Can the enemy release hypothesis explain the success of invasive alien predators and parasitoids?

Biological invasions are ecologically and economically costly. Understanding the major mechanisms that contribute to an alien species becoming invasive is seen as essential for limiting the effects of invasive alien species. However, there are a number of fundamental questions that need addressing such as why some communities are more vulnerable to invasion than others and, indeed, why some alien species become widespread and abundant. The enemy release hypothesis (ERH) is widely evoked to explain the establishment and proliferation of an alien species. ERH predicts that an alien species introduced to a new region should experience a decrease in regulation by natural enemies which will lead to an increase in the distribution and abundance of the alien species. At the centre of this theory is the assumption that natural enemies are important regulators of populations. Additionally, the theory implies that such natural enemies have a stronger regulatory effect on native species than they do on alien species in the introduced range, and this disparity in enemy regulation results in increased population growth of the alien species. However, empirical evidence for the role of the ERH in invasion success is lacking, particularly for invertebrates. Many studies equate a reduction in the number of natural enemies associated with an alien species to release without studying population effects. Further insight is required in relation to the effects of specific natural enemies on alien and native species (particularly their ability to regulate populations). We review the role of ecological models in exploring ERH. We suggest that recent developments in molecular technologies offer considerable promise for investigating ERH in a community context.     

Can enemy release explain the invasion success of the diploid <Emphasis Type="Italic">Leucanthemum vulgare</Emphasis> in North America?

Enemy release is a commonly accepted mechanism to explain plant invasions. Both the diploid Leucanthemum vulgare and the morphologically very similar tetraploid Leucanthemum ircutianum have been introduced into North America. To verify which species is more prevalent in North America we sampled 98 Leucanthemum populations and determined their ploidy level. Although polyploidy has repeatedly been proposed to be associated with increased invasiveness in plants, only two of the populations surveyed in North America were the tetraploid L. ircutianum. We tested the enemy release hypothesis by first comparing 20 populations of L. vulgare and 27 populations of L. ircutianum in their native range in Europe, and then comparing the European L. vulgare populations with 31 L. vulgare populations sampled in North America. Characteristics of the site and associated vegetation, plant performance and invertebrate herbivory were recorded. In Europe, plant height and density of the two species were similar but L. vulgare produced more flower heads than L. ircutianum. Leucanthemum vulgare in North America was 17 % taller, produced twice as many flower heads and grew much denser compared to L. vulgare in Europe. Attack rates by root- and leaf-feeding herbivores on L. vulgare in Europe (34 and 75 %) was comparable to that on L. ircutianum (26 and 71 %) but higher than that on L. vulgare in North America (10 and 3 %). However, herbivore load and leaf damage were low in Europe. Cover and height of the co-occurring vegetation was higher in L. vulgare populations in the native than in the introduced range, suggesting that a shift in plant competition may more easily explain the invasion success of L. vulgare than escape from herbivory.     

Is problematic mobile phone use explained by chronotype and personality?

In this study, the relationships among problematic mobile phone use, age, gender, personality and chronotype of Turkish university students were examined. The study included 902 university students (73% female, 27% male) and their participation in the study was anonymous and voluntary. Data were collected from each participant by assessing a demographic questionnaire, Composite Scale of Morningness (CSM) as a measure of chronotype, the Big Five Inventory (BIG-5) for personality assessment and Mobile Phone Problem Usage Scale (MPPUS). The most important result was that CSM scores were the best predictor for problematic mobile phone usage, and as a consequence, evening-oriented university students scored higher on the MPPUS. This result remained, even when compared with the most influential personality predictor, conscientiousness. In addition, while extraversion positively predicted, emotional stable and chronotype negatively predicted problematic mobile phone use. Lastly, age and gender were not predictors of problematic mobile phone use.     

Is invasion efficiency in malaria controlled by pre-invasion events?

The invasion efficiency of Plasmodium falciparum merozoites was found to decrease with increasing red blood cell density, a finding relevant to protection strategies against falciparum malaria. The mechanism of this 'density effect' remained unexplained. Searching for possible explanations, we studied selected video recordings of the dynamic events during merozoite invasion and identified a pre-invasion stage as a crucial mediator of invasion efficiency. We suggest that the role of the pre-invasion stage is to induce the apical alignment of the merozoite, and propose a working hypothesis on its mechanism, with a crucial role for elevated intracellular Ca2+.     

Death by transposition – the enemy within?

Here we present and develop the hypothesis that the derepression of endogenous retrotransposable elements (RTEs) – “genomic parasites” – is an important and hitherto under‐unexplored molecular aging process that can potentially occur in most tissues. We further envision that the activation and continued presence of retrotransposition contribute to age‐associated tissue degeneration and pathology. Chromatin is a complex and dynamic structure that needs to be maintained in a functional state throughout our lifetime. Studies of diverse species have revealed that chromatin undergoes extensive rearrangements during aging. Cellular senescence, an important component of mammalian aging, has recently been associated with decreased heterochromatinization of normally silenced regions of the genome. These changes lead to the expression of RTEs, culminating in their transposition. RTEs are common in all kingdoms of life, and comprise close to 50% of mammalian genomes. They are tightly controlled, as their activity is highly destabilizing and mutagenic to their resident genomes.     

Is Toxoplasma egress the first step in invasion?

The protozoan parasite Toxoplasma gondii maintains an intracellular lifestyle that requires careful timing and coordination when exiting one cell (egress) and entering another (invasion). Here it is argued that T. gondii uses similar molecular mechanisms for egress and invasion, based on common morphology, dependence on motility, and regulation by a calcium-dependent signal transduction pathway. In our view, this strategy is highly advantageous because it allows the parasite to egress rapidly from one cell and immediately invade an adjacent cell, thereby minimizing exposure to the extracellular environment where it could be destroyed by host immune mediators.     

Revisiting synchronous gamete release by fucoid algae in the intertidal zone: fertilization success and beyond?

In the marine environment, both external fertilization and settlementare critical processes linking adult and early juvenile life-historyphases. The success of both processes can be tightly linkedin organisms lacking a larval dispersive phase. This reviewfocuses on synchronous gamete release (= spawning) in fucoidalgae. These brown macroalgae are important components of temperateintertidal ecosystems in many parts of the world, and achievesynchronous gamete release by integrating various environmentalsignals. Photosynthesis-dependent sensing of boundary-layerinorganic carbon fluxes, as well as blue light and green lightsignals, possibly perceived via a chloroplast-located photoreceptor(s),are integrated into pathways that restrict gamete release toperiods of low water motion. Avoidance of turbulent and/or highflow conditions in the intertidal zone allows high levels offertilization success in this group. Temporal patterns and synchronyof spawning in natural populations are reviewed. Most species/populationshave a more or less semilunar periodicity, although phase differencesoccur both between and within species at different geographicallocations, raising the possibility that tidal and diurnal cuesare more important than semilunar cues in entraining the response.The ecological and evolutionary role(s) of synchronous spawningin the intertidal zone are considered, particularly with regardto hybridization/reproductive isolation in species complexes,and reproductive versus recruitment assurance in the intertidalzone, where synchronous spawning during calm periods may beimportant for recruitment assurance in addition to fertilizationsuccess. Ways in which the roles of spawning synchrony couldbe tested in closely related species with contrasting matingsystems (outcrossing versus selfing) are discussed.     

Can flower constancy in nectaring butterflies be explained by Darwin’s interference hypothesis?

 When foraging for nectar many insects exhibit flower constancy (a preference for flower species which they have previously visited) and frequently ignore rewarding flowers of other species. Darwin proposed the favoured explanation for this behaviour, hypothesizing that learning of handling skills for one flower species interferes with the ability to recall handling skills for previously learned species. A crucial element of this hypothesis is that savings in handling time resulting from constancy must exceed increases in travelling time necessitated by ignoring other suitable species. A convincing quantification of this trade-off has not been achieved and tests to date on bumblebees indicate that savings in handling time are too small to offset an increase in travelling time. To assess further the validity of Darwin’s hypothesis, handling and flight times of the butterfly, Thymelicus flavus, were measured under natural conditions, and the abundance and reward provided by the available flower species quantified to enable estimation of foraging efficiency. Butterflies exhibited a mean increase in handling time of 0.85 s per flower associated with switching between flower species, although the magnitude of this difference varied greatly among flower species. Switching was not associated with a decrease in travelling time, contrary to expectation. Switching was more frequent following a lower than average reward from the last flower visited. In butterflies, flights serve functions other than movement between nectar sources, such as mate location (unlike worker bees). Hence constancy may be a viable strategy to reduce time spent in handling flowers and increase time available for other activities. Although savings in handling time may be small, Darwin’s interference hypothesis remains a valid explanation for flower constancy in foraging butterflies. Received: 27 January 1997 / Accepted: 5 June 1997     

Can transgenerational plasticity contribute to the invasion success of annual plant species?

Adaptive transgenerational plasticity (TGP), i.e., significantly higher fitness when maternal and offspring conditions match, might contribute to the population growth of non-native species in highly variable environments. However, comparative studies that directly test this hypothesis are lacking. Therefore, we performed a reciprocal split-brood experiment to compare TGP in response to N and water availability in single populations of two invasive (Amaranthus retroflexus, Galinsoga parviflora) and two congeneric non-invasive introduced species (Amaranthus albus, Galinsoga ciliata). We hypothesized that the transgenerational effect is adaptive: (1) in invasive species compared with non-invasive adventives, and (2) in stressful conditions compared with resource-rich environments. The phenotypic variation among offspring was generated, in large part, by our experimental treatments in the maternal generation; therefore, we demonstrated a direct TGP effect on the offspring’s adult fitness. We found evidence, for the first time, that invasive and non-invasive adventive species differ regarding the expression of TGP in the adult stage, as adaptive responses were found exclusively in the invasive species. The manifestation of TGP was more explicit under resource-rich conditions; therefore, it might contribute to the population dynamics of non-native species in resource-rich sites rather than to their ecological tolerance spectra.     

Are there any consistent predictors of invasion success?

This article summarises the results of 49 studies that together test the significance of 115 characteristics in 7 biological groups: birds, finfish, insects, mammals, plants, reptiles/amphibians and shellfish. Climate/habitat match, history of invasive success and number of arriving/released individuals are associated with establishment success in at least four independent data sets, both within and across biological groups, and none are contraindicated by other studies. In the introduced-invasive control group, two species level characteristics—taxon and geographic range size—were significantly associated with establishment success across two biological groups. These characteristics, however, were not supported by independent data sets, or were contraindicated by these data sets, within the biological groups examined here. In the introduced-native control group, three species level characteristics—geographic range size, leaf surface area and fertilisation system (monoecious, hermaphroditic or dioecious)—were consistently supported within plants but were either not supported by independent data sets or contraindicated by datasets within or across other biological groups. Climate/habitat match is the only characteristic that is consistently significantly associated with invasive behaviour (in this case exotic range size) across biological groups. This finding, however, is not supported by two or more independent data sets within any of the biological groups examined here. Within plants there are a suite of characteristics, predominately associated with reproduction, that are significantly associated with a range of invasion metrics, predominately abundance in the invaded range. None of these characteristics, however, are supported across any other biological groups. We note the confounding effects of phylogeny, residence time and propagule pressure and suggest that site- and taxa-specific analysis will provide further useful insights.     

Is the increased vigour of invasive weeds explained by a trade-off between growth and herbivore resistance?

Blossey and Nötzold (1995) recently hypothesised that the increased vigour of certain invasive plant species has been at the expense of defences against natural enemies. A prediction of their evolution of increased competitive ability (EICA) hypothesis is that invasive genotypes are relatively poorly defended. We tested this prediction with herbivore bioassays and with direct quantification of plant secondary metabolites comparing non-indigenous genotypes of Lythrum salicaria L. (purple loosestrife) with indigenous forms. The herbivore bioassays revealed no significant intra-specific variation in herbivore resistance between indigenous and non-indigenous hosts. The phenolic content of L. salicaria leaves was significantly higher in indigenous genotypes, as predicted by the EICA hypothesis. The average phenolic content of leaves (regardless of their origin) was, however, low, implying that the role of plant phenolics in purple loosestrife anti-herbivore defence is probably limited. It is suggested that the EICA hypothesis, as tested in the current study, does not explain the increased vigour of L. salicaria in non-indigenous habitats.     

Is cell sorting caused by differences in the work of intercellular adhesion? A critique of the steinberg hypothesis

The differential adhesion hypothesis, developed by Malcolm Steinberg, proposes that the histotypic sorting out behavior of aggregated cells is mechanistically equivalent to certain aspects of liquid surface tension, specifically the spontaneous separation of immiscible liquids of differing surface tension. According to Steinberg's hypothesis, the adhesive forces between aggregated cells play essentially the same role in cell sorting as are played by intermolecular attractive forces in liquid surface tension.In this paper I discuss a number of crucial distinctions between intermolecular attraction (in liquids) and intercellular adhesion (in aggregates). First, liquid drops are closed systems thermodynamically whereas aggregates of living cells can generate an indeterminate amount of metabolic energy capable of altering cell positions and adhesions. Secondly, intercellular adhesions are more than just close range attractions since cells can be held together by forces in addition to those which originally pulled them together. Third, the breakage of intercellular adhesions need not be simply the reverse, thermodynamically, of the formation of those adhesions. And fourthly, because intercellular adhesion is generally concentrated at relatively small foci such as desmosomes, a maximization of intercellular adhesion does not necessarily require a maximization of intercellular contact area, or vice versa.In addition, several alternative hypotheses are proposed, each of which is theoretically capable of explaining cell sorting and the other surface tension-like aspects of cell aggregate behavior which Steinberg has sought to explain as consequences of differential adhesion. In particular, a differential surface contraction hypothesis is proposed, according to which cell sorting and related phenomena are the results of cell surface contractions induced to occur over those portions of the cell surface which are exposed to the surrounding culture medium. Because of the evidence that various invagination type movements of embryonic epithelia are caused by cell surface contractions, it is suggested that differential surface contraction is the most likely explanation of histotypic cell sorting. A number of experiments are suggested by which these various hypotheses might be tested.     

Does time since introduction influence enemy release of an invasive weed?

Release from natural enemies is considered to potentially play an important role in the initial establishment and success of introduced plants. With time, the species richness of herbivores using non-native plants may increase [species-time relationship (STR)]. We investigated whether enemy release may be limited to the early stages of invasion. Substituting space for time, we sampled invertebrates and measured leaf damage on the invasive species Senecio madagascariensis Poir. at multiple sites, north and south of the introduction site. Invertebrate communities were collected from plants in the field, and reared from collected plant tissue. We also sampled invertebrates and damage on the native congener Senecio pinnatifolius var. pinnatifolius A. Rich. This species served as a control to account for environmental factors that may vary along the latitudinal gradient and as a comparison for evaluating the enemy release hypothesis (ERH). In contrast to predictions of the ERH, greater damage and herbivore abundances and richness were found on the introduced species S. madagascariensis than on the native S. pinnatifolius. Supporting the STR, total invertebrates (including herbivores) decreased in abundance, richness and Shannon diversity from the point of introduction to the invasion fronts of S. madagascariensis. Leaf damage showed the opposite trend, with highest damage levels at the invasion fronts. Reared herbivore loads (as opposed to external collections) were greater on the invader at the point of introduction than on sites further from this region. These results suggest there is a complex relationship between the invader and invertebrate community response over time. S. madagascariensis may be undergoing rapid changes at its invasion fronts in response to environmental and herbivore pressure.     

Is vessel hull fouling an invasion threat to the Great Lakes?

Aim  Hull fouling is a leading vector for the introduction of marine, non‐indigenous species (NIS) worldwide, yet its importance to freshwater habitats is poorly understood. We aimed to establish the complement of NIS transported via this vector to the Great Lakes and to determine if they pose an invasion risk. Location  Laurentian Great Lakes. Methods  During 2007 and 2008, we collected scrapings from exterior surfaces as well as underwater video‐transects from 20 vessels shortly after their arrival in Great Lakes’ ports. Invertebrates present were sorted and identified in the laboratory. Results  Total estimated abundance averaged > 170,000 invertebrates per ship belonging to 109 taxa. Most (72%) of these taxa were freshwater species already present in the Great Lakes, whereas 11 and 31% were native to estuarine and marine habitats respectively, and would not be expected to survive in this habitat. Abundance was dominated by barnacles (51%), cladocerans (19%), bivalves (12%) and amphipods (11%). Sea‐chest grating and the rudder were hot‐spots for biofouling. Invertebrate diversity and total abundance were positively associated with total time spent in port during the last year and time in Pacific South American ports and negatively related to time in high latitudes and sailing speed. Although we found some live, established invaders such as Gammarus tigrinus and Dreissena rostriformis bugensis, only one individual of a freshwater NIS (Alexandrovia onegensis, Oligochaeta) not yet reported in the Great Lakes was detected. The animal’s poor condition and seemingly low population abundance indicated the risk of live introduction by this vector was likely quite low. Main conclusion  Our results indicate that hull fouling appears to pose a low risk of introductions of new species capable of surviving in the Great Lakes, unlike foreign‐sourced freshwater ballast water that historically was discharged by these transoceanic vessels.     

Is the firefly flash regulated by calcium?

The very different courtship flashes of Photuris versicolorand Photuris lucicrescens males mirror the pattern of neuralimpulses produced by their brain. Their lanterns luminescencevery differently, however, in response to direct, electricalstimulation. Whereas P. lucicrescens lanterns glow in responseto high frequency, continuous electrical stimulation, thoseof P. versicolor produce only rapid, triple-pulsed flashlettesthat resemble, but are not identical to, their courtship flashes.In addition, the exposed lantern tissue of P. versicolor males,when immersed in firefly saline high in potassium and calciumions, scintillates with hundreds of photocytes flashing in randomfashion. P. lucicrescens male lanterns, so treated, only glow.Tests of P. versicolor lanterns with salines of different compositionsuggest that calcium ions are essential in producing this intense,long lasting scintillation response and are therefore possiblyimplicated in the final stages of flash control in this species.     

Is the in-frame termination signal of the Escherichia coli release factor-2 frameshift site weakened by a particularly poor context?

The synthesis of release factor-2 (RF-2) in bacteria is regulated by a high efficiency +1 frameshifting event at an in-frame UGA stop codon. The stop codon does not specify the termination of synthesis efficiently because of several upstream stimulators for frameshifting. This study focusses on whether the particular context of the stop codon within the frameshift site of the Escherichia coli RF-2 mRNA contributes to the poor efficiency of termination. The context of UGA in this recoding site is rare at natural termination sites in E.coli genes. We have evaluated how the three nucleotides downstream from the stop codon (+4, +5 and +6 positions) in the native UGACUA sequence affect the competitiveness of the termination codon against the frameshifting event. Changing the C in the +4 position and, separately, the A in the +6 position significantly increase the termination signal strength at the frameshift site, whereas the nucleotide in the +5 position had little influence. The efficiency of particular termination signals as a function of the +4 or +6 nucleotides correlates with how often they occur at natural termination sites in E.coli; strong signals occur more frequently and weak signals are less common.