bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of Shrubs on Recruitment of Quercus Douglasii and Quercus Lobata in California
Authors: Callaway Ragan M.
Published: May 2006

Biologically derived safe sites appear to strongly affect spatial and temporal patterns in terrestrial plant communities; however, few experimental field studies have been conducted to investigate the importance of such interactions or the mechanisms by which they operate. Quercus douglasii and Quercus lobata are winter—deciduous oaks endemic to California. I examined association between seedlings of these oaks and several species of shrubs and conducted field experiments that tested for facilitative effects of two shrubs, Salvia leucophylla and Artemisia californica, on seedling survival. Naturally occurring Q. douglasii seedlings were relatively common (280 seedlings/ha) and were strongly associated with shrub canopies. In two experimental plantings, 30% and 55% of Q. douglasii seedlings that emerged under shrubs survived for >1 yr, whereas no seedlings in the open survived in either experiment. Survival of Q. douglasii where shrubs were removed was similar to survival in the open grassland, indicting that association between shrubs and oak seedlings was not due to share microsite requirements. Q. douglasii survival was not different between the two shrub species. Only three Q. lobata seedlings were found in 26 study plots, although reproductive adults were present. The seedlings were under shrub canopies. However, the survival of Q. lobata was not facilitated by shrub cover or shrub simulation and no other evidence was found for nurse—shrub interactions with this oak species. Artificial shade was critical to the survival of Q. douglasii seedlings in simulated shrub environments. When grown in artificial shade, photosynthetic capacities and root elongation rates of Q. douglasii seedlings were significantly higher than those of Q. lobata. These differences may partially explain why nurse—plant interactions exist between shrubs and Q. douglasii seedling mortality differed between the shrub and open grassland habitats. Under shrubs, acorn predation was the primary cause of mortality, whereas mortality due to shoot herbivory was much more frequent in the open grassland. Thus, propagules under shrubs that avoided predation until shoot emergence benefitted from canopy shade and protection from herbivores. The shade tolerance and nurse—plant utilization of Q. douglasii seedlings have parallels with shade tolerance and late successional roles of other oak species in temperature deciduous forests, and, as in these forests, may affect the long—term spatial dynamics in California woodlands.
Root Communication Mechanisms and Intracommunity Distributions of Two Mojave Desert Shrubs
Authors: Mahall B. E. , Callaway R. M.
Published: May 2006

Experimental studies using root observation chambers to observe the effects of encounters between individual roots on root elongation rates have revealed that the interactions among roots of Ambrosia dumosa and Larrea tridentata are more complex than simple competition for a limiting resource. Larrea roots inhibited elongation of either Larrea or Ambrosia roots in their vicinity, and Ambrosia roots inhibited elongation of contacted roots on other Ambrosia plants only. The purpose of the study reported here was to test the hypothesized involvement of inhibitory substances released by roots in these interroot encounters by attempting to remove such substances by adsorption to activated carbon. The presence of activity carbon caused a significant decrease in the inhibition of elongation of neighboring roots by Larrea roots, but activated carbon had no effect on the intraspecific responses of Ambrosia roots. These results support the hypotheses that the interaction mechanism of Larrea roots involves the release of a readily diffusible, generally inhibitory substance by Larrea roots into the soil, rather than a simple depletion of water or nutrients from around Larrea roots, and that the intraspecific, self—nonselfish—recognizing interaction mechanism of Ambrosia roots is mediated by contact and is fundamentally different from that of Larrea. These findings may enhance our understanding of Mojave desert community structure. The root—mediated allelopathy of Larrea may play a role in producing and maintaining the commonly occurring, regular distributions of Larrea. The complex communication mechanism of Ambrosia roots appears to constitute a detection and avoidance system that may allow this shrub to grow in clumped intraspecific distributions with little or no intraspecific competition for water. The interspecific interference between Larrea and Ambrosia in the field may be mechanistically asymmetrical due to their different root communication mechanisms.
Ant‐Herbivore Interactions: Reasons for the Absence of Benefits to a Fern with Foliar Nectaries Vanessa K. Rashbrook
Authors: Compton Stephen G. , Lawton John H.
Published: May 2006

We tested the hypothesis that ants, attracted to the foliar nectaries of bracken fern (Pteridium aquilinum) confer protection to the plant in South Africa. Using ant—exclusion experiments with unmanipulated and artificially augmented ant densities, we were unable to detect a positive effect of ants on bracken, as neither the number of herbivores nor the level of herbivory was significantly greater on ant—free fronds. We then examined possible explanations for these results by performing a number of laboratory experiments using the ant Cremtogaster peringueyi and two lepidopteran species, the major bracken—feeding herbivores. We showed that their eggs were susceptible to ant predation, but gained protection from their oviposition sites. We also demonstrated that bracken can benefit when densities of ants are high, such as when fronds are infested with honeydew—producing homopterans, as the lepidopteran larvae were more vulnerable to attack under these conditions. We propose that low ant density on the fronds is the primary factor limiting the occurrence of mutualism between bracken and ants in nature.
Foraging in Heteromyid Rodents: The Energy Costs of Scratch‐Digging
Authors: Morgan Kenneth R. , Price Mary V.
Published: May 2006

North American heteromyid rodents are alike in their primary seed harvesting behavior (scratch—digging), which is a potentially important component of their foraging energetics. The present study was designed to quantify the energy cost and efficiency ([Benefit — Cost]/Cost) of scratch—digging throughout the body size range represented among the Heteroyidae. The resting energy expenditure of bipeds was generally greater than that of quadrupeds on an equal body size basis, whereas their energy expenditure during scratch—digging (DMR) was equivalent. Seed concentration and substrate bulk—density did not markedly affect DMR, but seed concentration did alter the rate of seed harvest. Species differences in seed harvest rate were correlated with mode of locomotion, supporting the hypothesis that bipedality is correlated with specialization of the forelimbs for seed handling. In heteromyids, resting metabolic rate (RMR) was elevated by a factor of 1.5 over basal metabolic rate (BMR); DMR was elevated by a factor of 3.4 over BMR and by a factor of 2.3 over RMR. DMR, seed harvest rate, and the energy cost of scratch—digging on a per seed basis scaled with body mass to the 0.65, 0.31, and 0.33 powers, respectively. Although large bipedal heteromyids harvested seeds at more than twice the rates of small quadrupedal heteromyids, the unequal scaling of DMR and harvest rate makes them less efficient at scratch—digging. In comparison with other heteromyids, the kangaroo mouse (Microdipodops megacephalus) was exceptionally efficient at scratch—digging. Its higher efficiency may allow it to forage on sparsely distributed seeds in open areas where the energy return would be inadequate for larger bipeds. The pocket mouse (Chaetodipus baileyi) was exceptionally inefficient at scratch—digging. Its low efficiency in sand may be correlated with specialization for foraging in pebbly soils. For heteromyids of a given size, factors affecting pouching rate are most likely to determine foraging efficiency because DMR is essentially a fixed cost.
A Morphological Analysis of the Structure of Communities of Lizards in Desert Habitats
Authors: Ricklefs Robert E. , Cochran Donald , Pianka Eric R.
Published: May 2006

Using nine measurements of the head, body, and appendages we characterize the morphological relationships among lizards in deserts of western Australia, southern Africa, and western North America. Each species was placed in a morphological space whose dimensions were the logarithms of the original measurements. We calculated the dispersion of the species in each region along principal components to assess the number of dimensions and volume of morphological space occupied. Euclidean distances between all pairs of species were calculated to assess the density of species packing within the morphological space. We also generated random subsets of the species pool within each region and calculated matrices of Euclidean distance for them. In the combined sample of 83 species, the first three principal components accounted for 86, 8 and 3%, respectively, of the total variance in the dispersion of species in morphological space. Pairwise discriminant analysis revealed significant differences between the morphological distributions of the lizards in the three regions. When analyzed separately, the 54 Australian lizards occupied the largest volume, followed by the 11 North American species, and, lastly, the 18 southern African species. We used the average distance to the nearest neighbor (NND) as a measure of species packing. NND's in Australian localities (18—36 species) and in North American localities (4—10 species) were considerably greater than those in southern African localities (10—15 species). When assemblages were adjusted for number of species, the Australian lizards were more widely spaced morphologically than those in either the North American or African localities. Attributes of randomly generated assemblages did not differ morphologically from the subsamples of species found in each locality. Furthermore, the standard deviations of NND's, a measure of the regularity of species packing, were similar in natural and randomly generated communities. Hence the present analysis provides little indication of interaction between species according to morphological attributes. Our morphological analysis confirmed some patterns revealed by studies of ecological relationships in the same localities.
Seasonal Niche Relationships of Rain Forest Anoles
Authors: Lister Bradford C.
Published: May 2006

In this paper, seasonal changes in resource partitioning among Anolis lizards in the Luquillo rain forest in eastern Puerto Rico were investigated with the aim of understanding how climatic variation, competition and resource availability interact to affect niche relationships and community structure. Arthropod densities within the Luquillo forest were found to remain stable between the summer (July 1976) and winter (January 1977) seasons at lower levels of the forest but to decrease during the winter season at higher levels. Seasonal changes in the niche relationships of Anolis gundlachi, A. evermanni and A. stratulus included: (1) a winter season decrease in spatial overlap and spatial niche widths, (2) an expansion of the prey size and prey taxa components of the food niche during the winter. However, compared to anoles in more seasonal environments at Maricao and Guanica in southwestern Puerto Rico, anoles in the relatively aseasonal Luquillo forest exhibited greater niche stability between seasons and greater niche overlap during both seasons. At Maricao and Guanica, spatial niche widths and spatial overlap also decreased during the winter dry season but to a much greater extent than at Luquillo. Data on seasonal changes in food abundance, food consumption, and reproductive condition suggest that food competition is more intense during the prolonged dry seasons at Maricao and Guanica. Thus within Anolis communities, spatial overlap and spatial niche widths may be an inverse function of food competition and the degree of seasonal change in niche relationships may be directly correlated with the magnitude of seasonal fluctuations in food supply. Finally, loss rates of anoles from census areas at Luquillo were analyzed and suggested high rates of predation. The hypothesis was advanced that greater predation pressure may explain the generally higher levels of niche overlap among anoles in stable environments compared to more variable environments.
Competition Among Plants Sharing Hummingbird Pollinators: Laboratory Experiments on a Mechanism
Authors: Feinsinger Peter , Tiebout Harry M.
Published: May 2006

At Monteverde, Costa Rica, the plants Besleria triflora (Gesneriaceae) palicourea lasiorrachis, and Cephaelis elata (Both Rubiaceae) share hummingbirds (Lampornis calolaema) as pollinators. To investigate the effect that intervening visits to Cephaelis flowers have on pollen transfer between plants of self—compatible Besleria or of self—in—compatible Palicourea, we presented captive Lampornis with series of hand—held flowers: first, 2 or 15 flowers of either Besleria or Palicourea as pollen donors, followed by 0, 2, or 10 Cephaelis flowers, followed by 20 recipient flowers of the first species. In protandrous Besleria, we used male—phase flowers as donors and female—phase flowers as recipients. In distylous Palicourea, we used pin (long—styled) flowers as donors and thrum (short—styled) flowers as recipients. We assessed pollen transfer by counting pollen tubes in recipients' styles and, in Besleria, by counting pollen grains deposited on stigmas. Intervention of Cephaelis flowers strongly reduced pollen transfer among Palicourea flowers. Increasing the number of donor Palicourea flowers significantly increased the absolute amount of pollen transferred but did not overcome the negative effect of Cephaelis. Pollen transfer among Besleria flowers also declined with the intervention of Cephaelis flowers, the decline was less strongly significant than for Palicourea, perhaps because pollen loads on stigmas of some female Besleria flowers can contain self pollen left over from their male phase. Under controlled conditions, then, strong competition may exist where plants share pollinator individuals, a situation faced by the experimental species and many others at Monteverde. Other studies on these plants reveal that despite its demonstrability in the laboratory, however, competition among individual plants varies in its ability to explain density dependence in pollination among plant populations in the field. Furthermore, community—level patterns of coexistence among plant species at Monteverde feature few "ghosts of competition past." The collective results demonstrate that studying the role of particular interactions in community structure must involve examinations of mechanisms at the level of individuals, processes at the population level, and patterns of coexistence rather than just one level of this hierarchy. Studying mechanisms alone could exaggerate their importance to community structure, whereas studying coexistence patterns along could misjudge the strength of interactions among individuals.
Vegetation and Climate Change in Eastern North America Since the Last Glacial Maximum
Authors: Prentice I. Colin , Bartlein Patrick J. , Webb Thompson
Published: May 2006

Response surfaces describing the empirical dependence of surface pollen percentages of 13 taxa on three standard climatic variables (mean July temperature, mean January temperature, and mean annual precipitation) in eastern North America were used to infer past climates from palynological data. Inferred climates at 3000—yr intervals from 18 000 years ago to the present, based on six taxa (spruce, birch, northern pines, oak, southern pines, and prairie forbs), were used to generate time series of simulated isopoll maps for these taxa and seven others (hickory, fir, beech, hemlock, elm, alder, and sedge). The simulations captured the essential features of the observed isopoll maps for both sets of taxa, including differences in migration patterns during the past 10 000 yr that have previously been attributed to differential migration lag. These results establish that the continental—scale vegetation patterns have responded to continuous changes in climate from the last glacial maximum to the present, with lags ≤ 1500 yr. The inferred climatic changes include seasonality changes consistent with orbitally controlled changes in insolation, and shifts in temperature and moisture gradients that are consistent with modelled climatic interactions of the insolation changes with the shrinking Laurentide ice sheet. These results pose new ecological questions about the processes by which vegetated landscapes approach dynamic equilibrium with their changing environment.
Fine Litter Dynamics within the Tree Canopy of a Tropical Cloud Forest
Authors: Nadkarni Nalini M. , Matelson Teri J.
Published: May 2006

Fine litter deposition and decomposition within the upper tree canopy was measured in a neotropical cloud forest to determine the potential nutrient input to epiphyte communities from intercepted tree litterfall. A comparable amount of fine litter passed through the canopy (752 g°m—2°yr—1) as arrived on the forest floor (820 g°m—2°yr—1), but <1% of the biomass and nutrients of this "gross litterfall" was retained within the upper tree canopy. The standing crop of litter in the canopy (°170 g/m2 of branch surface area, 8.8 g/m2 of ground area) is equivalent to only 1% of the standing crop of litter on the forest floor. Measurements of leaf litter attrition (whole leaf loss from branches due to wind and other disturbances) with marked leaves documented that 70% of leaves deposited on branches are lost in the first 2 wk and nearly all are gone in 16 wk. Certain branch characteristics (branch angle, number of epiphyte stems and clumps) appear to affect the amount of litter retained at particular microsites. Decomposition of tethered, dead leaves within the canopy over a 12—mo period was half that of leaves on the forest floor (canopy litter turnover time = 2.8 yr). Assuming that litter accumulation within the canopy is at steady state, the biomass of fine litter retained and decomposed within the canopy was calculated as only 2.0 g°m—2°yr—1 and < 0.02 g°m—2°yr—1 for all nutrients. Nutrient replenishment of epiphyte communities appears to be decoupled from the litterfall pathway, as input from litterfall retained within the canopy is small relative to epiphyte productivity and nutrient requirements reported in other studies.
Group Membership Facilitates Feeding of The Herbivorous Sea Slug Placida Dendritica
Authors: Trowbridge Cynthia D.
Published: May 2006

The herbivorous sea slug Placida dendritica (Gastropoda: Opisthobranchia: Ascoglossa) formed feeding groups on the coenocytic green algae Codium setchellii and C. fragile, but not on the related green alga Bryopsis corticulans. The trophic implications of Placida congregation were examined in a series of laboratory experiments. The effect of group membership on Placida growth depended on the number and relative sizes of individuals in the group. Members of Placida congregations composed of similar—sized individuals grew significantly faster than single individuals. In mixed—sized congregations, however, trophic benefits were not shared equally among all members. Small slugs always benefited from the presence of conspecifics, small or large; large slugs, however, benefited only if conspecifics were comparable in size. Small Placida tended to reduce the growth rate of large conspecifics. Although the mechanisms of intraspecific facilitation and competition among group members were only partially elucidated, a slug's feeding not only stimulated other group members to feed but also modified algal condition, enhancing feeding and growth of conspecifics.
Constant vs. Variable Risk‐Aversion in Foraging Bananaquits
Authors: Wunderle Joseph M. , Cotto-Navarro Zopraida
Published: May 2006

Recent foraging theories predict that risk—averse foragers will trade off increases in mean reward against increases in reward variability. Different formulations of this trade—off have been proposed, with some models assuming constant risk—aversion and others assuming decreasing risk—aversion. A detailed analysis of indifference curves (when a forager shows equal preference for certain mean—variance combinations) can theoretically be used to discriminate between models. We follow the analysis of previous workers by equating decreasing risk—aversion with the predictions of the z—score model because those predictions suggest indifference curves that can be discriminated from the constant risk—aversion of the variance discounting model. Experiments conducted with captive Bananaquits (Coereba flaveola) foraging on nectar in an artificial floral patch indicate that they show a simple mean—variance trade—off. Bananaquit foraging behavior was consistent with the constant risk—aversion of the variance—discounting model, but was significantly different from decreasing risk—aversion represented by the z—score model.
Age‐Specific Patterns of Reproduction in the Glaucous‐Winged Gull: Increased Effort with Age
Authors: Reid Walter V.
Published: May 2006

I examined age—specific breeding patterns in the Glaucous—winged Gull (Larus glaucescens) in Washington and evaluated the importance of age—specific changes in reproductive effort as an explanation for improved breeding performance with age of the adult. Age was related to nest location, laying date, egg size, clutch size, hatching success, and territory size. Most variables influenced by age improved for at least the first two breeding seasons. Improved breeding performance is best explained by increased experience and, for two breeding variables, by colony growth. A decline in performance in several parameters (clutch size, egg size, and hatching success) among older birds may be a result of senescence. No evidence existed for an increase in reproductive effort with age in the Glaucous—winged Gull. During incubation, adult mass, relative to body size, was negatively correlated with age. However, the reduced mass of older birds is not evidence of increased reproductive effort for three reasons: (1) birds provided with supplemental food did not gain mass during incubation; (2) older birds were as likely as young birds to lay a fourth egg if the first was removed; and (3) birds were not limited in foraging time during incubation. During the nestling period, body mass, relative to size, was not correlated with adult age; however, older birds gained more mass than young birds between incubation and the end of the nestling period. The amount of time that adults spent foraging was unrelated to age. However, older birds spent more time resting and older females were significantly less aggressive than young females. Older birds also left the territory vacant for less time, indicative of better coordination of nesting activities. The absence of the predicted increase in reproductive effort may be due to direct effects of senescence combined with weak selection for an increase in effort.
Monotonic or Unimodal Diversity‐Productivity Gradients: What Does Competition Theory Predict?
Authors: Abrams Peter A.
Published: May 2006

This article discusses two types of proposed relationships between resource productivity and the diversity of coexisting consumer species. Monotonically increasing curves have recently been dismissed on both empirical and theoretical grounds, while unimodal ("hump—shaped") curves have been supported. Unimodal curves have been attributed to increased competitive exclusion, usually as the result of decreased heterogeneity in limiting resources at high productivities. This article argues that: (1) there are many viable mechanisms that can produce monotonic curves in the presence of competition; (2) there is little empirical support for any of the major variants of the hypothesis that productivity decreases heterogeneity, which increases competitive exclusion; and (3) there are alternative reasons for unimodal curves, some or all of which are consistent with previously observed productivity—diversity relationships. Additional theoretical and empirical work is required to understand what relationships are follow from different mechanisms of competition, and what relationships are most frequently observed under different observational protocols.
Aspen, Elk, and Fire in Northern Yellowstone Park
Authors: Romme William H. , Turner Monica G. , Wallace Linda L. , Walker Jennifer S.
Published: May 2006

Most stands of trembling aspen (Populus tremuloides) in northern Yellowstone National Park appear to have become established between 1870 and 1890, with little regeneration since 1900. There has been controversy throughout this century regarding the relative roles of browsing by elk (Cervus elaphus) and fire suppression in preventing aspen regeneration. Fires in 1988 burned 22% of the northern ungulate winter range in the park, and created an unusual opportunity to investigate interactions between fire, ungulate browsing, and aspen regeneration. We tested two hypotheses. (1) The fires would stimulate such prolific sprouting of new aspen stems in burned stands that many stems would escape ungulate browsing and regenerate a canopy of large aspen stems. (2) Browsing pressure would be so intense that it would inhibit aspen canopy regeneration in the burned stands, despite prolific sprouting, but increased forage production in the burned areas would attract elk so that they would not seek out remote aspen stands, and hence, aspen regeneration would occur in unburned aspen stands remote from the burned areas. We sampled aspen sprout density, height, growth form, and browsing intensity in six burned aspen stands six unburned stands close (< 1km) to the burned area, and six unburned stands remote (> 4 km) from the burned area. Density of sprouts was generally greater in the burned stands than in the unburned stands in spring 1990 (2 yr after the fires), but was approaching the density of unburned stands by fall 1991. There were no significant differences in browsing intensity (percent of aspen sprouts browsed by ungulates) in 1990 or 1991 among burned, unburned close, or unburned remote stands, nor were there differences in relation to growth form (juvenile vs. adult sprouts). Unbrowsed sprouts generally were lower than the depth of the snowpack, suggesting that elk browsed nearly all sprouts that were accessible. The age distribution of 15 aspen stands across the northern winter range indicated that regeneration of large canopy stems had been episodic even prior to the establishment of the park in 1872. The period 1870—1890, when the present—day aspen stands were generated, was historically unique: numbers of elk and other browsers were low, climate was relatively wet, extensive fires had recently occurred, and large mammalian predators of elk (e.g., wolf, Canis lupus) were present. This combination of events has not recurred since 1900. The recent paucity of aspen regeneration in northern Yellowstone National Park cannot be explained by any single factor (e.g., excessive elk numbers or fire suppression) but involves a complex interaction among factors.
Tropical Larval Anuran Fitness in the Absence of Direct Effects of Predation and Competition
Authors: Gascon Claude
Published: May 2006

I compare patterns of larval anuran species occurrences in aquatic habitats to results from exclosure experiments of tadpoles across all available habitat types. The purpose of this study was to investigate habitat effects on larval anuran fitness measures in the absence of direct effects of predators and competitors in a central Amazonian rain forest. In this sense, this study tests species—habitat associations established in an earlier study (Gascon 1991). Fifty—three primary forest aquatic sites of four distinct types (stream, streamside, peccary wallows, and terra firme sites) were surveyed for 2 yr for larval anuran species presence. In separate transplant experiments, I monitored growth, survival, and development of four larval anuran species reared in mesh cages as a function of habitat type. Tadpoles of Osteocephalus taurinus occurred in all types of habitats, but had a significantly higher mean number of cohorts in high—lying sites (terra firme and wallows) compared to low—lying sites (streams and streamside pools). Hyla geographica was restricted to streamside sites. Epipedobates femoralis and Phyllomedusa tomopterna occurred only in high—lying natural sites (wallows and terra firm sites). When transplanted, O. taurinus did significantly better in high—lying sites than in low—lying ones. Hyla geographica did not perform significantly better in sites where it naturally occurred. Phyllomedusa tomopterna showed significantly lower survival in low—lying sites compared to high—lying ones. Finally, E. fermoralis performed as well in streamside sites (where it never occurred naturally) as in high—lying sites. Occurrence patterns for P. tomopterna and O. taurinus were consistent with the transplant results. This suggests that these species occupy all habitats within their scope of physiological tolerance. Other factors, such as direct predation on larvae or interference competition, do not appear to account for the distribution of these species. For H. geographica and E. femoralis, results of transplants indicate that they do not perform better in their "preferred" habitat compared to unoccupied habitats.
Density Dependence Resolves the Stream Drift Paradox
Authors: Anholt Bradley R.
Published: May 2006

The downstream displacement of individuals by drifting in the current is a prominent feature in the population biology of stream invertebrates. To compensate for the loss of individuals it has been proposed that adults of aquatic insects preferentially fly upstream to oviposit and thereby maintain populations. The observation of adult flight biased in the upstream direction has been advanced as confirmation of the hypothesis. I argue here that upstream—biased dispersal is not sufficient for population persistence nor is it necessary. Population persistence can only be explained on the basis of density dependence at some point in the life cycle. Computer simulations showed that density dependence by itself is not sufficient for population persistence. Infrequent dispersal by randomly flying adults coupled with density dependence does allow population persistence. Upstream—biased dispersal can be explained by increased individual fitness. If upstream reaches are depopulated by drift, these reaches provide more rapid growth to successful colonists. Computer simulations of competition for space by two genotypes that differ only in the directionality of dispersal showed that genotypes with upstream—biased dispersal always drove random dispersers to extinction.
Growth Variation in a Naturally Established Population of Pinus Sylvestris
Authors: Stoll Peter , Weiner Jacob , Schmid Bernhard
Published: May 2006

We studied the variation in growth of individual trees in a population of Pinus sylvestris near Zurich in relation to tree age, size, and local competition. This population established naturally when the drainage pattern in a peat bog was altered after the Second World War. An unusual opportunity to study stand development was afforded when the trees were harvested in 1987 as part of a restoration program. All trees stumps were mapped, and growth since establishment was measured as cross—sectional area of wood in 5—yr intervals, using disks from the base of each tree. The distribution of cross—sectional area was lognormal over most of the period of growth. Size at the beginning of a time interval was the primary determinant of individual growth during that interval, but age and local competition also had significant effects. Younger, smaller trees were better fitted by an exponential growth model, whereas older, larger trees were better fitted by a Gompertz (sigmoidal) model. Some of the result were very different from previous studies: (1) The spatial distribution of trees was not significantly different from random. (2) Size variability decreased during the course of stand development, even though there was no self—thinning. This was because (3) the density of natural establishment was low, and competition, although clearly detectable, was still relatively weak and symmetric after 45 yr of stand development. Thus, there was no initial advantage in competition, and the younger trees, which were still growing exponentially, were able to catch up with the older trees, whose relative growth rates were declining.
Use and Misuse of Mixed Model Analysis of Variance in Ecological Studies
Authors: Bennington Cynthia C. , Thayne William V.
Published: May 2006

Analysis of variance is one of the most commonly used statistical techniques among ecologists and evolutionary biologists. Because many ecological experiments involve random as well as fixed effects, the most appropriate analysis of variance model to use is often the mixed model. Consideration of effects in an analysis of variance as fixed or random is critical if correct tests are to be made and if correct inferences are to be drawn from these tests. A literature review was conducted to determine whether authors are generally aware of the differences between fixed and random effects and whether they are performing analyses consistent with their consideration. All articles (excluding Notes and Comments) in Ecology and Evolution for the years 1990 and 1991 were reviewed. In general, authors that stated that their model contained both fixed and random effects correctly analyzed it as a mixed model. There were two cases, however, where authors attempted to define fixed effects as random in order to justify broader generalizations about the effects. Most commonly (63% of articles using two—way or greater ANOVA), authors neglected to mention whether they were dealing with a completely fixed, random, or mixed model. In such instances, it was not clear if the author was aware of the distinction between fixed and random effects, and it was often difficult to ascertain from the article whether their analysis was consistent with their experimental methods. These findings suggest several statistical guidelines that should be followed. In particular, the inclusion of explicit consideration of effects as fixed or random and clear descriptions of F tests of interest would provide the reader with confidence that the author has performed the analysis correctly. In addition, such an explicit statement would clarify the limits of the inferences about significant effects.
Variation in Palatability of Queen Butterflies (Danaus Gilippus) and Implications Regarding Mimicry
Authors: Ritland David B.
Published: May 2006

This study documents the existence of a food plant related palatability spectrum in Florida queen butterflies (Danaus gilippus). Abdomens of butterflies reared on four asclepiad food plants differed significantly in cardenolide content and palatability to a generalist avian predator (Red—winged Blackbird). However, unpalatability did not directly parallel food plant or butterfly cardenolide content. Queens reared on Sarcostemma clausum were cardenolide free and essentially palatable (85% eaten). Butterflies reared on Asclepias tuberosa and A. incarnata were also virtually cardenolide free, but were weakly and moderately unpalatable, respectively (62 and 46% eaten). Queens reared on the high cardenolide Asclepias curassavica contained 101—753 mg cardenolide and were very unpalatable (8% eaten). This results suggest that in addition to cardenolides, other asclepiad phytochemicals are directly or indirectly utilized by queens as components of their systemic chemical defense. The demonstrated variation in queen palatability is predicted to affect the aversive conditioning of predators and thus the outcome of queen—predator encounters. Queen populations feeding on more than one asclepiad species must exhibit automimicry, with attack rates on individual butterflies dependent upon both the proportion of palatable automimics and the noxiousness of unpalatable automodels in the population. The verification of a queen palatability spectrum also contributes to understanding the dynamics of mimicry between queens and viceroy butterflies (Limenitis archippus). This study predicts that, as a consequence of regional differences in asclepiad floras, the viceroy queen mimicry relationship in Florida switches between the Batesian and Mullerian mimicry modes. In the extreme, queens feeding on certain food plants may be Batesian mimics of sympatric, moderately unpalatable viceroys, reversing the traditionally cited mimicry roles of these butterflies.
Delayed Density Dependence as a Determinant of Vole Cycles
Authors: Hornfeldt Birger
Published: May 2006

Largely synchronous population fluctuations of Clethrionomys glareolus, C. rufocanus, and Microtus agrestis were monitored by snap—trapping in spring and autumn in 1971—1988 in a strongly seasonal environment near Umea, northern Sweden. All species were cyclic in the sense that they showed fairly regular (3—4 yr) fluctuations, but amplitudes (Nmax/Nmin) varied, averaging °200—fold in each species. This conclusion was supported by autocorrelation and spectral analysis, and by fitting time series data to a model for phase—forgetting cycles. By contrast, data did not conform to a model for phase—remembering cycles (with fixed period and amplitude). The transition between cycles, i.e., from the low to increase phase, was characterized by a distinct shift in rate of change in numbers from low to high or markedly higher values both in summer and winter. Generally, rate of change in summer declined continuously from the increase phase through each cycle. Moreover, there was a similar decrease of rate of change in winter, although rate of change (mainly in C. rufocanus and M. agrestis) first frequently increased early in the cycle. Rate of changes was delayed density dependent in all species both in summer and winter, as revealed by high negative correlations with density in previous autumn and spring for summer and winter changes, respectively. These new finding of delayed density dependence (DDD) support the suggestion that vole cycles are generated by a time—lag mechanism. Possible mechanisms of the DDD are discussed. Regression analyses of rate of change in the different voles suggest that, besides the strong dependence on previous density, rate of change in numbers was also affected by current seed supply (in C. glareolus) and/or weather variables (temperature and precipitation sums) that may have affected the quantity or quality of food.
Mass, Nitrogen, and Phosphorus Dynamics in Foliage and Root Detritus of Tallgrass Prairie
Authors: Seastedt T. R.
Published: May 2006

Mass nitrogen, and phosphorus of litterfall were measured for 4 yr on annually burned and unburned tallgrass prairie near Manhattan, Kansas. Decomposition and minearilization rates of foliage, flowering stem, and roots were concurrently studied with a series of litterbag and tethered litter experiments. Amounts of nitrogen and phosphorus in living and dead foilage and roots were measured for 2—yr on an annually burned site. Litterfall exhibited seasonal peaks on all sites, with most of the annual litter decompsition occurring in autumn. An average of 52 g°m—2°yr—1 of litter was deposited on the soil of the annually burned watersheds, while 142 g°m—2°—1 was collected on unburned sites. Nitrogen and phosphorus amounts were 0.53 and 0.07 g°m—2°yr—1, respectively, from burned sites and 1.94 and 0.19 g°m—2°yr—1, on unburned sites. Litterfall on unburned sites averaged °35% of aboveground maximum biomass estimates. Litter decay rates varied from a minimum of °10% yr for flowering stems in the canopy to a maximum of °50%/ yr for roots. Foilage and stems act as nitrogen and phosphorus sinks during the first 2 yr of decomposition. Dead roots and rhizomes also act as temporary nitrogen sinks, but unlike foilage, roots appear to minearlize significant amounts of phosphorus during the 1st yr of decay. Root nitrogen to phosphorus ratios may provide a proximate explanation for nitrogen limitation in tallgrass prairies.
Ontogeny of Time Budgets in Yellow‐eyed Juncos: Adaptation to Ecological Constaints
Authors: Sullivan Kimberly A.
Published: May 2006

Observational studies showed that among fledgling Yellow—eyed Juncos (Juncos phaeonotus) rates of scanning for predators, pecking rates, and feeding rates increased dramatically with age and converged on adult rates as the young reached independence. Juveniles adjusted their vigilance level in response to their social situation, maintaining higher vigilance in flocks of juveniles than when foraging with their parents. Independent juveniles spent a larger proportion of their time foraging, thus spent more time exposed to predators, and had lower survivorship than adults. Independent juveniles appeared to have difficulty in acquiring sufficient food when foraging time was limited by severe weather. Young juncos foraging in juvenile flocks spent less time on vigilance and pecked more often than juveniles foraging along. Social foraging by juveniles increased the probability of surviving after parental care was terminated.
The Combined Effects of Predation Risk and Food Reward on Patch Selection
Authors: Holbrook Sally J. , Schmitt Russell J.
Published: May 2006

Experiments were performed to examine the effects of predation risk on selection of food patches by a mobile forager, young black surfperch (Embiotoca jacksoni). The goals were to investigate (1) how predation risk affected relative selectivity among patches that differed in density of food, and (2) whether the forager's response depended on how variations in food level and predation risk were encountered. In the experiments, density of food (crustaceans on algae) was manipulated among spatially uniform patches of a foliose alga. Safety of the forager was varied by manipulating the presence in patches of kelp bass (Paralabrax clathratus), a principal predator of young surfperch. Patch choice was determined for an array of situations where food and risk were varied separately and together in space and time. The response of foragers to variations in food level and risk depended upon the specific manner in which the factors were encountered. When patches with a high food level contained a predator, foragers shunned the risky food—rich areas for safe food—poor patches. When food was equal between safe and risky patches, the degree to which patches with a predator were avoided remained constant over a range of absolute food densities. By constrast, selectivity between patches of differet food densities was dampened when all available patches contained a predator. Finally, predators enhanced relative selectivity for food—rich areas when foragers had simultaneous access to the complete combination of variation in rish and food level. Heightened selectivity in the presence of predators resulted from two hierarchical responses: foragers always took less risk to feed in the food—poor patch, and, when the difference in comparable value of available food patches was sufficiently great, they took more risk to eat where the density of food was higher. Overall, these results indicate that juvenile black surfperch possess an array of responses to food and predators, which together may act to minimize the sublethal effect of predators on feeding at a minimal increase in probability of death. Such plasticity may be a general characteristic of foragers that experience variation in food and risk at several spatial scales and temporal combinations.
A Test of the Pollinator Limitation Hypothesis for a Neotropical Herb
Authors: Horvitz Carol C. , Schemske Douglas W.
Published: May 2006

We hand—pollinated all flowers in inflorescences of the neotropical herb Calathea ovandensis (Marantaceae) and compared fruit and seed set of treatment plants to that of open—pollinated controls. Hand—pollination increased the number of initiated fruits by 24%, the number of mature fruits by 6.8%, and the number of seeds by 12.6%, but only the first of these differences was statistically significant. The proportion of flowers that matured fruits was very low compared with data reported for other species: 8.1% for the hand—pollinated treatment and 7.4% for open—pollinated controls. The proportion of flowers that initiated fruits was also quite low: 24.5% for hand—pollinated plants and 20.2% for open—pollinated plants. Mature fruit and seed production were significantly affected by nonpollinating animals that act at different developmental stages. Antguards attracted to extrafloral nectaries on the inflorescence appeared to stimulate flower production, while an ant—tended herbivore of reproductive tissues had a negative effect on fruit maturation. We searched for @`windows" of favorable biotic conditions during which full pollination could potentially improve fruit production, i.e., high antguard activity and low herbivore levels; while results were in the predicted direction, the effect was not significant. We conclude that while fruit initiation was pollen—limited, mature fruit and seed production were limited primarily by resources. The extremely low percentage of flowers maturing fruits may reflect selection for increasing fitness through male function. The remarkable pollination system of this species, with each flower having only a single opportunity to serve as a pollen donor, probably contributes to the unusually low fruit—to—flower ratio.
Leafcutting and Diet Selection: Relative Influence of Leaf Chemistry and Physical Features
Authors: Howard Jerome J.
Published: May 2006

The effects of leaf toughness, trichome characteristics, nutrient and water availability, and secondary chemistry on diet selection by the leafcutting ant Atta cephalotes were determined using individuals of 49 woody plant species from a tropical deciduous forest in Costa Rica. The palability of plants was measured by presenting whole leaves to leafcutting ant foragers on trails and counting the number of pieces cut from leaves. The effectiveness of physical barriers to leaf cutting was evaluated by measuring the speed with which ants were able to cut leaves of varying toughness, trichome densities, and trichome lengths. Plant secondary chemistry was highly correlated with the relative palatability of leaves, while palatability was only marginally correlated with trichome characteristics and plant nutritional quality and was correlated with leaf toughness and water content. Highly palatable leaves generally contained hydrolyzable tannins, while most unpalatable leaves possessed nonpolar extracts found to deter leaf cutting in laboratory bioassays. Within the subset of highly palatable plants, leaf protein content was positively correlated with the number of pieces cut, indicating that secondary chemistry and nutrient availability interact to determine palatability. Decreases in plant palatability over time paralleled decreases in protein content. Ant size was the most important variable determining the rate at which ants cut leaves. Trichome characteristics were uncorrelated with the rate at which leaves were cut, while leaf toughness explained a small amount of the variance in cutting rate. The results suggest that leaf toughness and trichome characteristics within the ranges encountered by leafcutting ants in this tropical deciduous forest do not generally deter attack by ants. Instead, the results are consistent with the idea that leafcutting ants use a combination of plant secondary chemistry and nutrient availability to evaluate the quality of leaves.