Is propagule size the critical factor in predicting introduction outcomes in passeriform birds?

Influential analyses of the propagule pressure hypothesis have been based on multiple bird species introduced to one region (e.g. New Zealand). These analyses implicitly assume that species-level and site-level characteristics are less important than the number of individuals released. In this study we compared records of passerine introductions with propagule size information across multiple regions (New Zealand, Australia, and North America). We excluded species introduced to just one of the three regions or with significant uncertainty in the historical record, as well as species that succeeded or failed in all regions. Because it is often impossible to attribute success to any single event or combination of events, our analysis compared randomly selected propagule sizes of unsuccessful introductions with those of successful introductions. Using Monte Carlo repeated sampling we found no statistical support for the propagule pressure hypothesis, even when using assumptions biased toward showing an effect.     

A population model for predicting the successful establishment of introduced bird species

One of the strongest generalities in invasion biology is the positive relationship between probability of establishment and the numbers of individuals introduced. Nevertheless, a number of significant questions remain regarding: (1) the relative importance of different processes during introduction (e.g., demographic, environmental, and genetic stochasticity, and Allee effects); (2) the relative effects of propagule pressure (e.g., number of introductions, size of introductions, and lag between introductions); and (3) different life history characteristics of the species themselves. Here, we adopt an individual-based simulation modeling approach to explore a range of such details in the relationship between establishment success and numbers of individuals introduced. Our models are developed for typical exotic bird introductions, for which the relationship between probability of establishment and the numbers of individuals introduced has been particularly well documented. For both short-lived and long-lived species, probability of establishment decreased across multiple introductions (compared with a single introduction of the same total size), and this decrease was greater when inbreeding depression was included. Sensitivity analyses revealed four predictors that together accounted for >95 % of model performance. Of these, R ₀ (the average number of daughters produced per female over her lifetime) and propagule pressure were of primary importance, while random environmental effects and inbreeding depression exerted lesser influence. Initial founder size is undoubtedly going to be important for ensuring the persistence of introduced populations. However, we found the demographic traits, which influence how introduced individuals behave, to have the greatest effect on establishment success.     

A reassessment of the role of propagule pressure in influencing fates of passerine introductions to New Zealand

Several studies have argued that principal factor in determining the fate of bird introductions is introduction effort. In large part, these studies have emerged from analyses of historical records from a single place—New Zealand. Here we raise two concerns about these conclusions. First, we argue that although many bird species were introduced repeatedly to New Zealand, in many cases the introductions apparently occurred only after the species were already successfully naturalized. The inclusion of such seemingly superfluous introductions may exaggerate the importance of propagule pressure. And second, we question the reliability of the records themselves. In many cases these records are equivocal, as inconsistencies appear in separate studies of the same records. Our analysis indicates that species were successful not because they were introduced frequently and in high numbers, but rather it is likely that they were introduced frequently and in high numbers because the initial releases were successful.     

A reassessment of historical records of avian introductions to Australia: no case for propagule pressure

Introduced species are widely believed to represent a significant threat to conservation of biological diversity. A better understanding of the ecological factors associated with successful species establishment should lead to improved management and mitigation of these introductions. The “propagule pressure hypothesis”, implying a greater chance of successful introduction with greater numbers introduced, has been widely accepted as a principal ecological factor in explaining establishment of exotic species. The historical record of bird introductions in a few locations, including the state of Victoria in Australia, has been advanced as the principal quantitative support for the hypothesis. We compiled lists of bird species introductions into Australia from several sources, and discovered inconsistencies in the records of introductions. In a series of comparisons, we found that the historical record of passerine introductions to Australia does not support the propagule pressure hypothesis unless superfluous introductions of already successful species are included. An additional problem with previous analyses is the inclusion of unsuccessful haphazard cage escapes.     

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

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.     

Strength in size not numbers: propagule size more important than number in sexually reproducing populations

Propagule pressure has consistently been identified as a primary factor in invader success, and reducing it can be one of the most effective methods for preventing the establishment of non-native species. However, when policy is implemented to reduce propagule pressure it almost exclusively focuses on the size of individual introduction events (‘propagule size’), with little confirmation that controlling this single aspect of propagule pressure is the most effective strategy. The number of introduction events (‘propagule number’) can play as much, or more, of a role in invader success, yet only a small portion of propagule pressure research has studied the relative importance of size and number. We investigated the relative roles of propagule size and number in the establishment of a sexually reproducing species using a field mesocosm experiment that introduced Hemimysis anomala (a non-native mysid) across a range of propagule sizes and numbers. We found that single, large introductions had higher abundances and probabilities of survival than smaller, more frequent additions. This experiment illustrated that, for sexual reproducers, focusing on lowering propagule size can be the most effective method for reducing non-native establishment.     

Propagule size and the relative success of exotic ungulate and bird introductions to New Zealand

We investigated factors affecting the success of 14 species of ungulates introduced to New Zealand around 1851-1926. The 11 successful species had a shorter maximum life span and were introduced in greater numbers than the three unsuccessful species. Because introduction effort was confounded with other life-history traits, we examined whether independent introductions of the same species were more likely to succeed when a greater number of individuals were introduced. For the six species with introductions that both succeeded and failed, successful introductions always involved an equal or greater number of individuals than unsuccessful introductions of the same species. For all independent introductions, there was a highly significant relationship between the number of individuals introduced and introduction success. When data for ungulate and bird introductions to New Zealand were combined, a variable categorizing species as ungulate or bird was a highly significant predictor of introduction success, after variation in introduction effort was controlled. For a given number of individuals introduced, ungulates were much more likely to succeed than birds.     

A population genetic signature of human releases in an invasive ladybeetle

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.     

Parsing propagule pressure: Number,not size,of introductions drives colonization success in a novel environment

Predicting whether individuals will colonize a novel habitat is of fundamental ecological interest and is crucial to conservation efforts. A consistently supported predictor of colonization success is the number of individuals introduced, also called propagule pressure. Propagule pressure increases with the number of introductions and the number of individuals per introduction (the size of the introduction), but it is unresolved which process is a stronger driver of colonization success. Furthermore, their relative importance may depend upon the environment, with multiple introductions potentially enhancing colonization of fluctuating environments. To evaluate the relative importance of the number and size of introductions and its dependence upon environmental variability, we paired demographic simulations with a microcosm experiment. Using Tribolium flour beetles as a model system, we introduced a fixed number of individuals into replicated novel habitats of stable or fluctuating quality, varying the number of introductions through time and size of each introduction. We evaluated establishment probability and the size of extant populations through seven generations. We found that establishment probability generally increased with more, smaller introductions, but was not affected by biologically realistic fluctuations in environmental quality. Population size was not significantly affected by environmental variability in the simulations, but populations in the microcosms grew larger in a stable environment, especially with more introduction events. In general, the microcosm experiment yielded higher establishment probability and larger populations than the demographic simulations. We suggest that genetic mechanisms likely underlie these differences and thus deserve more attention in efforts to parse propagule pressure. Our results highlight the importance of preventing further introductions of undesirable species to invaded sites and suggest conservation efforts should focus on increasing the number of introductions or reintroductions of desirable species rather than increasing the size of those introduction events into harsh environments.     

Freshwater fish diversity in Algeria with emphasis on alien species

Little is known about freshwater fish diversity in Algeria, especially after the broad national program of introduction of exotic species applied for over 20 years. This paper is an attempt to describe the current situation emphasizing the characteristics of the various introductions of non-autochthonous species, their current status and their possible impacts. The freshwater fish fauna of Algeria is composed of 48 species belonging to 15 families. Twenty-one species are autochthonous, of which, three are endemic (Haplochromis desfontainii, Aphanius saourensis, Aphanius apodus), two are endangered (H. desfontainii, Aphanius iberus), and one is critically endangered (Anguilla anguilla). Twenty-seven species were introduced, of which, 18 never became established and 9 are established. Four species are reported for the first time: Abramis brama, Aspius aspius, Carassius carassius, Perca fluviatilis. Since 1860, 303 introduction events have been recorded at almost 107 different sites in Algeria. The number of introduction events per site was between 1 and 10. Most of the introduced fish are found in Oubéïra Lake (seven species). Aquaculture development was the main reason for fish introductions (37.0%). The five most often introduced species were Cyprinus carpio (86 records), Hypophthalmichthys molitrix (65 records), Aristichthys nobilis (51 records), Oreochromis niloticus niloticus (33 records), and Ctenopharyngodon idella (18 records). Since the adverse effects of introduced fish are risky and demonstrated in several countries, the adoption of the precautionary principle is recommended when new introductions are planned. It is important, therefore, to adopt regulations and procedures which will minimize the risks arising from introductions.     

Massive genetic introgression in threatened northern crested newts (<Emphasis Type="Italic">Triturus cristatus</Emphasis>) by an invasive congener (<Emphasis Type="Italic">T. carnifex</Emphasis>) in Western Switzerland

Genetic pollution through introgressive hybridization of local species by exotic relatives is a major, yet neglected aspect of biological invasions, particularly in amphibians where human introductions are frequent. In Western Switzerland, crested newts make an interesting case: the Italian species Triturus carnifex was introduced at least a century ago within the range of the native and threatened T. cristatus. To understand the genetic consequences of this introduction and inform wildlife management authorities, we conducted a genetic survey on the remaining northern crested newt populations known in the area, using newly-developed species-diagnostic nuclear (microsatellites) and mitochondrial (control region) DNA markers. We documented massive nuclear introgression by the T. carnifex genome, which has completely replaced T. cristatus in most populations, especially in the Geneva area where the introduction was originally reported. However, many of these individuals retained the ancestral T. cristatus mtDNA, which could be explained by asymmetric introgression between the two species, stemming from demographic and/or selective processes. Analyses of genetic diversity support multiple events of T. carnifex releases, most-likely of proximate North Italian origin. We pinpointed the last indigenous populations in the region and recommend to prioritize their protection. Our study demonstrates the invasive potential of introduced taxa through introgressive hybridization, alerts about the underestimated rate of illegal amphibian translocations, and emphasizes the need for genetic analyses to monitor such invasions.     

How propagule size and environmental suitability jointly determine establishment success: a test using dung beetle introductions

Both the size of founding populations (propagule size) and environmental suitability are known to influence whether a species newly introduced to a location will establish a self-sustaining population. However, these two factors do not operate independently: it is the interaction between propagule size and environmental suitability that determines the probability an introduced population will establish. Here I use the example of dung beetle introductions to Australia to illustrate the importance of this interaction. I first describe equations that model establishment success jointly as a function of propagule size and environmental suitability. I then show how these equations provide insight into the different outcomes observed in two dung beetle species widely introduced to Australia. In one species, variation in propagule size had relatively little influence on establishment success due to large variation in environmental suitability, leading to an essentially bimodal outcome: sites were either very suitable for establishment and introductions succeeded, or sites were unsuitable and introductions failed regardless of propagule size. For the second species, there was much less variation among locations in environmental suitability, leading to propagule size having a strong influence on establishment success. These examples highlight how the interplay between environmental suitability and founding population size is central to determining the probability an introduced species will establish.     

A long-term study (1949–2005) of experimental introductions to an island; freshwater amphipods (Crustacea) in the Isle of Man (British Isles)

Aim Using data from field introduction experiments with Gammarus spp. conducted in the rivers of a small island, commencing in 1949, with resampling in the 60s, 70s, 80s, 90s and finally in 2005, we aimed to examine the long‐term interaction of the native freshwater amphipod Gammarus duebeni celticus with the introduced G. pulex. Using physico‐chemical data from a 2005 island‐wide survey, we also aimed to find what environmental factors could influence the distribution of the two species. Location The Isle of Man, British Isles. Methods We used species distribution data from 10 river sites from a field introduction study in 1949–69, resampling of these sites in 1974/5, 1988, 1995 and 2005. We sampled a further 82 sites in 2005, where physico‐chemical characteristics of each site were obtained. ANOVA was used to ascertain significant differences in environmental variables between native and introduced amphipod assemblages. Multiple discriminant analysis was used to relate Gammarus distribution to environmental gradients in rivers. Results Field introductions of G. pulex into previously G. d. celticus dominated sites failed, despite the large propagule pressure in terms of numbers of individuals released and repeated introductions. The island‐wide survey revealed that G. d. celticus was found in areas of higher water quality than G. pulex. Sites where the two species co‐occurred also tended to be of lower water quality than native only sites. Continuing metal pollution from past mining activity may be a major factor excluding all amphipods from many rivers. The North American amphipod Crangonyx pseudogracilis, which was first discovered on the Isle of Man in one river site in 1995, was found in a further five sites. Main conclusions This ecological experiment, through long‐term monitoring, thus continues to elucidate factors influencing the distributions of introduced and native species. Under some physicochemical regimes, G. d. celticus is able to withstand its usual displacement by G. pulex, whereas increased river pollution could promote replacement of the native by the introduced species.     

Propagule quality mediates invasive plant establishment

Propagule pressure is commonly considered a primary driver of invasive plant establishment and spread. However, the physical size or condition (i.e., quality) of propagules may also affect establishment, particularly under unfavorable habitat conditions such as low light environments. We used an outdoor mesocosm experiment to test the relative contribution of propagule size (number of individuals introduced) and quality (number of rhizome nodes) to the establishment and performance of the highly invasive cogongrass (Imperata cylindrica) under experimental sun and shade treatments. We found that the introduction of higher quality propagules (rhizome segments ≥3 nodes in length) significantly enhanced establishment across both light treatments, and increased final tiller count in the sun treatment. The sun treatment also enhanced rhizome growth, an effect that could increase spread rates and invasion success. Thus, while cogongrass is likely to establish in both sun and shade, introductions of large propagule sizes or large rhizomes in high light environments likely poses the greatest threat to native habitats. Our results demonstrate that propagule quality promoted both establishment and performance of a highly invasive grass species and suggest that propagule quality may play an important but underappreciated role in the invasion process.     

Weed biological control in California,USA: review of the past and prospects for the future

Weed biological control in California, USA began in 1940 with the release of a native scale insect on native Opuntia spp. on Santa Cruz Island, just offshore from mainland California. Since then, a total 39 weed species have been targets of biological control releases in California. Releases on 11 weed targets were transfer experiments where agents from related weed hosts were released on a new host. Most of the transfer experiment introductions failed but one weed was successfully controlled. Of the other 28 weeds, release sites for three species were destroyed and for six species releases are too recent to score, but for 19 weeds, their level of control was rated as: complete control (three species), substantial control (five species), and partial (six species), and negligible control (five species). Overall, 42% of the projects provide successful control, a result lower than observed in other countries worldwide. Since 1940, 77 species of agents have been released: 54 species established, 12 species failed to establish, six species had their release sites destroyed, and five species are too early to determine. Establishment rate was 82% but the rate differed among taxonomic orders. Individual agents were scored according to level of impact on their host and Coleoptera obtained the highest average impact score and Diptera the lowest. Mean impact scores over time showed a substantial drop in the 1980s but later increased. Future research efforts that emphasize introduction of high impact agents will further support development of this critical weed control method for California.     

Paradox lost: genetic diversity and the success of aquatic invasions

There is mounting evidence that reduced genetic diversity in invasive populations is not as commonplace as expected. Recent studies indicate that high propagule vectors, such as ballast water and shellfish transplantations, and multiple introductions contribute to the elimination of founder effects in the majority of successful aquatic invasions. Multiple introductions, in particular, can promote range expansion of introduced populations through both genetic and demographic mechanisms. Closely related to vectors and corridors of introduction, propagule pressure can play an important role in determining the genetic outcome of introduction events. Even low-diversity introductions have numerous means of avoiding the negative impact of diversity loss. The interaction of high propagule vectors and multiple introductions reveal important patterns associated with invasion success and deserve closer scrutiny.     

Animal trade and non-indigenous species introduction: the world-wide spread of squirrels

Aim  In this study, a dataset on world-wide squirrel introductions has been used to locate the relative pathways and to determine the factors correlated with species establishment.
Location  The world.
Methods  The analysis includes a chronological table of introductions, a biogeographical analysis and an assessment of the likelihood of establishment according to species, propagule pressure, area of origin and characteristics of the recipient area.
Results  The main vector of such introductions was the intentional importation of live animals. Introductions increased in developed countries and proportionately to the volume of imported mammals. Moreover, areas characterized by higher numbers of native squirrels were more invaded. Squirrels were often introduced deliberately and only to a smaller extent escaped from captivity. The likelihood of their establishment increased proportionately to the number of animals released and decreased proportionately to the increase of the latitudinal distance between the recipient area and the native range of the species. The likelihood that the release of one pair of either Sciurus or Callosciurus species would establish a new population was higher than 50%.
Main conclusion  Squirrels proved to be successful invaders and their importation should be restricted so as to prevent further introductions.     

Local-scale invasion pathways and small founder numbers in introduced Sacramento pikeminnow (Ptychocheilus grandis)

Given the general pattern of invasions with severe ecological consequences commonly resulting from multiple introductions of large numbers of individuals on the intercontinental scale, we explored an example of a highly successful, ecologically significant invader introduced over a short distance, possibly via minimal propagule pressure. The Sacramento pikeminnow (Ptychocheilus grandis) has been introduced to two coastal rivers in northern California where it poses a risk to threatened and endangered fishes. We assayed variation in seven microsatellite loci and one mitochondrial DNA gene to identify the source populations and estimate founder numbers for these introductions. Our analysis suggests that successful invasion of the Eel River was likely the result of a single transfer of 3–4 effective founders from nearby within the species’ native range: Clear Lake or its outflow Cache Creek. The other introduced population (Elk River), known from only seven individuals, likely represents secondary expansion from the introduced Eel River population. Our findings highlight the threat posed by close-range invaders and the ability of some fishes to rapidly invade ecologically suitable areas despite small effective founding numbers.     

Popularity and Propagule Pressure: Determinants of Introduction and Establishment of Aquarium Fish

Propagule pressure is frequently cited as an important determinant of invasion success for terrestrial taxa, but its importance for aquatic species is unclear. Using data on aquarium fishes in stores and historical records of fish introduced and established in Canadian and United States waters, we show clear relationships exist between frequency of occurrence in shops and likelihood of introduction and of establishment. Introduced and established taxa are also typically larger than those available from stores, consistent with the propagule pressure hypothesis in that larger fish may be released more frequently due to outgrowing their aquaria. Attempts to reduce the numbers of introductions may be the most practical mechanism to reduce the number of new successful invasions.     

Tracing the introduction history of the invasive swimming crab <Emphasis Type="Italic">Charybdis hellerii</Emphasis> (A. Milne-Edwards, 1867) in the Western Atlantic: evidences of high genetic diversity and multiple introductions

The swimming crab Charybdis hellerii is an invader with global distribution in warm waters. Native to the Indo-Pacific, this species invaded the eastern Mediterranean Sea after the Suez Canal opening. In 1987, it was first reported in the Western Atlantic, probably transported via ballast water of ships. Since then, it has been registered from many localities along the American coast from the USA to southern Brazil where it has rapidly established reproducing populations. Our main aim was to investigate the introduction history of this species along the American coast, using a phylogeographic approach. Additionally, we attempted to clarify the identity of this invasive species by molecular analyses and morphological assessment in order to provide a basis for our main investigation. C. hellerii was confirmed as a single species, but both cytochrome c oxidase subunit I and 16S rDNA revealed a genetic structure, splitting the potential source populations of American introductions into two groups: “western Indian Ocean and Mediterranean Sea” (WIO + MS) versus “eastern Indian + western Pacific oceans” (EIO + WPO). Most specimens from America clustered with the former group, supporting the hypothesis that the Mediterranean Sea represented the main source of Western Atlantic populations. However, the clustering of animals from southern Brazil with the latter group indicates that introductions from the eastern Indian or Pacific oceans must also have occurred. The existence of a third group, found exclusively within the American range and genetically related to EIO + WPO, also indicates an independent introduction from an unsampled locality from the native range. The haplotype and nucleotide diversities of American localities were comparable to those of source populations, contradicting a founder effect prediction. This finding might be related to the high propagule pressure associated with introductions via ballast water and the occurrence of multiple introductions from genetic distinct sources. The direct comparison of the haplotype numbers suggested no genetic bottleneck during introduction from the Mediterranean Sea, but a bottleneck might have occurred during introductions from the eastern Indian or Pacific oceans.