@article{BragaGomezAparicioHegeretal.2018,
  author    = {Braga, Raul Renno and Gomez-Aparicio, Lorena and Heger, Tina and Simoes Vitule, Jean Ricardo and Jeschke, Jonathan M.},
  title     = {Structuring evidence for invasional meltdown},
  series = {Biological invasions : unique international journal uniting scientists in the broad field of biological invasions},
  volume    = {20},
  journal   = {Biological invasions : unique international journal uniting scientists in the broad field of biological invasions},
  number    = {4},
  publisher = {Springer},
  address   = {Dordrecht},
  issn      = {1387-3547},
  doi       = {10.1007/s10530-017-1582-2},
  pages     = {923 -- 936},
  year      = {2018},
  abstract  = {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.},
  language  = {en}
}
@article{EilersHeger2019,
  author    = {Eilers, Elisabeth Johanna and Heger, Tina},
  title     = {Past Competition Affects Offspring Foliar Terpenoid Concentrations, Seed Traits, and Fitness in the Invasive Forb Erodium cicutarium (Geraniaceae)},
  series = {Frontiers in Ecology and Evolution},
  volume    = {7},
  journal   = {Frontiers in Ecology and Evolution},
  publisher = {Frontiers Research Foundation},
  address   = {Lausanne},
  issn      = {2296-701X},
  doi       = {10.3389/fevo.2019.00392},
  pages     = {12},
  year      = {2019},
  abstract  = {(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.},
  language  = {en}
}
@article{HegerBernardVerdierGessleretal.2019,
  author    = {Heger, Tina and Bernard-Verdier, Maud and Gessler, Arthur and Greenwood, Alex D. and Grossart, Hans-Peter and Hilker, Monika and Keinath, Silvia and Kowarik, Ingo and K{\"u}ffer, Christoph and Marquard, Elisabeth and Mueller, Johannes and Niemeier, Stephanie and Onandia, Gabriela and Petermann, Jana S. and Rillig, Matthias C. and Rodel, Mark-Oliver and Saul, Wolf-Christian and Schittko, Conrad and Tockner, Klement and Joshi, Jasmin Radha and Jeschke, Jonathan M.},
  title     = {Towards an Integrative, Eco-Evolutionary Understanding of Ecological Novelty: Studying and Communicating Interlinked Effects of Global Change},
  series = {Bioscience},
  volume    = {69},
  journal   = {Bioscience},
  number    = {11},
  publisher = {Oxford Univ. Press},
  address   = {Oxford},
  issn      = {0006-3568},
  doi       = {10.1093/biosci/biz095},
  pages     = {888 -- 899},
  year      = {2019},
  abstract  = {Global change has complex eco-evolutionary consequences for organisms and ecosystems, but related concepts (e.g., novel ecosystems) do not cover their full range. Here we propose an umbrella concept of "ecological novelty" comprising (1) a site-specific and (2) an organism-centered, eco-evolutionary perspective. Under this umbrella, complementary options for studying and communicating effects of global change on organisms, ecosystems, and landscapes can be included in a toolbox. This allows researchers to address ecological novelty from different perspectives, e.g., by defining it based on (a) categorical or continuous measures, (b) reference conditions related to sites or organisms, and (c) types of human activities. We suggest striving for a descriptive, non-normative usage of the term "ecological novelty" in science. Normative evaluations and decisions about conservation policies or management are important, but require additional societal processes and engagement with multiple stakeholders.},
  language  = {en}
}
@article{HegerNiklesJacobs2018,
  author    = {Heger, Tina and Nikles, Gabriele and Jacobs, Brooke S.},
  title     = {Differentiation in native as well as introduced ranges},
  series = {AoB PLANTS},
  volume    = {10},
  journal   = {AoB PLANTS},
  number    = {1},
  publisher = {Oxford Univ. Press},
  address   = {Oxford},
  issn      = {2041-2851},
  doi       = {10.1093/aobpla/ply009},
  pages     = {12},
  year      = {2018},
  abstract  = {Germination, a crucial phase in the life cycle of a plant, can be significantly influenced by competition and facilitation. The aim of this study was to test whether differences in cover of surrounding vegetation can lead to population differentiation in germination behaviour of an annual grassland species, and if so, whether such a differentiation can be found in the native as well as in the introduced range. We used maternal progeny of Erodium cicutarium previously propagated under uniform conditions that had been collected in multiple populations in the native and two introduced ranges, in populations representing extremes in terms of mean and variability of the cover of surrounding vegetation. In the first experiment, we tested the effect of germination temperature and mean cover at the source site on germination, and found interlinked effects of these factors. In seeds from one of the introduced ranges (California), we found indication for a 2-fold dormancy, hindering germination at high temperatures even if physical dormancy was broken and water was available. This behaviour was less strong in high cover populations, indicating cross-generational facilitating effects of dense vegetation. In the second experiment, we tested whether spatial variation in cover of surrounding vegetation has an effect on the proportion of dormant seeds. Contrary to our expectations, we found that across source regions, high variance in cover was associated with higher proportions of seeds germinating directly after storage. In all three regions, germination seemed to match the local environment in terms of climate and vegetation cover. We suggest that this is due to a combined effect of introduction of preadapted genotypes and local evolutionary processes.},
  language  = {en}
}
@misc{HegerNiklesJacobs2018,
  author    = {Heger, Tina and Nikles, Gabriele and Jacobs, Brooke S.},
  title     = {Differentiation in native as well as introduced ranges},
  series = {Postprints der Universit{\"a}t Potsdam : Mathematisch Naturwissenschaftliche Reihe},
  journal   = {Postprints der Universit{\"a}t Potsdam : Mathematisch Naturwissenschaftliche Reihe},
  number    = {650},
  issn      = {1866-8372},
  doi       = {10.25932/publishup-42464},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-424642},
  pages     = {12},
  year      = {2018},
  abstract  = {Germination, a crucial phase in the life cycle of a plant, can be significantly influenced by competition and facilitation. The aim of this study was to test whether differences in cover of surrounding vegetation can lead to population differentiation in germination behaviour of an annual grassland species, and if so, whether such a differentiation can be found in the native as well as in the introduced range. We used maternal progeny of Erodium cicutarium previously propagated under uniform conditions that had been collected in multiple populations in the native and two introduced ranges, in populations representing extremes in terms of mean and variability of the cover of surrounding vegetation. In the first experiment, we tested the effect of germination temperature and mean cover at the source site on germination, and found interlinked effects of these factors. In seeds from one of the introduced ranges (California), we found indication for a 2-fold dormancy, hindering germination at high temperatures even if physical dormancy was broken and water was available. This behaviour was less strong in high cover populations, indicating cross-generational facilitating effects of dense vegetation. In the second experiment, we tested whether spatial variation in cover of surrounding vegetation has an effect on the proportion of dormant seeds. Contrary to our expectations, we found that across source regions, high variance in cover was associated with higher proportions of seeds germinating directly after storage. In all three regions, germination seemed to match the local environment in terms of climate and vegetation cover. We suggest that this is due to a combined effect of introduction of preadapted genotypes and local evolutionary processes.},
  language  = {en}
}
@misc{HiggsHarrisHegeretal.2018,
  author    = {Higgs, Eric S. and Harris, Jim A. and Heger, Tina and Hobbs, Richard J. and Murphy, Stephen D. and Suding, Katharine N.},
  title     = {Keep ecological restoration open and flexible},
  series = {Nature Ecology \& Evolution},
  volume    = {2},
  journal   = {Nature Ecology \& Evolution},
  number    = {4},
  publisher = {Nature Publ. Group},
  address   = {London},
  issn      = {2397-334X},
  doi       = {10.1038/s41559-018-0483-9},
  pages     = {580 -- 580},
  year      = {2018},
  language  = {en}
}
@misc{JaricHegerMonzonetal.2019,
  author    = {Jaric, Ivan and Heger, Tina and Monzon, Federico Castro and Jeschke, Jonathan M. and Kowarik, Ingo and McConkey, Kim R. and Pysek, Petr and Sagouis, Alban and Essl, Franz},
  title     = {Crypticity in Biological Invasions},
  series = {Trends in Ecology \& Evolution},
  volume    = {34},
  journal   = {Trends in Ecology \& Evolution},
  number    = {4},
  publisher = {Elsevier},
  address   = {London},
  issn      = {0169-5347},
  doi       = {10.1016/j.tree.2018.12.008},
  pages     = {291 -- 302},
  year      = {2019},
  abstract  = {Ecological effects of alien species can be dramatic, but management and prevention of negative impacts are often hindered by crypticity of the species or their ecological functions. Ecological functions can change dramatically over time, or manifest after long periods of an innocuous presence. Such cryptic processes may lead to an underestimation of long-term impacts and constrain management effectiveness. Here, we present a conceptual framework of crypticity in biological invasions. We identify the underlying mechanisms, provide evidence of their importance, and illustrate this phenomenon with case studies. This framework has potential to improve the recognition of the full risks and impacts of invasive species.},
  language  = {en}
}
@article{LatimerJacobsGianolietal.2019,
  author    = {Latimer, Andrew M. and Jacobs, Brooke S. and Gianoli, Ernesto and Heger, Tina and Salgado-Luarte, Cristian},
  title     = {Parallel functional differentiation of an invasive annual plant on two continents},
  series = {AoB PLANTS},
  volume    = {11},
  journal   = {AoB PLANTS},
  number    = {2},
  publisher = {Oxford University Press},
  address   = {Oxford},
  issn      = {2041-2851},
  doi       = {10.1093/aobpla/plz010},
  pages     = {16},
  year      = {2019},
  abstract  = {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.},
  language  = {en}
}
@article{RyoJeschkeRilligetal.2020,
  author    = {Ryo, Masahiro and Jeschke, Jonathan M. and Rillig, Matthias C. and Heger, Tina},
  title     = {Machine learning with the hierarchy-of-hypotheses (HoH) approach discovers novel pattern in studies on biological invasions},
  series = {Research synthesis methods},
  volume    = {11},
  journal   = {Research synthesis methods},
  number    = {1},
  publisher = {Wiley},
  address   = {Hoboken},
  issn      = {1759-2879},
  doi       = {10.1002/jrsm.1363},
  pages     = {66 -- 73},
  year      = {2020},
  abstract  = {Research synthesis on simple yet general hypotheses and ideas is challenging in scientific disciplines studying highly context-dependent systems such as medical, social, and biological sciences. This study shows that machine learning, equation-free statistical modeling of artificial intelligence, is a promising synthesis tool for discovering novel patterns and the source of controversy in a general hypothesis. We apply a decision tree algorithm, assuming that evidence from various contexts can be adequately integrated in a hierarchically nested structure. As a case study, we analyzed 163 articles that studied a prominent hypothesis in invasion biology, the enemy release hypothesis. We explored if any of the nine attributes that classify each study can differentiate conclusions as classification problem. Results corroborated that machine learning can be useful for research synthesis, as the algorithm could detect patterns that had been already focused in previous narrative reviews. Compared with the previous synthesis study that assessed the same evidence collection based on experts' judgement, the algorithm has newly proposed that the studies focusing on Asian regions mostly supported the hypothesis, suggesting that more detailed investigations in these regions can enhance our understanding of the hypothesis. We suggest that machine learning algorithms can be a promising synthesis tool especially where studies (a) reformulate a general hypothesis from different perspectives, (b) use different methods or variables, or (c) report insufficient information for conducting meta-analyses.},
  language  = {en}
}
@misc{RyoJeschkeRilligetal.2020,
  author    = {Ryo, Masahiro and Jeschke, Jonathan M. and Rillig, Matthias C. and Heger, Tina},
  title     = {Machine learning with the hierarchy-of-hypotheses (HoH) approach discovers novel pattern in studies on biological invasions},
  series = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe},
  journal   = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe},
  number    = {1171},
  issn      = {1866-8372},
  doi       = {10.25932/publishup-51764},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-517643},
  pages     = {66 -- 73},
  year      = {2020},
  abstract  = {Research synthesis on simple yet general hypotheses and ideas is challenging in scientific disciplines studying highly context-dependent systems such as medical, social, and biological sciences. This study shows that machine learning, equation-free statistical modeling of artificial intelligence, is a promising synthesis tool for discovering novel patterns and the source of controversy in a general hypothesis. We apply a decision tree algorithm, assuming that evidence from various contexts can be adequately integrated in a hierarchically nested structure. As a case study, we analyzed 163 articles that studied a prominent hypothesis in invasion biology, the enemy release hypothesis. We explored if any of the nine attributes that classify each study can differentiate conclusions as classification problem. Results corroborated that machine learning can be useful for research synthesis, as the algorithm could detect patterns that had been already focused in previous narrative reviews. Compared with the previous synthesis study that assessed the same evidence collection based on experts' judgement, the algorithm has newly proposed that the studies focusing on Asian regions mostly supported the hypothesis, suggesting that more detailed investigations in these regions can enhance our understanding of the hypothesis. We suggest that machine learning algorithms can be a promising synthesis tool especially where studies (a) reformulate a general hypothesis from different perspectives, (b) use different methods or variables, or (c) report insufficient information for conducting meta-analyses.},
  language  = {en}
}