bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental effects of habitat fragmentation on rove beetles and ants: patch area or edge?
Authors: Golden David M. , Crist Thomas O.
Published: December 2004

The effects of habitat fragmentation may include the loss of species from isolated fragments or changes in species abundances among habitats that differ in area, structure, or edge characteristics. We measured the species richness and abundance of ground‐dwelling insects in a 1.14‐ha old field that was mowed to produce patches of unmowed vegetation which differed in size, degree of isolation, and the amount of habitat edge. Four treatments – ranging from unfragmented (169‐m2) to
highly fragmented (1‐m2) patches – were replicated four times in a Latin square design, and insects were sampled twice during 1995 using 177 pitfall traps. Species richness showed a non‐monotonic response to fragmentation, with the fewest species occurring in the slightly fragmented treatment. Responses of rove beetles and ants, the most species‐rich and abundant taxa, respectively, were similar to the overall insect community but ants had a stronger and more consistent treatment effect in both sample months. Ordinations of ant and rove‐beetle assemblages using nonmetric multidimensional scaling showed that the slightly fragmented treatment differed from other treatments in species occurrence and abundance. The lower species richness in the slightly fragmented treatment was primarily due to a subset of ant and rove beetle species that showed a lower abundance than in other treatments, possibly because this treatment had the greatest amount of habitat edge. We hypothesize that the non‐monotonic species response to fragmentation was due to the differential effects of habitat edge on species movements across the habitat boundary between unmowed patches and mowed areas. A greater effect due to the amount of habitat edge rather than total patch area, at least among the range of patch sizes studied, suggests that the length of habitat edge may be quite important to the distribution and abundance of ground‐dwelling animals in fragmented habitats.
Individual variability and population regulation: an individual‐based model
Authors: Uchmański Janusz
Published: December 2004

To study the influence of individual variability on population dynamics an individual‐based model of the dynamics of a single population consisting of different individuals is constructed. The model is based on differences in individual assimilation rates due to intraspecific competition and variability of initial weights. The model exhibits “imperfect regulation”, i.e., the number of individuals in the population oscillates and sooner or later the population becomes extinct. When individual variability is included, the model produces longer population extinction times than without individual variability. The average extinction time is not however a monotonic function of the degree of individual variability.
Stratification based on reproductive state reveals contrasting patterns of age‐related variation in demographic parameters in the kittiwake
Authors: Cam Emmanuelle , Monnat Jean Yves
Published: December 2004

Heterogeneity in individual quality can be a major obstacle when interpreting age‐specific variation in life‐history traits. Heterogeneity is likely to lead to within‐generation selection, and patterns observed at the population level may result from the combination of hidden patterns specific to subpopulations. Population‐level patterns are not relevant to hypotheses concerning the evolution of age‐specific reproductive strategies if they differ from patterns at the individual level. We addressed the influence of age and a variable used as a surrogate of quality (yearly reproductive state) on survival and breeding probability in the kittiwake. We found evidence of an effect of age and quality on both demographic parameters. Patterns observed in breeders are consistent with the selection hypothesis, which predicts age‐related increases in survival and traits positively correlated with survival. Our results also reveal unexpected age effects specific to subgroups: the influence of age on survival and future breeding probability is not the same in nonbreeders and breeders. These patterns are observed in higher‐quality breeding habitats, where the influence of extrinsic factors on breeding state is the weakest. Moreover, there is slight evidence of an influence of sex on breeding probability (not on survival), but the same overall pattern is observed in both sexes. Our results support the hypothesis that age‐related variation in demographic parameters observed at the population level is partly shaped by heterogeneity among individuals. They also suggest processes specific to subpopulations. Recent theoretical developments lay emphasis on integration of sources of heterogeneity in optimization models to account for apparently “sub‐optimal” empirical patterns. Incorporation of sources of heterogeneity is also the key to investigation of age‐related reproductive strategies in heterogeneous populations. Thwarting “heterogeneity's ruses” has become a major challenge: for detecting and understanding natural processes, and a constructive confrontation between empirical and theoretical studies.
Interactions between above‐ and belowground biota: importance for small‐scale vegetation mosaics in a grassland ecosystem
Authors: Blomqvist M. M. , Olff H. , Blaauw M. B. , Bongers T. , Van Der Putten W. H.
Published: December 2004

Grasslands are often characterised by small‐scale mosaics in plant community composition that contribute to their diversity. Although above‐ and belowground biota can both cause such mosaics, few studies have addressed their interacting effects. We studied multi‐trophic interactions between aboveground vertebrate grazers, subterranean ants, plant‐pathogenic soil biota (especially nematodes) and the vegetation in a temperate grassland. We found that when rabbits and cattle locally omit vegetation patches, yellow ants (Lasius flavus) respond to the
taller vegetation by digging up more sand from deeper soil layers (hence
making taller nest mounds), probably to maintain sufficiently high soil
temperatures. We found that this ant digging affects other soil biota, as
the mounds contain fewer plant‐parasitic and fungivore nematodes. Also,
the mounds have lower moisture content and soil bulk densities, and
higher pH and available nutrient content than the directly surrounding
soil. The clonal sedge Carex arenaria grows vigorously on the
mounds, producing more shoots and shorter rhizome internode lengths than
in surrounding vegetation. Other plant species, such as the grass
Festuca rubra, dominate the surrounding vegetation. A greenhouse
bioassay experiment revealed that harmful soil organisms (as
plant‐parasitic nematodes and pathogenic fungi) outweighed the effect of
beneficial organisms (e.g., mycorrhizae) in this system. Rhizome biomass
and shoot production of C. arenaria were indeed inhibited less by
biota in soil from ant mounds than by biota in soil from the surrounding
vegetation. However, the total biomass production of C.
arenaria was inhibited as strongly in both soil types. F. rubra was inhibited more strongly by biota in the surrounding soil. We suggest that various direct and indirect interactions between above‐ and belowground biota can contribute to community mosaics and hence diversity in grasslands.
Foraging information affects the nature of competitive interactions
Authors: Sandlin Elizabeth A.
Published: December 2004

If animals can learn environmental cues, how might their use of information affect competitive interactions between species? I used shared‐preference isoleg theory to generate four different predictions regarding possible ways information could affect density‐dependent habitat selection. To test these predictions, I conducted field experiments on the foraging behavior of three coexisting species of hummingbirds. I studied black‐chinned (Archilochus alexandri), blue‐throated (Lampornis clemenciae), and magnificent (Eugenes fulgens) hummingbirds at the Southwestern Research Station in the Chiricahua Mts. of Southeastern Arizona, USA. Blue‐throated hummingbirds behaviorally dominate the other two species. I found that birds foraging with complete information (via learned color cues) avoid some of the negative effects from competition. Birds that foraged with complete information remained highly selective on rich feeders even with high competitor densities. When birds suffered an information deficit, however, the two subordinate species shifted their foraging preferences. The dominant species did not shift preference. Each subordinate's shift reflects its unique place in this competitively structured guild. With high competitor densities, black‐chinneds shifted from selective to opportunistic foraging when they also suffered from an information deficit. Thus, an information deficit caused black‐chinneds to cross over their first isoleg. Rather than reduce foraging efficiency, the magnificents shortened their foraging time. I use the magnificents’ response to competition and an information deficit to explore the possibility that this guild of hummingbirds is centrifugally organized. My results require us to consider the information‐gathering (e.g., learning) abilities of individual decision‐makers when we evaluate density‐dependent habitat selection. Differences in the level of usable information can alter the way coexisting species respond to competitive interactions that potentially structure communities.
Plant diversity increases resistance to invasion in the absence of covarying extrinsic factors
Authors: Naeem Shahid , Knops Johannes M. H. , Tilman David , Howe Katherine M. , Kennedy Theodore , Gale Samuel
Published: December 2004

Biological invasion is a widespread, but poorly understood phenomenon. Elton's hypothesis, supported by theory, experiment, and anecdotal evidence, suggests that an important determinant of invasion success is resident biodiversity, arguing that high diversity increases the competitive environment of communities and makes them more difficult to invade. Observational studies of plant invasions, however, find little support for this hypothesis and argue strongly against it. Lack of control of extrinsic factors (e.g., disturbance, climate, or soil fertility) that covary with biodiversity and invasion in observational studies makes it difficult to determine if their findings truly refute Elton's hypothesis. We examined performance of Crepis tectorum (an invasive, annual composite weed) in experimental prairie grassland plots and greenhouse plant assemblages in which resident species richness was directly manipulated. Under these conditions, unlike observational studies, no covarying extrinsic factors could interfere with interpreting results. We found a strong inverse association between resident diversity and invader performance as predicted by Elton's hypothesis. Higher resident diversity increased crowding, decreased available light, and decreased available nutrients all of which increased the competitive environment of diverse plant assemblages and reduced C. tectorum success. Examination of individual resident species impacts on C. tectorum performance demonstrated that this diversity effect was not due to the sampling effect. These results suggest that both Elton's hypothesis and its competitive mechanism may operate in nature, but covarying extrinsic factors may obscure the negative impact of diversity on invader success.
The species richness–biomass relationship in herbaceous plant communities: what difference does the incorporation of root biomass data make?
Authors: Liira Jaan , Zobel Kristjan
Published: December 2004

So far, in all studies on the much‐discussed hump‐backed relationship between plant community productivity and species richness, productivity has been assessed through plant shoot biomass, i.e. it has been ignored that frequently most of the biomass is produced below ground. We revisited the 27 grassland and forest field‐layer communities, studied earlier by Zobel and Liira, to sample root biomass, plant total biomass and root/shoot allocation, and learn how the incorporation of below‐ground biomass data would affect the shape of the hump‐backed relationship. In order to avoid scaling artefacts we estimated richness as the average count of species per 500 plant ramets (absolute richness). We also included relative richness measures. Relative richness was defined as richness per 500 ramets/size of the actual species pool (the set of species present in the community), relative pool size was defined as size of the actual species pool/size of the regional species pool (the set of species available in the region and capable of growing in the given community).  The biomass‐absolute richness relationship was humped, irrespective of the biomass measure used, the hump being most obvious when plant total biomass was used as the independent variable. Evidently, the unimodal richness–productivity curve is not a sampling artefact, as suspected by Oksanen. However, relative richness was not related to community biomass (above‐ground, below‐ground or total). The hump‐backed curve is shaped by the sizes of actual species pools in communities, implying that processes which are responsible for small‐scale diversity pattern mainly operate on the community level.  Neither absolute nor relative richness were significantly related to root/shoot allocation. The presumably stronger (asymmetric) shoot competition at greater allocation to shoots appears not to suppress small‐scale richness. However, there is a significant relationship between relative pool size and root/shoot allocation. Relatively more species from regional species pools are able to enter and persist in communities with more biomass allocated into roots.
Testing sex‐allocation theory: flowers vs seeds in hermaphrodite plants
Authors: Rademaker Mariëlle C. J. , De Jong Tom J.
Published: December 2004

Plants of the self‐compatible Echium vulgare and Cynoglossum officinale frequently abort seeds, while all flowers contain pollen. Sex allocation then changes with the seed to flower ratio. Based on our field data, we can draw fitness sets. The analysis shows that the fitness set is bowed out and that hermaphroditism is evolutionarily stable in both species. It also predicts correctly the size‐dependent pattern of seeds per flower observed in C. officinale in the field, where small plants produce 0.39 and large plants 0.85 seeds per flower. The model fails to predict the pattern in E. vulgare.
Microhabitat use, giving‐up densities and temporal activity as short‐ and long‐term anti‐predator behaviors in common voles
Authors: Jacob Jens , Brown Joel S.
Published: December 2004

We used depletable food patches to determine the effect of microhabitat (mowed versus unmowed adjacent grasslands) and time (day versus night) on the foraging behavior of common voles (Microtus arvalis). The food remaining after 12‐h periods (giving‐up density, GUD) measured the vole's habitat selection under predation risk. In accord with several other rodent species and the effects of avian predators, voles had significantly lower GUDs in the unmowed than mowed portion of the grassland. GUDs in patches along the border between adjacent habitats were more similar to the risky mowed grassland than the safe unmowed grass. Time interacted strongly with microhabitat. In the mowed grass, voles had significantly higher GUDs at night than day. Whereas in the unmowed grass, GUDs were significantly higher during the day than night. Vole GUDs did not vary with time along the boundary. This suggests that predators are more abundant or effective in the mowed grass at night (owls?), and in the unmowed grass during the day (weasels?). In terms of predation risk, the voles perceived the mowed grass at night as the riskiest and the unmowed grass at night as the safest. Voles may have difficulties assessing resources under high predation risk: GUDs among patches were well equalized in the unmowed microhabitat whereas in the mowed grass only day GUDs did not vary significantly among patches. We linked these results to the vole's day‐night‐activity and life span. For the 533 voles live‐trapped at the study area, the ratio of day versus night captures for each individual served as an activity index and the span between first and last capture measured minimum life span. In accord with higher GUDs at night, very few individuals behaved selectively towards the night, but individual life expectancy increased with temporal opportunism. Microhabitat differences in GUDs reflect short‐term strategies of predator avoidance and the trapping data reflect long‐term patterns of anti‐predator behavior.
The effect of mammalian predator scent on the foraging behavior of Dipodomys merriami
Authors: Herman Cathy Soltz , Valone Thomas J.
Published: December 2004

Previous work has shown that kangaroo rats (Dipodomys) adjust foraging behavior to perceived risks of predation assessed via visual and auditory cues. A field study was performed in the Mojave Desert to determine if D. merriami also assesses predation risk via olfaction. The presence of mammalian predator urine was manipulated at food patches in two microhabitats (open and under shrubs) during two seasons and giving‐up densities (GUDs) were measured to quantify D. merriami's foraging behavior. In winter, D. merriami exhibited a preference for feeding under shrubs but the presence of predator scent in the shrub microhabitat significantly reduced foraging activity. In summer, D. merriami again exhibited a strong preference for the shrub microhabitat but no effect of predator scent was observed. These results indicate that predator scent can affect kangaroo rat behavior suggesting that olfaction is used by kangaroo rats to assess predation risk. The results thus provide a more complete description of how predators affect the foraging behavior of desert rodents.
Correlated evolution of the association between aphids and ants and the association between aphids and plants with extrafloral nectaries
Authors: Offenberg Joachim
Published: December 2004

The evolution of extrafloral nectaries (EFN) and the evolution of ant‐aphid associations may have influenced each other. Published records allowed me to determine whether aphid species are associated with ants and whether they are associated with host plant species with EFNs. On the basis of these results a comparative phylogenetic study was conducted on a subgroup of monoeocious aphid species living above the soil surface. As aphid phylogeny was unresolved below the family level, I analysed two families – Aphididae and Drepanosiphidae – separately. Within each family, a large number of random phylogenies were generated and each random tree was analysed with a standard phylogenetic approach. The results suggest, on the one hand, that being tended by ants increases the likelihood that an aphid species will evolve an association with host plants that produce EFNs, or on the other hand, that aphid species associated with host plants carrying EFNs were more likely than other species to evolve an association with ants. I present two new hypotheses – the host‐selection hypothesis and the host‐sharing hypothesis – to explain these evolutionary patterns. The hypothesis that ant‐attended homopterans may function as EFNs is rejected by the evolutionary patterns found in this study.
Spatial autocorrelation and the interpretation of patterns of tree seed and seedling predation by rodents in old‐fields
Authors: Manson Robert H.
Published: December 2004

Spatial autocorrelation is ubiquitous in ecological systems. However, consideration of this phenomenon in seed and seedling predation studies has been limited, perhaps explaining some of the variation observed between studies and the difficulty in generalizing about the impacts of herbivores on successional processes. I examined the prevalence and patterns of spatial autocorrelation in seed and seedling survival between neighboring tree propagules in old‐fields. All experiments were conducted within enclosures in which I manipulated the density of meadow voles (Microtus pennsylvanicus). Voles, and competitively inferior white‐footed mice (Peromyscus leucopus), are the dominant consumers of tree seedlings and seeds in old‐fields in the northeastern United States, respectively. An intermediate level of spatial autocorrelation was present in seed and seedling predation trials. Furthermore, the frequency of spatial autocorrelation increased under low‐vole density conditions and in study sites with greater heterogeneity in vegetation cover. Survival of neighboring tree propagules was generally positively autocorrelated, suggesting that foraging by mice and voles contributes to a clumped pattern of tree propagule establishment in old‐fields. A review of published predation studies found that only 8.3% attempted to test for spatial autocorrelation and that the minimum average spacing used between replicates is well below the average autocorrelation distance of 8.1±6.3 m observed in the current study. Patterns of autocorrelation appear somewhat predictable based on knowledge of the ecology of mice and voles and to provide insights into the factors influencing their foraging behavior and potential impacts on old‐field succession.
Resource partitioning of the host fungus Coriolus versicolor by two ciid beetles: the role of odour compounds and host ageing
Authors: Guevara Roger , Hutcheson Kerry A. , Mee Andrea C. , Rayner A. D. M. , Reynolds S. E.
Published: December 2004

The ciid beetles Octotemnus glabriculus and Cis boleti exploit different developmental stages of fruit bodies of their preferred host fungus Coriolus versicolor. Larvae of the smaller beetle, O. glabriculus, mainly use young, expanding, fruit bodies; adults of O. glabriculus are predominantly found in young fruit bodies. By contrast, adults and larvae of the larger beetle, C. boleti, are prevalent in fully developed fruit bodies of C. versicolor. Because fruit bodies of most genets emerge during spring and early summer and mature by autumn, O. glabriculus and C. boleti breed in separated seasons. Adults and larvae of O. glabriculus are abundant in spring and early summer. By contrast, the number of adults and larvae of C. boleti increases gradually from late spring to summer and peaks in autumn. We conducted a field experiment that suggests that the phenological dynamics of C. versicolor fruit bodies drive the separation of breeding seasons between O. glabriculus and C. boleti. Additionally, laboratory experiments revealed that O. glabriculus and C. boleti have differential behavioural responses to odour compounds from young and mature fruit bodies of C. versicolor. We conclude that age‐related changes in the chemical composition of fruit bodies may allow O. glabriculus and C. boleti to discriminate among C. versicolor, thus providing a mechanism for the partitioning of the resource.
Habitat‐dependent competition and the coexistence of Australian heathland rodents
Authors: Morris Douglas W. , Fox Barry J. , Luo Jia , Monamy Vaughan
Published: December 2004

When competing species depress one another's fitness in the habitats that they occupy, their competitive effects will emerge in each species’ pattern of density‐dependent habitat choice. Thus, a regression analysis of joint densities, corrected by the habitat effect, should reveal the magnitude of interspecific competition. We tested this idea by 1) demonstrating the connection between removal experiments and regression estimates of competition with those obtained from isodars (regressions that implicitly incorporate evolutionarily stable strategies of habitat selection) and 2) evaluating whether interspecific competition inferred from isodars corresponded with the inferences emerging from regression and field experiments. Previous removal experiments on two herbivorous rodents occupying coastal wet heathlands in eastern Australia documented that competition between Rattus lutreolus and Pseudomys gracilicaudatus is asymmetrically biased in favor of the much larger Rattus. The asymmetry in competition was also revealed by regression estimates of competition. Isodar analyses illustrate a habitat‐dependent mechanism for the asymmetry. Rattus compete effectively with Pseudomys in both ‘wetter’ and ‘drier’ patches of heath whereas Pseudomys appear to exert a competitive effect in only the drier sites. The magnitude of competition measured by a removal experiment in an area with more‐or‐less equal amounts of both habitats will be biased in favor of Rattus. More generally, one can use the isodar estimates to draw isolegs and isoclines of competitive coexistence. Isoclines for the two Australian rodents imply dynamic equilibria of stable competitive coexistence that vary with plant succession in fire‐dominated heathland ecosystems.
Competitive effects of shrubs and grasses in prairie
Authors: Köchy Martin , Wilson Scott D.
Published: December 2004

We investigated the relative contributions of size and growth form (biomass allocation) to competitive effects between grasses and shrubs in western Canada for two years. We measured the effects of grasses and shrubs on each other at the population level using removal experiments in natural vegetation. In prairie where shrub abundance was low, shrubs suppressed grasses as much as grasses suppressed shrubs, even though shrubs had six times more standing crop. In adjacent brush clumps, however, where shrub standing crop was 37 times grass standing crop, shrubs suppressed grasses strongly, whereas grasses did not suppress shrubs. Shrubs reduced available soil nitrogen more strongly than grasses did, but shrubs and grasses did not differ in their effects on light or soil water. On a per‐gram basis, however, shrubs had smaller effects on light, nitrogen, and water consumption than grasses did. In spite of their smaller per‐gram effects on resources, the secondary growth of shrubs allowed them to accumulate more mass and height, and to eventually displace grasses. During this process, competition between the woody and the herbaceous growth form changed from symmetric to asymmetric.
Do predators limit the abundance of alternative prey? Experiments with vole‐eating avian and mammalian predators
Authors: Norrdahl Kai , Korpimäki Erkki
Published: December 2004

Predation has been invoked as a factor synchronizing the population oscillations of sympatric prey species, either because predators kill prey unselectively (the Shared Predation Hypothesis; hereafter SPH), or because predators switch to alternative prey after a density decline in their main prey (the Alternative Prey Hypothesis; APH). A basic assumption of the APH is that the impact of predators on alternative prey depends more on the density of main prey than on the predator/alternative prey ratio. Both SPH and APH assume that the impact of predators on alternative prey is at least periodically strong enough to depress prey populations. To examine these assumptions, we utilized data from replicated field experiments in large areas where we reduced the breeding densities of avian predators during three years and the numbers of least weasels (Mustela nivalis) in two years when vole populations declined. In addition, we reduced the breeding densities of avian predators in two years when vole populations were high. The reduction of least weasels increased the abundance of their alternative prey, small birds breeding on the ground, but did not affect the abundance of common shrews (Sorex araneus). In years when vole populations declined, the reduction of avian predators increased the abundance of their alternative prey, common shrews and small birds. Therefore, vole‐eating predators do at least periodically depress the abundance of their alternative prey. At high vole densities, the reduction of avian predators did not increase the abundance of common shrews, although the ratio of avian predators to alternative prey was similar to years when vole populations declined, which supported APH. In contrast, the abundance of small birds increased after the reduction of avian predators also at high vole densities, which supported SPH. The manipulations had no obvious effect on the number of game birds, which are only occasionally killed by these small‐sized predators. We conclude that in communities where most predators are small or specialize on a single prey type, the synchronizing impact of predation is restricted to a few similar‐sized species.
Root decomposition and soil nutrient and carbon cycling in two temperate fen ecosystems
Authors: Scheffer Robbert A. , Aerts Rien
Published: December 2004

Although root litter contributes to a large extent to soil organic matter accumulation in peatlands, decomposition of root litter is often neglected in studies on litter decomposition and carbon and nutrient cycling in these ecosystems. In this study, decomposition of root and rhizome litter of Carex diandra and Carex lasiocarpa was determined in two temperate fens, one dominated by Sphagnum species (Sphagnum fen; soil pH=4.4) and one without a Sphagnum cover (Carex fen; soil pH=5.7). One‐year mass loss increased in the order: roots Carex diandra<roots Carex lasiocarpa<rhizomes Carex lasiocarpa. This ranking was in agreement with the generally established relationships between decomposition rate and litter chemistry, since rhizome litter had the lowest concentration of lignin and the highest N and P concentrations. The decomposition constants (k‐values) varied from 0.10 to 0.30 yr−1, which is at the low end of the range found for above‐ground litter of Carex species in other studies. Decomposition rates of root litter were lower in the Sphagnum site than in the Carex site, probably as a result of a lower pH and a lower nutrient status in the Sphagnum site. At both sites, N and P content of the litter decreased rapidly in the first three months indicating fast nutrient leaching and mineralization. Nutrient release per unit initial litter mass in the Carex site did not differ from that in the Sphagnum site. Estimated first‐year losses of N and P from Carex diandra roots in the Carex site were 2.31 g N m−2 and 0.20 g P m−2. First‐year nutrient losses from leaf litter of this species, as determined in another study, were only 0.06 g N m−2 and 0.02 g P m−2. These results show that roots do indeed play a major role in the cycling of carbon and nutrients in temperate fen ecosystems.
How host size may constrain the evolution of parasite body size and clutch size. The parasitic isopod Ichthyoxenus fushanensis and its host fish, Varicorhinus bacbatulus, as an example
Authors: Tsai Min‐Li , Li Jan‐Jung , Dai Chang‐Feng
Published: December 2004

The flesh‐burrowing parasitic isopod Ichthyoxenus fushanensis was found infecting the body cavity of a freshwater fish, Varicorhinus bacbatulus, in heterosexual pairs. Herein we investigate the question of how the host body size may constrain the parasite size and clutch size by analyzing the interactions among the body size and clutch size of the parasite, and host size. Due to the low transmission rate of I. fushanensis to its host and the positive relationship between clutch size and female size, selection may favor larger females with larger clutch sizes to compensate for massive losses of manca (the free‐living juveniles). The path model reveals that clutch size depends not only directly on female size, but also on the sizes of her host and mate. Female size also depends on the sizes of the host and her mate. A negative correlation exists between the body sizes of the paired males and females. This negative correlation may be regarded as a consequence of competition for limited available space or other resources provided by the host. The effects of host size on parasite size, however, act on the total volume of both sexes as a whole, not specifically on either the female or the male. In this case, the available space/resources may not allow both individuals of different sexes to evolve toward a larger size simultaneously. Under the constraint of host size, a strategy of reducing the body size of the paired male may provide a way to increase the body size of the paired female and achieve a larger clutch size.
Nestmate recognition mediated by intestinal bacteria in a termite, Reticulitermes speratus
Authors: Matsuura Kenji
Published: December 2004

Discrimination and aggressive responses toward non‐nestmates have been observed in a number of termite species, but the mechanism is poorly understood. Here I present the novel hypothesis that differential intestinal bacteria composition leads to production of colony‐specific chemical cues that enable nestmate recognition.  The intestinal microflora of a lower termite, Reticulitermes speratus, consisted of many bacteria species. The composition of the intestinal bacteria was exclusively colony‐specific. Termites that had adsorbed an unfamiliar odor of bacteria sampled from another colony were fiercely attacked by nestmates. Experimental manipulation of the composition of bacteria by antibiotics successfully changed the recognition behavior toward nestmates. These results indicate that intestinal bacteria play an important role in nestmate recognition.
Allee effect, sexual selection and demographic stochasticity
Authors: Møller Anders Pape , Legendre Stéphane
Published: December 2004

The negative frequency‐dependent effect of reproductive success in animals on population growth refers to a category of phenomena termed the Allee effect. The mechanistic basis for this effect and hence an understanding of its consequences has been obscure. We suggest that sexual selection, in particular female mate preferences, is a previously neglected component giving rise to the Allee effect. Lack of breeding and reduced reproductive success of females at low population densities are commonly described in situations where females have little or no opportunity to choose a mate, consistent with this suggestion. We developed a demographic model that incorporated the effects of lack of female choice on rates of reproduction. Using either a mating system with incompatibility or a system with a directional mate preference, we show that commonly encountered levels of reproductive suppression in the absence of suitable mates in a population, where sexual selection still operates, may increase the effects of demographic stochasticity considerably.
Decomposition of Carex and Sphagnum litter in two mesotrophic fens differing in dominant plant species
Authors: Scheffer R. A. , Van Logtestijn R. S. P , Verhoeven J. T. A.
Published: December 2004

Peatlands can be classified into fens and bogs based on their hydrology. Development of fens to bogs is accompanied by the invasion of Sphagnum species. The purpose of this study was to determine how the decomposition process in fens is influenced by the transition from a vascular plant‐dominated system to a Sphagnum‐dominated system. We carried out a reciprocal litter bag experiment, using litter of Carex diandra, C. lasiocarpa, Sphagnum papillosum and S. squarrosum in a fen dominated by Sphagnum species and a fen without Sphagnum. Decomposition rate and nitrogen and phosphorus dynamics of the plant litter were measured in a field experiment for two years. Decomposition rate was highest for the Carex litter types and lowest for the Sphagnum litter types. Surprisingly, decomposition rates hardly differed between the two sites. Nutrient dynamics, however, showed a clear site‐effect: In the Sphagnum site net mineralization was observed for all litter types whereas in the Carex site net immobilization was observed. These results show that carbon and nutrient cycles were coupled in a different way in a Sphagnum‐dominated and a Carex‐dominated site, respectively. Nutrient availability and adaptation of the microbial community to nutritional and other environmental conditions may be the main regulators of carbon and nutrient cycles in these peatlands.
Cost efficiency of nutrient acquisition and the advantage of mycorrhizal symbiosis for the host plant
Authors: Tuomi Juha , Kytöviita Minna‐Maarit , Härdling Roger
Published: December 2004

Mycorrhizal symbiosis involves reciprocal transfer of carbon and nutrients between shoots on the one hand and roots colonized by symbiotic fungi on the other. Mycorrhizas may improve the mineral nutrient acquisition rates, but simultaneously increase the belowground demand for carbon. Mycorrhizal plants will have a selective advantage over non‐mycorrhizal ones if they are more cost‐efficient in terms of carbon cost per unit of acquired mineral nutrient. However, we demonstrate here in a simple model system that this is not a necessary condition. Mycorrhizas may evolve even when they are less cost‐efficient, provided that photosynthesis and/or growth are strongly nutrient‐limited. This result implies a unique hypothesis for the evolution of mycorrhizal associations which may be inherently cost‐inefficient as compared to plant roots. Such symbioses may have evolved when the superior nutrient acquisition rate of fungi combines with the relatively high photosynthetic nutrient use efficiency of the host plant. Consequently, provided that mycorrhizas are really cost‐inefficient, the selective advantage of mycorrhizal plants will disappear when an increase in the nutrient acquisition rate is not associated with a sufficiently high nutrient use efficiency of photosynthesis, as at high soil nutrient levels or due to a loss of leaf area, shading or low temperatures.
Pollination of a core flowering shrub species in Mediterranean phrygana: variation in pollinator diversity, abundance and effectiveness in response to fire
Authors: Potts Simon G. , Dafni Amots , Ne'eman Gidi
Published: December 2004

Fire in Mediterranean‐type ecosystems produces catastrophic changes in plant‐pollinator systems; the recovery of which has been studied by comparing an unburnt mature forest habitat with that of an adjacent recently burnt area (eight years post‐fire). The composition, visitation profiles, and effectiveness of the taxonomically diverse pollinator assemblages found on a core nectar providing species (Saturejathymbra: Lamiaceae) were examined in these two contrasting habitats. S. thymbra in the freshly burnt area had low nectar standing crop and relatively less diverse bee community than an unburnt area which had twice the nectar standing crop and a higher bee diversity and abundance. Both sites supported bee assemblages dominated by the non‐native bumblebee Bombus terrestris. Spatio‐temporal heterogeneity of nectar standing crops and microclimatic conditions were sufficient to explain the form and magnitude of the diurnal foraging profiles at each site in relation to species specific foraging and flight abilities. B. terrestris, Apis mellifera and native solitary bees were the three primary guilds visiting S. thymbra and varied in the efficiency with which they delivered conspecific pollen grains to receptive stigmas. A pollinator effectiveness index for these three guilds was calculated based on floral visitation rates and pollen delivery efficiency and reflected the actual levels of effectiveness of each guild within and across the two habitat types. There was no overall inter‐community difference in pollination effectiveness as the bee assemblages in both habitats were sufficient to produce maximum fruit set in S.thymbra, though the relative contribution of each guild varied intra‐communally. Pollen limitation was not found to occur in either habitat.
The effects of soil fertilizer on amino acids in the floral nectar of corncockle, Agrostemma githago (Caryophyllaceae)
Authors: Gardener Mark C. , Gillman Michael P.
Published: December 2004

Plants of the meadow annual Agrostemma githago (Caryophyllaceae) were grown in 1‐m2 field plots prepared with three fertilizer treatments as follows: (a) “low”, no fertilizer, (b) “medium”, 75 g of slow release fertilizer granules, (c) “high”, 175 g of granular treatment. After sowing in spring the plants were left until flowering in late summer. Nectar was extracted using 5‐μl glass microcapillary tubes. The material was frozen and sampled at a later date. The samples were analysed by high performance liquid chromatography (HPLC) using the AccQtag (Waters Corp.) system. Analysis showed that the total concentration of amino acids increased significantly with increasing fertilizer treatment (P<0.05). Of the amino acids present glutamine showed a large and significant increase (P<0.01), proline showed an increase (P<0.05) and γ‐amino butyric acid (GABA) showed a decrease (P<0.05). No others showed a significant change. It is interesting to note that a common biosynthetic pathway (from α‐ketoglutarate) links the amino acids that altered in concentration. Due to these changes the relative abundance of about half of the amino acids in the nectar was significantly altered. Glutamine showed a significant increase (P<0.001) in percentage of the total with most of the remaining amino acids declining in relative abundance. The results show that, in contradiction to earlier work, soil conditions can affect the amino acid complement of nectar. This may have implications for plant‐insect interactions, as local populations of pollinators may benefit from the increased amino acid content of the nectar and preferentially visit plants growing in high nutrient conditions.
Variation in the outcome of competition between two aphid species on pecan: plants matter more than predators
Authors: Petersen Mette K. , Hunter Martha S.
Published: December 2004

The relative importance of resource‐mediated competition versus apparent competition was studied in two native aphids, Monellia caryella, and Melanocallis caryaefoliae, on pecan, Carya illinoensis. We compared reproduction and body size of the two aphid species when they were caged on pecan leaves as single or mixed species, and when green lacewing larvae (Chrysoperla comanche or Chyrsopa nigricornis) were present or absent. The experiment was repeated two times on mature compound leaves in the field in the autumn of 1997 and summer of 1998 and once in a greenhouse, on young seedlings with simple leaves. 
Our results suggest that interspecific competition was variable and asymmetric; in both the Summer 98 experiment and the greenhouse experiment, reproduction of M. caryaefoliae was significantly reduced in the mixed species treatments relative to the single species treatment, while M. caryella reproduction was significantly reduced only in the Summer 98 experiment. No evidence of an effect of competition on reproduction was found in the Autumn 97 experiment. M. caryella body size was reduced in one of the mixed species treatments in the Autumn 97 experiment. Body size of both aphid species was reduced in the mixed species treatments of the greenhouse experiment. 
The introduction of green lacewing larvae reduced the reproduction of aphids in two of the three experiments in comparison to controls. However, a significant interaction between aphid and predator treatment (M. caryella in the greenhouse experiment) was found in only one experiment. Variation in the outcome of competition was more likely to be due to aspects of plant quality, including leaf age and previous aphid feeding.
Using upper boundary constraints to quantify competitive response of desert annuals
Authors: Lessin Laura M. , Dyer Andrew R. , Goldberg Deborah E.
Published: December 2004

Using a target‐neighborhood approach with six annual dicot species in the Negev Desert of Israel, we tested whether neighborhood biomass constrained the upper limit of plant performance and if the slope of the upper boundary was correlated to species trait means, such as relative growth rate (RGR) and seed mass. Target individuals were measured in early spring and then collected at the onset of the dry season along with all naturally occurring neighbors within a 5‐cm radius of the target. Using a minimum of 50 samples for each of the six target species, we found no significant relationships between target performance and either density or aboveground biomass of neighbors, when including all target individuals in the regressions. However, aboveground neighbor biomass did influence the maximum potential size of targets. When regressions were restricted to the largest target plant within classes of neighbor biomass, significant negative relationships were found for all six species, with neighbor biomass explaining 55 to 94% of the variance in maximum target biomass. The slopes of the regressions were used as an index of competitive response. The correlations between competitive response and estimates of species traits were not significant with the possible exception that species with lower RGRmax may be better response competitors (i.e. were less sensitive to competition). These findings indicate the usefulness of the boundary regression technique for describing competitive interactions among neighbors, particularly in low productivity environments.
Why are leaf‐cutting ants more common in early secondary forests than in old‐growth tropical forests? An evaluation of the palatable forage hypothesis
Authors: Farji‐Brener Alejandro G.
Published: December 2004

I evaluated the hypothesis that leaf‐cutting ants are more common in early successional forests than in old‐growth forests because pioneer species, which dominate in early successional habitats, appear more susceptible to leafcutters than shade‐tolerant species, which dominate primary forests (palatable forage hypothesis). The relative importance of pioneer and shade‐tolerant species as plant resources for leaf‐cutting ant colonies was evaluated (1) by literature review of leaf‐cutting ants’ diet, and (2) experimentally, using field assays to determine leafcutter's selectivity. Pioneer species were harvested three times more frequently than shade‐tolerant species and made up the largest component of the diet in all the studies reviewed. The amount harvested was not correlated with the plant species abundance. In addition, leaves from pioneer plants were selected eight times more than leaves from shade‐tolerant species in the field assays. These results support the palatable forage hypothesis. Leafcutters probably select pioneer leaves because of their low level of chemical defenses and high nutrient content. The high availability of pioneer species in early successional forest probably decreases the cost to locate palatable resources. Therefore, early successional habitats support more ant colonies than old‐growth forests. On the other hand, the effective defense mechanisms of mature plant species and the high dispersion of palatable plants could explain the low density of leaf‐cutting ant colonies in old‐growth forests. The palatable forage hypothesis is compared with other hypotheses that explain leaf‐cutting ant density. The preference of foundress queens for forest clearings, the dependence of small colonies on herbs, and the importance of pioneer plant species for mature colonies (palatable forage hypothesis) can be considered complementary, because they focus on different stages of the colony's life history. Consequently, the availability of pioneer plants appears to be one of the most influential factors determining mature leaf‐cutting ant nest densities in Neotropical forests.