The effect of predation on the structure and invasibility of assembled communities

Summary Fifty four microcosmic communities were assembled over 4 months from a 28-species source pool of phytoplankton using nine different invasion patterns each replicated six times. Three communities from each set of replicates then were invaded with a cladoceran that feeds on phytoplankton. All communities were then treated identically for an additional 4 months. In all nine invasion categories species richness was greater in predated communities. Predation opened communities to invasion by increasing the representation of infrequently sampled species at the expense of more common species. Invasion rate was four times more influential than predation and over eleven times more important than either invasion order or the timing pattern of interspecific arrivals in determining species richness in this system of communitites.     

Different colonization patterns of phytophagous and predatory mites (Acari: Tetranychidae, Phytoseiidae) on three grape varieties: a case study

In a vineyard having three varieties of grape (Merlot, Trebbiano and Garganega) differently colonized by two phytoseiid species,Typhlodromus pyri Scheuten andAmblyseius andersoni (Chant), the dynamics of mite populations were monitored over 5 years (1989–1993) in order to study their colonization, interspecific competition and the control of spider mites, i.e.Panonychus ulmi (Koch). These aspects were also investigated by releasingT. pyri, A. andersoni andAmblyseius aberrans (Oudemans) on some of the above varieties. In most of the experimental years (1989–1992), selective pesticides were used in order to allow a successful release of phytoseiids, in particularA. aberrans. The use of non-selective insecticides was re-established during 1993 in order to test its effect on the new mite communities originating from 1989 onwards. In the first years of the experiments an apparent relationship between grape variety and phytoseiid species was observed: in the control plots,A. andersoni occurred on Merlot whereT. pyri was rare, while the latter species was largely dominant overA. andersoni on Trebbiano and Garganega.Panonychus ulmi populations reached moderate levels only on Merlot and in the first part of experiments. The variety-phytoseiid species relationship was temporary as, at the end of experiments,T. pyri was completely dominant on all varieties. This new situation started when prey occurrence and interspecific competition decreased in importance. The moderate success of theT. pyri release on Merlot contrasts with the results of previous experiments. Two factors could be involved in this phenomenon: low interspecific competition by phytoseiids and predation by macropredators.Amblyseius aberrans was able to displaceA. andersoni andT. pyri on grape varieties where the two species were more abundant and reached higher population densities on varieties with pubescent leaf undersurfaces. In the first experimental year, spider mite densities were reduced more effectively inA. aberrans release plots than in the control or inT. pyri release plots. One year later,P. ulmi reached lower levels in the release treatments than in the control.Typhlodromus pyri andA. aberrans persisted in conditions of prey scarcity. The high competitivity ofA. aberrans over the remaining two phytoseiid species constitutes a major factor in selecting predatory species for inoculative releases in vineyards.     

Escherichia coli K12 does not colonize the gastrointestinal tract of Fischer-344 rats

Summary The colonizing potential ofEscherichia coli K12 containing a vector coding for somidobove (bovine somatotropin) was determined. Treated male and female Fischer-344 rats were given a single oral gavage inoculum of sucrose with/without tetracycline (15 g/ml). Untreated control animals received similar drinking water regimes. All animals survived until termination. There were no clinical signs of toxicity observed and no treatment-related effect upon body weight, food consumption, or efficiency of food utilization. Fresh fecal samples were collected from each rat every 24 h following inoculation and the population of the marked strain was quantitated until no bacterial colonies were observed for two consecutive days. While all inoculated rats were positive at 24 h, by 72 and 96 h all had become negative for the test (marked) strain, as were the corresponding control group throughout the test. The frozen stock of the marked strain used as the positive control demonstrated that the agar plates were selective for the test strain. Fourteen days following inoculation, all groups of rats were killed and the gastrointestinal tracts removed and treated to recover the marked strain. There was no evidence of the marked strain in the gastrointestinal tract of any rat from any group. Thus, theE. coli K12 host/vector system used in this experiment does not colonize the gastrointestinal tract of Fischer-344 rats.     

Xylem colonization of the legume Sesbania rostrata by Azorhizobium caulinodans

A novel pathway of invasion of the legume Sesbania rostrata by Azorhizobium caulinodans is described that involves colonization of the root xylem, possibly following entry into the natural fissures created during emergence of lateral roots. Azorhizobia were detected microscopically, and their presence confirmed by the expression of a lacZ reporter gene. We have shown that rhizobial Nod factors are not required for either xylem colonization or for crack-entry of lateral roots. We discuss the extent to which this discovery of xylem colonization by azorhizobia is likely to improve our understanding of both symbiosis and of pathogenicity in plant–bacterial interactions.     

Population genetics of colonizing success of weedy rye in Northern California

Summary Genetic parameters of 11 weedy rye populations located in California's northern mountain area and the adjoining Oregon border were compared with those of the putative parents, wild species Secale montamim and cultivated rye S. cereale. All weedy populations exhibited high levels of genetic variation as determined by isozyme analysis. On average, 44% of the isozyme loci were polymorphic, total genetic diversity was 0.30; and number of alleles per locus was 1.65. High genetic identities, averaging 0.994 ± 0.005 between populations, indicated that little genetic differentiation has occurred among these weedy populations since the initial colonization. Lack of population differentiation could be attributed to a wind-pollinated, self-incompatible breeding system resulting in extensive gene flow among weedy populations, and between weedy populations and local cultivars of rye. Multilocus outcrossing rates of weedy populations ranged from 0.86 to 0.97. The estimated levels of gene flow using the private-alleles method were high among weedy populations, and between cv Merced and weedy populations, with estimated Nm values of 14.50 and 8.21, respectively. The colonizing success of weedy rye is discussed and a strategy for its conservation recommended.     

Colonization and emergence of midges (Chironomidae: Diptera) in slow sand filter beds

Colonization by midges, and temporal changes in their community structure, were examined in slow sand filter beds. The replicated beds allow the development of communities to be traced from a known starting point.The filter beds (rectangular concrete containers filled with water) have a substratum of sand on which a rich coating of organic particles develops during passage of the water through the bed. The containers (ponds) are drained from time to time and the organic layer is then scraped off the sand surface. This occurs on average, once a month. The length of time the ponds were filled with water (bed run) during the present study ranged from 16 to 77 days.In long bed runs small midges with a short aquatic phase (Cricotopus sylvestris, Psectrocladius limbatellus, Tanytarsus fimbriatus) produced adults after 16–20 days; other, larger midges,e.g. Psectrocladius barbimanus and the Tanypodinae required a longer aquatic phase. Of the Tanypodinae, the smallAblabesmyia phatta, had the shortest duration of the four species found, and was much the most numerous member of this subfamily. Some Chironomini only appeared when the organic coating had developed over the sand surface. Midges of this tribe frequently failed to complete their larval development within the duration of bed runs and were thus trapped on the substratum at the time of cleaning. When ponds were drained after short bed runs the succession in community structure observed in long runs was arrested.Three small midgesC. sylvestris, P. limbatellus andT. fimbriatus, were collected in high numbers throughout the life of all beds, except towards the end of the longest runs in the study. This suggests that small size, short life cycles, and the ability to colonize clean substrata, are important characteristics for the development of primary chironomid communities in short-lived temporary habitats.     

Colonization of artificial substrates by stream insects: Influence of substrate size and diversity

A field experiment was performed in a New Mexico (U.S.A.) stream to investigate the relation between size and diversity of substrate and numbers and diversity of colonizing insects. Baskets with either small gravel, large gravel, or a 1 : 1 mixture of the two sizes were placed in a riffle area and colonization was monitored for 19 days.Colonization was rapid, and both total number of individuals and number of species colonizing the substrates had stopped increasing by the end of the experiment. Fewer individuals colonized baskets downstream, suggesting that the drift is a major source of insect colonists. Small substrate supported more individuals and more species than the larger stones. Total number of individuals and the number of species on the mixed substrate were between numbers on small and large substrate. These results do not support previous generalizations on the relation between substrate size and complexity and the structure of invertebrate communities in streams. Results of the study are compared with the findings of recent experimental studies and it is suggested that earlier generalizations on the role of substrate size and complexity need to be reexamined.Order of authors was determined by the flip of a coin. Reprint requests may be sent to either author.     

Sterilization of Hulecoeteomyia japonica japonica (= Aedes japonicus japonicus) (Theobald, 1901) by high‐energy photon irradiation: implications for a sterile insect technique approach in Europe

Hulecoeteomyia japonica japonica (= Aedes japonicus japonicus) (Diptera: Culicidae) (Theobald 1901), a container‐breeding invasive species in North America and Europe, is attracting particular attention for its high local abundances and possible roles in the transmission of human and animal pathogens. The preferential habitats of this species are forested and bushy areas, which renders control measures extremely inefficient. Use of the sterile insect technique (SIT) may contribute to the implementation of area‐wide integrated pest management strategies, as has been successfully proven with other aedine mosquito species. The present study investigates the effects of irradiation at a dose of 40 Gy on fitness parameters in H. j. japonica. Irradiation was performed on 16–24‐h‐old pupae from a colonized strain (PA) using a TrueBeam linear accelerator. Males from the PA strain were crossed with females of the same colony or with field‐collected females. Irradiation induced a slight increase in mortality in male pupae, but did not alter the survival and mating abilities of emerging adult males. Rates of blood feeding and fertility were lower when PA strain males were kept with field‐collected females rather than PA females. Irradiated males induced reductions in fertility (residual fertility: 2.6%) and fecundity in mated females. The data indicate that the SIT is a suitable technique to enhance the control of this species.     

Seasonal variations in colonization dynamics of periphytic protozoa in coastal waters of the Yellow Sea,northern China

The colonization features of periphytic protozoa have proved to be a useful tool for indicating water quality status in aquatic ecosystems. In order to reveal the seasonal variations in colonization dynamics of the protozoa, a 1-year baseline survey was carried out in coastal waters of the Yellow Sea, northern China. Using glass slides as artificial substrates, a total of 240 slides were collected at a depth of 1 m in four seasons after colonization periods of 3, 7, 10, 14, 21, and 28 days. A total of 122 ciliate species were identified with 21 dominant species. The colonization dynamics of the protozoa were well fitted to the MacArthur-Wilson and logistic models in all four seasons (P < 0.05). However, the equilibrium species numbers (S_eq), colonization rates (G), and the time to 90% S_eq (T_90%) represented a clear seasonal variability: (1) more or less similar levels in spring and autumn (S_eq = 29/23; G = 0.301/0.296; T_90%=7.650/7.779); (2) with a significant difference in summer and winter (S_eq = 32/121; G = 0.708/0.005; T_90% = 3.252/479.705). Multivariate approaches demonstrated that the exposure time for the species composition and community structure of the protozoa to an equilibrium period were 10–14 days in spring and autumn, but less and more time periods were needed in summer and winter, respectively. Based on the results, we suggest that the colonization dynamics of periphytic protozoa were different within four seasons, and an optimal sampling strategy for monitoring surveys should be modified during different seasons in marine ecosystems.     

Early responses of bark and wood boring beetles to an outbreak of the hemlock looper Lambdina fiscellaria (Guenée) (Lepidoptera: Geometridae) in a boreal balsam fir forest of North America

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