bluuebunny/crossref_metadata_2025 huggingface dataset:
Facilitation by unpalatable weeds may conserve plant diversity in overgrazed meadows in the Caucasus Mountains
Authors: Callaway Ragan M. , Kikvidze Zaal , Kikodze David
Published: December 2004

Positive interactions among plants have been demonstrated in many communities around the world, and appear to play important roles in maintaining species coexistence, productivity, and species diversity. However, the potential for positive interactions to conserve biological diversity in ecosystems that are disturbed by humans is poorly understood and often overlooked. One of the most important positive effects one plant can have on another is protection from herbivory. By associating with an unpalatable neighbor, a tasty species may avoid being eaten and increase in size and reproductive fitness. We examined the role of two highly unpalatable plants, Cirsiumobvalatum and
Veratrum lobelianum, in subalpine meadow plant communities of the
central Caucasus Mountains in the Republic of Georgia, where intense
livestock grazing has occurred for over two thousand years. These two
species are avoided by livestock because of spines and toxicity,
respectively, and have increased dramatically in abundance recently due
to seasonal trans‐Caucasus migrations of vast herds of domestic sheep
during the Soviet era. The Gudauri region, bisected by the
Russian‐Georgian Military Road, was a focal point of these migrations,
and there we found that plant communities associated with Cirsium
and Veratrum were very different in composition than open meadows.
Forty‐four percent (15/34) of all species at our site were found at only
“trace” (<1.0%) cover values in the open meadow, but at
significantly higher covers under Cirsium or Veratrum. Of
the 38 species that were reproducing sexually at our site, eight were
found only under the unpalatable invaders. Communities associated with
Cirsium and Veratrum had 78–128% more species in flower
or fruit than open meadow communities, respectively, than open meadow
sites. Furthermore, community composition and reproductive output
differed substantially between Cirsium and Veratrum, indicating some degree of species‐specificity in their effects. These results indicate that unpalatable plants, which are generally indicators of unhealthy rangelands, have the potential to preserve plant diversity in overgrazed plant communities.
Correlations between measures of thermal stress resistance within and between species
Authors: Berrigan David
Published: December 2004

Resistance to an environmental stress such as elevated temperature can almost always be measured in several different ways. A number of recent studies in Drosophila show that lethal and non‐lethal measures of resistance to heat stress are genetically independent. In contrast, data reported here suggest that there are interspecific correlations between measures of thermal stress resistance. Together, these results show that studies of intraspecific variation in stress resistance must demonstrate the ecological relevance of the measures of stress resistance chosen for study. In contrast, interspecific comparisons could be based on any of several measures of resistance. It would be interesting to determine if this pattern holds for resistance to other major environmental stresses.
Size‐selective oviposition by parasitoids and the evolution of life‐history timing in hosts: fixed preferences vs frequency‐dependent host selection
Authors: McGregor Robert R. , Roitberg Bernard D.
Published: December 2004

The influence of size‐selective oviposition behaviour by parasitoids on the evolution of life‐history timing in their hosts was examined using an optimization model of a two‐stage life history similar to a genetic algorithm. Host populations with varying durations of early‐larval development were subjected to selection in scenarios where parasitoids had fixed preferences for oviposition on late‐stage larvae, or those where parasitoid attack was dependent on the relative frequencies of the two life stages present in the population. Fixed preference for oviposition on late‐stage larvae caused positive directional selection on the duration of early‐larval development. Surviving individuals remained for as long as possible in the first stage of development in order to avoid parasitoid attack. Frequency‐dependent parasitoid attack, in contrast, caused maintenance of variation in the duration of early‐larval development. The influence of the fitness payoffs of different life stages on the plasticity of size‐selective oviposition behaviour is discussed, as are possible implications of the model results for parasitoid‐host population dynamics.
Testing hypotheses of trophic level interactions: a boreal forest ecosystem
Authors: Sinclair A. R. E. , Krebs C. J. , Fryxell J. M. , Turkington R. , Boutin S. , Boonstra R. , Seccombe‐Hett P. , Lundberg P. , Oksanen L.
Published: December 2004

Models of community organization involve variations of the top‐down (predator control) or bottom‐up (nutrient limitation) hypotheses. Verbal models, however, can be interpreted in different ways leading to confusion. Therefore, we predict from first principles the range of possible trophic level interactions, and define mathematically the instantaneous effects of experimental perturbations. Some of these interactions are logically and biologically unfeasible. The remaining set of 27 feasible models is based on an initial assumption, for simplicity, of linear interactions between trophic levels. Many more complex and non‐linear models are logically feasible but, for parsimony, simple ones are tested first. We use an experiment in the boreal forest of Canada to test predictions of instantaneous changes to trophic levels and distinguish between competing models. Seven different perturbations systematically removed each trophic level or, for some levels, supplemented them. The predictions resulting from the perturbations were concerned with the direction of change in biomass in the other levels. The direct effects of each perturbation produced strong top‐down and bottom‐up changes in biomass. At both the vegetation and herbivore levels top‐down was stronger than bottom‐up despite some compensatory growth stimulated by herbivory. The combination of experiments produced results consistent with two‐way (reciprocal) interactions at each level. Indirect effects on one or two levels removed from the perturbation were either very weak or undetectable. Top‐down effects were strong when direct but attenuated quickly. Bottom‐up effects were less strong but persisted as indirect effects to higher levels. Although the ‘pure reciprocal’ model best fits our results for the boreal forest system different models may apply to different ecosystems around the world.
Aphid‐mediated coexistence of ladybirds (Coleoptera: Coccinellidae) and the wood ant Formica rufa: seasonal effects, interspecific variability and the evolution of a coccinellid myrmecophile.
Authors: Sloggett John J. , Majerus Michael E. N.
Published: December 2004

It is generally believed that most homopteran‐eating insects avoid ant‐tended colonies of Homoptera, due to the ant aggression they encounter there. However, because homopteran colonies which are ant‐tended often persist for longer than untended colonies, some homopteran‐eaters may utilise ant‐tended Homoptera when untended colonies are scarce. Furthermore, a few homopteran‐eaters are myrmecophilous, habitually coexisting with ants. To investigate these phenomena, a study was made of aphids and aphidophagous coccinellids (ladybirds) on Scots pine, Pinus sylvestris, growing in areas foraged and unforaged by
the wood ant Formica rufa. The non‐tended aphid Schizolachnus
pineti exhibited a marked population decline in late summer but
persisted in both areas at very low density. Facultatively tended
Cinara aphids exhibited higher population densities when
associated with F. rufa, and remaining colonies of these aphids
were only found associated with ants in late summer. Coccinellids
exhibited considerable interspecific variability in their level of
association with F. rufa, and there was some evidence of an
increase in certain species’ frequencies of occurrence with the ant
when Cinara aphids were all ant‐tended, in late summer.
Coexistence with ants appears to be associated with either an intolerance
of low aphid densities, in Coccinella septempunctata and
Harmonia quadripunctata, or with extreme dietary specialisation,
in Myzia oblongoguttata. Similar factors to those which bring
C. septempunctata into contact with ants were probably of
importance in the initial stages of the evolution of myrmecophily of its
congener, Coccinella magnifica.
Nutrient versus pollination limitation in Platanthera bifolia and Dactylorhiza incarnata (Orchidaceae)
Authors: Mattila Elisa , Kuitunen Markku T.
Published: December 2004

Availability of resources and pollination services have been demonstrated to have impact on reproductive success in some orchid species, but to our knowledge no studies have examined the effects of nutrient application and pollination limitation in the same experiment. In this study, factors limiting reproductive success were studied with two terrestrial orchid species in Central Finland during 1996 and 1997. In a field experiment using a factorial design, plants of nectar‐producing Platanthera bifolia and nectarless
Dactylorhiza incarnata were treated with nutrient application and
hand‐pollination. Inflorescence size was considered as an indicator of
attractiveness to pollinators as well as of the general condition of the
plant. In 1996, fertilizer treatment increased relative capsule
production in P. bifolia plants with small inflorescences,
indicating poorer store of resources in the underground corm and
dependence on nutrient availability during capsule maturation, which was
not found in large plants. Hand‐pollination and large
inflorescence size in P. bifolia did not affect capsule
initiation, but increased the proportion of mature capsules. P.
bifolia may thus be regarded as both resource‐ and
pollination‐limited within a year. Pollination success was observed to be
the only factor limiting reproduction of the nectarless D.
incarnata within a year, because hand‐pollination increased capsule
production, and there were no significant effects of fertilizer treatment
or inflorescence size in 1996. Capsule production in 1996 did not affect
the probability of P. bifolia flowering in the following year,
while high capsule production decreased the subsequent probability of
flowering in D. incarnata. Species with different pollination
strategies differed in the use of resources. The nectar‐producing P.
bifolia had a lower fruit/flower ratio than the nectarless D.
incarnata, also after hand‐pollination.
Dispersal in cactophilic Drosophila
Authors: Markow Therese Ann , Castrezana Sergio
Published: December 2004

Three species of Drosophila each breed in necrotic tissue of
specific columnar cacti endemic to the Sonoran Desert. Drosophila
pachea breeds in senita (Lophocereus schottii), D.
nigrospiracula breeds in saguaro (Carnegiea gigantea) or
cardón (Pachycereus pringlei), and D. mojavensis
uses organ pipe (Stenocereus thurberi) in Sonora, Mexico and
southern Arizona. Patches of these three host cacti have very different
spatial distributions, with those of senita being quite frequent and
close together, while those of the other hosts are much father apart.
Testing all three species simultaneously, we used
capture‐mark‐release‐recapture methods to ask if dispersal differed in
these species and if differences were those predicted by the spatial
availability of the host patches. D. pachea dispersed the shortest
distance in all experiments. Furthermore, D. pachea was the only
species showing sex‐biased dispersal, with male flies exhibiting the
greater propensity to disperse. The observations suggest that across
similar spatial scales, D. pachea should show greater population genetic structure than the other two species, and that mitochondrial DNA, because of its maternal inheritance, might show greater evidence of structure than nuclear markers.
Fertilization effects on species density and primary productivity in herbaceous plant communities
Authors: Gough Laura , Osenberg Craig W. , Gross Katherine L. , Collins Scott L.
Published: December 2004

Fertilization experiments in plant communities are often interpreted in the context of a hump‐shaped relationship between species richness and productivity. We analyze results of fertilization experiments from seven terrestrial plant communities representing a productivity gradient (arctic and alpine tundra, two old‐field habitats, desert, short‐ and tall‐grass prairie) to determine if the response of species richness to experimentally increased productivity is consistent with the hump‐shaped curve. In this analysis, we compared ratios of the mean response in nitrogen‐fertilized plots to the mean in control plots for aboveground net primary productivity (ANPP) and species density (D; number of
species per plot of fixed unit area). In general, ANPP increased and
plant species density decreased following nitrogen addition, although
considerable variation characterized the magnitude of response. We also
analyzed a subset of the data limited to the longest running studies at
each site (≥4 yr), and found that adding 9 to 13 g N
m−2 yr−1 (the consistent amount used at all sites) increased ANPP in all communities by approximately 50% over control levels and reduced species density by approximately 30%. The magnitude of response of ANPP and species density to fertilization was independent of initial community productivity. There was as much variation in the magnitude of response among communities within sites as among sites, suggesting community‐specific mechanisms of response. Based on these results, we argue that even long‐term fertilization experiments are not good predictors of the relationship between species richness and productivity because they are relatively small‐scale perturbations whereas the pattern of species richness over natural productivity gradients is influenced by long‐term ecological and evolutionary processes.
Ecosystem process rate increases with animal species richness: evidence from leaf‐eating, aquatic insects
Authors: Jonsson Micael , Malmqvist Björn
Published: December 2004

Effects of species number and identity on the breakdown rate of leaf litter were estimated in a laboratory experiment using leaf‐eating insects, three species of Plecoptera, as detritivores. We found significant differences between the different species on this process in single‐species experiment, but not when animal biomass was accounted for. When species were combined the effect of species identity was strongly reduced and rendered insignificant, whereas the number of species had a significant effect. This shows that rates of ecosystem processes may benefit from species richness even when all species belong to the same guild, which is in contrast to hypotheses predicting redundancy within guilds. Facilitation between species and negative interactions, where intraspecific interactions are greater than interspecific interactions, are two potential mechanisms which could explain increasing decomposition rates with species richness.
Preference and performance of a gall‐inducing sawfly: a test of the plant vigor hypothesis
Authors: Fritz Robert S. , Crabb Beau A. , Hochwender Cris G.
Published: December 2004

To test the Plant Vigor Hypothesis, we determined female oviposition preference of Phyllocolpa leavitii (Hymenoptera: Tenthredinidae)
for shoot lengths on 20 clones of Salix discolor (Salicaceae) and
examined larval performance by determining larval survival based on
clone, shoot length, and leaf length for each Phyllocolpa gall
observed. Sawfly galls were found on significantly longer shoots compared
to shoots without galls, and shoots with 2, 3, 4, and 5 galls were
successively longer than shoots with fewer galls. Furthermore, a much
higher proportion of longer shoots had galls that did shorter shoots.
These data demonstrate strong sawfly oviposition preference for long
shoots. However, when shoot length was adjusted for numbers of available
leaves per shoot, the number of galls per available leaf showed a weaker,
but still positive relationship with shoot size. Using a logistic
regression on survival of Phyllocolpa larvae, we detected highly
significant effects of clone, shoot length, leaf length, and the square
of leaf length on survival. Additionally, much higher sawfly survival on
the long shoots of one clone caused a significant clone by shoot‐length
interaction. Survival was positively correlated with shoot length,
indicating directional selection favoring oviposition on longer shoots;
still, shoot length explained only 11.8% of the variation in survival.
The directional component of selection favored oviposition on smaller
leaves as survival was inversely related to leaf length; however,
disruptive selection for leaf length oviposition preference was also
detected, with survival of Phyllocolpa galls lowest on intermediate leaf lengths. This study provided evidence supporting the preference prediction of the Plant Vigor Hypothesis. However, much of the data contradicted the performance prediction of the Plant Vigor Hypothesis, with only a modest amount of evidence supporting the performance prediction.
Linking above‐ground and below‐ground effects in autotrophic microcosms: effects of shading and defoliation on plant and soil properties
Authors: Mikola Juha , Barker Gary M. , Wardle David A.
Published: December 2004

Although factors affecting plant growth and plant carbon/nutrient balance – e.g., light availability and defoliation by herbivores – may also propagate changes in below‐ground food webs, few studies have aimed at linking the above‐ground and below‐ground effects. We established a 29‐week laboratory experiment (∼one growing season) using autotrophic microcosms to study the effects of light and defoliation on plant growth, plant carbon/nutrient balance, soil inorganic N content, and microbial activity and biomass in soil. Each microcosm contained three substrate layers – mineral soil, humus and plant litter – and one Nothofagus solandri var.
cliffortioides seedling. The experiment constituted of the
presence or absence of two treatments in a full factorial design: shading
(50% decrease in light) and artificial defoliation (approximately 50%
decrease in leaf area in the beginning of the growing season). At the end
of the experiment a range of above‐ground and below‐ground properties
were measured. The shading treatment reduced root and shoot mass,
root/shoot ratio and leaf production of the seedlings, while the
defoliation treatment significantly decreased leaf mass only. Leaf C and
N content were not affected by either treatment. Shading increased NO
3–N concentration and decreased microbial biomass in humus,
while defoliation did not significantly affect inorganic N or microbes in
humus. The results show that plant responses to above‐ground treatments
have effects which propagate below ground, and that rather
straightforward mechanisms may link above‐ground and below‐ground
effects. The shading treatment, which reduced overall seedling growth and
thus below‐ground N use and C allocation, also led to changes in humus N
content and microbial biomass, whereas defoliation, which did not affect
overall growth, did not influence these below‐ground properties. The
study also shows the carbon/nutrient balance of N. solandri var.
cliffortioides seedlings to be highly invariant to both shading and defoliation.
Population sex ratios, sex‐specific clonal traits and tradeoffs among these traits in the liverwort Marchantia inflexa
Authors: McLetchie D. Nicholas , Puterbaugh Mary N.
Published: December 2004

Unisexual, female‐ or male‐biased populations are common among some clonal plants. Within and among populations, the relative frequencies of males and females can be influenced by sex‐specific demographic patterns that, in turn, can be a consequence of life‐history characteristics. The objectives of this study were to describe (1) population sex‐ratio patterns among habitat patches along a river, and (2) sex‐specific patterns of and correlations among life‐history clonal traits including: growth rates, number of meristematic tips and asexual reproduction in the dioecious liverwort, Marchantia inflexa. In Trinidad, a section of a stream was surveyed for the occurrence of female and male sex expressing thalli among habitat patches, and habitat characteristics (canopy openness and patch size) were recorded. The numbers of female and male inflorescences were obtained also. Of the 209 patches observed in the field, 83% did not contain sex expressing thalli, 9% contained thalli of both sexes, and the rest contained only female‐ (4%) or male‐expressing (4%) thalli. Sex expression was less common among small patches, and there was a tendency for sex expression to be less likely among patches with the lowest canopy openness. The proportion of male inflorescences among the bisexual patches ranged from 0.22 to 0.80. In a greenhouse, we planted gemmae (asexual propagules) from field‐collected isolates: ten female and eight male. On six occasions, we harvested replicates of each isolate to estimate isolate trait means. Females grew faster, produced more meristematic tips and had lower levels of asexual reproduction than males. Number of meristematic tips was negatively correlated with asexual reproduction and positively correlated with growth rate. These sex‐specific patterns may lead to the high frequency of single sex patches as well as biased adult population sex ratios that are observed in the field.
Herbivore regulation and irreversible vegetation change in semi‐arid grazing systems
Authors: Van De Koppel Johan , Rietkerk Max
Published: December 2004

Models made to explain sudden and irreversible vegetation shifts in semi‐arid grasslands typically assume that herbivore density is independent of the state of the vegetation, e.g., under the control of humans. We relax this assumption and investigate the mathematical implications of vegetation‐regulated herbivore population dynamics. We show that irreversible vegetation change may also occur in systems where herbivore population dynamics are affected by changes in plant standing crop. Our analysis furthermore shows that irreversible vegetation change may occur for a larger set of soil and climatic conditions when herbivore numbers are independent of the vegetation, as compared to systems where vegetation density determines herbivore population size. Hence, our analysis suggests that irreversible vegetation change is less likely to occur in systems with natural herbivore population dynamics than in systems where humans control herbivore density.
Spatial arrangement of kin affects recruitment success in young male red grouse
Authors: MacColl Andrew D. C. , Piertney Stuart B. , Moss Robert , Lambin Xavier
Published: December 2004

Models have shown that population cycles might be driven by time lags resulting from positive feedback between kin structure and population change, coupled with negative feedback between density and population change. One such model operates through kin favouritism facilitating the recruitment of young cock red grouse. We investigated whether recruitment by young cocks depended on the presence and spatial arrangement of elder relatives in the territorial population. We used molecular genetic estimates of relatedness, and checked for effects of covariates including natal territory size, hatching date, body size, parasite burdens and local density. Philopatric recruitment by cock red grouse led to the formation of clusters of contiguous territories owned by kin. The probability that an individual young cock would establish a territory increased with the number of kin in his father's cluster. This pattern might have been due to genetic quality determining both recruitment success and the size of the paternal cluster. If so, there should have been a positive correlation between a young cock's probability of recruitment and the number of his relatives in the population, irrespective of their spatial distribution. This did not occur and so the effect of cluster size is unlikely to have been confounded by genetic quality. The only morphological measure correlated with recruitment success was supraorbital comb size. The results are consistent with the prediction that kin tolerance affects recruitment but were at the level of the individual within years, rather than the population among years. Hence an experimental test of the kin favouritism hypothesis for population cycles, by manipulation of relatedness in populations among years, is now required.
Ecosystem response of pasture soil communities to fumigation‐induced microbial diversity reductions: an examination of the biodiversity–ecosystem function relationship
Authors: Griffiths B. S. , Ritz K. , Bardgett R. D. , Cook R. , Christensen S. , Ekelund F. , Sørensen S. J. , Bååth E. , Bloem J. , De Ruiter P. C. , Dolfing J. , Nicolardot B.
Published: December 2004

A technique based on progressive fumigation was used to reduce soil microbial biodiversity, and the effects of such reductions upon the stability of key soil processes were measured. Mineral soil samples from a grassland were fumigated with chloroform for up to 24 h and then incubated for 5 months to allow recolonisation by surviving organisms. The diversity of cultivable and non‐cultivable bacteria, protozoa and nematodes was progressively reduced by increasing fumigation times, as was the number of trophic groups, phyla within trophic groups, and taxa within phyla. Total microbial biomass was similar within fumigated soils, but lower than for unfumigated soil. There was no direct relationship between biodiversity and function. Some broad‐scale functional parameters increased as biodiversity decreased, e.g. thymidine incorporation, growth on added nutrients, and the decomposition rate of plant residues. Other more specific parameters decreased as biodiversity decreased, e.g. nitrification, denitrification and methane oxidation. Thus specific functional parameters may be a more sensitive indicator of environmental change than general parameters. Although fumigation reduced soil microbial biodiversity, there was evidence to suggest that it selected for organisms with particular physiological characteristics. The consequences of this for interpreting biodiversity – function relationships are discussed. The stability of the resulting communities to perturbation was further examined by imposing a transient (brief heating to 40°C) or a persistent (addition of CuSO4) stress. Decomposition of grass residues was determined on three occasions after such perturbations. The soils clearly demonstrated resilience to the transient stress; decomposition rates were initially depressed by the stress and recovered over time. Resilience was reduced in the soils with decreasing biodiversity. Soils were not resilient to the persistent stress, there was no recovery in decomposition rate over time, but the soils with the highest biodiversity were more resistant to the stress than soils with impaired biodiversity. The study of functional stability under applied perturbation is a powerful means of examining the effects of biodiversity.
Effects of grazing on above‐ground biomass on a mountain snowbed, NW Finland
Authors: Virtanen Risto
Published: December 2004

The effects of grazing by the Norwegian lemming (Lemmus lemmus)
and reindeer (Rangifer tarandus) on biomass and vegetation
structure in a mountain snowbed habitat were studied using several
grazer‐exclosures kept for 5 or 15 yr. During the 5‐yr exclusion
experiment, the total biomass of mosses in particular, the main
winter‐food of lemmings, increased within fenced areas as compared to
open control areas. In the 15‐yr exclosures, Polytrichaceae mosses still
dominated, but graminoids (Anthoxanthum odoratum and Carex
bigelowii) had also increased. In these longer‐term exclosures, dead
plant material, consisting of Polytrichaceae moss and graminoid necromass
and other plant litter, had increased. By contrast, Kiaeria moss and lichens were absent or had low biomass after 15 yr of grazer exclosure, probably as a result of competitive exclusion. The results suggest that the plant biomass in such low productivity mountain snowbeds can be reduced to a low level by grazing. Grazing also determines the dominance relationships in the plant community and prevents accumulation of plant litter. The observed effects of grazing are inconsistent with hypotheses suggesting that herbivory is of little importance in low productivity environments, but rather support the exploitation ecosystems hypothesis predicting strong grazing impact in such low productivity environments.
Seasonal changes in cell‐mediated immunocompetence and mass gain in nestling barn swallows: a parasite‐mediated effect?
Authors: Merino Santiago , Møller Anders Pape , De Lope Florentino
Published: December 2004

Reproduction in seasonal environments is usually timed so peak demand for food by offspring coincides with peak availability. Hence, late breeders will encounter a scarcity of food. Since parasite populations grow during the reproductive season of their hosts, late reproducing animals will also face an increasing challenge by parasites. We hypothesised that seasonal decrease in food availability and seasonal increase in parasite abundance will cause a trade‐off between growth and immune function. This prediction was tested in nestling barn swallows (
Hirundo rustica) from first and second broods. Nestlings from second broods mounted stronger T cell mediated immune responses to a challenge with a novel antigen, but had lower rates of mass gain, than nestlings from first broods, consistent with the prediction. Broods in which at least one nestling died had lower levels of T cell mediated immune response, but not lower rates of mass gain, than broods without mortality, suggesting that brood reduction is mediated through an inability of offspring to defend themselves against parasites rather than an inability to grow. Possible mechanisms include scarcity of specific nutrients needed for immune responses, and/or parasites being concentrated on a single or few nestlings.
The Fractal Model: a new model to describe the species accumulation process and relative abundance distribution (RAD)
Authors: Mouillot David , Leprêtre Alain , Andrei‐Ruiz Marie‐Cécile , Viale Denise
Published: December 2004

Quantifying a sampled collection of taxa is a widespread problem in ecology. A number of diversity indices have been proposed and used in numerous works in spite of a lack of statistical characteristics and tests of comparison. These Relative Abundance Distributions (RAD) can also be described using graphic representations exhibiting both the number of taxa and the quantitative distribution of the individuals which constitute these taxa. Fitting these RADs to deterministic or stochastic models remains problematic for different reasons like scale dependence or complexity of the numerical procedures. In our study, we introduce a new model to describe RAD and ecological succession. This model is a Fractal Model based on the assumption that during an ecological succession/accumulation process, at each step of the succession, K
new species appear which are k times more abundant with
K=kd (where d is a fractal
dimension). We use a Monte‐Carlo procedure with a Kolmogorov‐Smirnov
distance to fit this model and to estimate parameters. Through the study
of entomological data from a Mediterranean Man And Biosphere reserve, we
demonstrate the qualities of the new Fractal Model. Thus, the Fractal
Model was rejected for none of the 13 RADs tested. In addition, the
parameters (K, k and d) are independent of the Hill family diversity indices and of three evenness indices and are able to provide additional information concerning both the diversity of the sampled ecosystem and development of this biodiversity during the species accumulation process in this ecosystem.
Survival of dispersing versus philopatric juvenile snowshoe hares: do dispersers die?
Authors: Gillis Elizabeth A. , Krebs Charles J.
Published: December 2004

We used radio‐telemetry to monitor the survival of dispersing and philopatric juvenile snowshoe hares (Lepus americanus) in southwestern Yukon Territory, Canada, during a cyclic population increase. Neither 28‐d survival nor the proportion of hares surviving to breed differed significantly between juvenile hares that dispersed and those that did not, nor was there a significant relationship between dispersal distance and fate (dead or alive). Our results indicate that the overall survival cost associated with natal dispersal is low for snowshoe hares during the early increase of the hare cycle.
Understanding the basis of pollinator selectivity in sexually dimorphic Fragaria virginiana
Authors: Ashman Tia‐Lynn , Swetz Jim , Shivitz Sandy
Published: December 2004

Pollinators of gender dimorphic (e.g., gynodioecious or dioecious) species often show a preference for hermaphrodite or male flowers over conspecific females, and such pollinator selectivity can have implications for the evolution of sexual dimorphism. However, because the sex morphs differ in several floral features, it is not readily apparent which specific traits generate differential pollinator service. In this study, we sought to understand the basis of pollinator preferences for hermaphrodites to females of gynodioecious Fragaria virginiana. We extend a previous study that revealed that lack of pollen‐filled anthers and shorter petals were not solely responsible for small bee and fly discrimination against females, by investigating the roles of other morphological features, reward characters, and floral context in bringing about pollinator discrimination. Using floral manipulations we revealed that small bees preferentially visit flowers with wide petals, more nectar, intact stamens, as well as flowers located in hermaphrodite‐dominated patches. Specifically, bees were able to remotely detect nectar presence. In addition, bees were attracted to both the anther sac and the filament regions of the stamen, indicating that not only anthers, but also filaments may function as visual or olfactory cues to pollen reward. Furthermore, pollinator preference for flowers in hermaphrodite‐dominated patches serves to intensify their discrimination against female flowers which occur predominantly in female‐dominated patches. For the most part, preferences of flies mirrored those of the bees with the exception that flies also showed a significant preference for flowers held high over those held closer to the ground. These results indicate that a suite of character differences contributes to pollinators’ preferences for hermaphrodite to female flowers. Furthermore, they suggest pollinators could mediate selection on both floral morphology and reward in this species.
Consequences of the reduction of plant diversity for litter decomposition: effects through litter quality and microenvironment
Authors: Hector A. , Beale A. J. , Minns A. , Otway S. J. , Lawton J. H.
Published: December 2004

Decomposition of plant litter is a key process for the flow of energy and nutrients in ecosystems that may be sensitive to the loss of biodiversity. Two hypothetical mechanisms by which changes in plant diversity could affect litter decomposition are (1) through changes in litter species composition, and (2) by altering the decomposition microenvironment. We tested these ideas in relation to the short‐term decomposition of herbaceous plant litter in experimental plant assemblages that differed in the numbers and types of plant species and functional groups that they contained to simulate loss of plant diversity. We used different litterbag experiments to separate the two potential pathways through which diversity could have an effect on decomposition. Our two litterbag trials showed that altering plant diversity affected litter breakdown differently through changes in decomposition microenvironment than through changes in litter composition. In the decomposition microenvironment experiment there was a significant but weak decline in decomposition rate in relation to decreasing plant diversity but no significant effect of plant composition. The litter composition experiment showed no effect of richness but significant effects of composition, including large differences between plant species and functional groups in litter chemistry and decomposition rate. However, for a nested subset of our litter mixtures decomposition was not accurately predicted from single‐species bags; there were positive, non‐additive effects of litter mixing which enhanced decomposition. We critically assess the strengths and limitations of our short‐term litterbag trials in predicting the longer‐term effects of changes in plant diversity on litter decomposition rates.
Constraints on patterns of covariation: a case study in strigid owls
Authors: Cartron Jean‐Luc E. , Kelly Jeffrey F. , Brown James H.
Published: December 2004

In a system of intercorrelated variables, the sign and strength of pairwise relationships must conform to a mathematical constraint affecting the entire system. The constraint exists as a property of all correlation matrices, the positive semi‐definite (PSD) criterion. We illustrate its implications with a case study of covariation among clutch size, body size, and latitude in the Strigidae. Our analyses indicate that clutch size and body size among strigid owls increase with latitude, while clutch size decreases with body size. The PSD provides a framework to explain the existence of this apparent macroevolutionary paradox. It also shows why the three relationships are never detected concurrently when using simple regressions or independent contrasts. In a system with three intercorrelated variables, the PSD criterion may allow one pairwise relationship to be negative even if the other two relationships are positive. However, the correlation coefficients cannot all be strong. Whenever the PSD criterion may constrain covariation, care should be exercised in inferring evolutionary or ecological processes from patterns of correlation.
Sexual dimorphism and sexual segregation in foraging strategies of northern giant petrels, Macronectes halli, during incubation
Authors: González‐Solís Jacob , Croxall John P. , Wood Andy G.
Published: December 2004

Giant petrels (Macronectes spp.) are the most sexually dimorphic of all seabirds. We used satellite‐tracking and mass change during incubation to investigate the influence of sexual size dimorphism, in terms of the intersexual food competition hypothesis, on foraging and fasting strategies of northern giant petrels at South Georgia. Females foraged at sea whereas males foraged mainly on the South Georgia coast, scavenging on seal and penguin carcasses. Foraging effort (flight speed, distance covered, duration of foraging trips) was greater for females than for males. In contrast, foraging efficiency (proportionate daily mass gain while foraging) was significantly greater for males than for females. Females were significantly closer to the desertion mass threshold than males and could not compensate for the mass loss during the incubation fast while foraging, suggesting greater incubation costs for females than for males. Both sexes regulated the duration and food intake of foraging trips depending on the depletion of the body reserves. In males the total mass gain was best explained by mass at departure and body size. We suggest that sexual segregation of foraging strategies arose from size‐related dominance at carcasses, promoting sexual size dimorphism. Our results indicate that sex‐specific differences in fasting endurance, contest competition over food and flight metabolic rates are key elements in maintenance of sexual size dimorphism, segregating foraging strategies and presumably reducing competition between sexes.
The changing relation of landscape patterns and jack pine budworm populations during an outbreak
Authors: Radeloff Volker C. , Mladenoff David J. , Boyce Mark S.
Published: December 2004

The influence of landscape patterns on ecological processes is generally acknowledged, but often difficult to quantify. The objective of our study was to quantify the relation of jack pine budworm (Choristoneura pinus pinus) population levels to both the
abundance of jack pine (Pinus banksiana) and of jack pine stand edges in the landscape. The 450 000 ha Pine Barrens region, located in northwestern Wisconsin, USA, experienced a severe jack pine budworm outbreak from 1990 to 1995. We calculated landscape indices on a landcover map derived from Landsat TM satellite imagery. Landscape indices were calculated on circular buffers (0.5, 0.75, 1, 1.5, and 2.5 km radius) centered on 143 budworm population sampling points for which annual budworm counts were available. Edge density was normalized for the proportion of jack pine in the landscape using random maps as a standard. Correlations between landscape patterns and budworm populations varied over time: proportion of jack pine showed strongest positive correlation with budworm population levels up to the peak of the outbreak (1993). Edge density exhibited positive correlation up to the peak of the outbreak, but negative correlation in the subsequent years as the outbreak declined. This may suggest that pollen‐bearing male cones, which are more abundant along edges, support higher budworm populations in the initial phase of the outbreak, but stronger predation on budworm along edges subsequently reduces populations. We provide insight into previously inconclusive results on the relation of jack pine budworm population density to jack pine stand edges. The effects of landscape patterns, such as edge density, may vary not only in magnitude, but also in direction, being positive and negative during different phases of an insect outbreak. Therefore, caution should be taken in relating landscape patterns to process at either a single scale or point in time.
Effects of agricultural soil loss on forest succession rates and tree diversity in east Tennessee
Authors: Lafon Charles W. , Huston Michael A. , Horn Sally P.
Published: December 2004

All land on what is today the Oak Ridge Reservation, Tennessee, was abandoned in 1942 when the United States government acquired the land for atomic weapons development. Most of the land now serves as a buffer around the government facilities and has not been managed, providing an opportunity to study forest succession on simultaneously abandoned fields. We sampled forest vegetation and soils on former pastures that had sustained varying levels of pre‐abandonment soil loss. The most eroded sites are still covered with forests of an early‐successional composition, with shortleaf pine (Pinus echinata) and Virginia
pine (P. virginiana) dominating. Hardwoods, particularly oaks
(Quercus) and hickories (Carya), dominate the least degraded former pastures. Canopy tree species diversity is highest at the least degraded sites, which also have the highest levels of soil nitrogen. These results suggest that soil degradation reduces the rate of secondary succession and thereby slows the increase in species diversity typical of early forest succession, while increasing the spatial complexity of the landscape.
Survivorship of an ant‐tended membracid as a function of ant recruitment
Authors: Morales Manuel A.
Published: December 2004

I used a host‐visitor modeling framework to examine the interaction between the treehopper Publilia concava and ants in the genus
Formica. In particular, I tested the functional relationship
between ant tending, the spatial distribution of treehoppers, and
treehopper density. The per‐capita density of ants at each host plant was
a decreasing function of treehopper density, distance from the ant nest
and the neighborhood density of treehoppers. Treehopper survivorship was
proportional to the per‐capita density of ants and the duration of ant
tending. Consequently, treehoppers in low‐density aggregations on
isolated host plants near the nest received maximum benefit from ant
tending. Treehoppers tended by the ant Formica integra were
abandoned as the summer progressed, although many of these treehoppers
were re‐colonized by other species of ants. While F. integra ultimately abandoned all treehoppers, treehoppers on host plants with fewer initial ants were abandoned first. Results from the present study are consistent with previous findings suggesting that patterns of density‐dependent benefit for homopterans are a function of the recruitment response of ants. Additionally, results suggest a tradeoff between maximizing the persistence or probability of ant‐tending and minimizing competition for ants when tended. In general, host‐visitor models of mutualism may provide a theoretical framework for understanding conditional outcomes in ant‐homopteran, and other host‐visitor mutualisms.
Predator satiation and extreme mast seeding in 11 species of Chionochloa (Poaceae)
Authors: Kelly Dave , Harrison Andrea L. , Lee William G. , Payton Ian J. , Wilson Peter R. , Schauber Eric M.
Published: December 2004

Variation in annual flowering effort is described for 16 long datasets from 11 species of Chionochloa (Poaceae) in New Zealand. All
populations exhibited extreme mast seeding. The most variable species was
C. crassiuscula (coefficient of variation, CV=3.02) over 26 years
at Takahe Valley, Fiordland, which is the highest published CV we know of
worldwide. The other populations also had high CVs (lowest CV=1.42, mean
CV=1.84) which were higher than for other well‐studied genera such as
Picea, Pinus and Quercus. There were also frequent
years of zero flowering (mean across all populations was 37.2% zero
years; maximum 53% for C. rubra and C. crassiuscula over 19
years) whereas zero years are rare in other published masting
datasets.Flowering was highly synchronous among species within a site
(mean r=0.886), and also (though significantly less so) among
sites. Among sites, synchrony was not significantly higher within‐species
(mean r=0.711) than between‐species (r=0.690). Warm summer
temperatures led to heavy flowering the following summer. Flowering
synchrony increased with increasing synchrony in local deseasonalised
summer temperatures, and decreased with increasing distance between
sites.Mast seeding has been shown in Chionochloa to reduce losses
to specialist flower or seed predators. Among‐species synchrony may be
adaptive if species share a common seed predator. Developing seeds of at
least 10 Chionochloa species are attacked by larvae of an
undescribed cecidomyiid. In Takahe Valley, where masting is most
pronounced, cecidomyiids attacked all six Chionochloa species in
all four years studied. Mean annual losses were almost constant (10.0 to
13.4%) while flowering effort varied 100‐fold. The invariant losses are
consistent with other evidence that the cecidomyiid may have extended
diapause, which would make it harder to satiate by mast seeding. We
hypothesise that one possible factor favouring such extremely high levels
of mast seeding in Chionochloa is that its seed predator is very hard to satiate.
Functional response of the least weasel, Mustela nivalis nivalis
Authors: Sundell Janne , Norrdahl Kai , Korpimäki Erkki , Hanski Ilkka
Published: December 2004

We investigated the functional response of the least weasel (Mustela nivalis nivalis) in a series of experiments conducted in
large outdoor enclosures (0.5 ha). Radiocollared Microtus voles were released in the enclosures at different densities (4, 8, 16 and 100/ha) three days before the release of a radiocollared weasel. During the three‐day experiment every vole killed was replaced with another one as soon as possible to retain constant prey density. The results demonstrated type II functional response with the predation rate reaching 50% of the asymptotic rate at a vole density of 15 individuals per ha. More voles were killed at the highest densities than would be expected from the known energy demands of weasels. Female and male voles were killed in proportion to their abundances in the enclosures, and no difference in predation risk was detected between voles released in the enclosure before the weasel (residents) and during the experiment (transients).