bluuebunny/crossref_metadata_2025 huggingface dataset:
The Effect of Neighbors on Root Distribution in a Creosotebush (Larrea Tridentata) Population
Authors: Brisson Jacques , Reynolds James F.
Published: May 2006

We excavated and mapped the lateral extension of 32 creosotebush shrubs (Larrea tridentata) in the Chihuahuan desert of New Mexico to examine the effect of neighborhood interaction on root distribution. The smallest closed—angle polygon encompassing all roots of an individual was taken as a representation of its root system. Several geometrical characteristics of these polygons were measured and compared to interference vectors based on the location and size of the neighbors. We found that root systems were more developed away from the maximum competitive pressure of neighbors. Relation between root system shape and pressure from neighbors was stronger when the competitive vectors were integrating effect from all neighbors. Size of neighbors did not appear to contribute significantly to the relation. The resulting spatial pattern tended to reduce the overlap between neighboring root systems. Two conceptual models of root growth response to neighbors appear to explain our results. Both models assume that when the root system of neighbors meet, root growth is impaired or ceases at the zone of contact. In the nonoverlapping, non—compensatory model, the decrease in root growth between two close neighbors in not compensated elsewhere, possibly affecting the overall plant performance. In the non—overlapping, compensatory model, a plant with a close neighbor responds by investing in root growth away from the competitive pressure or simply in zones free of neighbors. Under this model, two plants can be close to each other and not compete. Competition in the population is for space and only occurs when a plant root system is crowded on all sides.
Seedling Recruitment in Forests: Calibrating Models to Predict Patterns of Tree Seedling Dispersion
Authors: Ribbens Eric , Silander John A. , Pacala Stephen W.
Published: May 2006

Recruitment, the addition of new individuals into a community, is an important factor that can substantially affect community composition and dynamics. We present a method for calibrating spatial models of plant recruitment that does not require identifying the specific parent of each recruit. This method calibrates seedling recruitment functions by comparing tree seedling distributions with adult distributions via a maximum likelihood analysis. The models obtained from this method can then be used to predict the spatial distributions of seedlings from adult distributions. We calibrated recruitment functions for 10 tree species characteristics of transition oak—northern hardwood forests. Significant differences were found in recruit abundances and spatial distributions. Predicted seedling recruitment limitation for test stands varied substantially between species, with little recruitment limitation for some species and strong recruitment limitation for others. Recruitment was limited due to low overall recruit production or to restricted recruit dispersion. When these seedlings recruitment parameters were incorporated into a spatial, individual—based model of forest dynamics, called SORTIE, alterations of recruitment parameters produced substantial changes in species abundance, providing additional support for the potential importance of seedling recruitment processes in community structure and dynamics.
Chemical Defense Against Diverse Coral‐Reef Herbivores
Authors: Hay Mark E. , Fenical William , Gustafson Kirk
Published: May 2006

Five secondary metabolites from tropical marine algae and one related compound from an herbivorous sea—hare (Aplysidae) were coated, at approximately natural concentrations, onto the palatable seagrass Thalassia testudinum and placed on coral reefs where they could be eaten by the diverse group of herbivorous fishes that occur there. Laboratory feeding assays with the herbivorous sea urchin Diadema antillarum were also conducted. When compared to appropriate controls, the following terpenoid compounds significantly reduced the amount of Thalassia eaten by both Diadema and reef fishes: stypotriol, from the brown seaweed Stypopodium zonale; pachydictyol—A, which is produced by several genera of tropical (Dictyota and Dilophus) and warm—temperate (Pachydictyon and Glossophora) brown seaweeds; elatol, from the tropical red alga Laurencia obtusa; and isolaurinterol, which is produced by several tropical and warm—temperate species of Laurencia. Under very mild acid conditions, isolaurinterol is converted to a structurally similar compound, aplysin, found in high concentrations in sea—hares that feed on isolaurinterol—containing Laurencia species. Aplysin did not deter feeding by either type of herbivore. Cymopol, a terpenoid bromohydroquinone from the green alga Cymopolia barbata, significantly reduced feeding by reef fishes but significantly stimulated feeding by Diadema. Pharmacological and crude bioactivity tests suggest that several of these compounds function as generalized toxins. However, these generalized laboratory assays are not necessarily good predictors of how compounds will affect feeding by herbivores. For example, pachydictyol—A and stypotroil were equally effective at deterring fishes and Diadema, even though pachydictyol—A shows almost no bioactivity in laboratory assays while stypotriol and its oxidation product, stypoldione, are very bioactive. Herbivory on coral reefs is more intense than in any other habitat studied and the diversity of herbivore types is high. It appears that this intense grazing has provided strong selection for seaweeds that synthesize unique secondary metabolites that significantly reduce the consumption of plants exposed to attack by a diverse group of reef herbivores.
Coral Reef Recovery on Guam (Micronesia) After Catastrophic Predation by Acanthaster Planci
Authors: Colgan Mitchell W.
Published: May 2006

At Tanguisson Reef, Guam, in 1968—1969 a population explosion of the coral—eating sea star Acanthaster planci devastated the coral community. In the wake of this predation, coral species richness, density, and cover were drastically reduced, and the species composition was altered. In two of three reef zones examined, < 1% coral cover remained. At the time of disturbance, some considered the magnitude of this and similar Acanthaster disturbances unprecedented, and predicted long recovery times because reefs were viewed as mature, stable communities. This major disturbance offered an opportunity to study the processes and rate of community recovery. Using the data from previous studies in 1970, 1971, and 1974, and my data from 1980—1981, I present a long—term analysis of coral community development. I have focused on coral recruitment, survivorship, and species settlement patterns in relationship to the remnant surviving population. The preference of Acanthaster planci for certain prey (e.g., Montipora and Acropora) shifted the species composition to one in which nonpreferred prey predominated (e.g., Porites and Leptastrea). The predominance of nonpreferred prey was short—lived, and by 1980, preferred prey were the primary colonists of two zones. The three biological zones (reef front, submarine terrace, and seaward slope) had their own characteristic species assemblages soon after the disturbance. Nonrandom recruitment or survival of juvenile corals (diameter ≤ 4 cm) established these zones. Although the percentage of juvenile corals fell throughout the survey, their highest density was reached after adult colonies were established. In 1980, of the 34 species examined, 79% showed a significant coefficient of association between adult and juvenile conspecifics. This association caused clumped species dispersion patterns to develop within zones. As the result of recruitment, species diversity rose from 84 species in 1970 to 154 species in 1981. As surviving corals and new recruits developed, the distribution of coral growth forms became more diversified. There was a drastic increase in coral cover (e.g., submarine terrace zone, 0.9% in 1970 to 65% in 1981). This increase in cover corresponded to an increase in size and number of the coral colonies In 12 yr, species richness, cover, and composition reached or exceeded measurements of comparable reefs before the disturbance. The recovery occurred faster than predicted by a stable equilibrium model because long—term successional changes were not required. Recovery was accelerated because Acanthaster did not destroy the structural integrity of reef framework. This rapid recovery from a natural disturbance demonstrates that some coral communities have a greater resilience than was once believed.
Flowering, Shoot Production, and Vole Bark Herbivory in a Boreal Willow
Authors: Elmqvist Thomas , Ericson Lars , Danell Kjell , Salomonson Ann
Published: May 2006

Different intensities of vole bark herbivory were simulated on the boreal willow Salix myrsinifolia—phylicifolia in field experiments, and plant responses were followed during 4 yr. Intermediate damage (every second stem barked) resulted the 1st yr in a fourfold increase in the production of basal shoots per individual, and in the following years, a twofold increase in the number of catkins per intact stem compared with control willows. However, 1 yr after damage, the average number of seeds per individual was 21% lower than in control willows and seed germination was reduced 40%. Severe damage (all stems barked) caused a nearly 10—fold increase in the production of basal shoots the 1st yr and a reduction and delay of sexual reproduction. Mortality of damaged stems was high (100%), while genet mortality was < 5% in the group in which all stems were barked. Flowering in female willows was reduced more than in male willows, and males produced catkins on younger basal shoots than females. We suggest that in this system, where the principal herbivore fluctuates between years, compensatory reactions by damaged plants might be of particular benefit, as the period of growth following damage is more or less free from herbivores.
Direct and Indirect Effects of Host‐Plant Fertilization on an Insect Community
Authors: Strauss Sharon Y.
Published: May 2006

Artemisia ludoviciana plants that received ammonium nitrate fertilizer were visually censused for all insects at ° 11 —d intervals from 30 June to 1 September 1982. Community composition and species densities in fertilized plots were compared with those in control plots. Fertilization resulted in greater numbers of phloem— and seed—feeding insects and a concurrent increase in tending and patrolling by ants in fertilized plots. Chewing—insect densities did not respond uniformly to fertilization, and no chewing species exhibited the marked increase found in all of the sucking insects. Chrysomelid beetles and beetle damage decreased significantly in fertilized plots. A choice test between fertilized and control plants offered to beetles indicated that factors other than plant quality were responsible for the decrease in damage in fertilized plots. I suggest that increased densities of aggressive ants associated with honeydew—producing membracids and aphids caused this decline. Beetle damage was inversely correlated with aphid and membracid densities. Plots with ant nest had fewer plants with beetles and beetle damage than did those without ant nests. The indirect effect of increased ant densities from host—plant fertilization resulted in reduced levels of beetle damage to A. ludoviciana in fertilized plots. Since beetles removed large amounts of leaf area from A. ludoviciana in control plots, the indirect effect of higher ant densities should not be overlooked in assessing the general impact of fertilization on host plants.
Patch Size, Pollinator Behavior, and Pollinator Limitation in Catnip
Authors: Sih Andrew , Baltus Marie-Sylvie
Published: May 2006

We examined the effects of the patch size of catnip, Nepeta cataria, on pollinator visitation rates and pollinator limitation. The most important floral visitors were honey bees (Apis mellifera), solitary bees (Halictidae), and bumble bees (Bombus spp.). Our first goal was to see how spatial variation in patch size affected the rate at which individual flowers received pollinator visits (visitation rate). Visitation rate was higher in larger patches for honey bees and bumble bees, but lower for solitary bees. Patch size explained 74—83% of the variation in visitation rate. Intraspecific isolation also had an effect: isolated patches received relatively few visits. Visitation rate depended both on visitor abundance and on the proportion of flowers entered during one visit. All three visitor types wee more abundant in larger patches, i.e., flowers showed mutual attraction of pollinators. Relative to a visit to a small patch, during a visit to a large patch, honey bees visited more flowers but a lower proportion of flowers; solitary bees visited fewer flowers and thus a lower proportion of flowers; and bumble bees visited not only more flowers but a higher proportion of flowers as well. Thus within patches, flowers competed for visits from honey bees and solitary bees but showed facilitation regarding bumble bee visits. Our second goal was to relate patch size and visitation rates to immediate pollinator limitation. Comparisons of the percent of capsules setting seed (fruit set) of open—pollinated, hand—pollinated, and bagged flowers were used to evaluate pollinator limitation. The ranking of fruit set was: hand—pollinated > open—pollinated > bagged. Patch size did not affect fruit set in hand—pollinated or bagged flowers; however, for open—pollinated flowers, fruit set was lower in smaller patches. patch size explained 63% of the variation in pollinator limitation. The effect of patch size comes through its effects on visitation rates. A multiple regression model using visitation rates of the three types of visitors as independent variables explained 67% of the variation in pollinator limitation.
Flexible Life History Traits: Shifts by Scarlet Gilia in Response to Pollinator Abundance
Authors: Paige Ken N. , Whitham Thomas G.
Published: May 2006

In response to reduced levels of pollinators and pollination, scarlet gilia, Ipomopsis aggregata, shift from their normal semelparous mode of reproduction to iteroparous reproduction. When pollinators were excluded, plants were 5.5 times as likely to produce an ancillary rosette as controls that received natural pollinator levels. Similarly, when flowers were removed to prevent fruit production, experimental plants were 7.8 times as likely to produce and ancillary rosette as unmanipulated controls exposed to natural levels of pollination. Although ancillary rosette production is correlated with both season and a decline in pollinators, when seasonality was experimentally eliminated as a variable, changes in pollinator abundance still resulted in a corresponding change in rosette formation. Results suggest that there is a threshold below which an individual can be expected to switch from semelparity to iteroparity; the threshold appears to be between 30 and 40% fruit set. Our experiments also demonstrate a trade—off between cloning and fruit production at the population level. Herbivory had no effect on these life history traits. Plants clipped to simulate natural herbivory by deer and elk showed no change in rosette formation relative to undamaged controls. These and other data demonstrate that pollinators are limiting and suggest that pollination is the primary factor involved in these life history trait shifts by scarlet gilia. Midseason shifts in life history traits permit plants to adjust to current lows in pollinator services by producing an ancillary rosette that can then flower in a subsequent year when pollinators may be less limiting.
Disturbance and Predictability of Flowering Patterns in Bird‐Pollinated Cloud Forest Plants
Authors: Linhart Y. B. , Feinsinger P. , Beach J. H. , Busby W. H. , Murray K. G. , Pounds W. Zuchowski , Kinsman S. , Guindon C. A. , Kooiman M.
Published: May 2006

The distribution and flowering patterns of hummingbird—pollinated plants were compared from July 1981 to June 1983 in three patch types in cloud forest at Monteverde, Costa Rica. Study plots were: (1) four recent, large (1100—2500 m2) disturbances ("cutovers") produced by cutting vegetation, (2) six recent, smaller (200—600 m2) disturbances caused by treefalls, and (3) four plots (1600—1800 m2) of canopied forest. Based on published literature dealing with communities that characterize different regimes of disturbance, we tested one assumption and two hypotheses. Assumption: Plant species composition differs among the three patch types. Hypothesis 1: Phenotypic specialization by plants for co—evolved interactions with hummingbirds will be lowest in large gaps, highest in forest, and intermediate in treefalls. Hypothesis 2: Predictability of flowering phenologies and nectar production will be lowest in large gaps, highest in forest, intermediate in treefalls. Neither the assumption nor the hypotheses were supported by the results. The patch mosaic in this cloud forest was not associated with major differences in species composition of bird—pollinated plants. Most species studied were self—compatible. Most abundant in cutovers were species with long corollas, relatively specialized for attracting long—billed hummingbirds. Species with short corollas, which can be visited by many hummingbird species and some insects, were most abundant in treefalls and forest. Variation in phenological patterns showed no consistent trends among patch types. Predictability of flower and nectar production tended to be greatest in treefalls, which are foci of concentrated flowering activity by all species. Discrepancies between our results and previous studies can be ascribed to two facts. (1) Much of the literature dealing with ecological consequences of disturbance has dealt with large—scale anthropogenic disturbances such as old fields of the eastern USA, whereas we studied small, natural, or quasi—natural disturbances. (2) Studies of forest disturbance have focused on the tree layer, whereas we studied the understory herbs, shrubs, and epiphytes. Natural disturbance usually involves death and replacement of one or more trees, whereas individuals of other life forms may persist through the disturbance.
Proximate and Ultimate Roles of Food Amount in Regulating Egret Sibling Aggression
Authors: Mock Douglas W. , Lamey Timothy C. , Ploger Bonnie J.
Published: May 2006

In facultatively siblicidal bird species, the amount of food delivered by parent birds to their young ("food amount") has been assumed to be both an important proximate and ultimate cause of fatal aggression. The proximate "Food Amount Hypothesis" (FAH) contends that sibling aggression will vary inversely with the quantity of food delivered by the parents, presumably mediated by chick hunger. At the ultimate level, food shortages are expected to influence whether the combined effects of aggression and food control by older siblings will be fatal to the youngest brood member(s). We evaluated the effects of food amount on aggression and survival in facultatively siblicidal Ardeidae: (1) Sibling aggression in unmanipulated broods of Great Egrets (Casmerodius albus) and Great Blue Herons (Ardea herodias) did not decline with increasing amounts of food, as had been predicted; indeed, this relationship may even be weakly positive. (2) In egret broods, the strongest predictors of total brood success were competitive vigor of the youngest sib and fighting rate. (3) Wild egret broods whose food amount was experimentally doubled did not reduce fighting rates, but experienced significantly lower brood reduction than control broods. (4) Captive egret broods whose food amount was experimentally decreased did not increase fighting rates, but suffered significantly more brood reduction than control broods. (5) In the wild, food amount appears not to be a sufficiently stable parameter on which to base prudent sib—aggression strategies. We conclude that food amount has little direct influence on fighting behavior in these birds, though it consistently influences chick survival. Thus the proximate effects of this ecological variable must be divorced from its ultimate role, at least in species such as these ardeids and in obligate—siblicidal species. However, the FAH may well account for sib—fighting variation in other facultatively siblicidal birds, such as Blue—footed Boobies (Sula nebouxii), Black—legged Kittiwakes (Rissa tridactyla), Ospreys (Pandion haliaetus), and South Polar Skuas (Catharacta maccormicki). It is predicted that sibling aggression will be food—amount dependent in species whose food fluctuations are relatively slow. An attempt to manipulate prey monopolizability (bolus size) in captive Great Egrets was unsuccessful, either because sib aggression in the laboratory was so inflated as to swamp prey—size effects or because this species simply lacks sufficient developmental flexibility in its intrabrood fighting behavior. If the latter were true, it would reveal a striking difference between Great Egrets and Great Blue Herons, which have shown considerable (adaptive) plasticity in this regard.
Regulation of Lake Primary Productivity by Food Web Structure
Authors: Carpenter S. R. , Kitchell J. F. , Hodgson J. R. , Cochran P. A. , Elser J. J. , Elser M. M. , Lodge D. M. , Kretchmer D. , He X. , von Ende C. N.
Published: May 2006

We performed whole—lake manipulations of fish populations to test the hypothesis that higher trophic levels regulate zooplankton and phytoplankton community structure, biomass, and primary productivity. The study involved three lakes and spanned 2 yr. Results demonstrated hierarchical control of primary production by abiotic factors and a trophic cascade involving fish predation. In Paul Lake, the reference lake, productivity varied from year to year, illustrating the effects of climatic factors and the natural dynamics of unmanipulated food web interactions. In Tuesday Lake, piscivore addition and planktivore reduction caused an increase in zooplankton biomass, a compositional shift from a copepod/rotifer assemblage to a cladoceran assemblage, a reduction in algal biomass, and a continuous reduction in primary productivity. In Peter Lake, piscivore reduction and planktivore addition decreased zooplanktivory, because potential planktivores remained in littoral refugia to escape from remaining piscivores. Both zooplankton biomass and the dominance of large cladocerans increased. Algal biomass and primary production increased because of increased concentrations of gelatinous colonial green algae. Food web effects and abiotic factors were equally potent regulators of primary production in these experiments. Some of the unexplained variance in primary productivity of the world's lakes may be attributed to variability in fish populations and its effects on lower trophic levels.
Spatial Heterogeneity and the Design of Ecological Field Experiments
Authors: Dutilleul Pierre
Published: May 2006

Experimental design should be accommodated to spatial heterogeneity in nature as well as indoors, whether it is a nuisance or a characteristic of interest, combined or not with assessment of treatment effects. The following analysis—of—variance approach to quantification of spatial heterogeneity is based on the adequate design of ecological field experiments, according to the type and the scale of heterogeneity of concern (at small scale, patches, one— or two—dimensional gradients). There are no recipes for doing so and judgment must be exercised every time: the experimenter's knowledge about the experimental material, combined with premanipulation or control, then, provides a useful prerequisite. For patches and environmental gradients, in the presence of treatment assignment, recommended designs require the blocking principle of grouping similar experimental units, which allows avoidance of spurious treatment effects and inflated error mean square. Completely randomized designs should only be used in the very particular case of spatial homogeneity at large scale. Illustrations in ecological field experimentation are given and discussed.
Web‐Site Tenure in the Long‐Jawed Spider: Is It Risk‐Sensitive Foraging, or Conspecific Interactions?
Authors: Smallwood Peter D.
Published: May 2006

Long—jawed spiders (Tetragnatha elongata) present a paradox to behavioral ecologists, because spiders in prey—rich habitats move to new web sites almost every night, while those in poor habitats rebuild their webs on precisely the same spot for many nights in a row. Caraco and Gillespie built a risk—sensitive foraging model to explain this paradox. Their model demonstrates that it may be advantageous to relocate more often when experiencing high prey capture rates (under certain regimes of spatiotemporal heterogeneity). However, their model had not been tested. I tested the Caraco—Gillespie model by conducting food additions experiments in the field with spiders in the poor habitat. The results do not support the model. A reexamination of the Caraco–Gillespie model suggests that it implicitly assumes unlikely larger scale patterns in prey availability. I present an alternative hypothesis to explain the paradox. Since conspecific density is higher in rich habitats, I propose that more intense conspecific interactions in rich habitats cause these spiders to move to new web sites more often. I conducted the replicates of a density reduction experiment in a rich habitat, and the results support the conspecific density hypothesis. The results of these experiments challenge a widely cited example of risk—sensitive foraging, and underline the importance of explicitly considering multiple scales of variation in ecology.
Empirical Evidence of the Role of Heterogeneity in Ratio‐Dependent Consumption
Authors: Arditi Roger , Saiah Henni
Published: May 2006

Classical models describing the number of prey consumed by predators rest on an analogy with the law of mass action and, consequently, the functional response of predators depends only on the density of prey. An alternative model is that the functional response depends on the ratio of prey and predator densities. We hypothesize that the applicability of one or the other model depends on the degree of heterogeneity of predators and prey in space, the prey—dependent model being appropriate in homogeneous situations while the ratio—dependent model is appropriate in heterogeneous situations. We have designed experiments to test this hypothesis, using cladocerans filter—feeding on algae. The design is such that the two types of dependence can be discriminated by observation of equilibrium patterns. Daphnia magna and Simocephalus vetulus, the two cladoceran species tested, differ in their spatial distributions. Daphnia has homogeneous distribution whereas Simocephalus has heterogeneous distribution. Experimental results support the hypothesis: D. magna follows the prey—dependent model and S. vetulus follows the ratio—dependent model. By artificially modifying the environment of the two species, we forced D. magna to a heterogeneous distribution and S. vetulus to a homogeneous distribution. As a consequence, each species changed its dependence, further confirming our hypothesis.
Inducible Defenses and Allocation Shifts in a Marine Bryozoan
Authors: Harvell C. Drew
Published: May 2006

Most theoretical models of the evolution of conditional responses require some form of cost to balance the benefits of an intermittently deployed character. The inducible spines, keels, and helmets of marine and freshwater invertebrates have proven to be tractable subjects for testing this hypothesis. Membranipora membranacea, a marine bryopzoan, produces large spines within 2 d of detecting chemical cues from a predatory mollusc. The spine response, which increases with the level of predator attack and varies with colony size, is associated with reduced growth of colonies in short—term laboratory experiments. Because of the plastic nature of allocation shifts in colonial invertebrates, lifetime fitness consequences for inducibly defended colonies in the field were measured in this study. In two successive years, spined colonies grew somewhat more slowly, but also reproduced sooner than less—spined colonies. Spined colonies also senesced sooner than unspined colonies in the one year measured. The differences in growth trajectories between spined and less—spined colonies detected in the field were small relative to those predicted from short—term laboratory experiments, emphasizing the importance of field studies to evaluate the costs of defense in nature.
Habitat Heterogeneity and The Functional Signifiance of Fish in River Food Webs
Authors: Power Mary E.
Published: May 2006

Predation by fish (roach, Hesperoleucas symmetricus, and steelhead Oncorhynchus mykiss) produced strong cascading effects on biota associated with boulder—bedrock substrates in pools of a northern California river, but not on gravel—dwelling biota. Enclosure—exclosure experiments in the South Fork Eel River of northern California (39°44' N, 123°39'W) showed that fish, by suppressing densities of damselfly nymphs and other small predators, released algivorous chironomids (Pseudochironomus richardsoni) from predation. Chironomids in turn dramatically reduced algal standing crops. In contrast, fish had little effect on algae or invertebrates associated with gravel. Gravel—dwelling heptageniid mayflies were behaviorally inhibited from using tops of stones in fish enclosures, and stone surfaces had more chironomid tubes in fish enclosures than in fish exclosures. However, no effects on epilithic algae or densities of invertebrates comparable to those of biota on boulder—bedrock substrates were detected. These spatially varying predator effects in a river parallel results from marine benthic systems, where strong effects of large predators documented for rocky intertidal habitats and unvegetated soft bottoms are not conspicuous in seagrass beds.
Cascade Effects Among Four Trophic Levels: Bird Predation on Galls Affects Density‐Dependent Parasitism
Authors: Tscharntke Teja
Published: May 2006

Effects of herbivore density on parasitization, bird predation, and interference between parasitoids and predators were tested using midge gall clusters of Giraudiella inclusa (Diptera, Cecidomyiidae) from Phragmites habitats. Winter predation of Blue Tits (Parus caeruleus) did not simply cause an additional mortality of midge larvae, but affected both midge and parasitoid species. Fifty—six percent of galls were parasitized, so that the average of 70% gall predation resulted in only 30% additional midge mortality. Out of the eight frequent parasitoid species, two showed positive density—dependent responses, two showed negative density—dependent responses, and four showed density—independent responses. However, in aggregate, percent parasitism was density independent. Birds preferentially pecked open large gall clusters and thereby caused a particularly high mortality of parasitoid species concentrated in large clusters. Selection pressure by birds against parasitoids with density—dependent responses suppressed the parasitoids' presumed ability to regulate host populations and probably altered parasitoid coexistence. Host plant characteristics affected distribution and abundance of galls, and also the subsequent pattern of parasitization and the percentage of bird predation. Such four—trophic—level cascade effects can be complex: shoot damage by stem—boring caterpillars induced side shoots, thereby increasing gall abundance and facilitating the inverse density dependent endoparasitoid Aprostocetus gratus, since (1) gall clusters of side shoots were very small (enhancing percent parasitism), and (2) these small gall clusters were little preyed upon by birds.
Lack's Clutch Size Hypothesis: An Examination of the Evidence Using Meta‐Analysis
Authors: Vander Werf Eric
Published: May 2006

Meta—analysis is a method ecologists may find useful for quantitatively reviewing and systematically examining results from a large number of studies on a subject for which there is conflicting evidence. I used meta—analysis to integrate results from 42 independent brood—enlargement studies and tested Lack's hypothesis that clutch size in birds has evolved toward that which produces the most surviving offspring. Cumulative evidence did not support Lack's hypothesis. Significantly (two—tailed P = .016) more fledglings were produced in enlarged broods than in a normal—sized broods, indicating parents could raise more young than they had eggs. The standardized treatment effect of brood enlargement across all studies was a mean (± 1 se) increase of 0.55 (±0.22) standard deviations in the number of young produced. This result does not appear to be affected by a publishing bias and is unlikely to be reversed by inclusion of additional studies. I also examined differences in methodology and species studied as possible confounding factors and explanations for conflicting results. Longer studies and species with altricial young were more likely to show food limitation. Latitude and date of study, degree of brood enlargement, validity of a study, and annual adult survival did not affect results.
Interspecific and Intraspecific Interactions Via Plant Responses to Folivory: An Experimental Field Test
Authors: Faeth Stanley H.
Published: May 2006

I tested the hypothesis that phytophagous insect species compete interspecifically on shared host plants by inducing chemical, physical, and phenological changes in the plant that either directly reduce insect growth and survival or indirectly alter attack by natural enemies. I manipulated levels of simulated folivory on branches within eight trees of Quercus emoryi in a complete randomized block design. Survival and causes of mortality, including attack by natural enemies, of a dominant leafminer, Cameraria sp. nov., were monitored over two growing seasons. Survival varied significantly among trees but not among treatments. Causes of mortality also varied significantly among trees. Death from premature leaf abscission increased significantly with increasing levels of branch damage, while mortality from other causes, including death from bacterial and fungal attack, declined significantly with increasing levels of damage. However, neither of these changes in mortality was sufficient to cause changes in overall survival. Leaf damage did not influence attack by macroscopic natural enemies (hymenopteran parasitoids or vertebrate and invertebrate predators). By far, the most important factor affecting survival and causes of mortality within trees was the presence of co—occurring larvae on leaves (i.e., intraspecific competition). Simulated folivory in the first growing season did not affect folivory by Cameraria or other folivorous arthropods in the next season. Colonization, survival, rate of attack by natural enemies, and amount of herbivory by other arthropods in the second growing season were not affected by levels of leaf damage in the first one. Neither survival nor causes of mortality of Cameraria in the second growing season were associated with folivory by endemic leafchewing arthropods. Simulated folivory in the first growing season did, however, reduce leaf production in the second season. These results do not support the hypothesis that insect species complete interspecifically via induced plant responses and their effects on the third trophic level (natural enemies of the herbivores). The effects of simulated (first growing season) and insect (second growing season) folivory via induced plant responses on the Cameraria population was negligible relative to intraspecific interactions. Further, my results do not support the notion that folivore—induced responses are defensive and that plants have evolved these responses to manipulate the third trophic level. In natural settings, other factors that influence population dynamics of phytophagous insects, such as intraspecific competition and interactions with microparasites, may obscure any negative effects of herbivore—induced plant responses on interactions among herbivore species and their macroscopic natural enemies.
Salt Tolerances and The Distribution of Fugitive Salt Marsh Plants
Authors: Bertness Mark D. , Gough Laura , Shumway Scott W.
Published: May 2006

Fugitive species often live under harsh physical conditions, yet their ability to cope with environmental extremes has received little attention. We studied the association of fugitive plants and stressful physical conditions using salt marsh plants that live in hypersaline bare patches. In New England salt marshes, primary space is dominated by dense monospecific stands of perennial turfs, but disturbances frequently leave bare space that rapidly becomes hypersaline. Elevated salinities of bare patches result form direct exposure of soil to solar radiation and evaporative water loss. A number of fugitive plants that are rare in undisturbed salt marsh vegetation are commonly associated with bare patches before being displaced during secondary succession. Greenhouse studies show that plants that dominate undisturbed vegetation are stunted by high substrate salinities, whereas the photosynthesis and growth of patch—dependent fugitives are relatively independent of typical salinity variation. Moreover, in the field, watering more than doubled the survivorship and growth of seedlings in patches and more strongly affected dominants than patch—dependent fugitives. Our results show that physical conditions in salt marsh bare patches can limit plant colonization, and suggest that high salt tolerances may permit fugitive plants to utilize bare patches as refugia from competitors. Since fugitive plants are commonly restricted to physically stressful habitats, the ability to withstand harsh physical conditions is likely often an important aspect of their biology.
Habitat Selection by Two Species of Nectarivore: Habitat Quality Isolines
Authors: Possingham Hugh P.
Published: May 2006

I present a model predicting the distribution and abundance of two species of competing nectarivore exploiting nectar in two types of flower. The model uses a submodel of resource renewal and depletion, and the principle that individuals attempt to maximize individual fitness, to construct habitat isolines. The habitat isoline for a population of nectarivores is a line in an abundance phase—plane along which all individuals of the population, regardless of the habitat they are currently exploiting, have equal expected fitness. At equilibrium the habitat isoline determines the distribution of a population between two habitats given a fixed abundance of competitors. The habitat isolines for populations of two species exploiting two habitats can be used to predict the equilibrium distribution of both species. I illustrate the model using data about bumble bees foraging for nectar in flowers (Inouye 1978). An asset of the model is its ability to make a priori predictions about flower exploitation patterns using information about resource dynamics and the foraging abilities of the nectarivores. Two warnings regarding the empirical detection of exploitation competition between species emerge from the model analysis. Depending on the range over which density manipulations are carried out in exploitation competition systems, competition may or may not be detected, while interspecific competition may or may not appear to be more intense than intraspecific competition. Although the model is directed at nectarivores competing for nectar in flowers, the results have wider implications for all systems where species compete exploitatively for resources.
Alternative Permanent States of Ecological Communities
Authors: Law Richard , Morton R. Daniel
Published: May 2006

The states to which multispecies communities can tend has been an important issue to ecology, but one in which rather little progress has been made at a theoretical level through lack of a tractable global theory of the dynamics. This paper explores the use of a global theory called "permanence" that indicates whether the boundary of a phase space is a repellor to orbits in the phase space. The theory is used to try to identify, at a qualitative level, the states to which solutions tend in the phase space of an arbitrary pool of species. We define a "permanent state" of the pool as a subset of the species that is permanent in its own right and univadable by any other species from the pool. A simple assembly rule for communities that stems from this is that no permanent state can be a subset of another. Data on coexistence of drosophilid species and also on that of cuckoo doves, although incomplete, are consistent with this rule. A method is given for finding the permanent states of pools of species with Lotka—Volterra dynamics. Some properties of permanent states are illustrated by means of numerical examples from regional pools of species generated with Lotka—Volterra dynamics. These examples show three kinds of dynamics: a single permanent state, two or more alternative permanent state in which none are subsets of others, and an absence of any permanent states. The statistical distribution of these outcomes in pools of four and five species indicates that a single permanent state is the most likely one to occur, but that alternative states become more probable as the number of interactions among species increases. The implications of these results for understanding and modelling the process of succession driven by population dynamics are discussed.
Birch Seedling Responses to Daily Time Courses of Light in Experimental Forest Gaps and Shadehouses
Authors: Wayne P. M. , Bazzaz F. A.
Published: May 2006

This study examined the consequences of differing daily time courses of light availability, while controlling for total photon flux density (PFD, measured in moles per square metre), on the physiology, architecture, and growth of seedlings of two birch species (Betula populifolia and B. alleghaniensis) that differ in shade tolerance. In an experimental garden, seedlings were grown in two sets of contrasting diurnal light regimes, "gaps" and "shadehouses," and at four levels of total integrated PFD (°12, 27, 50, and 70% of full sun.) In gaps, seedlings received relatively more heterogeneous diurnal light regimes, with midday full sun peaks lasting between 40 and 280 min on sunny days, depending on gap size. In shadehouses, seedlings received daily total and average PFDs similar to that in gaps, but received no midday direct sunlight, and an overall more uniform distribution of light. The daily time course of light availability, independent of total PFD, significantly affected growth of seedlings. For both species, seedlings in shadehouses grew significantly larger than gap seedlings. Differences between diurnal treatments were greatest at lower integrated light levels in B. populifolia, but not for the more shade—tolerant B. alleghaniensis. Diurnal light regimes also significantly influenced the degree of plasticity in a seedling's sun—shade responses. However, the magnitude of effect due to diurnal light regime depended on the particular trait investigated, the integrated light level, and the species. At similar total PFDs most characters (e.g., specific leaf mass, leaf mass ratio) exhibited more of a sun—type response in shadehouses that in gaps, however, maximum net photosynthesis and chlorophyll a/b ratios reflected more of a sun—type response in gap light regimes. For most growth, physiological, and morphological characters, seedling responses to increasing total PFD were generally more sensitive in gap than in shadehouse regimes. Our results also show that differences between species were greater when compared along the total PFD gradient in gaps vs. shadehouses. Both these results suggest that experimental studies employing uniform light regimes, which incorporate little of the temporal variability experienced by seedlings in natural gaps, may underestimate plasticity within regenerating tree species, and the potential for species niche differences and coexistence.
Sex, Size and Interyear Variation in Flowering Among Dioecious Trees of the Malayan Rain Forest
Authors: Thomas S. C. , LaFrankie J. V.
Published: May 2006

In dioecious plants, intersex differences in the frequency and size dependence of flowering may often be important proximate determinants of a population's flowering sex ratio. Such differences have been little investigated among Southeast Asian rain forest trees, where dioecy is perhaps best represented in the world's flora. In this study we recorded flowering activity and sex of reproductive individuals in two separate flowering seasons for °2600 trees representing three species of Aporusa and two species of Baccaurea (all Euphorbiaceae) in a primary rain forest in peninsular Malaysia. We found neither sexually mixed trees nor sex switching of trees between years. Flowering sex ratios for the species of smallest stature. A. microstachya and B. parviflora, consistently exhibited a significant degree of male bias, which was greater in years with lower overall levels of flowering in the population. Two—year cumulative sex ratios were significantly male biased in these two species and in a second relatively small—statured species of Aporusa. The size distributions for male trees broadly overlapped those of female trees in all species. In the smaller statured species, male trees displayed a significantly greater degree of relative size variation than did female trees, suggesting that male trees begin flowering at a smaller size, but also grow to a larger size, than to females. Also, males were significantly more likely to flower in both years than were females in the smaller species studied. Sexual dimorphism in the frequency and size dependence of flowering has previously been explained as a result of higher energy costs of reproduction in females than in males. We suggest that this physiological constraint is most likely to play a strong role in energy limited environments. We therefore predict that, as found in this study, male—biased sex ratios and associated patterns of sexual dimorphism may generally be most pronounced among diminutive treelets of the rain forest understory.