bluuebunny/crossref_metadata_2025 huggingface dataset:
Scaling of photosynthetic production of aquatic macrophytes – a review
Authors: Sand‐Jensen Kaj , Binzer Thomas , Middelboe Anne Lise
Published: December 2007

Most studies on photosynthetic production of aquatic macrophytes have been made on detached leaves and algal thalli. This may have given the false impression that production is often saturated by light and that inorganic carbon and nutrients are more important limiting factors. However, studies on the more relevant ecological scale of macrophyte communities lead to a completely different perception because community production is light limited due to intense self‐shading. Relatively high irradiances are needed for photosynthesis to balance respiratory costs and not even maximum irradiances at noon during summer saturate photosynthetic production. The fundamental importance of light is confirmed by the close coupling to community light absorptance of both maximum production in high‐light environments and efficiency of light use in low‐light environments. The upper boundaries of light‐limited and light‐saturated production are distinctly and linearly related to community absorptance. Moreover, higher diversity in the community has a positive and stabilizing influence on light absorptance and production because different species supplement each other temporally and spatially. Close predictions of actual production rates in macroalgal communities throughout the year are possible solely from determinations of incident and absorbed irradiance. Along with the increasing regulating role of light from leaves or thalli to entire communities the importance of temperature, inorganic carbon and other resources decreases. Thus, ten‐fold rise of CO2 relative to atmospheric saturation does not enhance maximum production in dense communities of efficient HCO3−‐users and only doubles production of pure CO2‐users. A challenge therefore exists establishing the importance of light for photosynthetic production of macrophytes from individuals to communities and re‐evaluating the postulated main importance of inorganic carbon and temperature in the scenarios of globally rising CO2 and temperature.
Quantifying ecological filters: the relative impact of herbivory, neighbours, and sediment on an oligohaline marsh
Authors: Geho Ellen M. , Campbell Daniel , Keddy Paul A.
Published: December 2007

The objective of this experiment was to quantify the relative importance of herbivory, interactions with neighbours and sediment addition as filters in controlling the species present in an oligohaline marsh in southeastern Louisiana, USA. We planted 16 species of wetland plants in 3×3 m plots either inside or outside herbivore exclosures, with or without presence of established vegetation, and with or without added sediment. These species, representing 12 families and diverse morphologies, included herbaceous and woody plants. At the end of the growing season above‐ and below ground biomass were measured. In cleared plots (no neighbours) inside herbivore exclosures, 12 of the 16 species grew well, indicating that they could tolerate the physical conditions in the habitat. Competition significantly reduced the biomass of four of these species (A. calamus, C. occidentalis, P. hemitomon, P. cordata), suggesting that it is the strongest species‐specific filter operating in the marsh. These results confirm the general consensus that competition from existing plants is the strongest filter operating in high biomass habitats, and further imply that competition has the potential to remove 33% of the species from the community. Facilitation occurred for one species, R. corniculata. Herbivory had the largest general effect of reducing biomass, as determined in the full ANOVA model, but on a species‐specific basis, significantly reduced the biomass of only two species (T. distichum, T. domingensis). These results support the emerging view that herbivory is an important filter in coastal wetlands. Although several other studies have argued for the importance of sedimentation in controlling plant species composition in wetlands, we found no effect of sediment addition. Competition and herbivory appear to be two critical biological filters that control plant composition of coastal wetlands, and they must be considered in future studies and restoration efforts along the Gulf of Mexico.
Increase in mate availability without loss of self‐incompatibility in the invasive species Senecio inaequidens (Asteraceae)
Authors: Lafuma Lucile , Maurice Sandrine
Published: December 2007

The evolution of the strength of self‐incompatibility in Senecio inaequidens, a native of South Africa was investigated in relation to its invasion in Europe. Levels of self‐incompatibility were estimated with hand‐pollinations in five populations in greenhouse conditions. One population came from the native range of the species and four populations were sampled in Europe from two independent transects of colonization with old and recent populations. Contrary to Baker's law predictions, our results suggest that the species has a sporophytic self‐incompatible system maintained in all populations. We suggest that the ability of S. inaequidens to colonize new sites with a self‐incompatibility system is promoted by its ecological characteristics (perenniality, extended reproductive period, massive seed production, generalist pollinators). In addition, we found that mate availability was increased (1) in the introduced range compared to the native range, (2) in marginal versus central European populations. Possible explanations for this surprising result are discussed.
Up or down in space? Uniting the bottom‐up versus top‐down paradigm and spatial ecology
Authors: Gripenberg Sofia , Roslin Tomas
Published: December 2007

Why is the World green – what keeps herbivores, and herbivorous insects in particular, from consuming all of their food? When this question was first posed, the relative importance of top‐down and bottom‐up effects was hotly disputed. While modern ecologists may agree that impacts from several different directions will affect local insect densities, the bottom‐up vs top‐down jargon seems to be stuck in a unidimensional world. Here, we argue that the strength of almost every bottom‐up and top‐down force is likely to vary in space, and that in itself, spatial structure invokes new processes which defy classification in the traditional bottom‐up top‐down scheme. To understand the relative importance of different forces keeping herbivore numbers in check, we feel that we need a fresh synthesis between the novel paradigm of spatial ecology and the classical paradigms of top‐down and bottom‐up studies. This synthesis requires a consideration of forces beyond the standard framework of top‐down vs bottom‐up effects, and should be based on comparing the relative strength of such forces at several sites in a spatially explicit framework. Overall, we should switch our focus from whether the relative strength of top‐down and bottom‐up factors vary in space to why there is variation, how much variation there is, and at what spatial scale(s) it occurs.
A field guide to models of sex‐ratio evolution in gynodioecious species
Authors: Bailey Maia F. , Delph Lynda F.
Published: December 2007

Gynodioecious plant species, species in which individuals are females or hermaphrodites, are ideal systems for studying connections between genetics, ecology, and long‐term evolutionary changes because sex determination can be complex, involving cytoplasmic and/or nuclear genes, and sex ratio is often variable across landscapes. Field data are needed to evaluate the many theories concerning this breeding system. In order to facilitate the gathering of relevant data, this paper introduces the four types of gynodiocy (nuclear, nuclear‐cytoplasmic and stochastic gynodioecy plus subdioecy), describes example species and expected patterns, discusses the various forces that drive the evolution of female frequencies, and gives concrete advice on where to start collecting data for different systems. For species in which females are relatively rare, we recommend reciprocal crosses to determine if sex‐determination is nuclear or nuclear‐cytoplasmic along with a search for correlations between female frequencies and ecological factors. For species in which females are common and sex ratios are highly variable, we recommend looking at female offspring sex ratios to determine if females are primarily produced in ephemeral epidemics.

In the course of this discussion, we argue that the majority of natural gynodioecious species will have complex sex determination in which multiple cytoplasmic male sterility (CMS) genes interact with multiple nuclear restorers of fertility. Sex‐ratio evolution in such species will be primarily influenced by fitness differences among hermaphrodites (costs of restoration) and less influenced by fitness differences between the sexes (compensation). Metapopulation dynamics alone may explain population sex ratios of species in which females are associated with marginal environments or hybrid zones; however, we feel that in most cases equilibrium forces within populations and metapopulation dynamics among populations each explain portions of the sex‐ratio pattern.
Variable responses of natural enemies to Salix triandra phenotypes with different secondary chemistry
Authors: Hjältén Joakim , Niemi Lena , Wennström Anders , Ericson Lars , Roininen Heikki , Julkunen‐Tiitto Riitta
Published: December 2007

Plant phenotypes often differ in their resistance to natural enemies, but the mechanism for this has seldom been identified. The aim of this study was to determine if the spatial patterns of phenotype use of a highly specialized insect herbivore (the galling sawfly Pontania triandrae) in a natural willow population can be related to phenotypic variation in plant secondary chemistry. Furthermore, we tested if traits that confer resistance to one type of natural enemy, i.e. the galling sawfly, also confer resistance to others, in our case a leaf beetle Gonioctena linnaeana and the rust fungus Melampsora amygdalinae. We identified 18 phenotypes with high and 18 phenotypes with low gall density in our field population and determined gall densities, the degree of leaf damage and rust infection on each phenotype and collected leaves for chemical analyses. The concentration of phenolics was higher in phenotypes with high density of galls suggesting that this galling sawfly may use phenolics as oviposition cues. Rust infection showed the opposite pattern, with lower levels on clones with high concentration of phenolics, while leaf damage by G. linnaeana did not differ between clone types. This indicates that these important natural enemies may assert divergent selection on willow phenotypes and that this might provide a mechanism for maintaining phenotypic variation within willow populations.
Density compensation can cause no effect of biodiversity on ecosystem functioning
Authors: Jiang Lin
Published: December 2007

The role of density compensation (the decline of species density with increasing diversity), in the context of biodiversity and ecosystem functioning, has not been explicitly explored. I used aquatic microbial communities containing bacterivorous consumers (protozoans and rotifers) to investigate whether competition can lead to density compensation and whether density compensation can contribute to the relationship between biodiversity and ecosystem functioning. The experiment employed a nested design in which the consumer diversity gradient (0, 1, 2 or 4 species) was constructed by drawing all possible species or species combinations at each diversity level from a five‐species pool. All consumer species coexisted but there was little evidence for overyielding or species dominance, suggesting weak complementarity and sampling effects. Rather, increasing number of consumer species resulted in community‐wide density compensation, such that aggregate consumer biomass was unaffected by consumer diversity. Whereas culturable bacterial density declined as consumer diversity increased, total bacterial density showed no discernible response to changes in consumer diversity, a result probably due in part to heterogeneity in bacterial edibility. This study demonstrates the potential for density compensation to shape the relationship between biodiversity and ecosystem functioning.
Trophic supplements to intraguild predation
Authors: P. Daugherty Matthew , P. Harmon Jason , J. Briggs Cheryl
Published: December 2007

Intraguild predation (IGP) is a dominant community module in terrestrial food webs that occurs when multiple consumers feed both on each other and on a shared prey. This specific form of omnivory is common in terrestrial communities and is of particular interest for conservation biology and biological control given its potential to disrupt management of threatened or pest species. Extensive theory exists to describe the dynamics of three‐species IGP, but these models have largely overlooked the potential for other, exterior interactions, to alter the dynamics within the IGP module. We investigated how three forms of feeding outside of the IGP module by intraguild predators (i.e. trophic supplementation) affect the dynamics of the predators (both IG predator and IG prey) and their shared resource. Specifically, we examined how the provision of a constant donor‐controlled resource, the availability of an alternative prey species, and predator plant‐feeding affect the dynamics of IGP models. All three forms of trophic supplements modified the basic expectations of IGP theory in two important ways, and their effects were similar. First, coexistence was possible without the IG prey being a superior competitor for the original shared resource if the IG prey could effectively exploit one of the types of trophic supplements. However, supplements to the IG predator restricted the potential for coexistence. Second, supplements to the IG prey ameliorated the disruptive effects of the IG predator on the suppression of the shared resource, promoting effective control of the resource in the presence of both predators. Consideration of these three forms of trophic supplementation, all well documented in natural communities, adds substantial realism and predictive power to intraguild predation theory.
Control of phytoplankton–bacteria interactions by stoichiometric constraints
Authors: Danger Michael , Leflaive Joséphine , Oumarou Catherine , Ten‐Hage Loïc , Lacroix Gérard
Published: December 2007

In aquatic ecosystems, phytoplanktonic organisms are the major primary producers and bacteria the major decomposers. The interactions between phytoplankton and bacteria may be dependent on nutrient resources. Anthropogenic inputs, by modifying nutrient status and stoichiometry of lakes, might induce changes in these interactions, and thus, could have many consequences on some ecological processes such as primary production or importance of microbial recycling activity.

To test this hypothesis, we grew an axenic strain of a green alga, Scenedesmus obliquus, in a range of stoichiometric situations, in absence and in presence of a natural bacterial community. Here, we show that different phytoplankton limiting factors can generate between algae and bacteria either competition for nutrients in phosphorus‐limited conditions, commensalism in nitrogen‐limited conditions, or mutualism in eutrophic nutrient‐unlimited conditions. Causes of these different interaction types are discussed, in particular the hypothesis that in very eutrophic systems with high primary production, mutualism between algae and bacteria could be due to CO2 supply by heterotrophic respiration to inorganic carbon limited algae. Some probable consequences for aquatic ecosystems functioning are proposed.
Modes of speciation and the neutral theory of biodiversity
Authors: Etienne Rampal S. , Apol M. Emile F. , Olff Han , Weissing Franz J.
Published: December 2007

Hubbell's neutral theory of biodiversity has generated much debate over the need for niches to explain biodiversity patterns. Discussion of the theory has focused on its neutrality assumption, i.e. the functional equivalence of species in competition and dispersal. Almost no attention has been paid to another critical aspect of the theory, the assumptions on the nature of the speciation process. In the standard version of the neutral theory each individual has a fixed probability to speciate. Hence, the speciation rate of a species is directly proportional to its abundance in the metacommunity. We argue that this assumption is not realistic for most speciation modes because speciation is an emergent property of complex processes at larger spatial and temporal scales and, consequently, speciation rate can either increase or decrease with abundance. Accordingly, the assumption that speciation rate is independent of abundance (each species has a fixed probability to speciate) is a more natural starting point in a neutral theory of biodiversity. Here we present a neutral model based on this assumption and we confront this new model to 20 large data sets of tree communities, expecting the new model to fit the data better than Hubbell's original model. We find, however, that the data sets are much better fitted by Hubbell's original model. This implies that species abundance data can discriminate between different modes of speciation, or, stated otherwise, that the mode of speciation has a large impact on the species abundance distribution. Our model analysis points out new ways to study how biodiversity patterns are shaped by the interplay between evolutionary processes (speciation, extinction) and ecological processes (competition, dispersal).
Rules for macroorganisms applied to microorganisms: patterns of endemism in benthic freshwater diatoms
Authors: Kilroy Cathy , Biggs Barry J. F. , Vyverman Wim
Published: December 2007

Ecological theory based on the dynamic equilibrium model (DEM) suggests that maintenance of endemic taxa is most likely in stable, unproductive environments. We tested whether this hypothesis, which was developed mainly using terrestrial plant examples, held when applied to distributions of benthic freshwater diatoms in New Zealand. Given current arguments for the ubiquity of microbial organisms, with distributions determined mainly by environmental tolerances, demonstration that distinctive taxa with evidently restricted distributions conform to theory applicable to larger organisms would lend support to the opposite point of view, that barriers to dispersal do exist. We examined diatom communities from over 320 sites representing the entire spectrum of freshwater habitats in New Zealand and assessed relative abundances of the main taxa present. Each taxon distinguished was assigned to one of five distribution categories ranging from cosmopolitan to endemic. We derived indices of disturbance and productivity for each site using the River Environment Classification (REC), a GIS‐based classification system developed for New Zealand rivers. Diatom taxa assigned to endemic or distinctive potential endemic categories were significantly more abundant in low disturbance sites but occurred across a range of productivities. However, bogs and tarns, both of which fell mainly into low disturbance and productivity classes, were distinctive in supporting relatively high proportions of endemic and potential endemic diatoms. Thus our findings in general conformed to the patterns predicted by the DEM, thereby supporting the role of dispersal limitation in diatoms. At the same time, conformity with the DEM helps to explain the continued coexistence in New Zealand freshwaters of many common and apparently cosmopolitan taxa with endemic diatoms, since the DEM explanation for maintenance of endemism does not rely on geographic isolation of species.
Phase dependence in winter physiological condition of cyclic voles
Authors: Huitu Otso , Jokinen Ilmari , Korpimäki Erkki , Koskela Esa , Mappes Tapio
Published: December 2007

Lack of food resources has been suggested as a factor which limits the growth of cyclic vole populations. During peak phases of the cycle, vole population growth typically ceases during late autumn or early winter, and is followed by a decrease in density over the winter. To investigate whether this decrease is due to increased mortality induced by a depletion of food resources, we studied overwinter food consumption and physiological condition of field voles (Microtus agrestis) in western Finland in both an increase and a decrease phase of a three‐year population cycle. The growth rate of vole populations was negatively related both to prevailing vole densities and to densities six months earlier. The condition index of voles, as well as their blood levels of haematocrit, proteins, free fatty acids and immunoglobulin G, were positively related to population growth rate when populations were declining. When populations were increasing, these parameters tended to be negatively related to population growth rate. The overall physiological condition of voles was lower in the winter of the decrease phase as compared to the increase phase. The return rate of voles, a proxy of survival, was also lower in the decrease than in the increase phase of the cycle and positively related to haematocrit levels. Almost 90% of all green vegetation shoots were consumed by voles during the winter of the decrease phase while only two thirds were eaten in the increase phase. Our results suggest that the winter decrease phase of cyclic vole populations is associated with both a deterioration in the physiological condition of voles and a significant depletion of winter food resources. This implies that malnutrition induces poor physiological condition in voles, which in turn may increase mortality either directly through starvation or indirectly through increased susceptibility to predators and pathogens.
Biological filtering of correlated environments: towards a generalised Moran theorem
Authors: Ripa Jörgen , Ranta Esa
Published: December 2007

Many species from diverse taxa are known to display synchronous fluctuations across vast geographical ranges. It is often thought that climate factors influencing the growth of conspecific populations are correlated over large distances and hence produce the synchronous population dynamics – an effect known as the Moran effect. However, for species embedded in a food web the Moran effect needs not necessarily influence the focal species directly, but can act indirectly through other species. Such an indirect synchronization can also occur in an age‐structured population, where the correlated environment of one age‐class causes synchronous fluctuations of another. Here, we investigate this indirect Moran effect. We find first of all that synchrony is readily transferred through food webs or between age classes, which complicates the identification of the underlying synchronizing factor. Secondly, we find puzzling cases, where synchrony is enhanced as it is filtered through a food web or between age‐classes. Our results also apply to systems of different species, but with closely matching dynamics.
Influence of maternal characteristics and oceanographic conditions on survival and recruitment probabilities of Weddell seals
Authors: L. Hadley Gillian , J. Rotella Jay , A. Garrott Robert
Published: December 2007

For long‐lived animals, maternal age and breeding experience can vary widely and affect offspring survival and recruitment probabilities. In addition, these vital rates may be influenced by annual variation in environmental conditions. We evaluated various hypotheses regarding how offspring survival and recruitment probabilities vary as functions of maternal characteristics and oceanographic conditions, using 25 years of data from a study of individually‐marked Weddell seals in Erebus Bay, Antarctica. We predicted that survival and recruitment would be positively related to maternal age and experience up to some threshold value and considered three hypothesized shapes for the relationship beyond the threshold age (steadily increasing, pseudo‐threshold, or decreasing). We predicted an inverse relationship between maternal age at first reproduction and offspring survival and recruitment probabilities. We predicted that sea‐ice extent, which positively influences primary productivity, would be positively related to annual recruitment probabilities. Results revealed contrasting influences of maternal age on probabilities of survival and recruitment of young. Survival rate was best modeled by a pseudo‐threshold relationship with maternal age, e.g. in 1999, survival rate was estimated as 0.61, 0.69 and 0.72, respectively, for seals born to 6‐, 14‐ and 22‐yr‐old mothers. In contrast, estimated recruitment probability was highest for seals born to young mothers, e.g. recruitment probability for a 7‐yr‐old who had not yet had a pup was estimated as 0.51 vs 0.30, respectively, if she was born to a 6‐ versus a 14‐yr‐old mother. The combined results for offspring survival and recruitment suggest countervailing selection where genotypes favored for reproductive success are generally selected against as juveniles, resulting in high recruitment probabilities for individuals that had low juvenile survival rates. Finally, we found support for our prediction that oceanographic conditions affected annual recruitment rates, but not survival rates. Specifically, annual recruitment probability was positively related to the sea‐ice extent in September of the previous year.
Landscape geometry determines community response to disturbance
Authors: Starzomski Brian M. , Srivastava Diane S.
Published: December 2007

Ecological communities are impacted by anthropogenic changes in both habitat geometry (i.e. amount, shape, fragmentation and connectivity of habitat) and disturbance regime. Although the effect of each of these drivers on diversity is well‐documented, few studies have considered how habitat geometry and disturbance interact to affect diversity. We used a miniature landscape of moss patches to experimentally manipulate both habitat geometry and disturbance frequency on microarthropod communities. Species richness and abundance in local patches declined linearly with disturbance rate in all experimental landscapes, but the speed of this decline (a measure of ecological resilience) depended on the size and connectivity of the surrounding region. Reductions in region size had little effect on community resilience to disturbance until habitat loss resulted in complete loss of connectivity between patches, suggesting a threshold in community response to habitat loss. Beyond this threshold, repeated disturbance resulted in rapid declines in patch species richness and abundance and substantial changes in community composition. These effects of habitat geometry and disturbance on diversity were scale‐dependent. Gamma (regional‐scale) diversity was unaffected by habitat geometry, suggesting experimental reductions in alpha (local‐scale) diversity were offset by increases in beta diversity. There was no effect of body size, abundance, or trophic position in determining species response to disturbance. Taxonomic grouping had a weak effect, with oribatids less affected by drought. We conclude that, in this system, dispersal from the surrounding metacommunity is vital in allowing recovery of local communities from disturbance. When habitat loss and fragmentation disrupt this process, extinctions result. Studies that examine separately the effects of habitat alterations and disturbance on diversity may therefore underestimate their combined effects.
Proximate structural mechanisms for variation in food‐chain length
Authors: M. Post David , Takimoto Gaku
Published: December 2007

Food‐chain length is a central characteristic of ecological communities because of its strong influence on community structure and ecosystem function. While recent studies have started to better clarify the relationship between food‐chain length and environmental gradients such as resource availability and ecosystem size, much less progress has been made in isolating the ultimate and proximate mechanisms that determine food‐chain length. Progress has been slow, in part, because research has paid little attention to the proximate changes in food web structure that must link variation in food‐chain length to the ultimate dynamic mechanism. Here we outline the structural mechanisms that determine variation in food‐chain length. We explore the implications of these mechanisms for understanding how changes in food‐web structure influence food‐chain length using both an intraguild predation community model and data from natural ecosystems. The resulting framework provides the mechanisms for linking ultimate dynamic mechanisms to variation in food‐chain length. It also suggests that simple linear food‐chain models may make misleading predictions about patterns of variation in food‐chain length because they are unable to incorporate important structural mechanisms that alter food‐web dynamics and cause non‐linear shifts in food‐web structure. Intraguild predation models provide a more appropriate theoretical framework for understanding food‐chain length in most natural ecosystems because they accommodate all of the proximate structural mechanisms identified here.
Let the concept of trait be functional!
Authors: Violle Cyrille , Navas Marie‐Laure , Vile Denis , Kazakou Elena , Fortunel Claire , Hummel Irène , Garnier Eric
Published: December 2007

In its simplest definition, a trait is a surrogate of organismal performance, and this meaning of the term has been used by evolutionists for a long time. Over the last three decades, developments in community and ecosystem ecology have forced the concept of trait beyond these original boundaries, and trait‐based approaches are now widely used in studies ranging from the level of organisms to that of ecosystems. Despite some attempts to fix the terminology, especially in plant ecology, there is currently a high degree of confusion in the use, not only of the term “trait” itself, but also in the underlying concepts it refers to. We therefore give an unambiguous definition of plant trait, with a particular emphasis on functional trait. A hierarchical perspective is proposed, extending the “performance paradigm” to plant ecology. “Functional traits” are defined as morpho‐physio‐phenological traits which impact fitness indirectly via their effects on growth, reproduction and survival, the three components of individual performance. We finally present an integrative framework explaining how changes in trait values due to environmental variations are translated into organismal performance, and how these changes may influence processes at higher organizational levels. We argue that this can be achieved by developing “integration functions” which can be grouped into functional response (community level) and effect (ecosystem level) algorithms.
The role of model selection in describing stochastic ecological processes
Authors: Kuparinen Anna , Snäll Tord , Vänskä Simopekka , O'Hara Robert B.
Published: December 2007

A great deal of variation in ecological processes can be generated through variation in environmental conditions. Models describing the underlying processes should be able to account for this variation. However, models may not be flexible enough, so that different realizations of a process may be better described by different models. This may lead to uncertainty in model selection.

Here we examine the question of whether two empirical models can provide consistent fits to different realizations of a process affected by environmental variation. We further examine the sensitivity of the model predictions to the amount of data available and the selection of the model. To study this, we simulated pollen dispersal patterns under varying wind conditions and then investigated whether the datasets consistently supported the same model. The role of the model selection and the impact of the spatial range over which the dispersal distances were observed were assessed by comparing model predictions at long dispersal distances.

There was no consistent pattern of one model providing a better fit than the other across simulations. The model providing better fit varied depending on the range of distances over which the dispersal patterns were observed, and on the amount of long‐distance dispersal. The model predictions were found to be very sensitive to the selection of the model.

The variation between datasets produced with the same underlying mechanisms cannot be easily described using one model, which also limits our ability to reliably predict the underlying process. Therefore, the amount of information about a model choice provided by an individual field study may be rather limited. If we are to understand processes that are affected by environmental variation then we have to observe the range of possible outcomes of the processes under varying spatio‐temporal conditions.
Behavioral mechanisms underlying ants’ density‐dependent deterrence of aphid‐eating predators
Authors: P. Harmon Jason , Andow D. A.
Published: December 2007

Density‐dependent mutualisms have been well documented, but the behavioral mechanisms that can produce such interactions are not as well understood. We investigated interactions between predatory ladybirds and the ant Lasius neoniger, which engages in a facultative association with the aphid Aphis fabae. We found that ants disrupted predator aggregation and deterred foraging, but that this effect varied with aphid density. In the field, smaller aphid colonies had higher numbers of ants per aphid (higher relative ant density), whereas plants with larger aphid colonies had lower relative ant density. Ants deterred ladybird foraging when relative ant density was high, but when relative ant density was low, ladybirds aggregated to aphids and foraged more successfully. This difference in ladybird foraging success appeared to be driven by variation in the ants’ distribution on the plant and the ladybirds’ reaction to ants. When relative ant density was high, ants moved around the perimeter of the aphid colonies, which resulted in faster detection of predators and a greater likelihood of ladybirds leaving. However, when relative ant density was low, ants moved only in the midst of the aphid colonies and rarely around the perimeter, which allowed predators to approach the aphid colony from the perimeter and feed without detection. Such predators were less likely to leave the aphid colony when subsequently detected by ants. We suggest that differences in relative ant numbers, ant distribution, and predator reaction to detection by ants could lead to complex population‐level consequences including density‐dependent mutualisms and the possibility that predators act as prudent predators.
Environmental context influences the outcome of a plant–seed predator interaction
Authors: Kolb Annette , Leimu Roosa , Ehrlén Johan
Published: December 2007

Mutualistic and antagonistic interactions with animals are known to influence the performance of plants in many ways. Much less is known about how such effects are influenced by the environment and how they translate into effects on plant population dynamics. In this study, we first quantified how pre‐dispersal seed predation in the perennial herb Primula veris changes along a continuous gradient of canopy cover. We then used a deterministic demographic modeling approach to investigate how seed predation may influence population growth rate (λ) and how effects depend on environmentally‐induced variation in plant demography. Intensity of seed predation increased with the degree of canopy cover, while sensitivity of λ to changes in seed production decreased. This translated into non‐linear effects of seed predation on λ along the canopy cover gradient. Despite seed predation rates being highest in closed habitats, the negative effect of seed predation on λ was lower here than in slightly more open habitats. Our results demonstrate that knowledge of the intensity of plant‐herbivore interactions does not suffice to infer how animals influence the population dynamics of plants and their distribution. Plant demographic sensitivity and its dependence on the environment need also to be taken into account when assessing the importance of plant–animal interactions.
Tradeoffs involved in site selection and foraging in a wolf spider: effects of substrate structure and predation risk
Authors: Rypstra Ann L. , Schmidt Jason M. , Reif Brant D. , DeVito Jill , Persons Matthew H.
Published: December 2007

Understanding how animals weigh habitat features, exposure to predators and access to resources is important to determining their life history and distribution across the landscape. For example, when predators accumulate in structurally complex habitats, they face an environment with different competitive interactions, foraging opportunities and predatory risks. The wolf spider Pardosa milvina inhabits the soil surface of highly disturbed habitats such as agricultural fields throughout eastern North America. Pardosa displays effective antipredator behavior in the presence of chemical cues produced by a larger coexisting wolf spider, Hogna helluo. We used those cues to simulate predation risk in laboratory and field experiments designed to test the effects of habitat substrate and predation risk on site selection and prey consumption of Pardosa. In general, Pardosa preferred more complex substrates over bare dirt but those preferences were eliminated or reversed when cues from Hogna were present. Feeding trials revealed that substrate alone had few effects on Pardosa prey consumption, which we measured by documenting the change in the abdomen width. Although the presence of Hogna cues reduced prey consumption overall in field feeding trials, the negative effect of predation risk on prey consumption was only observed in grass and bare dirt substrates in the laboratory. We also found that prey capture was negatively affected by habitat complexity for both spider species but that same complexity offered Pardosa protection from predation by Hogna. This study provides insight into how two predator species interact to balance site selection and feeding in order to avoid predation. Shifts in foraging and distributional patterns of predators can have profound implications for their role in the food web.
Testing the species pool hypothesis for mire vegetation: exploring the influence of pH specialists and habitat history
Authors: Hájek Michal , Tichý Lubomír , Schamp Brandon S. , Zelený David , Roleček Jan , Hájková Petra , Apostolova Iva , Dítě Daniel
Published: December 2007

The Evolutionary species pool hypothesis (ESPH) predicts that historically more common habitats will be richer in species because they have had greater opportunity for the evolution of suitably adapted species. We explored the relationship between mire species richness and pH, an important environmental variable in mires, in two regions that differ in habitat pH distribution: the West Carpathians and Bulgaria. Mire habitats in both the West Carpathians and Bulgaria demonstrate support for the ESPH prediction that habitats with more common pH values host more species. We also explored the influence of habitat history by examining the distribution of generalists and specialists along gradients of habitat pH, using extensive community‐level vegetation data from European mires in these two regions. We found a striking pattern with the distribution of pH‐specialists having three distinct peaks in both regions, whereas the total species pool peaked in near neutral pH habitats in both regions. Because the peaks in specialist richness do not correspond to regional pH distribution patterns, we hypothesize that historical explanations may be important, and that habitats currently rich in pH‐specialists may have historically acted as pleniglacial refugia for many mire species. Our findings support the general predictions of the ESPH, but further suggest that historical processes such as patterns of glacial refugia, may significantly influence contemporary species distributions and the diversity of plant species in mire habitats.
Food quantity and quality regulation of trophic transfer between primary producers and a keystone grazer (Daphnia) in pelagic freshwater food webs
Authors: Persson Jonas , Brett Michael T. , Vrede Tobias , Ravet Joseph L.
Published: December 2007

The transfer of energy and nutrients from plants to animals is a key process in all ecosystems. In lakes, inefficient transfer of primary producer derived energy can result in low animal growth rates, accumulation of nuisance phytoplankton blooms and dissipation of energy from the ecosystem. Most research on carbon transfer efficiency in pelagic food webs has focused on either food quantity or food quality, with the latter considered separately as either elemental stoichiometry or biochemical composition. The natural occurrence and magnitude of these types of growth limitations and their combined effects on Daphnia, a keystone grazer in pelagic freshwater ecosystems, are largely unknown. Our empirical models predict that the strength and nature of food quantity and quality limitation varies greatly with lake trophic state (total phosphorus, TP) and that Daphnia growth rates and thus energy and nutrient transfer efficiency are highest in lakes with intermediate trophic status (TP 10–25 μg l−1). We predict that food availability place the greatest constraint on Daphnia growth in nutrient poor lakes (TP≤4 μg l−1). Phosphorus limitation of Daphnia growth increased with decreasing TP, but the overall effect was never predicted to be the dominant constraining factor. Eicosapentaenoic acid (EPA, 20:5ω3) limitation was predicted to occur in both nutrient poor and nutrient rich lakes and placed the primary constraint on food quality in the most productive lakes. Two contrasting EPA‐models gave different results on the magnitude of EPA‐limitation, implying that additional food quality factors decrease Daphnia growth at high TP. In conclusion, the model predicts that Daphnia growth should peak in mesotrophic lakes, food quantity will place the greatest constraint on growth in oligotrophic lakes and EPA will primarily limit growth in eutrophic lakes.
Non‐randomness in the species–area relationship: testing the underlying mechanisms
Authors: Fattorini Simone
Published: December 2007

The species–area relationship (SAR) is one of the best documented patterns in ecology. However, interrelations between the SAR and species distributions are largely unexplored. This research was aimed (1) to assess if the SAR for a group of sedentary insects is associated to a random or non‐random distribution of species across islands in a land‐bridge archipelago, and (2) to investigate possible factors responsible for the non‐randomness. Communities of tenebrionid beetles on the Aegean Islands (Greece) were studied as a case of a relict fauna. Three aspects of non‐randomness were analysed: (1) non‐random variation of species richness in the SAR, (2) degree of nestedness and (3) presence of special patterns of co‐occurrence. Species co‐occurrence and nestedness analyses indicated that historical aspects, as opposed to interspecific competition or distance‐mediated colonization events, have moulded these species distributions.
Null model tests of clustering of species, negative co‐occurrence patterns and nestedness in meta‐communities
Authors: Hausdorf Bernhard , Hennig Christian
Published: December 2007

We propose tests for patterns in meta‐community structure. The tests for clustering and nestedness of the occurrences of species and negative co‐occurrence patterns provide four important innovations. Firstly, they are not restricted to the analysis of communities along one‐dimensional gradients or to the main axis of variation. Secondly, abundance data can also be considered in the null model whereas most previous approaches could consider only presence/absence data. And thirdly, habitat suitability and spatial autocorrelation can be incorporated in the null model so that patterns that might be caused by biotic interactions can be distinguished from patterns which are the result of differences in the suitability or accessibility of sites for the examined organisms. Finally, the test for nestedness is also appropriate if there is more than one set of nested subsets. A re‐analysis of 35 data sets with these tests showed the importance of considering the autocorrelation of the occurrences of species in analyses of meta‐community structure and demonstrated the advantage of abundance data for tests of clustering of species. With abundance data it could be shown that there is a significant clustering of species, i.e. there are positive associations of species in most meta‐communities, even if an environmentally or spatially constrained null model is used for the test. Co‐occurrence patterns that might indicate interspecific competition were found in many of the analysed presence/absence data sets. Surprisingly the analysis of abundance data sets provides less evidence for interspecific competition. A hierarchical organization of communities, i.e. nestedness, turned out to be a rare pattern, if the autocorrelation of the occurrences of species is considered.
The impact of neutrality, niche differentiation and species input on diversity and abundance distributions
Authors: Zillio Tommaso , Condit Richard
Published: December 2007

We present a spatially‐explicit generalization of Hubbell's model of community dynamics in which the assumption of neutrality is relaxed by incorporating dispersal limitation and habitat preference. In simulations, diversity and species abundances were governed by the rate at which new species were introduced (usually called ‘speciation’) and nearly unaffected by dispersal limitation and habitat preference. Of course, in the absence of species input, diversity is maintained solely by niche differences. We conclude that the success of the neutral model in predicting the abundance distribution has nothing to do with neutrality, but rather with the species‐introduction process: when new species enter a community regularly as singletons, the typical J‐shaped abundance distribution, with a long tail of rare species, is always observed, whether species differ in habitat preferences or not. We suggest that many communities are indeed driven by the introduction process, accounting for high diversity and rarity, and that species differences may be largely irrelevant for either.
Social structure and winter survival in acorn ants
Authors: M. Herbers Joan , A. Johnson Christine
Published: December 2007

Fluctuating selection is a major theme in the evolutionary and ecological literature, yet attempts to measure how differential selection across time or space affects long‐term change in life history traits or behaviors are still rare. Social evolution among the insects has been broadly studied with respect to how key parameters such as queen number and relatedness vary to influence colony fitness. However, a primary focus on fertility selection in the warm months must be complemented by parallel investigations on survivorship selection during the cold months. Here we provide the first assessment of social structure and overwintering survivorship in the field. We studied the acorn ant, which stays aboveground throughout the cold winters in North America, by varying queen number and colony size over two consecutive winters. We found that winter survival was quite low but unconnected to variable colony structure. Therefore previous studies on how social structure affects fertility selection in acorn ants have not been confounded by countervailing selection during the cold months. Our data support the assumption of the larger literature that selective forces molding social behavior in ants act primarily on fertility selection during the reproductive season.