biology Review Impacts of the Invasive Impatiens glandulifera: Lessons Learned from One of Europe’s Top Invasive Species Stephanie Coakley and Carloalberto Petti * Department of Science and Health, EnviroCORE, Institute of Technology Carlow, R93 V960 Carlow, Ireland; [email protected] * Correspondence: [email protected] Simple Summary: Plants and animals are a part of a larger system, commonly referred to as an ecosystem. This generally implies a balance existing between prey and predators. The unintentional introduction of a species in a new environment can lead to a significant alteration of the ecosystem(s) and the uncontrolled spread of the species. When this takes place, the introduced species is referred to as invasive. Invasives can affect the ecosystem in profound ways, and generally, negatively impacting on the native species. This manuscript reviewed the current knowledge of one of Europe’s top invasives, the Himalayan balsam (Impatiens glandulifera). It provides insights on the species and what have we learned from this invasive species. Abstract: Biological invasions are renowned for their negative ecological and economic implications, however from studying invasions invaluable insights can be gained in the fields of ecology and evolution- potentially contributing towards conservation plans to deal, not only with biological inva- sion, but with other concerning issues, such as climate change. Impatiens glandulifera, or Himalayan balsam, is widely considered to be a highly problematic invasive, having spread across more than thirty countries during the past century. This paper will examine the findings which have arose from Citation: Coakley, S.; Petti, C. studying I. glandulifera and its impacts on the invaded ecosystem. Impacts of the Invasive Impatiens glandulifera: Lessons Learned from Keywords: invasive; Impatiens glandulifera; conservation biology; alien species; biological invasion One of Europe’s Top Invasive Species. Biology 2021, 10, 619. https:// doi.org/10.3390/biology10070619 1. Introduction Academic Editor: Philip J. Seddon Invasive species are the second greatest threat to biodiversity, capable of altering Received: 14 May 2021 the functioning of ecosystems and the provision of ecosystem services [1]. However, Accepted: 30 June 2021 biological invasions can also be viewed as “unintended large-scale experiments in nature”, Published: 3 July 2021 resulting in many valuable insights to ecology and evolution [2–4]. Considering the extend of the spread of I. glandulifera since its introduction to Europe from the Himalayas in Publisher’s Note: MDPI stays neutral 1839 (Figure1 ), there is an abundance of lessons which can be learned from studying with regard to jurisdictional claims in the invasion of I. glandulifera [5,6]. Impatiens glandulifera was listed as an invasive alien published maps and institutional affil- species of Union concern in the 2017 update to Regulation (EU) No. 1143/2014 [7,8]. iations. Understanding both the ecology of an invasive species, such as I. glandulifera and, the full range of its effects on the invaded ecosystem is essential in the formulation of effective control strategies and rehabilitation of invaded habitats [4,9–11]. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Biology 2021, 10, 619. https://doi.org/10.3390/biology10070619 https://www.mdpi.com/journal/biology Biology 2021Biology, 10,2021 x FOR, 10, 619PEER REVIEW 2 of 14 2 of 14 Figure 1. The distribution range of I. glandulifera in its native range (blue) and invaded range (red). The broad distribution Figure 1. The distribution range of I. glandulifera in its native range (blue) and invaded range (red). The broad distribution of I. glandulifera enables it to be studied in a multitude of different habitats across a broad latitudinal range, highlighting the of I. glandulifera enables it to be studied in a multitude of different habitats across a broad latitudinal range, highlighting effect of an invasive across a geographically diverse area. Furthermore, these insights may help develop assisted migration the effectconservation of an invasive plans. Theacross genus a Impatiensgeographicallyis often referred diverse to asar ‘poorea. 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Impatiens(the walleriana latter two found and nearbyImpatiensI. glandulifera parviflorain are the Himalayas)both considered considered invasive, potentially with Impatiens capensisinvasive, Impatiens [16]. balfourii Sources and of CreativeCommons Impatiens edgeworthii images: (theI. balfourii latter[ two17] CC found by-SA nearby 4.0, I. capensis I. glandulifera[18] CC by-NC-SAin the Himalayas) 2.0, I. consid- ered potentiallyedgeworthii invasive[19] CC by-SA-4.0, [16]. SourcesI. parviflora of CreativeCommons[20] CC by 2.0, I. noli-tangere images:[21] I. CC balfourii by-SA-2.0, [17]I. walleriana CC by-SA[22] 4.0, CC by-SAI. capensis 4.0. [18] CC by- NC-SA 2.0, I. edgeworthii [19] CC2. Lesson by-SA-4.0, Number I. parviflora One: Impatiens [20] CC glandulifera by 2.0, I. noli-tangereHas a Differential [21] CC Effectby-SA-2.0, on I. walleriana [22] CC by-SA 4.0. Invaded Ecosystems The biotic resistance hypothesis asserts that communities which are more diverse areResearch less susceptible has revealed to biological that invasionsthe soil microbiome than less diverse of coniferous communities forests [23,24]. is The more sensitive to theriparian influence habitats of dominated invasion by byI. glanduliferaI. glanduliferaare prone than to that invasives, of deciduous in part due forests, to their which may high disturbance regimes [25]. consequentlyResearch lead has revealed to a stronger that the soilimpact microbiome of I. glandulifera of coniferous [26]. forests Furthermore, is more sensitive open habitats (suchto the as influenceriverbanks of invasion and floodplains) by I. glandulifera tendthan to that experience of deciduous more forests, pronounced which may effects than shadedconsequently habitats, lead with to a I. stronger glandulifera impact exerting of I. glandulifera the strongest[26]. Furthermore, effect on open species habitats within habitats which(such are as riverbanksbright and and very floodplains) wet [27]. tendInterestingly, to experience Čuda more et pronounced al. [28] asserted effects thanthat the impacts of thisshaded invasive habitats, are with comparableI. glandulifera exertingin both theriparian strongest and effect forested on species habitats, within habitats with the extent of which are bright and very wet [27]. Interestingly, Cudaˇ et al. [28] asserted that the impacts theof impact this invasive depending are comparable on the indo bothminance riparian and and residence forested habitats, time of with I. glandulifera the extent in the in- vadedof the community. impact depending on the dominance and residence time of I. glandulifera in the invadedContrary community. to reports that Impatiens glandulifera can significantly reduce species rich- ness andContrary diversity to reports (by ~11 that toImpatiens 30 %), glandulifera there arecan reports significantly of this reduce invasive species having richness no/negligible and diversity (by ~11 to 30 %), there are reports of this invasive having no/negligible impactsimpacts on on native native floraflora [29 [29–33].–33]. As aforementioned,As aforementioned, the environmental the environmental requirements requirements of of thethe flora flora can can determine determine the the extent extent to which to whichI. glandulifera I. glanduliferacan impact can upon impact them. upon Light them. Light dependentdependent species species areare more more significantly significan impactedtly impacted by invasives by invasives such as I. such glandulifera as I. , glandulifera, whosewhose towering towering structures create create canopies canopies which which can impede can impede light filtering light through filtering to through to smallersmaller plants; plants; research research has has shown shown that that the manual the manual removal removal of I. glandulifera of I. glanduliferais likely to is likely to favour select plant species, for example species which are less-shade tolerant are likely to benefit from the removal of I. glandulifera. Whereas other species, such as those which are less drought-tolerant, will likely be eliminated following removal of I. glandulifera [10,34,35]. Although I. glandulifera is seen to alter the floristic composition of the invaded habitat to varying degrees [11,26,32,36], it is