Lichens are usually forgotten when the diversity of species in a particular area is investigated. They tend to be overlooked and are certainly misunderstood. It’s time for all of us to take out the magnifying glasses and learn a little more about them. They are among the most fascinating of all growth forms in our natural world and provide many benefits.
The Australian continent and Tasmania host 3503 described lichen species of which 36% are endemic, Queensland has 1864 species with 271 being endemic.
What are Lichens?
Lichens are among nature’s wonders, created by a partnership between a fungus and one or more algae and/or cyanobacteria. The fungal partner (mycobiont) names the lichen, provides structure and protects the algal partner against exposure to UV radiation and from moisture loss. The photobiont (alga or cyanobacterium) produces carbohydrates through photosynthesis and shares them with the fungus. This marriage between two completely different organisms works exceptionally well.
Types of Lichens
Lichens come in different growth forms. The shape of lichen body (thallus) can help with the identification. The shrubby or fruticose growth form attaches itself to the substrate at one point only. It takes on the appearance of a miniature shrub, or is pendulous and comes in either cylindrical form or in straplike shape. The foliose thallus form resembles leaf shapes, grows in lobes, and is loosely anchored to the substrate by rhizines which look like but are not roots. While both thallus types described above can be easily removed from the substrate, that is not the case with crustose lichens where the thallus is directly attached to it. Part of the substrate would come off if you attempted to remove the lichen. A not uncommon form is that of squamulose lichens. They have a primary thallus which consists of tiny scale structures from which secondary thalli (podetia) emerge.
The mycobiont in this amazing partnership reproduces via spores developed in fungal fruiting bodies (apothecia) which emerge from the lichen thallus in a variety of colours and shapes. The fungal reproduction is just like we know it from the world of fungi. In the case of lichenised fungi, the vast majority are sac fungi (ascomycetes) named after the way they produce their spores, a small number of lichenised fungi are club fungi (basidiomycetes). Here the fungal spores would have to find suitable algae or cyanobacteria to create new lichens, not the easiest method to achieve successful outcomes.
Vegetative reproduction is much simpler for a lichen as fungal hyphae (threads) already hold algal cells in formations called either soradia (soralia when clustered) or Isidia. Under the microscope or a good magnifying glass Isidia look like little protuberances coming out of the thallus, while soradia emerge from the thallus (often in groups) or along the thallus edge. Only one of the isidia or soradia needs to break off and find suitable conditions to start a new lichen.
Where do lichens grow?
Lichens need light and moisture and a substrate that stands still long enough to allow a thallus to grow. We can often see lichens on fence lines and roof tiles of houses, yet their place in nature is on tree trunks and branches, on rocks, on soil.
Are they easy to identify?
Some of them are. It helps to asses their structure, the way the thallus is shaped, what the surface texture is like, the reproductive method. Colour changes according to moisture and is therefore not that reliable. A so called spot test where minute amounts of specific chemicals are spotted onto the thallus or the medulla (the layer where the photosynthesis takes place) which may provide a colour reaction. Depending on the outcome, the lichen may be identifiable. If these test are not sufficient, further procedures can be initiated like thin layer chromatography where lichen extracts are transferred to a plate and undergo a series of treatments which are intended to identify their chemical components. There are further advancements from here with the last and most precise being DNA testing.
Do lichens have benefits?
Lichens are survivors as much as they are pioneers. They are found almost anywhere from the desert to Antarctica. Did you know that 350 lichens are known to grow in Antarctica but only two flowering plants. Did you know that they can withstand temperatures from +90°C in desert conditions to -195°C (liquid nitrogen in lab conditions)? The European Space Agency sent two lichen species known for their hardiness into the orbit. “...once at the correct orbital altitude, opened [the capsule] to exposure the samples inside to open space, i.e. exposed to vacuum, wide fluctuations of temperature, the complete spectrum of solar UV light and bombarded with cosmic radiation...” (www.esa.int). The lichens were returned to earth after 14.6 days and continued to function.
Lichens contain acids and these, over a very long time, break down rocks and help the formation of soil. Lichens are also the first to create what is termed soil crusts where soil is exposed. They do that together with hornworts, liverworts, mosses, and cyanobacteria and provide (i.e. moisture) protection and erosion control support. This is especially important in dry areas such as South Australia but can be observed easily in our catchments area. Lichens’ water absorption rate is 3 to 35% of their weight. Water is usually taken in during rainfall. Some lichens only absorb through fog and mist .
While it is known that invertebrates use lichens as camouflage and birds use lichen material for next building, it is not so much known that lichens provide an important food source not only for invertebrates (i.e. snails) but also for smaller mammals.
Lichens are being used as bio-indicators in many countries. They are ideal for monitoring climate change through observation of the movement of lichens from/to different climate zones. Knowing lichen species and their growth rates and knowing that they will be the first to establish themselves on exposed rocks, they can be/are being used to assess time frames for retreating glaciers.
Most countries in at least the northern hemisphere use lichens to monitor pollution because of the way they absorb not only water but also, for instance, sulphur dioxide and nitrogen compounds. Exhaust fumes, heavy industries with no or few environmental safeguards and fertilisers can all contribute to the disappearance of lichens. Compare the Inner City area to, for instance, locations with good air quality further out, and you will find a significant change in lichen flora.
Threats to Lichens
Lichens find their own suitable spots in a landscape. Rocks covered in beautiful lichens and transported into a garden are bound to lose their appeal. Hiking boots and mountain bikes in unauthorised areas may well disturb precious soil crusts described above. Fires, whether naturally occurring or through strategic back burning will most certainly come with lichen destruction. Some may well be lost for good, others will take years if not decades to re-establish themselves. Lichens are very slow growers. To hold a lichen thallus covering the palm of your hand may well mean that you are holding 50 plus years of growth. Therefore, unless for scientific purposes, do not remove lichens from the wild.