Biogeographical approach to plant invasions and processes underlying the naturalization of alien plants
Integration of hypotheses and theories explaining the ability of plant species to invade and vulnerability of regions/ecosystems to invasion is necessary for achieving progress in the theory of plant invasions; so far, these two facets were mostly considered separately (Richardson & Pyšek 2006, Pyšek et al. 2006). Biogeographical approach is a convenient tool for exploring the principles of naturalization (i.e. the ability of a species to sustain viable populations in the target area without help of humans (Pyšek & Richardson 2006). A global analysis of naturalization patterns shows that temperate mainland areas are more invaded than tropical mainlands, but islands in tropics suffer as much as islands in the temperate zone. Naturalization success decreases with latitude, indicating that for plant species it is easier to become naturalized in warmer climates. So far, studies used the number of naturalized species as a measure of naturalization success in a given region; in paper (Richardson & Pyšek 2006) we were the first who used a relative measure (percentage of the total number of alien plants introduced to the region that became successfully naturalized). This made it possible to reveal patterns that would otherwise remain hidden (Fig. 1).
Alongside with climate, propagule pressure (the number of propagules that are introduced to the target, i.e. invaded region) also crucially affects naturalization success. By using both climatic and propagule pressure variables in the same model (climatic match between source and target area was expressed by means of climate modelling, and the volume of trade and tourism between those two regions was used as a proxy for propagule pressure), we were able to explain present distribution and abundance of plants of South African origin invading in other parts of the world (Thriller at al. 2005).
Working on regional scale allows for a more precise evaluation of factors that determine naturalization as documented by a study considering the effects of propagule pressure and residence time. The approach adopted here was novel in that we worked with a source species pool (a vast majority of studies rely on plants in a target region, recruited from unknown species pool). Further, by focusing on woody plants we reduced potential bias associated with life forms (Křivánek e. Woody plants introduced to the Czech Republic for forestry purposes several centuries ago have significantly higher probability of escape from cultivation and subsequent naturalization than those introduced late (Fig. 2). The relative importance of residence time (i.e. time for which a species has been planted in CR) is significantly higher than that of the extent of planting (i.e. a proxy for propagule pressure). The paper indicates that forestry is an important historical pathways of alien plant invasions. Another study, using 180 woody species planted in the Czech Republic (Křivánek & Pyšek 2006), showed that invasive behaviour can be predicted based on species traits and concluded that a prediction scheme developed in Australia can be successfully used in the temperate zone of Central Europe.
Fig. 1. Based on data from 27 regions of the world , this figure documents that the naturalization rate (a relative measure of naturalization success based on the % of naturalized species of all introduced) decreases with latitude. If the number of naturalized species is plotted instead of naturalization rate, the pattern disappears
Fig. 2. The time for which the woody species has been planted (residence time) for forestry purposes in the Czech Republic has decisive influence on the probability of escape from cultivation, which is the first stage of the invasion process. For species that were introduced before 1800, there is a 95 % probability of escape
(Křivánek et al. 2006).
Pyšek P., Richardson D. M. 2006. The biogeography of naturalization in alien plants. J. Biogeogr. 33: 2040–2050.
Richardson D. M., Pyšek P. 2006. Plant invasions: Merging the concepts of species invasiveness and community invasibility. Progr. Phys. Geogr. 30: 409–431.
Pyšek P., Richardson D. M., Jarošík V. 2006. Who cites who in the invasion zoo: insights from an analysis of the most highly cited papers in invasion ecology. Preslia 78: 437–468.
Thuiller W., Richardson D. M., Pyšek P., Midgley G. F., Hughes G. O., Rouget M. 2005. Niche-based modelling as a tool for predicting the risk of alien plant invasions at a global scale. Global Change Biol. 11: 2234–2250.
Křivánek M., Pyšek P., Jarošík V. 2006. Planting history and propagule pressure as predictors of invasions by woody species in a temperate region. Conserv. Biol. 20: 1487–1498.
Křivánek M., Pyšek P. 2006. Predicting invasions by woody species in a temperate zone: a test of three risk assessment schemes in the Czech Republic (Central Europe). Diversity Distrib. 12: 319–327.
The role of arbuscular mycorrhiza in contaminated and degraded soils
Symbiosis of plants with arbucular mycorrhizal fungi (AMF) represents an important mechanism how to cope with stresses, e.g. deficiency in mineral nutrients, unfavorable water relations or soil pH, high concentrations of heavy metals etc. To study contribution of AMF to immobilization of cadmium in substrate, tobacco plants were grown non-inoculated or inoculated with AMF (Janoušková et al. 2006). Obtained substrates were amended with Cd and Cd toxicity was than assessed by a series of biotests using growth inhibition of lettuce roots as a sensitive indicator. Tests revealed lower toxicity of Cd in mycorrhizal than non-mycorrhizal substrate, with the difference increasing with concentration of Cd (Fig. 1). In a subsequent experiment, it was found that extraradical mycelium contained ten to twenty fold higher concentrations of Cd per biomass unit than tobacco roots. Hypothesis on immobilization of heavy metals by AMF confirm also results from the experiment conducted in a soil contaminated with lead, where AM inoculation led to higher Pb concentrations in tobacco roots, presumably due to metal accumulation in fungal mycelium (Sudová et al. 2007). These data point to a potential of AMF for fytostabilization of contaminated soils.
Composition of AMF community and length of the extraradical fungal mycelium significantly affected the coexistence of two dominant plant species co-occurring in highly alkaline anthropogenic sediment from PVC factory in Portugal, the shrub Salix atrocinerea and herbaceous Conyza bilbaoana (Fig. 2) (Oliveira et al. 2006). In another experiment focused on biomass production in substrate from a freshly recultivated spoil bank, it was found that dual inoculation with AMF and soil yeasts significantly increased biomass of maize depending on the combination of yeast strain and AMF isolate (Gollner et al. 2006).
Fig. 1. The effect of cadmium concentration in soil on root growth of germinating lettuce. Relative root length represents percentage of root length at concentration Cd 0 mg.kg-1. Empty symbols: substrate from mycorrhizal rhizosphere; full symbols: substrate from non-mycorrhizal rhizosphere.
Fig. 2. Coexistence ratio between Conyza bilbaoana and Salix atrocinerea (expresses the biomass of C. bilbaoana as percentage of the total biomass of both species). Plants were either non-inoculated, inoculated with G. intraradices BEG163, G. mosseae BEG198, G. claroideum BEG210, G. geosporum BEG199 or with a mixture of the four AMF.
Janoušková M., Pavlíková D., Vosátka M. 2006. Potential contribution of arbuscular mycorrhiza to cadmium immobilisation in soil. Chemosphere 65: 1959–1965.
Sudová R., Pavlíková D., Macek T., Vosátka M. 2007. The effect of EDDS chelate and inoculation with the arbuscular mycorrhizal fungus Glomus intraradices on the efficacy of lead phytoextraction by two tobacco clones. Appl. Soil Ecol. 35: 163–173.
Oliveira R. S., Castro P.M.L., Dodd J.C., Vosátka M. 2006. Different native arbuscular mycorrhizal fungi influence the coexistence of two plant species in a highly alcaline anthropogenic sediment. Plant Soil 287: 209–221.
Gollner M.J., Püschel D., Rydlová J., Vosátka M. 2006. Effect of inoculation with soil yeasts on mycorrhizal symbiosis of maize. Pedobiologia 50: 341–345.