Spatial patterns and associations of four species in an old-growth temperate forest

The spatial pattern of a tree species is an important characteristic of plant communities, providing critical information to explain species coexistence. The spatial distribution and association of four different successional species were analyzed among different life-history stages in an old-temperate forest. Significant aggregation patterns were found, and the degree of aggregation decreased with the scales and life-history stages. Significant interspecific spatial associations were detected. In comparing the relationships among the different life-history stages, positive associations were found at small scales in all of the juvenile species pairs. In the adult stage, negative associations were detected in coniferous vs. deciduous species pairs, while the deciduous species pairs, which have identical resource requirements, showed a positive association in this study. The coniferous species pairs showed a positive association at small scales. We infer that seed dispersal, competitive ability, or the requirement for specific topographic and light environments may contribute to the coexistence of these species.     

Spatial Distribution and Interspecific Associations of Tree Species in a Tropical Seasonal Rain Forest of China

Studying the spatial pattern and interspecific associations of plant species may provide valuable insights into processes and mechanisms that maintain species coexistence. Point pattern analysis was used to analyze the spatial distribution patterns of twenty dominant tree species, their interspecific spatial associations and changes across life stages in a 20-ha permanent plot of seasonal tropical rainforest in Xishuangbanna, China, to test mechanisms maintaining species coexistence. Torus-translation tests were used to quantify positive or negative associations of the species to topographic habitats. The results showed: (1) fourteen of the twenty tree species were negatively (or positively) associated with one or two of the topographic variables, which evidences that the niche contributes to the spatial pattern of these species. (2) Most saplings of the study species showed a significantly clumped distribution at small scales (0–10 m) which was lost at larger scales (10–30 m). (3) The degree of spatial clumping deceases from saplings, to poles, to adults indicates that density-dependent mortality of the offspring is ubiquitous in species. (4) It is notable that a high number of positive small-scale interactions were found among the twenty species. For saplings, 42.6% of all combinations of species pairs showed positive associations at neighborhood scales up to five meters, but only 38.4% were negative. For poles and adults, positive associations at these distances still made up 45.5% and 29.5%, respectively. In conclusion, there is considerable evidence for the presence of positive interactions among the tree species, which suggests that species herd protection may occur in our plot. In addition, niche assembly and limited dispersal (likely) contribute to the spatial patterns of tree species in the tropical seasonal rain forest in Xishuangbanna, China.     

Local‐scale spatial dynamics of ants in a temperate agroecosystem

At the local scale, spatial aggregations in ant distribution are often thought to be driven by competitive interactions among dominant ant species, although niche preferences and habitat heterogeneity might also lead to patchiness. Nevertheless, competitive interactions might be particularly important in agroecosystems that are structurally more homogeneous than natural habitats. The spatial patterns of ants in two Australian vineyards were investigated by intensive pitfall trapping to examine if non‐random patterns occur and whether these might be the result of competitive species interactions as well as the influence of woody vegetation adjacent to the vineyards. Null model analyses suggested competitive species interactions within ant assemblages that might have been driven by numerically dominant species, even though both positive and negative associations between these were found. Consistent spatial aggregations indicated significant spatial overlap in distributions of some species. Such overlap suggests that potential coexistence might be attributed to temporal partitioning or differences in foraging strategies. The presence of woody vegetation had a marked influence on ant assemblage structure and competitive interactions, and might facilitate coexistence by increasing resource heterogeneity. The implications of these findings for sampling strategies and ecological processes within vineyards are discussed.     

Determinant factors influencing the spatial distributions of subtropical lianas are correlated with components of functional trait spectra

Lianas are important vegetation components that control structure and function, especially in tropical and subtropical forests. To explore the spatial assembly mechanisms of a subtropical liana community, we tested the following hypotheses: spatial distributions of subtropical lianas are determined by forest structures and topographic features, which are surrogates for host/light availability and edaphic/water conditions, respectively, and these effects are mediated through species functional traits. We examined the spatial distribution of lianas in two plots (areas 9 and 16 ha) representing landscapes in an intact forest and a secondary forest, and analyzed spatial distribution pattern at the species level using a simple, spatially explicit model. We also examined the correlations between determinant factors for species distribution and species functional traits, including climbing habits, leaf traits and wood density. The spatial distribution of lianas was controlled mainly by topographic gradient. Most species had preferences for concave topographies, i.e., valley habitats. Any covariates related to the host (or to light) had little influence on the distribution of most liana species. Distributional responses to topography were different among species, and associated significantly with leaf nitrogen content and climbing habit, but not with wood density. The correlation between variation in habitat preferences and leaf economic spectrum suggests that an environmental filter for physiological response to topography is the important mechanism shaping the spatial patterns of this subtropical liana community.     

How do similarities in spatial distributions and interspecific associations affect the coexistence of Quercus species in the Baotianman National Nature Reserve,Henan, China

Congeneric species often have similar ecological characteristics and use similar resources. These similarities may make it easier for them to co‐occur in a similar habitat but may also lead to strong competitions that limit their coexistence. Hence, how do similarities in congeneric species affect their coexistence exactly? This study mainly used spatial point pattern analysis in two 1 hm² plots in the Baotianman National Nature Reserve, Henan, China, to compare the similarities in spatial distributions and interspecific associations of Quercus species. Results revealed that Quercus species were all aggregated under the complete spatial randomness null model, and aggregations were weaker under the heterogeneous Poisson process null model in each plot. The interspecific associations of Quercus species to non‐Quercus species were very similar in Plot 1. However, they can be either positive or negative in different plots between the co‐occurring Quercus species. The spatial distributions of congeneric species, interspecific associations with non‐Quercus species, neighborhood richness around species, and species diversity were all different between the two plots. We found that congeneric species did have some similarities, and the closely related congeneric species can positive or negative associate with each other in different plots. The co‐occurring congeneric species may have different survival strategies in different habitats. On the one hand, competition among congenerics may lead to differentiation in resource utilization. On the other hand, their similar interspecific associations can strengthen their competitive ability and promote local exclusion to noncongeneric species to obtain more living space. Our results provide new knowledge for us to better understand the coexistence mechanisms of species.     

Coexistence of two congeneric tree species of Lauraceae in a secondary warm‐temperate forest on Miyajima Island,south‐western Japan

We analyzed the spatial distributions of two congeneric tree species, Neolistea aciculata and Neolistea sericea (Lauraceae), in a warm‐temperate forest on Miyajima Island, south‐western Japan. Both species were mainly found in valley sites on the island. Hence, these species shared the same topographic habitat niche. However, we found a clear difference between the spatial distributions of the two species in relation to the light environment. Neolistea aciculata was predominantly found in stands with low light, such as beneath the canopy of dense evergreen broadleaved forest. In contrast, N. sericea was predominantly associated with ample light, such as in secondary Pinus densiflora forest. In stands with moderate light conditions, both species were found. This habitat niche segregation in relation to light conditions presumably allows the coexistence of these two species in the predominantly successional forest on Miyajima Island.     

Spatial patterns and interspecific associations of dominant tree species in two old-growth karst forests,SW China

Spatial patterns and interspecific associations of plant species in forests are important for revealing how species interact with each other and with the environment, and hence have important implications for optimal forest management and restoration in degraded forest ecosystems. In this paper, the O-ring statistics were used to characterize the spatial patterns and interspecific associations of eight dominant tree species in two 1-ha old-growth karst forest plots in Maolan National Natural Reserve, southwestern China. We found that most of the eight dominant tree species in two forests were continuously regenerating populations. Six species (Platycarya longipes, Acer wangchii, Clausena dunniana, Castanopsis carlesii var. spinulosa, Distylium myricoides, and Rhododendron latoucheae) exhibited significant aggregations at the majority of scales while others (Celtis biondii and Cyclobalanopsis myrsinaefolia) showed a random distribution pattern at most scales. Negative association was a dominant pattern for most species pairs in the two plots, while positive associations were found at most scales for only two species pairs (Platycarya–Clausena and Castanopsis–Rhododendron). Results also indicated that the two main factors of habitat complexity and heterogeneity—the elevation and rock-bareness rate—play important roles in determining spatial distribution patterns and interspecific associations of tree species in karst forests of Maolan. Thus, the observed spatial patterns among the eight tree species are influenced by habitat heterogeneity in the context of karst topographical variations. The partitioning of habitat niches contributes to the promoting species coexistence in species-rich karst forests. The differences of species features in spatial patterns and associations should be paid more attention when planning forest management and developing restoration strategies.     

Habitat specificity and diversity of tree species in an African wet tropical forest

Niche differentiation with respect to habitat has been hypothesized to shape patterns of diversity and species distributions in plant communities. African forests have been reported to be relatively less diverse compared to highly diversed regions of the Amazonian or Southeast Asian forests, and might be expected to have less niche differentiation. We examined patterns of structural and floristic differences among five topographically defined habitats for 494 species with stems ≥1 cm dbh in a 50-ha plot in Korup National Park, Cameroon. In addition, we tested for species–habitat associations for 272 species (with more than 50 individuals in the plot) using Torus translation randomization tests. Tree density and basal area were lowest in areas with negative convexity, which contained streams or were inundated during rainy periods and highest in moist well-drained habitats. Species composition and diversity varied along the topographical gradient from low flat to ridge top habitats. The low depression and low flat habitats were characterized by high diversity and similar species composition, relative to slopes, high gullies and ridge tops. Sixty-three percent of the species evaluated showed significant positive associations with at least one of the five habitat types. The majority of associations were with low depressions (75 species) and the fewest with ridge tops (8 species). The large number of species–habitat associations and the pronounced contrast between low (valley) and elevated (ridgetop) habitats in the Korup plot shows that niche differentiation with respect to edaphic variables (e.g., soil moisture, nutrients) contributes to local scale tree species distributions and to the maintenance of diversity in African forests.     

Topographic microclimates drive microhabitat associations at the range margin of a butterfly

The habitat associations of individuals underpin the dynamics of species distributions. Broad‐scale gradients in climate can alter habitat associations across species’ geographic ranges, but topographic heterogeneity creates local microclimates which could generate variation in habitat use at finer spatial scales. We examined the selection of microhabitats for egg‐laying by populations of a thermally‐constrained butterfly, the skipper Hesperia comma, across 16 sites with different regional temperatures and topographic microclimates. Using models of thermal microclimate, we examined how the association between eggs and warm bare ground microhabitats varied with ambient temperature, and predicted bare ground associations in 287 existing H. comma populations, to investigate the relative impacts of regional temperatures and topographic microclimates on microhabitat use. Eggs were most strongly associated with bare ground in relatively cool sites, indicating climate‐driven changes in microhabitat use. The majority of temperature variation between study sites was attributable to topographic microclimates rather than regional temperature differences, such that changes in microhabitat associations occurred principally between north‐ and south‐facing slopes within the same region. Predicted microhabitat associations across the UK distribution of H. comma showed that, due to the large temperature differences generated by topography, most of the between‐population variation in microhabitat use occurs locally within 5 km grid squares, with a smaller proportion occurring at a regional level between 5 km squares. Our findings show how microclimatic variation generated by topography alters the habitat associations of populations at fine spatial scales, suggesting that microclimate‐driven changes in habitat suitability could shape species’ distribution dynamics and their responses to environmental change.     

Environmental and neighbourhood effects on tree fern distributions in a neotropical lowland rain forest

Questions: To what extent are the distributions of tropical rain forest tree ferns (Cyatheaceae) related to environmental variation, and is habitat specialization likely to play a role in their local coexistence? Location: Lowland rain forest at La Selva Biological Station, Costa Rica. Methods: Generalized linear (GLM) and generalized additive (GAM) logistic regression were used to model the incidence of four tree fern species in relation to environmental and neighbourhood variables in 1154 inventory plots regularly distributed across 6 km² of old‐growth forest. Small and large size classes of the two most abundant species were modelled separately to see whether habitat associations change with ontogeny. Results: GLM and GAM model results were similar. All species had significant distributional biases with respect to micro‐habitat. Environmental variables describing soil variation were included in the models most often, followed by topographic and forest structural variables. The distributions of small individuals were more strongly related to environmental variation than those of larger individuals. Significant neighbourhood effects (spatial autocorrelation in intraspecific distributions and non‐random overlaps in the distributions of certain species pairs) were also identified. Overlaps between congeners did not differ from random, but there was a highly significant overlap in the distributions of the two most common species. Conclusions: Our results support the view that habitat specialization is an important determinant of where on the rain forest landscape tree ferns grow, especially for juvenile plants. However, other factors, such as dispersal limitation, may also contribute to their local coexistence.     

Lack of feral livestock interference with native guanaco during the dry season in a South American desert

Analyzing coexistence of exotic and native ungulates in arid areas is important from both a theoretical and a species conservation perspective. We assess the habitat use patterns and possible interference between guanaco (Lama guanicoe) and feral livestock (donkey and cattle) in arid environments of South America. To determine habitat use and niche overlap between exotic and native ungulate species, ten sites with different habitats and six natural waterholes were selected. Plots (20 at each site, ten around each waterhole) were randomly set up and characterized by environmental variables and relative use by cattle, donkey and guanaco through faecal pellet counts. Aggregation, niche breadth and niche overlap of the three herbivores were analyzed at habitat level (mesoscale). A direct redundancy analysis was used to examine the relationships between abundance of herbivore faeces and environmental variables at microscale. Mesoscale analyses showed (i) an extensive use of the area by all three species, with guanaco having the highest niche breadth followed by donkey and cattle and (ii) a large, broad guanaco–donkey and donkey–cattle habitat overlap. However, results at a finer scale showed high spatial aggregation of feral livestock species and an independent use of territory by guanacos. This study is the first to provide information about habitat partitioning between guanacos and feral livestock in the hyper-arid Monte Desert biome and points to an apparent lack of negative effects on the native ungulate.     

Aggregated spatial distributions of species in a subtropical karst forest,southwestern China

Aims Spatial distribution patterns of species reflect not only the ecological processes but also the habitat features that are related to species distribution. In karst topography, species distribution patterns provide more specific information about their environments. The objectives of this study are as follows: (i) to analyse and explain the spatial distribution patterns of conspecific trees in an old-growth subtropical karst forest; (ii) to investigate pattern changes at different spatial scales; (iii) to test the spatial pattern similarity (or dissimilarity) between trees at different abundances, diameter at breast height classes, canopy layers and different functional groups (shade tolerance and seed dispersal mode); (iv) to examine whether habitat heterogeneity has an important effect on the species spatial distribution.Methods The spatial distributions of woody species with ≥20 individuals in a 1-ha subtropical karst forest plot at Maolan in southwestern China were quantified using the relative neighbourhood density Ω based on the average density of conspecific species in a circular neighbourhood around each species.Important findings Aggregated distribution is the dominant pattern in the karst forest, but the ratio of aggregated species in total species number decreases with an increase in spatial scale. Less abundant species are more aggregated than most abundant species. Aggregation is weaker in larger diameter classes, which is consistent with the prediction of self-thinning. Seed dispersal mode influences spatial patterns, with species dispersed by animals being less aggregated than those dispersed by wind and gravity. Other species functional traits (e.g. shade tolerance) also influence the species spatial distributions. Moreover, differences among species habitat associations, e.g. with rocky outcrops, play a significant role in species spatial distributions. These results indicate that habitat heterogeneity, seed dispersal limitation and self-thinning primarily contribute to the species spatial distributions in this subtropical karst forest.     

Habitat associations with topography and canopy structure of tree species in a tropical montane forest on Mount Kinabalu,Borneo

Habitat associations with topography and canopy structure of 42 abundant tree species were studied in a 2.74-ha plot of tropical montane forest on Mount Kinabalu, Borneo. Many of these species belong to the same higher taxa including eight families and four genera. Analysis of intraspecific spatial distributions for stems ≥ 10 cm diameter revealed that 28 species (including all six species of Fagaceae) showed aggregated distributions at the 100-m² and/or 400-m² scales, and that 20 species showed habitat associations with topography by torus-translation tests; 17 species showed both characteristics. Species' associations with the local canopy structure were characterized by crown position index (CPI), which was defined relative to neighbour trees. The CPI differed greatly among individual stems at 10–40 cm diameter, and 19 species showed significantly different frequencies of crowns exposed vertically versus those shaded beneath the canopy. Mean growth rates at 10–40 cm diameter and size distributions of species were not related to topographic associations, but were explained by the associations with canopy structure; species with more exposed crowns grew faster and had less positively skewed distributions. Diversity in habitat associations was manifest between two genera (Syzygium and Tristaniopsis) in the family Myrtaceae and among species in these genera, but was less evident in other families and two genera (Garcinia and Lithocarpus). This revised version was published online in June 2006 with corrections to the Cover Date.     

Strong plant-soil associations in a heterogeneous subtropical broad-leaved forest

The relative importance of deterministic and neutral processes on community assembly is currently a topic of much debate among ecologists. Analyzing species-environment associations is an effective way to assess the importance of deterministic process such as niche differentiation, but both habitat association and dispersal limitation can produce similar patterns of spatial aggregation in species. Therefore, it is crucial to control for the impact of dispersal limitation on species distributions when analyzing species-environment associations. We sampled soil with high resolutions in a 24 ha stem-mapped subtropical forest and tested plant-soil associations. We controlled for the influence of dispersal limitation by employing the homogeneous Thomas process to simulate the effect of dispersal limitation on the aggregation of tree species. After controlling for the effect of dispersal limitation, we found that the spatial heterogeneity of soil properties was associated with distributions of 88.2% (90 of 102 species) of tree species in this subtropical forest. Furthermore, not only did soil properties influence the distribution of tree species, but also tree species tended to affect properties of the soil around them. The soil factors most strongly influencing species distributions were TC, TN, TP, K, Mg, Si, soil moisture, and bulk density. We found the spatial heterogeneity of soil properties to be strongly associated with tree species distributions. Niche partitioning of soil gradients contributed substantially to species coexistence in this subtropical forest.     

Spatial associations among major tree species in a cool-temperate forest community under heterogeneous topography and canopy conditions

Analysis of the spatial pattern of plants may provide insight into the processes and mechanisms that promote species coexistence and community organization. Using torus-translation tests and point-pattern analyses for a heterogeneous Poisson process, we investigated habitat association and intra- and inter-specific spatial relationships of six major tree species in a cool-temperate forest community. All stems ≥5 cm in diameter at breast height were mapped on a 1.4-ha (100 × 140 m) plot and the topographic conditions (convexity and slope degree) and canopy state were assessed. Our results showed that all six species exhibited habitat associations with topographic and/or canopy conditions except for Magnolia salicifolia. Intra-specific aggregation was found for Acer japonicum, M. salicifolia, and Hamamelis japonica var. obtusata. Community-wide analysis of the inter-specific spatial patterns showed mainly mixed or partially overlapped patterns at a scale of up to 30 m, whereas individual pairwise analyses of inter-specific patterns revealed that Fagus crenata was positively associated with two Acer species and M. salicifolia at a spatial scale of up to 5 m. These results highlight that scale-dependant ecologically important processes, such as species-specific habitat preference, regeneration mode, seed dispersal, facilitation and niche complementarity, may operate simultaneously to shape tree distributional patterns, although their presence/absence as well as relative importance vary among species. Given the complexity of the process and mechanisms promoting species coexistence and community organization, more attention should be given to the effect of spatial scale in analyzing the spatial patterns of tree species in forest communities.     

Patterns of shield darter, <Emphasis Type="Italic">Percina peltata</Emphasis>, distribution in the Eastern Piedmont of Maryland,USA

Detailed analyses of habitat associations with rare species are typically constrained by limited sample size and the availability of habitat data. The dense spatial coverage of stream sampling by the Maryland Biological Stream Survey provides ample data to quantitatively examine correlations between habitat and rare species distributions. The shield darter, Percina peltata, has a widespread distribution on the Atlantic Slope of the United States, but is uncommon throughout its range in Maryland. Associations of in situ physical habitat, water chemistry, and alterations in landscape with shield darter presence in the Eastern Piedmont physiographic province in Maryland were examined. Shield darter occurrence was associated with larger sized streams in concordance with the species’ known ecology. Shield darter distribution was further associated with stream segments with deep riffle habitats with diverse velocities, low concentrations of chloride and sulfate, low levels of urbanization in upstream catchments, and several pollution intolerant fish species. Although the exact mechanism of the effects is not clear, results indicate that the shield darter is sensitive to urban development and habitat and water quality alteration that typically accompanies urbanization. Shield darter conservation in Maryland necessitates the protection and restoration of minimally urbanized watersheds where they are known to occur. The results from this study indicate that habitat information on rare species may be important in elucidating important habitat associations that are not evident via examination of community level data.     

Is there a link between the type of habitat and the patterns of abundance of holothurians in shallow rocky reefs?

The presence of a mosaic of habitats, largely determined by sea urchin grazing, across shallow rocky reefs may potentially influence in differences in the distribution patterns of invertebrates. The aim of this paper was to assess, using a correlative approach, whether the type of habitat influences the abundance patterns of holothurians in the eastern Atlantic. We hypothesized that abundances of large (> 10 cm) holothurians varied among four types of habitat (3 vegetated habitats with low abundances of the sea urchin D. antillarum vs. ‘barrens’ with hyperabundances of sea urchins), and that these differences were consistent at a hierarchy of spatial scales, including two islands and several replicated sites within each type of habitat and island. Three species of large holothurians were found, accounting for a total of 300 specimens. We found remarkable differences in abundances of holothurians between the ‘barrens’ and the three vegetated habitats. This pattern was strongest for the numerically dominant species, Holothuria sanctorii. Total abundances of holothurians were between 5 and 46 times more abundant in ‘barrens’ compared with the vegetated habitats. Inter-habitat differences were species-specific with some inconsistent patterns from one island to the other. The total abundances of holothurians tended to increase with the abundance of sea urchins within ‘barrens’. Our study suggests that there may be a link, at least for the dominant species Holothuria sanctorii, between the distribution and abundances of large holothurians and the habitat across shallow-waters of the eastern Atlantic.     

Spatial Patterns and Associations between Species Belonging to Four Genera of the Lauraceae Family

Spatial distribution pattern of biological related species present unique opportunities and challenges to explain species coexistence. In this study, we explored the spatial distributions and associations among congeneric species at both the species and genus levels to explain their coexistence through examining the similarities and differences at these two levels. We first used DNA and cluster analysis to confirmed the relative relationship of eight species within a 20 ha subtropical forest in southern China. We compared Diameter at breast height (DBH) classes, aggregation intensities and spatial patterns, associations, and distributions of four closely related species pairs to reveal similarities and differences at the species and genus levels. These comparisons provided insight into the mechanisms of coexistence of these congeners. O-ring statistics were used to measure spatial patterns of species. Ω _0–10, the mean conspecific density within 10 m of a tree, was used as a measure of the intensity of aggregation of a species, and g-function was used to analyze spatial associations. Our results suggested that spatial aggregations were common, but the differences between spatial patterns were reduced at the genus level. Aggregation intensity clearly reduced at the genus level. Negative association frequencies decreased at the genus level, such that independent association was commonplace among all four genera. Relationships between more closely related species appeared to be more competitive at both the species and genus levels. The importance of competition on interactions is most likely influenced by similarity in lifestyle, and the habitat diversity within the species’ distribution areas. Relatives with different lifestyles likely produce different distribution patterns through different interaction process. In order to fully understand the mechanisms generating spatial distributions of coexisting siblings, further research is required to determine the spatial patterns and associations at other classification levels.     

Lizard habitat partitioning on islands: the interaction of local and landscape scales

Aim This study addresses how species resolve environmental differences into biological habitats at multiple, interacting spatial scales. How do patterns of local habitat use change along an elevation gradient? How do patterns of local habitat partitioning interact with partitioning at a landscape scale? Location Northern and southern Lesser Antilles islands, West Indies. Methods We document how Anolis Daudin, 1802 lizards partition habitat locally at sites along a landscape‐scale elevation gradient. We examine habitat partitioning both with and without interspecific interactions in the predominately flat northern Lesser Antilles islands and in the more mountainous southern islands. Results Anoles partition local habitat along perch‐height and microclimate axes. Northern‐group sympatric anoles partition local habitat by perch height and have overlapping distributions at the landscape scale. Southern‐group sympatric anoles partition local habitat by microclimate and specialize in particular habitats at the landscape scale. In both the northern and southern groups, species use different perch heights and microclimates only in areas of species overlap along the elevation gradient. Main conclusions We demonstrate the interaction between local‐ and landscape‐scale habitat partitioning. In the case of microclimate partitioning, the interaction results from the use of thermal physiology to partition habitat at multiple scales. This interaction prompts the question of whether habitat partitioning developed ‘local‐out’ or ‘landscape‐in’. We pose this dichotomy and present a framework for its resolution.     

The scale-dependent importance of habitat factors and dispersal limitation in structuring Great Lakes shoreline plant communities

Niche-based and neutral models of community structure posit distinct mechanisms underlying patterns in community structure; correlation between species’ distributions and habitat factors points to niche assembly while spatial pattern independent of habitat suggests neutral assembly via dispersal limitation. The challenge is to disentangle the relative contributions when both processes are operating, and to determine the scales at which each is important. We sampled shoreline plant communities on an island in Lake Michigan, varying the extent and the grain of sampling, and used both distance-based correlation methods and variance partitioning to quantify the proportion of the variation in plant species composition that was attributable to habitat factors and to spatial configuration independent of habitat. Our results were highly scale dependent. We found no distance decay of plant community similarity at the island scale (1−33 km). All of the explained variation (32%) in species composition among samples at this scale was attributed to habitat factors. However, at a site intensively sampled at a smaller scale (5−1,200 m), similarity of species composition did decay with distance. Using a coarse sampling grain (transects), habitat factors explained 40% of the variation, but the purely spatial component explained a comparable 22%. Analyzing plots within transects revealed variation in species composition that was still jointly determined by habitat and spatial factors (18 and 11% of the variance, respectively). For both grain sizes, most of the habitat component was spatially structured, reflecting an abrupt alongshore transition from sandy dunes to cobble beach. Space per se explained more variation in species composition at a second site where the habitat transition was more gradual; here, habitat acted as a less selective filter, allowing the signal of dispersal limitation to be detected more readily. We conclude that both adaptation to specific habitat factors and habitat-independent spatial position indicative of dispersal limitation determine plant species composition in this system. Our results support the prediction that dispersal limitation—a potentially, but not necessarily, neutral driver—is relatively more important at smaller scales.