Jean-François David

The ecology of saprophagous macroarthropods (millipedes, woodlice) in the context of global change

Millipedes (Diplopoda) and woodlice (Crustacea, Isopoda), with a total of about 15000 described species worldwide, contribute substantially to invertebrate biodiversity. These saprophagous macroarthropods, which are key regulators of plant litter decomposition, play an important role in the functioning of terrestrial ecosystems in tropical and temperate areas. Herein we review current knowledge on the effects of climate, food quality and land cover on millipede and woodlouse species to explore their potential responses to global change. Essentially similar trends are observed in the two taxa. Experiments have shown that climate warming could result in higher rates of population growth and have positive effects on the abundance of some temperate species. This is consistent with signs of northward expansion in Europe, although the mechanisms of dispersal remain unclear. The generality of this finding is evaluated in relation to the life histories and geographical distributions of species. At low latitudes, interactions with more severe droughts are likely and could affect community composition. Elevated atmospheric CO2 levels and changes in plant community composition are expected to alter leaf litter quality, a major determinant of macroarthropod fertility via the link with female adult body size. Although food quality changes have been shown to influence population growth rates significantly, it is proposed that the effects of warming will be probably more important during the coming decades. Land cover changes, mainly due to deforestation in the tropics and land abandonment in Europe, are critical to habitat specialists and could override any other effect of global change. Habitat destruction by man may be the main threat to macroarthropod species, many of which are narrow endemics. At the landscape scale, habitat heterogeneity could be a good option for conservation, even at the cost of some fragmentation. Two principal areas are identified which require further work: (i) the effects of climate change across broader geographic ranges, and on species with different ecologies and life histories; (ii) the effects of global change on both macroarthropods and their natural enemies (predators, parasites and pathogens), to improve predictions in field situations.

The importance of biotic factors in predicting global change effects on decomposition of temperate forest leaf litter

Increasing atmospheric CO2 and temperature are predicted to alter litter decomposition via changes in litter chemistry and environmental conditions. The extent to which these predictions are influenced by biotic factors such as litter species composition or decomposer activity, and in particular how these different factors interact, is not well understood. In a 5-week laboratory experiment we compared the decomposition of leaf litter from four temperate tree species (Fagus sylvatica, Quercus petraea, Carpinus betulus and Tilia platyphyllos) in response to four interacting factors: elevated CO2-induced changes in litter quality, a 3°C warmer environment during decomposition, changes in litter species composition, and presence/absence of a litter-feeding millipede (Glomeris marginata). Elevated CO2 and temperature had much weaker effects on decomposition than litter species composition and the presence of Glomeris. Mass loss of elevated CO2-grown leaf litter was reduced in Fagus and increased in Fagus/Tilia mixtures, but was not affected in any other leaf litter treatment. Warming increased litter mass loss in Carpinus and Tilia, but not in the other two litter species and in none of the mixtures. The CO2- and temperature-related differences in decomposition disappeared completely when Glomeris was present. Overall, fauna activity stimulated litter mass loss, but to different degrees depending on litter species composition, with a particularly strong effect on Fagus/Tilia mixtures (+58%). Higher fauna-driven mass loss was not followed by higher C mineralization over the relatively short experimental period. Apart from a strong interaction between litter species composition and fauna, the tested factors had little or no interactive effects on decomposition. We conclude that if global change were to result in substantial shifts in plant community composition and macrofauna abundance in forest ecosystems, these interacting biotic factors could have greater impacts on decomposition and biogeochemical cycles than rising atmospheric CO2 concentration and temperature.

Annual feeding rate of the millipede Glomeris marginata on holm oak (Quercus ilex) leaf litter under Mediterranean conditions

Summary The annual consumption and assimilation rates of Quercus ilex leaf litter by the millipede Glomeris marginata were estimated under Mediterranean conditions simulated in the laboratory. Measurements were made on six occasions throughout the year to take into account seasonal variations in (i) the quality of decomposing leaves, (ii) the physiological state of the animals, and (iii) temperature plus photoperiod. The mean weight-specific consumption rate was 14 g (dry weight) g−1 (live weight) y−1, which is much higher than published figures for Mediterranean oak leaf litter. The consumption rate was highest in spring (51 mg g−1 d−1) and lowest in summer (28 mg g−1 d−1), the summer consumption rate being even lower when determined with air-dried leaves (17 mg g−1 d−1). Assimilation efficiency averaged 6% for the year, with a maximum in spring (9%) and a minimum in winter (1%). A much higher assimilation efficiency (29%) was measured with freshly fallen leaves, but a food preference experiment showed that these leaves were generally avoided and that G. marginata preferred more decomposed leaves despite their lower digestibility. It is concluded that these saprophagous macroarthropods – with field biomasses of 7–11 g (live weight) m−2 – produce considerable amounts of faecal pellets in Mediterranean oak forests.

Belowground biodiversity in a Mediterranean landscape: relationships between saprophagous macroarthropod communities and vegetation structure

Millipedes and woodlice were sampled at 27 sites in a mosaic landscape in order to establish the extent to which the macroarthropod community changed with different plant formations. Multivariate analyses conducted on abundance data for ten species revealed four main types of macroarthropod communities. This classification was highly correlated with vegetation structure and particularly the degree of openness of the sites. Communities dominated by Ommatoiulus rutilans (Julidae) occurred in open grassland; those dominated by Glomeris marginata (Glomeridae) plus Porcellio gallicus (Porcellionidae) were found at the least open sites, with a high oak cover; communities with a high proportion of the endemic glomerid Glomeris annulata occurred in semi-open sites with a substantial cover of shrubs. Species diversity was significantly higher at the semi-open sites, this being interpreted as an edge effect. Population density and biomass were lower at wooded sites. In the context of a regional trend towards woodland expansion, the results are discussed from the viewpoint of conserving the pool of millipede and woodlouse species and of maintaining the abundance of saprophagous macroarthropods in the regions ecosystems.

The use of near infrared reflectance spectroscopy to study chemical changes in the leaf litter consumed by saprophagous invertebrates

Chemical changes in holm oak leaf litter that was consumed by the millipede Glomeris marginata were studied using near infrared reflectance spectroscopy. Compared to leaf litter, faecal pellets were significantly richer in lignin and poorer in nitrogen and readily assimilable constituents. The C:N ratio was significantly higher in the faeces. A NIRS-predicted index of litter decomposability was found to be lower in the faeces than in uneaten litter.

How to calculate leaf litter consumption by saprophagous macrofauna

Abstract Leaf litter, as food for saprophagous macrofauna, loses dry mass with time whether consumed or not, and this affects estimates of food consumption based on gravimetric methods. A model combining discontinuous consumption of food with continuous, exponential decomposition of litter makes it possible to compare the accuracy of various formulae used in calculating the amount of food consumed. Bococks formula yields an underestimate, unless consumption takes place at the very end of feeding tests. The Reiman formula yields an overestimate, unless consumption takes place at the very beginning of tests. A new formula giving an intermediate estimate is proposed and it is shown that this is the most accurate for most patterns of feeding. Calculations taking no account of decomposition (no control) may lead to substantial overestimates, especially if the amount of litter actually consumed is small in comparison with the initial amount.

Millipedes faced with drought: the life cycle of a Mediterranean population of Ommatoiulussabulosus (Linnaeus) (Diplopoda, Julida, Julidae).

Abstract Growth, development and life-cycle duration of the millipede Ommatoiulus sabulosus (f. aimatopodus) were studied in a Mediterranean shrubland of southern France and compared with previous data from northwest Europe. Changes in the proportions of stadia during the course of the year were analysed in several generations. The results show that stadia VII and VIII are consistently reached after the first year of growth, and stadia IX and X after the second year. First reproduction may occur at the age of two years in males reaching maturity at stadium X, but not until the age of three in those reaching maturity at stadia XI and XII. Reproduction cannot occur until at least the age of three in females, which carry mature eggs from stadium XI onwards. In comparison with more northern populations, life-cycle duration is not shorter in the Mediterranean population but there are marked differences in its phenology: the breeding period is in autumn, so that juveniles of stadia II to VI are never faced with the summer drought, and larger individuals are mostly inactive in summer; moreover, all individuals moult once every winter. The results illustrate how julid millipedes of humid temperate regions could respond to higher temperatures and drier summer conditions in the context of climate change.

Additional moults into ‘elongatus’ males in laboratory-reared Polydesmus angustus Latzel, 1884 (Diplopoda, Polydesmida, Polydesmidae) – implications for taxonomy

Abstract The number of stadia during post-embryonic development is supposed to be fixed in most species of the millipede order Polydesmida. For the first time since 1928, additional moults were observed in two males of Polydesmus angustus Latzel, 1884 reared in the laboratory. These ‘elongatus’ males sensu Verhoeff reached stadium IX instead of stadium VIII, with addition of a further podous ring (32 pairs of legs). One male had well-developed gonopods at stadium VIII, which regressed at stadium IX; the other had no gonopods at stadium VIII, which developed at stadium IX. The two cases correspond to the ‘regressionis’ and ‘progressionis’ forms described by Verhoeff in Polydesmus complanatus (Linnaeus, 1761), which confirms entirely his results. Additional moults appear to be associated with small body sizes and possible underlying mechanisms are discussed. Comparisons between millipede orders indicate that post-embryonic development is less strictly canalized in Polydesmida than in Chordeumatida. This implies that the adult number of body rings is of limited taxonomic value in Polydesmida and should not be viewed as a character of generic importance.

First Estimate of the Intrinsic Rate of Increase of a Millipede: Polydesmus angustus (Polydesmida: Polydesmidae) in a Seasonal Environment

ABSTRACT Although dramatic millipede outbreaks have been reported worldwide, there is no data available on the intrinsic rate of increase (rm) of these arthropods. This parameter was estimated in the west-European millipede Polydesmus angustus Latzel, a seasonally-breeding species with mixed 1-yr and 2-yr life cycles within populations. Individuals from 20 broods were reared throughout their life cycle under seasonal conditions simulated in the laboratory (monthly mean temperatures and naturally varying photoperiods), to determine age-specific survival and fertility in annual (1-yr life cycle) and biennial (2-yr life cycle) females. The finite rate of increase (&lgr;m) was calculated using a periodic matrix model and rm was found to be 5.85 per year. This estimate suggests that outbreaks of annual polydesmidan species in greenhouses or under favorable field conditions can be generated by a small number of fertilized females in 2 yr. The relevance of seasonal conditions to measure rm per year in long-lived arthropod species that breed seasonally is emphasized. Estimates such as that obtained in P. angustus integrate most seasonal adaptations of the species and are ecologically plausible in the absence of extrinsic sources of mortality. Finally, the results show that biennial females represent a very small proportion of individuals in the stable population structure, but analysis of short-term dynamics indicates that they may be more successful as colonists than annual females.