bluuebunny/crossref_metadata_2025 huggingface dataset:
Natal Philopatry, Competition for Resources, and Inbreeding Avoidance in Townsend's Voles (Microtus Townsendii)
Authors: Lambin Xavier
Published: May 2006

Individuals may leave their birthsite before reproducing or they may postpone reproduction because they are at risk of engaging in inbred matings with their close relatives or because they are exposed to severe competition for resources or for mates. I studied whether avoidance of inbreeding and intrasexual competition determine the seasonal changes in the rate of philopatry and the fate of individual Townsend's voles (Microtus townsendii). By marking most nestlings before weaning, I knew which recruits were related to one another and to which adult female. There was a pronounced sexual bias in the tendency to remain close to the birth site: 9% of all juvenile males 33% of all juvenile females entered the breeding population on their natal grid. Males settled farther from their birth site than females did. The proportion of males and females reproducing on their natal grid changed markedly over the breeding season. More males and females reproduced if they were born at the beginning of the breeding season or when breeding activity was depressed by botfly infestation. Seasonal changes in female philopatry were consistent with the hypothesis that females disperse in response to competition for resources with breeding females. Females born at high adult female density were less likely to be recruited than those born at other times. They also dispersed farther at low than at high density, probably in response to habitat saturation. Avoidance of extreme inbreeding influenced the dispersal tendency of maturing males. Males were less likely to join the breeding population if their mother or a littermate sister was present in their home range when they matured than if they had no female close relatives in their home range. Males with their mother alive at sexual maturation settled farther than males whose mother had disappeared by that time. I concluded that inbreeding avoidance influenced natal philopatry in males and that competition for resources influenced natal philopatry in females.
Influences of Trees on Savanna Productivity: Tests of Shade, Nutrients, and Tree‐Grass Competition
Authors: Belsky A. Joy
Published: May 2006

To determine why herbaceous productivity in tropical and subtropical savannas is often significantly higher under crowns of isolated trees than in adjacent grass—lands, experimental plots were established in three concentric zones, crown, tree—root, and grassland, surrounding isolated trees of Acacia tortilis in low—rainfall and high—rainfall savannas in Tsavo National Park, Kenya. Plots were fertilized (to determine the importance of nutrient enrichment by trees), shaded (to determine the importance of crown shade), fertilized and shaded (to identify fertilizer x shade interactions), or trenched (tree roots entering plots were severed to determine the importance of belowground competition between overstory trees and understory herbaceous plants). In addition, vertical root distributions of trees and herbaceous species were determined, and root systems of A. tortilis saplings were excavated. At both sites fertilization significantly increased herbaceous productivity in tree—root and grassland zones, but not in canopy zones; artificial shade had no effect on productivity at the low—rainfall site but increased productivity in the tree—root zone at the high—rainfall site; and severing tree roots had no effect on herbaceous productivity at the low—rainfall site, but increased productivity in the crown and tree—root zones at the high—rainfall site. Roots of herbaceous and woody species co—occurred within the same soil horizons, but tree roots extended farther into grasslands at the low—rainfall site than at the high—rainfall site. These studies suggest that savanna trees completed more intensely with understory plants at wetters sites, where their roots terminated in or near crown zones, than at drier sites, where their roots extended farther into open grassland. Nutrients added by trees to crown zones in the form of tree litter and animal droppings increased understory productivity by fertilizing nutrient—limited soils. Shade contributed more to regrowth after severe defoliation than to growth under more normal conditions.
Within‐Site Variability in Surveys of Wildlife Populations
Authors: Link William A. , Barker Ricard J. , Sauer John R. , Droege Sam
Published: May 2006

Most large—scale surveys of animal populations are based on counts of individuals observed during a sampling period, which are used as indexes to the population. The variability in these indexes not only reflects variability in population sizes among sites but also variability due to the inexactness of the counts. Repeated counts at survey sites can be used to document this additional source of variability and, in some applications, to mitigate its effects. We present models for evaluating the proportion of total variability in counts that is attributable to this within—site variability and apply them in the analysis of data from repeated counts on routes from the North American Breeding Bird Survey. We analyzed data on 98 species, obtaining estimates of these percentages, which ranged from 3.5 to 100% with a mean of 36.25%. For at least 14 of the species, more than half of the variation in counts was attributable to within—site sources. Counts for species with lower average counts had a higher percentage of within—site variability. We discuss the relative cost efficiency of replicating sites or initiating new sites for several objectives, concluding that it is frequently better to initiate new sites than to attempt to replicate existing sites.
Dynamic Models of Mass‐Dependent Predation, Risk‐Sensitive Foraging, and Premigratory Fattening in Birds
Authors: Bednekoff Peter A. , Houston Alasdair I.
Published: May 2006

The effect of mass—dependent predation on risk—sensitive foraging choices of small birds in two ecological contexts was examined using stochastic dynamic models. In the first context, nonmigratory foraging, predation risk dependent on the bird's body mass leads to risk—averse behavior in situations were risk—neutral behavior would otherwise be expected and generally reduces the amount of foraging and the level of reserves carried by foragers. In the second context, premigratory foraging, mass—dependent predation can lead to risk proneness during the buildup of reserves for migration. Risk proneness is favored if birds can reduce mass and hence reduce predation after migration, thereby giving them an advantage to rapid departure. Other advantages of early departure can also lead to premigratory risk proneness but require more specific ecological assumptions. The literature on risk sensitivity in amount of food reward suggests a connection between migration and risk proneness on negative energy budgets but more tests are needed before any conclusions can be drawn.
Perturbation Experiments in Community Ecology: Theory and Practice
Authors: Bender Edward A. , Case Ted J. , Gilpin Michael E.
Published: May 2006

We analyze perturbation experiments performed on real and ealized ecological communities. A community may be considered as a black box in the sense that the individual species grow and interact in complicated ways that are difficult to discern. Yet, by observing the response (output) of the system to natural or human—induced disturbances (inputs), information can be gained regarding the character and strengths of species interactions. We decline a perturbation as selective alteration of the density of one or more members of the community, and we distinguish two quite different kinds of perturbations. A PULSE perturbation is a relatively instantaneous alteration of species numbers, after which the system is studied as it "relaxes" back to its previous equilibrium state. A PRESS perturbation is a sustained alteration of species densities (often a complete elimination of particular species): it is maintained until the unperturbed species reach a new equilibrium. The measure of interest in PRESS perturbation is the net change in densities of the unperturbed species. There is a very important difference between these two approaches: PULSE experiments yield information only on direct interactions (e.g. terms in the interaction matrix), while PRESS experiments yield information on direct interactions mixed together with the indirect effects mediated through other species in the community. We develop mathematical techniques that yield measures of ecological interaction between species from both types of experimental designs. Particular caution must be exercised in interpreting results form PRESS experiments, particularly when some species are lumped into functional categories and others are neglected altogether in the experimental design. We also suggest mathematical methods to deal with temporal and random variation during experiments. Finally, we critically review techniques that rely on natural variation in numbers to estimate species interaction coefficients. The problems with such studies are formidable.
Habitat Segregation in Ground Crickets: The Role of Interspecific Competition and Habitat Selection
Authors: Howard Daniel J. , Harrison Richard G.
Published: May 2006

Habitat segregation of closely related species has been well documented among insects, but the factors responsible are poorly understood. We have examined the influence of habitat selection and interspecific competition on the habitat associations of the closely related ground crickets Allonemobius allardi and A. fasciatus. Allonemobius fasciatus is found in wet pastures and low—lying grassy areas; A. allardi occurs primarily in dry pastures and grasslands. In a large—scale field experiment, crickets were introduced into the centers of runways that extended from wet pasture at the edge of a pond to dry pasture on a south—facing slope. Each runway had been sprayed with insecticide, so that no Allonemobius were present prior to the introduction. We stocked the runways with either A. fasciatus alone, A. allardi alone, or a mixture of the two species. Twenty—four days after release, we collected from each of 15 segments within all runways. Analysis of distribution patterns in mixed— and single—species runways showed that neither species' distribution was influenced by the presence of the other. Thus there is no evidence that current interspecific competition is important in determining distributions. In single—species runways, A. fasciatus moved into wetter areas, whereas A. allardi displayed no clear tendency to move either up or down the runways. It appears that habitat selection by A. fiasciatus can account for its association with wet pasture. We discuss several scenarios for the evolution of habitat selection and its relationship to the processes of adaptation and speciation.
Reactions of Heracleum Lanatum to Floral Herbivory by Depressaria Pastinacella
Authors: Hendrix Stephen D.
Published: May 2006

Heracelum lanatum, an andromonoecious perennial plant, reacted to herbivorous destruction of its primary (first) umbel by Depressaria pastinacella by an increase in the proportion of hermaphroditic flowers in late—developing umbels and a subsequent increase in seed set on these umbels. However, these reactions were not sufficient to offset completely the damage sustained; control plants produced 40% more seeds than damaged plants and seed biomass was 53% greater. Plant size (measured by basal stem diameter of the flowering stalk) was significantly correlated with total flower production, seed number, and seed biomass in both control and damaged plants. Plant size was also significantly correlated with the number of hermaphroditic flowers in the primary umbel of control plants and secondary umbels of damaged plants. Hermaphroditic flower production by the secondary umbels of control plants was negatively correlated with flower number in these umbels. Pastinaca sativa, another andromonoecious host plant of D. pastinacella, shows the same type of reactions as H. lanatum, but in P. sativa, compensation, at least in terms of seed number, is complete. The difference between the two species in replacement of lost offspring is interpreted in terms of frequency of reproduction. The interoparous h. lanatum may maximize reproduction in later seasons by increasing vegetative growth rather than by replacing all lost offspring. In contrast, the semelparous P. sativa maximizes replacement of destroyed offspring during its single reproductive bout.
Niche Partitioning Between Two Phosphoglucoisomerase Genotypes in Amaranthus Retroflexus
Authors: Zangerl A. R. , Bazzaz F. A.
Published: May 2006

The problem of population variation and its relationship to ecological variability is one of continuing interest to ecologists. The problem is here addressed with experiments on the early successional annual, Amaranthus retroflexus. The germination responses of homozygous genotypes were compared over a range of oxygen concentrations. These genotypes were also compared with respect to their distributions along a soil moisture gradient. The germination behavior of the two PGI genotypes in response to oxygen availability was found to differ substantially, particularly at the lower oxygen concentrations. Homozygous fast (FF) genotypes were inhibited less by low oxygen availability relative to the homozygous slow (SS) genotypes. Similarly the performance of the two genotypes in mixture along the soil moisture gradient differed such that homozygous fast (FF) genotypes performed best at high soil moistures, while the homozygous slow (SS) genotypes performed best at moderate soil moistures. A field survey revealed that both alleles frequently co—occur in natural populations. Heterozygous individuals are, however, rare in both field and glasshouse populations, indicating a high level of inbreeding in the species. Inbreeding will tend to maintain the distinctiveness of genotypes within Amaranthus populations, perhaps leading to the large between—genotype component of niche breadth that was observed on the soil moisture gradient.
Overwinter Tissue Degrowth in Natural Populations of Freshwater Pulmonate Snails (Helisoma Trivolvis and Lymnaea Palustris)
Authors: Russell-Hunter W. D. , Browne Robert A. , Aldridge David W.
Published: May 2006

When populations of aquatic invertebrates including molluscs) are subjected to seasonal periods of starvation, many individuals survive by major shifts in metabolic budgeting. These shifts can result in degrowth, defined as decrease in individual biomass, in some cases involving loss of structural proteins. Snails after degrowth can be recognized in natural populations because their "oversize" shells provide evidence of former larger tissue biomass. Fall and spring samples were taken from four selected populations of Helisoma trivolvis and four of Lymnaea palustris in upstate New York. Both are freshwater pulmonate snails, with a widespread and relatively euryoecic distribution. Shell : tissue ratios were calculated, and overwinter changes are significant (P <.001) for six of the populations. Overwinter tissue degrowth is demonstrated in three populations of Helisoma and one of Lymnaea, with overwinter growth occurring in one population of Hlisoma and two of Lymnaea. The actual dry—mass values of tissues found in spring individuals can be compared with values predicted from their individual shell masses in spring (using regressions for the same populations in fall). Surviving individuals of Helisoma in the three populations with degrowth can have tissues at 19—116% of the predicted mass (with 56 out of 59 below 100%) and, in the population of Lymnaea showing degrowth individual values ranging from 58 to 169% (with 19 out of 22 below 100%). Mean values for the three Helisoma populations are 75.3, 71.7, and 58.7% and for the degrowth Lymnaea populations, 85.3%. This field demonstration of degrowth is discussed in relation to other laboratory studies of starvation in molluscs and in relation to actuarial bioenergetics of natural populations. Capacity for true degrowth can be claimed to confer adaptive advantages in certain environments and is of considerable ecological and evolutionary significance.
Induced Resistance in Ipomoea Purpurea
Authors: Rausher Mark D. , Iwao Keisuke , Simms Ellen L. , Ohsaki Naota , Hall David
Published: May 2006

A series of experiments indicate that induced changes occur in the foliage of the annual morning glory Ipomopea purpurea when subjected to prior damage. These changes reduce the growth rate, consumption rate, growth efficiency of larvae of the generalist lepidopteran Spodoptera eridania. They also alter the pattern of damage caused by specialist flea beetles, Chaetocnema confinis, and by generalist insect herbivores in the field. No effects of induction were detected on specialist tortoise beetles, Deloyala guttata and Metriona bicolor. Although total damage by both flea beetles and generalist insects did not differ between induced and noninduced plants, this result does not necessarily imply that the effects on these insects were nonadaptive. The observed effects of induced response are consistent with both "diffuse" and "pairwise" views of the selection imposed by herbivores on plant resistance. Finally, the existence of induced responses suggests that previous conclusions by Simms and Rausher that resistance to flea beetles and generalist insects is not costly in I. purpurea may be premature.
Competitive Effects of Grassland Annuals on Soil Water and Blue Oak (Quercus Douglasii) Seedlings
Authors: Gordon Doria R. , Rice Kevin J.
Published: May 2006

Four California annual grassland species were used to examine the hypothesis that different plant species have equivalent competitive effects. We investigated the effects of the annuals on soil water availability and the growth responses of blue oak (Quercus douglasii Hook & Arn.) to neighbor—induced water depletion. Neighborhoods of annuals were composed of species from California annual grassland with differing phenology and morphology that were hypothesized to show non—equivalent competitive effects on both a per—individual— and a per—unit—tissue basis. Three introduced species of winter annuals were sown at each of three densities (10, 30, or 100 seeds/dm2) around a single oak acorn. The grass Bromus diandrus Roth. and the forb Erodium botrys Cav. flower in early spring, while the grass B. mollis L. flowers slightly later. A native summer annual forb, Hemizonia luzulaefolia DC., was sown at only the intermediate density. Plants were grown outdoors in 15 cm diameter X 1 m deep tubes filled with soil from an oak woodland site. Identity and density of annual species had independent and interactive effects on the water resource level. Soil water potential was negatively density—dependent, remaining highest when oaks were grown without competitors. At the intermediate density soil water potentials decreased in the following order: B. mollis (slowest), B. diandrus, E. botrys, and H. luzulaefolia (most rapid). This order followed the pattern of root length developed in the neighborhoods, which was species— and density—dependent. The relationship between sowing density and neighborhood dry mass also differed among the annual species. Correlations between root length and soil water potential were stronger for the forbs than for the grasses. A given root length of the summer annual, H. Luzulaefolia, depleted soil water to a greater extent than did the same root length of any species. Oak seedling growth, gas exchange rates, and survivorship were negatively related to the rate and extent of soil water depletion. Additionally, the relationship between annual root length and oak seedling height or root length differed by neighborhood species. Thus, neighborhood species were non—equivalent in competitive effect on both a per—individual and a per—unit—root—length basis. Phenologically different species may represent different functional groups of competitors.
Temporal Overlap, Competition, and Priority Effects in Larval Anurans
Authors: Lawler Sharon P. , Morin Peter J.
Published: May 2006

Interspecific competitors often colonize communities at different times, but few studies have experimentally tested whether the strength of interspecific competition hinges on the order or temporal separation of species' arrivals. We added hatchlings of two sympatric anuran species (Hyla[=Pseudacris] crucifer and Bufo woodhousii) to artificial ponds on three different dates to manipulate the order and temporal separation of their arrival. Measurement of the growth and survival of each species in ponds where the second species arrived simultaneously (day 0), or after a delay of 7 or 14 d, indicated whether differences in the history of introductions affected interspecific competition. Other ponds contained H. crucifer alone, introduced on the same three dates, of B. woodhousii alone, introduced on the first and second date. These provided controls for seasonal differences in performance in the absence of competition from the other anuran species. Introductions of tadpoles at different times produced small differences in growth rates and larval periods when each anuran species occurred alone. In ponds containing both species, differences in the order and temporal separation of introductions had complex effects on the intensity of interspecific competition. When Bufo preceded Hyla, Hyla had prolonged larval periods and reduced mass and growth. When Hyla preceded Bufo, it was unaffected by Bufo. Hyla had no effects on Bufo, whether its introduction preceded or followed the introduction of Bufo. Asymmetric priority effects were not easily explained as a simple size—dependent competitive advantage of earlier arrivals. Early arrival increased the competitive impact of Bufo on Hyla, but failed to generate a competitive effect of Hyla on Bufo. Bufo tadpoles are more active than Hyla, and so may consume resources at higher rates that cannot be offset by a temporal advantage. Competition from Bufo was strongest when Hyla arrived 7 d later, suggesting that Hyla arriving 14 d after Bufo benefited from a longer period of reduced competition after Bufo metamorphosed from the ponds. The natural pattern, where Hyla breeds before or simultaneously with Bufo, permits Hyla to minimize competition from Bufo, while Bufo suffers no measurable cost from the size advantage obtained by competitively weaker Hyla tadpoles. Bufo may be prevented from breeding any earlier in the season by physiological constraints acting on eggs or breeding adults.
Specialization of Phytophagous Arthropod Communities on Introduced Plants
Authors: Andow D. A. , Imura O.
Published: May 2006

Arthropod herbivore communities vary in the proportion of plant family specialists and broad generalists. Examination of the literature of 23 annual crop plant species and the associated feeding relations of 498 arthropod species in Japan revealed that contemporary cropping area was only weakly associated with current arthropod species richness and that the proportion of plant family specialists was greater and the proportion of generalists was lower on plant species that had been in Japan for longer periods of time. These results suggest that arthropod communities become increasingly more specialized or less generalized the longer a plant coexists with the community, and that this process can occur over time periods of a few thousand years. Exotic arthropod species associated with these plants and known to have invaded Japan in the last 100—200 yr are more generalized than the native fauna, so species invasions could not contribute to the increased specialization or decreased generalization with time.
Effects of Floral Herbivory on Maternal Reproduction in Sanicula Arctopoides (Apiaceae)
Authors: Lowenberg Gregory J.
Published: May 2006

Although numerous studies have addressed plant compensatory responses to removal of vegetative tissue by herbivores, only a few have determined the effect of floral removal on subsequent seed production in natural populations. In Sanicula arctopoides, a perennial monocarp, removal of inflorescences (umbels), both naturally by deer (Odocoileus hemionus) and by artificial clipping early in the flowering season, led to no loss of maternal reproduction as measured by seed number and seed mass. Full compensation occurred when umbels bearing up to one—third of a plant's flowers were removed at the stage when plants are normally grazed. However, there are thresholds both in timing and severity of removal beyond which plants were unable to compensate fully. Compared to controls, artificial removal of developing fruits 20 d later than the normal stage resulted in a 42% decrease in seed production. Successive clippings that removed °55% of each plant's flowers resulted in a 52% decrease in seed production relative to controls; however, a single clipping of the same intensity resulted in full compensation. Differential reactions to clipping of different umbel ranks provided information about the mechanism of the response. Removal of secondary umbels resulted in decreased seed abortion rates in later developing umbels, while removal of tertiary umbels resulted in decreased abortion rates in both earlier and later formed umbels. These responses indicated that delayed abortion of developing seeds may be common in secondary umbels, that removal of secondary umbels by herbivores may relieve plants of some costs of abortion, and that S. arctopoides uses flexible allocation to achieve full compensation at typical levels of floral herbivory.
Bioenergetic Correlates of Foraging Mode for the Snakes Crotalus Cerastes and Masticophis Flagellum
Authors: Secor Stephen M. , Nagy Kenneth A.
Published: May 2006

Foraging mode may influence an array of ecological and bioenergetic characteristics, several of which we compared between two snakes: sympatric sit—and—wait foraging sidewinders, Crotalus cerastes, and widely foraging coachwhips, Masticophis flagellum. During the majority of surface activity, the nocturnal C. cerastes were either coiled on or partially buried in the sand waiting to ambush mobile prey, whereas the diurnal M. flagellum cruised through the habitat searching for active and sedentary prey. Average duration of daily surface activity of C. cerastes (X ± 1 SE = 7.2 ± 0.7 h/d) was nearly twice that of M. flagellum (3.9 ± 0.9 h/d). Body temperatures (Tb's) of active M. flagellum (33.1° ± 0.1°C) averaged higher, and were maintained within a narrower range, than those of active C. cerastes (25.3° ± 0.1°C). Field metabolic rates (FMR, as CO2 production), measured with doubly labeled water were significantly greater in M. flagellum (body mass = 124 ± 12 g [X ± SE]) than in C. cerastes (125 ± 6 g) during the active season (mid—April to mid—October; 0.154 ± 0.017 vs. 0.063 ± 0.005 mL°g—1°h—1), transition seasons (mid—March to mid—April, mid—October to mid—November; 0.058 ± 0.009 vs 0.028 ± 0.002 mL°g—1°h—1), and hibernation (mid—November to mid—March; 0.014 ± 0.002 vs. 0.007 ± 0.001 mL°g—1°h—1). Masticophis flagellum also possessed significantly greater rates of water influx than C. cerastes during the active (19.7 ± 2.9 vs. 7.6 ± 1.1 mL°kg—1°d—1) and transition seasons (7.1 ± 1.8 vs 2.5 ± 0.5 mL°g—1°d—1). Standard metabolic rates (SMR), measured at six Tb's (10°—35°C), of M. flagellum averaged 37 ± 4% greater than SMR of C. cerastes. The monthly metabolic cost of SMR, calculated by integrating 24—h Tb profiles with temperature—dependent regression equations for SMR, averaged 62 ± 4% greater for free—ranging M. flagellum than for C. cerastes. The difference between FMR and SMR is the energy allocated to activities and other energy—demanding functions such as digestion and represented 65 and 76%, respectively, of the yearly metabolic expenditure of C. cerastes and M. flagellum. Annually, M. flagellum spent 2.6 times the amount of energy on activity and other functions as did C. cerastes. Contributing to expenditures above SMR were digestion (19—43% of FMR) and movement (6—18% of FMR). Feeding rate (calculated from water influx) during the active season of M. flagellum (24.1 ± 5.5 g°kg—1°d—1) was more than twice that of C. cerastes (9.4 ± 1.4 g°kg—1°d—1). Over a full year, M. flagellum consumed 2.1 times as much assimilable energy as did C. cerastes, although both species gained similar energy profits (212 and 177 kJ/yr, respectively). The foraging strategy of the cryptic, ambushing C. cerastes balances low energy expenditure with low food intake, whereas widely foraging M. flagellum have greater expenditures but achieve greater energy consumption, as well.
A Probabilistic Analysis of Pollinator Behavior and Seed Production in Lobelia Deckenii
Authors: Burd Martin
Published: May 2006

Pollination of Lobelia deckenii, an African giant rosette lobelia, results from foraging by the Mountain Chat (Cercomela sordida) and the Scarlet—tufted Malachite Sunbird (Nectarinia johnstoni). I measured three components of bird behavior leading to fertilization and seed set: (1) the arrival rate of birds at focal inflorescences, (2) the number of probes for nectar made during a visit, and (3) the number of seeds set per probe. Predicted frequency distributions of seed set per flower, derived by compounding distributions for the three components, match some qualitative and quantitative features of actual seed set patterns. Stochastic variation in all three components, along with the modular packaging of ovules in individual flowers, results in high variance among flowers in their contribution to whole—plant female reproductive success, with most flowers receiving inadequate fertilizations for full seed set. The analysis points to the importance of viewing female reproductive success in plants as a composite entity. Although most flowers of Lobelia deckenii would set more seeds if additional pollen were acquired, sufficient control of pollinator behavior to achieve full fertilization in all fruits may not be possible. The inherent stochastic variation in fitness contributions among individual flowers may have a large influence on the evolution of reproductive traits, including packaging strategies and inflorescence morphologies.
Inferring Process from Pattern: Trophic Level Abundances and Imbedded Interactions
Authors: Sarnelle Orlando
Published: May 2006

Classical predator—prey models do not predict the positive relationships between the abundances of organisms on adjacent trophic levels seen in natural communities. Modifications that can relieve this disparity can be categorized as mechanisms of "predator damping," which can be expressed in a simple, analytical form as ratio dependence. It has been suggested that the strength of predator damping can be estimated from the shape of the relationship between prey and predator biomass using ratio—dependent models. I present a theoretical argument showing why this approach should only be applied when predator and prey are the top two trophic levels in an ecosystem. Using published data from temperate lakes that include large—scale fish manipulations, I also show that the relationship between phytoplankton biomass and Daphnia biomass suggests much weaker damping when higher trophic levels (fish) are absent than when fish are present. Thus, inferring the strength of predator damping using patterns of abundance in ecosystems is of dubious validity when interactions are imbedded within simple food chains.
Chemical Defense Against Different Marine Herbivores: Are Amphipods Insect Equivalents?
Authors: Hay Mark E. , Duffy J. Emmett , Pfister Catherine A. , Fenical William
Published: May 2006

The Structurally similar diterpenoid alcohols pachydictyol—A and dictyol—E are produced by the brown seaweed Dictyota dichotoma. This seaweed and several related species that also produce these compounds are known to be relatively low preference foods for tropical fishes and urchins. We evaluated the effect of various concentrations of these compounds on feeding by the three common types of herbivores that co—occur with Dictyota in coastal North Carolina. Fish (Diplodus holbrooki), sea urchins (Arbacia punctulata), and a mixed species group of gammarid amphipods were offered pieces of the palatable seaweed Gracilaria tikvahiae coated with either (1) dictyol—E or pachydictyol—A dissolved in diethyl ether or (2) diethyl ether alone. Dictyol—E significantly reduced consumption by fish and urchins at concentrations of 0.5 and 1.0% of algal dry mass, but had no effect on amphipod grazing. Pachydictyol—A significantly reduced fish grazing at the relatively high concentrations of 1.0 and 1.3% of plant dry mass; at 0.5% it tended to decrease grazing, but the effect was not significant (P = .07). Pachydictyol—A had no effect on urchin grazing and significantly increased amphipod grazing. When Pachydictyol—A was fed to fish as 1.0% of food dry mass, their growth rate was reduced by a significant 48%. In feeding preference experiments with several seaweeds, Dictyota ranks low for fish and urchins but high for amphipods. This is consistent with the hypothesis that the secondary metabolites produced by Dictyota play a major role in determining its susceptibility to herbivores. The ability of amphipods to circumvent the chemical defenses (Dictyota, and the fact that the two species of algae most readily consumed by amphipods (Codium and Dictyota) were the two species least readily consumed by fish, suggest that predation and herbivory by fishes may be major factors selecting for amphipods that can live on, and eat, seaweeds that are unpalatable to fishes. Amphipods that fed on Dictyota did not appear to sequester the Dictyota metabolites; when exposed to fish predation, Dictyota—fed amphipods were eaten as readily as amphipods that had fed on an alga with no defensive chemistry. Tubicolous amphipods and other small marine herbivores that may spend significant portions of their lives on only a few plants my be under very different evolutionary constraints than the larger, more mobile herbivores that commonly moved between many plants. Several characteristics of these smaller, less mobile, and much less studied, marine herbivores suggest that they may be ecologically similar to terrestrial insects and may play a large, but presently unappreciated, role in structuring marine plant communities.
Distribution‐Free and Robust Statistical Methods: Viable Alternatives to Parametric Statistics
Authors: Potvin Catherine , Roff Derek A.
Published: May 2006

After making a case for the prevalence of nonnormality, this paper attempts to introduce some distribution—free and robust techniques to ecologists and to offer a critical appraisal of the potential advantages and drawbacks of these methods. The techniques presented fall into two distinct categories, methods based on ranks and "computer—intensive" techniques. Distribution—free rank tests have features that can be recommended. They free the practitioner from concern about the underlying distribution and are very robust to outliers. If the distribution underlying the observations is other than normal, rank tests tend to be more efficient than their parametric counterparts. The absence, in computing packages, or rank procedures for complex designs may, however, severely limit their use for ecological data. An entire body of novel distribution—free methods has been developed in parallel with the increasing capacities of today's computers to process large quantities of data. These techniques either reshuffle or resample a data set (i.e., sample with replacement) in order to perform their analyses. The former we shall refer to as "permutation" or "randomization" methods and the latter as "bootstrap" techniques. These computer—intensive methods provide new alternatives for the problem of a small and/or unbalanced data set, and they may be the solution for parameter estimation when the sampling distribution cannot be derived analytically. Caution must be exercised in the interpretation of these estimates because confidence limits may be too small.
Nontraditional Regression Analyses
Authors: Trexler Joel C. , Travis Joseph
Published: May 2006

Least—squares linear regression and multiple regression are among the most commonly used analytical techniques of ecologists. However, these techniques only address a portion of the possible applications of regression methods. We discuss two less commonly used regression analyses that could find wide application in ecology, logistic regression and LOWESS regression. Logistic regression is appropriate in cases where the dependent variable is categorical, dichotomous, or polychotomus. It can be used with continuous and/or discrete independent variables. Logistic regression is motivated by the underlying binomial or multinomial distribution of dichotomous and polychotomous dependent variables and transforms the data to explicitly model these distributions. Locally weighted regression scatterplot smoothing or LOWESS regression is used to model the relationship between a dependent variable and independent variable when no single functional form will do. LOWESS regression is motivated by the assumption that neighboring values of the independent variable are the best indicators of the dependent variable in that range of independent values.
Freezing Tolerance and Water Relations of Opuntia Fragilis from Canada and the United States
Authors: Loik Michael E. , Nobel Park S.
Published: May 2006

To investigate the influence of winter climate on freezing tolerance at the population level, minimum January air temperatures in the field and cold acclimation determined in the laboratory were compared for Opuntia fragilis. Populations occurred at 20 locations as far north as 56°46' N latitude and at elevations up to 3029 m in Canada and the United States, most of which experience extreme freezing temperatures each winter. Low—temperature responses and water relations of stems were examined in the laboratory at day/night air temperatures of 25°/15°C and 14 d after the plants were shifted to a 5°/—5°C temperature cycle. Cold acclimation averaged 17°C and freezing tolerance averaged —29°C for the 20 populations following a shift to low day/night air temperatures, indicating that O. fragilis has the greatest cold acclimation ability and the greatest freezing tolerance reported for any cactus. Moreover, freezing tolerance and cold acclimation were both positively correlated (r2 ° 0.7) with the minimum temperatures at the 20 locations. Plants lost water during low—temperature acclimation, leading to 30% decreases in cladode and chlorenchyma thickness; the decrease in water content was greater for the five warmest populations than for the five coldest ones. Over the same period, the average osmotic pressure of the chlorenchyma increased from 1.42 to 1.64 MPa, and the relative water content (RWC) decreased from 0.58 to 0.49, but the average osmotic pressure of saturated chlorenchyma was unchanged, indicating no net change in solute content during acclimation. Although the role of water relations in freezing tolerance is unclear, the substantial freezing tolerance and cold acclimation ability of O. fragilis leads to its distribution into regions of Canada and the United States that experience minimum temperatures below —40°C during the winter.
Highly Structured Fish Communities in Australian Desert Springs
Authors: Kodric-Brown Astrid , Brown James H.
Published: May 2006

To assess the pattern and causes of community structure, we sampled 38 isolated springs in the Dalhousie Basin of South Australia to determine the distributions of five taxa of native fishes. We visited each spring on at least two consecutive days, used several collecting methods to determine the presence of absence of each taxon, and took standardized measurements of abiotic environmental variables. Community organization was highly predictable: (1) number of species increased with spring size, (2) each species occurred in nearly all springs larger than a certain size, and (3) species composition exhibited nearly perfect nestedness. These results suggest that much of the variation in the composition of other communities may not be stochastic. When the influence of historical and environmental factors can be assessed, the colonization—extinction processes and ecological relationships that determine community structure may be highly deterministic.
Enzymatic Adaption in a Specialist Herbivore for Feeding on Furanocoumarin‐ Containing Plants
Authors: Nitao James K.
Published: May 2006

Furanocoumarins are plant secondary compounds toxic or deterrent to a number of herbivores. The parsnip webworm, Depressaria pastinacella (Lepidoptera: Oecophoridae), however, feeds only on three genera of Umbelliferae, all of which contain furanocoumarins. The purpose of this study was to determine the tolerance of D. pastinacella to furanocoumarins occurring in its hosts, and to evaluate the adaptive significance of polysubstrate mono—oxygenases (PSMOs) in enabling this specialist to feed on plants chemically inaccessible to most other herbivores. The D. pastinacella larvae were offspring of adults collected as pupae on parsnips in the wild. The larvae were reared to pupation on a semidefined artificial diet with and without the furanocoumarins xanthotoxin and angelicin, under conditions of UV light appropriate to photoactivate xanthotoxin. Bioassays were performed on pupal mass for xanthotoxin and angelicin, and tested using one—way and two—way ANOVAs, respectively. No effect on larval growth and survivorship by xanthotoxin, a linear furanocoumarin toxic to polyphagous herbivores, was detected. PSMOs in the microsomal fraction of the midgut possess a high specific activity for this furanocoumarin. The importance of these enzymes in the detoxification of xanthotoxin was further suggested by the observation that xanthotoxin induces microsomal enzymes that in turn metabolize this allelochemical. Enzymatic adaption to xanthotoxin allows D. pastionacella to feed on plants unavailable to many generalist herbivores. Moreover, no effect of angelicin, an angular furanocoumarin, was found on parsnip webworm growth. Angelicin is toxic to at least one other herbivore adapted to feeding on xanthotoxin. Although the mechanism of the parsnip webworm's adaption to angelicin was not determined, the ability to tolerate this compound further suggests that D. pastinacell is a specialist on furanocoumarin—containing hosts.
Fugitive Coexistence in Sessile Species: Models with Continuous Recruitment and Determinate Growth
Authors: Armstrong Robert A.
Published: May 2006

I previously developed a model of growth and competition in sessile organisms, which I used to show that the sized of patches caused by disturbance could have an important effect on species diversity. When disturbance patch size is small, new individuals will be recruited from patches of many different ages and in different successional stages, such "regional" recruitment promotes coexistence. In contrast, when disturbance patch size is large and recruitment involves only individuals from a single successional stage, coexistence becomes less likely. That model contained the rather restrictive assumptions (1) that individuals grow radially at constant speed and (2) that all recruitment occurs in a short period of time following disturbance. In this paper I replace these assumptions with ones that allow for limited (determinate) growth to a fixed upper size; I also allow continuous recruitment from a well—mixed "reigonal" source. My conclusions are (1) that coexistence should be easier, and so diversity should be higher, in communities of organisms with determinate growth than in communities of organisms with indeterminate growth and (2) that continuous recruitment decreases the probability of coexistence.
The Importance of Scale to Predator‐Prey Spatial Correlations: An Example of Atlantic Fishes
Authors: Rose George A. , Leggett William C.
Published: May 2006

The signs and strengths of spatial correlations between densities of predators and their prey were scale dependent both in simulations and when calculated from acoustic transects across nearshore distributions of Atlantic cod (Gadus morhua), and its prey, capelin (Mallotus villosus). Spectral analyses of 1985 and 1986 transects indicated that spatial correlation (coherence) varied relative to: (1) the occupation of thermal refuge areas by capelin (sea temperatures <—0.5° or >8.5°C), (2) the aggregation dimensions of each species, and (3) the activity level of the predator. When capelin did not occupy thermal refuges, cod and capelin densities were coherent across a wide range of scales (2—30 km). At large scales (>4—10 km) densities were in phase (positively correlated), but at scales less than aggregation dimensions (3.5 km), densities were out of phase (negatively correlated). At the smallest scales measured (2—3 km), densities exhibited maximum negative correlation in both years. When capelin were aggregated within thermal refuges, predator—prey densities were coherent (in phase) only at scales larger than those of the refuges. When actually feeding, cod achieved coherent, in—phase distributions with capelin at scales as small as 3.5 m.
Population Regulation, Convergence, and Cannibalism in Notonecta (Hemiptera)
Authors: Orr Bruce K. , Murdoch William W. , Bence James R.
Published: May 2006

In a population convergence experiment, the initial densities of adults of the predatory backswimming bug Notonecta hoffmanni were set above and below a putative equilibrium density in stock tanks. The experiment was done at two constant rates of food supply (wingless Drosophila) for the larger instars (in natural pools Notonecta feed mainly on terrestrial arthropods that fall on the water surface). It was predicted that the densities of the resulting populations would converge on an equilibrium set by the rate of food supply for the larger instars. The tanks also contained zooplankton (mainly Daphnia), which were the main food supply for the smaller instars of Notonecta. The resulting overwintering populations converged towards the appropriate equilibrium densities, via density—dependent and food—dependent fecundity and then cannibalism. However, the populations overshot their equilibria, producing overconvergence. In natural populations such overconvergence might tend to produce 2—yr cycles in abundance. Overconvergence resulted from the insensitivity of the survivorship of the original adults (at least over the short term) to differences in food supply between treatments, allowing them to continue to affect (via cannibalism and reproduction) the eventual density of the new overwintering population. Because Notonecta population density was determined by the externally supplied, locally uncoupled food supply, even though the early instars depended for food largely upon dynamic populations of zooplankton, the dynamics of the Notonecta population were simpler than a description of the food web might suggest.