OPINIONS

The Melting Pot Below

I discovered a new species this week.

At least, I think I did. It’s always a little hard to tell when working with ectomycorrhizal fungi sampled from holes in the ground. Although many of these fungi will produce mushrooms at some point in their lives, I rarely get to see those reproductive structures. Instead, I use DNA sequences taken from the interface of the fungus and its host: tree roots. This DNA sequence, unique to each species, serves as a barcode for microscopic bits of fungus that are otherwise unremarkable.

Ectomycorrhizal fungi are a special breed. They’re what ecologists term “mutualists,” organisms that work closely with another species (in this case, a variety of plants) to the benefit of each partner. For the ectos I study, this means helping trees gather water and nutrients from the soil in exchange for sugars, which the tree provides by conducting photosynthesis. The more efficient a tree’s compliment of invisible nutrient-acquirers below, the more successful the host tree, and the happier the fungi it supports. It’s a win-win.

Well, most of the time.

Just like you’d expect in any group of humans, fungal society has its benefactors and its cheaters, its winners and its losers. And as far as we can tell, the host tree, which may be interacting with dozens of different fungal species at a time, is an imperfect referee. Over evolutionary time, it must be making generally the right calls with sanctions and rewards, or the mutualistic relationship would be overrun by sugar thieves. But in the moment, it must be hard to tell which fungus on what root tip is a freeloader and which one is just having a bad day.

Go work on an invasive tree species in New Zealand, and the story gets a whole lot more complicated. The island nation, about two thirds the size of California, has a spectacularly rich suite of native fungi, including dozens of ectomycorrhizae, some of which are yet to be identified (like those mystery DNA sequences showing up in my dataset this week). It’s also got hundreds of transfer species from all over the world, brought in accidentally in bits of soil (that’s why I can count on Kiwi border security for a free boot cleaning every time I fly in) or deliberately to aid the growth of foreign trees (that’s why I work on Douglas fir, a North American species planted for timber and known to be, well, a little promiscuous in its fungal associations). In my relatively limited sampling, I’ve come across fungi from nearby Tasmania, rather more distant China and Argentina and the far northern reaches of the Arctic.

Although we know relatively little about the mechanisms by which trees and fungi identify and exchange materials with their partners, we know that they must operate with some specificity to put the brakes on cheating. Which means that when you have fungal and tree species separated by millions of years of evolution that have just been suddenly thrown back into the mix, it’s hard to guess who will come out on top.

Will a flexible host like Douglas fir thrive in this Southern Hemisphere melting pot? Will the introduced fungal species stage hostile takeovers of defenseless native root systems? Will the remarkably biodiverse Kiwi fungi hold their ground with some as-yet-unrecognized special adaptations?

We know the answers in a few cases. There are some remarkably clean examples of “co-invasion,” in which foreign pine trees need to form associations with ectos from their native range to grow wild. And, at least in the early stages of fungal introduction, it seems that the foreigners are coexisting with the natives, though we don’t yet know the terms of those peace treaties.

Of course, the future of New Zealand’s fungi is just one chapter in a global book about biodiversity post-human globalization. We’ve spent centuries mixing up the geographic distributions of life on Earth. Sometimes, our relocations have been deliberate attempts to make a foreign land more familiar; in other cases they’ve been regrettable accidents. Today, though we give more lip service (and more security time) to invasive species risks, we also ship more people, goods and propagules around the world than ever before.

Regardless of our original intent, we now live in an epoch some biologists are calling “The Homogocene” while they wait to see whether we’ve turned an evolutionary mosaic into a melting pot and, if so, which scrappy species will emerge victorious.

Holly welcomes reader questions, comments and suggestions for additional Jurassic Park quotes to incorporate via email at hollyvm@stanford.edu.

About Holly Moeller

Holly is a Ph.D. student in Ecology and Evolution, with interests that range from marine microbes to trees and mushrooms to the future of human life on this swiftly tilting planet. She's been writing "Seeing Green" since 2007, and still hasn't run out of environmental issues to cover, so to stay sane she goes for long runs, communes with redwood trees and does yoga (badly).