Enzyme polymorphism in the European starling: Study on plasma esterases

In plasma of the European starling (Sturnus v. vulgaris L.), genetic variation was detected by polyacrylamide gel electrophoresis. Two esterase zones, of which one is polymorphic, had already been described earlier. In addition to these two zones (Es-1 and Es-3), we describe a third (Es-2) that also showed variations. The distribution of the phenotypes observed in the Es-2 zone closely fits the expected distribution according to the law of random mating so that the polymorphism of Es-2 is hereditary. This Es-2 locus is postulated to be a codominant system, controlled by two alleles.     

Determinants of within- and among-clutch variation in yolk corticosterone in the European starling

Maternal glucocorticoids are known to affect offspring phenotype in numerous vertebrate taxa. In birds, the maternal transfer of corticosterone to eggs was recently proposed as a hormonal mechanism by which offspring phenotype is matched to the relative quality of the maternal environment. However, current hypotheses lack supporting information on both intra- and inter-clutch variation in yolk corticosterone for wild birds. As such, we examined variation in yolk corticosterone levels in a wild population of European starlings (Sturnus vulgaris). Maternal condition, clutch size and nesting density were all negatively related to yolk corticosterone deposition; females with high condition indices, those laying larger clutches and those nesting in high-density associations deposited lower amounts of the hormone into eggs than those with low condition indices, laying small clutches and nesting in isolation. Alternatively, we found no effects of maternal age or human disturbance on yolk corticosterone deposition. Intra-clutch variation of yolk corticosterone was significant, with levels increasing across the laying sequence in all clutch sizes examined, with the difference between first and last-laid eggs being greater in large versus small clutches. Given the reported effects of yolk corticosterone on offspring size and growth, intra-clutch variation in yolk corticosterone has the potential to alter the competitive environment within a brood. Furthermore, our results indicate that variation in yolk corticosterone can originate from variation in both the mother's quality as well as the quality of her breeding environment. The presence of inter-female variation in particular is an important pre-requisite in testing whether the exposure of offspring to maternally-derived corticosterone is a mechanistic link between offspring phenotypic plasticity and maternal quality.     

Dominance, competition, and energetic reserves in the European starling, Sturnus vulgaris

We investigated the relationships between social dominance,competition for food, and strategies of body mass and fat regulationin the European starling (Sturnus vulgaris). In birds housedin groups of three, subdominant birds stored more fat than dominants.A removal experiment established a causal link between socialdominance and fat reserves; in groups that had the dominantindividual removed, the remaining birds reduced body mass andfat, relative to control groups that had the subordinate removed.In a second experiment, we investigated the influences of degreeof competition for food and dominance on body mass and fat reserves.Birds under high competition increased fat reserves and tendedto have higher body mass than birds under low competition. Theincrease in fat reserves was higher in the subdominants thanin the dominants. These results are consistent with hypothesesconcerning dominance-dependent access to food; subdominant birds,or birds under increased competition, may store more fat asan insurance against periods when food cannot be obtained. However,relations between dominance, body mass, and fat reserves mayalso arise through other proximate factors relating to dominance-dependentcosts and benefits of fat storage, such as predation risk andenergetic expenditure.     

Characterization of microsatellite loci in the European green crab (Carcinus maenas)

Carcinus maenas (Decapoda: Portunidae) has proven a highly successful invasive marine species whose potential economic and ecological impacts are of great concern worldwide. Here, we characterize 14 polymorphic microsatellite loci in C. maenas and its sister species Carcinus aestuarii. These markers will prove useful for fine‐scale genetic analyses of native and introduced populations, for assessment of the sources and routes of invasion and for evaluation of post‐invasion population dynamics.     

Response to enemies in the invasive plant Lythrum salicaria is genetically determined

Background and Aims

The enemy release hypothesis assumes that invasive plants lose their co-evolved natural enemies during introduction into the new range. This study tested, as proposed by the evolution of increased competitive ability (EICA) hypothesis, whether escape from enemies results in a decrease in defence ability in plants from the invaded range. Two straightforward aspects of the EICA are examined: (1) if invasives have lost their enemies and their defence, they should be more negatively affected by their full natural pre-invasion herbivore spectrum than their native conspecifics; and (2) the genetic basis of evolutionary change in response to enemy release in the invasive range has not been taken sufficiently into account.

Methods

Lythrum salicaria (purple loosestrife) from several populations in its native (Europe) and invasive range (North America) was exposed to all above-ground herbivores in replicated natural populations in the native range. The experiment was performed both with plants raised from field-collected seeds as well as with offspring of these where maternal effects were removed.

Key Results

Absolute and relative leaf damage was higher for introduced than for native plants. Despite having smaller height growth rate, invasive plants attained a much larger final size than natives irrespective of damage, indicating large tolerance rather than effective defence. Origin effects on response to herbivory and growth were stronger in second-generation plants, suggesting that invasive potential through enemy release has a genetic basis.

Conclusions

The findings support two predictions of the EICA hypothesis – a genetically determined difference between native and invasive plants in plant vigour and response to enemies – and point to the importance of experiments that control for maternal effects and include the entire spectrum of native range enemies.     

Testing the enemy release hypothesis: a review and meta-analysis

One of the most cited hypotheses explaining the inordinate success of a small proportion of introduced plants that become pests is the ‘natural enemies hypothesis’. This states that invasive introduced plants spread rapidly because they are liberated from their co-evolved natural enemies. This hypothesis had not been properly tested until recently. Previous reviews on this topic have been narrative and vote counting in nature. In this review, we carried out quantitative synthesis and meta-analysis using existing literature on plants and their herbivores to test the different components of the enemy release hypothesis. We found supporting evidence in that (1) insect herbivore fauna richness is significantly greater in the native than introduced ranges, and the reduction is skewed disproportionally towards specialists and insects feeding on reproductive parts; and (2) herbivore damage levels are greater on native plants than on introduced invasive congeners. However, herbivore damage levels are only marginally greater for plants in native than in introduced ranges, probably due to the small numbers of this type of study. Studies quantifying herbivore impacts on plant population dynamics are too scarce to make conclusions for either comparison of plants in native vs introduced ranges or of co-occurring native and introduced congeners. For future research, we advocate that more than two-way comparisons between plants in native and introduced ranges, or native and introduced congeners are needed. In addition, the use of herbivore exclusions to quantify the impacts of herbivory on complete sets of population vital rates of native vs introduced species are highly desirable. Furthermore, three-way comparisons among congeners of native plants, introduced invasive, and introduced non-invasive plants can also shed light on the importance of enemy release. Finally, simultaneously testing the enemy release hypothesis and other competing hypotheses will provide significant insights into the mechanisms governing the undesirable success of invasive species.     

Song perception in the European starling: hemispheric specialisation and individual variations

Hemispheric specialisation for speech in humans has been well documented. The lateralisation for song production observed in songbirds is reminiscent of this hemispheric dominance. In order to investigate whether song perception is also lateralised, we made multiunit recordings of the neuronal activity in the field L of starlings during the presentation of species-specific and artificial non-specific sounds. We observed a systematic stronger activation in one hemisphere than in the other one during the playback of species-specific sounds, with inter-subject variability in the predominant hemisphere for song perception. Such an asymmetry was not observed for artificial non-specific sounds. Thus, our results suggest that, at least at the individual level, the two hemispheres of the starlings' brain perceive and process conspecific signals differently.     

Ectoparasitism and the role of green nesting material in the European starling

Summary The use of green nesting material is widespred among birds. Recent evidence suggests that birds use secondary chemicals contained in green plants to control ectoparasites. We manipulated green nesting material and ectoparasites of European starlings (Sturnus vulgaris) to test two hypotheses: (1) ectoparasites adversely affect prefledging survival and morphometrics or postfledging survival, and (2) green nesting material ameliorates the effects of ectoparasites. We recorded fat score, numbers of scabs, tarsal length, body mass, and hematocrit level on each nestling 17 days after hatching. We also fitted each nestling with unique patagial tags and resighted the starlings for 6–8 weeks after fledging to estimate survival and sighting rates. Nests devoid of green nesting material and dusted with the insecticide, carbaryl, had fewer high ectoparasite infestations, and nestlings had significantly lower scab scores, and significantly higher body masses than nestlings in undusted boxes. However, there was no difference in postfledging survival between birds from carbaryl-treated and undusted nests. There also was no difference in prefledging survival and morphometrics or postfledging survival between nestlings from boxes with and without green nesting material. These results do not support the hypothesis that starlings use green nesting material to control nest ectoparasites. We suggest an alternative hypothesis; green nesting material is used for mate selection or pairbonding in the starling.     

Identifying the source of species invasions: sampling intensity vs. genetic diversity

Population geneticists and community ecologists have long recognized the importance of sampling design for uncovering patterns of diversity within and among populations and in communities. Invasion ecologists increasingly have utilized phylogeographical patterns of mitochondrial or chloroplast DNA sequence variation to link introduced populations with putative source populations. However, many studies have ignored lessons from population genetics and community ecology and are vulnerable to sampling errors owing to insufficient field collections. A review of published invasion studies that utilized mitochondrial or chloroplast DNA markers reveals that insufficient sampling could strongly influence results and interpretations. Sixty per cent of studies sampled an average of less than six individuals per source population, vs. only 45% for introduced populations. Typically, far fewer introduced than source populations were surveyed, although they were sampled more intensively. Simulations based on published data forming a comprehensive mtDNA haplotype data set highlight and quantify the impact of the number of individuals surveyed per source population and number of putative source populations surveyed for accurate assignment of introduced individuals. Errors associated with sampling a low number of individuals are most acute when rare source haplotypes are dominant or fixed in the introduced population. Accuracy of assignment of introduced individuals is also directly related to the number of source populations surveyed and to the degree of genetic differentiation among them ( F _ST). Incorrect interpretations resulting from sampling errors can be avoided if sampling design is considered before field collections are made.     

Mitochondrial DNA offers unique insights into invasion history of the common starling

Mitochondrial DNA (mtDNA) can be a powerful genetic marker for tracing origins and history of invasive populations. Here, we use mtDNA to address questions relevant to the understanding of invasion pathways of common starlings (Sturnus vulgaris) into Western Australia (WA) and discuss the utility of this marker to provide information useful to invasive species management. Mitochondrial sequence data indicate two geographically restricted genetic groups within Australia. Evidence of dispersal from genetically distinct sources outside the sampled range of starlings in Australia suggests increased vigilance by management agencies may be required to prevent further incursions from widely separated localities. Overall, genetic diversity in Australia was lower than in samples from the native range. Within Australia, genetic diversity was lowest in the most recently colonized area in the west, indicating that demographic bottlenecks have occurred in this area. Evidence of restricted dispersal between localities on the edge of the range expansion (ERE) in WA and other Australian sampling localities suggests that localized control within the ERE may be effective in preventing further range expansion. Signatures of spatial and demographic expansion are present in mismatch analyses from sampling localities located at the ERE, but neutrality indices did not support this finding, suggesting that the former may be more sensitive to recent expansion. Additionally, mismatch analyses support the presence of admixture, which is likely to have occurred pre-introduction. We compare our findings with those from a microsatellite study of the same samples and discuss how the mtDNA analyses used here offer valuable and unique insights into the invasion history of introduced species.     

The identification of sex in the starling Sturnus vulgaris using a molecular DNA technique

Female birds can be identified through the presence of a W-chromosome. We describe a procedure for amplifying a W-linked DNA marker in the starling (Sturnus vulgaris) by the polymerase chain reaction (PCR) so allowing the diagnosis of sex in this species. The technique is sensitive, allowing even the smallest chicks to be sexed from a blood sample. The method possesses a positive internal control to ensure accuracy. It is also applicable to the spotless starling (S. unicolor) but not to two bird species outside the genus. The nucleotide sequence of the female-specific PCR product is given.     

Colony structure and nest characteristics of European wasps, Vespula germanica (F.) (Hymenoptera: Vespidae), in Victoria, Australia

Abstract European wasps, Vespula germanica , are common across southern and south-eastern Australia and have a negative impact upon urban areas, primary industries, and natural ecosystems. Aspects of colony structure and nest characteristics are examined for nests located in the ground and collected from urban and rural sites in Victoria during two summer−autumn field seasons (1996 and 2001). On average, nests were located 28 cm beneath the surface (range 5−58 cm). The average number of combs in the nest and the total area of the nests increased from early February to late May. In addition, the proportion of cells used to produce workers and different life stages (larvae, pupae) also varied throughout the season; being consistent with studies from New Zealand. No differences in colony structure or nest character­istics were detected between urban and rural nests.     

Spatio-temporal variation in seed predation by a native weevil in the invasive Prunus serotina

Invasive species may escape the enemies from their native range (‘enemy release’), but they can also acquire new enemies in their introduced range, which will affect the invasion process. For the invasive tree species Prunus serotina, seed predation by the native weevil Furcipus rectirostris has been reported in forests in its introduced range. In this study, we quantified how common the infestation of P. serotina seeds by F. rectirostris is in a 4000 km² area in northern Belgium. Seeds were sampled on P. serotina trees in different habitats and in two years, i.e., with low and high P. serotina fruit production. Infestation was found throughout the study region, in 43 and 62% of the sampled trees in the two years of the study; the maximum infestation levels of infested seed samples were 50 and 69%. Overall, predation occurred in 4.4 and 10.8% of the sampled seeds. The level of infestation differed between habitats and years, and the number of fruits per raceme was inversely related to the infestation level. Notwithstanding the rather high incidence of F. rectirostris infestation in our study, the impact on P. serotina's invasiveness might remain low seeing the overall high seed production and dispersal capacity of the species.     

The implications of accepting untestedhypotheses: a review of the effectsof purple loosestrife (Lythrum salicaria)in North America

The acceptance of poorly tested hypotheses has adverse scientific consequences, and may have adverse ecological and social consequences. The hypothesis that purple loosestrife (Lythrum salicaria) has deleterious effects on North American wetlands is an example. We traced the history of purple loosestrife and its control in North America and found little scientific evidence consistent with the hypothesis that purple loosestrife has deleterious effects. The most commonly cited study of the effects of purple loosestrife on native flora and fauna produced inconclusive results. The general acceptance of this hypothesis, however, has resulted in the introduction of nonindigenous insects for biological control. Efforts to control purple loosestrife may be misplayed and may have long-term ecological consequences if loosestrife does not have the impact it is believed to have. The acceptance of this hypothesis using scientific justifications may affect future scientific credibil ity. Careful evaluation of the precautionary principle is necessary when considering the control of nonindigenous organisms.