Artificial night-time lighting and nutrient enrichment synergistically favour the growth of alien ornamental plant species over co-occurring native plant species

<ol> <li>Insights into ecological drivers of alien plant invasions can be gained through comparative studies of growth and fecundity of invasive alien plants versus those of co-occurring non-invasive alien plants and native plants across environmental conditions in common garden settings. Habitats that harbour alien plant species in many ecosystems globally are presently experiencing light pollution resulting from artificial light at night (ALAN) and increased rates of nutrient enrichment of the soil. However, the potential interactive effects of ALAN and nutrient enrichment on invasiveness of alien plant species remain unknown.</li> <li>Here, we performed a common-garden experiment to test the interactive effects of ALAN and soil nutrient enrichment on the growth of a random set of 10 alien (five invasive and five naturalized) and seven co-occurring native ornamental plant species that are commonly cultivated within urban and peri-urban areas of Nairobi city in Kenya. We predicted that a simultaneous increase in photoperiod via ALAN and nutrient enrichment will favor growth of invasive alien plant species over that of non-invasive alien and native plant species. We grew the 17 plant species under natural daylight (ALAN-) vs natural daylight followed by ALAN (ALAN+) and fully crossed with two levels of nutrient enrichment (low vs high) and competition (competition vs no-competition against a native plant <em>Ocimum</em> <em>gratissimum</em>) treatments.</li> <li>Under simultaneous high-nutrient and no-competition treatments, ALAN enhanced mean total biomass of invasive and naturalized alien species by 61.1% and 131.4%, respectively but decreased that of native plant species by 34%. In contrast, under simultaneous high-nutrient and competition treatments, ALAN enhanced mean total biomass of invasive alien plant species by 68.6% and that of naturalized alien species by 51.9% and native species by 35.4%. High-nutrient treatment enhanced flower formation more strongly in invasive and naturalized alien plants than in native plants. The invasive and naturalized alien species grew taller than native species across the light, nutrient, and competition treatments.</li> <li> <em>Synthesis</em>: The present findings suggest that light pollution and nutrient enrichment may jointly confer growth advantage to invasive alien plant species over that of co-occurring native plant species and enhance invasiveness of alien plant species.</li> </ol><p>Funding provided by: International Foundation of Science*<br>Crossref Funder Registry ID: <br>Award Number: D-6495-1</p><p>Funding provided by: National Geographic Society<br>Crossref Funder Registry ID: https://ror.org/04bqh5m06<br>Award Number: WW-155R-17</p><p>Funding provided by: Alexander von Humboldt Foundation<br>Crossref Funder Registry ID: http://dx.doi.org/10.13039/100005156<br>Award Number: 3.4-8151/GA-Nr. 19016</p><p>Funding provided by: German Academic Exchange Service<br>Crossref Funder Registry ID: https://ror.org/039djdh30<br>Award Number: 91787649</p><p>Here, we performed a common-garden experiment to test the interactive effects of ALAN and soil nutrient enrichment on the growth of a random set of 10 alien (five invasive and five naturalized) and seven co-occurring native ornamental plant species that are commonly cultivated within urban and peri-urban areas of Nairobi city in Kenya.</p&gt

Allelopathic effects of native and invasive Brassica nigra do not support the novel-weapons hypothesis

The novel-weapons hypothesis predicts that some plants are successful invaders because they release allelopathic compounds that are highly suppressive to naïve competitors in invaded ranges but are relatively ineffective against competitors in the native range. For its part, the evolution of enhanced weaponry hypothesis predicts that invasive populations may evolve increased expression of the allelopathic compounds. However, these predictions have rarely been tested empirically.publishe

Invasive plant species are locally adapted just as frequently and at least as strongly as native plant species

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In the presence of specialist root and shoot herbivory, invasive-range Brassica nigra populations have stronger competitive effects than native-range populations

1. The evolution of increased competitive ability (EICA) hypothesis predicts that release from specialist herbivores enables invasive plants to evolve increased growth. The most powerful tests of EICA hypothesis are provided by approaches that simultaneously assess the effects of specialist herbivory and competitive interactions. However, such approaches are extremely rare, and hence how simultaneous release from root and shoot herbivory influence competitive ability of invasive plants remains little understood.2. Here, we tested whether invasive-range Brassica nigra plants have evolved increased competitive ability, and whether expression of competitive ability depends on separate and simultaneous effects of specialist root and shoot herbivory. To do this, we grew B. nigra plants from eight invasive-range and eight native-range populations in the presence vs. absence of competition with a community of native plant species, and in the absence vs. presence of separate and simultaneous damage by a specialist root herbivore (Delia radicum) and a specialist shoot herbivore (Plutella xylostella). Brassica nigra performance was assessed by measuring biomass production and flowering of individual B. nigra plants.3. In partial support of the EICA hypothesis, invasive-range B. nigra had greater flowering than native-range conspecifics in the absence of competition. However, contrary to a prediction of the EICA hypothesis, invasiverange B. nigra produced less above-ground biomass than native-range B. nigra in the absence of shoot herbivory and competition. Moreover, with simultaneous root and shoot herbivory, invasive-range B. nigra suppressed a competing community more strongly than native-range B. nigra did.4. Synthesis. Our results suggest that invasiveness may be driven by mechanisms other than increased individual size. Simultaneous root and shoot herbivory in the invasive range may enhance suppressive effects of introduced plant species that have not completely escaped herbivore damage in the introduced range.publishe

The Interaction between Root Herbivory and Competitive Ability of Native and Invasive-Range Populations of <i>Brassica nigra</i>

<div><p>The evolution of increased competitive ability (EICA) hypothesis predicts that escape from intense herbivore damage may enable invasive plants to evolve higher competitive ability in the invasive range. Below-ground root herbivory can have a strong impact on plant performance, and invasive plants often compete with multiple species simultaneously, but experimental approaches in which EICA predictions are tested with root herbivores and in a community setting are rare. Here, we used <i>Brassica nigra</i> plants from eight invasive- and seven native-range populations to test whether the invasive-range plants have evolved increased competitive ability when competing with <i>Achillea millefolium</i> and with a community (both with and without <i>A</i>. <i>millefolium</i>). Further, we tested whether competitive interactions depend on root herbivory on <i>B</i>. <i>nigra</i> by the specialist <i>Delia radicum</i>. Without the community, competition with <i>A</i>. <i>millefolium</i> reduced biomass of invasive- but not of native-range <i>B</i>. <i>nigra</i>. With the community, invasive-range <i>B</i>. <i>nigra</i> suffered less than native-range <i>B</i>. <i>nigra</i>. Although the overall effect of root herbivory was not significant, it reduced the negative effect of the presence of the community. The community produced significantly less biomass when competing with <i>B</i>. <i>nigra</i>, irrespective of the range of origin, and independent of the presence of <i>A</i>. <i>millefolium</i>. Taken together, these results offer no clear support for the EICA hypothesis. While native-range <i>B</i>. <i>nigra</i> plants appear to be better in dealing with a single competitor, the invasive-range plants appear to be better in dealing with a more realistic multi-species community. Possibly, this ability of tolerating multiple competitors simultaneously has contributed to the invasion success of <i>B</i>. <i>nigra</i> in North America.</p></div

Results of likelihood-ratio model comparisons of nested linear mixed-effects models to test whether <i>B</i>. <i>nigra</i> (presence vs. absence), <i>B</i>. <i>nigra</i> range (invasive vs. native; fitted sequentially after <i>B</i>. <i>nigra</i>), Achillea presence (presence vs. absence of <i>Achillea millefolium</i>), and all possible interactions had a significant effect on the community aboveground biomass.

<p>^aRemoval of effect compared to: random part + <i>B</i>.<i>nigra</i> + <i>A</i>. <i>millefolium</i>.</p><p>^bRemoval of effect compared to: random part + <i>A</i>. <i>millefolium</i> + <i>B</i>. nigra + <i>B</i>. <i>nigra</i> range.</p><p>^cRemoval of effect compared to: random part + <i>A</i>. <i>millefolium</i> + <i>B</i>. <i>nigra</i> + <i>B</i>. <i>nigra</i> range + <i>A</i>. <i>millefolium</i> x <i>B</i>. <i>nigra</i>.</p><p>^dRemoval of effect compared to: random part + <i>A</i>. <i>millefolium</i> + <i>B</i>. <i>nigra</i> + <i>B</i>. <i>nigra</i> range + <i>A</i>. <i>millefolium</i> x <i>B</i>. nigra + <i>A</i>. <i>millefolium</i> x <i>B</i>. <i>nigra</i> Range.</p><p>Significant factors are marked in bold.</p

Mean (± 1SE) above-ground biomass of a community of four species (<i>Elymus glaucus</i>, <i>Nasella pulchra</i>, <i>Medicago lupulina</i> and <i>Sonchus oleraceus</i>) grown in the: a) absence (NoBr) versus presence of <i>B</i>.<i>nigra</i> plants from the invasive (Inv) or native (Nat)-range crossed with absence versus presence of <i>Achillea millefolium</i>, b) absence versus presence of root herbivory on invasive- or native-range <i>B</i>.<i>nigra</i> plants crossed with absence versus presence of <i>A</i>.<i>millefolium</i>.

<p>Mean (± 1SE) above-ground biomass of a community of four species (<i>Elymus glaucus</i>, <i>Nasella pulchra</i>, <i>Medicago lupulina</i> and <i>Sonchus oleraceus</i>) grown in the: a) absence (NoBr) versus presence of <i>B</i>.<i>nigra</i> plants from the invasive (Inv) or native (Nat)-range crossed with absence versus presence of <i>Achillea millefolium</i>, b) absence versus presence of root herbivory on invasive- or native-range <i>B</i>.<i>nigra</i> plants crossed with absence versus presence of <i>A</i>.<i>millefolium</i>.</p

Herbivores mediate different competitive and facilitative responses of native and invader populations of Brassica nigra

Differences in plant and herbivore community assemblages between exotic andnative ranges may select for different levels of plant traits in invasive and native populations of plant species. Little is currently known of how herbivores may mediate competitive and facilitative interactions between invasive and native populations of plant species and their plant neighbors. Here, we conducted a common-garden field experiment to test whether invasive and native populations of Brassica nigra differ in phenotypic expressions of growth (biomass and plant height) and reproductive (seed yield) traits under different plant neighbor treatments and ambient vs. reduced level of insect herbivore damage on the B. nigra plants. We found significant interactive effects of plant neighbor treatments, level of insect herbivore damage on B. nigra plants, and invasive status of B. nigra on the phenotypic trait expressions.Plant neighbor treatments had minimal effects on phenotypic trait expressions by invasive populations of B. nigra under either level of insect herbivore damage. In contrast, for native populations of B. nigra, ambient level of insect herbivore damage resulted in plant neighbors facilitating expression of the traits above, while reduced damage resulted in plant neighbors competitively suppressing trait expression. Our results suggest that insect herbivores and plant neighbors interactively shape expression of plant traits in native and exotic ranges of invasive plants. Such interactions could potentially lead to different selection pressures on traits that determine antiherbivore defenses and plant–plant interactions

Allelopathic and competitive interactions between native and alien plants

The novel-weapons and homeland-security hypotheses are based on the idea that aliens and natives are not adapted to each other’s allelochemicals as they did not co-evolve. However, as only a few studies have tested this, it remains unclear how important co-evolutionary history is in determining the strength of allelopathic interactions between aliens and natives. Here, we tested for potential pairwise allelopathic effects on each other of five alien and five native herbaceous species in China. We did a germination experiment and a competition experiment. In the germination experiment, we tested whether aqueous extracts of the ten study species had allelopathic effects on each other’s seed germination. In the competition experiment, we tested whether the alien and native species differed in their competitive effects and responses, and whether these were changed by the presence of activated carbon—a presumed allelopathy neutralizer– in the soil. Plant extracts had negative allelopathic effects on seed germination. This was particularly the case for extracts from the native species. Moreover, aqueous extracts had slightly stronger negative effects on germination of the aliens than on germination of the natives. In the competition experiment, on the other hand, the natives suffered more from competition than the alien species did, but we could not relate this to allelopathy. Alien plants had negative competitive and allelopathic effects on native plants, but the reverse was also true. These aliennative interactions, however, were not consistently stronger or weaker than native-native or alien-alien interactions.publishe