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Understanding the dynamics of biological invasions is crucial for managing numerous phenomena, from invasive species to tumors. While the Allee effect (where individuals in low-density populations suffer lowered fitness) is known to influence both the ecological and the evolutionary dynamics of an invasion, the possibility that an invader's susceptibility to the Allee effect might itself evolve has received little attention. Since invasion fronts are regions of perpetually low population density, selection should be expected to favor vanguard invaders that are resistant to Allee effects. This may not only cause invasions to accelerate over time but, by mitigating the Allee effects experienced by the vanguard, also make the invasion transition from a pushed wave, propelled by dispersal from behind the invasion front, to a pulled wave, driven instead by the invasion vanguard. To examine this possibility, we construct an individual-based model in which a trait that governs resistance to the Allee effect is allowed to evolve during an invasion. We find that vanguard invaders evolve resistance to the Allee effect, causing invasions to accelerate. This results in invasions transforming from pushed waves to pulled waves, an outcome with consequences for invasion speed, population genetic structure, and other emergent behaviors. These findings underscore the importance of accounting for evolution in invasion forecasts and suggest that evolution has the capacity to fundamentally alter invasion dynamics. [ABSTRACT FROM AUTHOR]

Spreading populations are subject to evolutionary processes acting on dispersal and reproduction that can increase invasion speed and variability. It is typically assumed that dispersal and demography traits evolve independently, but abundant evidence points to correlations between them that may be positive or negative and genetic, maternal, or environmental. We sought to understand how demography-dispersal correlations modify the eco-evolutionary dynamics of range expansion. We first explored this question with the beetle Callosobruchus maculatus , a laboratory model in which evolutionary acceleration of invasion has been demonstrated. We then built a simulation model to explore the role of trait correlations in this system and more generally. We found that positive correlations amplify the positive influence of evolution on speed and variability while negative correlations (such as we found empirically) constrain that influence. Strong negative genetic correlations can even cause evolution to decelerate invasion. Genetic and nongenetic (maternal and environmental) correlations had similar effects on some measures of invasion but different effects on others. Model results enabled us to retrospectively explain invasion dynamics and trait evolution in C. maculatus and may similarly aid the interpretation of other field and laboratory studies. Nonindependence of demography and dispersal is an important consideration for understanding and predicting outcomes of range expansion. [ABSTRACT FROM AUTHOR]

An introduced species' propagule pressure strongly influences the genetic diversity and evolutionary potential of its descendants and even the likelihood of biological invasion. We examined population genetic consequences arising from introduction of the invasive annual grass Bromus tectorum into the central United States (U.S.). The origin and frequency of introductions were investigated by assembling allozyme diversity data from 60 widely spaced populations. At least five introduction events contributed to the grass's genetic diversity in the central U.S. Populations in this region have fewer alleles (30 vs. 43) and polymorphic loci (5 vs. 13) than native populations, evidence of a genetic bottleneck. However , the populations are, on average, more genetically diverse and less structured than native populations. Assembly within central U.S. populations of previously allopatric genotypes may have allowed the formation, via outcrossing, of a rare multilocus genotype. Genetic admixtures may have occurred through any combination of east-to-west spread coincident with nineteenth-century arrival of European settlers, dispersal from southern Ontario via Great Lakes shipping and commerce, and direct introduction from the native range. Our results illustrate the population genetic consequences of relatively high propagule pressure (i.e., repeated immigrations to a new range from multiple sources). [ABSTRACT FROM AUTHOR]

The taxonomy of the Centaurea stoebe complex is controversial. Diploid and tetraploid plants occur in its native European range, but to date only tetraploids have been recorded from its introduced range in North America. We examined morphological differentiation of C. stoebe using multivariate and univariate approaches to clarify the taxonomic status of the known cytotypes. We measured more than 40 morphological traits on plants originating from 78 populations, grown from seed under uniform glasshouse conditions. The ploidy of almost 300 plants from 2 native and 20 introduced populations from Canada was assessed to test for the absence of diploids from North America. Finally, we explored whether postintroduction processes have resulted in phenotypic changes in introduced plants which may have contributed to the invasion success of C. stoebe. Morphometric analyses showed a clear separation of 2x and 4x plants and thus supported recognition of both cytotypes as separate taxa. Differences in the life cycle, the number of florets, the shape of capitula, and the shape of young rosette leaves were the best discriminant characters. Only minor differences were found between native and introduced tetraploids. All plants from the introduced range except for one hexaploid were found to be tetraploid. Rare diploids from Canada were identified as Centaurea diffusa or Centaurea psamogenna. [ABSTRACT FROM AUTHOR]

Generalized pollination systems and autonomous self-fertilization are traits that have been linked with plant invasiveness. However, whether specialized pollination requirements pose a significant barrier to plant invasions is not yet clear. Likewise, the contribution of pollinators to the fecundity of facultatively self-pollinating invasive plant species is poorly understood. We addressed these issues using the self-compatible and autonomously self-pollinating Lilium formosanum, which also has large, showy flowers that are adapted for pollination by hawk moths. We investigated the pollination of this lily-which is indigenous to Taiwan-in KwaZulu-Natal, South Africa, where it is invasive. The long-tongued hawk moth Agrius convolvuli was identified as the primary pollinator on the basis of field observations, pollen load analysis, presence of lepidopteran scales on stigmas, and higher seed production in emasculated flowers exposed at night than in those exposed during the day. Remarkably, this moth is native to much of the Old World, including Taiwan and South Africa. Autonomous self-pollination resulted in seed production, but at a reduced level relative to the seed production of open- and hand-pollinated flowers, which was significant in one out of two populations examined. Thus, pollinators potentially contribute to invasion by increasing seed production and genetic variability through cross-pollination, although contributions of pollinators to seed set versus that of autonomous self-pollination may vary between populations. We conclude that specialized pollination requirements do not present a barrier to invasions when plants are specialized to pollinators or pollinator functional groups with very wide distributions. [ABSTRACT FROM AUTHOR]

Colonization may favor self-compatibility (SC) in invasive plants, a process consistent with Baker's law. We investigated this hypothesis in invasive eastern North American populations of tristylous Lythrum salicaria L. (purple loosestrife) by controlled self- and cross-pollinations of 124 plants sampled from 12 populations grown under uniform glasshouse conditions. We evaluated whether populations at the northern front of the invasion exhibited higher levels of SC than southern populations, which are closer to source populations for the North American invasion. We also sought evidence for morph-specific differences in the strength of trimorphic incompatibility. We used the ASTER statistical program to jointly model three measures of SC: (1) the probability of producing at least one seed following self-pollination, (2) the number of fruits produced by self-pollination, and (3) the average number of seeds per fruit resulting from self-pollination. Just over one-quarter of plants set at least one fruit after self-pollination, and there was a significant effect of style morph on variation in SC, with the mid-styled morph most compatible. Although variation for SC was detected in 11 of the 12 populations, there was no evidence that SC increased toward the northern range limit as a result of the invasion process. [ABSTRACT FROM AUTHOR]

Biological invasions, including infectious disease outbreaks and biocontrol introductions, often involve small numbers of individuals arriving in spatially heterogeneous environments. Small numbers lead to demographic stochasticity, and spatial heterogeneity means that establishment success depends critically on the introduction sites and movement patterns of invaders. We present a general stochastic modeling framework to address how spatial heterogeneity and movement patterns determine establishment success, population growth, and rates of spatial spread. For dispersal-limited populations, our analysis reveals that spatial heterogeneity increases the expected population growth rate and that local reproductive numbers determine establishment success. Higher dispersal rates decrease the expected population growth rate but can enhance establishment success, particularly when movement patterns are positively correlated with local reproductive numbers. We also find that several small, randomly distributed propagules of invaders are more likely to succeed than a single large propagule. Even if invasions are ultimately successful, there may be substantial time lags before an invader reaches observable densities. These time lags are longer for invasions into patches where extinction risk is high and in landscapes where metapopulation-scale population growth rate is low, while the opposite holds true for rates of spatial spread. Sensitivity analysis of our models provides guidance for control efforts. [ABSTRACT FROM AUTHOR]

Many plant species have been introduced from their native ranges to new continents, but few have become naturalized or, ultimately, invasive. It has been predicted that species that do not require the presence of compatible mates and the services of pollinators for reproduction will be favored in establishment after longdistance dispersal. We tested whether this hypothesis, generally referred to as Baker's law, holds for South African species of Iridaceae (iris family) that have been introduced in other regions for horticultural purposes. Fruit and seed production of flowers from which pollinators had been experimentally excluded was assessed for 10 pairs of species from nine different genera or subgenera. Each species pair comprised one naturalized and one nonnaturalized species, all of which are used in international horticulture. On average, species of Iridaceae that have become naturalized outside their native ranges showed a higher capacity for autonomous fruit and seed production than congeneric species that have not become naturalized. This was especially true for the naturalized species that are considered to be invasive weeds. These results provide strong evidence for the role of autonomous seed production in increasing potential invasiveness in plants. [ABSTRACT FROM AUTHOR]

The spatial spread of invading organisms is a major contemporary concern. We focus here on invasions in inherently fragmented habitats, such as freshwater systems, and explore the usefulness of metapopulation models in this context. Maximum-likelihood methods allow the estimation of colonization and extinction rates, as functions of habitat patch sizes and positions, from time series of presence/absence data. This framework also provides confidence intervals of these estimates and hypotheses tests. We analyze a previously unpublished 12-year survey of the spread of the introduced snail Tarebia granifera in 47 Martinican rivers. Simple metapopulation models reproduce with reasonable accuracy several quantitative aspects of the invasion, including regional abundance, spatiotemporal structure, and site-by-site colonization dates. Sensitivity analysis reveals that the invasion sequence depended strongly on metapopulation size (number of sites) and spatial structure (distances among sites). The invasion history has also been accelerated by stochastic events, as illustrated by a large, central river that happened to be colonized very early and served as an invasion pool. Finally, we discuss the benefits of this approach for the understanding of invasions in fragmented landscapes. [ABSTRACT FROM AUTHOR]

Many mobile organisms exhibit resource-dependent movement in which movement rates adjust to changes in local resource densities through changes in either the probability of moving or the distance moved. Such changes may have important consequences for invasions because reductions in resources behind an invasion front may cause higher dispersal while simultaneously reducing population growth behind the front and thus lowering the number of dispersers. Intuiting how the interplay between population growth and dispersal affects invasions is difficult without mathematical models, yet most models assume dispersal rates are constant. Here we present spatial-spread models that allow for consumer-resource interactions and resource-dependent dispersal. Our results show that when resources affect the probability of dispersal, then the invasion dynamics are no different than if resources did not affect dispersal. When resources instead affect the distance dispersed, however, the invasion dynamics are strongly affected by the strength of the consumer-resource interaction, and population cycles behind the wave front lead to fluctuating rates of spread. Our results suggest that for actively dispersing invaders, invasion dynamics can be determined by species interactions. More practically, our work suggests that reducing invader densities behind the front may be a useful method of slowing an invader's rate of spread. [ABSTRACT FROM AUTHOR]

Many marine systems are subject to high nutrient loadings together with invasions by exotic species. Devising appropriate management responses is an increasing concern and one that has received relatively little attention from researchers. This paper considers the Black Sea anchovy fishery, which has benefited from the relaxation of a nutrient constraint, but has suffered from competition and predation by an invading comb-jelly (Mnemiopsis leidyi). We examine alternative hypotheses about the mechanism triggering outbreaks of the invader (sea temperatures versus nutrients), and the severity of these outbreaks, to see whether a constant escapement policy might be optimal for this fishery. If nutrient levels serve as the triggering mechanism, we argue a mixed blessing effect may be present, so that the effects of nutrient abatement for the anchovy fishery are uncertain. We specify our model empirically and show that a constant escapement policy would be viable under a scenario of reduced impacts from outbreaks of the invader and that nutrient abatement could be beneficial if nutrients trigger outbreaks. [ABSTRACT FROM AUTHOR]