Exotic invasive species in urban wetlands: environmental correlates and implications for wetland management
2008; Wiley; Volume: 45; Issue: 4 Linguagem: Inglês
10.1111/j.1365-2664.2008.01476.x
ISSN1365-2664
Autores Tópico(s)Coastal wetland ecosystem dynamics
ResumoJournal of Applied EcologyVolume 45, Issue 4 p. 1160-1169 Free Access Exotic invasive species in urban wetlands: environmental correlates and implications for wetland management Joan G. Ehrenfeld, Corresponding Author Joan G. Ehrenfeld *Correspondence author. E-mail: [email protected]Search for more papers by this author Joan G. Ehrenfeld, Corresponding Author Joan G. Ehrenfeld *Correspondence author. E-mail: [email protected]Search for more papers by this author First published: 09 July 2008 https://doi.org/10.1111/j.1365-2664.2008.01476.xCitations: 73AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Summary 1 Wetlands in urban regions are subjected to a wide variety of anthropogenic disturbances, many of which may promote invasions of exotic plant species. In order to devise management strategies, the influence of different aspects of the urban and natural environments on invasion and community structure must be understood. 2 The roles of soil variables, anthropogenic effects adjacent to and within the wetlands, and vegetation structure on exotic species occurrence within 21 forested wetlands in north-eastern New Jersey, USA, were compared. The hypotheses were tested that different vegetation strata and different invasive species respond similarly to environmental factors, and that invasion increases with increasing direct human impact, hydrologic disturbance, adjacent residential land use and decreasing wetland area. Canonical correspondence analyses, correlation and logistic regression analyses were used to examine invasion by individual species and overall site invasion, as measured by the absolute and relative number of exotic species in the site flora. 3 Within each stratum, different sets of environmental factors separated exotic and native species. Nutrients, soil clay content and pH, adjacent land use and canopy composition were the most frequently identified factors affecting species, but individual species showed highly individualistic responses to the sets of environmental variables, often responding in opposite ways to the same factor. 4 Overall invasion increased with decreasing area but only when sites > 100 ha were included. Unexpectedly, invasion decreased with increasing proportions of industrial/commercial adjacent land use. 5 The hypotheses were only partially supported; invasion does not increase in a simple way with increasing human presence and disturbance. 6 Synthesis and applications. The results suggest that a suite of environmental conditions can be identified that are associated with invasion into urban wetlands, which can be widely used for assessment and management. However, a comprehensive ecosystem approach is needed that places the remediation of physical alterations from urbanization within a landscape context. Specifically, sediment, inputs and hydrologic changes need to be related to adjoining urban land use and to the overlapping requirements of individual native and exotic species. Introduction Forested wetlands in developed landscapes are subject to a large number of stressors that alter their structure and function (Pickett et al. 2001; Ehrenfeld et al. 2003; Ehrenfeld 2004; Meyer, Paul & Taulbee 2005). These stressors include hydrological changes, inputs of nutrients from both aqueous and atmospheric sources, high frequencies of physical disturbance within sites, large propagule sources of exotic species from adjacent development, and enhanced dispersal of propagules from human and animal traffic on trails. Invasive and non-native species are likely to establish within such wetlands as a result of these stresses, as frequently demonstrated in comparisons of urban and non-urban landscapes (Kowarik 1995; Borgmann & Rodewald 2005). However, recent studies of urban ecology point out that urban landscapes are themselves highly heterogeneous (McGranahan & Satterthwaite 2003; Clergeau, Jokimäki & Snep 2006; Grove et al. 2006). This variability is not surprising, as 'urban development' is actually a very heterogeneous category of land use and includes a large range of conditions. In this study the importance of heterogeneity in environmental conditions was evaluated as an explanation for patterns of diversity and abundance of invasive species in urban forested wetlands. Developed landscapes may affect wetland plant communities through impacts emanating from the surrounding uplands. The types of adjacent land use (e.g. residential, commercial and industrial), road density and human population density in the surrounding region all affect water flow into and through the wetland (Booth & Jackson 1997; Reinelt & Taylor 2000; Paul & Meyer 2001; Allan 2004). These factors also influence nutrient and pollutant inputs, through both water and atmospheric deposition (Lovett et al. 2000; Brabec, Schulte & Richards 2002; Connor & Thomas 2003). Area and the amount of habitat edge are well-known correlates of species richness (Harper et al. 2005) and most sites in developed regions are small fragments of initially larger habitat. These factors generate other processes influencing invasion; for example, residential development may be a source of propagules from ornamental plants that are also commonly invasive in natural communities, such as Japanese barberry Berberis thunbergii DC, burning bush Euonymus alata (Thunb.) Sieb. and privet Ligustrum vulgare L. Direct impacts result from the presence of people within wetlands and involve a separate set of factors that can promote exotic invasions. These include : (i) dredging of channels and ditches within the wetland; (ii) presence of formally established or informally created paths, with concomitant disturbance from foot and vehicular traffic (e.g. bicycles and all-terrain vehicles); (iii) presence of pet animals that may serve as dispersers of seeds or as predators of native seed dispersers; (iv) frequent use of wetland edges as dumping sites for rubbish, garden waste, etc., all of which may enhance propagule transport into the wetlands or create disturbed areas; (v) other types of physical disturbance that are frequently observed, including excavations and earth mounds, often reflecting past abuse of the wetland for the disposal of material from adjacent development; (vi) erosional reductions of stream channel elevations (Booth & Jackson 1997; Groffman et al. 2003) that lowers the water table in the adjacent wetland surface. Urban disturbances also may modify the physical environment within wetlands. Soils may be enriched with nutrients, arriving in both soluble form (in floodwater and precipitation) and through deposition of sediments. In wetlands affected by ditches and eroded streams, formerly hydric soils may lose their distinctive properties if they are predominantly dry. Because the large majority of invasive species in these sites are understorey herbs and shrubs, invasion may also reflect the structural properties of vegetation, as canopy cover and shrub density will control light regimes, and plant densities will affect competitive interactions (Alston & Richardson 2006). Physical disturbances to both canopy and shrub strata may enhance invasion by permitting more light penetration and reducing competition from native species. This study used information on a previously described set of wetlands in the New Jersey section of the metropolitan area around New York City, USA (Ehrenfeld 2000, 2004, 2005; Ehrenfeld et al. 2003), to explore the relative importance of three sets of variables in explaining the degree of exotic invasion. These were (i) soil properties; (ii) descriptors of human impact, including adjacent land use and evidence of direct human disturbance within the wetlands; and (iii) descriptors of vegetation structure. Based on the considerations above, it was hypothesized that the diversity and abundance of exotic species will increase with (i) increasing evidence of direct human presence, (ii) increasing hydrologic disturbance, (iii) increasing residential development in the area surrounding the wetland, and decreasing area of the wetland, and (iv) disrupted canopies, evidenced by low tree densities and/or basal areas. Methods sites and data collection methods Sites were located throughout north-eastern New Jersey, which is part of the New York–Newark metropolitan urban region of 18·5 million people, the third-largest such agglomeration in the world (United Nations 2004). Sites were chosen following criteria described in Ehrenfeld (2000); briefly, all sites supported mature deciduous forested wetlands as mapped by the New Jersey Wetland Inventory maps (http://www.statc.nj.us/dep/gis/wetshp.html) and were distributed through the region by selecting one site per US Geological Survey 7·5° quadrangle maps (21 quadrangles). In each site, 10 circular plots, each 10 m in radius (314 m2), were distributed on transects orientated to cover the longest dimensions of each site. Data on vegetation, soils, hydrology and indicators of human disturbance were collected within each plot, as described more fully in Ehrenfeld (2004, 2005) and Ehrenfeld et al. (2003). Plants were separated into herbaceous (forbs, graminoids and pteridophytes), woody understorey (shrubs and woody vines), and trees for separate analyses, to test the hypothesis that the different strata are responding to different environmental factors. Taxonomy follows Kartesz & Meacham (1999). The soils data set included data that were averages of either samples collected at five of the 10 sample points of the top 20 cm of soil (pH, organic matter and texture) or randomly chosen samples from two points that were submitted to the Rutgers University Soil Testing Laboratory, New Brunswick, NJ, for analysis (Mehlich III extractable P, extractable cations and micronutrients). A separate set of eight samples per site was utilized for measurements of extractable inorganic nitrogen (NH4+, NO3− + NO2−), 2 m KCl extractions following standard methods, and 10-day incubations in the laboratory to determine potential net N mineralization and nitrification rates, normalized to the organic matter content, following standard protocols (Sollins et al. 1999). In addition, at each sample point field measurements were made of the depth of the organic layer, percentage of samples (n = 40 per site) with Munsell chromas of 2 or less, and percentage of samples with redoximorphic concentrations and depletions present in the top 30 cm of soil (Vepraskas 1996; Vepraskas & Faulkner 2001). Land-use data were based on measurements made using ARCInfo software. A buffer area of 300 m from the mapped wetland edge was analysed to determine the amount of residential development, a combination of industrial and commercial land use, and naturally vegetated area. These areas were then converted to percentages of the total buffer area for each site. Road density was quantified as kilometre of road surface (all road size categories) per km2 of buffer area. Population density of the surrounding municipality of each site (from census data for the year 2000 (http://www.census.gov,accessed July 2006) was used as an indicator of adjacent human population. The area of each wetland site (hectares), was determined from the New Jersey GIS file (http://www.state.nj.us/Department/gis/wetshp.html) based on contiguous polygons of forested wetlands at each site. The frequencies of occurrence at the 10 sample points of rubbish (e.g. fast food containers, plastic bags, shopping trolleys and broken furniture), ditches and trails were calculated as indicators of direct human impact. Descriptors of plant community structure included the mean diameter breast height (cm) of trees (n = 40 per site); tree density (number ha−1); shrub density (number of stems ha−1); basal area (m2 ha−1); an index of vegetation density [rank order observations of the density (absent, low and high) of four understorey strata, low herbaceous (< 0·5 m), tall herbaceous (0·5–1 m), low woody vegetation (1–3 m) and tall woody understorey vegetation (3–5 m)]; the total number of native species; and a measure of microtopographic heterogeneity (SD of six measures of height from a standard horizontal bar to the ground around a 2-m diameter circle). Shrubs included all species that had multiple woody stems originating from a single root base, whereas trees included stems of at least 2·5 cm d.b.h. of species that have a single main unbranched stem or trunk. Density of trees was calculated from point-quarter measurements made at the centre point of each of the 10 plots. Density of shrubs was determined as the average over the 10 plots of stem counts in a 5-m radius sample area at each point. statistical analyses Canonical correspondence analyses (CCA) were conducted to explore the relationships between species occurrences and the three sets of environmental factors (all analyses conducted in PC-ORD version 5·07; McCune & Mefford 1999). This method was chosen because it specifically and directly tests the importance of explanatory environmental variables in explaining the variance in species composition of a sample of sites (McCune & Grace 2002). In this study, the method was used to test the responses of groups of species (each stratum) to each set of environmental variables. This approach allowed both the important variables within each type of environmental data to be identified and permitted comparisons of the relative importance of the different types of environmental variables in explaining species composition. Species occurring in fewer than three of the 21 sites were eliminated from the analyses, leaving 105 herbaceous, 38 shrub and vine, and 42 tree species in the three data sets, respectively. Species were represented by their frequency of occurrence at the 10 sample points in each site. Most of the exotic species retained in the analyses were ranked as moderately to highly invasive and damaging to natural communities (NatureServe database; http://www.natureserve.org/explorer/index.htm,accessed July 2006). Results of each ordination are displayed as a graph of the species' locations with respect to the first two axes and a listing of factor loadings for the three axes identified by the ordination analysis. Exotic species are identified on each figure (1-3). Initial data sets including a larger number of variables were pruned to remove variables highly correlated with each other, so that each of these data sets included only variables with correlations of ≤ 0·50. When variables had correlations > 0·50, the variable with the largest number of correlations over 0·50 to other variables was dropped. All variables used in the ordinations are described in Table 1. Figure 1Open in figure viewerPowerPoint CCA of vegetation strata with respect to soil factors. Native species are indicated by filled circles and exotic species are indicated by triangles. Bi-plot arrows indicate environmental factors with correlation coefficients > 0·2; factor abbreviations are given in Table 1. Authorities for all names follow Kartesz & Meacham (1999). Exotic species are identified by numbers. (a) Herbs: 1, Alliaria petiolata; 2, Allium vineale; 3, Artemisia vulgaris; 4, Chenopodium album; 5, Hemerocallis fulva; 6, Lysimachia nummularia; 7, Microstegium vimineum; 8, Plantago major; 9, Polygonum cuspidatum; 10, Polygonum hydropiper; 11, Poa pratensis; 12, Prunella vulgaris; 13, Solanum dulcamara. (b) Shrubs: Celastrus orbiculatus; 2, Berberis thunbergii; 3, Euonymus alata; 4, Forsythia viridissima; 5, Humulus japonicus; 6, Ligustrum vulgare; 7, Lonicera japonica; 8, Lonicera tatarica; 9, Rhamnus cathartica; 10, Ribes rubrum; 11, Rosa multiflora; 12, Rubus phoenicolasius. (c) Trees: 1, Acer platanoides; 2, Ailanthus altissima; 3, Gleditsia triacanthos; 4, Malus pumila; 5, Morus rubra; 6, Prunus avium. Figure 2Open in figure viewerPowerPoint CCA of vegetation strata with respect to human factors. Species symbols and numbers and bi-plot arrows as in Fig. 1. Figure 3Open in figure viewerPowerPoint CCA of vegetation strata with respect to vegetation structural factors. Species symbols and numbers and bi-plot arrows as in Fig. 1. Table 1. Environmental variables included in the ordination analyses. For each, the abbreviation used in the ordination diagrams, a description of the units or method of determination, the mean ± SE and the range for the entire data set (21 sites) are given Data set Variable Units Abbreviation Mean ± SE Range Human impact Ditches Frequency (of 10 points site−1) DITCH 1·5 ± 0·3 0–5 Trash Frequency (of 10 points site−1) TRASH 3·6 ± 0·6 0–8 Trails Frequency (of 10 points site−1) TRAIL 1·2 ± 0·3 0–4 Population density Number km−2, 2000 census POPDEN 789·5 ± 106·2 261–2304 Area ha of forested wetland AREA 71·9 ± 23·7 5·2–488·6 Open area % of buffer in unplanted vegetation OPEN 61 ± 6 19–100 Residential % of buffer in residential land use RES 29 ± 6 0–81 Industrial % of buffer in industrial–commercial land use IND 8 ± 3 0–47 Roads km roads/km2 total buffer area ROADS 4·3 ± 0·6 0–10·5 Soil Organic horizon Thickness of organic horizon* OM 16·7 ± 4·7 0–70 cm pH pH units* pH 5·13 ± 0·10 4·3–6 Extractable NO3 mg NO3 kg organic matter−1† NO3 8·07 ± 1·7 0–35·4 Nitrification rate mg NO3 kg organic matter−1† NITR 3·5 + 0·5 0–7·44 day−1 % clay % clay fraction‡ CLAY 16·5 ± 3·0 0–45·5 % reduced chroma % of soil samples with chroma < 2§ REDCHR 64·7 ± 7·8 0–100 P kg Pextr ha−1§ P 97·2 ± 11·24 27·0–227·4 Zn mg Zn extr kg soil−1† Zn 17·3 ± 6·2 2·4–136·7 Vegetation Tree density Number stems ha−1 TREES 596·9 ± 81·0 231–1864 Shrub density Number stems ha−1 SHRUBS 5863·6 ± 690·4 51–12942 Diameter at breast height cm DBH 25·8 ± 1·23 14·7–35·1 Native richness Number species NATIVES 68·2 ± 5·1 26–110 Understorey density Values of 1–8 UND 0·71 ± 0·02 0·56–0·9 Microtopography Coefficient of variation MIC 6·64 ± 0·60 1·6–11·1 Basal area m2 ha−1 BA 30·5 ± 2·22 13·6–44·7 * n = 40 per site; surface soil to 20 cm depth. † n= 8 per site, top 20 cm of soil. ‡ n = 10 per site, top 30 cm of mineral soil. § n = 40 per site, top 30 cm of mineral soil. In order to test whether exotic species as a class were responding to environmental factors differently from native species, the ordination axis scores were compared using a t-test for each of the ordination analyses. Equality of variances of the two classes (exotics and natives) was tested and the appropriate F-statistic used to evaluate the results of each test. As a second test of the hypothesis that exotic and native species respond uniformly to environmental conditions, overall site invasion was indexed by (i) the total number of exotic species per site and (ii) the proportion of the site flora that was exotic; these two measures were then correlated with the environmental variables. The behaviour of individual exotic species relative to specific environmental variables was further examined by pooling data for the entire set of 210 plot measurements and using logistic regression to determine whether and how the probability of occurrence of each species was related to those environmental variables measured at each plot. Only exotic species that occurred in at least 20 plots were used for these analyses (12 species). While the 10 points within each site were not technically independent, they reflected a range of physical conditions (varying soil, hydrology, native plant diversity, proximity to boundaries, trails, streams, etc., within each site). For example, the range of diversities within sites (21·3 ± 2·1) was comparable with the mean diversity among sites (25·1 ± 1·6). Moreover, the average frequency of each species within sites was 32% (maximum 47%), indicating that no exotic occurred at all points within a site. The low frequencies of occurrences within sites suggested that spatial correlation of occurrence within sites was low or absent. Therefore, these analyses are presented as providing an indication of the behaviour of each exotic. All statistical tests other than the ordination analyses were conducted in SAS version 9·1 (SAS Institute Inc. 2002). Results species' ordinations The environmental variables against which the species were ordinated all covered a wide range of absolute values (Table 1). Canonical correspondence analyses of the three vegetation strata revealed environmental correlates of community composition for each of the sets of factors (1-3; Table 2). The ordinations all explained about one-third of the variance in the species matrices, suggesting that the three sets of environmental variables were similar in their influence on community structure. On only a few of the ordination axes were native and exotic species significantly different in their mean axis scores (Table 2). Table 2. Variables with correlations over ± 0·35 with each of the three axes of within each CCA (intraset correlations) and P-values of two-tailed t-test comparing mean values for exotics and natives (NS, not significantly different). (–) A negative correlation with the axis. Factors are listed in descending order of correlation coefficient. See Table 1 for factor abbreviations. The total percentage of variation explained is given for each ordination Stratum Soil axes P Human axes P Vegetation axes P Herbs 1: (–) Zn, (–) P, CLAY, (–) NITR 0·019 1: (–) POPDEN, (–) TRASH, RES 0·026 1: (–) DBH, TREES 0·012 2: (–) CLAY, (–) pH 0·038 2: ROADS, (–) OPEN, RES, (–) AREA NSNS 2: (–) NATIVES, MIC, UND 3: (–) OM, (–) Ca, (–) pH NS 3: IND, DITCH NS 3: BA, MIC, (–) UND 0·056 % variance 26·1 25·9 26·8 Shrubs and vines 1: (–) pH 0·004 1: (–) TRASH, RES 0·038 1: TREES, (–) d.b.h., MIC 0·060 2: (–) pH, (–) Ca,(–) OM, CLAY, NITR 0·042 2: (–) RES, DITCH, (–) TRAILS, TRASH, IND NS 2: MIC, (–) Natives NS 3: Zn, P, NITR NS 3: (–) IND, (–) POPDEN, (–) RES NS 3: UND 0·030 % variance 28·4 31·5 31·7 Trees 1: (–) pH, (–) NITR, Zn, REDCHR 0·015 1. (–) TRASH, (–) DITCH 0·005 1: (–) d.b.h., TREES, SHRUBS 0·001 2: (–) pH, (–) Zn, NITR, NS 2: DITCH NS 2: (–) NATIVES, UND 0·039 (–) NO3 3: P, Zn, NITR, (–) clay NS 3. POPDEN, IND, OPEN NS 3: MIC, (–) SHRUBS NS % variance 32·3 31·5 32·1 Ordination of the three vegetation strata with respect to soil variables (Fig. 1) showed that the species of each stratum responded to the soil factors differently, as did native and exotic groups of species (Table 2). For herbaceous species (Fig. 1a), gradients of nutrient availability and soil clay content distinguished exotic (higher nutrient availability and pH) and native species (lower nutrient availability, lower pH and higher clay content). Shrubs and vines, in contrast, were primarily arrayed along pH and calcium gradients, with lesser effects of soil texture and nutrient availability; pH was the primary factor separating native and exotic species (Fig. 1b). For the tree species, a combination of nutrients and pH, but also the redox status of the soil (as indicated by reduced chromas), was important in separating native and exotic species and in structuring the tree communities in general (Fig. 1c). Similarly, ordination of the three vegetation data sets with respect to the human environmental factors suggested that different sets of factors structured each of the strata (Fig. 2). Herbaceous species were most strongly related to a combination of population density, roads and residential land use outside the wetlands and the presence of rubbish within the sites, and these factors accounted for significant differences in the occurrence of exotic and native species. Herb communities were also affected by a separate gradient of industrial land use adjacent to the wetlands and ditches within the wetlands, but this gradient did not separate exotic from native species. For the shrubs and vines, only the presence of rubbish (an internal factor) and residential housing adjacent to the wetlands (an external factor) accounted for differences both between native and exotic species (Table 2). Other external (population density and industrial land use) and internal (ditches and trails) factors differentiated species' occurrences but did not distinguish exotic from native species. For trees, only within-wetland factors, the presence of rubbish and ditches, affected the occurrence of native and exotic species; adjacent land use affected community composition but did not distinguish exotic and native species presence. The influence of stand structure on the composition of the vegetation was more similar among the three strata (Fig. 3). For all three layers, tree size and density were a primary factor that both structured communities and accounted for differences in occurrence between native and exotic species. Microtopography influenced community composition of all strata, and contributed to an axis that was marginally significant in separating exotics and natives. Unexpectedly, the diversity of native species was not important in the distribution of exotic herbs but did significantly affect the distribution of exotic trees (Table 2). In all of the ordinations, most species clustered around the centres of the ordination spaces and did not form obvious clusters. In all cases, exotic species were prominently among those that were outliers at the ends of the various gradients (1-3). Certain exotics, notably Allium vineale L., Hemerocallis fulva (L.) L., Artemisia vulgaris L. and Polygonum cuspidatum Sieb. & Zucc., were frequently the outliers in several or all of the ordinations, whereas other exotics, such as Microstegium vimineum (Trin.) A. Camus, Solanum dulcamara L. and Polygonum hydropiper L., occurred near the centre of all of the ordinations, overlapping with many of the native species in response to the environmental gradients. site-based patterns Overall amount of invasion was measured by the absolute number of exotic species (species richness) and the relative fraction of the site flora represented by exotics. These two metrics may respond differently to environmental factors than the patterns of community composition captured by the ordination analyses. The total number of exotic species was responsive to wetland area and adjoining land use but the effects were mostly seen only when large sites (> 100 ha) were included in the analysis (Table 3 and Fig. 4). Surprisingly, the presence of adjacent industrial land reduced the number of exotic species over all sizes of wetlands. The fraction of the flora that was exotic was more strongly affected by area than absolute richness, as the relationship held for the set of small wetlands as well as the entire set of sites. In addition, the presence of vegetated upland adjacent to the wetland reduced relative invasion over all wetland sizes. Table 3. Spearman correlation coefficients (rho) and significance level (probability) between measures of species composition and environmental descriptors of the sites Invasion indicator Environmental factor Rho Probability Exotic richness Area (all sites) –0·503 0·02 Area (sites < 100 ha) NS Industrial land use (all sites) –0·503 0·02 Industrial land use (sites < 100 ha) –0·462 0·05 Open land use (all sites) –0·530 0·014 Open land use (sites < 100 ha) NS Fraction exotic Area (all sites) –0·721 0·002 Area (sites < 100 ha) –0·623 0·004 % sand 0·559 0·012 Open land use (all sites) –0·700 0·0004 Open land use (sites < 100 ha) –0·623 0·004 Native richness % sand –0·405 0·042 % clay 0·610 0·009 Area (all sites) 0·484 0·026 Area (sites ≤ 100 ha) 0·600 0·007 Figure 4Open in figure viewerPowerPoint Relationship of exotic species richness (number of species in 10 312-m2 plots site−1) and fraction of flora that is exotic with site area, for (a) and (c) all sites, and (b) and (d) sites of less than 100 ha in extent. Soil texture affected the fraction of the flora that was exotic but not the absolute number of exotic species. In contrast, both soil texture and site area affected the number of native species over the range of site areas. Soil texture had contrasting effects on native and exotic species. Native species increased with increasing clay and decreasing sand content, probably reflecting the increased presence of wetland-dependent plants such as sedges and rushes, whereas exotic species increased with increasing sand content, probably reflecting the better drainage in sandy soils and the fact that most of the exotics were only moderately to slightly tolerant of wetland conditions (Ehrenfeld et al. 2003). Notably, neither the total number of exotic species nor the fraction of exotics in the flora was significantly correlated with the number of native species. Analyses by vegetation stratum also failed to find any relationship between either measure of invasion and native species richness. In addition, neither measure of invasion was related to indicators of direct human presence (either adjacent population density or the frequency of trails within the sites). species' responses to environmental factors In striking contrast to the results of measures of overall site invasion (Table 3), the probability of occurrence of eight of the 12 species for which logistic regressions could be calculated were significantly related to the native richness of th
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