The invasion paradox describes the co-occurrence of independent lines of support for both a negative and a positive relationship between native biodiversity and the invasions of exotic species. The paradox leaves the implications of native-exotic species richness relationships open to debate: Are rich native communities more or less susceptible to invasion by exotic species? We reviewed the considerable observational, experimental, and theoretical evidence describing the paradox and sought generalizations concerning where and why the paradox occurs, its implications for community ecology and assembly processes, and its relevance for restoration, management, and policy associated with species invasions. The crux of the paradox concerns positive associations between native and exotic species richness at broad spatial scales, and negative associations at fine scales, especially in experiments in which diversity was directly manipulated. We identified eight processes that can generate either negative or positive native-exotic richness relationships, but none can generate both. As all eight processes have been shown to be important in some systems, a simple general theory of the paradox, and thus of the relationship between diversity and invasibility, is probably unrealistic. Nonetheless, we outline several key issues that help resolve the paradox, discuss the difficult juxtaposition of experimental and observational data (which often ask subtly different questions), and identify important themes for additional study. We conclude that natively rich ecosystems are likely to be hotspots for exotic species, but that reduction of local species richness can further accelerate the invasion of these and other vulnerable habitats.

Some theories and experimental studies suggest that areas of low plant species richness may be invaded more easily than areas of high plant species richness. We gathered nested-scale vegetation data on plant species richness, foliar cover, and frequency from 200 1-m 2 subplots (20 1000-m 2 modified-Whittaker plots) in the Colorado Rockies (USA), and 160 1-m 2 subplots (16 1000-m 2 plots) in the Central Grasslands in Colorado, Wyoming, South Dakota, and Minnesota (USA) to test the generality of this paradigm.At the 1-m 2 scale, the paradigm was supported in four prairie types in the Central Grasslands, where exotic species richness declined with increasing plant species richness and cover. At the 1-m 2 scale, five forest and meadow vegetation types in the Colorado Rockies contradicted the paradigm; exotic species richness increased with native-plant species richness and foliar cover. At the 1000-m 2 plot scale (among vegetation types), 83% of the variance in exotic species richness in the Central Grasslands was explained by the total percentage of nitrogen in the soil and the cover of native plant species. In the Colorado Rockies, 69% of the variance in exotic species richness in 1000-m 2 plots was explained by the number of native plant species and the total percentage of soil carbon.At landscape and biome scales, exotic species primarily invaded areas of high species richness in the four Central Grasslands sites and in the five Colorado Rockies vegetation types. For the nine vegetation types in both biomes, exotic species cover was positively correlated with mean foliar cover, mean soil percentage N, and the total number of exotic species. These patterns of invasibility depend on spatial scale, biome and vegetation type, spatial autocorrelation effects, availability of resources, and species-specific responses to grazing and other disturbances. We conclude that: (1) sites high in herbaceous foliar cover and soil fertility, and hot spots of plant diversity (and biodiversity), are invasible in many landscapes; and (2) this pattern may be more closely related to the degree resources are available in native plant communities, independent of species richness. Exotic plant invasions in rare habitats and distinctive plant communities pose a significant challenge to land managers and conservation biologists.

Some theories and experimental studies suggest that areas of low plant species richness may be invaded more easily than areas of high plant species richness. We gathered nested‐scale vegetation data on plant species richness, foliar cover, and frequency from 200 1‐m2 subplots (20 1000‐m2 modified‐Whittaker plots) in the Colorado Rockies (USA), and 160 1‐m2 subplots (16 1000‐m2 plots) in the Central Grasslands in Colorado, Wyoming, South Dakota, and Minnesota (USA) to test the generality of this paradigm. At the 1‐m2 scale, the paradigm was supported in four prairie types in the Central Grasslands, where exotic species richness declined with increasing plant species richness and cover. At the 1‐m2 scale, five forest and meadow vegetation types in the Colorado Rockies contradicted the paradigm; exotic species richness increased with native‐plant species richness and foliar cover. At the 1000‐m2 plot scale (among vegetation types), 83% of the variance in exotic species richness in the Central Grasslands was explained by the total percentage of nitrogen in the soil and the cover of native plant species. In the Colorado Rockies, 69% of the variance in exotic species richness in 1000‐m2 plots was explained by the number of native plant species and the total percentage of soil carbon. At landscape and biome scales, exotic species primarily invaded areas of high species richness in the four Central Grasslands sites and in the five Colorado Rockies vegetation types. For the nine vegetation types in both biomes, exotic species cover was positively correlated with mean foliar cover, mean soil percentage N, and the total number of exotic species. These patterns of invasibility depend on spatial scale, biome and vegetation type, spatial autocorrelation effects, availability of resources, and species‐specific responses to grazing and other disturbances. We conclude that: (1) sites high in herbaceous foliar cover and soil fertility, and hot spots of plant diversity (and biodiversity), are invasible in many landscapes; and (2) this pattern may be more closely related to the degree resources are available in native plant communities, independent of species richness. Exotic plant invasions in rare habitats and distinctive plant communities pose a significant challenge to land managers and conservation biologists.

Net primary production (NPP), the difference between CO2 fixed by photosynthesis and CO2 lost to autotrophic respiration, is one of the most important components of the carbon cycle. Our goal was to develop a simple regression model to estimate global NPP using climate and land cover data. Approximately 5600 global data points with observed mean annual NPP, land cover class, precipitation, and temperature were compiled. Precipitation was better correlated with NPP than temperature, and it explained much more of the variability in mean annual NPP for grass- or shrub-dominated systems (r2 = 0.68) than for tree-dominated systems (r2 = 0.39). For a given precipitation level, tree-dominated systems had significantly higher NPP (approximately 100-150 g C m(-2) yr(-1)) than non-tree-dominated systems. Consequently, previous empirical models developed to predict NPP based on precipitation and temperature (e.g., the Miami model) tended to overestimate NPP for non-tree-dominated systems. Our new model developed at the National Center for Ecological Analysis and Synthesis (the NCEAS model) predicts NPP for tree-dominated systems based on precipitation and temperature; but for non-tree-dominated systems NPP is solely a function of precipitation because including a temperature function increased model error for these systems. Lower NPP in non-tree-dominated systems is likely related to decreased water and nutrient use efficiency and higher nutrient loss rates from more frequent fire disturbances. Late 20th century aboveground and total NPP for global potential native vegetation using the NCEAS model are estimated to be approximately 28 Pg and approximately 46 Pg C/yr, respectively. The NCEAS model estimated an approximately 13% increase in global total NPP for potential vegetation from 1901 to 2000 based on changing precipitation and temperature patterns.

An increase in the number of citizen science programs has prompted an examination of their ability to provide data of sufficient quality. We tested the ability of volunteers relative to professionals in identifying invasive plant species, mapping their distributions, and estimating their abundance within plots. We generally found that volunteers perform almost as well as professionals in some areas, but that we should be cautious about data quality in both groups. We analyzed predictors of volunteer success (age, education, experience, science literacy, attitudes) in training-related skills, but these proved to be poor predictors of performance and could not be used as effective eligibility criteria. However, volunteer success with species identification increased with their self-identified comfort level. Based on our case study results, we offer lessons learned and their application to other programs and provide recommendations for future research in this area.

have generally supported a long-held ecological paradigm that, in natural areas, habitats of low plant diversity are more vulnerable to plant invasions than areas of high diversity (Elton 1958). This theory contends that, through the process of competitive exclusion (Grime 1973), species-rich areas are "immunized" against invasion by foreign plants through the preemption of resources by native species (Tilman 1999). One recent study by Kennedy et al. (2002) found that "diverse communities will probably require minimal maintenance and monitoring because they are generally effective at excluding undesirable invaders".It would be comforting to believe that areas with many plant species are less prone to invasion than those with fewer species. Botanical hotspots such as wetlands, riparian zones, Mediterranean environments, subtropical coastal areas, and tallgrass prairies might repel the frequent arrival of plants from other regions or countries. Mechanical, chemical, and biological control techniques might be limited to heavily invaded, species-poor areas, with little danger of compromising unique assemblages of native plant species.Recently, some ecologists have begun to question this perspective (Stohlgren et al. 1997(Stohlgren et al. , 1999Levine and D'Antonio 1999;Levine 2000). Casual observations have shown highly invasive plant species, including tamarisk (Tamarix spp.), Russian olive (Elaeagnus angustifolia), purple loostrife (Lythrum salicaria), and Chinese tallow (Sapium sebiferum), becoming widely established in species-rich riparian zones and wetlands. Until now, however, large, carefully collected data sets from natural landscapes have been unavailable. These are useful for comparing local, landscape, regional, and national patterns to those observed under carefully controlled conditions, such as heavily manipulated, small-scale, experimental plots protected from disturbance (Knops et al. 1997;Naeem et al. 2000;Kennedy et al. 2002). MethodsWe evaluated two large independent data sets on the distribution of native and non-native plant species. The first set was gathered from 316 large vegetation-monitoring plots in eight states, which are part of the USDA Forest Service 's Forest Health Monitoring Program (Cline et al. 1995). The plots are systematically spaced throughout the US (one every 63 942 ha), and the numbers vary by state: Colorado (33), Delaware (39), Michigan (71), Oregon (44), Pennsylvania (81), Virginia (15), Washington (12), and Wyoming (21). Each plot consists of four 168-m 2 subplots, with three 1-m 2 quadrats in each subplot. Between 1997 and 2001, all the plots were sampled once every summer. Data were collected on the presence and cover of native and non-native species in each quadrat and species presence in the subplots.The second data set was gathered over the past 20 years by the Biota of North America Program (www.bonap.org) at the University of North Carolina, Chapel Hill. Data were available for 44 of 50 US states, and were based on over 229 000 records of native and non-native p...

A standardized sampling technique for measuring plant diversity is needed to assist in resource inventories and for monitoring long-term trends in vascular plant species richness. The widely used 'Whittaker plot' (Shmida 1984) collects species richness data at multiple spatial scales, using 1 m 2, 10 m 2, and 100 m 2 subplots within a 20 m x 50 m (1000 m 2) plot, but it has three distinct design flaws involving the shape and placement of subplots. We modified and tested a comparable sampling design (Modified-Whittaker plot) that minimizes the problems encountered in the original Whittaker design, while maintaining many of its attractive attributes. We overlaid the two sampling methods in forest and prairie vegetation types in Larimer County, Colorado, USA (n = 13 sites) and Wind Cave National Park, South Dakota, USA (n = 19 sites) and showed that the modified design often returned significantly higher (p < 0.05) species richness values in the 1 m 2, 10 m 2, and 100 m 2 subplots. For all plots, except seven ecotone plots, there was a significant difference (p < 0.001) between the Whittaker plot and the Modified-Whittaker plot when estimating the total number of species in the 1000 m 2 plots based on linear regressions of the subplot data: the Whittaker plot method, on average, underestimated plant species richness by 34%. Species-area relationships, using the Modified-Whittaker design, conformed better to published semilog relationships, explaining, on average, 92% of the variation. Using the original Whittaker design, the semilog species-area relationships were not as strong, explaining only 83% of the variation, on average. The Modified-Whittaker plot design may allow for better estimates of mean species cover, analysis of plant diversity patterns at multiple spatial scales, and trend analysis from monitoring a series of strategically-placed, long-term plots.

Observations from islands, small‐scale experiments, and mathematical models have generally supported the paradigm that habitats of low plant diversity are more vulnerable to plant invasions than areas of high plant diversity. We summarize two independent data sets to show exactly the opposite pattern at multiple spatial scales. More significant, and alarming, is that hotspots of native plant diversity have been far more heavily invaded than areas of low plant diversity in most parts of the United States when considered at larger spatial scales. Our findings suggest that we cannot expect such hotspots to repel invasions, and that the threat of invasion is significant and predictably greatest in these areas.