bluuebunny/crossref_metadata_2025 huggingface dataset:
Feltleaf Willow‐Snowshoe Hare Interactions: Plant Carbon/Nutrient Balance and Floodplain Succession
Authors: Bryant John P.
Published: May 2006

This experiment was designed to determine if winter browsing by snowshoe hares is a potentially important control over primary succession on floodplains of Alaskan rivers. In a 2 x 2 x 2 factorial experiment, I increased nutrient availability by NPK fertilization, decreased light intensity with shade cloth, and clipped winter—dormant feltleaf willow (Salix alaxensis) to stimulate severe snowshoe hare browsing. After 3 yr of fertilization, 1 yr of clipping, and 1 yr of shading, the 240 experimental plants were measured for height growth and current—year twig production, and their winter—dormant twigs were bioassayed for palatability to snowshoe hares and analyzed chemically. With one exception, both fertilization and shading increased the palatability of twigs in comparison to twigs of control plants. Moreover, shade had a comparatively strong effect on the palatability of stump sprouts of clipped plants. Both fertilization and shade generally reduced the concentrations of soluble carbohydrate and phenolics in winter—dormant twigs of unclipped plants. However, only shade significantly affected the concentrations of soluble carbohydrate and phenolics in twigs of clipped plants. These results indicate (1) that the palatability and chemical defenses of winter—dormant S. alaxensis are sensitive to plant carbon/nutrient balance and (2) that the effects of soil fertility and light intensity on plant carbon/nutrient balance vary with respect to browsing history. Although severe clipping increased production of current—year twig biomass it greatly reduced plant height. These results imply that severe browsing of winter—dormant willow growing on floodplains of interior Alaskan rivers, such as occurs at some points during snowshoe hare cycles, may result in reduced height growth of willow. I suggest that such a reduction in height growth may result (1) in shading of floodplain willow by alder and (2) in a shade—caused increase in the palatability of willow stump sprouts. Furthermore, I suggest that this increase in the palatability of shaded willow may be a partial control over the shift from willow dominance to alder dominance that occurs in the late shrub stage of primary succession on floodplains of interior Alaskan rivers.
Population Ecology of a Desert Rodent: Dipodomys Merriami in the Chihuahuan Desert
Authors: Zeng Zongyong , Brown James H.
Published: May 2006

We show that the extremely flexible life history and other facultative behaviors of the kangaroo rat, Dipodomys merriami, facilitate adult survival and enable this small mammal to maintain remarkably stable populations despite wide, unpredictable fluctuations in its desert environment. Mark—recapture methods provided data on population density, growth, reproduction, dispersal, and survival for a population of D. merriami in the Chihuahuan Desert of southeastern Arizona from 1978 through 1984. Population density showed both annual and interannual fluctuations, but varied only from °3 to 15 individuals/ha. The life history is characterized by long survival (up to at least 3.5 yr), modest and variable reproductive effort (zero to at least two litters per year, and an average of two young per litter), slow growth and maturation of young (at least 3 mo from birth to reproductive maturity), and frequent, facultative dispersal of adults as well as juveniles (75% of the males and 59% of the females that lived >4 mo dispersed). This combination of life history traits enabled D. merriami to confine reproduction to favorable periods, with the result that although adults of both sexes showed a loss of body mass associated with reproduction, there was no evidence that they experienced increased risk of mortality. Frequent and male—biased dispersal of adults is probably not unique to this species. Most standard methods of analyzing mark—recapture data probably underestimate the frequency of dispersal and its effects on sex ratio and genetic structure.
Multiple‐Species Induction of Morphological Defenses in the Rotifer Keratella Testudo
Authors: Stemberger Richard S. , Gilbert John J.
Published: May 2006

Filtrates of cultures of 10 of 12 common freshwater zooplankton species induced posterior spines in the rotifer Keratella testudo. Filtrates of cultures of Asplanchna spp. and those of crustacean zooplankton, including cladocerans and cyclopoid and calanoid copepods, generally produced the strongest induction responses. The filtrates of Daphnia pulex culturers, at densities as low as 0.4 individuals/L, promoted significant posterior spine development. This is the first known case of a competitor—controlled developmental polymorphism in zooplankton. Filtrates of cultures of the herbivorous rotifer Synchaeta pectinata and the fourth instar larvae of the dipteran Chaoborus punctipennis were ineffective as spine—promoting agents. In some, but not all, interactions with inducing species, the posterior—spined Keratella phenotype and higher survivorships than the unspined one. Spined individuals were significantly more protected than unspined ones against injury by mechanical interference by Daphnia. Direct observations showed that the spined phenotype was more rapidly rejected after entering the branchial chambers of Daphnia. This decreased the retention time in the branchial chambers and, consequently, reduced the likelihood of injury by the filtering limbs and mouthparts of Daphnia. The ability of this rotifer to respond to many zooplankton species seems advantageous, because the spined phenotype is less vulnerable to predation and interference competition from a variety of larger, co—occurring zooplankton than is the unspined phenotype.
A Dynamic State Variable Model of Mate Desertion in Cooper's Hawks
Authors: Kelly Elizabeth J. , Kennedy Patricia L.
Published: May 2006

In a 4—yr study of the reproductive strategies of Cooper's Hawks (Accipiter cooperii) nesting in north—central New Mexico, >50% of the females deserted during the fledgling—dependency period and did not renest. A dynamic state variable model was developed to study the females' brood—rearing strategies. In this model a strategy consisted of combinations of staying at the nest, hunting, and deserting. The modeling assumptions were: a female's strategy during brood rearing maximizes her reproductive fitness, defined as the weighted average of the expected probability of survival of ether current offspring and her expected future reproduction; and the reproductive fitness function depends on the physical condition of the female and nestlings, the risk to the nestlings associated with each strategy, and the male's foraging capabilities. The model predictions were compared to the observations of female strategies in this population of Cooper's Hawks. To insure a valid comparison, the model parameters were estimated from sources other than the observed population. The best match between observations and predictions (84—96%) was obtained when the nestlings' survival and the female's future reproductive potential were equally weighted during the nestlings stage, but weighted in favor of the female's productive potential during the fledgling stage. A sensitivity analysis showed that the model predictions corresponded well with the observations of staying and hunting at all parameters bounds. However, those combinations of parameter values that reflected conditions with the least pressure to desert missed 70—85% of the desertions. The sensitivity analysis also indicated that a key factor influencing the female's choice of strategy was the interaction between the threat to her future reproduction due to her poor physical condition and the nestlings's risk of death from predation and exposure. The agreement of model predictions and observed strategies supported the modeling assumptions. These results combined with the sensitivity analysis indicated that dynamic state variable modeling is an excellent tool for studying mate desertion.
Dietary Protein and Energy as Determinants of Food Quality: Trophic Strategies Compared
Authors: Bowen Stephen H. , Lutz Ellen V. , Ahlgren Molly O.
Published: May 2006

The effects of dietary protein and energy on ingestion and growth were determined for nutrient ranges that correspond to primary foods in freshwaters: algae, aquatic macrophytes and organic detritus. Sixteen diets containing four levels of metabolizable energy (ME) (3.1, 6.7, 10.5, 14.1 kJ/g) and four levels of protein (3.0, 13.2, 23.2, 33.4 mg/kJ ME) were each fed ad libitum to four replicate groups of juvenile Tilapia aurea for 42 d. Protein, energy, and protein—energy interaction affected both ingestion and growth (two—way ANOVA, all P < 0.01). Increased ingestion largely compensated for lower energy levels within each protein level. Growth was proportional to diet protein content, and ingestion did not compensate for protein limitation. A second—order polynominal for growth as a function of diet protein content and energy assimilation rate fitted by linear regression accounts for 91% of variation in growth and provides a model for comparison of the relative importance of protein and energy as nutritional constraints for animals feeding on invertebrate prey, algae, aquatic macrophytes, and organic detritus. Protein appears to be the primary constraint to food value of macrophytes, and detritus, and we predict from our results that consumers of these materials will increase growth most by feeding selectively on the most protein—rich material available, as has been observed. In contrast, growth of animals feeding on algae will be increased most by increased ingestion. Omnivory is interpreted as a compromise strategy in which protein from scarce animal prey is complemented by energy from abundant primary foods.
Population Dynamics and Pattern Formation in Theoretical Populations
Authors: Molofsky Jane
Published: May 2006

Simple individual—based models of population dynamics and spatial patterns are developed using cellular automata theory. These models assume that individuals within a population are distributed in space and that important interactions among individuals take place over some predefined local scale. Local density dependence in the model produces dynamics that depend on the scale of the dispersal distance. When dispersal occurs over long distances, the population enters a stable limit cycle. However, when colonization is limited to the same local neighborhood where competition is taking place, the spatial and temporal patterns become chaotic. Habitat size also affects the dynamics. Chaotic dynamics become cyclical as habitat size decreased. These results illustrate how a wide array of complex dynamics can arise with simple spatially distributed models and that the resulting dynamics can depend on the habitat size.
Element Dynamics along a Decay Continuum in a Red Spruce Ecosystem in Maine, USA
Authors: Rustad Lindsey E.
Published: May 2006

Element dynamics (C, N, P, Ca, Mg, Mn, Fe, and Al) were examined along a decay continuum from freshly fallen litter to soil organic matter in a red spruce ecosystem in Maine. The continuum was defined using previously reported data on litterfall chemistry and the early stages (i.e., first 24 mo) of decay, combined with new studies on an additional 33 mo of decay and an evaluation of forest floor organic matter at the same site. Carbon concentrations decreased over time as fresh litter was transformed into soil organic matter; N, Fe, and Al concentrations increased; and P, Ca, Mg, and Mn concentrations showed variable patterns of increased and decrease. In general, homogeneity of the litter increased as fresh litter of different initial chemistries was transformed into chemically more uniform soil organic matter. Ecosystem budgets indicated that after 57 mo of decay, litter had lost 63% of its original mass and had released C (65%),N (17%), P (79%), Ca (50%), Mg (80%), and Mn (74%), and had accumulated Al (431%) and Fe (353%). Evidence is presented indicating that the retention of Al (and Fe) is primarily controlled by abiotic adsorption onto litter exchanges sites. Overall results are consistent with a two—phase model of decay: in the first phase (0 to 3—4 yr), mass loss fits a negative exponential model of decay, an inverse linear relationship exists between N concentration and mass loss, and rapid changes are observed in litter chemistry; the second phase (3—4+ yr) is characterized by markedly slower changes in mass loss and litter chemistry.
Consequences of Foraging in Clonal Plant Species
Authors: Cain Michael L.
Published: May 2006

Some clonal plant species decrease rhizome or stolon internode lengths and/or increase the frequency of branching when they grow in favorable environments. This foraging response is thought to be beneficial since it should allow ramets to concentrate in areas of favorable habitat. However, there have been few critical tests of the effectiveness with which ramets are placed in favorable habitat as a result of the foraging response. In this paper, I use empirically calibrated stochastic simulation and diffusion models to compare the growth of clones in favorable and unfavorable habitat. I ask whether observed changes in rhizome lengths and clonal branching patterns are likely to decrease significantly the distance clones move, and thus, to enable mets to remain for longer periods of time in favorable habitat. For the empirical data used in this study, results from the models indicate that the effectiveness of the foraging response is likely to be variable. In some cases, such as results from models based on the response of Glechoma hederacea to nutrients, there is no significant difference in the distance clones move in favorable and unfavorable habitat. Thus, even through rhizome lengths may be significantly shorter in favorable patches, this does not guarantee that clones disperse significantly less far and thereby remain longer in favorable habitat. The effectiveness of the foraging response depends strongly on the distribution of clonal growth angles, the pattern of clonal branching, and the variance in rhizome or stolon internode length. These results, particularly the importance of growth angles and the relatively limited effectiveness of the foraging response, differ from those in previously published models of foraging in clonal plant species. I conclude with a discussion of reasons that may underlie these differences.
Edge Detection Algorithms for Two‐Dimensional Ecological Data
Authors: Fortin Marie‐Josée
Published: May 2006

The widely advocated statement that ecotone movement may be useful in studies of the impacts of global warming implies not only the ecotones exist but that they can be delineated spatially. With sampled field data, the accuracy of the detected ecotone is related to the data type and its spatiotemporal resolution. In the present study, I introduce two edge detection algorithms for regularly (lattice—wombling) and irregularly (triangulation—wombling) two—dimensional sampled data. I investigate the reliability of these algorithms in detecting potential ecotones using simulated vegetation data that follow the individualistic, continuum—gradient, and community—type patterns. Ecotones were defined quantitatively as long narrow regions of high rates of change. Under this definition, significant ecotones were found mostly in the community—type patterns using either of the edge detection algorithms (lattice—wombling or triangulation—wombling) and a systematic or random sampling design, respectively.
Disease Reveals the Predator: Sarcoptic Mange, Red Fox Predation, and Prey Populations
Authors: Lindström Erik R. , Andrén Henrik , Angelstam Per , Cederlund Goran , Hörnfeldt Birger , Jäderberg Lars , Lemnell Per‐Arne , Martinsson Berit , Sköld Kent , Swenson John E.
Published: May 2006

An epizootic of sarcoptic mange was prevalent among Scandinavian red foxes (Vulpes vulpes) during the late 1970s and 1980s. By substantially reducing the population density of foxes, the epizootic created a natural experiment on the importance of fox predation for prey density. The fox population started to recover during the late 1980s. We monitored the populations of the fox and its prey [voles (Cricetidae), mountain hare (Lepus timidus), European hare (L. europaeus), Capercaillie (Tetrao urogallus), Black Grouse (T. tetrix), Hazel Grouse (Bonasa bonasia), and roe deer (Capreolus capreolus)] throughout the event, on a local (101—102 km2), a regional (104 km2), and a national scale. Methods included den counts, snap—trapping, pellet/dropping counts, counts of displaying birds, young/adult ratio from incidental observations of deer, regional questionnaires, and national hunting records. The study revealed red fox predation as a crucial factor in limiting the numbers of hares and grouse as well as fawns per doe of roe deer in autumn, and in conveying the 3—4 yr cyclic fluctuation pattern of voles to small game. The classical view, that predators take but a doomed surplus of their prey, was false for these species in Scandinavia.
Food Supply and Egg Mass Variation in the European Coot
Authors: Horsfall John A.
Published: May 2006

Female European Coots (Fulica atra) typically lay clutches of seven to eight, in which egg mass increases through the laying sequence. The proportions of yolk and albunmen do not change with egg mass, and hatchling mass is proportional to egg mass. Hatching is asynchronous, and laterhatced chicks often starve or die from exposure. These breeding characteristics are proposed to be related to the exploitation of a food supply showing both long— and short—term unpredictability in abundance. The higher mass of the later chicks within a brood may allow them to survive long enough to exploit any sudden increases in food abundance at little parental cost. Experimental manipulation of food supply during the prelaying period shows that this systematic variation in mass within a clutch may be due to a facultative response in females; individuals fed upon an extremely constant daily food supply laid less variable clutches than females exposed to the natural food supply. The function and mechanism of this response are discussed.
Water Balance of Terrestrial Anuran (Eleutherodactylus Coqui) Eggs: Importance of Parental Care
Authors: Taigen Theodore L. , Pough F. Harvey , Stewart Margaret M.
Published: May 2006

The terrestrial eggs of the coqui of Puerto Rico are brooded almost continuously by the male parent from the time of oviposition until the fully metamorphosed hatchlings emerge from the eggs 15—20 d later. The gelatinous layer surrounding each egg offers no resistance to the exchange of water by the egg, and rates of exchange are determined by microclimatic conditions, structural characteristics of the nest, and the behavior of the male frog . During development in natural nests, the eggs experience a three— to fourfold increase in mass. Laboratory experiments coupled with field observations indicate that this increase is the result of the transfer of liquid water from the incubating male to his eggs. The transfer is driven by a difference in water potential between the eggs and the body fluids of the male frog that is nearly constant throughout incubation, despite an increase in egg mass. Eggs must take up water during incubation. Eggs that do not experience an increase in mass during development either die or produce small hatchlings. A water uptake that doubles the initial mass of the egg is necessary to produce a full—size hatchling with normal tissue water content.
Controls over Nutrient Resorption from Leaves of Evergreen Mediterranean Species
Authors: Pugnaire Francisco I. , Chapin F. Stuart
Published: May 2006

Nutrient resorption from senescent leaves, by minimizing nutrient losses, could be just as important as uptake in determining nutrient balance, especially in low—nutrient environments. To examine the relative importance of phenotypic and genotypic factors in controlling nutrient resorption, we sampled several meditterranean trees and shrubs growing in serpentine and nonserpentine soils in northern California. Plants growing in serpentine soils had smaller, thicker leaves than those growing in richer sites and had lower N and P concentrations and pools on a unit—area basis. Pools of nutrients resorbed and nutrients left in litter in rich—site leaves were larger than pools in poor—site leaves, but N resorption efficiency (percent of total leaf N resorbed) was higher in less fertile sites (65 vs. 46%). Differences in efficiency may be due to the ratio of soluble/nonsoluble proteins, which is higher at low—N sites. Trends for phosphorus resorption efficiency were not significant (47 vs. 46% in poor and rich sites, respectively). There were no significant differences among species adapted to poor vs. rich soils, nor between needle vs. broad—leaved species. We conclude that a high nutrient resorption efficiency is a phenotypic response to low—nutrient environments.
Leaf Decomposition in Two Mediterranean Ecosystems of Southwest Spain: Influence of Substrate Quality
Authors: Gallardo Antonio , Merino Jose
Published: May 2006

We studied the influence of litter quality on the decomposition rate of leaves from nine Mediterranean shrubs and trees using litter bag methods for a 2—yr period at two ecosystems in southwest Spain. Linear and nonlinear regressions were calculated between mass loss and the concentrations of the major organic and inorganic constituents of leaves (lignin, cutin, cellulose, tannins, crude fat, soluble carbohydrates, nitrogen, and phosphorus) and between mass loss and leaf toughness in order to determine the best predictor of leaf litter decomposition in these ecosystems. In addition, ratios between some of the litter quality parameters were examined as mass loss predictors. Loss of soluble components from leaves was used to define two phases of decomposition: a leaching phase that lasted 2—4 mo and a postleaching phase. Leaf toughness and the ratio of toughness: phosphorus concentration were the best indicators of mass loss during the leaching phase in both ecosystems, and cutin: nitrogen or cutin: phosphorus ratios were the best predictors of mass loss in the postleaching phase, but only in the drier and more nutrient—poor ecosystem. When the two phases were combined, leaf toughness, toughness: nitrogen, and/or cutin: nitrogen significantly explained the mass loss in both ecosystems.
Energy Constraints and Ultimate Decisions During Egg‐Laying in the Blue Tit
Authors: Nilsson Jan-Ake , Svensson Erik
Published: May 2006

In a population of Blue Tits, Parus caeruleus, we performed two different food provisioning experiments; one starting before clutch initiaition and one starting when the first eggs were laid. We assessed the effect of these feeding experiments on four fitness—related factors, viz., laying date, clutch size, egg mass, and onset of incubation. Given that breeding birds are food and energy constrained, extra food during the breeding season should be invested in the fitness—related factor(s) that is most important for maximization of overall fitness. In the first feeding experiment, when breeding pairs had access to extra food both before and during egg—laying, the female converted this extra energy into earlier laying dates. When food was provided after clutch initiation only, the above preferred option was no longer available. In this situation, females used the extra energy to start incubating earlier in the laying sequence, with no influence on the other factors studied. Thus, both experiments resulted in surplus energy being used to increase the quality of the offspring instead of their number. In both cases the quality was connected with time savings leading to earlier hatching dates.
Forest Fragmentation, Pollination, and Plant Reproduction in a Chaco Dry Forest, Argentina
Authors: Aizen Marcelo A. , Feinsinger Peter
Published: May 2006

In a fragmented, dry subtropical forest in northwestern Argentina, we compared pollination levels, fruit set, and seed set among small (<1 ha) forest fragments, large (>2 ha) fragments, and continuous forest in 16 plant species representing a wide range of pollination systems, breeding systems, and growth forms. For three species, Prosopis nigra (Mimosoideae), Cercidium australe (Caesalpinoideae), and Atamisquea emarginata (Capparaceae), the three treatments were replicated across four sites; we achieved less replication for other species. Because comparisons between forest and fragment populations for different species took place in different sites, however, by treating all 6 species as a unit we lessened the potential bias of confounding site effects and could evaluate the overall impact of fragmentation. Significant or marginal (P < .10) fragmentation—related declines in number of pollen tubes per style, fruit set, and seed set occurred in 9 of 16, 5 of 15, and 3 of 14 species, respectively. Overall, significant or nonsignificant declines occurred in 81% (pollen tubes), 73% (fruit set), and 79% (seed set) of the species. In all cases these proportions were greater (P < .06) than the null binomial expectation of a 1:1 ratio of increases to decreases. Breeding system did not explain sensitivity to fragmentation: the magnitudes of declines in pollen tubes, fruit set, and seed set were virtually indistinguishable between self—compatible and self—incompatible species. At least 4 of the 10 self—incompatible species, however, were heavily visited in small fragments by Africanized honey bees, which may have compensated for a decline in visits by native pollinators. The exact nature of responses varied among plant species. In some, the absolute quantity of pollen grains transferred to stigmas decreased with fragmentation, and sometimes this was reflected in reduced fruit or seed set. In Cercidium, Prosopis, and Atamisquea, the quality of the grains transferred apparently changed: number of pollen tubes produced per pollen grain on the stigma declined with increasing fragmentation, and at least in the latter two species seed production declined as well. Overall, levels of pollination and seed production undoubtedly integrated many idiosyncratic effects of fragmentation on particular plant and animal populations, and indicated that “community health” of fragments suffered in comparison with that of continuous forest. Median decreases in pollination levels and seed output from forest to fragments approached 20%. The impact of these declines on plant recruitment is less clear, however, because cattle grazing and trampling of seedlings and saplings in fragments may constitute a much more serious short—term conservation problem.
Assessing How Fish Predation and Interspecific Prey Competition Influence a Crayfish Assemblage
Authors: Garvey James E. , Stein Roy A. , Thomas Heather M.
Published: May 2006

In northern Wisconsin lakes, the introduced crayfish Orconectes rusticus is replacing O. propinquus, a previous invader, and O. virilis, a native crayfish. Herein, we explore how fish predation and competition interact to drive this change in crayfish species composition. In outside pools, we conducted selective predation experiments exposing crayfish to largemouth bass, Micropterus salmoides, to quantify patterns of crayfish vulnerability. To determine how interactions among crayfish influence susceptibility, we quantified shelter use and behavioral interactions among crayfish in aquaria and outside pools. At equal size, O. virilis was more susceptible to fish predation than either of the invaders, O. rusticus and O. propinquus; the two invaders were equally susceptible to predation. However, sizes of these crayfish in the field are O. virilis > O. rusticus > O. propinquus. Because fish predators prefer small crayfish, at unequal size, small O. propinquus were more vulnerable to predation than large O. rusticus. Thus, O. rusticus can replace O. propinquus due to natural size differences. Although O. virilis grows larger than the invaders, it was more susceptible even when 3 mm larger. We hypothesized that O. virilis, although large, participated in behaviors that increased its risk to predation. When provided with unlimited shelters, all three species increased refuge use under predatory risk. When shelters were limiting and fish present, O. virilis was excluded from shelters by invaders. O. virilis also participated in risky behaviors, such as increased activity and swimming. Both agonistic interactions with congeners and approaches by largemouth bass increased risky behaviors in O. virilis. In addition, O. virilis was innately less aggressive than invaders. Given these behaviors, O. virilis was consumed at high rates and would be eventually replaced in lakes. In northern Wisconsin lakes, fish predation and crayfish—crayfish competition interact ti influence crayfish replacements. Based on our results, largemouth bass predation modifies the outcome of interference competition among the three crayfishes and, in turn competitive interactions among the crayfishes influence susceptibility to fish predation. We predict that O. virilis should suffer high mortality to fish predation in the presence, rather than in the absence, of the two invading species. Our results support the hypothesis that, in areas of sympatry where predators are selective and prey species compete, predation and competition interact to determine community structure.
Browsing by Moose on Scots Pine in Relation to Plant Resource Availability
Authors: Edenius Lars
Published: May 2006

The availability of resources for growth is believed to have a significant impact on the chemical defense of plants against herbivores. However, the means by which resource availability affects different plant traits, and the way in which these factors in turn affect diet selection, are not well understood. I addressed the relation between plant biomass, morphology, and tissue nutritional quality and browsing by moose (Alces alces (L.)) on Scots pine (Pinus sylvestris L.). In two experiments nutrient and light availability were manipulated and trees were subsequently exposed to free—ranging moose at feeding stations. Treatments included fertilization, shading, clipping, or root—cutting for 1—2 growing seasons. Treatments had a significant effect on tree height, twig biomass, shoot morphology, carbon/nitrogen ratio, and nitrogen concentration in current—year shoots. Twig biomass and shoot morphology explained most of the variation in per—tree twig biomass consumption by moose, whereas the carbon/nitrogen ratio had a significant impact only when between—treatment differences in biomass were removed. These findings show that variation in resource availability induces changes in a variety of plant traits other than tissue nutritional quality, to which moose respond by adjusting foraging behavior. This may have important implications for plant—animal interactions: the plants' chemical defense could be less effective against herbivores like moose that adjust foraging behavior at a variety of spatial and temporal scales.
Higher Order Interactions in Ecological Communities: What Are They and How Can They be Detected?
Authors: Billick Ian , Case Ted J.
Published: May 2006

The detection and significance of higher order interactions (HOIs) between species has been a matter of debate and experimentation in community ecology for several decades. HOIs are considered potentially significant because their presence is assumed to mean that the dynamic behavior of a full community of species is unpredictable based on observations of interactions between subsets of the species within the community. Despite such attention, the causal mechanisms that produce HOIs have been inadequately discussed. We discuss three different usages of the term HOIs and provide insight as to why HOIs might be found within a given community. HOIs may be detected for three reasons: inappropriate assumptions made concerning species interactions that influence statistical tests, unmeasured parameters and variables, and interaction modifications (i.e., a functional change in the interaction of two species caused by a third species. This confusion concerning the defining attributes of HOIs has made their detection problematic. While the statistical tests being used in the ecological experiments to detect HOIs are described in detail in most papers, the dynamic models underlying these tests are often not made explicit. Additionally, we demonstrate the equivalency of three different statistical tests: the Case and Bender (1981) test, analysis of variance, and a multiplicative test (Wootton 1994). However, the choice of a response variable (i.e., population densities, population growth rates, per—capita growth rates, etc.) and different data transformations applied to these response variables alter the underlying dynamics model that is being tested. The result is that the statistical test applied does not always perform the intended comparison but instead tests a different and sometimes unjustified or even inappropriate dynamic model. Finally, we review the relationship between indirect effects and HOIs. Whereas some researchers have lumped HOIs and indirect effects, we argue that the two represent completely unique and separate phenomena. Additionally, indirect effects can complicate detection of HOIs, and we review several methods by which to separate the two processes.
Xylem Embolism in Ring‐Porous, Diffuse‐Porous, and Coniferous Trees of Northern Utah and Interior Alaska
Authors: Sperry John S. , Nichols Kirk L. , Sullivan June E. M. , Eastlack Sonda E.
Published: May 2006

Xylem embolism was measured in nine tree species for one or more years. Species were ring—porous (Quercus sp.), diffuse—porous (Alnus, Betula, Populus spp.) or coniferous (Picea, Larix, Abies spp.). Intraspecific (Populus tremuloides) and intrageneric (Betula, Alnus) comparisons were made between sites in northern Utah and interior Alaska. Most embolism, >90% in some dicot species, occurred in winter. Within sites, dicot trees embolized more than conifers. Between sites, Alaskan dicot trees embolized less than their Utah counterparts. Differences were explained by vulnerability to embolism caused by freeze—thaw cycles. Most conifers were entirely resistant, whereas dicot trees were vulnerable. Less embolism in Alaskan dicot trees was associated with fewer freeze—thaw events in Alaska vs. Utah. Vulnerability was positively correlated with conduit volume and hydraulic conductance per unit xylem area (ks). Tracheids were superior to vessels in avoiding freeze—thaw—induced embolism, and had lower ks. At the other extreme, ring—porous xylem had the highest ks but lost >90% of hydraulic conductance after a single freeze—thaw event. Vulnerability to water—stress—induced cavitation was not correlated with conduit volume or ks. Dicot species either reversed winter embolism by refilling vessels with positive root pressures during spring (Betula, Alnus spp.), or tolerated it and relied on new xylem production to restore hydraulic conductance (Quercus sp.). Conifers reversed embolism by refilling tracheids in the absence of positive pressure. Populus species behaved inconsistently, showing some reversal one year but none the next. Even without embolism reversal, Populus species had hydraulic conductances per unit leaf area equal to other diffuse—porous species.
Importance of Stream Microfungi in Controlling Breakdown Rates of Leaf Litter
Authors: Gessner Mark O. , Chauvet Eric
Published: May 2006

Breakdown of seven leaf species covering a broad range of litter qualities (lignin: 7—31% of leaf dry mass; tannin: 0.0—6.7%; nitrogen: 0.5—2.6%; phosphorus: 0.017—0.094%) and dynamics of fungal biomass and reproductive activity were studied in a softwater mountain stream. Litter breakdown proceeded at exponential rates k ranging from 0.0042 d—1 (evergreen oak) to 0.0515 d—1 (ash). Fungal colonization of litter was generally rapid, with the fungus—specific indicator molecule ergosterol increasing from initially negligible concentrations to 375—859 mg/g of detrital mass. Using species—specific factors relating ergosterol concentrations to mycelial dry mass, maximum fungal biomass associated with litter was estimated as 61—155 mg/g of total system mass. Minimum estimates of net mycelial production during active growth varied between 0.3 and 3.8 mg°g—1°d—1, and maximum sporulation rates of aquatic hyphomycetes ranged from 760 to 7500 conidia°mg—1°d—1. Initially, reproductive activity was largely synchronized with increases in ergosterol concentrations, but it declined dramatically after peak sporulation rates were reached, whereas ergosterol concentrations levelled off or decreased at considerably slower rates. Periods of highest fungal productivity were thus limited to an initial breakdown stage of °2—8 wk. Strong correlations were found between the exponential breakdown coefficient and each of three parameters reflecting fungal activity in leaf litter, that is, maximum ergosterol concentration (P = 0.002, r = 0.96), net mycelial production (P = 0.02, r = 0.92), and sporulation rate (P < 0.001, r = 0.99). The initial lignin content of leaves was also significantly correlated with the rate constant k (P = 0.02, r = —0.83), suggesting that lignin was the primary factor determining the litter quality and thus breakdown rate. The correlation was even stronger when data were logarithmically transformed (P < 0.01, r = 0.95). Tannin concentration was significantly correlated with k only when two high—lignin species were excluded from the analysis (P = 0.19, r = 0.56 compared with P = 0.05, r = —0.88), while initial concentrations of phosphorus (P = 0.17, r = 0.58) and particularly nitrogen (P = 0.82, r = 0.06) were poor predictors of litter decomposability. These results suggest that the initial lignin content of leaves controlled litter breakdown rate through a kinetic limitation of carbon sources for saprotrophic microfungi. The decomposer activity of these organisms, in turn, would then have governed breakdown rates. In doing this, fungi produced substantial amounts of both mycelial and conidial biomass that was potentially available to higher trophic levels of the food web.
Determinants of Competitive Success in Clethrionomys Rufocanus
Authors: Ims Rolf Anker
Published: May 2006

This study introduces a new approach to identify determinants of competitive success in small mammals. An experimental population of Clethrionomys rufocanus consisting of laboratory—raised individuals with known origin and life history was introduced to a realistic competitive situation in the field. Successful females could be distinguished from nonsuccessful females on the basis of four individual attributes measured 25 d after they were released on the study plot, whereas males could not. The most consistent determinant of competitive success seemed to be the sex ratio of the litters in which females were born. Successful females originated from litters with a strongly female—biased sex ratio. These results are considered in the light of recent research on the consequences of prenatal conditions on female social behavior.
Dynamics of Nutrients and Leaf Mass in Central Himalayan Forest Trees and Shrubs
Authors: Ralhan Pushp K. , Singh Surendra P.
Published: May 2006

Leaf dynamics, nutrient (N, P, K, Ca, and Na) changes, and retranslocation from the leaves of 13 species were investigated in four representative forest communities of the Central Himalaya, India. In evergreens of the temperate regions, with leaf longevities of several years, there is negligible movement of substances from leaves during the first year. For evergreens of the Central Himalaya, by contrast, loss of leaf mass beings in autumn and often continues until summer, when new leaves are formed. The decrease in leaf dry mass during this period was 12.3—34.1% of the maximum leaf mass. The concentration of N, P, and K decreased from the initial peak, stabilized as leaves matured, and declined markedly during senescence, while Ca and Na concentrations increased throughout the life—span of leaves. Leaf mass loss was positively related to most of the nutrient retranslocation percentages. There were significant correlations between retranslocation levels of many nutrients. The dominant canopy species (chir pine), which is adapted to infertile sites, showed conspicuously greater levels of nutrient retranslocation than species adapted to fertile sites (oaks). However, species of different growth forms occupying the same sites differed markedly in the extent of nutrient retranslocation.
Seedling Recruitment Strategies in Obligate‐Seeding and Resprouting Banksia Shrubs
Authors: Zammit Charles , Westoby Mark
Published: May 2006

Banksia ericifolia is killed by fire and so depends upon seeds for persistence. In contrast, B. oblongifolia can persist after fire by resprouting from protected buds, as well as by recruiting seedlings. These serotinous species coexist in the fire—prone scrublands of the Sydney Basin. Here we report experiments that compared seed release from cones retained in the canopy, seed germination, and seedling establishment in these two species. B. oblongifolia seeds were released more quickly than B. ericifolia seeds after a fire, and B. oblongifolia follicles opened and shed seeds after exposure to lower temperatures than those of B. ericifolia. B. oblongifolia seeds germinated more quickly than those of B. ericifolia. The relative growth rates and leaf mass ratios (mean ratio of leaf dry mass to plant dry mass over sequential harvests) of the two species did not differ. Root : shoot ratios were significantly large and biomass allocation to roots was faster in B. oblongifolia seedlings. B. ericifolia seedlings survived low and intermediate levels of droughting better than B. oblongifolia under greenhouse conditions. In a field experiment in which water was not limiting, large B. oblongifolia seedlings had the highest survival rate. These results suggest that selection has acted on B. ericifolia to minimize the chance of zero seedling survival by distributing the risks over more than one germination episode. There was no consistent evidence to support the hypothesis that selection on this species has increased its probability of seedling survival during any one germination opportunity relative to its congener.
The Detection of Density‐Dependence from a Series of Annual Censuses
Authors: Pollard E. , Lakhani K. H. , Rothery P.
Published: May 2006

We report a distribution—free approach to the detection of density—dependence in the variation of population abundance, measured by a series of annual censuses. The method uses the correlation coefficient between the observed population changes and population size and proposes a randomization procedure to define a rejection region for the hypothesis of density—independence. It is shown that the use of the proposed statistic under the randomization approach is equivalent to the likelihood ratio test for a particular family of time series models. The randomization test is compared with two other recently proposed tests. Using computer—generated density—independent and density—dependent data, it is shown that, unlike the other tests, the randomization test is effective whether or not there is a marked trend in the observed data. Arguments are presented showing how one of the other two tests can be further improved. Caution is urged in the use and interpretation of any test for detecting density—dependence in census data because (a) the tests depend on assumptions about population processes, (b) errors of measurement may lead to spurious detection of density—dependence.
Spatial Autocorrelation: Trouble or New Paradigm?
Authors: Legendre Pierre
Published: May 2006

Autocorrelation is a very general statistical property of ecological variables observed across geographic space; it most common forms are patches and gradients. Spatial autocorrelation, which comes either from the physical forcing of environmental variables or from community processes, presents a problem for statistical testing because autocorrelated data violate the assumption of independence of most standard statistical procedures. The paper discusses first how autocorrelation in ecological variables can be described and measured, with emphasis on mapping techniques. Then, proper statistical testing in the presence of autocorrelation is briefly discussed. Finally, ways are presented of explicitly introducing spatial structures into ecological models. Two approaches are proposed; in the raw—data approach, the spatial structure takes the form of a polynomial of the x and y geographic coordinates of the sampling stations; in the matrix approach, the spatial structure is introduced in the form of a geographic distance matrix among locations. These two approaches are compared in the concluding section. A table provides a list of computer programs available for spatial analysis.
The Largest, Smallest, Highest, Lowest, Longest, and Shortest: Extremes in Ecology
Authors: Gaines Steven D. , Denny Mark W.
Published: May 2006

Biostatistics channels ecologists into thinking primarily about the mean and variance of a probability distribution. But many problems of biological interest concern the extremes in a variable (e.g., highest temperature, largest force, longest drought, maximum lifespan) rather than its central tendency. Such extremes are not adequately addressed by standard biostatistics. In these cases an alternative approach–the statistics of extremes–can be of value. In the limit of a large number of measurements, the probability structure of extreme values conforms to a generalized distribution described by three parameters. In practice these parameters are estimated using maximum likelihood techniques. Using this estimate of the probability distribution of extreme values, one can predict the expected time between the imposition of extremes of a given magnitude (a return time) and can place confidence limits on this prediction. Using data regarding sea—surface temperature, wave—induced hydrodynamic forces, wind speeds, and human life—spans we show that accurate long—term predictions can at times be made from a surprisingly small number of measurements if appropriate care is taken in the application of the statistics. For example, accurate long—term prediction of sea—surface temperatures can be derived from short—term data that are anomalous in that they contain the effects of an extreme EL Nino. In the cases of wave—induced forces and wind speeds, the probability distribution of extreme values is similar among years and diverse sites, indicating the possible existence of unifying principles governing these phenomena. Limitations and possible misuse of the method are discussed.