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

Nutrient enrichment promotes invasion success of alien plants via increased growth and suppression of chemical defenses

In support of the prediction of the enemy release hypothesis regarding a growth-defense trade-off, invasive alien plants often exhibit greater growth and lower anti-herbivory defenses than native plants. However, it remains unclear how nutrient enrichment of invaded habitats may influence competitive interactions between invasive alien and co-occurring native plants, as well as production of anti-herbivore defense compounds, growth-promoting hormones, and defense-regulating hormones by the two groups of plants. Here, we tested whether: (i) nutrient enrichment causes invasive alien plants to produce greater biomass and lower concentrations of the defense compounds flavonoids and tannins than native plants; and (ii) invasive alien plants produce lower concentrations of a defense-regulating hormone jasmonic acid (JA) and higher concentrations of a growth-promoting hormone gibberellic acid (GA3). In a greenhouse experiment, we grew five congeneric pairs of invasive alien and native plant species under two levels each of nutrient enrichment (low vs. high), simulated herbivory (leaf clipping vs. no-clipping), and competition (alone vs. competition) in 2.5-L pots. In the absence of competition, high-nutrient treatment induced a greater increase in total biomass of invasive alien species than that of native species, whereas the reverse was true under competition as native species benefitted more from nutrient enrichment than invasive alien species. Moreover, high-nutrient treatment caused a greater increase in total biomass of invasive alien species than that of native species in the presence of simulated herbivory. Competition induced higher production of flavonoids and tannins. Simulated herbivory induced higher flavonoid expression in invasive alien plants under low-nutrient than high-nutrient treatments. However, flavonoid concentrations of native plants did not change under nutrient enrichment and simulated herbivory treatments. Invasive alien plants produced higher concentrations of GA3 than native plants. Taken together, these results suggest that impact of nutrient enrichment on growth of invasive alien and co-occurring native plants may depend on the level of competition that they experience. Moreover, invasive alien plants might adjust their flavonoid-based defense more efficiently than native plants in response to variation in soil nutrient availability and herbivory pressure. Our findings suggest that large-scale efforts to reduce nutrient enrichment of invaded habitats may help to control future invasiveness of target alien plant species.

Invasive plant species that experience lower herbivory pressure may evolve lower diversities of chemical defense compounds in the exotic range

PREMISE

Invasive plant species often escape from specialist herbivores and are more likely to be attacked by generalist herbivores in the exotic range. Consequently, the shifting defense hypothesis predicts that invasive plants will produce higher concentrations of qualitative defense compounds to deter dominant generalist herbivores in the exotic range. Here, I additionally propose a reduced chemical diversity hypothesis (RCDH), which predicts that reduced herbivory pressure will select for invasive plant genotypes that produce lower diversities of chemical defense compounds in the exotic range.

METHODS

I tested whether (1) invasive Brassica nigra populations express a lower diversity and an overall higher concentration of glucosinolate compounds than native-range B. nigra; (2) Brassica nigra individuals that express high diversities and concentrations of glucosinolate compounds are more attractive to specialist and deterrent to generalist herbivores; and (3) tissues of invasive B. nigra are less palatable than tissues of native-range B. nigra to the generalist herbivores Theba pisana and Helix aspersa.

RESULTS

Invasive B. nigra populations produced a significantly lower diversity of glucosinolate compounds, a marginally higher concentration of total glucosinolates, and a significantly higher concentration of sinigrin (the dominant glucosinolate). Leaf tissues of invasive B. nigra were significantly less palatable to T. pisana and marginally less so to H. aspersa. Brassica nigra individuals that expressed high concentrations of total glucosinolate compounds were visited by a low diversity of generalist herbivore species in the field.

CONCLUSIONS

In line with the RCDH, the lower diversity of glucosinolate compounds produced by invasive B. nigra populations likely resulted from selection imposed by reduced herbivory pressure in the exotic range.

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

PREMISE

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.

METHODS

Here, we made aqueous extracts of roots and shoots of invasive (North American) and native (European) Brassica nigra plants. Seeds of nine species from North America and nine species from Europe were exposed to these extracts. As control solutions, we used pure distilled water and distilled water with the osmotic potential adjusted with polyethylene glycol (PEG) to match that of root and shoot extracts of B. nigra.

RESULTS

The extracts had a strong negative effect on germination rates and seedling root lengths of target species compared to the water-control. Compared to the osmolality-adjusted controls, the extracts had a negative effect on seedling root length. We found no differences between the effects of B. nigra plant extracts from the invasive vs. native populations on germination rates and seedling root growth of target plant species. Responses were largely independent of whether the target plant species were from the invaded or native range of B. nigra.

CONCLUSIONS

The results show that B. nigra can interfere with other species through allelochemical interactions, but do not support predictions of the novel-weapons hypothesis and evolution of increased allelopathy.

Can polyploidy confer invasive plants with a wider climatic tolerance? A test using Solidago canadensis

Polyploidy can cause variation in plant functional traits and thereby generate individuals that can adapt to fluctuating environments and exploit new environments. However, few empirical studies have tested for an association between ploidy level and climatic tolerance of invasive cytotypes relative to conspecific native-range cytotypes. Here, we used an invasive plant Solidago canadensis to test whether invasive populations had a higher proportion of polyploids, greater height and stem-base diameter, and occupied a wider range of climatic conditions than conspecific native-range populations. We also tested whether the invasive populations had overcome genetic founder effects. We sampled a total of 80 populations in parts of the invaded range in China and native range in North America for in situ measurements of plant height and stem-base diameter in the field and for population genetic and cytotype analyses. To examine climatic correlates, we augmented our field-sampled data with occurrence records obtained from Global Biodiversity Information Facility. All, except one, of the populations that we sampled in China occurred in a humid subtropical climate. In contrast, the North American populations occurred in humid continental, humid subtropical, and semi-arid climatic zones. All populations of S. canadensis in China were purely hexaploid, while the North American populations were diploid, tetraploid, and hexaploid. The invasive hexaploids were significantly taller and had a larger stem-base diameter than native hexaploids. Native hexaploids were significantly taller and had larger stem-base diameter than native diploids. Climatic correlate assessment found that invasive and native populations occupied different climatic envelopes, with invasive populations occurring in warmer and less seasonal climates than native populations. However, there was no significant correlation between ploidy level and climatic envelope of S. canadensis. Molecular phylogeography data suggest reduced genetic founder effects in the invaded range. Overall, these results suggest that polyploidy does not influence S. canadensis climatic tolerance.

No difference in the competitive ability of introduced and native Trifolium provenances when grown with soil biota from their introduced and native ranges

The evolution of increased competitive ability (EICA) hypothesis could explain why some introduced plant species perform better outside their native ranges. The EICA hypothesis proposes that introduced plants escape specialist pathogens or herbivores leading to selection for resources to be reallocated away from defence and towards greater competitive ability. We tested the hypothesis that escape from soil-borne enemies has led to increased competitive ability in three non-agriculturalTrifolium(Fabaceae) species native to Europe that were introduced to New Zealand in the 19th century.Trifoliumperformance is intimately tied to rhizosphere biota. Thus, we grew plants from one introduced (New Zealand) and two native (Spain and the UK) provenances for each of three species in pots inoculated with soil microbiota collected from the rhizosphere beneath conspecifics in the introduced and native ranges. Plants were grown singly and in competition with conspecifics from a different provenance in order to compare competitive ability in the presence of different microbial communities. In contrast to the predictions of the EICA hypothesis, we found no difference in the competitive ability of introduced and native provenances when grown with soil microbiota from either the native or introduced range. Although plants from introduced provenances of two species grew more slowly than native provenances in native-range soils, as predicted by the EICA hypothesis, plants from the introduced provenance were no less competitive than native conspecifics. Overall, the growth rate of plants grown singly was a poor predictor of their competitive ability, highlighting the importance of directly quantifying plant performance in competitive scenarios, rather than relying on surrogate measures such as growth rate.

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

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 Brassica nigra plants from eight invasive- and seven native-range populations to test whether the invasive-range plants have evolved increased competitive ability when competing with Achillea millefolium and with a community (both with and without A. millefolium). Further, we tested whether competitive interactions depend on root herbivory on B. nigra by the specialist Delia radicum. Without the community, competition with A. millefolium reduced biomass of invasive- but not of native-range B. nigra. With the community, invasive-range B. nigra suffered less than native-range B. nigra. 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 B. nigra, irrespective of the range of origin, and independent of the presence of A. millefolium. Taken together, these results offer no clear support for the EICA hypothesis. While native-range B. nigra 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 B. nigra in North America.