bluuebunny/crossref_metadata_2025 huggingface dataset:
Body size mediated coexistence of consumers competing for resources in space
Authors: Basset Alberto , Angelis Donald L.
Published: December 2007

Body size is a major phenotypic trait of individuals that commonly differentiates co‐occurring species. We analyzed inter‐specific competitive interactions between a large consumer and smaller competitors, whose energetics, selection and giving‐up behaviour on identical resource patches scaled with individual body size. The aim was to investigate whether pure metabolic constraints on patch behaviour of vagile species can determine coexistence conditions consistent with existing theoretical and experimental evidence. We used an individual‐based spatially explicit simulation model at a spatial scale defined by the home range of the large consumer, which was assumed to be parthenogenic and semelparous. Under exploitative conditions, competitive coexistence occurred in a range of body size ratios between 2 and 10. Asymmetrical competition and the mechanism underlying asymmetry, determined by the scaling of energetics and patch behaviour with consumer body size, were the proximate determinant of inter‐specific coexistence. The small consumer exploited patches more efficiently, but searched for profitable patches less effectively than the larger competitor. Therefore, body‐size related constraints induced niche partitioning, allowing competitive coexistence within a set of conditions where the large consumer maintained control over the small consumer and resource dynamics. The model summarises and extends the existing evidence of species coexistence on a limiting resource, and provides a mechanistic explanation for decoding the size‐abundance distribution patterns commonly observed at guild and community levels.
Evaluation of species‐area functions using Sonoran Desert plant data: not all species‐area curves are power functions
Authors: Stiles Arthur , Scheiner Samuel M.
Published: December 2007

Ecologists have been studying the relationship between species richness and area for about a century. As area increases, more species are typically observed. Many mathematical functions have been proposed to describe the pattern of increase. Numerous researchers have assumed that the relationship is a power function despite the fact that there are many possible alternatives. There has been limited work in evaluating which species‐area functions are most appropriate for field data. This study examines which of a variety of functions best describe how Sonoran Desert plant species richness of remnant habitat patches in the Phoenix metropolitan area vary with sampled area and the area of entire patches. No single species‐area function was adequate for describing all empirical datasets. Sample curves of woody species were most frequently best described by the sigmoid logistic, Hill, and Lomolino functions, whereas herbaceous datasets were best fit by the sigmoid logistic or convex rational functions. A curve depicting the relationship between patch‐level woody species richness and patch area was best fit by the convex exponential function. The power function provided the best fit for only one case. This study demonstrates the utility of testing alternative functions for statistical fit rather than assuming that any particular equation adequately describes the species‐area relationship.
Life‐history consequences of vegetative damage in scarlet gilia, a monocarpic plant
Authors: Brody Alison K. , Price Mary V. , Waser Nickolas M.
Published: December 2007

Although herbivory can occur throughout a plant's life, little is known about relative fitness impacts of damage at different life stages. In the long‐lived monocarpic wildflower, Ipomopsis aggregata (scarlet gilia), for example, the response to browsing by ungulates in the year of flowering has been studied extensively, whereas damage and its fitness consequences during the preceding years of vegetative growth remain largely unexplored. As part of a long‐term demographic study of I. aggregata, we mapped 5324 individual seedlings belonging to two annual cohorts in three natural populations, and followed these plants throughout their lives. Of the 30.4% of plants that survived past seedling stage, 15.3% suffered observable damage to the apical meristem of their single rosette of leaves, usually resulting in multiple rosettes in vegetative plants and multiple flowering stalks in plants that survived to flower. Vegetative damage reduced survival to flowering by ca 30%, and slowed growth, leading to an average delay in flowering of over one yr relative to undamaged plants. On average, damaged plants were 136% larger than undamaged members of their cohort in the summer before flowering, but produced only 86% as many flowers, and achieved 78% the overall fitness compared to undamaged plants when damage effects were integrated across their life history. Previously published studies from the same sites suggested that plants suffering browsing during only the year of flowering, achieved 47% of the fitness of unbrowsed plants. Our results indicate that plants are better able to recover from vegetative damage than from loss of their initial reproductive effort, perhaps because of the longer time over which recovery is possible in the former case. These results underscore the importance of studying the consequences of damage across the entire life history.
How is diversity related to species turnover through time?
Authors: Shurin Jonathan B.
Published: December 2007

Empirical studies across a wide range of taxa show that the slopes of species–time relationships often decline as average species richness increases, indicating that more diverse communities have greater temporal stability in species composition. I explored potential explanations for this observation using two simple model formulations for species temporal dynamics. In the Abiotic model, species turnover is governed by the degree of heterogeneity in the environment and the range of species’ tolerances. In this case, more variable conditions lead to lower species richness and higher turnover, but only if the distribution of species’ niche widths and the size of the species pool are independent of the degree of environmental variability. The Biotic model represents direct effects of diversity on turnover through positive or negative feedbacks between diversity and species’ colonization and extinction rates. Declining turnover with increasing richness occurred when higher diversity either facilitated colonization by new species or reduced extinction rates of extant species. Both models could produce the observed pattern of declining turnover at higher diversity under some circumstances, however the conditions for this outcome in the Abiotic model were restrictive and potentially unrealistic. The models provide a process‐based framework for understanding the connection between diversity and species turnover through time.
Diversity effects on invasion vary with life history stage in marine macroalgae
Authors: White Laura F. , Shurin Jonathan B.
Published: December 2007

Most experimental studies of diversity effects on invasibility have reported negative relationships while observational studies have often found positive correlations between the numbers of exotic and native taxa. Nearly all of these studies have been done with terrestrial plants or aquatic invertebrates. We investigated effects of native macroalgal diversity on invasion success of the introduced macroalgaSargassum muticum(Yendo) Fensholt (Phaeophyceae: Fucales) on the west coast of Vancouver Island. We conducted both observational field surveys of the correlation between native diversity and exotic cover, and experimental manipulations of native diversity in constructed 25×25 cm communities. Field surveys found higher cover ofS. muticumin plots with low native diversity, suggesting a negative relationship between diversity and invasibility at the neighbourhood scale. The experiment found initial cover ofS. muticumgermlings was highest in plots with greater diversity. Over the duration of the experiment cover of settled germlings increased fastest in the low diversity plots, so that there was a weak negative effect of diversity on final cover of the invader after 77 days. The slope of the relationship reversed over time, with field patterns and experimental results converging at the end of the experiment. Our results suggest native diversity has contrasting effects on different stages of invasion. Diversity facilitates invader recruitment ofS. muticumbut decreases growth and or survivorship.
Covariation and composition of arthropod species across plant genotypes of evening primrose, Oenothera biennis
Authors: Johnson Marc T. J. , Agrawal Anurag A.
Published: December 2007

Genetic variation in plants has broad implications for both the ecology and evolution of species interactions. We addressed how a diverse community of arthropod species covary in abundance among plant genotypes of a native herbaceous plant (Oenothera biennis), and if these effects scale‐up to shape the composition, diversity, and total abundance of arthropods over the entire lifetime of plants (two years). In a field experiment, we replicated 14 plant genotypes of O. biennis across five field habitats and studied the arthropod communities that naturally colonized plants. Genetic variation in O. biennis affected the abundance of 45% of the eleven common species in 2002, and 75% of sixteen common species in 2003. We examined the strength of correlations in mean abundance of arthropod species among plant genotypes and found that species responded independently to variation among genotypes in the first year of the study, whereas species formed positively covarying clusters of taxa in the second year (rmean=0.35). The strength of these correlations did not consistently correspond to either taxonomy or functional attributes of the different species. The effects of plant genetic variation on the abundance and covariation of multiple arthropod species was associated with cascading effects on higher levels of community organization, as plant genotype and habitat interacted to affect the species composition, diversity, and total abundance of arthropods in both 2002 and 2003, though the specific effects varied across years. Our results suggest that plants may employ generalized resistance strategies effective against multiple herbivores, but such strategies are unlikely to be effective against entire functional groups of species. Moreover, we show that genotypic variation in plants is an important ecological factor that affects multiple levels of community organization, but the effects of plant genotype vary in both space and time.
Post‐invasion evolution of native plant populations: a test of biological resilience
Authors: Mealor Brian A. , Hild Ann L.
Published: December 2007

Contemporary evolution may explain the success of some exotic plant invasions. However, the evolutionary response of recipient native plant populations to exotic invasion has received relatively little attention. Because plant populations are genetically variable, contemporary evolution may also occur in native populations following entry of invasive species. Previously, we documented molecular differences in native populations; here we extend these studies to evaluate growth of native species in a common garden experiment. We seek to determine if three populations of two native grass species (Hesperostipa comata and Sporobolus airoides) demonstrate evidence of contemporary evolution in response to invasion by Acroptilon repens. We obtained 50 genets of the two native grass species from communities long‐invaded (25–80 years) by A. repens and from adjacent, noninvaded areas, and planted five transplants of each genet into two A. repens infestations (Laramie and Fort Steele, Wyoming, USA) to document their growth and survival. Cumulative differences between collections from invaded and noninvaded communities were species‐specific. S. airoides displayed a consistent positive response to long‐term coexistence with A. repens, whereas the performance of H. comata originating from invaded communities was not different from H. comata collected from noninvaded communities. In general, genets from invaded communities had fewer tillers than genets from noninvaded communities, but their relative tiller production (percent increase) was greater for genets from invaded communities at both field transplant sites for both grass species. Basal area increase and overall performance of collections from invaded and noninvaded communities of origin depended on transplant site and grass species. The results suggest that native species have the potential for adaptation to coexist with exotic invasives, although that potential may differ among species.
Reproductive interference, priority effects and the maintenance of parapatry inSkistodiaptomuscopepods
Authors: A. Thum Ryan
Published: December 2007

Reproductive interactions among species, such as attempts to mate with heterospecifics, can have profound impacts on patterns of geographic range and co‐occurrence. For example, several theoretical models demonstrate that reproductive interference‐the negative influence of heterospecifics on the abilities of conspecifics to successfully mate‐can lead to parapatry, even across homogeneous landscapes, when the parapatric species are initially allopatric. However, the potential consequences of reproductive interactions on patterns of range limits and co‐occurrence have been largely ignored by ecologists. Here, I use a combination of laboratory mating experiments and a genetic survey for two parapatric species ofSkistodiaptomuscopepods to evaluate the potential importance of interspecific reproductive interactions on the maintenance of parapatry. The genetic survey demonstrates phylogenetic exclusivity of these species, suggesting that gene flow has not occurred between them. Moreover, laboratory crosses between both species demonstrate that their parapatric boundary is not maintained as a hybrid zone because the two species cannot form viable hybrids. However,Skistodiaptomus oregonensisandS. pygmaeusmales mate indiscriminately and interfere with the ability of heterospecific females to successfully reproduce. I suggest that reproductive interference results in priority effects that strongly influence the maintenance of this parapatric boundary and stress that reproductive traits are important and often‐overlooked in ecological models for species range limits.
Metapopulation structure and habitat quality in modelling dispersal in the butterflyIolana iolas
Authors: G. Rabasa Sonia , Gutiérrez David , Escudero Adrián
Published: December 2007

Quantifying dispersal is fundamental to understanding the effects of fragmentation on populations. Although it has been shown that patch and matrix quality can affect dispersal patterns, standard metapopulation models are usually based on the two basic variables, patch area and connectivity. In 2004 we studied migration patterns among 18 habitat patches in central Spain for the butterflyIolana iolas,using mark–release–recapture methods. We applied the virtual migration (VM) model and estimated the parameters of emigration, immigration and mortality separately for males and females. During parameter estimation and model simulations, we used original and modified patch areas accounting for habitat quality with three different indices. Two indices were based on adult and larval resources (flowers and fruits) and the other one on butterfly density. Based on unmodified areas, our results showed that both sexes were markedly different in their movements and mortality rates. Females emigrated more frequently from patches, but males that emigrated were estimated to move longer daily dispersal distances and suffer higher mortality than females during migration. Males were more likely to emigrate from small than from large patches, but patch area had no significant effect on female emigration. The effects of area on immigration rate and the within‐patch mortality were similar in both sexes. Based on modified areas, the estimated parameter values and the model simulation results were similar to those estimated using the unmodified patch areas. One possible reason for the failure to significantly improve the parameter estimates of the VM model is the fact that resource quantity and butterfly population sizes were strongly correlated with patch area. Our results suggest that the standard VM modelling approach, based on patch area and connectivity, can provide a realistic picture of the movement patterns ofI. iolas.
Disentangling the complexities of vertebrate sex allocation: a role for squamate reptiles?
Authors: Wapstra Erik , Uller Tobias , Pen Ido , Komdeur Jan , Olsson Mats , Shine Richard
Published: December 2007

Sex allocation is an important field in evolutionary biology, both historically and currently. However, while sex allocation theory has successfully predicted sex ratio bias in some taxa, most notably parasitic wasps, vertebrates are notorious for their poor fit to theoretical models. We argue that this arises from the use of very complex model systems to test relatively simple theoretical models. We further argue that squamate reptiles – lizards and snakes – have unduly been neglected in sex allocation studies and in fact may conform more readily to the underlying assumptions of existing theoretical models than many other vertebrates. We provide a five‐point argument in favor of the use of squamates as model systems in sex allocation based on their diversity in sex determining mechanisms, life history biology, and ease of experimental manipulations.
The characterization of complex continuous norms of reaction
Authors: M. Simons Andrew , Wagner Ioan
Published: December 2007

Recent focus on the array of phenotypes expressed under differing environmental conditions, or phenotypic plasticity, has led to increased understanding of its genetic basis as well as its adaptive significance. However, the quantification of plasticity has proven difficult, hampered by both the limited number of environments over which plasticity may typically be assessed and by the need to assume, a priori, the general form of reaction norms under study. Our understanding of the shapes of continuous norms of reaction and, consequently, the subtle differences that may exist in shapes among genotypes or populations is rudimentary. Here, we propose the use of the loess smoothing function to analyze complex norms of reaction and to quantify total plasticity over many environments. A thermogradient incubator offers an ideal means to provide many environments for a demonstration of the use of the loess method. We test seed germination in three populations of two monocarpic plant species for population differentiation in plasticity to temperature. First, we test for differentiation in norms of reaction to 30 temperature environments among three populations of the monocarpic perennial, Lobelia inflata. The second demonstration assesses plasticity to eight temperature environments of three populations of the arctic‐alpine annual, Koenigia islandica. Our demonstration shows that the loess technique can detect significant genetic differentiation among populations in complex norms of reaction for both species studied, and suggests that the use of this procedure should be considered where the form of norms of reaction might be complex. The general applicability of the approach is discussed.
Different carbon support for respiration and secondary production in unproductive lakes
Authors: Karlsson Jan
Published: December 2007

This study investigates the allocation of allochthonous organic carbon (AlloOC) to pelagic respiration and biomass production in unproductive lakes. Metabolic process rates and stable isotopic composition (δ13C) of crustacean zooplankton and respired CO2 were measured in the epilimnion of 13 forest lakes in northern Sweden. The δ13C of zooplankton was low (−31.2 to −38.0‰) compared to that of respired CO2 (−28.4 to −30.6‰), implying that the relative importance of AlloOC was lower for zooplankton (ca 40%) than for respiration (ca 80%). Combining δ13C and carbon flux data revealed that a large amount of metabolized AlloOC was lost in respiration, compared to the amount transferred to zooplankton (<3%). Thus, despite large respiratory losses, AlloOC was still important for zooplankton growth, implying a high supply of AlloOC in comparison to phytoplankton generated organic carbon in the lakes.
Alternative logical states
Authors: K. Didham Raphael , A. Norton David
Published: December 2007

N. W. H. Mason, J. B. Wilson and J. B. Steel argue that there is no logical conceptual basis, and no empirical data, to support an association between environmental adversity and the occurrence of alternative stable states. While we agree that robust debate on the relative frequency of occurrence of alternative stable states is valuable, any apparent logic in their argument is confounded by misinterpretation and direct misrepresentation of our earlier arguments (Didham et al. 2005), despite the fact that we clarified many of the same issues in response to T. Fukami and W. G. Lee (Didham and Norton 2006). Opinion, not logic or evidence, underlies Mason, Wilson and Steel's argument, and this does little to further our understanding of why some systems exhibit alternative stable states that are resilient to restoration management efforts.
Chemical diversity – highlighting a species richness and ecosystem function disconnect
Authors: Epps Kimberly Y. , Comerford Nicholas B. , Reeves, III James B. , Cropper, Jr. Wendell P. , Araujo Quintino R.
Published: December 2007

The lack of predictability in litter‐mix studies may result from the low correlation between species number and the traits that drive the processes under observation. From the standpoint of litter‐quality‐dependent ecological processes, we propose that litter chemical qualities are functional traits and introduce a multivariate index of chemical diversity (CDQ) based on Rao's quadratic entropy to describe the compositional heterogeneity of litter and foliar mixtures. Using published data from temperate and tropical forest systems to illustrate the relationship between species richness and chemical diversity, we show the variation of chemical diversity based on profiles of total nutrient concentrations (N, P, K, Ca and Mg) with species richness. We discuss how this behavior may explain the idiosyncratic responses exhibited in litter‐mix experiments and how it may contribute to the observed dominance of species identity over species diversity. As a summary of resource heterogeneity relevant to detritivore and microbial processes, the chemical diversity index is potentially a better predictor of diversity effects on nutrient dynamics than species richness. Finally, we propose the use of infrared spectroscopy techniques for a rapid and more comprehensive determination of foliar and litter chemical composition to provide a more information‐rich index.
Contrasting effects of natural habitat loss on generalist and specialist aphid natural enemies
Authors: Rand Tatyana A. , Tscharntke Teja
Published: December 2007

The greater susceptibility of higher trophic levels to habitat loss has been demonstrated to disrupt important trophic interactions such as consumer control of prey populations. This pattern is predicted to break down for generalist species that can use matrix habitats, yet empirical studies comparing generalist and specialist enemy pressure in response to natural habitat loss are lacking. Here we examined the effects of landscape simplification resulting from habitat conversion to agriculture on nettles, Urtica dioica, their specialized aphid herbivore, Microlophium carnosum, and associated natural enemies that varied broadly in their degree of specialization. Both nettles and their specialized aphid herbivore were significantly more abundant in complex than simple landscapes. Different enemy groups showed contrasting responses. Aphid specialists (parasitic wasps and cecidomyiid midges) reached higher densities in complex than simple landscapes, and this effect was primarily related to shifts in local resource abundance (i.e. nettle aphid densities). In contrast, densities of generalists (coccinellid beetles and spiders) were significantly higher in simple landscapes, presumably due to spillover of generalists from surrounding cropland habitats. Natural enemy‐prey ratios did not differ significantly across landscape types for specialist groups but were significantly higher in simple than complex landscapes for generalist groups, suggesting that enemy pressure on nettle aphids likely increases with landscape simplification. This was supported by our finding that aphid population growth rates were lower in simple than complex landscapes, and declined significantly with increasing coccinellid densities. Thus, in marked contrast to previous work, our results suggest that natural habitat loss may augment rather than disrupt consumer–prey interactions, and this will depend greatly on the degree of specialization of functionally dominant natural enemies.
Food quality, nutrient limitation of secondary production, and the strength of trophic cascades
Authors: Hall Spencer R. , Shurin Jonathan B. , Diehl Sebastian , Nisbet Roger M.
Published: December 2007

Recent meta‐analyses confirm that the strength of trophic cascades (indirect positive effects of predators on plant biomass through control of herbivores) varies among ecosystem types. In particular, most terrestrial systems show smaller cascades than most aquatic ones. Ecologists still remain challenged to explain this variation. Here, we examine a food quality hypothesis which states that higher quality plants should promote stronger trophic cascades. Food quality involves two components: digestion resistance of plants and magnitude of stoichiometric imbalance between plants and herbivores (where stoichiometry involves ratios of nutrient:carbon ratio of tissues). Both factors vary among ecosystems and could mediate conversion efficiency of plants into new herbivores (and hence control of plants by herbivores). We explored the food quality hypothesis using two models, one assuming that plant stoichiometry is a fixed trait, the other one allowing this trait to vary dynamically (but with a minimal nutrient:carbon ratio of structural mass). Both models produce the same suite of results. First, as expected, systems with more easily digested plants promote stronger cascades. Second, contrary to expectations, higher (fixed or minimal) nutrient:carbon ratio of plants do not promote stronger cascades, largely because of the net result of ecosystem feedbacks. Still, the model with dynamic stoichiometry permits positive correlations of realized plant nutrient:carbon ratio and cascade strength (as predicted), mediated through digestion resistance. Third, lower nutrient:carbon ratio of herbivores promotes stronger cascades. However, this result likely cannot explain variation in cascade strength because nutrient:carbon stoichiometry of herbivores does not vary greatly between terrestrial and aquatic ecosystems. Finally, we found that predation promotes nutrient limitation of herbivores. This finding highlights that food web processes, such as predation, can influence stoichiometry‐mediated interactions of plants and herbivores.
Seed‐size variation determines interspecific differential predation by mammals in a neotropical rain forest
Authors: Mendoza Eduardo , Dirzo Rodolfo
Published: December 2007

It has been suggested that the anthropogenically driven loss of herbivorous mammals might lead to changes in the recruitment patterns of tropical rain forest plants, but few studies have examined the explicit mechanisms that might account for this effect. Here we propose a conceptual model linking differential mammalian defaunation and differential plant recruitment. We posit that in the absence of medium/large herbivores but with small rodent granivores still present (i.e. differential defaunation), predation pressure will be greater on small‐seeded species than on large‐seeded species. We tested such differential predation hypothesis (DPH) by means of a series of experiments directed to assess: 1) seed consumption by small rodents in laboratory cages; 2) seed‐removal rates in small rodent enclosures in the field and 3) removal of seeds placed on the forest floor and exposed to either the full complement of mammals or only small rodents. Seeds used in the experiments were arranged in pairs consisting of species from the same taxonomic family but with a contrasting size (large, small). We found: 1) a significantly greater consumption of smaller seeds (a 2.3‐ to 20.5‐fold difference) in cages and 2) a significantly greater removal of small‐seeded species (a 3.7‐ to 65‐fold difference) in field enclosures. Results of seed removal experiments in free‐access plots and selective exclosures were more complex, with a general absence of significant differences among treatments but we found that predation was in general concentrated on small‐seeded species and small rodents were the predominant visitors to the plots. This, together with the overall short distance of dispersal among large seeds suggest that in differentially defaunated forests large‐seeded species are more likely to escape predation. We posit that such size related differential predation may lead to the floristic impoverishment observed in some defaunated forests.
Consequences of sequential attack for resistance to herbivores when plants have specific induced responses
Authors: Viswanathan D. V. , Lifchits O. A. , Thaler J. S.
Published: December 2007

Plants in nature are attacked sequentially by herbivores, and theory predicts that herbivore‐specific responses allow plants to tailor their defenses. We present a novel field test of this hypothesis, and find that specific responses of Solanum dulcamara lead to season‐long consequences for two naturally colonizing herbivores, irrespective of the second herbivore to attack plants. This result indicates that responses induced by the initial herbivore made plants less responsive to subsequent attack. We show that initial herbivory by flea beetles and tortoise beetles induce distinct plant chemical responses. Initial herbivory by flea beetles lowered the occurrence of conspecifics and tortoise beetles relative to controls. Conversely, initial herbivory by tortoise beetles did not influence future herbivory. Remarkably, the experimentally imposed second herbivore to feed on plants did not modify consequences (induced resistance or lack thereof) of the first attacker. Induction of plant chemical responses was consistent with these ecological effects; i.e. the second herbivore did not modify the plant's initial induced response. Thus, canalization of the plant resistance phenotype may constrain defensive responses in a rapidly changing environment.
Variation in developmental time affects mating success and Allee effects
Authors: Robinet Christelle , Liebhold Andrew , Gray David
Published: December 2007

A fundamental question in biological conservation and invasion biology is why do some populations go extinct? Allee effects, notably those caused by mate location failure, are potentially key factors leading to the extinction of sparse populations. Several previous studies have focused on the inability of males and females to locate each other in space when populations are at low densities but here we investigate the effects of differences in the timing of male and female maturation on mating success. We develop a generalized model to clarify the role of protandry (the appearance of males before female emergence) and variability in adult maturation times. We show that temporal asynchrony can substantially reduce the probability of successful mating. We then apply this generalized model to estimate mating success in invading populations of the gypsy moth in North America in relation to local climate and its associated seasonality. Considerable geographic heterogeneity was observed in simulated mating success and this variability was not correlated with previous evaluations of bioclimatic requirements and habitat suitability. Furthermore, we found that the generalized model of temporal asynchrony provided reliable predictions and that detailed modeling of gypsy moth development was not necessary.
Early is better: seasonal egg fitness and timing of reproduction in a zooplankton life‐history model
Authors: Varpe Øystein , Jørgensen Christian , Tarling Geraint A. , Fiksen Øyvind
Published: December 2007

Timing of reproduction influences future prospects of offspring and therefore the reproductive value of parents. Early offspring are often more valuable than later ones when food availability and predation risk fluctuate seasonally. Marine zooplankton have evolved a diversity of life history strategies in response to seasonality. We present a state‐dependent life history model for the annual and herbivorous high‐latitude copepodCalanoides acutus. Individuals are characterised by four states; developmental stage, structural size, energy reserves and vertical location. There are two habitats, a surface habitat with seasonal predation risk and food availability, and a safer deep habitat with no food and low metabolism (diapause). Optimal life histories (diapause and energy allocation strategies) are found by dynamic programming. Seasonal egg fitness (reproductive value) emerges from the model and peak values are typically before the feeding season. Disentangling the fitness components, we conclude that seasonality in egg fitness is caused both by environmental seasonality in food and predation risk and by time‐constraints on development and diapause preparation. Realised egg production, as predicted from population simulations, does not match the seasonal peak in offspring fitness but is delayed relative to peak egg fitness. We term this an ‘internal life history mismatch’ as constraints and tradeoffs cause sub‐optimal birth dates for most eggs whereas mothers maximise their reproductive value by high number of eggs rather than few and optimally timed eggs. The earliest eggs have a disproportionately high contribution to population recruitment, emphasising the importance of early eggs and the need to understand seasonal patterns in offspring fitness.
Dendrograms and measuring functional diversity
Authors: Petchey Owen L. , Gaston Kevin J.
Published: December 2007

Patterns and changes in functional diversity can inform about spatial and temporal variation in trait diversity, about the processes that drive assembly, and whether assemblages are likely to contain redundant species. We recently provided a new measure (termed FD) and detailed its advantages over previous ones. Since then an increasing amount of research effort has been directed towards both developing appropriate measures of functional diversity and critiquing previous ones, including FD.Podani and Schmera (2006)attempt to do both, though here we argue that they accomplish neither. First, they suggest that a particular distance measure and clustering method are appropriate. We suggest that this is not the case, and show that they may have little effect on quantitative patterns in FD. Second, they suggest that values of functional diversity must be insensitive to the number of functional traits used. We do not agree because we can envisage no relevant ecological question. Third, they observe that we originally defined an FD of zero for an empty assemblage, whereas it is more appropriate for single species assemblages to have FD of zero. We agree. Their solution, however, is to create a measure of functional diversity which violates set monotonicity. Our solution is a revised version of FD for which single species assemblages have FD=0, and which does not violate set monotonicity. In conclusion, we are confident that FD behaves appropriately and note that it remains the measure of functional diversity with greatest power to explain variation in ecosystem functioning.
Leaf pubescence mediates intraguild predation between predatory mites
Authors: Seelmann L. , Auer A. , Hoffmann D. , Schausberger P.
Published: December 2007

Plant morphological traits such as leaf pubescence may affect herbivores and their natural enemies at the individual, population and community levels. Leaf pubescence has been repeatedly shown to mediate predator‐herbivore interactions whereas the influence of leaf pubescence on predator–predator interactions such as intraguild predation (IGP) has seldom been investigated. Using a three‐pronged approach we assessed the influence of leaf pubescence on the predatory mites Kampimodromus aberrans and Euseius finlandicus. Both predators occur on broad‐leaved trees in Europe. Euseius finlandicus is mostly found on trees with glabrous leaves whereas K. aberrans mainly occurs on trees with pubescent leaves. We hypothesized that leaf pubescence mediates IGP between K. aberrans and E. finlandicus and thereby determines their dominance and proportional abundance. A field survey on apple revealed that the abundance of K. aberrans and E. finlandicus is negatively correlated, with the former predominating on cultivars with strongly pubescent leaves and the latter predominating on cultivars with little pubescent or glabrous leaves. Microhabitat choice tests showed that K. aberrans preferentially resides on pubescent leaves whereas E. finlandicus preferentially resides on glabrous leaves. The effects of leaf pubescence on survival and development of immature IG predators and IG prey were reversed for K. aberrans and E. finlandicus. In the presence of the IG predator E. finlandicus, immature K. aberrans had higher survival probabilities on pubescent leaves than on glabrous ones. In contrast, the survival chances of immature E. finlandicus were higher on glabrous leaves than on pubescent ones when the IG predator K. aberrans was present. Artificial leaf pubescence enhanced IG prey capture by immature K. aberrans and prolonged their longevity but impaired IG prey capture by immature E. finlandicus and shortened their longevity. We conclude that leaf pubescence mediates IGP strength and symmetry and discuss the implications to natural and biological control.
No genetic bottleneck or associated microparasite loss in invasive populations of a freshwater amphipod
Authors: Wattier Rémi A. , Haine Eleanor R. , Beguet Jérémie , Martin Guenaël , Bollache Loïc , Muskó Ilona B. , Platvoet Dirk , Rigaud Thierry
Published: December 2007

Understanding what factors determine the success of an invasive species in its adopted range is crucial from an evolutionary ecology point of view, because it can provide insights into which biological characteristics are required for survival in varied environmental conditions. Successful establishment may depend on both maintaining genetic diversity, which will allow the species to evolve and/or adapt to new environments, and the presence or absence of natural enemies such as parasites. We tested these two hypotheses by studying populations of the amphipod crustacean Dikerogammarus villosus. This Ponto‐Caspian invader has rapidly and successfully invaded western Europe and threatens macroinvertebrate biodiversity in its adopted ranges. It is a unique system to study since both its colonisation history and its geographic origins are well‐known. Using samples from the whole geographic range of the invasion route, and using four molecular markers, we found no evidence for genetic bottlenecks during the invasion of D. villosus in western Europe, despite slight variations in allelic proportions according to spatio‐temporal subdivisions of our dataset. In addition, we analysed the prevalence and diversity of parasites across its native and adopted range. We found no macro‐parasites, and no significant parasite loss of microsporidian parasites during the invasive process. Our data suggest that D. villosus invasion was either massive, or recurrent, or both, allowing a parasitic cortege to follow the host. The maintenance of genetic diversity may have contributed to its success, including the variation in resistance in the face of the natural enemies.
In response to the continuum model for fauna research: a hierarchical, patch‐based model of spatial landscape patterns
Authors: Dunn Adam G. , Majer Jonathan D.
Published: December 2007

Models of nature are implicitly influenced by the scale of observation of the processes on which they are founded. The continuum model and the hierarchical patch‐based model are two alternate approaches for the spatial modelling of fauna distribution. The continuum model aggregates continuous approximations to individual landscape characteristics, whereas the hierarchical patch‐based model constructs a hierarchy in which classifications of landscape characteristics describe an interconnected series of patches. We propose the hierarchical patch‐based theory for models of population distributions and landscapes in which the spatial patterns can be effectively represented by mosaics at the variety of levels within the set of individual process models. Given that observations are typically made as points or pixels, and that discrete boundaries exist in both natural and human‐modified landscapes, we suggest that the hierarchical patch‐based method has many applications in conservation and management.
Amphibian species richness across environmental gradients
Authors: Werner Earl E. , Skelly David K. , Relyea Rick A. , Yurewicz Kerry L.
Published: December 2007

Large‐scale field patterns are a fundamental source of inferences on processes responsible for variation in species richness among habitats. We examined species richness of larval amphibian communities in 37 ponds over seven years on the Univ. of Michigan's E. S. George Reserve. Ordination of the community incidence matrix indicated a strong major axis of variation in species associations that was correlated with pond hydroperiod, surface area and forest canopy cover. Communities were significantly nested with those species found in ponds with high canopy cover, small area and short hydroperiod being nested subsets of those found in ponds with contrasting characteristics. Presence of fish had strong negative effects on species richness; relaxation of this effect also was apparent when fish were extirpated from ponds by drought. We employed a model selection analysis to identify the most appropriate statistical model for predicting the long‐term average species richness of these ponds from local abiotic and biotic (predator and competitor density) factors. A model including only the abiotic factors was overwhelmingly superior for the anurans; hierarchical partitioning indicated that area and canopy cover alone accounted for over 70% of the independent effects of predictor variables. The global model including both abiotic and biotic factors was the best supported model for the caudates, and correspondingly hierarchical partitioning suggested that area, hydroperiod, invertebrate predators and caudate biomass all accounted for 9–16% of the independent effects. Overall, biotic factors accounted for much less of the variation in species richness than abiotic factors. The patterns in larger, open‐canopy ponds provided little evidence of competitive effects on species richness, though there were patterns consistent with competitive effects in small, closed‐canopy ponds. The unusual temporal and spatial extent of these data enabled us to critically evaluate ideas regarding patterns in larval amphibian communities, and the effects of area, disturbance (hydroperiod) and productivity (canopy cover) on species richness of these communities. These results have important implications to the conservation of amphibian species richness in freshwater wetlands, which are among the most threatened ecosystems worldwide.
Browse biomass production an.regrowth capacity after biomass loss in deciduous and coniferous trees: responses to moose browsing along a productivity gradient
Authors: Persson Inga‐Lill , Bergström Roger , Danell Kjell
Published: December 2007

To understand how large herbivores interact with their food resources and the ecosystem, it is necessary to understand plant growth responses to biomass loss. Tolerance to biomass loss, and the ability to regrow new photosynthetic tissue after herbivore attack, is one such important mechanism. Here we experimentally studied how year‐round browsing corresponding to four different levels of moose, Alces alces, population density affected total annual browse biomass production, which also serves as a proxy for production of new photosynthetic tissue, in birch (Betula pubescens and B. pendula) and Scots pine, Pinus sylvestris, along a habitat productivity gradient. Our results revealed that habitat productivity index was the most important variable determining annual browse biomass production for birch, and that a high moose density and biomass removals of 25–40% annually actually could stimulate birch above‐ground productivity in more productive sites. Contrary to birch, pine was not affected by habitat productivity index, and appeared to have much lower regrowth capacity than birch. For pine, percent biomass removed was the most important factor affecting annual browse biomass production, which decreased with increasing levels of biomass removal. We also tested if birch and pine responded to biomass removal by allocating more resources to browse biomass production at in the lower portion of the stem (0–0.5 m), which was protected from biomass removal in winter. Birch did not allocate more resources to biomass production in the lower portion of the stem, whereas pines subjected to high biomass removal had more biomass in the lower portion of the stem than those protected from losses. Our study thus shows that plant responses can be linear or non‐linear along the range of herbivore densities we studied, and that responses can change from negative to positive along underlying habitat productivity gradients.
Trophic interception: how a boundary‐foraging organism influences cross‐ecosystem fluxes
Authors: Marczak Laurie B. , Hoover Trent M. , Richardson John S.
Published: December 2007

The rate at which subsidies move between habitats is a function not only of those factors that facilitate such transfers, but also of factors that limit or restrict the movement of subsidies. The interruption or redirection of subsidies by organisms foraging at the boundary between habitats, or trophic interception, has the potential to substantially restructure the food webs of recipient habitats. In this study we describe how a waterstrider, Aquarius remigis, limits the transfer of a subsidy across the land‐water boundary. Prey interception varied with the type of stream habitat; on average, waterstriders in isolated pools intercepted 71.8% of experimental prey inputs of all sizes compared with 21.5% in connected pools and 0.8% in riffles. Across all stream habitat types, waterstriders consumed 43.2% of the experimental inputs of the smallest prey used, the prey size class most similar to natural inputs in our study area. We estimate that foraging waterstriders may intercept 0.3–1.2 kg of terrestrial prey subsidy for every 100 m of stream channel during three summer months, resources that could otherwise support 13–58 young‐of‐the‐year salmonids. In controlled trials, waterstriders significantly altered the amount of terrestrial prey directly consumed by fish, while fish also altered waterstrider interception of those prey. Interestingly, when waterstriders and fish were present together, more terrestrial prey were lost to the streambed than when either or both of these consumers were absent, making this resource available to benthic detritivores, and facilitating the direct incorporation of terrestrial nutrients into stream detrital webs. Overall, we demonstrate that organisms that forage at habitat boundaries can control the quantity and quality of subsidies arriving in recipient habitats, potentially altering food web structures within those habitats.