bluuebunny/crossref_metadata_2025 huggingface dataset:
Discrimination Between Exploitative and Interference Competition Between Cladocera and Keratella Cochlearis
Authors: MacIsaac Hugh J. , Gilbert John J.
Published: May 2006

Rotifers and cladocerans are important components of most freshwater plankton communities. Large cladocerans can suppress rotifers through both mechanical interference competition (IC) and exploitative competition (EC) for shared food resources. We assessed the relative extent by which small— and large—bodied cladocerans suppressed the rotifer Keratella cochlearis by these mechanisms, using circulating algal suspensions in dual—chamber laboratory cultures and natural animal densities. The small—bodied cladoceran Ceriodaphnia dubia suppressed Keratella exclusively through EC, while a slightly larger species (Daphnia ambigua) did it primarily by this mechanism. Adults (Daphnia galeata mendotae and Daphnia pulex) and subadults (Daphnia pulex) of two much larger bodied cladocerans suppressed Keratella primarily through IC. The ratio of initial Keratella to adult Daphnia pulex density had little effect on the relative importance of the mechanisms–IC always greatly exceeded EC. Suppression by EC occurred in all treatments only after food was depleted below the concentration necessary to support maximal rotifer population growth, while suppression by IC occurred at both high and low food concentrations. These two mechanisms by which cladocerans interact with rotifers should affect the composition of natural rotifer communities in markedly different ways; IC should favor invulnerable rotifer species, many of which are large bodied, while EC should favor rotifer species with low food requirements, which tend to be small bodied.
Mechanisms of Competitive Dominance Between Crustose Coralline Algae: An Herbivore‐Mediated Competitive Reversal
Authors: Steneck Robert S. , Hacker Sally D. , Dethier Megan N.
Published: May 2006

Competitive superiority among encrusting species is established when the margin of one species consistently overgrows another. By measuring overgrowth patterns for the two most abundant tidepool species of crustose coralline algae (Lithophyllum impressum and Pseudolithophyllum whidbeyense) in the San Juan Islands of Washington State, we documented a reversal in competitive dominance that occurs at about the +1 m level; L. impressum wins in upper zones and P. whidbeyense in lower zones. Regardless of tidepool elevation or species, thicker crusts overgrow thinner ones and crusts of equal thickness are competitively equal. Both crust species are grazed by limpets (primarily Lottia pelta and Tectura scutum), which are the dominant herbivores in tidepools. Laboratory experiments show that limpet grazing can reduce crust thickness. Laboratory and field analyses of limpet grazing indicate that corallines in tidepools in the upper intertidal zone are subject to high frequency (bite rate per unit area) and low intensity (penetration depth per bite) limpet grazing, whereas those in low tidepools are subjected to opposite grazing characteristics. L. impressum is a thick crust, which has a multicellular covering over its meristem. This covering protects the growing portion of the plant from the frequent, low—intensity herbivory that occurs in the higher tidepools. In contrast, P. whidbeyense lacks a multilayered epithallus and is more susceptible to injury in high tidepools. In low pools, meristems of both crusts are injured. L. impressum heals these deep wounds by regenerating vertically, but the net result is a thinner plant that is easily overgrown. P. whidbeyense dominates the low zone because it is capable of much more rapid lateral growth including over its own deep wounds; the crust overgrows itself, thereby developing a thicker thallus and improving its competitive success over L. impressum. We suggest that a continuum of defensive and regenerative characteristics exists among organisms; the adaptive nature of these depends on the levels of production and the frequency and intensity of disturbance in their environment. It is evident that herbivory is important in this system, not by the traditional mechanism of releasing limiting resources (e.g., primary space), but by mediating competitive abilities through the relative susceptibility to, and recovery from, herbivory—induced disturbance.
Interactions Between Predation Risk and Competition: A Field Study of Kangaroo Rats and Snakes
Authors: Bouskila Amos
Published: May 2006

The effects of predation risk from snakes on microhabitat use of kangaroo rats (Dipodomys deserti and D. merriami) was studied in the Mojave Desert. I concentrated on the effects of the predator on the foraging behavior of the rodents and eliminated effects of prey capture on prey population size. Foraging stations contained three seed trays, one in each of three microhabitats (Open, Bush, Grass). The amount of seed left in each tray after one night of foraging was used to assess the responses to predation risk and other foraging costs; additional data were collected during direct observations and by trapping. To investigate the effect of snakes on foraging and microhabitat use of kangaroo rate I manipulated snake presence at the stations. I studied the interactions between predation risk from snakes and moonlight by conducting experiments near full and new moon nights. Both species of kangaroo rats preferred to forage in the Open and avoided the Bush. The preference is opposite to the preference of the main rodent—eating snake at the study site, the sidewinder (Crotalus cerastes), which prefers the Bush over the Open. At stations with snakes, D. deserti reduced its foraging and avoided the Bush more than in control plots. However, D. merriami foraged more at stations with snakes. D. merriami thus foraged at the trays that were avoided by D. deserti and it reduced the risk of interference from the dominant D. deserti. In contrast to studies by other investigators, moonlight (which is associated with increased risk from owls) did not reduce the foraging or affect the micro habitat use of kangaroo rates in summer; moonlight effect was seen only in the fall, when snakes were not active. Unlike risk from owls, risk from snakes is high under bushes and during dark nights. The activity or rodents in summer is a combined reaction to the different predation risks posed by snakes and by owls.
Plant Architecture and Allocation in Different Neighborhoods: Implications for Competitive Success
Authors: Tremmel D. C. , Bazzaz F. A.
Published: May 2006

A plant's competitive ability is determined by its capacity to acquire resources from resource pools shared with neighbors. Resource acquisition abilities depend on plant part placement in relation to available resources. Using a model system of annuals, we examined how the architecture of and allocation to plant parts was affected by different neighbor species. Species having different morphologies were grown as "targets" (target plants) in continuous canopies of each of four neighbor species in the greenhouse. Architectural and allocational traits of Abutilon theophrasti did not vary significantly with neighbor identity. Several architectural and allocation traits of Dutura stramonium and Polygonum penslvanicum plants did vary significantly among neighbor identities; however, in all but two cases, analyses of covariance revealed that these differences were due to differences in shoot biomass among neighbors. We could therefore not relate changes in architecture and allocation to the effects of specific neighbors, and therefore to competitive ability under specific conditions. In all species part masses were more variable than part lengths or areas, suggesting that allocational flexibility in these species allowed them to maximize part size at the expense of part mass to some extent. When we examined within plant biomass allocation patterns among plants of different size classes, regardless of neighbor, we found differences among species in how these patterns changed with plant size. Abutilon and Datura plants in the smallest size class showed a marked reduction in allocation to stems in the upper canopy, while producing similar numbers of nodes, when compared to plants in the largest size class. By contrast, Polygonum plants in the smallest size classes produced many fewer nodes than, but had a relative allocation pattern similar to, plants in the largest size class. These differences had consequences for plant shape, and so suggest differences among species in architectural response to shading. Any such response that allowed a plant to increase its resource—gathering ability would represent an important mechanism of competition.
Long‐Term Soil Water Dynamics in the Shortgrass Steppe
Authors: Sala O. E. , Lauenroth W. K. , Parton W. J.
Published: May 2006

To assess the temporal and spatial dynamics of soil water at a shortgrass steppe site in northcentral Colorado, we evaluated the precipitation regime for a 33—yr period and ran a simulation model for this period. Small precipitation events accounted for a large fraction of the total number of events and represented a source of water with small interannual variability. The difference between wet and dry years was related to the occurrence of a few large events. Average daily precipitation was concentrated during the warmest months of the year with a maximum in late spring. Water in the surface soil layers had a short residence time and no seasonal pattern. Intermediate layers reflected the seasonal pattern of precipitation. Maximum soil water availability occurred in the late spring, but this was also the period with the highest interannual variability. The wettest layer was at 4—15 cm of depth. The frequency of wet conditions decreased above this layer because of the strong influence of evaporation and below because recharge was infrequent. No deep percolation events were recorded. During dry years distribution of soil water was very shallow and during wet years wet conditions reached to depths of 120—135 cm. This shallow distribution of soil water matches the distribution of processes and structural elements in the steppe suggesting that there is a cause—effect relationship between them. We speculate that the pattern of water availability interacts with biotic constraints and determines the rate of ecosystem processes. The depth distribution of water in dry and wet years is compared to the root distribution of grasses, shrubs, herbs, and succulents to suggest the response of each group to modal and extreme conditions. Comparison of long—term soil water patterns and traits of the major species allows us to suggest why Bouteloua gracilis is the dominant species in the shortgrass steppe.
Effects of the Argentine Ant on Arthropod Fauna of Hawaiian High‐Elevation Shrubland
Authors: Cole F. Russell , Medeiros Arthur C. , Loope Lloyd L. , Zuehlke William W.
Published: May 2006

Human—caused biological invasions by an alien species are a worldwide phenomenon. They are particularly significant on isolated oceanic islands and represent a serious threat to endemic biota. The Argentine ant (Iridomyrmex humilis) has become established in portions of the high—elevation shrubland of Haleakala National Park, Maui, Hawaii, over the past 25 yr. This ecosystem lacks native ants but possesses many locally endemic and rare anthropod species. Pitfall trapping and under—rock surveys were conducted to determine the effects of I. humilis on the local arthropod fauna. More than 180 taxa were sampled, mostly Arthropoda. Presence of the Argentine ant is associated with reduced populations of many native and non—native anthropod species, including important predator species and major pollinators of native plants. Effects of ant invasion were particularly severe at higher elevations of Haleakala volcano where endemic species normally exist at low densities. Some taxa, primarily alien species, were more abundant in the presence of ants. Invasion of the Argentine ant has locally reduced the abundance of many endemic species in the shrubland ecosystem. Although the spread of this ant species is slow, I. humilis appears to have the potential to invade a much larger area of Haleakala National Park than it now occupies. Active management of Argentine ant populations will be necessary if the endemic fauna is to be preserved.
Competition, pH, and the Ecology of Larval Hyla Andersonii
Authors: Pehek Ellen L.
Published: May 2006

I investigated the effects of an abiotic factor, pH, and a biotic factor, interspecific competition, on breeding success in a low—pH tolerant anuran, Hyla andersonii. I tested whether low pH would indirectly facilitate H. andersonii by lessening the impact of competition from two less acid—tolerant species. I also tested whether the competitive effect of an allotopic, closely related species (Hyla versicolor) would be greater than that of a syntopic, more distantly related species (Rana sphenocephala). I exposed H. andersonii tadpoles to two levels of pH (3.9, ambient) and three levels of competition (H. versicolor, R. sphenocephala, and none). Reduced pH had no direct effect on any of the species and did not influence the outcome of interspecific competition. from R. sphenocephala decreased survival and mass of H. andersonii, whereas competition from H. versicolor only decreased mass, without affecting survival of H. andersonii. These results suggest that some other correlate of low pH in natural ponds limits H. andersonii to acidic ponds. Further, the assumptions that co—occurrence or level of relatedness can be used as a gauge of competitive strength are shown to be unreliable.
Diameter, Height, Crown, and Age Relationship in Eight Neotropical Tree Species
Authors: O'Brien Sean T. , Hubbell Stephen P. , Spiro Peter , Condit Richard , Foster Robin B.
Published: May 2006

Diameter, height, crown shape, and crown area were measured on 23—42 trees ranging in size from saplings to large adults for each of eight common dicotyledonous tree species in a neotropical forest on Barro Colorado Island, Panama. Six species were canopy trees, one species was an emergent tree, and the remaining species was an understory tree. Crown areas and shapes were quantified by eight radii measured every 45° from the trunk to the vertically projected edge of the crown. areas were calculated from the areas of circles with the average radius; crown shapes were measured by the coefficients of variation of the eight crown radii. Age—diameter relationships were estimated from diameter growth increments over an 8—yr period. Observed height—diameter relationships were compared to expectations based on the theories of elastic similarity, constant stress, and geometric similarity. Slopes of log—transformed height—diameter relationships differed from the theoretically expected value of 2/3 for elastic similarity in three of eight species; two canopy species had higher slopes, but not as high as the expected value of 1.0 for geometric similarity; one canopy species was shorter, but the slope was greater than the slope of 0.5 predicted by the constant stress theory. The theory of elastic similarity also predicts that canopy mass to trunk mass ratio should remain constant during tree growth. Observed ratios based on proxy variables for the masses were constant in six of eight species–the two deviant species had heavier crowns in large trees than expected from their diameters. Crown shapes were much more variable in some species than others. The understory species had much lower r2 values for height—diameter and crown—diameter relationships, suggesting that these relationships may be more variable in trees that live in the less windy understory than in trees that reach the canopy. The allometric relationships for the species in this study were not unique for each species, suggesting that it may be possible to model the allometric relationships of many species with fewer equations than species. If the primary adaptive forces acting on tree species in a diverse forest are their physical environment and the sum of competitive interactions with an ever—changing mix of neighbors, then different species may have similar resource allocation patterns. Thus, using the age—size relationships, we found three groups of species among the seven canopy species. Two groups had three species each and one species (Ocotea whitei) was in a group by itself. The understory species, Faramea occidentalis, was also in a group by itself.
Fish Predation and The Evolution of Invertebrate Drift Periodicity: Evidence from Neotropical Streams
Authors: Flecker Alexander S.
Published: May 2006

Drift activity of stream invertebrates typically is greatest during the nighttime hours in running waters throughout the world. Such diel periodicity may be an adaptive response that minimizes exposure to visually hunting, drift—feeding fishes. I tested this risk—of—predation hypothesis by examining drift behavior of mayflies in a series of Andean mountain and piedmont streams that vary in the abundance of drift—feeding fishes. Drift was primarily nocturnal in piedmont streams with natural populations of visually hunting predators. In contrast, mayfly drift activity did not differ between day and night in mountain streams that historically lack drift—feeding fishes. However, in naturally fishless Andean streams containing introduced rainbow trout, nocturnal peaks in drift were observed for the mayfly Baetis, suggesting a rapid evolutionary change in behavior in response to an exotic predator. When drift periodicity was examined along a gradient of predation regimes, activity was found to be increasingly restricted to the nighttime hours as predation risk became more intense. Diel periodicity was observed even when fish were experimentally excluded, suggesting that nocturnal activity has evolved as a fixed behavioral response to predation, and is not a direct ecological consequence of diurnal feeding by fishes. These observations support the hypothesis that predation risk is important in determining the timing of prey drift behavior.
Adaptive Life Histories Effected by Age‐Specific Survival and Growth Rate
Authors: Hutchings Jeffrey A.
Published: May 2006

Life history data for three unexploited populations of brook trout, Salvelinus fortinalis, were used to test the predictions of life history theory that, relative to juveniles, (1) high adult survival favors low reproductive effort and delayed reproduction, and (2) increased juvenile growth rate favors high effort and early reproduction. Field data supported both predictions. The population having the highest adult—to—juvenile survival ratio expended the least effort, reproduced latest in life, and experienced the lowest survival cost of reproduction. Among populations a high juvenile—to—adult growth rate was associated with early reproduction, high reproductive effort, and high reproductive cost. Early reproduction was also associated with increased growth within populations. The adaptive significance of interpopulation variation in life history was assessed by comparing the fitness, r, of observed life histories with those of potentially alternative strategies. Empirically derived fitness functions supported the hypothesis that population differences in life history were adptive. Observed combinations of age—specific survival and fecundity were those that maximized fitness. Within populations the fitness advantages associated with reproducing early in life favored reduced age at reproduction for the fastest—growing individuals. The results are consistent with the predictions of life history theory and demonstrate empirically how survival and growth rate can independently and interactively influence life history evolution.
Fruit Rot, Antifungal Defense, and Palatability of Fleshy Fruits for Frugivorous Birds
Authors: Cipollini Martin L. , Stiles Edmund W.
Published: May 2006

We tested several predictions concerning fungal rot and antifungal defense of vertebrate—dispersed fruit. (1) Fungal fruit to should be generally deterrent to frugivores. But because fungi may vary in their negative effects, antifungal defenses should be allocated in relationship to net effects of particular species on dispersal. (2) Defenses of fruits under selection for high palatability should be directed primarily toward microbes ("microbespecific" defenses). Lower quality fruits, for which high palatability is not crucial to dispersal, may be defended by more general defenses. (3) Preferred fruits should be defended to a lower degree than less preferred fruits, as a result of consistently high dispersal rates (Removal Rate model). We tested our hypotheses using real fruits and artificial—pulp media of six temperate ericaceous plants in feeding trials with captive birds and in fungal growth trials. Plants had been assigned to two temperature fruiting classes (Summer Small—seeded, SS, Fall—Low—quality, FL), and differed in ripening phenology, persistence times, and fungal resistance in the field. We compared feeding rates on fruits rotted by members of a suite of fruit rot fungi. We also determined feeding rates on artifical—pulp media that either contained ripe fruit extracts or did not, and we determined fungal growth rates on identical media. We found that: (1) fungal species depressed feeding rates in general accordance with their reported vertebrate toxicity. Relative to nontoxic fungi, toxic fungi were inhibited more strongly by secondary fruit pulp extracts. (2) Artificial—pulp media of SS fruits were preferred in feeding trials, due both to basic nutritional composition and nondeterrent (microbe—specific) secondary defense. (3) In accordance with the Removal—Rate model, artificial—pulp media of the preferred SS fruits showed significant antifungal activity, but at a much lower level than that of FL fruits. Our results suggest that a primary function of secondary chemicals in ripe fruits may be to defend against destructive fungi that compete with seed dispersers for the pulp. Our results generally support the hypothesis that selective pressures may result in ripe fruit defenses that are directed primarily against organisms having strong negative effects upon dispersal. As such, factors influencing risk of pulp consumption by nondispersers may be useful predictors of fruit defense patterns.
Cooperative and Competitive Interactions in the Recruitment of Marsh Elders
Authors: Bertness Mark D. , Yeh Su Ming
Published: May 2006

While most studies of plant recruitment focus on competitive interactions, recruitment can be influenced by both positive and negative interactions. We studied recruitment in the marsh elder, Iva frutescens, to examine the relationship between positive and negative forces in recruitment. Marsh elder seedling establishment was competitively precluded in undisturbed habitats by the dense perennial turfs that dominate marsh habitats. Adult marsh elders, however, passively trapped tidally transported plant debris that kills underlying vegetation. As a result, adult Iva created bare space and positively influenced seedling recruitment by facilitating the formation of seedling safe sites. Within bare patches, Iva seedling survivorship was high under adult conspecifics and at high seedling densities. In contrast, solitary Iva seedlings without adult neighbors suffered extremely high mortality. These positive associations among neighbors were experimentally shown to result from neighbor buffering of hypersaline soil conditions. Salinities in unshaded bare patches were elevated due to increased exposure to radiation and surface evaporation. Both adult nurse plants and high seedling densities shaded the soil, ameliorated high soil salinities, and thus had positive effects on seedling performance. Under adult canopies or when soil salinities were experimentally reduced by shading or watering, positive associations did not occur and all interactions among seedlings or between seedlings and adults were competitive. Our results indicate that marsh plant recruitment is dictated by a balance between positive and negative forces. Moreover, since positive interactions were only seen once densities and physical forces had been experimentally manipulated, our results when that positive interactions may be important forces in assemblages even where they are not conspicuous. These findings contribute to growing evidence that positive interactions, particularly those mediated by neighbor amelioration of harsh physical conditions, are important community level processes in a wide range of plant and animal assemblages.
A Simulation Experiment to Quantify Spatial Heterogeneity in Categorical Maps
Authors: Li Habin , Reynolds James F.
Published: May 2006

Spatial heterogeneity (SH) is generally defined as the complexity and variability of a system property (e.g., plant biomass, cover) in space. Despite its importance in both theoretical and applied ecology, a formal and rigorous definition of SH is lacking. Such a definition is needed to facilitate quantitative analyses. To this end, we suggest that SH must be defined in terms of its underlying components. For each categorical maps, SH is the complexity in five components: (1) number of patch types, (2) proportion of each type, (3) spatial arrangement of patches, (4) patch shape, and (5) contrast between neighboring patches. To illustrate the use of these components to develop a quantitative definition of SH, we used statistical models to produce categorical maps with known underlying SH characteristics. These simulated maps were analyzed in a factorial experiment to examine the effectiveness and sensitivity of four indices (i.e., fractal, contagion, evenness, patchiness) to detect patterns in these underlying components of SH. Our results show that any definition of SH is strongly dependent on the underlying variables and the methods used, that many indices depict different aspects of SH, that significant interactions exist among the five components of SH, and that some indices are strongly correlated. Quantification of spatial heterogeneity is essential to our understanding of the relationships between spatial heterogeneity and landscape functions and processes. However, all techniques to measure SH must be evaluated against systems with known characteristics of SH. Such a system is developed and used in this study.
Competition vs. Facilitation of Tree Seedling Growth and Survival in Early Successional Communities
Authors: Berkowitz Alan R. , Canham Charles D. , Kelly Victoria R.
Published: May 2006

Competition has been widely assumed to be one of the principal mechanisms underlying the resistance of shrub and herbaceous communities to invasion by trees. However, there are potential mechanisms by which low—growing species, particularly in physically stressful sites, could enhance growth or survival of tree seedlings (facilitation). The balance of inhibition and facilitation will determine the net effects of a community on tree seedling growth and survival. We conducted a large—scale field experiment to quantify the net effects of four major physiognomic types (shrub tickets, shrubby grass meadows, grass meadows, and herbaceous meadows) commonly found both in old fields and along utility rights—of—way on the growth and survival of three common tree species: sugar maple (Acer saccharum), red maple (Acer rubrum), and gray birch (Betula populifolia). The eight community types were distributed in 23 sites representative of the range of upland environments present in the Hudson Valley of New York. Growth of planted seedlings of all three tree species was uniformly slow in all of the community types and environments. In all cases, the net effect of intact vegetation was to inhibit the growth of the tree seedlings. The intensity of competition varied substantially among the 23 sites but was not consistently related either to community type or to the biomass of the intact community. Instead, variation in the intensity of competition was related to the underlying favorability of the site for the growth of a particular target tree species. On the physically unfavorable sites, tree seedlings grew slowly because of physical stress rather than competition with the intact vegetation. As site quality increased, the intensity of competition increased. Thus, competition and physical stress traded off along a site–quality gradient, with the result that tree seedling growth was uniformly slow. In contrast to the results for growth, there were cases in which the survival of these young tree seedlings was enhanced (facilitated) by the presence of intact vegetation. In particular, survival of the highly shade tolerant but drought intolerant sugar maple seedlings was facilitated by intact vegetation at many sites, especially for the 1988 cohort, which experienced a drought during its first growing season. Since we detected no consistent differences among communities in the intensity of competitive effects on seedling growth, the apparent differences among early successional communities in resistance to tree seedling establishment may be caused by variation in the duration of competition resulting from differences in height and canopy structure of the low—growing communities.
Sampling Methods to Estimate Foliage and Other Characteristics of Individual Trees
Authors: Gregoire Timothy G. , Valentine Harry T. , Furnival George M.
Published: May 2006

The total foliar area or mass of a tree is difficult to measure, as is its bark or cambial area, and various other components of aboveground biomass. A variety of sampling methods is proposed and estimators of these characteristics are presented. Based on probability precepts, all estimators are unbiased. An unbiased estimator of variance for each estimator also is presented. The basis in probability rather than a fitted regression equation provides some important safeguards, and is a useful alternative when fitted regression functions are unavailable for a particular species and physiographic condition.
Assessing the Impact of Competition on Community Assembly: A Case Study using Small Mammals
Authors: Kelt Douglas A. , Taper Mark L. , Meserve Peter L.
Published: May 2006

Competition has been shown to be an important factor in natural systems. Yet, its effect on community structure remains poorly understood and is often disputed. Null models have been employed by researchers in recent years, but these generally suffer from a lack of power estimation and unclear distinction between statistical significance and biological importance. Models that employ an appropriate alternative hypothesis allow for powers estimates and for quantitative evaluation of the effects of competition. We have developed methodologies to (1) allow for testing the significance of competition in natural communities, (2) determine the power of these estimates, and (3) produce maximum likelihood estimates of interaction parameters or these communities. Using these methologies we show that small mammal communities at the interface between Valdivian temperate rain forest and Patagonian steppe in southern South America appear significantly and importantly structured by competition, although we cannot distinguish between resource competition, interference competition, and apparent competition. Secondarily, we show that the geographic origin of the component species (Valdivian vs. Patagonian) is relatively more important than their habitat affinities in structuring these communities.
Persistence of Maternal Nutrient Effects in Plantago Major: The Third Generation
Authors: Miao S. L. , Bazzaz F. A. , Primack R. B.
Published: May 2006

Plants of Plantago major were grown for two generations in different environments. Seed germination of the second generation was examined for the effects of maternal environments, as well as subsequent plant growth and reproduction in the presence of inter— and intraspecific competitors. Maternal treatments were (1) plants that had received nutrient pulses in both the first and second generations (PP plants), (2) plants that had received a nutrient pulse only in the first generation (PC plants), (3) plants that had received a nutrient pulse only in the second generation (CP plants), and (4) plants that did not receive a nutrient pulse in either generation (CC plants). Under favorable conditions (with out interaction with a leaf canopy and root system of grasses), seeds of CC plants generally germinated earlier and had a higher final germination percentage. In contrast, under a Poa canopy, seeds of PP plants germinated earlier and had a higher final germination percentage. For adult plants, there were significant interactions between maternal environment and neighbor density. The effects of maternal environments on leaf area generally were evident before flowering but disappeared after flowering. The greatest difference in leaf area occurred between CC and PP plants, and these were often different from CP and PC plants. There was a significant three—way interaction among the type of competition, maternal treatment, and neighbor density of biomass characters. In intraspecific competition with one neighbor, the spike biomass was highest in PP plants followed by CP plants, PC plants, and CC plants, with the last significantly lower than the first two. In general, the juvenile maternal effect did not necessarily result in an adult reproductive advantage. The influence of the maternal environment differed among plant characters. These results demonstrate that maternal environmental factors can affect plant phenotypic characters for at least three generations. The expression and the extent of these maternal effects varied with the resource condition experienced by the progeny, the intensity of competitive interactions, and life history stage.
Measuring Realized Niche Spaces: Climatic Response of Chaparral and Coastal Sage Scrub
Authors: Westman Walter E.
Published: May 2006

Two methods for estimating climatic response functions by measuring realized climatic niche spaces are compared. The first, direct gradient analysis, utilizes measures of abundance of the species at sites spanning the climatic range of the species' occurrence. The second, geographic distribution analysis, quantifies the proportion of the species' total areal occurrence that is found within each climatic zone. A geographic information system is used to digitize species distribution maps and climatic isoline maps, and to compute areas of coincidence. The methods are compared, using independent data sets, on eight species of Californian coastal sage scrub. The position of species maxima along climatic gradients coincided for 75% of the 24 cases in which climatic response from the two methods was compared. This result suggests that the quicker and less expensive geographic distribution analysis method may have value in generating first approximations to climatic response functions. The geographic distribution analysis method is also used to characterize the realized climatic niche space for an additional eight species of Californian hard chaparral. Most species' response surfaces from data throughout the species' range were approximately bell shaped along climatic axes, providing some confirmation of theoretical expectation. The smoothness of such curve shapes will, however, be somewhat sensitive to the scale of sample resolution.
Patterns of Island Occupancy Explained by Colonization and Extinction Rates in Shrews
Authors: Peltonen Anu , Hanski Ilkka
Published: May 2006

This is the first study reporting patterns of island occupancy in a guild of mammals with detailed information on the processes affecting these patterns: dispersal, colonization, and extinction. We surveyed the presence of three species of shrews, Sorex araneus, S. caecutiens, and S. minutus, on 108 islands in three lakes in Finland. A significant compensatory effect between island area and isolation was detected in one species, but most islands were so little isolated (< 1 km) that isolation played no great role in this study. The position and the slope of the incidence function varied greatly among the species. Fitting the model J = 1/(1 / y/Ax) to the observed incidence functions, and assuming that the annual extinction probability is 1 on islands of A = 0.5 ha, yielded quantitative predictions of the rates of colonization and extinction in the three species. Observed rates agreed remarkably well with the predicted ones (median difference 20%). Overwater dispersal rate and/or ability was lowest in the smallest species (S. minutus), probably because of its short starvation time and slow swimming rate. Unexpectedly, this species had the highest colonization ability (rate of successful colonization conditional on arriving at an island). There were no significant differences between the species in colonization rate, but interspecific differences in extinction rate were highly significant and negatively correlated with body size. We suggest that these differences are due to increasing importance of environmental stochasticity with decreasing body size in shrews. This hypothesis is supported by larger coefficients of variation of abundance in small than in large species on the mainland.
Nutrient Limitation and Metrosideros Forest Dieback in Hawaii
Authors: Gerrish G. , Mueller-Dombois D. , Bridges K. W.
Published: May 2006

Large numbers of Metrosideros polymorpha trees have died in the montane rain forest on the Island of Hawai'i, but previous research has failed to identify a principal cause. This paper describes an experiment that tests the hypothesis that nutrient deficiency is the principal cause of tree death and stand—level dieback. Treatments were fertilizing, stand thinning, and a combination of the two. No significant change in the mortality rate was caused by the treatments; however, stem diameter growth of surviving trees was found to be nutrient limited at all sites. We conclude that nutrient deficiency is not the principal cause of most Metrosideros dieback, but it may be a contributing factor.
Growth and Canopy Architecture of Shade‐Tolerant Trees: Response to Canopy Gaps
Authors: Canham Charles D.
Published: May 2006

Patterns of aboveground growth, branching, and leaf display were examined in saplings of Acer saccharum Marsh. (sugar maple) and Fagus grandifola Ehrh. (beech) to determine (1) the responses of aboveground growth rates to variation in forest light regimes and (2) the importance of branching and leaf display patterns to the ability of saplings of these two species to respond to changes in forest light regimes produced by canopy gaps. For both species, rate of height growth, lateral growth, and the production of new shoots in even the low gap light levels created by small canopy gaps (15—75 m2) were as much as an order of magnitude greater than growth rates of saplings beneath closed canopies. However, saplings of both species showed little response to further increases in gap light levels. The strong response of maple saplings to low gap light levels was correlated with an increased in sapling leaf area index and the efficiency of leaf display (measured as leaf area per unit length or surface area of branches). In beech, the more modest response to low light levels in small gaps and the higher growth rates than maple beneath a closed canopy were paralleled by a lack of significant increase in beech leaf area indices in small gaps and a higher efficiency of leaf display beneath a closed canopy than in small gaps. Thus, the magnitude of the response of these two species to small canopy gaps is correlated with their degree of plasticity in patterns of branching and leaf display. Both species can be considered small—gap specialists in the sense that their combinations of shade tolerance, growth responses, and canopy architecture make them particularly successful at exploiting small canopy gaps. However, the two species differ in their placement on a gradient in the degree to which woody plants respond to canopy disturbances.
Seed‐Harvester Ant Association with Shrubs: Competition for Water in the Mohave Desert?
Authors: Rissing Steven W.
Published: May 2006

Nests of the desert seed—harvester ant Pogonomyrmex rugosus are located preferentially near shrubs in some microhabitats at a Mohave Desert site, while nests of the same species are located preferentially away from shrubs at a Sonoran Desert site. At the Mohave site, nearest neighboring shrubs to ant nests show greater mortality than expected at random; this effect is reduced at the Sonoran site. The ants appear to be the cause of shrub mortality; workers defoliate shrubs and make no apparent use of severed leaves or stems. Shrub viability at the Mohave site is dependent upon distance from a P. rugosus nest and size of that nest. In the Mohave Desert, shrubs may initially provide valuable shade to starting colonies, yet later in colony ontogeny represent a significant competitor for water.
A Carbon Flow Model of Epipelic Algal Productivity in Alaskan Tundra Ponds
Authors: Stanley Donald W.
Published: May 2006

Epipelic algal production in arctic tundra ponds (near Barrow, Alaska) was simulated by a mathematical model utilizihg experimental data and measured initial conditions. There was close agreement between the model's estimate of total annual production and an estimate calculated from measured data points (e.g., 9.4 g C°m(—2)°yr(—1) and 10.1 g C°m(—2)°yr(—1) for one pond in 1971). A hypothetical annual carbon budget for the epipelic algae, produced by summing all the hourly transfer rates from one summer simulation run, suggested that most of the surface algal production (97%) was lost by burial in the pond sediments and that only 15.1% was consumed by benthic grazer organisms in the top 5 cm. The model simulations also strongly suggest that much of the year—to—year variation in total epipelic production in these ponds can be explained solely by variations in temperature and light.
Two Costa Rican Bat‐Generated Seed Shadows of Andira Inermis (Leguminosae)
Authors: Janzen D. H. , Miller G. A. , Hackforth-Jones J. , Pond C. M. , Hooper K. , Janos D. P.
Published: May 2006

Bat—generated seed shadows of two mature Andira inermis (W. Wright) DC. trees in the deciduous forest lowlands of Guanacaste Province, Costa Rica, are highly heterogeneous; very unequal numbers of seeds are deposited beneath the trees used as feeding roosts. It is calculated that a 45—g Artibeus jamaicensis bat may potentially obtain as many as 1,766 joules [= 422 calories] per Andira fruit per round trip of 270 m between the parent tree and the feeding roost. Seed predation by Cleogonus weevils was found to be highest below the parent tree, intermediate under bat feeding roosts, and lowest among fruits dropped accidentally by bats between the parent tree and the feeding roosts.
Diet Mixing: Nonadditive Interactions of Diet Items in an Omnivorous Freshwater Turtle
Authors: Bjorndal Karen A.
Published: May 2006

Diet selection and optimization may be affected by nonadditive interactions (or associative effects) among ingested diet items. Associative effects occur when one diet item affects the digestion (either positively or negatively) of another diet item. Feeding trials were conducted with an opportunistic omnivore, the yellow—bellied slider turtle (Trachemys scripta scripta) on three diets: duckweed, insect larvae, and duckweed/larvae mix. Diet has a significant effect on digestibilities and intake, but not on transit time. There was a significant associative effect in the duckweed/larvae diet. For all diet components except lipids, the digestive efficiencies measured in the duckweed/larvae diet were significantly greater than those predicted from the digestive efficiencies for duckweed and larvae diets when fed alone. The presence of larvae in the digesta significantly improved the digestive processing of duckweed, probably by supporting greater numbers or diversity of cellulolytic microbes. A given mass of duckweed provided °70% more energy and 20% more nitrogen to the turtle when ingested with insect larvae than when ingested alone. Associative effects should be incorporated in optimal foraging models and studies of diet selection because the value of a diet item can vary with the foods with which it is ingested. The assigned value must include not only the direct nutritional gain from the item, but also the indirect nutritional gain (or loss) through positive (or negative) effects on digestion of food items with which it is ingested.
Resilience of Lotic Ecosystems to a Light‐Elimination Disturbance
Authors: Steinman Alan D. , Mulholland Patrick J. , Palumbo Anthony V. , Flum Terry F. , DeAngelis Donald L.
Published: May 2006

Resilience of eight laboratory stream ecosystems was evaluated following a 92—d light elimination disturbance. Prior to the disturbance, four treatments (snails/once—through flow, snails/recirculated flow, no snails/once—through flow, no snails/recirculated flow) were imposed on the streams, resulting in systems with different biomass levels, nutrient concentrations, and recycling indices. Based on results from models of ecosystem response to disturbance, we hypothesized a priori that once—through streams would recover more quickly than recirculated streams within each grazing regime and that grazed streams would recover more quickly than ungrazed streams within each flow regime. Our results indicated that once—through streams did have a higher resilience than recirculated streams when snails were absent, but not when snails were present. Indeed, most parameters recovered faster in streams without snails than those with them, irrespective of flow regime, in contrast to our prediction. Despite the faster initial recovery rates in once—through than recirculated streams without snails, final biomass levels were similar between these streams. Measurements of phosphorus recycling indices suggested that higher rates of nutrient recycling near the end of the experiment in recirculated streams compensated for the lower inputs of new nutrients in the incoming water, allowing biomass to reach levels similar to those in once—through streams. Results from this empirical work are consistent with the theory that resilience is positively associated with the rate of nutrient input to a nutrient—limited system, but are inconsistent with the idea that resilience is positively associated with the addition of a herbivore trophic level to the system. Grazing intensity (i.e., density), as well as behavioral (e.g., vagility) and life history characteristics may need to be considered in future models.