bluuebunny/crossref_metadata_2025 huggingface dataset:
Low‐Level Forest Disturbance Effects on Primary Production, Leaf Chemistry, and Lemur Populations
Authors: Ganzhorn Jorg U.
Published: May 2006

This study quantifies the effects of low—intensity logging (affecting < 10% of the forest surface) on leaf chemistry, leaf and fruit production, and their effects on lemurs in a dry deciduous forest in western Madagascar. Logging exposed the remaining trees to more sunlight. Within a single tree among trees, this resulted in higher protein concentrations in sun—exposed leaves, while fiber content remained constant. Concentrations of condensed tannins were also higher at higher sun exposure, but they leveled off at lower light intensities before protein and sugar concentrations. Within the 2—yr study period, crown extension due to higher availability of sunlight could compensate only for parts of the crown area lost due to logging. Thus, the biomass of leaves available was reduced, while average leaf quality (measured as the ratio of protein to fiber) increased. Fruit production was also positively correlated with sun exposure to the tree crown. Since fruits of the main timber trees (Commiphora spp.) are rarely consumed by frugivorous lemurs, removal of these trees increases sun exposure of other potential food trees for frugivores. At the described level of logging intensity, sightings of all lemur species increased (significantly in three out of seven species at P < 0.05) compared to the pre—logging state and two control plots. This increase was attributed to increased fruit production and higher protein concentration in sun—exposed leaves. In natural forests, treefall gaps may have effects similar to those of logging in the present study. Low level disturbances, in general, may be important for the carrying capacity for folivorous and frugivorous lemurs and possibly for most primates.
Morphological and Physiological Consequences of a Positive Plant Interaction
Authors: Hacker Sally D. , Bertness Mark D.
Published: May 2006

It is known that plants can create acceptable environments for other plant species that may be unable to physiologically adjust to harsh physical conditions in isolation. In this study, we explore the morphological and physiological responses of a positive interaction on a common New England salt marsh shrub, Iva frutescens L. In previous experiments, we have shown that Iva benefits from the presence of the black rush, Juncus gerardi Loisel. Juncus removal in the lower marsh caused increases in soil salinity and anoxia, which resulted in lower Iva biomass, growth, and survival. We hypothesize that the positive effect of Juncus on Iva is a consequence of (1) the turf morphology of Juncus shading soil surfaces, thereby minimizing salt accumulation, and (2) the increased soil oxygen content surrounding Juncus caused by radial oxygen loss from its rhizopshere. However, the specific morphological and physiological benefits conferred on Iva are unknown. We conducted experiments in the salt marsh to isolate the effects of Juncus neighbors, waterlogging, and salt water stress on Iva. We found that Iva, when exposed to salty, oxygen—deprived soil conditions created in the absence of Juncus, responded more negatively to salinity than waterlogging, although the interaction between salinity and waterlogging was often significant. Iva without neighbors showed decreases in plant height, leaf density, leaf area, and flower density compared to controls or plants with fresh—water additions. Leaves of Iva without neighbors responded to salt stress and waterlogging by increasing leaf thickness and succulence. The increase in soil salinity of plants without neighbors caused plants to have correspondingly low water potentials, indicating that they were severely water stressed. Physiologically, Iva plants had a low mean photosynthetic rate, transpiration rate, water—use efficiency, and stomatal conductance compared to controls or Iva with fresh—water additions. Low water—use efficiency in Iva without neighbors was correlated with stomatal closure but also with high intercellular CO2 concentrations. The mechanisms behind this apparent anomaly may include both nonstomatal inhibition of photosynthesis as well as patchy stomatal closure, although it is uncertain which predominates from our data. Our results show that the presence of Juncus, with its superior ability to withstand waterlogging and salt stress, can create a hospitable environment for Iva, extending its distribution to lower intertidal habitats in which it does not have the morphological or physiological plasticity to normally cope.
Plant Competition and Herbivory in Relation to Vegetation Biomass
Authors: Bonser S. P. , Reader R. J.
Published: May 2006

We tested whether effects of competition and herbivory on plant growth depend on the aboveground biomass of vegetation. Our objective was to test theoretical predictions about the site—dependence of biotic mechanisms of plant population regulation. Biomass ranged from a mean of 64 to 776 g/m2 at the eight study sites, which included six old fields and two herbaceous plant communities near water. To measure effects of competition, 4—5 wk old plants of a perennial grass (Poa compressa) were transplanted into plots where neighboring plants were either left intact or removed and after 4 mo the shoot mass of transplants was compared in the two treatments. Transplants were caged in both treatments to exclude herbivores. Removing neighbors increased transplant shoot mass significantly at all sites, indicating that transplants experienced less competition for resources when their neighbors were removed. Regression analysis indicated that the increase in shoot mass due to neighbor removal was significantly greater at sites with higher biomass. This result supports theories that predict competition should increase with increasing biomass. To measure effects of herbivory, the growth of caged and uncaged plants was compared. Caging transplants did not increase their shoot mass significantly at the two sites with the lowest biomass. However, shoot mass was significantly greater for caged transplants than for uncaged transplants at all other sites. Caged transplants lost less tissue to herbivores such as small mammals and molluscs. Regression analysis indicated that the increase in shoot mass due to caging was significantly greater at sites with higher biomass. The combined effect of herbivory and competition was measured by comparing transplant growth in plots where transplants were caged and neighbors were removed with the growth of transplants in plots where transplants were not caged and neighbors were left intact. Transplant shoot mass was significantly greater where herbivores were excluded and neighbors were removed, especially at sites with high biomass. Our results indicate that competition and herbivory each have a greater effect on plant growth at sites with higher biomass and that herbivory has less effect than competition on plant growth at sites with relatively low biomass.
Implications of Spatial Heterogeneity for the Paradox of Enrichment
Authors: Scheffer Marten , De Boer Rob J.
Published: May 2006

We analyze a simple model to show that spatial heterogeneity of zooplankton can explain discrepancies between the behavior of classical predator—prey models and the patterns observed in natural planktonic systems. We use a Lotka—Volterra type model of Daphia and algae. Daphnia occupies only a part of total volumes whereas the algae grow in the entire volume and diffuse between the two compartments. This simple spatial structure suffices to explain the observations that (1) natural Daphnia—algae systems tend to be relatively stable up to high nutrient values, and that (2) in the presence of Daphnia edible algae do increase with enrichment. Additionally, the model trivially explains confusing observations of oscillating Daphnia densities in the presence of a practically constant density of edible algae. The model is supported by the results of a laboratory experiment with a cascade of zooplankton—phytoplankton containers, devised originally to test ratio—dependent foraging. We derive minimalizations of our model, which no longer explicitly account for the spatial structure, but still preserve the essential behavior of the full model.
Sexual Differences in Life History and Resource Utilization by the Hermit Crab
Authors: Asakura Akira
Published: May 2006

Evolutionary causes and ecological consequences of sexual differences in hermit crabs were examined through their life history patterns, habitats, behavioral responses to seasonal and size differences in the utilization of vacant shell resources and intraspecific competitive ability. The sand—dwelling hermit crab, Diogenes nitidimanus, and the sand snail, Umbonium moniliferum, were extremely abundant in the study area, comprising a virtually single resource and utilizer system. Size—frequency distributions of the crabs were nearly flat for males but had a prominent peak for larger females, suggesting different patterns of growth and mortality for the two sexes. No conspicuous peak in the size range of small crabs was obvious in size—frequency histograms for each sex from September to the following June. Males were larger than females. The number and size of vacant shells produced by the population of U. moniliferum, estimated from their survivorshop and growth curves, indicated that both extremely small and large shells were readily available, whereas medium—sized shells suitable for small crabs were relatively rare. This resulted in small crabs using shells that were damaged, much smaller than the sizes they preferred, and of less common species or species that did not inhabit the sand flat. Habitat use by makes was more diverse than by females. The males used shells of U. moniliferum and other snails inhabiting the sand flat and shells of preferred size more frequently than did females, and also tended to use shells that were larger and more variable in size. Shells that were broken or epibiota covered were less preferred by both sexes, although epibiota—covered shells were used more frequently by males. Broken shells were used at similar frequency both sexes. Crabs supplied with shells of a similar size distribution to that available in the field were cultured for 1. yr. Males tended to grow faster than females, exhausting the limited supply of medium—sized shells. The smaller crabs restrained their growth until such shells became available, explaining the absence of a peak of small crabs in the field population. Although the sex ratio of the field population was female biased, that of the laboratory population. Although the sex ratio of the field population was female biased, that of the laboratory population was 1:1, indicating that males had a higher mortality rate in the field. These results suggest that males are better intraspecific competitors than females in the acquisition of vacant shells. The quality enables males to grow larger, which is advantageous in the acquisition of females. However, shell scarcity produced greater stress in males resulting in their higher mortality rate.
Maintenance of an Exceptional Intertidal Grazer Biomass in South Africa: Subsidy by Subtidal Kelps
Authors: Bustamante R. H. , Branch G. M. , Eekhout S.
Published: May 2006

Dense populations of patellid limpets characterize the mid— to low—intertidal communities of the southwestern rocky shores of South Africa. Patella argenvillei and P. granatina reach average densities up to 200/m2, representing in some cases °10—13 kg/m2 of whole wet biomass, and attain size up to 100—mm shell length. Traditionally these limpets have been regarded as generalized grazers, but in facts they have highly specialized feeding mechanisms. P. granatina is found primarily on the mid— to low shore of sheltered boulder—bays, while P. argenvillei forms a conspicuous monospecific belt low on semiexposed shores in association with adjacent kelp beds. Both species are the dominant occupiers of space in their respective habitats. We have investigated the main pathways and supply of food necessary to maintain limpet biomasses as high as this. Epilithic micro— and macroalgae and the kelps Ecklonia maxima and Laminaria pallida are common sources of food for these two limpet species. Gut contents show that kelp presents >50% of the total diet of both species. An abundant source of food is provided to P. granatina by drifting kelp and seaweed debris, and to P. argenvillei by nearly attached kelp plants, which they actively prune. Differences in the timing and method of feeding exist. P. granatina captures drifting kelp and seaweeds while underwater during incoming and outgoing tides, and P, argenvillei prunes the fronds of nearby attached kelp plants during the rising tide. P. argenvillei feed collectively, several individuals sharing a piece of kelp trapped by a single limpet. This overcomes the intraspecific competition normally experienced in high density limpet populations, and the maximum size of P. argenvillei rises with density. P. granatina does not feed collectively and its maximum size declines with density. An experimental exclusion of kelp from the limpets' diets resulted in significantly increased mortality and a reduction in body mass. Subtidal production of kelp fronds subsidizes populations of both limpet species and is vital to the maintenance of their remarkably high biomasses. Supplied with a suberabundance of food, both species achieve such high "packing" that primary space becomes their limiting factor. The trophic subsidy provided by kelps has important secondary consequences since the dense limpet populations powerfully influence intertidal community structure and function.
Drought Tolerance and Xylem Embolism in Co‐Occurring Species of Coastal Sage and Chaparral
Authors: Kolb Kimberley J. , Davis Stephen D.
Published: May 2006

Some species of coastal sage and chaparral shrubs of California are extremely tolerant of tissue dehydration, surviving water potentials as low as —9 MPa during dry summer months. Such low water potentials (high tensions on xylem water) are known to cause severe embolism formation in the xylem vessels of woody plants, blocking water transport and potentially causing shoot dieback. Thus drought hardly species of coastal sage and chaparral are either extremely resistant to water stress—induced embolism or they become severely embolized during summer drought. We compared the seasonal changes in xylem water potential and xylem embolism (percent loss in hydraulic conductivity of stem segments due to air emboli) between co—occurring Salvia mellifera (coastal sage) and Ceanothus megacarpus (chaparral) growing in the Santa Monica Mountains of southern California. We also determined the relative sensitivity of each species to water stress induced embolism by artificially dehydrating branches and measuring percent loss in hydraulic conductivity of xylem tissues at a given water potential. We found that both species experienced the same minimum in seasonal water potentials (—8MPa) but the xylem of S. mellifera lost 78% in hydraulic conductivity whereas the xylem of C. megacarpus lost only 17% in hydraulic conductivity. These values for a natural plant community were within 10% of those predicted by our artificial dehydration curves. Our estimate of susceptibility to water stress—induced embolism indicated that 50% loss in hydraulic conductivity would occur at —4.5 MPa for S. mellifera but at ‐11 MPa for C. megacarpus. Irrigation of S. mellifera for one summer reduced loss in conductivity from 78 to 38% and increased leaf areas 10—fold, indicating that xylem embolism and leaf drop were drought induced. Our results show that xylem tissues of S. mellifera are more sensitive to water stress and tissue dehydration than those of co—occurring C. megacarpus. The observed ability of S. mellifera to inhabit drier sites than C. megacarpus may result from drought deciduousness in summer and high growth rates in spring that facilitate rapid construction of new xylem and leaf tissues. It may be that facultative drought deciduousness in coastal sage is tightly coupled to drought—induced embolism of xylem tissues.
Fungal Endophytes in Stems and Leaves: From Latent Pathogen to Mutualistic Symbiont
Authors: Carroll George
Published: May 2006

Endophytes are fungi that form inapparent infections within leaves and stems of healthy plants. Closely related to virulent pathogens but with limited, if any, pathogenic effects themselves, many endophytes protect host plants from natural enemies. Animal herbivores and, in some cases, pathogenic microbes are poisoned by the mycotoxins produced by endophytes. "Constitutive mutualism" is the relatively faithful association, usually with grasses, of endophytes that infect host ovules and are propagated in host seed; substantial fungal biomass with probable high metabolic cost develops throughout the aerial parts of the host plant. "Inducible mutualist" endophytes are not involved with host seed and disseminate independently through air or in water. Infecting only vegetative parts of the host and remaining metabolically inactive for long periods with relatively little fungal biomass, inducible mutualists grow rapidly and produce toxins against herbivores when damaged host tissues provide new sites for infection. I surmise that endophytes may be as common among plants as are mycorrhyzae.
Fungal Endophytes of Grasses: A Defensive Mutualism between Plants and Fungi
Authors: Clay Keith
Published: May 2006

Many grasses are infected by systemic fungal endophytes (family Clavicipitaceae, Ascomycetes) that produce pysiologically active alkaloids in the tissues of their hosts. Infection makes grasses toxic to domestic mammals and increases resistance to insect herbivores. Some grasses are sterilized by endophyte infection while remaining vegetatively vigorous; other infected grasses remain completely fertile. Experiments demonstrate that plant growth and seed production can be increased by infection. This symbiotic association may be a defensive mutualism in which the fungi defend their hosts against herbivory, thereby defending their own resources. Recent studies suggest that defensive mutualism of endophytes with grasses may be widespread.
Causes of Fire Effects in Tallgrass Prairie
Authors: Hulbert Lloyd C.
Published: May 2006

Eleven experimental treatments were applied to 2 x 2 m plots over 2 yr at Konza Prairie Research Natural Area, Riley County, Kansas, to ascertain why burning tallgrass prairie causes increased production and flowering. Warming of the soil in unburned plots resulted in an increase in both total production and flower stalk production of dominant tall grasses, primarily big bluestem (Andropogon gerardii) and Indian grass (Sorgastrum nutans), but the increase was small (34% increase in biomass; 78% increase in number of flower stalks) compared with that in burned plots (151% increase in biomass; 435% increase in flower stalks). Increased surface light intensity also appears to be a factor affecting changes in productivity following burning as suggested by the combined responses of increased productivity with removal of standing dead, whether by clipping or burning, and decreased productivity with shading. Further, the addition of ammonium nitrate increased yield 41% and flowering 168% for the dominant grasses, suggesting that any factor increasing nitrogen availability would affect these vegetative parameters. Neither ash left from burning nor heating of the soil surface during burning produced detectable effects on subsequent vegetative growth. Different results for some parameters between years and between species suggest that many complex interactions operate to affect the grassland's response to burning, but surface light, soil surface temperature, and nitrogen appear to be particularly important factors.
Utilization of Substitutable Carbon and Phosphorus Sources by the Mixotrophic Chrysophyte Ochromonas Sp
Authors: Rothhaupt Karl O.
Published: May 2006

In laboratory experiments, I studied the influences of bacterial density and light on the ingestion and growth rates, pigment contents, and the carbon and phosphorus turnover rates of the mixotrophic flagellate Ochromonas sp. The investigated strain is a bacterivorous flagellate that can enhance its photosynthetic apparatus and grow phototrophically when bacterial densities are low. This was also evident from significantly higher chlorophyll $a$ contents during active photosynthetic growth phases. Moderate phototrophic growth should be possible even if bacteria were absent. Bacterial ingestion rates increased hyperbolically with bacterial density, and there was no difference between light— and dark—adapted cells. Ochromonas released soluble reactive phosphorus (SRP) when growth was predominantly phagotrophic, but it took up SRP when growth was phototrophic. The mixotrophic strategy in Ochromonas appears to be bound up with costs and trade—offs: Ochromonas needs high bacterial densities to reach maximum growth rates, its basic metabolic costs are higher than for obligately phagotrophic flagellates, and its phototrophic growth rates are lower than for obligately phototrophic phytoplankton of comparable size.
Altered Behavior of Parasitized Killifish Increases Susceptibility to Predation by Bird Final Hosts
Authors: Lafferty Kevin D. , Morris A. Kimo
Published: May 2006

Parasites that are transmitted from prey to predator are often associated with altered prey behavior. Although many concur that behavior modification is a parasite strategy that facilitates transmission by making parasitized prey easier for predators to capture, there is little evidence from field experiments. We observed that conspicuous behaviors exhibited by killfish (Fundulus parvipinnis) were associated with parasitism by larval trematodes. A field experiment indicated that parasitized fish were substantially more susceptible to predation by final host birds. These results support the behavior—modification hypothesis and emphasize the importance of parasites for predator—prey interactions.
Seed Banks in Desert Annuals: Implications for Persistence and Coexistence in Variable Environments
Authors: Pake Catherine E. , Venable D. Lawrence
Published: May 2006

It is widely believed that desert annual plants maintain between—year seed banks, yet few field studies actually have measured the proportion of the viable seed bank that remains dormant through a season. Dormancy and germination fractions were quantified for a guild of winter annuals on a creosote flat in the Sonoran Desert for three years. Predictions from two types of theoretical models applicable to temporally variable environments were examined: (1) the evolution of life history traits promoting persistence in the face of temporal variation and (2) the role of temporal variation in mediating species coexistence. The density of ungerminated seeds was estimated by collecting soil samples after germination, but prior to new seed set. Seedlings were followed in nearby plots to estimate the density of germinated seedlings and their reproductive success. Long—term data collected from permanent plots over a 10—yr period were used to calculate temporal variation in reproductive success for each species. Species with higher temporal variation in reproductive success had lower germination fractions and smaller seeds, consistent with the theory that seed dormancy and large seed size are partially substitutable bet—hedging strategies. The data also suggested that this system possesses traits that are necessary for temporal variation to promote coexistence. First, between—year seed banks, necessary to buffer populations in unfavorable years, were documented for 17 species. Second, there was a strong tendency for year—to—year variation in germination fractions to vary among species. Finally, plants germinated more in years of higher reproductive success. We discuss how a correlation between germination and reproductive success enhances the role of temporal variance in success hierarchies in promoting species coexistence.
Spatial Pattern and Process in Plant‐‐Pathogen Interactions
Authors: Real Leslie A. , McElhany Paul
Published: May 2006

An individual host's likelihood of acquiring an infectious disease depends, in large part, on the location of the host relative to sources of infection, proximity to other hosts, and occupation of specific microhabitats that confer increased susceptibility. Spatial organization of the host and pathogen populations then may be critical in determining patterns of disease occurrence and dynamics. We discuss three fundamental problems associated with understanding the interactions of plants, pathogens, and spatial structure: (1) how one characterizes spatial patterns, (2) how we determine spatial dynamic processes from a given spatial pattern, and (3) how we then simulate the spatial dynamics presumed to be dominating a given host—pathogen interaction. To demonstrate methods for the characterization of spatial pattern, we analyzed spatial maps of a Silene latifolia population infected with the anther smut Microbotryum violaceum, using join—count statistics, continuous spatial autocorrelation, and Mantel's test. Different statistical techniques provided different interpretations of the same data set, indicating the value of using multiple methods. We described spatial structure in the plant population, the pathogen population (factoring out the plant population structure), and spatial cross correlation between two variables (plant gender and disease status). Each of these tests provides information on how the disease may be spreading in the population. We are not very optimistic about the prospect of determining underlying disease—spread processes purely from analysis of spatial pattern. Most spatial patterns can potentially be generated by a number of biological processes (e.g., true vs. apparent contagion), and it is not possible to distinguish between hypotheses without additional information about the system. A difficulty in modeling spatial processes is simulation of populations with given spatial patterns. Simulated spatially structured populations are useful for predicting disease dynamics in a spatial context. We present a number of techniques for creating these spatially structured populations, including a method we developed for modeling the effect of vector behavior on the spread of a plant virus.
Generalization in Pollination Systems, and Why it Matters
Authors: Waser Nikolas M. , Chittka Lars , Price Mary V. , Williams Neal M. , Ollerton Jeff
Published: May 2006

One view of pollination systems is that they tend toward specialization. This view is implicit in many discussions of angiosperm evolution and plant—pollinator coevolution and in the long—standing concept of pollination syndromes. But actual pollination systems often are more generalized and dynamic than these traditions might suggest. To illustrate the range of specialization and generalization in pollinators' use of plants and vice versa, we draw on studies of two floras in the United States, and of members of several plant families and solitary bee genera. We also summarize a recent study of one local flora which suggests that, although the colors of flowers are aggregated in phenotype space, there is no strong association with pollinator types as pollination syndromes would predict. That moderate to substantial generalization often occurs is not surprising on theoretical grounds. Plant generalization is predicted by a simple model as long as temporal and spatial variance in pollinator quality is appreciable, different pollinator species do not fluctuate in unison, and they are similar in their pollination effectiveness. Pollinator generalization is predicted when floral rewards are similar across plant species, travel is costly, constraints of behavior and morphology are minor, and/or pollinator lifespan is long relative to flowering of individual plant species. Recognizing that pollination systems often are generalized has important implications. In ecological predictions of plant reproductive success and population dynamics it is useful to widen the focus beyond flower visitors within the correct" pollination syndrome, and to recognize temporal and spatial fluidity of interactions. Behavioral studies of pollinator foraging choices and information—processing abilities will benefit from understanding the selective advantages of generalization. In studies of floral adaptation, microevolution, and plant speciation one should recognize that selection and gene flow vary in time and space and that the contribution of pollinators to reproductive isolation of plant species may be overstated. In conservation biology, generalized pollination systems imply resilience to linked extinctions, but also the possibility for introduced generalists to displace natives with a net loss of diversity.
Temperature‐Driven Variation in Substrate Oxygenation and the Balance of Competition and Facilitation
Authors: Callaway Ragan M. , King Leah
Published: May 2006

Emergent wetland plants often alleviate the effects of anaerobic soils on root respiration by passively transporting oxygen belowground through continuous air spaces (aerenchyma) in leaves and roots. Oxygen leaked from the roots into the rhizosphere may oxidize minerals in the soil or become available to other plants. Some spatial patterns in marsh plant communities suggest interspecific facilitative interactions, but there is little experimental evidence for interplant facilitation via soil oxygenation. We investigated the capability of the widespread, highly aerenchymous wetland plant, Typa latifolia, to aerate sediments and affect the growth of two non—aerenchymous neighbors, Salix exigua and Myosotis laxa, both in greenhouse experiments and in a natural pothole pond in western Montana. At soil temperatures of 11°—12°C in the greenhouse, mean dissolved oxygen (DO) ranged from 2.75±0.41 to 4.43±1.11 mg/L (mean ± 1 SE) in pots with Typha, whereas in pots without Typha DO content ranged from 0.40 ± 0.06 to 0.65 ± 0.06 mg/L. At soil temperatures of 18°—20°C, DO in the Typha treatment was lower than in the low—temperature experiment and did not differ significantly from DO in the pots without Typha. At substrate temperatures 11°—12°C, all rooted Salix cuttings survived when planted with Typha, whereas non survived when planted alone. At the same substrate temperature, Myosotis transplants grew significantly larger when planted with Typha than when alone, also indicating that facilitation occurred. When grown with Typha at soil temperatures of 18°—20°C, however, Myosotis root mass was significantly less than when grown alone, suggesting that competition occurred. At the pond margins where Typha and Myosotis coexist in water—saturated soils, naturally occurring Myosotis plants adjacent to transplanted Typha had significanlty larger shoot and fruit mass than did control Myosotis without nearby Typha. In this experiment DO levels in soil water and available soil nitrogen did not differ among treatments. These experiments indicate that aerenchymous wetland plants have the potential to facilitate neighbors via soil oxygenation, and that facilitaton and competition may shift in importance with changes in the physical environment.
Luxury Uptake and Storage of Nitrogen in the Rhizomatous Alpine Herb, Bistorta Bistortoides
Authors: Lipson David A. , Bowman William D. , Monson Russell K.
Published: May 2006

The patterns of luxury uptake and storage of nitrogen were studied in the rhizomatous alpine herb, Bistorta bistortoides (Pursh.). Nitrogen pools in the rhizome and shoot were measured in fertilized and nonfertilized plants over two growing seasons (3—4 growing seasons after fertilization began). Fertilized plants stored more N in the rhizome throughout both seasons than did nonfertilized plants. Inflorescence, foliar, and preformed bud N concentrations and biomass production rate did not increase in response to fertilization. The increased storage of N without a concomitant influence on biomass production demonstrates that luxury uptake occurs in this species. Increased N storage in fertilized plants was accomodated byan increase in the amino acids arginine, the nonprotein amino acid, δmdash;acetylornithine, glutamine, and glutamate. Luxury uptake was accommodated entirely through filling of existing rhizome cells; no increased cell proliferation was found. Sucrose concentration was significantly lower in the recently produced rhizome tissue of fertilized plants, possibly reflecting the carbon costs of storing the additional N. During the first half of the season, fertilized plants relied more on stored N to drive growth, with translocation from reserves accounting for 56% and 100% of the aboveground N requirement in nonfertilized plants, respectively. In fertilized plants, the complete reliance on stored N indicates a decrease in current—season uptake of N during the period normally associated with net N uptake in this species. This is interpreted as either (1) a seasonal shift in N uptake, or (2) a negative feedback between stored N and uptake that would constrain the continued accumulation of excess N and eliminate any long—term benefits of luxury N uptake in this species.
An Explicit Genetic Model for Ecological Character Displacement
Authors: Doebeli Michael
Published: May 2006

Realistic models for ecological character displacement should incorporate population genetics. In Slatkin's pioneering model (Slatkin 1980), the genetics of the quantitative character determining the competitive interactions are modeled by assuming that the character is normally distributed in each generation. Only the mean and the variance of the character distributions change over time. With symmetric ecological assumptions for the two competing species, and with normally distributed resources that are equally used by both species, this model did not yield significant displacement. This has led to the belief that ecological asymmetries or constraints on resource use, e.g., by constraining the phenotypic variances, are necessary for character displacement. I argue that the reason for the negative result in Slatkin's original model is that the genetics are modeled too rigidly. With a more flexible genetic model, obtained by explicitly modeling many loci with additive effects, character displacement occurs as a rule even for symmetric ecological assumptions and without constraints on the phenotype distributions. The model can also be used in other contexts than competition for resources. For example, character displacement in a host—parasite system can lead to parasite specialization. The results suggest that more detailed genetic models yield a finer resolution of the interaction between population genetics and ecological dynamics. Explicit genetics lead to more insights than the usual quantitative genetic assumption of normal character distribution.
Plant Tolerance of Gall‐Insect Attack and Gall‐Insect Performance
Authors: Fay Philip A. , Hartnett David C. , Knapp Alan K.
Published: May 2006

We examined plant tolerance of gall—insect attack and gall—insect performance in rosinweed (Silphium integrifolium, Asteraceae) and its apical meristem galler Antistrophus silphii (Hymenoptera: Cynipidae). Gall densities were varied in field rosinweed populations, while gall densities, water, and nutrients were varied for rosinweed in an experimental garden. Field plants grew under prevailing resource and competitive conditions, but garden plants grew free from competition, so gall—insect impacts, rosinweed regrowth, and gall—insect performance were observed under widely different growing conditions. Seasonal measures of rosinweed growth and leaf physiology, and end—of—season measures of biomass, reproduction, gall—wasp emergence, growth, sex ratios, and parasitism were made for both experiments. Rosinweed poorly tolerated Antistrophus gall damage in the field. Galls reduced plant height, leaf area, and inflorescence production. Rosinweed diverted biomass to stems, but produced no regrowth from axillary meristems. In the garden, rosinweed was much more tolerant of Antistrophus gall damage. Galls initially reduced plant height and leaf area, but axillary meristems grew profusely after gall formation, producing nearly all galled plant inflorescences and more than replacing leaf area initially lost to gall formation. Water— and nutrient—supplemented rosinweed were most tolerant of gall damage, experiencing little loss of total biomass or reproductive output. Field rosinweed failed to mount a tolerance—enhancing regrowth response because galls, resource availability, and competition combined to constrain axillary meristem growth. Gall—wasp performance was largely independent of rosinweed tolerance. Emergence, growth, sex ratios, and parasitism were comparable in field and garden, and only slightly affected by resource availability. Gall—insect performance may be buffered from environmental variation, disconnecting plant and herbivore population dynamics. Rosinweed's poor tolerance of gall damage may typify forb responses to herbivory in highly competitive grassland plant communities.
Wind Dispersal of Seeds from a Forest Into a Clearing
Authors: Greene D. F. , Johnson E. A.
Published: May 2006

Although there are many studies of wind dispersal of seeds from a forest into an adjacent clearing, no physical model has yet been advanced. The model constructed here calculates the trajectories of seeds from individual trees in the area source to a line of seed traps (in the clearing) oriented perpendicular to the forest edge. The model uses a log—normal distribution of horizontal wind velocities at a nearby reference station to evaluate wind velocities at any point in the forest and clearing as a function of both height above the ground and distance from the forest edge. The model predicts that (1) the slope of the area source dispersal curve (seed density vs. leeward distance) approximates a negative exponential; and (2) the great majority of seed deposition in the clearing is contributed by source trees lying within a few tree heights of the forest edge. An evaluation of previously published empirical data shows that the area source model adequately characterizes the decline in seed density with leeward distance.
Source‐Sink Population Dynamics may Complicate the Interpretation of Long‐ Term Census Data
Authors: Brawn Jeffrey D. , Robinson Scott K.
Published: May 2006

We examined the reproductive success and long—term population dynamics of Neotropical migrant birds in the fragmented landscapes of Illinois. Our primary objective was to assess whether annual variation in abundances and persistence of populations within woodlots was related to variation in reproductive success. Nesting success of migrants was so low in the woodlots where we sampled that it is unlikely that these populations are self—sustaining. Moreover, populations frequently disappear from and recolonize these woodlots. These data suggest that the source—sink metaphor for population dynamics within spatially structured populations is applicable to migratory birds in Illinois. The scale of this dynamic is unknown, but may be regional (i.e., midwestern U.S.). An important implication of this result is that census data must be interpreted carefully. For one woodlot, we detected no systematic trend in the overall abundances of migratory birds over much of the 20th century. Yet the viability of populations within that woodlot likely decreased dramatically over that time owing to increasing rates of nest predation and brood parasitism. Lack of data on dispersal is a major gap in understanding the population dynamics of Neotropical migrants and prescribing effective conservation measures.
Geostatistics as a Tool for Examining Hypothesized Declines in Migratory Songbirds
Authors: Villard Marc-Andre , Maurer Brian A.
Published: May 2006

Data from the North American Breeding Bird Survey (BBS) indicate significant declines in the populations of several species of songbirds, including several Neotropical migrants. These declines have been attributed to habitat destruction and fragmentation on the breeding grounds, in strategic migratory stopover sites, and on the wintering grounds. Using BBS data from the 1967—1989 period and universal kriging, we produced maps of abundance change for two declining species of wood warblers to test hypothetical spatial scenarios of decline over entire breeding ranges. These species were the Cerulean Warbler (Dendroica cerulea) and the Prairie Warbler (D. discolor). We found considerable variability in the location of areas of decline when comparing successive 5—yr periods. In some comparisons, areas of decline were concentrated in the centers of abundance of these species, and in others, they were scattered throughout their range. We also found that the direction and intensity of population trends was quite sensitive to the methods used for calculating abundance. Our results indicate that, even for species where significant long—term declines have been reported, considerable variation exists in the direction of abundance change, both geographically and temporally. Although most of the long—term declines reported in particular species at local and regional scales are undeniable, gaps in our knowledge still prevent us from incorporating these trends into a global model of the annual cycle of Neotropical migrants.
The Role of History in Community Dynamics: A Modelling Approach
Authors: Tanner Jason E. , Hughes Terence P. , Connell Joseph H.
Published: May 2006

Recent history plays an important role in the physiology, behavior, and ecology of individuals, and in the dynamics of populations and assemblages of species. In this paper, we examine the impact of history on the species composition of intertidal reef corals, by comparing simulation models that incorporate four different levels of knowledge about the recent past (over a time scale of 1—27 yr). The models are Markov or semi—Markov transition probability matrix models, based on rates of colonization, persistence, and species replacement measured from a long—term study spanning three decades at Heron Island, Great Barrier Reef. Rates of colonization (transitions from free space) varied 20—fold for different species groups, while mortality (transitions to free space) ranged fivefold, reflecting a wide range of life histories among the coral assemblage. Virtually all species groups could undergo reciprocal transitions (e.g., from A to B, and B to A) in a single time interval, indicating the lack of a single competitive dominant that was capable of outcompeting all or most other species. Transition probabilities changed markedly as a function of history. For most species groups, the probability of persisting (i.e., transitions from A to A) increased with time. Thus, a colony that had occupied space for some time was generally more likely to continue to do so than a new arrival. This result is consistent with an escape in size for older colonies from mortality agents such as competition and predation. However, three species groups showed the opposite pattern. Algae, Pocilloporid corals, and fragile tabular Acropora showed marked increases in transitions to free space after 3—5 yr, reflecting a more ephemeral suite of life history traits. Similarly, free space that had recently been generated had a higher rate of colonization than substratum that had been unoccupied for some time. These results falsify a major assumption of standard first—order models, i.e., that transition probabilities are constant, and that history is irrelevant. Although the changes in transition probabilities as a function of history were often striking, the four different models we employed show only minor variation in community composition in both transitory and climax (equilibrium) phases. Thus, while recent history was important in determining transition probabilities, it had little effect on community dynamics and structure in this system. This discrepancy is due to the rapid turnover of corals and algae on shallow reef crests, where only a small proportion of colonies survive long enough to display effects of history. All models agreed that the length of time required for this system to reach an equilibrium community structure is far longer than the observed interval between recurrent disturbances from tropical cyclones.
Complex Dynamics in Experimental Populations of an Annual Plant, Cardamine Pensylvanica
Authors: Crone Elizabeth E. , Taylor Douglas R.
Published: May 2006

To study the numerical dynamics of plant populations, 12 experimental populations of an annual greenhouse weed, Cardamine pensylvanica, were maintained for 15 generations in controlled—environment growth chambers by growing plants in an array of pots and allowing seed for the next generation to disperse into an adjacent array of fresh pots. Discrete generations were enforced by harvesting mature plants after seed dispersal, but germination, recruitment, competition, and dispersal occurred naturally. The numerical dynamics of the experimental populations cycled from high to low density with a period of four to five generations, as indicated by negative autocorrelations in population size at lags of two and three generations. Demographic data collected during the experiment indicate that population density affected plant growth and seed set. Independent estimates of low—density recruitment were also high enough to predict complex population dynamics from simple models of direct density—dependent population regulation. However, simple population models fit to the time series data predicted stable dynamics. Similar models including time—lagged density dependence qualitatively reproduced the dynamics of the experimental populations. Delayed feedback through maternal effects or interacting herbivores or pathogens may be possible causes of the observed dynamics. This suggests that although plant population dynamics may be stabilized by direct density dependence, delayed density dependence could destabilize dynamics.
Biological Control of Marine Pests
Authors: Lafferty Kevin D. , Kuris Armand M.
Published: May 2006

Biological control, as used in terrestrial systems, may hold promise for use against exotic marine species. We first review some marine pests, displaying their diversity, the damage they cause, and possible controls. We then contrast approaches for marine and terrestrial pest control, providing guidelines for adapting terrestrial controls to the marine environment. Although several of the same principles apply in terrestrial and marine environments, marine systems differ with respect to the types of control agents available, the degree of pest—population reduction needed for effective control, the spatial scale over which biological control must operate effectively, the practicality of implementation, and the nature and degree of concern over safety. As an example, we propose a strategy for developing a biological control program against the European green crab, Carcinus maenas, which has had substantial negative impacts where previously introduced (New England, Atlantic Canada, South Africa, south Australia) and which has recently been introduced to central California, and to Tasmania. We conclude that biological control may be possible for some marine pests, but that existing strategies and expectations will require modification.