1. The structure of biological communities reflects the influence of both local environmental conditions and processes such as dispersal that create patterns in species' distribution across a region. 2. We extend explicit tests of the relative importance of local environmental conditions and regional spatial processes to aquatic plants, a group traditionally thought to be little limited by dispersal. We used partial canonical correspondence analysis and partial Mantel tests to analyse data from 98 lakes and ponds across Connecticut (northeastern United States). 3. We found that aquatic plant community structure reflects the influence of local conditions (pH, conductivity, water clarity, lake area, maximum depth) as well as regional processes. 4. Only 27% of variation in a presence ⁄ absence matrix was explained by environmental conditions and spatial processes such as dispersal. Of the total explained, 45% was related to environmental conditions and 40% to spatial processes. 5. Jaccard similarity declined with Euclidean distance between lakes, even after accounting for the increasing difference in environmental conditions, suggesting that dispersal limitation may influence community composition in the region. 6. The distribution of distances among lakes where species occurred was associated with dispersal-related functional traits, providing additional evidence that dispersal ability varies among species in ways that affect community composition. 7. Although environmental and spatial variables explained a significant amount of variation in community structure, a substantial amount of stochasticity also affects these communities, probably associated with unpredictable colonisation and persistence of the plants.

Invasive species richness often is negatively correlated with native species richness at the small spatial scale of sampling plots, but positively correlated in larger areas. The pattern at small scales has been interpreted as evidence that native plants can competitively exclude invasive species. Large-scale patterns have been understood to result from environmental heterogeneity, among other causes. We investigated species richness patterns among submerged and floating-leaved aquatic plants (87 native species and eight invasives) in 103 temperate lakes in Connecticut (northeastern USA) and found neither a consistently negative relationship at small (3-m2) scales, nor a positive relationship at large scales. Native species richness at sampling locations was uncorrelated with invasive species richness in 37 of the 60 lakes where invasive plants occurred; richness was negatively correlated in 16 lakes and positively correlated in seven. No correlation between native and invasive species richness was found at larger spatial scales (whole lakes and counties). Increases in richness with area were uncorrelated with abiotic heterogeneity. Logistic regression showed that the probability of occurrence of five invasive species increased in sampling locations (3 m2, n = 2980 samples) where native plants occurred, indicating that native plant species richness provided no resistance against invasion. However, the probability of three invasive species' occurrence declined as native plant density increased, indicating that density, if not species richness, provided some resistance with these species. Density had no effect on occurrence of three other invasive species. Based on these results, native species may resist invasion at small spatial scales only in communities where density is high (i.e., in communities where competition among individuals contributes to community structure). Most hydrophyte communities, however, appear to be maintained in a nonequilibrial condition by stress and/or disturbance. Therefore, most aquatic plant communities in temperate lakes are likely to be vulnerable to invasion.

Invasive plants alter community structure, threatening ecosystem function and biodiversity, but little information is available on whether invasive species richness responds to environmental conditions in the same way that richness of native plants does. We surveyed submerged and floating-leaved plants in 99 Connecticut (northeast USA) lakes and ponds, collecting quantitative data on abundance and frequency. We used multiple linear and logistic regression to determine which environmental conditions were correlated with species richness of invasive and native plants. Independent variables included lake area, maximum depth, pH, alkalinity, conductivity, phosphorus concentration, productivity, and dominance (the proportional abundance of the most abundant and frequently found species), plus two estimates of human activity. Species richness of both native and invasive richness was correlated with alkalinity and human activity. Native richness also increased with water clarity, lake area, and productivity; invasive species richness also rose with pH. We found no evidence that richness of one group affected richness of the other. We also investigated patterns of dominance and found that native plants were as likely to become dominant as invasive species. Dominance occurred overwhelmingly in shallow lakes with high productivity.Résumé : Les plantes envahissantes modifient la structure des communautés en menaçant les fonctions des écosystèmes et leur biodiversité, mais on connaît peu de chose à savoir si la richesse en espèces envahissantes réagit aux conditions du milieu de la même façon que la richesse en plantes indigènes. Les auteurs ont suivi les plantes submergées et à feuilles flottantes dans 99 lacs et étang du nord-est des É tats-Unis, en réunissant les données d'abondance et de fréquence. Ils ont utilisé la régression linéaire multiple et logistique pour déterminer quelles conditions environnementales montrent une corrélation avec les richesses en espèces des plantes envahissantes et indigènes. Les variables indépendantes comprennent la surface des lacs, la profondeur maximum, le pH, l'alcalinité, la conductivité, la teneur en phosphore, la productivité et la dominance (l'abondance proportionnelle des espèces les plus fréquentes et les plus abondantes), ainsi que deux valeurs estimées pour l'activité humaine. La richesse en espèces indigènes aussi bien qu'en espèces envahissantes montre une corré-lation avec l'alcalinité et l'activité humaine. La richesse en espèces indigènes augmente également avec la clarté de l'eau, la surface du lac et la productivité; la richesse en espèces adventices augmente aussi avec le pH. On ne perçoit aucune preuve que la richesse d'un groupe affecte la richesse de l'autre. Les auteurs ont également examiné le patron de la dominance pour constater que les plantes indigènes ont autant de chance de devenir dominantes que les espèces envahissantes. On retrouve la dominance surtout dans les lacs peu profonds à forte productivité.