bluuebunny/crossref_metadata_2025 huggingface dataset:
The Relationship Between Gap Size and Performance of Several Colonizing Annuals
Authors: McConnaughay K. D. M. , Bazzaz F. A.
Published: May 2006

Several colonizing annual species comprising a range of seed masses were grown in artificially created disturbances in an old field. Gaps of 5, 10, 20, and 40 cm in diameter were established in a Poa pratensis L. background. Undisturbed vegetation was used as a control. Although survivorship was greater in larger gaps only for small—seeded species, the probability of reaching reproductive maturity increased with gap size for six of the species tested. Other measures of species performance, such as plant mass and number and mass of seeds produced, followed similar trends. Ipomoea hederacea, a climbing plant, was unique in its lack of response to gap size. Many differences in responses among species suggest that the size of a disturbance might direct the events of colonization.
Role of Fire in Seed Germination of Woody Taxa in California Chaparral
Authors: Keeley Jon E.
Published: May 2006

Germination behavior of 45 tree, shrub, subshrub, and liana taxa from fireprone coastal sage scrub and chaparral was investigated. Seeds were sown on filter paper or potting soil, in the light or dark, with or without powdered charred wood (charate), and after heating treatments of 1 h at 70°, 5 min at 100°, 5 min at 120°C, or no heat. Germination medium had a significant effect on >70% of the species, although the pattern of response to other treatments remained the same on both media. Germination of over half of the species was inhibited in the dark, but two species were light inhibited. Charate significantly enhanced the germination of approximately a quarter of the species. A similar proportion of species exhibited heat—stimulated germination. In only a few species did both heat and charate play a role in stimulating germination. Nearly a third of the species had seeds that germinated readily upon wetting, and germination was not further stimulated by any fire—related cue. These different germination syndromes are correlated with different life history syndromes. Most coastal sage subshrubs germinate readily in the absence of fire—related stimuli and thus are capable of colonizing other forms of disturbance. For many of these species, germination was inhibited in the dark. This may result in a portion of the seed pool remaining dormant until fire since, in the case of several species, dark inhibition is overcome by charate. Chaparral shrubs and trees that germinate readily upon wetting seldom establish seedlings after fire. Their persistance in these fire—prone environments is due to resprouting after fire. Seedling establishment and population expansion for such species is dependent upon extended fire—free periods. In contrast, woody species that fail to germinate without some fire—related cue have seedling establishment and potential population expansion restricted to postfire conditions. The present mixture in species composition may be maintained by variable burning regimes.
Linking Predation Risk Models with Behavioral Mechanisms: Identifying Population Bottlenecks
Authors: Williamson Craig E.
Published: May 2006

A comprehensive understanding of the factors controlling predation risk in natural communities is dependent upon knowledge of both the patchiness of predator and prey populations in nature, and on the outcome of behavioral interactions between predator and prey individuals. While both of these areas of predation ecology are rich in theory and data, there are no models that permit these two fundamental components of predation risk to be combined in a quantitative way. Here a model is developed to combine these density risk and prey vulnerability components of predation. The model is used to estimate the contribution of predator and prey population overlap to predation risk, and to quantify both density risk and prey vulnerability components of predation risk for two planktonic invertebrate predators and five rotifer prey. Behavioral data from the literature are used to interpret the prey vulnerability values and demonstrate the utility of the model. The predation risk value obtained for each prey species also are compared to the intrinsic population growth rates (°max), taken from previous studies. In addition, I test the a priori hypothesis that prey population growth will be negative when predation risk exceeds °max. The results reveal the presence of severe bottleneck periods where predation risk exceeds the °max of four of the five prey species, thus supporting the a priori prediction of negative population growth or extremely how prey populations during these periods.
Patterns of Leaf Development and Herbivory in a Tropical Understory Community
Authors: Aide T. Mitchell
Published: May 2006

Most plant—herbivore studies have demonstrated that young leaves are preferred over mature leaves, and that the majority of lifetime damage occurs during the 1st mo. In spite of this, we know little about how young leaves are defended. Young leaves are an ephemeral resource, and herbivore damage may be reduced by (1) producing leaves at times of low herbivore abundance, (2) synchronous leaf production, or (3) rapid leaf expansion. To address these hypothesis I examined patterns of leaf phenology, leaf expansion, and herbivory in saplings of 32 species in a Panamanian lowland moist forest. Leaf phenology ranged from continuous to highly synchronous, and mean monthly damage rates varied from 6 to 60%. Within species, leaves produced during the dry season, a period of relatively low insect abundance, received less damage than leaves produced during the west season. Leaves produced during synchronous flushes received less damage than leaves produced out of synchrony. Species with continuous or synchronous leaf production had lower damage rates than intermediate species, but growth rates and survivorship were not correlated with leaf production. It appears that higher damage rates in intermediate species may be compensated by reduced investment in chemical defense compared with continuous species, and a less restricted growing season and reduced storage costs compared with synchronous species. Previous studies have identified leaf expansion rate as an important young leaf defense, but in this study there was no correlation between expansion rate and herbivory.
Responses of Arctic Tundra to Experimental and Observed Changes in Climate
Authors: Chapin F. Stuart , Shaver Gaius R. , Giblin Anne E. , Nadelhoffer Knute J. , Laundre James A.
Published: May 2006

We manipulated light, temperature, and nutrients in moist tussock tundra near Toolik Lake, Alaska to determine how global changes in these parameters might affect community and ecosystem processes. Some of these manipulations altered nutrient availability, growth—form composition, net primary production, and species richness in less than a decade, indicating that arctic vegetation at this site is sensitive to climatic change. In general, short—term (3—yr) responses were poor predictors of longer term (9—yr) changes in community composition. The longer term responses showed closer correspondence to patterns of vegetation distribution along environmental gradients. Nitrogen and phosphorus availability tended to increase in response to elevated temperature, reflecting increased mineralization, and in response to light attenuation, reflecting reduced nutrient uptake by vegetation. Nutrient addition increased biomass and production of deciduous shrubs but reduced growth of evergreen shrubs and nonvascular plants. Light attenuation reduced biomass of all growth forms. Elevated temperature enhanced shrub production but reduced production of nonvascular plants. These contrasting responses to temperature increase and to nutrient addition by different growth forms "canceled out" at the ecosystem level, buffering changes in ecosystem characteristics such as biomass, production, and nutrient uptake. The major effect of elevated temperature was to speed plant response to changes in soil resources and, in the long term (9 yr), to increase nutrient availability through changes in N mineralization. Species within a growth form were similar to one another in their responses to changes in resources (light or nutrients) but showed no consistent response to evelated temperature. Species richness was reduced 30—50% by temperature and nutrient treatments, due to loss of less abundant species. Declines in diversity occurred disproportionately in forbs, which are important for animal nutrition, and in mosses, which maintain soil thermal regime. There was no increased abundance of initially rare species in response to any treatment. During our 9—yr study (the warmest decade on record in the region), biomass of one dominant tundra species unexpectedly changed in control plots in the direction predicted by our experiments and by Holocene pollen records. This suggests that regional climatic warming may already be altering the species composition of Alaskan arctic tundra.
Size‐Specific Shelter Limitation in Stone Crabs: A Test of The Demographic Bottleneck Hypothesis
Authors: Beck Michael W.
Published: May 2006

Habitat structural complexity can facilitate species persistence by providing refuges and the availability of refuges may control the size of many populations. In organisms that increase greatly in size during ontogeny the size and abundance of refuges is particularly important, because population—regulating factors may operate strongly on only one size class or ontogenetic stage within a population, creating a demographic bottleneck. I test the demographic bottleneck hypothesis in stone crabs by first supplementing shelter with five different sizes of polyvinyl chloride (PVC) pipe at eight sites in St. Joseph Bay, Florida to determine the size class most likely to be sheltered limited. I then used tethering experiments, cage studies, and natural observations to ask if habitat structural complexity regulates the stone crab population through its size—specific effects on growth, survival, and fecundity. Shelter availability appears to create a demographic bottleneck that affects the growth and fecundity of large crabs. In both 1991 and 1992, the largest PVC pipes were occupied in significantly greater proportions than smaller pipes, suggesting that this shelter size class is limiting. Predation in size specific; smaller individuals were at significant risk but no large crabs were taken by predators. However, shelter is particularly important for the growth of large crabs. There was a fivefold increase in molting individuals on shelter—supplemented sites relative to natural densities suggesting that the bottleneck size class has been identified correctly and that shelter additions can alleviate the bottleneck. Supplemental shelters also particularly attracted gravid females and, in tethering experiments, females with shelter produce egg masses twice as quickly as females without shelter. These results differ from most prior results in marine populations in two ways. First, it is factors affecting adults and not juveniles that may regulate the stone crab populations. Second, refuge use in those size classes likely to be shelter limited is not explained primarily by predation, but by effects on growth and fecundity. The evidence suggests the merits of the demographic bottleneck hypothesis; habitat structural complexity may affect both the size of populations and the size of individuals within the population.
Competition and Biodiversity in Spatially Structured Habitats
Authors: Tilman David
Published: May 2006

All organisms, especially terrestrial plants and other sessile species, interact mainly with their neighbors, but neighborhoods can differ in composition because of dispersal and mortality. There is increasingly strong evidence that the spatial structure created by these forces profoundly influences the dynamics, composition, and biodiversity of communities. Nonspatial models predict that no more consumer species can coexist at equilibrium than there are limiting resources. In contrast, a similar model that includes neighborhood competition and random dispersal among sites predicts stable coexistence of a potentially unlimited number of species on a single resource. Coexistence occurs because species with sufficiently high dispersal rates persist in sites not occupied by superior competitors. Coexistence requires limiting similarity and two—way or three—way interspecific trade—offs among competitive ability, colonization ability, and longevity. This spatial competition hypothesis seems to explain the coexistence of the numerous plant species that compete for a single limiting resource in the grasslands of Cedar Creek Natural History Area. It provides a testable, alternative explanation for other high diversity communities, such as tropical forests. The model can be tested (1) by determining if coexisting species have the requisite trade—offs in colonization, competition, and longevity, (2) by addition of propagules of propagules to determine if local species abundances are limited by dispersal, and (3) by comparisons of the effects on biodiversity of high rates of propagule addition for species that differ in competitive ability.
Predicting Direct and Indirect Effects: An Integrated Approach Using Experiments and Path Analysis
Authors: Wootton J. Timothy
Published: May 2006

Determining the strengths of interactions among species in natural communities presents a major challenge to ecology. Using an approach combining experimental perturbations and path analysis, I examined the mechanisms by which birds directly and indirectly affected other members of an intertidal community, evaluated alternative causal hypotheses, and predicted whether interactions among the unmanipulated species would be strong or weak. Comparing treatments with t tests indicated that excluding bird predators with cages caused increases in Pollicipes polymerus, and declines in Nucella spp., Mytilus californianus, and Semibalanus cariosus. However, these conclusions provided no insight into the underlying mechanisms causing the differences. Path analysis permitted insight into the causal mechanisms by making a variety of predictions about the strength of direct interactions: (a) Bird predation negatively affects Pollicipes, but not Nucella, Leptasterias, or Mytilus; (b) Pollicipes reduces Semibalanus and Mytilus abundance because of space competition; (c) Mytilus reduces Semibalanus cover through competition for space; and (d) as prey species, Semibalanus and Pollicipes enhance Nucella density, but Nucella predation does not have important effects on Semibalanus of Pollicipes. Based on the estimated strength of direct interactions, the importance of indirect effects among species could also be predicted. In experiments manipulating Nucella, Pollicipes, Semibalanus, and birds independently of one another, I tested 11 of the interactions predicted by the path analysis; all were supported. Path analysis in conjunction with limited experiments may provide an efficient means to predict important direct and indirect interactions among unmanipulated species within ecological communities.
Effects of Top and Intermediate Predators in a Terrestrial Food Web
Authors: Spiller David A. , Schoener Thomas W.
Published: May 2006

To determine the effects of lizards and web spiders on species in lower trophic levels, we manipulated their abundances within large field enclosures on Staniel Cay, Bahamas, from 1989 to March 1992. The experimental design (2 x 2 factorial) measured the separate effects of lizards and spiders and compensatory predation (lizard x spider interaction). In treatments where web spiders were unaltered, mean number of web spider individuals was 1.4 times higher in enclosures with lizards removed than in those with lizards present at natural densities. Total biomass of aerial arthropods caught in sticky traps was 1.4 times higher in treatments with web spiders removed than in treatments with web spiders unaltered. Lizards had no significant effect on aerial arthropods. Total amount of herbivore damage on sea grape leaves was 3.3 times higher in treatments with lizards removed than in treatments with lizards unaltered. Web spiders had no significant effect on leaf damage. The lizard x spider interaction was not significant in each analysis, indicating that compensatory predation was weak. Our results support a model in which the interaction between top predators (lizards) and herbivores is strong, whereas the interaction between intermediate predators (web spiders) and herbivores is weak. Consequently, the net effect of top predators on producers is positive.
Herbivory and Structure in a Midlittorial Rocky Community: A Case in Southern Chile
Authors: Jara H. Fernando , Moreno Carlos A.
Published: May 2006

The abundance of the red alga Iridaea boryana and the role of herbivores in structuring the mid—intertidal community were studied during a 2—yr period at a semiprotected site in Mehuin, Chile. Herbivore densities wee manipulated on three contiguous rocky areas having similar initial communities. In one treatment, herbivores were totally removed every 15 d.; herbivores were maintained at double the control density in a second treatment; and an undisturbed area was used as a control. Percent cover of sessile organisms was measured at monthly intervals from September 1978 to September 1980. Changes which occurred in the 2—yr period are interpreted as follows. Herbivore removal allowed high cover (>80%) of I. boryana. Herbivore addition and a natural increase in herbivores on the control caused a drastic decline in this alga, and the substrate became dominated by barnacles and crustose algae. Ephemeral algae (e.g., Petalonia and Scytosiphon) were frequent in the presence of herbivores (control and addition), while in the absence of herbivores, invasion by ephemerals was precluded by the high abundance of I. boryana. In winter, herbivores and high wave action caused a significant reduction in algal abundance, which did not occur on the removal rock. Light seemed to be the main limiting resource in the absence of herbivores, where I. boryana was the competitive dominant. Two patterns of community development were found after new space (burned and scraped) was created. (1) Succession following spring denudations began with algal colonization leading to a dominance of I. boryana. (2) Succession following autumn denudations began with barnacles and terminated in a community dominated by barnacles and crustose algae. The natural regional mosaic of I. boryana dominance and barnacles plus crustose algae depends, locally, on the season of the last disturbance and on herbivore abundance. Differences between this southern Chilean and equivalent North American intertidal communities are discussed.
Interaction of Factors Governing the Distribution of a Predatory Aquatic Insect
Authors: Vodopich Darrell S. , Cowell Bruce C.
Published: May 2006

We investigated the interaction of factors regulating the larval distribution of the predatory chironomid Procladius culiciformis in Lake Thonotosassa, Florida. Hypotheses were tested experimentally to relate sediment grain size, organic content, prey density, diet and feeding selectivity to larval growth, feeding habits, and distribution. The distribution of P. culiciformis was determined by monthly sampling (June 1978 to May 1979) of the benthic macroinvertebrate community along a littoral zone transect. Experimental and field observations were compared for final analysis. The distribution of P. culiciformis, within 0.2—1.5 m depth, correlated positively with density of total organisms, density of total chironomids, and median grain size of the sediment. However, the distribution was correlated negatively with density of oligochaetes, water depth, and sediment organic content. The annual mean density of P. culiciformis across all transect stations was 371 larvae/m2 and was highest (1024 larvae/m2) at the shoreline station (0.7 m depth). The influence of food on distribution was measured by larval preference for naturally occurring prey and by larval growth on four common diets. Feeding selectively by P. culiciformis was positive for chironomids, ostracods, and cladocerans, and negative for rotifers. Survival and growth of larvae were highest on a diet of oligochaetes, significantly lower on diets of chrionomids and zooplankton, and lowest on a diet of algae—detritus. Sediment grain size and organic content were tested for larval preference and their effect on predation rate. Larvae preferred sediment with high organic content, but no preference was found among natural sediments with different median grain sizes. Large—grain sand and high prey density promoted high predation rates in the laboratory. However, the presence of organic matter decreased the predation rate and nullified the effect of grain size. Multiple environmental factors, especially those influencing prey availability, interact to regulate the larval distribution of P. culiciformis.
Litter Decomposition in a Subarctic Spruce‐Lichen Woodland, Eastern Canada
Authors: Moore T. R.
Published: May 2006

The litter bag technique was used to examine the decomposition of six litter types (representing Picea mariana, Cladina stellaris, Betula glendulosa, and Ledum groenlandicum) and standard cellulose. The decomposition was measured over a 2—yr period, with tissue samples placed on a recently burnt site and on top of and beneath a lichen mat in a mature spruce—lichen woodland in northern Quebec. Mass losses after 2 yr were between 10 and 60%, with the largest losses occurring in B. glandulosa and L. groenlandicum leaves and the smallest in C. stellaris; 60—90% of the lst—yr mass loss occurs during the winter (September to June). The exponential decay constant k ranged from —0.05 to —0.53. There were few statistically significant differences in decomposition rate among the three sites. Of the chemical constituents analyzed, original concentrations of Ca, carbohydrate, P.K. and N were closely correlated either with the decomposition parameter k or with mass remaining after 1 or 2 yr. Ca, Mg, and k are rapidly lost from the decomposing tissues, except for C. stellaris. All the tissues, except B. glandulosa, showed an accumulation of N. associated with high C:N ratios. The results emphasize the importance of N to this ecosystem, in that it tends to remain immobilized in the litter, at least over the first 2 yr.
Successional Status, Seed Size, and Responses of Tree Seedlings to CO^2, Light, and Nutrients
Authors: Bazzaz F. A. , Miao S. L.
Published: May 2006

We studied how an enriched CO2 atmosphere, in a fully crossed design of light nutrients, influenced 1st—yr seedling growth in six New England deciduous forest tree species. The species, in the order of increasing shade tolerance, were gray birch (Betula populifolia), ash (Fraxinus americana L.), red maple (Acer rubrum L.), Red Oak (Quercus rubra L.), yellow birch (Betula alleghaniensis Britton), and striped maple (Acer pensylvanicum). Elevated CO2 environments significantly stimulated the seedling growth of all six species. Generally this was more pronounced in low light. The greatest stimulation was found under the condition of low light and high nutrients. However, individual species responded differently to elevated CO2 levels. Among the three early—successional species, gray birch, ash, and red maple, a significant increase in seedling growth under elevated CO2 conditions was found only with high nutrients. The three late—successional species grown under elevated CO2 conditions (red oak, yellow birch, and striped maple) showed a greater percentage increase in seedling growth in low light than in high light. Thus, for the early—successional species, the degree of enhancement of seedling growth by elevated CO2 levels was more sensitive to nutrient levels, while in the late—successional species the enhancement was more sensitive to the level of light. Moreover, species with large seeds (e.g., red oak) exhibited a greater response to elevated CO2 levels under low light than species with small seeds (e.g., gray birch). The results emphasize the importance of plant species as well as other environmental resources in modifying the response of plants to elevated CO2. Considering the light and nutrient environment observed in forest gaps of various sizes, the results of the present experiment suggest seedling regeneration in New England deciduous forests may be altered in a future high CO2 environment.
Intraguild Predation and Competition Between Larval Dragonflies: Direct and Indirect Effects on Shared Prey
Authors: Wissinger Scott , McGrady Jill
Published: May 2006

We conducted manipulative field experiments in artificial ponds to quantify the predatory impact of larvae of a migratory dragonfly (Tramea lacerata) on a common resident dragonfly species (Erythemis simplicicollis), and on damselflies as shared prey of the two dragonflies. We found that the combined predatory effects of these two dragonflies on damselflies were not additive. To determine the underlying cause of non—additive predation rates in the field, we conducted a second experiment in laboratory aquaria to isolate the impact of each predator on the consumption rates of the other. Dragonfly consumption rates of damselflies in single—predator treatments were compared to those in the presence of heterospecifics or conspecifics with their menta (mouthparts) surgically modified so that they could not capture prey. In the laboratory experiment, de—mented Tramea reduced the consumption rates of Erythemis to less than half of that observed when Erythemis foraged alone. Erythemis numbers were also reduced by Tramea predation. Erythemis had neither effect on Tramea. Both of the negative effects of Tramea on Erythemis will have indirect positive effects on damselflies. The "behavioral" component (reduced Erythemis foraging rate) should be more important than the "trophic link" (reduced Erythemis numbers) indirect effect. Together these indirect positive effects will allay, but not completely compensate for, the direct negative effects of Tramea predation on damselflies. These results illustrate how an asymmetric potential for intraguild predation can lead to asymmetries in interference competition and to non—additive effects on prey mortality. The addition of removal of predators that interact in this manner to or from communities should have only a small net effect on prey because of compensating direct and indirect effects. This may explain why predator manipulations have often had unpredictable or undetectable effects on freshwater benthic communities.
Floral Character Displacement Generates Assemblage Structure of Western Australian Triggerplants (Stylidium)
Authors: Armbruster W. Scott , Edwards Mary E. , Debevec Edward M.
Published: May 2006

We studied the pollination ecology and assemblage structure of 31 species of Stylidium (Stylidiaceae) at 25 sites in Western Australia. The number of species per study site varied between two and size. Stylidium species are pollinated by a variety of nectar—seeking solitary bees and bombyliid flies. Within and among species there is significant variation in nectar—tube length (and therefore in the insects that visit the flowers) and in pollen placement on pollinators. Pollen is placed “explosively” on the insect by a motile column of fused staminate and pistillate tissues; the position and reach of the column varies within and among species, thereby causing variation in site of pollen deposition. When discrete pollination niches were defined for all species, only one niche overlap was observed across the 86 interacting pairs of Stylidium species at the 25 sites. To determine whether this was a nonrandom assemblage structure we compared our observation with the outcome of null models. We developed three null models to cover the most likely structuring processes: that communities are organized by (1) ecological sorting, (2) evolution of plant phenotypes, or (3) both processes. We concluded that it was unlikely (P = .055—.002) that so few overlaps in pollination niches would occur by chance. We developed another null model to test whether chance could have created the apparent pattern of character displacement in pollination niches exhibited by the nine species showing intraspecific variation. The analysis indicated that character displacement has probably occurred (P = .014). This study is one of the clearest demonstrations to date of reproductive interactions generating assemblage structure and character displacement in plants.
Structural Characteristics of Old‐Growth Hemlock‐Hardwood Forests in Relation to Age
Authors: Tyrrell Lucy E. , Crow Thomas R.
Published: May 2006

We characterized the structure of 25 old—growth hemlock—hardwood forests in northern Wisconsin and adjacent Michigan in order to examine our working hypotheses that differences in their structure are related to stand age (i.e., stage of development) and that changes in stand structure continue after old—growth status is achieved. Estimates of stand age, i.e., age of oldest tree cored, based on 10 cores taken from hemlocks of a range of diameters in each stand, ranged from 177 to 374. By investigating the patterns of live tree structure, coarse woody debris (CWD), tip—up mounds, and canopy gaps in relation to stand age, we were able to infer changes that occur during stand development. Along the gradient of old—growth stand development, some changes in structural features, including total volume of CWD, snag (Standing dead tree) basal area, and total area and average size of canopy gaps were continuous, linear increases over time. In contrast, changes in live tree and snag density, density of large trees, volume of well—decayed hemlock logs, and diameter—age relationships occurred after a threshold stand age of 275—300 yr was reached. Area and density of tip—up mounds and density of large seedlings and saplings were not correlated with stand age. Old—growth hemlock—hardwood stands at the upper end of the age continuum (>275—300 yr) have accumulated both gradual and threshold structural changes, acquiring most of the following characteristics: (1) a strong correlation between age and diameter of trees, (2) low densities of live trees distributed across all size classes, (3) trees >70 cm dbh (diameter at breast height), (4) dead wood >120—150 m3/ha, with logs >80 m3/ha, (5) hemlock logs present in all decay classes, and (6) canopy gaps occupying >10% of the stand, with the average gap size >50 m2, some gaps >200 m2, and no more than 30% of the gaps <10 m2. The two oldest stands may be at or near equilibrium, where maximum volumes of CWD are maintained through a balance of tree growth and mortality, and wood decay; and oldest tree age approaches 400+ yr, the maximum longevity typical for hemlock.
Predation of Zebra Mussels by Diving Ducks: An Exclosure Study
Authors: Hamilton Diana J. , Ankney C. Davison , Bailey Robert C.
Published: May 2006

Zebra mussels (Dreissena polymorpha) are a novel and abundant potential food source for several species of diving ducks in the Great Lakes region. Using predator exclusion cages, behavioral observations, and analyses of duck gizzard contents, we examined the predator—prey relationship between zebra mussels and their duck predators during the fall migratory period in Lake Erie at Point Pelee, Ontario. Diving ducks feeding on zebra mussels in the area reduced mussel biomass by 57% during the period of heaviest feeding, but had no measurable impact on mussel numbers. Birds were size—selective predators, preferentially taking medium and large mussels over the more common small ones, thereby altering the size structure of the mussel population. Ducks were abundant at Point Pelee only during late fall, and differences between cages and control areas had disappeared by the following spring. Overall, ducks had little lasting impact on mussel populations, but mussel abundance may have determined duck concentration in the area. Duck staging populations were higher in 1990, when mussels were more abundant, than in 1991, when mussels were much scarcer. Effective predator control of zebra mussels in the portion of the Great Lakes region that freezes over winter is unlikely because diving ducks are unable to overwinter in the area. However, mussels have become an important food source for diving ducks, and temporary reductions of mussel density through predation are likely in areas where ducks feed during migratory periods. Similarly, in regions where ducks are resident through winter, greater and longer lasting effects may be observed.
Species Richness of Experimental Productivity Gradients: How Important is Colonization Limitation?
Authors: Tilman David
Published: May 2006

The biodiversity of a site should depend on the interplay of local colonization (gain) and extinction (loss) rates, but few theoretical explanations of diversity patterns on productivity gradients have included effects of productivity on colonization. In an 11—yr study, experimental increases in productivity via nitrogen addition generally led to decreased species richness in four grassland fields. Decreased diversity in productive plots was caused as much by lower rates of species gain as by greater rates of loss of existing species. Annual grasses and forbs had high gain and loss rates, but these were independent of productivity. In contrast, the rates of gain of perennial grass and forb species declined with productivity, and their rates of loss increased. Species richness was dependent on litter mass and on light penetration, but not on aboveground living plant mass, suggesting that there was no direct effect of productivity on diversity. Diversity in periodically burned prairie, which had low litter mass, was independent of productivity by the 10th and 11th yr of the study. Results suggest that diversity is lower in productive grasslands because accumulated litter, and possibly lower light penetration, inhibit germination and/or survival of seedlings, and thus decrease rates of establishment by new species. Higher productivity also leads to higher rates of loss, presumably via competitive displacement, of existing species. Results do not support the hypothesis that soil N heterogeneity controls diversity on the temporal and spatial scales of this study.
Resistance of Creosotebush to Mammalian Herbivory: Temporal Consistency and Browsing‐Induced Changes
Authors: Ernest Kristina A.
Published: May 2006

Interactions between genotype and environment determine the underlying constitutive (background—level) resistance of individual plants to herbivory. Resistance, however, may change in response to herbivory (induced resistance). This study was designed to investigate temporal fluctuations in resistance, and to test the relative influences of constitutive and browsing—induced resistance on patterns of mammalian herbivory. A natural population of creosotebush, Larrea tridentata, browsed by black—tailed jackrabbits, Lepus californicus, served as the study system. Creosotebushes varied considerably in their resistance to jackrabbit browsing. Some individual plants were fed upon lightly and infrequently, while others had up to 90% of their living branches clipped during a single month, and were browsed several times per year. Resistance levels of individual shrubs were similar between seasons, and over time scales > 2 yr. Shrubs with a history of heavy browsing (low constitutive resistance) were more likely to be browsed again than were individuals with a history of light browsing (high constitutive resistance). In an experiment designed to test jackrabbit response to constitutive resistance and recent browsing damage (artificially clipped vs. unclipped), the importance of both factors was confirmed. Jackrabbits browsed more heavily on plants with low than with high consecutive resistance, but less heavily on clipped shrubs than on controls. The enhanced resistance of clipped shrubs is interpreted as an induced response by individuals to herbivory, and may be responsible for variation in browsing levels of individuals shrubs over time scales of 1—2 yr.
Effects of Tench and Perch on Interactions in a Freshwater, Benthic Food Chain
Authors: Bronmark Christer
Published: May 2006

The purpose of this study was to determine the importance of cascading trophic interactions in freshwater, benthic food chain and if the strength of the indirect interactions depends on the degree of specialization in the predator. The effects of predation by two benthivorous fishes, tench (Tinca tinca) and Eurasian perch (Perca fluviatilis), on benthic macroinvertebrates, epiphytic algae, and submerged macrophytes were studied in a field experiment, using cages (2m x 3m x 0.8 m) placed in a eutrophic pond in southern Sweden. Cages were assigned to four different treatments: fishless controls, tench, perch, and tench + perch. Core samples were taken to assess the density of benthic macroinvertebrates and submerged macrophytes, and artificial substrates were used to study the dynamics of periphyton. Nonmolluscan benthic macroinvertebrates were not greatly affected by the presence of fish, whereas predation by tench dramatically reduced the biomass of snails and bivalves. Tench had an indirect, positive effect on the biomass of periphyton through a reduction of grazing pressure by snails, whereas perch had no demonstrable direct effect on molluscs or indirect effect on periphyton. Further, in the cages with low snail and high periphytic biomass (tench and tench + perch cages), growth of the dominant submerged macrophyte (Elodea canadensis) was reduced, probably due to shading by periphyton. This experiment confirms that a predator can have profound effects on interactions in benthic food chains and that the strength of the indirect interactions is dependent on the strength of the direct interactions. The strong direct effect of tench on snails indirectly affected periphyton and submerged macrophytes, whereas perch had no direct effect on benthic macroinvertebrates and, consequently, no indirect effect on periphyton and submerged macrophytes.
Biomass and Nutrient Accumulation During Stand Development in Wyoming Lodgepole Pine Forests
Authors: Pearson John A. , Knight Dennis H. , Fahey Timothy J.
Published: May 2006

Accumulation rates of biomass and nutrients during stand development in lodgepole pine (Pinus contorta ssp. latifolia [Engelm. ex Wats.] Critchfield) forests were calculated using dendrochronological reconstructions of biomass in conjunction with information on detrital immobilization of nutrients. Dead wood, forest floor, and live tree Including root) biomass were examined. Maximum total biomass accumulation rates of 2.5—3.2 Mg°ha—1°yr—1 were reached between ages 40 and 60 yr in even—aged stands, but an uneven—aged stand did not achieve a maximum accumulation rate (1.5 Mg°ha—1°yr—1) until after 80 yr of development. The results suggest (1) a major role for forest floor detritus in the retention of N, P, Ca, and Mg; (2) the importance of living vegetation for K retention; and (3) the fundamental differences in biomass and nutrient dynamics between even—aged stands that develop after fire and stands that develop through gradual invasion of a meadow.
Wind Dispersal Distances in Dimorphic Achenes of Ragwort, Senecio Jacobaea
Authors: McEvoy Peter B. , Cox Caroline S.
Published: May 2006

A mark—recapture study of wind—dispersed achenes of Senecio jacobaeo conducted in western Oregon showed that the proportion of achenes dispersing a given distance varied significantly with changes in site (inland vs. coastal), surroundings (mown vs. unmown), height of release (0—50, 50—100, 100—150, 150—200 cm), time of release (early vs late in the season), direction of dispersal, and achene type (disk vs. ray achenes). Influences of height of release, direction, achene type, and time of release were strongly conditioned by site and surroundings. The majority of achenes dispersed very short distances. Of 53 301 achenes falling in the recapture area, 31% travelled only 1 m, 89% travelled 5 m or less, and none were collected > 14 m from the source. Thus, while it is theoretically possible for these wind—dispersed achenes to travel long distances, actual dispersal distances are short due to local conditions of humidity, wind, and vegetation structure.
A Null Model for Competitive Hierarchies in Competition Matrices
Authors: Shipley Bill
Published: May 2006

To evaluate the tendency of plant species to be arranged in hierarchies of competitive ability, Keddy and Shipley (1989) introduced a definition of competitive transitivity in multispecies competition matrices and developed an inferential statistic to test for such a pattern. Here, I introduce a more demanding definition of competitive transitivity ("complete transitivity") that defines a strict hierarchy of competitive relationships and that has more desirable properties for inferring competitive exclusion. A null model, and its accompanying Monte Carlo test, is developed that can be used in analyzing empirical competition matrices. Ten published competition matrices are analyzed; nine show clear evidence of complete transitivity.
Effects of Bird Predation on Grasshopper Densities in an Arizona Grassland
Authors: Bock Carl E. , Bock Jane H. , Grant Michael C.
Published: May 2006

In a 4—yr field experiment, we tested the hypotheses that insectivorous birds (1) controlled densities of herbivorous grasshoppers in an ungrazed semiarid grassland in southeastern Arizona, and (2) functioned as keystone predators, by limiting abundances of grasshoppers that otherwise might change vegetation cover and species composition, and/or by mediating the effects of otherwise competitively superior members of the grasshopper assemblage. We measured grasshopper densities and vegetation on 32 464—m2 grassland plots for 1 yr, then enclosed 16 of these plots with bird exclosures and continued data collection for 3 yr. Eight of the 16 experimental plots were further modified in the last 2 yr of the study by installing fine—mesh 1 m high barriers designed to retard grasshopper dispersal. Microclimates of caged plots differed only slightly from open plots. Lizards and rodents increased inside the exclosures, but they were removed and released elsewhere such that their average abundances did not differ among treatments. By the final year of the study, mean annual adult grasshopper density was >2.2 times higher on plots from which birds were excluded, and where grasshoppers were enclosed by dispersal barriers, than on unmanipulated control plots. Mean nymph density was >3.0 times higher in the same comparison. Grasshoppers were significantly more abundant in bird exclosures with insect dispersal barriers, indicating that experimental plots were dispersal sources rather than sinks. Seven of 12 common grasshopper species were more abundant inside the bird exclosures, while none was less abundant. Among the more abundant taxa, those responding most positively were grass feeders: Eritettix simplex, Opeia obscura, Paropomala wyomingensis, and Phoetaliotes nebrascensis. We found no evidence that grasshoppers competed with one another under increased densities inside the bird exclosures. Although the amount of insect herbivory was somewhat higher inside the bird exclosures, and was positively correlated with grasshopper density across all 32 plots (r = 0.87), overall vegetation cover and species composition did not differ among treatments by the end of the study. Dactylotum variegatum, an aposematic species apparently immune to avian predation, showed no significant responses to the experiment. Birds clearly limited grasshoppers in this grassland ecosystem, but they failed to qualify as keystone predators, at least in the short term, for two reasons: (1) in their absence, increased grasshopper densities had no appreciable impact on vegetation cover or species composition; and (2) there was no evidence that birds mediated competition among grasshoppers.
Effects of Preventing Nutrient Resorption on Plant Fitness and Foliar Nutrient Dynamics
Authors: May Jeffrey D. , Killingbeck Keith T.
Published: May 2006

To determine whether autumnal nutrient resorption contributes significantly to plant fitness, resorption was prevented by defoliating scrub oak (Quercus ilicifolia) plants ("defoliate plants") just prior to the onset of foliar senescence in the years 1985 through 1987. At the same time, to account for the potential influence of the reduced photosynthetic season on defoliated plants, another group ("shaded plants") was subjected to darkness, resulting in senescence in the absence of concomitant photosynthesis. In 1988, senescence and abscission were allowed to proceed normally in all study plants. In the three years that followed treatment years, 1986—1988, radial stem growth and yearly increases in foliar biomass of defoliated plants averaged 41 and 54% less than in controls. Acorn production in defoliated plants in 1989 was only 10% of that in either controls or shaded plants. In contrast to such marked responses in growth and reproduction, content of nitrogen, phosphorus, copper, manganese, and zinc in presenescent (green) leaves exhibited little response to defoliation in the previous year. In 1988, when resorption efficiencies could be measured in plants previously subjected to defoliation, there were no significant differences between control and defoliated plants for nitrogen, copper, manganese, or zinc. The efficiency of phosphorus resorption was 10% lower in defoliated plants. In shaded plants, resorption efficiencies of nitrogen, phosphorus, and copper, but not of manganese and zinc, were substantially reduced relative to controls in treatment years, indicating both quantitative and qualitative differences between dark—induced and normal senescence. The significant negative effects on growth and reproduction that resulted from preventing resorption in Q. ilicifolia offer strong support for the hypothesis that resorption contributes significantly to plant fitness.
Compositional Analysis of Habitat Use From Animal Radio‐Tracking Data
Authors: Aebischer Nicholas J. , Robertson Peter A. , Kenward Robert E.
Published: May 2006

Analysis of habitat use based on radio—tagged animals presents difficulties inadequately addressed by current methods. Areas of concern are sampling level, data pooling across individuals, non—independence of habitat proportions, differential habitat use by groups of animals, and arbitrary definition of habitat availability. We advocate proportional habitat use by individual animals as a basis for analysis. Hypothesis testing of such nonstandard multivariate data is done by compositional analysis, which encompasses all MANOVA/MANCOVA—type linear models. The applications to habitat use range from testing for age class effects or seasonal differences, to examining relationships with food abundance or home range size. We take as an example the comparison of habitat use and availability. The concepts are explained and demonstrated on two data sets, illustrating different methods of treating missing values. We compare utilized with available habitats in two stages, examining home range selection within the overall study area first, then habitat use within the home range. At each stage, assuming that use differs from random, habitats can be ranked according to relative use, and significant between—rank differences located. Compositional analysis is also suited to the analysis of time budgets or diets.