bluuebunny/crossref_metadata_2025 huggingface dataset:
The Effects of Parasitoids on Sympatric Host Races of Rhagoletis Pomonella (Diptera: Tephritidae)
Authors: Feder Jeffrey L.
Published: May 2006

Enemy—free space is a potentially important factor affecting host plant use by phytophagous insects. In particular, escape from parasitoids, predators and competitors could mediate a successful host shift by compensating for decreased feeding performance on a new plant. Here I investigate the effects that two braconid wasp parasitoids (Opius lectus and Biosteres melleus) have on sympatric host races of Rhagoletis pomonella (Diptera: Tephritidae) infesting apple (Malus punila) and hawthorn (Crataegus spp.) at a field site near Grant, Michigan, USA. Levels of parasitoid attack were much lower for fly larvae infesting the derived host apple (13%) compared to the ancestral host hawthorn (46%). At least three factors contributed to apples being a relatively safe haven from parasitoids for maggots. First, apples were larger (mean fruit width = 5.2 cm) than hawthorns (mean width = 1.6 cm). Apples therefore provided greater physical protection for fly larvae than hawthorns because the ovipositors of female wasps were too short to reach a majority of maggots feeding within apples. Second, apples had an earlier fruiting phenology than hawthorns and the seasonal distribution of O. lectus parasitoids was asynchronous with the development of larvae in apples. Third, hawthorn fruits were infested by various species of caterpillars (tortricids and an agonoxenid) and a curculionid weevil to a much greater degree than apples (65% compared to 11%). These heterospecific insects forced fly larvae to feed near the surface of hawthorn fruits where they were more prone to detection and attack from braconids (73% of larvae feeding in hawthorns with heterospecific insects were parasitized compared to 34% of larvae infesting hawthorns without caterpillars and weevils). Data from other Rhagoletis and tephritid species suggest that escape from parasitoids may be a common theme in host range expansion and race formation for these flies.
The Ecology of Cooperation in Wasps: Causes and Consequences of Alternative Reproductive Decisions
Authors: Nonacs Peter , Reeve Hudson K.
Published: May 2006

Polistes wasps can initiate colonies by themselves, but spring foundresses often cooperate in constructing a shared nest and raising a common brood. The relative benefits for such cooperation vary across environmental and social contexts and thus foundresses should shift reproductive strategies when contexts change. An experimental study of 10 field populations of P. dominulus provides evidence for such strategy shifts. P. dominulus females displayed three basic alternative reproductive strategies: initiate a nest alone or as a dominant on a multifoundress nest, join as a subordinate, or refrain from nesting and instead "sit—and—wait" for reproductive opportunities to open up later in the season. In our populations, joining likely yields inclusive fitness benefits by increasing colony survival and resistance to usurpation. Nevertheless, joiners and other foundresses readily moved to improve their situations by (1) adopting orphaned nests, (2) switching to larger nests, and (3) usurping small nests, which may have had a weakened foundress or a foundress with relatively low motivation for nest defense. Joining occurring more often early in the season, and adopting and usurping occurring more often later were consistent with inclusive fitness maximization. Lost past investments in nests did not affect future decisions. Overall, residents of nests were larger than the wasps that joined them, but this difference decreased over the course of the season. Wasps that had cooperated in the past to form multifoundress associations were more likely to renest (usually together) after removal of their original nest than were single foundresses. After nest loss, single foundresses were more likely than multiple foundresses to take over another wasp's nest by adoption or usurpation. Cooperation within multifoundress associations is facultative and the balance between cooperation and conflict theoretically depends on the asymmetry in reproduction resulting from the asymmetry in dominance. P. dominulus cofoundresses closer in size (less asymmetrical in dominance) added cells to their nests more slowly and produced significantly smaller nests and fewer workers than associations with more size variation, as would be predicted by greater conflict between wasps that are closer in size. The decline in size differences between joining wasps and residents suggests that late—season joiners were relatively large wasps seeking to dominate residents. Conflict on the nest may also explain why multiple foundresses disappeared from nests at a higher rate than did single foundresses. In sum, the multiple reproductive options of P. dominulus lead to a dynamic and flexible balance between cooperation and conflict in their social interactions.
Ratio‐Dependent Predation: An Abstraction That Works
Authors: Akcakaya H. Resit , Arditi Roger , Ginzburg Lev R.
Published: May 2006

Recent papers opposing ratio dependence focus on four main criticisms: (1) the empirical evidence we present is insufficient or biased, (2) ratio—dependent models exhibit pathological behavior, (3) ratio dependence lacks a logical or mechanistic base, and (4) more general models incorporate both prey and ratio dependence and there is no need for either of the two simplifications. We review these arguments in the light of empirical evidence from field and experimental studies. We argue that (1) empirical evidence shows that most natural systems are closer to ratio dependence than to prey dependence, (2) "pathological" dynamics in a mathematical sense is not only realistic, but the lack of such dynamics in prey—dependent models actually makes them pathological in a biological sense, (3) the mechanistic base of ratio dependence is (direct and indirect) interference and resource sharing, and (4) although more general models (with extra parameters) can never fit natural patterns worse that either prey— or ratio—dependent models, there are theoretical, practical, and pedagogical reasons for attempting to find simpler models that can capture the essential dynamics of natural systems.
The Influence of Light and Nutrients on Foliar Phenolics and Insect Herbivory
Authors: Dudt Jan Frederic , Shure Donald J.
Published: May 2006

We examined the changes in leaf phenolic chemistry and insect herbivory from saplings of two temperate deciduous species, Liriodendron tulipifera (tulip poplar) and Cornus florida (dogwood), planted in five microenvironments in Gilmer County, Georgia, USA. The experimental design permitted comparisons between saplings grown in an open field, under shade cloth within the field, on the edge between field and forest, in forest understory, and within canopy gaps established within the forest. Half of the trees in each microenvironment were fertilized. Leaves from each tree were sampled at the end of the growing season and 1989 and 1990 and analyzed for toughness, percent dry mass, total phenolics, hydrolyzable tannins, condensed tannins, and insect herbivory (percent leaf area damaged). The shade—tolerant dogwood saplings contained higher levels of total phenolics and hydrolyzable tannins than the shade—intolerant tulip poplar saplings. Dogwood generally had lower levels of herbivory. These results support earlier studies suggesting that slow—growing, shade—tolerant species tend to have higher levels of phenolics and experienced lower levels of herbivory than fast growing, shade—intolerant species. However, dogwood leaves contained lower levels of condensed tannins and were as tough as tulip poplar leaves. Sunlight availability had a significant positive influence on levels of phenolics in both species. Leaf phenolics generally increased with greater insolation from forest to field and when sunlight was greater within field for forest habitats. However, the levels of tannins in dogwood saplings only dropped significantly in the deep shade of the forest. The similar levels of dogwood phenolics in most microenvironments are indicative of the relatively high photosynthetic efficiency of this species in reduced light environments. Overall, these results are consistent with carbon/nutrient balance theory that predicts trade—offs in the allocation of photosynthate from defense to growth as light declines. Levels of insect herbivory and total phenolics were inversely related for dogwood. However, the relationship with tannins was less apparent. Herbivory on tulip poplar was unrelated to changes in phenolics, possibly reflecting the greater chemical diversity of that species. Fertilization increased the biomass of both species, but had no apparent influence on levels of leaf phenolics or insect hervivory. The lack of a fertilization effect was unexpected in light of previous suggestions that fertilization results in reduced phenolics and increased herbivory.
Dynamics of Gross Nitrogen Transformations in an Old‐Growth Forest: The Carbon Connection
Authors: Hart Stephen C. , Nason G. E. , Myrold David D. , Perry David A.
Published: May 2006

We conducted a 456—d laboratory incubation of an old—growth coniferous forest soil to aid in the elucidation of C controls on N cycling processes in forest soils. Gross rates of N mineralization, immobilization, and nitrification were measured by 15N isotope dilution, and net rates N mineralization and nitrification were calculated from changes in KCl—extractable inorganic N and NO3—@ON pool sizes, respectively. Changes in the availability of C were assessed by monitoring rates of CO2 evolution and the sizes of extractable organic C and microbial biomass pools. Net and gross rates of N mineralization (r2 = 0.038, P = .676) and nitrification (r2 = 0.403, P = .125) were not significantly correlated over the course of the incubation, suggesting that the factors controlling N consumptive and productive processes do not equally affect these processes. A significant increase in the NO3— pool size (net nitrification) only occurred after 140 d, when the NO3— pool size increased suddenly and massively. However, gross nitrification rates were substantial throughout the entire incubation and were poorly correlated with these changes in NO3— pool sizes. Concurrent decreases in the microbial biomass suggest that large increases in NO3— pool sizes after prolonged incubation of coniferous forest soil may arise from reductions in the rate of microbial immobilization of NO3—, rather than from one of the mechanisms proposed previously (e.g., sequestering of NH4+ by microbial heterotrophs, the deactivation of allelopathic compounds, or large increases in autotrophic nitrifier populations). Strong correlations were found between rates of CO2 evolution and gross N mineralization (r2 = 0.974, P < .0001) and immobilization (r2 = 0.980, P < .0001), but not between CO2 evolution and net N mineralization rates. Microbial growth efficiency, determined by combining estimates of gross N immobilization, CO2 evolution, and microbial biomass C and N pool sizes, declined exponentially over the incubation. These results suggest the utilization of lower quality substrates as C availability declined during incubation. Results from this research indicate the measurement of gross rates of N transformations in soil provides a powerful tool for assessing C and N cycling relationships in forests.
Assessing Benefits to Both Participants in a Lycaenid‐Ant Association
Authors: Cushman J. Hall , Rashbrook Vanessa K. , Beattie Andrew J.
Published: May 2006

We examined interactions between the ant Iridomyrmex nitidiceps and the lycaenid butterfly Paralucia aurifera in southeastern Australia, and present data supporting the hypothesis that both participants benefit from their association. In the field, lycaenids persisted only on those host plants that ants subsequently colonized. In the laboratory, lycaenid larvae reared with ants were 31—76% heavier, developed 37% faster, and commonly completed one or two fewer instars than larvae reared without ants. Ant tending also resulted in 20% heavier pupae, 69% shorter pupal duration, and 5% larger adults as measured by forewing length; adults were not significantly different as measured by body length. We hypothesize that these positive effects occurred largely because ant—tended lycaenid larvae spent more time feeding than did untended larvae. Field data documented that ants colonized host plants only after lycaenid larvae were present, indicating that ants actively maintained the association. In laboratory experiments, 40% more ant workers survived when lycaenid larvae were present than when they were absent, although ant mass was not significantly affected. We hypothesize that the survivorship effects occurred because ants consumed the lycaenid's nectary gland secretions, which contained considerable amounts of glucose and amino acids. Our results showed that lycaenids can benefit from ants in ways other than, or in addition to, protection from natural enemies and that they incur minimal developmental costs from associating with ants.
Foraging Responses of Kudus to Seasonal Changes in Food Resources: Elasticity in Constraints
Authors: Owen-Smith Norman
Published: May 2006

Foraging behavior provides the mechanistic link between species performance and food resources in the environment. I document the foraging responses of a browsing ruminant, the kudu (Tragelaphus strepsiceros), to changes in food abundance and quality over the seasonal cycle and interpret the results in the context of optimal foraging theory. The study was carried out on hand—reared but free—ranging animals in a savanna region with summer wet season and winter dry season in Transvaal, South Africa. During the dry season, kudus expanded their diet to include evergreen and unpalatable deciduous woody species neglected during the wet season. The acceptability rating of woody plants was correlated with food value expressed in terms of a protein—condensed tannin index, relative to handling time during ingestion or digestion. However, dietary expansion alone was inadequate to satisfy the daily energy requirement during this period. The animals also increased the fraction of trees of palatable species that was accepted for feeding, extended feeding duration at feeding stations, and increased encounter rate with evergreen trees still retaining leaves. Both total time active and proportion of active time spent foraging increased over the dry season. Digestive capacity was apparently increased to accommodate a higher daily food intake, which compensated for reduced diet quality. Only in September at the end of the dry season, when little foliage remained, were compensatory adjustments inadequate to meet energy requirements. The functional response relating consumption rate to seasonal changes in food abundance showed little variation, despite a decline in foliage biomass by more than an order of magnitude. The kudus were neither energy maximizers, nor time minimizers, but rather targeted on their energy requirements with least overall cost. Findings demonstrate that the foraging time and digestive capacity constraints assumed in optimal diet models are somewhat elastic. Dietary predictions obtained using average parameter settings may be misleading, because constraints may become effective only under extreme conditions. Further research is needed to establish the costs associated with stretching physiological constraints towards their upper tolerance limits.
Long‐Term Memory for Food Patches and Implications for Ideal Free Distributions in Sticklebacks
Authors: Milinski Manfred
Published: May 2006

Population foraging in a patchy environment can lead to an ideal free distribution of competitors among the patches. If patches profitabilities do not change too quickly, individuals may remember patches in which they had been successful and use their memory when they have to deal with new competitors. I investigated whether three—spined sticklebacks (Gasterosteus aculeatus) expected to find food in a patch in which they had foraged between 1 and 4 d ago. Long—term memory for a patch in which the fish had formerly been fed existed for at least 8 d. In another experiment I tested whether sticklebacks, use their long—term memory in population foraging. A group of four sticklebacks, each of which had been fed singly in one of two patches, was offered food in both patches simultaneously 3 d later. The four fish achieved an ideal free distribution, with those that had fed in the left patch preferring the left patch and those that had fed in the right patch preferring the right patch, although all fish sampled both patches frequently. The preference for the former patch in the ideal free distribution was similar when there had been a pause of 6 between feeding and test. Implications of long—term memory for competitive foraging are discussed.
The Search for Resources by Cabbage Butterflies (Pieris Rapae): Ecological Consequences and Adaptive Significance of Markovian Movements in a Patchy Environment
Authors: Root Richard B. , Kareiva Peter M.
Published: May 2006

Unlike many insects, the caterpillars of the cabbage butterfly, Pieris rapae, are less abundant on host plants that grow in large dense stands than on hosts that are scattered sparsely in diverse vegetation. In this study we describe how the flight behavior of ovipositing P. rapae results in the placement of more eggs on isolated hosts. The flight paths of females were measured in terms of move lengths and tuning angles; this permitted us to analyze the flight as a sequence of movement vectors punctuated by landings and oviposition. A variety of tests revealed that the butterflies conformed with the assumptions required to describe their flight as a Markov process. Females ovipositing in potato—collard bicultures behave identically to females oviposition in collard monocultures. In gardens containing collards planted at different spacings, females make shorter moves and land more frequently when encountering dense stands of collars. Under all circumstances, however, ovipositing butterflies tend to follow linear routes of flight. As a result of this directionality, the number of eggs laid per plant declines with increases in host plant density, even when these increases in host concentration prompt shorter moves. Females foraging for nectar, on the other hand, readily abandon their linear flight paths. By engaging in tight turning behavior in clumps of flowers, nectar—feeding butterflies concentrate their activities in resource patches. The differences between the nectar—feeding and ovipositional flights highlight an "egg—spreading syndrome." In particular, three features of P. rapae oviposition behavior enhance each female's tendency to spread her eggs widely: (1) eggs are usually laid singly on plants, (2) females tend to follow linear flight paths, and (3) females typically pass over many suitable hosts. Such behavior cannot be the product of natural selection acting solely to increase energetic efficiency or competitive ability. We evaluate several adaptive explanations for this egg—spreading syndrome and exclude all but one. The remaining hypothesis is that P. rapae scatter their eggs widely to "spread the risk" and minimize the generation—to—generation variance in replacement rates. To estimate the potential advantages accrued by egg spreading, we use data on the spatial variability in survivorship to simulate the replacement rates that would be achieved by scattering eggs on different plants within a garden and in different gardens within a region. Although this simulation cannot prove the risk—spreading hypothesis, it does demonstrate the effectiveness of egg spreading in averaging out observed variations in larval survivorship. The variability in P. rapae survivorship appears to be associated with the highly unpredictable milieu (local herbivore load, plant condition, etc.) of a developing caterpillar. The implications of such unpredictability to evolution and community organization are discussed.
A Comparison of Surface and Buried Larrea Tridentata Leaf Litter Decomposition in North American Hot Deserts
Authors: Santos Perseu F. , Elkins Ned Z. , Steinberger Yosef , Whitford Walter G.
Published: May 2006

We conducted studies of mass losses from surface and buried litter bags in four North American hot desert areas to test the following hypotheses: (1) leaf litter disappearance in hot deserts is independent of actual evapotranspiration, (2) buried litter disappearance is a function of actual evapotransporation, (3) the pattern of microarthropod colonization of buried leaf litter is a function of the stage of decomposition, and (4) elimination of microarthropods results in reduced rates of decomposition and increased numbers of free—living nematodes. Mass losses from surface Larrea tridentata leaf litter bags ranked highest to lowest: Chihuahuan desert, Sonoran desert, Mojave desert, Coloradan desert. Mass losses from buried litter bags were essentially equal. °40%, in each of the deserts for bags buried from March to October. There was low correlation between rainfall and mass loss of buried litter and surface litter in the North American hot deserts. Mass losses from insecticide—treated buried bags were lower than from untreated bags. There was a greater abundance of nematodes in insecticide—treated bags than in untreated bags. Tarsonemid mites were found only in litter bags from the Chihuahuan desert. The most abundant microarthropods in buried leaf litter in the other deserts were predatory raphignathids, tydeids, and arctacarids. Decomposition (litter disappearance) in North American hot deserts was highly correlated with long—term rainfall patterns, which we hypothesize have served as the selective agents for the soil biota active in the decomposition process. Thus litter disappearance does not respond to annual fluctuations in rainfall amounts.
Costs and Benefits of Induced Resistance and Plant Density for a Native Shrub, Gossypium Thurberi
Authors: Karban Richard
Published: May 2006

Many plants employ induced defenses that are activated after attack rather than constitutive defenses that are always present. Current theory predicts that one benefit of induced defenses is to optimize the plant's allocation to defense, growth, and reproduction. According to this reasoning induced defenses are expected under the following conditions: (1) Attack is unpredictable and not fatal. (2) Defense is costly; when herbivores are not present, the plant can save resources by not being defended and use them for growth and reproduction. (3) When herbivores are present, activation of the defense increases plant growth and reproduction. This study tested the allocation hypothesis by examining the effects of early—season damage to a desert shrub, Gossypium thurberi, in environments that either included or did not include its most abundant herbivore, Bucculatrix thurberiella. A companion paper demonstrated that induction occurred. Plants damaged early in the season supported fewer miners than controls that were not previously induced. If there was a benefit of induced resistance, I expected that survival, growth, and seed production would be greater for plants that had been induced experimentally by caterpillars early in the season compared to those that had not. I found no evidence that plants benefited by responding to damage early in the season, despite reductions in the populations of herbivores on damaged plants. If there were costs of induced resistance, I predicted that survival, growth, and seed production would be lower for induced plants than for controls when herbivores were excluded from both treatments. I found no evidence that induced resistance cost the plant anything in terms of survival, growth, or seed production. Finally, to determine if plant crowding was costly and if crowding interacted with induced resistance, survival, growth, and seed production were compared for plants that had no neighbors within 50 cm vs. those experiencing endemic densities of neighbors. Plants that experienced reduced density grew more and produced twice as many seeds as controls that experienced endemic densities of neighbors. No interactions between damage and plant density were observed. There are many different ways to interpret these negative results. I measured only a subset of the plant's lifetime fitness and the statistical power of my tests may have been insufficient. My experiment to test the benefits of induced resistance was much weaker than my tests of the costs of damage and of crowding. Herbivory may be unimportant in this system, although indirect evidence suggests otherwise. This and other empirical results suggest that the cost—benefit model may not be appropriate to explain the advantages of induced resistance over constitutive resistance. I discuss three alternative hypotheses to explain the existence of induced resistance.
Effects of Herbivores on Growth and Reproduction of their Perennial Host, Erigeron Glaucus
Authors: Karban Richard , Strauss Sharon Y.
Published: May 2006

Experiments were conducted over several years to assess the effects of the three most abundant herbivores (a thrips, a spittlebug, and a plume moth) on the growth and reproduction of their long—lived, iteroparous host plant. In a greenhouse experiment, thrips reduced root biomass of seedlings, but not shoot biomass. No correlation was found between density of thirps and change in aboveground area of plants after monitoring for 3 yr in the field. Thirps fed on the phyllaries and corollas of flower heads, as well as on leaves, and were particularly damaging to ray petals. Placed in artificial arrays, flower heads with rays damaged by thirps were less likely to be visited by pollinators than were undamaged flower heads. Plants in a common garden were subjected to three seasons of feeding by either spittlebugs, plume moth caterpillars, both, or neither. Survival, growth, and flower production were measured for those 3 yr and for an additional 3 yr following the herbivory treatments. Plant survival was unaffected by these insect herbivores, although gophers (Thomomys bottae) were a major source of mortality. Plants with caterpillars produced more rosettes than those without moths. By consuming apical buds the caterpillars released axillary buds, thereby causing plants to produce more side rosettes. This difference in architecture did not result in an increase in flower production. Plants that had experienced 3 yr of spittlebug or caterpillar attack produced fewer flower heads than controls during their 3 yr. One year later, and after the herbivory treatments were discontinued, plants that had been damaged by spittlebugs (but not plume moth caterpillars) still produced fewer flowers than those in other treatments. Thereafter, no long—terms effects of herbivory could be detected. Al three herbivores were found to adversely affect some components of plant growth and reproduction. Because they are consistently abundant at Bodega Bay, these herbivores may reduce the fitness of their host plant.
Mechanisms Controlling Invasion of Coastal Plant Communities by the Alien Succulent Carpobrotus Edulis
Authors: D'Antonio Carla M.
Published: May 2006

The South African succulent Carpobrotus edulis has been invading native and non—native plant assemblages in California since its introduction in the early 1900s. This study was designed to determine mechanisms controlling invasion by this species into three different communities: coastal grassland, backdune, and coastal scrub. The communities differed in several factors that influenced invasion by Carpobrotus: soil disturbance, herbivory, and identity of competitors. Disturbances of the soil was required for establishment of Carpobrotus at the grassland site. Soil disturbance did not promote Carpobrotus establishment in the coastal scrub or backdune. Establishment in these sites was restricted by herbivory. The influence of neighboring vegetation on Carpobrotus establishment was site dependent. At the grassland site, Carpobrotus seedling survival was reduced by grasses, and the only surviving seedlings were on gopher mounds. Growth of larger Carpobrotus plants, however, was unaffected by grasses. Proximity to shrubs, the dominant plant type at the backdune and coastal scrub sites, did not affect Carpobrotus seedling survival. It did, however, decrease both root and shoot biomass of seedlings and cuttings. Invasion is expected to occur most rapidly at the grassland site. Although germination and early survival there depend on the occurrence of rodent disturbances, these occur frequently, and seedling recruitment in the grassland was observed during this study. Once established, clonal growth is great and is not affected by herbivory or competition. In contrast, at the coastal growth, intense herbivory prevented successful establishment during this study. Cage experiments suggested that herbivory counteracted the high growth potential of Carpobrotus at this site. At the dune site, even caged plants showed very poor growth. Thus, although seedling recruitment does not occur there, invasion is expected to be slow due to harsh physical conditions that limit both survival and growth. These findings demonstrate strong community differences in the factors governing invasability by Carpobrotus edulis.
Processes Regulating Soil Emissions of NO and N^2O in a Seasonally Dry Tropical Forest
Authors: Davidson E. A. , Matson P. A. , Vitousek P. M. , Riley R. , Dunkin K. , Garcia-Mendez G. , Maass J. M.
Published: May 2006

While much is known about control of production of NO and N2O by nitrifying and denitrifying bacteria at the cellular level, application of this knowledge to field studies has not yielded unifying concepts that are widely applicable and that foster understanding of global sources of these atmospheric trace gases. We applied a simple conceptual model to the investigation of sources of NO and N2O and the environmental factors affecting fluxes in a drought—deciduous forest of Mexico. Fluxes of NO and N2O were higher in the wet season than the dry season, but addition of water to dry soil caused large pulses of CO2, NO, and N2O emissions. Immediate increases of extractable soil NH4+ and high rates of gross N mineralization and gross nitrification also were observed following wetting of dry soil NO2— had accumulated during the dry season, and that NO2— plus the pulse of increased soil NH4+ were mostly consumed within 24 hours of wetting. This dynamic microbial processing of soil inorganic N coincided with the pulses of NO and N2O production following wetting of dry soil. Acetylene inhibition experiments indicated that NO production was dependent on nitrification, that nitrification was the dominant source of N2O when the soil was wetted at the end of the dry season, and that dentrification might be an important source of N2O during the wet season. Post—wetting soil moisture was correlated negatively with NO fluxes and positively with N2O fluxes. These results support a conceptual model in which N trace gas production is generally constrained by the rates of N mineralization and nitrification, while the specific ratios of NO and N2O fluxes and the contributions from nitrifying and denitrifying bacteria are controlled largely by soil moisture.
Population Regulation in Theory and Practice
Authors: Murdoch William W.
Published: May 2006

Population regulation is a fundamental process related to most phenomena in ecology, including evolutionary ecology. I review the conceptual basis for defining regulation as bounded fluctuations in abundance, in contrast to the unbounded fluctuations of random—walk populations. Regulation arises as a result of potentially stabilizing density—dependent processes, even when brought about by “non—equilibrium” mechanisms. Although the phenomena is unambiguous in theory, detecting regulation by finding evidence for density dependence in a series of population estimates faces unsolved statistical problems. So, while there is growing evidence for widespread regulation, severe detection problems remain. I illustrate these with data from bird populations. Whether regulation is typically achieved by local stabilizing mechanisms or via metapopulation dynamics remains to be determined. I summarize recent studies on a particularly well—regulated system–red scale (Aonidiella aurantii) and its controlling parasitoid, Aphytis melinus. We tested and failed to find evidence for eight hypotheses that might account for the system's stability, including spatial heterogeneity in attack rates, a refuge, and metapopulation dynamics. We also failed to find evidence for density—dependent parasitism, but such density dependence might be still be present. Recent laboratory and modeling studies have uncovered a number of other potentially stabilizing mechanisms centering on the response of individual Aphytis to their size—structured host. This plethora of size— and stage—dependent interactions leads naturally to a consideration of the factors controlling Aphytis' size—dependent behavioral decisions, and consequently to the elaboration of size—structured models. The latter provide a vehicle for bringing together investigations of selection of life histories, and population dynamics. This is illustrated by a model of Aphytis and red scale dynamics that can explain a dramatic case of competitive displacement. The red scale/Aphytis system exemplifies a particularly challenging problem in population regulation, namely to account for the co—occurrence of stability and severe suppression of the prey population. A potentially generic solution is to assume stabilizing density dependence in the parasitoid or predator population; however, this has the consequence of increasing the host or prey population equilibrium. My colleagues and I have shown that observed prey densities in a plankton system are too low for such a mechanism to be operating. Further work is needed to test this and other hypotheses.
Litter Decomposition on the Mauna Loa Environmental Matrix, Hawai'i: Patterns, Mechanisms, and Models
Authors: Vitousek Peter M. , Turner Douglas R. , Parton William J. , Sanford Robert L.
Published: May 2006

We determined controls on litter decomposition and nutrient release for the widespread native tree Metrosideros polymorpha in 11 sites arrayed on gradients of elevation, precipitation, and substrate age on Hawaiian lava flows. The effects of site characteristics were evaluated using three common substrates (Metrosideros leaf litter from one of the sites, wood dowels, and filter paper) decomposed in each of the sites, and the inherent decomposability of tissue (substrate quality) was evaluated using Metrosideros leaf litter from each of the sites decomposed in a common site. Site characteristics were responsible for most of the variation in rates of decomposition in the range of sites and substrates examined. Common substrates decomposed much more rapidly in warm, low elevation sites; apparent Q10 values, calculated on the basis of variation in mean annual temperature with elevation on individual lava flows, ranged from 4 to 11. Litter decomposed slowly in the dry sites, but leaf litter produced in the dry sites decomposed more than twice as rapidly as litter from wet sites when both were measured in the same site. The higher substrate quality of litter from dry sites could be due to trade—offs among nutrient—use efficiency, water—use efficiency, and carbon gain by water—limited Metrosideros. We used these results to test a revision of the CENTURY soil organic matter model that had been designed to simulate the decomposition of surface litter. Simulations accurately matched the pattern but underestimated the magnitude of among—site differences in the decomposition of common substrates in a range of sites. Analyses of both field and simulation results suggested that the decomposition of Metrosideros leaf litter could be limited by nitrogen availability.
Ungulate Population Models with Predation: A Case Study with the North American Moose
Authors: Messier Francois
Published: May 2006

In this article I examine moose—wolf interactions over a broad spectrum of moose densities with the primary objective to test empirically whether wolf predation can regulate moose numbers. I also present four conceptual models of moose population dynamics and outline their specific predictions. Based on 27 studies where moose were the dominant prey species, the functional and numerical responses of wolves to changing moose density were derived using an hyperbolic, Michaelis—Menten function. Per capita killing rate was strongly related (P = .01) to moose density, as was the density of wolves (P < .01). Killing rate plateaued at 3.36 moose°wolf—1°(100 days)—1 when predators were fully satiated. The asymptotic value for wolf density was 58.7 animals/1000 km2. Wolf predation rate, as derived from the total predator response, proved to be density dependent from 0 to 0.65 moose/km2, and inversely density dependent at higher moose densities. Predator: prey ratios reflected wolf predation rate poorly because they did not integrate the functional response. An empirical model based on these results suggests that moose would stabilize at 2.0 moose/km2 in the absence of predators, and at °1.3 moose/km2 in the presence of a single predator, the wolf. Density—dependent food competition creates these two high—density equilibrium conditions. If moose productivity is diminished through either deteriorating habitat quality or bear—induced early calf mortality, then a low—density equilibrium (0.2—0.4 moose/km2) is predicted. The model also suggests that when a low equilibrium develops, a “predator pit” is absent or extremely shallow, thus arguing against the appropriateness of a predation—food two—state model. Further research on the density relationship of bear predation, on the effect of alternate prey on wolf total response, and on the regulatory impact of food competition at high moose densities, is required for a full understanding of moose demography.
Effects of Lake Acidification and Recovery on the Stability of Zooplankton Food Webs
Authors: Locke Andrea , Sprules W. Gary
Published: May 2006

The effect of food web structure on community stability and resilience has rarely been examined using empirical data. Yet there is a practical application for such studies insofar as resistance stability determines the ability of a system to “absorb” anthropogenic stress and adjustment stability determines the reversibility of resulting damage. The stability of zooplankton food webs in 46 Precambrian Shield lakes was examined using data collected in the 1970s, when pH ranged from 3.8 to 7.0, and in 1990, when pH had increased by up to two units in some lakes. Acidification overcame resistance stability at pH < 5.0, as evidenced by decreases in species richness, numbers of predatory and competitive links, directed connectance, predator generalization, and linkage density, identified by analysis of variance. Adjustment stability was demonstrated by changes in food web attributes in lakes with higher pH in 1990 than in the 1970s. Species richness, numbers of predatory and competitive links, linkage density, and predator generalization all increased relative to the 1970s values. Food web attributes of “recovering” lakes were statistically indistinguishable from those of lakes of similar pH that had not been more acidic in the 1970s. Similar trajectories of food web change were followed during environmental degradation and recovery. Planktonic food webs of anthropogenically acidified lakes may eventually recover to resemble their pre—acidification condition, given sufficient time without acidic inputs. Whether adjustment stability is a general feature of anthropogenically stressed systems remains to be determined.
The Impact of Fungi on Soil Seeds: Implications for Plants and Granivores in a Semiarid Shrub‐Steppe
Authors: Crist Thomas O. , Friese Carl F.
Published: May 2006

Fungi are unbiquitous in soil and may affect seed survivorship directly by decomposition or pathogenesis, or indirectly through the effects of fungi on seed preference of granivores. We studied the role of fungi in the persistence of soil seeds in a shrub—steppe ecosystem using two experimental approaches. In the first, we examined survivorship of 900 seeds of five shrub—steppe species in nylon bags over a 10—mo period and determined variable seed losses to germination, decomposition, and pathogens. The proportion of seeds representing a viable seed bank decreased from 84.4% in the May pre—experimental conditions to 4.4% in March. Decomposition and attack by fungi were responsible for the greatest decrease from fall to winter (0% in August, 32.0% in November, 35.2% in March) while germination accounted for the greatest decrease from winter to spring (18.5% in August, 21.9% in November, 55.0% in March). Small seeds were more likely to be lost to decomposition and pathogens, whereas larger seeds had greater losses to germination. A subset of 80 experimental seeds was cultured for seed—infecting fungi. We found fungi in virtually all seeds; the most common fungal isolates were cosmopolitan soil fungi, such as species of Penicillium and Mucor. These species are known producers of extracellular enzymes and mycotoxins. Two sterile (nonfruiting) forms of fungi were also frequently isolated, as were two pathogenic species of Fusarium. In a second experiment we placed seeds in fungal cultures obtained from soil seeds and presented moldy seeds along with control seeds to a shrub—steppe granivore, the western harvester ant (Pogonomyrmex occidentalis). Ants harvested control seeds at nearly twice the rate of moldy seeds, suggesting that they avoided seeds infected with fungi. In this way, fungi may indirectly affect seed mortality due to predation by granivores, in addition to the direct effects fungi can have on seed survivorship. We suggest that fungi may play an important role in soil seed dynamics, but one that is poorly understood.
Methane Emissions from Wetlands in the Midboreal Region of Northern Ontario, Canada
Authors: Bubier J. L. , Moore T. R. , Roulet N. T.
Published: May 2006

Methane (CH4) fluxes were measured by a static chamber technique from May to October 1991 at 19 wetland sites near Cochrane, northern Ontario, representative of the Clay Belt Midboreal region of central Canada. Seasonal average fluxes of CH4 from the peatlands ranged from 0.4 to 67.5 mg°m—2°d—1 (0.06—10.1 g°m—2°yr—1). Beaver ponds showed the highest fluxes of CH4, from both the open water section (seasonal average 290 mg°m—2°d—1, 44g°m—2°yr—1) and the adjacent marsh areas where the water table rose close to or above the soil surface (91—350 mg°m—2°d—1, 13—35 g°m—2°yr—1). Seasonal mean water table position, particularly at the microtopographic scale of hummock and hollow, explained most of the variability in CH4 emission among wetlands (r2 = 0.74). Trophic status, such as pore—water Ca, Mg, and pH had little correlation with CH4 emissions. Broad—scale peatland classifications that do not account for hydrological differences at the microtopographic level are inadequate for predicting CH4 flux in boreal wetlands, particularly in forested ecosystems where conifer swamps are diverse and comprise the major peatland class. Based on areal estimates of the different wetland types in the Clay Belt, we estimate an annual CH4 flux of 3.4 g°m—2°yr—1, generally lower than that used in extrapolations to continental— or global—scale methane budgets.
Zooplankton Assemblages in Fishless Bog Lakes: Influence of Biotic and Abiotic Factors
Authors: Arnott Shelley E. , Vanni Michael J.
Published: May 2006

Some bog lakes in northern Wisconsin and Michigan lack fish because of low pH and low winter oxygen concentrations. The lakes are characterized by a dominance of large zooplankton species. Small zooplankton species are scarce, whereas in nearby lakes with fish they are common. The importance of biotic and abiotic factors in preventing the successful invasion of small zooplankton species into these fishless lakes was determined using a series of field manipulations. The relative importance of competition with large herbivores (the cladoceran Daphnia pulex), predation by large invertebrates (the dipteran Chaoborus americanus and the copepod Diaptomus leptopus), and abiotic conditions including pH were examined experimentally. Abiotic and/or resource conditions suppressed the population growth rate of 70% of the small zooplankton species introduced into enclosures in the fishless lakes relative to those introduced into enclosures in their resident lake (a lake with fish). Fewer small zooplankton species were suppressed in a similar experiment when pH was experimentally raised to 7, suggesting that pH is one factor that inhibits the successful invasion of small zooplankton into these lakes. Manipulations of large herbivores and invertebrate predators showed that predation was more important than competition in restricting the distribution of small species. Predation by Chaoborus primarily reduced the densities of crustaceans while predation by Diaptomus reduced the densities of rotifers. Competitive suppression of small zooplankton by Daphnia was not detected. The results suggest that both predation by Chaoborus and Diaptomus and abiotic conditions, including pH, are important factors determining zooplankton community structure in these fishless bog lakes.
Putting the Pieces Together: Testing the Independence of Interactions among Organisms
Authors: Wootton J. Timothy
Published: May 2006

An important challenge in ecology is determining the degree to which the dynamics at one level of organization can be predicted by the behavior of its component parts at a lower level of organization. This endeavor is likely to succeed only if the behavior of the component parts remains essentially the same regardless of the state of other elements in the system. For example, the behavior of multi—species communities is predictable from a knowledge of interactions among species pairs only if the functions describing the pair—wise interactions are not modified by the abundance of other species in the community. Here I discuss some of the problems in identifying empirically whether such interaction modifications (“higher order interactions”) occur. Four persistent problems include (1) confusion about the definition of “higher order interactions,” (2) discrepancies between the currency typically used in theoretical work (instantaneous rates) and that required in empirical work (the states of variables [e.g., abundances] after a discrete time period), (3) frequent incongruities between statistical procedures and the underlying theory being tested, and (4) a failure to explicitly consider the theoretical framework being tested. Progress may be more rapid in the future if the theory being tested is specified, and if empirical investigations shift their focus toward identifying possible mechanisms by which interactions can be modified.
Vegetational Cover Predicts the Sex Ratio of Hatchling Turtles in Natural Nests
Authors: Janzen Fredric J.
Published: May 2006

I monitored natural nests of an Illinois population of the western painted turtle (Chrysemys picta bellii), a species with temperature—dependent sex determination, for 4 yr to investigate the relationships between vegetational cover at the nest site and hatchling sex ratio. Most nests produced hatchlings all of one sex (66% of 116 nests). Hatchling sex ratio within nests was predictably related to the nesting beach and to the quantity of solar exposure at the nest mainly from the south. The North beach tended to produce more all—male nests and an overall male—biased sex ratio than did the South beach. Within each beach, nests with more vegetational cover on the southern and perhaps western aspects produced more males than did more exposed nests. However, the relationship between solar exposure and nest sex ratio was absent during two atypically cool summers. This study suggests a possible mechanism by which female turtles choose the thermal environment of nests and, hence, the sex ratio of their offspring: they may assess vegetational cover on the nest site at oviposition.