bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of stoat's presence and auditory cues indicating its presence on tree seedling predation by meadow voles
Authors: Pusenius Jyrki , Ostfeld Richard S.
Published: December 2004

Predators may control the impact of herbivores on their plant resources by 1) decreasing herbivore numbers, 2) imposing predation risk affecting foraging behavior. The goal of the present study was to examine the effects of a predator and auditory cues indicating its presence on the rate of tree seedling (Acer rubrum, Betula lenta) consumption by meadow voles (Microtus pennsylvanicus). The first of our experiments involved introduction of a stoat (Mustela erminea) into an enclosed vole population and the second a playback of recordings of vole distress calls, movements of a stoat and its vocalizations. In both experiments we manipulated vegetation cover and the availability of food next to the experimental seedlings to assess the effects of microhabitat under the different risk situations. The results of the first unreplicated experiment suggested an increased rate of seedling predation in the presence of the stoat. Consistent with these results, the playback of predator sounds in the second replicated experiment caused an increased rate of seedling predation compared to control plots with no recordings. A mowed circle around a seedling station, representing increased risk of predation on the voles, decreased seedling consumption. This effect was modest in the playback treatment. We suggest the results to be due to displacement behavior by the voles exposed to prolonged risk and conflicting demands of foraging and avoiding predators. Alternatively, as suggested by the model of Lima and Bednekoff, prolonged risk of predation forced the voles to decrease their levels of vigilance during low‐risk playback breaks. The modest inhibitory effect of cover removal on seedling predation in the playback treatment is consistent with this interpretation. The results confirm recent evidence for trophic cascades mediated by behavioral interactions between predator and prey. They are novel in suggesting that the presence of predation risk can increase the inhibitory effects of consumers on their resources.
Gregariousness and repellent defences in the survival of phytophagous insects
Authors: Hunter Alison F.
Published: December 2004

Group living has both costs and benefits for plant‐feeding insects, but defence against predators is the most widely acknowledged benefit. Gregarious folivores typically have warning coloration and elaborate anti‐predator defences. Do these defences protect these species from predation? To see if protection from predators generally results from gregariousness, I compared the shapes of published survivorship curves of externally feeding, gregarious and solitary Lepidoptera and Symphyta. Gregarious species are less likely than solitary species to die in the larval stages. However, solitary species that have anti‐predator defences do not have higher larval survival compared to gregarious species. This result, along with evidence from experimental manipulations of group size, suggests that repellent defences per se do not increase survival of gregarious larvae. Group behaviour is undoubtedly important in affecting the higher larval survival of gregarious species, but we currently cannot determine whether predator learning, dilution of risk, or rapid development contribute most to increasing survival.
The effects of familiarity on competitive interactions between threespined sticklebacks
Authors: Utne‐Palm Anne C. , Hart Paul J. B.
Published: December 2004

In this study, pairs of juvenile sticklebacks either familiar with each other or pairs unfamiliar with each other met to share a food source. The study showed that sticklebacks were less aggressive towards a partner when sharing a common food source with a familiar conspecific, compared to when sharing a food source with an unfamiliar conspecific. The results showed that the aggressive behaviour was built up and broken down gradually depending on how long the two competitors had been together or apart. A decrease in aggressiveness was found after the sticklebacks had been together two weeks, with a further decrease after four weeks together. Conversely there was an increase in aggressiveness after the sticklebacks had been parted for two weeks or four weeks. The chance of food being equally distributed between the competing pairs increased with time spent together and decreased with time parted. The observed decrease or increase in the amount of aggression is discussed as a build‐up or a breakdown of rank knowledge or of a co‐operative partnership.
A method to determine rates and patterns of variability in ecological communities
Authors: Collins Scott L. , Micheli Fiorenza , Hartt Laura
Published: December 2004

It is well known that ecological communities are spatially and temporally dynamic. Quantifying temporal variability in ecological communities is challenging, however, especially for time‐series data sets of less than 40 measurement intervals. In this paper, we describe a method to quantify temporal variability in multispecies communities over time frames of 10–40 measurement intervals. Our approach is a community‐level extension of autocorrelation analysis, but we use Euclidean distance to measure similarity of community samples at increasing time lags rather than the correlation coefficient. Regressing Euclidean distances versus increasing time lags yields a measure of the rate and nature of community change over time. We demonstrate the method with empirical data sets from shortgrass steppe, old‐field succession and zooplankton dynamics in lakes, and we investigate properties of the analysis using simulation models. Results indicate that time‐lag analysis provides a useful quantitative measurement of the rate and pattern of temporal dynamics in communities over time frames that are too short for more traditional autocorrelation approaches.
Impact of natural enemies on obligately cooperative breeders
Authors: Courchamp F. , Grenfell B. T. , Clutton‐Brock T. H.
Published: December 2004

Obligately cooperative breeders (cooperators) display a negative growth rate once they fall below a minimum density. Constraints imposed by natural enemies, such as predators or competitors, may push cooperator groups closer to this threshold, thus increasing the risk that stochastic fluctuations will drive them below it. This may indirectly drive these groups to extinction, thereby increasing the risk of population extinction. In this paper, we construct mathematical models of the dynamics of groups of cooperators and non‐cooperators in the presence of two types of enemies: enemies whose dynamics do not depend on the dynamics of their victim (e.g., amensal competitor, generalist predator) and those whose dynamics do. In the latter case, we distinguish positive (e.g., specialist predator) and negative (e.g., bilateral competitor) reciprocal effects. These models correspond to the classical amensal, predation and competition models, in the presence of an Allee effect. We then develop the models to study consequences at the population level. By comparing models with or without an Allee effect, we show that enemies decrease the group size of cooperators more than that of non‐cooperators, and this increases their group extinction risk. We also demonstrate how an Allee effect at a lower dynamical level can have consequences at a higher level: inverse density dependence at the group level generated lower population sizes and higher risks of population extinction. Our results also suggest that demographic compensation can be achieved by cooperators through an increased intrinsic growth rate, or by decreasing the enemy constraint. Both of these types of compensation have been observed in empirical studies of cooperators.
Dung beetle movements at two spatial scales
Authors: Roslin Tomas
Published: December 2004

To understand the dynamics of spatially structured populations, we need to know the level of movements at different spatial scales. This paper reports on Aphodius dung beetle movements at two scales: movements between dung pats within pastures, and movements between pastures.

 First, I test an assumption common to many recent models of spatially structured populations – that the probability of an individual moving between habitat patches decreases exponentially with distance. For dung beetles, I find sufficient evidence to reject this assumption. The distribution of dispersal distances was clearly leptokurtic, with more individuals moving short and long distances than expected on the basis of an exponential function. In contrast, the data were well described by a power function. I conclude that dung beetle movements include an element of non‐randomness not captured by the simplistic exponential model. The power function offers a promising alternative, but the actual mechanisms behind the pattern need to be clarified.

 Second, I compare several species of Aphodius to each other. Although these species occur in the same network of habitat patches, their movement patterns are different enough to result in a mixture of different spatial population structures. Movements between pastures were more frequent the larger the species, the more specific its occurrence in relation to pat age, and the more specialized it is on cow dung and open pasture habitats. Within pastures, all species form “patchy” populations, with much movement among individual pats.
The effects of trophic structure and soil fertility on the assembly of plant communities: a microcosm experiment
Authors: Buckland S. M. , Grime J. P.
Published: December 2004

Using closed, ventilated outdoor microcosms, plant communities were allowed to assemble from a pool of 48 herbaceous species comprising a wide range of plant functional types. The experiment involved factorial manipulation of soil fertility, invertebrate herbivores (slugs, grass aphids) and their predators (ground beetles, ladybirds). In the absence of herbivores the vegetation on soils of high, moderate and low fertility was dominated by plant species which under natural field conditions are restricted to fertile soils. At high fertility, the rate of competitive exclusion was rapid compared with communities persisting at lower fertility which remained species‐rich with high densities of individuals.  The effects of herbivory were profound and attributable mainly to the slug Deroceras reticulatum. At moderate and low soil fertility the main effect of herbivores was to change the ranking of plant species; palatable grasses (e.g. Poa annua, Poa trivialis and Lolium perenne) were suppressed and plant species of slower growth rate (e.g. Festuca rubra, Anthoxanthum odoratum, Festuca ovina) were promoted. More subtle impacts of herbivory altered the size structure of plant populations suggesting the preference, by slugs, for seedlings and small (suppressed) plants. Although ladybirds and their larvae persisted in low numbers in some microcosms, carnivory was more strongly in evidence (lower densities of D. reticulatum and reduced consumption of lettuce discs) in relation to the activities of the ground beetle Pterostichus melanarius. Only in the low fertility treatment, however, was this potential translated into effects on vegetation; for several plant species the frequency of large individuals increased in the presence of P. melanarius.  We conclude that outdoor microcosms provide a valuable ecological tool and, in particular, allow investigation of (a) the role of herbivores in promoting slow‐growing plant species in the vegetation of infertile habitats and (b) the protection of palatable plants against herbivores by carnivores.
Processes structuring communities: evidence for trait‐mediated indirect effects through induced polymorphisms
Authors: Raimondi Peter T. , Forde Samantha E. , Delph Lynda F. , Lively Curtis M.
Published: December 2004

Trait‐mediated indirect effects (TMIs) are changes in the density of one species that are caused by induced changes in one or more traits of an intervening species. For example, induced defense in a prey species may alter the nature of indirect effects that are mediated through that prey species. In the present study, we investigated the TMIs that stem from an interaction between a carnivorous whelk (Acanthina angelica) and an intertidal barnacle (Chthamalus anisopoma). Depending on the timing of the interaction, the predator either kills the barnacle or induces a predation‐resistant morph. Based on previous work that addressed the direct interactions between Chthamalus and other species in the community we predicted and subsequently found that community structure varies as a function of these differing effects. Specifically, we found that Acanthina has a positive indirect effect on mussels when it interacts with (kills) undefended adult barnacles. In contrast, the predator has a positive indirect effect on an encrusting algae (Ralfsia) when it comes into contact with juvenile barnacles, causing the induction of the predator‐resistant morph. We suggest that further research should consider the role of environmentally induced polymorphisms in structuring communities.
Exploitative competition between two seed parasites on the common sedge, Carex nigra
Authors: Ingvarsson Pär K. , Ericson Lars
Published: December 2004

We present results from a two‐year experiment studying the competitive interaction between the gall mite Phytoptus caricis and the smut fungus Anthracoidea heterospora. A factorial addition/removal experiment showed that a negative interaction occurs between A. heterospora and P. caricis, presumably due to competitive interactions in the early infection process of their shared resource, developing utricles on the common sedge, C. nigra. Strong effects of interspecific competition for both species were only evident in 1995 while A. heterospora showed some signs of suffering negative effects from competition also in 1994. Results thus suggest that significant effects of competitive interaction may only be apparent when densities of the two competitors are fairly high. The interaction was strongly asymmetric, with A. heterospora being more affected by the presence of P. caricis than vice versa. Estimates of interaction strength for A. heterospora were about twice as large compared to that for P. caricis. The difference in interaction was significant when data were pooled over the two years of the study. The study highlights the need to consider interspecific interactions between different plant parasites when studying the effects of plant pathogens.
Phenols in reproductive and somatic structures of lichens: a case of optimal defence?
Authors: Hyvärinen Marko , Koopmann Ricarda , Hormi Osmo , Tuomi Juha
Published: December 2004

Optimal defence theory (ODT) attempts to explain variation in plant secondary compounds between different species, different growth conditions and different parts of individual plants. The theory is widely applied to vascular plants and more recently also to seaweeds. Surprisingly, ODT has gained little attention as potential explanation on the distribution of lichen secondary metabolites. In the present study, we analysed intrathalline variation in total phenol content and phenol spectra between reproductive and somatic structures of three foliose lichens, Xanthoria parietina, Vulpicida pinastri and Hypogymnia physodes. The results showed that the concentration of phenolic compounds is higher in sorediate than in non‐sorediate lobe ends of V. pinastri and H. physodes as well as in apothecia of X. parietina compared to other parts of the thallus. These results were in accordance with ODT predicting higher allocation of phenols in structures that are most important for the fitness of an individual genet or ramet. This pattern was parallel in all species regardless whether the compounds originate from either acetate‐mevalonate or shikimic acid pathways. Moreover, both sexual (X. parietina apothecia) and asexual (soralia of V. pinastri and H. physodes) reproductive structures were higher in phenols compared to somatic tissue.
Spite, xenophobia, and collaboration between hosts and parasites
Authors: Rózsa Lajos
Published: December 2004

Results of current computer simulations suggest that a pathogen‐mediated version of Hamilton's spite hypothesis may play an unexpected role in the evolution of host‐parasite interactions. Theory predicts that hosts’ ability to influence parasite transmission to conspecifics is a neutral character provided that transmission routes are random. There are two possible deviations from this. Firstly, vertical transmission is biased in favour of kin, and this selects hosts to decrease pathogen transfer. Secondly, horizontal transmission is biased against kin and this selects hosts to increase pathogen transfer to conspecifics. Unfortunately, we typically do not know transmission statistics in relation to genetic kinship within a host population, thus cannot easily predict whether there is a selective pressure upon hosts either to transmit pathogens or not to transmit. Here I aim to show that even simple descriptive information on host and parasite life‐histories may be used to make rough estimations about transmission routes and selection pressures. Interspecific predictions suggest that pelagic hosts, passively aerial hosts, and the intermediate hosts of pathogens transmitted along the predator‐prey transmission routes are not selected to influence pathogen transmission. Intraspecific predictions suggest that parasitism and disease should facilitate emigration, and immigrants surrounded by non‐related conspecifics should adaptively enhance pathogen transmission to conspecifics. This behaviour may select social animals, like humans, for an adaptive xenophobia in the context of an intraspecific arms race.
Are spring mass migrations of bumblebees and wasps driven by vole cyclicity?
Authors: Vepsäläinen Kari , Savolainen Riitta
Published: December 2004

In search for factors which could drive the spectacular spring migrations of pre‐reproductive queens of bumblebees and social wasps, we made a literature search of potential mortality factors of individuals and colonies. Although the role of parasitism cannot be ruled out, we highlight the potential of vole cyclicity in explaining the migrations. Because the spatial scale of vole‐cycle asynchrony is located at the same level (γ scale) as the range of migratory movements, spring flights could rid bumblebees and wasps of strong nest‐site competition among colony‐founding queens, and of severe predation by voles in high vole‐density regions, to take advantage of abundant, empty vole burrows as nest sites in low‐predation, relatively vole‐free areas. As a corollary of the migratory movements, relaxed parasite pressure is expected.
Condition‐dependent expression of virulence in a trypanosome infecting bumblebees
Authors: Brown M. J. F. , Loosli R. , Schmid‐Hempel P.
Published: December 2004

Parasite virulence affects both the temporal dynamics of host‐parasite relationships and the degree to which parasites regulate host populations. If hosts can compensate for parasitism, then parasites may exhibit condition‐dependent virulence, with high virulence being seen only when the host is under conditions of stress. Despite their usually low level of virulence, theory suggests that such parasites may still affect host population dynamics. We tested whether a trypanosome intestinal parasite of bumblebees, Crithidia bombi, expresses condition‐dependent virulence. Hosts were infected with the parasite and then kept under either favourable or starvation (stressed) conditions. Under favourable conditions the infection caused no mortality, while when hosts were starved the infection increased the host mortality rate by 50%. In addition, we found a parasite‐related change in host resource allocation patterns. Infected bees invested relatively more resources into their fat body and less into their reproductive system than did non‐infected bees. Whether this reallocation is parasite‐driven, to enhance transmission, or a host‐response to parasitism, remains unknown.
A food web/landscape interaction model for microtine rodent density cycles
Authors: Lidicker, Jr. William Z.
Published: December 2004

I present here a conceptual model that combines several factors known individually to be important in microtine population dynamics, but are insufficient separately as causal agents. Specifically, I bring together insights from trophic web theory, particularly the role of generalist and specialist predators, with landscape theory, particularly the influence of habitat patchiness on demographic processes. The resulting trophic/ROMPA interaction model (TRIM) is sufficiently complex and realistic to be used as a substrate for building even more complex and/or locally specific models. Yet, it remains simple enough to be general and widely applicable. The model suggests that predators are an important but not sufficient factor in causing multi‐annual cycles greater than two years in length. In a patchy system, generalist predators have a slightly more severe impact than do specialists, and spillover predation plays a significant role. The ratio of optimal to marginal patch areas (ROMPA) influences the tendency for multi‐annual cycles to occur such that medium ratios generate the strongest tendency toward cycling. However, cycles can occur with high ROMPA and specialist predators. Communities with more than three trophic levels and greater species diversity will be less likely to show cycles. Finally, it should be noted that the model combines factors intrinsic to microtines, extrinsic food‐web interactions, and contextual landscape processes into an integrated framework. A plethora of null hypotheses are thereby generated against which population data can be compared.
Ontogenetic niche shifts in two populations of juvenile threespine stickleback, Gasterosteus aculeatus, that differ in pelvic spine morphology
Authors: Sillett Kristine B. , Foster Susan A.
Published: December 2004

Most research on ontogenetic niche shifts has focused on changes in habitat or resource use related to food resource distribution and heterospecific size‐limited predation. Cannibalism, an intraspecific interaction, can also affect habitat selection or resource use by vulnerable size classes. Morphological defenses, such as spines, increase the effective size of an individual, making it more difficult to consume. The importance of such defense structures in affecting niche shifts in early life history stages is unclear. Using a combination of field observations and experiments in aquaria and wading pools, we examined the relative roles of cannibalism and morphology in determining juvenile habitat use in two populations of threespine stickleback that differ in pelvic spine morphology. Juveniles were categorized into three size classes: small (5–10 mm), medium (11–15 mm), and large (15–25 mm). In experiments assessing the relative vulnerability of juveniles to cannibalism by adults, we documented a significant difference among size classes in the number of juveniles eaten such that more large juveniles were eaten from the population lacking pelvic spines. The natural distribution of small and large juveniles in two distinct littoral microhabitats, open water and vegetation, was determined in each lake. In both populations, small juveniles were more abundant in vegetation. In the population with pelvic spines, a greater proportion of large juveniles was observed in open water than in vegetation. In the population without pelvic spines, the proportion of large juveniles did not differ between the two habitats. Experiments comparing juvenile habitat use in the presence or absence of adult conspecifics suggest that differences in habitat use may not only depend on the size of the individual, or the size of the individual relative to the size of the adult predator, but also on the degree of development or expression of defensive structures.
Why do pollinators visit proportionally fewer flowers in large patches?
Authors: Goulson D.
Published: December 2004

Pollinators collect resources that are patchy, since flowers are usually aggregated on several spatial scales. Empirical studies have established that pollinators almost invariably visit a smaller proportion of flowers as patch size increases. This has not been adequately explained. Here I present data on the payoff curve achieved by bumblebees, Bombus lapidarius, when visiting patches containing different numbers of inflorescences, and use the marginal value theorem to predict the optimum duration of stay within patches. The data demonstrate that visiting a declining proportion of inflorescences as patch size increases is an optimal strategy, if we assume that bees are attempting to maximise their rate of reward acquisition. I argue that this occurs because searching for the remaining unvisited inflorescences is easier in a small patch. On large patches, bees visited more inflorescences per patch than predicted (although still visiting a declining proportion). I suggest that this may occur because bees are using simple departure rules which result in near‐optimal behaviour. I show that a departure rule based on two successive encounters with empty inflorescences closely predicts observed behaviour.
Food limitation in Australian desert rodents: experiments using supplementary feeding
Authors: Predavec Martin
Published: December 2004

Supplementary feeding experiments were used to directly test the hypotheses that populations of two species of Australian desert rodents (Pseudomys hermannsburgensis and Notomys alexis) are food limited. Addition of sunflower seeds to a series of trap plots resulted in an increase in the captures of P. hermannsburgensis. Captures of P. hermannsburgensis were related directly to the amount of supplementary food provided and the amount of food consumed. Supplementary feeding was not, however, able to reverse a population decline, yet it did slow the overall rate of the decline in P. hermannsburgensis. Notomys alexis showed an increase in mean body mass in response to supplementary feeding, but population numbers did not change. Neither species showed a long‐term response to the dispersion of supplementary food (clumped vs spread). The combination of pattern analyses and experimental manipulations suggests that the population of P. hermannsburgensis is limited by the availability of food, whereas the population of N. alexis is not. This contrasts with natural patterns of increase, which suggest similar mechanisms are involved in both species, highlighting the need for experimental evidence to support correlative studies.
Site‐to‐site differences in pollinator visitation patterns in a Louisiana iris hybrid zone
Authors: Emms Simon K. , Arnold Michael L.
Published: December 2004

We studied pollinator visitation rates and movement patterns in experimental arrays of irises at two sites within a Louisiana iris hybrid zone. Arrays contained single‐flowered stems of red‐flowered I. fulva, blue‐flowered I. hexagona, and purple‐flowered F1 hybrids. At one site, where I. hexagona was the only wild‐growing iris, queen bumble bees were the most common pollinator, and the rank order of pollinator visit rates was I. hexagonaI. fulva. At a second site, where I. fulva predominated in the wild, hummingbirds were the most common pollinators, and this order was reversed: I. hexagona<F1<I. fulva. Thus, at both sites, the naturally occurring iris was visited most frequently and F1 flowers were visited more frequently than the non‐native species. Pollinators used both long‐distance and short‐distance cues to discriminate among flowers, and differences in visit rates seemed to be based on differences in the ease of access to pollen and nectar rewards on flowers of different size. Analysis of movement patterns between flowers showed that pollinators were more likely to make parental‐F1 transitions than heterospecific transitions. These data suggest that pollinator behavior acts as a partial barrier to initial hybridization, although this barrier largely breaks down after F1 formation. Our results help to explain the existence of many introgressed populations of Louisiana iris, despite the fact that F1s are extremely rare in the wild.
How can maladaptive habitat choice generate source‐sink population dynamics?
Authors: Remeš Vladimı´r
Published: December 2004

Several theoretical models have been proposed to describe population dynamics in a spatially heterogeneous environment. The source‐sink model is among the most popular. Diffendorfer recently summarized its assumptions and predictions. Given the model reviewed, he argued that source‐sink population dynamics arises if dispersal is somehow constrained. I offer an additional mechanism by suggesting that source‐sink population dynamics can be generated by anthropogenic changes in landscapes that occur so quickly that organisms no longer make optimal habitat selection decisions. Individuals select the same habitats as their ancestors but these decisions no longer provide high fitness because of human‐induced changes in habitat quality, such as increased rates of predation and/or parasitism. Provided that some of the habitats selected are turned by human‐induced changes into sink habitats, source‐sink population dynamics can emerge.
Running with the Red Queen: reflections on ‘Sex versus non‐sex versus parasite’
Authors: Wilkinson David M.
Published: December 2004

The scientific output of W. D. Hamilton can be broadly split into two parts; the first dealing with inclusive fitness and the evolution of social behaviour while the second part focused on the role of parasites in the evolution of sex. This change in research interest was marked by a classic paper published in this journal in 1980, ‘Sex versus non‐sex versus parasite’. To mark Hamilton's death I consider the historical background and influence of this paper.
Population dynamics of large and small mammals
Authors: Erb John , Boyce Mark S. , Stenseth Nils Chr.
Published: December 2004

We offer an evaluation of the Caughley and Krebs hypothesis that small mammals are more likely than large mammals to possess intrinsic population regulating mechanisms. Based on the assumption that intrinsic regulation will be manifest via direct density‐dependent feedbacks, and extrinsic regulation via delayed density‐dependent feedbacks, we fit autoregressive models to 30 time series of abundance for large and small mammals to characterize their dynamics. Delayed feedbacks characterizing extrinsic mechanisms, such as trophic‐level interactions, were detected in most time series, including both small and large mammals. Spectral analyses indicated that the effect of such delayed feedbacks on the variability in population growth rates differed with body size, with large mammals exhibiting predominantly reddened and whitened spectra in contrast with predominantly blue spectra for small mammals. Large mammals showed less variance and more stable dynamics than small mammals, consistent with, among other factors, differences in their potential population growth rates. Patterns of population dynamics in small versus large mammals contradicted those predicted by the Caughley and Krebs hypothesis.
Size of the local species pool determines invasibility of a C4‐dominated grassland
Authors: Smith Melinda D. , Knapp Alan K.
Published: December 2004

The size of the local species pool (i.e., species surrounding a community capable of dispersal into that community) and other dispersal limitations strongly influence native plant community composition. However, the role that the local species pool plays in determining the invasibility of communities by exotic plants remains to be evaluated. We hypothesized that the richness and abundance of exotic species would be greater in C4‐dominated grassland communities if the local species pool included a larger proportion of exotic species. We also predicted that an increase in the exotic species pool would increase the invasibility of sites thought to be resistant to invasion (annually burned grassland). To test these hypotheses, study plots were established within two long‐term (>20 yr) fire experiments at a tallgrass prairie preserve in NE Kansas (USA). Study plots were surrounded by either a small pool of exotic species (small species pool (SSP) plots; six species) or a larger exotic species pool (large species pool (LSP) plots; 18 species). We found that richness and absolute cover of exotic species was significantly (P<0.001) lower (∼70 and 90%, respectively) in annually burned compared to unburned plots, regardless of the size of the exotic species pool. As predicted, exotic species richness was higher (P<0.001) for LSP plots (3.9 per 250 m2) than for SSP plots (0.7 per 250 m2); however, absolute cover was unaffected by the size of the exotic species pool. In the absence of fire, plots with a LSP had four times as many exotic species than SSP plots. An increase in the local exotic species pool also increased the invasibility of annually burned grassland. Indeed, richness of exotic plant species in annually burned LSP plots did not differ from unburned plots with a SSP, indicating that a larger pool of exotic species countered the negative effects of fire. These findings have important implications for predicting how the invasion of plant communities may respond to human‐induced global changes, such as habitat fragmentation. Community characteristics or factors such as frequent fires in grasslands may impart resistance to invasions by exotic species in large, intact ecosystems. However, when a large pool of exotic species is present, frequent fire may not be sufficient to limit the invasions of exotic plants in fragmented landscapes.
Sexually transmitted parasites and sexual selection in the milkweed leaf beetle, Labidomera clivicollis
Authors: Abbot Patrick , Dill Larry M.
Published: December 2004

Milkweed leaf beetles (MLBs; Labidomera clivicollis Chrysomelidae) are parasitized by a subelytral mite, Chrysomelobia labidomerae (Tarsonemina: Podapolipidae). We show that C. labidomerae is transmitted between MLBs when they copulate, and can reduce the survival of nutritionally stressed beetles. We investigated the effect of this sexually transmitted parasite on mate choice and male‐male competition in MLBs, and the consequences of variation in these behaviours for mite transmission. We found no evidence of parasite avoidance by MLBs, and evidence for high rates of parasitism in the MLB populations we surveyed. In the absence of females, parasitized males contacted unparasitized males more often and for longer than controls, and they tended to displace rival males from females more often than did unparasitized males, a result consistent with the interpretation that parasitized beetles compensate for loss of fitness by increasing reproductive effort. These changes can also benefit mites, because longer and possibly more contacts between beetles provide more opportunities for transmission, but there is no evidence that these changes in male behaviour result from parasite manipulation.
How does environmental variation translate into biological processes?
Authors: Laakso Jouni , Kaitala Veijo , Ranta Esa
Published: December 2004

Birth and death rates, as so many other biological processes, are usually not linearly related to environmental variation. Common examples of non‐linear response forms include unimodal “optimum‐type” responses and various saturating responses. These responses filter the signal coming from the environment to a corresponding biological process. We explored how different types of environmental signal may be transformed to a biological process. We were interested in the effect of the filter on modulation of (1) the variance of the signal, on (2) the variance‐covariance structure between the signal and the filtered signal, and on (3) the match between the power spectra of the signal and the filtered signal. We found that the filters will change the frequency distribution (mean, variance, modality) of the signal. Especially symmetric filters that have a single peak of optimum will change signal structure so that there either exists or does not exist a correlation between the signal and the filtered signal. When the correlation exists it may be either positive or negative depending on the signal's mode relative to the filter structure. Also, the power spectrum properties of the signal may be dramatically transformed after passing the filter, e.g., blue noise can turn into red noise. Our results strongly suggest that studies on the influence of external signals on biological processes, such as population dynamics, should explicitly consider how the signal is transferred to biological processes.
Separating the chance effect from other diversity effects in the functioning of plant communities
Authors: Lepš Jan , Brown Valerie K. , Diaz Len Tomas A. , Gormsen Dagmar , Hedlund Katarina , Kailová Jana , Korthals Gerard W. , Mortimer Simon R. , Rodriguez‐Barrueco Claudino , Roy Jacques , Santa Regina Ignacio , Van Dijk Cornelis , Van Der Putten Wim H.
Published: December 2004

The effect of plant species diversity on productivity and competitive ability was studied in an experiment carried out simultaneously in five European countries: Czech Republic (CZ), the Netherlands (NL), Sweden (SE), Spain (SP), and United Kingdom (UK). The aim was to separate the ‘chance’ or ‘sampling effect’ (increasing the number of sown species increases the probability that a species able ‘to do a job’ will be included) from the complementarity effect (species‐rich communities are better able to exploit resources and to take care of ecosystem functions than species‐poor communities). In the experiment, low diversity (LD) and high diversity (HD) mixtures of grassland species were sown into fields taken out of arable cultivation. The HD mixture consisted of five grass species, five legumes and five other forbs. The LD mixtures consisted of two grasses, one legume and one other forb, with different plant species combinations in each replicate block. The design of the experiment was constructed in such a way that the total number of seeds of each species over all the replications was exactly the same in HD and LD treatments, and the total number of grass seeds, leguminous seeds and other forb seeds were the same in both LD and HD. The responses measured were the total above‐ground biomass (as a measure of productivity) and the average number of naturally establishing species in a plot (as a measure of the competitive ability of the mixture), both measured in the third year of the experiment.  The results show that, on average, the HD plots performed better (i.e., attained higher biomass, had better weed suppression), but that the best LD mixture was as good as the best HD mixture. On the contrary, the worst LD mixture was always less successful than the worst HD replicate. The performance of particular species in the HD mixtures was a good predictor of the success of a certain species combination in a LD mixture (explaining 61% of variability between particular LD mixtures). In all sites, the LD mixture composed of species which were the most abundant in HD mixtures was as efficient in suppressing weeds as the HD mixture itself. It is argued that the performance of a species assemblage is influenced mostly by the identity of species and the diversity effect is mainly due to the ‘chance’ or ‘sampling’ effect: with increasing number of species the probability that an important species will be included in the mixture increases. Caution is urged in interpreting experiments with manipulated diversity and the possible limitations of such experiments are discussed.
Predator‐induced life history changes in amphibians: egg predation induces hatching
Authors: Chivers Douglas P. , Kiesecker Joseph M. , Marco Adolfo , Devito Jill , Anderson Michael T. , Blaustein Andrew R.
Published: December 2004

The timing of transitions between life history stages should be affected by factors that influence survival and growth of organisms in adjacent life history stages. In a series of laboratory experiments, we examined the influence of predation risk as a cue to trigger a life history switch in amphibians. In the Oregon Cascade Mountains, some populations of Pacific treefrogs (Hyla regilla) and Cascades frogs (Ranacascadae) are under intense egg predation by predatory leeches (families Glossiphonidae and Erpobdellidae). We document that both treefrogs and Cascades frogs show plasticity in hatching characteristics in response to the threat of egg predation. Pacific treefrogs hatch sooner and at an earlier developmental stage when either predatory leeches or non‐predatory earthworms are allowed direct contact with the developing egg mass. The same response is elicited even without direct contact. Chemical cues of predatory leeches and chemicals released from injured eggs appear to elicit the same early hatching response in treefrogs. For Cascades frogs, cues of leeches, but not those of injured eggs, elicit an early hatching response. Hatching early in response to egg predators may reduce predation. Plasticity of hatching characteristics has rarely been examined. However, we suspect that it may be common, particularly in populations or species that experience high variability in predation pressure between years.
Reconstructing the evolution of birds on islands: 100 years of research
Authors: Grant Peter R.
Published: December 2004

Birds, especially those on islands, have contributed disproportionately to the development of theories that explain the origin and maintenance of organic diversity. This essay surveys the major ideas of how bird species are thought to evolve, speciate and multiply in adaptive radiations on islands. I begin with the nineteenth century view that speciation is the result of natural selection slowly causing divergence of geographically separated populations. I then trace the course of discovery and deeper understanding during the twentieth century, from the influence of Mendelian genetics at the beginning to the modern, molecular era at the end. The dominant and pivotal evolutionary question has been how new species are formed. An initial, nearly exclusive, attention to adaptive processes has been replaced by a search for additional reasons for allopatric divergence, coupled with the conditions that permit coexistence in sympatry after allopatric divergence has occurred. Recent studies have shown that pre‐mating barriers to gene exchange caused by natural selection and sexual selection often arise before post‐mating genetic incompatibilities have evolved. Therefore behavioural and ecological factors play a predominant role in this stage of speciation. Imprinting and imprinting‐like processes of early learning contribute to pre‐mating barriers by constraining mate choice. Ecological factors determine coexistence potential and the fates of hybrids. Genetical factors at all stages, from the founding of a new population to the evolution of genetic incompatibilities, are still poorly known. But molecular genetics is now helping us to reconstruct evolutionary history, thereby transforming ideas about systematic relationships, colonization routes and both the tempo and mode of evolution. As patterns of evolution within radiations become better known, more attention needs to be given to the task of explaining the sequential build‐up of avian communities; by evolutionary radiation and additional immigration, offset to some extent by extinction. The new and substantial challenge is to understand evolutionary radiations in relation to changing environments. Until that challenge is met we cannot claim that the problem of explaining adaptive radiations of birds or any other group of organisms is solved.
A model of triploid endosperm evolution driven by parent‐offspring conflict
Authors: Härdling Roger , Nilsson Patric
Published: December 2004

The parental investment in angiosperms comprises the endosperm, a nutrient reserve that is used during seed development. The endosperm contains genes from both parents. The most common endosperm form is the 3n Polygonum‐type with more maternal genetic influence than paternal, i.e. with two maternal nuclei and one paternal nucleus. The evolutionary original state is thought to be a diploid endosperm with equal influence of the parents. We focus on the evolution of the triploid endosperm and show that a gene for triploid endosperm would have an initial advantage in a population of diploid endosperm type plants, and increase to fixation. We assume that endosperm amount is controlled by endosperm genes. Then a gene causing triploid endosperm will increase the influence of the mother plant on parental investment. The production of endosperm with two copies of the maternal genes will modify the inheritance of endosperm amount and cause an increased production of seeds.
Costs and benefits for phytophagous myrmecophiles: when ants are not always available
Authors: Stadler Bernhard , Fiedler Konrad , Kawecki Tadeusz J. , Weisser Wolfgang W.
Published: December 2004

Costs and benefits in mutualistic associations between ants and phytophagous myrmecophiles are context dependent. We collected information from the literature on costs and benefits of myrmecophily in aphids, coccids, membracids and lycaenids. A key result of the literature survey is that investment in mutualism with ants entails costs paid not only when ants are present (direct costs) but also when they are absent (indirect costs). We incorporated such a trade‐off in a model that investigates the fitness consequences of the decision of a potential myrmecophile whether or not to invest in cooperation with ants. The model shows that whether myrmecophily should be favoured depends on the rate of increase of the population, and, if there are indirect costs, on the frequency of habitats with ants. Both direct and indirect costs can limit or prevent the evolution of myrmecophily even when ants are abundant. To understand the patterns of associations in the field we therefore need to measure the benefits and costs of myrmecophily both in the presence and in the absence of ants.