bluuebunny/crossref_metadata_2025 huggingface dataset:
Sea Otter Predation and Community Organization in the Western Aleutian Islands, Alaska
Authors: Estes James E. , Smith Norman S. , Palmisano John F.
Published: May 2006

Predation by the sea otter (Enhydra lutris) limits epibenthic invertebrates, especially sea urchins (Strongylocentrotus polyacanthus), in turn allowing a luxuriant development of the macroalgal canopy. Where sea otters are abundant, sea urchins are small and scarce in shallow water, and the association of fleshy macroalgae apparently is regulated by competition. Sea urchins are larger and more abundant in deeper water, where they are less accessible to sea otters. Macroalgae are most abundant, and competition in the plant association is severest, near the sublittoral fringe where sea otters can remove sea urchins most efficiently. In deep water, competition among maccroalgae is reduced because the light intensity is lower and grazing by sea urchins increases. On islands where sea otters are absent, sea urchins are abundant, large, and are probably limited by intraspecific competition; and they have eliminated fleshly macroalgae. Available data suggest that the association of Laminaria spp. and Agarum cribrosum contributes most to primary production in nearshore areas of the western Aleutian Islands. Where sea otters are absent and sea urchins have eliminated this plant association, some higher trophic forms also are absent or less abundant than where sea otters are common and the plant association is well developed. Earlier studies of sea otter food suggested that low—density populations of sea otters consume primarily sea urchins and mollusks in the western Aleutian Islands. Later studies of high—density populations showed a wider variety of foods consumed, with fish an important component of the diet. These studies support our observations on the differences in availability of these foods between islands with and without sea otters.
Drosophila Population Dynamics: Chaos and Extinction
Authors: Philippi Thomas E. , Carpenter M. Patricia , Case Ted J. , Gilpin Michael E.
Published: May 2006

Earlier work in our laboratory and others suggests that the discrete population dynamics of serial transfer Drosophila systems are stable, despite observed fluctuations about the carrying capacity. To investigate the robustness of this conclusion, we have radically reduced the available food in the system (from 40 to 3 mL of media). The number of flies in such systems fluctuates to such an extent that they go extinct. To investigate whether this is due to chaotic population dynamics, as opposed to environmental or demographic stochasticity, we fitted the discrete Q—logistic growth equation to data from input—output" experiments (i.e., experiments in which we recorded survivorship and recruitment in systems at eight starting densities) and compared the calculated parameters to previous work in our laboratory. Surprisingly, the resulting parameters for the mean behavior of our thin—food systems indicated greater stability than those for previous thick—foodsystems. Thus, the observed fluctuations and extinctions may be explained on the basis of a reduced carrying capacity and an increased variance of growth dynamics (environmental stochasticity). We have corroborated this with an empirically based computer model incorporating stochasticity. This work has implications for the problem of minimum viable population size
Virus Spread and Vector Dynamics in Genetically Diverse Plant Populations
Authors: Power Alison G.
Published: May 2006

Little is known about the influence of genetic diversity in plant populations on the dynamics of plant viruses, particularly those transmitted by insects. For these viruses, plant genetic diversity may affect virus incidence through impacts on the population dynamics of the vector insects or through impacts on vector feeding behavior, which determines transmission of the virus. This study was designed to explore the influence of plant genetic diversity on virus dispersal by aphid vectors and to examine the biological mechanisms responsible for that influence. In a set of field experiments using the aphid—transmitted barley yellow dwarf virus, I examined the influence of genetic diversity in oat (Avena sativa L.) populations on the spread of the virus and on the population dynamics and movement behavior of aphid vectors of the virus. Only at relatively high aphid abundance were the densities of aphid vectors influenced by plant genetic diversity. In one year out of three, densities of the oat—bird cherry aphid, Rhopalosiphum padi (L.), were significantly lower in the genetically diverse stand than in the genetically homogeneous stands. In no year were densities of the English grain aphid, Sitobion avenae (F.), influenced by the host—plant population. Despite these weak or absent effects on vector abundance, the incidence of the virus was consistently lower in the genetically diverse oat populations. Disease reduction in the diverse populations appears to depend upon changes in aphid movement behavior that affect the efficiency of virus transmission. Mark—release experiments with S. avenae demonstrated that movement rates were significantly higher and plant tenure times were significantly lower in the genetically diverse oat populations. Because the barley yellow dwarf virus requires several hours of aphid feeding for effective transmission, these reduced tenure times and increased travel time among plants led to a reduction of virus transmission. While plant genotype can clearly influence herbivorous insects dramatically, this study suggests that the effects on insects of genetic diversity per se in the host—plant population are likely to be subtle and not easily detected using standard field sampling techniques, except at high insect densities. Yet even at low vector densities, behavioral responses to plant genetic diversity can lead to significant effects on the spread of pathogens.
Island Ecology: Reduced Hummingbird Diversity and the Pollination Biology of Plants, Trinidad and Tobago, West Indies
Authors: Feinsinger Peter , Wolfe James A. , Swarm Lee Ann
Published: May 2006

To examine effects of pollinator diversity on pollination biology of plants, we studied early successional, hummingbird—pollinated plants for 13 mo on Trinidad and Tobago. A large, land—bridge island just off the Venezuelan coast, Trinidad has 16 hummingbird species. Smaller and more isolated, Tobago has a subset of five of these. Study sites in the Arima Valley of Trinidad had 17 species of plants adapted for hummingbird pollination; 12 of these, plus one other, existed on comparable Tobago sites. From hypotheses of island ecology and plant—pollinator coevolution, we developed four predictions; results supported the first three: (1) Overall, among the eight plant species observed at least 4 d (up to 28 d) on each island, Tobago flowers received significantly fewer visits than conspecific Trinidad flowers from hummingbirds and the Bananaquit Coereba flaveola, the only other avian visitor. A two—way ANOVA showed that the treatment (island) effect significantly exceeded even the tremendous month—to—month, plant—to—plant variation in visits received within each plant population. (2) Pollen loads carried by hummingbirds and Bananaquits on Tobago tended to be mixed, whereas birds on Trinidad often carried single—species loads. (3) In two of three plant species studied intensively, Tobago flowers secreted greater volumes of nectar than Trinidad flowers. The third species showed a slight but significant increase as well. An indirect measure of nectar standing crop, the time a hummingbird spent probing a flower, suggested that Tobago plants as a group secreted higher nectar volumes than Trinidad conspecifics. We found no significant differences in sugar concentration, however. (4) Contrary to predictions that flowering seasons should overlap less on Tobago than on Trinidad, there was no discernible difference in flowering seasons between Trinidad and Tobago conspecifics. Evidently, pollination service on Tobago is currently more erratic than on Trinidad, and has been sufficiently erratic in the past to select for increased nectar production but not so erratic as to override other, stabilizing, selective forces on flowering times.
Selection of Seed Distribution Types by Dipodomys Merriami and Perognathus Amplus
Authors: Reichman O. J. , Oberstein David
Published: May 2006

Laboratory experiments were conducted to investigate the foraging responses of kangaroo rats (Dipodomys merriami) and pocket mice (Perognathus amplus) to aggregates of seeds which differed in size, depth, and degree of clumping. Kangaroo rats effectively used clumps of seeds, whereas the pocket mice did not differentially select clumps of seeds over a scattered distribution of seeds. In addition to distribution, clump size and depth were important criteria for both species in determining foraging efforts. These data, coupled with recent information which suggests wide arrays of seed distribution types in the desert, lead us to propose that rodents select seeds on the basis of spatial distribution of the resource. This is an attractive hypothesis because it can accommodate both of the primary mechanism previously suggested for desert rodent coexistence, seed size selection and habitat selection.
Evaluation of a Density Estimator Based on a Trapping Web and Distance Sampling Theory
Authors: Wilson Kenneth R. , Anderson David R.
Published: May 2006

A new density estimation technique using a trapping web and distance sampling theory was evaluated using Monte Carlo methods. A sophisticated computer algorithm simulated movement of small mammals according to four different home range utilization distributions. Trapping of small mammal populations was simulated with four spatial patterns, two population densities, and three realistic average capture probabilities; in total, 8151 replications were evaluated. Percent relative bias of the estimator of density ranged from —4 to 29% for average capture probabilities (°) of .16 and .24, and from —17 to 30% at ° =.09. Average coefficients of variation were 7.8—9.8% at a density (D) of 100 individuals/ha and 15.1—19.1% at D = 25 individuals/ha. Achieved confidence interval coverage for the density estimator ranged from 32 to 94%, under the assumption of a 95% nominal level. An improved variance estimator for the trapping web was developed that should improve the confidence interval coverage. The trapping web approach appears very promising in aiding the manager or researcher to obtain reliable density estimates for small mammal populations. The statistical theory underlying this estimation method is well developed and is based on relatively few assumptions.
Patterns of Morphological Evolution in Bird Genera of New World and Old World Peatlands
Authors: Niemi Gerald J.
Published: May 2006

Morphological convergence is defined in evolutionary ecology as evolution toward similar dimensions of traits in unrelated animals that live in the same or similar environments. Because the genus is a taxonomic unit in which differences among species represent directional changes in morphology from a common ancestor, I look for evidence of morphological convergence by comparing radiations in sets of congeneric species of birds that occur in peatland habitats of northern Minnesota and eastern Finland. Through discriminant function analysis, I determined that habitats selected by species in peatland shrub areas of Minnesota were similar to habitats selected by species in peatland shrub areas Finland. This pattern held for the species in peatland forests, also, but not for those found in open peatlands. Open peatlands in Finland had more water pools and less vegetation than those in Minnesota. The morphological analysis used 15 characters: size and 14 shape variables (skeletal measurements adjusted for size) pertaining to three functional complexes (bill, flight apparatus, and hind limb). Special emphasis was placed on four genera in Minnesota, Zonotrichia, Spizella, Geothlypis, Dendroica, and one genus in Finland, Emberiza, because these genera have species that occur both in shrub and coniferous forest peatland habitats. In all five genera, species in shrub vegetation are consistently smaller, have relatively longer pelves and legs, and have wider and shallower sterna than their respective congeners forests. Furthermore, with the exception of Emberiza and Spizella, the bill, sternum, and humerus are relatively longer in the shrub—dwelling peatland species than in their respective congeners in coniferous forest. The most abundant species in the shrub vegetation in Minnesota (Cistothorus platensis) and in Finland (Acrocephalus schoenobaenus) are among the most extreme in this respect; they are relatively small, with relatively wide and shallow sterna, relatively long pelves and legs. Also, warblers breeding in shrub peatlands have relatively deeper and wider bills, but sparrows have relatively shallower and narrower bills than their respective congeners in coniferous forests. These morphological relationships indicated significant (P = .03, sign test) directional patterns of differentiation among distantly related genera (warbleres and sparrows) for morphology associated with movement through the habitat (e.g., hind limb structure, size, and sternal shape). Morphological differences associated with feeding, however, showed trends at the family level that probably reflect differences in diet and behavior between sparrows and warblers. The consistency of these patterns among the genera studied emphasizes (1) the sensitivity of species adaptations in morphology to habitat structure, and (2) the likelihood of a common underlying mechanisms for these results.
Behavioral Interactions Between Two Desmognathine Salamander Species: Importance of Competition and Predation
Authors: Roudebush Roger E. , Taylor Douglas H.
Published: May 2006

Behavioral interactions were examined in two species of salamanders. Desmognathus quadramaculatus and D. monticola, in artificial stream tanks. Substrate choice for D. monticola was altered in the presence of D. quadramaculatus, but the converse was not observed. The substrate choice of small D. monticola was altered in the presence of large D. monticola. Interspecific competition for substrate was not detected, and those species—specific avoidance of D. quadramaculatus by D. monticola may thus represent predator avoidance. Intraspecific competition for substrate was detected in D. monticola. Competition and predation both appear to structure desmognathine salamander communities.
Predator Foraging Characteristics and Prey Population Structure on a Sheltered Shore
Authors: Robles Carlos
Published: May 2006

I investigated predation on mussels occurring at upper levels of a sheltered rocky shore on Santa Catalina Island, USA. Despite continuous recruitment, the mussels Mytilus californianus, M. edulis, and Septifer bifurcatus seldom survived to lengths >1.5 cm., and they remained inconspicuous beneath a canopy of perennial algae. Surveys using scuba at high tide and gut—content analyses showed that intertidal mussels were preyed upon by whelks (Ceratostoma nuttali and Maxwellia gemma), diurnal fishes (Halichoeres semisinctus and Semicossyphus pulcher), and nocturnal spiny lobsters (Panulirus interruptus). Whelks and fishes foraged primarily on mid to low shore levels, the lower half of the mussels' vertical range. However, the lobsters foraged over the entire intertidal zone and were the only predator to specialize on mussels. Exclusion of lobsters from upper shore levels caused significant increases in the densities and sizes of the mussels and herbivorous molluscs. Increases in the percent cover of mussels coincided with decreases in the percent cover understory algae. Previous studies open temperate shores indicate that mussel beds form in upper shore refugia where brief submergence intervals preclude predators that require long foraging times and are intolerant of exposure. On some sheltered temperate shores, predators that move quickly with the tides may influence upper shore zonation. Pronounced variation in prey population structure can result from shifts in the relative effectiveness of different foraging characteristics within diverse predator faunas.
Response of Eriophorum Vaginatum to Elevated CO_2 and Temperature in the Alaskan Tussock Tundra
Authors: Tissue David T. , Oechel Walter C.
Published: May 2006

Small greenhouses were used in the arctic to maintain Erioporum vaginatum—dominated tussock tundra for 10 wk at ambient CO2 (340 μL/L), elevated CO2 (510 or 680 μL/L), or elevated CO2 and 4°C above ambient temperature (680 μL/L, ambient + 4°). These treatments represent present levels of atmospheric CO2 and temperature, and those predicted for the next century. Within 3 wk, plants maintained at elevated CO2 exhibited a physiological adjustment of their photosynthetic rate so that plants grown at ambient and elevated CO2 levels had similar photosynthetic rates at their respective growth CO2 concentrations. The reduction in photosynthetic capacity for plants grown at elevated CO2 levels did not appear to be due to stomatal closure or end—product inhibition. Other possible mechanisms were not explored. Transpiration rates and water use efficiency did not differ among treatments in the generally wet environment of tussock tundra. Relative leaf growth rate and the seasonal pattern of growth were also unaltered, suggesting that the growth of mature tillers is not, under normal ambient conditions, limited by temperature or carbohydrate. However, new tiller production was significantly increased at elevated CO2, suggesting that the long—term effect of CO2 enhancement in this sedge may be the production of a greater number of new tillers rather than an increase in the size or productivity of existing tillers. Our results are consistent with the notion that growth of Eriophorum vaginatum in the field is more limited by nutrient supply than by photosynthesis. We further suggest that photosynthetic rates reflect the sink activity. It is therefore very difficult to assign cause and effect between growth rates and photosynthetic rates.
The Effect of Adult Food Limitation on Life History Traits in Speyeria Mormonia (Lepidoptera: Nymphalidae)
Authors: Boggs Carol L. , Ross Charles L.
Published: May 2006

Variation in food availability is likely to occur in the wild, and may affect resource allocation to various life history traits. Quantitative adult diet restriction had no effect on life—span or mean individual egg mass, but reduced fecundity in the butterfly Speyeria mormonia. The sum of fecundity plus unlaid oocytes remaining in the ovaries at death declined in direct proportion to the reduction in the adult diet. This indicates that oocytes were resorbed and resources re—allocated away from reproduction under resource stress, since the sum of laid and unlaid eggs for butterflies fed ad libitum did not differ from the number of oocytes present in the ovaries at eclosion. In this nectivorous species, then, life—span is conserved at the expense of reproduction under adult resource stress. Further, for butterflies fed ad libitum, the volume of honey—water imbibed declined with age for both sexes. Daily volume imbided by females fed ad libitum was directly correlated with daily egg production and life—span, suggesting that factors as yet unexplored may be affecting both resources intake and life history traits when resources are available ad libitum.
Defining the Western Spruce Budworm's Nutritional Niche With Response Surface Methodology
Authors: Clancy Karen M. , King Rudy M.
Published: May 2006

A response surface design was used to help define the "nutritional niche" of the western spruce budworm, Choristoneura occidentalis. We evaluated how calcium, magnesium, and phosphorus interacted to influence budworm fitness on artificial diets containing five different levels of the minerals. We quantified survival rates for several life stages using a three—generation bioassay. Data from the bioassay were used to estimate population growth over three complete generations. Performance of the budworm was affected by Mg (linear response for F1 generations; 0.83—1.95 mg/g tested) and P (quadratic response for P1 [parental] and F1 generations; 2.74—4.95 mg/g tested) but was largely unaffected by Ca (0.50—6.81 mg/g tested). Overall, the results suggested budworm performance is best when Mg is at low concentrations and P is at moderate concentrations, and that very high levels of Mg and P are detrimental to the budworm. On average, host trees had too much Mg and too little P for optimal response by budworms. Interactions between Mg and P imply that balances or ratios of minerals are important in budworm nutritional ecology. However, detectable effects from Mg, P, and Ca in the diets diminished as the bioassay continued into the F2 generation, and, consequently, the estimated number of larvae alive at the beginning of the F2 and F3 generations showed no relationship to concentrations of the test minerals. This result probably reflected: (1) the loss of some design points in the second and third generations of the biomass because larvae did not survive on diets that were markedly suboptimal and (2) increased variability in the population growth response as the experiment contributed into the F2 and F3 generations.
Relating Wet and Dry Year Ecophysiology to Leaf Structure in Contrasting Temperate Tree Species
Authors: Abrams Marc D. , Kubiske Mark E. , Mostoller Scott A.
Published: May 2006

Leaf structure in 17 temperate, broad—leaved tree species on xeric, mesic, and wet—mesic sites in central Pennsylvania (USA) was compared to leaf gas exchange and tissue—water relations data for these species measured during wet (1990) and dry (1991) growing seasons. We tested the hypothesis that ecophysiological responses across contrasting species, sites, and growing—season conditions can be related to specific aspects of leaf structure. Species guard—cell length (GCL), leaf mass per area (LMA), and leaf thickness were positively correlated and increased as sites became drier. Mesic species had lower stomatal density than xeric and west—mesic species. During the wet year, xeric and mesic species had higher area—based net photosynthesis (A) and leaf conductance (gwv) and lower osmotic potentials (π) than wet—mesic species. Unlike the xeric species, mesic and wet—mesic species had decreased A and gwv in the dry vs. the wet year. Thus, gas exchange in mesic species resembled that of xeric species in the wet year and that of wet—mesic species in the dry year, which may be due to differences in leaf structure. Maximum A rates and mean gwv were positively related to species LMA, whereas wet and dry year mean A and gwv increased with increasing leaf thickness and GCL across the study sites. Minimum and mean midday leaf water potential and π at zero turgor decreased with increasing LMA, GCL, and leaf thickness. Stomatal density and ecophysiology were not significantly related across the study sites, but they were related within sites. Intrasite comparisons indicated that, in general, gas exchange parameters increased and tissue water parameters decreased with increases in leaf parameters, trends that were consistent with intersite comparisons. These results suggest that both gas exchange and tissue—water relations during wet and dry years were related to specific aspects of species leaf morphology, and that leaf structure has an important predictive role in evaluating ecophysiological responses to environmental stress at the community and landscape level.
Community Structure in a Desert Perennial Community
Authors: Silvertown Jonathan , Wilson J. Bastow
Published: May 2006

A battery of null models is used to test for the presence of six aspects of community structure in the pattern of plant species distributed in patches around focal or “nurse” plants at a site in the Bolson de Mapimi, Chihuahua Desert, Mexico. Significant community structure was found in five of the six types of test. There were significantly more positive associations between species than expected for a random assemblage. Examined pairwise, negative associations wee almost all with one of the two main focal plants: Larrea tridentata or Opuntia rastrera. The other species were generally positively associated with each other, because of their rarity in small patches. Species—poor patches contained nonrandom subsets of the flora of species—richer patches (i.e., the species/patch matrix was significantly nested). This is interpreted as indicating the existence of species whose establishment is dependent on conditions that can be provided by other species. There was also checkerboarding in the species/patch matrix, indicating the possibility of some competitive exclusion. All species tended to occur more often on species—rich patches, but relative to other species Larrea occurred significantly more often on small patches. Therefore we suggest that the presence of this shrub may be required before other species, such as Opuntia rastrera, Jatropha dioica, and Opuntia schottii/grahamii, which tended to occur on two—species patches, may colonize a patch. Some species, most of them cacti, showed a significant tendency to occur in the largest patches, suggesting that they were able to invade a patch only after preconditions for dispersal or establishment had been satisfied by habitat changes induced by other species. The two main foci, Larrea and O. rastrera, differed in the overall species composition of their associated flora. Many species showed a significant tendency to occur in adjacent patches, indicating the importance of dispersal, probably by rodents, in determining distribution. We conjecture that these patterns result from colonization processes in which facilitation, inhibition, and tolerance all play a role.
The Dynamics of Juvenile Dispersal: Why Field Ecologists Must Integrate
Authors: Gaines Steven D. , Bertness Mark
Published: May 2006

The impact of dispersing juveniles on population and community dynamics is a topic of considerable debate, particularly for marine species with broadly dispersing larvae. We argue that sampling problems continue to pose a serious impediment to progress. The root of the problem is the large variation in dispersal dynamics over space and time. For short—distance dispersal the variance in dispersal has been largely ignored. By contrast, for long—distance dispersal the variance in dispersal may have been overemphasized due to the inappropriate time scales of sampling. We present data for an intertidal barnacle, Semibalanus balanoides, to contrast conclusions reached from standard sampling of long—distance juvenile dispersal with those reached by techniques that more appropriately integrate the dynamics of dispersers over time. The standard sampling approach misrepresents the true juvenile dynamics.
Consequences for Carolina Chickadees of Foraging with Tufted Titmice in Winter
Authors: Cimprich David A. , Grubb Thomas C.
Published: May 2006

The purpose of this investigation was to accomplish a simultaneous evaluation of both the costs and benefits of mixed—species foraging for a socially subordinate species. Specifically, we studied Carolina Chickadees that commonly forage in winter with a socially dominant congener, the Tufted Titmouse. We prevented these species from foraging together by removing titmice from five central Ohio woodlots. We then compared the biology of chickadees on these sites to that of chickadees on four sites where titmice had not been removed. Where titmice had been removed, chickadees foraged in more titmouse—like locations. Although this shift was statistically significant for two of the five niche parameters measured, there was a consistent pattern of shifts toward the titmouse niche in all five. We were unable to detect a change in chickadee vigilance in the absence of titmice. Chickadees showed statistically greater induced feather growth in the absence of titmice, indicating better nutritional condition, but we detected no differences in survival between treatment groups. We conclude that the foraging niche results provide evidence of a cost to chickadees of foraging with titmice. Furthermore, based on the feather growth results, we conclude that the costs and benefits for chickadees of this interaction sum to a net cost in foraging efficiency if not in survivorship.
Effects of Insect Herbivory and Fungal Endophyte Infection on Competitive Interactions among Grasses
Authors: Clay Keith , Marks Susan , Cheplick Gregory P.
Published: May 2006

Interactions among plants may be influenced by pests of parasites that differentially affect one competitor. The purpose of this study was to determine the effects of fungal parasitism and insect herbivory, alone in concert, on plant competitive interactions. The effects of fungal endophyte (Acermonium spp.) infection and fall armyworm (Spodoptera frugiperda) herbivory on competitive interactions in one— and two—species mixtures of the grasses tall fescue (Festuca arundinacea), red fescue (F. rubra), and perennial ryegrass (Lolium perenne) were examined in greehouse experiments. In general, herbivory reduced plant biomass whereas endophyte infection increased plant biomass. Endophyte infected (E+) plants were less damaged by herbivory than uninfected (E—) plants of the same species. Studies on fall armyworm larval feeding and choice tests with the five grass species were generally consistent with the outcome of the competition experiments; E+ plants were less nutritious and less preferred than E— plants of the same species. There were significant interactions among factors so that the outcome of competition depended on the species identities and the presence or absence of endophytes and herbivores. In competition with Kentucky bluegrass (Poa pratensis), E+ and E— perennial ryegrass produced similar biomass in the absence of herbivory, but E+ and E— perennial ryegrass had nearly twice the biomass of E— plants when herbivores were present. E+ and E— tall fescue were poor competitors with orchard grass (Dactylis glomerata) when herbivores were absent, but E+ tall fescue was a better competitors than E— plants and orchard grass when herbivores were present. This study indicates that competitive hierarchies among grasses are altered by interactions with insect herbivores and fungal endophytes, which have typically been ignored in past studies.
Physiological Basis of Ratio‐Dependent Predator‐Prey Theory: The Metabolic Pool Model as a Paradigm
Authors: Gutierrez A. P.
Published: May 2006

Ratio—dependent models of predators and prey are approximations of the biology of resource acquisition and allocation and their consequences for population birth and death rates. A demand—driven functional response model that has a physiological basis in mass (energy) dynamics is reviewed (i.e., the metabolic pool model), and its obvious links to the logistic model are outlined. To demonstrate the utility of this approach, a distributed maturation time age—structure model of the dynamics of A. J. Nicholson's classic laboratory population data on the sheep blow fly (Lucilia cuprina Weidman) is developed. The model provides sufficient information on the dynamics of the intermediate life stages to show that the blow fly oscillations were due to the effects of larval competition for food on size, fecundity, and pupation success. These results agree with Nicholson's conclusions. The advantage of this model, in contrast to prior models, is that the dynamics emerge by considering the processes of resource acquisition and allocation as they affect growth, reproduction, and survival. No explicit time delays, which automatically lead to oscillations, were included. Lastly, the notions of the metabolic pool model are found in Nicholson's original model for equilibrium population density. The metabolic pool paradigm in an age—structure setting is used to model the tri—trophic dynamics of Acyrthosiphon aphids in an alfalfa ecosystem. The model explains the role of the various natural enemies in the regulation of the aphids.
Community Structure in Time Past: Influence of Human Fishing Pressure on Algal‐Urchin Interactions
Authors: Levitan Don R.
Published: May 2006

The sea urchin Diadema antillarum increases the relative size of Aristotle's lantern (length of demipyramid) when food limited. This provides a tool for investigating algal abundance and biotic interactions in the past. A field collection of D. antillarum provided a baseline relationship between demipyramid length and body size (test diameter). Experimental manipulations of food availability and population density resulted in shifts in relative demipyramid length. The baseline and experimental data provide a range of responses that was compared to museum specimens collected over the past 100 yr from 30 locations in the Caribbean. The relative demipyramid lengths from the museum specimens were also compared over time, and with human population density, human fishing pressure, and geographic region. The results indicate that relative demipyramid size is indirectly influenced by human activity and varies with geographic region. This suggests that human fishing pressure has influenced Caribbean coral reef community structure by affecting predator—herbivore relationships. However, this human impact may be small relative to naturally occurring variation in demipyramid size.
Density Dependence in the Common Lizard: Demographic Consequences of a Density Manipulation
Authors: Massot Manuel , Clobert Jean , Pilorge Thierry , Lecomte Jane , Barbault Robert
Published: May 2006

Density dependence plays an important role in the regulation of most populations. Descriptive studies provide only limited evidence, while density manipulations are thought to be a more powerful tool. Here, we describe such a manipulation. We experimentally analyzed demographic responses to changes in density in common lizard (Lacerta vivipara) populations. Two neighboring sites were studied from 1986 to 1988. At the end of the first year, density was artificially decreased at one site and increased at the other. The evolution of demographic parameters over time (before and after manipulation) was compared between the two sites. We found that: (1) density in each age and sex class quickly converged to pre—experimental values at both sites; (2) survival rates (introduced individuals excluded) were unchanged, except that juvenile mortality was positively related to density; (3) emigration rates remained low and did not seem to be influenced by density, while immigration rates were negatively influenced by density; (4) the proportion of young reproductive females was negatively influenced by density; (5) body length and growth rate were negatively influenced by sensitivity only in yearlings, and at least in females, no catchup on growth is apparently possible; no difference in reserve storage was found; and (6) female reproduction was modified by the manipulation through clutch size and hatching success, which were negatively influenced by density; however, no difference was detected for hatching date, number of abortive (clear) eggs, prenatal mortality, reproductive investment, and body mass of live neonates. Rapid density readjustments were mostly explained by immigration into the site where density was decreased, and by high mortality of introduced individuals and of native juveniles where density was increased. Other parameters that were influenced by the manipulation could only have induced delayed effects on population density. Different age and sex classes showed different responses to density. These responses indicate that density dependence plays a key role in shaping the demography of this lizard species.
Effects of Food, Genotype, and Maternal Size and Age on Offspring Investment in Daphnia Magna
Authors: Glazier Douglas S.
Published: May 2006

This laboratory study of Daphnia magna had two major aims: (1) to examine the effect of food quantity on offspring size and number and how this effect varies with genotype and maternal size, age, and metabolic demand; and (2) to examine the relationship of investment per offspring to total reproductive investment. Differences in the response of offspring size to food quantity between the two clones in this study and between Daphnia studies in the literature are explained by nonlinear model that explicitly considers the effect of food quantity relative to the metabolic demands of the mother. According to this model, offspring size covaries positively with food quantity/maternal demand at very low food levels because of reproductive constraints, but it covaries negatively with food quantity/maternal demand at middle food levels as an adaptive response, and it remains constant at very high food levels because of a lower limit on viable offspring size. Effects of genotype (clone) and ration level on trade—offs between egg size and number and on the maternal size—dependence of egg mass, brood mass, and brood size are described. Strong genotype—environment interactions are evident for egg size and number. Trade—offs between egg size and number are seen at high low ration levels, but only in second (or third) to sixth broods, and not in first broods, which had relatively high variation in total mass. Results of this study that contradict predictions or assumptions of optimal offspring investment theory include: (1) at both ration levels both egg size and number were strongly positively correlated with brood mass, independently of maternal mass, (2) although, at low ration, variability of brood mass usually exceeded the variability of egg mass, as expected, the opposite was true at high ration, and (3) the interclonal difference in offspring size was not correlated with survival ability under starvation. Offspring size covaried positively with maternal size and age, partly independently of culture conditions. This relationship is explained as the result of selection in growing parthenogenetic populations for increased fecundity in young small females, which are constrained to produce relatively small eggs because of spatial limitations of the brood pouch.
Shelter Selection by Spiny Lobster Under Variable Predation Risk, Social Conditions, and Shelter Size
Authors: Eggleston David B. , Lipcius Romuald N.
Published: May 2006

Shelter use patterns of den—dwelling Caribbean spiny lobster, Panulirus argus, appear to be regulated by predation risk. The risk of predation may be modified by (1) social structure, which alters the effectiveness of communal defense, and (2) the scaling between lobster size and shelter size, which enhances the protective capacity of the den. These hypotheses were tested with field enclosure experiments using artificial lobster shelters, which examined the effects of predation risk (i.e., presence or absence of a major predator, the nurse shark Ginglyostoma cirratum), spiny lobster size, social condition (i.e., presence or absence of conspecifics), and shelter size upon den choice by juvenile and adult P. argus. To corroborate the findings of the enclosure experiments we also quantified seasonal, size—specific abundance patterns of P. argus in the field by deploying artificial lobster shelters (casitas) of different sizes in two habitats that differed primarily in the potential for gregarious interactions: an inner—bay, sand seagrass flat with high lobster densities, and an outer—bay, seagrass bed adjacent to coral reefs with sparsely distributed lobsters. The experimental and observational field results were strikingly similar–social condition and the scaling of lobster size to shelter size jointly regulated den choice patterns of adult and juvenile Panulirus argus, particularly under high predation risk. When conspecific density and predation risk were low, lobsters resided primarily in shelters whose dimensions were scaled to their own; when conspecific density was high and predation risk was low, lobsters resided predominantly in large shelters offering the highest potential for gregariousness; when conspecific density and predation risk were high, lobsters shifted to gregarious habitation in smaller, scaled shelters; and, when predation risk was high and conspecific density was low, lobsters occupied smaller shelters. The frequency of gregariousness in the field was much higher at the inner—bay site, where lobsters were dense, than at the outer—bay site, where lobsters were sparse, even accounting for the difference in lobster density between sites. This study indicates that the density of conspecifics in a given habitat can enhance gregariousness in spiny lobsters, which in turn influences the relative impact of lobster size, shelter size, and predation risk upon den choice. In defining the critical determinants of den choice for P. argus, we also provide an empirical and conceptual framework for identifying how variation in the availability of resources, such as conspecifics and appropriately scaled refuges, influence the distribution and abundance of social, shelter—dwelling species.
Effects of Soil Resources on Plant Invasion and Community Structure in Californian Serpentine Grassland
Authors: Huenneke Laura Foster , Hamburg Steven P. , Koide Roger , Mooney Harold A. , Vitousek Peter M.
Published: May 2006

Non—native annual grasses dominate most Californian mediterranean—climate grasslands today. However, native Californian grassland flora persists in grasslands on serpentine—derived soils. We manipulated soil nutrient resources to explore the links between nutrient availability, plant productivity, and the relative abundances of native and non—native species in serpentine grassland. Factorial combinations of nitrogen, phosphorus, and other components of a nutritionally complete formula were added to field plots over two growing seasons. Fertilization with nitrogen and phosphorus increased biomass of the resident vegetation substantially in the first season, and within two years allowed the invasion and dominance of non—native annual grasses in patches originally dominated by native annual forbs. Species richness declined with fertilization, as the increased biomass production by invaders suppressed some native forbs. Increased macronutrient availability can increase production on serpentine—derived soil, even when other serpentine characteristics (such as low Ca/Mg ratios and high heavy—metal concentrations) have not been mitigated. Observed changes in community structure and composition demonstrate that the invasibility of plant communities may be directly influenced by nutrient availability, independent of physical disturbance.
Determinants of Herbivore Carrying Capacity: Plants, Nutrients, and Equus Asinus in Northern Australia
Authors: Freeland W. J. , Choquenot David
Published: May 2006

Populations of feral donkeys, Equus asinus, in northern Australia are limited by density—dependent mortality during the first 6 mo of individuals' lives. A female's ability to raise her offspring successfully increases with age and is dependent on her maintaining high levels of stored mineral nutrients (calcium, phosphorus, and sodium). A low proportion of offspring was successfully raised in a population at carrying capacity where females had low levels of stored mineral nutrients. Females in a growing population that had been reduced to 45% of carrying capacity had high levels of stored minerals and successfully raised a higher proportion of offspring. Females in the population at carrying capacity ingested a species—poor diet (predominantly a grass, Sehima nervosa) containing low levels of nitrogen and mineral nutrients, and high levels of crude fibre. Females in the growing population ingested a more species—rich diet with higher levels of nitrogen and minerals, and less crude fibre. Levels of calcium, phosphorus, and potassium in the faeces of females in the population at carrying capacity were higher than those in the food ingested. The reverse was true for females in the growing population. At the time of sampling (early dry season), 17% of females in the population at carrying capacity and 0% of females in the growing population were eating or had recently eaten clay. Increased salivary secretion and gastrointestinal irritation caused by high—fibre diets (even when lubricated by clay) may be responsible for mineral depletion of females at carrying capacity, and hence population limitation through the inability of females to raise offspring.
Ecosystem Metabolism and Turnover of Organic Carbon along a Blackwater River Continuum
Authors: Meyer Judy L. , Edwards Rick T.
Published: May 2006

Variation in ecosystem metabolism and organic carbon turnover length was examined in the Ogeechee River (Georgia, USA) basin, a sixth—order blackwater river, to determine if the patterns observed in other rivers are also seen along a river continuum characterized by warm water temperatures, low stream gradient, extensive riparian swamps, and tea—colored water. The study was in two parts: an intensive study of seasonal variation in metabolism and organic carbon turnover length at a fourth—order site, which can be compared with an earlier study at a sixth—order site, and a study of the variation in system metabolism at five second— through sixth—order sites along the continuum. Metabolism varied seasonally at the fourth—order site that was studied most intensively; mean gross primary production (GPP, measured as O2) was 0.8 g°m—2°d—1 and mean community respiration (CR, also measured as O2) was 4.1 g°m—2°d—1. The community production/respiration ratio (P/R) ranged from 0.02 to 0.4 with an annual mean of 0.2. This is clearly a heterotrophic stream reach, where high rates of community respiration are supported by inputs of organic matter from extensive riparian swamps. Mean annual organ carbon turnover length at the fourth—order site was 570 km, although on most dates the length was much less, with a median of 114 km. Turnover lengths are significantly less than those at the sixth—order site, but much greater than observed in other rivers of similar size, probably because most of the organic carbon export in this river is dissolved organic carbon (DOC), which is less efficiently retained. Seasonal variation in turnover length reflected variation in transport. At five sites along the Ogeechee River continuum ranging from second to sixth order, GPP, CR, and P/R increased with stream order, as observed in other rivers; yet P/R was always <1, and net daily metabolism was always negative and became more negative with increasing stream order. In contrast to other river continua studied, metabolism in this low—gradient blackwater river becomes more dependent on allochthonous organic carbon with increasing stream size despite downstream increases in GPP; floodplains and riparian swamps appear to be the source of allochthonous carbon. Riparian influences increase rather than decrease along the continuum in this river. Although first—through third—order streams represent 32% of the channel area in the Sixth—order basin, only 4% of basin—wide GPP and 9% of basin—wide CR occurs in them. Higher order streams are less efficient in organic carbon processing (longer turnover lengths), but are nonetheless responsible for most of the metabolic activity in this river basin.