Sten Nordstrom

Population density and tissue metal concentration of lumbricids in forest soils near a brass mill

Abstract The effect of metal pollution on species composition, population density, and age structure of earthworms was studied in coniferous forests near a brass mill in SE Sweden. Samples were taken in two biotapes, dominated by Deschampsia flexuosa and Vaccinium myrtillus, respectively, from eight sites at varying distances (175–20 000 m) from the mill. pH and CEC were found to increase in the soil close to the mill. A decrease in the concentration of extractable metals (Zn, Cu, Pb) could be followed up to 1·2 km from the emission source. Copper and lead were analysed in pharynx, gizzard, muscles, clitellum, seminal vesicles and cerebral ganglion of different earthworm species. Generally, the concentration of copper in tissues was higher compared with lead and inversely proportional to the distance from the mill. A substantial metal uptake was found in the vital tissues, such as seminal vesicles and cerebral ganglion. Juveniles tended to have higher metal concentrations compared with adults. Density and biomass of earthworms was proportional to the distance from the mill, especially in the Deschampsia-dominated biotopes. Dendrobaena octaedra was the most abundant lumbricid species but was absent in soils within 1 km from the mill. Data indicate that the reduced density of earthworms near the mill was due to metal toxicity, but food deprivation could not be ruled out as a possible factor. Reasons for decreased density of earthworms at more remote sites are discussed and data on density and soil zinc concentrations are used to predict the density of earthworms at a metal polluted site.

Effect of Bird Predation on Lumbricid Populations

The effects of predation by golden plover Pluvialis apricaria (L.) on populations of Allolobophora caliginosa (Sav.) and Lumbricus rubellus Hoffm. in early spring on a hayfield in Iceland were estimated by two independent methods. Field experiments (lasting for 22 days) with nets preventing bird predation showed that the mean abundance of lumbricids was 238 ind m-2 on protected sites and 107 ind m-2 on exposed sites (P < 0.001). Observations of feeding plovers showed that about 4.5 lumbricids were taken per m2 and day; i.e. about 100 ind m-2 in the experimental period. This agrees closely with the result obtained by the first method. No selective predation could be confirmed but certain differences between the two species of lumbricids are discussed.

Methods of Sampling Lumbricids

The study was carried out on a permanent pasture and in a beech wood in southern Sweden in April and August 1967. Times and localities were chosen to obtain differences in soil physical properties and in associations, abundance and size classes of earthworms. Hand sorting was compared with extraction methods (formaldehyde and potassium permanganate solutions). The extraction methods were tested with and without added detergent. The formaldehyde method was as efficient as hand sorting when estimating populations of Lumbricus terrestris L. during periods of activity. Hand sorting was the best method for studying Allolobophora spp., whereas Dendrobaena spp. and L. rubellus Hoffm. can be studied using all the tested methods. There was a tendency for hand sorting to under-estimate small specimens. Detergent did not increase the efficiency of the extraction methods.

Selective predation on lumbricids by golden plover Pluvialis apricaria.

Predation by golden plovers Pluvialis apricaria (L.) on lumbricids was studied on a hayfield in Iceland in early June 1977 by means of stomach analyses and formalin sampling. Lumbricids, Allolobophora caliginosa (Sav.) and Lumbricus rubellus Hoffm., constituted about 90% (fresh wt) of food remains in stomachs. The plovers had taken relatively more A. caliginosa than L. rubellus and a relatively larger portion of medium-sized (by biovolume) than smaller and larger lumbricids. Thus the predation provided a selective advantage to L. rubellus over A. caliginosa and to the smallest and largest over the medium-sized individuals. The results are discussed in terms of degree of pigmentation (cryptic colouration), burrowing behaviour, activity, and density of the lumbricids. It is suggested that the selective effects of predation may vary considerably from one year to another.

Short-Term Colonization Success of Lumbricid Founder Populations

Small founder units (two or four individuals) of adult Allolobophora caliginosa (Sav.) and Lumbricus rubellus Hoffm. kept in netbags were introduced into a hayfield in NE Iceland. After 14 months 82% of the A. caliginosa units and 35% of the L. rubellus units still contained live adults, and the rates of individual survival were of the same magnitude. Differences in survival between units of A. caliginosa were presumably associated with position of the netbags in the field (exposure, etc.) and initial density of founders; thus, survival seemed to decrease with increasing density. A. caliginosa produced significantly more cocoons and newly hatched individuals than L. rubellus. Colonization success is discussed in relation to the experimental conditions and the difference between the two species in terms of differences in their ecology. It is suggested that A. caliginosa achieves a relatively high post-dispersal colonization success by having a relatively long life-span as adult rather than having a high birth rate.