Effects of Tamarix removal on the community dynamics of riparian birds in a semiarid grassland

Invasion of riparian habitats by non‐native plants is a global problem that requires an understanding of community‐level responses by native plants and animals. In the Great Plains, resource managers have initiated efforts to control the eastward incursion of Tamarix as a non‐native bottomland plant (Tamarix ramosissima) along the Cimarron River in southwestern Kansas, United States. To understand how native avifauna interact with non‐native plants, we studied the effects of Tamarix removal on riparian bird communities. We compared avian site occupancy of three foraging guilds, abundance of four nesting guilds, and assessed community dynamics with dynamic, multiseason occupancy models across three replicated treatments. Community parameters were estimated for Tamarix‐dominated sites (untreated), Tamarix‐removal sites (treated), and reference sites with native cottonwood sites (Populus deltoides). Estimates of initial occupancy (ψ₂₀₀₆) for the ground‐to‐shrub foraging guild tended to be highest at Tamarix‐dominated sites, while initial occupancy of the upper‐canopy foraging and mid‐canopy foraging guilds were highest in the treated and reference sites, respectively. Estimates of relative abundance for four nesting guilds indicated that the reference habitat supported the highest relative abundance of birds overall, although the untreated habitat had higher abundance of shrub‐nesters than treated or reference habitats. Riparian sites where invasive Tamarix is dominant in the Great Plains can provide nesting habitat for some native bird species, with avian abundance and diversity that are comparable to remnant riparian sites with native vegetation. Moreover, presence of some native vegetation in Tamarix‐dominated and Tamarix‐removal sites may increase abundance of riparian birds such as cavity‐nesters. Overall, our study demonstrates that Tamarix may substitute for native flora in providing nesting habitat for riparian birds at the eastern edge of its North American range.     

Sublethal effects on reproduction in native fauna: are females more vulnerable to biological invasion?

Although invasive species are a major threat to survivorship of native species, we know little about their sublethal effects. In soft-sediment marine systems, mat-forming invasive species often have positive effects, facilitating recruitment and enhancing the diversity and abundance of native invertebrates. However, because mat-forming invasive species change the habitat in which they invade, and benthic invertebrates are sensitive to environmental disturbance, important sublethal effects on native species may exist. Using a model marine system we show that the widespread mat-forming invasive alga Caulerpa taxifolia (Vahl) C. Agardh has strong negative effects on the reproductive traits of a native bivalve Anadara trapezia (Deshayes, 1840) (e.g. timing of reproductive development and spawning, and follicle and gamete production) even though the invader has positive effects on recruitment. Moreover, gender specific responses occurred and indicated that females were more susceptible to invasion than males. Our results indicate that sublethal effects of an invasive species on reproductive traits will have severe consequences for fitness of the native species.     

Reduced performance of native infauna following recruitment to a habitat-forming invasive marine alga

Despite well-documented negative impacts of invasive species on native biota, evidence for the facilitation of native organisms, particularly by habitat-forming invasive species, is increasing. However, most of these studies are conducted at the population or community level, and we know little about the individual fitness consequences of recruitment to habitat-forming invasive species and, consequently, whether recruitment to these habitats is adaptive. We determined the consequences of recruitment to the invasive green alga Caulerpa taxifolia on the native soft-sediment bivalve Anadara trapezia and nearby unvegetated sediment. Initially, we documented the growth and survivorship of A. trapezia following a natural recruitment event, to which recruitment to C. taxifolia was very high. After 12 months, few clams remained in either habitat, and those that remained showed little growth. Experimental manipulations of recruits demonstrated that all performance measures (survivorship, growth and condition) were significantly reduced in C. taxifolia sediments compared to unvegetated sediments. Exploration of potential mechanisms responsible for the reduced performance in C. taxifolia sediments showed that water flow and water column dissolved oxygen (DO) were significantly reduced under the canopy of C. taxifolia and that sediment anoxia was significantly higher and sediment sulphides greater in C. taxifolia sediments. However, phytoplankton abundance (an indicator of food supply) was significantly higher in C. taxifolia sediments than in unvegetated ones. Our results demonstrate that recruitment of native species to habitat-forming invasive species can reduce growth, condition and survivorship and that studies conducted at the community level may lead to erroneous conclusions about the impacts of invaders and should include studies on life-history traits, particularly juveniles.     

Year-to-year Oscillations in Demography of the Strictly Biennial <Emphasis Type="Italic">Pedicularis sylvatica</Emphasis> and Effects of Experimental Disturbances

In 1998–2001, I studied disturbance effects on the population structure and dynamics of a grassland strict biennial Pedicularis sylvatica, and on the species demography (monthly dynamics of seedling recruitment in 1998 and within- and between-year survival in 1998–2000). In two Czech populations, I established three experimental disturbance regimes: (1) a gap treatment, that simulated grazing by clipping vegetation and creating small gaps, (2) a mowing treatment, where I clipped the vegetation, and (3) a no management treatment, where I left the vegetation untreated. The number of recruiting seedlings varied greatly by year, and demographic structure of populations showed significant year-to-year oscillations in mean seedling numbers, from low (3 ± 0.7 s.e. per 0.25 m² plot) to high (103 ± 20). Inversely in the same years and plots, mean adult numbers in populations oscillated from high (12 ± 2) to low (0.7 ± 0.3). Disturbance effects were only important for seedling recruitment in early census dates in all years. In 1998, most seedlings recruited in April–May in gaps in both sites, but most died before winter. Within- and between-year survival was not affected by disturbance regimes but fluctuated significantly among years. Between-year survival increased with increasing size of the overwintering bud and was higher in disturbance treatments. Since the oscillations in population structure did not significantly vary in response to experimental disturbances, population dynamics may be driven endogenously rather than by disturbance events. The weak disturbance effects on species demography may also indicate population resilience to changes in habitat quality. However, since disturbances promoted seedling recruitment, grazing or mowing regimes are strongly recommended, as they create regeneration opportunities and maintain habitat quality, meeting the species long-term conservation goals.     

Restoring habitat for native and endemic plants through the introduction of a fungal pathogen to control the alien invasive tree <Emphasis Type="Italic">Miconia calvescens</Emphasis> in the island of Tahiti

Success of biological control programs is commonly assessed by studying the direct negative impacts of released agents on the target invasive species. Very few quantitative studies have focused on the indirect positive effects on native biodiversity. In this study, we monitored the response of the plant community (both native and alien species) in permanent plots located in four different sites in montane rainforests of the tropical island of Tahiti (South Pacific) severely invaded over decades by the alien invasive tree Miconia calvescens DC (Melastomataceae), after the release of a defoliating fungal pathogen Colletotrichum gloeosporioides f. sp. miconiae Killgore & L. Sugiyama. Results of five years of monitoring showed that total native and endemic species richness and plant cover increased in all sites and plots. Partial defoliation of miconia canopy trees (between 6% and 36%) led to significant recruitment of light-demanding pioneer species, but also to the appearance of some semi-shade and shade tolerant rare endemic species. Native ferns and angiosperms remained dominant (ca. 80%) in the forest understorey during the monitoring period. Colonization by a small number of alien plant species occurred in one permanent plot located at the lower elevation. We conclude that biological control may be considered a tool for partial habitat restoration and recovery of native and endemic species, but long-term monitoring is needed to confirm the stability and resilience of the “novel plant assemblage”.     

The interaction between propagule pressure,habitat suitability and density‐dependent reproduction in species invasion

Seedling recruitment limitations create a demographic bottleneck that largely determines the viability and structure of plant populations and communities, and pose a core restriction on the colonization of novel habitat. We use a shade‐tolerant, invasive grass, Microstegium vimineum, to examine the interplay between seed and establishment limitations – phenomena that together determine recruitment success but usually are investigated individually. We add increasing amounts of seed to microhabitats containing variable levels of leaf litter thickness – with reduced leaf litter simulating disturbance – to investigate whether reduced seed limitation overcomes the establishment limitation posed by litter cover. We do this across gradients in understory light, moisture and temperature, and quantify germination, survival, and then per capita adult biomass and reproduction in order to understand the implications for invasion across the landscape. We find that the combined effects of seed and establishment limitation influence recruitment; however, propagule pressure overwhelms the inhibitory effects of leaf litter thickness. Leaf litter reduces germination by 22–57% and seedling survival by 13–15% from that observed on bare soil. However, density‐dependent reproduction compensates as 1–3 plants can produce far more seeds (approx. 525) than are required for persistence. As such, just a few plants may establish in understory forest habitat and subsequently overwhelm establishment barriers with copious propagule production. These results, for a widespread, invasive plant, are consistent with the emerging perspective for native plants that seed and establishment limitation jointly influence recruitment. The ability for an exotic plant species to compensate for low population densities with high per capita seed production, that then overrides establishment limitations, makes its invasive potential daunting. Further work is required to test if this is a common mechanism underlying plant invasions.     

Plantlet recruitment is the key demographic transition in invasion by Kalanchoe daigremontiana

Biological invasions have a great impact on biodiversity and ecosystem functioning worldwide. Kalanchoe daigremontiana is a noxious invasive plant in arid zones. Besides being toxic for domestic animals and wildlife, this species inhibits the growth of native plants. Its rapid proliferation in Cerro Saroche National Park (Venezuela) is of great concern because this area hosts several species endemic to the scarce arid zones in the Caribbean. The traits of K. daigremontiana that contribute to its invasive success are unknown. Based on empirical data, we derived a stage structured, stochastic and density-dependent model, to identify characteristics relevant for its establishment. Sensitivity analyses revealed that the establishment of K. daigremontiana depends exclusively on plantlet recruitment. Because asexual plantlets reproduce in less than 1 year populations are able to increase rapidly during the initial phases of invasion, when extinction risks are higher. Sexual seedlings, on the contrary, require a minimum of 3 years to reproduce. As a result, seedling recruitment contributes little to the transient dynamics of the population and therefore cannot warrant the successful establishment of the species. Simulations of various management strategies show that eradication through plant removal may only be achieved if harvest begins shortly after introduction. If a rapid response is not possible, reducing the survival and growth rates of plantlets through biological control is an alternative option. Thus, a strict control of dispersal of plantlets by humans and a continuous monitoring of new invasions should be the first priority for reducing further impact on native species.     

Evaluating nurse plants for restoring native woody species to degraded subtropical woodlands

Harsh habitats dominated by invasive species are difficult to restore. Invasive grasses in arid environments slow succession toward more desired composition, yet grass removal exacerbates high light and temperature, making the use of “nurse plants” an appealing strategy. In this study of degraded subtropical woodlands dominated by alien grasses in Hawai'i, we evaluated whether individuals of two native (Dodonaea viscosa, Leptocophylla tameiameia) and one non‐native (Morella faya) woody species (1) act as natural nodes of recruitment for native woody species and (2) can be used to enhance survivorship of outplanted native woody species. To address these questions, we quantified the presence and persistence of seedlings naturally recruiting beneath adult nurse shrubs and compared survival and growth of experimentally outplanted seedlings of seven native woody species under the nurse species compared to intact and cleared alien‐grass plots. We found that the two native nurse shrubs recruit their own offspring, but do not act as establishment nodes for other species. Morella faya recruited even fewer seedlings than native shrubs. Thus, outplanting will be necessary to increase abundance and diversity of native woody species. Outplant survival was the highest under shrubs compared to away from them with few differences between nurse species. The worst habitat for native seedling survival and growth was within the unmanaged invasive grass matrix. Although the two native nurse species did not differentially affect outplant survival, D. viscosa is the most widespread and easily propagated and is thus more likely to be useful as an initial nurse species. The outplanted species showed variable responses to nurse habitats that we attribute to resource requirements resulting from their typical successional stage and nitrogen fixation capability.     

Restoring native understory to a woodland invaded by Euonymus fortunei: multiple factors affect success

Invasive species are capable of causing change in native plant communities, but invasion is often associated with other anthropogenic impacts on natural areas, such as habitat fragmentation and associated dispersal limitation for native species. Consequently, invasive species removal alone may not always be sufficient to meet restoration objectives. We tested if invasion and dispersal limitation interact to limit plant community restoration within a forest fragment invaded by Euonymus fortunei. Removal of Euonymus alone did not lead to the recolonization of native plant species. However, planting seedlings increased total native cover in invaded, Euonymus removal, and uninvaded control treatments. The consistent establishment of native plant seedlings across all treatments indicates that Euonymus invasion may have limited ability to displace established plants. In contrast, plant species that we added as seed were unable to establish in invaded plots, indicating that Euonymus invasion limits recruitment of native plant species from seed. Over the course of our experiment, a number of setbacks and surprises occurred, including high levels of herbivory, a windstorm, and extreme drought, all of which likely limited restoration success. Overall, our results indicate that Euonymus may contribute to native species declines, but other factors are important. Thus, invasive species removal alone may not be sufficient to reestablish a diverse native plant community. Instead, impacts on natural areas may need to be mitigated along with invasive species removal for restoration to be successful.     

Climate warming increases biological control agent impact on a non‐target species

Climate change may shift interactions of invasive plants, herbivorous insects and native plants, potentially affecting biological control efficacy and non‐target effects on native species. Here, we show how climate warming affects impacts of a multivoltine introduced biocontrol beetle on the non‐target native plant Alternanthera sessilis in China. In field surveys across a latitudinal gradient covering their full distributions, we found beetle damage on A. sessilis increased with rising temperature and plant life history changed from perennial to annual. Experiments showed that elevated temperature changed plant life history and increased insect overwintering, damage and impacts on seedling recruitment. These results suggest that warming can shift phenologies, increase non‐target effect magnitude and increase non‐target effect occurrence by beetle range expansion to additional areas where A. sessilis occurs. This study highlights the importance of understanding how climate change affects species interactions for future biological control of invasive species and conservation of native species.     

Soil conditioning effects of Phragmites australis on native wetland plant seedling survival

Interactions between introduced plants and soils they colonize are central to invasive species success in many systems. Belowground biotic and abiotic changes can influence the success of introduced species as well as their native competitors. All plants alter soil properties after colonization but, in the case of many invasive plant species, it is unclear whether the strength and direction of these soil conditioning effects are due to plant traits, plant origin, or local population characteristics and site conditions in the invaded range. Phragmites australis in North America exists as a mix of populations of different evolutionary origin. Populations of endemic native Phragmites australis americanus are declining, while introduced European populations are important wetland invaders. We assessed soil conditioning effects of native and non‐native P. australis populations on early and late seedling survival of native and introduced wetland plants. We further used a soil biocide treatment to assess the role of soil fungi on seedling survival. Survival of seedlings in soils colonized by P. australis was either unaffected or negatively affected; no species showed improved survival in P. australis‐conditioned soils. Population of P. australis was a significant factor explaining the response of seedlings, but origin (native or non‐native) was not a significant factor. Synthesis: Our results highlight the importance of phylogenetic control when assessing impacts of invasive species to avoid conflating general plant traits with mechanisms of invasive success. Both native (noninvasive) and non‐native (invasive) P. australis populations reduced seedling survival of competing plant species. Because soil legacy effects of native and non‐native P. australis are similar, this study suggests that the close phylogenetic relationship between the two populations, and not the invasive status of introduced P. australis, is more relevant to their soil‐mediated impact on other plant species.     

Invasive Plants can Inhibit Native Tree Seedlings: Testing Potential Allelopathic Mechanisms

The mechanisms by which invasive species affect native communities are not well resolved. For example, invasive plants may influence other species through competition, altered ecosystem processes, or other pathways. We investigated one potential mechanism by which invasive plants may harm native species, allelopathy. Specifically, we explored whether native tree species respond differently to potential allelopathic effects of two invasive plant species. We assessed the separate effects of Lolium arundinaceam (tall fescue) and Elaeagnus umbellata (autumn olive) on three common successional tree species: Acer saccharinum (silver maple), Populus deltoides (eastern cottonwood), and Platanus occidentalis (sycamore). Tall fescue and autumn olive are widely planted and highly invasive or persistent throughout North America where they often grow in forest edges, old fields, and other sites colonized by pioneering tree species. In an exploratory greenhouse experiment, we applied aqueous extracts derived from soil, leaf litter, or live leaves to native trees. We compared these treatments to a sterile water control and also to minced leaves leached in water, a common, but potentially less realistic method of testing for allelopathy. For all tree species, minced leaves from tall fescue reduced the probability that seedlings emerged, and minced leaves of autumn olive reduced the number of days to emergence. During other demographic stages, the three native tree species diverged in their responses to the invasive plants. Platanus occidentalis exhibited the widest range of responses, with reduced root biomass due to minced tissue from both invasive species, reduced days to emergence and marginally reduced survival from minced tall fescue, and reduced leaf biomass from tall fescue leaf litter. Populus deltoides appeared insensitive to most extracts, although survival was marginally increased with application of minced or fresh leaf extracts from autumn olive. In addition, minced tall fescue shortened the time to seedling emergence for Acer saccharinum, potentially a positive effect. Overall, results suggest that allelopathy may be one mechanism underlying the negative impacts of tall fescue and autumn olive on other plant species, but that effects can depend strongly upon the source of allelochemicals and the tree species examined.     

Seedling Ecology and Restoration of Blackbrush (Coleogyne ramosissima) in the Mojave Desert,United States

Increases in fire frequency are disrupting many ecological communities not historically subjected to fire. In the southwestern United States, the blackbrush (Coleogyne ramosissima) community is among the most threatened, often replaced by invasive annual grasses after fire. This long‐lived shrub is vulnerable because it recruits sporadically, partially due to mast seeding and the absence of a seed bank. The goal of this study was to evaluate if shrub restoration can be enhanced by identifying and ameliorating recruitment limitations. Specifically, we tested the effect of encapsulating seeds in predation‐deterring “seed balls.” We also tested the effects of nurse plants and mammalian exclusion cages on seedling emergence, growth, and survivorship. These experiments were conducted in a full‐factorial design across three sites differing in elevation. Over 2 years, 13% of all planted seeds emerged and the effect of seed balls was overwhelmingly negative because of low emergence. Nurse plants had overall positive effects at Low Elevation, but negative effects at Mid‐ and High Elevation. Emergence and survival were highest in caged plots everywhere, and effect sizes increased with elevation. Interactions between the cage and the nurse plant treatments indicated that nurse plants tended to attract mammalian predators, lowering emergence and seedling survivorship, particularly at higher elevations. Findings conform to the stress‐gradient hypothesis in that interactions among seedlings and mature plants shifted from facilitation to competition as environmental stress decreased with increasing elevation, suggesting that they are transferable to ecologically similar communities elsewhere. Knowledge of site‐specific recruitment limitations can help minimize ineffective restoration efforts.     

Differential population responses of native and alien rodents to an invasive predator,habitat alteration and plant masting

Invasive species and anthropogenic habitat alteration are major drivers of biodiversity loss. When multiple invasive species occupy different trophic levels, removing an invasive predator might cause unexpected outcomes owing to complex interactions among native and non-native prey. Moreover, external factors such as habitat alteration and resource availability can affect such dynamics. We hypothesized that native and non-native prey respond differently to an invasive predator, habitat alteration and bottom-up effects. To test the hypothesis, we used Bayesian state-space modelling to analyse 8-year data on the spatio-temporal patterns of two endemic rat species and the non-native black rat in response to the continual removal of the invasive small Indian mongoose on Amami Island, Japan. Despite low reproductive potentials, the endemic rats recovered better after mongoose removal than did the black rat. The endemic species appeared to be vulnerable to predation by mongooses, whose eradication increased the abundances of the endemic rats, but not of the black rat. Habitat alteration increased the black rat''s carrying capacity, but decreased those of the endemic species. We propose that spatio-temporal monitoring data from eradication programmes will clarify the underlying ecological impacts of land-use change and invasive species, and will be useful for future habitat management.     

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.     

Reproductive strategy of an invasive thistle: effects of adults on seedling survival

Invasive species are known for their ability to form monocultures that exclude native species, yet intraspecific interactions among invasives have not been well studied. Cynara cardunculus (L.) is an invasive perennial thistle that establishes high-density populations in coastal California grasslands. We examined the natural distribution of C. cardunculus seedlings in an established population and found that nearly 100% of seedlings grew within 2 m of adults despite an expected distribution peak at 3 m from source plants based on measured dispersal distances. We then investigated the role of mature plants in seedling survival and establishment with regard to live vegetation, litter, and seedling distance by planting seedlings at increasing distances around adults and applying removal treatments to the focal adult rosettes. We applied control (no removal), adult rosette removal (live leaves), litter removal (dead leaves), and adult rosette plus litter removal (all aboveground plant material) treatments. Seedlings experienced a higher rate of survival, measured by senescence date, and establishment, measured by return rate the following year, with all adult rosette removal treatments. Inhibition by adult rosettes was reduced with distance to 60–80 cm from the rosette, and there was little effect of adult plants between 80 and 200 cm. These results suggest that adult rosettes may inhibit conspecific seedlings at very close distances but provide a favorable environment for seedlings within nearby interspaces. This pattern may contribute to the creation and maintenance of high-density populations in C. cardunculus. Land managers seeking to control this species may improve long-term effectiveness by expanding management efforts to include a 2 m radius around adult plants and treating within 5 months of seedling emergence to prevent recruitment rather than treating adults alone.     

Capsule Treatments to Enhance Seedling Emergence of Gaura neomexicana ssp. coloradensis

Management of riparian vegetation is difficult because these communities are frequently impacted by herbivores, invasive weeds, and altered hydrologic regimes. Multiple and intertwined factors affecting rare species recruitment are particularly difficult to identify. Gaura neomexicana ssp. coloradensis Munz (Gaura) is a short‐lived perennial forb endemic to riparian areas in mixed‐grass prairies of Wyoming, Nebraska, and Colorado, U.S.A. It became a federally listed threatened species in October 2000. Because the species is a recruitment‐limited monocarpic perennial, we studied the effects of six capsule‐collection dates, a 2‐month cool‐moist stratification, 24‐hr leaching, and 24‐hr imbibition on Gaura seedling emergence. Seedling emergence did not vary with collection date. Capsules collected from Gaura plants grown at the Bridger Plant Materials Center in Montana exhibited greater emergence than capsules harvested from endemic populations near Cheyenne, Wyoming, suggesting that maternal plant growing conditions impact dormancy. Because cool‐moist stratification enhanced seedling emergence of Gaura and leaching did not, sufficient moisture during cool temperatures may be more critical than leaching of germination inhibitors as might occur with normal stream flows. Spring flooding may enhance Gaura recruitment by increasing the availability of riparian sites that are inundated during periods of cool temperatures. If so, hydrologic and climatic regimes must be considered in restoring the unique conditions needed for germination of this rare riparian endemic.     

Native and invasive plant interactions in wetlands and the minimal role of invasiveness

The effects of invasive plants on plants native to areas that are being invaded can be quite variable, depending on the species of the invasive plant involved as well as the physical characteristics of the location being invaded. My study focuses on the effects of Phragmites australis Linnaeus (common reed) and Lythrum salicaria L. (purple loosestrife) on the same native plant community. Uninvaded plots dominated by native plants Typha angustifolia L. (narrowleaf cattail) and Typha latifolia L. (broadleaf cattail) served as the control. I surveyed percent cover of species during early summer and midsummer for 3 years in six Hudson River freshwater tidal wetlands (sites). Differences in species richness, composition and abundance were small, but significant among invaded and uninvaded plots and among sites. However, these differences remained significant when data for dominant species (invasive and native) were removed. Differences in native plant species abundance were attributed to invasive plant species-specific characteristics and differences in species richness and composition were attributed to physical location (zonation) in these freshwater tidal marshes. “Invasive” status of a dominant plant species was less important in invasive plant–native plant interactions than species-specific characteristics and zonation. Further research into the effects of site and land-use on invasive plant impacts is recommended.     

Long-term demography of a zebra mussel (Dreissena polymorpha) population

1. We used long‐term data and a simulation model to investigate temporal fluctuations in zebra mussel populations, which govern the ecological and economic impacts of this pest species. 2. The size of the zebra mussel (Dreissena polymorpha) population in the Hudson River estuary fluctuated approximately 11‐fold across a 13‐year period, following a cycle with a 2–4 year period. 3. This cycling was caused by low recruitment during years of high adult population size, rapid somatic growth of settled animals, and adult survivorship of 50% per year. 4. Adult growth and body condition were weakly correlated with phytoplankton biomass. 5. The habitat distribution of the Hudson's population changed over the 13‐year period, with an increasing proportion of the population spreading onto soft sediments over time. The character of soft‐sediment habitats in the Hudson changed because of large amounts (mean = 34 g DM m^?2) of empty zebra mussel shells now in the sediments. 6. Simulation models show that zebra mussel populations can show a range of long‐term trajectories, depending on the balance between adult space limitation, larval food limitation, and disturbance. 7. Effective understanding and management of the effects of zebra mussels and other alien species depend on understanding of their long‐term demography, which may vary across ecosystems.     

Invasive slugs as under-appreciated obstacles to rare plant restoration: evidence from the Hawaiian Islands

Introduced slugs have invaded many parts of the world where they were recognized as important pests of gardens and agriculture, but we know little about the effects of introduced slugs on rare plants in natural areas. The Hawaiian Islands have no native slugs, but over a dozen introduced slug species are now established. We reviewed Rare Plant Recovery Plans produced by the U.S. Fish and Wildlife Service for Hawaii and found that introduced slugs were specifically mentioned as threats or potential threats to 59 rare plant species (22% of all endangered and threatened plants), based mainly on anecdotal observations by field biologists. We then initiated an experimental field study to assess the impact of slug herbivory on the growth and survival of two endangered plant species (Cyanea superba, and Schidea obovata), one non-endangered native species (Nestegis sandwicensis) and two co-occurring invasive plant species (Psidium cattleianum and Clidemia hirta). In mesic forest on the Island of Oahu, we tracked the fate of outplanted seedlings in replicated 1 m² plots, with and without slug control. Slugs decreased seedling survival of the endangered species by 51%, on average. Slugs did not significantly affect survival of the non-endangered or invasive plant species. Introduced slugs seem to be under-appreciated as a direct cause of plant endangerment. Invasive slugs may also facilitate the success of some invasive plant species by reducing competition with more palatable, native plant competitors. Slug control measures are relatively inexpensive and could facilitate rare plant establishment and population recovery.