bluuebunny/crossref_metadata_2025 huggingface dataset:
Enemy‐mediated apparent competition: empirical patterns and the evidence
Authors: Chaneton Enrique J. , Bonsall Michael B.
Published: December 2004

Apparent competition arises when two victim species negatively affect each other (−, −) by enhancing the equilibrium density or changing the foraging behaviour of a shared natural enemy. Shared enemies can also mediate non‐reciprocal (−, 0) indirect effects, i.e. indirect amensalism, whenever one prey species is not affected by the presence of alternative prey. We review 34 studies on terrestrial and freshwater systems to evaluate the extent to which apparent competition has been perceived as a reciprocal (−, −) or non‐reciprocal (−, 0) interaction. We found only three studies showing reciprocal effects between apparent competitors. Indirect amensalism was documented in 10 studies and could be inferred for 16 other cases (76% in total). The remaining five studies provided insufficient data to determine the form of indirect interaction. The apparent prevalence of non‐reciprocal enemy‐mediated interactions resembles that observed for resource‐based interspecific competition. Amensal indirect effects via shared predation may result from differences in population size, nutritional value, susceptibility to attack, or asynchronous dynamics of alternative prey, or the predator's feeding preferences. Moreover, experimental protocols may confound the actual form of apparent competition through short‐term observations, incomplete designs, or biased consideration of conspicuous interactions, leading to reciprocal effects being overlooked. We conclude that, at present, it is still difficult to determine the relative role of apparent competition vs indirect amensalism in natural food webs because most published studies have failed to document in full interactions via shared enemies.
Stability and coexistence in a lawn community: mathematical prediction of stability using a community matrix with parameters derived from competition experiments
Authors: Roxburgh Stephen H. , Wilson J. Bastow
Published: December 2004

Community matrix theory has been proposed as a means of predicting whether a particular set of species will form a stable mixture. However, the approach has rarely been used with data from real communities. Using plant competition experiments, we use community matrix theory to predict the stability and competitive structuring of a lawn community.  Seven species from the lawn, including the six most abundant, were grown in boxes, in conditions very similar to those on the lawn. They were grown alone (monocultures), and in all possible pairs.  The species formed a transitive hierarchy of competitive ability, with most pairs of species showing asymmetric competition. Relative competitive ability (competitive effect) was positively correlated with published estimates of the maximum relative growth rate (RGRmax) for the same species.  A seven‐species community matrix predicted the mixture of species to be unstable. Simulations revealed two topological features of this community matrix. First, the matrix was closer to the stability/instability boundary than predicted from a range of null (random) models, suggesting that the lawn may be close to stability. Second, the tendencies of the lawn species to compete asymmetrically, and to be arranged in competitive hierarchies, were found to be positively associated with stability, and hence may be contributing factors to the near‐stability seen in the matrix.  The limitations of using competition experiments for constructing community matrices are discussed.
Stability and coexistence in a lawn community: experimental assessment of the stability of the actual community
Authors: Roxburgh Stephen H. , Wilson J. Bastow
Published: December 2004

Calculation of a community matrix for a lawn, based on pairwise competition experiments between lawn species, had predicted that the community would be unstable, but that it was close to the stability/instability boundary.  To test for stability in the actual community, three types of ‘pulse’ perturbation were applied: shading for six weeks, mechanical perturbation (removal of vegetation) and a herbicide specific to grasses. Following shade and mechanical perturbation, the abundances of the species recovered towards that of the undisturbed community, although full recovery of the whole community was not realised. For the herbicide perturbation, recovery was still quite incomplete after 4.5 years, apparently because the compensatory growth of dicots made it difficult for the grasses to reinvade. Based on species presence/absence, the lawn fully recovered from all three perturbations.  The overall pattern of recovery of the lawn was consistent with the prediction of marginal instability derived from the community matrix analysis. However, care must be taken in the interpretation of these results, as a pre‐requisite for valid application of community matrix theory is that the community should be at equilibrium, but in the lawn this assumption was violated.  A transitive hierarchy of competitive dominance was identified in the pairwise competition experiments; however, this hierarchy was unrelated to the rankings of the species in field abundance in the undisturbed community.  The problems in applying community matrix theory to real communities are reviewed. It is concluded that whilst the theory may have biological relevance to at least simple communities, such as the lawn system used here, difficulties in its practical application, and more importantly, the restrictive assumptions on which the theory is based, will limit its relevance in most natural systems. These results call into question the generality of a large volume of theoretical studies based on these methods.
Rodents, plants, and precipitation: spatial and temporal dynamics of consumers and resources
Authors: Ernest S. K. Morgan , Brown James H. , Parmenter Robert R.
Published: December 2004

Resource/consumer dynamics are potentially mediated by both limiting resources and biotic interactions. We examined temporal correlations between precipitation, plant cover, and rodent density, with varying time lags using long‐term data from two sites in the Chihuahuan desert of North America: the Sevilleta Long‐term Ecological Research site (LTER), New Mexico, USA and a site near Portal, Arizona, USA. We also calculated the spatial correlations in precipitation, plant cover, and rodent dynamics among six sites, five at Sevilleta and one at Portal. At Sevilleta, all three variables were temporally correlated, with plant cover responding to precipitation during the same growing season and rodent populations lagging at least one season behind. At Portal, plant stem count was also correlated with precipitation during the same growing season, but there was no significant correlation between rodents and either precipitation or plant growth. Spatial correlations in plant cover and rodent populations between sites reflected the localized nature of summer rainfall, so that sites with highly correlated summer precipitation exhibited higher correlations in plant cover and rodent populations. In general, our results indicate that limiting resources influence consumer dynamics, but these dynamics also depend crucially on the biotic interactions in the system.
A re‐examination of the expected effects of disturbance on diversity
Authors: Mackey Robin L. , Currie David J.
Published: December 2004

Disturbance is often cited as one of the main factors determining patterns of species diversity. Several models have predicted qualitatively that species richness should be highest at intermediate intensities and/or frequencies of disturbances, but none indicate whether this effect should be strong (statistically accounting for much variability in diversity) or only subtle. Empirical evidence on the point is very mixed. This study examines Markov models of the dynamics of six real communities. We derive the predicted changes in species richness and evenness when these communities are subjected to quantified disturbance frequency and intensity gradients. We also use several different sampling intensities (i.e. numbers of individuals counted) to determine how this affects richness‐disturbance relationships. Our models predict that peaked responses of diversity to disturbance should be less common than monotonic ones. Species richness should vary, on average, by only 3% over gradients of no disturbance to complete disturbance. In the most extreme case, richness varied two‐fold over this gradient. Moreover, richness may increase monotonically, decrease monotonically, or be a peaked function of disturbance, interacting in a non‐intuitive fashion with both the sampling intensity and the community in question. These results are broadly consistent with a review of published richness‐disturbance relationships. Evenness varies somewhat more strongly along disturbance gradients, but the effect is still small. We conclude that extant models provide little reason to believe that disturbance should play more than a subtle role in determining patterns of diversity in nature, contrary to most contemporary literature.
On the correlation between allocation to defence and regrowth in plants
Authors: De Jong Tom J. , Van Der Meijden Ed
Published: December 2004

Because storage of resources to regrow after damage and investment in defence draw upon the same resource pool, it has been argued that they should show a negative correlation. We sketch a model for optimal allocation to defence and to storage for regrowth. In the model generalist herbivores exert a constant herbivore pressure against which the plant can defend itself. With discrete intervals, disturbance occurs by an external cause against which the plant cannot defend. This could be an abiotic disturbance or the outbreak of a specialist herbivore that is unaffected by the defence. If we compare genotypes or species, each adapted to its own habitat, then a positive correlation or no correlation between allocation to defence and to regrowth is to be expected. The parameter space in which plants should both defend and store resources for regrowth is limited. Especially under favourable growing conditions, plants should only allocate to growth. We discuss some experimental measures of the regrowth capacity of plants in the context of our model and argue that these should be used with caution.
Is there safety‐in‐numbers for prey?
Authors: Connell Sean D.
Published: December 2004

The abundance of prey affects the rate of predation, but little consensus exists on whether this enhances or reduces per capita mortality. Studies of aggregating prey in marine habitats generally emphasise that the probability of predation of any individual is the reciprocal of the number of prey within a school. A field experiment tested the alternative hypotheses that predation by predatory fish on schooling prey (1) increased with an increase in the number of prey per school and that this caused (2) survival to be lower in schools with more individuals. The number of prey (juvenile Acanthochromis
polyacanthus) per school was manipulated in replicate treatments with natural densities of large predatory fish (open plots) and treatments without large predatory fish (exclusion cages). Large predatory fish preyed on juveniles in a density‐dependent manner and this was the key source of density‐dependent mortality in plots open to all predators. There was some suggestion that small predatory fish also prey on juveniles in a density‐dependent manner, but this was weak and did not translate into density‐dependent mortality of juveniles. It would appear that aggregation of prey may be a successful strategy against predation from some predators, but not always every predator, or all predators in combination.
Prey vulnerability to a gape‐size limited predator: behavioural and morphological impacts on northern pike piscivory
Authors: Nilsson P. Anders , Brönmark Christer
Published: December 2004

The northern pike (Esox lucius) is a selective and important predator in lake ecosystems. Prey size in pike is limited by pike gape size, which is a linear function of pike body length. Here we show that the absolute gape‐size limit in pike is greater than previously considered, and that maximum ingestible prey size is limited by prey body depth. Further, we experimentally show that pike prefer shallow‐bodied roach before deeper‐bodied common bream, and small prey sizes within each prey species. Handling time in pike increases with prey body depth, and since common bream are deeper‐bodied than roach, handling time is longer for bream than for roach of the same length, but equal considering body depth. Prey handling time is suggested to be a major cost to the pike, since it increases the risk of losing the prey, as well as exposure to predation, kleptoparasitism and cannibalism. Consequently, prey vulnerability is determined by risk of predation and intraspecific interactions, and behavioural preferences in the pike, and not by pike gape‐size limits. The consequences for natural populations is evaluated by analysing size structures of predator and prey fish populations in a eutrophic lake.
Distribution and abundance of parasite nematodes: ecological specialisation, phylogenetic constraint or simply epidemiology?
Authors: Morand Serge , Guégan Jean‐François
Published: December 2004

We investigate the patterns of abundance‐spatial occupancy relationships of adult parasite nematodes in mammal host populations (828 populations of nematodes from 66 different species of terrestrial mammals). A positive relationship between mean parasite abundance and host occupancy, i.e. prevalence, is found which suggests that local abundance is linked to spatial distribution across species. Moreover, the frequency distribution of the parasite prevalence is bimodal, which is consistent with a core‐satellite species distribution. In addition, a strong positive relationship between the abundance (log‐transformed) and its variance (log‐transformed) is observed, the distribution of worm abundance being lognormally distributed when abundance values have been corrected for host body size.  Hanski et al. proposed three distinct hypotheses, which might account for the positive relationship between abundance and prevalence in free and associated organisms: 1) ecological specialisation, 2) sampling artefact, and 3) metapopulation dynamics. In addition, Gaston and co‐workers listed five additional hypotheses. Four solutions were not applicable to our parasitological data due to the lack of relevant information in most host‐parasite studies. The fifth hypothesis, i.e. the confounded effects exerted by common history on observed patterns of parasite distributions, was considered using a phylogeny‐based comparison method. Testing the four possible hypotheses, we obtained the following results: 1) the variation of parasite distribution across host species is not due to phylogenetic confounding effects; 2) the positive relationship between mean abundance and prevalence of nematodes may not result from an ecological specialisation, i.e. host specificity, of these parasites; 3) both a positive abundance‐prevalence relationship and a negative coefficient of variation of abundance‐prevalence relationship are likely to occur which corroborates the sampling model developed by Hanski et al. We argue that demographic explanations may be of particular importance to explain the patterns of bimodality of prevalence when testing Monte‐Carlo simulations using epidemiological modelling frameworks, and when considering empirical findings. We conclude that both the bimodal distribution of parasite prevalence and the mean‐variance power function simply result from demographic and stochastic patterns (highlighted by the sampling model), which present compelling evidence that nematode parasite species might adjust their spatial distribution and burden in mammal hosts for simple epidemiological reasons.
Tritrophic interactions in a soil community enhance decomposition rates
Authors: Hedlund Katarina , Öhrn Maria Sjögren
Published: December 2004

Microbivorous soil fauna can influence decomposition rates by regulating biomass and composition of the microbial community. The idea that predators at higher trophic levels regulate population densities of microbivorous fauna and thus indirectly increase microbial growth and activity has often been suggested but rarely examined in soil ecosystems. In this paper the effects of tritrophic interactions on decomposition processes in the soil are studied and expressed as soil respiration, hyphal lengths, cellulase and chitinase activities. The experiments were carried out in soil microcosms in a factorial design with three fungal species (Alternaria alternata, Fusarium oxysporum,
Trichoderma viride), the fungivorous collembolan Folsomia
fimetaria and the predatory mite Hypoaspis aculeifer. The
respiration rate was significantly higher with three trophic levels than
in those with two and lowest in those with only fungi present. This
indicates that a low level of grazing stimulates microbial respiration
more than a high level or no grazing at all. The effect was similar for
all three fungal species but most pronounced in microcosms with the
fungus A. alternata which was a preferred food source by the
collembolans. Hyphal lengths were in all cases but with T. viride
reduced in the presence of collembolans and predatory mites. T.
viride had a slightly higher chitinase activity than the other fungi but increased numbers of trophic levels did not affect the enzymatic activities of any of the fungi.
Regulation of the mutualism between yuccas and yucca moths: intrinsic and extrinsic factors affecting flower retention
Authors: Humphries Shelley A. , Addicott John F.
Published: December 2004

Yuccas initiate far more flowers than they can mature as fruit, thereby providing opportunities for them to mature flowers of the highest quality. Flower quality in yuccas has both intrinsic and extrinsic components. Intrinsic components relate to flower morphology and inflorescence architecture. Yucca moths (Tegeticula spp.,
Incurvariidae), the sole pollinators and primary seed predators of most
yuccas (Yucca spp., Agavaceae), mediate extrinsic components of
flower quality through their ovipositions in flowers, and the quantity
and quality of pollen that they transfer. In addition, intrinsic and
extrinsic components interact as a function of flowering phenology and
moth activity within inflorescences. 
We investigated selective abscission of flowers in Y. kanabensis
with respect to various combinations of intrinsic and extrinsic factors.
First, we considered the effect of high and low pollen loads delivered to
different subsets of flowers and in different presentation orders. In the
absence of moth ovipositions, Y. kanabensis is sensitive to the amount of pollen that moths deliver and tends to retain high pollen flowers, even when all flowers receive sufficient pollen for full fertilization. However, pollen delivery sequence and the position of flowers with an inflorescence modify this high pollen effect. We then considered the interplay between high and low pollen combined with moth ovipositions and found that the number of ovipositions dominated the pollen effect. Finally, we considered number of ovipositions in conjunction with different flowers in the blooming sequence while controlling pollen levels and found that the clear effect of ovipositions on flower fate can be tempered by where the flower is in the blooming sequence.  These results have implications for the regulation of the mutualism between yuccas and yucca moths, indicating that yuccas are capable of regulating costs, retaining flowers of relative high quality and selectively abscising the rest. Yucca sensitivity to several intrinsic and extrinsic factors allows the plant to respond flexibly to the pollination environment and several species of moths.
Explicit links among physical stress, habitat heterogeneity and biodiversity
Authors: Therriault Thomas W. , Kolasa Jurek
Published: December 2004

We tested the links among biodiversity, habitat heterogeneity and physical stress in a system of artificial rock pools on the north coast of Jamaica that mimic natural aquatic invertebrate communities. The experimental design consisted of three tiers of small plastic pools arranged at increasing distances from the shore. As a result of community development over six months (January to June 1997), we observed considerable differentiation of physical conditions among replicate habitats at the benign end of the physical gradient, with a concurrent increase in biodiversity (species richness per habitat unit). The most probable explanation for this observed gradient is self‐generated habitat heterogeneity that, in turn, promotes biodiversity, likely through species interactions. Using additional analyses, including randomization techniques, we excluded the effects of sample size and external factors as sources for the observed increase in biodiversity in the third tier (furthest from the sea). We interpret this result as evidence for the complex causal relationship among physical stress, habitat heterogeneity and biodiversity.
Patterns of species density and productivity at different spatial scales in herbaceous plant communities
Authors: Gross Katherine L. , Willig Michael R. , Gough Laura , Inouye Richard , Cox Stephen B.
Published: December 2004

A major challenge in evaluating patterns of species richness and productivity involves acquiring data to examine these relationships empirically across a range of ecologically significant spatial scales. In this paper, we use data from herb‐dominated plant communities at six Long‐Term Ecological Research (LTER) sites to examine how the relationship between plant species density and above‐ground net primary productivity (ANPP) differs when the spatial scale of analysis is changed. We quantified this relationship at different spatial scales in which we varied the focus and extent of analysis: (1) among fields within communities, (2) among fields within biomes or biogeographic regions, and (3) among communities within biomes or biogeographic regions. We used species density (D=number of species per m2) as our measure of diversity to have a comparable index across all sites and scales. Although we expected unimodal relationships at all spatial scales, we found that spatial scale influenced the form of the relationship. At the scale of fields within different grassland communities, we detected a significant relationship at only one site (Minnesota old‐fields), and it was negative linear. When we expanded the extent of analyses to biogeographic regions (grasslands or North America), we found significant unimodal relationships in both cases. However, when we combined data to examine patterns among community types within different biogeographic regions (grassland, alpine tundra, arctic tundra, or North America), we did not detect significant relationships between species density and ANPP for any region. The results of our analyses demonstrate that the spatial scale of analysis – how data are aggregated and patterns examined – can influence the form of the relationship between species density and productivity. It also demonstrates the need for data sets from a broad spectrum of sites sampled over a range of scales for examining challenging and controversial ecological hypotheses.
High divorce rates in Corsican blue tits: how to choose a better option in a harsh environment
Authors: Blondel Jacques , Perret Philippe , Galan Marie‐José
Published: December 2004

We investigate which hypothesis, the “better mate hypothesis” or the “better territory hypothesis” best explains the unusually high divorce rate (59%) in a population of blue tits (Parus caeruleus) living in a sclerophyllous habitat characterised by severe environmental constraints (trophic, parasitic, climatic) on the island of Corsica, France. Using data from the breeding seasons 1985–1998 and from a brood size experiment (1990–1993) we examined the causes of divorce and their consequences on breeding performance, mate assortment and territory choice. Breeding performance had no significant effect on whether birds re‐united or divorced in the next breeding season. Re‐uniting pairs did better than divorced females and the latter improved their breeding performance compared to prior to divorce, but this was mainly due to age and territory effects. There were no differences in male performance depending on whether they re‐united or divorced. The age combination of pairs did not differ between re‐uniting and divorcing pairs, but mate assortment changed after divorce with males re‐mating more often with older partners than females. Manipulation of brood size showed a trend for birds with enlarged broods to divorce more. Pairs responded significantly to territory quality by divorcing more often in poor than in good breeding sites. Both faithful pairs and male divorcees had shorter breeding dispersal distances than female divorcees. Divorce rates were determined by the large differences in quality among breeding sites. Males, whatever their status, usually retained their previous territory whereas divorced females moved significantly longer distances and improved their breeding site. Moving to a better territory after divorce benefits only females which appear to be the choosing sex in the decision to divorce. This study strongly supports the “habitat mediated hypothesis” and we suggest that the large observed intraspecific variation in the magnitude of divorce rates in many species of birds is mostly determined by habitat characteristics.
Specificity of induced resistance in wild radish: causes and consequences for two specialist and two generalist caterpillars
Authors: Agrawal Anurag A.
Published: December 2004

Inducible plant resistance against herbivores is becoming a paradigm of plant–herbivore ecology. Fundamental to understanding induced resistance and its evolutionary ecology is specificity of “induction” and “effects”. Specificity in the induction of resistance refers to whether plant damage by various herbivores causes the same response in plants. Specificity in the effects of induced resistance refers to whether induction has the same consequences (i.e., reduced preference or performance) for various herbivores. I examined both specificity of induction and effect employing four lepidopteran herbivores and wild radish plants, a system for which fitness benefits and costs of induction have been documented for the plant. Variation in the specificity of induction and effects of induced plant resistance was found; however, this variation was not associated with diet specialization in the herbivores (i.e., specialists vs generalists). Induction caused by Plutella (specialist) and
Spodoptera (generalist) resulted in general resistance to all of
the herbivores, induction caused by Pieris (specialist) induced
resistance only to Spodoptera (generalist) and Pieris, and
plant damage by Trichoplusia (generalist) failed to induce
resistance and reduce the performance of any of the herbivores. To the
contrary, plants damaged by Trichoplusia supported enhanced growth
of subsequently feeding Trichoplusia compared to uninduced controls. These results add a novel level of complexity to interactions between plants and leaf chewing caterpillars. Within the same guild of feeders, some herbivores cause strong induced resistance, no induced resistance, or induced susceptibility. Similarly, caterpillar species were variable in the level to which induced resistance affected their performance. Such interactions limit the possibility of pairwise coevolution between plants and herbivores, and suggest that coevolution can only be diffuse.
Vegetation modification and resource competition in grazing ungulates
Authors: Murray Martyn G. , Illius Andrew W.
Published: December 2004

The prevalence of interspecific competition in animal communities is the subject of a long‐running debate, chiefly because the underlying processes of resource exploitation and resource supply are often poorly understood. To provide some insight into these processes within a guild of grazing herbivores, two hypothetical mechanisms of exploitation competition were tested by measuring food intake of topi (Damaliscus
lunatus) and wildebeest (Connochaetes taurinus) when foraging on different sward structures in the Serengeti National Park. According to our bite quantity hypothesis, wildebeest, which have relatively wide mouths, can graze down vegetative swards to a height below that which can be tolerated by topi; and according to our bite quality hypothesis, the narrower‐mouthed topi can reduce the leafy component of differentiated swards (i.e. swards in which seed‐bearing stems have developed) through selective feeding to a level below that which can be tolerated by wildebeest.  On differentiated swards with erect growth form, the topi selected 20% more green leaf in their diet, as measured by a calibrated visual technique, and also obtained higher short‐term intake rates. Greater selectivity alone provided topi with a metabolisable energy intake estimated to be 16% higher than that of wildebeest. On vegetative swards, it was estimated that wildebeest could maintain positive energy balance on 2‐cm swards, 1 cm shorter than the threshold height for topi. Our findings indicate the conditions under which each ungulate species may limit the other's use of natural pastures through interspecific competition: bite quantity competition may apply on short grazing lawns; bite quality competition is expected on differentiated swards with a limited supply of green leaf.  We suggest that herbivory by one species can modify the vegetation in a way that makes it less profitable to competing species. In effect the vegetation is ‘captured’ as a resource by one species. Thus modification of vegetation is argued to be a critical component of resource competition in herbivores.
Assessing top‐down and bottom‐up control in a litter‐based soil macroinvertebrate food chain
Authors: Ponsard Sergine , Arditi Roger , Jost Christian
Published: December 2004

The relative importance of top‐down and bottom‐up control in setting the equilibrium abundances within trophic levels is examined in a comparative study on the litter‐based food chain of a temperate deciduous forest. During two consecutive years, we estimated the abundances of macroinvertebrate detritivores and their predators on a natural gradient of annual litterfall. Detritus‐based food chains are thought to be classical examples of donor‐controlled systems. Indeed, both trophic levels showed higher abundances on sites with higher annual litterfall. Therefore, they appear to be bottom‐up controlled. Using the Errors‐in‐Variables regression technique, we quantitatively compared our data with the equilibrium predictions of a set of simple trophic chain models including bottom‐up effects with different types of functional responses (Beddington‐DeAngelis, Hassell‐Varley, and ratio‐dependent). The model with a Hassell‐Varley type functional response yielded the best adjustment to the data, although with a very high value of the mutual interference parameter suggesting the existence of overcompensating density dependence. Several changes to the structure of this model were considered. Their adjustment to the data consistently yielded such high values of the interference parameter.
Seed size and shape and persistence in the soil in the New Zealand flora
Authors: Moles Angela T. , Hodson Dave W. , Webb Colin J.
Published: December 2004

Seed size and shape predict seed persistence in the soil for British and Argentinian herbaceous plant species. Those species with small, rounded seeds tend to have persistent seeds while those with larger, more elongate or flattened seeds usually lack persistence. It has been suggested that the mechanism underlying this pattern may be ease of burial, as small, rounded seeds are incorporated into the soil more easily than large, elongate or flattened seeds and are therefore less likely to be eaten by seed predators. We tested whether seed size and shape were related to persistence in the soil for 47 species native to New Zealand forests. There was a tendency for species with persistent seeds to have smaller seeds than species with transient seeds. However, species with large and/or elongate or flattened persistent seeds were relatively common. This indicates that seed size and shape are not related to persistence in New Zealand in the same way as in Britain and Argentina. A similar negative result has been found in Australia. The underlying cause of the patterns observed is unlikely to be ease of burial, since incorporation of seeds into the soil is likely to operate in all countries in a similar manner on seeds without specialised seed burial mechanisms. Data from all four floras studied to date also suggest that species with small, rounded seeds that do not germinate immediately must have the ability to survive periods of burial.
Mechanisms of interspecific competition among an invasive and two native fire ants
Authors: Morrison Lloyd W.
Published: December 2004

The mechanisms of interspecific competition among an invasive and two native Solenopsis fire ant forms were investigated in a series of
laboratory experiments. In separate trials each with a different food
resource, the native S. geminata×xyloni retrieved the
greatest amount of a protein‐ and lipid‐rich artificial food resource and
a high protein natural food resource, and the native S. geminata
retrieved the greatest amount of a high carbohydrate food resource. In
trials investigating aspects of interference competition at the colony
level, the invasive S. invicta proved to be initially more
aggressive than S. geminata, but less aggressive than S.
geminata×xyloni. Solenopsis invicta eventually
controlled more of the foraging arenas against both native forms when
colonies were equivalent by worker biomass, but not when colonies were
equivalent by worker number. When paired with S. invicta, S.
geminata suffered a significantly greater proportional reduction in
both workers and entire colonies when colonies were initially
standardized by worker biomass, but not when colonies were standardized
by worker number. When paired with S. invicta, a significantly
greater proportional reduction of workers occurred in S.
geminata×xyloni, regardless of how colonies were
standardized. In pairwise trials at the individual level, majors always
exhibited significantly less mortality than minors, regardless of the
Solenopsis form. The majors of both native forms suffered
significantly less mortality than those of S. invicta. Superiority
in colony‐level interference ability appears to be an important mechanism
allowing S. invicta to displace native Solenopsis forms.
The ability of S. invicta to reach high population densities, because of intrinsic biological characteristics or an escape from natural enemies, plays an important contributory role. Similar mechanisms may underlie the success of other invasive ant species.
Sex‐related differences in reaction norms in the butterfly Lycaena tityrus (Lepidoptera: Lycaenidae)
Authors: Fischer Klaus , Fiedler Konrad
Published: December 2004

We investigate sexual differences in reaction norms in directly developing individuals of the copper butterfly Lycaena tityrus predicted from sexual selection theory. As recent studies on butterflies revealed a high degree of adaptive plasticity in growth and development, which may undermine the basic trade‐offs assumed in life‐history theory, we focus on effects of temperature, trying to drive growth rates to their physiological upper limit and thus disclosing otherwise potentially concealed responses. Development time strongly depended on temperature, leading in accordance with a central assumption in life‐history theory to a larger size at low temperatures, and vice versa. At all temperatures larval development time of males was significantly shorter compared to females, as was predicted by protandry theory. This was partially due to an invariably higher growth rate of males. However, sexes responded in different ways to developmental time constraints caused by increasing temperatures. Despite the shorter larval time of males, both sexes achieved similar body sizes at lower temperatures, because males avoided a reduction in weight due to plastic growth. At high temperatures, in contrast, males were forced to make a trade‐off in which they favoured early emergence over large size, leading to a dramatic weight loss. Weight of females, however, remained similar throughout showing no trade‐off. These different reaction norms reflect divergent selective pressures acting on males and females, which can be explained in relation to the reproductive system. The strong selection for early emergence in males is likely to be due to monandry, discrete non‐overlapping generations (as was already predicted by theory), and territoriality, because prior ownership of a territory seems to be a major advantage for successful reproduction. On the other hand, the preference of females for large body size was expected due to the close relationship between this trait and fecundity. Thus, our results highlight the extraordinary importance of the specific reproductive system, which can influence central life‐history traits in manifold ways.
Ecosystem functioning and intrinsic value of biodiversity
Authors: Ghilarov Alexei M.
Published: December 2004

Trying to show the importance of biodiversity for ecosystem functioning, ecologists are repeatedly looking for a possible connection between species diversity and intensity of various ecosystem processes. However it appears that simple “proof” of such a connection cannot easily be demonstrated and involves a lot of contingencies. The different meanings of “ecosystem functioning” may be crucial for attempts to show the importance of biodiversity and to assess possible redundancy. If “ecosystem functioning” means only total production of organic matter or consumption of CO2 then some degree of redundancy in species diversity of autotrophs will be obvious in most cases. However if “ecosystem functioning” includes the synthesis of all compounds that plants, animals and other organisms of a given community contain in their bodies or release in the environment, then any decrease in species diversity will be meaningful and any redundancy will be impossible by definition. In accordance with such a definition biodiversity obviously cannot be diminished without some loss of ecosystem functioning. I emphasize that attempts to conserve biodiversity do not need special justification in possible relationships between diversity and ecosystem services. If biodiversity has intrinsic value it means that it could in principle be “useless” for human needs or for “ecosystem functioning”.
Genetic differentiation of life‐history traits within populations of the clonal plant Ranunculus reptans
Authors: Prati Daniel , Schmid Bernhard
Published: December 2004

We tested the hypothesis that environmental heterogeneity can maintain genetic variation in life‐history traits within populations of the clonal plant Ranunculus reptans. This may be important for the further
evolution and potential adaptation to environmental change. Moreover, we
asked to what extent environmental heterogeneity can also reveal
trade‐offs among fitness components, in particular between sexual
reproduction and clonal growth. In the natural habitat of R.
reptans two distinct types of environment can often be found within
10 m distance: vegetation‐sparse zones close to the water
(“lake”) and zones of competition with grasses (mostly
Agrostis stolonifera) more inland (“land”). We grew
vegetative offspring of 16 land genotypes and 16 lake genotypes from four
populations in a plant room. Cuttings from each genotype were grown with
and without competition by the grass A. stolonifera and on two
different substrates, sand and gravel. 
We found considerable environmental (E) and genetic (G) variation in
life‐history traits. Competition strongly reduced growth, branching,
flowering, and final biomass. Lake genotypes invested more into sexual
reproduction (measured as proportion of ramets that flowered), whereas
land genotypes invested more into vegetative reproduction (proportion of
rooted ramets). Significant G×E interactions were consistent with a
home‐away effect: land‐genotypes performed better if grown under
competition, lake‐genotypes without competition. These results indicate
that genetic variation and phenotypic plasticity in life‐history traits
of R. reptans are maintained in a heterogeneous environment,
thereby supporting the main hypothesis stated above. They further
demonstrate that genetic differentiation occurs even over very short
distances (10 m). We also found a significant genetic trade‐off between
sexual and vegetative reproduction (negative genetic correlation between
the proportion of flowering ramets and the proportion of rooted ramets).
In response to the supplementary question posed above, this suggests that
the environmental heterogeneity finds its correspondence in the
“genetic integration” of life‐history traits within the
phenotype of R. reptans.
Life history plasticity and population regulation in sea otters
Authors: Monson Daniel H. , Estes James A. , Bodkin James L. , Siniff Donald B.
Published: December 2004

We contrasted body condition, and age‐specific reproduction and mortality between a growing population of sea otters (Enhydralutris) at Kodiak Island and a high‐density near‐equilibrium
population at Amchitka Island, Alaska. We obtained data from marked
individuals, population surveys, and collections of beach‐cast carcasses.
Mass:length ratios indicated that females (but not males) captured in
1992 at Amchitka were in poorer condition than those captured at Kodiak
in 1986–1987. In 1993, the condition of females at Amchitka
improved in apparent response to two factors: (1) an episodic influx of
Pacific smooth lumpsuckers, Aptocyclus ventricocus, from the
epi‐pelagic zone, which otters consumed; and (2) an increase in the
otters’ benthic invertebrate prey resulting from declining otter
numbers. Reproductive rates varied with age (0.37 [CI=0.21 to 0.53]
births female−1 yr−1 for 2–3‐yr‐olds, and 0.83 [CI=0.69 to 0.90] for females ≥4 yr old), and were similar at both areas. Weaning success (pups surviving to ≥120 d), in contrast, was almost 50% lower at Amchitka than at Kodiak and for females ≥4 yr of age was 0.52 (CI=0.38 to 0.66) vs 0.94 (CI=0.75 to 0.99), respectively. Sixty‐two percent of the preweaning pup losses at Amchitka occurred within a month of parturition and 79% within two months. Postweaning survival was also low at Amchitka as only 18% of instrumented pups were known to be alive one year after mother‐pup separation. Adult survival rates appeared similar at Amchitka and Kodiak. Factors affecting survival early in life thus are a primary demographic mechanism of population regulation in sea otters. By maintaining uniformly high reproductive rates over time and limiting investment in any particular reproductive event, sea otters can take advantage of unpredictable environmental changes favorable to pup survival. This strategy is consistent with predictions of “bet‐hedging” life history models.
Biodiversity and ecosystem functioning: recent theoretical advances
Authors: Loreau Michel
Published: December 2004

The relationship between biodiversity and ecosystem functioning has emerged as a major scientific issue today. As experiments progress, there is a growing need for adequate theories and models to provide robust interpretations and generalisations of experimental results, and to formulate new hypotheses. This paper provides an overview of recent theoretical advances that have been made on the two major questions in this area: (1) How does biodiversity affect the magnitude of ecosystem processes (short‐term effects of biodiversity)? (2) How does biodiversity contribute to the stability and maintenance of ecosystem processes in the face of perturbations (long‐term effects of biodiversity)?  Positive short‐term effects of species diversity on ecosystem processes, such as primary productivity and nutrient retention, have been explained by two major types of mechanisms: (1) functional niche complementarity (the complementarity effect), and (2) selection of extreme trait values (the selection effect). In both cases, biodiversity provides a range of phenotypic trait variation. In the complementarity effect, trait variation then forms the basis for a permanent association of species that enhances collective performance. In the selection effect, trait variation comes into play only as an initial condition, and a selective process then promotes dominance by species with extreme trait values. Major differences between within‐site effects of biodiversity and across‐site productivity–diversity patterns have also been clarified. The local effects of diversity on ecosystem processes are expected to be masked by the effects of varying environmental parameters in across‐site comparisons.  A major reappraisal of the paradigm that has dominated during the last decades seems necessary if we are to account for long‐term effects of biodiversity on ecosystem functioning. The classical deterministic, equilibrium approaches to stability do not explain the reduced temporal variability of aggregate ecosystem properties that has been observed in more diverse systems. On the other hand, stochastic, nonequilibrium approaches do show two types of biodiversity effects on ecosystem productivity in a fluctuating environment: (1) a buffering effect, i.e., a reduction in the temporal variance; and (2) a performance‐enhancing effect, i.e., an increase in the temporal mean. The basic mechanisms involved in these long‐term insurance effects are very similar to those that operate in short‐term biodiversity effects: temporal niche complementarity, and selection of extreme trait values. The ability of species diversity to provide an insurance against environmental fluctuations and a reservoir of variation allowing adaptation to changing conditions may be critical in a long‐term perspective.  These recent theoretical developments in the area of biodiversity and ecosystem functioning suggest that linking community and ecosystem ecology is a fruitful avenue, which paves the way for a new ecological synthesis.