The western honeybee, Apis mellifera, has been globally transported for honey production and pollination for hundreds of years and is often kept in large numbers in beekeeping operations. Concern has been expressed that it might act as an invasive species with large impact on biodiversity. However, although the honeybee has spread into the wild and has established feral populations in Australia and the Americas, the extent to which introduced honeybees alter biodiversity remains controversial. Here we focus on the best documented cases of A. mellifera invasions and discuss the effects on biodiversity under three different conditions: 1) regions where other subspecies of A. mellifera are endemic (Europe, Africa, and western Asia), 2) regions where A. mellifera is not endemic, but other species of Apis naturally occur (central and eastern Asia), 3) regions where Apis species are not endemic (America, Australia). Although some studies show an impact on native bee survival, fecundity, or population density in response to large A. mellifera aggregations (e.g., on apiaries), there are no reports that feral honeybees caused the extinction of native bee pollinators, which are the most likely competing group of organisms. Honeybee introductions have had or still may have negative effects only within the genus Apis, primarily interfering with beekeeping activities. Although honeybee invasions seem to have had little if any effect on biodiversity of native pollinators so far, we nevertheless caution against transporting honeybees around the globe, and we particularly advise against importing foreign Apis species into tropical ecosystems.

Biological invasions are characterized by remarkable spatio-temporal dynamics, with many species having extended their distribution range from within a single region to much of the globe within the last century. The comparative analysis of the spatio-temporal dynamics of over 100 taxa from studies undertaken worldwide provides the basis of a critical assessment of current knowledge. At the scale of single habitats, simple reaction-diffusion models may be accurate enough to predict the spread of new invaders without recourse to complex life history parameterization. Average rates of local spread reported for invasive species in the literature range from 2 m · y⁻¹ to a maximum of 370 m · y⁻¹. Average rates of long-distance dispersal are at least two orders of magnitude greater than estimates of local dispersal, with a maximum of 167 km · y⁻¹. While local-scale studies do pick up dispersal events of several kilometres, study sites are rarely sufficiently large or monitored for long enough to characterize these events accurately. Long-distance dispersal events may occur during periods of negligible population increase and appear to bear little relationship to population size. At regional scales, invasive species rarely move across the landscape as a continuous front and both local and long-distance dispersal determine spatial patterns. At these larger spatial scales, both local and long-distance dispersal require parameterization, and this has been achieved through spatially explicit individual-based simulation models using two or more dispersal functions. It is doubtful whether a single estimate of spread encapsulates the spatio-temporal dynamics of invasive species at this scale. Thus, estimates of spread drawn from successive distribution maps will tend to be biased towards long-distance dispersal events. The frequency and distribution of introduction events play a key role in invasion trajectories, and the stochastic nature of such events may explain why the longer a species has been introduced into a region the greater the likelihood that it becomes invasive. However, cumulative counts of localities or samples only provide one perspective on the invasion process and need to be associated with spatial information to depict spread more realistically. This review highlights that monitoring of invasive species must be approached from a hierarchical perspective with data gathered at more than one spatial scale. Such an approach will improve predictions and integrate landscape attributes into invasion dynamics.

Time lags can be found throughout the invasion process, including in the arrival, establishment, and impacts of invaders. While we often lack the information necessary to generate quantitative expectations of invader performance, some types of lags are not surprising. For example, populations often grow exponentially in the early phases of invasion, and this will give rise to an inherent lag. More broadly, early rates of anthropogenic invasion were much slower than what we are now witnessing, but as the vectors of invasion have also increased dramatically over time, this early lag is not unexpected. Many other lags, however, appear dramatically prolonged, and can come to an end with changes to the invader or its environment. For example, exotics can exist in relatively low numbers for decades before exploding, or invaders can become more aggressive over time and increase their impacts on native species. Invasion-related lags are critical for our efforts to manage invaders, as they may lead us to make inaccurate assessments of the risks posed by invaders as well as miss critical windows for action. Recognition of the phenomenon of long lags before sudden changes in invader dynamics also suggests that we adopt a strict precautionary principle: we should assume that any invader has the potential for undesirable effects and that long periods of seemingly consistent behaviour can be poor predictors of what invaders will do in the future.

Remote sensing is becoming a vital tool for understanding the changing vegetation patterns that are associated with broad-scale plant invasions. The establishment of North American east coast native Spartina alterniflora (smooth cordgrass) in Willapa Bay, Washington is a specific example of broad-scale invasion following local introduction. In this study, we examined a 120-y historical record of aerial photographs, oral histories, museum records, and publications to reconstruct the spatial, temporal, and historical elements of this invasion. We conclude that the most likely means for S. alterniflora to have reached Willapa Bay was the transport and translocation of oysters from New York harbour. Our data and analysis suggest that multiple areas were colonized between 1894 and 1920 coincident with sustained import of oysters from the Atlantic seaboard. We have evidence that S. alterniflora had been long established and growing in multiple widely spread locations by 1945, which is in contrast to a widely reported single introduction. Multiple foci would not only explain the colonization patterns we have observed, but could also increase the heterogeneity of the founding populations, helping to overcome barriers to reproduction that may have initially slowed the colonization in isolated populations during the first 50–70 y.

We postulated that dispersal through streams is an important factor in the spread of nonindigenous aquatic species to uninvaded lakes. We tested this hypothesis with zebra mussels (Dreissena polymorpha), whose planktonic larvae are particularly prone to transport through streams. To examine this potential mechanism of spread, we (1) assessed populations of zebra mussels in 2000 and 2003 in coupled lake-stream systems of the St. Joseph River basin (Indiana and Michigan, USA) and (2) examined the interconnectedness of lake-stream systems by evaluating all invaded inland lakes and reservoirs in the United States. We compared observed patterns in zebra mussel populations in 2000 and 2003 to patterns predicted by two proposed models of spread: the static source–sink model and the progressive downstream-march model. Adult zebra mussel densities in lake outflows declined with distance downstream of invaded lakes. Maximum downstream occurrences of adults were variable over the years surveyed, but did not increase through time, suggesting that the source–sink model best fit zebra mussel distributions in these lake-stream couplets. For the conterminous US, we examined the connectedness of inland lakes in close proximity to invaded lakes to determine if stream connections were related to invasions. We also measured the distances between invaded lakes and downstream lakes that were potential recipients of colonists to examine the importance of stream distance in relation to zebra mussel invasions. Lakes connected to invaded lakes were more likely to be invaded than non-connected lakes, and the probability of becoming invaded increased with the proximity between lakes. Our results suggest that a better understanding of the role that streams play as pathways for new biological invasions is crucial for directing management and prevention efforts.

The spread of Phragmites australis between 1980 and 2002 was documented from seven series of aerial photographs and remote sensing images covering the Grandes Battures Tailhandier (Boucherville Islands, St. Lawrence River, Québec, Canada). Over the 23-y period, the colonized surface rose exponentially from 0.86 to 32.6 ha, corresponding to an 18% annual increase. This increase resulted mostly from vegetative growth, although the establishment of new colonies — most likely resulting from seed germination — allowed longer-range dispersion. Hydrological factors, especially the water level and duration of flooding over the growth season (July 1 to October 31) of the previous year, favoured the spread of colonies. Gains were highest the year following low water-level conditions and in a southerly direction, whereas they were reduced when plants grew at more than 1.5 m above mean water level or when they were flooded for more than 100 d during the previous growing season. The rate of surface colonization observed at Boucherville Islands was compared to that recorded at four other fluvial sites. Between Cornwall and Trois-Rivières, the noticeable increase in the number of colonized sites since 1980 suggests that low water levels in 1995, 1999, and 2001 favoured the establishment of colonies of P. australis along the shores of the St. Lawrence River.

An estuarine snail (Batillaria attramentaria), introduced to northern California marshes, is displacing a native confamilial mudsnail (Cerithidea californica) through superior competition for shared, limiting food resources. Batillaria, however, is absent from similar marsh habitats in southern California. I tested whether regional-scale variation in relative performance (growth) of the snails may have influenced Batillaria's invasion pattern. I quantified growth using RNA:DNA ratios (a growth index that I ground-truthed with direct growth measurements) for snails collected throughout their entire collective North American distribution. Batillaria exhibited a high growth rate that was more than double Cerithidea's growth rate in sympatric populations. A broad-scale relationship of species' growth rates against latitude projected an amply adequate growth rate for Batillaria in southern California where it is presently absent. Furthermore, growth rates of Cerithidea did not increase in southern California, suggesting that Batillaria would maintain its dramatic relative performance advantage. Thus, even if resources are limiting at southern latitudes, biotic resistance through competition with Cerithidea does not explain Batillaria's absence. Among alternative, untested hypotheses for Batillaria's absence, insufficient propagule inoculation has strongest support. Because transplant experiments with nonindigenous species are unethical, examination of species' performance over geographic scales provides a powerful alternative approach for invasion studies.

Reed canarygrass (Phalaris arundinacea) is one of the most invasive plant species in North American wetlands, but there are few historical data regarding the spread of this species, particularly at its northeastern distribution limit. We reconstructed the spread of reed canarygrass in Québec using herbarium specimens and a method that accounts for some of the biases associated with this type of historical record. We hypothesized that the beginning of the spread of the species coincided with the introduction in Canada of reed canarygrass cultivars for forage. Reed canarygrass specimens collected during the 19^th century in places as remote as Lake Mistassini and Anticosti Island provide strong evidence that this species is native to northeastern North America. The spatial distribution of specimens collected before 1925 suggests that reed canarygrass probably occupied most of its present-day area at that time. There is no strong evidence that reed canarygrass expanded its distribution limits in Québec during the 20^th century. However, the species colonized several new sites within its distribution limits in recent decades, especially from 1963 to 1978. The spread was probably associated with nitrate pollution and road construction in southern Québec, and with water level fluctuations of the St. Lawrence River. Although herbarium specimens suffer from sampling biases, their use may provide new spatio-temporal insights into the spread of invasive species and facilitate the identification of probable causes of invasiveness.

Invasion of exotic species may be regulated either by regional factors (dispersal limitation) or by local site characteristics that influence colonization success (invasibility). To address the latter question, we surveyed crustacean zooplankton and abiotic features of 171 lakes in Missouri, Arkansas, and Oklahoma and examined features associated with presence of the exotic cladoceran Daphnia lumholtzi. The data set was also used to examine variation in zooplankton species composition in this region. Canonical Correspondence Analysis revealed that zooplankton distributions were most strongly correlated with lake fertility and conductivity. Logistic regression analyses showed that D. lumholtzi was more likely to invade larger and more fertile lakes and appeared unaffected by native zooplankton species richness or presence of particular species. Thus, the current study suggests that physico-chemical characteristics are better predictors than zooplankton communities for explaining the regional spread of D. lumholtzi. Nevertheless, the underlying causes for these trends are unclear, as landscape features correlated to the same physical features may also influence dispersal processes, and other unknown differences between lakes may also be important for colonization success.

While it is widely assumed that there has been a great increase in the proportion of exotic species over the last century for regions within North America, there has been little direct evidence for such a trend. I analyzed 1,410 vascular floras published between 1875 and 2004 and found a small increase in the percentage of exotic species, from slightly less than to slightly more than 10%. The increase is most noticeable (but still weak) at broad spatial scales, especially when latitude, area, and elevation are statistically factored out. The observed increase in exotics may be due, in part, to subtle biases in the data.

This study explores the correlates of alien plant species richness in South Africa at the scale of quarter-degree squares (QDS; ≈ 25 × 27 km; 675 km²). We considered all alien plant species for which we had records and a subset of these – those that invade natural and semi-natural vegetation. The main source of data for species richness of indigenous and alien plant species was a national database based on herbarium specimens. For invasive alien species, data were from a national atlassing project. First, we explored the importance of energy availability and habitat heterogeneity as correlates of indigenous, alien, and invasive alien plant species richness. Linear regression models showed that species richness in the three groups of plants was explained by the same variables: a principal component of climatic factors and topographic roughness were the top-ranking variables for all groups. Next, we examined the role of indigenous species richness together with a range of environmental and human-activity variables in explaining species richness of alien and invasive alien plants. Results reveal an interplay of natural features and variables that quantify the dimension of human activities. If indigenous species richness is ignored, human-activity variables are more strongly correlated with alien species richness than with invasive alien species richness. Numbers of alien and invasive species in QDSs are significantly correlated with indigenous plant species richness in the 1,597 QDSs selected for analysis, a pattern consistent with findings from other parts of the world. Analysis of residuals between observed and predicted values showed that patterns differed between biomes. The results are useful for planning long-term intervention policy at the national scale; they suggest that areas with rich native biodiversity will face a sustained onslaught from invasive alien species and that ongoing management actions will be required to reduce and mitigate impacts from biological invasions in these areas.

Since its introduction to Nova Scotia in the late 1980s, the invasive green alga Codium fragile subsp. tomentosoides has spread from rocky subtidal habitats to tidepools on the Atlantic coast. We monitored recruitment, growth, and survival of C. fragile, and potential biotic and abiotic factors that regulate these processes in three tidepools at different tidal heights on a wave-exposed rocky shore over 4 y (2000–2003). Large seasonal and interannual fluctuations in population density (up to 520 plants · 0.25 m⁻²) were driven by recruitment of small thalli (≤ 2 cm length) in summer and subsequent mortality of larger plants in fall and winter. Variation in the timing and magnitude of recruitment among years may reflect differences in the mode of reproduction, with intensive recruitment via dispersing propagules establishing the dense populations that in subsequent years produce new thalli vegetatively. Growth rates of new recruits increased with water temperature between June and September. Survival of marked plants steadily decreased during summer and fall (to ≤ 20% by November 2001) and was greater in recruits transplanted to deeper and more wave-sheltered microhabitats within pools. Two pools in the low intertidal zone had lower temperatures, greater water movement, and fewer grazers than a third pool high on the shore. These environmental differences may account for variation in growth and survival of C. fragile among pools at different intertidal heights.

We studied the riparian vegetation on the agricultural floodplain of the Middle Garonne River (SW France) in order to compare native and exotic plant community patterns. In total, we investigated the vegetation of 1,824 plots during four seasons along 50-m-long transects delineated transversely from the banks of five types of water bodies (subsystems) differing by their exposure to natural and to human-induced disturbance. Exotic species accounted for 21% of the total of 726 species identified. We characterized native and exotic plant communities of each subsystem on the basis of Grime's ecological strategies, species lifespan, species richness, and species cover. The communities of each subsystem were compared with respect to the landscape structure (35 patch types) and to their distribution according to the distance to the bank of each water body, the distance to the main channel of the river, the annual duration of inundation, the total plant cover, the total species richness, and the proportion of exotics. Although significant contrasts exist between the community structures of each water body, we found a strong correlation between the attributes of native and exotic species pools. Native and exotic species covers were negatively correlated, while native species richness was positively correlated to exotic species richness, both at local and large scale. This positive correlation remained significant when comparing plots within each patch type. Few or no differences were detected between the distribution of native and exotic species according to the six variables of interest, the effect of the origin (exotic or native) of the plants being negligible as a discriminant attribute. The possible role of landscape complexity and the role of combined natural and human-induced disturbances are discussed to explain these patterns.

The rates and patterns of spread of 63 species in the Czech Republic and seven in Britain and Ireland were determined from quadrat records, mapovací pole (11- × 12-km mapping zones in the Czech Republic), and 10- × 10-km hectads in Britain and Ireland. Species that have straight sections on logarithmic, square root, and arithmetic plots were found; the last group (10 of them) are generally casuals. Straight square root plots (52 found) are expected from Fisher-Skellam reaction diffusion models, logarithmic equivalents (36 found) where the dispersal kernel has a thicker tail or where spread involves scattered colonies. The greater frequency of the straight square root plots is unexpected. The straightness of so many plots shows that there is no significant variation in recorder bias. About a third of the species showed lags, and many species showed a later slowing of the rate of spread, called here bends. Lags may indicate an initial casual phase as well as slower growth; bends, a lack of further areas to spread to. Typically, with much variation, areas of occupancy double in about 10 y (logarithmic mode) or spread at about 2 km · y⁻¹ (square root mode). Both speeds indicate that most spread is from human activity. The reasons for the major phenomena (the occurrence of the two types of spread predicted by models, their distinctness, and the distribution in rates) may reflect economic and landscape features as well as biological ones. The straightness, speed, and patterns found are all new results for invasion biology.