Diplopoda — Ecology

CHAPTER 12

DIPLOPODA — ECOLOGY

JEAN-FRANÇOIS DAVID

HABITATS Range of habitats

Millipedes are terrestrial arthropods that typically live in decomposing plant litter on the forest floor (Golovatch & Kime, 2009), i.e. an environment that provides food for detritivores and is relatively buffered against temperature and moisture fluctuations. However, while their feeding habits have been highly conserved during evolution (Shear & Selden, 2001), these arthropods are currently represented in a wide variety of habitats and microhabitats. Many juliformian millipedes which are capable of burrowing have successfully colonized more open, less buffered habitat types. In South Africa, dozens of spirostreptidan species live in savannah, five genera being confined to this biome (Hamer et al., 2006). In Europe, many Julida thrive in grasslands (Kime & Golovatch, 2000). Millipedes also occur in more extreme environments such as deserts and high altitude areas above the tim- berline. The most common species in warm deserts are large Spirostreptida, whereas the fauna of high mountain habitats is more diverse and dominated by either Chordeumatida or Polydesmida depending on the geographical area (Golovatch & Kime, 2009). Little is known about the biology and ecology of the great majority of species living in extreme environments but, clearly, they must be capable of withstanding long periods of inactivity due to drought or cold. For example, the spirostreptid Orthoporus ornatus in deserts of the southwest USA remains dormant for about eight months of dry season (Crawford et al., 1987). By contrast, many species have evolved in an opposite direction and colonized caves and other subterranean habitats, in which diurnal and seasonal variations in temperature and humidity are much reduced. Millipedes are often numerically dominant in these environments, which are inhabited by both surface-dwelling species that enter caves sporadically or seasonally (troglophiles) and obligate cave dwellers (troglobionts) (Culver

& Pipan, 2009). Troglobiotic millipedes, although they remain imperfectly known, have been described in large numbers in some countries, e.g. 19 species and subspecies in Bulgaria (16% of the total diplopod fauna) (Stoev, 2007), 37 species in France (Geoffroy, 1997) and 85 species in the USA (Culver & Shear, 2012). Many show morphological adaptations usually found in cave-dwelling organisms, including elongated legs and antennae, and reduced eyes and pigmentation. The ‘climatic relict hypothesis’ posits that many troglobionts have evolved from surface-dwelling ancestors that sought refuge from climatic stress, notably during the periods of glaciation of the Quartenary (Culver & Pipan, 2009). However, other factors may have been involved in the colonization of caves by millipedes, especially in tropical areas. For example, many troglophilic and troglobiotic cambalopsids from southeast Asia are strongly associated with the presence of guano (Deharveng & Bedos, 2012), and this food source may have been as important as buffered climatic conditions in determining their colonization success. The presence of millipedes in a variety of unusual habitats has often been reported (Hopkin & Read, 1992; Golovatch & Kime, 2009). A few species are arboreal, even large pill millipedes such as the sphaerotheriids Sphaerotherium punctulatum in South Africa and Zoosphaerium arboreale in Madagascar, which climb trees more or less regularly for feeding (Haacker & Fuchs, 1972; Wesener & Sierwald, 2005). Other millipedes have adapted to semi-aquatic or aquatic environments. A few species, such as Thalassisobates littoralis (Nemasomatidae) and Dolichoiulus tongiorgii (Julidae), are exclusively littoral (Barber, 2009). A larger number of species live near rivers and some have adapted to survive periodical flooding. In Amazonian forests subject to seasonal inundation, most millipedes escape flooding by migrating for weeks or months on emerged plants and trees (Adis et al., 1996). The Amazonian pyrgodesmid Myrmecodesmus adisi appears to be unique in being capable of surviving under water for months: the species retreats under the bark of submerged tree trunks, where it breathes through a thin air layer trapped along the cuticle (plastron respiration) (Adis et al., 1996). This type of underwater respiration is however used by other polydesmidan species to resist shorter submersions. In caves of northern Italy, for example, the polydesmid Serradium semiaquaticum that lives near subterranean streams enters water spontaneously, where it breathes without difficulty (Adis et al., 1997). A number of habitat analyses were conducted at different spatial scales, mostly in temperate areas, to identify the abiotic and biotic characteristics that best explain the distribution of species in nature (Haacker, 1968; Kime & Wauthy, 1984; David, 1990; Branquart et al., 1995; Lee, 2006; Voigtländer, 2011). They have revealed significant relationships between environmental variables and the presence of particular species, as detailed below. However, as several environmental variables may covary in field studies, laboratory experiments are often necessary to identify which variable(s), singly or in combination, really influence(s) distribution.

Climatic and microclimatic inﬂuences Irrespective of the geographic distribution of species in the major climate zones of the Earth (Chapter 13), differences in climatic conditions within a species’ range are