of three size classes (0—1, 1—3, and 3—5 cm diameter) was placed on forest floors of a control hardwood watershed and on mesic and xeric sites of a clear—cut watershed at Coweeta Hydrologic Laboratory, North Carolina. Exponential decay coefficients for mass loss on the control were .1524, .1728, and .0912 yr—1 for 0—1, 1—3, and 3—5 cm branches, respectively. Coefficients for 0—1, 1—3, and 3—5 cm branches were. 1752, .0756, and .1644 yr—1 on the mesic and site and .0456, .0948, and .0377 yr—1 on the xeric site. The effect of site differences on decomposition rate was greater than the effect of diameter, although an inverse relationship between diameter and decay coefficient is suggested. Time in the field, temperature, moisture, and microarthropod abundance also appeared to influence decomposition rate. Microarthropods dominated the animal community on decaying wood with oribatid mites and collembolans the most numerous. Microarthropod densities were highest on 0—1 cm twigs and lowest on 3—5 cm branches. Microarthropod densities were generally highest on the control, slightly depressed on the mesic site, and greatly depressed on the xeric site. Time in the field and state of decomposition both positively correlated with microarthropod abundance. Calcium concentration and total calcium exhibited transitory increases but little net change at the end of 1 yr in experimental branches. Total potassium decreased on all sites for 6 mo and for 12 mo on the control and xeric sites, with °60% remaining after 1 yr. Total potassium increased rapidly on the mesic site over the last 6 mo and reached 110% of the initial amount. Temperature, moisture, microbes, and microarthropods appeared to control nutrient dynamics. Wood litter dynamics were more important in terms of nutrient conservation on the clear—cut watershed than on the control and contributed to the resilience of the system. Removal or destruction of woody debris after clear—cutting would decrease the nutrient conservation properties of decaying wood and would probably contribute to watershed output of nutrients.
Guild Structure of Surface‐Gleaning Bats in Panama
Authors: Humphrey Stephen R. , Bonaccorso Frank J. , Zinn Terry L.
Published: May 2006

Food habits data from a complex guild of surface—gleaning. Neotropical bats over an annual cycle show that prominent consumption of beetles is the feature common to all species. Other insect orders, arachnids, vertebrates, and fruits are minor dietary components of various subsets of the guild. Multivariate analyses confirm the unity of the guild and show that at least five types of food constitute important niche dimensions for guild members. A consistent but not ubiquitous pattern of interspecific differences in size of the dentary apparatus indicates that food item size may be another important niche dimension. These results, added to previously documented differences in vertical and horizontal microhabitat use, invoke at least eight niche axes to differentiate these gleaning omnivores.
The Role of Density Dependence in a Marine Infaunal Community
Authors: Wilson W. Herbert
Published: May 2006

Abundance data from a marine sand flat in the Pacific Northwest reveal a strong negative correlation between abundances of the spionids Pygospio elegans and Pseudopolydora kempi, and hence suggest that competition is occurring between these two deposit—feeding polychaetes. The data also suggest that the specific winner of a competitive encounter is determined by the initial densities of the two species. A density—dependence experiment was performed in the field to test this hypothesis, in which the density of each species was independently varied over four densities in experimental containers. Immigration, emigration, and mortality combined to cause Pygospio densities in all experimental treatments to converge to ambient density after 8 wk. After 8 wk, Pseudopolydora densities were converging on the ambient density; the densities of some experimental treatments were significantly different from each other, but in no case did an experimental density differ significantly from ambient. The rate of convergence was strongly density dependent in both species. In four different experiments, conducted for 1, 2, 4, and 8 wk, Pygospio survivorship was strongly affected by its own density but was weakly affected, if at all, Pseudopolydora density. Pseudopolydora survivorship was affected only by its own density except for the 1—wk experiment, in which Pygospio density also had a significant effect. The field results conflict with the results from a laboratory experiment where Pygospio rapidly migrated away from Pseudopolydora but not from conspecifics. I propose a spacing hypothesis which predicts that Pygospio relocates as often as necessary to minimize contact with Pseudopolydora. Static spatial data from the field provide some support for the hypothesis. In the laboratory, Pygospio underwent asexual reproduction only at low densities and in the absence of Pseudopolydora. This response is interpreted as an adaptation for the rapid utilization of resources.
Pollination Effectiveness of Specialist and Generalist Visitors to a North Carolina Population of Claytonia Virginica
Authors: Motten Alexander F. , Campbell Diane R. , Alexander David E. , Miller Helen L.
Published: May 2006

We measured the pollination effectiveness and visitation rates of major insect visitors of Claytonia virginica, an obligately insect—pollinated spring wildflower, in a North Carolina deciduous forest. Seed set in the population was not pollinator—limited except during rainy weather and very early in the flowering season. The solitary bee Andrena erigeniae and the bee fly Bombylius major were responsible for more than 75% of the visits to C. virginica. Andrena erigeniae is a specialist on C. virginica, while B. major is a common visitor to many plant species. We measured the effectiveness of a pollinator by the probability that a visit resulted in fruit (capsule) formation. For those flowers that were successfully pollinated and thus produced a capsule, number of seeds did not vary with visitor identity or the total number of visits received. Although B. major and female A. erigeniae differ greatly in morphology and foraging behavior, a visit by either insect results in equally high seed set. As B. major is about two—thirds as abundant as A. erigeniae females on C. virginica, both insects contribute substantially to seed set in our population. With the visitation frequency and pollination effectiveness we measured, the generalist B. major alone has the potential to pollinate three—quarters of the flowers.
Spatiotemporal Variation in Insect Mutualists of a Neotropical Herb
Authors: Horvitz Carol C. , Schemske Douglas W.
Published: May 2006

We observed striking spatiotemporal variation in the antguard and pollinator mutualist assemblages of a neotropical herb. Fifteen different ant taxa (in five subfamalies) and five pollinator taxa (four euglossine bees and one anthrophorid bee) were associated with the plants during censuses at four sites. Ants and pollinators were censused for 4 and 3 yr, respectively. The mean number of taxa per site per year was 6.8 for ants and 3.6 for pollinators. The ant assemblage varied more than the pollinator assemblage, paticularly spatially. The ant assemblage varied more through space than through time (means of the proportional similarities for pairwise comparisons of assemblages were 0.37 and 0.51, respectively), while the pollinator assemblage varied equally through space and time (means of the proportional similarities were 0.53 and 0.50, respectively). For individual taxa of both assemblages, the coefficient of variation in relative abundance was large (>75%) for the majority of spatial and temporal comparisons. In each assemblage there was a single taxon (in the antguard, a rare taxon, but in the pollinators, an abundant taxon) that was the least variable, surprisingly in both time and space, in its relative abundance. These results, coupled with our previous findings of significant variation among mutualist taxa in the magnitude of their beneficial effects, indicate that plants may be subject to highly variable selection by their mutualist assemblages. The most beneficial antguard was consistently rare; the most beneficial pollinator was abundant in one site—year but rare or absent in most site—years. The taxon that was consistently abundant was not an efficient pollinator. Evolutionary specialization of plants on particular animals may be constrained by lack of constancy in the relative abundance of animals and the opportunity for specialization may differ greatly between interactions (e.g., antguards and pollinators) due to divergent patterns of constancy, rarity, and quality.
Food and Microhabitat Utilization by Grasshoppers from Arid Grasslands: Comparisons with Neutral Models
Authors: Joern Anthony , Lawlor Lawrence R.
Published: May 2006

Patterns of food and microhabitat use in arid grassland grasshopper communities are examined by comparing actual resource use with neutral models. A variety of neutral models is included in the analysis ranging from completely randomized resource utilization to models retaining much of the original community structure. Comparisons with resource utilization which are completely randomized indicate that biotic interactions are present for both diet and microhabitat niche dimensions. Average similarity in resource use of actual communities is much lower than in the analogous, completely randomized communities which suggests resource utilization by these species is hyperdispersed (on the average). However, mean niche overlap is not as low as possible which suggests other factors such as phylogenetic constraints on resource use or coevolved interactions which affect overall use of resources by a community are important.
Developmental Responses to Food Limitation as Indicators of Environmental Conditions for Ephydra Cinerea Jones (Diptera)
Authors: Collins Nicholas C.
Published: May 2006

Adaptive developmental responses to suboptimal conditions should preserve the life history traits most important to fitness at the expense of less critical traits. This paper compares responses to food limitation of Ephydra cinerea Jones (new data) to those of other Diptera (literature data) and concludes that the differences are indeed related to differences in demographic situations and other environmental characteristics. All the flies surveyed respond to larval food limitation by sacrificing adult body size, fecundity, and development time; these responses will moderate increases in prereproductive mortality, which demographic analyses identify as the most influential component of fitness for all the species. Differences between E. cinerea and the other flies in the degree to which: (a) body size is reduced, (b) development time is extended, and (c) mass—relative reproductive investment is increased under larval food limitation are associated with differences in: (a) the importance of size—related adult social interactions and range of larval food availability, (b) the safety of larval environments, and (c) demographic conditions. Similar comparisons generated predictions that, in comparison to other higher Diptera, adult size should be unusually important to fitness in house flies (Musca domestica), and the risks of mortality late in the larval period should be unusually high in the natural environments of blowflies (Chrysomyia spp., Lucilia spp.). Adult life—span and fecundity of fly species with poor access to adult food are relatively insensitive to reductions in adult food availability. Similarly, starvation resistance of newly enclosed larvae is correlated with the difficulty larvae have in locating food in nature, while absolute and relative egg size are not. In contrast to some other insects and mites, all the flies surveyed maintain nearly constant egg size (and where measured, egg quality) over a wide range of larval food limitation, suggesting that reduced egg size would have drastic effects on larval success. Where environmental data are available for the flies, conditions would favor evolution of near minimal investment per offspring.
Direct and Indirect Effects of Predators on Two Anuran Species along an Environmental Gradient
Authors: Werner Earl E. , McPeek Mark A.
Published: May 2006

This study examines direct and indirect interactions that influence the distribution of larvae of the bullfrog (Rana catesbeiana) and green frog (R. clamitans) along the environmental gradient of permanent to temporary ponds. The bullfrog is found in permanent ponds that typically contain fish, whereas the green frog is widely distributed along the gradient and is most successful in ponds where the bullfrog is absent. In a set of experimental ponds, bullfrogs were most abundant in ponds containing fish (bluegill, Lepomis macrochirus) and were rare in permanent pond that lacked fish. In contrast, green frogs were most abundant in ponds that had been drained the previous fall, second most abundant in those that lacked fish, and sparse in ponds containing bluegill. When presence and absence of bluegill were experimentally manipulated in divided ponds, bullfrogs survived well in the presence of bluegill, whereas no individuals survived in the absence of bluegill. Green frogs survived in low numbers under both treatments with a tendency to better survivorship in the absence of bluegill. A series of laboratory choice experiments was conducted with three major predator types: the dragonfly larva Anax junius, the salamander Ambystoma tigrinum, and L. macrochirus. Bullfrog larvae were more vulnerable to Anax and Ambystoma than were green frogs (because of higher activity levels), and green frogs were more vulnerable to bluegill than were bullfrogs (because tadpoles were less noxious). We argue that the high densities of bullfrogs found in the presence of bluegill reflect and indirect facilitation through two pathways: bluegill had strong negative effects on invertebrate and salamander predators of bullfrogs and on the green frog, which is a bullfrog competitor. The direct and indirect effects of these suites of predators appear to explain the differences in species abundance along the environmental gradient. Finally, we discuss the trade—offs at the individual level contributing to these differences in species performance along the gradient.
Plant Species Migration in a Mixed‐History Forest Landscape in Eastern North America
Authors: Matlack Glenn R.
Published: May 2006

The understory flora of successional forest in the Delaware/Pennsylvania Piedmont zone (USA) is species poor relative to old regrowth stands in the region. Impoverishment may reflect (a) limitations on dispersal of potential colonists, or (b) the unsuitability of successional stands for establishment. To examine the importance of dispersal, herb and shrub distributions were surveyed in successional stands of varying age and spatial arrangement. In successional stands contiguous with species—rich old—regrowth forest, understory species richness declined with distance into the successional stands, implying contagion across the old—regrowth ecotone. Individual species showed evidence of contagion in 67% of site x species combinations. Mean rates of migration ranged from to measurable movement (e.g., Carex laxiflora, Cimicifuga racemosa) to >2 m/yr in some species (e.g., Galium aparine, Potentilla canadensis). Significant differences in rate of migration were observed among seed dispersal modes: ingested > adhesive ° wind @> ants @> none. By contrast, there were no significant differences in rate between clonal and exclusively sexual species. In stands disjunct from old regrowth, the understory was species poor relative to old—regrowth forest due to a lack of species dispersed by ants and spores and those with no obvious dispersal vector. Understory species richness was greater in older disjunct stands and in stands closer to potential sources of propagules than in young and isolated stands, trends that were also noted in distributions of individual species. These results suggest that accessibility to colonists plays an important role in determining understory composition of successional stands. Medium— and long—range migration appears to be an issue of seed and spore dispersal rather than vegetative propagation. The extremely low migration rates of some species threaten their continued existence in the second—growth forest landscape.
Growth, Production, and Decomposition Dynamics of Sphagnum under Natural and Experimentally Acidified Conditions
Authors: Rochefort Line , Vitt Dale H. , Bayley Suzanne E.
Published: May 2006

Annual linear growth, net primary production, and decomposition of Sphagnum fuscum, Sphagnum magellanicum, and Sphagnum angustifolium were measured under experimental acidification and natural conditions in a poor fen at the Experimental Lakes Area (ELA), Ontario, Canada. Acidification increased growth and production of most species (2 out of 3 in the oligotrophic zone, and 2 out of 3 in the minerotrophic zone) in the first 2 yr. After 2—3 yr of artificial acidification, growth and production were not stimulated to the same extent in experimental areas as in earlier years. After 4 yr, growth and production in the experimental area declined so that they were the same as controls for 4 of the 6 treatments. Therefore, the effect of the acid treatment changed over the years. With 4 yr of simulated “acid rain,” decomposition was unaffected. Our results suggest that the fertilizing effect of SO42— and NO3— in North American acid precipitation as suggested previously in the literature is a very short—term one, at least for this type of peatland. Under natural conditions, in the oligotrophic central zone of mire 239, production for hollow species was somewhat greater than for hummock species for the 4 yr studied. Since the decomposition rate ratio between hummock—top, mid—hummock, and hollow species is roughly in the ratio 7:9:13, the rate of peat accumulation should be higher in hummocks. Hence hummocks appear to be maintained in this poor fen due to low decomposition rates rather than relatively high production. For 3 of the 4 yr studied (1985—1987) in the minerotrophic edge zone, hummock production (303, 175, and 156 g°m—2°yr—1, respectively) was greater than hollow (198, 100, and 97 g°M—2°yr—1) and mid—hummock production (103, 59, and 52 g°m—2°yr—1). The relative decomposition rates of the species had the same ratio as in the oligotrophic zone. Thus hummocks appear to be expanding in this minerotrophic edge zone at a faster rate than in the oligotrophic zone.
The Distribution and Growth of Salicornia Europaea on an Inland Salt Pan
Authors: Ungar Irwin A. , Benner David K. , McGraw David C.
Published: May 2006

The nature of the distribution of plant species along a soil salinity gradient at Rittman, Ohio, indicates that there is a definite relationship between soil salinity concentrations and the position of plant species along this environmental gradient. Only a single species of flowering plant, Salicornia europaea, was found growing at the most extreme portion of the salinity gradient. Soil samples collected from sites dominated by S. europaea had salinity levels which yielded in situ electrical conductivities ranging from 18.9 to 142.8 mmhos/cm, Atriplex triangularis grew at sites with intermediate levels of soil salinity, 3.9 to 40.0 mmhos/cm, and Hordeum jubatum dominated the least saline sites, 1.1 to 14.0 mmhos/cm, which were directly adjacent to a nonsaline meadow. Salicornia europaea had high levels of mortality during the spring and early summer months of the growing season, with the mean number of seedlings ranging from 58 plants per 100—cm2 quadrat in May to a mean value of 18 mature plants per quadrat at the end of the growing season in October. At the most highly saline sites containing S. europaea, seedling numbers ranged from a mean of 5 plants per 100—cm2 quadrat in May to 1 mature individual per quadrat by October. Transplants of S. europaea had the highest standing crop biomass levels in the Atriplex and Hordeum zones, with dry weight production decreasing in zones of both higher and lower soil salinity. Laboratory investigations indicated that S.europaea reached its optimal growth in nutrient solution treatments containing 170 meq/1 and 340 meq/1 NaC1. Germination of seeds of S. europaea under field conditions occurred over an extended period of time, from February through June, when soil salinity levels were lowest. Seed germination was not stimulated by treatments with salt and maximum germination percentages occurred in distilled water controls.
Estimates of Net Annual Aboveground Production: Sensitivity to Sampling Frequency
Authors: Dickerman Joyce A. , Stewart Arthur J. , Wetzel Robert G.
Published: May 2006

We used frequently repeated measurements of growth characteristics in undisturbed study plots, and replicated harvests of shoots from nearby plots, to measure biomass accumulation of Typha latifolia in 1978 and 1979. The data were used to calculate net annual aboveground production (NAAP) using seven computational procedures: peak biomass; the Milner and Hughes; Smalley; Valiela et al.; a modified Wiegert and Evans; Allen curve; and the summed shoot maximum which we devised for this study. The procedures were evaluated with respect to their sensitivity to differences in sampling frequency by repetitively calculating NAAP using different sequential subsets of the study plot and harvesting data sets. Year—to—year differences in the relative accuracy of some methods (peak biomass; Milner and Hughes; Smalley; and Valiela et al.) were affected by cohort structure of the population. The Allen curve NAAP estimates were not affected by the differences in cohort structure. Our modified Wiegert and Evans method overestimated NAAP, and is inappropriate for use with Typha. As apparent sampling frequency declined, NAAP estimates for most methods declined and were more variable, especially for the more complicated methods. Relative to the standard used in this study (the summed shoot maximum), most methods underestimated NAAP by 18—38%, even with 1—3 wk sampling intervals. Underestimates were due to inadequate corrections for early shoot mortality, leaf turnover, and losses of portions of individual leaves. The procedure of estimating each cohort's production by plotting shoot number vs. mean biomass per shoot over time (Allen curve method) has inherent advantages relative to the other methods examined and was comparatively insensitive to changes in sampling frequency.
Coexistence of Three Congeneric Salamanders: The Importance of Habitat and Body Size
Authors: Southerland Mark T.
Published: May 2006

In the southern Appalachians three congeneric salamanders–Desmognathus quadramaculatus, D. monticola, and D. ochrophaeus–are broadly sympatric. D. monticola is intermediate to the others species in body size and in degree of terrestrial habit. This study analyzed the roles of competition, predation, and habitat conditions in limiting the abundance of this "ecologically intermediate" species and in directing its evolution. In field observations, D. monticola regularly used both stream—bank and forest—floor habitats; experimental exclusion from either of these habitats resulted in poor survival. Field enclosure experiments also showed that growth, survival, and habitat selection of D. monticola were altered by the presence of congeners. Interference from conspecifics and from congeners of equal size decreased growth and reduced survival of D. monticola. Predation by large D. quadramaculatus sharply reduced survival of juveniles and caused shifts in activity and habitat selection. I suggest that predation has controlled the evolution of body size and habitat selection in D. monticola and that populations of this intermediate species are limited by the availability of streambank cover as a refuge from predation. Competition may also be important among species of similar size when food is limited.
A Process Model of Fire Ecology and Succession in a Mixed‐Conifer Forest
Authors: Kercher J. R. , Axelrod M. C.
Published: May 2006

A forest succession simulator, SILVA, has been developed for the mixed—conifer forest (seven major species) of the Sierra Nevada, California, to simulate the effects of fire on forest dynamics. SILVA is an extensive modification of a simulator for forests of the northeastern United States. The simulation includes the time development of the growth in tree diameter, tree height, and leaf—area index. Recruitment and mortality are modeled stochastically. Modifications include fire ecology, temporal seed—crop patterns, and seedling—survival factors unique to Sierra Nevada forests. The probability of mortality from fire is determined by the height of crown scorch (a function of fire intensity, diameter at breast height, and bark thickness). The model simulates the dynamic and structural responses of communities to many factors. For 500—yr simulations from an initial clear—cut condition, the time—averaged basal—area ratios of Pinus ponderosa to Abies concolor were 5.2:1 and 1:16 for elevations of 1524 m and 1829 m, respectively. At 1524 m, the ratio of P. ponderosa to A. concolor decreased 59% when fire suppression was introduced. Fire provides P. ponderosa with a strong competitive advantage. Its growth form and growth rate are significant factors in its ability to evade fire. Rank correlations of species were compared with data for stands of ponderosa pine and white fir. Correlations were significant at 1% and 10% levels, respectively.
Peripheral Foraging by Territorial Rufous Hummingbirds: Defense by Exploitation
Authors: Paton David C. , Carpenter F. Lynn
Published: May 2006

Rufous Hummingbirds in the Sierra Nevada Mountains were observed to forage in the edge areas of their territories during the first few hours of the day, evenly over their territories during midday, and in the territory cores in late afternoon. This distribution of foraging effort appeared to be inconsistent with maximizing their rates of net energy gain during foraging. If Rufous Hummingbirds were to maximize their net energy gain during individual foraging bouts, foraging effort would have to be distributed evenly over the territory at all times of the day. We present a model which suggests that the observed feeding behavior of territorial Rufous Hummingbirds leads to higher daily net energy gains and lower food losses than if the birds maximized only net energy gain during each foraging bout during the day. Intrusion pressure accounts for the early—morning bias of foraging of edge areas. Intruders attempted to rob nectar from flowers in the edge areas of territories much more frequently than in the cores, and a higher proportion of these attempts were successful. Intruders frequently returned within 5—10 min to the sites that they had just robbed, and returned most frequently to those territories where they were able to rob for the longest time (on average) before being detected and chased. Nectar levels on territories were highest at dawn, and since edges were more prone to food losses than were cores, the territory owner minimized losses by feeding in the edge areas early in the morning. After chasing intruders from their territories, owners often returned and fed in the site just robbed by the intruder, which also reduced food losses by depleting nectar levels in areas prone to intrusion. According to the model, reduction of reintrusion rates by lowering the yield per intrusion and thereby discouraging robbers contributes more to the owner's benefit than does simple pre—emption of the resource by the owner. Thus, the peripheral foraging behavior of hummingbirds and their feeding response following intrusions are components of territory defense. Such defense by exploitation may be common when food resources are being defended, and should be considered along with aggressive exclusion of intruders (defense by interference) in the overall strategies of territorial animals. We close with a speculation that the late—afternoon foraging bias to the core may raise foraging efficiency during the time of day when the birds are heaviest and therefore have the highest foraging cost.
Mechanistic Constraints and Optimality Models: Thermoregulatory Strategies in Colias Butterflies
Authors: Kingsolver Joel G. , Watt Ward B.
Published: May 2006

To explore mechanistic constraints on the evolution of optimal phenotypes, we develop an optimality model for thermoregulation in Colias butterflies. The model identifies the thermoregulatory characteristics of butterflies that maximize time available for flight activity subject to an overheating constraint, and was tested in three Colias populations along an elevational gradient. Model simulations and field results showed that environmental variation limits available flight time in higher elevation populations, even for optimally designed butterflies. Optimal predicted thermoregulatory characteristics were within two standard deviations of the observed characteristics in each population. Model simulations and field transplant experiments showed that large departures from optimality would have deleterious consequences for flight and overheating. This represents the first test of a strategy model for optimal thermoregulatory characteristics in animals. Our results illustrate how the constraints imposed by environmental variation and the mechanisms of adaptation affect the evolution of locally optimal phenotypes. We argue that, used in this mechanistic context, optimality modeling remains a powerful tool in studying adaptation.
Odonate Guild Composition: Experiments with Colonization History and Fish Predation
Authors: Morin Peter Jay
Published: May 2006

Fewer than half of the anisopteran odonate species that oviposited in a North Carolina farm pond successfully metamorphosed from that pond. Odonates metamorphosed during two periods: early April though early June, and late May through early September. I used an experiment designed for variance analysis to measure the impact of fish and early—breeding odonates on the abundance and species composition of late—breeding odonates. Odonates recruited naturally in open—topped screened pens that either excluded or contained fish. I placed pens in the pond either during or after the early species breeding season to alter the abundance of larval early breeders. Fish exclusion increased the combined abundances of all anisopteran species 5—10 times above abundances in pens containing fish. However, fish also facilitated the recruitment of one species, Perithemis tenera. Placement of fish exclusion pens in late June instead of early May reduced abundance of early breeders and increased abundances of two late breeders, Perithemis tenera and Pachydiplax longipennis. Early—breeders did not reduce late—breeder recruitment in pens with fish. Abundances of early and late breeders were significantly negatively correlated in pens without fish, but abundances of the same species were not significantly correlated in pens with fish. Different histories of colonization by early breeders influenced interspecific interactions among odonates only if fish were absent. Most variation in the composition of experimental communities was explained by a hierarchy of negative effects of fish on odonate abundance, and negative effects of early—breeders on late—breeders in the absence of fish.
Herbivory and the Seasonal Abundance of Algae on a High Intertidal Rocky Shore
Authors: Cubit John D.
Published: May 2006

This paper examines the contribution of herbivory to seasonal variations in algal coverage at the uppermost intertidal levels of a rocky shore on the outer coast of Oregon, USA. In this habitat, simple, fast—growing algae are abundant during wetter, less sun—exposed, and cooler weather of the winter months, but disappear in the drier, sunnier, and warmer conditions of the summer. Acmaeid limpets, the predominant herbivores in the habitat, are present throughout the year. Previous studies of the high intertidal zone have suggested that physical stresses alone prevent the survival of algae through the summer months, and that grazing affects algal abundance when physical conditions are more benign. To test these hypotheses, cageless methods were used to exclude the limpets from plots on the uppermost intertidal rocks. The experiments were repeated at 3—4 mo intervals over a period of several years. This partial reduction in herbivory resulted in the development and persistence of dense covers of algae through the summer months, as well as increasing algal abundance during the other times of year. Apparently the primary cause of the seasonal fluctuations in algal abundance is variation in rates of algal production rather than change in absolute rates of algal loss to herbivory or physical stresses. Evidence from this study and the literature indicates that the limpets in this habitat are probably consuming more, not less, algae in the winter months. However, higher rates of algal production in winter more than compensate for the increased herbivory, and the algae become more abundant. During the summer, drier conditions cause rates of production by the algae to fall below rates of consumption by limpets, and the limpets remove most of the standing crop of algae. Thus, increased grazing pressure coincides with increased physical stress. To be a perennial or summer annual in this habitat, an alga not only must be resistant to desiccation, insolation, and high temperatures, but also must be inaccessible to herbivores or be resistant to herbivory. In the uppermost intertidal zone, the abundance of algae in winter evidently depends on the pattern of changes in primary production over the entire year. Scarcity of forage in the summer months reduces populations of the resident limpets, and the favorable conditions for primary production in winter months allow the transient species of algae to establish rapidly, grow, and reproduce before the populations of limpets recover. If even the most favorable physical conditions for algal growth remained constant through the year, the transient algae would probably disappear. It is speculated that under present conditions the establishment of perennial algae in this habitat is limited by the intensity of herbivory in the summer and by competition with transient algae in the winter. If this is correct, less fluctuating physical conditions, even if severe, would change the species composition of this high intertidal community by favoring the establishment of perennial algae.