Temporal dynamics of range expander and congeneric native plant responses during and after extreme drought events.

Climate change is causing range shifts of many species to higher latitudes and altitudes and increasing their exposure to extreme weather events. It has been shown that range-shifting plant species may perform differently in new soil than related natives; however, little is known about how extreme weather events affect range-expanding plants compared to related natives. In this study we used outdoor mesocosms to study how range-expanding plant species responded to extreme drought in live soil from a habitat in a new range with and without live soil from a habitat in the original range (Hungary). During summer drought, the shoot biomass of the range-expanding plant community declined. In spite of this, in the mixed community, range expanders produced more shoot biomass than congeneric natives. In mesocosms with a history of range expanders in the previous year, native plants produced less biomass. Plant legacy or soil origin effects did not change the response of natives or range expanders to summer drought. During rewetting, range expanders had less biomass than congeneric natives but higher drought resilience (survival) in soils from the new range where in the previous year native plant species had grown. The biomass patterns of the mixed plant communities were dominated by Centaurea spp.; however, not all plant species within the groups of natives and of range expanders showed the general pattern. Drought reduced the litter decomposition, microbial biomass, and abundances of bacterivorous, fungivorous, and carnivorous nematodes. Their abundances recovered during rewetting. There was less microbial and fungal biomass, and there were fewer fungivorous nematodes in soils from the original range where range expanders had grown in the previous year. We concluded that in mixed plant communities of range expanders and congeneric natives, range expanders performed better, under both ambient and drought conditions, than congeneric natives. However, when considering the responses of individual species, we observed variations among pairs of congenerics, so that under the present mixed-community conditions there was no uniformity in responses to drought of range expanders versus congeneric natives. Range-expanding plant species reduced soil fungal biomass and the numbers of soil fungivorous nematodes, suggesting that the effects of range-expanding plant species can trickle up in the soil food web.

Single introductions of soil biota and plants generate long-term legacies in soil and plant community assembly.

Recent demonstrations of the role of plant-soil biota interactions have challenged the conventional view that vegetation changes are mainly driven by changing abiotic conditions. However, while this concept has been validated under natural conditions, our understanding of the long-term consequences of plant-soil interactions for above-belowground community assembly is restricted to mathematical and conceptual model projections. Here, we demonstrate experimentally that one-time additions of soil biota and plant seeds alter soil-borne nematode and plant community composition in semi-natural grassland for 20 years. Over time, aboveground and belowground community composition became increasingly correlated, suggesting an increasing connectedness of soil biota and plants. We conclude that the initial composition of not only plant communities, but also soil communities has a long-lasting impact on the trajectory of community assembly.

A novel indirect defence in Brassicaceae: structure and function of extrafloral nectaries in Brassica juncea.

While nectaries are commonly found in flowers, some plants also form extrafloral nectaries on stems or leaves. For the first time in the family Brassicaceae, here we report extrafloral nectaries in Brassica juncea. The extrafloral nectar (EFN) was secreted from previously amorphic sites on stems, flowering stalks and leaf axils from the onset of flowering until silique formation. Transverse sections at the point of nectar secretion revealed a pocket-like structure whose opening was surrounded by modified stomatal guard cells. The EFN droplets were viscous and up to 50% of the total weight was sugars, 97% of which was sucrose in the five varieties of B. juncea examined. Threonine, glutamine, arginine and glutamate were the most abundant amino acids. EFN droplets also contained glucosinolates, mainly gluconapin and sinigrin. Nectar secretion was increased when the plants were damaged by chewing above- and belowground herbivores and sap-sucking aphids. Parasitoids of each herbivore species were tested for their preference, of which three parasitoids preferred EFN and sucrose solutions over water. Moreover, the survival and fecundity of parasitoids were positively affected by feeding on EFN. We conclude that EFN production in B. juncea may contribute to the indirect defence of this plant species.

Interactions to the fifth trophic level: secondary and tertiary parasitoid wasps show extraordinary efficiency in utilizing host resources.

1. Parasitoid wasps are highly efficient organisms at utilizing and assimilating limited resources from their hosts. This study explores interactions over three trophic levels, from the third (primary parasitoid) to the fourth (secondary parasitoid) and terminating in the fifth (tertiary parasitoid). 2. Host utilization and adult body mass of the secondary and tertiary parasitoid Gelis agilis was determined when developing on pre-pupae of its primary parasitoid host, Cotesia glomerata, and from pre-pupae of another secondary parasitoid, Lysibia nana that had developed initially on pre-pupae of C. glomerata. 3. In both C. glomerata and G. agilis, the body mass of emerging adult parasitoids was strongly positively correlated with initial cocoon mass. For a given cocoon mass at parasitism, emerging adult G. agilis wasps were almost 90% as large as C. glomerata adults developing in healthy cocoons of comparable mass. Furthermore, G. agilis adults were still 75% as large as C. glomerata adults even when developing on L. nana that in turn had developed on C. glomerata. Otherwise, in terms of adult body mass per unit of host resources, there was no apparent difference in the quality of C. glomerata or L. nana hosts for the development of G. agilis. 4. Analyses of carbon and nitrogen in body tissues of the parasitoids over the third to the fifth trophic level revealed that percentage nitrogen was higher and carbon lower in G. agilis and L. nana than in C. glomerata. Furthermore, percentage carbon was lower in adult G. agilis wasps that had developed from L. nana than from C. glomerata. 5. We argue that the remarkable efficiency which characterizes the development of secondary and tertiary parasitoids is based on the very high nutritional quality of resources that increase in quality up the food chain and rigid selection optimizing allocation of limited host resources. Consequently, food webs involving secondary and tertiary parasitoids can go to levels hitherto unexplored thus far in empirical studies. The use of molecular markers in future studies may reveal just how long food chains involving these insects may extend.

Growth reduction in Plantago lanceolata in relation to biotic factors in the soil environment.

To investigate the influence of soil-borne pathogens on the population biology of a grassland species in natural vegetation, soil samples were collected, partly disinfected or sterilized and then planted with Plantago lanceolata. Those on some of the untreated soils showed growth reduction due to a biotic factor in the soil. This growth reduction was not associated with the density of P. lanceolata plants at the sampling sites. It was only visible when differences in abiotic factors - especially nutrient levels -were eliminated. In the natural situation the nutritional status of the soil has a far greater impact than biotic soil factors. Micro-organisms harmful to Plantago roots are considered to be of minor importance in the distribution of the individuals in the P. lanceolata population studied.

Differing Host Exploitation Efficiencies in Two Hyperparasitoids: When is a 'Match Made in Heaven'?

Host exploitation behavior in two hyperparasitoids, Lysibia nana and Gelis agilis, was compared in single cocoon clusters of their primary parasitoid host, Cotesia glomerata. L. nana reproduces sexually, is fully winged, does not host-feed and matures eggs quite rapidly after eclosion, whereas G. agilis possesses opposite traits. Cohorts of individual hyperparasitoid females of differing age and physiological state were given access to single cocoon clusters of C. glomerata that also varied in age. These results reveal that the reproductive biology of L. nana is well matched to exploit cocoon broods in C. glomerata, suggesting strong a co-evolutionary history with this host. By contrast, G. agilis is much less efficient at exploiting host cocoons and is probably a generalist species that attacks other hosts in nature.

Development of an insect herbivore and its pupal parasitoid reflect differences in direct plant defense.

In nature, plants defend themselves by production of allelochemicals that are toxic to herbivores. There may be considerable genetic variation in the expression of chemical defenses because of various selection pressures. In this study, we examined the development of the small cabbage butterfly, Pieris rapae, and its gregarious pupal ectoparasitoid, Pteromalus puparum, when reared on three wild populations (Kimmeridge, Old Harry, Winspit) of cabbage, Brassica oleracea, and a Brussels sprout cultivar. Wild plant populations were obtained from seeds of plants that grow naturally along the south coast of Dorset, England. Significant differences in concentrations of allelochemicals (glucosinolates) were found in leaves of plants damaged by P. rapae. Total glucosinolate concentrations in Winspit plants, the population with the highest total glucosinolate concentration, were approximately four times higher than in the cultivar, the strain with the lowest total glucosinolate concentration. Pupal mass of P. rapae and adult body mass of Pt. puparum were highest when reared on the cultivar and lowest when developing on Kimmeridge plants, the wild strain with the lowest total glucosinolate concentration. Development of male parasitoids was also more negatively affected than female parasitoids. Our results reveal that plant quality, at least for the development of 'adapted' oligophagous herbivores, such as P. rapae, is not based on total glucosinolate content. The only glucosinolate compound that corresponded with the performance of P. rapae was the indole glucosinolate, neoglucobrassicin. Our results show that performance of ectoparasitoids may closely reflect constraints on the development of the host.