Woody exotic plant invasions and fire: reciprocal impacts and consequences for native ecosystems

Fire regimes influence and are influenced by the structure and composition of plant communities. This complex reciprocal relationship has implications for the success of plant invasions and the subsequent impact of invasive species on native biota. Although much attention has been given to the role of invasive grasses in transforming fire regimes and native plant communities, little is known about the relationship between woody invasive species and fire regime. Despite this, prescribed burning is frequently used for managing invasive woody species. In this study we review relationships between woody exotic plant invasions and fire in invaded ecosystems worldwide. Woody invaders may increase or decrease aspects of the fire regime, including fire frequency, intensity and extent. This is in contrast to grass invaders which almost uniformly increase fire frequency. Woody plant invasion can lead to escape from a grass-fire cycle, but the resulting reduction in fire frequency can sometimes lead to a cycle of rare but more intense fires. Prescribed fires may be a useful management tool for controlling woody exotic invaders in some systems, but they are rarely sufficient to eliminate an invasive species, and a dearth of controlled experiments hampers evaluation of their benefits. Nevertheless, because some woody invaders have fuel properties that differ substantially from native species, understanding and managing the impacts of woody invaders on fire regimes and on prescribed burns should become an important component of resource and biodiversity management.     

Bryophytes display allelopathic interactions with tree species in native forest ecosystems

Bryophytes are widespread in terrestrial ecosystems but little is known about their influence on vascular species. Water‐soluble leachates (0%, 1%, 5%, 10% concentration) derived from 18 species of bryophytes (mosses 11 species; liverworts 7 species) were tested on the germination and seedling growth of Lactuca sativa and two common trees Melicytus ramiflorus (Violaceae) and Fuchsia excorticata (Onagraceae) in southern New Zealand forests. Bryophyte water soluble extracts (BWSE) have minor impact on seed germination of Lactuca, stimulatory effects on radical growth at low (1%) concentrations and inhibitory effects at higher concentrations (5–10%). For Melicytus the BWSE had variable effects, with evidence of strong stimulatory (Dendrohypopterygium filiculiforme) and inhibitory (Lepidozia concinna) effects on germination, but generally inhibited radical growth. BWSE at all test concentrations consistently inhibit both germination and seedling radicle growth in Fuchsia. The toxicity effect of water‐soluble leachates varies significantly between bryophyte species but not consistently between mosses and liverworts. Bryophyte species exhibiting strongest inhibition effects under control conditions were associated with significantly reduced densities of broadleaved tree seedlings in forest ecosystems. Our results demonstrate that some bryophyte species via allelopathic interactions can inhibit seedling establishment and growth of forest trees. This mechanism provides an additional factor constraining the spatial distribution of the regeneration niche in forest communities.     

Dispersal and recruitment limitation in native versus exotic tree species: life‐history strategies and Janzen‐Connell effects

Life‐history traits of invasive exotic plants are typically considered to be exceptional vis‐à‐vis native species. In particular, hyper‐fecundity and long range dispersal are regarded as invasive traits, but direct comparisons with native species are needed to identify the life‐history stages behind invasiveness. Until recently, this task was particularly problematic in forests as tree fecundity and dispersal were difficult to characterize in closed stands. We used inverse modelling to parameterize fecundity, seed dispersal and seedling dispersion functions for two exotic and eight native tree species in closed‐canopy forests in Connecticut, USA. Interannual variation in seed production was dramatic for all species, with complete seed crop failures in at least one year for six native species. However, the average per capita seed production of the exotic Ailanthus altissima was extraordinary: > 40 times higher than the next highest species. Seed production of the shade tolerant exotic Acer platanoides was average, but much higher than the native shade tolerant species, and the density of its established seedlings (≥ 3 years) was higher than any other species. Overall, the data supported a model in which adults of native and exotic species must reach a minimum size before seed production occurred. Once reached, the relationship between tree diameter and seed production was fairly flat for seven species, including both exotics. Seed dispersal was highly localized and usually showed a steep decline with increasing distance from parent trees: only Ailanthus altissima and Fraxinus americana had mean dispersal distances > 10 m. Janzen‐Connell patterns were clearly evident for both native and exotic species, as the mode and mean dispersion distance of seedlings were further from potential parent trees than seeds. The comparable intensity of Janzen‐Connell effects between native and exotic species suggests that the enemy escape hypothesis alone cannot explain the invasiveness of these exotics. Our study confirms the general importance of colonization processes in invasions, yet demonstrates how invasiveness can occur via divergent colonization strategies. Dispersal limitation of Acer platanoides and recruitment limitation of Ailanthus altissima will likely constitute some limit on their invasiveness in closed‐canopy forests.     

Effects of resource availability and propagule supply on native species recruitment in sagebrush ecosystems invaded by Bromus tectorum

Resource availability and propagule supply are major factors influencing establishment and persistence of both native and invasive species. Increased soil nitrogen (N) availability and high propagule inputs contribute to the ability of annual invasive grasses to dominate disturbed ecosystems. Nitrogen reduction through carbon (C) additions can potentially immobilize soil N and reduce the competitiveness of annual invasive grasses. Native perennial species are more tolerant of resource limiting conditions and may benefit if N reduction decreases the competitive advantage of annual invaders and if sufficient propagules are available for their establishment. Bromus tectorum, an exotic annual grass in the sagebrush steppe of western North America, is rapidly displacing native plant species and causing widespread changes in ecosystem processes. We tested whether nitrogen reduction would negatively affect B. tectorum while creating an opportunity for establishment of native perennial species. A C source, sucrose, was added to the soil, and then plots were seeded with different densities of both B. tectorum (0, 150, 300, 600, and 1,200 viable seeds m⁻²) and native species (0, 150, 300, and 600 viable seeds m⁻²). Adding sucrose had short-term (1 year) negative effects on available nitrogen and B. tectorum density, biomass and seed numbers, but did not increase establishment of native species. Increasing propagule availability increased both B. tectorum and native species establishment. Effects of B. tectorum on native species were density dependent and native establishment increased as B. tectorum propagule availability decreased. Survival of native seedlings was low indicating that recruitment is governed by the seedling stage.     

The roles of exotic grasses and forbs when restoring native species to highly invaded southern California annual grassland

Many semi-arid shrublands in the western US have experienced invasion by a suite of exotic grasses and forbs that have altered community structure and function. The effect of the exotic grasses in this area has been studied, but little is known about how exotic forbs influence the plant community. A 3-year experiment in southern California coastal sage scrub (CSS) now dominated by exotic grasses was done to investigate the influence of both exotic grasses (mainly Bromus spp.) and exotic forbs (mainly Erodium spp.) on a restoration seeding (9 species, including grasses, forbs, and shrubs). Experimental plots were weeded to remove one, both, or neither group of exotic species and seeded at a high rate with a mix of native species. Abundance of all species varied with precipitation levels, but seeded species established best when both groups of exotic species were removed. The removal of exotic grasses resulted in an increase in exotic and native forb cover, while removal of exotic forbs led to an increase in exotic grass cover and, at least in one year, a decrease in native forb cover. In former CSS now converted to exotic annual grassland, a competitive hierarchy between exotic grasses and forbs may prevent native forbs from more fully occupying the habitat when either group of exotics is removed. This apparent competitive hierarchy may interact with yearly variation in precipitation levels to limit restoration seedings of CSS/exotic grassland communities. Therefore, management of CSS and exotic grassland in southern California and similar areas must consider control of both exotic grasses and forbs when restoration is attempted.     

Overstorey tree species regulate colonization by native and exotic plants: a source of positive relationships between understorey diversity and invasibility

The North American woody species, Prunus serotina Ehrh., is an aggressive invader of forest understories in Europe. To better understand the plant invasion process, we assessed understorey plants and Prunus serotina seedlings that have colonized a 35-year-old replicated common-garden experiment of 14 tree species in south-western Poland. The density and size of established (> 1 year old) P. serotina seedlings varied among overstorey species and were related to variation in light availability and attributes of the understorey layer. In a multiple regression analysis, the density of established P. serotina seedlings was positively correlated with light availability and understorey species richness and negatively correlated with understorey species cover. These results suggest that woody invader success is adversely affected by overstorey shading and understorey competition for resources. Simultaneously, however, invader success may generally be positively associated with understorey species richness because both native and invasive plant colonization respond similarly to environmental conditions, including those influenced by overstorey tree species. Identification of characteristics of forests that increase their susceptibility to invasion may allow managers to target efforts to detect invasives and to restore forests to states that may be less invasible.     

Modelling the impacts of two exotic invasive species on a native butterfly: top-down vs. bottom-up effects

1. Exotic invasive species can influence population dynamics of native species through top-down or bottom-up forces. The present study examined separate and interactive effects of multiple exotic species invasions on the native mustard white butterfly, Pieris napi oleracea Harris (Lepidoptera: Pieridae), using a stochastic simulation model. 2. P. n. oleracea populations in North America have decreased regionally since the 1860s. Competition with an exotic congener (P. rapae L.), loss of native host plants and parasitism by the introduced broconid wasp (Cotesia glomerata L.), have been suggested to be independently responsible for its decline. The present study examined these hypotheses, as well as an alternative, invasion by an exotic crucifer, garlic mustard (Alliaria petiolata[Bieb.] Cavara & Grande). 3. A stochastic simulation model of P. n. oleracea population dynamics revealed that decreasing the number of host plants available for oviposition and larval development (i.e. habitat loss), sharply reduced the probability of populations persistence and decreased population size for those that persisted. 4. Simulated invasion by garlic mustard also substantially decreased both probability of persistence (= 0 at approximately 50% cover) and mean population size. Persistence probability never reached zero under any C. glomerata scenarios, even when larval mortality in the second generation due to parasitism was 100%. The impact of garlic mustard was intensified by the addition of C. glomerata parasitism. 5. Results suggest that bottom-up forces, loss of host plants through forest understorey loss and/or garlic mustard invasion are the most important forces driving P. n. oleracea population decline. Parasitism by C. glomerata may interact to reduce P. n. oleracea populations more rapidly, but appears insufficient alone to cause local extinction.     

Seedling growth and soil nutrient availability in exotic and native tree species: implications for afforestation in southern China

Background and aims

The relationship between tree species and soil nutrient availability is critical for evaluating plantation succession and promoting forest restoration. This study was conducted to evaluate the impact of exotic and native tress species on soil nutrient availability.

Methods

Four exotic species (Eucalyptus urophylla, E. tereticornis, Acaia auriculaeformis, A. mangium) and four native species (Castanopsis fissa, Schima superba, C. hystrix, Michelia macclurei) were planted and grown for one-year. Soil solution (DOC, DON, NH₄?N, NO₃?N) was sampled and analyzed during the study. After the experiment, soil properties were determined, and plant tissues were analyzed.

Results

DOC levels were greater in soils with trees planted than controls without trees. Compared to native species, exotic species had much faster growth rates and greatly reduced DON and NO₃?N concentrations. Exotic species always had less P concentrations in leaves and stems than native species. Furthermore, N-fixing A. auriculaeformis led to greater soil available P compared to other species.

Conclusions

Based on these findings, we provide some recommendations for afforestation practice. This study highlights that a better understanding of the pros and cons of exotic species would be beneficial to advance afforestation in China and the world.     

Effect of a native tree on seedling establishment of two exotic invasive species in a semiarid ecosystem

Theory predicts that in more stressful environments, positive plant-plant interactions should be more important than negative ones. For instance, in arid and semiarid regions, amelioration of soil drought produced by the shade of established plants could facilitate establishment of other species, in spite of light reduction. However, this theory has not been tested widely in the context of plant invasion. In this paper we evaluated the hypothesis that in a semiarid ecosystem of central Chile, the native tree, Lithrea caustica, should facilitate through positive shading effects, the seedling establishment of two widely planted and invasive forestry species, Pinus radiata and Eucalyptus globulus. We assessed the seedling establishment examining two processes: seedling recruitment (including germination) and subsequent seedling survival. We sowed seeds (to assess recruitment) and planted 8 months old seedlings (to assess seedling survival) of each exotic species under Lithrea patches, open sites and under an artificial shade mimicking Lithrea shading. The study was repeated in a north-facing and a south-facing slope in the study area located in a xeric zone within the distribution range of plantations of these species in central Chile. Our results show that in a north-facing slope Lithrea had positive effects on recruitment of both species, which was produced by shading. These effects were counteracted by negative effects on seedling survival but through a different mechanism, which suggests that Lithrea would have no significant effect on the whole seedling establishment process of Pinus radiata nor Eucalyptus globulus in this habitat. In turn, in a south-facing slope Lithrea had no significant effect on recruitment but had a negative effect on seedling survival, which was not produced by shading. This suggests that in this habitat Lithrea has a negative effect on the seedling establishment of these exotic species. Our results suggest that the effect of the native Lithrea caustica on the seedling establishment of these exotic species is dependent upon the life-cycle phase (recruitment or seedling survival) and habitat even within the same semiarid ecosystem. In contrast to the expected positive effects Lithrea is unlikely to facilitate seedling establishment of these exotic species in this area, and in fact in some habitats this effect could be negative. However, our results also suggest that a common mechanism proposed to resist invasion in forest ecosystems such as shading, probably is not sufficient to inhibit invasion in a semiarid region.     

Complex patterns of hybridization between exotic and native North American poplar species

? Premise of the study: Poplars and their hybrids are seen as important candidates for bioenergy initiatives. However, many concerns have been raised about large-scale plantations of new poplar cultivars. The deployment of such plants with novel traits brings the risk of potential spread of novel genome regions (including exotic genes, transgenes, or other heritable modifications) into natural populations of related species. The possibility of introgression is especially high in poplars because reproductive barriers between species are weak. Knowledge of the frequency of hybridization between cultivated trees and natural populations is one important step in the risk-assessment process. ? Methods: We studied the rate of spontaneous hybridization from two sexually mature poplar plantations into adjacent natural populations of Populus deltoides and P. balsamifera. The two plantations, both in eastern Canada, contain many different complex hybrid clones with components from exotic species, mostly P. nigra, P. trichocarpa, and P. maximowiczii. We analyzed 12 species-specific single nucleotide polymorphisms from six different genes in 5373 offspring sampled from the natural populations. ? Results: Contributions from all three exotics were found in the offspring, confirming low reproductive barriers among poplar species in these sections. The frequency of hybrid offspring varied among pollen donors, recipient populations, and years. ? Conclusions: The remarkably high rate of hybridization that was found in the smallest natural population sampled suggests that small peripheral populations carry a higher risk of introgression. These results could be used as a starting point for developing regulatory guidelines for the introduction of plants with novel traits.     

Exotic vertebrate and invertebrate herbivores differ in their impacts on native and exotic plants: a meta-analysis

Herbivores modify various ecological processes including interactions between native and exotic plants that may affect invasion success by the exotic plants. It is unknown whether different types of exotic herbivores have similar effects on native and exotic plants. Using two distinct data sets, we ran meta-analyses to compare exotic vertebrate and invertebrate herbivore preferences for, and effects on performance and population sizes of native and exotic plants. We found that exotic vertebrate herbivores have positive effects on exotic plant performance and population sizes, and no significant effects on native plants. Exotic invertebrates have significant negative effects on performance and population sizes of both exotic and native plants. Vertebrates prefer to feed on native plants relative to exotic plants, while invertebrates prefer the exotic plants to native plants. Thus the exotic vertebrate herbivores may aid invasiveness of exotic plants, in accordance with the invasional meltdown hypothesis, while exotic invertebrate herbivores probably have no net effect on invasion process of the exotic plants. Invertebrate herbivore preferences for exotic plants support the biotic resistance hypothesis, as the native plants probably resist the invertebrate herbivory. We also tested an evolutionary logic that posits that herbivores with similar evolutionary history as plants will affect the plants less negatively than plants with which they have not co-evolved. Our results indicate that there is no consistent pattern in effects of exotic vertebrate and invertebrate herbivores on exotic plants with or without which they have co-evolved.     

Decline in exotic tree density facilitates increased plant diversity: the experience from Melaleuca quinquenervia invaded wetlands

The Australian tree Melaleuca quinquenervia (melaleuca) formed dense monocultural forests several decades after invading parts of Florida and the Caribbean islands. These dominant forests have displaced native vegetation in sensitive wetland systems. We hypothesized that native plant diversity would increase following recent reductions in density of mature melaleuca stands in south Florida. We therefore examined data on changes in melaleuca densities and plant species diversity derived from permanent plots that were monitored from 1997 to 2005. These plots were located within mature melaleuca stands in nonflooded and seasonally-flooded habitats. Two host-specific biological control agents of melaleuca, Oxyops vitiosa and Boreioglycaspis melaleucae, were introduced during 1997 and 2002, respectively. Also, an adventive rust fungus Puccinia psidii and lobate-lac scale Paratachardina pesudolobata became abundant during the latter part of the study period. Overall melaleuca density declines in current study coincided with two to four fold increases in plant species diversity. The greatest declines in melaleuca density as well as the greatest increases in family importance values and species diversity indices occurred in nonflooded as compared to seasonally-flooded habitats. Most pioneer plant species in study sites belonged to Asteraceae, Cyperaceae, Poaceae, and Ulmaceae. The rapid reduction in melaleuca density and canopy cover during the study period may be attributed to self-thinning accelerated by the negative impact of natural enemies. Densities of other woody plants, particularly Myrica and Myrsine, which were sparsely represented in the understory by a few suppressed individuals also declined during the same period, possibly due to infestation by the generalist lac-scale. These findings indicate that natural-enemy accelerated self-thinning of melaleuca densities is positively influencing the native plant diversity and facilitating the partial rehabilitation of degraded habitats.     

Differential use of trophic resources between an exotic and a coexisting native snail

Limnology - Knowing the interactions between exotic and native species is essential to establish possible threats to the local fauna. In this study, we assessed the use of food resources and diet...     

The effect of soil nitrogen on competition between native and exotic perennial grasses from northern coastal California

The invasion of European perennial grasses represents a new threat to the native coastal prairie of northern California. Many coastal prairie sites also experience anthropogenic nitrogen (N) deposition or increased N availability as a result of invasion by N-fixing shrubs. We tested the hypothesis that greater seedling competitive ability and greater responsiveness to high N availability of exotic perennial grasses facilitates their invasion in coastal prairie. We evaluated pairwise competitive responses and effects, and the occurrence of asymmetrical competition, among three common native perennial grasses (Agrostis oregonensis, Festuca rubra, and Nassella pulchra) and three exotic perennial grasses (Holcus lanatus, Phalaris aquatica, and Festuca arundinacea), at two levels of soil N. We also compared the root and shoot biomass and response to fertilization of singly-grown plants, so we could evaluate how performance in competition related to innate plant traits. Competitive effects and responses were negatively correlated and in general varied continuously across native and exotic species. Two exceptions were the exotic species Holcus, which had large effects on neighbors and small responses to them, and competed asymmetrically with all other species in the experiment, and the native grass Nassella, which had strong responses to but little effect on neighbors, and was out-competed by all but one other species in the experiment. High allocation to roots and high early relative growth rate appear to explain Holcus’s competitive dominance, but its shoot biomass when grown alone was not significantly greater than those of the species it out-competed. Competitive dynamics were unaffected by fertilization. Therefore, we conclude that seedling competitive ability alone does not explain the increasing dominance of exotic perennial grasses in California coastal prairie. Furthermore, since native and exotic species responded individualistically, grouping species as ‘natives’ and ‘exotics’ obscured underlying variation within the two categories. Finally, elevated soil N does not appear to influence competition among the native and exotic perennial grasses studied, so reducing soil N pools may not be a critical step for the restoration of California coastal prairie.     

Demographic changes and the development of competition in a native Australian eucalypt forest invaded by exotic pines

Data from the second half of a 14-year study of a eucalypt dry sclerophyll forest invaded by Pinus radiata D. Don shows further progress in the development of a mixed eucalypt-pine stand. Earlier (Chilvers and Burdon 1983) it was clear that pines invading from an adjacent commercial plantation were starting to contribute their own progeny to the mixed stand. Initially the recruitment of established pines (> 1 m high) from these seedlings was very slow. However, since 1984, increasing numbers of seedlings have survived beyond their first year, suggesting the possibility of another wave of successful establishment in the near future. The established fraction of the pine population continued to grow rapidly and incurred no mortality. Many of these plants have now emerged through the top of the eucalypt canopy. In contrast, both juvenile and adult eucalypt numbers continued to decline. Despite these demographic and visual changes, comparative analysis of the growth of established pines over two consecutive 7-year periods (1974–1981; 1981–1988) showed that competition between neighbouring plants, especially pines, is beginning to affect individual growth rates. Growth rates of pines having other pines as nearest neighbours declined 40% between 1974–1981 and 1981–1988, while that of pines with eucalypts as nearest neighbours fell by 28%. In pine-pine nearest neighbour (NN) pairs, growth rates were significantly affected by the size (diameter) of the nearest neighbour. In pine-eucalypt NN pairs growth rates of the pines were not affected by the size of the eucalypt, but these were significantly negatively correlated with the inverse of the distance to the eucalypt. More broad-based assessment of 43 pines showed a significant relationship between their growth rates and the summed competitive effect of other pines within a 5-m radius. No such relationship was found between growth rate and the summed competitive effect of eucalypts in the 5-m zone.     

Photosynthesis, photoinhibition, and nitrogen use efficiency in native and invasive tree ferns in Hawaii

Photosynthetic gas exchange, chlorophyll fluorescence, nitrogen use efficiency, and related leaf traits of native Hawaiian tree ferns in the genus Cibotium were compared with those of the invasive Australian tree fern Sphaeropteris cooperi in an attempt to explain the higher growth rates of S. cooperi in Hawaii. Comparisons were made between mature sporophytes growing in the sun (gap or forest edge) and in shady understories at four sites at three different elevations. The invasive tree fern had 12-13 cm greater height increase per year and approximately 5 times larger total leaf surface area per plant compared to the native tree ferns. The maximum rates of photosynthesis of S. cooperi in the sun and shade were significantly higher than those of the native Cibotium spp (for example, 11.2 and 7.1 µmol m^-2 s^-1, and 5.8 and 3.6 µmol m^-2 s^-1 respectively for the invasive and natives at low elevation). The instantaneous photosynthetic nitrogen use efficiency of the invasive tree fern was significantly higher than that of the native tree ferns, but when integrated over the life span of the frond the differences were not significant. The fronds of the invasive tree fern species had a significantly shorter life span than the native tree ferns (approximately 6 months and 12 months, respectively), and significantly higher nitrogen content per unit leaf mass. The native tree ferns growing in both sun and shade exhibited greater photoinhibition than the invasive tree fern after being experimentally subjected to high light levels. The native tree ferns recovered only 78% of their dark-acclimated quantum yield (F_v/F_m), while the invasive tree fern recovered 90% and 86% of its dark-acclimated F_v/F_m when growing in sun and shade, respectively. Overall, the invasive tree fern appears to be more efficient at capturing and utilizing light than the native Cibotium species, particularly in high-light environments such as those associated with high levels of disturbance.     

Responses to nitrogen pulses and growth under low nitrogen availability in invasive and native tree species with differing successional status

Invasive species are frequently found in recently disturbed sites. To examine how these disturbance-dependent invasive species exploit resource pulses resulting from disturbance, twelve physiological and morphological traits, including age-dependent responsiveness in leaf traits to nitrogen pulse, were compared between Bischofia javanica, an invasive tree species in Ogasawara islands, and three native Ogasawara species, each having a different successional status. When exposed to a nitrogen pulse, invasive B. javanica showed higher increases in photosynthetic capacity, leaf area, epidermal cell number and cell size in leaves of broad age classes, and root nitrogen absorption ability than two native mid-/late or late-successional species, but showed no particular superiority to a native pioneer species in these responses. Under low nitrogen, however, it showed the largest relative growth rate among the four species, while the native pioneer showed the lowest growth. From these results, we concluded that the combination of moderately high responsiveness to resource pulses and the ability to maintain steady growth under resource limitations may give B. javanica a competitive advantage over a series of native species with different successional status from early to late-successional stages.     

Pioneer effects on exotic and native tree colonizers: Insights for Araucaria forest restoration

We integrate analyses of spatial association with empirical measures of juvenile performance to develop a strategy for Araucaria forest restoration in Brazil. Because pioneer species can differentially facilitate or inhibit target species, we measure native Araucaria angustifolia and exotic Pinus taeda responses to two common pioneer species, Baccharis uncinella and Vernonia discolor in a clear-cut pine plantation. P. taeda establishment and growth was greater than that of A. angustifolia in all treatments, showing its potential for invasiveness. B. uncinella showed significant positive spatial associations with P. taeda and negative spatial associations with A. angustifolia. V. discolor showed significant negative spatial associations with P. taeda but neutral spatial associations with A. angustifolia, it also supported a higher diversity of native species under its canopy than B. uncinella. Experimental results show that these patterns are due to differential establishment restrictions imposed by both pioneers on target species. We conclude that B. uncinella promotes pine invasion while V. discolor restrains it. We suggest strategies for Araucaria forest restoration and highlight the broad applicability of our approach to restoration programs.     

Feeding behavior of two exotic aphid species on their original hosts in a new invaded area

Greenidea ficicola Takahashi and Greenidea psidii van der Goot (Aphididae: Greenideinae) are Asian aphid species newly introduced in Brazil associated with Moraceae and Myrtaceae. The feeding behavior of G. ficicola and G. psidii was investigated on their respective host plants, Ficus benjamina (Moraceae) and Psidium guajava (Myrtaceae), using the Electrical Penetration Graph (EPG). Fifteen females of each aphid species were monitored during 24h using a DC-EPG GIGA-4 monitor. The time spent in phloem phase (waveforms E1 and E2) was 13.6% of the total recording time for G. ficicola and 0.8% for G. psidii. The average time in the pathway phase (waveforms C and pd) represented 50% of the total time for both species. Aphids spent more time in non-penetration and stylet pathway activities than in the phloem phase or actual feeding. The principal component analysis (PCA) showed that the two species formed different groups in relation to EPG parameters, despite some overlapping. The probing patterns with multiple penetrations of short duration in the sieve elements for both species may indicate apparent unsuitability for sustained feeding on their respective host plants. These results suggest that these two exotic species are in the process of adaptation to their host plants in their new environment and/or the plants may present either chemical or physical barriers against these insects.     

Foraging interactions between native and exotic bumblebees: enclosure experiments using native flowering plants

To assess the impact of Bombus terrestris invasion on the foraging efficiency of native Japanese bumblebees, consumption and acquisition of floral resources during foraging on flowers of native Japanese plant species were investigated using enclosures with three treatments: one with only B. terrestris (exotic), one with both B. terrestris and native Japanese bumblebee species (mixed), and one with only Japanese species (native), but with the bumblebee density held constant. Changes in the body mass of queens and the nest mass of colonies for two days did not significantly differ among four combinations of the species and treatment, B. terrestris in the exotic and mixed treatments and Japanese species in the mixed and native treatments. Thus, it is not clear that B. terrestris has higher foraging efficiency than native species and that B. terrestris individuals more negatively affect the foraging efficiency of native species than individuals of the native species themselves. The nectar standing crop of Cirsium kamtschaticum was smaller in the exotic treatment than in the mixed and native treatments. However, this may have been an artifact of differences in the numbers of flowers in the various treatments. T. Nagamitsu and T. Kenta contributed equally to this work