bluuebunny/crossref_metadata_2025 huggingface dataset:
Annual Carbon Mineralization and Belowground Production of Spartina Alterniflora in a New England Salt Marsh
Authors: Howes Brian L. , Dacey John W. H. , Teal John M.
Published: May 2006

The annual rates and depth distribution of organic carbon mineralization to CO2 were determined in sediments supporting stands of short Spartina alterniflora. Carbon dioxide production was estimated by two independent techniques. We constructed a CO2 budget based on measurements of CO2 emission from the sediment to the atmosphere, export of dissolved inorganic carbon in porewater exchange, and changes in the porewater pool of dissolved inorganic carbon throughout the year. We also measured CO2 production in salt marsh sediments by monitoring changes in total inorganic carbon in cores. The estimates obtained by the two methods were similar, giving a total annual CO2 production of between 67 and 70 mol°m—2°yr—1. We also measured the losses of organic matter as methane (O.1—0.3 mol°m—2°yr—1) and dissolved organic carbon (0—3 mol°m—2°yr—1) and burial (7.4 mol°m—2°yr—1) in order to construct a carbon budget for the sediments. These data, when combined with the estimates of carbon mineralization, gave an estimate for the organic carbon loading to the sediments of 68—78 mol°m—2°yr—1. About 95% of the annual carbon input either decomposes to CO2 in situ or is buried, and <5% is exported from the sediment. We estimated that belowground C production in short S. alterniflora in this Massachusetts marsh is 58—75 mol°m—2°yr—1.
Streamflow Regulation and Fish Community Structure
Authors: Bain Mark B. , Finn John T. , Booke Henry E.
Published: May 2006

Many regulated streams are characterized by high variable and unpredictable flow regimes. Since changes in streamflow directly modify physical habitat, streams with such highly variable flows provide highly unstable aquatic habitats. We evaluated the effect of artificial streamflow fluctuations on stream fish communities by comparing fish densities, in species and habitat groups, between two rivers differing in daily flow regime: on with with a natural flow, and one with highly regulated flows. We developed a simple model describing the relationship between available streams habitat and its use by 15 species or size classes of fish in the natural river. Species and size classes that used a specific set of microhabitat conditions were identified by comparing habitat characteristics for samples with and without each type of fish; for fish that used a particular type of microhabitat, we grouped species and size classes according to similarity in microhabitat use. Next, we categorized stream habitat samples in both the natural and regulated rivers into groups on the basis of fish habitat use criteria. Fish densities for each fish and habitat group were ten individually compared between the two rivers. An abundant (>90% of all fish) and diverse (nine species) group of small—fish species and size classes were restricted to microhabitat that was characterized as shallow in depth, slow in current velocity, and concentrated along stream margins. This group of fish was reduced in abundance in the regulated river and absent at the study site with the greatest flow fluctuation. Another fish group included species and size classes that used either a broad range of habitat or a microhabitat that was deep, fast, or both, and was concentrated in midstream areas. The density of fish in this group was higher in the regulated river and peaked at the sites with the greatest fluctuations in flow. Highly variable and unpredictable flow regimes appear to be a high—frequency disturbance that effects fish differently depending on the way they use stream habitat and acts to reduce community complexity.
Interactions for Pollination among Coflowering Shrubs
Authors: Rathcke Beverly
Published: May 2006

To examine competition for pollination in a natural community, I measured pollinator sharing, pollinator dependence, and limitation of fruit set for four colflowering shrub species in The Great Swamp, Rhode Island. These species included all potential competitors for pollination and have had the long, extensive associations deemed necessary for competitive coevolution. The four shrubs shared the major pollinators (bees), and three of the four species depended upon pollinators for fruit set; but pollination limitation due to competition was detected in only one species. Despite the lack of competition, the shrubs interacted negatively and stronly for bumble bees, which were major pollinators. The species differentially attracted bumble bees, and their relative attractive (competitive) abilities were predicted by an index based on average daily sucrose production per flower x local floral density as: Ilex opaca > Kalmia latifolia > Gaylussacia frondosa > Kalmia angustifolia. Bumble bees rarely visited the less attractive species during flowering overlap in 3 of 4 yr. Competitive avoidance was achieved, not by specialization and divergence as is commonly hypothesized, but by having traits that confer tolerance of rare pollinator visits (e.g., long—lived flowers or self—pollination) or by having diverse pollinator assemblages (small bees and bumble bees). K. latifolia was the only shrub specialized for pollination (by bumble bees), and this specialization increased, rather than decreased, the risk of pollination limitation because K. latifolia was a poorer competitor for bumble bees than Ilex. Pollination limitation was prevented or reduced by the extended longevity of unpolinated flowers. Flowers could remain viable > 2 wk until Ilex ceased flowering and bumble bees switched to K. latifolia K. angustifolia, the poorest competitor, was visited by many species of small bees and could also self—pollinate, so fruit set was maintained even in the absence of bumble bees, Gaylussacia lacked these traits, and it was pollination limited: flowers are short—lived and cannot self—pollinate. For this species small bees were important in maintaining fruit set when bumble bees switched to more attractive species. These traits for competitive avoidance are advantageous when pollination is uncertain for any reason, such as unfavorable weather for pollinator foraging. As a result, their presence cannot be unambiguously attributed to past competition, although they currently are important in reducing competition among these shrubs. Such traits as generalization for resource use of tolerance of sparse resources may be as important as specialization or greater competitive ability for the coexistence of species in competitive communities.
Functional Response of an Ambush Predator: Chaoborus americanus Predation on Daphnia pulex
Authors: Spitze Ken
Published: May 2006

Estimations of invertebrate predation parameters usually rely on behavioral observations and on feeding rates in single, arbitrary prey densities. In this study, predation parameters were estimated directly from functional response data. This type of analysis relies on many prey densities and therefore should lead to more widely applicable parameter values. Functional response curves of third—and fourth—instar Chaoborus americanus preying on five size—classes of Daphnia pulex in the laboratory were examined. Data for each size—class were fitted to Rogers' random predator equation, a modification of Holling's type II functional response equation. The data fit the model well. The parameters a' (instantaneous attack rate) and Th (handling time) were calculated for each prey size from these curves. Attack rate reached a maximum, indicating highest vulnerability, at an intermediate size. Deduced handling times were greater than simple observation would suggest and were highly correlated with prey dry mass, suggesting digestion as the primary component. Results of mixed—size—class experiments were compared to predictions based on the parameters from single—size—class experiments. The ratio of predation—caused mortality rates for prey size—classes was consistent with the predictions, suggesting that no active behavioral selection was occurring. Feeding rates in mixed—sized—class experiments were underestimated when experiments were run a month later than the single—size—class experiments during the 1st yr of this study. However, when single— and mixed—size—class experiments were run simultaneously during the 2nd yr, predictions were very close to observed feeding rates. This indicates the possibility that predator age and seasonal physiological changes affect functional response parameters. Attack rate increased with temperature, probably due to increased encounter rates as Daphnia swimming speed increased. Handling time decreased with increased temperature, probably due to increased digestive rate. The biological significance of Holling's parameters and those of other more behaviorally based models is discussed.
Dynamics of Nitrogen Competition Between Successional Grasses
Authors: Tilman David , Wedin David
Published: May 2006

Pairwise competition experiments were performed for 3 yr on an experimental nitrogen gradient at Cedar Creek Natural History Area, Minnesota, where N is the major limiting resource during early succession. Agrostis scabra, and early successional grass, competed against another early (Agropyron repens), a later (Schizachyrium scoparium), and an even later (Andropogon gerardi) successional species. On low N soils, Agrostis was competitively displaced by each of the later successional species, but persisted with Agropyron. On high N soils, Agrostis was displaced by all three of the other species. The inferior competitive ability for N of the early successional species refutes the resource ratio hypothesis of succession. Rather, the high allocation of Agrostis to seed and its rapid colonization of field support a colonization—competition hypothesis of succession. For two of three cases, the outcome of competition on low nitrogen soils was predicted by R*, the nitrogen concentration to which monocultures of each species reduced extractable soil nitrate and ammonium on N—limited soils. In these cases, the species with the significantly lower R* for nitrogen displaced the other species. In the third case, the species had more similar R* values, and the species with the lower R* had not displaced the other species within 3 yr (but it had done so after 5 yr; see Note added in proof).
Seasonal Changes in Thermoregulation by the Frillneck Lizard, Chlamydosaurus Kingii, in Tropical Australia
Authors: Christian Keith A. , Bedford Gavin S.
Published: May 2006

The frillneck lizard Chlamydosaurus kingii is an arboreal lizard that is a conspicuous component of the reptile fauna of the wet—dry tropics of northern Australia during the wet season. During the dry season, however, they are secretive, and a previous study revealed that during this season they remain perched in trees and have field metabolic rates only 28% of the wet season levels. Body temperatures (Tb's) of the lizards were measured by ratio telemetry throughout the day during the wet and dry seasons. The midday Tb's during the wet season high (grand mean = 36.7°C) and typical for heliothermic lizards, but the dry season midday Tb's were significantly lower (grand mean = 32.8°C). Microclimatic data and physical characteristics of the lizards were used in a biophysical model to calculate the operative temperatures (Te) of lizards in the shade, in the sun on a horizontal plane, and normal to the sun at each hour of the day for the two seasons. The Te's revealed the physical possibility for the lizards to achieve much higher Tb's during the dry season than were measured. Thus, the lower Tb's in the dry season represent a shift in preference rather than an inability to attain a high Tb's during the cooler dry season. Inspection of the Tb's and Te's revealed that although the lizards remained cooler in the dry season, they did not thermoregulate at the lowest possible Tb's. During both seasons the lizards basked in the sun early and late in the day, but during the dry season the lizards stopped intensive basking at a Tb °4°C lower than in the wet season. An index of the extent to which the lizards exploit the available thermal environment indicates that they thermoregulate carefully in both seasons. Tb's were also measured in a laboratory thermal gradient during both seasons, and the Tb's selected during the dry season were significantly lower than those selected in the wet season. This suggests that the seasonal shift in thermal preference is acclimatization response or an endogenous seasonal cycle rather than a response to a simple thermal cue. The lower Tb's in the dry season result in a conservation of energy and water during a season when these resources are relatively scarce. However, the fact that the lizards do not thermoregulate at the lowest possible Tb's suggests that the dry season Tb's represent a compromise between conservation of resources and the ability to perform other functions such as escape predators and/or digest food.
A Behavioral Trade‐Off and Its Consequences for the Distribution of Pseudacris Treefrog Larvae
Authors: Skelly David K.
Published: May 2006

Larval chorus frogs (Pseudacris triseriata) and spring peepers (P. crucifer) show marked differences in their distribution patterns, with chorus frogs tending to occur in less permanent ponds containing lower predator densities, and spring peepers tending to occur in more permanent ponds with higher predator densities. In order to understand this pattern I performed experiments at two levels. First, at the community level, I evaluated the importance of competition, predation, and pond permanence for patterns of larval performance. Second, at the behavioral level, I asked whether a hypothesized trade—off between gain and risk underlies ecological interactions, and ultimately, patterns of performance and distribution of these species. I employed a field manipulation to asses the contribution of competition and predation within different ponds that varied in permanence. In order to examine the effect of interspecific competition, enclosures contained each tadpole species either alone or in the presence of an equal density of the other species; these enclosures were placed in two temporary ponds, two intermediate ponds, and two permanent ponds. Tadpole stocking densities reflected local natural densities. This competition treatment was crossed with a predator treatment manipulating the absence or presence of predators (with "presence" representing the naturally occurring density within each pond). Patterns of relative performance were consistent with relative distribution patterns across the pond permanence gradient: chorus frogs performed best relative to spring peepers in temporary ponds while spring peepers performed best relative to chorus frogs in permanent ponds (performance was measured as survivorship and growth). The dominant sources of mortality were pond drying in temporary ponds and predation in more permanent ponds. In contrast to most previous experimental studies of larval anurans, competition had small effects on tadpole performance. Results from the field experiment suggest that pond drying and predation may be important determinants of performance, and that the two species are differentially susceptible to these mortality agents. In a set of laboratory experiments I evaluated the hypothesis that a trade—off between foraging gain and predation risk mediated through activity (proportion of time spent moving) might explain patterns of Pseudacris distribution. In both species, individuals facultatively reduced activity in the present of a caged predator with a corresponding drop in growth rate. In general, larval chorus frogs also tend to be more active and faster growing and developing than spring peepers. These results provide support for the existence of the trade—off and suggest that interspecific differences in activity may contribute to patterns of larval performance and distribution among ponds. The general nature of this trade—off suggests that it may also be important in other systems.
A Model of Multiple‐Element Limitation for Acclimating Vegetation
Authors: Rastetter Edward B. , Shaver Gaius R.
Published: May 2006

In this paper we present a simple model of multiple—element limitation of plant production and biomass accumulation. The primary aim of this models is to develop a theoretical framework for examining multiple—element limitation vs. single—element limitation and for examining the relationship between short—term and long—term responses to changes in element availability. In the model we assume that there is an "optimal" ratio of mineral elements in vegetation biomass, and that the vegetation continually adjusts its relative element uptake capacities to compensate for shifts away from this optimum. We examine the responses of this model to changes in element availability in the plant environment, where "availability" is defined either as fixed concentrations of non—depletable elements or as fixed replenishment rates of depletable elements. The model results suggest that the nature of the controls on element availability has a major impact on whether single— or multiple—element limitation prevails, even when plants can acclimate so as to maintain an "optimal" nutritional balance. Single—element limitation occurs when the replenishment rate of an essential element to the available pool is limited and sustainable plant uptake of that element equals the replenishment rate. Furthermore, when single—element limitation prevails, there is little or no correlation between short—term responses to a change in element availability and long—term, equilibrium responses. In general, previous experimental studies and models of plant growth in response to changes in relative availability of multiple, essential elements have either not specified how those resources are controlled, or have examined only one type of control. Our results help to explain the diversity of results of past studies of multiple—element limitation, suggest some improvements in experimental design for future studies, and have important implications for the extrapolation of the results of controlled experiments to field situations.
Surfgrass (Phyllospadix Torreyi) Reproduction: Reproductive Phenology, Resource Allocation, and Male Rarity
Authors: Williams Susan L.
Published: May 2006

Little is known about the reproductive ecology of aquatic angiosperms, despite their wide distribution and ecological importance. Surfgrass (Phyllospadix torreyi) is a clonal marine angiosperm that lives on turbulent rocky shores. Surfgrass exhibits a strongly female—biased sex ratio in flowering shoots throughout its distribution along the Pacific coast of North America. The ecological causes for this biased sex ratio were examined over 3 yr in a surfgrass population off Catalina Island in southern California, USA. Abundances, biomass, and C:N content of male and female flowering ramets were measured, and the reproductive phenology, reproductive losses to abortions, herbivore damage to fruits and seeds, and clonal survival and growth were quantified. Each year, a quarter of the vegetative ramets reproduced sexually and seed production in the population was prolific. The population sex ratio was female biased across the entire depth distribution of surfgrass, but males were more abundant deeper in the bed. Despite male rarity, pollen was abundant in the population, and virtually all ovules were fertilized, with few losses to abortions or herbivory before seed release. Potential female fitness (seeds produced) was highest at shallow depths where light availability was greatest and abortions were reduced. The spatial segregation of the sexes was not due to environmental sex lability but to greater ramification of female vs. male clones in shallow depths. Female flowering shoots had significantly higher biomass than males across water depth, despite no obvious cost in terms of vegetative growth, or advantage in terms of plant size. If current theory on sex allocation than the sex ratio should be biased toward the gender with the lower reproductive cost is true for surfgrass, then surfgrass provides an example of how a subtle resource allocation difference can result in a dramatic population bias. An alternative explanation for the female bias in surfgrass population is greater male mortality due to significantly weaker attachment to the substratum. This study demonstrates the potential role hydrodynamics can play in aquatic plant reproduction and in structuring their population.
Tropical Forest Litter Dynamics and Dry Season Irrigation on Barro Colorado Island, Panama
Authors: Wieder R. Kelman , Wright S. Joseph
Published: May 2006

Moisture seasonality may control forest floor decomposition rates in tropical forest. We used a mass balance model and 5 yr (December 1986 through December 1990) of weekly litterfall and monthly forest floor mass measurements from control and dry season irrigated plots to test this hypothesis on Barro Colorado Island, Panama. Litterfall and forest floor mass were greater in the dry season that in the wet season. Irrigation affected neither the timing nor the quantity of litterfall. In contrast, dry season irrigation reduced forest floor mass throughout the year, not just during the dry season. Forest floor decomposition during the dry season was enhanced by irrigation. During the dry season, net decomposition (in grams per square metre per day) and exponential decay coefficients (per day) averaged 48 and 42% greater, respectively, in irrigated plots than in controls plots. As a consequence, seasonal differences in decomposition rates were more pronounced in the control plots than in the irrigated plots. Net decomposition rates, for examples, a averaged 105 and 22% greater during the wet season than during the dry season on control and irrigated plots, respectively. Net decomposition was positively correlated with rainfall in the control plots, but not in the irrigated plots. These results support the hypothesis that moisture seasonality controls forest floor decomposition in tropical moist forests.
Search and Prey Detection by Foraging Sparrows
Authors: Getty Thomas , Pulliam H. Ronald
Published: May 2006

The conventional ecological approach to predation provides a limited framework for understanding perceptual and tactical mechanisms of search and detection and their dynamical consequences. In constrast, models developed to guide naval search operations are "spatiotemporal dynamic interaction models," analogous to those developed in plant population ecology. Our application of these modes to foraging sparrows indicates that perceptual and tactical constraints play a critical role in the allocation of search effort across habitat patches, in the choice of tactics within patches, and in the predation dynamics that emerge over time. Sparrows search where prey are most detectable rather than where they are most abundant, and they tune search tactics to preceptual operating conditions. The approach describes and predicts details of search and detection, cumulative harvest rates, and selectivities more accurately than the conventional search—efficiency approach.
Effects of Nitrogen Availability and Herbivory on Eelgrass (Zostera Marina) and Epiphytes
Authors: Williams Susan L. , Ruckelshaus Mary H.
Published: May 2006

Although the growth of eelgrass (Zostera marina) is controlled by resources as well as higher order interactions with epiphytes and their herbivores, these constraints rarely are considered together. The ability to utilize both water column and sediment nutrient sources in a complex habitat may provide eelgrass with a partial release from nutrient competition with epiphytes that have more efficient uptake kinetics and can reduce eelgrass growth, particularly in eutrophic habitats. We investigated the relative effects of dissolved inorganic nitrogen in the water column vs. the sediments, and herbivory by the common isopod Idotea resecata, on eelgrass growth and epiphyte biomass in an intertidal eelgrass bed in Padilla Bay, Washington. In the field, we fertilized the sediments and/or the water column with ammonium and measured eelgrass growth and epiphyte biomass. We also monitored epiphyte biomass and water column nutrient concentrations and censused isopod densities. Laboratory experiments focused on the effects of I. resecata, fertilization of the water column and sediments, and depletion of sediment nutrients on eelgrass growth and epiphyte biomass. Most simply, we hypothesized that epiphytes would respond positively to increased water column nutrients as eelgrass would to increased sediment nutrients, and that herbivory on epiphytes could mitigate deleterious effects of epiphytes on eelgrass. We demonstrated that eelgrass growth is affected both by sediment nitrogen resources and the higher order effects of epiphytes and their control by Idotea resecata. During our field experiments, growth of eelgrass leaves tended to increase in response to sediment fertilization; this trend was significant in April 1988 but not in August 1987. At both times, leaf growth rates demonstrated a saturation—type response to sediment ammonium concentrations >100 μmol/L, providing further support for nitrogen limitation of eelgrass growth over much of the range in ambient concentrations (30—137 μmol/L) in the sediment porewaters. Together, sediment ammonium concentrations and epiphyte biomass explained a significant portion (71%) of the variance in eelgrass leaf growth in August 1987. Consideration of sediment nitrogen, epiphytes, or herbivores alone is unlikely to yield a predictable understanding of the control of eelgrass primary productivity in nature, particularly given the complexity of the eelgrass habitat with respect to its dual nutrient sources. For example, epiphyte biomass was predicted by our laboratory experiments and other data to be nitrogen limited in Padilla Bay, yet it was not correlated with ambient nitrogen concentrations nor did it increase with fertilization of the water column. These results can be reconciled by considering herbivory by Idotea resecata. In the laboratory, the isopod reduced epiphyte biomass by one—third and in its absence, epiphyte biomass increased with increasing nitrogen concentrations in the water column and negatively affected eelgrass growth.
Costs of Reproduction in Water Skinks
Authors: Schwarzkopf Lin
Published: May 2006

Cost of reproduction measured under laboratory conditions may have little bearing on actual cost observed in the field, because both environmental conditions and behavior can modify the manifestation of these costs. I studied survival, growth, and reproduction in a wild population of southern water skinks (Eulamprus tympanum) over 3 yr to measure costs of reproduction in females, to examine how the environment influences these costs, and to assess the extent to which females are able to reduce costs. The proportion of mature females that reproduced each year varied from 30—60%. Frequent reproduction appeared costly in terms of both survival rates and growth. Growth rates and survival rates were negatively correlated with reproduction during the 3—yr study. In the coolest and wettest year of the study, when food intake by females was relatively low, gravid females and females that had reproduced in the previous years survived well than nonproducing females. There was no evidence of a survival cost of reproduction in the other two years of the study. Reproduction also imposed a fecundity cost, as overall, mean growth rates of reproducing females were reduced relative to those of nonreproducing females of the same body size. Reduced growth rate translates to reduced litter size in these skinks, because fecundity increases with body size. These results suggest that reproduction can be costly in terms of both survival and future reproduction in these skinks, but that survival costs are not consistent from year to year and are mediated by environmental conditions. Females appear to adjust their frequency of reproduction to minimize these costs, as those females that skipped opportunities for reproduction were those expected to gain the greatest growth and fecundity benefit by skipping.
Dietary and Digestive Constraints on Basal Energy Metabolism in a Small Herbivorous Rodent
Authors: Veloso Claudio , Bozinovic Francisco
Published: May 2006

McNab (1986, 1988) has hypothesized that mammals using food with low energy content should exhibit basal metabolic rates (BMR) lower than those expected on the basis of their body mass (mb). That is, those species that exploit food with low energy content and/or high cost of digestion tend to have low, mass—independent metabolic rates. To date there is not an experimental test of this pattern. The aim of this work was to examine experimentally the effect of diet quality on BMR, digestive efficiency, and the relationship between digestion and energy expenditure in a small herbivorous mammal. We used as a model the herbivorous caviomorph burrowing rodent Octodon degus (mb nearly 200 g), an inhabitant of semi—arid and mediterranean communities of northern and central Chile. Individuals maintained during 27 wk with a diet high in dietary fiber showed significantly lower BMRs (28%) than those feeding on low fiber. Daily food intake and ingestion rates (energy and dry matter) of individuals under a high—fiber diet were significantly higher than animals maintained with a low—fiber diet. The same pattern was obtained for total feces production and rate of feces production. The total intake and rate of ingestion of proteins were not significantly different between treatments. However, a significantly higher amount of protein was excreted by the individuals exposed to a high—fiber diet. Apparent digestibility of dry matter, energy, and protein were consistently lower in individuals maintained with high fiber. However, nonsignificant differences were observed between gut contents in the two treatments (P > .58), but significantly higher digesta turnover rate was observed in animals exposed to a high—fiber diet (P >.05). A significant correlation was found between digestibility and the basal metabolic rate of individuals (rs = 0.781, P >.01), suggesting that elevated digestibilities on high—quality diets allow increased basal rates of metabolism. We suggest that, although small mammals like degus may select sparsely distributed plants of high quality in their habitat, their capability to drop their metabolic demands may help them meet their nutritional and energy requirements when nutritional conditions in the environment deteriorate.
Interactions Between Greater Snow Geese and Their Rearing Habitat
Authors: Manseau Micheline , Gauthier Gilles
Published: May 2006

For herbivores, food quality is a major component of habitat quality. However, food quality is complex to assess because it must integrate not only the availability of nutrients but also their digestibility and the requirements of the animal. To evaluate the quality of food and to determine the feeding constraints faced by young herbivores, we compared two brood—rearing habitats used by the Greater Snow Goose (Chen caerulescens atlantica) in the High Arctic using three levels of investigating. The first was based on a quantitative and qualitative analysis of the plants; the second was behavioral, and compared the feeding effort of human—imprinted goslings; and the third was physiological and compared food selection and plant nutrient metabolizability by the goslings. The two habitats grazed by goslings were a sedge meadow (dominated by Carex aquatilis and Dupontia fisheri) and a grass meadow (dominated by Dupontia fisheri and Eriophorum spp.). Plant biomass was higher in the sedge than in the grass meadow. Plant quality declined during the summer but Eriophorum spp. had a significantly higher nitrogen and lower fiber content than the other two species at all times. However, when combining data on quality and relative abundance, the overall plant quality was similar in both meadows. Goslings spent more time feeding and had a higher pecking rate in the grass than in the sedge meadow. Comparison of diet quality to overall plant quality indicated a higher nitrogen and lower fiber content of the diet in the grass meadow but not in the sedge meadow, probably because goslings fed selectively on Eriophorum in the former habitat. Apparent metabolizability of organic matter and nitrogen, and nitrogen assimilation efficiency by goslings, were lower in the sedge than in the grass meadow at some periods. However, goslings did not compensate by an increased rate of food intake when their nitrogen intake was low which suggests that they had reached their gastrointestinal capacity. Under some circumstances, food quality is therefore more important than food abundance for herbivores. We show that integration of our three levels of investigation (plant food, animal behavior, and physiology) is essential to fully evaluate habitat quality for an herbivore.
Regeneration Mode in Neotropical Piper: Habitat and Species Comparisons
Authors: Greig Nancy
Published: May 2006

Clonal growth or vegetative propagation is widespread among plants, yet we still have no clear understanding of the circumstances under which it is favored over sexual reproduction (via seeds). Within some growth forms of rainforest plants (lianas, understory shrubs), vegetative propagation may be the primary means of regeneration. In this study, the mode and frequency of vegetative propagation in 30 co—occurring species of the tropical shrub genus Piper at the La Selva Biological Station in Costa Rica was assessed through field observations made over several months. Piper species were found to propagate by several means, including stem sprouting, root sprouting, layering, and fragmentation. The mode and frequency of propagation varied significantly among habitats and among species. Shade—tolerant Piper species typical and mid— and late—successional habitats regenerated primarily by vegetative means, especially layering and fragmentation, while shade—intolerant, early—successional species appeared to reproduce and disperse primarily by seed. These differences among Piper species in regeneration mode were due not only to environment, but also to a genetic component, as indicated by experiments investigating the importance of one means of vegetative propagation, fragmentation, to Piper species from different successional habitats. A shadehouse experiment showed that early successional species were less likely than late—successional species to regenerate from detached fragments, independent of light regime. This outcome was corroborated by a transplant experiment in forest understory and large gaps comparing fragment vs. seedling survival among five Piper species from different stages of succession over a 7—mo period. Almost none of the cuttings from early—successional species survived in either habitat. Species of later successional stages survived better as fragments in the understory than in gaps; however, surviving fragments grew larger in gaps. Almost no seedlings from experimentally planted seeds of any species survived in either gaps or understory. Natural seedlings of early—successional Piper species were frequently found in large treefall gaps and other disturbances, while seedlings of species from late succession were seldom found in any habitat.
Species Coexistence, Keystone Species, and Succession: A Sensitivity Analysis
Authors: Tanner Jason E. , Hughes Terence P. , Connell Joseph H.
Published: May 2006

One of the major questions in ecology is, what controls the structure of communities? We used projection matrix models to examine community dynamics and patterns of succession. The inputs of the model are transition probabilities of species replacements that were measured repeatedly during a long—term (1962—1989) study of diverse coral assemblages on Heron Island, Great Barrier Reef. Transitions varied strikingly among species and sites, reflecting differences in recruitment, growth, longevity (persistence), and the rate of replacement of one species by another. Species that had a poor ability to persist (e.g., algae and Pocilloporid corals) were generally good colonists. The observed number of transitions expressed as a proportion of the maximum number possible provides an index of the complexity of interactions in an assemblage, analogous to the concept of connectance in food—web analysis. Transitions occurred to and from nearly every species group, indicating that there was no competitive dominant in this system. We use the models in simulations to track transitory changes in species abundance and community composition following a major disturbance (e.g., due to a cyclone or outbreak of crown—of—thorns starfish). Some species showed a rapid initial increase followed by a decline to lower equilibrium levels, while others increased smoothly to a generally higher equilibrial abundance. The length of time required to reach a climax assemblage using the same matrix recurrently (°20 yr) is far greater than the observed interval between major disturbances, supporting nonequilibrium theories of coral reef communities. Climax assemblages were highly diverse and varied in composition from site to site. The intermediate disturbance “hypothesis” does not fully predict successional changes in these shallow—water coral assemblages since diversity remained very high at equilibrium (i.e., long after major disturbance). Competitively inferior species were not eliminated because routine mortality ensured that some space always remained available for colonization. We also present a novel method for quantifying the relative importance of each species interaction to community composition and the rate of succession, based on a sensitivity analysis of the transition matrix. The analysis shows that the importance of a species to the dynamics of a community may be unrelated to its abundance at equilibrium, with some rare species groups having a greater impact than more common ones. Sensitivity analysis of this type will provide a powerful means of identifying “keystone” species in complex assemblages where experimental manipulation of each species is impossible.
Rainfall, Biomass Variation, and Community Composition in the Park Grass Experiment
Authors: Silvertown Jonathan , Dodd Mike E. , McConway Kevin , Potts Jacqueline , Crawley Mick
Published: May 2006

We used data on grassland plant community composition over a 90—yr period in the history of the Park Grass Experiment, England to look for relationship between variation in composition and annual variation in rainfall and biomass. This was investigated by regressions of biomass and rainfall on each other, and of these variables separately on each of three different measures of variation in plant community composition. Two of these measures, principal components analysis scores based on variation in species abundance and the ratio by mass of nongrass/grass species, showed significant relationships with biomass variation on many experimental plots, although relationships with rainfall were relatively slight or nonexistent. The third measure employed similarity indices to detect changes in species composition in response to variation in biomass, but failed to find any. Biomass was significantly increased by rainfall on all plots. We propose that variation in community composition was more closely related to biomass variation than to rainfall because rainfall selectively favored the grasses in the community, which we believe competed asymmetrically (for light) with the other species when rainfall was high. The severity of this competition would depend upon biomass more directly than upon rainfall, although it is rainfall that enhanced grass growth. In effect, asymmetric competition magnified the effect of rainfall on community composition.
Edge Effects in Central Amazonian Forest Fragments
Authors: Malcolm Jay R.
Published: May 2006

Current descriptive models of edge effects describe little more than the relationship between perimeter and area. A more realistic model, one that incorporates the additive nature of edge effects, is presented and tested. The total edge effect at a location within a patch is modelled as the sum of edge effects at points along the edges of the patch, weighted by the distances from the edge points to the location. At four sites in the central Amazon °80 km north of Manaus, Brazil, I measured vertical stratification of foliage in three primary forest habitats: (1) continuous forest close to linear edges, (2) 10—ha forest fragments, and (3) 1—ha forest fragments. Foliage thickness (foliage density along a vertical line) was scored in six height intervals (0—2, 2—5, 5—10, 10—20, 20—30, and 30—40 m). With decreasing distance from a linear edge, understory (0—5 m) foliage thickness increased and overstory (10—30 m) foliage thickness decreased, and the relationship between foliage thickness and distance to the edge agreed with that predicted by the model. Using parameter estimates obtained from the best “fit” of the model to these data from linear edges, understory and overstory thickness in 1— and 10—ha fragments was predicted. Foliage thickness varied extensively among fragments, but agreed quite closely with that predicted by the model, and the additive model performed better than a curvilinear non—additive one.
How Quality of Host Plant Affects a Predator‐Prey Interaction in Biological Control
Authors: Walde Sandra J.
Published: May 2006

In cases of successful biological control the pest population is maintained at low densities by a natural enemy, but the mechanisms by which the predator or parasitoid manages to control the pest are often unknown. This study was undertaken to determine the response of a predator to different population growth rates of its prey within a biological control system. Population growth rate of the phytophagous mite Panonychus ulmi was varied by altering the nutritional status of the host plant (apple trees [Pyrus malus]). NPK fertilizer was added to the soil, resulting in higher nitrogen, lower phosphorus, and lower potassium levels in the foliage of the trees. There appeared to be nonlinear relationship between nitrogen and P. ulmi densities, with density increasing and then decreasing as nitrogen concentration was increased. Two predaceous mites, Typhlodromus pyri and Zetzellia mali, were implicated in the control of P. ulmi, but the effectiveness of both was reduced at high nitrogen levels. The predators therefore did not fully compensate for increases in the population growth rate of their prey, although P. ulmi densities were maintained well below the economic threshold in all cases. However, the probability of persistence of the prey populations was not significantly affected by the nutrient regimes, suggesting that the fundamental dynamics of the predator—prey interaction were not altered by the range of population growth rates in this study.
Induced Resistance and Susceptibility to Herbivory: Plant Memory and Altered Plant Development
Authors: Karban Richard , Niiho Chikako
Published: May 2006

Systems of induced resistance for both vertebrates and invertebrates possess a memory; individuals that are attacked by a particular pathogen are able to respond more quickly and more effectively the second time they encounter that pathogen. Many plants have been found to be more induced by repeated herbivory than by a single bout of herbivory. In these experiments the frequency and magnitude of damage were confounded so that it was not clear if memory was involved of if more overall damage amplified the induced response. We conducted an experiment to test the memory of induced resistance in cotton to repeated attacks by spider mites. We held the overall amount of damage (herbivore—days) constant but varied the number of times that plants were exposed to damage. We did not find that plants damaged a second time responded more effectively than those damaged only once. Power analysis indicated that a large of memory was unlikely to have been overlooked in these experiments if it existed. Following observations that different types of damage caused different responses in mountain birch (Haukioja et al. 1990), we damaged either cotyledonary leaves or apical buds of young cotton plants and challenged them with spider mites. We found that damage to cotyledons induced resistance but damage to apical buds induced susceptibility. Damage to cotyledons accelerated the rate of abscission of cotyledons, a favored tissue for spider mites; damage to buds delayed abscission of cotyledons. Cotyledon abscission was sufficient to induce resistance but was not necessary condition since induced resistance has been observed when cotyledon abscission was not found. Our results were consistent with the hypothesis that damage to these two tissues differentially affected hormonal regulation as well as the alternative hypothesis that these forms of damage had opposite effects on plant ontogeny.
A Model of Species Density in Shoreline Vegetation
Authors: Shipley Bill , Keddy Paul A. , Gaudet Connie , Moore Dwayne R. J.
Published: May 2006

Predicting the species density (number of species per unit area) of communities is a major goal of ecology. We present a regression model of species density on a local (0.25 m2) scale for the vegetation of freshwater shorelines in southwestern Quebec, Canada. Two attributes of the vegetation, the amount of aboveground biomass (in grams) and the proportion of the vegetation composed of obligate perennial species, predicted 76% of the variation in species density. The success of the predictor variables suggests that competitive intensity, as reflected in biomass levels, and the time elapsed since the last disturbance event, as reflected in the proportion of the vegetation composed of obligate perennials, are important determinants of local variation in species density. The model was then tested against independent data from shoreline vegetation in southeastern Ontario, Canada. There were no significant differences in the two data sets in their response to the two independent variables in the full model. However, only 42% of the variance in species density was explained in the combined data set.