The distribution of native and exotic plants in a naturally fragmented sagebrush-steppe landscape

We tested two general hypotheses for the diversity of native and exotic plants in an undisturbed, naturally fragmented sagebrush-steppe landscape in SE Idaho, USA, evaluating whether the MacArthur–Wilson hypothesis of island biogeography or a suite of environmental variables explained the distributions of native and exotic plants. We also tested a third hypothesis, which incorporated assumptions about the origin of exotic plants and their interaction with native plants. Of the three hypotheses we tested, the hypothesis that included exotic species best explained the diversity of the native plant community. The MacArthur–Wilson model of island biogeography did not explain the diversity of native (R ² = 0.13) or exotic plants well (R ² = 0.11), and the model fit the data poorly. A model of environmental variables better explained the diversity of native (R ² = 0.48) and exotic plants (R ² = 0.57), but it also fit the data poorly. Instead, proximity to a railroad explained the cover (R ² = 0.59) and richness of exotic plants (R ² = 0.63), which then explained the species richness of native plants (R ² = 0.34), and the model fit was adequate and had the lowest AIC value. This suggests that the transportation corridor had a significant, though indirect, effect on the native plant community, even in this undisturbed area. Moreover, explained variance, model fit, and the AIC model selection criteria favored the model with the railroad and exotic species over the M–W and environmental models. Since the habitat patches we studied were largely undisturbed by people and their activities, our results further suggest that the transportation corridor influenced the distribution of exotic plants by serving as a vector for colonization, rather than as a source of disturbance. Additionally, the results suggest that exotic plant species have had a negative effect on the diversity of the native plant community and have changed its composition. The results also support the inference that the nascent exotic plant community is influenced by source-sink (Pulliam in Am Nat 132:652–661, 1988) and assembly dynamics. In contrast, the native plant community appears to be more strongly influenced by environmental conditions associated with an elevational gradient, but there is evidence that the native community also has undergone directional change in species composition, associated with the invasion by non-native species.     

Competition,legacy, and priority and the success of three invasive species

Competitive ability, the ability to generate legacy effects, and the potential to benefit from priority, individually or interactively, are traits that may increase the invasive potential of plants. In this project we examine these three traits in three invasive species (Agropyron cristatum, Bromus tectorum, and Taeniatherum caput-medusae). Specifically in this study, we examine competitive effects of these invasive species, the ability of these invasive species to generate legacy effects (as plant–soil feedback), and the potential of these three species benefit from priority (being sown concurrently, 30 days before, and 30 days after the restoration species) in a greenhouse study using field collected soil. Our results suggest that all three invasive species can benefit from priority and all three have high competitive ability. However, only A. cristatum benefited from legacy effects of plant–soil feedback.     

The Imperial Palm (Roystonea oleracea (Jacq.) O.F. Cook) as an invasive species of a wetland in Brazilian Atlantic forest

The aim of this research was to study the population structure of an exotic palm, Roystonea oleracea (Arecaceae), map its spatial distribution and investigate its invasive status in an Atlantic forest wetland. A total of 130 live and 7 dead standing palms ≥10 cm DBH were sampled. Data showed a trend towards population expansion, with most seedlings and saplings occurring within a radius of ~12 m from the mother plants but some larger trees (DBH >10 cm) up to 440 m from the parents. This exotic species is established and has the potential to become invasive, especially near wetlands. Its control or eradication should be undertaken and we suggest the immediate removal of reproductive individuals, and constant monitoring of young individuals leading to their future removal. Use of this species as an ornamental should be made with care to prevent or reduce possible harmful effects of this exotic and potentially invasive species in Brazilian wetlands.     

Invasive alien plants increase CH4 emissions from a subtropical tidal estuarine wetland

Methane (CH₄) is an important greenhouse gas whose emission from the largest source, wetlands is controlled by a number of environmental variables amongst which temperature, water-table, the availability of substrates and the CH₄ transport properties of plants are most prominent and well characterised. Coastal wetland ecosystems are vulnerable to invasion by alien plant species which can make a significant local contribution to altering their species composition. However the effect of these changes in species composition on CH₄ flux is rarely examined and so is poorly understood. Spartina alterniflora, a perennial grass native to North America, has spread rapidly along the south-east coast of China since its introduction in 1979. From 2002, this rapid invasion has extended to the tidal marshes of the Min River estuary, an area that, prior to invasion was dominated by the native plant Cyperus malaccensis. Here, we compare CH₄ flux from the exotic invasive plant S. alterniflora with measurements from the aggressive native species Phragmites australis and the native species C. malaccensis following 3-years of monitoring. CH₄ emissions were measured over entire tidal cycles. Soil CH₄ production potentials were estimated for stands of each of above plants both in situ and in laboratory incubations. Mean annual CH₄ fluxes from S. alterniflora, P. australis and C. malaccensis dominated stands over the 3 years were 95.7 (±18.7), 38.9 (±3.26) and 10.9 (±5.26) g m^?2 year^?1, respectively. Our results demonstrate that recent invasion of the exotic species S. alterniflora and the increasing presence of the native plant P. australis has significantly increased CH₄ emission from marshes that were previously dominated by the native species C. malaccensis. We also conclude that higher above ground biomass, higher CH₄ production and more effective plant CH₄ transport of S. alterniflora collectively contribute to its higher CH₄ emission in the Min River estuary.     

Optimal prescribed burn frequency to manage foundation California perennial grass species and enhance native flora

Grasslands can be diverse assemblages of grasses and forbs but not much is known how perennial grass species management affects native plant diversity except in a few instances. We studied the use of late-spring prescribed burns over a span of 11 years where the perennial grass Poa secunda was the foundation species, with four additional years of measurements after the final burn. We evaluated burn effects on P. secunda, the rare native annual forb Amsinckia grandiflora and local native and exotic species. Annual burning maintained P. secunda number, resulted in significant expansion, the lowest thatch and exotic grass cover, the highest percentage of bare ground, but also the lowest native forb and highest exotic forb cover. Burning approximately every 3 years maintained a lower number of P. secunda plants, allowed for expansion, and resulted in the highest native forb cover with a low exotic grass cover. Burning approximately every 5 years and the control (burned once from a wildfire) resulted in a decline in P. secunda number, the highest exotic grass and thatch cover and the lowest percentage of bare ground. P. secunda numbers were maintained up to 4 years after the final burn. While local native forbs benefited from burning approximately every 3 years, planted A. grandiflora performed best in the control treatment. A. grandiflora did not occur naturally at the site; therefore, no seed bank was present to provide across-year protection from the effects of the burns. Thus, perennial grass species management must also consider other native species life history and phenology to enhance native flora diversity.     

Nitrogen acquisition by annual and perennial grass seedlings: testing the roles of performance and plasticity to explain plant invasion

Differences in resource acquisition between native and exotic plants is one hypothesis to explain invasive plant success. Mechanisms include greater resource acquisition rates and greater plasticity in resource acquisition by invasive exotic species compared to non-invasive natives. We assess the support for these mechanisms by comparing nitrate acquisition and growth of invasive annual and perennial grass seedlings in western North America. Two invasive exotic grasses (Bromus tectorum and Taeniatherum caput-medusae) and three perennial native and exotic grasses (Pseudoroegneria spicata, Elymus elymoides, and Agropyron cristatum) were grown at various temperatures typical of autumn and springtime when resource are abundant and dominance is determined by rapid growth and acquisition of resources. Bromus tectorum and perennial grasses had similar rates of nitrate acquisition at low temperature, but acquisition by B. tectorum significantly exceeded perennial grasses at higher temperature. Consequently, B. tectorum had the highest acquisition plasticity, showcasing its ability to take advantage of transient warm periods in autumn and spring. Nitrate acquisition by perennial grasses was limited either by root production or rate of acquisition per unit root mass, suggesting a trade-off between nutrient acquisition and allocation of growth to structural tissues. Our results indicate the importance of plasticity in resource acquisition when temperatures are warm such as following autumn emergence by B. tectorum. Highly flexible and opportunistic nitrate acquisition appears to be a mechanism whereby invasive annual grasses exploit soil nitrogen that perennials cannot use.     

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.     

Alterations in flood frequency increase exotic and native species richness of understorey vegetation in a temperate floodplain eucalypt forest

The delivery of environmental flows for biodiversity benefits within regulated river systems can potentially contribute to exotic weed spread. This study explores whether exotic plants of a floodplain forest in Victoria, Australia, are characterised by specific functional groups and associated plant traits linked to altering hydrological conditions over time. Permanently marked 20 × 20 m² plots from five wetland sites in Eucalyptus camaldulensis floodplain forest were sampled twice, first in the early 1990s (1993–1994) and then 15 years later (2007–2008). Species cover abundance data for understorey vegetation communities were segregated by season and analysed using ordination analysis. Exotic species richness was modelled as a function of site flooding history and native species richness using general linear models. Site ordinations by detrended correspondence analysis showed differential community compositions between survey dates, but native and exotic species were not clearly differentiated in terms of DCA1 scores. Most exotics belonged to functional groups containing annual species that germinate and reproduce under drier conditions. Exotics reproducing under wetter conditions were in the minority, predominantly perennial and capable of both sexual and asexual reproduction. Site flooding history and native species richness significantly predicted exotic species richness. Vegetation changes are partially structured by reduced flood frequency favouring increased abundance of exotic, sexually reproducing annuals at drier sites. Sites of low flood frequency are more sensitive to future exotic weed invasion and will require targeted management effort. Flow restoration is predicted to benefit propagule dispersal of species adopting dual regeneration strategies, which are predominantly natives in this system.     

Does restoration help the conservation of the threatened forest of Robinson Crusoe Island? The impact of forest gap attributes on endemic plant species richness and exotic invasions

Invasive plant species are major drivers of biodiversity losses, especially on islands which are prone to invasions and extinctions. In the “endemic montane forest” of Robinson Crusoe Island (Pacific Ocean, Chile) invasive exotic plant species threaten conservation efforts by establishing in gaps and outcompeting native tree species regeneration. We compared gap attributes and ground vegetation cover in three gap types: those dominated by native species (<5 % cover of invasive species), invaded gaps (>30 % cover by invasive species), and treated gaps (invasive species removed). We examined (a) which gap attributes favored native and exotic species, (b) the relationship between gap size and species richness, and (c) species responses to invasion and treatment. Gaps ranged in size from 46 to 777 m² caused mainly by uprooted and snapped trees. Multi response permutation procedures showed a different floristic composition between natural, invaded and treated gaps. The presence of Myrceugenia fernandeziana (native species) and Aristotelia chilensis (invasive species) as gap border trees was positively and negatively correlated with native species richness, respectively. New gaps had more native species than old gaps, and smaller gaps contained relatively more native species than larger ones. An increase in invasive species cover was related to a decline in native species cover and richness. 1–6 years after treatment gaps tended to recover their native floristic composition. Highly effective conservation management programs will concentrate on monitoring gap creation, early control of invasive species, and by treating smaller gaps first.     

Consumers and establishment limitations contribute more than competitive interactions in sustaining dominance of the exotic herb garlic mustard in a Wisconsin, USA forest

The understory is a diverse component of temperate forest ecosystems, contributing significantly to forest ecosystem services. Despite their importance, many native understories face stresses from current and past land use, habitat fragmentation, invasive species, and overabundant herbivores. We established a four block, three factor experiment to evaluate the relative contribution of native plant establishment, competitive effects from the invasive herb garlic mustard (Alliaria petiolata), and herbivory from white-tailed deer (Odocoileus virginianus) to better understand the mechanisms promoting low native plant richness and cover and understory dominance by the biennial exotic herb garlic mustard in a NE Wisconsin, USA forest. Four years of garlic mustard removal failed to increase native plant richness or cover in non-restored plots. However, deer access and the introduction of native plants (restoration treatment) both significantly enhanced native plant cover and richness, with restored species cover in fenced plots approximately 216 % that of open-access plots, and the majority of these species flowered at significantly higher proportions inside of fenced areas. In contrast, deer access did not significantly alter the cover, or seed production of garlic mustard. We also found no significant effect of garlic mustard presence on the cover or flowering of restored native species. We conclude that multiple factors, including limited natural establishment by native species and selective herbivory drove low native, high exotic dominance at our site, suggesting that a shift in focus from invasive plant removal to combined native plant restoration and herbivore control is needed to maximize the recovery of this degraded forest understory.     

Invasion by Fallopia spp. in a French upland region is related to anthropogenic disturbances

Within Europe, mountain ecosystems are generally less invaded by exotic plant species than are lowland areas. This pattern is commonly attributed to climatic harshness, which limits invasive species presence, and higher propagule pressure and rates of disturbance in lowlands, which favours dissemination. However, the extent to which anthropogenic and natural disturbances contribute to invasive species presence in mountain and lowland environments remains unclear. We conducted field observations in a lowland and an upland region in France and measured environmental variables, estimated the natural and anthropogenic disturbance of plots invaded by Fallopia spp. and compared them to non-invaded plots. Based on generalised linear mixed models, the predictors of Fallopia spp. presence in the upland area only included anthropogenic elements such as the presence of a road or trail and frequentation by humans, whereas both anthropogenic parameters and natural components (light penetration, slope, presence of a road and of a watercourse) were retained as predictors for the lowland region. We calculated the odds of Fallopia spp. presence for the increase of one unit of each predictor. We conclude that the spread of Fallopia spp. in upland areas was mainly linked to human activity whereas dissemination of the species occurred both through humans and in natural ways in lowland areas, and this may be due to a more recent colonisation in the mountains. We therefore advise stakeholders to undertake actions in mountain areas to specifically limit the dissemination of exotic species by humans and to monitor areas of high invasion risk by exotic species, such as areas neighbouring trails and roads highly frequented by humans.     

Impact of invasive plants in Mediterranean habitats: disentangling the effects of characteristics of invaders and recipient communities

While the effects of an invasive alien plant that has become dominant in a community may seem obvious, there are few studies that attempt to understand how impacts vary according to the characteristics of invaders and recipient communities. For this purpose, the vegetation of invaded and non-invaded plots was sampled for eight different invasive species in a variety of habitats within the French continental Mediterranean region. Most of the observed impact variation was species-specific, with greater effects on community-level metrics found for Carpobrotus spp. and Reynoutria × bohemica and lower effects for Amorpha fruticosa, Ambrosia artemisiifolia and Phyla filiformis. Some trends were consistent with competition-driven processes, with higher impact found in the presence of rhizomatous and creeping perennial invasive species compared to annuals, or in habitats with sparse vegetation. The importance of community characteristics such as the cover of the invasive plant or the differences in cover between the invader and the native dominant species confirmed previous results obtained in Central Europe. Therefore, such variables, easy to measure and with a generic value, could be profitably integrated into risk assessment methods to improve the prediction of the most threatened habitats. Beyond the overall decline in species diversity, the presence of some invasive species was associated with significant changes in species composition, with a filtering toward more shade-tolerant and nitrophilous ruderal species. Managers should consider replacement of resident species by species with different ecological preferences together with simple community-level metrics, to decide whether management is justified.     

Influence of different landscape design styles on plant invasions in Central Europe

When considering the most frequent invasive exotic plants on an international scale, it is evident that the majority are ornamentals and that they were deliberately introduced in relation to landscape design. Although there are worldwide numerous lists of invasive exotic plants—which means plants that have harmful effects on native biodiversity—an assessment of the contribution of different landscape design styles on plant invasions has not been done. We used the extensive database on the history of introduction and naturalization of alien plants into natural habitats of Central Europe (Lohmeyer and Sukopp, Agriophyten in der Vegetation Mitteleuropas in: Schriftenreihe Vegetationskunde 25, 1992; Nachtrag: Braunschweiger Geobotan Arbeiten 8:179–220, 2001) to examine how many alien ornamental plants there are in the different natural habitats; and how many of them are invasive exotic plants and—in contrast—how many have not spread significantly. Also, we researched contributions by different landscape design styles to these plant invasions since medieval times. Of the estimated 12,000 alien plant species introduced into Central Europe since the Neolithic period, 279 taxa (2.3 %) are currently identified as being naturalized in natural habitats; 103 (0.86 %) of these naturalized taxa are ornamentals, and of these, 40 (0.33 %) are invasive exotic plants. Our investigation has shown a correlation between the frequency of plant invasions and changes in landscape-design styles. Evaluating the impact of plant invasions through horticulture and landscape design on native biodiversity, our study illustrates that it is significantly lower in Central Europe than in other parts of the world.     

Limits to local spatial spread in a highly invasive annual grass (Microstegium vimineum)

The potential roles of seed and microsite limitation in local spatial spread of the invasive grass Microstegium vimineum were experimentally investigated in a woodland and open lawn in central New Jersey, USA. Plots (30 × 30 cm) in three sites previously unoccupied by M. vimineum for at least 8 years (woodland interior, woodland edge, and open lawn) were sown with ~262 M. vimineum seeds in early spring 2008. Seedling emergence, density, summer growth and autumn reproduction were compared to plots in a nearby control population where natural recruitment occurred. Seedling emergence was greatest in the open lawn (54% of seeds sown) where plants showed the greatest growth and reproduction due to high light availability. Seedling emergence was lowest in the woodland interior (24%) and edge sites (9%), and growth and reproduction were greatly reduced there (relative to the control). Plots in the open lawn supported a consistently high density of M. vimineum (>1,000 plants per m²) through the growing period (April to October). The number of seeds in both cleistogamous and chasmogamous spikelets was correlated with shoot dry mass and thus, total seed production was greatest in the sunny open lawn where plants were largest, despite high density there. Across all sites, plants in plots at the highest densities produced the most seeds. Total seed production correlated with levels of light, but not soil moisture. Both seed availability and microsite limitation may reduce the probability of establishment of new M. vimineum populations into previously unoccupied sites. Intraspecific density does not negatively affect survival or reproduction. Light and soil moisture can be limiting abiotic factors in some areas, but poor, natural seed dispersal limits the distribution of this invasive species on a local scale.     

Growth and phenology of Dittrichia graveolens, a rapidly spreading invasive plant in California

Dittrichia graveolens is a rapidly spreading invasive plant in California. While populations are observed primarily in disturbed areas, there is concern it may expand into adjacent undisturbed areas, particularly grasslands and riparian corridors. In a field experiment conducted in two successive years, we compared plant growth and phenological development of fall, winter, and spring sown seeds. Plants establish equally well in disturbed upland sites in both above and below average precipitation years but the absence of late spring rainfall negatively affected total plant biomass. In a greenhouse experiment, we compared growth in four light environments (100, 50, 27 and 9 % available light). Total plant growth decreased exponentially with decreasing light. This suggests that D. graveolens is not competitive in low light environments, such as woodlands and riparian forests. All plants flowered in early- to mid-September, coinciding with flowering in field grown plants, suggesting that photoperiod is the primary signal for reproductive growth. Using a minirhizotron system, we measured root growth over time in D. graveolens and three common California annual grassland species, two non-natives, Centaurea solstitialis and Bromus hordeaceus, and the native forb Holocarpha virgata. Root growth of D. graveolens began later in the season than the other species, reaching depths >1 m by late May. Roots of C. solstitialis and H. virgata reached >1 m earlier in the season. The temporal difference in root growth suggests that D. graveolens may be less competitive for soil moisture with other early season annuals than other deep-rooted broadleaf species found in grasslands.     

Frugivory by introduced black rats (Rattus rattus) promotes dispersal of invasive plant seeds

Oceanic islands have been colonized by numerous non-native and invasive plants and animals. An understanding of the degree to which introduced rats (Rattus spp.) may be spreading or destroying seeds of invasive plants can improve our knowledge of plant-animal interactions, and assist efforts to control invasive species. Feeding trials in which fruits and seeds were offered to wild-caught rats were used to assess the effects of the most common rat, the black rat (R. rattus), on 25 of the most problematic invasive plant species in the Hawaiian Islands. Rats ate pericarps (fruit tissues) and seeds of most species, and the impacts on these plants ranged from potential dispersal of small-seeded (≤1.5 mm length) species via gut passage (e.g., Clidemia hirta, Buddleia asiatica, Ficus microcarpa, Miconia calvescens, Rubus rosifolius) to predation where <15% of the seeds survived (e.g., Bischofia javanica, Casuarina equisetifolia, Prosopis pallida, Setaria palmifolia). Rats consumed proportionally more seed mass of the smaller fruits and seeds than the larger ones, but fruit and seed size did not predict seed survival following rat interactions. Although invasive rat control efforts focus on native species protection, non-native plant species, especially those with small seeds that may pass internally through rats, also deserve rat control in order to help limit the spread of such seeds. Black rats may be facilitating the spread of many of the most problematic invasive plants through frugivory and seed dispersal in Hawaii and in other ecosystems where rats and plants have been introduced.     

Invasive and socially parasitic ants are good bioindicators of habitat quality in Mediterranean forest remnants in northeast Spain

We surveyed ants in 16 forest remnants in the Vallès lowlands north of Barcelona, Spain: eight with invasive exotic ants (IE = Linepithema humile and/or Lasius neglectus) present, seven with native parasitic ants (P = Lasius meridionalis, Lasius carniolicus, Plagiolepis xene, Chalepoxenus muellerianus, and/or Polyergus rufescens) present, and one remnant with both invasive exotic and parasitic ants present. Forest remnants with IE ants were smaller, more isolated, had greater perimeter/area, lower ant species density, and lower ant species richness than remnants with P ants. The community composition was also significantly different, with greater dissimilarity within remnants with alien, invasive species. The presence of some species is bio-indicator of low disturbance, whereas others are indicative of high disturbance. Our findings underscore the value of different types of ants as bio-indicators of fragmentation and habitat quality.     

A fast-track for invasion: invasive plants promote the performance of an invasive herbivore

With the greater frequency of biological invasions worldwide there is an increased likelihood that exotic species will interact with each other, and such interactions could enhance one another??s invasion potential. Although direct and indirect interactions between exotic species have been well documented for plant-herbivore interactions, the majority of studies have focused on a single interaction and on plant rather than herbivore performance. In this study we investigated whether invasive exotic plants could contribute to the invasion of California by an exotic generalist herbivore (Epiphyas postvittana). We tested this expectation in the greenhouse by monitoring the performance of larval and pupal stages of E. postvittana on six pairs of congeneric invasive and native plants. Larval survivorship and pupal weight of E. postvittana were both greater on the invasive species, and larval development time was shorter on the invasive plant species for two of the plant genera. Our results suggest that prior invasion of exotic plants could function as a catalyst for the subsequent invasion of an exotic insect herbivore, at least in the case where they have shared some history, thereby accelerating the invasion process and expansion of its novel geographic range.     

Promoting Native Vegetation and Diversity in Exotic Annual Grass Infestations

Exotic plant invasions are especially problematic because reestablishment of native perennial vegetation is rarely successful. It may be more appropriate to treat exotic plant infestations that still have some remaining native vegetation. We evaluated this restoration strategy by measuring the effects of spring burning, fall burning, fall applied imazapic, spring burning with fall applied imazapic, and fall burning with fall applied imazapic on the exotic annual grass, medusahead (Taeniatherum caput‐medusae (L.) Nevski), and native vegetation at six sites in Oregon for 2 years post‐treatment. Medusahead infestations included in this study had some residual native perennial bunchgrasses and forbs. Burning followed by imazapic application provided the best control of medusahead and resulted in the greatest increases in native perennial vegetation. However, imazapic application decreased native annual forb cover the first year post‐treatment and density the first and second year post‐treatment. The spring burn followed by imazapic application produced an almost 2‐fold increase in plant species diversity compared to the control. The fall burn followed by imazapic application also increased diversity compared to the control. Results of this study indicate that native plants can be promoted in medusahead invasions; however, responses vary by plant functional group and treatment. Our results compared to previous research suggest that restoration of plant communities invaded by exotic annual grass may be more successful if efforts focus on areas with some residual native perennial vegetation. Thus, invasive plant infestations with some native vegetation remaining should receive priority for restoration efforts over near monocultures of invasive plant species.     

Integration of exotic seeds into an Azorean seed dispersal network

Seed dispersal plays a central role in plant ecology. Among animals, birds are particularly important seed dispersers, often incorporating exotic plants into their diets and facilitating their integration in the communities. Network theory offers a highly informative framework to study the structural and functional attributes of complex interactions networks. We used information from bird fecal samples to build a quantitative seed dispersal network for the last fragment of native laurel forest in the island of São Miguel—Azores with three specific objectives: (1) to assess the integration of exotic seeds into seed dispersal; (2) to evaluate the impact of exotic plants in network structure; (3) to test the potential of an exotic species to reduce the seed dispersal of a co-occurring native, via competition for seed dispersers. The seed dispersal network was based on the analysis of 1,121 droppings and described 74 unique interactions between 41 plant species and 7 bird species. Exotic seeds deeply infiltrated into the seed dispersal network forming the majority (59 %) of seeds in the droppings and including those of three globally invasive plants. Overall, birds depended equally on native and exotic fruits despite the lower abundance of the latter in the study area. In an experiment, birds did not show a preference for fruits of the exotic Leycesteria formosa over the native Vaccinium cylindraceum consuming them equally. However, the presence of the exotic plant negatively affected the number of native seeds dispersed, by diverting some of the consumers of the native fruits. Taken altogether the results reveal an alarming invasion level of seed dispersal systems in one of the last remnant native forests of the Azores.