Showing papers on "Propagule pressure published in 2012"

Continent-wide risk assessment for the establishment of nonindigenous species in Antarctica

Abstract: Invasive alien species are among the primary causes of biodiversity change globally, with the risks thereof broadly understood for most regions of the world. They are similarly thought to be among the most significant conservation threats to Antarctica, especially as climate change proceeds in the region. However, no comprehensive, continent-wide evaluation of the risks to Antarctica posed by such species has been undertaken. Here we do so by sampling, identifying, and mapping the vascular plant propagules carried by all categories of visitors to Antarctica during the International Polar Year's first season (2007–2008) and assessing propagule establishment likelihood based on their identity and origins and on spatial variation in Antarctica's climate. For an evaluation of the situation in 2100, we use modeled climates based on the Intergovernmental Panel on Climate Change's Special Report on Emissions Scenarios Scenario A1B [Nakicenovic N, Swart R, eds (2000) Special Report on Emissions Scenarios: A Special Report of Working Group III of the Intergovernmental Panel on Climate Change (Cambridge University Press, Cambridge, UK)]. Visitors carrying seeds average 9.5 seeds per person, although as vectors, scientists carry greater propagule loads than tourists. Annual tourist numbers (∼33,054) are higher than those of scientists (∼7,085), thus tempering these differences in propagule load. Alien species establishment is currently most likely for the Western Antarctic Peninsula. Recent founder populations of several alien species in this area corroborate these findings. With climate change, risks will grow in the Antarctic Peninsula, Ross Sea, and East Antarctic coastal regions. Our evidence-based assessment demonstrates which parts of Antarctica are at growing risk from alien species that may become invasive and provides the means to mitigate this threat now and into the future as the continent's climate changes.

Invasions: the trail behind, the path ahead, and a test of a disturbing idea

Abstract: Summary 1. We provide a brief overview of progress in our understanding of introduced plant species. 2. Three main conclusions emerge from our review: (i) Many lines of research, including the search for traits that make species good invaders, or that make ecosystems susceptible to invasion, are yielding idiosyncratic results. To move forward, we advocate a more synthetic approach that incorporates a range of different types of information about the introduced species and the communities and habitats they are invading. (ii) Given the growing evidence for the adaptive capacity of both introduced species and recipient communities, we need to consider the implications of the long-term presence of introduced species in our ecosystems. (iii) Several foundational ideas in invasion biology have become widely accepted without appropriate testing, or despite equivocal evidence from empirical tests. One such idea is the suggestion that disturbance facilitates invasion. 3. We use data from 200 sites around the world to provide a broad test of the hypothesis that invasions are better predicted by a change in disturbance regime than by disturbance per se .N either disturbance nor change in disturbance regime explained more than 7% of the variation in the % of cover or species richness contributed by introduced species. However, change in disturbance regime was a significantly better predictor than was disturbance per se, explaining approximately twice as much variation as did disturbance.

Humans introduce viable seeds to the Arctic on footwear

Abstract: Expanding visitation to Polar regions combined with climate warming increases the potential for alien species introduction and establishment. We quantified vascular plant propagule pressure associated with different groups of travelers to the high-Arctic archipelago of Svalbard, and evaluated the potential of introduced seeds to germinate under the most favorable average Svalbard soil temperature (10°C). We sampled the footwear of 259 travelers arriving by air to Svalbard during the summer of 2008, recording 1,019 seeds: a mean of 3.9 (±0.8) seeds per traveler. Assuming the seed influx is representative for the whole year, we estimate a yearly seed load of around 270,000 by this vector alone. Seeds of 53 species were identified from 17 families, with Poaceae having both highest diversity and number of seeds. Eight of the families identified are among those most invasive worldwide, while the majority of the species identified were non-native to Svalbard. The number of seeds was highest on footwear that had been used in forested and alpine areas in the 3 months prior to traveling to Svalbard, and increased with the amount of soil affixed to footwear. In total, 26% of the collected seeds germinated under simulated Svalbard conditions. Our results demonstrate high propagule transport through aviation to highly visited cold-climate regions and isolated islands is occurring. Alien species establishment is expected to increase with climate change, particularly in high latitude regions, making the need for regional management considerations a priority.

When should exotic forest plantation tree species be considered as an invasive threat and how should we treat them

Abstract: The area of forests planted with exotic tree species is increasing worldwide in order to fulfill various economic and environmental demands. Numerous species currently used in forest plantations are considered to be invasive in many parts of the world. Exotic plantation tree species are endowed with a series of life-history traits that are characteristic of invasive species: easy establishment, fast growth, high propagule pressure, and low or intermediate shade tolerance. In addition, plantation forestry presents many features that increase ecosystem invasibility, including a regime of frequent and intensive canopy disturbances, a dense network of roads and trails with continuous traffic, and wide plantation areas. Exotic trees planted for production purposes have strong direct positive economic impacts on the local and national economies of many countries, often leading to notable conflicts of interest when the species becomes invasive, as well as to negative impacts on the ecosystem. Studies and management programs mainly focus on Pinus and Acacia, which are used as model species. Complementary management strategies have been established to control these species at different stages of the invasion process. Knowledge gained is useful to predict problems in other regions of the world with the same species and to guide research or management actions on other problematic but less studied tree species.

The Interaction Between Propagule Pressure, Habitat Suitability and Density‐Dependent Reproduction in Species Invasion

Abstract: 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 shadetolerant, 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.

Native and naturalized range size in Pinus: relative importance of biogeography, introduction effort and species traits

Abstract: Aim Pine trees (genus Pinus) represent an ancient lineage, naturally occurring almost exclusively in the Northern Hemisphere, but introduced and widely naturalized in both hemispheres. As large trees of interest to forestry, they attract much attention and their distribution is well documented in both indigenous and naturalized ranges. This creates an opportunity to analyse the relationship between indigenous and naturalized range sizes in the context of different levels of human usage, biological traits and the characteristics of the environments of origin. Location Global. Methods We combined and expanded pre-existing data sets for pine species distributions and pine species traits, and used a variety of regression techniques (including generalized additive models and zero-inflated Poisson models) to assess which variables explained naturalized and indigenous range sizes. Results Indigenous and naturalized range sizes are positively correlated but there are many notable exceptions. Some species have large indigenous ranges but small or no naturalized ranges, whereas others have small indigenous ranges, but have naturalized in many regions. Indigenous range is correlated to factors such as seed size (−), age at first reproduction (−), and latitude (+, supporting Rapoport's rule), but also to the extent of coverage of species in the forestry literature (+). Naturalized range size is strongly influenced by the extent of coverage of species in the forestry literature (+), a proxy for propagule pressure. Naturalization was also influenced by average elevation in the indigenous range (−) and age at first reproduction (−). Main conclusions The macroecological and evolutionary pressures facing plant groups are not directly transferable between indigenous and naturalized ranges. In particular, there are strong biases in species naturalization and expansion in invasive ranges that are unrelated to factors determining indigenous range size. At least for Pinus, a new set of macroecological patterns are emerging which are profoundly influenced by humans.

Truth and consequences: the bioinvasion of the Mediterranean Sea.

Abstract: Over 660 alien marine multicellular species have been recorded in the Mediterranean Sea, with many establishing viable populations and subsequently dispersing from their points of entry. A brief overview is presented of the current state of knowledge of bioinvasions in the Mediterranean Sea, with particular attention paid to gelatinous invasive species. A database of alien species records was used to characterize their identity, pattern of expansion and native region, and to discuss the relative importance of different taxa and vectors in the various regions of the sea. The temporal and spatial spread of 4 alien scyphozoans and 2 alien ctenophores is outlined. Although thermophilic species constitute the majority of Mediterranean aliens, Erythrean aliens are predominant in the east, with shipping and mariculture being mainly responsible for introductions in the northwest. The propagule pressure driving the Erythrean invasion is powerful in the establishment and spread of alien species in the eastern Mediterranean. The implications of the enlargement of the Suez Canal, reflecting patterns in global trade and economy, are briefly discussed. Finally, as alien species inventories play a pivotal role in informing regional policy and management decisions and identifying resource priorities, the scientific community is called upon to pay particular attention to their accuracy and veracity.

Alien and native plant life‐forms respond differently to human and climate pressures

Abstract: Aim To investigate whether differences in the elevational trend in native and alien species richness were dependent on climate or human pressures. Specifically we tested whether life-form and/or alien/native status modifies the response of plant species richness to human population and temperature along: (1) a complete elevational gradient, and (2) within separate elevational bands that, by keeping temperature within a narrow range, elucidate the effects of human pressures more clearly. Location Two provinces (c. 7507 km 2 ) on the southern border of the European Alps (Italy), subdivided into 240 contiguous sampling cells (c. 35.7 km 2 ). Methods We used an extensive dataset on alien and native species richness across an elevation gradient (20‐2900 m a.s.l.). Richness of natives and naturalized aliens were separately related to temperature, human population and Raunkiaer life-form using general linear mixed models. Life-form describes different plant strategies for survival during seasons with adverse cold/arid conditions. Results The relationship between species richness and temperature for natives was strongly dependent on life-form, while aliens showed a consistent positive trend. Similar trends across alien and native life-forms were found for the relationship between species richness and human population along the whole gradient and within separate elevational bands. Main conclusions The absence of life-form-dependent responses amongst aliens supports the hypothesis that the distribution of alien plant species richness was more related to propagule pressure and availability of novel niches created by human activities than to climatic filtering. While climate change will potentially contribute to relaxing species thermal constraints, the response of alien species to future warming will also be contingent on changes in anthropogenic pressures.

What determines pine naturalization: species traits, climate suitability or forestry use?

Abstract: Aim Species attributes, biogeographic features and human factors have all been shown to discriminate between invasive and non-invasive plant species. However, the relative importance of these factors, their generality in determining invasion outcomes across different regions and their ability to discriminate success and failure at different stages of the invasion process have not been established. Location New Zealand (NZ) and Great Britain (GB). Methods For species in the genus Pinus, we used boosted regression trees to identify factors associated with success or failure at the introduction and naturalization stages in each region. Results Human factors, notably the forestry use index, were the strongest determinants of which species from the global pool were introduced to both NZ and GB. Species with a close climate match were also more likely to be introduced to NZ but not to GB. Human factors and climate match were also the strongest determinants of which introduced species became naturalized in both NZ and GB, although the order of importance differed (human factors followed by climate match for NZ and vice versa for GB). Species attributes (life-history traits and the Z-score) had much less ability to discriminate successful and failed species at both the introduction and naturalization stages in these two regions. Main conclusions We show for the first time that human factors are more important than either species or biogeographic traits in determining the likelihood of a species being introduced or becoming naturalized. The similarity between two different regions in the factors found to be important in success at both these invasions stages points to potentially general mechanisms underlying these processes. The strong human component to introduction and naturalization highlights a potential conflict between future afforestation using alien species with conservation and management aims in the surrounding landscape, given that the factors desirable for forestry species are also those that may promote invasion.

Parasites and invasions: a biogeographic examination of parasites and hosts in native and introduced ranges

Abstract: Aim To use a comparative approach to understand parasite demographic patterns in native versus introduced populations, evaluating the potential roles of host invasion history and parasite life history. Location North American east and west coasts with a focus on San Francisco Bay (SFB). Methods Species richness and prevalence of trematode parasites were examined in the native and introduced ranges of two gastropod host species, Ilyanassa obsoleta and Littorina saxatilis. We divided the native range into the putative source area for introduction and areas to the north and south; we also sampled the overlapping introduced range in SFB. We dissected 14,781 snails from 103 populations and recorded the prevalence and identity of trematode parasites. We compared trematode species richness and prevalence across the hosts’ introduced and native ranges, and evaluated the influence of host availability on observed patterns. Results Relative to the native range, both I. obsoleta and L. saxatilis have escaped (lost) parasites in SFB, and L. saxatilis demonstrated a greater reduction of trematode diversity and infection prevalence than I. obsoleta. This was not due to sampling inequalities between the hosts. Instead, rarefaction curves suggested complete capture of trematode species in native source and SFB subregions, except for L. saxatilis in SFB, where infection was extremely rare. For I. obsoleta, infection prevalence of trematodes using fish definitive hosts was significantly lower in SFB compared to the native range, unlike those using bird hosts. Host availability partly explained the presence of introduced trematodes in SFB. Main conclusions Differential losses of parasite richness and prevalence for the two gastropod host species in their introduced range is probably the result of several mechanistic factors: time since introduction, propagule pressure, vector of introduction, and host availability. Moreover, the recent occurrence of L. saxatilis’ invasion and its active introduction vector suggest that its parasite diversity and distribution will probably increase over time. Our study suggests that host invasion history and parasite life history play key roles in the extent and diversity of trematodes transferred to introduced populations. Our results also provide vital information for understanding community-level influences of parasite introductions, as well as for disease ecology in general.

Relationship between propagule pressure and colonization pressure in invasion ecology: a test with ships' ballast.

Abstract: Increasing empirical evidence indicates the number of released individuals (i.e. propagule pressure) and number of released species (i.e. colonization pressure) are key determinants of the number of species that successfully invade new habitats. In view of these relationships, and the possibility that ships transport whole communities of organisms, we collected 333 ballast water and sediment samples to investigate the relationship between propagule and colonization pressure for a variety of diverse taxonomic groups (diatoms, dinoflagellates and invertebrates). We also reviewed the scientific literature to compare the number of species transported by ships to those reported in nature. Here, we show that even though ships transport nearly entire local communities, a strong relationship between propagule and colonization pressure exists only for dinoflagellates. Our study provides evidence that colonization pressure of invertebrates and diatoms may fluctuate widely irrespective of propagule pressure. We suggest that the lack of correspondence is explained by reduced uptake of invertebrates into the transport vector and the sensitivity of invertebrates and diatoms to selective pressures during transportation. Selection during transportation is initially evident through decreases in propagule pressure, followed by decreased colonization pressure in the most sensitive taxa.

Ancient and recent alien species in temperate forests: steady state and time lags

Abstract: Temperate forests are relatively little affected by invasions globally. However, it remains unclear if these low invasion levels can be attributed to high invasion resistance, or if particularly long time lags in forests have lead to wide-spread invasion debt at the landscape and habitat scales, thus masking the invasibility of forests. Using linear and generalized linear mixed-effects models, we tested which factors determine the level of invasion of old (archaeophytes, pre-1500 alien species) and new (neophytes, post-1500 alien species) invaders across 30 study sites (5 ha size) and 16 transects in lowland forests in eastern Austria, Central Europe. We analysed the importance of different forest types, adjacent habitats, introduction pathways, species’ local residence time and proximity to human settlements as proxy for propagule pressure. We recorded 109 neophyte and 50 archaeophyte species, with significant differences between both groups with respect to region of origin, pathways, and life form. The number of neophyte species per study site varied by more than an order of magnitude and was dependent on both proximity to settlements and residence time in a non-linear manner, with proximity to settlements showing a particularly strong effect for recently introduced neophytes. In contrast, archaeophyte species numbers were only marginally affected by distance to the nearest settlement. Within habitats, transect plots close to the forest edge and adjacent to settlements showed significantly higher numbers and cover of neophyte species, whereas for archaeophytes and native species no such effect could be detected. Our results suggest that time lag phenomena stemming from dispersal limitation (e.g. aggravated by isolation of forest patches), heterogeneity in species’ local residence time as well as increases in local levels of propagule pressure (e.g. created by expanding settlements), may result in wide-spread invasion debt at the landscape and habitat scales, hence masking invasibility of forests. Spread of new invaders may ultimately, although possibly only over long time periods, result in increased frequency and impact of neophyte species in temperate forests.

The vulnerability of habitats to plant invasion: disentangling the roles of propagule pressure, time and sampling effort

Abstract: Aim To quantify the vulnerability of habitats to invasion by alien plants having accounted for the effects of propagule pressure, time and sampling effort. Location New Zealand. Methods We used spatial, temporal and habitat information taken from 9297 herbarium records of 301 alien plant species to examine the vulnerability of 11 terrestrial habitats to plant invasions. A null model that randomized species records across habitats was used to account for variation in sampling effort and to derive a relative measure of invasion based either on all records for a species or only its first record. The relative level of invasion was related to the average distance of each habitat from the nearest conurbation, which was used as a proxy for propagule pressure. The habitat in which a species was first recorded was compared to the habitats encountered for all records of that species to determine whether the initial habitat could predict subsequent habitat occupancy. Results Variation in sampling effort in space and time significantly masked the underlying vulnerability of habitats to plant invasions. Distance from the nearest conurbation had little effect on the relative level of invasion in each habitat, but the number of first records of each species significantly declined with increasing distance. While Urban, Streamside and Coastal habitats were over-represented as sites of initial invasion, there was no evidence of major invasion hotspots from which alien plants might subsequently spread. Rather, the data suggest that certain habitats (especially Roadsides) readily accumulate alien plants from other habitats. Main conclusions Herbarium records combined with a suitable null model provide a powerful tool for assessing the relative vulnerability of habitats to plant invasion. The first records of alien plants tend to be found near conurbations, but this pattern disappears with subsequent spread. Regardless of the habitat where a species was first recorded, ultimately most alien plants spread to Roadside and Sparse habitats. This information suggests that such habitats may be useful targets for weed surveillance and monitoring.

Online trading tools as a method of estimating propagule pressure via the pet-release pathway

Abstract: The increasing amount of internet trade in live animals has facilitated the sale and circulation of exotic species all over the world. This is an area of concern, as the deliberate or accidental release of pets is an important pathway by which exotic species are often introduced into new environments, often with negative effects on the local species and ecosystems. Internet trading sites were used to determine the distribution and magnitude of propagule pressure of red-eared slider turtles (RES; Trachemys scripta elegans) within the New Zealand pet trade. Sites were tracked daily from October 1, 2007 to September 30, 2009 and information on age, sex, season, and location was recorded. More than 1,500 sales and 80 reports of lost/found RES were recorded. Unsurprisingly, the highest number of sales and lost/found RES was in Auckland, the region with the highest human population. Females were more often reported as lost or found than males, despite a similar sex ratio of sales. The type and quality of information gathered in this manner is not perfect, as it only provides an estimate of minimum numbers of animals that are being traded/lost into the environment, but nonetheless, provides useful data when planning a management or eradication plan for feral turtles in New Zealand. Of concern, our results highlighted areas where turtles were most often being released in New Zealand, being those areas predicted to be the most climatically-suitable for this species, and in which incubation conditions were most likely to be met. Monitoring online sales of exotic species provides useful demographic information, as well as an indication of propagule pressure via the pet-release pathway. This technique is applicable to other species and may be a useful tool to help determine locations at risk of the establishment of other exotic species.

Manipulating the intrinsic parameters of propagule pressure: implications for bio-invasion

Abstract: Much effort has been devoted to understanding the factors promoting species colonisation, and processes that may cause small incipient populations to fail. Post colonisation and community level processes are proving to be less reliable predictors of initial population size and propagule pressure is now posited as a key predictor of species establishment. Yet, empirical studies manipulating the intrinsic parameters of propagule pressure, such as arrival frequency (number), and intensity (size) are lacking. Understanding colonisation and incipient population survival is particularly important in the field of bio-invasions. Propagule pressure is now often cited as a key determinant of invasion success, yet we have few empirical tests manipulating the intrinsic parameters of propagule pressure. We conducted a series of factorial field experiments utilizing larvae of the cosmopolitan invasive oyster Crassostrea gigas. A newly developed technique was used to quantitatively alter the size of propagules and the frequency at which they arrive. When total propagule pressure was held constant, and the size and arrival frequency was altered, frequent small inoculations dramatically increased incipient population size >65% relative to infrequent large introductions. We also found that smaller, less dense inoculations resulted in >85% greater proportional settlement and colonisation of this cosmopolitan species. Predation altered the overall survival of incipient oyster populations, but did not alter this pattern. Our findings may help explain patterns of ballast water and hull-fouling introductions, as ports and harbours that receive frequent exposure to invasive propagules from the same source location will be at greater risk of invasion.

Predator-driven biotic resistance and propagule pressure regulate the invasive apple snail Pomacea canaliculata in Japan

Abstract: Species richness in local communities has been considered an important factor determining the success of invasion by exotic species (the biotic resistance hypothesis). However, the detailed mechanisms, especially the role of predator communities, are not well understood. We studied biotic resistance to an invasive freshwater snail, Pomacea canaliculata, at 31 sites in an urban river basin (the Yamatogawa) in western Japan. First, we studied the relationship between the richness of local animal species and the abundance of P. canaliculata, demonstrating a negative relationship, which suggests that the intensity of biotic resistance regulates local snail populations. This pattern was due to the richness of native predator communities rather than that of introduced species or non-predators (mainly competitors of the apple snail). Local snail abundance was also affected by immigration of snails from nearby rice fields (i.e. propagule pressure), where few predators occur. Second, we assessed short-term predation pressure on the snail by means of a tethering experiment. Predation pressure was positively correlated with the number of individual predators and negatively correlated with snail abundance. The introduced crayfish Procambarus clarkii was responsible for the variance in predation pressure. These results indicate that the predator community, composed of both native and introduced species, is responsible for resistance to a novel invader even in a polluted urban river.

Interactions between multiple recruitment drivers: post-settlement predation mortality and flow-mediated recruitment.

Abstract: Background Dispersal is a primary driver in shaping the future distribution of species in both terrestrial and marine systems. Physical transport by advection can regulate the distance travelled and rate of propagule supply to a habitat but post-settlement processes such as predation can decouple supply from recruitment. The effect of flow-mediated recruitment and predation on the recruitment success of an intertidal species, the eastern oyster Crassostrea virginica was evaluated in two-replicated field experiments. Two key crab species were manipulated to test predator identity effects on oyster mortality. Findings Recruitment was ∼58% higher in high flow compared to low flow, but predation masked those differences. Predation mortality was primarily attributed to the blue crab Callinectes sapidus, whilst the mud crab Panopeus herbstii had no effect on recruit mortality. Recruit mortality from predation was high when recruit densities were high, but when recruit density was low, predation effects were not seen. Under high recruitment (supply), predation determined maximum population size and in low flow environments, recruitment success is likely determined by a combination of recruitment and resource limitation but not predation. Conclusions Four processes are demonstrated: (1) Increases in flow rate positively affect recruitment success; (2) In high flow (recruitment) environments, resource availability is less important than predation; (3) predation is an important source of recruit mortality, but is dependent upon recruit density; and (4) recruitment and/or resource limitation is likely a major driver of population structure and functioning, modifying the interaction between predators and prey. Simultaneous testing of flow-mediated recruitment and predation was required to differentiate between the role of each process in determining population size. Our results reinforce the importance of propagule pressure, predation and post-settlement mortality as important determinants of population growth and persistence, but demonstrate that they should not be considered mutually exclusive.

Interactions between propagule pressure and seed traits shape human-mediated seed dispersal along roads.

Abstract: Human-mediated dispersal along the road network is a crucial process in the population dynamics of roadside vegetation and during plant invasions. The potential for a species to be dispersed by vehicles is, however, difficult to quantify. The predictive power of categorical classification schemes of human-mediated dispersal is limited as many species that are usually attributed to particular primary dispersal vectors may become subject to very different secondary dispersal vectors owing to human activity. Analysing seed traits that promote seed transport by human dispersal vectors could overcome these limitations. However, the analysis has to account for the divergent chance of seed transport that results from different propagule pressures along the transport corridor. To reveal the effects of seed traits and their interplay with propagule pressure on the chance and magnitude of human-mediated dispersal by vehicles, we compared traits and regional frequencies of a set of species that were dispersed by vehicles to a control set not dispersed but present in the same study area. We then used the same traits for a comparison of intentionally and unintentionally introduced species with the flora of Berlin. Different traits influenced the chance of vehicle dispersal and its magnitude. While propagule pressure was most important for determining the magnitude of seed transport, small seed mass and size best predicted the absolute chance of species dispersal by vehicles. The dispersal of nonnative species was least dependent on propagule pressure. Seed traits that were important in vehicle dispersal were similarly reflected in unintentionally introduced species in the Berlin flora. Mean seed size of these species was lower compared to the entire Berlin flora, whereas it was higher for intentionally introduced species. This suggests that unintentional introduction of nonnative plant species pre-selects for seed traits that promote further spread by human-mediated adhesive dispersal. Probability and magnitude of adhesive seed transport by vehicles can be predicted by dispersal-related plant traits. However, the effect size of plant traits on dispersal strongly depends on regional propagule pressure. This highlights the need to analyse interactions between species traits and propagule pressure.

A population genetic signature of human releases in an invasive ladybeetle.

Abstract: Biological invasions have been accelerated by a variety of human activities. Propagule pressure, the number of introduced individuals and independent introductions, is probably to be influenced by these human activities and may be an important factor for successful range expansion in new environments. We tested whether the current distribution of the predatory ladybeetle Coccinella septempunctata in the introduced range (USA) is the result of multiple historical human introductions or natural range expansion from the first established populations in the USA. To test this hypothesis, we compared historical records of propagule size, propagule number, specific introduction locations and the date of each introduction, with estimates of genetic variation in mitochondrial DNA (cytochrome oxidase I). Our results indicated that genetic diversity in the introduced range was positively correlated with historical records of propagule size and number and negatively correlated with distance to nearest introduction point, suggesting that multiple human releases were successful. Higher genetic diversity in populations found near introduction points suggest that initial founder effects were limited, but lower genetic diversity found farther from introduction points is probably the result of serial founder effects during secondary range expansion. These results suggest that the current distribution of C. septempunctata in the introduced range is the result of a combination of human releases and short-range expansion from multiple established populations in the introduced range.

Propagule pressure and colony social organization are associated with the successful invasion and rapid range expansion of fire ants in China

Abstract: We characterized patterns of genetic variation in populations of the fire ant Solenopsis invicta in China using mitochondrial DNA sequences and nuclear microsatellite loci to test predictions as to how propagule pressure and subsequent dispersal following establishment jointly shape the invasion success of this ant in this recently invaded area. Fire ants in Wuchuan (Guangdong Province) are genetically differentiated from those found in other large infested areas of China. The immediate source of ants in Wuchuan appears to be somewhere near Texas, which ranks first among the southern USA infested states in the exportation of goods to China. Most colonies from spatially distant, outlying areas in China are genetically similar to one another and appear to share a common source (Wuchuan, Guangdong Province), suggesting that long-distance jump dispersal has been a prevalent means of recent spread of fire ants in China. Furthermore, most colonies at outlier sites are of the polygyne social form (featuring multiple egg-laying queens per nest), reinforcing the important role of this social form in the successful invasion of new areas and subsequent range expansion following invasion. Several analyses consistently revealed characteristic signatures of genetic bottlenecks for S. invicta populations in China. The results of this study highlight the invasive potential of this pest ant, suggest that the magnitude of international trade may serve as a predictor of propagule pressure and indicate that rates and patterns of subsequent range expansion are partly determined by the interplay between species traits and the trade and transportation networks.

Urbanized landscapes favored by fig-eating birds increase invasive but not native juvenile strangler fig abundance.

Abstract: Propagule pressure can determine the success or failure of invasive plant range expansion. Range expansion takes place at large spatial scales, often encompassing many types of land cover, yet the effect of landscape context on propagule pressure remains largely unknown. Many studies have reported a positive correlation between invasive plant abundance and human land use; increased propagule pressure in these landscapes may be responsible for this correlation. We tested the hypothesis that increased rates of seed dispersal by fig-eating birds, which are more common in urban habitats, result in an increase in invasive strangler fig abundance in landscapes dominated by human land use. We quantified abundance of an invasive species (Ficus microcarpa) and a native species (F. aurea) of strangler fig in plots spanning the entire range of human land use in South Florida, USA, from urban parking lots to native forest. We then compared models that predicted juvenile fig abundance based on distance to adult fig seed sources and fig-eating bird habitat quality with models that lacked one or both of these terms. The best model for juvenile invasive fig abundance included both distance to adult and fig-eating bird habitat terms, suggesting that landscape effects on invasive fig abundance are mediated by seed-dispersing birds. In contrast, the best model for juvenile native fig abundance included only presence/absence of adults, suggesting that distance from individual adult trees may have less effect on seed limitation for a native species compared to an invasive species undergoing range expansion. However, models for both species included significant effects of adult seed sources, implying that juvenile abundance is limited by seed arrival. This result was corroborated by a seed addition experiment that indicated that both native and invasive strangler figs were strongly seed limited. Understanding how landscape context affects the mechanisms of plant invasion may lead to better management techniques. Our results suggest that prioritizing removal of adult trees in sites with high fig-eating bird habitat may be the most effective method to control F. microcarpa abundance.

Natural range expansion and human-assisted introduction leave different genetic signatures in a hermaphroditic freshwater snail

Abstract: Colonization events like range expansion or biological invasions can be associated with population bottlenecks Small population size may lead to loss of genetic diversity due to random genetic drift, to loss of heterozygosity due to increased inbreeding and should leave a signature on the genetic polymorphism and genetic structure of populations The mating system might additionally influence the outcome of such a process Here, we compare invasive and native populations of the hermaphroditic freshwater snail Lymnaea stagnalis In the native range we included populations that were ice-free during the last glaciation period and populations that were glaciated and are located at the edge of the species’ native distribution range The microsatellite data show substantial loss of genetic variation in the introduced range and no signs of high propagule pressure or admixture The expressed polymorphism was so low that mating system analysis was not possible In the native region, all populations display strong levels of differentiation (global F ST: 0341) independent of colonization history and exhibit no significant pattern of inbreeding However, the populations in more recently colonized habitats show diminished genetic diversity Overall, these results illustrate how dramatic the reduction in genetic diversity can be for hermaphroditic animals and that gene flow in the native range can be surprisingly low despite short distances

Biological control as an invasion process disturbance and propagule pressure affect the invasion success of Lythrum salicaria biological control agents

Abstract: Understanding the mechanisms behind the successful colonization and establishment of introduced species is important for both preventing the invasion of unwanted species and improving release programs for biological control agents. However, it is often not possible to determine important introduction details, such as date, number of organisms, and introduction location when examining factors affecting invasion success. Here we use biological control introduction data to assess the role of propagule pressure, disturbance, and residence time on invasion success of four herbivorous insect species introduced for the control of the invasive wetland plant, Lythrum salicaria, in the Columbia River Estuary. Two sets of field surveys determined persistence at prior release sites, colonization of new sites, and abundance within colonized sites. We quantified propagule pressure in four ways to examine the effect of different measurements. These included three measurements of introduction size (proximity to introduction site, introduction size at a local scale, and introduction size at a regional scale) and one measure of introduction number (number of introduction events in a region). Disturbance was examined along a tidal inundation gradient (distance from river mouth) and as habitat (island or mainland). Statistical models and model averaging were used to determine which factors were driving invasion success. In this study we found: (1) sparse evidence for the positive influence of propagule pressure on invasion success; (2) disturbance can negatively affect the invasion success of herbivorous insects; (3) the effects of disturbance and propagule pressure are species specific and vary among invasion stages, and (4) not all measures of propagule pressure show the same results, therefore single measures and proxies should be used cautiously.

Modeling biofouling from boat and source characteristics: a comparative study between Canada and New Zealand

Abstract: In marine systems subject to vessel traffic, the likelihood of an invasion by an exotic species typically increases with the arrival of each infested boat In this paper, recreational boating activity patterns and boat fouling by tunicates were compared between eastern Canada and central New Zealand, and the relative importance of boat characteristics on fouling was evaluated using Boosted Regression Tree analysis For Canada, we also determined the relative importance of boat characteristics and propagule exposure (ie, the interaction between tunicate density in source region and time in water) on patterns of boat fouling Approximately half of boats examined during the fall in eastern Canada and during the summer in central New Zealand were fouled by tunicates Although there was a greater richness of tunicate species on New Zealand boats, the two countries had several species or genera in common, including Botryllus schlosseri, Ciona spp and Botrylloides spp The time since last boat maintenance was longer in New Zealand than in Canada However, boat fouling and boat-mediated spread may be facilitated in Canada by boating activity patterns there, as the movement of boats among multiple marinas is considerably greater than in New Zealand Among the boat characteristics, voyage type, the time that boats spent in water (Canada) and time since last application of antifouling paint (New Zealand) were among the best predictors of boat fouling However, our results from Canada showed that propagule exposure was more important than boat characteristics in predicting the presence of the invasive colonial tunicate, B schlosseri This study shows the importance of small boats as potential vectors for tunicates and demonstrates that predictive models for the spread of biofouling species should be based on regional boating patterns, boating characteristics, and local propagule exposure

Alien plant species and factors of invasiveness of anthropogenic vegetation in the Northwestern Balkans - a phytosociological approach.

Abstract: We studied the anthropogenic vegetation of the Northwest Balkans in order to determine its susceptibility to invasion by alien plant species. We compiled a dataset of 3089 vegetation plots sampled between 1939 and 2009, recording a set of variables for each sample plot in order to determine which factors have the most effect on a habitat’s vulnerability to invaders. We calculated the proportion of native species, archaeophytes and neophytes for each plot. We used regression tree models to determine the site conditions of the most invaded anthropogenic habitats. The sample plots contained an average of 12.7% alien plant species, with a low proportion of archaeophytes (4.3%) and 8.4% neophytes. Local habitat conditions proved to have the largest effect, rather than climatic variables or propagule pressure. The proportion of archaeophytes follows a different pattern than that seen in central and northern Europe, indicating that macroecological factors are more important. Neophytes show a similar distribution to other European locations.

Historical records of passerine introductions to New Zealand fail to support the propagule pressure hypothesis

Abstract: Blackburn et al. (Biodiver Conserv 20:2189–2199, 2011) claim that a reanalysis of passerine introductions to New Zealand supports the propagule pressure hypothesis. The conclusions of Blackburn et al. (2011) are invalid for three reasons: First, the historical record is so flawed that there is no sound basis for identifying the mechanisms behind extinction following introduction, or whether species were successful because they were introduced in large numbers or were introduced in large numbers because earlier releases succeeded. Second, the GLIMMIX analysis of Blackburn et al. (2011) is biased in favor of the propagule pressure hypothesis. Third, the population viability analysis presented by Blackburn et al. (2011) is based on unjustified and questionable assumptions. It is likely that the outcome of passerine bird introductions to New Zealand depended on species characteristics, site characteristics, and human decisions more than on a simple summing of the numbers introduced.

Propagule pressure and functional diversity: interactive effects on a macroalgal invasion process

Abstract: Invasive species are considered an increasing global threat to marine ecosystems. Understanding which factors can accelerate or reduce invasion success is one of the main goals of invasion ecology. The present study investigated the interactive effects of propagule pressure and native functional diversity on the invasion success of the invasive alga Sargassum muticum (Yendo) Fensholt over a 1 yr invasion process (i.e. settlement, recruitment and colonisation). The invasibility of macroalgal assemblages with different functional diversity was assessed under high and low propagule pressure. Synthetic assemblages resembling natural marine macroalgal assemblages were built and used as model systems. Recruitment discs were used to record early settlement in the assemblages. Recruitment and colonisation success were monitored in the field 2 and 10 mo after exposure to propagule rain in the laboratory. The availability of resources (space and photosynthetically active radiation (PAR)) was also monitored for each assemblage and used as predictive variables in the analyses. The effects of propagule pressure and functional diversity varied among the invasion stages. Space availability did not play a significant role in the recruit- ment success of the invader. However, PAR was a key resource in its invasion success. Overall, the invasion success of S. muticum was influenced differently by different traits of the recipient assemblages. Hence, invasion success may be better explained by the interaction of environmen- tal factors that co-vary with species diversity and identity at different invasion stages.

Hotspots of exotic free-spawning sex: man-made environment facilitates success of an invasive seastar

Abstract: Summary 1. The introduction of exotic species to new areas poses a major threat to the environment. For those introduced species that establish and survive beyond the short term, the opportunities to manage the risk of continuing spread often rest with limiting reproductive output. The introduced northern Pacific seastar Asterias amurensis is an ecologically important pest that has established in the Derwent Estuary (Australia). Driven by oversupply of bivalve prey, it is persistent, abundant and fecund when associated with man-made structures such as docks, marinas, jetties and piers, that is, ‘wharves’. 2. As a free-spawning invertebrate, fertilization by A. amurensis is a critical life-history stage constrained by strong Allee effects. Eggs must be released in close proximity to sperm sources because the chances of achieving fertilization drastically reduce with increasing distances of spawner separation. 3. Investigation of zygote production in the Derwent Estuary using a spatially explicit model of free-spawning fertilization shows that A. amurensis at wharves, while representing <10% of the total population in the estuary and concentrated in 90% of total zygote production. Given the seastars’ long-lived and highly dispersive larvae, we show that wharves not only represent important sites of invasion but also facilitate propagule pressure promoting secondary invasions. 4. Synthesis and applications. In the absence of effective pest control solutions, focusing on reproductive hotspots has the potential to reduce further spread of established marine pests and to alleviate ongoing ecological impacts. In the case of the northern Pacific seastar, elimination of highly localized wharf populations annually prior to spawning can reduce overall zygote production by up to estimated 90%. The long-termprotection of key sources of larval production is a common goal for marine reserve design and fisheries management. However, the same concept but in reverse, whereby larval production is minimized at key sources, could be effective in the management of introduced pests in subtidal marine environments.

Invasibility of a nutrient-poor pasture through resident and non-resident herbs is controlled by litter, gap size and propagule pressure.

Abstract: Since inference concerning the relative effects of propagule pressure, biotic interactions, site conditions and species traits on the invasibility of plant communities is limited, we carried out a field experiment to study the role of these factors for absolute and relative seedling emergence in three resident and three non-resident confamilial herb species on a nutrient-poor temperate pasture. We set up a factorial field experiment with two levels each of the factors litter cover (0 and 400 g m(-2)), gap size (0.01 and 0.1 m(2)) and propagule pressure (5 and 50 seeds) and documented soil temperature, soil water content and relative light availability. Recruitment was recorded in spring and autumn 2010 and in spring 2011 to cover initial seedling emergence, establishment after summer drought and final establishment after the first winter. Litter alleviated temperature and moisture conditions and had positive effects on proportional and absolute seedling emergence during all phases of recruitment. Large gaps presented competition-free space with high light availability but showed higher temperature amplitudes and lower soil moisture. Proportional and absolute seedling recruitment was significantly higher in large than in small gaps. In contrast, propagule pressure facilitated absolute seedling emergence but had no effects on proportional emergence or the chance for successful colonisation. Despite significantly higher initial seedling emergence of resident than non-resident species, seed mass and other species-specific traits may be better predictors for idiosyncratic variation in seedling establishment than status. Our data support the fluctuating resource hypothesis and demonstrate that the reserve effect of seeds may facilitate seedling emergence. The direct comparison of propagule pressure with other environmental factors showed that propagule pressure affects absolute seedling abundance, which may be crucial for species that depend on other individuals for sexual reproduction. However, propagule batch size did not significantly affect the chance for successful colonisation of disturbed plots.