Life competes for all kinds of natural resources, whether they be food, light, water or shelter. But competition is only a part of the picture. Cooperation and mutual benefit are also a foundation of countless interactions in nature.
‘Symbiosis’ comes from Greek, and means ‘living together’. In its technical sense, ecologists use the word to talk about a range of interactions:
- Parasitism is where one organism feeds on another, without necessarily killing the host.
- Commensalism is when one organism harmlessly ‘hitches a ride’ on another.
- Mutualism refers to those interactions in which both organisms benefit. In popular usage, when we talk about symbiosis we usually mean mutualistic relationships. Let’s explore these win-win partnerships and the vital role they play in the forest.
Lichens rest near the foundations of many ecosystems. These amazing organisms are actually made up of a fungus and an alga. Sometimes a third player called a cyanobacteria is also involved. The fungus forms the structure of the lichen, while the alga provides the energy through photosynthesis. This is a very close alliance called endosymbiosis. It means one organism lives inside the cells or body of another.
There are many species of lichen within the Caledonian forest. Among the more common ones are map lichen – a crust-like species that grows on rocks. Amazingly, it secretes acids which can dissolve rock. This helps to start the process of soil formation. Lichens can form a substrate on which other plants can grow. They are often habitat for tiny mites, spiders and other invertebrates.
Fungi are another crucial, but often under-appreciated part of forest ecosystems. Mycorrhizas are symbiotic relationships between certain fungi and the roots of plants. The fine fungal threads (called hyphae) wrap around or penetrate the host plant’s roots. The fungus helps the plant to extract nutrients and water from the soil. It also protects its host against harmful organisms. In return the fungus receives sugars via the plant’s photosynthesis.
As with most mutualistic relationships, each partner grows better with the other than it would alone. Birch has a number of these partnerships, the most familiar being with the red-with-white-spots fly agaric. The sought-after chanterelle is also partner of birch. Scots pine has mycorrhizal associations with over 200 species of fungi in Scotland. In fact, the majority of plants in the Caledonian forest benefit from mycorrhizal relationships. Mycorrhizas helped plants to colonise the land, millions of years ago.
Symbiosis and cells
Indeed, many scientists believe that most major evolutionary leaps were ‘jump-started’ by symbiosis. Plant and animal cells contain organelles. These are structures that perform special functions within the cell. They evolved from endosymbiotic relationships, with one bacterium living inside another cell. A key organelle within plant cells is the chloroplast, which is responsible for photosynthesis. Chloroplasts evolved from cyanobacteria – primitive bacteria that can themselves photosynthesise. Looking around at the tapestry of green, we can really see the scale on which symbiosis has influenced the forest.
Symbiosis works on many different scales. For example there is a relationship between alder and a bacterium called Frankia alni. In this case, Frankia lives inside nodules on alder roots. The bacteria absorb nitrogen from the atmosphere and fix it in the soil. This benefits the alder, which via photosynthesis provides the bacteria with sugars. The soil becomes enriched as a result of this process. In fact people have used alder in various parts of the world, to restore depleted soils.
Ruminants are hoofed mammals that digest their food in two stages. Examples in the Caledonian forest include red deer and the now-extinct aurochs. Ruminants have a complex digestive system, and depend on symbiosis for their survival. After their food is regurgitated to be chewed as ‘cud’, it then enters one of four stomach chambers. Bacteria break down the otherwise indigestible cellulose in the plant material. The bacteria get glucose as part of the bargain. They produce volatile fatty acids, providing their host mammal with energy.
Pollination is a form of symbiosis that is easy to observe. Flowers act as powerful adverts to insects, offering energy-rich nectar. The insect, having fed upon the sugary liquid, then unknowingly carries pollen to fertilise other flowers. This benefits the overall population of that particular plant species.
Many insects are choosy about the plants they visit. Certain bee species have a longer ‘tongue’ than others, and this affects their choice of flower. The white-tailed bumblebee for example, chooses deeper flowers such as foxglove. The shorter-tongued bees can only drink nectar from flowers that are not as deep, such as goat willow.
Pollination probably evolved in response to early insects eating the pollen itself. Plants that offered nectar as an alternative to pollen would stand a better chance of reproducing. The pollen was not only spared, but carried from plant to plant. Insects could still feed, and flowering plants evolved and thrived.
Berries evolved in a similar way, with the plant adapting to cope with animals feeding on its seeds. The bird or mammal gains a meal, while the plant’s seed is not only unharmed, but dispersed. The seeds of some plants (such as rowan) are often ‘activated’ by passing through the rigours of a digestive system.
Wood ants have symbiotic relationships with a number of other organisms in the forest. Some species of flowering plant in the forest depend on ants for their dispersal. Cow-wheat seeds have a fatty attachment on them. The ants take the seeds to their nests, and feed the fatty bit to their larvae, thus helping to disperse the plant. In deforested areas, such plants cannot return easily without the aid of ants. This is important to know if we want to restore ground flora to areas of new woodland.
Wood ants have a fascinating partnership with certain aphids. The ants stroke the aphids, stimulating them to release a waste product known as honeydew. This liquid provides a meal for the ants, while the aphids benefit by gaining the ants’ protection from predators. The ants also protect the aphids’ food source from competing sapsuckers.
A worm called Dendrodrilus rubidus often lives in the nests of northern wood ants. Here the conditions are very suitable for worms, with an abundant food supply. The worms benefit the ants by helping prevent the mounds from becoming overgrown with moulds and fungi.
Wolves and ravens
There is a close relationship between ravens and wolves. Wolves not only make carcasses available, but also open up tough hides that the birds would not be able to penetrate by themselves. Wolves often howl before going on hunt and ravens have learned to respond to this cue. It may be that wolves have also learned to listen to raven calls indicating that there are deer about. Perhaps similar interactions took place in the Caledonian forest before humans wiped out wolves.
Mutually beneficial partnerships occur on many different levels and their influence is huge. Some of the most fundamental processes, from photosynthesis to the survival of herbivores, happen thanks to these alliances.
Sources and further reading
- Campbell, N.A. (1993) (3rd ed.) Biology. Benjamin Cummings: Redwood City.
- Heinrich, B. (1999) Mind of the Raven. Harper Collins: New York
- Margulis, L. (1998) The Symbiotic Planet. Phoenix: London.
- Steinhart, P. (1995) The Company of Wolves. Vintage Books: New York.
- Wilson, E.O. (1992) The Diversity of Life. Penguin: London