Prunus serotina unleashed: invader dominance after 70 years of forest development

Propagule pressure and disturbance have both been found to facilitate invasion. Therefore, knowledge on the history of introduction and disturbance is vital for understanding an invasion process, and research should focus on areas in which the invasive species has not been deliberately introduced or managed to study unconfounded colonization patterns. Comparing the outcome of such spontaneous colonization processes for different ecosystems might provide a useful framework for setting management priorities for invasive species that enter new, uninvaded areas. We focused on the 70-year spontaneous spread of the invasive tree species Prunus serotina in a pine forest in the Netherlands. To reconstruct the invasion pattern, we combined historical maps, tree ring analysis, spatially explicit tree inventory data, seed density data, and regeneration data for both native and non-native species. Prunus serotina was the only species that showed successful regeneration: the species was present throughout the forest in the tree, shrub, and herb layer. Native species were not able to outgrow the seedling stage. Our data demonstrate that P. serotina is a gap-dependent species with high seed production that builds up a seedling bank. We also compared the results of this study with a similar study on P. serotina colonization in a deciduous forest in Belgium, where P. serotina invasion was not successful. The sharp contrast between the outcomes of the two invasion processes shows the importance of studying an invasive species and the recipient ecosystem jointly and made us raise the hypothesis that herbivore pressure may facilitate P. serotina invasio

Local approximation of a metapopulation's equilibrium

We consider the approximation of the equilibrium of a metapopulation model, in which a finite number of patches are randomly distributed over a bounded subset $\Omega$ of Euclidean space. The approximation is good when a large number of patches contribute to the colonization pressure on any given unoccupied patch, and when the quality of the patches varies little over the length scale determined by the colonization radius. If this is the case, the equilibrium probability of a patch at $z$ being occupied is shown to be close to $q_1(z)$, the equilibrium occupation probability in Levins's model, at any point $z \in \Omega$ not too close to the boundary, if the local colonization pressure and extinction rates appropriate to $z$ are assumed. The approximation is justified by giving explicit upper and lower bounds for the occupation probabilities, expressed in terms of the model parameters. Since the patches are distributed randomly, the occupation probabilities are also random, and we complement our bounds with explicit bounds on the probability that they are satisfied at all patches simultaneously

A conceptual model of community dynamics during the transport stage of the invasion process: a case study of ships’ ballast

Aim: After J. L. Lockwood, P. Cassey and T. Blackburn (2009, Diversity and Distributions, 15, 904–910) first described a theoretical relationship between propagule pressure and colonization pressure, two empirical studies demonstrated that the transport stage of the invasion process can profoundly influence the strength of the relationship among multiple events, as well as predictions of introduction risk. However, studies exploring dynamics of transported communities are rare, as repeated-measures sampling during transportation by any vector is logistically difficult. We constructed a conceptual model of community dynamics during transportation and supported it by empirical data for propagule pressure and colonization pressure of plankton. Location: Global. Methods: A conceptual model of community dynamics was developed based on lognormal species abundance distribution and the simulation model of J. L. Lockwood, P. Cassey and T. Blackburn (2009, Diversity and Distributions, 15, 904–910). We considered four cases: case ‘A’ – no reduction in propagule nor colonization pressure; case ‘B’ – strong reduction in propagule and mild reduction in colonization pressure; case ‘C’ – mild reduction in propagule and strong reduction in colonization pressure; and case ‘D’ – strong reduction in both propagule and colonization pressures. Results: The cases ‘B’, ‘C’ and ‘D’ were supported by empirical data for invertebrates, dinoflagellates and diatoms from ships’ ballast tanks, respectively. Propagule pressure of invertebrates, dinoflagellates and diatoms decreased 99.95%, 80% and 94% in 25 days, respectively, while colonization pressure decreased 34%, 57% and 64%. Main conclusions: Transport affects both propagule pressure and colonization pressure of taxa, with the magnitude of change dependent on length of transport and taxon-specific survival and reproduction. Our model demonstrates that introduction risk varies substantially across and within taxa depending on the occurrence and severity of selection pressures during transportation which serve to change species abundance distributions

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

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

Temporal introduction patterns of invasive alien plant species to Australia

We examined temporal introduction patterns of 132 invasive alien plant species (IAPS) to Australia since European colonisation in 1770. Introductions of IAPS were high during 1810–1820 (10 species), 1840– 1880 (51 species, 38 of these between 1840 and 1860) and 1930–1940 (9 species). Conspicuously few introductions occurred during 10-year periods directly preceding each introduction peak. Peaks during early European settlement (1810–1820) and human range expansion across the continent (1840-1860) both coincided with considerable growth in Australia’s human population. We suggest that population growth during these times increased the likelihood of introduced plant species becoming invasive as a result of increased colonization and propagule pressure. Deliberate introductions of IAPS (104 species) far outnumbered accidental introductions (28 species) and were particularly prominent during early settlement. Cosmopolitan IAPS (25 species) and those native solely to South America (53 species), Africa (27 species) and Asia (19 species) have been introduced deliberately and accidentally to Australia across a broad period of time. A small number of IAPS, native solely to Europe (5 species) and North America (2 species), were all introduced to Australia prior to 1880. These contrasting findings for native range suggest some role for habitat matching, with similar environmental conditions in Australia potentially driving the proliferation of IAPS native to southern-hemisphere regions. Shrub, tree and vine species dominated IAPS introduced prior to 1840, with no grasses or forbs introduced during early colonisation. Since 1840, all five growth forms have been introduced deliberately and accidentally in relatively large numbers across a broad period of time. In particular, a large number of grass and forb IAPS were deliberately introduced between 1840 and 1860, most likely a direct result of the introduction of legislation promoting intensive agriculture across large areas of the continent. Since the 1980s, only three IAPS have been introduced (all deliberately introduced forbs). The decline in IAPS introductions is most likely a reflection of both increased surveillance and biosecurity efforts and the likelihood that many potential IAPS are still within a pre-expansion lag period

The impact of antibiotic use on transmission of resistant bacteria in hospitals: Insights from an agent-based model

Extensive antibiotic use over the years has led to the emergence and spread of antibiotic resistant bacteria (ARB). Antibiotic resistance poses a major threat to public health since for many infections antibiotic treatment is no longer effective. Hospitals are focal points for ARB spread. Antibiotic use in hospitals exerts selective pressure, accelerating the spread of ARB. We used an agent-based model to explore the impact of antibiotics on the transmission dynamics and to examine the potential of stewardship interventions in limiting ARB spread in a hospital. Agents in the model consist of patients and health care workers (HCW). The transmission of ARB occurs through contacts between patients and HCW and between adjacent patients. In the model, antibiotic use affects the risk of transmission by increasing the vulnerability of susceptible patients and the contagiousness of colonized patients who are treated with antibiotics. The model shows that increasing the proportion of patients receiving antibiotics increases the rate of acquisition non-linearly. The effect of antibiotics on the spread of resistance depends on characteristics of the antibiotic agent and the density of antibiotic use. Antibiotic's impact on the spread increases when the bacterial strain is more transmissible, and decreases as resistance prevalence rises. The individual risk for acquiring ARB increases in parallel with antibiotic density both for patients treated and not treated with antibiotics. Antibiotic treatment in the hospital setting plays an important role in determining the spread of resistance. Interventions to limit antibiotic use have the potential to reduce the spread of resistance, mainly by choosing an agent with a favorable profile in terms of its impact on patient's vulnerability and contagiousness. Methods to measure these impacts of antibiotics should be developed, standardized, and incorporated into drug development programs and approval packages

Regulation of surface architecture by symbiotic bacteria mediates host colonization

Microbes occupy countless ecological niches in nature. Sometimes these environments may be on or within another organism, as is the case in both microbial infections and symbiosis of mammals. Unlike pathogens that establish opportunistic infections, hundreds of human commensal bacterial species establish a lifelong cohabitation with their hosts. Although many virulence factors of infectious bacteria have been described, the molecular mechanisms used during beneficial host–symbiont colonization remain almost entirely unknown. The novel identification of multiple surface polysaccharides in the important human symbiont Bacteroides fragilis raised the critical question of how these molecules contribute to commensalism. To understand the function of the bacterial capsule during symbiotic colonization of mammals, we generated B. fragilis strains deleted in the global regulator of polysaccharide expression and isolated mutants with defects in capsule expression. Surprisingly, attempts to completely eliminate capsule production are not tolerated by the microorganism, which displays growth deficits and subsequent reversion to express capsular polysaccharides. We identify an alternative pathway by which B. fragilis is able to reestablish capsule production and modulate expression of surface structures. Most importantly, mutants expressing single, defined surface polysaccharides are defective for intestinal colonization compared with bacteria expressing a complete polysaccharide repertoire. Restoring the expression of multiple capsular polysaccharides rescues the inability of mutants to compete for commensalism. These findings suggest a model whereby display of multiple capsular polysaccharides provides essential functions for bacterial colonization during host–symbiont mutualism