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 fascinating interactions in nature.
‘Symbiosis’ is derived from Greek, and means ‘living together’. Applying it in its strictest sense, ecologists use the word to refer to a range of different interactions. Parasitism is where one organism feeds on another, harming, but not necessarily killing the host; commensalism is when an organism lives on a host, ‘hitching a ride’, but not really causing any harm; mutualism refers to those interactions in which both organisms benefit. In more popular usage, symbiosis usually refers to mutualistic relationships, and it is these, and the vital role they play in the Caledonian Forest, that we will explore below.
Lichens rest near the foundations of many ecosystems. These intriguing organisms are actually made up of a fungus and an alga. The fungal partner forms the solid structure of the lichen, while the alga provides the energy through photosynthesis – the process of converting the sun’s energy into sugars. This is a very close alliance called endosymbiosis, a term used to describe a symbiotic relationship in which one organism lives within the cells or body of another.
There are many species of lichen within the Caledonian Forest. Among the more frequently seen ones are map lichen (Rhizocarpon geographicum) – a crust-like species that grows on rocks. Amazingly, it secretes acids which can dissolve rock, and so helps to start the process of soil formation. Lichens can form a substrate on which other plants can grow, and they are often habitat for tiny mites, spiders and other invertebrates.
Fungi are another crucial, but often under-appreciated part of all forest ecosystems. Mycorrhizas are symbiotic relationships between certain fungi and the roots of plants. The fine fungal threads (called hyphae) either ensheath or penetrate the host plant’s roots. The fungus helps the plant to extract nutrients and water from the soil, and also protects its host against harmful organisms. In return, and in common with lichens, the fungus receives sugars via the plant’s photosynthesis.
As with most mutualistic relationships, each partner grows better in association with the other than it would individually. Birch (Betula spp.) has a number of these partnerships, the most familiar being with the red and white fly agaric (Amanita muscaria), as well as with the chanterelle (Cantharellus cibarius). Scots pine has mycorrhizal associations with over 200 species of fungi in Scotland, including another kind of chanterelle (Cantharellus lutescens). In fact, the majority of plants in the Caledonian Forest benefit from mycorhizzal relationships, and it is thought that 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, structures that perform special functions within the cell. These are thought to have 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 are considered to have 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, as is clearly illustrated by the relationship between alder (Alnus glutinosa) and a bacterium (Frankia alni). In this case, Frankia lives within special nodules on the roots of the alder (another example of endosymbiosis), and absorbs nitrogen from the atmosphere, ‘fixing’ 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, and alder has been used in ecological restoration projects in various parts of the world, to restore depleted soils.
Ruminants are hooved mammals that digest their food in two stages. Examples in the Caledonian Forest include red deer (Cervus elaphus), roe deer (Capreolus capreolus) and the now-extinct aurochs (Bos primigenius). 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 where bacteria break down the otherwise indigestible cellulose in the plant material. While these bacteria use nearly all of the glucose from the cellulose, they produce volatile fatty acids, providing their host mammal with energy.
Many of these relationships are difficult to see, but pollination is a form of symbiosis that can be observed quite easily. There are many flowering plants in the Caledonian Forest. Flowers act as powerful advertisements to insects, offering energy-rich nectar. The visiting insect, having fed upon the sugary liquid, then goes on to carry pollen to fertilise other flowers, benefiting the overall population of that particular plant species.
Some insects are fairly specific in their choice of plant. Certain bee species have a longer ‘tongue’ than others, and this affects their choice of flower. The three banded white-tail bumblebee (Bombus hortorum) (a species found in Glen Affric) for example, chooses deeper flowers such as foxglove (Digitalis purpurea). The shorter-tongued bees can only drink nectar from flowers that are not as deep, such as raspberry (Rubus idaeus) and goat willow (Salix caprea).
Butterflies are also very visible in their role as pollinators. For example, the pearl-bordered fritillary (Boloria euphrosyne) can be found in open deciduous woodland where it visits spring flowers such as bugle (Ajuga reptans), birds-foot trefoil (Lotus corniculatus) or dandelion (Taraxacum officinale), for their nectar.
Pollination probably evolved in response to early insects eating the pollen itself. Plants that offered organic matter (ie nectar) as an alternative to pollen for insects to feed on would increase their reproductive success – the pollen was not only spared, but carried from plant to plant. Insects could still feed, and flowering plants evolved and thrived.
Berries probably evolved in a similar way, with the plant adapting to cope with animals feeding on its seeds. As a result of this development, the bird or mammal gains a meal, while the plant’s seed is not only unharmed, but dispersed, and is often ‘activated’ by passing through the rigours of a digestive system.
The berries of many plants such as rowan (Sorbus aucuparia), holly (Ilex aquifolium) and bird cherry (Prunus padus), all take part in symbiotic relationships with birds. Some mammals such as the pine marten (Martes martes) also feed on, and disperse, berries.
Wood ants (Formica spp.) 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 (Melampyrum pratense) seeds have a fatty attachment on them called an elaiosome. The ants take the seeds to their nests, and feed the elaiosome to their larvae, thus helping to disperse the plant. In areas that have been deforested, such plants cannot return easily without the aid of ants, and this understanding is crucial when attempting to restore woodland ground flora to areas that have been deforested for a long time.
Wood ants have a fascinating partnership with aphids such as Symydobius oblongus. The ants stroke the aphids, stimulating them to release a waste product known as honeydew. This liquid provides the ants with energy and nutrients, while the aphids benefit by gaining the ants’ protection from predators. As the aphids feed on tree sap, their food source is also given protection against other competing sapsucking insects by the ants.
A species of worm (Dendrodrilus rubidus) often frequents the nests of northern wood ants (Formica aquilonia) where the conditions are very suitable for worms, with an abundant food supply. The worms provide a service to the ants by helping prevent the mounds from becoming overgrown with moulds and fungi.
Wolves and ravens
In North America, a theory has developed surrounding a mutualistic relationship between ravens (Corvus corax) and wolves (Canis lupus). Ravens will often associate with wolf packs, as the wolves not only make food (such as deer carcasses) available, but also open up tough hides that the birds would not be able to penetrate very easily by themselves. Ravens have been observed to reveal the location of prey by calling, thereby making hunting easier for the wolves, and improving the chances of a meal for themselves. It is quite likely that similar interactions took place in the Caledonian Forest before these carnivores were extirpated.
Mutually beneficial relationships occur on many different levels and their influence is immense; some of the most fundamental processes in the forest, from photosynthesis to the survival of herbivores, are a result of these alliances.
Sources & further reading
- Campbell, N.A. (1993) (3rd ed.) Biology. Benjamin Cummings: Redwood City.
- 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
- Living larders for bumblebees. Natural History Museum.