Spread Of Exotic Plant Species Research Articles

Land‐use intensity and relatedness to native plants promote exotic plant invasion in a tropical biodiversity hotspot

Abstract Exotic plant invasions threaten biodiversity and are costly to farmers. Land use is a major pathway promoting the spread of exotic plant species; however, little is known about the processes underlying the success of exotic plants in tropical agricultural landscapes. Focussing on the heterogeneous smallholder landscapes of north‐eastern Madagascar, we studied exotic plants of understorey communities across a land‐use intensity gradient from unburned lands (old‐growth forests, forest fragments and forest‐derived vanilla agroforests) to burned ones (fallow‐derived vanilla agroforests, woody fallows and herbaceous fallows). We quantified the absolute species richness, abundance and cover of exotic plants across land‐use types and their proportional contribution to community richness, abundance and cover as indicators of exotic plant invasion. We tested for the effects of land‐use parameters, namely land‐use history, canopy closure and landscape‐level forest cover, on exotic plants. Additionally, we tested whether the phylogenetic relatedness between exotic and native species in the same plot affected invasion success, testing Darwin's naturalisation and pre‐adaptation hypotheses. All indicators of exotic plant invasion were lowest in old‐growth forests and forest fragments and highest in fallow‐derived vanilla agroforests, woody fallows and herbaceous fallows. Absolute and proportional exotic richness were negatively affected by canopy closure, and landscapes with high forest cover had lower proportions of exotic plant richness. High phylogenetic relatedness between exotics and natives was associated with lower proportional richness but higher proportions of exotics in abundance and cover. However, individual exotic species showed contrasting responses to land‐use parameters and relatedness to natives. Synthesis and applications: Our results indicate that maintaining unburned lands, land‐use types with dense canopies and landscapes with high forest cover prevents the spread of exotic plants within the agricultural landscapes of north‐eastern Madagascar. Supporting Darwin's pre‐adaptation hypothesis, exotic plants that are phylogenetically closely related to native plants are more likely to become successful invaders in terms of abundance and cover. Nevertheless, individual species show different responses to land‐use changes and phylogenetic relatedness. Therefore, land‐use decisions and management choices can be tailored to limit the spread of exotic species and to preserve native plants in this global biodiversity hotspot.

Assessing the diet and seed dispersal ability of non-native sambar deer (Rusa unicolor) in native ecosystems of south-eastern Australia.

Understanding the influence of non-native herbivores on ecosystems by means of dietary foraging and seed dispersal is important for understanding how non-native species can alter an invaded landscape, yet requires multiple methodologies. In south-eastern Australia, introduced sambar deer (Rusa unicolor) are rapidly expanding in range and placing native ecosystems at risk through browsing and as vectors for seed dispersal. We simultaneously investigated sambar deer dietary composition and seed dispersal using DNA sequencing and germination trials, from faecal pellets collected in alpine and wet forest ecosystems. This allowed us to contrast the dietary impacts of introduced sambar deer in different environments, and to explore the potential for habitat-specific variation in diet. DNA sequencing of the trnL, ITS2 and rbcL gene regions revealed a diverse plant species dietary composition comprising 1003 operational taxonomic units (OTUs). Sambar deer exhibited intermediate feeder behaviours dominated by forbs in alpine and shrubs in wet forest ecosystems. A large proportion of plant OTUs were considered likely to be native, however, the proportion of exotic species in the diet in both ecosystems was greater than would be expected based on the proportion of exotic species in each of the two landscapes. Seed germination trials indicated that sambar deer can disperse a substantial number of native and exotic species in both alpine and wet forest ecosystems. In alpine ecosystems, an individual sambar deer was estimated to disperse on average 816 (±193) seeds per day during the study period, of which 652 (±176) were exotic. Synthesis and applications. Our results suggest that native plant species comprise the majority of sambar deer diets in Australian ecosystems and that the introduced species is dispersing both native and exotic plant species via endozoochory. However, exotic species seedling germination numbers were significantly higher in alpine ecosystems, and given the large daily movements of sambar deer, represents a significant vector for the spread of exotic plant species. Management of native plant species and vegetation communities of conservation significance, or at risk to sambar deer browsing is of high priority, through either the removal of sambar deer or implementation of exclusion-based methods.

Goats foster endozoochoric dispersal of exotic species in a seasonally dry tropical forest ecosystem

Goats at high densities are known to degrade vegetation in arid environments around the world. Their role as potential dispersers of seeds of native and exotic plant species, however, still remains uncertain. This study assesses whether goats, through endozoochorous seed dispersal, may potentially change the plant assemblage of seasonally dry tropical forests. We used seedling emergence as a method to analyze which plant species germinated from goat feces and thus may be dispersed by goats after passing through their intestinal tract. Seedling emergence data were analyzed with respect to the plot vegetation, water availability (permanent, temporary and no water courses), and altitude (as an indicator of anthropogenic pressure). We identified 18 species belonging to 14 families. Feces from the plots near permanent water with the largest anthropogenic pressure were characterized by the highest number of germinated plants. The most common plant species by far found in the experiment was the exotic Prosopis juliflora. This high abundance of P. juliflora seedlings shows that non-native grazers facilitate the spread of exotic plant species in threatened dry forest habitats. We therefore conclude that free-roaming goats may play an important ecological role in changing plant species assemblages of even remote areas.

Ant preference for seeds without awns increases removal of exotic relative to native grass seeds

1. The removal of seeds by granivorous ants can affect plant recruitment through either seed loss from predation or the dispersal and recruitment of seeds that are removed but not consumed. Consequently, variation in ant selection preferences can influence patterns of seed removal and affect plant community composition, including the spread of exotic plant species.2. We conducted a seed removal experiment to determine whether: 1) rates of removal by ants differed between three native and three exotic grass species in an Australian temperate grassland; and 2) differences in removal rates were associated with the presence or absence of awns.3. We found that seeds of the three exotic species, none of which had awns, were removed by ants at a higher rate than those of the three native species, all of which had awns. Removal rates of native species increased when awns were manually removed, suggesting the awns of native species acted as a removal barrier.4. While we do not know the fate of seeds removed from our experiment, differences among species in removal rates mirrored differences in their spatial spread in a separate seed addition experiment. Exotic species removed by ants at a higher rate in the removal experiment had more widely dispersed seedlings than native species in the seed addition experiment, potentially indicating a role for granivorous ants in dispersing exotic seeds. Identifying ant selection preferences and directly linking removal to seed fate could help explain how exotic grass species move around the landscape.

A recipe for weed disaster in islands: a super-generalist native pollinator aided by a ‘Parlourmaid’ plant welcome new arrivals in Fiji

In isolated islands with low pollinator diversity, it has been suggested that native pollinators should evolve into super-generalists that could facilitate the spread of exotic plant species that might otherwise rely on specialist pollinators. Consequently, in the absence of exotic pollinators isolated islands may still be particularly vulnerable to a wide variety of introduced plants. Fiji has a highly abundant and diverse introduced plant fauna, as well as a variety of introduced bee species, but has extremely low endemic bee fauna diversity. We examined bee–plant pollination networks in lowland regions of Viti Levu (Fiji) where there is only one endemic bee species, Homalictus fijiensis. We show that this bee is a super-generalist for introduced plants, and whilst introduced bee species can show high intensities of floral visits, they do not substantially increase the breadth of weeds receiving bee visits. Surprisingly, one introduced plant species, Sphagneticola trilobata, receives high visitations from introduced bee species, even though it spreads vegetatively. We regard this species as a ‘Parlourmaid’ weed that likely augments the spread of exotic bees without gaining pollination benefits. Our results indicate a ‘twofold’ promotion of invasive species, namely, super-generalist native pollinators can promote the spread of diverse introduced plants, and Parlourmaid plants provide resources that can promote the spread of introduced pollinators.

Climate change and resilience of deciduous Nothofagus forests in central–east Chilean Patagonia over the last 3200 years

ABSTRACTWe examine the response of Nothofagus forests to climate change and disturbance regimes over the last 3200 years near Coyhaique (45°S), central–east Chilean Patagonia, using fine‐resolution pollen and charcoal records from lake sediment cores. Closed‐canopy deciduous Nothofagus forests have dominated the region with little variation until the arrival of Chilean–European settlers, suggesting a predominance of cool‐temperate and wet conditions. Within this state we identify centennial‐scale episodes of forest fragmentation, increase in littoral macrophytes and volcanic/paleofire disturbance between 2700 and 3000 cal a BP, 2200 and 2500 cal a BP and over the last ∼250 years, which we interpret as intervals with negative hydrologic balance. Natural variability caused little impact on the physiognomy and composition of the vegetation in pre‐European time, in contrast to the accelerated shift that started during the late 19th century associated with deforestation, homogenization and synchronization of ecosystem changes at the landscape level, and spread of exotic plant species brought by Chilean and European settlers during a warm/dry interval. The resilience of deciduous Nothofagus forests to natural disturbance regimes and climate change was exceeded by large‐scale human disturbance since the late 19th century by fire, timber exploitation and livestock grazing. These disturbances caused an ecosystem shift towards artificial meadows and scrublands with frequent high‐magnitude fires. Copyright © 2017 John Wiley & Sons, Ltd.

Characterizing Pathways of Invasive Plant Spread to Alaska: II. Propagules from Imported Hay and Straw

AbstractThe extent and nature of spread of exotic plant species to and within Alaska by shipment of hay and straw was studied. The amounts of hay and straw imported into Alaska and the amounts and types of seed in imported and locally produced hay and straw was determined We purchased alfalfa hay, wheat straw, ryegrass straw, and timothy hay produced in Washington and Oregon (WA–OR) and locally produced straw and hay. The hay and straw were shaken over screens, and the remaining fines were mixed with sterile potting soil and incubated in the greenhouse. Forty-nine plant species were identified from hay and straw, nine of which are ranked as invasive in Alaska, including downy brome, foxtail barley, hare barley, narrowleaf hawksbeard, and quackgrass—a prohibited weed in Alaska. The number of seeds ranged from 0 to 6,205 seeds kg−1, with an average of 585 seeds kg−1, and the number of species ranged from 0 to 12, with an average of 4.2 species per bale. Crop seed comprised a large proportion of the germinated seeds in ryegrass straw, wheat straw, and timothy/brome hay (98, 78, and 62%, respectively), but was less prevalent (ranging from 0 to 38%) in the other three hay and straw crop types. Hay and straw from Alaska contained more total seeds and species than hay and straw from WA-OR, but the difference was not significant when only weed seed was used in the analysis. Alaska-grown timothy/brome hay contained significantly more total seed than alfalfa hay and wheat straw from WA–OR and Alaska-grown barley straw. The grower or distributor of the hay and straw also influenced the number of seeds and species among some crop types. Results of this study document that large numbers of alien plant species are transported by movements of hay and straw into and within Alaska.

Diversity and conservation status of native Australian bees

Australia’s idiosyncratic bee fauna is characterised by a combination of numerous endemic taxa and by the complete absence of some families. Many species, and in particular several oligolectic species, remain undescribed and more than half the named taxa are in need of revision. The main threats to the native bee fauna include removal of nesting and foraging opportunities through land clearing and agriculture, the spread of exotic plant species and the consequences of climate change. Early steps to conserve the native bee fauna include commercial applications, the raising of public awareness and preservation of natural habitat. However, these actions are severely hampered by a lack of both identification keys and taxonomic expertise. Considerable investment in taxonomic research is needed to improve this state of affairs.

Gradient Analysis of Exotic Species in Pinus radiata Stands of Tenerife (Canary Islands)

Identifying the factors that influence the spread of exotic species is essential for evaluating the present and future extent of plant invasions and for the development of eradication programs. We randomly established a network of 250 plots on an exotic Pinus radiata D. Don plantation on Tenerife Island in order to determine if roads and urban centers are favouring the spread of exotic plant species into the forest. We identified four distinct vegetation groups in the P. radiata stands: advanced laurel forest (ALF), undeveloped laurel forest (ULF), ruderal (RU), and Canarian pine stand (CPS). The groups farthest from roads and urban nuclei (ALF and CPS) have the best conserved vegetation, characterizing by the main species of the potential vegetation of the area and almost no exotic and ruderal species. On the other hand, the groups nearest to human infrastructures (ULF and RU) are characterized by species from potential vegetation's substitution stages and a higher proportion of exotic and ruderal species. The results indicate distance to roads and urban areas are disturbance factors favouring the presence of exotic and ruderal species into the P. radiata plantation. We propose the eradication of some dangerous exotic species, monitoring of the study area in order to detect any intrusion of alien species in the best conserved areas and implementation of management activities to reduce the perturbation of the ULF and RU areas.

Exotic grasses and feces deposition by an exotic herbivore combine to reduce the relative abundance of native forbs

Increased resource availability can facilitate establishment of exotic plant species, especially when coincident with propagule supply. Following establishment, increased resource availability may also facilitate the spread of exotic plant species if it enhances their competitive abilities relative to native species. Exotic Canada geese (Branta canadensis) introduce both exotic grass seed and nutrients to an endangered plant community on the Gulf Islands of southwestern British Columbia, Canada. I used greenhouse experiments to assess the competitive advantage of the exotic grasses relative to native and exotic forbs in this community and to test the impacts of nutrient addition from goose feces on competitive outcomes. I grew experimental communities varying in their proportion of forbs versus exotic grasses, and added goose feces as a nutrient source. I found that both native and exotic forbs produced significantly more biomass in competition with conspecifics than in competition with the grasses, and that the proportional abundance of two out of three native forbs was lowest in the combined presence of exotic grasses and nutrient addition. In a second experiment, I found that in monoculture all species of forbs and grasses showed equal growth responses to nutrients. The exotic species did not convert additional nutrients into additional biomass at a higher rate, but did germinate earlier and grow larger than the native species regardless of nutrient availability. This suggests that the exotic species may have achieved their competitive advantage partly by pre-empting resources in community mixtures. Small and late-germinating native forbs may be particularly vulnerable to competitive suppression from exotic grasses and forbs and may be at an even greater disadvantage if their competitors are benefiting from early access to additional nutrients. In combination, the input of exotic propagules and additional nutrients by nesting geese may compromise efforts to maintain native community composition in this system.

Allelopathic effects of fruits of the Brazilian pepper Schinus terebinthifolius on growth, leaf production and biomass of seedlings of the red mangrove Rhizophora mangle and the black mangrove Avicennia germinans

Exotic plant species can negatively affect native flora and fauna by changing the diversity and productivity of the system and altering successional processes. The novel weapons hypothesis has been suggested as one mechanism for invasion and spread of exotic plant species. It states that exotic species with “new weapons”, such as allelopathic chemicals, may have greater impacts on species in the invaded range without co-evolved defense strategies against the exotic chemicals. In Florida, one successful exotic species, Schinus terebinthifolius (Brazilian pepper), has invaded nearly every habitat throughout the state. Past studies have documented allelopathic properties of S. terebinthifolius negatively impacting the growth of native terrestrial flora. Here we document the effect of S. terebinthifolius for the first time on mangrove systems. We examined growth and survival of Rhizophora mangle (red mangrove) and Avicennia germinans (black mangrove) seedlings when exposed to a range of densities of intact and crushed S. terebinthifolius fruits (0, 25, 50) at two different salinities (15, 30 ppt). We also documented the natural density of S. terebinthifolius fruits beneath the tree canopy after fruit production in mangrove habitat. Growth and biomass were significantly reduced in A. germinans when exposed to the highest density of intact S. terebinthifolius fruits growing in 30 ppt saltwater. Trials with intact fruits had no significant effects on R. mangle; however, crushed fruits significantly decreased growth, and leaf production. The ecological importance of the different effects of crushed and intact fruits is supported by field observations which found equal or greater numbers of crushed fruits compared to intact fruits beneath the tree canopy of female S. terebinthifolius. In addition, abiotic factors, such as salinity, may interact with allelopathic chemicals in natural systems and needs to be addressed when planning future allelopathic studies.

Purple Loosestrife (Lythrum salicaria) Control with Herbicides: Single-Year Application1

The introduction and spread of exotic plant species is one of the most serious threats to biodiversity. It is estimated that over 40 million ha of U.S. land are already infested with invasive plants. Purple loosestrife is one such species that is currently invading wetlands and waterways in Midwestern states including 5,000 ha in Nebraska. Field studies were conducted in 2000 and 2001 at two locations in each year with the primary objective to evaluate performance of a single application of 16 herbicide treatments. A secondary objective was to document the impact of those herbicides on the total ground cover of local vegetation for two seasons after herbicide application. Evaluation at 10 wk after treatment showed that excellent season-long control (≥90%) of purple loosestrife was achieved with glyphosate at 3.36 kg ae/ha; 2,4-D dimethylamine at 2.66 kg ae/ha; triclopyr at 2.1 kg ae/ha; imazapyr at 1.68 kg ae/ha; and with the two mixtures of 2,4-D plus triclopyr at 1.33 1.26 kg/ha and 2,4-D plus metsulfuron at 1.33 ae/ha 0.042 kg ai/ha. Evaluation at 1 yr after treatment (YAT) showed excellent control (≥90%), with imazapyr at 0.33, 0.66, 1.12, and 1.68 kg/ha and metsulfuron at 0.084 and 0.168 kg/ha. Evaluation at 2 YAT suggested that excellent control (≥90%) was achieved with imazapyr at 1.12 and 1.68 kg/ha and metsulfuron at 0.084 and 0.168 kg/ ha. High rates of imazapyr caused negative effects on the local vegetation, indicating that use of these rates should be limited. Results suggest that a single application of more than half of the tested herbicides did not provide satisfactory control of purple loosestrife that lasts more than two seasons. However, the selective herbicides integrated with other control methods (e.g., mechanical, biocontrol) may be a valuable strategy for longer term control. Further studies are needed to evaluate the long-term effects of multiyear herbicide applications.Nomenclature: 2,4-D; glyphosate; imazapyr; metsulfuron; triclopyr; purple loosestrife, Lythrum salicaria L. #3 LYTSA.Additional index words: Integrated weed management, invasive species, noxious weeds.Abbreviations: WAT, weeks after treatment; YAT, years after treatment.

The Relationship of Apis mellifera with Exotic and Native Plants in Boulder County, Colorado

Since honeybees (Apis mellifera) were introduced to North America in the 1600’s, their influence has been profound and widespread. As pollinators, honeybees are extremely valuable economically and are vital to crop pollination. However, their presence has affected native ecosystems, including the plains ecosystem in Colorado. Using recruitment and other unique foraging characteristics, honeybees may be out-competing native pollinators for nectar and pollen resources. Our study was designed to determine if A. mellifera has a preference for exotic or native plants. We observed patches of exotic plants and patches of native plants and recorded the type of bee (exotic or native) that visited each flowering head. We also examined data from Kearns and Oliveras (unpublished), which illustrates that invasive plant species are also popular with native bees, and may draw vital pollinators away from native plants. Our results indicate that honeybees prefer to visit exotic invasive plant species to native plants. Consequently, honeybees may contribute to the spread of exotic plant species and the decline of native plant species, reducing biodiversity. Thus, native plants are doubly jeopardized. The invasion of exotic flora and fauna into native ecosystems on the plains of Colorado is part of a worldwide phenomenon of species invasion, and researchers must continue to investigate interspecies relationships to minimize the potential negative effects of invasive species.

Geographical Analysis of the Distribution and Spread of Exotic Plant Species in Grand Teton National Park, Wyoming

The effects of introduced exotic species in natural environments are becoming important issues in conservation biology and natural resource management and recent scientific literature reveals increasing concern regarding the spread of invasive exotic plant species (Allen, 1996; Vitousek et al. 1996; Walker and Smith, 1997). Ecological consequences of these species include increased competition for space, water, and nutrients with native plants (which could result in a decrease in biodiversity), decreased forage quality for native ungulates, and changes in the microenvironments where the establishments took place (Woods, 1997). Sheley et al (1998) list several ecologically and economically detrimental impacts of exotic species. The National Park Service recognizes the need to protect ecosystems from exotic species (National Park Service, 1997) through management based on the ability to predict species distributions and spread, and monitoring in areas that are most susceptible to invasion. Recommended strategies for preventing the spread of exotic species include developing an early warning system to identify and eradicate new infestations of exotic plants in National Parks, and continued inventory and monitoring of exotic plants (National Park Service, 1997). These strategies will be based on assessment of the distribution and spread of exotic plants (National Park Service, 1997) using remote sensing and Geographic Information Systems (GIS) technologies for mapping and monitoring exotic plants, and models to predict the invasiveness and spread of exotic plants. In Grand Teton National Park (GTNP), exotic species are a great concern for park managers (National Park Service, 1997). Of the 1000 species of flowering plants within GTNP, there are also four (possibly five) rare plants that may be threatened as a result of competition with exotics (Wyoming Rare Plant Technical Committee, 1994): Draba borealis (Boreal draba), Epipactis gigantea (Giant helleborine), Lesquerella carinata var. carinata (Keeled bladderpod), Lesquerella paysonni (Payson's bladderpod), and possibly Draba densifolia var. apiculata (Rockcress draba). The continued survival of these sensitive plants in GTNP increases the need for management of exotic plants. GTNP has implemented a classification system for exotic plant species that consists of three priority levels (GTNP, 1997a). Priority 1 species are designated as "noxious" since they are capable of invading natural ecosystems and disrupting or displacing native vegetation. Currently, there are thirteen exotic plant species with a Priority 1 status within GTNP (Table 1 ).