Collembola, alga and lichen
Collembola, colloquially known as springtails, are ubiquitous and abundant tiny invertebrates, with bodies often no more than about five millimetres long. They are common in leaf litter where they are well-protected against drying out (to which Collembola are very sensitive) and where they may feed on a wide variety of decaying organic matter or micro-organisms. Various Collembola are also found living in the shelter provided by lichens and this case study deals with such a Collembola-lichen association in the Netherlands.
In that country the only large expanses of acidic rock are provided by the stones in megalithic monuments and there are 54 such megaliths, most of them more than 5,000 years old and most covered by sand until about 300 years ago. These megalithic sites host various lichens that are otherwise rare in the Netherlands (or not known there), though they may be more common in rocky areas in nearby countries. The megaliths can also support other growths, especially algae. During one survey of the megalith lichens all specimens collected were sampled for Collembola. In most cases no more then a few Collembola per sample were found but one specimen of the foliose lichen Xanthoparmelia conspersa yielded many tens of specimens of the Collembola species Anurophorus laricis. The lichen looked quite healthy but on the rock around where the lichen had been growing there was an algal-free zone.
To the researchers this suggested a mutually beneficial association between lichen and Collembola. At times the exposed rock surfaces of the megaliths can become quite dry and hot but the surface lichens create niches with milder micro-habitats in which micro-invertebrates such as Collembola can shelter. In cooler, more humid conditions the Collembola could then leave the lichens to feed on the nearby algae. Algae are competitors for space and surface nutrients with lichens so the Collembola are helping keep such competition at bay.
Of the several hundred specimens sampled, conspicuous algal-free zones were found only around specimens from one particular monument. Moreover, such algal-free zones were found only around lichens that supported numerous Collembola. Given these facts the researchers concluded that the zones were best explained as due to grazing by Collembola, rather than by any other hypothesis (such as the inhibition being caused by a toxic lichen secondary metabolite). In the Netherlands Xanthoparmelia conspersa is rarely found on substrates other than the megalithic monuments and is also a species in decline in that country. The researchers noted that greater rainfall would have favoured algal growth. In such circumstances the presence of the Collembola would, as well as helping existing lichen colonies compete against algae, also provide bare rock substrate on which new colonies can become established. Indeed, the photo that appeared with the paper listed below showed some small thalli in the approximately two centimetre band of bare rock around a large and long-established thallus of Xanthoparmelia conspersa.