Ectomycorrhizal Invasions
In the age of globalization, things get moved around. Species are no exception. For fungi, however, patterns of global of transport and establishment are sketchily documented, with the possible exception of a few well-known pathogens. Lack of this basic information makes understanding (and ameliorating) the impact of these newcomers on native ecosystems difficult. For this reason, two recently published papers on ectomycorrhizal introductions represent an important scientific step forward. In the first paper, Else Vellinga and colleagues (2009) survey a large body of literature to assess our current state of knowledge on global ectomycorrhizal introductions. In the second, Anne Pringle and colleagues (2009) use genetic techniques to document the time and tempo of the Amanita phalloides invasion in western North America.
Because ectomycorrhizal fungi are obligate biotrophs, the vast majority of known ectomycorrhizal introductions are associated with the establishment of exotic timber plantations. Thus, 84% of the documented introductions found by Vellinga and colleagues (2009) are associated with establishment of plantations of Pine (Pinaceae = 57%) or Eucalpytus (Myrtaceae = 27%), many of these in the southern hemisphere (e.g. New Zealand). Our own Monterey pine (Pinus radiata), being one of the most extensively planted culprits in this process. For this reason, the most commonly introduced genera of fungi tend to be pine associates, the top three being Suillus, Amanita, and Rhizopogon. Other commonly introduced ectomycorrhizal fungi include Scleroderma, Laccaria, Lactarius and Hydnagium. While fungal spores can travel long-distances, it is nearly certain that the vast majority of these introductions are the direct result of human transport of roots and soil and not aerial dispersal. For example, Monterey pine in the southern hemisphere is associated with European species, such as Suillus luteus, rather than north American natives like Suillus pungens, reflecting the origins of the plantation stock in European nurseries.
Amanita phalloides © Michael Wood
In total, Vellinga and colleagues document the introduction of 200 species of ectomycorrhizal fungi. However, this is certainly an underestimate, as many introductions likely go unobserved. As the authors show in their study, the number of introductions in a particular region is strongly correlated with the number of publications from that region. Thus, we know little or nothing about introductions in poorly studied regions. In addition, given the cryptic nature of fungal fruiting habits and our poor knowledge of most mycofloras, proving a species is introduced takes a great deal of work for all but the simplest cases (for example, an ectomycorrhizal species in a country that previously had no ectomycorrhizal plants). Who would notice a European species of Inocybe or Cortinarius in a California foray? In this respect, the case of Amanita phalloides and the research by Anne Pringle and her colleagues is an exemplar.
First, Pringle set the historical record straight by sequencing DNA from herbarium collections attributed as A. phalloides in California going back to 1911. They found that the earliest records were misidentifications of A. ocreata and A. pantherina. The first molecularly verified records of A. phalloides come from the Del Monte Hotel grounds in Monterey (1938) and UC Berkeley campus (1945), both sites with extensive collections of exotic plants. Second, they developed molecular markers to analyze the genetic diversity of A. phalloides fruit bodies collected from Europe, eastern North America, and western North America. Genetic diversity in A. phalloides populations from western North America was much lower than in Europe and appeared to represent a subset of the alleles present in Europe. This is the pattern one might expect if populations in western North America were recently derived from one or a few individuals from Europe – i.e. an introduction. In addition, while most of the genetic markers showed affinity with European populations from mainland France, alleles from Norway and Corsica were also present within single sites in California. This pattern suggests multiple introductions of A. phalloides to California, and also that populations genetically isolated in Europe may be mixing in California.
Vellinga and colleagues point out that most ectomycorrhizal introductions do not lead to invasions - that is the introduced fungi do not associate with native trees or spread within native ecosystems. A. phalloides is one of the few cases where a true invasion is known to be happening. Using historical records of fruit body collections, Pringle and colleagues estimate that A. phalloides has spread from its original introduction point through native California forests at a rate of about 10 km per year. A. phalloides is a high profile mushroom and if we know anything about it, it is because the price of this invasion can be measured in human lives. However, we still know next to nothing about what the invasion of A. phalloides means for local ectomycorrhizal communities, let alone the effects of less obvious ectomycorrhizal invaders. Hopefully these two studies will pave the way for greater exploration in this important topic.
Pringle A, Adams RI, Cross HB, Bruns TD. 2009. The ectomycorrhizal fungus Amanita phalloides was introduced and is expanding its range on the west coast of north america. Molecular Ecology 18: 817-833. (PDF)
Vellinga EC, Wolfe BE, Pringle A. 2009. Global patterns of ectomycorrhizal introductions. New Phytologist 181: 960-973. (PDF)