Antscape: ghosts in the graveyard

Despite the title, it's early March and so couldn't be much further from Halloween, but wildlife respects its own calendar, not ours... Anyhow, having gone for a wander in the woods a few days ago, I took my usual shortcut through the nearby churchyard and noticed something intriguing. On many of the graves, especially on the corner-stones, tussocks of soil and grass had developed. I must have seen this dozens of times before but it had never really grabbed my attention - this time however, I stopped to take a closer look...

Tussocks on the corners of a grave.

From a distance, these tussocks just look like long grass that the mower has missed. However, close up it is clear that they are actually formed of fairly loose soil with grass and moss growing on it. Like any good ecologist, I began to delve...

Yellow Meadow Ants (Lasius flavus) in a churchyard 'tussock'

As soon as pulled open one of these tussocks, it was clear how they had formed - they are actually anthills of the Yellow Meadow Ant (Lasius flavus). L. flavus is a common species, associated with the formation of 'antscapes' comprised of hundreds of closely spaced anthills on undisturbed grasslands (although sometimes they don't form anthills). In Britain it is the only species building long-term nests of the 'unthatched mound' type - a couple of others such as L. niger produce occasional small, temporary mounds, while the Wood Ant Formica rufa produces much bigger mounds covered ('thatched') with bits of twig etc. (Skinner & Allen 1996). L. flavus nests are up to about 30 cm tall, the mound effectively being the spoil heap from soil dug out when the colony builds its underground nest of tunnels and chambers. Being subterranean, L. flavus is rarely seen unless sought out, despite probably being Britain's commonest ant - the underground habit has led to reduced eye development and even soil is brought to the surface at night (Pontin 2005), so daytime activity is unlikely to be seen in passing - certainly I saw no ants on the surface. This also keeps different species apart - in this case, L. flavus lives in the same areas as L. niger, but the latter lives on the surface forming much larger territories (active L. flavus mounds can be as little as 2 m apart, below which inter-colony competition is too intense and queens are attacked prior to establishment of a new colony) (Pontin 2005). As the ants cannot make mounds in mowed (or heavily grazed) areas, it is clear that their use of the graves as a focus for their anthills allows them to avoid disturbance by mowers. In meadows, I have seen newly forming nests being built around plant stems - possibly to provide an initial scaffolding for the loose soil particles.

Like many ant species, L. flavus feeds on the sugary 'honeydew' produced by aphids that are tended by the colony. However, the ant-aphid relationship is more complex than this as L. flavus has 'collected' a number of subterranean aphid species (e.g. of the genera Forda, Geoica, Aphis, Tetraneura and Baizongia) which, in Britain, have lost their sexual generation associated with woody plants and are now entirely asexual on the roots and stolons of grasses. Others such as Sappaphis bonomii lay over-wintering eggs on plants above ground, and L. flavus tends them as if they were ant eggs (at the wrong time of year). Not only that, but as well as using aphids as sources of honeydew, L. flavus also uses them as prey to feed its larvae (Donisthorpe 1927, Pontin 2005). The aphids rarely disperse openly above ground level, suggesting that despite the possibility of being eaten by ant larvae, the protection of subterranean ants outweighs this risk. Further, this may effectively form a type of 'culling', keeping aphid density below levels where they tend to start producing winged forms for dispersal.

So, although this is a common and widespread species, it hides a symbiotic lifestyle that is rarely seen despite taking place beneath our feet as we walk across many an old pasture or other undisturbed grassland. I'm certainly tempted to ask permission to investigate these churchyard mounds more closely and maybe learn more about the aphids and other 'guest' invertebrates that can be found within. As the church (St. Mary's, Bishopstoke) was consecrated in 1891, there has been plenty of time for the ant community to develop. Lastly, I want to look briefly at the landscape, or 'antscape', effects of L. flavus. In an old pasture, the anthills are close together (active ones may abut or overlap old inactive ones), but their arrangement is effectively patchy and random. In the churchyard, the association with graves means that this is not the case and the anthills are arranged more or less in a grid matching the positions of graves - even where these have since disappeared - in such cases the pattern of anthills marks the outlines of old graves - ghosts in the graveyard!

Pattern of L. flavus anthills following the locations of current and missing graves.

References

Donisthorpe, H.St.J.K. (1927). Guests of British Ants. Routledge, London. [An old classic; can be a bit expensive, and a lot of the taxonomy has changed, but still worth having].
Pontin, J. (2005). Ants of Surrey. Surrey Wildlife Trust, Woking. [Maps of ant distribution in Surrey, but plenty of other more widely applicable information about British ants].
Skinner, G.J. & Allen, G.W. (1996). Ants. Richmond, Slough. [An excellent little book in the Naturalists' Handbooks series. Includes species-level keys to the British ant fauna. If you want just one book on British ants, I recommend this].

Further reading

Agosti, D., Majer, J.D., Alonso, L.E. & Schultz, T.R. (2000). Ants. Standard Methods for Measuring and Monitoring Biodiversity. Smithsonian, Washington DC. [Takes a global approach with particular emphasis on the tropics and Americas, but covers a range of widely applicable techniques and ideas].