My thoughts on niche partitioning in fungi
With so many plants and fungi jostling for space on a forest floor or open grassland, you’d think that they should all be in fierce competition, evolving strategies to outdo each other. As much as this makes for good TV documentaries the truth is far tamer. By and large, fungi have a ‘live and let live’ strategy for getting on with their neighbours. Perhaps they recognise that diversity is the key to holistic systems!
Mycologists who study fungi think that high diversity in ectomycorrhizal fungi communities involve strategies that allow competitive coexistence in a small, uniform host environment. The host being a tree or plant at the centre of the fungal web and the uniform environment being the soil directly around it. The terms they are using for these strategies are: niche partitioning, disturbance-related patch dynamics, density-dependent mortality and competitive networks.
Niche partitioning involves a lottery strategy. The lucky winning fungal spore move into rapidly colonise a vacant space on a ‘first come, first serve’ basis. They quickly become the dominant mycelium, attaching themselves to fine roots, and keep other species out.
However, they’ve found that various ectomycorrhizal fungi species might also coexist, overlapping in a small space. They can do this and survive because fluctuating environmental conditions allow different species to thrive at different times. Some years, certain species may seem almost dormant, a coexistence called ‘the storage effect’. These fungi produce spores that can persist in soils, waiting to germinate until the conditions are right. They can wait for very long periods of time.
Some appear to take it in turns, for example to focus on fruiting versus growing underground, known as a trade-off. For example, after a forest fire in a bishop pine forest, small islands of trees were left standing, on which mushrooms fruited. When a study by Peay (2007) analysed their DNA, the pungent slippery jack Suillus pungens was found in most (43%) of the fruit body samples but in the least (13%) of the root tip samples. Alternatively, the Camembert brittlegill Russula amoenolens was in less (23%) of fruit body samples, but it made up more (35%) of root tip samples. This might explain why some years a particular mushroom will fruit in large quantities but hardly at all the following year, while another species is scarce that first year and abundant the next.
Interestingly, in Monterey pine forests, the pungent slippery jack is almost always found in the company of the pine spike Chroogomphus vinicolor [Stevens, n.d.].This relationship is explained because the pine spike is parasitic on the mycorrhizal root-tips of the slippery jack. We tend to think of ‘parasitic’ as a negative, destructive action. A more modern word is ‘biotroph‘ – an organism that eats its host but also helps to keep it alive – as it cannot survive in a dead host. In this case, the mutualism is in everyone’s best interest.
One of the problems with many studies on ectomycorrhizal fungi (EMF) communities is that so often the information is framed from a human perspective. For example, ‘Another way that EMF species can gain a competitive advantage is by colonizing resources ahead of their competitors.’ This language assumes that having the most resources, like inhabiting the most space, is the winning objective for every species. That is a very human way of looking at the world. Perhaps, each species is happy to fulfil its niche and doesn’t care to be the best resourced, or largest occupant if it is at the expense of the health and longevity of the forest ecosystem? Growth at all costs has been a dominant human strategy and ended up threatening the existence of our species and many others on this earth. A more humble aim of living within our means, and taking turns, has still not occurred to enough people to halt the Great Extinction.
What we do know is that the world of plants and fungi is far more complex than we had ever imagined. And that, far from just being a ‘survival of the fittest’, ‘dog-eat-dog’ world, nature has many different ways of cooperating so that everyone gets a turn to thrive. We would do well to take note.