What are ecological mechanisms? Suggestions for a fine-grained description of causal mechanisms in invasion ecology

Invasion ecology addresses the spread of species outside of their native ranges. A central aim of this field is to find mechanistic explanations for why species are able to establish and spread in an area in which they did not evolve. Usually it remains unclear, however, what exactly is meant by ‘mechanistic explanation’ or ‘mechanism’. The paper argues that the field can benefit from the philosophical discussion of what a mechanism is. Based on conceptions of mechanisms as processes in concrete systems, causal mechanisms can be defined as one type of mechanism, representing recurring networks of causal relationships. With the example of a well-known hypothesized mechanism in invasion ecology, namely enemy release, the paper demonstrates how such causal mechanisms can be depicted as causal network diagrams. This approach could facilitate the development of step-by-step explanations, enhance clear argumentation and allow for more precise linkage of empirical tests to theory. Challenges to assessing the empirical relevance of hypothesized mechanisms are discussed, and suggestions are made concerning how the proposed approach could help in overcoming some of them

The role of eco-evolutionary experience in invasion success

Invasion ecology has made considerable progress in identifying specific mechanisms that potentially determine success and failure of biological invasions. Increasingly, efforts are being made to interrelate or even synthesize the growing number of hypotheses in order to gain a more comprehensive and integrative understanding of invasions. We argue that adopting an eco-evolutionary perspective on invasions is a promising approach to achieve such integration. It emphasizes the evolutionary antecedents of invasions, i.e. the species’ evolutionary legacy and its role in shaping novel biotic interactions that arise due to invasions. We present a conceptual framework consisting of five hypothetical scenarios about the influence of so-called ‘eco-evolutionary experience’ in resident native and invading non-native species on invasion success, depending on the type of ecological interaction (predation, competition, mutualism, and commensalism). We show that several major ecological invasion hypotheses, including ‘enemy release’, ‘EICA’, ‘novel weapons’, ‘naive prey’, ‘new associations’, ‘missed mutualisms’ and ‘Darwin’s naturalization hypothesis’ can be integrated into this framework by uncovering their shared implicit reference to the concept of eco-evolutionary experience. We draft a routine for the assessment of eco-evolutionary experience in native and non-native species using a food web-based example and propose two indices (xpFocal index and xpResidents index) for the actual quantification of eco-evolutionary experience. Our study emphasizes the explanatory potential of an eco-evolutionary perspective on biological invasions

Species from different taxonomic groups show similar invasion traits

Invasion ecology tends to treat taxonomic groups separately. However, given that all invasive species go through the same stages of the invasion process (transport, escape, establishment, spread), it is likely that – across taxa – comparable traits help to successfully complete this process ("invasion traits"). Perhaps not all invasive species have the same invasion traits, but different combinations of invasion traits can be found among invaders, corresponding to different possibilities to become a successful invader. These combinations of invasion traits might be linked to taxonomic affiliation, but this is not necessarily the case. We created a global dataset with 201 invasive species from seven major taxonomic groups (animals, green plants, fungi, heterokonts, bacteria, red algae, alveolates) and 13 invasion traits that are applicable across all taxa. The dataset was analysed with cluster analysis to search for similarities in combinations of invasion traits. Three of the five clusters, comprising 60% of all species, contain several major taxonomic groups. While some invasion trait frequencies were significantly related to taxonomic affiliation, the results show that invasive species from different taxonomic groups often share similar combinations of invasion traits. A post-hoc analysis suggests that combinations of traits characterizing successful invaders can be associated with invasion stages across taxa. Our findings suggest that there are no universal invasion traits which could explain the invasion success of all invaders, but that invaders are successful for different reasons which are represented by different combinations of invasion traits across taxonomic groups

What are ecological mechanisms? Suggestions for a fine-grained description of causal mechanisms in invasion ecology

Invasion ecology addresses the spread of species outside of their native ranges. A central aim of this field is to find mechanistic explanations for why species are able to establish and spread in an area in which they did not evolve. Usually it remains unclear, however, what exactly is meant by ‘mechanistic explanation’ or ‘mechanism’. The paper argues that the field can benefit from the philosophical discussion of what a mechanism is. Based on conceptions of mechanisms as processes in concrete systems, causal mechanisms can be defined as one type of mechanism, representing recurring networks of causal relationships. With the example of a well-known hypothesized mechanism in invasion ecology, namely enemy release, the paper demonstrates how such causal mechanisms can be depicted as causal network diagrams. This approach could facilitate the development of step-by-step explanations, enhance clear argumentation and allow for more precise linkage of empirical tests to theory. Challenges to assessing the empirical relevance of hypothesized mechanisms are discussed, and suggestions are made concerning how the proposed approach could help in overcoming some of them

Parallel functional differentiation of an invasive annual plant on two continents.

Rapid local adaptation frequently occurs during the spread of invading species. It remains unclear, however, how consistent, and therefore potentially predictable, such patterns of local adaptation are. One approach to this question is to measure patterns of local differentiation in functional traits and plasticity levels in invasive species in multiple regions. Finding consistent patterns of local differentiation in replicate regions suggests that these patterns are adaptive. Further, this outcome indicates that the invading species likely responds predictably to selection along environmental gradients, even though standing genetic variation is likely to have been reduced during introduction. We studied local differentiation in the invasive annual plant Erodium cicutarium in two invaded regions, California and Chile. We collected seeds from across strong gradients in precipitation and temperature in Mediterranean-climate parts of the two regions (10 populations per region). We grew seeds from maternal families from these populations through two generations and exposed the second generation to contrasting levels of water and nutrient availability. We measured growth, flowering time and leaf functional traits across these treatments to obtain trait means and plasticity measures. We found strong differentiation among populations in all traits. Plants from drier environments flowered earlier, were less plastic in flowering time and reached greater size in all treatments. Correlations among traits within regions suggested a coordinated evolutionary response along environmental gradients associated with growing season length. There was little divergence in traits and trait intercorrelations between regions, but strongly parallel divergence in traits within regions. Similar, statistically consistent patterns of local trait differentiation across two regions suggest that local adaptation to environmental gradients has aided the spread of this invasive species, and that the formation of ecotypes in newly invaded environments has been relatively consistent and predictable

Zukunftsbild Langsam – Naturerleben

Naturzerstörung, einseitige Monokulturlandschaften, stark überfüllte Urlaubsgegenden und immer unattraktivere Städte führen zu einem Mangel an erfüllender Naturbegegnung und zu einer Flucht in virtuelle Welten. – Projekt Zukunftsbilder: Wissenschaftler:innen stellen auf Basis aktueller Forschungsergebnisse und Studien dar, wie eine nachhaltige Welt aussehen könnte. Dieser Text beschreibt eine von über 40 Facetten des Lebens und Wirtschaftens in einem von vier Zukunftsbildern. Weitere Texte finden sich in der Zenodo-Community des Projekts ‘Zukunftsbilder’. Neben einem Weiter-so-Bild (Titel: „Langsam“) werden drei verschiedene positive Zukünfte geschildert (Titel: „Groß“, „Fokussiert“ und „Graswurzel“), die soziale Ziele erreichen und planetare Grenzen (zum Beispiel die Pariser Klimaziele oder Biodiversitätsziele) einhalten. Unsere Texte sind keine formellen wissenschaftlichen Zukunftsszenarien, sondern Beschreibungen von möglichen Zukünften. Sie wurden gemeinsam von Wissenschaftler:innen und Menschen aus sozialen Bewegungen erarbeitet und von Wissenschaftler:innen begutachtet. Die Texte stellen die Sichtweise der Autor:innen dar und sind nicht innerhalb aller beteiligten Organisationen abgestimmt. Das Projekt dient dazu, eine neue, konstruktive Diskussion um unsere Zukunft mit möglichst vielen Menschen anzuregen. Mehr Informationen unter www.zukunftsbilder.net. Bitte die aktuellste Version beachten

Past competition affects offspring foliar terpenoid concentrations, seed traits, and fitness in the invasive forb Erodium cicutarium (Geraniaceae)

Eilers E, Heger T. Past competition affects offspring foliar terpenoid concentrations, seed traits, and fitness in the invasive forb Erodium cicutarium (Geraniaceae). Frontiers in Ecology and Evolution. 2019;7: 392.(1) Environmental conditions experienced in the past may lead to intraspecific differences in ecological and chemical traits of plants, which likely affect future responses to altered or new environments. Whether competition by neighbors is such a trait-shaping factor is not yet well-known. We aimed to understand how the level of ancestral plant competition affects traits related to plant fitness and resource allocation, reproduction, and (phyto-)toxin accumulation in offspring, and whether a potential differentiation in these traits can be found in different geographic origins of which one belongs to the native and one to the invaded range. (2) We compared differentiation of the following traits in offspring plants of multiple populations in Erodium cicutarium (Geraniaceae): biomass, seed production, seed traits related to dispersal and germination, and concentrations of foliar mono- and sesquiterpenes. We tested the allelopatic potential of aqueous extracts of the same E. cicutarium plants on seeds of five different plant families. (3) In plants originating from populations that experienced high levels of competition, we found twice as high monoterpene concentrations. These plants also produced more biomass and a higher proportion of ripe to unripe seeds until harvesting. Seeds originating from high competition sites were shorter. Aqueous E. cicutarium leaf extracts with high terpenoid content reduced radicle length of Zea mays and radicle and hypocotyl length of E. cicutarium seedlings. (4) The results of this study provide first evidence that the surrounding vegetation may shape chemo-ecological plant traits that may be fundamental for competitive ability. Our study calls for more research testing whether competition experienced in the native range may lead to an enhanced capability of plants to establish populations and spread in a new range

Zukunftsbild Graswurzel – Naturerleben

Wir schätzen Natur. Nicht nur weil sie uns nützt, sondern auch als Wert an sich. Auch Tiere haben Rechte. Wir leben, wirtschaften und entwickeln neue Technologien heutzutage immer mit Blick auf die Auswirkungen auf die Natur. – Projekt Zukunftsbilder: Wissenschaftler:innen stellen auf Basis aktueller Forschungsergebnisse und Studien dar, wie eine nachhaltige Welt aussehen könnte. Dieser Text beschreibt eine von über 40 Facetten des Lebens und Wirtschaftens in einem von vier Zukunftsbildern. Weitere Texte finden sich in der Zenodo-Community des Projekts ‘Zukunftsbilder’. Neben einem Weiter-so-Bild (Titel: „Langsam“) werden drei verschiedene positive Zukünfte geschildert (Titel: „Groß“, „Fokussiert“ und „Graswurzel“), die soziale Ziele erreichen und planetare Grenzen (zum Beispiel die Pariser Klimaziele oder Biodiversitätsziele) einhalten. Unsere Texte sind keine formellen wissenschaftlichen Zukunftsszenarien, sondern Beschreibungen von möglichen Zukünften. Sie wurden gemeinsam von Wissenschaftler:innen und Menschen aus sozialen Bewegungen erarbeitet und von Wissenschaftler:innen begutachtet. Die Texte stellen die Sichtweise der Autor:innen dar und sind nicht innerhalb aller beteiligten Organisationen abgestimmt. Das Projekt dient dazu, eine neue, konstruktive Diskussion um unsere Zukunft mit möglichst vielen Menschen anzuregen. Mehr Informationen unter www.zukunftsbilder.net. Bitte die aktuellste Version beachten

Germany’s first Action plan on the pathways of invasive alien species to prevent their unintentional introduction and spread

The increase in global trade and traffic networks contributes to the introduction and spread of invasive alien species, posing a threat to biodiversity. EU Regulation 1143/2014 addresses the prevention and management of invasive alien species and requires an action plan on the priority pathways of unintentional introduction and spread of invasive alien species by each member state. To this end, the first German action plan was developed for the German government in cooperation with relevant Ministries and authorities, scientists, administrative experts, stakeholder working groups and political as well as public deliberation processes. As a result, 14 priority pathways of unintentional introduction, escape or release and spread were identified, resulting in 24 targeted measures. Nineteen sectors will be involved in the implementation of these measures. Here we describe the development process and outcome of Germany’s first action plan. By this, we aim making the process public and transparent, which can facilitate a revision of the action plan required at least every six years, and trigger broader European as well as national cooperation in the future

Structuring evidence for invasional meltdown: broad support but with biases and gaps

Negative interactions have been suggested as a major barrier for species arriving in a new habitat. More recently, positive interactions drew attention from community assembly theory and invasion science. The invasional meltdown hypothesis (IMH) introduced the idea that positive interactions among non-native species could facilitate one another’s invasion, even increasing their impact upon the native community. Many studies have addressed IMH, but with contrasting results, reflecting various types of evidence on a multitude of scales. Here we use the hierarchy-of-hypotheses (HoH) approach to differentiate key aspects of IMH, organizing and linking empirical studies to sub-hypotheses of IMH. We also assess the level of empirical support for each sub-hypothesis based on the evidence reported in the studies. We identified 150 studies addressing IMH. The majority of studies support IMH, but the evidence comes from studies with different aims and questions. Supporting studies at the community or ecosystem level are currently rare. Evidence is scarce for marine habitats and vertebrates. Few sub-hypotheses are questioned by more than 50% of the evaluated studies, indicating that non-native species do not affect each other’s survival, growth, reproduction, abundance, density or biomass in reciprocal A ↔ B interactions. With the HoH for IMH presented here, we can monitor progress in empirical tests and evidences of IMH. For instance, more tests at the community and ecosystem level are needed, as these are necessary to address the core of this hypothesis