bluuebunny/crossref_metadata_2025 huggingface dataset:
Disturbance and Population Structure on the Shifting Mosaic Landscape
Authors: Clark James S.
Published: May 2006

A stochastic model of plant population dynamics is developed and analyzed to determine how density and age structure depend on thinning rates and disturbance regimes. Probability distributions of age and density are derived from the distribution of regeneration niches on a landscape and the thinning rates of cohorts on patches created by adult mortality or larger disturbances. The theory is then extended to different types of disturbances that operate at different scales and are interdependent, such as treefalls that only become important as the early—successional trees that initially colonize an area affected by a larger scale disturbance become mature. In general, landscapes that provide frequent regeneration niches support high—density young stands. Density distributions are negatively skewed. Decreasing frequency of regeneration niches results in lower mean density, higher variance, and increased (less negative) skewness. When regeneration niches are rare, density is low, variance is low, skewness is positive, and the age classes are highly variable. In more complex cases, regeneration niches may depend on the time since the last large disturbance; for example, canopy gaps can become more frequent as postfire cohorts become senescent. Then age class distributions on a given disturbed area become increasingly platykurtic with time; skewness is negative for early—successional species and positive for late—successional species. The exponential distribution of age classes commonly observed in late—successional species is the asymptotic result of a more general distribution that depends on time since the last disturbance. Across a landscape that supports these interdependent disturbance processes, infrequent disturbances result in density distributions for late—successional species with large mean, negative skewness, and low variance. An early—successional species is present at low density with positive skewness and low variance. Density distributions for both species types are more platykurtic and have higher variance when disturbance frequency ° thinning rate. Frequent disturbances produce higher densities of early—successional species with positive skewness and lower densities of late—successional species. The landscape distributions of age classes are J—shaped for both species. With frequent disturbance, age distributions are leptokurtic for both species, but more so for late—successional species. Age class distributions are increasingly platykurtic with less frequent disturbance. The "intermediate" disturbance frequency that maximizes the probability of being reproductively mature at the time of the next disturbance event (Clark 1991a) is also that which maximizes the density of reproductive individuals on this shifting mosaic landscape.
Effects of Soil Moisture on Shrub Seedling Survival in Semi‐Arid Grassland
Authors: Harrington Graham N.
Published: May 2006

Grazing of wooded grasslands by domestic stock has led to dominance by woody plants, in many parts of the world. In semi—arid grasslands in Australia Dodonaea attenuata is a common, invasive shrub of perennial grasslands on sandy soils. There is evidence that cohorts of D. attenuata establish infrequently in vigorous grasslands and more frequently where the perennial grass has been damaged by overgrazing. This study examined the influence of herbaceous growth on mortality of D. attenuata seedlings, by growing them in plots with controlled moisture input. The herbaceous layer in the plots was either natural, clipped, or killed. Without summer irrigation all the seedlings in the natural grassland died. Seedling survival was inversely related to the amount of herbaceous growth on the other treatments. Summer irrigation maintained D. attenuata seedlings and produced a large biomass of perennial grass, which left the seedlings susceptible to grass fires. A water—balance model was run for the experimental site for the 97 yr of climatic records. The soil—moisture patterns associated with the establishment of three known cohorts of D. attenuata were identified. The model output was interrogated for similar soil moisture patterns in other years. It was estimated that only six widespread D. attenuata establishment events have occurred in 97 yr. In intact grasslands D. attenuata rarely establishes densely, and when it does so it is susceptible to grass fires. Modern management has increased establishment frequency by increasing the availability of summer soil moisture by overgrazing the grasslands and has suppressed grass fires. This has changed open grassland to dense shrubland.
Prey Refuges Affecting Interactions Between Piscivorous Perch and Juvenile Perch and Roach
Authors: Persson Lennart , Eklov Peter
Published: May 2006

In size—structured populations, interactions are strongly dependent on size—specific foraging and anti—predator capacities of the organism. Conflicting size—specific selection pressures over the ontogeny often have different effects on different species leading to asymmetries in competitive and predator—prey interactions. Habitat complexity is likely to affect such asymmetric interactions due to species/size—specific competitive abilities in different habitats and due to the fact that habitat structural complexity may act differently as a prey refuge for different species. We experimentally analyzed the impact of a piscivorous predator (adult perch, Perca fluviatilis) on performance of juvenile perch and roach (Rutilus rutilus) at different levels of structural complexity (no structure, structure forming a partial refuge, and structure forming a complete refuge) in enclosures in an experimental pond. We measured predator diet and growth, prey fish habitat use, survival, diet and growth, and prey resource levels in different habitats. Prey fish (perch and roach) were found in the diet of piscivorous perch in no refuge and partial refuge treatments. Growth rate of the piscivorous perch decreased with increased refuge efficiency. Juvenile perch increased their proportional use of the structurally complex refuges in the presence of piscivorous perch and the survival increased with increased refuge efficiency (from partial to complete refuge). The diet of juvenile perch changed from predominantly cyclopoid copeponds in the absence of predators to predominantly macroinvertebrates in the presence of predators. There was no effect of predator—induced habitat restriction on growth of juvenile perch. Roach survival also increased with increased refuge efficiency in the presence of predators, and roach survival in the refuge treatments did not differ from each other or from the treatments with predators absent. Predator—induced habitat restriction in roach was associated with a decreased growth of roach. Our results suggest that, compared to juvenile roach, juvenile perch may compensate more for lost foraging opportunity in the open water via increased exploitation of structure—associated prey in refuges. As a result, predation—induced habitat shifts by juvenile perch and roach may alter competitive interactions between the species. On the other hand, structural complexity may form an almost complete refuge for juvenile roach from predators and thereby affect the predator—prey relationships between piscivorous perch and juvenile perch and roach to the advantage of juvenile roach. The demonstrated effects of structural complexity on competitive and predator—prey interactions between perch and roach can be related to the two species' distributions in lakes with different degrees of structural complexity.
Juvenile Groups in a Coral‐Reef Damselfish: Density‐Dependent Effects on Individual Fitness and Population Demography
Authors: Booth David J.
Published: May 2006

Costs and benefits to group living in animals may affect the fitness of individual group members and also demography of the population. The effects of grouping on the growth, survival, and attainment of maturity of juveniles of an Hawaiian coral—reef damselfish (Dascyllus albisella) were evaluated from 1987 through 1989. In this species, pelagic larvae settle (at °14 mm total length) on coral heads, joining temporary groups of up to 15 juveniles. Groups members establish a dominance hierarchy based on size, and fish leave these groups upon reaching mature size (70 mm total length) to enter the nearby adult population. Previously, I had demonstrated that larvae preferentially join larger groups and I expected to find clear advantages to group membership. Survival, especially of smaller fish, was enhanced in large groups, but growth, especially by individuals of low social status, was reduced. Consequently, the time to reach maturity increased with group size in both years of the study, suggesting a more rapid entry into the adult population of fish in smaller groups or living alone. However, the probability of reaching mature size (a function of size—specific growth and survival) increased with group size in 1988 but not in 1987, thus indicating a benefit to group living during only one of the two years of the study. Although this study demonstrated density—dependent juvenile growth and survival, with consequent fitness effects of group living, it has also highlighted considerable inter— and intra—annual variability in these relationships. Such variability will have major consequences for predictions of the effects of juvenile ecology on the demography of organisms.
Resource Consumption Variance Within and Among Individuals: On Coloniality in Spiders
Authors: Caraco Thomas , Uetz George W. , Gillespie Rosemary G. , Giraldeau Luc-Alain
Published: May 2006

Several models for the economics of sociality demonstrate that the probability distribution of an individual's resource consumption could depend on the size of its social group. When groups share food clumps discovered by any member, expected resource consumption may increase or decline with group size, but the individual's resource consumption variance will ordinarily decline as group size increases. Hence, the formation and dissolution of social groups may represent risk—sensitive responses to foraging success. A series of field studies report greater mean prey consumption and decreased prey consumption variance as group size in colonial spiders increases. Several hypotheses attempt to explain this effect. Coloniality may promote acquisition of information concerning temporal variation in prey availability. Colony membership may permit individuals to steal food when the number of captured prey varies spatially. Finally, colonality can enhance food acquisition because prey (that might otherwise escape) ricochet from one spider's web to another group member's web. Our purpose was to quantify each hypothesis and ask if the resulting models predict reduced resource consumption variance as group size increases. We model each mechanism and conclude that stealing prey is the simplest explanation for variance reduction in spider colonies. We emphasize that variance among individuals need not provide a good estimate of the within—individual resource consumption variance, and it is the latter variance to which risk sensitivity responds.
Riparian Vegetation Recovery Patterns Following Stream Channelization: A Geomorphic Perspective
Authors: Hupp Cliff R.
Published: May 2006

Hundreds of kilometres of West Tennessee streams have been channelized since the turn of the century. After a stream is straightened, dredged, or cleared, basinwide ecologic, hydrologic, and geomorphic processes bring about an integrated, characteristic recovery sequence. The rapid pace of channel responses to channelization provides an opportunity to document and interpret vegetation recovery patterns relative to otherwise long—term, concomitant evolution of river geomorphology. Nearly 150 sites along 15 streams were studied in the Obion, Forked Deer, Hatchie, and Wolf River basins. Channels of these streams, except that of the Hatchie River main stem, have undergone major modifications along all or parts of their courses. This paper presents the eco—geomorphic analyses and interpretation of a large multidisciplinary study, with special reference to their interrelated hydrogeomorphic aspects of channel recovery. Quantitative plant ecological analyses were conducted to infer relative bank stability, to identify indicator recovery species, and to determine patterns of vegetation development through the course of accelerated channel evolution. Binary—discriminant and ordination analyses show that distinctive riparian—species patterns reflect a six—stage model of channel evolution and can be used to infer channel stability and hydrogeomorphic conditions. Woody vegetation initially establishes on low— and mid—bank surfaces at the same location and time that bank accretion begins, and corresponds to the site of initial geomorphic restabilization. The linkage of channel bed aggradation, woody vegetation establishment, and bank accretion all lead to recovery of the channel. Pioneer species are hardy and fast growing, and can tolerate moderate amounts of slope instability and sediment deposition; these species include river birch (Betula nigra), black willow (Salix nigra), boxelder (Acer negundo), and silver maple (Acer saccharinum). High stem densities and root—mass development appear to enhance bank stability. Tree—ring analyses suggest that on average 65 yr may be required for recovery after channelization.
CO_2 and Temperature Effects on Leaf Area Production in Two Annual Plant Species
Authors: Ackerly D. D. , Coleman J. S. , Morse S. R. , Bazzaz F. A.
Published: May 2006

We studied leaf area production in two annual plant species, Abutilon theophrasti and Amaranthus retroflexus, under three day/night temperature regimes (18°/14°, 28°/22°, and 38°/31°C) and two concentrations of carbon dioxide (400 and 700 @mL/L). The production of whole—plant leaf area during the first 30 d of growth was analyzed in terms of the leaf initiation rate, leaf expansion, individual leaf area, and, in Amaranthus, production of branch leaves. Temperature and CO2 influenced leaf area production through effects on the rate of development, determined by the production of nodes on the main stem (the plastochron index), and through shifts in the relationship between whole—plant leaf area and the number of main stem nodes. In Abutilon, leaf initiation rate was highest at 38°, but area of individual leaves was greatest 28°. Total leaf area was greatly reduced at 18° due to slow leaf initiation rates. Elevated CO2 concentration increased leaf initiation rate at 28°, resulting in an increase in whole—plant leaf area. In Amaranthus, leaf initiation rate increased with temperature, and was increased by elevated CO2 at 28°. Individual leaf area was greatest at 28°, and was increased by elevated CO2 at 28° but decreased at 38°. Branch leaf area displayed a similar response to CO2, but was greater at 38°. Overall, wholeplant leaf area was slightly increased at 38° relative to 28°, and elevated CO2 levels resulted in increased leaf area at 28° but decreased leaf area at 38°. The effects on leaf area closely parallel rates of biomass accumulation in the same experiment, suggesting that responses of developmental processes to elevated CO2 and interacting factors may play an important role in mediating effects on plant growth.
Long‐Term Changes in Survival Rates of Lesser Snow Geese
Authors: Francis Charles M. , Richards Miriam H. , Cooke Fred , Rockwell Robert F.
Published: May 2006

Survival rates of adult and juvenile Lesser Snow Geese (Anser caerulescens caerulescens) were estimated based on hunter recoveries from over 80 000 geese banded between 1970 and 1988 at La Perouse Bay, near Churchill, Manitoba. Adult survival rate increased significantly from °78% in 1970 to nearly 88% in 1987. Similar increases in adult survival, although not significant, occurred between 1969 and 1979 at a second colony of Snow Geese at Cape Henrietta Maria in Northern Ontario. These increases coincided with a decline in the proportion of Snow Geese being shot each year on the flyways, suggesting that reduced mortality from hunting may be responsible for the increased survival. In contrast, survival rates of fledglings over their first year decreased significantly from a mean of 60% in 1970 to °30% in 1987, despite the reduction in hunting pressure. The indicates that young geese have been suffering increasing rates of nonhunting mortality, most likely prior to leaving the breeding grounds or on their first autumn migration. Their increased mortality appears to be related to slower growth rates and reduced body size induced by deteriorating feeding conditions on the breeding grounds. This study shows that different processes can influence mortality rates at different stages of the life cycle. The long—term changes indicate that mean values of age—specific survival rates are not adequate to describe the dynamics of this population.
The Maintenance of Taxon Diversity in an Asexual Assemblage: An Experimental Analysis
Authors: Wilson Christopher C. , Hebert Paul D. N.
Published: May 2006

Clonal complexes can be viewed as model systems that permit an assessment of factors organizing a guild whose members show extreme ecological similarity. The present study couples a description of distributional and abundance patterns of unpigmented clones of Daphnia pulex at a low—arctic site with manipulation experiments that test the environmental and biological variables responsible for generating these patterns. The descriptive studies, carried out in 1985 and 1989, revealed 36 clones in 150 habitats with individual ponds containing an average of 1.7 and a maximum of 5 clones. Distributional patterns and relative abundances of the clones were stable over the 4—yr interval. Three clones dominated the assemblage, comprising >75% of the animals. One of these clones dominated saline habitats while the other two displayed a checkerboard distribution in low—salinity ponds. The latter distributional pattern appeared linked to the presence of a predatory copepod, Hesperodiaptomus arcticus, in some ponds. Manipulation experiments showed that abiotic variables (e.g., salinity) played a role in excluding clones from some habitats, but that many habitats were invasible by a broader spectrum of clones than was ever detected in them. Experiments showed that clones were ordinarily superior in their home environments, supporting the role of competition in limiting local richness. The experimental results were not totally congruent with the clones' natural distributions, as one clone became dominant in all of the low—salinity manipulation ponds. This outcome was reversed by introducing H. arcticus into ponds. The results confirm that clonal diversity is maintained by physical and biological variation in the system, and demonstrate the substantial taxon diversity can be sustained in the absence of marked ecological divergence.
Direct and Indirect Effects of Omnivory in a Littoral Lake Community
Authors: Diehl Sebastian
Published: May 2006

In spite of the ubiquity of omnivory in nature, its consequences for population dynamics have received little attention from theoretical and experimental ecologists. Having three direct consumer—resource links, three indirect numerical links, and a potential for indirect effects mediated by size structure and/or behavioral flexibility, a three—species omnivory system may exhibit complex population dynamics. In 2 X 3 m field enclosures in the littoral zone of a lake, I manipulated the dominating native benthivorous fish (>2—yr—old perch, Perca fluviatilis, omnivorous top consumer) and the dominating native benthic invertebrate predators (Sialis lutaria, Megaloptera, and odonates, intermediate consumers) in three gradients: increasing densities of perch in the near absence of Sialis and odonates, and increasing densities of Sialis and odonates in either the presence or absence of perch. The densities of their common prey (mainly chironomids) were left unmanipulated. Macroinvertebrate abundance, biomass, and size structure, as well as gut contents of perch and Sialis were sampled monthly over a 3—mo summer period. In the absence of perch, the experimental gradient of Sialis densities remained unchanged over time. In the presence of perch, Sialis decreased by about half at high initial densities, but remained unchanged at low initial densities. Perch also had a negative effect on odonates. In the near absence of Sialis and odonates, perch had a strong, negative effect on chironomids. Compared to enclosures without perch, chironomid abundance was strongly reduced at the lowest perch density, but leveled off with further increases in perch density. Sialis and odonates did not affect chironomid abundance when perch was absent. In contrast, chironomid abundance was positively affected by Sialis and odonates when perch were present. The overall effect of perch on chironomid abundance in the presence of Sialis and odonates was negative. The combined predatory and competitive effects of perch on Sialis and odonates raise the issue how Sialis and odonates coexist with perch. The lack of effect of perch on Sialis and chironomids at low densities of these prey suggests that prey refuges contribute significantly to their persistence in natural systems. The indirect positive effect of Sialis and odonates on chironomids indicates density dependence in per capita interaction coefficients and is most likely to be explained by a behavioral response of perch or chironomids to Sialis and odonates. I suggest that the theoretical and empirical investigation of model systems (such as three—component omnivory systems), which are sufficiently simple to be analytically and experimentally tractable, but still display a richness of common indirect effects, will contribute to our understanding of the dynamics of more complex food webs. Their study will benefit from experimental manipulations of more than one population at more than two densities.
Rodent Dispersal of Vesicular‐Arbuscular Mycorrhizal Fungi in Amazonian Peru
Authors: Janos David P. , Sahley Catherine T. , Emmons Louise H.
Published: May 2006

We quantitatively assessed rodent dispersal of vesicular—arbuscular mycorrhizal fungi in a lowland tropical rain forest. We examined fecal pellets from seven rodent species of the genera Proechimys, Oryzomys, and Mesomys trapped each month from August 1983 through July 1984 at the Cocha Cash field station in Manu National Park, Peru. We found sporocarps of Sclerocystis coremioides and spores of four Glomus species in 69.3% of fecal samples, with 37.5% of samples containing 2—4 fungus species. There were median numbers of 8.6 S. coremioides sporocarps and 712 Glomus spores/0.1 g feces. S. coremioides and all Glomus species comprised up to 4.1 and 5.4% of fecal mass, respectively. We did not detect differences in fungus consumption by rodent species or sex, but did find significant differences between rodent genera. More than 60% of Proechimys samples contained glomalean fungi in contrast to °37% of Oryzomys samples. Occurrence of the two fungus genera in feces differed seasonally but was generally congruent between Proechimys and Oryzomys. Glomus presence in feces is significantly positively associated with the dry season. Both rodent genera together annually pass 2.96 x 105 S. coremioides sporocarps and 7.30 x 107 Glomus spores/ha at Cocha Cashu. We concluded that Proechimys and Oryzomys are likely to be important agents of vesicular—arbuscular mycorrhizal fungus spore dispersal.
Regional Patterns of Normalized Difference Vegetation Index in North American Shrublands and Grasslands
Authors: Paruelo Jose M. , Lauenroth William K.
Published: May 2006

We characterized the seasonal pattern of Normalized Difference Vegetation Index (NDVI) of 49 grassland and shrubland sites corresponding to areas with low human impact. Each site was characterized as a vector of 21 elements, where each element corresponded to the average NDVI for one date. We used the integral of the NDVI as an estimate of the total production, and the difference between the maximum and minimum NDVI over the year as a measure of the seasonality. Based on a Principal Component Analysis, the integral of the NDVI accounted for 63% of the variability of the SITE x DATE matrix. The difference between the maximum and minimum NDVI throughout the year accounted for 19%. Mean annual precipitation explained 67% of the variability of the integral of the NDIV curves (P < 0.01). The slope of the relationship between precipitation and integrated NDVI was significantly steeper for grasslands than for shrublands. The difference between the extreme values of NDVI increased with mean annual precipitation and the thermal amplitude, and decreased with mean annual temperature (P < 0.01).
Causes and Ecosystem Consequences of Multiple Pathways of Primary Succession at Glacier Bay, Alaska
Authors: Fastie Christopher L.
Published: May 2006

The classic account of primary succession inferred from a 220—yr glacial retreat chronosequence at Glacier Bay National Park, Alaska was compared to reconstructions of stand development based on tree—ring records from 850 trees at 10 sites of different age. The three oldest sites (deglaciated prior to 1840) differ from all younger sites in the early recruitment of Sitka spruce (Picea sitchensis), the presence of western hemlock (Tsuga heterophylla), and the inferred importance of early shrub thickets. The nitrogen—fixing shrub Sitka alder (Alnus sinuata) has been an important and long—lived species only at sites deglaciated since 1840. Black cottonwood (Populus trichocarpa) has been an overstory dominant only at sites deglaciated since 1900. These single—species additions or replacements distinguish three pathways of vegetation compositional change which are segregated spatially and temporally. The communities of different age at Glacier Bay do not constitute a single chronosequence and should not be used uncritically to infer long—term successional trends. Among—site differences in texture and lithology of soil parent material cannot account for the multiple pathways. However, distance from each study site to the closest seed source of Sitka spruce at the time of deglaciation explains up to 58% of the among—site variance in early spruce recruitment. Multiple pathways of compositional change at Glacier Bay appear to be a function of landscape context, which, in conjuction with species life history traits (dispersal capability and generation time), affects seed rain to newly deglaciated surfaces and thereby alters the arrival sequence of species. Differences among the pathways probably include long—term differences in ecosystem function resulting from substantial accumulation of nitrogen at sites where nitrogen—fixing shrubs are important.
Succession and Fire Season In Experimental Prairie Plantings
Authors: Howe Henry F.
Published: May 2006

Fire season influenced the cover of species and flowering guilds of plants in replicated grass and forb plantings of tallgrass prairie species in Wisconsin. Over two burn cycles at 3—yr intervals, cover increased in the rhizomatous perennials Andropogon gerardii, Aster simplex and Solidago altissima in plots burned in spring, or summer, or left unburned, but cover showed dramatic positive or negative responses to spring or summer burns in Agropyron repens, Erigeron annuus, Panicum virgatum, Phalaris arundinacea, and Rudbeckia hirta. Aggregate response of flowering guilds was stronger. By the last (1993) census, the phenological guild flowering before mid—July (summed cover values for nine species) accounted for 4% cover in unburned plots and <2% cover in plots burned in early spring, but 32% cover in plots burned in midsummer. The late—flowering guild (summed values for 21 species) accounted for 93% cover in unburned plots and 97% cover in plots burned in early spring, but 66% cover in plots burned in midsummer. Remaining space was taken by a mid—season guild (11 species), only one of which (E. annuus) was common enough for individual statistical analysis. These results have implications for succession of prairie vegetation under different seasonal fire regimes. Over 8 yr, summer fires that simulated the timing of lightning fires retarded progression to dominance of large, late—flowering C4 grasses, allowing early—flowering species eliminated in other treatments to persist or even prosper. Primaeval lightning fires in midsummer may have produced quite different communities than anthropogenic fires, which are rarely set during the middle of the growing season.
Statistical Sirens: The Allure of Nonparametrics
Authors: Johnson Douglas H.
Published: May 2006

Although nonparametric statistical methods have a role to play in the analysis of data, often their virtues are overstated and their deficiencies overlooked. A recent Special Feature in Ecology advocated nonparametric methods because of an erroneously stated advantage that they require no assumptions regarding the distribution underlying the observations. The present paper points out some often—ignored feature of nonparametric tests comparing two means, and advocates parameter estimation as a preferred alternative to hypothesis testing in many situations.
The Plant Stress Paradox: Effects on Pinyon Sawfly Sex Ratios and Fecundity
Authors: Mopper Susan , Whitham Thomas G.
Published: May 2006

Plant stress theory predicts and many observational studies support the notion that abiotic stress can increase plant quality and benefit the performance of phytophagous insects. Paradoxically, numerous experiments contradict this assumption. To test the theory, we conducted experiments to determine the influence of pinyon pine (Pinus edulis) stress on pinyon sawfly (Neodiprion edulicolis) performance. In a natural pinyon population, we transferred first—instar sawfly larvae to 41 trees divided into four groups of 11; each group received either water, fertilizer, water and fertilizer, or no supplements. Upon completion of their penultimate larval instar, we collected sawflies and determined their fecundity and sex ratios. Although sex ratios are rarely used to estimate insect performance in natural systems, we believe that their sensitivity to changes in plant quality may play an overlooked role in insect—plant interactions. Most of our results refuted theoretical predictions, because sawflies performed better on trees receiving both water and fertilizer. However, in some cases, sawflies did benefit from pinyon stress. We contend that "simultaneous plant stress," such as low precipitation while the insect is feeding or ovipositing, can reduce performance. In contrast, "sustained plant stress," such as chronically poor soil conditions or prolonged drought, can benefit insect performance. These different effects on herbivores may parsimoniously explain some discrepancies between theoretical and empirical plant stress studies.
Nutrient Enrichment and Grazer Effects on Phytoplankton in Lakes
Authors: Sarnelle Orlando
Published: May 2006

I derived predictions about how the magnitude of Daphnia effects on total phytoplankton biomass should vary across a gradient of enrichment (expressed as algal carrying capacity) using two, simple predator—prey models. These predictions were then compared with data from a survey of field experiments in temperate lakes. Algal responses to Daphnia manipulation were quantified as an Algal Response Factor (ARF), defined as total algal biomass in the low—Daphnia treatment divided by total algal biomass in the high—Daphnia treatment. Total phosphorus concentration (TP) was used as an index of algal carrying capacity, ranging from 10 to 460 mg/L over the 22 experiments surveyed. The Algal Response Factor ranged from 1 to 40 and was a positive, linear function of TP: ARF = ‐0.14 + 0.08(TP), r2 = 0.81; log ARF = ‐0.81 + 0.83(log TP), r2 = 0.75. Thus, algal carrying capacity, as quantified by TP, explains much of the variation in Daphnia effects on total algal biomass across lakes. The survey results supported the prediction of the simpler model, a two—species, Lotka—Volterra model of pure exploitation. Incorporating the additional complexity of inedible algae into this model did little to improve its predictive power. The conclusion that inedible algae are not of major importance to the prediction of Daphnia effects across an enrichment gradient was supported by a survey of Daphnia effects on the proportionate biomass of inedible algae. There was no evidence that Daphnia grazing typically favors dominance by inedible algae at equilibrium in highly enriched lakes. My results suggest that the well—established positive relationship between TP and equilibrium algal biomass in lakes is more likely to be a consequence of increases in Daphnia's death rate with enrichment, rather than decreases in Daphnia's feeding and assimilation rates. When zooplanktivorous fish are rare, Daphnia should be able to prevent phytoplankton biomass from responding to nutrient addition at equilibrium. As a consequence, the success of biomanipulation in eutrophic lakes should critically depend upon the effectiveness of strategies aimed at reducing zooplanktivory.
Life History Traits of Open‐ vs. Cavity‐Nesting Birds
Authors: Martin Thomas E. , Li Pingjun
Published: May 2006

We re—examined the longstanding dogma that cavity—nesting birds have larger clutch sizes than open—nesting species because of lower nest predation rates, which allow longer developmental periods. We provide data on nesting success of open—nesting species plus excavator and nonexcavator species (cavity—nesters that do vs. do not create their own cavities) of birds that coexist in the same habitat and use natural nest sites; studies were conducted in central Arizona in high—elevation forest drainages from 1987 to 1989. In comparisons among species, nest success increased in the order open—nesting < nonexcavator ° excavator species. Length of the nestling period increased, whereas number of broods decreased, with increased nest success across these nest types. Contrary to conventional expectation, clutch size did not increase directly with increased nest success and length of the nestling period, and these patterns were not simply a result of phylogenetic effects. A literature review of data on adult survival in land birds indicated that nonexcavators have significantly lower adult survival than the other two nest types. The lower adult survival of nonexcavators was associated with greater annual productivity than for the other two nest types. Annual productivity appeared to be associated with nest site attributes, whereas number of broad attempts was related to nest failure rate. Clutch size was a byproduct of annual productivity and number of brood attempts. Thus, the long—standing paradigm relating clutch size directly to nesting failure and developmental period was not supported.
Salt Marsh Plant Zonation: The Relative Importance of Competition and Physical Factors
Authors: Pennings Steven C. , Callaway Ragan M.
Published: May 2006

In Carpinteria Salt Marsh, Salicornia virginica (pickleweed) grows at lower marsh elevations than does Arthrocnemum subterminalis (Parish's glasswort). Standing biomass of both species was greatest immediately adjacent to their abrupt border, suggesting that conditions for plant growth were best here. We utilized field experiments, in which growth rates of naturally occurring and transplanted individuals of both species were measured in four marsh zones, to investigate the role of edaphic factors and competition in maintaining this zonation pattern. The frequency of flooding, and hence soil waterlogging, was greatest at lower marsh elevations, whereas salinity was highest at higher marsh elevations. Consequently, it was not clear, a priori, which part of the marsh had the most severe physical conditions. In our field experiments, both Salicornia and Arthrocnemum grew better in the two middle marsh zones (high Salicornia zone and Arthrocnemum zone) than in either the low marsh (low Salicornia zone), where flooding was frequent and soils were waterlogged, or the high marsh (transition zone), where soil salinity was extremely high during much of the year and plant water potentials very low. However, Salicornia appeared better able to tolerate flooding, and so persisted in the low Salicornia zone, whereas Arthrocnemum appeared better able to tolerate high salinities, and so persisted in the transition zone. Interspecific competition was most important in the relatively benign middle marsh zones, where each species excluded the other from a portion of this prime habitat. In this marsh, flooding, soil salinity, and competition all interacted to determine plant zonation patterns, but the relative importance of these factors varied at different elevations.
Selectivity and Growth of the Generalist Herbivore Dolabella Auricularia Feeding Upon Complementary Resources
Authors: Pennings Steven C. , Nadeau Masatomo T. , Paul Valerie J.
Published: May 2006

An assumption of most optimal diet theory is that different resources are substitutable, that is, that they are identical in all relevant aspects and so can be ranked in value using a single currency, such as energy. However, this assumption is probably not valid in many cases. The sea hare Dolabella auricularia (Gastropoda: Opisthobranchia: Anaspidea) grew far better upon an ad libitum mixture of four species of algae than upon any of the algal species alone, suggesting that algae were complementary resources. When offered three pairs of algae in 4:1 and 1:9 ratios, Dolabella changed its foraging behavior so that the rarer alga was preferred. Consequently, the mixtures of algae consumed were more similar between treatments (ratios) than the mixtures of algae offered. Replicate Dolabella were maintained on six single algal diets for 6—7 d and then offered the maintenance alga and another alga at equal abundances. Preference for a species of algae was always higher when it was not the maintenance diet compared to when it was. These results suggest that Dolabella actively maintains a mixed diet because it is nutritionally superior to any single diet. An alternative hypothesis (the nonadditive toxin hypothesis) suggests that herbivores might consume a mixed diet because of mixture of plant secondary metabolites is less toxic than any one metabolite alone; consequently, herbivores can consume more total biomass of a mixed than of a single diet. However, sea hares fed single— and mixed—species diets did not consume more of mixed diets than of single—species diets, and animals fed a mixed diet grew better than those fed single diets even when the biomass of algae offered was held constant. If resources are complementary, as suggested here, several important implications follow: (1) consumers will not rank resources in any absolute way because the value of each will depend upon what else is consumed, (2) consumer specialization will be opposed by the benefits of a mixed diet, and (3) consumer foraging will tend to destabilize the resource community, because rarer resources will be disproportionately consumed.
The Ecology of Sediment Bacteria in Lakes and Comparisons with Other Aquatic Ecosystems
Authors: Schallenberg Marc , Kalff Jacob
Published: May 2006

We have analyzed our own data set as well as a number of previously published ones to examine factors related to the numbers and biomass of sediment bacteria in a variety of ecosystems. We have chosen to express sediment variables either on an areal or per unit sediment volume basis to avoid bias due to the more common standardization to sediment dry mass (also known as gram dry weight). Cross—system comparisons show that the amount of organic matter present and the sediment water content are important to sediment bacterial abundance. The C:N ratio of the sediment organic matter, possibly reflecting organic matter quality, was found to explain variation in bacterial numbers in addition to that explained by organic content. Over a broad range, sediment water content is related to sediment bacterial numbers in a unimodal fashion indicating dilution of sediments by water at high (>80%) water content sites and dilution by inorganic particles at low (<50%) water content sites. In lakes, the most important factors explaining sediment bacterial biomass were sediment water content and hydraulic flushing rate, a finding supported by analysis of an independent data set of English lakes. Planktonic chlorophyll a and planktonic bacterial numbers (both reflecting autochthonous organic matter levels) were not significantly related to sediment bacterial biomass. We found that sediments overlain by epilimnetic water had significantly higher bacterial biomass than those overlain by hypolimnetic water. A strong positive relationship between macrobenthos and sediment bacterial biomass was found for lakes in Ontario and Quebec. Calculations of annual bacterial production and P:B ratios necessary to sustain the macrobenthic population, based on the observed bacterial and macrobenthic biomass and temperature at our sites, indicate that the bacterial biomass present is more than sufficient to maintain the macrobenthos. In addition, based on the observed ratios of bacterial to detrital carbon at our sites and on a range of published assimilation efficiencies for macrobenthos grazing on these carbon sources, we calculated that the mean percentage of carbon of bacterial origin assimilatable by macrobenthos at our sites is 53% with a maximum of 79% and a minimum of 12%. Our results suggest a tighter trophic linkage between sediment bacteria and macrobenthos in our lakes than in most marine habitats, as indicated by published studies. From these analyses, the general picture emerges of a decoupling between pelagic and benthic bacterial processes in lakes, the pelagic ones being driven mainly by autochthonous production and the benthic ones by allochthonous inputs.
Diet Heterogeneity: Implications for Growth of a Generalist Herbivore, the Gypsy Moth
Authors: Stockhoff Brian A.
Published: May 2006

Within—plant variation in food nutritional quality is a fundamental feature in the foraging environment of herbivores, but little is known as to how such small—scale variability affects individuals or structures populations of herbivores. The selective pressure favoring choosy herbivores may depend on the relative performance of random foragers, which will experience chronic changes in food quality, vs. non—random foragers. To determine the effect of involuntary variation in nitrogen intake on larvae of a polyphagous insect, gypsy moth (Lymantria dispar (L.)), I reared individuals on one of three diet treatments: (1) Constant nitrogen: larvae received 3.0% N diet continuously; (2) Low variance: larvae were switched each day in an alternating sequence of 2.25% and 3.75% N diet; and (3) High variance: as in Low variance, except larvae were alternated between 1.25% and 4.75% N diet. Despite equal relative consumption rates (RCR) and nitrogen consumption rates (RNCR) across all treatments, larvae experiencing variation in the diet suffered reduced pupal mass and extended development time. Evidently growth performance cannot be predicted based solely on mean diet nitrogen concentration, which was equal among all treatments (3.0% N). Further, growth on a mixture of foods cannot be predicted based on known responses to the individual foods. Reduced growth associated with the chronic variation likely was caused, in part, by nonlinearity in the relationship between nitrogen and food utilization efficiency, and by disruption of the compensatory feeding response. These results suggest that intraplant variation in food quality has the potential to exert strong selective pressure for regulation of variation in the total diet.
Effects of Habitat Size on Tadpole Populations
Authors: Pearman Peter B.
Published: May 2006

While density dependence has been well documented in many animal species, the effects of absolute size of habitat have not been thoroughly explored. This study investigates the effects of habitat size by examining the response of the distribution of several fitness correlates. I used populations of tadpoles of Bufo americanus and Rana clamitans in artificial ponds as a model system. The natural histories of these species suggested that they might respond differently to variation in habitat size, which was defined in terms of pond surface area and depth. These dimensions were factors in two completely crossed factorial experiments, one for each species. Ponds were of three areas: 529 cm2, 2116 cm2, and 8464 cm2 and three depths: 10 cm, 20 cm, and 40 cm. Tadpole populations were established at a single density (number per unit volume). The use of artificial ponds controlled variation in nutrient content, shape, and vegetation structure that may be confounded with pond size in nature. Increases in pond area resulted in Bufo populations that had lower survival, growth rates, and mass at metamorphosis. Populations in ponds of large area showed greater within—population variation in larval period, although the mean of this parameter was only weakly affected. Size distributions of individuals showed positive skewness in small—area ponds. Increasing depth led to lower survival but greater mass at metamorphosis. In experiments with Rana, increased area and depth both led to lower survival but greater growth by survivors. There were statistically significant interactions between area and depth in their effects on early Bufo growth and on Rana survival. The fitness of Bufo showed a significant positive relationship with increasing surface—area—to—volume ratio while Rana showed a mixed response. The results suggest that competition among Bufo tadpoles intensifies with habitat size under conditions of constant initial density by volume because of a decreasing ratio of edge to interior habitat. The form of competition appears to shift towards stronger interference competition in smaller habitats. Individuals may compensate for early, negative effects of habitat size. The variability of the several fitness correlates, while different among populations, showed no consistent pattern with population size. Theory based on assumptions of equal levels of variation in fitness in large and small populations may not be widely applicable to populations in nature. Evidence of changes in competition and fitness distributions should be sought in other systems characterized by variation in habitat size.