Arbuscular mycorrhizal fungi promote small-scale vegetation recovery in the forest understorey

Root-associating arbuscular mycorrhizal (AM) fungi foster vegetation recovery in degraded habitats. AM fungi increase nutrient availability for host plants; therefore, their importance is expected to be higher when nutrient availability is low. However, little is known about how small-scale variation in nutrient availability influences plant and AM fungal communities in a stable ecosystem. We conducted a 2-year field study in the understorey of a boreonemoral forest where we examined plant and AM fungal communities at microsites (15 cm diameter) with intact vegetation cover and at disturbed microsites where vegetation was cleared away and soil was sterilized to remove soil biota. We manipulated soil nutrient content (increased with fertilizer, unchanged, or decreased with sucrose addition) and fungal activity (natural or suppressed by fungicide addition) at these microsites. After two vegetation seasons, manipulations with nutrient content resulted in significant, although moderate, differences in the content of soil nutrients (e.g. in soil phosphorus). Suppression of fungal activity resulted in lower richness, abundance and phylogenetic diversity of AM fungal community, independently of microsite type and soil fertility level. Plant species richness and diversity decreased when fungal activity was suppressed at disturbed but not in intact microsites. The correlation between plant and AM fungal communities was not influenced by microsite type or soil fertility. We conclude that small-scale variation in soil fertility and habitat integrity does not influence the interactions between plants and AM fungi. The richness, but not composition, of AM fungal communities recovered fast after small-scale disturbance and supported the recovery of species-rich vegetation.

Soil heterogeneity increases plant diversity after 20 years of manipulation during grassland restoration

The "environmental heterogeneity hypothesis" predicts that variability in resources promotes species coexistence, but few experiments support this hypothesis in plant communities. A previous 15-yr test of this hypothesis in a prairie restoration experiment demonstrated a weak effect of manipulated soil resource heterogeneity on plant diversity. This response was attributed to a transient increase in richness following a post-restoration supplemental propagule addition, occasionally higher diversity under nutrient enrichment, and reduced cover of a dominant species in a subset of soil treatments. Here, we report community dynamics under continuous propagule addition in the same experiment, corresponding to 16-20 yr of restoration, in response to altered availability and heterogeneity of soil resources. We also quantified traits of newly added species to determine if heterogeneity increases the amount and variety of niches available for new species to exploit. The heterogeneous treatment contained a factorial combination of altered nutrient availability and soil depth; control plots had no manipulations. Total diversity and richness were higher in the heterogeneous treatment during this 5-yr study due to higher cover, diversity, and richness of previously established forbs, particularly in the N-enriched subplots. All new species added to the experiment exhibited unique trait spaces, but there was no evidence that heterogeneous plots contained a greater variety of new species representing a wider range of trait spaces relative to the control treatment. The richness and cover of new species was higher in N-enriched soil, but the magnitude of this response was small. Communities assembling under long-term N addition were dominated by different species among subplots receiving added N, leading to greater dispersion of communities among the heterogeneous relative to control plots. Contrary to the deterministic mechanism by which heterogeneity was expected to increase diversity (greater variability in resources for new species to exploit), higher diversity in the heterogeneous plots resulted from destabilization of formerly grass-dominated communities in N-enriched subplots. While we do not advocate increasing available soil N at large scales, we conclude that the positive effect of environmental heterogeneity on diversity can take decades to materialize and depend on development of stochastic processes in communities with strong establishment limitation.

Seed and Seedling Detection Using Unmanned Aerial Vehicles and Automated Image Classification in the Monitoring of Ecological Recovery

Monitoring is a crucial component of ecological recovery projects, yet it can be challenging to achieve at scale and during the formative stages of plant establishment. The monitoring of seeds and seedlings, which represent extremely vulnerable stages in the plant life cycle, is particularly challenging due to their diminutive size and lack of distinctive morphological characteristics. Counting and classifying seedlings to species level can be time-consuming and extremely difficult, and there is a need for technological approaches offering restoration practitioners with fine-resolution, rapid and scalable plant-based monitoring solutions. Unmanned aerial vehicles (UAVs) offer a novel approach to seed and seedling monitoring, as the combination of high-resolution sensors and low flight altitudes allow for the detection and monitoring of small objects, even in challenging terrain and in remote areas. This study utilized low-altitude UAV imagery and an automated object-based image analysis software to detect and count target seeds and seedlings from a matrix of non-target grasses across a variety of substrates reflective of local restoration substrates. Automated classification of target seeds and target seedlings was achieved at accuracies exceeding 90% and 80%, respectively, although the classification accuracy decreased with increasing flight altitude (i.e., decreasing image resolution) and increasing background surface complexity (increasing percentage cover of non-target grasses and substrate surface texture). Results represent the first empirical evidence that small objects such as seeds and seedlings can be classified from complex ecological backgrounds using automated processes from UAV-imagery with high levels of accuracy. We suggest that this novel application of UAV use in ecological monitoring offers restoration practitioners an excellent tool for rapid, reliable and non-destructive early restoration trajectory assessment.

Abiotic and Landscape Factors Constrain Restoration Outcomes Across Spatial Scales of a Widespread Invasive Plant

The natural recolonization of native plant communities following invasive species management is notoriously challenging to predict, since outcomes can be contingent on a variety of factors including management decisions, abiotic factors, and landscape setting. The spatial scale at which the treatment is applied can also impact management outcomes, potentially influencing plant assembly processes and treatment success. Understanding the relative importance of each of these factors for plant community assembly can help managers prioritize patches where specific treatments are likely to be most successful. Here, using effects size analyses, we evaluate plant community responses following four invasive Phragmites australis management treatments (1: fall glyphosate herbicide spray, 2: summer glyphosate herbicide spray, 3: summer imazapyr herbicide spray, 4: untreated control) applied at two patch scales (12,000 m2 and 1,000 m2) and monitored for 5 years. Using variation partitioning, we then evaluated the independent and shared influence of patch scale, treatment type, abiotic factors, and landscape factors on plant community outcomes following herbicide treatments. We found that Phragmites reinvaded more quickly in large patches, particularly following summer herbicide treatments, while native plant cover and richness increased at a greater magnitude in small patches than large. Patch scale, in combination with abiotic and landscape factors, was the most important driver for most plant responses. Compared with the small plots, large patches commonly had deeper and more prolonged flooding, and were in areas with greater hydrologic disturbance in the landscape, factors associated with reduced native plant recruitment and greater Phragmites cover. Small patches were associated with less flooding and landscape disturbance, and more native plants in the surrounding landscape than large patches, factors which promoted higher native plant conservation values and greater native plant cover and richness. Herbicide type and timing accounted for very little of the variation in native plant recovery, emphasizing the greater importance of patch selection for better management outcomes. To maximize the success of treatment programs, practitioners should first manage Phragmites patches adjacent to native plant species and in areas with minimal hydrologic disturbance.

Abiotic factors drives floristic variations of fern's metacommunity in an Atlantic Forest remnant

We analyzed floristic variations in fern's metacommunity at the local scale and their relationship with abiotic factors in an Atlantic Forest remnant of northeastern Brazil. Floristic and environmental variations were accessed on ten plots of 10 × 20 m. We performed cluster analyses, based on Bray-Curtis dissimilarity index to establish the floristic relationship. The influence of abiotic factors: luminosity, temperature, relative air humidity and relative soil moisture was evaluated from a redundancy analysis. We found 24 species belonging to 20 genera and 12 families. The fern's flora showed high floristic heterogeneity (>75% for most of the plot's associations). The fern's metacommunity was structured along an abiotic gradient modulated by temperature, luminosity, and relative soil moisture.

Predicting species establishment using absent species and functional neighborhoods

Species establishment within a community depends on their interactions with the local environment and resident community. Such environmental and biotic filtering is frequently inferred from functional trait and phylogenetic patterns within communities; these patterns may also predict which additional species can establish. However, differentiating between environmental and biotic filtering can be challenging, which may complicate establishment predictions. Creating a habitat‐specific species pool by identifying which absent species within the region can establish in the focal habitat allows us to isolate biotic filtering by modeling dissimilarity between the observed and biotically excluded species able to pass environmental filters. Similarly, modeling the dissimilarity between the habitat‐specific species pool and the environmentally excluded species within the region can isolate local environmental filters. Combined, these models identify potentially successful phenotypes and why certain phenotypes were unsuccessful. Here, we present a framework that uses the functional dissimilarity among these groups in logistic models to predict establishment of additional species. This approach can use multivariate trait distances and phylogenetic information, but is most powerful when using individual traits and their interactions. It also requires an appropriate distance‐based dissimilarity measure, yet the two most commonly used indices, nearest neighbor (one species) and mean pairwise (all species) distances, may inaccurately predict establishment. By iteratively increasing the number of species used to measure dissimilarity, a functional neighborhood can be chosen that maximizes the detection of underlying trait patterns. We tested this framework using two seed addition experiments in calcareous grasslands. Although the functional neighborhood size that best fits the community's trait structure depended on the type of filtering considered, selecting these functional neighborhood sizes allowed our framework to predict up to 50% of the variation in actual establishment from seed. These results indicate that the proposed framework may be a powerful tool for studying and predicting species establishment.

Dispersal and neutral sampling mediate contingent effects of disturbance on plant beta-diversity: a meta-analysis

A major challenge in ecology, conservation and global-change biology is to understand why biodiversity responds differently to similar environmental changes. Contingent biodiversity responses may depend on how disturbance and dispersal interact to alter variation in community composition (β-diversity) and assembly mechanisms. However, quantitative syntheses of these patterns and processes across studies are lacking. Using null-models and meta-analyses of 22 factorial experiments in herbaceous plant communities across Europe and North America, we show that disturbance diversifies communities when dispersal is limited, but homogenises communities when combined with increased immigration from the species pool. In contrast to the hypothesis that disturbance and dispersal mediate the strength of niche assembly, both processes altered β-diversity through neutral-sampling effects on numbers of individuals and species in communities. Our synthesis suggests that stochastic effects of disturbance and dispersal on community assembly play an important, but underappreciated, role in mediating biotic homogenisation and biodiversity responses to environmental change.

Local epiphyte establishment and future metapopulation dynamics in landscapes with different spatiotemporal properties

Understanding the relative importance of different ecological processes on the metapopulation dynamics of species is the basis for accurately forecasting metapopulation size in fragmented landscapes. Successful local colonization depends on both species dispersal range and how local habitat conditions affect establishment success. Moreover, there is limited understanding of the effects of different spatiotemporal landscape properties on future metapopulation size. We investigate which factors drive the future metapopulation size of the epiphytic model lichen species Lobaria pulmonaria in a managed forest landscape. First, we test the importance of dispersal and local conditions on the colonization-extinction dynamics of the species using Bayesian state-space modelling of a large-scale data set collected over a 10-yr period. Second, we test the importance of dispersal and establishment limitation in explaining establishment probability and subsequent local population growth, based on a 10-yr propagule sowing experiment. Third, we test how future metapopulation size is affected by different metapopulation and spatiotemporal landscape dynamics, using simulations with the metapopulation models fitted to the empirical data. The colonization probability increased with tree inclination and connectivity, with a mean dispersal distance of 97 m (95% credible intervals, 5-530 m). Local extinctions were mainly deterministic set by tree mortality, but also by tree cutting by forestry. No experimental establishments took place on clearcuts, and in closed forest the establishment probability was higher on trees growing on moist than on dry-mesic soils. The subsequent local population growth rate increased with increasing bark roughness. The simulations showed that the restricted dispersal range estimated (compared to non-restricted dispersal range), and short tree rotation length (65 yr instead of 120) had approximately the same negative effects on future metapopulation size, while regeneration of trees creating a random tree pattern instead of an aggregated one had only some negative effect. However, using the colonization rate obtained with the experimentally added diaspores led to a considerable increase in metapopulation size, making the dispersal limitation of the species clear. The future metapopulation size is thus set by the number of host trees located in shady conditions, not isolated from occupied trees, and by the rotation length of these host trees.

Scaling disturbance instead of richness to better understand anthropogenic impacts on biodiversity

A primary impediment to understanding how species diversity and anthropogenic disturbance are related is that both diversity and disturbance can depend on the scales at which they are sampled. While the scale dependence of diversity estimation has received substantial attention, the scale dependence of disturbance estimation has been essentially overlooked. Here, we break from conventional examination of the diversity-disturbance relationship by holding the area over which species richness is estimated constant and instead manipulating the area over which human disturbance is measured. In the boreal forest ecoregion of Alberta, Canada, we test the dependence of species richness on disturbance scale, the scale-dependence of the intermediate disturbance hypothesis, and the consistency of these patterns in native versus exotic species and among human disturbance types. We related field observed species richness in 1 ha surveys of 372 boreal vascular plant communities to remotely sensed measures of human disturbance extent at two survey scales: local (1 ha) and landscape (18 km2). Supporting the intermediate disturbance hypothesis, species richness-disturbance relationships were quadratic at both local and landscape scales of disturbance measurement. This suggests the shape of richness-disturbance relationships is independent of the scale at which disturbance is assessed, despite that local diversity is influenced by disturbance at different scales by different mechanisms, such as direct removal of individuals (local) or indirect alteration of propagule supply (landscape). By contrast, predictions of species richness did depend on scale of disturbance measurement: with high local disturbance richness was double that under high landscape disturbance.

Effect of rotational shepherding on demographic and genetic connectivity of calcareous grassland plants

Response to habitat fragmentation may not be generalized among species, in particular for plant communities with a variety of dispersal traits. Calcareous grasslands are one of the most species-rich habitats in Central Europe, but abandonment of traditional management has caused a dramatic decline of calcareous grassland species. In the Southern Franconian Alb in Germany, reintroduction of rotational shepherding in previously abandoned grasslands has restored species diversity, and it has been suggested that sheep support seed dispersal among grasslands. We tested the effect of rotational shepherding on demographic and genetic connectivity of calcareous grassland specialist plants and whether the response of plant populations to shepherding was limited to species dispersed by animals (zoochory). Specifically, we tested competing dispersal models and source and focal patch properties to explain landscape connectivity with patch-occupancy data of 31 species. We fitted the same connectivity models to patch occupancy and nuclear microsatellite data for the herb Dianthus carthusianorum (Carthusian pink). For 27 species, patch connectivity was explained by dispersal by rotational shepherding regardless of adaptations to zoochory, whereas population size (16% species) and patch area (0% species) of source patches were not important predictors of patch occupancy in most species. [Correction made after online publication, February 25, 2014: Population size and patch area percentages were mistakenly inverted, and have now been fixed.] Microsite diversity of focal patches significantly increased the model variance explained by patch occupancy in 90% of the species. For D. carthusianorum, patch connectivity through rotational shepherding explained both patch occupancy and population genetic diversity. Our results suggest shepherding provides dispersal for multiple plant species regardless of their dispersal adaptations and thus offers a useful approach to restore plant diversity in fragmented calcareous grasslands.

Staged invasions across disparate grasslands: effects of seed provenance, consumers and disturbance on productivity and species richness

Exotic plant invasions are thought to alter productivity and species richness, yet these patterns are typically correlative. Few studies have experimentally invaded sites and asked how addition of novel species influences ecosystem function and community structure and examined the role of competitors and/or consumers in mediating these patterns. We invaded disturbed and undisturbed subplots in and out of rodent exclosures with seeds of native or exotic species in grasslands in Montana, California and Germany. Seed addition enhanced aboveground biomass and species richness compared with no-seeds-added controls, with exotics having disproportionate effects on productivity compared with natives. Disturbance enhanced the effects of seed addition on productivity and species richness, whereas rodents reduced productivity, but only in Germany and California. Our results demonstrate that experimental introduction of novel species can alter ecosystem function and community structure, but that local filters such as competition and herbivory influence the magnitude of these impacts.

Effects of agricultural tillage and sediment accumulation on emergent plant communities in playa wetlands of the U.S. High Plains

Identifying community assembly filters is a primary ecological aim. The High Plains, a 30 million ha short-grass eco-region, is intensely cultivated. Cultivation disturbance, including plowing and eroded soil deposition down-slope of plowing, influences plant composition in depressional wetlands, such as playas, within croplands. We evaluated influences of wetland cultivation and sediment deposition on plant composition in playas embedded within croplands (46 plowed and 32 unplowed) and native grasslands (79) across 6 High Plains' states. Sediment accumulation ranged from 7 to 78 cm in cropland and 1 to 35 cm in grassland playas. Deeper sediments and plowing each decreased wetland plant richness, 28% and 70% respectively in cropland wetlands. Sediment depth reduced richness 37% in small grasslands playas while it increased richness 22% in larger ones, suggesting moderate disturbance increased richness when there were nearby propagule sources. Sediment depth was unrelated to species richness in plowed wetlands, probably because plowing was a strong disturbance. Plowing removed perennial plants from vegetation communities. Sediment accumulation also influenced species composition in cropland playas, e.g., probability of Eleocharis atropurpurea increased with sediment depth, while probability of Panicum capillare decreased. In grassland playas, observed lighter sediment depths did not influence species composition after accounting for wetland area. Sediment accumulation and plowing shift wetland plant communities toward annual species and decrease habitat connectivity for wetland-dependent organisms in cropland playas over 39,000 and 23,400 ha respectively. Conservation practices lessening sediment accumulation include short-grass buffer strips surrounding wetlands. Further, wetland tillage, allowed under federal agricultural conservation programs, should be eliminated.

Positive diversity-invasibility relationship in species-rich semi-natural grassland at the neighbourhood scale

BACKGROUND AND AIMS

Attempts to answer the old question of whether high diversity causes high invasion resistance have resulted in an invasion paradox: while large-scale studies often find a positive relationship between diversity and invasibility, small-scale experimental studies often find a negative relationship. Many of the small-scale studies are conducted in artificial communities of even-aged plants. Species in natural communities, however, do not represent one simultaneous cohort and occur at various levels of spatial aggregation at different scales. This study used natural patterns of diversity to assess the relationship between diversity and invasibility within a uniformly managed, semi-natural community.

METHODS

In species-rich grassland, one seed of each of ten species was added to each of 50 contiguous 16 cm(2) quadrats within seven plots (8 × 100 cm). The emergence of these species was recorded in seven control plots, and establishment success was measured in relation to the species diversity of the resident vegetation at two spatial scales, quadrat (64 cm(2)) within plots (800 cm(2)) and between plots within the site (approx. 400 m(2)) over 46 months.

KEY RESULTS

Invader success was positively related to resident species diversity and richness over a range of 28-37 species per plot. This relationship emerged 7 months after seed addition and remained over time despite continuous mortality of invaders.

CONCLUSIONS

Biotic resistance to plant invasion may play only a sub-ordinate role in species-rich, semi-natural grassland. As possible alternative explanations for the positive diversity-invasibility relationship are not clear, it is recommended that future studies elaborate fine-scale environmental heterogeneity in resource supplies or potential resource flows from resident species to seedlings by means of soil biological networks established by arbuscular mycorrhizal fungi.

Ecosystem development in roadside grasslands: biotic control, plant-soil interactions, and dispersal limitations

Roadside grasslands undergoing secondary succession are abundant, and represent ecologically meaningful examples of novel, human-created ecosystems. Interactions between plant and soil communities (hereafter plant-soil interactions) are of major importance in understanding the role of biotic control in ecosystem functioning, but little is known about these links in the context of ecosystem restoration and succession. The assessment of the key biotic communities and interactions driving ecosystem development will help practitioners to better allocate the limited resources devoted to roadside grassland restoration. We surveyed roadside grasslands from three successional stages (0-2, 7-9, and >20 years) in two Mediterranean regions of Spain. Structural equation modeling was used to evaluate how interactions between plants, biological soil crusts (BSCs), and soil microbial functional diversity (soil microorganisms) affect indicators of ecosystem development and restoration: plant similarity to the reference ecosystem, erosion control, and soil C storage and N accumulation. Changes in plant community composition along the successional gradient exerted the strongest influence on these indicators. High BSC cover was associated with high soil stability, and high soil microbial functional diversity from late-successional stages was associated with high soil fertility. Contrary to our expectations, the indirect effects of plants, mediated by either BSCs or soil microorganisms, were very weak in both regions, suggesting a minor role for plant-soil interactions upon ecosystem development indicators over long periods. Our results suggest that natural vegetation dynamics effectively improved ecosystem development within a time frame of 20 years in the grasslands evaluated. They also indicate that this time could be shortened if management actions focus on: (1) maintaining well-conserved natural areas close to roadsides to enhance plant compositional changes towards late-successional stages, (2) increasing BSC cover in areas under strong erosion risk, to avoid soil loss, and (3) enhancing soil microbial functional diversity in resource-limited areas, to enhance soil C and N accumulation.

Seed arrival and ecological filters interact to assemble high-diversity plant communities

Two prominent mechanisms proposed to structure biodiversity are niche-based ecological filtering and chance arrival of propagules from the species pool. Seed arrival is hypothesized to play a particularly strong role in high-diversity plant communities with large potential species pools and many rare species, but few studies have explored how seed arrival and local ecological filters interactively assemble species-rich communities in space and time. We experimentally manipulated seed arrival and multiple ecological filters in high-diversity, herbaceous-dominated groundcover communities in longleaf pine savannas, which contain the highest small-scale species richness in North America (up to > 40 species/m2). We tested three hypotheses: (1) local communities constitute relatively open-membership assemblages, in which increased seed arrival from the species pool strongly increases species richness; (2) ecological filters imposed by local fire intensity and soil moisture influence recruitment and richness of immigrating species; and (3) ecological filters increase similarity in the composition of immigrating species. In a two-year factorial field experiment, we manipulated local fire intensity by increasing pre-fire fuel loads, soil moisture using rain shelters and irrigation, and seed arrival by adding seeds from the local species pool. Seed arrival increased species richness regardless of fire intensity and soil moisture but interacted with both ecological filters to influence community assembly. High-intensity fire decreased richness of resident species, suggesting an important abiotic filter. In contrast, high-intensity fire increased recruitment and richness of immigrating species, presumably by decreasing effects of other ecological filters (competition and resource limitation) in postfire environments. Drought decreased recruitment and richness of immigrating species, whereas wet soil conditions increased recruitment but decreased or had little effect on richness. Moreover, some ecological filters (wet soil conditions and, to a lesser extent, high-intensity fire) increased similarity in the composition of immigrating species, illustrating conditions that influence deterministic community assembly in species-rich communities. Our experiment provides insights into how dispersal-assembly mechanisms may interact with niche-assembly mechanisms in space (spatial variation in disturbance) and time (temporal variation in resource availability) to structure high-diversity communities and can help guide conservation of threatened longleaf pine ecosystems in the face of habitat fragmentation and environmental change.

Experimental evaluation of seed limitation in alpine snowbed plants

BACKGROUND

The distribution and abundance of plants is controlled by the availability of seeds and of sites suitable for establishment. The relative importance of these two constraints is still contentious and possibly varies among species and ecosystems. In alpine landscapes, the role of seed limitation has traditionally been neglected, and the role of abiotic gradients emphasized.

METHODOLOGY/PRINCIPAL FINDINGS

We evaluated the importance of seed limitation for the incidence of four alpine snowbed species (Achillea atrata L., Achillea clusiana Tausch, Arabis caerulea L., Gnaphalium hoppeanum W. D. J. Koch) in local plant communities by comparing seedling emergence, seedling, juvenile and adult survival, juvenile and adult growth, flowering frequency as well as population growth rates λ of experimental plants transplanted into snowbed patches which were either occupied or unoccupied by the focal species. In addition, we accounted for possible effects of competition or facilitation on these rates by including a measure of neighbourhood biomass into the analysis. We found that only A. caerulea had significantly lower seedling and adult survival as well as a lower population growth rate in unoccupied sites whereas the vital rates of the other three species did not differ among occupied and unoccupied sites. By contrast, all species were sensitive to competitive effects of the surrounding vegetation in terms of at least one of the studied rates.

CONCLUSIONS/SIGNIFICANCE

We conclude that seed and site limitation jointly determine the species composition of these snowbed plant communities and that constraining site factors include both abiotic conditions and biotic interactions. The traditional focus on abiotic gradients for explaining alpine plant distribution hence appears lopsided. The influence of seed limitation on the current distribution of these plants casts doubt on their ability to readily track shifting habitats under climate change unless seed production is considerably enhanced under a warmer climate.

Spatial niches and coexistence: testing theory with tarweeds

Competitive coexistence in a spatially heterogeneous environment is traditionally attributed to niche differences, but several recent theories have proposed important additional roles for propagule limitation and chance (e.g., neutral theory, stochastic niche theory, spatial storage effect). We tested whether propagule supply and timing of disturbance affected the coexistence of three ecologically similar plants that replace one another with partial overlap along a local soil gradient. We asked what prevents the species that dominates the most common habitat (Holocarpha virgata, open hillsides) from invading the habitats where the other two species are dominant (Calycadenia pauciflora, rocky hilltops; Hemizonia congesta, clay-rich bottomlands). We added abundant Holocarpha seeds into Calycadenia and Hemizonia habitats that were experimentally disturbed at different times of year. Initial Holocarpha seedling densities in Calycadenia and Hemizonia habitats equaled or exceeded those in unmanipulated Holocarpha habitat, but Holocarpha survival, adult size, and fecundity were much lower outside its own habitat. Holocarpha persisted in Calycadenia and Hemizonia habitats for three years, and springtime disturbance promoted this expansion. However, outside its own habitat Holocarpha showed below-replacement fitness and little competitive effect on the other two species. Our results were most consistent with a deterministic view of spatial niches. Nonetheless, chance events may often cause natural communities to include some transient populations at any given time, leading them to appear "unsaturated" with species.