bluuebunny/crossref_metadata_2025 huggingface dataset:
Patterns of Herbivory Along a Prouductivity Gradient: An Empirical and Theoretical Investigation
Authors: van de Koppel Johan , Huisman Jef , van der Wal Rene , Olff Han
Published: May 2006

According to classical exploitation theory, an increase of primary productivity should result in increased herbivore grazing pressure, thus maintaining a low plant standing crop. However, field data obtained from a salt marsh revealed a maximal grazing pressure by hares, rabbits, and geese at intermediate levels of plant standing crop. Grazing pressure was relatively low in the more productive parts. We argue that this pattern is due to a low foraging efficiency of these herbivores in dense vegetation. In order to investigate this hypothesis, we examine a plant—herbivore model where grazing becomes less efficient in dense vegetation, and analyze the behavior of this model along a gradient of primary productivity. In systems of intermediate productivity, the model predicts that a plant—herbivore system may have two stable states. In one state, the herbivore maintains a low standing crop. The other state is dominated by a dense vegetation unsuitable for herbivore grazing. In systems of high productivity, the herbivore is unable to keep plant growth in check and a dense vegetation develops. Thus, in line with our field data but in contrast to classical exploitation theory, our model does not predict top—down control in productive environments.
Community Structure of Desert Small Mammals: Comparisons Across Four Continents
Authors: Kelt Douglas A. , Brown James H. , Heske Edward J. , Marquet Pablo A. , Morton Stephen R. , Reid Julian R. W. , Rogovin Kontantin A. , Shenbrot Georgy
Published: May 2006

Presence/absence data for the small—mammal species at sites in seven deserts were analyzed for evidence of similarity in community structure. The deserts studied were located in North and South America (268 and 118 sites, respectively), Australia (245 sites), Israel (54 sites), and greater Eurasia (Thar, 15 sites; Turkestan, 36 sites; Gobi, 98 sites). Patterns observed in all deserts included: (1) low α diversity (2—4 species per site); (2) high β diversity (species turnover between sites); and (3) local coexistence of 20—30% of the species in the regional pool. Additionally, the number of species with which a species co—occurred increased with the number of sites at which that species occurred. Although these results suggested that some features of community structure were similar across deserts, other aspects, especially trophic structure, differed widely. Deserts in the northern hemisphere possessed more granivores, and the Turkestan Desert more folivores, than other deserts. Carnivorous small mammals were most strongly represented in Australia, and omnivores in South America, Australia, and the Thar. The structure of desert small—mammal communities is strongly influenced by historical factors; different taxonomic groups with distinctive trophic adaptations proliferate in different desert regions where they are subject to some common structuring processes of community assembly.
Detection of Critical Densities Associated with Pinon‐Juniper Woodland Ecotones
Authors: Milne Bruce T. , Johnson Alan R. , Keitt Timothy H. , Hatfield Colleen A. , David John , Hraber Peter T.
Published: May 2006

The interfaces between pinon—juniper canopies and grasslands in the south—western USA present an opportunity to use the modern theory of spatial phase transitions as a formal characterization of ecotone structure. The theory requires an estimation of a critical value of tree cover at which the woodlands switch abruptly from a fragmented collection of small patches of trees to a network of connected canopies. Presumably, this transition is associated with critical environmental conditions that regulate the ecologies of trees vs. grasses. We developed and tested a new method to estimate the critical cover value of woodlands on complex terrain. The method was based on multiscale assessments of the associations between local tree coverage and two types of patch edge. Tests on artificial gradient percolation maps revealed an interaction between the type of edge used (hull edge, which is based on only the orthogonal connections between canopy—occupied cells, vs. accessible edge, which is based on both orthogonal and diagonal connections between canopy—occupied cells) and the neighborhood rule used to define a cluster (von Neumann 5—cell or Moore 9—cell). When applied to digitized, geographically referenced aerial photographs, the method indicated that areas ≤ 6.6 ha exhibited the theoretical critical value of 59.3% cover predicted for square lattices and the 5—cell neighborhood. Construction of both edge types on a given map can reveal locations of steep environmental gradients that may be buffered against modest climate fluctuations. The edges can be used in the calibration of independent variables to predict tree cover. The agreement between the expected and observed critical densities will motivate extensions of phase transition theory to studies of ecotones in real landscapes.
Size‐Specific Interactions in Lake Systems: Predator Gape Limitation and prey Growth Rate and Mortality
Authors: Persson Lennart , Andersson Jens , Wahlstrom Eva , Eklov Peter
Published: May 2006

To study the effects of different predators on the behavior and dynamics of their prey, we compared the performance of perch (Perca fluviatilis) and the abundance of their prey resource in four lakes with two different types of piscivory. In two of the lakes, perch were the only fish species present and piscivory was restricted to cannibalism. In the other two lakes, perch co—occurred with piscivorous pike (Esox lucius). Pike grow to larger sizes and can capture larger prey than piscivorous perch. Therefore, perch reach a size refuge earlier in the lakes without pike. Perch in the lakes lacking pike used both the epilimnion and hypolimnion habitats of the lakes, and a high proportion of the perch occurred below the thermocline. In contrast, perch in the lakes with pike were mainly captured along the littoral zone and were never captured below the thermocline. This difference in habitat use was reflected in different diets of perch in the different lakes. Perch in the lakes with pike were also less abundant than those in the lakes lacking pike and the growth rates of individual perch and the densities of predator—sensitive prey (pelagic and littoral) were higher, indicating that the presence of pike resulted in decreased intraspecific competition in perch. Evidence for a behavioral response to predation risk was found in 1 + (1 yr old) perch, which was suggested to be due to predation risk from both larger perch and pike. It is hypothesized that the use of the cold—water hypolimnion habitat by perch in the lakes lacking pike was a response of perch to reduce metabolic demands under conditions of severe resource limitation. Differences in gape limitation between predators species are suggested to have major effects on size—dependent mortalities and growth rates in prey and thereby on overall community dynamics. Evidence for this latter effect was observed in differences in zooplankton size structure between the lakes with and without pike.
Phylogenetic Perspectives on Community Ecology
Authors: Losos Jonathan B.
Published: May 2006

Ecologists have long been interested in the differences that exist among communities. If species adapted rapidly and without constraint, and if any lineage could occur in any community, then we would expect differences in community structure to be indicative of environmental differences. Because lineages differ in their evolutionary potential and are geographically restricted, however, comparisons of community structure must take account of communities' histories. Phylogenetic information about the constituent lineages in a community can allow lineage effects to be factored out, thus allowing an assessment of environmental determinants of community structure. In addition, phylogenetic information permits understanding of how communities have evolved through time and suggests hypotheses that may be tested using extant communities. Methodological difficulties with the application of these methods to community ecological issues are also discussed.
Facilitation between Higher Plant Species in a Semiarid Environment
Authors: Pugnaire Francisco I. , Haase Peter , Puigdefabregas Juan
Published: May 2006

A major role of facilitation between higher plant species, particularly in stressful environments, has recently been reported in several papers that suggest that beneficial interactions could be more important in the dynamics of plant communities than has been recognized to date. In a semiarid region in southeastern Spain we determined the effect of a leguminous shrub, Retama sphaerocarpa, on its environment, testing the hypothesis that facilitation by the shrub of one species of its understory, Marrubium vulgare, is reciprocal and that the shrub benefits from sheltering herbs beneath its canopy. Marrubium plants under Retama had greater specific leaf area, leaf mass, shoot mass, leaf area, more flowers, a higher nitrogen (N) concentration in leaf tissue, and more N per plant than isolated plants, suggesting a facilitative effect of Retama on Marrubium including increased availability of resources. Biomass of 1—yr—old cladodes, total biomass, total nitrogen content of 3—yr—old branches, and shoot water potential at midday were higher for Retama shrubs with Marrubium beneath them than in shrubs without Marrubium. Retama strongly improved its own environment, facilitated the growth of Marrubium and other species underneath its canopy, and at the same time obtained benefits from sheltering herbs underneath. The interaction between these two species was indirect, associated with differences in soil properties and with improved nutrient availability under shrubs compared with plants growing on their own. We propose that the mutual benefit of the association between Retama and Marrubium is best termed a facultative mutualism. Each partner benefits from greater availability of resources in the “island of fertility” that results from their association. If beneficial plant—plant interactions as described here are widespread, positive relationships may have a major role in determining the pattern and structure of plant communities.
Mechanisms of Hummingbird‐Mediated Selection for Flower width in Ipomopsis Aggregata
Authors: Campbell Diane R. , Waser Nickolas M. , Price Mary V.
Published: May 2006

The form of angiosperm flowers is thought to have evolved in part via selection that excludes ineffective pollinators and increases the efficiency of pollen transfer by effective ones. In previous studies with the montane plant Ipomopsis aggregata, we documented pollinator—mediated selection on several aspects of floral form, including flower width. This character varies continuously within natural populations of I. aggregata, and individuals with wider flowers export more pollen per flower to surrounding plants. We previously showed that this component of phenotypic selection is due primarily to the pervisit effectiveness of hummingbirds, the most important pollinator in our study populations. Here we investigate mechanisms of differential visit effectiveness in greater detail. First, counts of pollen in unvisited flowers showed that pollen production itself increases with width. In aviary experiments, hummingbirds also removed a greater proportion of available pollen as width increased. We next videotaped visits to flowers that varied in width either naturally, or by experimental treatment, and found that hummingbirds inserted their bills more deeply into wider flowers. Finally, we directly manipulated how deeply birds could insert their bills, and found that more pollen was removed after deep insertion. Thus, several mechanisms appear to underlie selection on corolla width via visit effectiveness in pollen export. One involves a phenotypic correlation with pollen production; this underscores the value of experiments for untangling indirect from direct selection. Another mechanism involves direct selection due to the depth of insertion of a hummingbird's bill; this is rare evidence for one form of selection involving the “fit” between pollinator and flower.
Effects of Nutrients and Planktivorous Fish on the Phytoplankton of Shallow and Deep Aquatic Systems
Authors: Proulx M. , Pick F. R. , Mazumder A. , Hamilton P. B. , Lean D. R. S.
Published: May 2006

The response of phytoplankton to nutrient (N, P) and planktivorous fish additions (Phoxinus sp.) was compared between two sets of enclosures (8 m in diameter) installed at different depths in a dimictic temperate lake. One set of eight enclosures was located in the deep part of the lake (12 m) where water thermally stratifies during the summer. The other set of eight enclosures was located in a shallow area of the lake (3.5 m) where the water does not thermally stratify. During the summer months, phytoplankton cell number and biomass increased significantly with both nutrient additions (P = 0.004) and the presence of planktivorous fish (P = 0.011) in the deep, thermally stratified enclosures. On average, total algal biomass in the nutrient and fish treatments was 2.6 times higher than in the nutrient—only treatments. In contrast, in the shallow enclosures, total phytoplankton biomass increased with nutrient additions (P = 0.04) but was not significantly affected by the addition of fish (P = 0.34). However, fish additions increased the number of algal cells in both the shallow and deep enclosures, independently of nutrients, because of an increase in small cells: the proportion of nanoplankton cells was significantly greater in the presence of fish. Fertilization increased the large nanoplankton fraction (10—20 μm) in the deep enclosures, but had no significant effect on the size distribution of algal biomass in the shallow enclosures. In general, large (> 64 μ m) cells were a minor component of the phytoplankton biomass in all treatments. Fertilization stimulated the growth of Chlorophyta in the deep (P = 0.018) and in the shallow (P = 0.012) enclosures. Cyanobacteria were conspicuous in the fertilized shallow enclosures, but the effect on total biomass was not statistically significant. The presence of fish stimulated a significant increase in Cryptophyta at both depths and Pyrrophyta were more common. Nutrients and planktivorous fish had different effects on phytoplankton depending on the thermal regime. In terms of total algal biomass there was no significant difference between deep and shallow enclosures within a given treatment, but depth did affect the size distribution of algal biomass among treatments. The effects of fish were more evident with community structure variables (size or taxonomic distribution) than with coarser variables such as algal biomass.
Effects of Stoneflies on Local Prey Populations: Mechanisms of Impact Across Prey Density
Authors: Kratz Kim W.
Published: May 2006

Research on predator impact in streams has generally ignored density—dependent effects. Although predator and prey behaviors are known to influence the impact of stream predators on local prey populations, no study has simultaneously addressed how predator impacts, and behavioral mechanisms contributing to those impacts, vary with prey density. I assessed the effect of a stream invertebrate predator (stonefly) on local prey (baetid mayfly) density across a gradient of six prey densities in in situ channels with natural substrata. Calculations of predator foraging rates and prey responses were based, in part, on an empirically based model assuming an exponential decline in prey densities. Total predator impact (measured as the negative natural log of the ratio of prey remaining in predator vs. predator—free channels) across prey densities followed a unimodal curve, with reduced predator effects at high and low, relative to intermediate, prey densities. Although direct predator consumption was more important than predator—induced emigration in explaining overall prey losses across all prey densities (65 vs. 35%), the relative importance of each loss process varied across individual prey densities. Total baetid drift increased with, and per capita drift was independent of, baetid benthic density in all predator treatments. Total and per capita drift rates of baetids were increased at least 3.5 times in the presence of predatory stoneflies. Few baetids were observed on the tops of rocks in either control or predator treatments during the day; however, baetid numbers on upper rock surfaces increased substantially at night and were significantly reduced by stonefly presence. The proportion of baetid populations on surface substrata was low and unaffected by predator presence during the day (0.3%) but was reduced in predator relative to predator—free channels at night (4 vs. 14%). Predatory stoneflies exhibited a type III functional response, and stonefly emigration decreased as initial baetid density increased. These results suggest that discrepancies in predator impacts between previous studies may be owing to prey density and reflect prey and predator behaviors that vary with prey density.
Phenotypic Variability within and between Fish Shoals
Authors: Krause Jens , Godin Jean-Guy J. , Brown David
Published: May 2006

Theory predicts that individuals within social groups should be more similar (less variable) phenotypically than individuals between groups. We tested this hypothesis by comparing the variability of certain phenotypic characters (body size, parasite load) of individual fish within and between sampled shoals with the phenotypic variability of randomly constituted shoals of the same size. We caught 34 entire fish shoals in a lake, and for each shoal member, we identified the species, measured its body length, and counted the number of black spots (indicating the presence of a parasitic trematode worm) visible through their skin. We also recorded two habitat characteristics, water depth and distance to cover, at the location where each of the 34 shoals was caught to ascertain whether (and to which degree) the spatial distributions of different fish species and fish size classes were determined by certain habitat characteristics. A comparison between the observed and the randomly assorted shoals (obtained through computer simulations) revealed that fish at our study site exhibited a significant association with conspecifics and with individuals of similar body size (within and across species); that is, shoals were assorted by species and by individual body size. Parasite—assortative shoaling was not observed. Different species (and to a lesser degree different size classes within species) showed a tendency to occur in different microhabitats. Nevertheless, despite some degree of habitat segregation, there remained considerable spatial overlap between the different species and fish body size classes. Furthermore, given the spatial proximity of different habitats and the relatively weak habitat association of most species, we therefore suggest that active shoal choice by individual fish was the most important determinant of shoal composition.
Evidence of Chemical Communication During the Gametogenesis of Holothuroids
Authors: Hamel Jean-Francois , Mercier Annie
Published: May 2006

This research provides evidence for chemical mediation in the initiation of gametogenesis and interindividual fine—tuning among populations of the holothurian Cucumaria frondosa. Initiated in the laboratory by increased day length, the gametogenesis of C. frondosa developed normally when individuals were transferred to an environment in which temperature and photoperiod were held constant and food was withheld. Gonad development and spawning of these sea cucumbers occurred simultaneously with animals that were fed. Individuals collected from the deep aphotic zone showed gonadal indices and histological development of gametes synchronous with populations found in the shallower photic zone. These data strongly suggest that day length and temperature were not the sole factors controlling the onset and development of gametogenesis, but rather that they act synergistically with other mechanisms. Laboratory experiments showed that gametogenic synchrony was less for individuals separately maintained under natural environmental conditions than it was for similarly treated individuals kept in groups. Finally, gametogenesis was initiated by exposure to more developed individuals, even without the photoperiod cue. This induction, effective only between animals of the same sex, became more evident with increasing maturity of the more developed individuals.
Effect of Wind on Field Metabolic Rates of Breeding Northern Fulmars
Authors: Furness Robert W. , Bryant David M.
Published: May 2006

The field metabolic rate (FMR) of nesting Northern Fulmars (Fulmarus glacialis) was measured using the doubly labeled water technique. Although some labeled Northern Fulmars showed marked differences in behavior compared to controls. FMR did not change correspondingly. At—sea FMR averaged 1444 kJ/d, equivalent to 4.5 X basal metabolic rate (BMR). As a multiple of BMR it was independent of sex, wing length, body mass, mass change, and duration of the period at sea, but was strongly dependent on wind speed, being higher during slack winds. There was also a trend for higher wingbeat frequency at winds to low for sustained dynamic soaring. The high energetic cost of flapping flight during windless conditions may explain patterns of nest attendance: in particular, why nest site attendance by Northern Fulmars in winter is greater during calm weather. Lack of wind might limit the breeding range of this species and other Procellariiformes with high flapping—flight costs.
The Long‐Term Stability of the Boreal Forest Limit in Subarctic Quebec
Authors: Lavoie Claude , Payette Serge
Published: May 2006

Using black spruce (Picea mariana) macrofossil remains at the forest—tundra to tundra transition in norther Quebec, Canada, we address the question: did the boreal forest limit shift in unison with climate change during the Holocene? Logs buried in peatlands, charcoal in the organic soil layers on well—drained sites, and dry peat underlying stunted clonal spruce were sampled across a 60—km transect south and north of the modern forest limit. Of 146 spruce logs sampled south of the modern forest limit, 34 were 14C dated between 4580 and 1540 yr BP, and 34 additional logs were cross—dated with 14C—dated spruces (3010—1540 yr BP) using diagnostic tree rings. South of the forest limit, charcoal with abundant spruce fragments was widespread in dry—mesic sites and dated between 1920 and 1140 yr BP. No spruce logs or charcoal were found in all the surveyed sites north of the forest limit. The residence time of humified peat confined to the area occupied by tundra clonal spruces ranged between 3040 and 630 yr BP. The absence of spruce macrofossils in the tundra zone, together with the geographic coincidence of the charcoal limit and forest limit, strongly suggest that the forest limit remained stable during the last 2000—3000 yr BP. Old—aged peat produced by tundra spruces is additional evidence for the stability of the forest limit. Because only spruce is able to accumulate such a thick organic layer, the 14C dates of basal peat indicate that spruce clones developed continuously in the absence of external disturbance at least over the last 3000 yr BP. The stability of the forest limit during war (≈ 2000 yr BP, medieval times, and this century) and cold (≈ 3000 and 1300 yr BP, and the Little Ice Age) periods of the late Holocene demonstrates that mechanisms allowing forest limit advance or retreat are not easily triggered by climatic change.
Predicting the Outcome of Competition Using Experimental Data: Maximum Likelihood and Bayesian Approaches
Authors: Pascual Miguel A. , Kareiva Peter
Published: May 2006

Lotka—Volterra (LV) equations have been used extensively to explore the possible dynamic outcomes of interspecific competition. But while there have been hundreds of papers on the mathematical properties of Lotka—Volterra models, there have been only a handful of papers that explore techniques for fitting these models to actual data, and no papers that explore the interface of experimental design and statistical inference when fitting LV equations to census data. In this paper we present a statistical analysis of Gause's experimental cultures of Paramecium aurelia and P. caudatum, using analytical methods based on maximum likelihood and Bayesian statistics. We compare the effectiveness of these two approaches in addressing several questions about competition from experimental data: Are the mutual effects of competing populations substantial? Are these competitive effects symmetrical? Are two populations expected to coexist or to eliminate each other by competition? We show that even a laboratory—derived data set with minimal variability can entail significant levels of uncertainty about the nature of the competitive interaction. We assess the errors involved in estimating the strength and symmetry of competition, and find that one's conclusions depend critically on assumptions about sources of variability in the data. We also estimate the probabilities of alternative dynamic behaviors for competing species. We use simulations to evaluate how particular experimental designs might improve our power to characterize the dynamic outcome of competition. We show that much more information is gained by running competition experiments at different starting conditions than by replicating the same experiment for a particular starting condition.
Temporal Scales of Foraging in a Marine Predator
Authors: Boyd I. L.
Published: May 2006

The pattern of prey distribution can profoundly affect the foraging behavior and success of a predator. In pelagic marine ecosystems, where prey is often patchily distributed, predators must be able to adapt quickly to changes in the spatial patterning of prey. Antarctic fur seals feed primarily on krill, which is patchily distributed. When combined with information about swimming speed on the surface, the time taken for a fur seal to locate a new patch after leaving an old one is an indication of the distance between patches. The frequency distribution of intervals between bouts of foraging showed that fur seals foraged at two spatial distributions: (1) a fine—scale (median distance 0.18—0.27 km) represented by short (<5 min) travel durations between patches; and (2) a coarser or meso—scale (median distance 1.3—1.6 km) represented by longer (>5 min) travel durations. In a study lasting 5 yr, the distributions of travel durations between bouts of feeding changed between years. These changes suggested that the structure and/or the spatial distribution of krill swarms varied between years. The behavior of fur seals suggested that there was overall clumping of prey at the fine—scale, but there was a more even spacing of prey patches at the meso—scale level. Only in 1 yr of the study (1990/1991) were there indications that fur seals had difficulty in finding enough food. Fur seal behavior suggested that there was no reduction in the number of prey patches available in that year but that prey patches were of poorer quality. The study showed how predator behavior can provide valuable information about the functional relationship between prey dispersion and predator performance.
Pollen Digestion by Darwin's Finches and Its Importance for Early Breeding
Authors: Grant B. Rosemary
Published: May 2006

This paper examines the role of pollen digestion in the early breeding of Darwin's finches. Pollen is a rich source of protein, but the extent to which birds digest pollen has been debated. On Isla Daphne Major, Galapagos, Geospiza scandens, the Cactus Finch, and G. fortis, the Medium Ground Finch, consume pollen and nectar of the prickly pear cactus, Opuntia echios. I found that >90% of ingested pollen was digested during passage through the gut of adult and nestling G. scandens and adult G. fortis. This value is twice as high as the maximum reported for birds, and comparable to the most efficient digestion known for specialist marsupials. Nectar is ingested with pollen by the finches and may stimulate germination of pollen in the crop, facilitating digestion. Opuntia pollen grains germinated in vitro in a nectar and water solution. Breeding of finches typically follows shortly after the arrival of the first heavy rains of the year. In 6 of 12 years, however, some G. scandens pairs bred and successfully fledged young before onset of the rains. At these times, Opuntia flowers were abundant, arthropods were scarce, and the diet of G. scandens was predominantly Opuntia pollen and nectar. As a result of breeding early, pre—rains breeders produced more clutches and fledged more young per year than post—rains breeders. Although G. fortis take pollen and nectar from Opuntia flowers and efficiently digest pollen grains, they feed on flowers to a much lesser extent than G. scandens, and did not breed before the rains. These results have implications for similar vertebrate pollinator and plant systems in arid tropical regions.
Consequences for Plantain Chemistry and Growth When Herbivores are Attacked by Predators
Authors: Stamp Nancy E. , Bowers M. Deane
Published: May 2006

The indirect effect of insect predators on the chemistry and performance of host plants of their prey is a central issue in the study of tritrophic—level interactions. Few experimental studies evaluate chemistry and performance of host plants fed upon by insect herbivores in the presence and absence of predators. To examine these relationships, we conducted a field experiment using the plant Plantago lanceolata, which contains the iridoid glycosides aucubin and catalpol. Each plot had two plants, designated the "colonization" plant upon which four insect herbivores were initially placed and the "dispersal" plant, to which the herbivores could move. We contrasted the effects of two insect herbivores, Junonia coenia (Nymphalidae), a specialist feeder, vs. Spilosoma congrua (Arctiidae), a more generalized feeder, and the effects of variation in their densities caused by two different types of insect predators, Podisus maculiventris stink bugs and Polistes fuscatus wasps, on host plant growth and phytochemistry. We contrasted these to plots without herbivores. The two herbivore species had different impacts on iridoid glycoside concentration of their host plants. Iridoid glycoside concentrations were higher in plants that had been used by the specialist J. coenia compared to those fed upon by the generalist S. congrua. In contrast to a previous study, herbivory did not induce higher iridoid glycoside concentration. The two herbivore species had similar impacts on plant mass. Predation by wasps and stink bugs had different indirect effects (via the interaction of predators and herbivores) on iridoid glycoside concentration and plant mass. Aucubin concentration was lower for plants that had had wasps than those with stink bugs. For plants with the specialist J. coenia, catalpol concentration was lower for plants that had had predators, and iridoid glycoside concentrations were lower for plants that had had wasps present than those that had stink bugs. Leaf mass was higher when predators were present than when predators were absent. For the plants with the specialist J. coenia, scape mass was higher for those that had predators present than those without predators. We discuss the potential for an indirect facultative mutualism between the host plant plantain and the insect predators, and conclude that such a mutualism is transitory, at best.
Adaptation to Predators in a New Community: Swimming Performance and Predator Avoidance in Damselflies
Authors: McPeek Mark A. , Schrot Ann K. , Brown Jonathan M.
Published: May 2006

Species evolve in response to the abiotic and biotic interactions they experience in a community, and so studying the adaptation of species in a community context is a necessary component for understanding the development of community structure. Previous studies have suggested that Enallagma damselflies (Odonata: Coenagrionidae) invaded aquatic habitats with large dragonflies as the top predators at least twice, and large changes in morphological structures (e.g., caudal lamellae and abdomen), which should increase swimming speed, are associated with these invasions. This is significant because these species swim away from attacking predators, whereas species in the ancestral habitat (habitats with fish as the top predators) do not swim from attacking predators. We quantified swimming speed over a large ontogenetic size range for larvae of seven Enallagma species, two in dragonfly lakes and five in fish lakes. Results of analysis of covariance and a modification of Felsenstein's method of evolutionary contrasts indicate that a large evolutionary increase in the slope of the ontogenetic relationship between swimming speed and loge(body length) is associated with invasions into dragonfly lakes, and this shift results in larvae of dragonfly—lake species being faster swimmers, especially in larger size classes than larvae of fish—lake species. We also present experimental results demonstrating that damselflies with larger lamellae are faster swimmers and have higher survival when exposed to dragonfly predation. The results of this study indicate that the change in selective environment associated with habitat shifts into dragonfly lakes promoted adaptations for increased swimming speed in Enallagma lineages. The potential ramifications of this adaptation for community structure are discussed.
Ant Community Structure: Effects of Predatory Ant Lions
Authors: Gotelli Nicholas J.
Published: May 2006

This study examined the responses of ground—foraging ants to larval ant lions (Neuroptera: Myrmeleontidae). In central Oklahoma, these sit—and—wait arthropod predators are restricted by abiotic factors to sheltered cliff bases. A high—density ant lion zone forms an effective "minefield" of predation for local ant assemblages. The density of ant—nest entrances and the number of pitfall—trap captures of ant foragers were significantly lower in the ant lion zone than in the adjacent forest or grassland. Differences in ant abundance could not be attributed to differences in thermal microhabitat within and outside the ant lion zone. Over a 24—h period, ants foraged continuously at tuna—fish baits placed on the forest floor, but never utilized baits placed within the ant lion zone. Field behavioral tests with individual ant foragers confirmed that the risk of predation from ant lions was high for common ant species in the assemblage. Among species, predation risk was negatively correlated with worker body mass. Manipulative field experiments tested whether ants use biotic or abiotic cues to avoid ant lion aggregations. Ants foraged readily at baits on the forest floor and baits placed in the center of experimental sand patches, but they avoided baits placed in sand patches that contained ant lions. Ant foraging was also substantially lower in patches from which ant lions had been removed 3 h prior to the start of the experiment. Ant foraging was slightly reduced in patches with artificial ant lion pits that had never contained predators. These experiments suggest that ants use biotic cues associated with the presence or recent presence of predators. Previous studies have not implicated predation as an important factor structuring ant communities. This study suggests that predators can have important community—wide impacts on the distribution, abundance, and behavior of ground—foraging ants.
The Effect of Landscape Heterogeneity on the Probability of Patch Colonization
Authors: Gustafson Eric J. , Gardner Robert H.
Published: May 2006

The effect of landscape heterogeneity on the dispersal of organisms between habitat islands is poorly understood. Preferred pathways for dispersal (i.e., corridors), as well as dispersal barriers, are difficult to identify when the landscape matrix is composed of a complex mixture of land cover types. We developed an individual—based dispersal model to measure immigration and emigration rates between habitat islands within heterogeneous landscapes. Dispersing individuals of a model organism were simulated as self—avoiding random walkers (SAW) traversing a digital land cover map, with each habitat type assigned a priori a probability that the SAW would enter that habitat type. Each individual began the dispersal process on a random site at the edge of a deciduous forest patch and was allowed to move until it reached a different deciduous forest patch. Visualization of the movement patterns across the landscape was achieved by tabulating the frequency of visitation of successful dispersers to each grid cell on the map. The model was used to estimate the probabilities of disperser transfer between patches by varying the a priori probabilities of movement into each habitat type in order to: (1) estimate the effect of changing landscape heterogeneity on the transfer probabilities, and (2) visualize dispersal corridors and barriers as perceived by model organisms operating by specific movement rules and at specific scales. The results show that 89% of the variability in dispersal success can be accounted for by differences in the size and isolation of forest patches, with closer and larger patches having significantly greater exchange of dispersing organisms. However, changes in the heterogeneity of the landscape matrix could significantly enhance or decrease emigration success from an individual patch, depending on the landscape. Changes in emigration success from an individual patch resulting from changes in matrix heterogeneity were not predictable, and transfer rates between patches were not symmetrical due to differences in the proximity of neighboring patches, and differences in the funneling attributes of certain landscape patterns. Visualizations showed that corridors are often diffuse and difficult to identify from structural features of the landscape. A wide range of organisms with differing movement capabilities can be simulated using the approach presented to increase our understanding of how landscape structure affects organism dispersal.
Predator‐Induced Behavioral Indirect Effects: Consequences to Competitive Interactions in Anuran Larvae
Authors: Werner Earl E. , Anholt Bradley R.
Published: May 2006

This study examines the non—lethal effects of an odonate predator (Anax junius) on the competitive interactions among several size classes of anuran larvae. In an outdoor experiment using cattle watering tanks, we estimated the effects of both large and small bullfrogs (Rana catesbeiana) on themselves, on each other, and on small green frogs (R. clamitans) in the absence and non—lethal (caged) presence of Anax. The presence of Anax depressed both growth and survivorship of small bullfrogs and green frogs. In contrast, the presence of Anax had positive effects on growth rates and size at metamorphosis of the large bullfrogs. Increasing density of competitors also decreased survivorship of small classes, and growth rates of all classes. The per—unit—biomass competitive effects of the small bullfrogs on target classes were much greater than those of large bullfrogs. The presence of Anax significantly altered the per—unit—biomass competitive effects of small bullfrogs but not large bullfrogs, presumably because individuals in the small class reduced their activity rates in the presence of Anax. Overall production of new tadpole biomass was quite similar across experimental units, with decreases in production of small size classes in the presence of Anax compensated for by increases in production of the large size class. Thus the non—lethal presence of Anax had substantial effects on the nature of competitive interactions in this system, and we discuss the implications of such behavioral indirect effects in the study of ecological communities. Our results also illustrate the futility of attempting to partition the effects of competitors and predators, as both competitors and the non—lethal presence of predators significantly affected growth rates and death by starvation of small larvae. Finally, our results illustrate how individual behavioral responses may be translated to community and ecosystem properties.
Plant‐Specific Response to Herbivory: Simulated Browsing of Suppressed Balsam Fir on Isle Royale
Authors: McLaren Brian E.
Published: May 2006

Balsam fir (Abies balsamea) is a shade—tolerant conifer that experiences striking growth suppression from browsing of the understory of mixed, boreal—type forests of Isle Royale, Michigan, where it forms an important component of moose (Alces alces) diet in winter. Suppression increases with increasing canopy cover and forest age. However, response to an individual, severe browsing episode cannot be generalized in terms of suppression. Such shorter—term responses are best investigated by experimentation. Artificial removal of foliage from understory balsam fir in two sites on Isle Royale and one mainland site showed that damage such as reduced terminal growth and complete mortality was actually highest among least suppressed trees. Canopy cover produced a secondary effect, in which trees experiencing >60% cover were unable to compensate for removal by clipping. Among less suppressed trees, competition for limited light produced an apically oriented morphology that countered the laterally oriented morphology associated with suppression. Higher vulnerability to the artificial browsing occurred because trees were no longer protected by snow cover. By the second growing season following clipping, surviving clipped trees even in the most damaged sites resumed the same growth as controls. Over the long term, suppression may be a means by which fir remains alive when subjected to continuous browsing. Over the shorter term, delayed growth recovery following severe browsing in an unsuppressed understory may also contribute to sustainability in Isle Royale's plant—herbivore system.
Above‐ and Belowground Competition Intensity in Two Contrasting Wetland Plant Communities
Authors: Twolan-Strutt Lisa , Keddy Paul A.
Published: May 2006

A fundamental question in plant ecology is if and how the intensity of competition changes with productivity. This question has been the source of considerable discussion during the last two decades, yet few experiments have tested whether competition intensity changes with productivity in nature. Even fewer studies have separated competition into its above— and belowground components in the field. We used a field experiment to measure total competition intensity and its above— and belowground components in two wetlands that represent extremes in habitat productivity: an infertile sandy shoreline and a fertile bay. Transplants of Lythrum salicaria and Carex crinita were grown with no neighbors, with roots of neighbors only, and with roots and shoots of neighbors; their growth rates were used to measure competition intensity (CI). The experiment was carried out to answer the following main questions: (1) Is there a difference in total, above— and belowground competition intensity in two wetlands that differ in standing crop? and (2) Is there an effect of standing crop on total, above— and belowground competition intensity when the data from the two wetlands are combined? Results based on the average of both species show that total and aboveground competition intensity were greater in the high standing crop wetland, but belowground competition did not differ between wetlands (CITOTAL: P < 0.00001, CIABOVE: P = 0.0013, CIBELOW: P = 0.58). Total and aboveground competition intensity were significantly affected by standing crop in the wetlands studied but belowground competition intensity was not (CITOTAL: P = 0.0001, CIABOVE: P = 0.0001, CIBELOW: P = 0.89). Results based on the two species separately show that species vary in their sensitivity to competition in wetland communities.
Regional Dynamics and the Status of Amphibians
Authors: Hecnar Stephen J. , M'Closkey Robert T.
Published: May 2006

Increasing attention is being devoted to the dynamics of interacting local populations of species, especially the role of changes in incidence that affect regional persistence. The status of species in a regional context may be determined more by metapopulation dynamics than by purely demographic birth and death processes. The contrast between local and regional views of species' persistence is illustrated today by discussion of the status of amphibians. The amphibian fauna may provide an important indicator of the impact of anthropogenic disturbance to wetland ecosystems. We assessed the status of 11 amphibian species in southwestern Ontario, Canada, by estimating species richness, changes in presence and absence, and incidence at 97 ponds from 1992 to 1994. We detected a significant reduction in amphibian species richness in one of three regions. This loss of diversity relative to the historical species complement is a consequence of the land use history. We observed surprisingly high turnover of species at ponds, with increased incidence varying from 0.07 to 0.29 species per pond per year, and decreased incidence ranging from 0.16 to 0.30 species per pond per year. The incidence of common species across years included both declines (leopard frog, Rana pipiens) and increases (American toad, Bufo americanus). For eight relatively rare species, losses exceeded gains between 1992 and 1993, but this pattern was reversed between 1993 and 1994. Understanding environmental factors that determine the status of species will require an expanded, large—scale view of groups of populations (metapopulations) and their spatial dynamics.
Tests of Ecological Mechanisms Promoting the Stable Coexistence of Two Bacterial Genotypes
Authors: Turner Paul E. , Souza Valeria , Lenski Richard E.
Published: May 2006

A series of competition experiments with two genotypes of Escherichia coli showed that each genotype was favored when it was the minority, allowing their coexistence in a stable polymorphism. In these experiments, glucose was the sole source of carbon provided, and its concentration was limiting to population density. Thus, the stable polymorphism did not conform to a simple model of competitive exclusion. In similar experiments also with glucose as the sole resource, we considered two hypotheses that might explain the observed coexistence: (1) a strictly demographic trade—off, such that one genotype is competitively superior when glucose is abundant whereas the other genotype is the better competitor for sparse glucose; and (2) a cross—feeding interaction, whereby the superior competitor for glucose excretes a metabolite that acts as a second resource for which the other genotype is the better competitor. Although there was a demographic tradeoff, the advantage to the superior competitor at high glucose concentrations was too large (given the initial concentration of glucose used in these experiments) to allow the second genotype to invade when rare at the observed rate. Therefore, the second genotype must have had some other advantage that allowed it to readily invade a population of the superior competitor for glucose. Indeed, the second genotype could increase in abundance after glucose was depleted, but only in the presence of the superior competitor for glucose, thus implicating a cross—feeding interaction. These results confirmed earlier studies showing that populations of E. coli can maintain ecologically relevant genetic diversity even in a simple environment.