bluuebunny/crossref_metadata_2025 huggingface dataset:
Within‐group variation in the willingness to risk exposure to a predator: the influence of species and size
Authors: Abrahams Mark V. , Cartar Ralph V.
Published: December 2004

The costs and benefits associated with feeding in the vicinity of a predator should vary within and among species, depending on an individual's vulnerability to a predator. In this paper, we investigate how willingness to risk exposure to a predator in order to gain access to food is expressed within groups of fish. We provided groups of six individually marked brook sticklebacks (Culeainconstans) and fathead minnows (Pimephalespromelas) with the opportunity to feed in safety, or in the
presence of a single predator (walleye, Stizostedionvitreum ). Sticklebacks and fathead minnows are commonly found in the same locations at the same time, but sticklebacks also possess armour and spines that provide defence against some predators. Despite this morphological variation, patterns of individual behaviour were consistent between species. While feeding close to the predator, there was a positive relation between the size of the individual and the proportion of food consumed. No relation existed on the side farthest from the predator. These data suggest that fish may risk exposure to predators to exploit size‐related differences in vulnerability to gain a competitive advantage for access to food.
Density compensation in New World bat communities
Authors: Stevens Richard D. , Willig Michael R.
Published: December 2004

Understanding the role of competitive interactions in shaping the structure of communities has been one the most unrelenting challenges to ecology. Traditionally, competitive interactions were assumed to be the most important agent of deterministic structure, with overdispersed morphological patterns based on body size and trophic status as their hallmark. However, models of community organization based solely on morphology have yielded only equivocal results for many taxa. Fortunately, morphological patterns may not be the only indications of competitively induced deterministic structure. Herein, we explore the degree to which the structure of five feeding guilds (aerial insectivores, frugivores, gleaning animalivores, molossid insectivores, and nectarivores) from 15 New World bat communities reflects density compensation. Nonrandom associations between abundance and morphological distance were detected in five communities, in three feeding guilds, and with respect to four competitive scenarios. Nonetheless, patterns consistent with the hypothesis of density compensation were neither pervasive nor consistent in New World bat communities. Competitively induced community structure may exist under only narrow temporal and environmental conditions, and may not be characteristic of organization in most situations.
Shapes and sizes of badger territories
Authors: Blackwell P. G. , Macdonald D. W.
Published: December 2004

We examine closely the models, methods and conclusions of Doncaster and Woodroffe (1993; Oikos 66: 88–93 who argued that den or main sett sites of clans of badgers, Meles meles, are particularly important in determining
territory shape and size, and hence influence the size of social group.
We consider a realistic alternative hypothesis which allows the key
assertion by Doncaster and Woodroffe to be directly tested. We show that
a Dirichlet tessellation model that does not give a major role to the main setts fits data from several studies – two of those considered by Doncaster and Woodroffe, and a more recent and extensive one – significantly better than Doncaster and Woodroffe's model. For the majority of territories, especially in the most extensive data set, differences in territory shape and size under the two models are substantial, suggesting that a different biological mechanism is at work, as well as or instead of dependence on main sett locations.
Resprouting as a life history strategy in woody plant communities
Authors: Bellingham Peter J. , Sparrow Ashley D.
Published: December 2004

Resprouting is an efficient means by which woody plants regain biomass lost during disturbance, but there is a life history trade‐off that occurs in all disturbance regimes between investment in the current generation through resprouting vs investment in future generations at the same or more distant sites. The relative allocation to resprouting vs seeding in woody plant communities is dictated by the nature of disturbance regimes. Resprouting is the predominant response to the least severe disturbance regimes, but is also a common response in disturbance regimes of high severity, those that destroy most or all above‐ground biomass, and which occur at medium to high frequency. The response to disturbance either by resprouting or seeding is dictated by the site's productivity. We present a comprehensive model for relative allocation to resprouting vs seeding across a range of disturbance regimes. Competition between plants that mostly seed vs those that mostly resprout should accentuate differences in allocation along a gradient of disturbance frequency. However the patchy nature of disturbance in time and space, coupled with gene flow among populations undergoing different disturbance regimes, ensures that it is unlikely that either resprouting or seeding will be the sole response in most plant communities at most disturbance frequencies. Additional influences on resprouting in woody plant communities include changes in allocation during the lifespan of individual plants and phylogenetic constraints that are expressed as biogeographic patterns.
Influence of plant quality on pine sawfly population dynamics
Authors: Larsson Stig , Ekbom Barbara , Björkman Christer
Published: December 2004

The contribution of plant quality to the population dynamics of herbivorous insects has been an issue of much controversy. Many studies have documented how variable plant quality differentially influences the survival and fecundity of insect individuals. Whether or not such effects can be translated to the level of insect populations is, however, not clear. In order to test this hypothesis one needs to combine processes at both the level of the individual and the population. This is difficult with an empirical approach, but could be achieved by means of modeling given that appropriate data exist for both levels of organization. In this paper we report on a model developed to analyze whether altered Scots pine (Pinus sylvestris) quality can contribute to the
build‐up of populations of the European pine sawfly (Neodiprion
sertifer). Experimental data on responses of sawfly larvae to
variable plant quality, i.e. needle concentrations of resin acids, were
used to parameterize the model. Larval survival and sawfly fecundity are
reduced at high resin acid concentrations. However, high resin acid
concentrations are, at the same time, beneficial because larval defense
against predators is enhanced. In the model, data on individual responses
were combined with literature data at the population level; a type III
functional response related to cocoon predation was presumed to be the
density‐dependent process regulating sawfly populations. The analysis
showed that the risk for an outbreak is high when needle resin acid
concentration (r) or larval predation pressure (p) is low.
When r or p is high there is no risk. By analyzing
different scenarios it was found that small changes in r and
p can result in the sawfly population moving from low to high
outbreak risk. Changes of the same, or larger, magnitude in r have
been observed in empirical studies. The role of tritrophic interactions
was also considered. This was done by removing the positive effects of
resin acids on larval performance in the model. It was found that the
anti‐predator defense of N.sertifer makes it prone to
outbreak under wider combinations of r and p than an insect without the defense. We conclude that small changes in a density‐independent factor, such as needle chemistry, can have significant effects on herbivore population dynamics because increased fecundity and survival caused by needle quality may allow the population to escape the control of density‐dependent factors, such as cocoon predation.
Pollinator abundance and behavior on Aconitum lycoctonum (Ranunculaceae): an analysis of the quantity and quality components of pollination
Authors: Utelli Anna‐Barbara , Roy Barbara A.
Published: December 2004

The yellow‐flowered monkshood Aconitum lycoctonum has variable
degrees of inbreeding among populations, despite being primarily
bee‐pollinated. Here we examined the pollination ecology of A.
lycoctonum. We investigated pollinator community and frequency in
four populations at two altitudes over two years. We found that flowers
were more often visited at low elevation than at high elevation. However,
because flowers lived longer at high elevation than at low elevation, and
plants at high elevation had more flowers than at low elevation, the
overall chance of a plant being visited may be greater at high elevation.
Breeding system experiments showed that at least some populations of
A. lycoctonum were self‐compatible and also were not completely
protandrous. Thus selfing, especially by geitonogamous pollen, will be
common if pollinators often visit several flowers within a plant, as we
found to be true for the main pollinator (bumblebees) of A.
lycoctonum. Although the stereotypical behavior of bees is to move
upwards, 17% of the within‐plant movements of the main pollinator
(Bombus gerstaeckeri) were downwards, i.e. from a male‐phased to a
female‐phased flower because the flowers open from the bottom to the top
of the plant. Other pollinators of A. lycoctonum moved less often
within plants. We conclude that in addition to differences in
self‐compatibility, differences in pollinator abundance and behavior
could have led to variation in the realized mating system in different
A. lycoctonum populations.
Combining field experiments and individual‐based modeling to identify the dynamically relevant organizational scale in a field system
Authors: Schmitz Oswald J.
Published: December 2004

Community ecologists continually strive to build analytical models that realistically describe long‐term dynamics of the systems they study. A key step in this process is identifying which details are relevant for predicting dynamics. Currently, this remains a limiting step in development of analytical theory because experimental field ecology, which provides the key empirical insight, and theoretical ecology, which translates empirical knowledge into analytical theory, remain weakly linked. I illustrate how an individual‐based computational model of species interactions is a useful way to bridge the gulf between empirical research and theory development. I built a computational model that reproduced key natural history and biological detail of an old‐field interaction web composed of a predator species, a herbivore species and two plant groups that had been the subject of extensive previous field research. I examined, using simulation experiments, how individual behavior of herbivores in response to changing resource and predator abundance scaled to long‐term population‐level and community‐level dynamics. The simulation experiments revealed that the long‐term community dynamics could be highly predictable because of two counterintuitive reasons. First, seasonality was a strong forcing variable on the system that removed the possibility of serial dependence in population abundance over time. Second, because of seasonality, short‐term behavioral responses of herbivores played a much stronger role in shaping community structure than longer‐term processes such as density responses. So, simply knowing the short‐term responses of herbivores at the evolutionary ecological level was sufficient to forecast the long‐term outcome of experimental manipulations. This study shows that an individual‐based model, once it is calibrated to the real‐world field system, can provide key insight into the biological detail that analytical models should include to predict long‐term dynamics.
Reproductive versus ecological advantages to larger body size in female snakes, Vipera aspis
Authors: Bonnet Xavier , Naulleau Guy , Shine Richard , Lourdais Olivier
Published: December 2004

Body size can influence an organism's microevolutionary fitness either via ecological factors (ecological selection) or changes in reproductive output (sexual or fecundity selection). Published studies on sexual dimorphism in reptiles have generally focussed on sexual‐selective forces on males, under the implicit assumption that the intensity of fecundity selection in females (and hence, overall selection on female body size) is likely to be relatively consistent among lineages. In this paper, we explore the degree to which larger body size enhances ecological attributes (e.g., food intake, growth, survival) and reproductive output (reproductive frequency, litter size, offspring size, offspring viability) in free‐ranging female aspic vipers, Vipera
aspis. The less‐than‐annual reproductive frequency of these animals allows us to make a direct comparison between females in years during which they concentrate on “ecological” challenges (especially building energy reserves) versus reproductive challenges (producing a litter). Because female snakes have limited abdominal space to hold the clutch (litter), we expect that fecundity should depend on body size. However, our data show that larger body size had a more consistent effect on ecological attributes (such as feeding rates and “costs of reproduction”) than on reproductive output per se. Indeed, total reproductive output was maximised at intermediate body sizes. These results suggest that variation in female body size among and within species (and hence, in the degree of sexual dimorphism) may be driven by the ecological as well as reproductive consequences of body size variation in both sexes.
Rare species in communities of tropical insect herbivores: pondering the mystery of singletons
Authors: Novotný Vojtech , Basset Yves
Published: December 2004

The host specificity, taxonomic composition and feeding guild of rare species were studied in communities of herbivorous insects in New Guinea. Leaf‐chewing and sap‐sucking insects (Orthoptera, Phasmatodea, Coleoptera, Lepidoptera and Hemiptera‐Auchenorrhyncha) were sampled from 30 species of trees and shrubs (15 spp. of Ficus, Moraceae, six
spp. of Macaranga and nine species of other Euphorbiaceae) in a lowland rain forest. Feeding trials were performed with all leaf‐chewers in order to exclude transient species. Overall, the sampling produced 80 062 individuals of 1050 species. The species accumulation curve did not attain an asymptote, despite 950 person‐days of sampling. Rare species, defined as those found as single individuals, remained numerous even in large samples and after the exclusion of transient, non‐feeding species. There was no difference among plant species in the proportion of rare species in their herbivore communities, which was, on average, 45%. Likewise, various herbivore guilds and taxa had all very similar proportions of rare and common species. There was also no difference between rare and common species in their host specificity. Both highly specialised species and generalists, feeding on numerous plants, contributed to the singleton records on particular plant species. Predominantly, a species was rare on a particular host whilst more common on other, often related, host species, or relatively rare on numerous other host plants, so that its aggregate population was high. Both cases are an example of the “mass effect”, since it is probable that such rare species were dependent on a constant influx of immigrants from the other host plants. These other plants were found particularly often among congeneric plants, less so among confamilial plants from different genera and least frequently among plants from different families. There were also 278 very rare species, found as one individual on a single plant species only. Their host specificity could not be assessed; they might have been either very rare specialists, or species feeding also on other plants, those that were not studied. The former possibility is unlikely since monophagous species, collected as singletons at the present sampling effort, would have existed at an extremely low population density, less than 1 individual per 10 ha of the forest.
Size of environmental grain and resource matching
Authors: Ranta Esa , Lundberg Per , Kaitala Veijo
Published: December 2004

For most animals their foraging environment consists of a patch network. In random environments there are no spatial autocorrelation at all, while in fine‐grained systems positive autocorrelations flip to negative ones and back again against distance. With increasing grain size the turnover rate of spatial autocorrelation slows down. Using a cellular automaton with foragers having limited information about their feeding environment we examined how well consumer numbers matched resource availability, also known as the ideal free distribution. The match is the better the smaller the size of the environmental grain. This is somewhat contrary to the observation that in large‐grained environments the spatial autocorrelation is high and positive over long distances. In such an environment foragers, by knowing a limited surrounding, should in fact know a much larger area because of the spatially autocorrelated resource pattern. Yet, when foragers have limited knowledge, we observed that the degree of undermatching (i.e., more individuals in less productive patches than expected) increases with increasing grain size.
Climatic gradients in woody plant (tree and shrub) diversity: water‐energy dynamics, residual variation, and topography
Authors: O'Brien Eileen M. , Field Richard , Whittaker Robert J.
Published: December 2004

Recent studies at the macro‐scale have demonstrated that geographic gradients in the richness of plants, in particular of woody plants such as trees and shrubs, can be viewed as by‐products of water‐energy dynamics. According to this view, they are climatic rather than latitudinal/longitudinal gradients, relating to coincident and predictable variations in planetary surface‐atmosphere thermal dynamics and consequent patterns in biological activity. Previous analyses have shown that a two‐variable model capturing the dynamic relationship between energy (heat/light) and water (rainfall) accounts for most of the variation in woody plant richness across southern Africa at species, genus, and family levels. Here we move towards a more complete explanation, while demonstrating how geographic analysis of residuals can be used to identify the type and sequence of additional variables for inclusion, either at the same or at more discrete scales of analysis. Residual geographic variation in richness from the two‐variable model displays a geographic pattern unrelated to longitude and latitude. Regional clusters of under‐ and over‐prediction point to macro‐scale variation in topographic relief as a significant factor. When topographic relief is added as a third variable, the explanatory power (R2) increases by 7 to 12%, and the subsequent pattern of variation in residuals becomes even more unpredictable. What clustering remains points to other macro‐, and meso‐ or micro‐scale variables that need to be considered. Such a top‐down, trans‐scalar approach permits systematic and objective development of more complete explanations, while the three‐variable macro‐scale model developed herein is the basis for a powerful research tool for ecologists, biogeographers, conservationists and bio‐climatologists alike.
When species accumulation curves intersect: implications for ranking diversity using small samples
Authors: Lande Russell , DeVries Philip J. , Walla Thomas R.
Published: December 2004

Diversity in biological communities frequently is compared using species accumulation curves, plotting observed species richness versus sample size. When species accumulation curves intersect, the ranking of communities by observed species richness depends on sample size, creating inconsistency in comparisons of diversity. We show that species accumulation curves for two communities are expected to intersect when the community with lower actual species richness has higher Simpson diversity (probability that two random individuals belong to different species). This may often occur when comparing communities that differ in habitat heterogeneity or disturbance, as we illustrate using data from neotropical butterflies. In contrast to observed species richness, estimated Simpson diversity always produces a consistent expected ranking among communities across sample sizes, with the statistical accuracy to confidently rank communities using small samples. Simpson diversity should therefore be particularly useful in rapid assessments to prioritize areas for conservation.
Interpopulation variation in reproductive traits of female masu salmon, Oncorhynchus masou
Authors: Tamate Tsuyoshi , Maekawa Koji
Published: December 2004

Several life history models predict that larger eggs and lower fecundity should be favored in a low‐growth environment. We applied the model of Sibly et al. to seven Japanese populations of masu salmon (Oncorhynchus masou) in order to test the predictions that populations in which individual growth rate is low are characterized by larger eggs and lower fecundity.  Two populations, Shumarinai and Shikaribetsu, had the lowest growth rates of individuals, largest egg sizes and lowest fecundities of all populations examined. In contrast, the Shiribetsu, Chitose and Uono populations had highest growth rates of individuals, smallest egg sizes and highest fecundities. The Shibetsu and Toya populations were intermediate between these two groups. The gonadsomatic index (GSI) of the Shumarinai and Shikaribetsu populations was smaller than that of all other populations. Correlation analysis indicated that populations with lower individual growth rates had larger eggs and lower fecundities. The results were consistent with the predictions of the Sibly et al. model: increased egg size which results in decreased fecundity is probably an adaptation to low‐growth environments. Therefore, in masu salmon, growth differences among populations may explain interpopulation variation in egg size and fecundity.
Mate quality influences multiple maternity in the sex‐role‐reversed pipefish Syngnathus typhle
Authors: Jones Adam G. , Rosenqvist Gunilla , Berglund Anders , Avise John C.
Published: December 2004

In the pipefish Syngnathus typhle, pregnant males provide all
parental care. Females are able to produce more eggs than males can
brood, and consequently females compete more intensely for mates than do
males, a phenomenon defined as sex‐role reversal. As the genetic mating
system influences the operation of sexual selection, we investigate
variation in one phenotypic component of mate quality, female body size,
as a possible proximate influence on mating system variation in S.
typhle. Breeding trials were employed, each consisting of a single
receptive male with four adult females. In each replicate, a focal male
was paired either with a set of small or with a set of large females.
Males were allowed to mate freely, and after several weeks of brood
development, maternity of the progeny was resolved using three
microsatellite loci. Males with access either to small or to large
females successfully mated with a mean of 2.1 or 1.3 females,
respectively, a significant difference. Results indicate that variation
in female size can affect the mating system and thereby influence sexual
selection in pipefish. Thus, the high rate of multiple mating by S.
typhle males in the wild may be explained in part by the extensive size variation in naturally occurring, sexually mature females.
The effects of gerbil foraging on the natural seedbank and consequences on the annual plant community
Authors: Lortie Christopher J. , Ganey David T. , Kotler Burt P.
Published: December 2004

We explicitly tested the following predictions using Gerbillus
allenbyi and G. pyramidum foraging on artificial arrays of natural seedbank on a semi‐stabilized sand dune: 1. that the gerbils will forage using a quitting harvest rate rule, 2. that larger seeds are preferred due to higher encounter rates, and 3. that there are community level consequences for the annual plants as a direct result of foraging by the desert rodents. Natural seedbank, separated into two size classes, was placed in seed trays in the field at four different densities (1/16×, 1/4×, 1×, and 2×normal). Following exposure to granivory, the remaining seeds were weighed and germinated to test for community level effects. Only the 1× and 2×normal density plots were heavily foraged, and at both seed sizes, which suggests that the gerbils employed a quitting harvest rate rule. In support of our second prediction, the two species of gerbils tended to consume more of the larger seeds, particularly at higher densities. The mean number and total number of plant species that germinated in plots exposed to granivores was not significantly different from unexposed samples. At the community level however, there was no net association of germinated plant species in the four treatment groups exposed to granivores, but a significant net positive association in the unexposed control seedbank. Gerbil foraging on annual plant seedbank may thus subtly change the entire community structure (from positive to neutral), although not necessarily shift the species distributions significantly. Our results corroborate other studies involving artificial food types such as millet and suggest that foraging decisions may affect the plant community.
Mass‐dependent mass loss: how to get the null hypothesis wrong
Authors: Ruf Thomas
Published: December 2004

In ecological studies there is often considerable interest in the question whether the magnitude of mass loss in animals depends on their initial mass. Cichon et al. have recently questioned the validity of conventional statistical testing of relations between mass loss and initial mass, and derived a new null hypothesis to be used for a “correct” statistical analysis. Here, I demonstrate that the method employed by Cichon et al. is wrong and leads to false conclusions. Hence, there is no need to reverse the biological interpretation of previously published results.
Patch type influences on regeneration in a western Pennsylvania, USA, catastrophic windthrow
Authors: Peterson Chris J. , Pickett Steward T. A.
Published: December 2004

We investigated the influence of differing patch types on the germination, growth, and survival of two species of tree seedlings in a catastrophic, old‐growth forest windthrow. We documented environmental conditions, and experimentally introduced seeds, transplanted seedlings, and manipulated litter and herbivore access to determine the effects of patch types during initial forest regrowth.  Among patch types defined by substrate condition and exposure, we found significant differential influence of patch type on germination of seeds of a common colonizer, Betulaalleghaniensis. Patch types
that provided exposed mineral soil allowed greater germination than other
patch types, regardless of light availability. Germination of Acerrubrum varied among patch types in a fashion similar to that of
B. alleghaniensis, although differences among patch types were
not significant. 
Litter can also be the basis of patch classification. Both type and
amount of litter significantly influenced emergence of seedlings, with
broad‐leaf litter (in amounts typical of intact forest) producing greater
inhibition of emergence than amounts of conifer needles typical of intact
forest. Survival and performance of fenced and unfenced Acerrubrum seedlings transplanted into different patch types reflected
influences of both herbivory and competition with existing
vegetation. 
Survival was lowest where the Acer seedlings were most apparent to
mammalian herbivores. Growth of Acer seedlings was greater when they were protected from herbivory and free of competition from surrounding vegetation.  These findings demonstrate that patch types offer different suitability as regeneration microsites during early tree life history stages. Because these patch types are common and easy to recognize, the differences in species’ responses may be sufficiently general to apply in other disturbances of this type. If general species responses to patch types can be combined with knowledge of propagule abundance and patch abundance, it could provide a basis for improved understanding of post‐disturbance regeneration, via patch influences.
Do delayed effects of overgrazing explain population cycles in voles?
Authors: Klemola Tero , Norrdahl Kai , Korpimäki Erkki
Published: December 2004

Theoretical models predict that delayed density‐dependent processes with a time‐lag of approximately nine months are sufficient to generate regular 3–5 year fluctuations in densities of northern small rodents. To examine whether this time‐lag could be generated by plant‐herbivore interactions, we studied delayed effects of overgrazed food plants on voles. We introduced field voles (Microtus
agrestis) in four large predator‐proof enclosures that had suffered
heavy grazing during the preceding autumn and winter, and compared them
with voles introduced to previously ungrazed control areas. We found no
detrimental effects of previous grazing on population growth,
reproduction or body condition of voles. Chemical analyses did not show
consistent effects of grazing on nutritional components of common food
plants (grasses). These results suggest that short‐term population cycles
of Microtus voles in grassland habitats are not primarily driven by delayed effects of plant‐herbivore interactions.
Do small leaves expand faster than large leaves, and do shorter expansion times reduce herbivore damage?
Authors: Moles Angela T. , Westoby Mark
Published: December 2004

Leaves are most vulnerable to herbivory during expansion. We hypothesised that one factor favouring small leaves could be that smaller‐leaved species have shorter expansion times and are therefore exposed to high levels of herbivory for a shorter period than large leaves. In order to test this hypothesis, leaf expansion time and leaf area loss were measured for 51 species from Sydney, Australia. Strong positive correlations were found between leaf length and area and leaf expansion time, confirming that small leaves do expand in a shorter time than large leaves. The amount of leaf area lost was highly variable (from 0.5 to 90% of total leaf area), but was significantly related to both leaf expansion time and log leaf area. The amount of leaf area lost was not significantly correlated with specific leaf area nor with the presence of distasteful substances in the leaves, but was lower on species with hairy expanding leaves.
Hidden diversity and productivity patterns in mixed Mediterranean grasslands
Authors: Troumbis Andreas Y. , Dimitrakopoulos Panayiotis G. , Siamantziouras Akis‐Stavros D. , Memtsas Dimitris
Published: December 2004

A detailed study of the variation in productivity across a diversity gradient in an experimental Mediterranean grassland examines the effects of a dominant perennial grass species upon the overall diversity–productivity relationship. The experiment took place at the Greek site of the European‐wide BIODEPTH programme. The experimental design is characterized by the use of a number of communities containing annuals and perennials within the total set of manipulated plots. The main results are: 1) a log‐linear relationship between diversity and productivity exists in Mediterranean grasslands synthesized by annuals only, 2) in mixed communities where multiple growth forms coexist, the performance of a dominant or keystone species may reverse or hide the diversity–productivity pattern of a functional or growth form group of species taken separately, and 3) the introduction of the dominant grass in the low‐diversity mixtures creates an ‘inverted’ sampling effect which can produce as an artefact a constant productivity response across the diversity gradient.
The quality and the timing hypotheses evaluated using data on great reed warblers
Authors: Hansson Bengt , Bensch Staffan , Hasselquist Dennis
Published: December 2004

The seasonal decline in reproductive success observed in many animal species may be caused by timing per se (timing hypothesis) or by variation in phenotypic quality between early and late breeding females (quality hypothesis). To distinguish between these two hypotheses, several studies of birds have used clutch removal experiments to manipulate breeding date. However, removal experiments also increase the females’ previous reproductive effort due to the production of an extra clutch and a longer incubation period. According to life‐history theory an increase in reproductive effort lowers future reproduction. Hence, life‐history theory predicts lowered success of replacement broods for other reasons than expected from the timing hypothesis. Female great reed warblers, Acrocephalus arundinaceus, studied in Sweden are frequently exposed to nest predation, after which many lay replacement clutches. In order to examine possible effects of previous reproductive effort on different fitness components, we analysed the re‐laying frequency and the reproductive success of replacement broods in relation to time of the season and previous reproductive effort (measured as the length of the previous breeding attempt, LPB). In clutches of re‐laying females both the number of fledglings and the proportion of recruits were negatively correlated with LPB, whereas re‐laying frequency and clutch size were not related to LPB. We expect such relationships to be present also among other species. Consequently, the use of replacement clutches, as for example in clutch removal experiments, in evaluations of the cause of the often observed seasonal decline in various fitness components, might exaggerate the importance of the timing hypothesis over the quality hypothesis.
Behavioural flexibility predicts invasion success in birds introduced to New Zealand
Authors: Sol Daniel , Lefebvre Louis
Published: December 2004

A fundamental question in ecology is whether there are evolutionary characteristics of species that make some better than others at invading new communities. In birds, nesting habits, sexually selected traits, migration, clutch size and body mass have been suggested as important variables, but behavioural flexibility is another obvious trait that has received little attention. Behavioural flexibility allows animals to respond more rapidly to environmental changes and can therefore be advantageous when invading novel habitats. Behavioural flexibility is linked to relative brain size and, for foraging, has been operationalised as the number of innovations per taxon reported in the short note sections of ornithology journals. Here, we use data on avian species introduced to New Zealand and test the link between forebrain size, feeding innovation frequency and invasion success. Relative brain size was, as expected, a significant predictor of introduction success, after removing the effect of introduction effort. Species with relatively larger brains tended to be better invaders than species with smaller ones. Introduction effort, migratory strategy and mode of juvenile development were also significant in the models. Pair‐wise comparisons of closely related species indicate that successful invaders also showed a higher frequency of foraging innovations in their region of origin. This study provides the first evidence in vertebrates of a general set of traits, behavioural flexibility, that can potentially favour invasion success.
Predation‐mediated shifts in size distribution of microbial biomass and activity during detritus decomposition
Authors: Jürgens Klaus , Sala Maria Montserrat
Published: December 2004

Many experimental studies on detritus decomposition revealed a comparable microbial succession after the addition of a substrate pulse: from small, freely suspended single bacteria at the beginning, to more complex and larger growth forms during a later stage, accompanied by the appearance of bacterivorous protists. We examined in three model experiments with different organic carbon sources whether this shift in bacterial size structure is linked to the grazing impact of bacterivores. In short‐term (8–10 d) microcosm experiments we added natural dissolved and particulate detritus (macrophyte leaves and leachate, dead phytoplankton cells) as an organic substrate source. By the use of size‐fractionated inocula and eucaryotic inhibitors we obtained treatments without protists, in which bacteria developed without predation. These were compared, by measurements of bacterial activity and microscopical analysis of bacterial size structure, to incubations in which either cultured heterotrophic nanoflagellates or a natural protist assemblage was included in the inoculum. The presence of bacterial grazers resulted in a 50–90% reduction of bacterial biomass compared to grazer‐free trials. The selective removal of freely suspended bacteria produced a very different relative composition of bacterial biomass: it became dominated by large, grazing‐resistant forms such as filaments and cells attached to particles or clustered in small aggregates. In grazer‐free treatments, bacterial biomass was always dominated (>80%) by free‐living, single bacterial cells. The time course of the bacterial development suggested different underlying mechanisms for the appearance of predation resistant filamentous and of aggregated or attached bacteria. As bacterial aggregates developed in approximately similar amounts with and without grazers no specific growth stimulation by protists could be detected. In contrast, concentrations of filamentous bacteria were 2–10 times higher in treatments with protists, thus indicating a stimulation of this growth form during enhanced grazing pressure. Measurements of ectoenzymatic activity and
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H‐leucine uptake indicated that microbial activity was also shifted to larger size fractions. In most cases more than 50% of bacterial activity in treatments with protists was associated with the size fraction>10 μm whereas this value was <2% without grazers. Grazing by protists also enhanced the specific activity of the bacterial assemblage which is in contrast to an assumed lower competitive ability of complex bacterial growth forms. The results imply that the selective force of bacterivory in nutrient‐rich environments changes the structure and possibly the function of aquatic bacteria and their position in the food web, making protist‐resistant bacteria more vulnerable to metazoan filter feeders and detritivores, and possibly also subject to sedimentation.
Ecological significance of leaf life span among Central European grass species
Authors: Ryser Peter , Urbas Pille
Published: December 2004

Interspecific variation in leaf life span reflects the variation in nutrient conservation ability among different plant species and is considered to be associated with nutrient availability in the characteristic habitat. As defoliation interferes with nutrient conservation by the long‐lived leaves, we hypothesized that disturbance rate is another important environmental factor working as a selective force on interspecific variation in leaf life span. In order to investigate this, we measured leaf life span of 32 grass species in mature garden‐grown individuals. Variation in leaf life span was compared to measured leaf traits, to available data on species occurrence along gradients of nutrient availability and disturbance, and to published relative growth rates of the species.  Leaf life span was associated positively with leaf tissue mass density and negatively with specific leaf area. Leaf life span correlated negatively with the disturbance rate in the characteristic habitat of a species, but not with nutrient availability. The latter relationship did not come about due to the long leaf life spans of species from nutrient‐rich habitats with a relatively low disturbance rate, and to some extent also due to the short leaf life spans of annual species from relatively nutrient‐poor sites. We conclude that although leaf longevity is an important means of reducing nutrient losses, this is a selective advantage only if the plant is not subjected to frequent defoliation. The frequently postulated association between leaf life span of a species and nutrient availability in its characteristic habitat may occur among species of habitats with positively correlated nutrient availability and disturbance rate. Leaf life span is negatively associated with seedling RGR, but there may be deviations in this relationship due to species with contrasting characteristics at seedling stage and at maturity.
Effects of demographic parameters on metapopulation size and persistence: an analytical stochastic model
Authors: Nachman Gösta
Published: December 2004

An analytical stochastic metapopulation model is developed. It describes how individuals will be distributed among patches as a function of density‐dependent birth, death and emigration rates, and the probability of successful dispersal. The model includes demographic stochasticity, but not catastrophes, environmental stochasticity or variation in patch size and suitability. All patches are equally likely to be colonized by migrants. The model predicts: (a) mean and variance of the number of individuals per patch; (b) probability distribution of individuals per patch; (c) mean number of individuals in transit; and (d) turn‐over rate and expected persistence time of a single patch. The model shows that (a) dispersal rates must be intermediate in order to ensure metapopulation persistence; (b) the mean number of individuals per patch is often well below the carrying capacity; (c) long transit times and/or high mortality during dispersal reduce the mean number of individuals per patch; (d) density‐dependent emigration responses will usually increase metapopulation size and persistence compared with density‐independent dispersal; (e) an increase in the per capita net growth rate can both increase and decrease metapopulation size and persistence depending on whether dispersal rates are high or low; (f) density‐independent birth, death, and emigration rates lead to a spatial pattern described by the negative binomial distribution.
Spatially aggregated parasitism on pea aphids, Acyrthosiphon pisum, caused by random foraging behavior of the parasitoid Aphidius ervi
Authors: Olson Anders C. , Ives Anthony R. , Gross Kevin
Published: December 2004

We investigated the role of the foraging behavior of the parasitoid wasp Aphidius ervi in producing nonrandom spatial patterns of parasitism among pea aphids, Acyrthosiphon pisum. We measured spatial variability in percent parasitism by determining the number of aphids and percent parasitism in 40 sampling plots (0.65‐m2 circles) located within a homogeneous alfalfa field. In one replicate of this experiment, mean parasitism of aphids was 18.7%, and percent parasitism showed density‐independent aggregation (i.e., greater than random variability in percent parasitism among sampling plots). In the other replicate, mean parasitism was 56.3%, and percent parasitism was not aggregated among plots. We used a combination of field observations of parasitoid foraging and mathematical models to explore these results. In particular, we asked whether the presence or absence of density‐independent aggregation at different mean percent parasitism can be explained even if parasitoids forage randomly, without changing their behavior in response to encounters with aphids. Observations show that parasitoids tend to move short distances between nearby alfalfa stems (mean=10.8 cm), and the turning angle between successively visited stems was uniformly distributed. We incorporated this behavior into both simulation and analytical models of parasitoid foraging. The models show the same pattern as that observed in the field: parasitism is aggregated in a density‐independent fashion when mean percent parasitism is low but not when mean percent parasitism is high. Therefore, density‐independent aggregation in percent parasitism does not necessarily imply behavioral responses of parasitoids to host encounters and previously parasitized hosts.
C‐S‐R triangle theory: community‐level predictions, tests, evaluation of criticisms, and relation to other theories
Authors: Wilson J. Bastow , Lee William G.
Published: December 2004

Grime's C‐S‐R triangle theory has been discussed in plant ecology for two decades, but it has rarely been tested, and not often dispassionately evaluated. We consider the theory from a community viewpoint, and attempt to develop and test predictions for plant communities. C‐S‐R assumes that in high‐disturbance (ruderal, R) patches or habitats, competition will be absent, or low in intensity. Testing this is problematic because of the difficulty of defining the intensity of competition, and we could find no rigorous evidence to support or refute the prediction. The theory also implies that in high‐disturbance habitats there will be no difference in species composition between ‘competition’ and ‘stress’ sites, but from available evidence this does not seem to be true. C‐S‐R assumes that in stressful (S) habitats, competition will be low. This assumption is difficult to define or test, because of the overall difference in plant growth rate between habitats. A prediction from the theory is that in stressful habitats autosuccession should occur, i.e. that the climax species should regenerate directly, with no specialist secondary pioneer (R) species. There is some evidence that autosuccession occurs under the most extreme stresses of various types. Previous criticisms of C‐S‐R are evaluated. Only a few are considered valid, mainly those that emphasise that C‐S‐R theory is a considerable simplification of reality. Previous tests of C‐S‐R theory appear to be inconclusive. C‐S‐R theory is basically a combination of r/K theory and Leaf Amortisation theory. We conclude that there is limited support for the C‐S‐R theory. Whether the gain in generality that the theory offers justifies the loss via simplification that it involves is still an open question. As formulated, it has limited utility as a predictive model in community ecology. Yet, it is currently the most comprehensive and coherent theory for community ecology.