Earthworms in relict natural woodlands on the westernmost Canary Islands: species richness and ecological preferences

Abstract

This is an investigation of the earthworm species identified in the evergreen laurel forests of La Palma and Hierro, characterised by the presence of Palaearctic species and to a lesser extent by an exotic component of different origins such as Amynthas corticis (native to Southeast Asia), Microscolex phosphoreus (South America) and Ocnerodrilus occidentalis (Central America). There is little evidence of local speciation, confirming that this ancient forest (more than 300 000 years old) does not harbour endemic species. Soil pH and moisture influence the spatial ordination of some earthworm species. However, anthropogenic disturbance, greatly intensified since the fifteenth century, emerges as the strongest factor to explain their distribution, particularly of the tropical invasives, e.g. A. corticis and O. occidentalis, which once established are difficult to remove. Species such as the Eiseniella tetraedra and Heraclescolex moebii complex maintain narrow microhabitat preferences and their current distribution and perhaps subsistence is threatened, mainly due to the overexploited water resources. The colonisation history of earthworms can be linked with the human settlers who began to arrive almost three millennia ago, bringing rootstocks of some of their habitual crops and domestic animals. Faunistic similarities are found with those Southern Iberian, Mediterranean basin and Maghreb areas that during the Miocene harboured humid subtropical Tertiary–Tethian flora, now largely extinct.