Floristic regions of the world, from Takhtajan [ 27 ] . Region names and associated statistics are listed in Tables 2 and 3. Those shown here do not include several largely oceanic or archipelagic regions ignored in the present analysis. doi:10.1371/journal.pone.0003630.g001 

Floristic regions of the world, from Takhtajan [ 27 ] . Region names and associated statistics are listed in Tables 2 and 3. Those shown here do not include several largely oceanic or archipelagic regions ignored in the present analysis. doi:10.1371/journal.pone.0003630.g001 

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Biogeographic patterns of species invasions hold important clues to solving the recalcitrant 'who', 'where', and 'why' questions of invasion biology, but the few existing studies make no attempt to distinguish alien floras (all non-native occurrences) from invasive floras (rapidly spreading species of significant management concern), nor have invas...

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... (annually disturbed) ecosystems (34%), forests (29%), and wetland, riparian, and aquatic systems (13%, 13%, and 4%, respectively; Table 3). More than half of the woody invasive taxa (58%) are from the Eastern Asiatic region, and significantly more of the East Asian taxa (56%) are woody than expected based on the overall representation of woody invaders. At the other extreme, the invaders of four regions were significantly more likely to be herbaceous than the overall invasive pool, including those from the Sudano-Zambezian (100% herbaceous), Macronesian (96%), Saharo-Arabian (86%), and Mediterranean regions (80%; all P , 0.05; Table 3). Several regions exhibited significant habitat bias in their invasive representation (Table 3, Fig. 2). Annually disturbed, managed ecosystems are far more prone to invasion from Mediterranean plants than plants from the Eastern Asiatic region (P , 0.001; Fig. 2). Conversely, 41% (74/180) of the invasive taxa from East Asia invade forests, compared to only 7% (4/53) invaders from Macronesia and 29% of the invasive taxa overall (Table 3). Significant deviations in habitat representation among invaders from different source floras also include a greater representation of East Asian taxa in riparian habitats and very few Irano-Turanian species in aquatic habitats (Table 3). Major floristic patterns of the invasive pool of all habitat types are illustrated in Fig. 2. The composition of species growth form and duration is significantly different among native, alien, and invasive EUS floras, with departures being greatest between the invasive and alien pools (Table 4). Compared to the native flora, the alien flora is overrepresented by annuals, biennials, and vines, and under- represented by shrubs and grasses. Other growth form categories have remarkably similar representation in the native and alien floras, including the overall split between woody and herbaceous taxa (about 1 woody species in 5). In contrast, the growth form and duration composition of the invasive flora shows a strong departure from the alien flora. Invaders were significantly more likely to be perennial trees, shrubs, and vines, and thus much less likely to be herbaceous (65%) than both the native or alien floras (Table 4). The typical alien vascular plant of the Eastern Deciduous Forest biome of the Eastern U.S. is a European forb, either from the Circumboreal northern and central regions of Europe or the southern Mediterranean region. The clear European bias in non- native plants has been documented in many global floras by plant biogeographers [6,7,29] and ecological historians [30,31] and is referred to as the Imperialist Dogma [30]. This model asserts that the spread of European cultures since the Age of Discovery, including crops, weeds, and commensals, explains both the greater historical transport of European species to global floras and the greater ability of co-evolved European weeds to persist in landscapes dominated by agricultural practices that originated in southern Europe and the eastern Mediterranean [29,30,32]. When applied to all alien species, the Imperialist Dogma is supported in the present study by the biased representation of European species (including Circumboreal, Mediterranean, and Macronesian regions) in the non-native EUS flora; furthermore, the vast majority of invasive species in frequently disturbed habitats (weeds) stem from these regions. A central contribution of the present study, however, is to suggest that the Imperialist Dogma cannot be a general framework for plant invasions, because 1) alien species from Europe are less likely to be invasive than those from East Asia; 2) European species only dominate anthropogenic habitats such as managed agricultural areas, disturbed fields, waste places, and roadsides (Fig. 2)—and nonetheless as forests have greatly expanded in EUS over the past 150 years, plant invasions have increased; and 3) although alien species are typically European, the invasive flora is better described as Eurasian and is nearly as likely to come from central and east Asia as Europe (Table 2). Taken together, these observations suggest that the prevailing view of Europe as the ancestral cradle of plant invasions is only useful in so far as it describes the recent co-evolution of ‘weedy’ plants in historically novel human-dominated ecosystems [32]—a restricted set of conditions when viewed in the full context of plant invasions in a variety of disturbed and natural ecosystems worldwide. In contrast to the total alien flora, EUS alien invaders are commonly woody species from East Asia, perhaps better reflecting EUS landscapes as dominated by closed secondary forests. Indeed, if the composition of the alien flora is used as a null model for invader composition, taxa from some regions are significantly more likely to invade (Table 2). It should come as no surprise that invasive taxa are most likely to come from areas with climates that resemble those of EUS (Fig. 1)—all floristic regions of greater- than-expected invader representation ( . 17%) are those of extra- tropical distribution. However, climate similarity is not sufficient to predict the bias in invader distribution among floristic regions. Part of this variance is attributable to native range size, in that species with native ranges that span continents are represented in many historically isolated floras, and native range size is well correlated with invasive potential [11,33] (Fig. 3). This is particularly true of the high invader contribution of more arid temperate regions (including the Saharo-Arabian, Irano-Turanian, and Mediterranean)—regions with almost no EUS invaders endemic to them. Of particular interest is that, although overall those alien taxa endemic to particular floristic regions are very unlikely to be invaders (13%), those endemic to East Asia are nearly as invasive as the entire invader pool from East Asia (25% compared to 29%). Of those other regions with at least 20 EUS alien taxa that are natively endemic, only two—the Circumboreal and Chile-Patagonian regions—have at least 1 in 10 of those as invasive (11% and 10%, respectively), despite similar climates to the EUS existing on all continents [34]. Why is the flora—and in particular the woody forest flora—of East Asia so unusually invasive in the Eastern U.S.? From a broad historical perspective, colonization of EUS mesic forest habitat by East Asian plants is hardly novel. The late Pleistocene origin of the Eastern Deciduous Forest is thought largely to stem from the southern Appalachians and adjacent Cumberland Plateau [26,35], and floristic similarities between this region and the forests of Japan and central China have been of great interest to botanists for centuries [36,37]. These regions were connected via Beringia for much of the Tertiary, and taxonomic disjunctions, largely at the genus level, have resulted from periods of isolation following continental drift, increasing aridity in the Western U.S., and cool and dry conditions associated with major glaciation events in EUS [38,39]. Interestingly, White [38] found these disjunct genera to be overrepresented by woody understory taxa, similar to the qualities of overrepresented invasive taxa reported here. Furthermore, most of the major woody forest EUS invaders endemic to East Asia have congeners in the EUS native flora, including Berberis thunbergii (native is B. canadensis ), Celastrus orbiculatus (near-endemic to East Asia, native is C. scandens ), Elaeagnus umbellata ( E. commutata ), Euonymus alatus (several native bush Euonymus ), Lonicera morrowii ( L. canadensis ), Rosa multiflora (several natives), Viburnum dilatatum (several natives), and Wisteria sinensis ( W. futescens ), among others. It is therefore tempting to suggest that the modern invasion of EUS forests is only the latest chapter in a long history of highly (pre)adapted East Asian lineages colonizing mesic temperate forests worldwide. Consistent with this view, few if any woody understory species from EUS (or Europe) made a list of 126 non-native plant species in China [40]. If true, it suggests that forest invasion mechanisms can be deconstructed by comparative ecophysiological studies of East Asian-EUS sister taxa. It also qualitatively supports patterns of biotic interchanges throughout geologic history, in that modern invasions are similarly characterized by certain regions donating more invaders to particular habitats [2]. An important component of invasive species management is the prevention or early detection of species that exhibit strong invasive tendencies [41], and the association of invaders from certain regions with particular habitats (Fig. 2) suggests several guidelines for natural area management in the EUS. First, although the European bias in alien species persists for those invaders of open and managed habitats, European species are significantly less likely to pose significant management concern in forested natural areas of the Eastern U.S. Instead, managers should be particularly concerned about current and future introductions of woody plants from East Asia that already account for the majority of woody species that dominate forest understories. Second, native endemism can be an important tool for screening plant invasive potential. It is already well appreciated that species of larger native ranges are more likely to become pests in their introduced range [11,42]; the present study confirms this and adds greater detail by classifying endemism according to specific regions. For example, although a significant number of EUS invaders are sub-Saharan African in origin (particularly warm-season grasses), there is not a single EUS invader endemic to an African floristic region (Table 2). On the other hand, there are only four EUS alien taxa endemic to the Indo-Chinese region of southeast Asia, and yet two of these are invasive, again attesting to the strong invasive potential of ...
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... southeast Asia); the floating freshwater aquatic Eichhornia crassipes (water hyacinth, from the Amazon basin); many fast-spreading shrubs of open, disturbed woodlands such as Lonicera tartarica (Tartarian honey- suckle, from the steppes of central Asia); major crop pests like Cyperus rotundus (purple nut sedge, probably from India); and canopy dominants that threaten to replace entire forest stands such as Triadaca sebifera (Chinese tallow tree). Is there any underlying pattern to which global floras contribute invaders to particular habitats? Are the provenances of those species that become invasive an unbiased subset of total alien flora, or are invaders more likely to come from particular evolutionary centers of origin? To invade, a species must be introduced, establish, and spread [18], and processes specific to each of these stages could bias non- native floras toward particular provenances. For example, introduction attempts of non-native species to a focal region may vary according to geographic origin due to historical differences in rates of trade and travel between regions [19]. Furthermore, introduced species that become naturalized should preferentially come from areas that match certain climate, soil, or disturbance conditions that allow a species to reproduce without human assistance [20]. Finally, on top of floristic biases in both introduction attempts and naturalizations, species that become invasive—those that spread naturally and compete successfully with native vegetation—may preferentially come from certain regions where species have achieved superior levels of fitness under competition in a given environment, what Darwin [1] referred to as a ‘‘higher stage of perfection or dominating power’’, and others have referred to as ‘preadaptation’ [21]. This hierarchy based on different mechanisms of introduction, establishment, and spread suggests that comparing floristic patterns of different components of non-native floras (e.g., the provenances of alien species versus the subset of those that become invasive) could help refine studies of biological attributes that allow a typically small subset of introduced species to become invasive. The hierarchy also suggests that non-native floristic associations should vary strongly by habitat type [3,9,22,23], given 1) modes of introduction vary by habitat type, as accidental introductions are often agronomic and follow the spread of agricultural operations, whereas ornamental introductions span a larger range of potential environments (sun versus shade, xeric versus mesic); 2) global floristic regions vary greatly in habitat representation, and some floras lack major habitat types entirely (there is no mesic deciduous forest in the Sahara); and 3) superior competitive abilities are more important to invader success in some habitats, particularly those of low disturbance intensities [24,25]. In this paper I analyze the alien and invasive vascular floras of the EUS coincident with the Eastern Deciduous Forest biome of North America [26] to determine whether alien and invasive plant species of this region are more likely to come from particular source floras, using the Takhtajan [27] global floristic regions as source areas that correspond to global centers of plant diversification (Fig. 1). Due to the prevailing view that strategies for plant success depend strongly on habitat qualities, which in turn suggests that global floras should preferentially contribute species to certain habitats, I conducted the analysis for invasive species using a habitat classification (Table 1) based on environmental differences that select for well known differences in plant strategies (disturbance regime and resource availability [25]). Two plant strategies associated with habitat type that are widely available for floristic-based analyses include species growth form (trees, forbs, etc.) and duration (annual, biennial, perennial); for these attributes I also asked whether native, alien, and invasive components of the EUS flora exhibit regular differences in attribute composition associated with floristic and habitat patterns. The primary objective of this study was to address whether modern plant invasions are qualitatively any different from biotic interchanges throughout the history of biotic migrations [4,5,28]—that is, whether biogeographic patterns of modern invasions reveal new evolutionary-based insights that provide a general framework for predicting where invaders come from and which areas are preferentially invaded. The alien flora of the EUS includes 2629 vascular plant taxa, 449 of which (17%) are documented as invasive (Table 2). Infraspecific taxa (subspecies and varieties) account for 304 of the alien taxa and 14 of the invasive taxa. Alien taxa of the EUS come from all major global floristic zones (Table 2). Nearly half (45%) of the alien taxa have native ranges that overlap the Circumboreal floristic region (including central and northern Europe; Fig. 1), followed in representation by the Mediterranean (39%), Irano- Turanian (31%), and Eastern Asiatic (24%) regions. Of the world floristic regions where nativity could be reliably assigned, the Neozeylandic region is the smallest donor to the EUS alien flora (7 taxa), and 5% of the alien taxa are derived from cultivation (many crops and ornamental plants). Of the 2629 taxa analyzed here, about 50 could not be reliably categorized into native floristic regions, due to lacking nativity information, highly questionable non-native status, or native-nonnative hybrid origin; eight of these were reported invasive (see Supplemental Dataset S1). The subset of 449 invasive EUS taxa is not a random sample of native floristic regions of the alien taxa (Table 2, Fig. 2). Twenty- nine percent of the alien taxa with native ranges that include the Eastern Asiatic region are reported invasive, compared to 22% and 20% of the alien taxa from Circumboreal and Mediterranean regions. Alien taxa present in the Saharo-Arabian and Irano- Turanian regions also include high proportions of invasive taxa (42% and 27%, respectively). However, when alien taxa were instead restricted to those that only occur in a single native region (region endemics), the amount of invasive taxa from Saharo- Arabian and Irano-Turanian regions essentially disappeared (0% and 3%, resp.), as did those from the Mediterranean (2%). This in part reflects the clear relationship between native range size, measured as the number of floristic regions inhabited, and invasion potential (Fig. 3). Despite the smaller overall invasive proportion of region-endemic alien taxa (13%; Table 2), endemics from East Asia have nearly as high an invasive percentage as non- endemics (25%), whereas the percentage of endemic invaders from the Circumboreal region is cut in half (11%, compared to 22% non-endemic invaders). The Neozeylandic region is the only region to lack any invasive contribution to the EUS flora. A majority of the invasive flora (74%) is found in open habitats of irregular disturbance, followed by roadsides (44%), managed (annually disturbed) ecosystems (34%), forests (29%), and wetland, riparian, and aquatic systems (13%, 13%, and 4%, respectively; Table 3). More than half of the woody invasive taxa (58%) are from the Eastern Asiatic region, and significantly more of the East Asian taxa (56%) are woody than expected based on the overall representation of woody invaders. At the other extreme, the invaders of four regions were significantly more likely to be herbaceous than the overall invasive pool, including those from the Sudano-Zambezian (100% herbaceous), Macronesian (96%), Saharo-Arabian (86%), and Mediterranean regions (80%; all P , 0.05; Table 3). Several regions exhibited significant habitat bias in their invasive representation (Table 3, Fig. 2). Annually disturbed, managed ecosystems are far more prone to invasion from Mediterranean plants than plants from the Eastern Asiatic region (P , 0.001; Fig. 2). Conversely, 41% (74/180) of the invasive taxa from East Asia invade forests, compared to only 7% (4/53) invaders from Macronesia and 29% of the invasive taxa overall (Table 3). Significant deviations in habitat representation among invaders from different source floras also include a greater representation of East Asian taxa in riparian habitats and very few Irano-Turanian species in aquatic habitats (Table 3). Major floristic patterns of the invasive pool of all habitat types are illustrated in Fig. 2. The composition of species growth form and duration is significantly different among native, alien, and invasive EUS floras, with departures being greatest between the invasive and alien pools (Table 4). Compared to the native flora, the alien flora is overrepresented by annuals, biennials, and vines, and under- represented by shrubs and grasses. Other growth form categories have remarkably similar representation in the native and alien floras, including the overall split between woody and herbaceous taxa (about 1 woody species in 5). In contrast, the growth form and duration composition of the invasive flora shows a strong departure from the alien flora. Invaders were significantly more likely to be perennial trees, shrubs, and vines, and thus much less likely to be herbaceous (65%) than both the native or alien floras (Table 4). The typical alien vascular plant of the Eastern Deciduous Forest biome of the Eastern U.S. is a European forb, either from the Circumboreal northern and central regions of Europe or the southern Mediterranean region. The clear European bias in non- native plants has been documented in many global floras by plant biogeographers [6,7,29] and ecological historians [30,31] and is referred to as the Imperialist Dogma [30]. This model asserts that the spread of European cultures since the Age of Discovery, including crops, weeds, and commensals, explains both the greater historical transport of European species to global ...
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... despite many invasive lists being derived from agricultural activities where woody species are less common, suggests conservationists and natural resource managers in the EUS focus energies on preventing the introduction and local establishment of non-native woody species [17,43]. It is also important to consider that many alien species only become invasive after significant time lags [44], suggesting that Asian woody taxa considered non-invasive in the present study nonetheless be treated carefully in horticultural practice. The clear floristic distinction between alien and invasive plant taxa in the EUS, and the strong biases in habitat representation between invaders of different origin and life history attributes, is further rationale for more careful delimitation of the focal species pool in invasion studies [23]. In particular, the failure of plant ecologists to identify easily screened attributes of ‘invaders’ should be expected if analyses include all alien (non-native, exotic) taxa [45]. Species that successfully naturalize do share attributes relating to long-distance dispersal ability and reproductive potential [7,10,11], but the present study suggests critical attributes of those that become invasive are specific to particular environmental circumstances rather than universal across habitat types. If other global regions show floristic biases in the invasive species pools of particular habitats that resemble those described in the present study, there should be renewed motivation for comparative studies of the biology of plants from different floras. As modern invasions increasingly appear to qualitatively resemble past biotic interchanges [3,4,5], such comparative studies may also help paleobiologists better understand the historical development of modern plant assemblages. I constructed a database (Supplemental Dataset S1, with associated metadata in Supplemental Text S1) of all alien vascular plant taxa present in the Eastern Deciduous Forest of the Eastern U.S. (state occurrences from MN to LA, east to the coasts of ME to GA, excluding presences unique to FL) using the USDA PLANTS database [46]. I defined as alien those taxa listed as ‘‘Introduced’’ by USDA PLANTS residing in the above states. Taxa such as Phragmites australis with native and exotic populations listed as ‘‘Native and Introduced’’ were not included. I included unam- biguous non-native infraspecific taxa (e.g., Taraxacum officinalis ssp. officinalis , Ranunculus acris var. acris , Viburnum opulus var. opulus ) that are tracked by PLANTS. Alien plant species in the U.S. are only tracked by PLANTS if their native range is wholly outside the contiguous U.S., preventing analysis of those alien taxa native to the Western U.S. Alien plants were categorized as ‘‘invasive’’ if they were represented on the USDA PLANTS ‘‘Weedy and Invasive Plants’’ lists for Eastern U.S. regions, including the Northeast [47], Kentucky [48], Tennessee/Southeast [49], and Wisconsin [50], plus any remaining alien taxa that were indicated as present in the selected states in the WeedsUS database maintained by the U.S. National Park Service [51]. The invasive plant definition used here is thus an alien in the Eastern U.S. of significant management concern. All species were assigned growth form and duration attributes according to the USDA PLANTS database. Growth form attributes included the non-exclusive forms ‘‘tree’’, ‘‘shrub’’ (including ‘‘subshrub’’), ‘‘vine’’, ‘‘graminoid’’, and ‘‘forb/herb’’; ‘‘herbaceous’’ and ‘‘woody’’ classes were derived from lumping ‘‘graminoid’’ and ‘‘forb/herb’’ forms (which includes all herbaceous vines) and ‘‘tree’’, ‘‘shrub’’, and ‘‘vine’’ forms (using only those vines which were not also listed as forbs). A small set of species are semi-woody and are included in herbaceous and woody categories. Duration attributes included annual, biennial, and perennial designations. The composition of the alien flora with respect to these attributes was compared to the EUS native flora using a species-level query of contiguous U.S.-native plants residing in the above selected states from PLANTS. The subset of alien species defined as invasive was further assigned habitat designations describing the environmental circumstances of their occurrences in EUS. Detailed habitat descriptions were first obtained from major EUS floras [52,53,54]; these idiosyncratic descriptions (e.g., ‘‘wet meadows’’, ‘‘bottomland hardwood forests’’) were then grouped into seven habitat classes meant to describe important environmental correlates (disturbance regime, light availability, soil moisture status). Table 1 summarizes this classification, as illustrated in Fig. 2. Non-invasive alien species are typically rare in their introduced ranges, preventing any reliable assessment of foreign habitat affinity for these taxa. All alien taxa in the EUS flora were assigned membership to native source floras using the floristic region designations of Takhtajan [27] (Fig. 1). Takhtajan’s system is based on geographic patterns of endemism, particularly at the species and genus levels, and is meant to represent patterns of historical isolation and evolutionary divergence in the global distribution of vascular plants [55]. Along with the antecedent work of Good [56], to which it closely coincides, it remains the only attempt to categorize the world’s flora phylogenetically at the sub-continental scale [55]. For studies of plant species behavior based on aspects of their evolutionary history, Takhtajan’s regions thus represent a clear advantage over native biogeographic units based on geopolitical boundaries. Each alien taxon was assigned to one or more Takhtajan regions according to documented native range descriptions from source floras. The majority of these assignments were accomplished with taxon queries in the online Germplasm Resources Information Network [57], a central location of floristic distribution information compiled from world floras. In some cases where GRIN records were unavailable, a number of other source floras were consulted. In general, the assignment of native ranges to floristic regions for those taxa distributed close to region boundaries was conservative. A list of native floristic regions for each taxon, along with additional bibliographic information, is available as Supplement Dataset S1 and Text S1. Due to small spatial resolution and sample sizes of alien taxa, three Takhtajan regions for the southern tip of Africa were combined into a single region, as were the three floristic regions of Australia (Table 2). Alien taxa were also essentially absent from small island or archipelago regions, and are ignored in the present analysis. Contingency tables of floristic region vs. habitat and floristic region vs. growth form were analyzed for independence with Pearson chi-square tests in R [58]. Significant residuals were identified with the Freeman-Tukey deviate statistic [59], with a threshold of an expected count of at least 5 for significance [60]. Dataset S1 Native floristic zones of alien plant taxa of the Eastern U.S. database. Found at: doi:10.1371/journal.pone.0003630.s001 (0.92 MB XLS) Text S1 Native floristic zones of alien plant taxa of the Eastern U.S. database: metadata. Found at: doi:10.1371/journal.pone.0003630.s002 (0.04 MB ...
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... history, plant and animal assemblages have evolved in isolated biotas that have occasionally come into contact with one another, and the resulting interchange has usually been the near-wholesale replacement of one region’s species with another’s [1–4]. Modern, human-assisted plant invasions are a clear analogue of historical biotic interchanges [3,5], and yet relatively few invasion biologists have asked whether there are regular patterns of global dominance of plants from particular floristic regions [6,7]. If such patterns exist, they would be of prime importance to the management community concerned with invasions, as resources for prevention and control could be focused on those regions most likely to be sources of future invaders [8]. Such patterns would also be a significant advance for ecologists and evolutionary biologists still struggling to identify generalizations concerning which plants invade [9,10,11] and which communities are most susceptible to invasion [12–15]. The Eastern U.S. (EUS) has seen an unprecedented spread of invasive species in nearly all major habitats over the past century [16,17], and these non-native species represent nearly all of the world’s floristic regions. The diversity of invader habitats and their provenances includes the turfgrass pest Poa annua (annual bluegrass, from Europe); the mesic forest understory grass Microstegium vimineum (Japanese stiltgrass, from southeast Asia); the floating freshwater aquatic Eichhornia crassipes (water hyacinth, from the Amazon basin); many fast-spreading shrubs of open, disturbed woodlands such as Lonicera tartarica (Tartarian honey- suckle, from the steppes of central Asia); major crop pests like Cyperus rotundus (purple nut sedge, probably from India); and canopy dominants that threaten to replace entire forest stands such as Triadaca sebifera (Chinese tallow tree). Is there any underlying pattern to which global floras contribute invaders to particular habitats? Are the provenances of those species that become invasive an unbiased subset of total alien flora, or are invaders more likely to come from particular evolutionary centers of origin? To invade, a species must be introduced, establish, and spread [18], and processes specific to each of these stages could bias non- native floras toward particular provenances. For example, introduction attempts of non-native species to a focal region may vary according to geographic origin due to historical differences in rates of trade and travel between regions [19]. Furthermore, introduced species that become naturalized should preferentially come from areas that match certain climate, soil, or disturbance conditions that allow a species to reproduce without human assistance [20]. Finally, on top of floristic biases in both introduction attempts and naturalizations, species that become invasive—those that spread naturally and compete successfully with native vegetation—may preferentially come from certain regions where species have achieved superior levels of fitness under competition in a given environment, what Darwin [1] referred to as a ‘‘higher stage of perfection or dominating power’’, and others have referred to as ‘preadaptation’ [21]. This hierarchy based on different mechanisms of introduction, establishment, and spread suggests that comparing floristic patterns of different components of non-native floras (e.g., the provenances of alien species versus the subset of those that become invasive) could help refine studies of biological attributes that allow a typically small subset of introduced species to become invasive. The hierarchy also suggests that non-native floristic associations should vary strongly by habitat type [3,9,22,23], given 1) modes of introduction vary by habitat type, as accidental introductions are often agronomic and follow the spread of agricultural operations, whereas ornamental introductions span a larger range of potential environments (sun versus shade, xeric versus mesic); 2) global floristic regions vary greatly in habitat representation, and some floras lack major habitat types entirely (there is no mesic deciduous forest in the Sahara); and 3) superior competitive abilities are more important to invader success in some habitats, particularly those of low disturbance intensities [24,25]. In this paper I analyze the alien and invasive vascular floras of the EUS coincident with the Eastern Deciduous Forest biome of North America [26] to determine whether alien and invasive plant species of this region are more likely to come from particular source floras, using the Takhtajan [27] global floristic regions as source areas that correspond to global centers of plant diversification (Fig. 1). Due to the prevailing view that strategies for plant success depend strongly on habitat qualities, which in turn suggests that global floras should preferentially contribute species to certain habitats, I conducted the analysis for invasive species using a habitat classification (Table 1) based on environmental differences that select for well known differences in plant strategies (disturbance regime and resource availability [25]). Two plant strategies associated with habitat type that are widely available for floristic-based analyses include species growth form (trees, forbs, etc.) and duration (annual, biennial, perennial); for these attributes I also asked whether native, alien, and invasive components of the EUS flora exhibit regular differences in attribute composition associated with floristic and habitat patterns. The primary objective of this study was to address whether modern plant invasions are qualitatively any different from biotic interchanges throughout the history of biotic migrations [4,5,28]—that is, whether biogeographic patterns of modern invasions reveal new evolutionary-based insights that provide a general framework for predicting where invaders come from and which areas are preferentially invaded. The alien flora of the EUS includes 2629 vascular plant taxa, 449 of which (17%) are documented as invasive (Table 2). Infraspecific taxa (subspecies and varieties) account for 304 of the alien taxa and 14 of the invasive taxa. Alien taxa of the EUS come from all major global floristic zones (Table 2). Nearly half (45%) of the alien taxa have native ranges that overlap the Circumboreal floristic region (including central and northern Europe; Fig. 1), followed in representation by the Mediterranean (39%), Irano- Turanian (31%), and Eastern Asiatic (24%) regions. Of the world floristic regions where nativity could be reliably assigned, the Neozeylandic region is the smallest donor to the EUS alien flora (7 taxa), and 5% of the alien taxa are derived from cultivation (many crops and ornamental plants). Of the 2629 taxa analyzed here, about 50 could not be reliably categorized into native floristic regions, due to lacking nativity information, highly questionable non-native status, or native-nonnative hybrid origin; eight of these were reported invasive (see Supplemental Dataset S1). The subset of 449 invasive EUS taxa is not a random sample of native floristic regions of the alien taxa (Table 2, Fig. 2). Twenty- nine percent of the alien taxa with native ranges that include the Eastern Asiatic region are reported invasive, compared to 22% and 20% of the alien taxa from Circumboreal and Mediterranean regions. Alien taxa present in the Saharo-Arabian and Irano- Turanian regions also include high proportions of invasive taxa (42% and 27%, respectively). However, when alien taxa were instead restricted to those that only occur in a single native region (region endemics), the amount of invasive taxa from Saharo- Arabian and Irano-Turanian regions essentially disappeared (0% and 3%, resp.), as did those from the Mediterranean (2%). This in part reflects the clear relationship between native range size, measured as the number of floristic regions inhabited, and invasion potential (Fig. 3). Despite the smaller overall invasive proportion of region-endemic alien taxa (13%; Table 2), endemics from East Asia have nearly as high an invasive percentage as non- endemics (25%), whereas the percentage of endemic invaders from the Circumboreal region is cut in half (11%, compared to 22% non-endemic invaders). The Neozeylandic region is the only region to lack any invasive contribution to the EUS flora. A majority of the invasive flora (74%) is found in open habitats of irregular disturbance, followed by roadsides (44%), managed (annually disturbed) ecosystems (34%), forests (29%), and wetland, riparian, and aquatic systems (13%, 13%, and 4%, respectively; Table 3). More than half of the woody invasive taxa (58%) are from the Eastern Asiatic region, and significantly more of the East Asian taxa (56%) are woody than expected based on the overall representation of woody invaders. At the other extreme, the invaders of four regions were significantly more likely to be herbaceous than the overall invasive pool, including those from the Sudano-Zambezian (100% herbaceous), Macronesian (96%), Saharo-Arabian (86%), and Mediterranean regions (80%; all P , 0.05; Table 3). Several regions exhibited significant habitat bias in their invasive representation (Table 3, Fig. 2). Annually disturbed, managed ecosystems are far more prone to invasion from Mediterranean plants than plants from the Eastern Asiatic region (P , 0.001; Fig. 2). Conversely, 41% (74/180) of the invasive taxa from East Asia invade forests, compared to only 7% (4/53) invaders from Macronesia and 29% of the invasive taxa overall (Table 3). Significant deviations in habitat representation among invaders from different source floras also ...
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... vascular floras of the EUS coincident with the Eastern Deciduous Forest biome of North America [26] to determine whether alien and invasive plant species of this region are more likely to come from particular source floras, using the Takhtajan [27] global floristic regions as source areas that correspond to global centers of plant diversification (Fig. 1). Due to the prevailing view that strategies for plant success depend strongly on habitat qualities, which in turn suggests that global floras should preferentially contribute species to certain habitats, I conducted the analysis for invasive species using a habitat classification (Table 1) based on environmental differences that select ...
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... invasive (Table 2). Infraspecific taxa (subspecies and varieties) account for 304 of the alien taxa and 14 of the invasive taxa. Alien taxa of the EUS come from all major global floristic zones (Table 2). Nearly half (45%) of the alien taxa have native ranges that overlap the Circumboreal floristic region (including central and northern Europe; Fig. 1), followed in representation by the Mediterranean (39%), Irano- Turanian (31%), and Eastern Asiatic (24%) regions. Of the world floristic regions where nativity could be reliably assigned, the Neozeylandic region is the smallest donor to the EUS alien flora (7 taxa), and 5% of the alien taxa are derived from cultivation (many crops and ...
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... landscapes as dominated by closed secondary forests. Indeed, if the composition of the alien flora is used as a null model for invader composition, taxa from some regions are significantly more likely to invade (Table 2). It should come as no surprise that invasive taxa are most likely to come from areas with climates that resemble those of EUS (Fig. 1)-all floristic regions of greater- than-expected invader representation (.17%) are those of extra- tropical distribution. However, climate similarity is not sufficient to predict the bias in invader distribution among floristic regions. Part of this variance is attributable to native range size, in that species with native ranges that ...
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... alien taxa in the EUS flora were assigned membership to native source floras using the floristic region designations of Takhtajan [27] (Fig. 1). Takhtajan's system is based on geographic patterns of endemism, particularly at the species and genus levels, and is meant to represent patterns of historical isolation and evolutionary divergence in the global distribution of vascular plants [55]. Along with the antecedent work of Good [56], to which it closely coincides, it remains ...

Citations

... Ligustrum lucidum and L. sinense are widespread globally, the former particularly dominant in South America (Fernandez et al., 2020). In North America, more than 20 non-native shrubs and small trees are commonly found in native deciduous forests (Fridley, 2008;Fridley et al., 2022), ...
... Some native floras have experienced more extreme climatic histories than others of similar latitude, creating 'empty niches' that have been colonized by functionally distinct invaders (Dansereau, 1964), often facilitated by climate change. For example, Fridley (2013) and Zohner and Renner (2017) suggested that the environmental history of North American deciduous forests may have predisposed them to invasions from Eurasian woody species, which leaf out earlier and senesce later as a result of more predictable spring and autumnal climate across Eurasia (Fridley, 2012;Zohner & Renner, 2017). Fridley et al. (2022) showed this strategy of extended leaf phenology to be associated with greater shade tolerance through longer leaf life span in invaders despite their greater growth and photosynthetic rates compared with natives; leaf phenology is implicated in other deciduous forest invasions as well, including the spread of H. dulcis in subtropical Brazil (Rejmánek, 2013) and A. platanoides in temperate North America (Kloeppel & Abrams, 1995). ...
Article
1. Although closed‐canopy forests are characterized by low light availability and slow population dynamics, many are under threat from non‐native, invasive woody species that combine high colonization ability and fast growth potential with high low‐light survival. This ‘superinvader’ phenotype contravenes expected tradeoffs predicted by successional niche theory, posing a challenge to both invasion and forest succession theory. 2. We propose a parsimonious conceptual model based on the whole‐plant light compensation point (WPLCP) that, across a variety of plant strategies and growth forms, can explain greater competitive abilities of forest invaders in the context of both high‐light growth rate and shade tolerance. The model requires only that non‐native species experience relatively fewer carbon costs than native species, enabling resource‐acquisitive species to establish in low‐light conditions. 3. We review evidence for lower carbon costs in invasive species resulting from 1) enemy release, 2) recent environmental changes that favor less stress‐tolerant phenotypes, and 3) phylogenetically constrained native floras. We also discuss implications of invader shade tolerance in the context of other life history strategies that, combined with canopy disturbances, facilitate their rapid numerical dominance. 4. Synthesis. An invasion framework driven by carbon dynamics suggests renewed focus on whole‐plant carbon costs, including belowground respiration and tissue turnover, which are rarely measured in functional studies of forest invaders.
... At the time of species selection (2006), few physiological studies had been conducted on NA native and invasive shrubs (Harrington et al., 1989), and we had little to no information on leaf properties, branching strategies, xylem anatomy or root behaviour of our focal species. Our selection of woody understorey invaders includes nearly all those managed as high-impact invaders in the Northeast U.S. (Fridley, 2008). To select appropriate native species, we considered both their ecological importance and their phylogenetic relatedness to invaders, to protect against misinterpretation of trait differences as a result of evolutionary distance alone. ...
... To select appropriate native species, we considered both their ecological importance and their phylogenetic relatedness to invaders, to protect against misinterpretation of trait differences as a result of evolutionary distance alone. Although some species are classified primarily as small trees, we excluded species that attain canopy status because forest invasions in NA are strongly biased towards understorey shrubs and lianas (Fridley, 2008;Webster et al., 2006). Growth form classifications of all focal species therefore included 'shrub' or 'vine' (USDA & NRCS, 2020). ...
... We treated species as non-native if they were introduced to NA after 1500 C.E. Of the 32 non-native species, 24 are managed as invaders by one or more U.S. management agencies because they (1) have spread naturally and widely from points of introduction (Fridley, 2013), and (2) have impacted native populations (Fridley, 2008). We treat eight non-native focal species as naturalised rather than invasive, because they remain uncommon more than a century after introduction and have not been placed on management agency invasive lists. ...
Article
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Successful control and prevention of biological invasions depend on identifying traits of non‐native species that promote fitness advantages in competition with native species. Here, we show that, among 76 native and non‐native woody plants of deciduous forests of North America, invaders express a unique functional syndrome that combines high metabolic rate with robust leaves of longer lifespan and a greater duration of annual carbon gain, behaviours enabled by seasonally plastic xylem structure and rapid production of thin roots. This trait combination was absent in all native species examined and suggests the success of forest invaders is driven by a novel resource‐use strategy. Furthermore, two traits alone—annual leaf duration and nuclear DNA content—separated native and invasive species with 93% accuracy, supporting the use of functional traits in invader risk assessments. A trait syndrome reflecting both fast growth capacity and understorey persistence may be a key driver of forest invasions. We show that non‐native, invasive woody plants of North American deciduous forests express a unique functional syndrome combining traits associated with both high growth potential and high shade tolerance. This syndrome was absent in all native species examined and involves an integrated growth strategy reflecting leaf, stem and root traits.
... The garden included three genera of shrubs and lianas of ENA forests, with each genus represented by at least one native and one invasive species (Table 1). All non-native species included in the study are considered invasive in ENA forests (Fridley, 2008) and are hereafter referred to as invaders or invasive species. The common garden was established in an existing mowed field within an~40 × 40 m area of homogenous flat topography that was evenly tilled. ...
Article
Premise: Invasive species tend to possess acquisitive plant traits that support fast growth and strong competitive ability. However, the relevance of symbioses with arbuscular mycorrhizal fungi (AMF) to the fast growing, acquisitive strategy of invasive species is still unclear. Methods: We measured AMF colonization in roots of five congeneric pairs of invasive and native Eastern North American woody species (10 species total; 4 lianas, 6 shrubs) that were grown in a monoculture common garden experiment in Syracuse, NY. We then examined the relationships of AMF colonization to above and belowground traits of these species. Key results: Total AMF colonization and arbuscule colonization were greater in invasive compared to native woody species, a pattern that was more distinct in congeneric shrubs than congeneric lianas. AMF colonization was also positively correlated with traits indicative of rapid plant growth and nutrient uptake. Conclusions: The concordance of a resource-acquisitive strategy with higher AMF colonization suggests that symbioses with AMF may be part of the strategy by which invasive woody plants of Eastern North America are able to maintain fast growth rates and outcompete their native counterparts. This article is protected by copyright. All rights reserved.
... Thailand is located where two major phytogeographical regions and three zoological regions overlap, making it a perfect hub for animal and plant diversity (Hughes et al., 2003;Fridley, 2008). In the past, Thailand was covered with more than 50 % of forest, but only 31.5% remains today (Royal Forest Department or RFD, 2017). ...
... Studies of species invasiveness have often focused on the inherent competitive ability of invaders, reasoning that the success of some invaders may derive from evolutionary processes in their native range (Alpert, 2006;Buckley & Catford, 2016;Fridley, 2013;Hejda et al., 2009;Mack, 2003). Drawing on Darwin's (1859) ideas about the innate competitive superiority of species from large mainland areas, Fridley and Sax (2014) reframed this argument as the evolutionary imbalance hypothesis (EIH), and showed that introduced species from larger, more phylogenetically diverse native regions were more likely to be invasive in several well-known non-native floras. ...
... Due to the importance of matching species' traits to environmental context, invasiveness risk assessments perform better when applied to invaders of particular habitats. decline in importance in highly disturbed, human-impacted ecosystems where establishment is less dependent on innate competitive ability (Fridley, 2008;Grime, 1973), their regional-scale analysis did not differentiate invasiveness on the basis of habitat characteristics, likely contributing to the relatively weak effect size of native range phylogenetic diversity (PD) in predicting invasiveness at the regional scale. ...
... Although the use of habitat data in studies of species invasions is common (e.g. Chytrý, Jarošík, et al., 2008;Chytrý, Maskell, et al., 2008;Diez et al., 2008;Divíšek et al., 2018;Fridley, 2008Fridley, , 2013Hejda et al., 2009Hejda et al., , 2014Lambdon & Hulme, 2006;Pyšek et al., 2015), we are not aware of the use of habitat data to inform theory on the role of competition in invasions, despite the general expectation that invasion mechanisms are habitat specific (Fridley, 2013;Lambdon & Hulme, 2006;Procheş et al., 2008). Specific to our study system, Diez et al. (2008) showed that patterns of taxonomic relatedness were consistent with a competition mechanism using abundance in the Auckland region as the metric of invasiveness. ...
Article
Full-text available
Many invasion theories invoke resource competition as the primary mechanism of invader advantage. These include Darwin's naturalization hypothesis (DNH), which treats phylogenetic similarity as a proxy for niche overlap and competitive intensity, and the evolutionary imbalance hypothesis (EIH), which suggests the phylogenetic diversity (PD) of an introduced species’ native range is an indicator of its competitive ability. Few tests of invasion theory, however, consider habitat characteristics associated with the role of competition in community assembly. In particular, plant invasions of habitats characterized by high environmental stress and disturbance levels should rarely be driven by competition. This suggests tests of EIH and DNH are habitat dependent, and their relative importance in invasiveness models should be predictable based on habitat qualities related to competitive intensity. Using a dataset of plant invasions in New Zealand (NZ) natural areas that distinguishes naturalized species according to both habitat type and community impact, we evaluated the predictive ability of factors related to EIH, DNH and covariates including year of introduction, introduction mode and life history attributes, in driving species invasiveness. We hypothesized that EIH and DNH would be more important predictors of invasiveness in forested habitats and decline in importance as communities shifted towards those more dominated by herbaceous species and/or more sparsely vegetated. We found mixed support for the role of competition linked to DNH and EIH as a driver of invasions in relation to habitat type. Native range PD was among the best predictors of invasiveness in forests, and declined in importance in more disturbed habitats, supporting EIH. In contrast, phylogenetic nearest neighbour distance (PNND) of invaders to native communities was more important in disturbed environments, suggesting competition does not drive DNH. Further, for most habitats and across all of NZ, neither PD nor PNND was as important as year of introduction or life history and growth form attributes in predicting invasiveness. Synthesis. Although both native range PD and PNND predict the invasiveness of naturalized plants in NZ, the results of our habitat‐specific models indicate that only PD is consistent with an invasion mechanism based on competitive ability. Effects of PNND were greatest in grasslands that have been extensively modified by fire and grazing, suggesting they are more likely driven by invader pre‐adaptation to modified habitat conditions. Due to the importance of matching species' traits to environmental context, invasiveness risk assessments perform better when applied to invaders of particular habitats. Invasive iceplant (Carpobrotus sp.) smothering a native Coprosma shrub on a shingle beach on the South Island of New Zealand. Habitat‐specific models of plant invasiveness in New Zealand natural areas are more accurate than models that ignore ecosystem type, because they better reflect the interaction of species traits and habitat qualities. In particular, those traits associated with invader competitive ability, such as the phylogenetic diversity of an invader's native range, are strong predictors of invasiveness in less‐disturbed habitats such as forests. Credit: Jason Fridley.
... All species are common to oldfield ecosystems in southwestern Pennsylvania and are comprised of six native and five nonnative forbs across four plant families (Asteraceae, Clusiaceae, Polygonaceae, and Ranunculaceae; Table 1). All species are of European or Eurasian origin, which is consistent with the origin of most old-field nonnatives (Fridley 2008). We scored all specimens of each species that were collected within southwestern Pennsylvania. ...
Article
Full-text available
Native and nonnative plant species can exhibit differences in the timing of their reproductive phenology and their phenological sensitivity to climate. These contrasts may influence species' interactions and the invasion potential of nonnative species; however, a limited number of phenology studies expressly consider phenological mismatches among native and nonnative species over broad spatial or temporal scales. To fill this knowledge gap, we used two complementary approaches: First, we quantified the flowering phenology of native and nonnative plants at five old‐field sites across a spatially extensive range of eastern North America. Second, we used herbarium records to compare the sensitivity of flowering and fruiting phenology to climate across a 114‐yr time period in a subset of common old‐field species in southwestern Pennsylvania. Across the study region, nonnatives reproduced substantially earlier in the growing season than natives, suggesting that nonnatives occupy a unique phenological niche (0.55 months earlier flowering across the North American study sites; 50.1 d earlier flowering and 17.5 d earlier fruiting in southwestern Pennsylvania). Both natives and nonnatives advanced their reproductive phenology between 1900 and 2014 but exhibited contrasting phenological sensitivity to climate factors. During the flowering stage of phenology, nonnatives were more sensitive to changes in precipitation than natives and generally delayed flowering in wetter years. Nonnative plants had greater sensitivity and advanced fruiting when the month preceding fruiting was warmer, while native plants had greater sensitivity and advanced fruiting when the three‐month period preceding fruiting was warmer. Our findings suggest that nonnative old‐field species occupy an earlier phenological niche relative to native species, which may facilitate their invasion into old‐field communities. However, given the different sensitivities of native and nonnative plants to climate factors, present‐day patterns of phenology are likely to shift with future climate changes, potentially leading to novel species interactions that may influence the outcomes of invasion.
... The establishment of O. cornifrons, and abundance of crop and wild plant species from East Asia in North America [26,27], makes this an ideal system to determine how floral preferences of recently introduced pollinators may shift in new environments. Using pollen metabarcoding, we analysed pollen species identity and relative abundance in O. cornifrons larval pollen provisions. ...
... The northeastern United States, where this study occurred, shares the same temperate forest vegetation biome as East Asia (and Northern Europe) and is now home to many East-Asian exotic plant species [26,27]. The pollen provisions of O. cornifrons exhibited a heterogeneous mix of East-Asian, European and North American species of Prunus, Rubus and Cercis (genera with native species to each geographic region; figures 1 and 2). ...
... The pollen provisions of O. cornifrons exhibited a heterogeneous mix of East-Asian, European and North American species of Prunus, Rubus and Cercis (genera with native species to each geographic region; figures 1 and 2). Although only 25% of all plant species found in our dataset were introduced to North America from East Asia [26,27], O. cornifrons collected significant amounts of pollen from plants from this geographic origin (approx. 40% across landscapes, χ 2 = 840.1, ...
Article
Full-text available
Studying the pollen preferences of introduced bees allows us to investigate how species use host-plants when establishing in new environments. Osmia cornifrons is a solitary bee introduced into North America from East Asia for pollination of Rosaceae crops such as apples and cherries. We investigated whether O. cornifrons (i) more frequently collected pollen from host-plant species they coevolved with from their geographic origin, or (ii) prefer host-plant species of specific plant taxa independent of origin. To address this question, using pollen metabarcoding, we examined the identity and relative abundance of pollen in larval provisions from nests located in different landscapes with varying abundance of East-Asian and non-Asian plant species. Our results show that O. cornifrons collected more pollen from plant species from their native range. Plants in the family Rosaceae were their most preferred pollen hosts, but they differentially collected species native to East Asia, Europe, or North America depending on the landscape. Our results suggest that while O. cornifrons frequently collect pollen of East-Asian origin, the collection of pollen from novel species within their phylogenetic familial affinities is common and can facilitate pollinator establishment. This phylogenetic preference highlights the effectiveness of O. cornifrons as crop pollinators of a variety of Rosaceae crops from different geographic origins. Our results imply that globalization of non-native plant species may ease the naturalization of their coevolved pollinators outside of their native range.
... The establishment of O. cornifrons, and abundance of crop and wild plant species from East Asia in North America [26,27], makes this an ideal system to determine how floral preferences of recently introduced pollinators may shift in new environments. Using pollen metabarcoding, we analysed pollen species identity and relative abundance in O. cornifrons larval pollen provisions. ...
... The northeastern United States, where this study occurred, shares the same temperate forest vegetation biome as East Asia (and Northern Europe) and is now home to many East-Asian exotic plant species [26,27]. The pollen provisions of O. cornifrons exhibited a heterogeneous mix of East-Asian, European and North American species of Prunus, Rubus and Cercis (genera with native species to each geographic region; figures 1 and 2). ...
... The pollen provisions of O. cornifrons exhibited a heterogeneous mix of East-Asian, European and North American species of Prunus, Rubus and Cercis (genera with native species to each geographic region; figures 1 and 2). Although only 25% of all plant species found in our dataset were introduced to North America from East Asia [26,27], O. cornifrons collected significant amounts of pollen from plants from this geographic origin (approx. 40% across landscapes, χ 2 = 840.1, ...
Article
Full-text available
Studying the pollen preferences of introduced bees allows us to investigate how species use host-plants when establishing in new environments. Osmia cornifrons is a solitary bee introduced into North America from East Asia for pollination of Rosaceae crops such as apples and cherries. We investigated whether O. cornifrons (i) more frequently collected pollen from host-plant species they coevolved with from their geographic origin, or (ii) prefer host-plant species of specific plant taxa independent of origin. To address this question, using pollen metabarcoding, we examined the identity and relative abundance of pollen in larval provisions from nests located in different landscapes with varying abundance of East-Asian and non-Asian plant species. Our results show that O. cornifrons collected more pollen from plant species from their native range. Plants in the family Rosaceae were their most preferred pollen hosts, but they differentially collected species native to East Asia, Europe, or North America depending on the landscape. Our results suggest that while O. cornifrons frequently collect pollen of East-Asian origin, the collection of pollen from novel species within their phylogenetic familial affinities is common and can facilitate pollinator establishment. This phylogenetic preference highlights the effectiveness of O. cornifrons as crop pollinators of a variety of Rosaceae crops from different geographic origins. Our results imply that globalization of non-native plant species may ease the naturalization of their coevolved pollinators outside of their native range.
... The relationship between post-assembly host and parasite richness was contingent on the final abundance of exotic host species that resulted from community assembly. Many of the exotic hosts that dominate Southeastern US old fields were introduced by humans from fertilized pastures (Fridley, 2008), benefit from experimental fertilization (Heckman et al., 2016(Heckman et al., , 2017, and are sensitive to initial host diversity. Previous analyses indicate that exotic hosts also contributed most to parasite abundance in communities that they dominated, suggesting that exotic host species may contribute disproportionately to parasite transmission (Halliday et al., 2019). ...
Article
Host and parasite richness are generally positively correlated, but the stability of this relationship in response to global change remains poorly understood. Rapidly changing biotic and abiotic conditions can alter host community assembly, which in turn, can alter parasite transmission. Consequently, if the relationship between host and parasite richness is sensitive to parasite transmission, then changes in host composition under various global change scenarios could strengthen or weaken the relationship between host and parasite richness. To test the hypothesis that host community assembly can alter the relationship between host and parasite richness in response to global change, we experimentally crossed host diversity (biodiversity loss) and resource supply to hosts (eutrophication), then allowed communities to assemble. As previously shown, initial host diversity and resource supply determined the trajectory of host community assembly, altering post‐assembly host species richness, richness‐independent host phylogenetic diversity, and colonization by exotic host species. Overall, host richness predicted parasite richness, and as predicted, this effect was moderated by exotic abundance: communities dominated by exotic species exhibited a stronger positive relationship between post‐assembly host and parasite richness. Ultimately, these results suggest that, by modulating parasite transmission, community assembly can modify the relationship between host and parasite richness. These results thus provide a novel mechanism to explain how global environmental change can generate contingencies in a fundamental ecological relationship: the positive relationship between host and parasite richness.
... However, it is unclear how disturbance events such as herbivore outbreaks affect invasive species following their establishment-are invasive species also more resilient to disturbances than co-occurring native species? In Eastern North America (ENA), many shade-tolerant, nonnative invasive woody plants have spread through forests for over a century (Mack 2003, Fridley 2008. Despite studies commonly highlighting productivity differences between ENA native and invasive woody species (Stewart and Graves 2006, Herron et al. 2007, Lapointe and Brisson 2012, Paquette et al. 2012, few have evaluated the differences in survival following defoliation (Gleason andAres 2004, Vanderklein et al. 2015). ...
Article
Many non-native, invasive woody species in mesic forests of North America are both shade tolerant and more productive than their native counterparts, but their ability to tolerate disturbances remains unclear. In particular, complete defoliation associated with herbivory and extreme weather events may have larger impacts on invaders if natives maintain greater resource reserves to support regrowth. On the other hand, invaders may be more resilient to partial defoliation by means of upregulation of photosynthesis or may be better able to take advantage of canopy gaps to support refoliation. Across a light gradient, we measured radial growth, new leaf production, non-structural carbohydrates (NSCs), chlorophyll content, and survival in response to varying levels of defoliation in saplings of two native and two invasive species that commonly co-occur in deciduous forests of Eastern North America. Individuals were subjected to one of four leaf removal treatments: no defoliation controls, 50% defoliation over three growing seasons, 100% defoliation over one growing season, and 100% defoliation over two growing seasons. Contrary to our hypothesis, native and invasive species generally did not differ in defoliation responses, although invasive species experienced more pronounced decreases in leaf chlorophyll following full defoliation and native species' survival was more dependent on light availability. Radial growth progressively decreased with increasing defoliation intensity, and refoliation mass was largely a function of sapling size. Survival rates for half-defoliated saplings did not differ from controls (90% of saplings survived), but survival in fully defoliated individuals over one and two growing seasons was reduced to 45% and 15%, respectively. Surviving defoliated saplings generally maintained control NSC concentrations. Under high light, chlorophyll concentrations were higher in half-defoliated saplings compared to controls, which may suggest photosynthetic upregulation. Our results indicate that native and invasive species respond similarly to defoliation, despite the generally faster growth strategy of invaders.