Co-occurring Native Plant Research Articles

Herbivory and elevated levels of CO2 and nutrients separately, rather than synergistically, impacted biomass production and allocation in invasive and native plant species.

Large parts of the Earth are experiencing environmental change caused by alien plant invasions, rising atmospheric concentration of carbon dioxide (CO2 ), and nutrient enrichments. Elevated CO2 and nutrient concentrations can separately favour growth of invasive plants over that of natives but how herbivory may modulate the magnitude and direction of net responses by the two groups of plants to simultaneous CO2 and nutrient enrichments remains unknown. In line with the enemy release hypothesis, invasive plant species should reallocate metabolites from costly anti-herbivore defences into greater growth following escape from intense herbivory in the native range. Therefore, invasive plants should have greater growth than native plants under simultaneous CO2 and nutrient enrichments in the absence of herbivory. To test this prediction, we grew nine congeneric pairs of invasive and native plant species that naturally co-occurred in grasslands in China under two levels each of nutrient enrichment (low-nutrient vs. high-nutrient), herbivory (with herbivory vs. without herbivory) and under ambient (412.9 ± 0.6 ppm) and elevated (790.1 ± 6.2 ppm) levels of CO2 concentrations in open top chambers in a common garden. Elevated CO2 and nutrient enrichment separately increased total plant biomass, while herbivory reduced it regardless of the plant invasive status. High-nutrient treatment caused the plants to allocate a significantly lower proportion of total biomass to roots, while herbivory induced an opposite pattern. Herbivory suppressed total biomass production more strongly in native plants than invasive plants. The plants exhibited significant interspecific and intergeneric variation in their responses to the various treatment combinations. Overall, these results suggest that elevated CO2 and nutrients and herbivory may separately, rather than synergistically, impact productivity of the invasive and co-occurring native plant species in our study system. Moreover, interspecific variation in resource-use strategies was more important than invasive status in determining plant responses to the various treatment combinations.

Nitrogen acquisition strategy and its effects on invasiveness of a subtropical invasive plant.

Preference and plasticity in nitrogen (N) form uptake are the main strategies with which plants absorb soil N. However, little effort has been made to explore effects of N form acquisition strategies, especially the plasticity, on invasiveness of exotic plants, although many studies have determined the effects of N levels (e.g. N deposition). To address this problem, we studied the differences in N form acquisition strategies between the invasive plant Solidago canadensis and its co-occurring native plant Artemisia lavandulaefolia, effects of soil N environments, and the relationship between N form acquisition strategy of S. canadensis and its invasiveness using a 15N-labeling technique in three habitats at four field sites. Total biomass, root biomass, and the uptakes of soil dissolved inorganic N (DIN) per quadrat were higher for the invasive relative to the native species in all three habitats. The invader always preferred dominant soil N forms: NH4 + in habitats with NH4 + as the dominant DIN and NO3 - in habitats with NO3 - as the dominant DIN, while A. lavandulaefolia consistently preferred NO3 - in all habitats. Plasticity in N form uptake was higher in the invasive relative to the native species, especially in the farmland. Plant N form acquisition strategy was influenced by both DIN levels and the proportions of different N forms (NO3 -/NH4 +) as judged by their negative effects on the proportional contributions of NH4 + to plant N (f NH4 +) and the preference for NH4 + (β NH4 +). In addition, total biomass was positively associated with f NH4 + or β NH4 + for S. canadensis, while negatively for A. lavandulaefolia. Interestingly, the species may prefer to absorb NH4 + when soil DIN and/or NO3 -/NH4 + ratio were low, and root to shoot ratio may be affected by plant nutrient status per se, rather than by soil nutrient availability. Our results indicate that the superior N form acquisition strategy of the invader contributes to its higher N uptake, and therefore to its invasiveness in different habitats, improving our understanding of invasiveness of exotic plants in diverse habitats in terms of utilization of different N forms.

Nutrient enrichment promotes invasion success of alien plants via increased growth and suppression of chemical defenses.

In support of the prediction of the enemy release hypothesis regarding a growth-defense trade-off, invasive alien plants often exhibit greater growth and lower anti-herbivory defenses than native plants. However, it remains unclear how nutrient enrichment of invaded habitats may influence competitive interactions between invasive alien and co-occurring native plants, as well as production of anti-herbivore defense compounds, growth-promoting hormones, and defense-regulating hormones by the two groups of plants. Here, we tested whether: (i) nutrient enrichment causes invasive alien plants to produce greater biomass and lower concentrations of the defense compounds flavonoids and tannins than native plants; and (ii) invasive alien plants produce lower concentrations of a defense-regulating hormone jasmonic acid (JA) and higher concentrations of a growth-promoting hormone gibberellic acid (GA3). In a greenhouse experiment, we grew five congeneric pairs of invasive alien and native plant species under two levels each of nutrient enrichment (low vs. high), simulated herbivory (leaf clipping vs. no-clipping), and competition (alone vs. competition) in 2.5-L pots. In the absence of competition, high-nutrient treatment induced a greater increase in total biomass of invasive alien species than that of native species, whereas the reverse was true under competition as native species benefitted more from nutrient enrichment than invasive alien species. Moreover, high-nutrient treatment caused a greater increase in total biomass of invasive alien species than that of native species in the presence of simulated herbivory. Competition induced higher production of flavonoids and tannins. Simulated herbivory induced higher flavonoid expression in invasive alien plants under low-nutrient than high-nutrient treatments. However, flavonoid concentrations of native plants did not change under nutrient enrichment and simulated herbivory treatments. Invasive alien plants produced higher concentrations of GA3 than native plants. Taken together, these results suggest that impact of nutrient enrichment on growth of invasive alien and co-occurring native plants may depend on the level of competition that they experience. Moreover, invasive alien plants might adjust their flavonoid-based defense more efficiently than native plants in response to variation in soil nutrient availability and herbivory pressure. Our findings suggest that large-scale efforts to reduce nutrient enrichment of invaded habitats may help to control future invasiveness of target alien plant species.

Eastern skunk cabbage (Symplocarpus foetidus), a common native wetland plant, shows reduced size and seedling abundance near invasive Japanese knotweed (Reynoutria japonica) 1

Nonnative plants may shift community composition and reduce diversity through competition with native neighbors, or invasion patterns may reflect edaphic gradients that create suitable habitat for colonization. We tested the impacts of invasive Reynoutria japonica (Japanese knotweed) on performance of a co-occurring native plant, Symplocarpus foetidus (eastern skunk cabbage), to determine if invasion has potential to change community composition by altering population-level processes. We tested two hypotheses: (1) Reynoutria japonica invasion is correlated with reduced population performance in S. foetidus; (2) Reynoutria japonica invasion is correlated with environmental variation, and contrasting habitat preferences allow R. japonica and S. foetidus to coexist. Sampling took place in an urban riparian site in Pittsburgh, PA, USA. Data were collected from 26 1-m2 plots with R. japonica present (n = 13) or absent (n = 13). We recorded pH, texture, and moisture. Size and flowering of all S. foetidus individuals and R. japonica cover and abundance were recorded in each plot. Symplocarpus foetidus size was significantly reduced in invaded plots, and both size and seedling abundance were significantly negatively associated with R. japonica cover. Abundance was negatively associated with percent clay and positively associated with soil moisture; seedling abundance was also predicted by lower soil pH. In contrast, R. japonica was most abundant and had highest cover with high pH and lower percent sand. Flowering of S. foetidus did not differ significantly between invaded and uninvaded plots. We conclude that R. japonica invasion both suppresses S. foetidus and correlates with environmental heterogeneity. Reduced performance of S. foetidus in invaded plots suggests competitive suppression. While R. japonica negatively impacted S. foetidus, distinct microhabitat preferences may allow them to co-occur.

Competitiveness of Rosa multiflora Thunb. in varying plant communities and light regimes compared with co-occurring native species

Multiflora rose (Rosa multiflora Thunb.) is a non-native invasive shrub of central lower Michigan that is broadly distributed throughout much of the non-mountainous U.S. Like many non-native invasive species, multiflora rose (MR) colonizes a variety of habitats where it competes with and replaces native plant species. The goal of this research was to compare the light and water use efficiency of MR with co-occurring native woody plants in mature forests with minimal light penetration to the forest floor, young forests with substantial light penetration, and edge or meadow habitats with full sunlight exposure. Photosynthesis rate, transpiration rate, and leaf chlorophyll content were measured for MR and two co-occurring native woody plants. Black cherry and Virginia creeper are native woody plants found to co-occur with MR in every replicate location of all three habitats assessed. MR was significantly more light use efficient than Black cherry at light intensities greater than 750 W/m2 and exhibited a much broader light use efficiency advantage over Virginia creeper at light intensities greater than 100 W/m2. These results indicate land managers should prioritize surveillance of high value landscapes with high levels of incident light when there is little or no MR infestation, choosing first to remove MR from quality forest habitats with less light because mitigation efforts in these areas are more likely to be successful. Removal or death of overstory plants will increase the risk of encroachment by MR which could also inhibit juvenile recruitment by desired tree and forest floor ephemeral species. Encouraging the growth of overstory plants to shade MR in forests to minimize the amount of light reaching it will reduce MR competitiveness even after MR has been established.

Native herbivores indirectly facilitate the growth of invasive Spartina in a eutrophic saltmarsh.

Current theory (e.g., consumer-controlled theory) predicts that nutrient enrichment typically amplifies herbivory and thereby suppresses the growth and expansion of invasive plants. Herbivores can facilitate plant regrowth in the native community by stimulating complementary growth or ameliorating habitat conditions (e.g., by increasing soil oxygen and nutrient availability), but whether they have similar positive effects on invasive plants, especially under nutrient enrichment, remains unknown. Using a field nitrogen (N)-enrichment × crab exclusion experiment, we evaluated and compared the effects of both N enrichment and crab herbivory on the growth performance of a global invasive cordgrass, Spartina alterniflora, and a co-occurring native plant, Phragmites australis. We found that crabs consistently suppressed P. australis by density and aboveground biomass regardless of N enrichment. In contrast, for S. alterniflora, the negative effects of crabs under ambient N were replaced by positive effects under N enrichment, with crabs stimulating complementary increases in density and aboveground biomass. The differing effects between the N treatments were driven by crab burrowing activity, which increased soil N availability, and the nutrient-use efficiency of S. alterniflora. Our findings revealed that native herbivores can have opposing effects on native and invasive plants, which broadens our understanding of how exotic plants can achieve dominance in a changing world.

Ecological impacts of the invasive plant Xanthium strumarium and the impacts of three aboveground herbivores on the invader

Exotic plant invasions are generally assumed to negatively impact biodiversity of recipient communities. However, experimental evidence for this assumption is little, especially in developing countries. In addition, there is hardly applicable method to control invasive plants. Enemy release is often hypothesized to facilitate exotic plant invasions, thus re-association with natural enemies of invasive ranges may inhibit invasions of exotic plants, and these enemies may have the potential to be used as biological control agents for these invaders. To address above problems, we determined abundance of the invasive plant Xanthium strumarium, its impacts on co-occurring native plants, and abundances of three herbivores and their impacts on the invader at five sites in the rice tract of Punjab, Pakistan. Relative abundance of the invader varied greatly among the study sites, ranging from 3.5% to 50.4%. The invader significantly reduced abundance of co-occurring native plants at all study sites, and their richness and evenness at some sites with high abundances of the invader. The polyphagous insects Phenacoccus solenopsis, Podisus maculiventris and Spodoptera frugiperda were all found on the invader (not on crops) at all study sites, which significantly reduced growth of the invader. The abundance of P. solenopsis on the invader was much higher than those of P. maculiventris and S. frugiperda. The abundances of the herbivores were significantly higher at sites with high relative to low relative abundances of the invader. Our study indicates that natural enemies may accumulate on invasive plants with the increase of their abundance and/or invasion time, inhibiting further invasions, and provides evidence for enemy release hypothesis. More studies are needed to determine whether these herbivores, especially P. solenopsis, could be used as biological control agents for the invader.

Phytotoxicity of root exudates of invasive Solidago canadensis on co-occurring native and invasive plant species

Phytotoxicity of root exudates of invasive Solidago canadensis on co-occurring native and invasive plant species

Fungal endophytes increase biomass production in pale swallow-wort (Vincetoxicum rossicum (Kleopow) Barbar.)

Pale swallow-wort, Vincetoxicum rossicum (Kleopow) Barbar., is an invasive weed in the lower Great Lakes Basin of North America. We investigated the relationship between V. rossicum productivity and several environmental variables across 54 established V. rossicum populations in southern Ontario. Variables included climate measurements, soil characteristics (pH, texture, and nutrient status), habitat type, plant community, and the diversity of root-associated fungi. Vincetoxicum rossicum roots were collected at all 54 sites, and associated fungi were compared using terminal restriction fragment length polymorphism. Aboveground biomass of V. rossicum was measured at 23 sites with similar light regimes, allowing comparison between populations. Results suggested that abiotic soil variables, and habitat type have little effect on V. rossicum productivity. Aboveground biomass production was significantly correlated with precipitation, and the presence of fungal pathogens and dark septate endophytes. Annual precipitation and soil clay content were also positively correlated with the abundance of soil pathogens. Accumulation of these microbes may negatively affect co-occurring native plants and associated fungal partners. The presence of these fungal species in soil could be used as an indicator of site susceptibility to invasion by V. rossicum and assist in the development of management plans for this exotic vine.

Plant height and leaf size: Which one is more important in affecting the successful invasion of Solidago canadensis and Conyza canadensis in urban ecosystems?

It is still debated whether invasive alien species (IAS) have superior functional traits, especially plant height and leaf size, than co-occurring native plant species (NPS). Meanwhile, as two important indicators affecting the competitiveness for sunlight acquisition, plant height and leaf size, which one is more important in affecting the successful invasion of IAS? Thus, it is vital to assess the differences in the functional trait between IAS and co-occurring NPS and the significance of plant height and leaf size in the successful invasion to further elucidate the mechanisms underlying successful invasion especially with diverse degrees of invasion, especially in urban ecosystems. This study aims to assess the differences in the functional trait between IAS and co-occurring NPS and the significance of plant height and leaf size in the successful invasion of two notorious IAS [i.e., Solidago canadensis L. and Conyza canadensis (L.) Cronq.] with different degrees of invasion (i.e., low and heavy degrees of invasion based on their relative abundance in the invaded habitats) in urban ecosystems by using field sampling experiments. The competitiveness for sunlight acquisition of IAS distinctly increases as the degree of invasion increases. The competitiveness for sunlight acquisition of IAS is evidently higher than that of NPS but contrary to the leaf photosynthetic area. Plant height is more strongly related to the relative abundance of the two IAS than leaf size. The contribution intensity of plant height of the two IAS to their relative abundance is obviously higher than leaf size. Thus, plant height, rather than leaf size, was the most important determinant of the successful invasion of the two IAS, especially under heavy degree of invasion. Thus, the invasiveness of the two IAS is largely contributed by the higher competitiveness for sunlight acquisition rather than the larger leaf photosynthetic area, especially under heavy degree of invasion.

Mixed evidence for plant-soil feedbacks in forest invasions.

Plant-soil feedbacks (PSFs) are plant-mediated changes to soil properties that ultimately influence plant performance, and can, thus, determine plant diversity, succession, and invasion. We hypothesized that PSFs influence invasion processes and that PSF mechanisms are largely driven by changes in soil properties produced by specific plant species. To test these hypotheses, we studied the effects of different soils collected from under common plant species on the growth of the invasive plant Phytolacca americana. We found that PSFs may interfere with invasion resistance because P. americana seedlings showed reduced growth (lower biomass) in soils collected from underneath some native species compared with soils collected from underneath P. americana and two non-native plants. We then selected eight co-occurring native and non-native plant species, and examined PSF dynamics and mechanisms in a pairwise conditioned soil greenhouse experiment. Plant species-specific conditioning effects regarding soil nutrients and enzyme activities were observed. Phytolacca americana had a high ability to use soil N, which may be related to its high invasion ability. Soil P was significantly lower in Quercus acutissima-conditioned soil, indicating that low P availability in Q. acutissima forests may enhance resistance to plant invasion. However, surprisingly, some native plants did not produce PSF effects that decreased the relative performance of invasive plants, nor did the invasive plants produce PSF effects that increased their own performance. We speculate that these PSF findings from greenhouse experiments cannot be extrapolated to field conditions because the litter and allelochemicals of some plants may be important for invasion resistance.

Release from below- and aboveground natural enemies contributes to invasion success of a temperate invader

Efforts to concurrently test for enemy release of both above- and belowground enemies for invasive plants in non-native range are limited. Moreover, direct evidence for the causal link between enemy release and performance is rare for invasive plants. Thus, we sought to investigate if above- and/or belowground enemies are involved in the enemy release mechanisms at various life stages of a temperate invader and their consequences on plant community properties. We conducted field surveys of enemy damage and plant performance, and plant-soil feedback experiments in the glasshouses using Ambrosia trifida in both its non-native and native ranges across the seed, seedling, and adult life stages. Field surveys showed that seeds of A. trifida were more severely damaged by aboveground enemies in the native relative to the non-native range, while the difference in leaf damage between ranges was small. Plant-soil feedback experiments showed that release from belowground enemies was also important, as seed germination and plant growth were significantly reduced in soils from the native range compared with soils from the non-native range. Consistent with the above results, A. trifida was larger, produced more seeds, had higher density, and exerted stronger impacts on co-occurring native plants in the non-native relative to the native range. Our results demonstrated that release from both above- and belowground enemies at various life stages contributed to the invasion success of A. trifida in its non-native range and highlighted the importance of considering both above- and belowground enemy release when studying plant invasions.

Lesser leaf herbivore damage and structural defense and greater nutrient concentrations for invasive alien plants: Evidence from 47 pairs of invasive and non-invasive plants

Empirical evidence of enemy release is still inconsistent for invasive alien plant species, although enemy release is the key assumption for both the enemy release hypothesis (ERH) and the evolution of increased competitive ability hypothesis (EICA). In addition, little effort has been made to test this assumption in terms of defense investment using a multi-species comparative approach. Using a phylogenetically controlled within-study meta-analytical approach, we compared leaf herbivore damage, structural defenses and nutrients between 47 pairs of invasive versus native and/or non-invasive alien plants in China. The invasive relative to the co-occurring native or non-invasive (native and non-invasive alien) plants incurred lesser leaf herbivore damage, had lesser leaf concentrations of cellulose, hemicellulose, lignin and carbon, lesser leaf density and carbon or lignin to nitrogen ratio but greater nutrients, which may facilitate success of the invasive plants. The lesser structural investment did not result in lesser leaf construction costs for the invaders, which may be associated with their greater leaf nitrogen concentration. However, the invasive plants were not significantly different from the non-invasive alien plants in any trait. Our results provide strong evidence for ERH, also are consistent with EICA, and indicate that enemy release may be an important factor in alien plant invasions.

The enhancement of root biomass increases the competitiveness of an invasive plant against a co-occurring native plant under elevated nitrogen deposition

Nitrogen (N) is one of the most important nutrition controlling plant functioning and ecological stability, which is predicted to increase rapidly in the future. Despite numerous attempts to clarify the effects of N deposition on the growth and physiological processes of plants, the difference in the effects of N deposition on biomass allocation between invasive and native species have not been fully understood. We conducted simulated N deposition experiments on a co-occurring species pair in a greenhouse to examine the plant invasion mechanisms from the patterns in plant biomass allocation. The results showed that the invasive Solidago canadensis and native Artemesia argyi have the similar response trend in N addition, where leaf mass fraction (LMF) significantly decreased and stem mass fraction (SMF) significantly increased. The root mass fraction (RMF) strongly increased in invasive S. canadensis but there was a decrease in native A. argyi respond to N addition. These findings indicated that increased nutrients, such as soil N, may shift the plant allocation structure due to unequal effects of N among different organs and species. And provided an invasion mechanism where a change in the environment leads to the potential exclusion of native competitors via a significant increase in root biomass fraction of invasive plants. We also found that the optimal partitioning theory rather than the allometric partitioning theory, better explains the N-induced changes in biomass allocation strategies in the poor-resource environment. These could lead to an increased competitive ability of invasive plants for resource acquisition and profit from neighboring species, coupled with greater biomass accumulation and higher competitiveness in its new habitat (N addition in poor resource environment) compared to A. argyi, could give S. canadensis a substantial advantage over its co-occurring native species.

Invasive alien clonal plants are competitively superior over co-occurring native clonal plants

Alien plant invasions are a major component of global change, and can severely threaten local floras and global biodiversity. A striking pattern of invasive floras is that many of the world’s worst invasive plants are clonal species. However, it is still unclear whether invasive clonal plant species are in general competitively superior over native clonal plant species and whether this pattern can be affected by environmental conditions. To test whether nitrogen (N) availability and suppression of enemies can alter the competitive outcome between invasive alien and resident native clonal plants, we selected five pairs of invasive alien and co-occurring native clonal plant species in China. We grew them both in monocultures (i.e. with intraspecific competition) and in two-species mixtures (i.e. with interspecific competition) with or without N addition, and with or without suppression of enemies (using insecticide and fungicide application). Overall, invasive alien clonal plants produced significantly more biomass than native clonal plants, and took more advantage of N addition. Invasive clonal plants tended to grow better with interspecific than with intraspecific competition, whereas the reverse was true for native clonal plants. This species-origin-by-competition interaction, however, was weaker with than without N addition. Without enemy suppression, native clonal plants experienced more leaf damage than invasive aliens when they grew in interspecific competition with each other, but only in the absence of N addition. Nevertheless, enemy suppression benefited growth of invasive alien and native clonal plants to similar degrees. Our results show that already under strongly N-limited conditions, invasive clonal plants in China are competitively superior over natives, and that this competitive superiority of invasive clonal plants becomes even stronger after N addition. This implies that the ongoing eutrophication of many habitats is likely to further stimulate invasions by clonal plants.

Experimental species introduction shapes network interactions in a plant-pollinator community

Invasive species that form mutualistic interactions can perturb resident communities by creating new interactions, or weakening the strength of existing interactions via competition. We hypothesized that introducing a super-generalist plant species to bee-plant networks would lead to (1) increases in the weighted and unweighted average degree, nestedness, and connectance and (2) decreases in the modularity. We constructed visitation networks of bees to five native plant species in experimental research plots, in the presence and absence of an invasive thistle and while varying thistle abundance and time/duration of introduction. Species introduction did not change the visitation rate of bees to co-occurring native plants, or the interaction structure between the native plant species and bee visitors; seed set of a focal native plant species was also unaffected. We found the number of bee species with which the introduced species interacted (generality) correlated with significant increases in unweighted and weighted average degree, nestedness, and connectance, but not modularity. When comparing the impact of the introduced species either early or late in the season, we found similar relationships between introduced species generality and weighted and unweighted average degree and connectance; there was a significant negative relationship between introduced species generality and modularity early in the season, and a significant positive relationship with nestedness late in the season, suggesting introduction timing within a season may affect some measures differently. Overall, the native community was robust to the introduction of the super-generalist; our experimental test of network theory predictions demonstrates an improving mechanistic understanding of how mutualistic networks respond to ecological perturbations.

Microhabitat characteristics and impacts of invasive species for the triple-ribbed milkvetch (Astragalus tricarinatus [Fabaceae]) in the San Bernardino Mountains, California.

Astragalus tricarinatus A.Gray is a federally-listed short lived perennial herb endemic to southern California. Its primary habitat is along the ecotone of the Mojave and Colorado Deserts in the San Bernardino and Little San Bernardino Mountains between 390 and 1525 m. Discovery of this species' upland habitat has prompted research into the ecology of populations across its range, as the life history and threats may be different from those of the previously-known wash populations. Our study sought to further document A. tricarinatus distribution and abundance within its range, as well as quantify impacts to its growth and reproductive efforts associated with co-occurring native and invasive plant species. We mapped all known localities in our study area on aerial imagery then used this to visually recognize its typical edaphic habitat and identify new potential sites. Where A. tricarinatus was found, we established plots in which plant size, the number of reproductive structures, and microhabitat characteristics were recorded. Two new occurrences (defined here as >1 km away from known localities) were recorded within its existing range as well as a previously undocumented locality between two known occurrences containing an estimated 2600 plants. Most plants were found on south and east aspects, and on slopes ranging from 30–45°. We found that increases in invasive cover correlated with a decrease in production of reproductive structures. As a federally-listed species occurring in steep, remote habitats with few threats, these findings suggesting an association between invasive species presence and reduced reproductive structures should be further investigated experimentally, as they affect population dynamics and conservation of this species.

Diminishing importance of elaiosomes for acacia seed removal in non-native ranges

Myrmecochorous plants produce seeds with lipid-rich appendages (elaiosomes) which act as a reward for seed-dispersing ants. Seed dispersal is important for exotic species, which often need to establish new mutualistic interactions in order to colonize new non-native habitats. However, little is known about the importance of elaiosomes for seed removal in many of their non-native ranges. We studied ant–seed interactions of elaiosome-bearing and elaiosome-removed seeds of the Australian trees Acacia dealbata and Acacia longifolia in order to assess the relative importance of elaiosomes for seed removal between their native (Australia) and non-native (Portugal) ranges. In Portugal, we also studied the co-occurring native plant species with myrmecochorous seeds, Pterospartum tridentatum and Ulex europaeus, across three contiguous levels of acacia invasion: control (i.e. no acacia), low, and high acacia tree density. Acacia seeds were successfully removed by ants in their non-native region by a diversified assemblage of ant species, even in sites where native plants interacted with only one specialized ant species. In the invaded range, diminishing relative importance of elaiosomes was associated with changes in the ant community due to acacia invasion, and for A. dealbata, ant species richness decreased with increasing acacia tree density. Although seed removal was high for both acacia species, the importance of elaiosomes was proportionally lower for A. dealbata in the non-native region. Native plant species experienced significant reductions in seed removal in areas highly invaded by acacia, identifying another mechanism of displacement of native plants by acacias.

An invasive plant provides refuge to native plant species in an intensely grazed ecosystem

Invasion by exotic plant species and herbivory can individually alter native plant species diversity, but their interactive effects in structuring native plant communities remain little studied. Many exotic plant species escape from their co-evolved specialized herbivores in their native range (in accordance with the enemy release hypothesis). When these invasive plants are relatively unpalatable, they may act as nurse plants by reducing herbivore damage on co-occurring native plants, thereby structuring native plant communities. However, the potential for unpalatable invasive plants to structure native plant communities has been little investigated. Here, we tested whether presence of an unpalatable exotic invader Opuntia ficus-indica was associated with the structure of native plant communities in an ecosystem with a long history of grazing by ungulate herbivores. Along 17 transects (each 1000 m long), we conducted a native vegetation survey in paired invaded and uninvaded plots. Plots that harboured O. ficus-indica had higher native plant species richness and Shannon–Wiener diversity H′ than uninvaded plots. However, mean species evenness J was similar between invaded and uninvaded plots. There was no significant correlation between native plant diversity and percentage plot cover by O. ficus-indica. Presence of O. ficus-indica was associated with a compositional change in native community assemblages between paired invaded and uninvaded plots. Although these results are only correlative, they suggest that unpalatable exotic plants may play an important ecological role as refugia for maintenance of native plant diversity in intensely grazed ecosystems.

Initial Response by a Native Beetle, Chrysochus auratus (Coleoptera: Chrysomelidae), to a Novel Introduced Host-Plant, Vincetoxicum rossicum (Gentianales: Apocynaceae).

Native insects can form novel associations with introduced invasive plants and use them as a food source. The recent introduction into eastern North America of a nonnative European vine, Vincetoxicum rossicum (Kleopow) Barbar., allows us to examine the initial response of a native chrysomelid beetle, Chrysochus auratus F., that feeds on native plants in the same family as V. rossicum (Apocynaceae). We tested C. auratus on V. rossicum and closely related or co-occurring native plants (Apocynum spp., Asclepias spp., and Solidago canadensis L.) using all life stages of the beetle in lab, garden, and field experiments. Experiments measured feeding (presence or absence and amount), survival, oviposition, and whether previous exposure to V. rossicum in the lab or field affected adult beetle feeding. Beetles fed significantly less on V. rossicum than on native Apocynum hosts. Adult beetles engaged in exploratory feeding on leaves of V. rossicum and survived up to 10 d. Females oviposited on V. rossicum, eggs hatched, and larvae fed initially on the roots; however, no larvae survived beyond second instar. Beetles collected from Apocynum cannabinum L. field sites intermixed with V. rossicum were less likely to feed on this novel nonnative host than those collected from colonies further from and less likely to be exposed to V. rossicum (>5 km). Our experimental work indicates that V. rossicum may act as an oviposition sink for C. auratus and that this native beetle has not adapted to survive on this recently introduced novel host plant.