Competition and the Effect of Spatial Resource Heterogeneity on Evolutionary Diversification

A model is presented to explore how the form of selection arising from competition for resources is affected by spatial resource heterogeneity. The model consists of a single species occupying two patches connected by migration, where the two patches can differ in the type of resources that they contain. The main goal is to determine the conditions under which competition for resources results in disruptive selection (i.e., selection favoring a polymorphism) since it is this form of selection that will give rise to the evolutionary diversification of resource exploitation strategies. In particular, comparing the conditions giving rise to disruptive selection when the two patches are identical to the conditions when they contain different resources reveals the effect of spatial resource heterogeneity. Results show that when the patches are identical, the conditions giving rise to disruptive selection are identical to those that give rise to character displacement in previous models. When the patches are different, the conditions giving rise to disruptive selection can be either more or less stringent depending upon demographic parameters such as the intrinsic rate of increase and the migration rate. Surprisingly, spatial resource heterogeneity can actually make forms of evolutionary diversification such as character displacement less likely. It is also found that results are dependent on how the resource exploitation strategies and the spatial resource heterogeneity affect the population dynamics. One robust conclusion however, is that spatial resource heterogeneity always has a disruptive effect when the migration rate between patches is low.     

Soil Particle Heterogeneity Affects the Growth of a Rhizomatous Wetland Plant

Soil is commonly composed of particles of different sizes, and soil particle size may greatly affect the growth of plants because it affects soil physical and chemical properties. However, no study has tested the effects of soil particle heterogeneity on the growth of clonal plants. We conducted a greenhouse experiment in which individual ramets of the wetland plant Bolboschoenus planiculmis were grown in three homogeneous soil treatments with uniformly sized quartz particles (small: 0.75 mm, medium: 1.5 mm, or large: 3 mm), one homogeneous treatment with an even mixture of large and medium particles, and two heterogeneous treatments consisting of 16 or 4 patches of large and medium particles. Biomass, ramet number, rhizome length and spacer length were significantly greater in the treatment with only medium particles than in the one with only large particles. Biomass, ramet number, rhizome length and tuber number in the patchy treatments were greater in patches of medium than of large particles; this difference was more pronounced when patches were small than when they were large. Soil particle size and soil particle heterogeneity can greatly affect the growth of clonal plants. Thus, studies to test the effects of soil heterogeneity on clonal plants should distinguish the effects of nutrient heterogeneity from those of particle heterogeneity.     

Effects of Spatial Scale of Soil Heterogeneity on the Growth of a Clonal Plant Producing Both Spreading and Clumping Ramets

Soil nutrients are commonly heterogeneously distributed at different spatial scales. Although numerous studies have tested the effects of soil nutrient heterogeneity on growth of clonal plants producing either spreading ramets or clumping ramets, no study has examined the effects on the growth of clonal plants producing both spreading and clumping ramets and how spatial scale affects such effects. To test these effects, clones of Buchloe dactyloides, a stoloniferous clonal plant that produces both clumping and spreading ramets, were grown in six heterogeneous environments with different patch sizes and one homogeneous environment containing the same quantity of nutrients. Total biomass, total number of ramets, number of clumping ramets, number of spreading ramets, spacer length, or root:shoot ratio of the whole plants did not differ significantly among the seven treatments. However, at the patch level there were significant effects of patch size by nutrient level on biomass, number of ramets, number of spreading ramets, and number of clumping ramets, and these four variables were significantly larger in the nutrient-rich patches than in the nutrient-poor patches in the heterogeneous treatment with the largest patch size, but not in the other five heterogeneous treatments with smaller patch sizes. Neither nutrient level nor patch size significantly affected spacer length or root:shoot ratio. Based on our results, we propose that B. dactyloides can efficiently exploit nutrient-rich patches by a plastic response of clumping ramets and spreading ramets at larger spatial scales of soil heterogeneity but not at smaller ones.     

Phosphate Addition and Plant Species Alters Microbial Community Structure in Acidic Upland Grassland Soil

Agricultural improvement (addition of fertilizers, liming) of seminatural acidic grasslands across Ireland and the UK has resulted in significant shifts in floristic composition, soil chemistry, and microbial community structure. Although several factors have been proposed as responsible for driving shifts in microbial communities, the exact causes of such changes are not well defined. Phosphate was added to grassland microcosms to investigate the effect on fungal and bacterial communities. Plant species typical of unimproved grasslands (Agrostis capillaris, Festuca ovina) and agriculturally improved grasslands (Lolium perenne) were grown, and phosphate was added 25 days after seed germination, with harvesting after a further 50 days. Phosphate addition significantly increased root biomass (p < 0.001) and shoot biomass (p < 0.05), soil pH (by 0.1 U), and microbial activity (by 5.33 mg triphenylformazan [TPF] g⁻¹ soil; p < 0.001). A slight decrease (by 0.257 mg biomass-C g⁻¹ soil; p < 0.05) in microbial biomass after phosphate addition was found. The presence of plant species significantly decreased soil pH (p < 0.05; by up to 0.2 U) and increased microbial activity (by up to 6.02 mg TPF g⁻¹ soil) but had no significant effect on microbial biomass. Microbial communities were profiled using automated ribosomal intergenic spacer analysis. Multidimensional scaling plots and canonical correspondence analysis revealed that phosphate addition and its interactions with upland grassland plant species resulted in considerable changes in the fungal and bacterial communities of upland soil. The fungal community structure was significantly affected by both phosphate (R = 0.948) and plant species (R = 0.857), and the bacterial community structure was also significantly affected by phosphate (R = 0.758) and plant species (R = 0.753). Differences in microbial community structure following P addition were also revealed by similarity percentage analysis. These data suggest that phosphate application may be an important contributor to microbial community structural change during agricultural management of upland grasslands.     

Bacterial Activity, Community Structure, and Centimeter-Scale Spatial Heterogeneity in Contaminated Soil

In an anthropogenically disturbed soil (88% sand, 8% silt, 4% clay), 150-mg samples were studied to examine the fine-scale relationship of bacterial activity and community structure to heavy metal contaminants. The soils had been contaminated for over 40 years with aromatic solvents, Pb, and Cr. Samples from distances of <1, 5, 15, and 50 cm over a depth range of 40–90 cm underwent a sequential analysis to determine metabolic potential (from ¹⁴C glucose mineralization), bacterial community structure [using polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE)], and total extractable Pb and Cr levels. Metabolic potential varied by as much as 10,000-fold in samples <1 cm apart; log–log plots of metal concentration and microbial metabolic potential showed no correlation with each other. Overall, metal concentrations ranged from 9 to 29,000 mg kg⁻¹ for Pb and from 3 to 8500 mg kg⁻¹ for Cr with small zones of high contamination present. All regions exhibited variable metal concentrations, with some soil samples having 30-fold differences in metal concentration in sites <1 cm apart. Geostatistical analysis revealed a strong spatial dependence for all three parameters tested (metabolic activity, Pb, and Cr levels) with a range up to 30 cm. Kriging maps showed that in zones of high metal, the corresponding metabolic activity was low suggesting that metals negatively impacted the microbial community. PCR-DGGE analysis revealed that diverse communities were present in the soils with a random distribution of phylotypes throughout the sampling zones. These results suggest the presence of spatially isolated microbial communities within the soil profile.     

Heterogeneity Profoundly Alters Emergent Stress Fields in Constrained Multicellular Systems

Stress fields emerging from the transfer of forces between cells within multicellular systems are increasingly being recognized as major determinants of cell fate. Current analytical and numerical models used for the calculation of stresses within cell monolayers assume homogeneous contractile and mechanical cellular properties; however, cell behavior varies by region within constrained tissues. Here, we show the impact of heterogeneous cell properties on resulting stress fields that guide cell phenotype and apoptosis. Using circular micropatterns, we measured biophysical metrics associated with cell mechanical stresses. We then computed cell-layer stress distributions using finite element contraction models and monolayer stress microscopy. In agreement with previous studies, cell spread area, alignment, and traction forces increase, whereas apoptotic activity decreases, from the center of cell layers to the edge. The distribution of these metrics clearly indicates low cell stress in central regions and high cell stress at the periphery of the patterns. However, the opposite trend is predicted by computational models when homogeneous contractile and mechanical properties are assumed. In our model, utilizing heterogeneous cell-layer contractility and elastic moduli values based on experimentally measured biophysical parameters, we calculate low cell stress in central areas and high anisotropic stresses in peripheral regions, consistent with the biometrics. These results clearly demonstrate that common assumptions of uniformity in cell contractility and stiffness break down in postconfluence confined multicellular systems. This work highlights the importance of incorporating regional variations in cell mechanical properties when estimating emergent stress fields from collective cell behavior.     

Competition for Light in a Plant Monoculture Modelled as a Spatial Stochastic Process

In a glasshouse experiment closely spaced plants were regularlysub-irrigated with nutrient solution and it was assumed thatcompetition between individuals was for light. Interaction betweenneighbours is modelled as a spatial process in relation to differencesin plant height. The influence one plant has on another is calculatedas the ratio of the angle ß between their apices,and an angle which represents the intensity of competitionin the population as a whole. When ß < suppressionis considered to occur, i.e. tall plants suppress short plantsbut not vice versa, competition is one-sided. The status ofeach plant within the population is calculated by multiplicationof its individual plant neighbour interaction terms where ß< . This need not be restricted to nearest neighbours. Plantswith status = 1 are free from competition, those with status= 0 are totally suppressed. It is acknowledged that other processes than that modelled ascompetition may result in variation in relative growth rates.To account for this plant status is used to modify a relativegrowth rate drawn from a population of mean µ and standarddeviation . Plants for which status µ is less than anestimated are considered to die. Parameter values were estimatedfor a sequence of fortnightly harvests of glasshouse grown Tagetespatula. There was a decrease in µ and , increased andplants came to compete with their second nearest neighboursas well as their nearest as the community developed. The greatest was at an intermediate stage, suggesting that more frequentassessments of the competition process are required at somestages than at others. Model parameters fitted to one data setwere used in a Monte Carlo testing procedure with a second,independent data set. The importance of this technique is stressedbecause plants in a single community are not independent realizationof the competition process. Simulations with the model reproducedboth bimodality in the frequency distributions of plant sizeand an even spatial distribution of large or surviving plants,features which have been observed in a range of plant monocultures.To achieve these features it was essential that plant interactionbe modelled as a one-sided process. Tagetes patula L., competition, light, monoculture, growth model     

Spatial Heterogeneity of Denitrification in Semi-Arid Floodplains

Riparian ecosystems are recognized as sinks for inorganic nitrogen (N). Denitrification, a heterotrophic microbial process, often accounts for a significant fraction of the N removed. Characteristics of both riparian soils and hydrologic vectors may constrain the locations where denitrification can occur within riparian ecosystems by influencing the distribution of substrates, water, and suitable redox conditions. We employed spatially explicit methods to quantify heterogeneity of soil characteristics and potential rate of denitrification in semi-arid riparian ecosystems. These results allow us to evaluate the relative contributions of hydrologic vectors (water courses that convey materials) and soil resources (materials required by biota) to spatial heterogeneity of denitrification. During dry and monsoon seasons we contrasted a mesic site, characterized by shallow groundwater and annual inundation by floods, with a xeric site that is inundated less often and has a deeper water table. Potential denitrification was detected throughout the mesic floodplain and the average rate of denitrification was greater at the mesic site than at the xeric site, indicating the influence of water availability on denitrification. At the xeric reach, sharp declines in pools of soil resources and rate of denitrification occurred away from the stream, demonstrating the importance of the stream in determining spatial patterns. Using geographically weighted regression analysis, we determined that soil organic matter and soil nitrate were significant predictors of denitrification at the xeric site, but that factors influencing denitrification varied spatially. Spatial heterogeneity of carbon (C) and N substrates in soils likely influenced spatial patterns of denitrification, but distribution of C and N substrates was ultimately organized by hydrologic vectors. Droughts will increase the abundance of reaches with hydrogeomorphic templates similar to the xeric reach studied here. Consequences of such a transition may include a reduced rate of denitrification and patchy distribution of denitrification in floodplain soils, which will decrease the contribution of riparian ecosystems to N removal. TKH designed and completed the study and wrote the paper; EAW contributed methods and edited the paper; NBG designed the study and edited the paper.     

Identification of Soil Microbes Capable of Utilizing Cellobiosan

Approximately 100 million tons of anhydrosugars, such as levoglucosan and cellobiosan, are produced through biomass burning every year. These sugars are also produced through fast pyrolysis, the controlled thermal depolymerization of biomass. While the microbial pathways associated with levoglucosan utilization have been characterized, there is little known about cellobiosan utilization. Here we describe the isolation and characterization of six cellobiosan-utilizing microbes from soil samples. Each of these organisms is capable of using both cellobiosan and levoglucosan as sole carbon source, though both minimal and rich media cellobiosan supported significantly higher biomass production than levoglucosan. Ribosomal sequencing was used to identify the closest reported match for these organisms: Sphingobacterium multivorum, Acinetobacter oleivorans JC3-1, Enterobacter sp SJZ-6, and Microbacterium sps FXJ8.207 and 203 and a fungal species Cryptococcus sp. The commercially-acquired Enterobacter cloacae DSM 16657 showed growth on levoglucosan and cellobiosan, supporting our isolate identification. Analysis of an existing database of 16S rRNA amplicons from Iowa soil samples confirmed the representation of our five bacterial isolates and four previously-reported levoglucosan-utilizing bacterial isolates in other soil samples and provided insight into their population distributions. Phylogenetic analysis of the 16S rRNA and 18S rRNA of strains previously reported to utilize levoglucosan and our newfound isolates showed that the organisms isolated in this study are distinct from previously described anhydrosugar-utilizing microbial species.     

In Situ Dynamics and Spatial Heterogeneity of Soil Bacterial Communities Under Different Crop Residue Management

The effect of the location of wheat residues (soil surface vs. incorporated in soil) on their decomposition and on soil bacterial communities was investigated by the means of a field experiment. Bacterial-automated ribosomal intergenic spacer analysis of DNA extracts from residues, detritusphere (soil adjacent to residues), and bulk soil evidenced that residues constitute the zone of maximal changes in bacterial composition. However, the location of the residues influenced greatly their decomposition and the dynamics of the colonizing bacterial communities. Sequencing of 16S rRNA gene in DNA extracts from the residues at the early, middle, and late stages of degradation confirmed the difference of composition of the bacterial community according to the location. Bacteria belonging to the γ-subgroup of proteobacteria were stimulated when residues were incorporated whereas the α-subgroup was stimulated when residues were left at the soil surface. Moreover, Actinobacteria were more represented when residues were left at the soil surface. According to the ecological attributes of the populations identified, our results suggested that climatic fluctuations at the soil surface select populations harboring enhanced catabolic and/or survival capacities whereas residues characteristics likely constitute the main determinant of the composition of the bacterial community colonizing incorporated residues.     

Competition Alters Temporal Dynamics of Sporulation in the Wheat Stem Rust Fungus

To investigate the effects of competition on the timing of pathogen reproduction, urediniospores of two strains of Puccinia graminis f.sp. tritici (SR22 and SR41) were inoculated onto leaves of wheat seedlings singly and in 1 : 1 mixture at three inoculum densities. On randomly sampled leaves, uredinia were counted 9 days after inoculation and urediniospores were collected and quantified every other day from the seventh to the 29th day after inoculation. Increases in inoculum density resulted in progressively smaller increases in uredinial numbers. However, total urediniospore production per leaf was not significantly affected by inoculum, and hence uredinial, density over a range of approximately 10-300 uredinia on the leaf. Total urediniospore production per uredinium generally decreased with increasing inoculum or uredinial density. At high densities, sporulation per uredinium peaked earlier in the sporulation period, had a less distinct peak, and dropped off earlier than for the lower densities. Logistic model fits to cumulative sporulation curves over time revealed that strain SR41 had a greater epidemic rate parameter (r) than SR22 at low and intermediate inoculum or uredinial densities, while SR22 had a higher r-value than SR41 at high density. Both strains also exhibited greater r-values in the presence of the other strain than when alone. Results suggest that strains may have different ecological strategies in their timing of reproduction, and that both intra- and interstrain competition can have complex effects on the temporal dynamics of sporulation in pathogen strains.     

Spatial Heterogeneity of Crenarchaeal Assemblages within Mesophilic Soil Ecosystems as Revealed by PCR-Single-Stranded Conformation Polymorphism Profiling

Microbial ecologists have discovered novel rRNA genes (rDNA) in mesophilic soil habitats worldwide, including sequences that affiliate phylogenetically within the division Crenarchaeota (domain Archaea). To characterize the spatial distribution of crenarchaeal assemblages in mesophilic soil habitats, we profiled amplified crenarchaeal 16S rDNA sequences from diverse soil ecosystems by using PCR-single-stranded-conformation polymorphism (PCR-SSCP) analysis. PCR-SSCP profiles provide a measure of relative microbial diversity in terms of richness (number of different phylotypes as estimated from the number of unique PCR-SSCP peaks) and evenness (abundance of each phylotype as estimated from the relative area under a peak). Crenarchaeal assemblages inhabiting prairie, forest, turf, and agricultural soils were characterized at six sampling locations in southern and central Wisconsin. Phylotype richness was found to be more stable than evenness among triplicate samples collected within 30 cm at each sampling location. Transformation of the PCR-SSCP data by principal-component analysis, followed by statistical testing (analysis of variance [P < 0.0001] and least-significant-difference analysis [α = 0.5]), supported the conclusion that each location exhibited a unique profile. To further characterize the spatial distribution of crenarchaeal assemblages at one location, additional soil samples (a total of 30) were collected from agricultural field plots at the Hancock Agricultural Research Station. PCR-SSCP revealed a patchy spatial distribution of crenarchaeal assemblages within and between these plots. This mosaic of crenarchaeal assemblages was characterized by differences in phylotype evenness that could not be correlated with horizontal distance (15 to 30 m) or with depth (0 to 20 cm below the surface). Crenarchaeal 16S rDNA clone libraries were produced and screened for unique SSCP peaks. Clones representing the dominant phylotypes at each location were identified, sequenced, and found to group phylogenetically with sequences in crenarchaeal clade C1b.     

The Plant Oncogene rolB Alters Binding of Auxin to Plant Cell Membranes

We measured auxin-binding capacity of the membrane preparationsfrom tobacco cells transformed by rolB as compared to untransformedcontrols. In the transformed cells, the overall auxin-bindingactivity is severalfold enhanced through an increase in a bindingactivity removable from the membranes at 0.5 M salt, while thebinding activity still attached to the membranes after saltwashes remains unchanged. Antibodies against the 22 kDa maizeauxin binding protein (ABP) depress most of the membrane-attachedbinding activity in both normal and rolB-transformed cells,while they do not affect the salt-washable binding activity.In contrast, antibodies against the RolB protein prevent completelybinding of auxin to the latter activity in both normal and transformedcells, while substantially unaffecting the membrane-associatedbinding. These results point to the presence, in untransformedmembranes, of an auxin-binding activity associated with a proteinimmunologically related to RolB. This activity is much increasedin rolB cells. In contrast, the auxin-binding protein analogousto maize ABP present in tobacco membranes does not increasein the rolB-transformed cells. (Received October 1, 1993; Accepted April 22, 1993)     

Spatial Measures of Genetic Heterogeneity During Carcinogenesis

In this work we explore the temporal dynamics of spatial heterogeneity during the process of tumorigenesis from healthy tissue. We utilize a spatial stochastic model of mutation accumulation and clonal expansion in a structured tissue to describe this process. Under a two-step tumorigenesis model, we first derive estimates of a non-spatial measure of diversity: Simpson’s Index, which is the probability that two individuals sampled at random from the population are identical, in the premalignant population. We next analyze two new measures of spatial population heterogeneity. In particular we study the typical length scale of genetic heterogeneity during the carcinogenesis process and estimate the extent of a surrounding premalignant clone given a clinical observation of a premalignant point biopsy. This evolutionary framework contributes to a growing literature focused on developing a better understanding of the spatial population dynamics of cancer initiation and progression.     

Soil Moisture Alters the Response of Soil Organic Carbon Mineralization to Litter Addition

Increasing rainfall and longer drought conditions lead to frequent changes in soil moisture that affect soil organic carbon (SOC) mineralization. However, how soil moisture affects response of SOC mineralization to litter addition in forest ecosystems remains unexplored. We added ¹³C-labeled litter to subtropical forest soils with three mass water contents (L, 21%; M, 33%; H, 45%). Carbon dioxide production was monitored, and the composition of soil microbial communities was determined by phospholipid fatty acid (PLFA). When no litter was added, SOC mineralization was greater in the M-treated soil. Litter addition promoted SOC mineralization, but this promotion was altered by soil moisture and litter type. Priming effects induced by P. massoniana leaf litter in the M-moistened soil were significantly (P < 0.05) higher than those in other treatments. Litter-derived C was approximately 55% incorporated into 18:1ω9c and 16:0 PLFAs, and this proportion was not significantly affected by soil moisture. Soil moisture affected the distribution of litter-¹³C in i15:0, i17:0, and cy19:0 individual PLFAs. The primed C evolution was significantly related to the ratio of Gram-positive to Gram-negative bacteria. These results suggest that changes in soil moisture could affect SOC mineralization in forest ecosystems.     

Spatial Heterogeneity and Soil Nitrogen Dynamics in a Burned Black Spruce Forest Stand: Distinct Controls at Different Scales

We evaluated spatial patterns of soil N and C mineralization, microbial community composition (phospholipid fatty acids), and local site characteristics (plant/forest floor cover, soil pH, soil %C and %N) in a 0.25-ha burned black spruce forest stand in interior Alaska. Results indicated that factors governing soil N and C mineralization varied at two different scales. In situ net N mineralization was autocorrelated with microbial community composition at relatively broad scales (∼ ∼8 m) and with local site characteristics (`site' axis 1 of non-metric scaling ordination) at relatively fine scales (2–4 m). At the scale of the individual core, soil moisture was the best predictor of in situ net N mineralization and laboratory C mineralization, explaining between 47 and 67% of the variation (p < 0.001). Ordination of microbial lipid data showed that bacteria were more common in severely burned microsites, whereas fungi were more common in low fire severity microsites. We conclude that C and N mineralization rates in this burned black spruce stand were related to different variables depending on the scale of analysis, suggesting the importance of considering multiple scales of variability among key drivers of C and N transformations.