We compared herbivory pressure in the native Jacobaea vulgaris (formely Senecio jacobaea) and the alien invasive S. inaequidens in sites where they co-occur in Belgium. We predicted that the alien species experience s relaxed herbivory pressure by specialist herbivores (enemy release hypothesis ERH) whereas it is still attacked by generalist herbivores. Impacts of two generalist (gastropods and rabbits) and one specialist (the caterpillar Tyria jacobaeae) herbivores were assessed with field observations and exclusion experiments. The generalist herbivores had a higher impact on the bio mass and survival of the seedlings of the alien S. inaequidens than on the native J. vulgaris. On the contrary, the spe cialist Tyria jacobaeae attacked exclusively the adults of the native species, supporting one of the main predictions of the ERH. These results are discussed in relation to differences in pyrrolizidine alkaloid profiles between the two spe cies.
Natural enemies play a key role in hypotheses explaining the invasive success of alien plant species when introduced outside their native range [1-5]. First the enemy release hypothesis (ERH) proposes that the proliferation of an introduced plant species is due to the relaxed pressure by coevolved herbivores and pathogens [6,7]. The ERH has been tested in many different ways including comparisons between native and alien congeners (community approach), or between aliens in their native and introduced range (biogeographical approach) [5,8,9]. The ERH has been confirmed in many cases, as the invasive plants were less attacked in their new range than indigenous counterparts or than in their native range [2,9-13]. In contrast, the biotic resistance hypothesis (BRH) predicts that native generalist herbivores will suppress alien plants because these are naive, i.e. have not been selected to deter those herbivores [6,14]. Alien species are in this case considered to be more vulnerable than native species to attacks by native herbivores [1,15-18]. These two hypotheses could be complementary as differences in herbivore damage can exist between generalist and specialist herbivores [19-21]. In the introduced range, generalist and specialist herbivores are expected to differently interact with a new alien plant. Generalists feed on a wide range of plants, possibly including introduced species [
In the present study, we compared damage of generalist versus specialist herbivores on two closely related Senecionae species (Asteraceae): Jacobaea vulgaris Gaert. (syn. Senecio jacobaea L.) native to NW Europe and the invasive Senecio inaequidens DC. We chose these species because they occasionally co-occur in open grasslands in Europe. S. inaequidens is troublesome in Europe while the native European species is invasive in other parts of the world. They thus represent a adequate system for testing hypotheses about herbivore influence on alien species success.
Senecio inaequidens, the narrow-leaved ragwort, native to South Africa and Lesotho, was accidentally introduced at several locations in Europe at the end of the 19th century as a contaminant of sheep wool [23,24]. The species is now considered as one of the most aggressive alien invasive species in Europe [25,26]. The species is a perennial polycarpic suffrutescent herb, mostly occurring, in its introduced range, as a pioneer of well-drained soil, also occurring occasionally in closed mesic grassland [23, 24].
Jacobaea vulgaris (formerly Senecio jacobaea), the tansy ragwort, is native to Europe. It is a biennial to shortlived perennial monocarpic species which forms a rosette during the first year and a flowering shoot in the second year [
In this paper, we assessed herbivore damage at different life stages for generalist and specialist herbivores. We focused on two types of generalist herbivores, i.e. gastropods and rabbits that are known to feed on seedlings [30, 31]. On the other hand, we assessed damages due to the specialist caterpillar Tyria jacobaeae (Lepidoptera, Arctiidae), the cinnabar moth, which attacks J. vulgaris shoots mainly at the flowering stage. We did not estimate damages from other herbivores as they remained rare with very low damages on our sites (even the specialists Botanophila seneciella, Diptera, seed-head fly or Longitarsus jacobaeae, Coleoptera, flea beetle or Aphis jacobaeae, Hemiptera, aphid).
All Senecionae species contain pyrrolizidine alkaloids (PAs) of the senecionine type as constitutive defences against herbivores [32,33]. Some adapted co-evolved insects can detoxify and even sequester and use PAs for their own defence against predators [
We addressed three questions: 1) Is the alien S. inaequidens less attacked by generalists and/or specialists than the native J. vulgaris? Based on the ERH and the BRH, we predict that S. inaequidens should experience lower rates of foliar damage than the native congener by specialist herbivores and equal to higher pressure by generalist herbivores; 2) Are the two species equally affected by herbivores throughout their whole life cycle? We predict that the most critical life stages (seedlings) are those affected by generalist herbivores; 3) Can differences in the pattern of herbivory between the two species be explained by their profiles of pyrrolizidine alkaloids? We predict that the concentrations of PAs are higher in the alien than in the native species.
Observations and field experiments were conducted in two field sites where both species co-occurred. Such sites were chosen to ensure that the two species are subjected to the same herbivore community. The first field site was a rough mesic mown grassland on a SW-facing slope on a roadside situated in Nossegem (50˚52'18.30''N, 4˚30' 39.44''E), central Belgium. Vegetation was dominated by S. inaequidens, J. vulgaris, Bromus mollis and Daucus carota, with scattered alien shrubs (Buddleja davidii). This site was studied in 2005 and was thereafter destroyed by road works. Therefore, a second site was selected in Antwerp (51˚14'36.40''N, 4˚23'15.03''E), northern Belgium, for field observations and experiments in 2006 and 2007. There, the vegetation was a rough grassland on sandy soil dominated by S. inaequidens, J. vulgaris, Festuca rubra, Plantago lanceolata and Cirsium arvense.
We collected seeds in a total of seven populations of either species in Belgium. Seeds were pooled per species and sown in 3 L pots filled with a mixture of sand and compost (1:3, v:v) in a glasshouse. After emergence, the number of seedlings per pot was equalized to 30 and transferred to the field.
In June 2007, 80 pots were distributed in five blocks in the field site at Antwerp, separated from each other by approximately 10 m. In each block, four treatments were applied, i.e. exposed to both gastropods and rabbits, exposed to gastropods only, exposed to rabbits only and exposed to neither gastropods nor rabbits. Rabbit protection consisted of 60 cm high fences (mesh: 2.5 cm), buried to 20 cm deep, forming an exclosure of 70 × 120 cm around the pots. Gastropod protection consisted of copper tapes (Adalia®) glued around the pots, thus preventing gastropods from reaching the seedlings. The fully protected (neither gastropods nor rabbits) pots were protected both by the rabbit fence and the copper tape. The unprotected pots had no copper tape and were placed outside the fence. Surviving seedlings per pot were counted twice a week. After six weeks, surviving seedlings in each pot were cut at soil level, pooled in a paper bag, and dried at 50˚C for 6 days. The mean individual biomass was calculated as the total biomass per pot divided by the final number of surviving seedlings.
The number of flowering individuals of J. vulgaris and S. inaequidens attacked by Tyria jacobaeae caterpillars, and the number of caterpillars per plant, were counted from July to October, in the sites Nossegem (in 2005, 400 individuals per species) and Antwerp (2006 and 2007). In Antwerp the populations of both species were too large for an exhaustive survey. Plants were thus monitored within five permanent plots of approximately 0.75 m2 for each species. These plots initially contained a total of 50 individuals per species. Damage of T. jacobaeae was estimated (percent leaf damage) and the number of remained flowering heads per plant was counted.
We sampled both species at different life stages. The seedlings were collected twice, i.e. at 2 weeks old (one true leaf) and 2 weeks later (2 - 4 true leaves), from plants used in the generalist herbivore field experiment. For later stages, plant material was randomly sampled at the Antwerp site in early July 2008. Leaves were harvested from non flowering plants (6 plants per species) and 5 flowering heads (all florets open) on flowering plants. All samples were individually dried at 50˚C for 6 days, except for seedlings pooled in 2 bags per species.
Chemical analyses of pyrrolizidine alkaloids (PAs) were performed by Klaas Vrieling at the Institute of Biology, Leiden University, The Netherlands, using previously described method. Composition and concentration in PAs were determined by gas chromatography (HP 6890, 30 m × 0.25 µm HP-1) with split injector (1 - 30), Exterlut columns, PND detector and N2 as carrier gas at 0.9 ml/min. The temperature program was 0-22-5-250˚C and pressure 56 kPa. The total PA concentration was calculated by summing the area of all the peaks in the chromatogram.
Survival rate and remaining biomass were tested by 4- way ANOVA with all factors fixed. The effects of species, rabbit protection, gastropod protection, block and interactions were tested. Survival rates of seedlings were arcsin transformed and seedling biomass data were logtransformed to achieve normality. PA composition was tested with 2-way ANOVA with species and life stage as main factors. All statistical analyses were performed with SAS Enterprise version 4.1. Data are shown as means ± SD.
In fully protected pots and in pots exposed to gastropods only, nearly all seedlings of the two species survived. In fully exposed pots and in pots exposed to rabbits, survival rates were significantly lower (
vulgaris (significant rabbit * species interaction;
Gastropod grazing, obviously not very important in our study site, had no effect on the remaining biomass of both species (
No caterpillar was observed feeding on S. inaequidens over the three years at both field sites. In contrast, caterpillars were often observed feeding on J. vulgaris but their impact varied highly among years and sites (
heads have been eaten, with only stems remaining ungrazed. Approximately half of these individuals showed compensatory growth, and produced secondary flowering heads and set fruit in September, when caterpillars had pupated. In Antwerp in 2007, caterpillars were very abundant and attacked nearly all the J. vulgaris plants (
For all stages pooled, a total of 13 and 14 different PAs were detected in S. inaequidens and J. vulgaris, respectively and 7 were shared (
The PA concentration significantly increased (F = 38.2, p < 0.001) across the successive life stages from seedlings to flowering heads, except for the rosettes of J. vulgaris which contained higher PA than adult flowering plants. The composition also varied with life stages as 7 - 8 different PAs were detected in seedlings and 12 - 13 different alkaloids were present in flowering heads (
Previous work showed that the alien Senecio inaequidens does not escape herbivory in Europe, with more than 62 species of insects feeding on it in Germany [34, see also 35]. Heteropteran species and rabbits had a significant negative effect on fitness [
On the contrary, the specialist caterpillar Tyria jacobaeae had no effect on the alien species while it can destroy all the flowering population of the native species, thus having a great negative impact on the reproductive
success. These results are in agreement with some studies [
Explanation for differences in herbivore preferences may lie in PA concentrations for flowering life stages. PA defences are synthesized in roots and translocated to shoots; as for most plant defences, reproductive organs usually show the highest concentrations [
For both species, seedlings contained only traces of PA and represented the most palatable stage. Therefore, differences in rabbit damages between the two species cannot be ascribed to PA defences. Differences in seedling morphology might be more important. Seedlings of S. inaequidens possess erect leaves while those of J. vulgaris form a rosette closely pressed onto the soil, thus being less easily grazed by mammalian herbivores including rabbits [
Based upon assessments of herbivore damage, we tentatively calculated an overall estimate of the global effect of herbivory on the whole life cycle for both species (
lower flowering head production, high damage due to the specialist caterpillar and lower seed set per flowering head explain the mean seed production that only reaches 3650 seeds after two years. This huge difference in seed production can explain the invasive success of S. inaequidens in Europe.
Several other studies already suggested that invasiveness may be the result of different combinations of particular functional traits such as high reproductive success [
Senecio inaequidens suffers from herbivory mostly at the seedling stage, due to generalist herbivores, i.e. rabbits in this study. For J. vulgaris, the flowering stage was the most heavily affected, due to the attacks from the specialist moth Tyria jacobaeae. Finally, the global estimation of herbivory pressure throughout the life cycle suggests that populations of S. inaequidens can still grow faster compared to J. vulgaris, and that herbivore pressure cannot control the invasion. This conclusion is in line with the rapid expansion of S. inaequidens in Europe.
This work was financed by the Belgian Science Policy (framed within INPLANBEL project), the Université catholique de Louvain (FSR project) and the F.N.R.S. (FRFC contract number 2.4605.06). V. Vanparys had FRIA fellowship (Fonds pour la formation à la Recherche en Industrie et en Agriculture). We would like to acknowledge gratefully C. Dechamps, K. Wart, J. Vermander and the technical staff of the Jardin Jean Massart for their help in the construction of the experimental designs, K. Vrieling and his team for PA analysis. Finally, we thank N. Noret, C. Mayer, A. Paulet, P. V. Baret and A. Vervoort for their help in improving the manuscript.