Backwash intensity and frequency impact the microbial community structure and function in a fixed-bed biofilm reactor

Linkages among bioreactor operation and performance and microbial community structure were investigated for a fixed-bed biofilm system designed to remove perchlorate from drinking water. Perchlorate removal was monitored to evaluate reactor performance during and after the frequency and intensity of the backwash procedure were changed, while the microbial community structure was studied using clone libraries and quantitative PCR targeting the 16S rRNA gene. When backwash frequency was increased from once per month to once per day, perchlorate removal initially deteriorated and then recovered, and the relative abundance of perchlorate-reducing bacteria (PRB) initially increased and then decreased. This apparent discrepancy suggested that bacterial populations other than PRB played an indirect role in perchlorate removal, likely by consuming dissolved oxygen, a competing electron acceptor. When backwash intensity was increased, the reactor gradually lost its ability to remove perchlorate, and concurrently the relative abundance of PRB decreased. The results indicated that changes in reactor operation had a profound impact on reactor performance through altering the microbial community structure. Backwashing is an important yet poorly characterized procedure when operating fixed-bed biofilm reactors. Compared to backwash intensity, changes in backwash frequency exerted less disturbance on the microbial community in the current study. If this finding can be confirmed in future work, backwash frequency may serve as the primary parameter when optimizing backwash procedures.     

Identifying “vital attributes” for assessing disturbance–recovery potential of seafloor communities

Despite a long history of disturbance–recovery research, we still lack a generalizable understanding of the attributes that drive community recovery potential in seafloor ecosystems. Marine soft‐sediment ecosystems encompass a range of heterogeneity from simple low‐diversity habitats with limited biogenic structure, to species‐rich systems with complex biogenic habitat structure. These differences in biological heterogeneity are a product of natural conditions and disturbance regimes. To search for unifying attributes, we explore whether a set of simple traits can characterize community disturbance–recovery potential using seafloor patch‐disturbance experiments conducted in two different soft‐sediment landscapes. The two landscapes represent two ends of a spectrum of landscape biotic heterogeneity in order to consider multi‐scale disturbance–recovery processes. We consider traits at different levels of biological organization, from the biological traits of individual species, to the traits of species at the landscape scale associated with their occurrence across the landscape and their ability to be dominant. We show that in a biotically heterogeneous landscape (Kawau Bay, New Zealand), seafloor community recovery is stochastic, there is high species turnover, and the landscape‐scale traits are good predictors of recovery. In contrast, in a biotically homogeneous landscape (Baltic Sea), the options for recovery are constrained, the recovery pathway is thus more deterministic and the scale of recovery traits important for determining recovery switches to the individual species biological traits within the disturbed patch. Our results imply that these simple, yet sophisticated, traits can be effectively used to characterize community recovery potential and highlight the role of landscapes in providing resilience to patch‐scale disturbances.     

Textural fingerprints: a comprehensive descriptor for biofilm structure development

Automated tools to determine biofilm structure are necessary to interpret large time series of biofilm images. Image analysis based on the evaluation of Spatial Gray Level Dependence Matrices (SGLDM) enabled us to monitor biofilm structure development in response to external disturbances (i.e., periodic increases of wall shear stress) at a large scale (i.e., >1 mm). We applied our method to an experiment conducted in an annular reactor over a 10-week period. Six states of biofilm development were differentiated by their unique structure. Previous exposure to rapidly increased shear influenced the resulting biofilm structure after additional shear increases. In addition, on the scale of the biofilm images, the biofilm structure after a shear increase was spatially heterogeneous and resulted in spatially differentiated regrowth after detachment at different locations in the biofilm. SGLDM was developed further as an alternative to approaches based on image binarization as binarization leads to information loss for low-magnification and low-resolution images. During post-processing of image data, structural states of biofilm development were identified by K-means clustering and data display in Principal Component plots. Quantitatively selected representative images were used to illustrate the meaning of the clusters. Post-treatment of image data was essential for managing several thousands of raw biofilm images and therefore improved the usefulness of the image analysis.     

Disturbance and diversity at two spatial scales

The spatial scale of disturbance is a factor potentially influencing the relationship between disturbance and diversity. There has been discussion on whether disturbances that affect local communities and create a mosaic of patches in different successional stages have the same effect on diversity as regional disturbances that affect the whole landscape. In a microcosm experiment with metacommunities of aquatic protists, we compared the effect of local and regional disturbances on the disturbance–diversity relationship. Local disturbances destroyed entire local communities of the metacommunity and required reimmigration from neighboring communities, while regional disturbances affected the whole metacommunity but left part of each local community intact. Both disturbance types led to a negative relationship between disturbance intensity and Shannon diversity. With strong local disturbance, this decrease in diversity was due to species loss, while strong regional disturbance had no effect on species richness but reduced the evenness of the community. Growth rate appeared to be the most important trait for survival after strong local disturbance and dominance after strong regional disturbance. The pattern of the disturbance–diversity relationship was similar for both local and regional diversity. Although local disturbances at least temporally increased beta diversity by creating a mosaic of differently disturbed patches, this high dissimilarity did not result in regional diversity being increased relative to local diversity. The disturbance–diversity relationship was negative for both scales of diversity. The flat competitive hierarchy and absence of a trade-off between competition and colonization ability are a likely explanation for this pattern.     

Functional biotic homogenization of bird communities in disturbed landscapes

Aim  Worldwide, functional homogenization is now considered to be one of the most prominent forms of biotic impoverishment induced by current global changes. Yet this process has hardly been quantified on a large scale through simple indices, and the connection between landscape disturbance and functional homogenization has hardly been established. Here we test whether changes in land use and landscape fragmentation are associated with functional homogenization of bird communities at a national scale.
Location  France.
Methods  We estimated functional homogenization of a community as the average specialization of the species present in that community. We studied the spatial variation of this community specialization index (CSI) using 1028 replicates from the French Breeding Bird Survey along spatial gradients of landscape fragmentation and recent landscape disturbance, measured independently, and accounting for spatial autocorrelation.
Results  The CSI was very sensitive to both measures of environmental degradation: on average, 23% of the difference in the CSI values between two sample sites was attributed to the difference in fragmentation and the disturbance between sites. This negative correlation between CSI and sources of landscape degradation was consistent over various habitats and biogeographical zones.
Main conclusions  We demonstrate that the functional homogenization of bird communities is strongly positively correlated to landscape disturbance and fragmentation. We suggest that the CSI is particularly effective for measuring functional homogenization on both local and global scales for any sort of organism and with abundance or presence–absence data.     

Environmental stress alters native-nonnative relationships at the community scale

The invasion of natural habitats by nonnative species is affected by both native biodiversity and environmental conditions; however few tests of facilitation between native community members and nonnative species have been conducted along disturbance and stress gradients. There is strong evidence for an increase in facilitation between native plant species with increasing levels of natural environmental stress, however it is unknown whether these same positive interactions occur between nonnative invaders and native communities. I investigated the effects of natural stress on community interactions between native heathland species and nonnative species with two field studies conducted at the landscape and community scale. At the landscape scale of investigation, nonnative species richness was positively related to native species richness. At the community level, nonnative invaders experienced facilitation with natives in the most stressful zones, whereas they experienced competition with native plants in the less stressful zones of the heathlands. Due to the observational nature of the landscape scale data, it is unclear whether nonnative diversity levels are responding positively to extrinsic factors or to native biodiversity. The experimental component of this research suggests that native community members may ameliorate stressful environmental conditions and facilitate invasion into high stress areas. I present a conceptual model which is a modification of the Shea and Chesson diversity-invasibility model and includes both facilitation as well as competition between the native community and nonnative invaders at the community level, summing to an overall positive relationship at the landscape scale.     

Natural disturbances are spatially diverse but temporally synchronized across temperate forest landscapes in Europe

Natural disturbance regimes are changing substantially in forests around the globe. However, large‐scale disturbance change is modulated by a considerable spatiotemporal variation within biomes. This variation remains incompletely understood particularly in the temperate forests of Europe, for which consistent large‐scale disturbance information is lacking. Here, our aim was to quantify the spatiotemporal patterns of forest disturbances across temperate forest landscapes in Europe using remote sensing data and determine their underlying drivers. Specifically, we tested two hypotheses: (1) Topography determines the spatial patterns of disturbance, and (2) climatic extremes synchronize natural disturbances across the biome. We used novel Landsat‐based maps of forest disturbances 1986–2016 in combination with landscape analysis to compare spatial disturbance patterns across five unmanaged forest landscapes with varying topographic complexity. Furthermore, we analyzed annual estimates of disturbances for synchronies and tested the influence of climatic extremes on temporal disturbance patterns. Spatial variation in disturbance patterns was substantial across temperate forest landscapes. With increasing topographic complexity, natural disturbance patches were smaller, more complex in shape, more dispersed, and affected a smaller portion of the landscape. Temporal disturbance patterns, however, were strongly synchronized across all landscapes, with three distinct waves of high disturbance activity between 1986 and 2016. All three waves followed years of pronounced drought and high peak wind speeds. Natural disturbances in temperate forest landscapes of Europe are thus spatially diverse but temporally synchronized. We conclude that the ecological effect of natural disturbances (i.e., whether they are homogenizing a landscape or increasing its heterogeneity) is strongly determined by the topographic template. Furthermore, as the strong biome‐wide synchronization of disturbances was closely linked to climatic extremes, large‐scale disturbance episodes are likely in Europe's temperate forests under climate changes.     

Modeling biocide action against biofilms

A phenomenological model of biocide action against microbial biofilms was derived. Processes incorporated in the model include bulk flow in and out of a well-mixed reactor, transport of dissolved species into the biofilm, substrate consumption by bacterial metabolism, bacterial growth, advection of cell mass within the biofilm, cell detachment from the biofilm, cell death, and biocide concentration-dependent disinfection. Simulations were performed to analyze the general behavior of the model and to perform preliminary sensitivity analysis to identify key input parameters. The model captured several general features of antimicrobial agent action against biofilms that have been observed widely by experimenters and practitioners. These included (1) rapid disinfection followed by biofilm regrowth, (2) slower detachment than disinfection, and (3) reduced susceptibility of microorganisms in biofilms. The results support the plausibility of a mechanism of biofilm resistance in which the biocide is neutralized by reaction with biofilm constituents, leading to a reduction in the bulk biocide concentration and, more significantly, biocide concentration gradients within the biofilm. Sensitivity experiments and analyses identified which input parameters influence key response variables. Each of three response variables was sensitive to each of the five input parameters, but they were most sensitive to the initial biofilm thickness and next most sensitive to the biocide disinfection rate coefficient. Statistical regression modeling produced simple equations for approximating the response variables for situations within the range of conditions covered by the sensitivity experiment. The model should be useful as a tool for studying alternative biocide control strategies. For example, the simulations suggested that a good interval between pulses of biocide is the time to minimum thickness. (c) 1996 John Wiley & Sons, Inc.     

Influence of detachment,substrate loading and reactor scale on the formation of biofilms in airlift reactors

 For a stable and reliable operation of the biofilm airlift suspension reactor (BAS reactor) means to control biomass concentration, biofilm thickness and biofilm morphology are required. For this reason, the influence of applied detachment forces and surface substrate loading on the formation of heterotrophic biofilms in laboratory-scale BAS reactors was studied. Detachment forces were altered by variation of the initial bare carrier concentration or the superficial air velocity. In addition, the dynamics of biofilm formation during start-up of a full scale BAS reactor (300 m³) was monitored and compared with the laboratory-scale start-up (3 l). This study shows that the biofilm morphology and strength were influenced to a large extent by the surface substrate loading and applied detachment forces. A moderate surface substrate loading and a high detachment force yielded smooth and strong biofilms. The combination of a high surface substrate loading and low detachment forces did lead to rough biofilms, but did not lead to the expected high amount of biomass on the carrier, apparently because of the formation of weaker biofilms. The strength of the bio-films appeared to be related to the detachment forces applied during biofilm formation, in combination with the surface substrate loading. The biofilm morphology and biomass on carrier in the BAS reactor can be controlled using the carrier concentration, substrate loading rate and the superficial air velocity as parameters. The dynamics of biofilm formation during the start-up of a full-scale BAS reactor proved to be similar to heterotrophic biofilm formation in laboratory-scale reactors. This indicates that a model system on the laboratory scale can successfully be applied to predict dynamic phenomena in the full-scale reactor. Received: 31 March 1995/Received revision: 11 August 1995/Accepted: 22 August 1995     

Self-recovery of plant communities: examples, mechanisms, and approaches for description

Autogenic (self-recovering) plant communities that need no transitional stages of succession for their recovery after damage are widespread in different areas of the Globe. They are typical for tundras, but also common in boreal forests, alpine belts, deserts and Mediterranean biomes. Such communities usually dominate the landscape. The authors examine in details the communities of cottongrass (Eriophorum vaginatum) wet tussock tundras of North-Eastern Asia and Alaska, the dryad (Dryas punctata) lichen dry patchy tundras of the same region and Scots pine (Pinus sylvestris) lichen-feathermoss forests on fluvial-glacial deposits of Fennoscandia and Russian Plain. Although authogenic plant communities are serial, they exist side by side with proper edaphic variants of climaxes but don't appear to be replaced by the latter, being even more stable. Moreover, they even seem to "compete" for the space with the climax communities. Stabilization of autogenic communities is primarily induced by the impact of abiotic disturbance factors, like periodic fires, permafrost action or scree sliding, and afterwards maintained by several mechanisms of community structure organization. Such mechanisms of self-recovery act at the levels of both plant population and community as a whole. Specific adaptions of dominant species to typical kinds of disturbance (at a scale of either the community, or its local parts) illustrate the first level, whereas close co-adaptations between dominants and accompanying species refer to the second one. Such co-adaptations are present as complementarity of plant life strategies and growth forms within a community and/or as a shifting-mosaic steady state which leads to quick recovery of local disturbances. Among different possible approaches to simulation of community autogenesis those of combinatorial simulation seem to be especially perspective.     

Functional spatial scale of community composition change in response to windthrow disturbance in a deciduous temperate forest

Community dynamics in local habitats are affected by landscape characteristics such as the area and connectivity of surrounding habitats at a functional spatial scale where the community responds to landscape structure. However, the functional spatial scale at which community composition is affected by landscape structure has never been explored. We assessed the functional spatial scales of composition change in birds and in three types of arthropod communities (canopy, forest-floor and flying ones) with regard to landscape heterogeneity resulting from a large typhoon in a temperate forest of Japan. We examined the effects of tree-fall disturbance on the communities at various spatial scales, with special attention to compositional evenness. The spatial scale of the best-fitting model, which was selected from models fitted to the disturbance area at stepwise spatial scales, was interpreted as the community-specific functional spatial scale. The composition of all communities studied was all significantly dependent on gap area. The functional spatial scale was highest in birds (370 m in radius), intermediate in flying arthropods (90 m) and lowest in canopy and forest-floor arthropods (10 m). This result may reflect typical dispersal ability and the spatial range of resource use in the community. Compositional changes in each community were consistent with theory regarding traits and responses of component taxa, although the enhancement of evenness was observed only in the arthropod communities. These results imply that management and reserve selection based on functional spatial scales can be effective in the conservation of biodiversity and ecosystem services at the community level.     

Structure and function of wildfire and mountain pine beetle forest boundaries

Landscape boundaries, in particular those created by natural disturbances, are fundamental structures in landscape functioning. At the stand scale, forest boundaries show a wide range of characteristics varying from sharp to diffuse, wavy to straight, enclosed areas (patches) to open ones; all of these have different effects of forest vegetation. The objectives of this study were 1) to characterize the structure of forest boundaries (width, steepness, heterogeneity) and 2) to investigate forest vegetation functional response in the boundary zones created by wildfires and mountain pine beetle (MPB) outbreaks. We studied 11 mountain pine beetle and 7 wildfire boundary zones (4–7 y since disturbance) using field‐based two‐dimensional grids in southern central British Columbia (Canada). Boundary zone delineation was determined through spatially constrained clustering algorithms at three sampling unit resolutions (5×5 m, 10×10 m, and 20×20 m). Then, to characterize boundary width and shape, we developed two new boundary indexes (maximized and minimized boundary zones). The identified boundary widths ranged from 0 to 127 m with a mean width of 51 m at the 20×20 m resolution for both fire and MPB. Although the widths were comparable between disturbance types, fires generally had steeper boundaries (more pronounced) than MPB, largely due to higher peak tree mortality within the disturbances (89 vs 43%). Most of the forest vegetation response variables (understory vegetation diversity, plot‐level species richness, evenness, and multivariate community measures) in the boundary zones tended not to be intermediate in value between the intact forest and disturbance area, in spite of intermediate tree mortality. Tree mortality heterogeneity in the boundary zones was often highest in fires and was equal to the internal heterogeneity in MPB disturbances. Using historical natural disturbance patterns as a conservation strategy, this study proposes that forest management should create boundary complexity (width and shape), thereby creating landscape heterogeneity similar to landscapes influenced primarily by natural disturbances.     

Stability in a Denitrifying Fluidized Bed Reactor

This study evaluates changes in the microbial community structure and function of a pilot-scale denitrifying fluidized bed reactor during periods of constant operating conditions and periods of perturbation. The perturbations consisted of a shutdown period without feed, two disturbances in which biofilms were mechanically sheared from carrier particles, and a twofold step increase in feed nitrate concentration. In the absence of perturbations, nitrate removal was stable and consistently greater than 99%. The structure and dynamics of the microbial community were studied using cloning and sequencing techniques and terminal restriction fragment length polymorphism (T-RFLP) of the SSU rRNA gene. Under unperturbed operating conditions, stable function was accompanied by high constancy and low variability of community structure with the majority of terminal restriction fragments (T-RFs) appearing throughout operation at consistent relative abundances. Several of the consistently present T-RFs correlated with clone sequences closely related to Acidovorax (98% similarity), Dechloromonas (99% similarity), and Zoogloea (98% similarity), genera recently identified by molecular analyses of similar systems. Significant changes in community structure and function were not observed after the shutdown period. In contrast, following the increase in loading rate and the mechanical disturbances, new T-RFs appeared. After both mechanical disturbances, function and community structure recovered. However, function was much more resilient than community structure. The similarity of response to the mechanical disturbances despite differences in community structure and operating conditions suggests that flexible community structure and potentially the activity of minor members under nonperturbation conditions promotes system recovery.     

Scale-dependent effects of windthrow disturbance on forest arthropod communities

The effect of disturbance on local communities may operate within the context of the spatial landscape. We examined the scale-dependent effects of windthrow disturbance caused by a large typhoon on three arthropod communities in a temperate forest of Japan. Canopy arthropods were collected by beating foliage, forest-floor arthropods were collected by sweeping the vegetation, and flying arthropods were collected in Malaise traps. To assess the “functional spatial scale” at which arthropods responded to tree-fall disturbance, the gap rate was quantified at different spatial scales by sequentially enlarging the radius of a circular landscape sector in 10-m increments from 10 to 500 m. We then analyzed the responses of order richness and abundance to the gap rate for each arthropod community. The spatial scale of the significant best-fitting model, which was selected from the models fitted to the gap rate at stepwise spatial scales, was regarded as the arthropod-specific functional spatial scale. Arthropod order richness was not dependent on the gap rate. In contrast, arthropod order abundance depended significantly on the gap rate in many orders, but varied in the response direction and functional spatial scale. These order-specific, scale-dependent responses to tree-fall gaps could complicate interactions among organisms, leading to complex community organization. An understanding of the spatial processes that link the use of space by organisms with the spatial scale at which ecological processes are experienced is required to elucidate the responses of populations, communities, and biotic interactions to disturbances in a spatial landscape context.     

Recurrent disturbances and the degradation of hard coral communities in taiwan

Recurrent disturbances can have a critical effect on the structure and function of coral reef communities. In this study, long-term changes were examined in the hard coral community at Wanlitung, in southern Taiwan, between 1985 and 2010. In this 26 year interval, the reef has experienced repeated disturbances that include six typhoons and two coral-bleaching events. The frequency of disturbance has meant that species susceptible to disturbance, such as those in the genus Acropora and Montipora have almost disappeared from the reef. Indeed, almost all hard coral species have declined in abundance, with the result that total hard coral cover in 2010 (17.7%) was less than half what it was in 1985 (47.5%). In addition, macro-algal cover has increased from 11.3% in 2003 to 28.5% in 2010. The frequency of disturbance combined with possible chronic influence of a growing human population mean that a diverse reef assemblage is unlikely to persist on this reef into the future.     

The microbial community in a moving bed biotrickling filter operated to remove hydrogen sulfide from gas streams

The information available on the microbial communities responsible for pollutant degradation is increasingly accessible. Its use to optimize process design and operation is an important challenge in the field of effluent treatment research. Therefore, a prototype of a moving bed biotrickling filter (MBBTF) reactor was designed and, for the first time, operated at full-scale for the removal of sulfides desorbing from tannery industrial wastewater. The bacterial community operating in this innovative reactor was studied, and its composition and response to different operating conditions were characterized. A stable biomass, dominated by sulfur-oxidizing bacteria of the genus Acidithiobacillus was selected from inside the MBBTF reactor, and temperature, pH and bed rotation were shown to be the main factors driving the community structure. Moreover, data from different approaches indicated an uneven spatial distribution of biofilm inside the studied reactor, due to the combined effect of fluid dynamics and substrate gradients within the bed volume. Despite the high removal efficiency achieved by this innovative prototype (80% on average), the data suggested that the result could be improved by adopting solutions for a more stable and even biofilm distribution. It was shown that short frequent bed rotations, rather than long scattered rotations, ensured biomass stability. Furthermore, diversifying biofilm support media as a function of expected local pollutant concentrations should be considered. Data obtained from the bacterial community can therefore provide indications for possible further improvement of MBBTF reactor design and performance.     

空间直观景观模型LANDIS Ⅰ．运行机制

空间直观景观模型是指在异质景观中模拟景观尺度上生态过程的空间直观模型．LANDIS是一个用于模拟森林景观干扰、演替和管理的空间直观景观模型．通过在样地尺度上跟踪以10年为间隔的物种年龄级,半定量化地描述火和风倒,及使用位数组表示物种年龄结构,LANDIS能同时在物种、样地和景观尺度上模拟各种生态过程及其相互关系．详细论述了LANDIS模型对种子传播、火、风倒和砍伐等生态过程的模拟,并讨论了模型中存在的一些不足．     

Disturbance effects on plant community diversity: spatial scales and dominance hierarchies

It is proposed that evaluations of disturbance effects upon community diversity will be influenced by two factors currently overlooked in models addressing disturbance-diversity relationships: (1) the spatial scale of inquiry, and (2) the level of the species abundance (dominance) hierarchy at which the search for diversity is done. We analyzed how two disturbance types—cattle grazing and large flooding—affected community diversity at two spatial scales (stand and patch) and three levels of species dominance in a grassland of the Flooding Pampa, Argentina. The effect of disturbance interaction was also examined. Species diversity at the stand scale was reduced by either grazing or flooding. Both disturbances decreased community spatial heterogeneity. At the patch scale, diversity declined with flooding but was enhanced by grazing. Flooding increased diversity under grazing conditions. However, stand diversity was highest in the undisturbed grassland; pattern diversity was the salient feature in this condition. The combination of disturbances yielded the highest patch-scale diversity; grazing increased richness whilst flooding enhanced evenness. Comparisons among grassland conditions appeared scale-dependent. Moreover the extent of disturbance effects varied with the level of dominance hierarchy considered. We point out the relevance of site history and initial conditions, encompassing the possibility of disturbances interaction, to the patters produced by disturbance events. Effects perceived at different spatial scales, or in species positioned at separate dominance levels, may parallel meaningful changes in the relative importance of factors controlling species coexistence and community organization.     

Landscape partitioning among Triodia spp. (Poaceae) in the fire prone Kimberley,north‐west Australia

The processes which determine the structure of plant communities vary across spatial and temporal scales. Climatic factors are more likely to influence community structure at a regional scale with more transient environmental effects such as disturbance or demographic interactions having a greater influence at local scales. Understanding these differences is important for managing communities at a landscape scale. Triodia spp. grasslands are the most extensive plant community in Australia, covering 1.4 million km², and yet little is known about the processes which structure these communities. We collected data on six sympatric Triodia spp. at the regional, landscape and local scale across the 325 000 ha property, Mornington Wildlife Sanctuary, in the Kimberley region of northern Western Australia to investigate the processes which structure this community. Regionally we looked for correlations between species distributions and substrate or rainfall. At the landscape scale we collected data on substrate, drainage and vegetation type and at the local scale we determined the extent to which individuals form mono‐specific stands both along and across the contour gradient. Only one species, T. aeria, was found to be substrate specific and only T. epactia was restricted to the drier southern end of the property. The other species were not restricted by substrate or rainfall at the regional scale and were found to be habitat generalists at the landscape scale. All species grew in mono‐specific stands with little to no mixing at shared boundaries. However, this pattern broke down when crossing the contour gradient on hillsides. The results suggest rainfall may influence the distribution of some Triodia spp. at a regional scale with interspecific competition, due to differences in post‐fire regeneration niches, structuring the community at the local scale. At the landscape scale community structure appears to be influenced by feedback mechanisms involving differences in the post‐fire regeneration strategies of sympatric species and subsequent competition for establishment microsites.     

Pattern,process, and natural disturbance in vegetation

Natural disturbances have been traditionally defined in terms of major catastrophic events originating in the physical environment and, hence, have been regarded as exogenous agents of vegetation change. Problems with this view are: (1) there is a gradient from minor to major events rather than a uniquely definable set of major catastrophes for each kind of disturbance, and (2) some disturbances are initiated or promoted by the biotic component of the system. Floras are rich in disturbance-adapted species. Disturbances have probably exerted selective pressure in the evolution of species strategies. Heathland cyclic successions and gap-phase dynamics in forests have been viewed as endogenous patterns in vegetation. When death in older individividuals imposes a rhythm on community reproduction, dynamics may indeed be the result of endogenous factors. However, documented cases of senescence in perennial plants are few and many cyclic successions and cases of gap-phase dynamics are initiated by physical factors. Forest dynamics range from those that are the result of individual tree senescence and fall, through those that are the result of blowdown of small groups of healthy trees, to those that are the result of large windstorms which level hectares of forest. The effect of wind ranges from simple pruning of dead plant parts to widespread damage of living trees. Wind speed is probably inversely proportional to occurrence frequency. Disturbances vary continuously. There is a gradient from those community dynamics that are initiated by endogenous factors to those initiated by exogenous factors. Evolution has mediated between species and environment; disturbances are often caused by physical factors but the occurrence and outplay of disturbances may be a function of the state of the community as well. Natural disturbances in North American vegetation are: fire, windstorm, ice storm, ice push on shores, cryogenic soil movement, temperature fluctuation, precipitation variability, alluvial processes, coastal processes, dune movement, saltwater inundation, landslides, lava flows, karst processes, and biotic disturbances. Disturbances vary regionally and within one landscape as a function of topography and other site variables and are characterized by their frequency, predictability, and magnitude. The landscape level is important in assessing disturbance regime. Disturbances and cyclic successions belong to the same class of events—that of recurrent dynamics in vegetation structure—irrespective of cause. Dynamics may result from periodic, abrupt, and catastrophic environmental factors or they may result from an interaction of the changing susceptability of the community and some regular environmental factor. In any case, the dynamics result in heterogeneous landscapes; the species adapted to this heterogeneity are numerous, suggesting their long time importance. The importance of disturbance regime as part of the environmental context of vegetation means that allogenic and autogenic models of vegetation are difficult to apply. Species composition can be seen to be a function of disturbance regime, as well as other environmental variables. Competitive replacement in succession occurs, then, only as disturbances cease to operate and can be viewed as allogenic adjustment to a new disturbance-free environment. Competitive divergence, separation of role, and competition avoidance may, in fact, underlie successional patterns traditionally viewed as the competitive replacement of inferior species by superiorly adapted climax species. The importance of ongoing dynamics is also difficult to reconcile with the concept of climax, founded as it is on the idea of autogenesis within a stable physical environment. Climax composition is relative to disturbance regime. Climax is only arbitrarily distinguished from succession. Climax as an organizing paradigm in plant ecology has obscured the full temporal-spatial dimensions important in understanding the vegetated landscape and the evolution of species which contribute to the landscape patterns. Whittaker’s coenocline concept is accepted with modifications: (1) natural disturbance gradients and Whittaker’s complex gradient are intimately related, (2) temporal variation in the community should be viewed as an added axis of community pattern, and (3) ongoing dynamics have important effects on specificity of species to site relations and the predictability of vegetation patterns. Recent work has suggested an r-K continuum in species strategy. In general, colonizing ability is seen as a trade-off against specialization. Frequent disruption of the community and the creation of open sites seems to result in mixes of species that are fleeting in time and do not repeat in space. Species in such mixes are often tolerant of wide environmental extremes but are compressed into early successional time if disturbance ceases. The composition of such communities is not predictable from site characteristics. Even communities with low disturbance frequency lack complete environmental determinism, and historical events are important in understanding present composition. Communities vary in level of environmental determinism and species differ in niche breadth and degree of site specificity. Management implications of vegetation dynamics are discussed.