Habitat Invasibility and Dominance by Alien Annual Plants in the Western Mojave Desert

Patterns of habitat invasibility and alien dominance, respectively measured as species richness and biomass of alien annual plants, were evaluated in association with four habitat factors at the Desert Tortoise Research Natural Area (DTNA) in the western Mojave Desert, USA. Habitat factors varied in levels of disturbance outside (high) and inside (low) the DTNA, and in levels of soil nutrients in washlet (high) and hummock (low) topographic positions, in Larrea-north (high), Larrea-south (medium), and interspace (low) microhabitats near creosote bushes (Larrea tridentata), and during 1995 when rainfall was 207% (high) and 1994 when rainfall was 52% (low) of the long-term average. Dominant alien plants included the annual grasses Bromus rubens, Bromus trinii, and Schismus spp., and the forb Erodium cicutarium. Species richness and dominance of alien annual plants were slightly higher where disturbance was high, and much higher where soil nutrients were high. B. rubens and B. trinii were most dominant in washlets and in the Larrea-north microhabitats during both years. These two species evolved in mesic ecosystems, and appeared to be particularly limited by soil nutrients at this site. Schismus spp. and E. cicutarium were also most dominant in washlets, but their dominance varied between interspaces in 1994 and the Larrea-south microhabitat in 1995. Monitoring to detect the invasion of new annual plants should focus on regions of high rainfall and nitrogen deposition and on washes and beneath-canopy microhabitats. The ecological range of each alien species should be evaluated separately, because their evolutionary origins may greatly affect their patterns of invasion and dominance in the Mojave Desert. This revised version was published online in July 2006 with corrections to the Cover Date.     

Niche opportunities and invasion dynamics in a desert annual community

Although many factors influence the ability of exotics to invade successfully, most studies focus on only a few variables to explain invasion; attempts at theoretical synthesis are largely untested. The niche opportunities framework proposes that the demographic success of an invader is largely affected by the availability of resources and the abundance of its enemies. Here, we use a 31‐year study from a desert ecosystem to examine the niche opportunities framework via the invasion of the annual plant Erodium cicutarium. While the invader remained rare for two decades, a decline in granivory combined with an ideal climate window created an opportunity for E. cicutarium to escape control and become the dominant annual plant in the community. We show that fluctuations in consumption and resources can create niche opportunities for invaders and highlight the need for additional long‐term studies to track the influence of changing climate and community dynamics on invasions.     

Does Invasion of Exotic Plants Promote Invasion of Exotic Flower Visitors? A Case Study from the Temperate Forests of the Southern Andes

Habitat disturbance, particularly of human origin, promotes the invasion of exotic plants, which in turn might foster the invasion of alien-interacting animals. Here we assess whether the invasion of exotic plants – mostly mediated by habitat disturbance – facilitates the invasion of exotic flower visitors in temperate forests of the southern Andes, Argentina. We recorded visit frequencies and the identity of visitors to the flowers of 15 native and 15 exotic plant species occurring in different highly disturbed and less disturbed habitats. We identified three alien flower visitors, the hymenopterans Apis mellifera, Bombus ruderatus, and Vespula germanica. We found significantly more visitation by exotic insects in disturbed habitats. This pattern was explained, at least in part, by the association between alien flower visitors and flowers of exotic plants, which occurred more frequently in disturbed habitats. However, this general pattern masked different responses between the two main alien flower visitors. Apis mellifera exploited almost exclusively the flowers of a subset of herbaceous exotic plants that thrive under disturbance, whereas B. ruderatus visited equally flowers of both exotic and native plants in both disturbed and undisturbed habitats. We did not find any strong evidence that flowers of exotic plants were more generalist than those of native plants, or that exotic flower visitors were more generalist than their native counterparts. Our results suggest that alien plant species could facilitate the invasion of at least some exotic flower visitors to disturbed habitats. Because flowering plants as well as flower visitors benefit from this mutualism, this association may enhance, through a positive feedback, successful establishment of both exotic partners.     

Interactions among unrelated species: Granivorous rodents,a parasitic fungus,and a shared prey species

Summary Granivorous rodents and a parasitic fungus in the Sonoran Desert utilize a common prey species, Erodium cicutarium, a desert annual plant. Experimental removal of rodents from field exclosures resulted in significantly higher densities of E. cicutarium. Fungal infection was significantly higher in the absence of rodents, suggesting that, while they do not interact directly, rodents and the fungus affect each other's densities by their use of a common prey species.     

Potential for higher invasiveness of the alien Oxalis pes-caprae on islands than on the mainland

The higher vulnerability of islands to invasions compared to mainland areas has been partially attributed to a simplification of island communities, with lower levels of natural enemies and competitors on islands conferring vacant niches for invaders to establish and proliferate. However, differences in invader life-history traits between populations have received less attention. We conducted a broad geographical analysis (i.e. 1050 km wide transect) of plant traits comparing insular and mainland populations to test the hypothesis that alien plants from insular populations have the potential for higher invasiveness than their alien mainland counterparts. For this purpose plants of the annual geophyte Oxalis pes-caprae were grown from bulbs collected in the Balearic islands and the Spanish mainland under common greenhouse conditions. There were no significant differences in bulb emergence and plant survival between descendants from insular and mainland populations. However, Oxalis descendants from insular populations produced 20% more bulbs without reducing allocation to bulb size, above-ground biomass or flowering than descendants from mainland populations. Based on the lack of sexual reproduction in Oxalis and the dependence of invasion on bulb production, our study suggests that the higher occurrence of Oxalis in the Balearic islands than in the Spanish mainland can partially be explained by genetically based higher propagation potential of insular populations compared to mainland populations.     

苋科(广义)入侵植物墙生藜在中国的新记录

外来物种的归化和入侵对全球环境和社会发展造成了严重影响，已成为当今各国生物多样性管理和生态保护中所面临的全球性问题。我国是遭受外来入侵危害最为严重的国家之一，在外来物种入侵的预警、管理和治理等方面形势严峻。基于野外调查和文献研究，该文报道了苋科(广义)入侵植物墙生藜[Chenopodiastrum murale(L.) S. Fuentes, Uotila&Borsch]在中国的新记录。墙生藜原产于地中海地区，现已扩散到欧洲、美洲、非洲和大洋洲的40多个国家，是一种危害较大的外来入侵植物，同时也是我国海关和检验检疫部门明确规定禁止入境的检疫性有害生物，现于云南省昆明市呈贡区发现该外来入侵植物。该文对其形态特征进行了详细描述，简要介绍了其分类历史，并提供了可供鉴定比对的野外生态照片；此外，对墙生藜可能的传入途径进行了分析，对其危害和风险作了简要评估。该物种的新发现说明我国外来入侵生物的本底调查还存在不足。     

Predicting plant invasions under climate change: are species distribution models validated by field trials?

Climate change may facilitate alien species invasion into new areas, particularly for species from warm native ranges introduced into areas currently marginal for temperature. Although conclusions from modelling approaches and experimental studies are generally similar, combining the two approaches has rarely occurred. The aim of this study was to validate species distribution models by conducting field trials in sites of differing suitability as predicted by the models, thus increasing confidence in their ability to assess invasion risk. Three recently naturalized alien plants in New Zealand were used as study species (Archontophoenix cunninghamiana, Psidium guajava and Schefflera actinophylla): they originate from warm native ranges, are woody bird‐dispersed species and of concern as potential weeds. Seedlings were grown in six sites across the country, differing both in climate and suitability (as predicted by the species distribution models). Seedling growth and survival were recorded over two summers and one or two winter seasons, and temperature and precipitation were monitored hourly at each site. Additionally, alien seedling performances were compared to those of closely related native species (Rhopalostylis sapida, Lophomyrtus bullata and Schefflera digitata). Furthermore, half of the seedlings were sprayed with pesticide, to investigate whether enemy release may influence performance. The results showed large differences in growth and survival of the alien species among the six sites. In the more suitable sites, performance was frequently higher compared to the native species. Leaf damage from invertebrate herbivory was low for both alien and native seedlings, with little evidence that the alien species should have an advantage over the native species because of enemy release. Correlations between performance in the field and predicted suitability of species distribution models were generally high. The projected increase in minimum temperature and reduced frosts with climate change may provide more suitable habitats and enable the spread of these species.     

Ecological dynamics of Quercus dominated woodlands in California and southern Spain: a state-transition model

There are many similarities between Spanish and Californian Quercus woodlands and savanna. Both are located in Mediterranean climate zones, and are used predominantly for livestock grazing. The Californian overstory is dominated by one or a combination of five Quercus species and their hybrids: Quercus douglasii H.&A., Q. agrifolia Nee., Q. wislizenii A.DC., Q. lobata Nee., and Q. englemennii Greene (blue, coast live, interior live, valley, and Englemann oaks). In southern Spain and Portugal, Quercus woodland overstory is predominantly one or a combination of two Quercus species, Quercus ilex L. (holm oak) and Quercus suber L. (cork oak). The underlying natural and semi-natural ecological dynamics of the Quercus woodlands of Spain and California are different, and it follows that the management practices employed also differ. The greatest point of contrast between California and Spain is in the intensity and diversity of management goals and practices. A state-transition model for comparing the ecological dynamics of Quercus woodlands and savanna in California and southern Spain is developed and examined. The highly simplified model is an analytic tool of use in organizing research and developing management practices. States are reached and maintained in different ways in Spain and California, but their appearance and their function in each landscape are quite similar.     

Phenotypic trait differences between Iris pseudacorus in native and introduced ranges support greater capacity of invasive populations to withstand sea level rise

Aim

Tidal wetlands are greatly impacted by climate change, and by the invasion of alien plant species that are being exposed to salinity changes and longer inundation periods resulting from sea level rise. To explore the capacity for the invasion of Iris pseudacorus to persist with sea level rise, we initiated an intercontinental study along estuarine gradients in the invaded North American range and the native European range.

Location

San Francisco Bay-Delta Estuary; California, USA and Guadalquivir River Estuary; Andalusia, Spain.

Methods

We compared 15 morphological, biochemical, and reproductive plant traits within populations in both ranges to determine if specific functional traits can predict invasion success and if environmental factors explain observed phenotypic differences.

Results

Alien I. pseudacorus plants in the introduced range had more robust growth than plants in the native range. The vigour of the alien plants was reflected by expression of higher leaf water content, fewer senescent leaves per leaf fan, and more carbohydrate storage reserves in rhizomes than plants in the native range. Moreover, alien plants tended to show higher specific leaf area and seed production than native plants. I. pseudacorus plants in the introduced range were less affected by increasing salinity and were exposed to deeper inundation water along the estuarine gradient than those in the native range.

Main Conclusions

Functional trait differences suggest mature populations of I. pseudacorus in the introduced range have greater adapted capacity to adjust to environmental stresses induced by rising sea level than those in the native range. Knowledge of these trait responses can be applied to improve risk assessments in invaded estuaries and to achieve climate-adapted conservation goals for conservation of the species in its native range.     

Genetic and historical evidence disagree on likely sources of the Atlantic amethyst gem clam Gemma gemma (Totten, 1834) in California

Aim Historical information about source populations of invasive species is often limited; therefore, genetic analyses are used. We compared inference about source populations from historical and genetic data for the oyster‐associated clam, Gemma gemma that invaded California from the USA Atlantic coast. Location Mid‐Atlantic (North Carolina, Maryland), Northeastern (New Jersey, New York, Massachusetts) and the California coasts (Elkhorn Slough, San Francisco Bay, Bolinas Lagoon, Tomales Bay, Bodega Harbor). Methods The documented history of transplantation of Eastern oysters to California was reviewed. Cytochrome c oxidase subunit I (COI) sequences from recent and archived clams were examined in a haplotype network. We used AMOVA to detect geographic genetic structure and a permutation test for significant reductions in diversity. Results Chesapeake Bay oysters were transplanted to New York prior to shipment to San Francisco Bay and from there to peripheral bays. Gemma in the Northeastern and Mid‐Atlantic regions were genetically differentiated. In California, populations in Bodega Harbor and Tomales Bay were genetically similar to those in the Mid‐Atlantic area while clams in San Francisco Bay, Elkhorn Slough and Bolinas Lagoon resembled populations in the Northeastern region. In California, genetic variation was not highest in San Francisco Bay despite greater magnitude of oyster plantings. Haplotypes varied over time in native and introduced populations. Main Conclusions Historical records and inferences from genetics agree that both Northeastern and Mid‐Atlantic regions were sources for Gemma in California. Only complex genetic hypotheses reconcile the strong segregation of haplotypes in California to the historical evidence of mixing in their proximate source (New York). These hypotheses include sorting of mixtures of haplotypes or selection in non‐native areas. Haplotype turnover in San Francisco and Massachusetts samples over time suggests that the sorting hypothesis is plausible. We suggest, however, that Gemma was introduced independently and recently to Tomales Bay and Bodega Harbor.     

Trophic consequences of non‐native species for commercial fish in a backwater bay of the Three Gorges Reservoir,Southwest China

Trophic niche overlap in native and alien fish species can lead to competitive interactions whereby non‐native fishes outcompete indigenous individuals and eventually affect the viability of natural populations. The species Erythroculter mongolicus and Erythroculter ilishaeformis (belonging to the Culterinae), which are two commercially important fish species in the backwater bay of the Pengxi River in the Three Gorges Reservoir (TGR), were threatened by competition from the non‐native Coilia ectenes (lake anchovy). The latter is an alien species introduced into the lower reaches of the Yangtze River in China and now widespread in the TGR. The trophic consequences of non‐native lake anchovy invasion for E. mongolicus and E. ilishaeformis were assessed using stable isotope analysis (δ¹³C and δ¹⁵N) and associated metrics including the isotopic niche, measured as the standard ellipse area. The trophic niche of native E. mongolicus had little overlap (<15%) with the alien fish species and was significantly reduced in size after invasion by lake anchovy. This suggests that E. mongolicus shifted to a more specialized diet after invasion by lake anchovy. In contrast, the trophic niche overlap of native fish E. ilishaeformis with the alien fish species was larger (>50%) and the niche was obviously increased, implying that fish in this species exploited a wider dietary base to maintain their energetic requirements. Thus, marked changes for the native E. mongolicus and E. ilishaeformis were detected as the trophic consequences of invasion of non‐native lake anchovy.     

Predicting the outcome of competition along a nitrogen gradient – a case study with Erodium cicutarium and Geranium pusillum

In order to understand the effect of a nitrogen gradient on the intraspecific and interspecific competition between plants, the two species Erodium cicutarium and Geranium pusillum were grown in a response-surface competion experiment at three different densities along a nitrogen gradient consisting of four different nitrogen levels. Seed set data were estimated from biomass measurements and fitted to a generalized hyperbolic competition model, and the probabilities of different ecological scenarios were calculated. The model predicted that E. cicutarium is competitively superior along most of the nitrogen gradient. This prediction was contrary to our prior expectations which were based on the occurance of the two species in natural habitats.     

Are boreal ecosystems susceptible to alien plant invasion? Evidence from protected areas

Although biological invasion by alien species is a major contributor to loss of indigenous biological diversity, few studies have examined the susceptibility of the boreal biome to invasion. Based on studies of other ecosystems, we hypothesized that alien plants will be restricted to disturbed areas near human activity and will not be found in natural areas of boreal ecosystems in Gros Morne National Park (Canada), a protected area experiencing a wide range of disturbance regimes. The distribution of alien plants in the region was evaluated using surveys, and study sites were established in naturally and anthropogenically disturbed habitats that had been invaded. Within study sites, randomization tests evaluated the importance of disturbance to alien plant invasion by examining changes in environmental conditions and species abundance within various disturbance regimes, while the importance of site characteristics limiting the distribution of alien plants were examined using Canonical Correspondence Analysis. Consistent with studies in a variety of biomes, areas of high disturbance and human activity had the greatest abundance of resources and the highest percentage of alien species. However, contrary to our hypothesis, natural areas of boreal ecosystems were found susceptible to alien plant invasion. Vegetation types vulnerable to invasion include forests, riparian areas, fens, and alpine meadows. Natural disturbance occurring in these vegetation types caused increases in bare ground and/or light availability facilitating alien plant invasion. Although high soil pH was associated with alien plants in these areas, disturbance was not found to cause changes in soil pH, suggesting susceptibility to invasion is pre-determined by bedrock geology or other factors influencing soil pH. Moose (Alces alces), a non-native herbivore, acts as the primary conduit for alien plant invasion in GMNP by dispersing propagules and creating or prolonging disturbance by trampling and browsing vegetation. The recurrent nature of disturbance within the boreal biome and its interaction with site conditions and herbivores enables alien plants to persist away from areas of high human activity. Managers of natural lands should monitor such interactions to decrease the invasion potential of alien plants.     

Plant zonation at salt marshes of the endangered cordgrass <Emphasis Type="Italic">Spartina maritima</Emphasis> invaded by <Emphasis Type="Italic">Spartina densiflora</Emphasis>

The South American cordgrass Spartina densiflora is invading European salt marshes getting into contact with the indigenous and endangered low-marsh dominant, Spartina maritima. This work describes the evolution of the plant zonation during 7 years in a marsh of S. maritima invaded by S. densiflora. S. maritima appeared throughout the whole intertidal gradient from 1.72 to 3.33 m over Spanish Hydrographic Zero (SHZ), showing its higher biomasses and shoot densities at low elevations. In contrast, S. densiflora only invaded upper areas (>+2.59 m SHZ) at the centre of circular tussocks of S. maritima. Above-ground biomass of S. maritima dropped drastically at maximum occupation of space by the alien, and its shoot density and above-ground biomass decreased at S. densiflora zone during the study. The competitive potential of S. densiflora was reflected in high above- and below-ground biomass and shoot densities, accompanied by elevated wrack accumulation and the absence of other marsh plants presented together with S. maritima from areas dominated by S. densiflora. S. densiflora altered the native vegetational zonation pattern through the invasion of the centre of S. maritima tussocks; however, the alien invasion may be limited by the presence of the autochthonous cordgrass at lower elevations. Handling editor: Luis Mauricio Bini     

不同入侵程度下飞机草对喀斯特地区土壤理化性质的影响

外来生物入侵威胁着全球的生物多样性和生态系统的功能,研究外来入侵植物对土壤理化性质的影响有助于理解外来入侵植物的入侵机制。以广西喀斯特地区飞机草（Chromolaena odorata）入侵生境为研究对象,比较分析了不同飞机草入侵程度下（对照、轻度入侵、中度入侵、重度入侵）土壤物理和化学指标变化特征。结果表明：随着飞机草的入侵程度加重,土壤容重显著增加,轻度入侵、中度入侵和重度入侵生境下土壤容重分别比对照增加了10.3%、16.7%、22.3%,土壤非毛管孔隙度、总孔隙度和土壤含水量显著降低,土壤毛管孔隙度无显著变化;飞机草入侵对土壤pH值无显著影响,随着入侵程度加重,土壤速效磷和速效钾的含量显著升高,在重度入侵生境下土壤速效磷和速效钾含量分别比对照增加了4.3倍、3.8倍,土壤全氮和有机质含量显著降低。飞机草入侵显著改变了土壤理化性状,导致喀斯特地区土壤物理结构退化,生态系统水土保持能力下降,同时飞机草通过改变土壤养分循环提高了土壤可直接利用养分的水平,创造对自身有利的土壤环境,进而促进其生长和扩散。     

The diet of native and invasive fish species along the eastern Mediterranean coast (Osteichthyes)

The opening of the Suez Canal in 1869 enabled the invasion of more than 100 alien fish species into the Mediterranean. The aim of the present study was to compare the diet of native and alien fish species and to identify possibly shared food resources. We examined the diet composition of 13 of the most abundant fish species (6 alien, 7 native) on shallow soft bottom off southern Israel. All 13 species are omnivorous/carnivorous. The native fish exhibited a wider diversity of food types than the aliens. Alien fish prey upon and are preyed by native species as well as by other alien fish. A high level of diet overlap was found among some species, the aliens Saurida lessepsianus and Scomberomorus commerson overlapped with the native Synodus saurus; and the alien Nemipterus randalli with the native species Pagrus caeruleostictus, Lithognathus mormyrus and Pagellus erythrinus. The identified diet overlap is discussed, and the possibility of competitive interactions between these species is considered.     

Clarifying marine invasions with molecular markers: an illustration based on mtDNA from mistaken calyptraeid gastropod identifications

Species invasions are occurring at an increasing rate in coastal environments. Accurately identifying introductions is a critical issue to take full advantage of the new invasion databases. Further, life history differences between morphologically comparable species may require that different management strategies be instigated to effectively control different species. Facing this problem, we used molecular approaches and documented a case of mistaken identification in a group of marine invertebrates, the calyptraeid gastropods. Members of this group have repeatedly and successfully invaded new habitats after anthropogenic introduction, especially in estuaries and bays on the west coast of the United States of America. For example, Crepidula fornicata, native to the east coast of the USA, has been reported from at least five USA west coast estuaries. We sequenced a fragment of the COI gene of a sample of putative C. fornicata from Humboldt Bay, California. By constructing a phylogeny of these and other calpytraeid gastropod sequences, we discovered that the individuals were C. convexa, the convex slippershell. In contrast to C. fornicata, C. convexa has large, demersal eggs and larvae are well developed at hatching. Its potential for dispersal is therefore lower as compared to C. fornicata and therefore any strategy to manage the invasion should take this into account. In the present study, we demonstrated the utility of molecular tools that can be used by non-taxonomic experts, to quickly and accurately identify alien species – an important first step in any study of invasion biology.     

Western pine beetle voltinism in a changing California climate

1. Recent hot droughts in California resulted in ponderosa pine (Pinus ponderosa) mortality attributed to drought and western pine beetle (WPB, Dendroctonus brevicomis). While drought alone can cause tree death, direct warming effects on WPB are a contributing factor. Research on WPB generation timing (voltinism), however, is lacking.
2. We monitored WPB tree attacks and adult emergence timing at two California sites and developed a degree-day model from field-observed data. Historical, contemporary, and future temperatures for several California sites were used with the model to examine trends in WPB voltinism.
3. Field data showed a single summer and an overwinter generation at a northern California site. As summer temperatures increased beyond 1900–1980 averages, the predicted number of full and partial WPB generations by 2021 had increased from ~2 annual (one summer and one overwinter) generations historically to ~2.3 at two northern California sites and from ~2.3 to ~3.2 at two warmer California sites.
4. Historical and contemporary data suggest winter warming was not sufficient for an additional generation overwinter. Instead, increases in generations were driven by summer and fall temperatures.
5. Unconstrained increases in the number of future annual generations will be limited by complex, but not well understood, WPB thermal adaptations. Increased knowledge of temperature-driven WPB population growth will improve forest vegetation models aimed at predicting ponderosa pine mortality in a changing climate.

     

Exotic plant invasions in an endemic‐rich habitat: The spread of an introduced Australian grass,Agrostis avenacea J. F. Gmel., in California vernal pools

Abstract The present abundance and historical spread of an exotic grass, Agrostis avenacea, is documented for California, USA, and for the vernal pools of San Diego County in particular. Agrostis avenacea is native to Australia where it is a common grass of ephemeral and fluctuating wetlands. California vernal pools, by reason of their extreme variability and high endemism, have been thought to be relatively resistant to invasion. The recent expansion of A. avenacea in San Diego County demonstrates, however, that the alleged resistance is probably a function of a relatively small pool of invaders and a low probability of targeted dispersal. Although A. avenacea is now abundant in pools with little current disturbance, human disturbance appears to have been a major factor that facilitated its initial establishment. This specific instance adds to the mounting evidence that there are probably few, if any, habitats immune to invasion.     

Comparing the reproductive biological characteristics of the alien invasive Coreopsis lanceolata to those of the non-invasive alien congener Coreopsis tinctoria

It is important to determine the characteristics associated with the success of alien species. However, there is no universal syndrome of invasion reproductive characteristics to explain the success of invasiveness has not been identified. Comparative studies on congeneric successful or non-successful alien species are beneficial for revealing the drivers of invasion success and predicting future distributions. Reproductive biological characteristics, including the mating system, phenology of flowering, floral syndrome, pollination biology and seed germination have been investigated in the invasive Coreopsis lanceolata and non-successful congener alien species Coreopsis tinctoria. The results revealed that C. lanceolata significantly attracted insect visitors by 11.67 times more than its congener, and the corolla density of C. lanceolata was positively correlated with the number of visiting insects. The high attraction of local insect bees Apis cerana guarantees the reproductive success and high seed production of C. lanceolata. Nevertheless, there was no versatile mating system, because its congener non-successful alien species C. lanceolata still successfully invaded due to the combination strategies of self-compatibility and presence of rhizomes. Furthermore, the ability of seed germination of C. lanceolata was lower than that of C. tinctoria, especially under drought and Al³⁺ stress, suggesting that germination performance can serve as a reference indicator. However, this was not an absolute trait for invasiveness. Hence, these germination traits might slow down the invasiveness of C. lanceolata in south China. These present findings highlight that the combination of self-incompatibility and rhizomes should be given attention in the risk assessment protocols for plant invasiveness.