Ground-feeding migratory songbirds as cellular slime mold distribution vectors

Cosmopolitan species of cellular slime molds occur continents apart in both tropical and temperate zones of the world though the spore masses are too heavy to be wind borne, and water dispersal is limited to the watercourses. A highly mobile distribution vector was found in ground-feeding migratory song birds. Nine ubiquitous species and 2 ecologicially distinct species of dictyostelid cellular slime molds were isolated from the feces of ground-feeding eastern North American migratory thrushes, finches, sparrows and warblers, both on breeding and winter grounds. Three propagules of slime molds, amoebae, spores and macrocysts survive passage through the avian digesive tract and remain in the gut long enough to be transported by major bird migrations. Habitats with the greates species diversity of both cellular slime molds and ground-feeding passerines concur in both eastern North America and Central America. Birds actively seek their prefered habitats; the cellular slime molds have arrived at these habitats as passengers. Rare slime molds can serve as a marker to the habitats that migratory birds have visited, or birds with known habitats can provide clues as to the distribution of rare species of cellular slime molds.     

Residence time calculation for chemotactic bacteria within porous media

Local chemical gradients can have a significant impact on bacterial population distributions within subsurface environments by evoking chemotactic responses. These local gradients may be created by consumption of a slowly diffusing nutrient, generation of a local food source from cell lysis, or dissolution of nonaqueous phase liquids trapped within the interstices of a soil matrix. We used a random walk simulation algorithm to study the effect of a local microscopic gradient on the swimming behavior of bacteria in a porous medium. The model porous medium was constructed using molecular dynamics simulations applied to a fluid of equal-sized spheres. The chemoattractant gradient was approximated with spherical symmetry, and the parameters for the swimming behavior of soil bacterium Pseudomonas putida were based on literature values. Two different mechanisms for bacterial chemotaxis, one in which the bacteria responded to both positive and negative gradients, and the other in which they responded only to positive gradients, were compared. The results of the computer simulations showed that chemotaxis can increase migration through a porous medium in response to microscopic-scale gradients. The simulation results also suggested that a more significant role of chemotaxis may be to increase the residence time of the bacteria in the vicinity of an attractant source.     

Migration in Dictyostelium polycephalum

By comparing two species of cellular slime molds that have stalkless migration stages it is possible to gain interesting insights into how the cells move. In contrast to the familiar behavior of Dictyostelium discoideum, Dictyostelium polycephalum slugs can travel greater distances through soil and even can migrate through agar. In addition to the interest in the differences, these differences shed light on the mechanism of slug movement. Unlike D. discoideum, D. polycephalum does not have prestalk and prespore zones and severed sections of any part of these slugs move at a rate proportional to their length. This leads to the hypothesis that longer slugs move faster because the amoebae aligned along the inside of the slime sheath each contribute a forward push and the more extended the amoebae line is the faster the slug moves.     

Ecological role of energy taxis in microorganisms

Motile microorganisms rapidly respond to changes in various physico-chemical gradients by directing their motility to more favorable surroundings. Energy generation is one of the most important parameters for the survival of microorganisms in their environment. Therefore it is not surprising that microorganisms are able to monitor changes in the cellular energy generating processes. The signal for this behavioral response, which is called energy taxis, originates within the electron transport system. By coupling energy metabolism and behavior, energy taxis is fine-tuned to the environment a cell finds itself in and allows efficient adaptation to changing conditions that affect cellular energy levels. Thus, energy taxis provides cells with a versatile sensory system that enables them to navigate to niches where energy generation is optimized. This behavior is likely to govern vertical species stratification and the active migration of motile cells in response to shifting gradients of electron donors and/or acceptors which are observed within microbial mats, sediments and soil pores. Energy taxis has been characterized in several species and might be widespread in the microbial world. Genome sequencing revealed that many microorganisms from aquatic and soil environments possess large numbers of chemoreceptors and are likely to be capable of energy taxis. In contrast, species that have a fewer number of chemoreceptors are often found in specific, confined environments, where relatively constant environmental conditions are expected. Future studies focusing on characterizing behavioral responses in species that are adapted to diverse environmental conditions should unravel the molecular mechanisms underlying sensory behavior in general and energy taxis in particular. Such knowledge is critical to a better understanding of the ecological role of energy taxis.     

Montane outcrop vegetation of Banks Peninsula, South Island, New Zealand

Species composition patterns and vegetation?environment relationships were quantified for montane volcanic outcrops on Banks Peninsula. The flora of these habitat islands comprises 346 vascular plant species including 82 exotic species and 52 species that are nationally rare, regionally rare, or regional endemics. Both MDS ordination analysis and TWINSPAN results illustrated the high compositional and environmental heterogeneity across the outcrops. Multi-dimensional scaling revealed that primary environmental factors related to community composition comprise both regional-scale gradients of altitude and outcrop-scale gradients of slope steepness, soil pH, area available to plants, maximum vegetation height, and the percentage of the surrounding vegetation that is forest. Accordingly, TWINSPAN separated four outcrop communities that occur on steeper slopes, have less fertile soils and tend not to face north from three outcrop communities that have shallower slopes, more fertile soils and tend to face north. Types in the first group are more likely to be bordered by forest or taller shrublands, whereas those in the second group occur on outcrops primarily bordered by grasslands and support more exotic species. Within these broader groups, communities differ in their altitude and the size, soil depth and shading of the outcrops on which they occur. We describe the vegetation of the seven communities; this ranges from predominance of stunted trees and taller statured species such as Podocarpus hallii and Phormium cookianum to vegetation of shrubby species such as Heliohebe lavaudiana and Hebe strictissima, to short vegetation of native woodland and grassland species such as Polystichum vestitum and Rytidosperma corinum, to exotic pasture of clovers and exotic grasses. The percentage of species on an outcrop face that are exotic is well modelled by site factors, with exotics increasing as the surrounding matrix becomes more disturbed, slopes become more gentle, the percentage of shade on the outcrop decreases, and soil fertility increases. In contrast, nearby disturbance has little influence on the percentage or number of species that are rare on an outcrop face; rather rare species richness is more strongly related to outcrop area and lack of shade, echoing patterns observed for rare outcrop species elsewhere in the world. These results highlight the importance of considering the high compositional heterogeneity among outcrops and the influence of disturbance to surrounding ecosystems in guiding conservation planning.     

pH optima for Danish forest species compared with Ellenberg reaction values

Species distribution depends on the physiological and ecological niche where a species can exist and regenerate in resource competition with other species (niche limitation). The realized niche is influenced by local biotic processes that influence species behaviour and the shape of the response curves relative to environmental gradients. Processes on larger scales also influence the species niche through source-sink mechanisms (dispersal limitation) and the species richness of an area (pool limitation). Despite the growing evidence of skewed or irregular species response curves along gradients, many ecologists still assume symmetric, unimodal response curves along gradients in ecological interpretation. Ellenberg’s indicator system is probably the most common example. However, the assumption is not ecologically or statistically valid, due to the many different processes affecting the distribution of plant species. Here I present the results of Huisman-Olff-Fresco (HOF) regressions for 209 Danish forest species. HOF modelling is chosen to avoid the classical drawbacks of assuming symmetric, unimodal response patterns. I calculate the optima for all species with unimodal responses to soil pH and compare these with the Ellenberg indicator values for reaction (R), which are often used as a substitute for soil pH measurements. I demonstrate that the assumption of symmetric, unimodal species behaviour is violated in 54% of the cases and that pH optima and R indicator values for species are not always compatible. Ellenberg reaction scale has been used byEwald (Folia Geobot. 38: 357–366, 2003) as an indicator of which species are calcicole, i.e., whether they can grow and reproduce on calcareous soils. Such affinities of species, however, are related to both local niche properties and processes on large scales and cannot be generalized from a single empirical variable such as pH, nor from Ellenberg semi-ordinal indicator scale. I conclude that while the determination of whether species are calcicole or calcifuge requires more research, it is evident that Denmark contains a fairly balanced number of calciphytic and acidophytic species. This is probably due to the nearly equal areas with acidic and alkaline soils in Denmark, which also contribute to the high species richness of more than 500 vascular plant species in Danish forests.     

Using ants to manage sustainable grazing: Dynamics of ant faunas along sheep grazing gradients conform to four global patterns

Ants are considered an important faunal group for the functioning of arid rangelands, they have a long history of use for environmental monitoring, and exhibit four global patterns in grazing lands: (i) soil and vegetation type are primary determinants of ant community composition, and have a far greater effect on ant community composition than grazing; (ii) grazing induces species compositional change, but does not necessarily affect species richness or abundance; (iii) a species response to grazing is not necessarily consistent across habitats; and (iv) approximately one‐quarter to one‐half of species that are common enough for statistical analysis have significant responses to grazing. Here we report the patterns of arid zone ant faunas as they exist after several decades of sheep grazing in southern Australia, and examine the extent to which they conform to the four global patterns. We measured ant faunas along grazing gradients (varying distance to water) in seven paddocks containing two soil and two vegetation types on five pastoral properties. Total site abundance and richness of ants did not differ significantly with distance from water, but the abundance of 10 (34%) of the 29 most common species did differ; three were increasers, three were decreasers, and four had mixed responses dependent on soil/vegetation type. Rare species showed no trend with grazing intensity. The ant fauna of the more structurally complex vegetation types appeared to be the most vulnerable to grazing effects. Multivariate analysis showed soil type was the primary factor influencing ant faunal composition, followed by vegetation structure; however, grazing treatment effects were present. This study fully supports the recently identified global patterns of ant responses to grazing. It also shows that sampling regional ant faunas using widely dispersed traps can detect ant faunal patterns comparable to studies that use smaller‐scale grids of traps.     

The effects of dictyostelids on the formation and maturation of myxomycete plasmodia

Dictyostelids (cellular slime molds) and myxomycetes (plasmodial slime molds) are two groups of mycetozoans usually present and often abundant in the soil and litter microhabitats of terrestrial ecosystems. Because they utilize the same food resource and occur together in a spatially limited and clearly defined microhabitat, the potential for ecological interactions would seem to exist. However, relatively few previous studies have considered this aspect of mycetozoan ecology. In the present study twenty-eight isolates (8 species) of dictyostelids were co-cultured in all possible pair-wise combinations with fourteen isolates (7 species) of myxomycetes to determine if there were any effects on the production of fruiting bodies. Dictyostelids showed little or no delay in culmination and only random and inconsistent reductions in sorocarp abundance when co-cultured with myxomycetes. In contrast, myxomycetes displayed a number of specific effects. The heterothallic isolates exhibited delays in plasmodial formation and/or maturation, with some pairings showing little to no effect, while others displayed nearly complete inhibition of plasmodial formation or maturation. Apomictic isolates, in general, were much less affected, with only a few combinations displaying significant delays in both formation and maturation of plasmodia.     

Multivariate analysis of vegetational variation in different habitats at Omayed,Egypt

The application of indicator species analysis has proved useful in classifying stands into groupings coinciding with topographic variations. 41 indicator pseudo-species are identified in the vegetation along a phytosociological gradient: some are characteristic of specific habitats, and the others indicate transition between groups of habitats.The use of qualitative estimations (transformed density records) in multivariate analysis in the present study indicates that they may be more preferable than quantitative estimations, because of their easy and fast recording in the field, and the less comprehensive computations, while yielding precise results.The X-axis of the reciprocal averaging ordination is related to the salinity and fertility gradients, while the Y-axis reflects soil texture.The phytosociological gradients of the canonical variate axes reflect essentially contrasts between groups of species, each correlated with one or the other of the environmental gradients.Comparing the results obtained by the two ordination methods (reciprocal averaging and canonical variates) we find that the first method detects the overall phytosociological changes along strong environmental gradients which would be helpful in studying large surveys. On the other hand, the second technique, which is mainly predictive, is more sensitive to changes within both the phytosociological and environmental gradients and can detect the impact of these changes on the overall variance of each gradient.Nomenclature follows Täckholm (1964).The authors are grateful to Dr J. Jeffers and Mr and Mrs P. Howard of the Institute of Terrestrial Ecology, England, for their help with applying the multivariate techniques to the vegetation data.     

Determination of the active portion of the folic acid molecule in cellular slime mold chemotaxis.

From earlier work it is known that folic acid attracts the amoebae of various species of cellular slime molds (11). Here we have tested a wide variety of pteridines, pyrimidines, and pyrazines to determine what part of the folic acid molecule is chemotactically active. It was shown that the activity lies in the pteridine ring itself. Furthermore, the cell-free supernatants of slime mold amoebae contain an enzyme that renders pterin and folic acid chemotactically inactive, which apparently increases the chemotactic sensitivity of the amoebae to those compounds. Despite the fact that slime mold amoebae secrete small amounts of folic acid-related compounds, there is no evidence that folates are acrasins; rather it is postulated that attraction to folates may be a food-seeking device for the amoebae which prey on folate-secreting bacteria in the soil.     

Covariation in plant functional traits and soil fertility within two species-rich forests

The distribution of plant species along environmental gradients is expected to be predictable based on organismal function. Plant functional trait research has shown that trait values generally vary predictably along broad-scale climatic and soil gradients. This work has also demonstrated that at any one point along these gradients there is a large amount of interspecific trait variation. The present research proposes that this variation may be explained by the local-scale sorting of traits along soil fertility and acidity axes. Specifically, we predicted that trait values associated with high resource acquisition and growth rates would be found on soils that are more fertile and less acidic. We tested the expected relationships at the species-level and quadrat-level (20 × 20 m) using two large forest plots in Panama and China that contain over 450 species combined. Predicted relationships between leaf area and wood density and soil fertility were supported in some instances, but the majority of the predicted relationships were rejected. Alternative resource axes, such as light gradients, therefore likely play a larger role in determining the interspecific variability in plant functional traits in the two forests studied.     

Precipitation gradients,plant biogeography,and the incidence of drip-tips in Cerrado plant species

Examining how both climate and species distribution patterns correlate with leaf morphology can give insights into the ecological and evolutionary patterns that drive adaptive selection of leaf form and function. Drip-tips are a common feature of leaves in rain forest tree species; they are thought to be an adaptation that aids leaf drying and maximizes photosynthesis in areas with high-rainfall climates. We tested whether this macroecological pattern holds true across the precipitation gradients in a non-rain forest region—the woodland savannas of Brazil known as the Cerrado—and compared our results with previous studies from Amazonia. Drip-tips were, as expected, less common overall in the drier Cerrado than in Amazonia. In addition, within the Cerrado, drip-tips were more prevalent in areas with higher rainfall as well as in Cerrado sites that were closer to Amazonia. Moreover, species that occurred across both the Cerrado and Amazonia had drip-tips more often than species that were found only in the Cerrado. These findings support the hypothesis that drip-tips are adaptive and that either the cost of retaining drip-tips is low or that in drier regions they have other benefits.     

Diversity and Frequency of Clonal Traits Along Natural and Land-Use Gradients in Grasslands of the Swiss Alps

The frequency of clonal plants in different vegetation types is known to be influenced by environmental and land-use factors. However, the underlying behavior of individual clonal traits or clonal trait diversity has received little attention. Here, we assess for species- and trait-diverse grasslands of the Swiss Alps the relative importance of temperature, soil moisture, land use and species richness on the diversity and frequency of individual compared with all clonal traits. We further analyzed how cover-weighted data alters the relationships found with commonly used presence-absence data. We combined species compositional, land-use and environmental data from 236 28-m² grassland plots with clonal trait information for 527 species following the Clonal Growth Organ (CGO) classification. Test results are based on linear models, ANOVAs and ANCOVAs. The grassland sites were 84% dominated by clonal species. Drought-prone grasslands harbored the least clonal species. No increase in clonality was detected with decreasing temperature (= altitude). Mown or pastured grasslands had more clonal species than fallows. Certain sets of traits were correlated. Rhizomatous species especially reacted strongly to climatic and land-use gradients and had highest frequencies in cold, moist and disturbed sites. Clonal diversity was strongly dependent on species richness. Cover-weighted and presence-absence based estimates were largely similar. Overall, our data outlined that common clonal traits react differently to natural and land-use gradients as well as differently to the sum of clonal traits. Also, soil moisture was more decisive than temperature (= altitude) for the presence of clonal species. Lastly, the strong correlation between species-richness and clonal trait diversity needs to be accounted for when interpreting the functional role of clonal traits.     

Within and between species variation in response to environmental gradients in Polygonum pensylvanicum and Polygonum virginianum

Summary The responses of Polygonum pensylvanicum L., an early successional annual, and of Polygonum virginianum L., a late successional perennial, were examined along discontinuous gradients of soil moisture, light and nutrient availability. In the case of P. virginianum both individuals grown from seed and individuals grown from rhizomes were examined. The results show that variation in the response of individuals of a species of different age to environmental variation is as great as that found between the two congeneric species of different successional habitats. In general, individuals of the two species were more similar to one another in response to the resource gradients when both were started from seed, than were individuals of P. virginianum grown from seed and from rhizomes. Potential differences in stored reserves (starting capital) between rhizomes and seeds appeared to have little effect upon responses to resource availability. A number of plant characters were found to vary along the gradients in ways that were unique to the character, the gradient, and the age of the individual. These characters included aspects of leaf size, shape, and orientation, as well as whole plant architecture. Many if not all of these characters are likely to have significant effects upon the functioning of plants. The origin of this difference in response to the gradients of individuals of P. virginianum of differing age may be ontogenetic or may reflect differences in genetic composition created by recombination.     

Vegetation changes on formerly overgrazed hill slopes in semi-arid central Tanzania

Abstract. Composition of hill slope vegetation was studied in a semi-arid part of upland Tanzania where all grazing had been banned for 12 yr. The hills had been severely overgrazed previously and suffered from heavy gully and sheet erosion. Eight vegetation types are described. Floristic gradients revealed by numerical ordination techniques were found to be related mainly to degree of erosion, soil type and succession. The more or less bare soil that prevailed after grazing had ceased is now covered by grassland, woodland and immature secondary forest. The grasslands are still characterized by early successional species and they will probably remain open grassland as long as frequent burning continues. Brachystegia woodlands may have developed during earlier periods when the field layer was sparse due to grazing. The grazing had reduced the frequency of fire which in turn promoted the establishment of Brachystegia spp. Secondary forests are believed to have developed mainly where fires were not frequent, particularly at higher altitudes.     

Evolution of conditional dispersal: a reaction–diffusion–advection model

To study evolution of conditional dispersal, a Lotka-Volterra reaction-diffusion-advection model for two competing species in a heterogeneous environment is proposed and investigated. The two species are assumed to be identical except their dispersal strategies: both species disperse by random diffusion and advection along environmental gradients, but one species has stronger biased movement (i.e., advection along the environmental gradients) than the other one. It is shown that at least two scenarios can occur: if only one species has a strong tendency to move upward the environmental gradients, the two species can coexist since one species mainly pursues resources at places of locally most favorable environments while the other relies on resources from other parts of the habitat; if both species have such strong biased movements, it can lead to overcrowding of the whole population at places of locally most favorable environments, which causes the extinction of the species with stronger biased movement. These results provide a new mechanism for the coexistence of competing species, and they also imply that selection is against excessive advection along environmental gradients, and an intermediate biased movement rate may evolve.     

Demography in relation to population density in two herbivorous marsupials: testing for source–sink dynamics versus independent regulation of population size

We compared demography of populations along gradients of population density in two medium-sized herbivorous marsupials, the common brushtail possum Trichosurus vulpecula and the rufous bettong Aepyprymnus rufescens, to test for net dispersal from high density populations (acting as sources) to low density populations (sinks). In both species, population density was positively related to soil fertility, and variation in soil fertility produced large differences in population density of contiguous populations. We predicted that if source–sink dynamics were operating over this density gradient, we should find higher immigration rates in low-density populations, and positive relationships of measures of individual fitness—body condition, reproductive output, juvenile growth rates and survivorship—to population density. This was predicted because under source–sink dynamics, immigration from high-density sites would hold population density above carrying capacity in low-density sites. The study included 13 populations of these two species, representing a more than 50-fold range of density for each species, but we found that individual fitness, immigration rates and population turnover were similar in all populations. We conclude that net dispersal from high to low density populations had little influence on population dynamics in these species; rather, all populations appeared to be independently regulated at carrying capacity, with a balanced exchange of dispersers among populations. These two species have suffered recent reductions in range, and they are ecologically similar to other species that have declined to extinction in inland Australia. It has been argued that part of the cause of the vulnerability of species like these is that they exhibit source–sink dynamics, and disturbance to source habitats can therefore cause large-scale population collapses. The results of our study argue against this interpretation.     

Comparing species interaction networks along environmental gradients

Knowledge of species composition and their interactions, in the form of interaction networks, is required to understand processes shaping their distribution over time and space. As such, comparing ecological networks along environmental gradients represents a promising new research avenue to understand the organization of life. Variation in the position and intensity of links within networks along environmental gradients may be driven by turnover in species composition, by variation in species abundances and by abiotic influences on species interactions. While investigating changes in species composition has a long tradition, so far only a limited number of studies have examined changes in species interactions between networks, often with differing approaches. Here, we review studies investigating variation in network structures along environmental gradients, highlighting how methodological decisions about standardization can influence their conclusions. Due to their complexity, variation among ecological networks is frequently studied using properties that summarize the distribution or topology of interactions such as number of links, connectance, or modularity. These properties can either be compared directly or using a procedure of standardization. While measures of network structure can be directly related to changes along environmental gradients, standardization is frequently used to facilitate interpretation of variation in network properties by controlling for some co‐variables, or via null models. Null models allow comparing the deviation of empirical networks from random expectations and are expected to provide a more mechanistic understanding of the factors shaping ecological networks when they are coupled with functional traits. As an illustration, we compare approaches to quantify the role of trait matching in driving the structure of plant–hummingbird mutualistic networks, i.e. a direct comparison, standardized by null models and hypothesis‐based metaweb. Overall, our analysis warns against a comparison of studies that rely on distinct forms of standardization, as they are likely to highlight different signals. Fostering a better understanding of the analytical tools available and the signal they detect will help produce deeper insights into how and why ecological networks vary along environmental gradients.     

Vegetation and microclimatic edge effects in two mixed-mesophytic forest fragments

Forest edges are known to consist of microenvironments that may provide habitat for a different suite of species than forest interiors. Several abiotic attributes of the microenvironment may contribute to this change across the edge to center gradient (e.g., light, air temperature, soil moisture, humidity). Biotic components, such as seed dispersal, may also give rise to changes in species composition from forest edge to interior. We predicted that abiotic and biotic measures would correlate with distance from forest edge and would differ among aspects. To test these predictions, we measured abiotic and biotic variables on twelve 175 m transects in each of two 24 ha forest fragments in east-central Illinois that have remained in continuous isolation for upwards of 100 years. Both univariate and multivariate techniques were used to best describe the complex relationships among abiotic factors and between abiotic and biotic factors. Results indicate that microclimatic variables differ in the degree to and distance over which they show an edge effect. Relative humidity shows the widest edge, while light and soil moisture have the steepest gradients. Aspect influences are evidenced by the existence of more pronounced edge effects on south and west edges, except when these edges are protected by adjacent habitat. Edges bordered by agricultural fields have more extreme changes in microclimate than those bordered by trees. According to PCA results, species richness correlates well with microclimatic variation, especially light and soil moisture; however, in many cases species richness had a different depth of edge influence than either of these variables. The herbaceous plant community is heavily dominated by three species. Distributions of individual species as well as changes in plant community composition, estimated with a similarity index, indicate that competition may be influencing the response of the vegetation to the edge to interior gradient. This study indicates that edge effects must be considered when the size and potential buffering habitat of forest preserves are planned.