Sven Bacher

Disentangling the role of environmental and human pressures on biological invasions across Europe

The accelerating rates of international trade, travel, and transport in the latter half of the twentieth century have led to the progressive mixing of biota from across the world and the number of species introduced to new regions continues to increase. The importance of biogeographic, climatic, economic, and demographic factors as drivers of this trend is increasingly being realized but as yet there is no consensus regarding their relative importance. Whereas little may be done to mitigate the effects of geography and climate on invasions, a wider range of options may exist to moderate the impacts of economic and demographic drivers. Here we use the most recent data available from Europe to partition between macroecological, economic, and demographic variables the variation in alien species richness of bryophytes, fungi, vascular plants, terrestrial insects, aquatic invertebrates, fish, amphibians, reptiles, birds, and mammals. Only national wealth and human population density were statistically significant predictors in the majority of models when analyzed jointly with climate, geography, and land cover. The economic and demographic variables reflect the intensity of human activities and integrate the effect of factors that directly determine the outcome of invasion such as propagule pressure, pathways of introduction, eutrophication, and the intensity of anthropogenic disturbance. The strong influence of economic and demographic variables on the levels of invasion by alien species demonstrates that future solutions to the problem of biological invasions at a national scale lie in mitigating the negative environmental consequences of human activities that generate wealth and by promoting more sustainable population growth.

A unified classification of alien species based on the magnitude of their environmental impacts.

We present a method for categorising and comparing alien or invasive species in terms of how damaging they are to the environment, that can be applied across all taxa, scales, and impact metrics.

Brain Size Predicts the Success of Mammal Species Introduced into Novel Environments

Large brains, relative to body size, can confer advantages to individuals in the form of behavioral flexibility. Such enhanced behavioral flexibility is predicted to carry fitness benefits to individuals facing novel or altered environmental conditions, a theory known as the brain size–environmental change hypothesis. Here, we provide the first empirical link between brain size and survival in novel environments in mammals, the largest‐brained animals on Earth. Using a global database documenting the outcome of more than 400 introduction events, we show that mammal species with larger brains, relative to their body mass, tend to be more successful than species with smaller brains at establishing themselves when introduced to novel environments, when both taxonomic and regional autocorrelations are accounted for. This finding is robust to the effect of other factors known to influence establishment success, including introduction effort and habitat generalism. Our results replicate similar findings in birds, increasing the generality of evidence for the idea that enlarged brains can provide a survival advantage in novel environments.

No saturation in the accumulation of alien species worldwide

Although research on human-mediated exchanges of species has substantially intensified during the last centuries, we know surprisingly little about temporal dynamics of alien species accumulations across regions and taxa. Using a novel database of 45,813 first records of 16,926 established alien species, we show that the annual rate of first records worldwide has increased during the last 200 years, with 37% of all first records reported most recently (1970–2014). Inter-continental and inter-taxonomic variation can be largely attributed to the diaspora of European settlers in the nineteenth century and to the acceleration in trade in the twentieth century. For all taxonomic groups, the increase in numbers of alien species does not show any sign of saturation and most taxa even show increases in the rate of first records over time. This highlights that past efforts to mitigate invasions have not been effective enough to keep up with increasing globalization.

When are eradication campaigns successful? A test of common assumptions

Eradication aims at eliminating populations of alien organisms from an area. Since not all eradications are successful, several factors have been proposed in the literature (mainly by referring to case studies) to be crucial for eradication success, such as infestation size or reaction time. To our knowledge, however, no study has statistically evaluated which factors affect eradication success and attempted to determine their relative importance. We established a unique global dataset on 136 eradication campaigns against 75 species (invasive alien invertebrates, plants and plant pathogens) and statistically tested whether the following factors, proposed by others were significantly related to eradication success: (1) the reaction time between the arrival/detection of the organism and the start of the eradication campaign; (2) the spatial extent of the infestation; (3) the level of biological knowledge of the organism; and (4) insularity. Of these, only the spatial extent of the infestation was significantly related to the eradication outcome: local campaigns were more successful than regional or national campaigns. Reaction time, the level of knowledge and insularity were all unrelated to eradication success. Hence, some factors suggested as being crucial may be less important than previously thought, at least for the organisms tested here. We found no differences in success rates among taxonomic groups or geographic regions. We recommend that eradication measures should generally concentrate on the very early phase of invasions when infestations are still relatively small.

Mutualistic interaction between a weevil and a rust fungus, two parasites of the weed Cirsium arvense.

We present a mutualism between a stem-boring weevil, Apion onopordi Kirby (Coleoptera: Apionidae), and a rust fungus, Puccinia punctiformis (Str.) Röhl. (Uredinales), both parasites of the creeping thistle, Cirsium arvense (L.) Scop. (Asteraceae). Females, but not males, of A. onopordi induced systemic rust infections of thistle shoots in the season after they were attacked by the weevil, indicating that insect oviposition is a crucial stage in pathogen transmission. Adult weevils emerged from systemically infected thistle shoots were heavier than weevils from healthy C. arvense shoots. Heavier females had a higher fecundity and laid larger eggs. The weevil preferred to deposit eggs in systemically rust-infected over healthy thistle shoots, which seemed to be a sub-optimal host. This is to our knowledge the first report of a mutualistic interaction between an herbivorous insect and a biotrophic plant pathogen. The mechanism responsible for the advantage of rust-infected shoots for A. onopordi causes a different outcome in other thistle herbivores, and therefore can not be explained by a general enhancement of nutritional quality in rust-infected tissue. This mutualism likely has evolved from a competitive relationship. Unlike other thistle herbivores A. onopordi seems to be better suited as mutualist for P. punctiformis because of its small impact on the host plant and its feeding niche on plant parts not directly associated with pathogen reproduction.

Effects of defoliation by horse chestnut leafminer ( Cameraria ohridella ) on reproduction in Aesculus hippocastanum

In large parts of Europe horse chestnut trees (Aesculus hippocastanum) suffer from severe defoliation by an alien invasive species, the specialist leaf mining moth Cameraria ohridella (Lepidoptera; Gracillariidae). Heavily infested trees have a drastically shortened period for photosynthesis. We quantified the effect of moth infestation on reproduction of horse chestnut trees in two different cities in central Europe, Bern and Munich. C. ohridella negatively affected seed and fruit weight of A. hippocastanum at both locations. In Munich, seed weight of heavily damaged trees was reduced by almost half. However, the number of seeds per fruit, the number of fruits per inflorescence, and the number of inflorescences per tree did not change due to herbivory. We conclude that C. ohridella mining affects seed quality but not seed quantity. The reduced seed weight may severely impair growth and survival of horse chestnut seedlings and thus may endanger the long-term persistence of A. hippocastanum in its endemic forests in south-east Europe.

Gaps in border controls are related to quarantine alien insect invasions in Europe

Alien insects are increasingly being dispersed around the world through international trade, causing a multitude of negative environmental impacts and billions of dollars in economic losses annually. Border controls form the last line of defense against invasions, whereby inspectors aim to intercept and stop consignments that are contaminated with harmful alien insects. In Europe, member states depend on one another to prevent insect introductions by operating a first point of entry rule – controlling goods only when they initially enter the continent. However, ensuring consistency between border control points is difficult because there exists no optimal inspection strategy. For the first time, we developed a method to quantify the volume of agricultural trade that should be inspected for quarantine insects at border control points in Europe, based on global agricultural trade of over 100 million distinct origin-commodity-species-destination pathways. This metric was then used to evaluate the performance of existing border controls, as measured by border interception results in Europe between 2003 and 2007. Alarmingly, we found significant gaps between the trade pathways that should be inspected and actual number of interceptions. Moreover, many of the most likely introduction pathways yielded none or very few insect interceptions, because regular interceptions are only made on only a narrow range of pathways. European countries with gaps in border controls have been invaded by higher numbers of quarantine alien insect species, indicating the importance of proper inspections to prevent insect invasions. Equipped with an optimal inspection strategy based on the underlying risks of trade, authorities globally will be able to implement more effective and consistent border controls.

Comparing impacts of alien plants and animals in Europe using a standard scoring system

Summary 1. Alien species can change the recipient environment in various ways, and some of them cause considerable damage. Understanding such impacts is crucial to direct management actions. This study addresses the following questions: Is it possible to quantify impact across higher taxa in a comparative manner? Do impacts differ between taxonomic groups? How are environmental and socio-economic impacts related? Can impacts be predicted based on those in other regions? 2. To address these questions, we reviewed literature describing the impacts of 300 species from five major taxonomic groups: mammals, birds, fish, terrestrial arthropods and plants. To make very diverse impact measures comparable, we used the semi-quantitative generic impact scoring system (GISS) which describes environmental and socio-economic impacts using twelve categories. In each category, scores range from zero (no impact known or detectable) to five (the highest possible impact). 3. Using the same scoring system for taxa as diverse as invertebrates, vertebrates and plants, we found that overall, alien mammals in Europe have the highest impact, while fish have the lowest. Terrestrial arthropods were found to have the lowest environmental impact, while fish had relatively low socio-economic impact. 4. Overall, the magnitude of environmental and socio-economic impacts of individual alien species is highly correlated. However, at the species level, major deviations are found. 5. For mammals and birds, the impacts in invaded ranges outside of Europe are broadly similar to those recorded for alien species within Europe, indicating that a consideration of the known impacts of a species in other regions can be generally useful when predicting the impacts of an alien species. However, it should be noted that this pattern is not consistent across all mammal and bird orders, and thus, such information should be considered with caution. 6. Synthesis and applications. Comparing the impacts of alien species across taxa is necessary for prioritizing management efforts and effective allocation of resources. By applying the generic impact scoring system (GISS) to five major taxonomic groups, we provide the basis for a semiquantitative cross-taxa listing process (e.g. ‘black lists’ or 100-worst-lists). If more data are collated from different geographical regions and habitats using standard GISS protocols, risk assessments for alien species based on rigorous measures of impact could be improved by taking into account local variation, and context dependence of impacts. This would also allow studies at lower taxonomic levels, and within-taxon analyses of functional groups and guilds.

Which factors affect the success or failure of eradication campaigns against alien species

Although issues related to the management of invasive alien species are receiving increasing attention, little is known about which factors affect the likelihood of success of management measures. We applied two data mining techniques, classification trees and boosted trees, to identify factors that relate to the success of management campaigns aimed at eradicating invasive alien invertebrates, plants and plant pathogens. We assembled a dataset of 173 different eradication campaigns against 94 species worldwide, about a half of which (50.9%) were successful. Eradications in man-made habitats, greenhouses in particular, were more likely to succeed than those in (semi-)natural habitats. In man-made habitats the probability of success was generally high in Australasia, while in Europe and the Americas it was higher for local infestations that are easier to deal with, and for international campaigns that are likely to profit from cross-border cooperation. In (semi-) natural habitats, eradication campaigns were more likely to succeed for plants introduced as an ornamental and escaped from cultivation prior to invasion. Averaging out all other factors in boosted trees, pathogens, bacteria and viruses were most, and fungi the least likely to be eradicated; for plants and invertebrates the probability was intermediate. Our analysis indicates that initiating the campaign before the extent of infestation reaches the critical threshold, starting to eradicate within the first four years since the problem has been noticed, paying special attention to species introduced by the cultivation pathway, and applying sanitary measures can substantially increase the probability of eradication success. Our investigations also revealed that information on socioeconomic factors, which are often considered to be crucial for eradication success, is rarely available, and thus their relative importance cannot be evaluated. Future campaigns should carefully document socioeconomic factors to enable tests of their importance.