bluuebunny/crossref_metadata_2025 huggingface dataset:
Integration of ecosystem engineering and trophic effects of herbivores
Authors: Wilby Andrew , Shachak Moshe , Boeken Bertrand
Published: December 2004

Herbivores affect vegetation in a variety of ways, involving both trophic and ecosystem engineering interactions, but the study of these different interaction types has rarely been integrated. The aim of this study was to investigate both the trophic and engineering effects of herbivores on plant communities in the Negev desert, Israel, and to promote an integrative approach to the study of herbivore effects in ecosystems. First, we summarise previous studies of the Indian crested porcupine (Hystrix indica), which show that in digging for food, porcupines excavate soil pits, which accumulate resources and seeds resulting in marked changes in plant species richness, density and biomass. By contrast, their trophic effect, via consumption of bulbs, has little impact on populations of perennial plants. Second, we present an empirical study of the trophic and ecosystem engineering effects of harvester ants (Messor spp.). An exclusion experiment, using barriers to restrict ant access, failed to reveal any significant effect of seed collection by harvester ants on plant species incidence (proportional occurrence in samples) or abundance (number of individuals). However, we show that vegetation on nest mounds of M. ebeninus differs in plant density, species richness and biomass from that on undisturbed soil. An analysis of incidence and abundance responses of individual plant species suggests that the observed differences in vegetation resulted from multiple interacting mechanisms.  The case studies highlight that many interactions between herbivores and plant communities can occur simultaneously, and that ecosystem engineering and trophic processes can be closely associated, resulting from single actions of herbivores. We propose a conceptual framework that classifies the range of possible trophic and engineering interactions between herbivores and plant communities with respect to the level of association between trophic and engineering effects. The framework is presented as an aid to the design and interpretation of studies of interactions between herbivores and plant communities, and promotes integrative research into the roles of herbivores in ecosystems.
A simple landscape‐scale test of a spatially explicit population model: patch occupancy in fragmented south‐eastern Australian forests
Authors: Lindenmayer David B. , McCarthy Michael A. , Possingham Hugh P. , Legge Sarah
Published: December 2004

The results of a landscape‐scale test of ALEX, a widely used metapopulation model for Population Viability Analysis (PVA), are described. ALEX was used to predict patch occupancy by the laughing kookkaburra and the sacred kingfisher in patches of eucalypt forest in south‐eastern Australia. These predictions were compared to field surveys to determine the accuracy of the model. Predictions also were compared to a “naïve” null model assuming no fragmentation effects.  The naïve null model significantly over‐predicted the number of eucalypt patches occupied by the sacred kingfisher, but the observed patch occupancy was not significantly different from that predicted using ALEX. ALEX produced a better fit to the field data than the naïve null model for the number of patches occupied by the laughing kookaburra. Nevertheless, ALEX still significantly over‐predicted the number of occupied patches, particularly remnants dominated by certain forest types – ribbon gum and narrow‐leaved peppermint. The predictions remained significantly different from observations, even when the habitat quality of these patches was reduced to zero. Changing the rate of dispersal improved overall predicted patch occupancy, but occupancy rates for the different forest types remained significantly different from the field observations. The lack of congruence between field data and model predictions could have arisen because the laughing kookaburra may move between an array of patches to access spatially separated food and nesting resources in response to fragmentation. Alternatively, inter‐specific competition may be heightened in a fragmented habitat. These types of responses to fragmentation are not incorporated as part of traditionally applied metapopulation models. Assessments of predictions from PVA models are rare but important because they can reveal the types of species for which forecasts are accurate and those for which they are not. This can assist the collection of additional empirical data to identify important factors affecting population dynamics.
Metapopulation dynamics of the bog fritillary butterfly: movements between habitat patches
Authors: Petit Sandrine , Moilanen Atte , Hanski Ilkka , Baguette Michel
Published: December 2004

Recent studies on metapopulation dynamics have emphasized the need for improved methods for quantifying individual movements between local populations and habitat patches. In this paper, we report on a 6‐yr study in which a network of 12 habitat patches occupied by the bog fritillary, Proclossiana eunomia, was surveyed, with special focus on quantifying movements between the habitat patches. We applied the Virtual Migration model which has been designed to estimate survival and migration parameters in a metapopulation of several connected local populations. The model was parameterized using mark‐release‐recapture data collected during 6 yr. Generally, the estimated parameter values indicated a high level of movements, with roughly half of butterfly‐days spent outside the natal patch. Mortality within patches was higher in males than in females. Females tended to be more mobile and spent more time outside their natal patch than males. Further analysis of the MRR data shows that in this protandrous species males tend to move very little between habitat patches before substantial numbers of females have emerged.
Competition for mutualists in an ant–homopteran interaction mediated by hierarchies of ant attendance
Authors: Fischer Melanie K. , Hoffmann Klaus H. , Völkl Wolfgang
Published: December 2004

Workers of the ant species Lasius niger showed marked preferences when collecting honeydew from three aphid species living on tansy, Tanacetum vulgare. Colonies of Metopeurum fuscoviride were visited most intensively, Brachycaudus cardui had an intermediate position, and colonies of Aphis fabae were visited least intensively. L. niger abandoned colonies of the less preferred species as soon as colonies of a higher‐ranked species were available on neighbouring tansy plants. As a consequence, the abandoned colonies suffered a higher predation by aphid predators and persisted for a shorter time than attended colonies of the same species. If two aphid species were present in a mixed colony on the same shoot, the preferred species increased in numbers, while the less preferred species decreased due to predation by L. niger. The results show that there is a competition for the mutualistic services of ants between aphid species exploiting the same resource. The outcome of this competition depends on the aphid's selective attractiveness to ants, which corresponded with already reported differences in honeydew quality and quantity.
Flexible oviposition strategies in túngara frogs and their implications for tadpole spatial distributions
Authors: Marsh David M. , Borrell Brendan J.
Published: December 2004

Because oviposition site selection is often studied in a single ecological context, little is known about flexibility in oviposition strategies. We studied the oviposition site selection strategies of túngara frogs (Physalaemus pustulosus) with respect to conspecific eggs and larvae in two different ecological contexts‐arrays of artificial ponds and natural stream pools. We fit maximum likelihood models to sequential pond use data for both systems and found that strategies for oviposition differed greatly between the two systems. Within arrays of artificial ponds, túngara frogs avoided both conspecific egg masses and conspecific larvae. However, within a set of natural stream pools, túngara frogs consistently oviposited in the same pools from night to night, while many nearby ponds went unused. These variable strategies produced very different distributions of tadpoles among ponds, such that tadpoles tended to be evenly spread among artificial ponds, but highly clumped within stream pools. The difference between the two strategies appears to be an overriding effect of habitat quality. Surface area and the amount of leaf litter were all significant predictors of whether stream pools were used. Additionally, an experiment in which egg masses were either transplanted to, or removed from, selected stream pools provided no evidence that frogs in these pools used egg masses per se as a cue for oviposition. These results illustrate that oviposition strategies may be very flexible, allowing species to adjust strategies to different ecological contexts. Furthermore, they demonstrate that variation in oviposition strategies may have strong effects on egg and larval spatial distributions in patchy environments.
Environmentally constrained null models: site suitability as occupancy criterion
Authors: Peres‐Neto Pedro R. , Olden Julian D. , Jackson Donald A.
Published: December 2004

Null models have proven to be an important quantitative tool in the search for ecological processes driving local diversity and species distribution. However, there remains an important concern that different processes, such as environmental conditions and biotic interactions may produce similar patterns in species distributions. In this paper we present an analytical protocol for incorporating habitat suitability as an occupancy criterion in null models. Our approach involves modeling species presence or absence as a function of environmental conditions, and using the estimated site‐specific probabilities of occurrence as the likelihood of species occupancy of a site during the generation of “null communities”. We validated this approach by showing that type I error is not affected by the use of probabilities as a site occupancy criterion and is robust against a variety of predictive performances of the species‐environmental models. We describe the expected differences when contrasting classical and the environmentally constrained null models, and illustrate our approach with a data set of Dutch dune hunting spider assemblages. Together, an environmentally constrained approach to null models will provide a more robust evaluation of species associations by facilitating the distinction between mutually exclusive processes that may shape species distributions and community assembly.
The method of synthesis in ecology
Authors: Ford E. David , Ishii Hiroaki
Published: December 2004

Synthesis of results from different investigations is an important activity for ecologists but when compared with analysis the method of synthesis has received little attention. Ecologists usually proceed intuitively and this can lead to a problem in defining differences between the syntheses made by different scientists. It also leads to criticism from scientists favoring analytical approaches that the construction of general theory is an activity that does not follow the scientific method.  We outline a methodology for scientific inference about integrative concepts and the syntheses made in constructing them and illustrate how this can be applied in the development of general theory from investigations into particular ecological systems. The objective is to construct a causal scientific explanation. This has four characteristics. (1) It defines causal and/or organizational processes that describe how systems function. (2) These processes are consistent – under the same conditions they will produce the same effect. (3) A causal scientific explanation provides general information about events of a similar kind. (4) When experiments are possible then a designed manipulation will produce a predictable response.  The essential characteristic of making synthesis to construct a causal scientific explanation is that it is progressive and we judge progress made by assessing the coherence of the explanation using six criteria: acceptability of individual propositions including that they have been tested with data, consistency of concept definitions, consistency in the type of concepts used in making the explanation, that ad hoc propositions are not used, that there is economy in the number of propositions used, that the explanation applies to broad questions.  We illustrate development of a causal scientific explanation for the concept of long‐lived pioneer tree species, show how the coherence of this explanation can be assessed, and how it could be improved.
Relative roles of top‐down and bottom‐up forces in terrestrial tritrophic plant–insect herbivore–natural enemy systems
Authors: Walker Matthew , Jones T. Hefin
Published: December 2004

Whether resources (bottom‐up forces), natural enemies (top‐down forces), or both, determine the abundance of insect herbivore populations in plant–insect herbivore–natural enemy systems remains a major issue in population ecology. Unlike recent surveys of the tritrophic literature we do not seek to quantify whether top‐down or bottom‐up forces predominate in any given set of experimental systems. Acknowledging the dearth of empirical synthesis we employ two contrasting literature surveys to determine whether the plant–insect herbivore–natural enemy literature is currently adequate to form a conceptual synthesis of the relative roles of top‐down and bottom‐up forces.  The emergence of a synthesis of the relative roles of top‐down and bottom‐up forces in plant–insect herbivore–natural enemy systems appears to have been largely prevented by (1) the absence of appropriate empirical data; (2) failure to appreciate the merits of existing data; (3) a continued desire to emphasise either top‐down or bottom‐up forces to the exclusion of the other; and (4) confusion regarding which processes regulate and which influence the abundance of insect herbivores.
Changes in transmission of Baylisascarisprocyonis to intermediate hosts as a function of spatial scale
Authors: Page L. Kristen , Swihart Robert K. , Kazacos Kevin R.
Published: December 2004

Physical changes in landscapes alter the abundance and distribution of species. Higher‐order effects can occur when changes in ecological processes result in altered interspecific interactions and subsequent changes in a species’ abundance or persistence. Baylisascarisprocyonis, a roundworm parasite of raccoons (Procyonlotor), is pathogenic to numerous small vertebrates that serve as intermediate hosts, including white‐footed mice (Peromyscusleucopus). Raccoons have responded positively to agriculturally induced changes in landscapes, with potential consequences for intermediate hosts of B. procyonis. We examined white‐footed mice from a homogeneous, predominantly forested landscape in south‐central Indiana and a heterogeneous, predominantly agricultural landscape in northwestern Indiana for presence of larval B. procyonis. We compared prevalence of infection, intensity of infection, and average number of larvae per mouse between the landscapes, and among forest patches within the highly fragmented agricultural landscape. Prevalence, intensity of infection, and average number of larvae were significantly higher in the highly fragmented landscape. Within the agricultural landscape, regression models predicted probability of infection, intensity of infection, and average number of larvae per mouse per patch as functions of forest patch area and isolation. We conclude that positive responses of raccoons to agriculturally induced fragmentation of forests have resulted in increased encounter rates between white‐footed mice and infective stages of B. procyonis between and within landscapes, resulting in increased transmission of the parasite to intermediate hosts.
Root traits associated with nutrient exploitation following defoliation in three coexisting perennial grasses in a semi‐arid savanna
Authors: Busso C. A. , Briske D. D. , Olalde‐Portugal V.
Published: December 2004

Experiments were conducted to evaluate root traits associated with nutrient exploitation following defoliation in three coexisting perennial grasses in a semi‐arid savanna. Root length density was determined within soil cores directly beneath plants, nitrogen uptake was evaluated by excised‐root assay with (15NH4)2SO4, and mycorrhizal root colonization was estimated by observation of root segments. Root length density was lowest for Bouteloua curtipendula, intermediate for Eriochloa sericea, and highest for Aristida purpurea indicating that root length density was a more important trait for the mid‐seral than the late‐seral species. Rates of 15N uptake were greatest in the least grazing tolerant late‐seral species, E. sericea, intermediate in the mid‐seral species, A. purpurea, and lowest in the most grazing tolerant late‐seral species, B. curtipendula. Two successive defoliations reduced 15N uptake 60% in the late‐seral species with the greatest uptake rate (E. sericea), but not in species with lowest uptake rates (B. curtipendula). Root length colonization was consistently high (33–61%) in all three species suggesting that these C4 perennial grasses may function as obligate mycotrophs. Contrasting responses among the two late‐seral species indicate that the least grazing tolerant species, E. sericea, appears best adapted for nutrient exploitation while the most grazing tolerant species, B. curtipendula, appears best adapted for efficient nutrient retention. Contrasting responses of nitrogen uptake to short‐term defoliation parallel the population responses of these two coexisting late‐seral species to long‐term herbivory. These data indicate that herbivory may shift interspecific competitive interactions by mediating nutrient exploitation and that a trade‐off may exist between nutrient exploitation and herbivory tolerance in these species.
Context dependent effects of ectomycorrhizal species richness on tree seedling productivity
Authors: Jonsson Lena M. , Nilsson Marie‐Charlotte , Wardle David A. , Zackrisson Olle
Published: December 2004

While there has been much recent interest about the relationships between plant diversity and plant productivity, much remains unknown about how the diversity of mycorrhizal fungi affects plant productivity. We investigated the effects of ectomycorrhizal fungal community composition and diversity on the productivity and growth characteristics of seedlings of two tree species (Pinus sylvetris and Betula pendula) as well as their interactions with each other. This involved setting up a mycorrhizal fungal diversity gradient from one to eight species using a design previously demonstrated to be able to separate diversity effects from compositional effects. We found that the eight mycorrhizal fungal species differed in their effects on seedling productivity and that the nature of effects was determined by the fertility of the substrate. Fungal species richness effects were also important in affecting seedling productivity over and above what could be explained by “sampling effect” but only in some situations. For B. pendula in a low fertility substrate there were clear positive causative effects between fungal species richness and productivity with the eight species treatment having over double the productivity of any of the eight monoculture treatments; no diversity effects were, however, detected in a high fertility substrate. For P. sylvestris in a high fertility substrate there were significant negative effects of fungal diversity on productivity while in a low fertility substrate no effects were apparent. The possible mechanistic bases for these results are discussed. The growth of P. sylvestris relative to that of B. pendula when grown in combination was unaffected by mycorrhizal treatments. Our results provide clear evidence that effects of mycorrhizal fungal diversity on productivity are context dependent and may be positive, negative or neutral depending on the situation considered.
Partial life cycle analysis: a model for pre‐breeding census data
Authors: Oli Madan K. , Zinner Bertram
Published: December 2004

Matrix population models have become popular tools in research areas as diverse as population dynamics, life history theory, wildlife management, and conservation biology. Two classes of matrix models are commonly used for demographic analysis of age‐structured populations: age‐structured (Leslie) matrix models, which require age‐specific demographic data, and partial life cycle models, which can be parameterized with partial demographic data. Partial life cycle models are easier to parameterize because data needed to estimate parameters for these models are collected much more easily than those needed to estimate age‐specific demographic parameters. Partial life cycle models also allow evaluation of the sensitivity of population growth rate to changes in ages at first and last reproduction, which cannot be done with age‐structured models. Timing of censuses relative to the birth‐pulse is an important consideration in discrete‐time population models but most existing partial life cycle models do not address this issue, nor do they allow fractional values of variables such as ages at first and last reproduction. Here, we fully develop a partial life cycle model appropriate for situations in which demographic data are collected immediately before the birth‐pulse (pre‐breeding census). Our pre‐breeding census partial life cycle model can be fully parameterized with five variables (age at maturity, age at last reproduction, juvenile survival rate, adult survival rate, and fertility), and it has some important applications even when age‐specific demographic data are available (e.g., perturbation analysis involving ages at first and last reproduction). We have extended the model to allow non‐integer values of ages at first and last reproduction, derived formulae for sensitivity analyses, and presented methods for estimating parameters for our pre‐breeding census partial life cycle model. We applied the age‐structured Leslie matrix model and our pre‐breeding census partial life cycle model to demographic data for several species of mammals. Our results suggest that dynamical properties of the age‐structured model are generally retained in our partial life cycle model, and that our pre‐breeding census partial life cycle model is an excellent proxy for the age‐structured Leslie matrix model.
Is assemblage variability related to environmental variability? An answer for riverine fish
Authors: Oberdorff Thierry , Hugueny Bernard , Vigneron Thibault
Published: December 2004

We examine the variability of riverine fish assemblages in terms of assemblage stability (i.e. variability of numbers of individuals within species over time and variability of assemblage total density), assemblage persistence, and assemblage species richness using data from a 9‐yr survey of 27 sites within 18 coastal streams of North‐western France. To do so, we test a hypothesized directional model for the expected relationships between environmental variability, assemblage variability, assemblage persistence, and assemblage species richness: 1) environmental variability within a given system is likely to generate variable local population size within this system, thus increasing local assemblages variability; 2) environmental variability should increase extinction rates (or, under constant colonization rates, decrease persistence), because the more population sizes vary within an assemblage, the more likely they are to become zero in some period of time; 3) assemblage variability should reduce assemblage species richness by increasing extinction rates within populations composing these assemblages. Results are compatible with our starting hypotheses and show that assemblage variability increased with environmental variability (i.e. discharge variability), that assemblage persistence decreased with environmental variability, and that species richness decreased with assemblage variability after environmental factors were controlled for. Thus, disturbance regimes, in our case, can alter the stability properties of assemblages and extrinsic determinants of assemblage variability may be an important determinant of assemblage species richness. These results have important conservation and management implications, due to the strong impact of river regulation on flow regimes.
Great tits, Parus major, lay too many eggs: experimental evidence in mid‐boreal habitats
Authors: Rytkönen Seppo , Orell Markku
Published: December 2004

This study shows that great tits lay too large clutches in mid‐boreal habitats. First, breeding success, measured with number of fledglings or proportion of eggs that produce fledglings, in northern Finland (65°N) is much poorer than in central and western Europe. Second, brood size manipulations (ca ±30% of the natural mean) revealed that reduced broods produced equal numbers of and larger‐sized fledglings than control and enlarged broods, giving thus the best fitness value for reduced broods. Third, parents of enlarged broods could not adjust (i.e. increase) their feeding effort to the greater number of nestlings. Fourth, extra feeding (about 1/3 of the theoretical maximal needs of the nestlings) during the nestling period resulted in more numerous and larger‐sized fledglings in comparison to control broods. We suggest that the ultimate explanation for the too large clutches is gene flow from the southern population, which prevents local adaptations in the north. Consequently, the main reason for food limitation during the nestling period is that northern great tits apply “southern” decision rules for timing of breeding, clutch size and foraging behaviour. Thus, they tend to breed too early in comparison to the food abundance peak, lay too large clutches in comparison to the level of resources and, perhaps, forage on a too narrow diet (75% caterpillars). Since the late broods that matched the local food abundance peak did not succeed better than the mismatched earlier ones, the most crucial fault of northern great tits seems to be that they overestimate food abundance during peak demands and lay too large clutches. Another explanation for this could be that northern great tits have adopted a brood reduction strategy. However, the long‐term data reveal that years of high breeding success, which would maintain large clutches in the population, are very rare in the north. Therefore, it is unlikely that a brood reduction strategy per se could explain the phenomenon. Instead, it could work together with the gene flow against local adaptation for clutch adjustment.
Shape and sources of variations of the functional response of wildfowl: an experiment with mallards, Anas platyrhynchos
Authors: Fritz Hervé , Durant Daphné , Guillemain Matthieu
Published: December 2004

Understanding the variations of the functional response of an organism, i.e. the predation rate in relation to prey density, is necessary to understand the interactions between the animal and its food supply. This has received little attention in dabbling ducks so we investigated experimentally the shape of the functional response of mallard feeding on poultry pellets, and assessed the influence of several factors such as the size of food items, sex or individual performance on this functional response. Individual differences in intake rate are of crucial importance in group or gregarious foraging species.  We used two approaches of the functional response: 1) the relation between feeding rate (pellets/s) and pellet densities (pellets/m2), and 2) the relationship between instantaneous intake rate (g/s) and biomass density (g/m2). For both approaches, we found that the Type II functional response gave better estimates than a Type I linear functional response but explained only a third of the variance. Our results show that pellet size has a large effect on instantaneous intake rate. The comparison of the functional response parameters suggest that handling time per prey may not reflect the real constraints on intake rate, but that handling time per gram ingested may be more appropriate to integrate the effect of item size in the functional response. We then discuss the possible mechanisms involved. We also found individual variations in the functional response for each of the experiments, with some consistency in the hierarchy regarding feeding efficiency. We did not find any differences between males and females.  Our results provide an evaluation of individual variations in intake rate in interference‐free conditions, which has rarely been done, and call for more controlled experiments to allow a finer understanding of the mechanisms of food acquisition in dabbling ducks.
Suitability of stressed and vigorous plants to various insect herbivores
Authors: Inbar Moshe , Doostdar Hamed , Mayer Richard T.
Published: December 2004

We conducted a controlled experiment to test the plant vigor and the plant stress hypotheses. The two hypotheses associate plant physiological conditions to insect feeding mode and performance. We exposed tomato, Lycopersicon esculentum, to different types of growing conditions: optimal (vigorous plants), resource based stress (water and/or nutrient deficit), and physical stress (punched hole in terminal leaflets). Plant performance, foliar nutritional value for insects and chemical defenses were analyzed after 14 d. These plants were offered to insects belonging to distinct feeding guilds: the silverleaf whitefly, Bemisia argentifolii, a phloem feeder; the leafminer, Liriomyza trifolii; and the corn earworm, Heliothis zea, a leaf chewing caterpillar.

 The experimental conditions generated a gradient of plant growth in the following order: optimal (vigorous)>control=hole punched>no fertilizer>no water>no water and no fertilizer. The last two treatments resulted in plants with poor nutritional value (based on %water, C/N, %N) and higher levels of defensive compounds (i.e., peroxidase and total phenolics) compared with control and the vigorous plants. Hole‐punching neither affected plant growth nor any of the phytochemicals measured. In a choice experiment adult whitefly ovipositioning was not affected by either vigor or punching but was reduced on the other plants (P<0.01). Leafminer feeding and oviposition and corn earworm larval growth rates were higher on the vigorous plants and lower on the punched, no fertilizer, no water, and no water and no fertilizer host plants (P<0.01).  Regardless of insect species or bioassay method, the results in the tomato system support the plant vigor hypothesis that predicts positive association between insect performance and plant growth. The results contradict the plant stress hypothesis that rank stressed plants as better hosts for insects. The mechanisms involved are a combination of poor nutritional value and chemical defenses. We demonstrate a negative association between plant growth and chemical defense. However, induced response triggered by hole‐punching was not cost effective to the plants.
Coexistence, saturation and invasion resistance in simulated plant assemblages
Authors: Moore J. L. , Mouquet N. , Lawton J. H. , Loreau M.
Published: December 2004

A popular hypothesis is that species‐rich systems are less susceptible to invasion. This hypothesis is based on the idea that species richness correlates with community saturation so that establishment by a new species is more difficult in saturated communities. Little attention has been focussed on how changing assumptions about the processes regulating species richness will alter community properties such as invasion resistance. Here, we simulate plant community assembly using four models that have different underlying coexistence mechanisms (and so differ in the amount of available niche space) and subject them to invasion. We created species richness gradients by comparing between models or by considering the output of a single model with different parameter values. We found that the relation between species richness and invasion resistance depends critically on the model considered and the cause of the species richness gradient. Overall, our results suggest that species richness does not necessarily correlate with saturation and is likely to be a poor predictor of invasion resistance. These results provide a possible explanation for the variety of outcomes reported in recent experimental and observational studies that examine the relationship between species richness and invasion resistance. We conclude that consideration of the processes regulating species richness is crucial for a successful understanding of invasion resistance along species richness gradients.
Flammability and serotiny as strategies: correlated evolution in pines
Authors: Schwilk Dylan W. , Ackerly David D.
Published: December 2004

Fire may act as a selective force on plants both through its direct effects by killing or wounding susceptible individuals and through its effect on the environment: the post‐fire environment may select specific physiological traits or life histories. We used phylogenetic independent contrasts to test the hypothesis that fire has selected for correlated evolution among alternative suites of traits in pines: a survival/avoidance suite characterized by thick bark, height, and self‐pruning of dead branches; and a fire‐embracing strategy in which plants invest little into survival, exhibit traits which enhance flammability, and use fire as a means to cue seedling establishment to the post‐fire environment through serotinous cones. We created a set of alternative ‘supertree’ phylogenies for the genus Pinus from published sources. Using these alternative phylogenies, published ecological data for 38 pine species, and newly collected morphological data, we demonstrate that much variation in trait evolution occurs along a fire‐surviving/fire‐embracing axis. Pines vary in their susceptibility to ignition since a tree that retains dead branches is more likely to carry a fire into the canopy than a tree that self‐prunes. The evolution of increased flammability may have altered evolutionary trajectories prompting an evolutionary switch from a fire‐surviving to a fire‐embracing life history. Alternatively, the fire‐embracing strategy may in fact select for increased flammability to ensure canopy ignition and the realization of serotinous seed‐release.
Dynamics of habitat distribution in breeding mallards: assessing the applicability of current habitat selection models
Authors: Pöysä Hannu
Published: December 2004

The applicability of ideal free, ideal despotic and ideal preemptive habitat selection models to explain dynamics of habitat distribution of breeding mallards (Anas platyrhynchos) was explored. Data from 35 lakes studied between 1985 and 2000 were used to examine overall habitat distribution of breeding pairs, breeding success in different habitats, within‐year order of habitat occupation, and density dependence of habitat distribution and breeding success. Two habitat types, rich and poor, were defined based on the structure and luxuriance of shore vegetation; each lake belonged to one or the other of the habitat types. Breeding pairs used the rich habitat more than expected, breeding density also being higher there than in the poor habitat. Both average brood density and breeding success were higher in the rich habitat than in the poor. Breeding success was not density dependent, neither when analysed separately for the habitat types, nor in the study area in general. Within season, arriving mallard pairs did not occupy rich lakes earlier than poor lakes. An isodar analysis based on between‐year variation of the breeding density in rich and poor habitats revealed that habitat distribution of breeding pairs was not density dependent. By contrast, density in the rich habitat increased and proportional use of the poor habitat decreased with increasing overall population density, i.e. the rich habitat got increasingly crowded. None of the habitat selection models considered was applicable to explain the dynamics of habitat distribution of breeding mallards.
How simple grazing rules can lead to persistent boundaries in vegetation communities
Authors: Farnsworth Keith D. , Anderson Alexander R. A.
Published: December 2004

Natural landscape boundaries between vegetation communities are dynamically influenced by the selective grazing of herbivores. Here we show how this may be an emergent property of very simple animal decisions, without the need for any sophisticated choice rules etc., using a model based on biased diffusion. Animal grazing intensity is coupled with plant competition, resulting in reaction‐diffusion dynamics, from which stable boundaries spontaneously emerge. In the model, animals affect their resources by both consumption and trampling. It is assumed that forage consists of two heterogeneously distributed competing resource species, one that is preferred (grass) over the other (heather) by the animals. The solutions to the resulting system of differential equations for three cases a) optimal foraging, b) random walk foraging and c) taxis‐diffusion are presented. Optimal and random foraging gave unrealistic results, but taxis‐diffusion accorded well with field observations. Persistent boundaries between patches of near‐monoculture vegetation were predicted, with these boundaries drifting in response to overall grazing pressure (grass advancing with increased grazing and vice versa). The reaction‐taxis‐diffusion model provides the first mathematical explanation for such vegetation mosaic dynamics and the parameters of the model are open to experimental testing.
Metapopulation processes and persistence in remnant water vole populations
Authors: Telfer S. , Holt A. , Donaldson R. , Lambin X.
Published: December 2004

We examined the spatial distribution of water vole populations in four consecutive years and investigated whether the regional population processes of extinction, recolonisation and migration influence distribution and persistence. We examined how such regional processes are influenced by spatial variation in habitat quality. In addition, we assessed the relevance of metapopulation concepts for understanding the dynamics of species that deviate from classical metapopulation assumptions and developing conservation measures for them. Populations were patchy and discrete, and the patchy distribution was not static between years. Population turnover occurred even in the absence of predatory mink, which only influenced the network of populations at the end of the study. Most populations were clustered close together in the upper tributaries. Local population persistence was predominantly influenced by population size: large populations were more persistent. Recolonisation rates were influenced by isolation and habitat quality. The isolation estimates which best explained the distribution of water vole populations incorporated straight‐line distances, suggesting water voles disperse overland. The distribution of recolonised sites indicated that dispersing voles actively selected habitat on the basis of its quality. Water voles depart from some of the assumptions made by frequently used metapopulation models. In particular there is no clear binary distinction between suitable and non‐suitable habitat. Accounting for variation in habitat quality before investigating temporal changes in population distribution allowed us to demonstrate that the key metapopulation processes were important. The significance of regional population processes relative to local population processes may have increased in declining, fragmented populations compared to pristine regional populations. We hypothesise that although mink predation is likely to eventually cause regional extinction in many areas, metapopulation processes have delayed this decline. Consequently, conservation measures should take into account mink predation rates and regional population processes, before considering aspects of habitat quality.
Within‐population craniometric variability of insular populations of deer mice, Peromyscus maniculatus, elucidated by landscape configuration
Authors: Landry Pierre‐Alexandre , Lapointe François‐Joseph
Published: December 2004

Within‐population genetic variability of twelve insular and four mainland populations of deer mice (Peromyscus maniculatus) was assessed using craniometric characters, and compared to results previously obtained from RAPD data. An index of Craniometric Variance (CVar) was computed from pairwise distances among all specimens. Variations in CVar measures were then compared to landscape variables using a linear regression approach. Our results suggest that CVar decreases in presence of large number of a competitive species (the boreal redback vole, Clethrionomys gapperi; r=−0.527, p<0.037) in deer mouse populations. Island remoteness (r=−0.251, p<0.220) and the geometry of the bank opposite to each island (r=−0.459, p<0.067) were marginally correlated with CVar, but the linear combination of these two variables, forming a composite isolation index, represented the major factor explaining the observed CVar (r=−0.648, p<0.011). Using a multiple regression model, 76.3% of the CVar was explained by a combination of this isolation index and the competitors’ abundance. These results suggest that taking into account landscape barriers as well as the dispersal behavior of small mammals might provide sounder ecological variables than geographical distances alone for predicting within‐population genetic variability in a network of habitat patches.
Evolutionarily stable allocation to vegetative and sexual reproduction in plants
Authors: Olejniczak Paweł
Published: December 2004

I apply the resource allocation approach to construct an ESS model aimed at predicting the rate of vegetative and sexual reproduction in plants. The model provides an integrated explanation of why the mode of reproduction should vary between populations and species, and what factors should affect the trade‐off between ramet and seed production. It follows that seed production, usually considered to be less effective and more costly than ramet production, may be maintained in the population. The production of ramets should be favored when their recruitment rate is high or the costs are low. The intensity of vegetative reproduction is also related to conditions which affect seed production directly. The rate of ramet production should be inversely correlated with factors favoring sexual reproduction, such as lower cost of seeds or high opportunity for colonization of new patches, etc. Under such conditions it is beneficial to allocate more resources available for reproduction into seeds, at the expense of ramet production. The model emphasizes the need to consider both modes of plant reproduction whenever one of them is theoretically or empirically studied.
Experimental demonstration that plant diversity reduces invasibility – evidence of a biological mechanism or a consequence of sampling effect?
Authors: Wardle David A.
Published: December 2004

Several recent studies have claimed to present experimental evidence from synthesised plant communities in which diversity was varied as a treatment that diversity reduces community invasibility by other plant species. This type of result contrasts from that of many observational studies which find diversity and invasibility to be positively correlated in nature, but some recent literature has claimed that these observational studies are confounded by extrinsic covarying factors while experimental studies are not. In this article I evaluate each of eight experiments from six recent publications in which the effect of varying plant diversity on the success of invasive species was investigated. In each case that invasibility was identified by the authors as being adversely affected by plant species richness, the result can be explained by factors that covaried with diversity in the experiment, most notably as a consequence of “sampling effect” (in which the most competitive species or species combination in the total species pool has a greater probability of occurring as species richness is increased), or through the incorrect use of statistical techniques. It is proposed that the apparent discrepancy between the results of many observational and experimental studies at least in grasslands is because: (1) in observational studies competitive dominant species are often associated with the most productive plots, and these dominants both reduce diversity through competitive exclusion of subordinates and competitively suppress invasive species; and (2) in recent experimental studies “sampling effect” results in the most competitive species (and therefore those most likely to suppress the invader) occurring with greater frequency as diversity is increased. Both observational and experimental studies therefore point to the role of competitive dominants in reducing invasibility, and in both situations species richness of the plant community need not be invoked as an explanation for the results.
Interactions between algae and the microbial loop in experimental microcosms
Authors: Hulot Florence D. , Morin Peter J. , Loreau Michel
Published: December 2004

We conducted a short‐term microcosm experiment to study the direct and indirect effects of a bacterivore on bacteria and the dynamics of two species of green algae. We introduced Scenedesmus, Chlorella and Colpidium, a bacterivorous ciliate, successively in a carbon‐rich medium. Bacteria were introduced with Scenedesmus, Chlorella and Colpidium. The experiment lasted 40 days, preventing us from detecting whether the populations had reached equilibrium. The bacterivore had a positive effect on both species of algae by limiting the abundance of bacteria. In absence of the bacterivore, bacteria did not exclude the two algal species, despite the high carbon:nutrient ratio of the medium. Unexpectedly, by the end of the experiment the bacterivore declined in all microcosms. Also, Chlorella growth was impeded by the presence of Scenedesmus. These two observations might be explained by allelopathic interactions. Our experiment suggests that the functioning of such a simple community is far more complex than assumed in previous theoretical and experimental models. Studying the dynamics of the system, however, allowed us to disentangle species interactions.
Group size versus territory size in group‐living badgers: a large‐sample field test of the Resource Dispersion Hypothesis
Authors: Johnson Dominic D. P. , Macdonald David W. , Newman Chris , Morecroft Michael D.
Published: December 2004

Badgers (Meles meles) have been the focus for the development of a pervasive model of social grouping behaviour, relevant to a number of carnivore species and other taxonomic groups – the Resource Dispersion Hypothesis (RDH). The RDH hypothesises that the dispersion and richness of resources in the environment provide a passive mechanism for the formation of groups, even without any direct benefits of group living. However, few studies have tested the RDH in the field. The principal prediction is that, as opposed to enlargement of territory sizes to accommodate more members, territory size (TS) is independent of group size (GS). Instead, TS is determined by the spatial dispersion of resources, while GS is independently determined by the richness of those resources. However, these predictions provide only weak correlative tests, especially in non‐experimental field studies. The first predicts an absence of correlation and is therefore prone to Type II error, especially given the small sample sizes and errors in estimating TS and GS of mammals in the field. We tested for independence of territory size and group size in all years with available data since the beginning of the long‐term badger study in Wytham Woods in 1974. We used two methods of TS estimation, a sequential Bonferroni technique to adjust for multiple inference tests, a combined analysis and an analysis with pooled data. This prediction of the RDH could not be rejected on the basis of any of these analyses. Given this evidence that other processes are independently determining group size and territory size, further predictions of the RDH will be worth investigating in considerable detail.