This blog is the third in a series, and follows on from Part 1, in which I wrote about the role of fungi in helping to break down dead wood, and Part 2, which focussed on slime moulds. Here I’ll describe some of the invertebrates associated with dead wood, including springtails and mites.
A variety of small invertebrates can usually be found by close examination of dead wood, and amongst the most common of these are springtails. Known in scientific terms as Collembola, they are hexapods (ie creatures with six legs), but are not insects – they are differentiated from that group by having internal mouthparts (whereas insects have external mouthparts).
Springtails derive their common name from the presence of a forked tail-like appendage called a furcula on the underside of their bodies. The furcula is folded in on itself and held under tension against the abdomen. When the tension is released, the furcula flicks against the surface the springtail is resting on, enabling the invertebrate to spring away suddenly and rapidly. Having no wings, springtails are unable to fly, but this evolutionary adaptation enables them to escape from potential predators. The photographs below show a springtail with its furcula in its folded position (left) and extended after the tension has been released (right).
Springtails prefer to live in moist conditions so are commonly found in rotting wood and old logs etc. They do not break down the dead wood directly themselves, but are part of the detritivore community, and many species feed on the hyphae and spores of fungi. By doing so, they help with the redistribution and circulation of nutrients that have been freed up from the dead wood by the action of fungi.
Springtails are very small in size and mostly range from about 1 mm to a maximum of 6 mm in length, with a few species reaching 10 mm. As a result they are rarely noticed by the general public, and are relatively unknown compared to other invertebrates such as insects. However, they are claimed to be some of the most abundant creatures on Earth, with estimates indicating there can be as many as 100,000 individuals in a square metre of soil!
Springtails are preyed upon by many other creatures including spiders, mites, beetles, flies and pseudoscorpions. They are an integral part of the food web within the living ecosystem that is a woodland and although they are out of sight most of the time they play a vital role within the overall forest community.
Another group of organisms that can be found in dead wood are mites. These are very small creatures, with most being 1 mm or less in body length, and in scientific terms they are classified as arachnids, because they have 8 legs, like spiders and harvestmen. Mites actually occur in a range of habitats, with some of them inducing galls on the leaves of trees, whilst others are parasites and some even live in water. Here I’m focussing only on those species that can be found on dead wood.
The mites that occur on dead wood and in leaf litter on soils are mainly those known as Oribatid mites. They are decomposers and consume dead plant material, as well as fungal material and even some lichens. Because of their small size they are usually not seen by most people, but dedicated searching of the right habitat will reveal them, sometimes in great abundance.
To date, biologists have identified and classified over 48,000 species of mites, but there are large numbers of undescribed species, possibly a million or more. Amongst those that have been described, some of the most remarkable are the Linopodes mites. These have exceptionally long front legs (that are often mistaken for antennae!), and they are used by the mite for feeling its way around, in a somewhat similar fashion to how many insects use their antennae.
Some of the most common mites I’ve encountered here on the Findhorn Hinterland are those in the genus Cepheus. These are small black mites with bodies that are roughly circular in shape and legs that are dark red in colour. I’ve seen them mostly crawling around on fungi on dead logs so I presume they are fungivorous in their diet. What is most dramatic about them though is the behaviour of their nymphs.
They retain their exuviae, or cast skins, as they grow in size, and position them on their backs to form a distinctive spiny-looking shape. This is presumably a defence mechanism to provide some protection from predators that might otherwise eat the nymphs. They are rather bizarrely-shaped mini-beasts moving slowly across the fungi-covered surfaces of old logs, and in some instances at least it seems as though some of them get engulfed by the fungi, as the following images illustrate:
Here’s some brief video footage of one of these black mites (Cepheus sp.) and a nymph of the same species on some white fungus on the underside of an old fallen dead branch of a Scots pine on the Findhorn Hinterland:
Because of their tiny size, these mites have a very different experience of the world to larger creatures such as birds or humans. Even the smallest bumps on the substrates that they live on create challenges for them to negotiate, and fungi such as the swarming spine fungus (Mucronella calva) provide a veritable obstacle course that needs to be traversed carefully, as can be seen in the short video clip below.
Other Orders of mites, apart from the Oribatid mites, have very different roles in the ecosystem, and their diets are correspondingly varied. Some of them are predatory, feeding on other very small organisms that live on, or inside, the habitat provided by dead wood. Some, like this predatory Rhagidiid mite with its semi-translucent white and yellow body, are quite exotic-looking in their appearance.
The mites live alongside a variety of other mini-beasts in what is for them the vast habitat provided by a log or a standing dead tree. Those include millipedes, centipedes, spiders and ants and I’ll be featuring those and others in Part 4 of this blog.
My thanks and appreciation to Peter Shaw and Frans Janssens for their help and expertise in identifying the springtails in my photographs in this blog, and to Joanna Makol for identifying the Rhagidiid mite.