Cellular basis for polarized light perception in the spider crab,Libinia

Summary Differential increases in the numbers of pinocytotic vesicles, multivesicular bodies and total complex bodies occurred in the cytoplasm of specific photoreceptor cells in the compound eye of the crab Libinia exposed for six hours to polarized light with various e-vector orientations. These data coupled with previous results on the same species proved that the seven retinular cells in each ommatidium formed two functional groups selectively light adaptable by e-vectors oriented 90° apart. One group (Channel I, comprising Cells 1, 4 and 5) was more affected by horizontal polarization; the other (Channel II, comprising Cells 2, 3, 6 and 7) was more affected by vertical polarization.This confirmed by a quite independent technique the conclusion reached from electrophysiological experiments on the crab Cardisoma that decapod compound eyes have two orthogonal polarization analyzer channels. In addition the present data showed that both channels occur in each ommatidium as hypothesized on previous electron microscopic evidence and that the axes of maximum absoprtion in the two retinal channels were parallel to the long axes of their cells' rhabdom microvilli, horizontal in Channel I and vertical in Channel II. The latter relations in turn supported the hypothesis that the dichroism of rhodopsin was fundamental to the analyzer mechanism.This research has been supported by U. S. Air Force Grant AFOSR 1064 and NASA Grant NGR 07-004-055. The authors wish to thank Professor Joseph G. Gall for generously sharing his electron microscope facilities.     

Sapling growth as a function of light and landscape-level variation in soil water and foliar nitrogen in northern Michigan

Interspecific differences in sapling growth responses to soil resources could influence species distributions across soil resource gradients. I calibrated models of radial growth as a function of light intensity and landscape-level variation in soil water and foliar N for saplings of four canopy tree species, which differ in adult distributions across soil resource gradients. Model formulations, characterizing different resource effects and modes of influencing growth, were compared based on relative empirical support using Akaike’s Information Criterion. Contrary to expectation, the radial growth of species associated with lower fertility (Acer rubrum and Quercus rubra) was more sensitive to variation in soil resources than the high fertility species Acer saccharum. Moreover, there was no species tradeoff between growth under high foliar N versus growth under low foliar N, which would be expected if growth responses to foliar N mediated distributions. In general, there was functional consistency among species in growth responses to light, foliar N, and soil water availability, respectively. Foliar N influenced primarily high-light growth in F. grandifolia, A. rubrum, and Q. rubra (but was not significant for A. saccharum). In A. saccharum and A. rubrum, for which soil water availability was a significant predictor, soil water and light availability simultaneously limited growth (i.e., either higher light or water increased growth). Simple resource-based models explained 0.74–0.90 of growth variance, indicating a high degree of determinism. Results suggest that nitrogen effects on forest dynamics would be strongest in high-light early successional communities but that water availability influences growth in both early successional and understory environments.     

Where to from here? The mechanisms enabling the movement of first instar caterpillars on whole plants using Helicoverpa armigera (Hübner)

Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) is an economically-important, polyphagous herbivore in Old World countries. The distribution of larvae within various host plants has been described, but few studies have tried to determine the behavioural mechanisms by which the given distributions arose. Our aim was to determine the mechanisms which enable larval movement on pea plants, starting with first instars. Observations and bioassays determined larval movement in response to light and angled surfaces, as well as the effect of feeding and plant volatiles on these responses. The majority (68–72%) of 1st instars were positively phototactic towards blue, green and white light and 42% towards UV light. In the dark, larvae showed negative geotaxis. The angle of their substrate also had a kinetic effect on larvae; the steeper the angle from horizontal the more larvae moved under all conditions. Phenylacetaldehyde (a flower volatile) suppressed larval movement except at 90°. (Z)-3-Hexenyl acetate (a green leaf volatile) reversed the direction of movement at the flattest angle. Feeding lessened the probability of moving. We suggest that phototaxis and geotaxis are behaviours common to larval lepidopterans (caterpillars), and that these basic behaviours are modulated by environmental, larval, and plant factors to give observed distributions. Using a multinomial model approach, we created a flow chart to qualitatively and quantitatively represent the decision-making process of first instar H. armigera in response to the factors influencing movement.     

Nutrient concentrations in a Littorella uniflora community at higher CO2 concentrations and reduced light intensities

1. Oligotrophic softwater lakes represent a special type of aquatic ecosystem with unique plant communities where generalisations from other aquatic plant communities to rising CO₂ in the water column may not apply. 2. In the present study, we set up large in situ mesocosms and supporting laboratory experiments with isoetid vegetation (Littorella uniflora) where water column CO₂ and light could be manipulated in order to test whether (i) light and CO₂ availability affect nutrient concentrations in isoetid vegetation, and (ii) if changes in light and CO₂ climate affect fluxes of inorganic nitrogen (N) and phosphorus (P) from sediment to water column, which potentially could result in increased growth of epiphytic algae. 3. The results showed that the standing stocks of phosphorus and nitrogen in the L. uniflora vegetation were significantly influenced by CO₂ concentration and light intensity. Both standing stocks of P and N were significantly higher in the mesocosm treatments with high CO₂ concentration than in those at low CO₂ concentration. Similarly, standing stocks of P and N enhanced with increasing light intensity. 4. Measurements of nutrient fluxes both in the field and the laboratory did not show any significant release of nutrients to the water column from plants or sediments at any of the light or CO₂ treatments. However, mats of epiphytic algae developed from the beginning of June to late September and caused a light reduction for the isoetid vegetation. 5. Increasing CO₂ concentrations in the water column may over time potentially result in a change in soft water plant communities.     

Canopy herbivore community structure: large-scale geographical variation and relation to forest composition

Abstract. 1. Geographical distributions of individual foliage‐feeding forest herbivore species have been found to be aligned closely with the distribution of their host trees, however little is known about the extent to which broad herbivore communities are geographically associated with distinct host communities. 2. Large‐scale geographic variation in canopy herbivore communities in a 80 000 km² area (the state of Maine, U.S.A.) was characterised using historical insect survey data. Variation in insect communities was compared with corresponding variation in forest over‐storey composition, which was quantified using data from a regional forest inventory survey. 3. Principal components analysis was used to characterise associations among herbivore and tree species. Analysis of the herbivore data identified three main insect groups: group A corresponded to a single species (Choristoneura fumiferana), group B corresponded to pine‐feeding species including Semiothisa sp., and group C corresponded to the spruce‐feeding species Gilpinia hercyniae and Pikonema alaskensis. Analysis of the forest inventory data characterised three important forest types: northern hardwoods, eastern white pine, and northern spruce‐fir forest types. 4. Spatial analyses were carried out on the first two components of each of the principal components analyses. Factor 1 of the insect data showed a trend of decreasing values from south to north, while factor 2 of the forest inventory data showed an opposite trend. These inverse trends reflected the distribution of the main contributing species to the principal components analysis, C. fumiferana and Pinus strobus respectively. These distributions were highlighted further by the significant negative cross‐correlations that were found between the two factors up to distances of 120 km. 5. Analyses indicated a parallel between the geographic variation in the insect guilds associated with conifers and the geographic pattern of their conifer hosts. Hardwood‐feeding caterpillars, on the other hand, did not form well‐defined guilds and showed varying geographical distributions. The survey data showed varying quality in defining large‐scale associations in the structures of herbivore and host communities. 6. Implications for biodiversity management are discussed.     

Plant distribution can be reflected by the different growth and morphological responses to water level and shade in two emergent macrophyte seedlings in the Sanjiang Plain

In the Sanjiang Plain (North East China), narrowleaf small reed (Deyeuxia angustifolia) usually distributes widely in typical meadow or marsh, while reed (Phragmites australis), the concomitant species, is distributed sparsely in the D. angustifolia communities or relative open sites. To date, the mechanisms responsible for their different distribution patterns are far from clear. Both water level and light are important factors determining plant distribution in wetland ecosystems and therefore, the aim of this paper was to identify the role of these two factors and their potential interaction on plant distribution in this plain. Growth responses and biomass allocation of the two macrophytes were investigated by growing them in three irradiances (300, 100, 20 μmol m⁻² s⁻¹) and two water levels (0 and 5 cm) under greenhouse conditions. Biomass accumulation, mean relative growth rate (RGR), height and mean relative elongation rate (RER) of both species significantly decreased with the reduction of light availability. Biomass accumulation, RGR, height and RER of P. australis were significantly inhibited by higher water level. However, water level had no effect on the growth of D. angustifolia. Stem mass fraction was higher at 0-cm water level in D. angustifolia, and was not affected by water level in P. australis. These data suggest that D. angustifolia has a higher adaptive ability to acclimate to flooding and shade stresses than does P. australis, which might be an important reason for their different distribution patterns.     

Black abalone (Haliotis cracherodii) population structure shifts through deep time: Management implications for southern California's northern Channel Islands

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Effects of Spilled Oil on Bacterial Communities of Mediterranean Coastal Anoxic Sediments Chronically Subjected to Oil Hydrocarbon Contamination

The effects of spilled oil on sedimentary bacterial communities were examined in situ at 20 m water depth in a Mediterranean coastal area. Sediment collected at an experimental site chronically subjected to hydrocarbon inputs was reworked into PVC cores with or without a massive addition of crude Arabian light oil (∼20 g kg⁻¹ dry weight). Cores were reinserted into the sediment and incubated in situ at the sampling site (20 m water depth) for 135 and 503 days. The massive oil contamination induced significant shifts in the structure of the indigenous bacterial communities as shown by ribosomal intergenic spacer analysis (RISA). The vertical heterogeneity of the bacterial communities within the sediment was more pronounced in the oiled sediments particularly after 503 days of incubation. Response to oil of the deeper depth communities (8–10 cm) was slower than that of superficial depth communities (0–1 and 2–4 cm). Analysis of the oil composition by gas chromatography revealed a typical microbial alteration of n-alkanes during the experiment. Predominant RISA bands in oiled sediments were affiliated to hydrocarbonoclastic bacteria sequences. In particular, a 395-bp RISA band, which was the dominant band in all the oiled sediments for both incubation times, was closely related to hydrocarbonoclastic sulfate-reducing bacteria (SRB). These bacteria may have contributed to the main fingerprint changes and to the observed biodegradation of n-alkanes. This study provides useful information on bacterial dynamics in anoxic contaminated infralittoral sediments and highlights the need to assess more precisely the contribution of SRB to bioremediation in oil anoxic contaminated areas.     

Environmental variability influences the structure of benthic algal communities in an oligotrophic lake

Theoretical models suggest that environmental variability can promote biodiversity, both in terms of richness and evenness of species. These findings have been supported experimentally in simplified communities with simplified regimes of environmental variability. Here we test the importance of environmental variability in a more natural setting by comparing the composition of benthic algal communities along a natural gradient of environmental variability created by internal waves in an elongated lake basin. Algal communities were grown on tiles at five upwind and five downwind sites in the lower littoral zone of South Arm, in Lake Opeongo, Ontario, Canada. Upwind sites were 1°C colder, on average, than downwind sites and had more variable water temperatures. The ranges of mean water temperatures and of water temperature variability were greater among upwind sites than among downwind sites. We used water temperature variability as a proxy for environmental variability. Total algal biovolume was similar at all sites, but the composition of these communities varied systematically with temperature variability. An observed shift in the relative dominance between Achnanthidium minutissimum, Surirella sp. and Pinnularia spp. is consistent with what we know about these species. Diatom richness increased from 12 to 15 genera with increasing variability at upwind (p=0.10, r²=0.65) and downwind sites (p=0.11, r²=0.63). Community evenness also increased with increasing variability (p=0.03, r²=0.82 at upwind sites; p=0.0002, r²=0.99 at downwind sites). These relationships were observed despite low nutrient and light availability, as well as the presence of grazers. Furthermore, environmental variability affected diatom communities in the presence of several competing factors (temperature, light, nutrients and disturbance). Our results suggest that increased environmental variability can lead to higher biodiversity in a complex natural system.     

Some Factors Controlling the Distribution of Two Pond-Dwelling Ciliates with Algal Symbionts (Frontonia vernalis and Euplotes daidaleos)1

The vertical distributions of two pond-dwelling zoochlorellae-bearing ciliates (Euplotes daidaleos Diller & Kounaris, 1966 and Frontonia vernalis Ehrenberg, 1838) were monitored over a 24-h period. Both species maintained peak abundance at a low O₂ level (usually < 1 mg/liter). They did not migrate in response to the changing light level. Experiments with laboratory cultures indicated that the characteristic distribution in an O₂ gradient in the dark was largely controlled by the oxygen tension. The increased motility in anoxia and high pO₂ was independent of large changes in pCO₂ and pH. Ciliates living in anoxia or a very low pO₂ would migrate out of the dark and into the dimly lit (10 μE m_-2 sec_-1) part of a glass cell because there they could photosynthesize, produce O₂, and create a suitable oxygenated microenvironment; a further increase in the light level caused a slow migration out of the light. Similar migrations were observed when the light level remained low but the pO₂ was artificially raised. Ciliates suspended in 1 μM DCMU (an inhibitor of photosynthetic O₂ evolution) took longer to migrate into the light and they did not avoid high light levels (> 100, μE m_-2 sec_-1)- Frontonia suspended in water with a pO₂ of 1% aggregated at a low light level (1 μE m_-2 sec_-1); peak daytime abundance in the pond occurred at about this light level. Frontonia vernalis tends to swim vertically upwards (anterior end up) when suspended in anoxic water. This apparent negative geotaxis compensates for the high sedimentation velocity (0.36 mm sec_-1) of this large ciliate and facilitates its aggregation at the metalimnion. The O₂ tension appears to be the principal factor controlling the vertical distributions of both species. Occasional, enhanced convection within the metalimnion has a secondary influence. Light influences the vertical profile only if it promotes photosynthesis and increases the intracellular pO₂.     

Community fluctuations and local extinction in a planktonic food web

Determining statistical patterns irrespective of interacting agents (i.e. macroecology) is useful to explore the mechanisms driving population fluctuations and extinctions in natural food webs. Here, we tested four predictions of a neutral model on the distribution of community fluctuations (CF) and the distributions of persistence times (APT). Novel predictions for the food web were generated by combining (1) body size–density scaling, (2) Taylor's law and (3) low efficiency of trophic transference. Predictions were evaluated on an exceptional data set of plankton with 15 years of weekly samples encompassing c. 250 planktonic species from three trophic levels, sampled in the western English Channel. Highly symmetric non‐Gaussian distributions of CF support zero‐sum dynamics. Variability in CF decreased while a change from an exponential to a power law distribution of APT from basal to upper trophic positions was detected. Results suggest a predictable but profound effect of trophic position on fluctuations and extinction in natural communities.     

Consistent patterns in diatom assemblages and diversity measures across water‐depth gradients from eight Boreal lakes from north‐western Ontario (Canada)

1. Until recently, the distribution of diatom species assemblages and their attributes (e.g. species richness and evenness) in relation to water depth have been identified but not quantified, especially across several lakes in a region. Here, we examined diatom assemblages in the surface sediment across a water‐depth gradient in eight small, boreal lakes in north‐western Ontario, minimally disturbed by human activities. 2. Surface‐sediment diatom assemblages were collected within each lake along a gentle slope from near‐shore to the centre deep basin of the lake, at a resolution of ～1 m water depth. Analysis of sedimentary samples provided an integrated view of assemblages that were living in the lake over several years and enabled a high‐resolution analysis of many lakes. The study lakes ranged in water chemistry, morphology and size and are located along an east–west transect approximately 250 km long in north‐western Ontario (Canada). 3. The majority of diatom species were distributed along a continuum of depth, with those taxa having similar habitat requirements forming distinct, though overlapping, assemblages. Three major zones of diatom assemblages in each lake were consistently identified: (i) a near‐shore assemblage of Achnanthes (sensu lato), Nitzschia, Cymbella (sensu lato) and other benthic species; (ii) a mid‐depth assemblage of small Fragilaria (sensu lato)/small Aulacoseira and various Navicula taxa; and (iii) a deep‐water assemblage of planktonic origin (mainly Discotella spp.). 4. The depth of the transition between assemblage zones varied between the eight lakes. The boundary between the deep‐water planktonic zone and the mid‐depth benthic zone varied according to water chemistry and was probably related to light attenuation. The boundary was deeper in lakes with the lower dissolved organic carbon and total phosphorus (TP) (i.e. less light attenuation) and vice versa. 5. Generally, species richness, species evenness and turnover rate of species as a function of depth were significantly lower in the planktonic assemblage zone in comparison with the two zones nearer the shore. Reproducibility of species and assemblage distributions across the depth gradient of the lakes illustrated that, despite potential for sediment transport, detailed ecological characterisation of diatom species can be gleaned from sedimentary data. Such data are often lacking, particularly for near‐shore benthic species.     

Phylogenetics of the allodapine bee genus Braunsapis: historical biogeography and long-range dispersal over water

Aim A previous study of the allodapine bee genus Braunsapis suggested an African origin, with dispersal events into Madagascar and Asia, and from Asia into Australia. We re‐examine the phylogeny of this genus, using an expanded set of taxa from Madagascar and Malawi and additional sequence data, in order to determine the number of dispersals and the timeframe over which they occurred. Location Africa, Madagascar, Malawi, Asia and Australia. Methods One nuclear (EF‐1α F2) and two mitochondrial (CO1 and Cyt b) gene regions were sequenced for 36 allodapine bee species (including members of the genera Braunsapis, Nasutapis, Allodape, Allodapula, and Macrogalea) and one ceratinine species (Ceratina japonica). We used Bayesian analyses to examine phylogenetic structure and a penalized likelihood approach to estimate approximate ages for key divergences in our phylogeny. Results Our analyses indicate a tropical African origin for Braunsapis in the early Miocene followed by very early dispersal into Asia and then a subsequent dispersal, following Asian diversification, into Australia during the late Miocene. There have also been two dispersals of Braunsapis from Africa to Madagascar and this result, when combined with phylogenetic and biogeographical data for other allodapines, suggests that these bees have the ability to cross moderately large ocean expanses. These dispersals may have been aided by the West Wind Drift, but rafting across the Mozambique Channel is also possible, and could be aided by the existence of developmental stages that require minimal or no feeding and by tolerance to sea water and spume. Accumulating evidence suggests that many biogeographical patterns in the southern hemisphere may be better explained by dispersal than by Gondwanan vicariance hypotheses. Our results add to this growing body of data and raise the possibility that some puzzling trans‐Indian Ocean distributions may also be explained by historical dispersal events across oceanic barriers that now seem insuperable.     

Qualitative effects of patchy stomatal conductance distribution features on gas-exchange calculations

The qualitative influence of patchy stomatal conductance distributions on the values of photosynthesis (A) and intercellular CO₂ concentration (c_i) as determined by gas-exchange measurements were investigated using computer modelling. Gas-exchange measurements were simulated for different conductance distributions by modelling photosynthesis explicitly for each patch, summing these rates, and inferring c_i from a diffusion equation. Qualitative relationships are presented between conductance distribution features and the difference between assimilation rates measured for patchy and homogeneous leaves at the same c_i (A_p and A_h, respectively). These data show that, although most conductance distributions have little effect on the value of A measured for a given c_i, some distribution features (which we have termed ‘bimodality’, ‘position’, ‘skewness’ and ‘range’) play a key role in controlling the magnitude of these effects. Distributions that are more nearly bimodal, span regions of lower conductance, are right-skewed, or have broader conductance ranges are associated with larger effects on the A(c_i) relationship. To clarify our mathematical analysis and illustrate some of the trends it predicts, we present conductance distributions and gas-exchange data from leaves of Malus dolgo var. Spring Snow Dial were treated with ABA. The results are discussed in the light of recent controversy over the effect of patchy stomatal conductance on gas-exchange data.     

Mortality,movement and behaviour of native mussels during a planned water‐level drawdown in the Upper Mississippi River

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Deep-water aquatic plant communities in an oligotrophic lake: physiological responses to variable light

1. Oligotrophic Lake Waikaremoana, New Zealand, is used for hydroelectric power generation and the lake levels are manipulated within an operating range of 3 m. There was concern that rapidly changing water levels adversely affected the littoral zone by decreasing light availability in two ways: local turbidity caused by shoreline erosion at low water levels; and decreased light penetration to the deep littoral zone caused by high water levels in summer. 2. The littoral zone was dominated by native aquatic plants with vascular species to 6 m and a characean meadow below this to 16 m. The biomass and heights of the communities in the depth zone 0–6 m were reduced at a site exposed to wave action relative to those at a sheltered site. However, the community structure below 6 m was similar at exposed and sheltered sites. The lower boundary of the littoral zone was sharply delimited at 16 m and this bottom boundary remained constant throughout the year despite large seasonal changes in solar radiation and the 3 m variation in lake level. 3. There was evidence that the deep-water community consisting of Chara corallina had adapted physiologically to low-light conditions. Net light saturated photosynthesis (CO₂ exchange) per unit chlorophyll a (Chl a) was reduced to 1.7 μg C (μg Chl a)^?1 h^?1 at the lower boundary, half of that recorded at 5 m. The concentration of Chi a per gram of biomass (dry weight), was considerably greater at the lower boundary than higher in the profile [c. 7 mg Chl a (g dry wt)^?1 at 16 m vs. 4 mg Chl a (g dry wt)^?1 at 5 m]. Chl b also increased with depth and there was no change in the ratio of Chl a and Chl b with increasing depth. The saturation light intensity (I_k) of the community at the lower boundary was only 78 μmol photons m^?2 s^?1. Photosynthetic parameters (I_k and α) as well as the Chl a content remained relatively constant throughout the seasonal and short-term changes in radiation. 4. The photosynthetic characteristics of the littoral community were therefore not greatly affected by the lake level change caused by the present hydroelectric operations. However, the sharpness of the lower boundary and its extreme shade characteristics imply that the deep-water community would be sensitive to any further changes in underwater light availability.     

Gradient versus cluster analysis of fossil assemblages: a comparison from the Ordovician of southwestern Virginia

Studies in modern ecology indicate that most species are distributed independently along environmental gradients according to their individual requirements. Steep gradients often produce species associations separated by discontinuities; gradual gradients produce broadly-overlapping distributions. Approaching the distribution of species populations as a continuum, using gradient analysis, avoids artificial subdivision of totally intergrading distributions, yet permits discontinuities to emerge where present. Faunas of the Martinsburg Formation (Ordovician) in southwestern Virginia offer an excellent opportunity to test the applicability of gradient analysis in a paleoecological setting. A broad spectrum of environments, from nearshore to open-marine, clastic to carbonate-dominated facies, provide both temporal and geographic variation against which to evaluate changes in species distributions. Variations of five classical, Petersen-type communities were recognized in the Martinsburg using cluster analysis: (1) Lingula, (2) bivalve, (3) Rafinesquina, (4) Onniella, and (5) Sowerbyella-dominated communities. Two gradient analysis techniques, ordination and Markov analysis, revealed the same basic associations. However, ordination and Markov analysis permit arrangement of these associations along one or more interpreted environmental gradients. Factors related to water depth and distance from clastic source areas, particularly bottom stability and disturbance frequency, appear to have been the most important of a complex of interrelated physical parameters. The high-stress, nearshore end of the Martinsburg gradient complex was occupied by a Lingula association, followed seaward by an association of bivalves adapted to less-stressed environments. Low-stress, open-shelf environments were occupied by Rafines-quina, Onniella, or Sowerbyella-dominated associations. Broad overlaps among these articulate brachio-pod associations reflect variations in the open-shelf habitat.     

Neighboring plant influences on arbuscular mycorrhizal fungal community composition as assessed by T-RFLP analysis

Controls on root colonization by arbuscular mycorrhizal fungi (AMF) include host nutrient status, identity of symbionts and soil physico-chemical properties. Here we show, in the field, that the subset of the AMF community colonizing the roots of a common grass species, Dactylis glomerata, was strongly controlled by neighboring roots of a different plant species, Centaurea maculosa, an invasive forb, thus adding a biological spatial component to controls on root colonization. Using an AMF-specific, 18s rDNA-based terminal restriction fragment length polymorphism (T-RFLP) analysis method, significant differences were found between AMF community fingerprints of samples derived from roots of grasses with (G_Cm) and without (G₀) neighboring C. maculosa. There were also significant differences between samples derived from C. maculosa roots (C_mac) and both G_Cm and G₀ roots. Sample ordination indicated three generally distinct groups consisting of C_mac, G_Cm and G₀, with G_Cm samples being of intermediate distance between G₀and C_mac. Our results indicate that, with the presence of C. maculosa, AMF communities of D. glomerata shift to reflect community composition associated with C. maculosa roots. These results highlight the importance of complex spatial distributions of AMF communities at the scale of a root system. An additional dimension to our study is that C. maculosa is an aggressively invasive plant in the intermountain West. Viewed in this light, these results suggest that pervasive influences of this plant on AMF communities, specifically in roots of its competitors, may represent a mechanism contributing to its invasive success. However, further work is clearly required to determine the extent to which AMF genotypic alteration by neighboring plants influences competitive relationships.     

The Role of Light and Soil Moisture in Plant Community Resistance to Invasion by Yellow Starthistle (Centaurea solstitialis)

To resist establishment by an invasive plant, a community may require one or more species functionally similar to the invader in their resource acquisition pattern. In this study, communities consisting of native winter annual forbs, non‐native annual grasses, native perennials, or a combination of the two native communities were established with and without Centaurea solstitialis to determine the effect of soil moisture and light availability on plant community invasion resistance. The annual plant communities were unable to resist invasion by C. solstitialis. In the native winter annual forb community, senescence in late spring increased light penetration (>75%) to the soil surface, allowing seeded C. solstitialis to quickly establish and dominate the plots. In addition, native annual forbs utilized only shallow soil moisture, whereas C. solstitialis used shallow and deep soil moisture. In communities containing native perennials, only Elymus glaucus established well and eventually dominated the plots. During the first 2 years of establishment, water use pattern of perennial communities was similar to native annual forbs and resistance to invasion was associated with reduced light availability during the critical stages of C. solstitialis establishment. In later years, however, water use pattern of perennial grass communities was similar or greater than C. solstitialis‐dominated plots. These results show that Central Valley grasslands that include E. glaucus resist C. solstitialis invasion by a combination of light suppression and soil water competition. Spatiotemporal resource utilization patterns, and not just functional similarity, should be considered when developing restoration strategies to resist invasion by many non‐native species.     

The role of aquatic macrophytes in microhabitatual transformation of physical-chemical features of small water bodies

The paper presents the results of an examination of the phycical-chemical parameters of water together with an analysis of the chlorophyll a concentration of 12 small water bodies situated within urban and suburban areas of the city of Poznań (mid-west Poland)—typical mid-forest, strongly anthropogenically modified in the urban landscape, strongly antropogenically modified in an agricultural area and clay-pits. There were zones of open water (Unvegetated Zone) as well as zones of rush and aquatic vegetation (Vegetated Zone) distinguished in the examined ponds. The influence of the rush vegetation, nymphaeids and elodeids on the abiotic parameters of an aquatic environment was examined. Water samples were taken during the summer of 2004 from 12 stations within the open water and 24 within macrophytes. The plant matter was randomly collected in triplicate from the central part of the vegetated stand. The influence of macrophytes on the abiotic features of water was estimated using the parameter of the plant length (cm l⁻¹) and the plant biomass (g l⁻¹). In the studied ponds 12 aquatic macrophyte communities were distinguished. A salient feature of submerged macrophytes was a great density of plant stems along with considerebly low biomass, however, the rush vegetation (Phragmitetum communis, Typhetum latifoliae) when compared to nymphaeids (Polygonetum natantis, Potametum natantis) and elodeids (Potametum lucentis) was characterised by lower stem densities and higher biomass. The water bodies were alkaline and of pronounced hardness. In most of them high trophy conditions were found with especially high concentrations of phosphorus (96 μg l⁻¹ on average). There was significant differentiation in the water chemistry (mainly in respect to mineral compounds) between the Vegetated and Unvegetated Zones as well as between particular aquatic macrophyte communities.