Barry Muir | Guest Contributor
In Part 1 (Muir 2024) of this series discussing Globally Decreasing Bird Numbers, I dealt with climate change myths. Then In Part 2 (Muir 2024), the impact of climate change on endophytes, those ubiquitous but unseen upholders of plant and animal life: birds in particular because of their role in fruit, nectar and pollen production. In Part 3 of this series I discuss another type of fungi: mycorrhizae and their connections with birds, and the impact of climate change on those fungi.
Mycorrhizae (literally “fungus-root” in Latin) are highly specialised fungi that allowed plants to leave the oceans millions of years ago and invade land. They are instrumental and essential to the survival and health of almost all plants globally. About 90–96% (depending on definitions and authors) of all the plants on Earth are mycorrhizal. Some plants can survive without mycorrhizae but, unless they have evolved without a mycorrhizal association, they struggle or die. Dispersal of mycorrhizal fungi spores actively shapes plant communities.
There are two types of mycorrhizae: endomycorrhizae which live inside plant roots and are mostly microscopic and unnoticed (Photo 1 below)….
Photo 1. Mycorrhizal fungi attached to the roots of a plant. Photo from Cotton et al (undated), Orchard of Flavours.
…. and ectomycorrhizae which attach to the outside of plant roots and many of which produce some of the well-known mushrooms and toadstools of garden and bushland.
Photo 2. Fruiting bodies of Cortinarius ochraceofulvus, a typical mycorrhizal mushroom. Photo by Barry Muir.
To avoid getting more complicated than necessary, I will call the whole lot just “mycorrhizae”.
The main role of mycorrhizae is to collect soil nitrogen, trace elements (phosphorous in particular) and water, and pass this to the plant. Mycorrhizae are particularly good at this because their hyphal threads may cover a huge area. There may be many kilometres of fungal hyphae in a square metre of soil: much more than the plant’s roots.
The plant ‘pays’ for this mycorrhizal service with sugar that it manufactures using chlorophyll and sunlight. The plants and fungi in this symbiotic relationship ‘talk’ to each other. If the plant needs water or trace elements it cuts back on its sugar supply to the fungus, and the fungus ‘picks up its game’ and passes these requirements across to the plant. Similarly, if the fungus is a bit poor in energy, it cuts back on its trace elements supply to the plant, and the fungus gets some more sugar as ‘encouragement’.
Thus, indirectly, all foliage, flowers, nectar, fruit, seeds, and even woody tissues of mycorrhizal plants depend on the fungi that occupy its roots. Without the fungi, the plant either cannot survive or has a difficult time obtaining the nutrients it needs to grow.
Mycorrhizae also help protect the plants from the poisonous effects of pollutants. The fungi act as a sort of filter, retaining heavy metals that would otherwise be absorbed by the plant.
Further, plants with mycorrhizae show increased tolerance towards various stresses such as drought. The plants are less susceptible to frost and more resistant to disease-causing microorganisms in the soil. The mycorrhizae also produce hormones that promote plant growth.
Climate change, in particular, is adversely affecting mycorrhizal fungi (Boddy et al. 2013; Gange et al. 2007; Pickles et al. 2011; Simões 2021) and, at the same time, making life easier for pathogenic fungi that attack living plants (Desprez-Loustau et al. 2007).
Consequences for birds are obvious. Firstly, as the climate warms the continued existence of certain plants that are hosts for prey insects, and the sources of nectar, pollen, fruits and seeds, may lessen, or may disappear altogether.
Alternatively, their distribution may gradually ‘move’. Obviously, plants cannot get up and walk, but by expanding their range into cooler areas as the climate warms (and dying where the habitat is no longer suitable), they ‘move’ over time to higher latitudes or higher elevations. If the plants’ life-cycle is too long, and global warming too fast, these plants and the heaths, shrublands, woodlands and forests of which they are part, may just die. Even if the plants survive, there is no guarantee that the animals that the plants support can adapt and move to keep up with the pace of change.
Some birds, such as Australian Brush-turkey, Orange-footed Scrubfowl, Lyrebird, Cassowary, Chowchilla, Emu, and even Pale-headed Robin, have been observed eating truffles, the underground fruiting bodies of certain mycorrhizal fungi. The birds play a vital role in spreading the fungal spores, thereby supporting the plants dependent on these species. Examination of Cassowary droppings, for example, has identified many species of fungi (Muir 2023).
Birds may not be able to exploit features of new environments created by plants being in new locations. Birds that occupy the foothills of mountains, for example, may follow their preferred plants and habitats to higher altitudes until they run out of anywhere to go. This has already been observed with certain plants in the mountains north of Cairns (unpublished data). Plants that used to be common in the foothills no longer occur there, but now grow higher up the mountains. Any animals associated with those plants are also forced to move. If they cannot adapt, they die.
Another change already observed in England, and Europe generally, is that fruiting times for mycorrhizal fungi, and even the ability of the fungi to cooperate with their host plants, are changing (van der Linde et al. 2018).
One issue is that industrial and vehicle pollution is increasing the amount of nitrates in the atmosphere (Aihua Zhao et al. 2020; Dasgupta 2018). Mycorrhizae are suppliers of nitrogen to plants and if the plants get their nitrogen via rainfall from the air, they depend less on the mycorrhizae. There is evidence that this reduction in dependence for nitrogen is changing the fungi and resulting in less phosphorous and water supply. If the fungi (or the plants) change their biology too much, they may no longer be able to cooperate with other relationships and either or both will die, resulting in loss of bird habitat, food plants and even nest sites.
References
Available on email request to the author at unit57.may@gmail.com
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