Mutualisms, in which two or more species provide each other with services or resources that they can’t produce on their own, are everywhere you find living things. Mutualists offer protection, help transport pollen, and provide key nutrients.
Even when a mutualist’s services aren’t absolutely vital, they can help make stressful environments tolerable. That’s the insight behind a new study that finds the help from one group of mutualists could allow an unremarkable-looking species of grass to colonize more than 25,000 square kilometers (almost 10,000 square miles) of territory where it otherwise wouldn’t survive.
The study, which is online ahead of print at the journal Ecology Letters, focuses on Bromus laevipes, or Chinook brome, a grass species native to the U.S. west coast. Many populations of Chinook brome are heavily infected with a fungus in the genus Epichloë, but they’re not sick. The fungus is a mutualistic endophyte, a member of a class of fungi that help, rather than hurt, the plants they infect.
Mutualistic endophytes are essentially parasites that made themselves at home. Like many parasitic fungi, they grow throughout the tissues of their host plants, insinuating themselves into the spaces between the host’s cells and making a living on the sugars the plant produces by photosynthesis. (“Endophyte” literally means “within plant.”) Unlike parasites, though, mutualistic endophytes are usually vertically transmitted—passed on from a parent plant to its offspring—and they repay their hosts by producing chemicals that fight off damaging parasites and herbivores, or by protecting the host in stressful conditions.
The latter is the case for Chinook brome, as Michelle Afkhami, Patrick McIntyre, and Sharon Strauss found when they surveyed populations of the plant across California. Patches of Chinook brome tended to be either overwhelmingly infected by fungal endophytes or completely free of them. When Afkhami and her coauthors cross-referenced the climate conditions with the infection rate, they found that highly-infected Chinook brome populations were in different climate conditions than uninfected populations. To make a formal comparison, they created species distribution models—statistical models of climates where a species is found—based on the temperature and rainfall where they found either infected or uninfected Chinook brome.
As you can see from the figure, the distribution models identified overlapping, but far from identical, areas as suitable for Chinook brome with and without the endophyte. Afkhami et al. estimated that the differences between these areas were greater than expected by chance—and, examining the climates occupied by other species of brome found in California, they saw that the differences between climates occupied by infected and uninfected Chinook brome were similar to the differences between climates occupied by entirely different brome species. Infected brome generally occupied drier areas, and the authors estimated that 25,277 square kilometers of California were suitable for Chinook brome with the endophyte, but not without it.
All of this, however, is observational evidence. To really demonstrate the importance of the endophyte to its host, you need an experiment comparing infected and uninfected plants grown in the same conditions. Afkham et al. did just that, growing infected and uninfected brome in common gardens across a range of natural rainfall rates, and subjecting them to artificial drought stress in a greenhouse experiment. Endophyte-infected plants were more likely to survive in low-rainfall gardens than uninfected plants—and while infected plants tolerated water deprivation in the greenhouse better than either naturally uninfected plants, or plants from infected populations that were treated with fungicide to kill off the endophyte.
Other endophyte species have been shown to protect their hosts against drought stress, and it looks like this experimentally-established benefit lines up well with the observed differences in sites where Afkhami et al. found Chinook brome with and without endophytes. What’s particularly striking about this study’s results is the scale of the habitat that its endophyte makes available to Chinook brome—enough to make it almost like a different species.
Afkhami, M. E., P. J. McIntyre, and S. Y. Strauss. 2014. Mutualist-mediated effects on species’ range limits across large geographic scales. Ecology Letters. doi: 10.1111/ele.12332.
Clay, K., and C. Schardl. 2002. Evolutionary origins and ecological consequences of endophyte symbiosis with grasses. The American Naturalist. 160 Suppl:S99–S127. doi: 10.1086/342161.