Adaptive plasticity of egg size in response to competition in the cowpea weevil,Callosobruchus maculatus (Coleoptera: Bruchidae)

Life history theory predicts that larger propagules should be produced when the offspring are expected to experience intense competition. This study tested whether female cowpea weevils responded to high larval or adult density by producing larger eggs. In a splitbrood design I measured the effect of density experienced by females at their larval stage (1 vs. 4–6 larvae/cowpea) on the size of eggs produced just after emergence. The females were then kept either at low adult density (1 female+1 male per vial), or at high adult density (10 females+10 males) for 2 days, and tested for the effect of this adult density treatment on the size of eggs laid subsequently. I measured egg length and width, as well as the diameter of the entrance tunnel made by the larva, which can be regarded as a crude measure of larval size. Females that experienced high adult density subsequently laid slightly wider eggs than those kept at low density. This difference, albeit small (about 1–4% after correction for female weight and the effect of family, depending on the statistical model used), was statistically significant and robust to alterations of the statistical model. It may be a remnant of a larger plastic response of egg size to competition that has become eroded during many generations in high-density laboratory cultures. There was no difference in egg length or the diameter of the entrance tunnel. Eggs laid just after emergence by females reared at high larval density also tended to be wider than those produced by females that had no competitors. This effect was only marginally significant, however, and sensitive to the statistical model. Both egg length and width and the diameter of the entrance tunnel increased with female weight and decreased with female age. The tunnel diameter was positively correlated with both egg length and width, but the effect of width was larger.     

Minimum threshold for establishment and dispersal of Lilioceris cheni (Coleoptera: Chrysomelidae): a biological control agent of Dioscorea bulbifera

The successful establishment or failure of a new population is often attributed to propagule pressure, the combination of the number of independent introduction events, and the number of individuals released at each event. The design of optimal release strategies for biological control agents benefits from an understanding of the impact of propagule pressure on the species being released. The dispersal rate of individuals from nascent population foci can also affect establishment success. We assessed the minimum threshold for establishment and measured dispersal of Lilioceris cheni (Coleoptera: Chrysomelidae), a biological control agent for Dioscorea bulbifera (Dioscoreales), air potato. Replicated releases of 10, 50, and 100 adults of L. cheni were conducted on the east and west coasts of south Florida. Dispersal was measured from 19 of these sites plus 19 additional release locations in south and central Florida. Lilioceris cheni established populations from all three release sizes with no apparent influence of site location. Releases of 10, 50, and 100 adults resulted in 50%, 67%, and 85% establishment, respectively. Beetles dispersed an average of 1.41?±?0.515?km/yr. Dispersal distance was significantly affected by the time since release but not the number of individuals released. Our results suggest that future releases of 100 individuals could be spaced several kilometres apart on the landscape to facilitate rapid colonisation of D. bulbifera infestations.     

Influence of soil fumigation and source of strawberry plants on population densities of spores and infective propagules of endogonaceous mycorrhizal fungi

Summary Fumigation of soil with methyl bromide-chloropicrin (2∶1, w/w) killed all detectable infective propagules of endogonaceous fungi in one field and reduced those in another to a trace. However, spore population densities were reduced by only half in the first field and not affected in the second. These effects persisted overwinter in the first field and through the growing season in the second. Apparently spores killed by fumigation do not degrade in soil readily. No evidence was obtained that commercial strawberry transplants were an important source for introduction of endogonaceous fungi.     

Saving camels from straws: how propagule pressure-based prevention policies can reduce the risk of biological invasion

Nonnative species that harm or have the potential to cause harm to the environment, economy, or human health are known as invasive species. Propagule pressure may be the most important factor in establishment success of nonnative species of various taxa in a variety of ecosystems worldwide, and strong evidence is emerging that propagule pressure determines both the scale of invasion extent and impact. In a limited way, the US government is applying a “propagule pressure approach” in a variety of prevention policy contexts aimed at minimizing the impact of harmful organisms. However, there are also readily apparent opportunities for enacting propagule pressure-based measures to fill current gaps in invasive species prevention and control at national, state, and local levels. An explicit focus on propagule pressure-based policies could substantially increase the effectiveness of US efforts to prevent the introduction of invasive species through by intentional and unintentional introductions. The views expressed in this paper are solely those of the authors and do not necessarily reflect those of the US government. “As the last straw breaks the laden camel’s back...” -Charles Dickens, Dombey and Son     

Altitudinal distribution of alien plant species in the Swiss Alps

In summer 2003 we recorded the presence and abundance of alien plant species at 232 sites (107 railway stations and 125 road sites) along mountain passes in the Swiss Alps. The altitudinal distribution of species was related to the current abundance of the species in Switzerland and time since introduction. A total of 155 alien taxa were recorded. Numbers of species per site declined exponentially with altitude, and only a few species were found in the alpine zone (>2000 m). In contrast, species richness among comparable native taxa appeared to be nearly independent of altitude over the range investigated. Maximum altitude reached by alien species was related positively to both total area occupied in Switzerland and to time since introduction. A comparison of the results with earlier records suggests that many species, particularly those previously restricted to low or intermediate altitudes, have advanced their altitudinal limits over the past few decades. Various hypotheses are presented to explain the declining abundance of alien species with altitude: low-altitude filter effects, low propagule pressure, and genetic swamping of peripheral populations at higher altitudes. However, at present we do not have sufficient evidence to determine the relative importance of these effects. We conclude that invasion into mountain areas such as the Swiss Alps tends to proceed rather slowly, though the process may be accelerated by climatic warming. For this reason, further research to investigate the processes determining how plants invade mountain areas is urgently needed. And more generally, investigations into the distribution of alien species along strong altitudinal gradients may provide valuable insights into the mechanisms driving the spread of alien organisms.     

Three propaguliferous species of Pohlia (Mielichhoferiaceae,Musci) in China

Abstract

As a result of a literature survey and examination of specimens, we believe that the identification of three species of propaguliferous Pohlia in China, Pohlia flexuosa, Pohlia proligera, and Pohlia leucostoma needs to be clarified. Pohlia flexuosa has two differing types of propagule: one short and oblong and the other long and thread-like; both propagules have inconspicuous leaf primordia, and can occur on the same stem. Propagules of P. proligera are shortly clavate to long vermicular, with one to three unicellular or multicellular prong-like leaf primordia arising at the apex of the propagule, and the leaf primordia are approximately one-fourth of the total length of the propagules. Pohlia leucostoma has long filiform propagules, with two or three short peg-like leaf primordia about one-tenth of total propagule length.     

Local and landscape effects on temporary pond zooplankton egg banks: conservation implications

Understanding landscape correlates of local habitat integrity and community structure and the identification of spatial scales at which these associations operate are relevant for management and conservation of unique but globally threatened temporary ponds. We use a multivariate variance decomposition approach to determine taxon-specific associations of zooplankton communities banked in dry pond soils (rotifers, cladocerans, copepods) with local habitat features and landscape characteristics across four spatial scales (100 buffer strip, 1, 5, and 10 km scales). Results show similar degrees of correlation between rotifer and cladoceran communities with local habitat conditions (chiefly water quality). This is interpreted according to life-history traits of component species of wetland propagule banks. Associations with landscape features varied between communities with rotifers correlating with landscape structural features only at the buffer scale while cladocerans showed no significant correlations with landscape characteristics across all scales. Copepods were neither significantly associated with local nor landscape characteristics. The results of this study contrast strikingly with our previous population-based study, where populations of Triops cancriformis and Branchinecta orientales were significantly correlated with landuse features at the broadest scale. The combined results suggest that a three-way management scheme could be useful for conservation of zooplankton resting egg banks in this remnant wetland complex. These schemes could focus on the restoration of wetland water quality, the establishment of vegetated buffer strips around the ponds to counteract degradation resulting from runoff, and reforestation and/or the creation of hedgerows in agricultural catchments to avoid impacts resulting from broad-scale diffuse pollution fluxes. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Long term dynamics of Rosa multiflora in a successional system

Long term studies of invasion dynamics are critical in developing a more complete understanding of the factors that influence species spread. To address this issue, the dynamics of the non-native invasive plant, Rosa multiflora, were examined using a 40-year record of successional change. The roles of biotic and abiotic factors in regulating R. multiflora invasion were also assessed. The invasion showed an initial 9-year time lag, followed by a 20-year period of population expansion and an ultimate decline as succession progressed. During all phases of R. multiflora’s invasion, there was continuous turnover within plots. Rainfall during the previous season was found to increase R. multiflora colonization during population expansion while tree species inhibited the invader’s growth. During expansion and decline of R. multiflora, common associated species were often positively or negatively correlated with changes in R. multiflora cover. Though early population dynamics were regulated by propagule pressure, the major influence on R. multiflora late in succession was canopy closure. Although the invasion of this species was largely self-limiting in this system, the species is likely to persist within late successional systems and may require management intervention.     

French wasps in the New World: experimental biological control introductions reveal a demographic Allee effect

Many populations introduced into a novel environment fail to establish. One underlying process is the Allee effect, i.e., the difficulty of individuals to survive and reproduce when rare, and the consequently low or negative population growth. Although observations showing a positive relation between initial population size and establishment probability suggest that the Allee effect could be widespread in biological invasions, experimental tests are scarce. Here, we used a biological control program against Diuraphis noxia (Mordvilko) (Hemiptera: Aphididae) in the United States to manipulate initial population size of the introduced parasitoid Aphelinus asychis Walker (Hymenoptera: Aphelinidae) originating from France. For eight populations and three generations after introduction, we studied spatial distribution and spread, density, mate-finding, and population growth. Dispersal was lower in small populations during the first generation. Smaller initial population size nonetheless resulted in lower density during the three generations studied. The proportion of mated females and the population sex ratio were not affected by initial population size or population density. Net reproductive rate decreased with density within each generation, suggesting negative density-dependence. But for a given density, net reproductive rate was smaller in populations initiated with few individuals than in populations initiated with many individuals. Hence, our results demonstrate a demographic Allee effect. Mate-finding is excluded as an underlying mechanism, and other component Allee effects may have been overwhelmed by negative density-dependence in reproduction. Impact of generalist predators could provide one potential explanation for the relationship between initial population size and net reproductive rate. However, the continuing effect of initial population size on population growth suggests genetic processes may have been involved in the observed demographic Allee effect.     

Founder population size and number of source populations enhance colonization success in waterstriders

Understanding the factors that underlie colonization success is crucial both for ecological theory and conservation practices. The most effective way to assess colonization ability is to introduce experimentally different sets of individuals in empty patches of suitable habitat and to monitor the outcome. We translocated mated female waterstriders, Aquarius najas, into 90 streams that were not currently inhabited by the species. We manipulated sizes of propagules (from 2 to 16 mated females) and numbers of origin populations (one or two). Three origin populations were genetically different from each other, but they were less than 150 km from the streams of translocation. The results demonstrate clearly that both the larger propagule size and the high number of source populations have positive effects on the probability of colonizing a new stream. Thus, in addition to the stochastic factors related to the propagule size it may be essential to consider also the diversity of genetic origin for colonization success.Due to an error in the citation line, this revised PDF (published in December 2003) deviates from the printed version, and is the correct and authoritative version of the paper.