Paper Reviewed
Afkhami, M.E., McIntyre, P.J. and Strauss, S.Y. 2014. Mutualist-mediated effects on species' range limits across large geographic scales. Ecology Letters 17: 1265-1273.

In introducing their study of the subject, Afkhami et al. (2014) state that "understanding the processes determining species range limits is central to predicting species distributions under climate change," noting, for example, that "if individuals with mutualists have different environmental tolerances from those without, they could occupy unique portions of the range, resulting in a larger species range." And they add that "given the ubiquity of such facultative mutualisms in nature (Bronstein, 1994; Bruno et al., 2003), studying their effects is essential to developing a deeper understanding of the processes determining range limits, especially in light of more stressful future climates (Kivlin et al., 2013)."

Taking their own advice, and working with a grass (Bromus laevipes) native to California, the three researchers used (1) field surveys of 92 populations, (2) ten common garden experiments that were conducted throughout B. laevipes current range, (3) preexisting species distribution models and (4) several greenhouse experiments, in order to demonstrate - in the case of the grass they studied - that "mutualistic fungal endophytes ameliorate drought stress and broaden the geographic range of their native grass host by thousands of square kilometers (~20% larger) into drier habitats."

This huge effort revealed, as Afkhami et al. describe it, that "positive biotic interactions may be under-appreciated in determining species' ranges and species' responses to future climates across large geographic scales." And they note that its great importance in this regard is demonstrated by the fact that "endophytic fungi are extremely widespread, occurring in every major plant lineage (Rodriguez et al., 2009) with systemic fungal endophytes of the genera Neotyphodium and Epichloe (Clavicipitaceae) residing in the aboveground tissue of an estimated 20-30% of the approximately 10,000 grass species" that have been reported by Leuchtmann (1992) to be found growing throughout the world. Without a proper understanding and incorporation of these important biotic interactions, projections of species range reductions -- such as those anticipated by the IPCC -- as a result of future climate change should be reassessed.

References
Bronstein, J.L. 1994. Our current understanding of mutualism. Quarterly Review of Biology 69: 31-51.

Bruno, J.F.,Stachowicz, J.J. and Bertness, M.D. 2003. Inclusion of facilitation into ecological theory. Trends in Ecology and Evolution 18: 119-125.

Kivlin, S.N., Emery, S.M. and Rudgers, J.A. 2013. Fungal symbionts alter plant responses to global change. American Journal of Botany 100: 1445-1457.

Leuchtmann, A. 1992. Systematics, distribution, and host specificity of grass endophytes. Natural Toxins 1: 150-162.

Rodriguez, R.J., White, J.F., Arnold, A.E. and Redman, R.S. 2009. Fungal endophytes: diversity and functional roles. New Phytologist 182: 314-330.