Recolonization of laser-ablated bacterial biofilm

The recolonization of laser-ablated bacterial monoculture biofilm was studied in the laboratory by using a flow-cytometer system. The marine biofilm-forming bacterium Pseudoalteromonas carrageenovora was used to develop biofilms on titanium coupons. Upon exposure to a low-power pulsed irradiation from an Nd:YAG laser, the coupons with biofilm were significantly reduced both in terms of total viable count (TVC) and area cover. The energy density used for a pulse of 5 ns was 0.1 J/cm(2) and the durations of irradiation exposure were 5 and 10 min. When placed in a flow of dilute ZoBell marine broth medium (10%) the laser-destructed bacterial film in a flow-cytometer showed significant recovery over a period of time. The flow of medium was regulated at 3.2 ml/min. The increase in area cover and TVC, however, was significantly less than that observed for nonirradiated control (t-test, P< 0.05). The coupons were observed for biofilm area cover and TVC at different intervals (3, 6, and 9 h) after irradiation. While the biofilm in the control coupon at the end of 9 h of exposure showed 95.6 +/- 4.1% cover, the 5- and 10-min irradiated samples after 9 h showed 60.3 +/- 6.5 and 37.4 +/- 12.1% area cover, respectively. The reduced rate of recolonization compared to control was thought be due to the lethal and sublethal impacts of laser irradiation on bacteria. This observation thus provided data on the online recolonization speed of biofilm, which is important when considering pulsed laser irradiation as an ablating technique of biofilm formation and removal in natural systems.     

A spatially explicit model for an Allee effect: why wolves recolonize so slowly in Greater Yellowstone

A reduced probability of finding mates at low densities is a frequently hypothesized mechanism for a component Allee effect. At low densities dispersers are less likely to find mates and establish new breeding units. However, many mathematical models for an Allee effect do not make a distinction between breeding group establishment and subsequent population growth. Our objective is to derive a spatially explicit mathematical model, where dispersers have a reduced probability of finding mates at low densities, and parameterize the model for wolf recolonization in the Greater Yellowstone Ecosystem (GYE). In this model, only the probability of establishing new breeding units is influenced by the reduced probability of finding mates at low densities. We analytically and numerically solve the model to determine the effect of a decreased probability in finding mates at low densities on population spread rate and density. Our results suggest that a reduced probability of finding mates at low densities may slow recolonization rate.     

Population demography and the evolution of helping behaviors

Limited dispersal may favor the evolution of helping behaviors between relatives as it increases their relatedness, and it may inhibit such evolution as it increases local competition between these relatives. Here, we explore one way out of this dilemma: if the helping behavior allows groups to expand in size, then the kin-competition pressure opposing its evolution can be greatly reduced. We explore the effects of two kinds of stochasticity allowing for such deme expansion. First, we study the evolution of helping under environmental stochasticity that may induce complete patch extinction. Helping evolves if it results in a decrease in the probability of extinction or if it enhances the rate of patch recolonization through propagules formed by fission of nonextinct groups. This mode of dispersal is indeed commonly found in social species. Second, we consider the evolution of helping in the presence of demographic stochasticity. When fecundity is below its value maximizing deme size (undersaturation), helping evolves, but under stringent conditions unless positive density dependence (Allee effect) interferes with demographic stochasticity. When fecundity is above its value maximizing deme size (oversaturation), helping may also evolve, but only if it reduces negative density-dependent competition.     

Range expansion of a selfing polyploid plant despite widespread genetic uniformity

Background and Aims

Ongoing and previous range expansions have a strong influence on population genetic structure of plants. In turn, genetic variation in the new range may affect the population dynamics and the expansion process. The annual Ceratocapnos claviculata (Papaveraceae) has expanded its Atlantic European range in recent decades towards the north and east. Patterns of genetic diversity were investigated across the native range to assess current population structure and phylogeographical patterns. A test was then made as to whether genetic diversity is reduced in the neophytic range and an attempt was made to identify source regions of the expansion.

Methods

Samples were taken from 55 populations in the native and 34 populations in the neophytic range (Sweden, north-east Germany). Using amplified fragment length polymorphism markers an analysis was made of genetic variation and population structure (Bayesian statistical modelling) and population differentiation was quantified. Pollen/ovule ratio was analysed as a proxy for the breeding system.

Key Results

Genetic diversity at population level was very low (mean H_e = 0·004) and two multilocus genotypes dominated large parts of the new range. Population differentiation was strong (F_ST = 0·812). These results and a low pollen/ovule ratio are consistent with an autogamous breeding system. Genetic variation decreased from the native to the neophytic range. Within the native range, H_e decreased towards the north-east, whereas population size increased. According to the Bayesian cluster analysis, the putative source regions of the neophytic range are situated in north-west Germany and adjacent regions.

Conclusions

Ceratocapnos claviculata shows a cline of genetic variation due to postglacial recolonization from putative Pleistocene refugia in south-west Europe. Nevertheless, the species has expanded successfully during the past 40 years to southern Sweden and north-east Germany where it occurs as an opportunistic neophyte. Recent expansion was mainly human-mediated by single long-distance diaspore transport and was facilitated by habitat modification.     

Langfristige Auswirkungen ehemaliger Bewirtschaftungsvarianten auf die Diversit?t von Blütenpflanzen, Moosen und Flechten eines Kalkmagerrasens

Jeschke M., Kiehl K., Pfadenhauer J. and Gigon A. 2008. Long-term effects of former management on the diversity of vascular plants, mosses and lichens in a calcareous grassland. Bot. Helv. 118: 95 – 109. Long-term effects of grassland management on the species richness of vascular plants, mosses and lichens were studied in a calcareous grassland near Schaffhausen (Switzerland). Experimental plots were treated for 22 years (1977 – 1999) with different mowing regimes, annual burning in late winter, or abandonment. From 2000 onwards, all plots were mown annually in autumn. Five years later (2004), we studied the after-effects of the previous treatments by recording the cover of vascular plants, mosses and lichens in nested plots with sizes from 0.01 m² to 16 m². Vascular plant species richness as well as the number of species characteristic for calcareous grasslands were significantly lower in formerly abandoned or burnt plots than in regularly mown plots, with larger differences on small subplots. There was no significant effect of the previous mowing time (July vs. October) or mowing frequency (annual or every two years) on vascular plants. Mosses and lichens had been eliminated by annual burning between 1977 and 1999, and only few moss species re-colonized the formerly burnt plots until 2004. The species richness of mosses did not differ between formerly abandoned and mown plots, but there were more mesophytic and fewer xerophytic species in the formerly abandoned plots. Mesophytic mosses were also more abundant in plots that used to be mown every second year than in those mown annually. Our data indicate that even after five years of similar management, i.e. annual mowing in autumn, the previous long-term management had strong after-effects on species richness and species composition of calcareous grasslands. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Eingereicht am 19. M?rz 2008, Angenommen am 30. August 2008, Redaktion: Sabine Güsewell     

Life history variation between species of the relictual genus Borderea (Dioscoreaceae): phylogeography, genetic diversity, and population genetic structure assessed by RAPD markers

The genus Borderea consists of two species, B. pyrenaica and B. chouardii, taxa which have been previously considered as conspecific due to their overall close morphology. These two sole species of the rare genus of Dioscoreaceae are endemic to the Pyrenees (Spain, France). This mountain range likely operated as a refugium for these plants during the last glaciations. B. chouardii is only known from a single population in the Spanish Prepyrenees and has been classified as at risk of extinction in the Red List of Endangered Species (IUCN); B. pyrenaica shows a narrow distribution range in the central Pyrenees and Prepyrenees. We analysed genetic variation, population structure and differentiation in these two taxa using RAPD markers. Our study was conducted on the same seven populations for which very low levels of genetic differentiation were detected previously through allozyme analysis. By contrast, high levels of genetic variability were detected through the RAPD hypervariable markers. Twelve RAPD primers produced 112 distinct bands in the 397 surveyed individuals, totalling 395 different RAPD phenotypes. Only four bands were monomorphic across all samples of Borderea, whereas 21 of the polymorphic bands were species‐specific (20 for B. chouardii, and one for B. pyrenaica). The largest genetic distances were those between the B. chouardii and the B. pyrenaica phenotypes. An analysis of molecular variance showed greater variance between groups (B. chouardii vs. B. pyrenaica, 76.08%) than within groups (3.60%). RAPD band specificity, phenotypic distances, and the partitioning of variance all support the taxonomic separation of the two species. Statistical evaluation of within‐ and among‐population RAPD genetic variability in B. pyrenaica showed that genetic variability was higher within populations (>80%) than among them. No clear pattern of RAPD differentiation could be observed among the six studied populations of this taxon though slight differences in genetic diversity could be observed in the more isolated Prepyrenean populations compared with the more widespread Pyrenean ones. These results suggest a recent postglacial origin of the present B. pyrenaica populations. © 2003 The Linnean Society of London, Biological Journal of the Linnean Society, 2003, 80 , 483–498.     

Phylogeography of five Polytrichum species within Europe

Using allozymes and microsatellites we have analysed the genetic structure among European populations for several Polytrichum species to infer relevant factors, such as historical events or gene flow, that have shaped their genetic structure. As we observed low levels of genetic differentiation among populations, and no decreasing levels of genetic variation with increasing latitude within most of the examined species, no genetic evidence was obtained for a step-wise recolonization of Europe from southern refugia after the latest glacial period for P. commune , P. uliginosum , P. formosum and P. piliferum . The near absence of population substructuring within these species does indicate that extensive spore dispersal is the most important factor determining the genetic structure among European Polytrichum populations. Gene flow levels have apparently been sufficient to prevent genetic differentiation among populations caused by genetic drift, and to wipe out any genetic structure caused by the postglacial recolonization process. On the other hand, increased genetic differentiation of alpine P. formosum populations suggests that mountain ranges might restrict gene flow significantly among Polytrichum populations. In contrast to most examined Polytrichum species, P. juniperinum showed high levels of genetic differentiation and a profound genetic structure. Assuming that gene flow is not more restricted in P. juniperinum , these findings suggest that this species has recolonized Europe after the latest glacial period from two different refugia, one possibly being the British Isles.  © 2003 The Linnean Society of London, Biological Journal of the Linnean Society 2003, 78, 203–213.     

Seed limitation,not soil legacy effects,prevents native understorey from establishing in oak woodlands in Scotland after removal of Rhododendron ponticum

Following removal of the invasive species Rhododendron ponticum, the native understorey plant community typically fails to reestablish itself. Potential explanations for this failure include (1) lack of an appropriate native seed source; (2) inability of seed to penetrate a dense bryophyte layer; and (3) persistence of chemical “legacy effects” in the soil. We established an experiment to test these competing hypotheses in an Atlantic oak woodland where R. ponticum had been removed. The following experimental treatments were applied singly and in combination: (1) addition of a native seed mix to test for seed limitation; (2) removal of the established ground vegetation at the start of the experiment (which principally consisted of bryophytes) to test for the impact of a barrier layer; (3) addition of activated carbon to test for chemical legacy effects in the soil; and (4) fertilization as an additional measure to promote the establishment of native vascular plants. Application of the native seed mix was revealed to be an effective way to increase the cover of native vascular plants and was particularly effective when applied after the removal of the bryophyte layer. The application of activated carbon and/or fertilizer, however, had no effect on the cover of native vegetation. We conclude that reports of R. ponticum exerting chemical legacy effects long after its removal may have been overstated and that seed limitation and inability to successfully establish in a dense bryophyte layer provided the strongest barriers to natural recolonization by the native plant community following R. ponticum removal.     

THE EFFECT OF DELAYED POPULATION GROWTH ON THE GENETIC DIFFERENTIATION OF LOCAL POPULATIONS SUBJECT TO FREQUENT EXTINCTIONS AND RECOLONIZATIONS

I investigated the effects of delayed population growth on the genetic differentiation among populations subjected to local extinction and recolonization, for two different migration functions; (1) a constant migration rate, and (2) a constant number of migrants. A delayed period of population growth reduces the size of the newly founded populations for one or several generations. Whether this increases differentiation among local populations depends on the actual pattern of migration. With a constant migration rate, fewer migrants move into small populations than into large, thus providing ample opportunity for drift to act within a population. A prolonged period of population growth thus makes the conditions for enhanced differentiation between local populations less restrictive and also inflates the actual levels of differentiation. The effect depends on the relative magnitudes of k_e, the effective number of colonizers and k, the actual number of colonizers. When there is a constant number of migrants into a population per generation, migration into small populations is increased. This increase of migration in small populations counteracts the effects of genetic drift due to small population size. It increases the rate by which populations approach equilibrium, as small populations are swamped by migrants from larger populations closer to genetic equilibrium, and overall levels of differentiation are thus reduced. I also discuss situations for which the results of this paper are relevant.