Biological Conservation 29 (1984) 241-252

Earthworm Populations in Soils Disturbed by Trampling

T. G. Piearce

Department of Biological Sciences, University of Lancaster, Bailrigg, Lancaster LAI 4YQ, Great Britain

ABSTRACT

Heavy trampling by livestock in pasture entrances considerably reduced earthworm density and biomass, as did vehicular compaction on a human path through pasture. Surface species were most severely affected, whilst the deep burrowing Allolobophora longa proved particularly resistant. A ll species showed a down ward shift in vertical distribution. A laboratory experiment demonstrated the great effectiveness of A. longa in ameliorating the effects of soil compaction. Earthworm population changes on trampling are likely to facilitate recovery of soil drainage once trampling ceases.

INTRODUCTION

Numerous studies have demonstrated the effects of trampling on physical and chemical properties of soils (see Briggs, 1978; Saini, 1980) and on the nature and abundance of vegetation and fauna (review by Liddle, 1975). Earthworm abundance and biomass have been shown to decline as a result of trampling by humans and vehicular compaction (Chappell et al., 1971; Aritajat et al., 1977), poaching by cattle (Briggs, 1978) and raising sheep density in pasture (Hutchinson & King, 1980). Earthworms have a major influence on soil structure (review by Edwards & Lofty, 1977), so that there are important implications for soil properties. Baker (1981), for example, showed that soil drainage was impaired when earthworm activity on sports pitches was diminished by application of chlordane.

241 Biol. Conserv. 0006-3207/84/$03.00 © Elsevier Applied Science Publishers Ltd, England, 1984. Printed in Great Britain

242 T. G. Piearce

To assess fully the effects of trampling it is necessary to take into account differences between species in their response to trampling and influence on the soil. In this paper the abundance, biomass and vertical distribution of earthworm populations in several trampled soils are described and the relative abilities of selected species to ameliorate the effects of soil compaction compared.

SITES AND METHODS

Pasture entrances were selected as areas showing severe effects of poaching by livestock, and a path through pasture as a site of moderate human trampling. The soil in each case was a fine loam with some gleying, and the vegetation a mixture of common pasture grasses, including species of Agrostis, Poa, Phleum and Holcus. Lolium perenne L. was abundant in the sward at all sites, and conspicuous in the more heavily trampled areas, where Poa annua L. and a variety of dicotyledons, notably Matricaria matricarioides (Less.) Porter, Plantago major L., Polygonum aviculare agg., TriJolium repens L. and Ranunculus acris L. and R. repens L. were also particularly abundant.

In September 1979 samples were taken at Hazelrigg Farm (SD 492574) and Barker House Farm sites A (SD 481567) and B (SD 484566) (near Lancaster) in three zones at increasing distances from the gateway,judged from the appearance of vegetation and soil to be heavily, moderately and lightly trampled. At the time of sampling Barker House Farm site A was subject to the heaviest trampling with daily passage of scores of cattle; traverses by livestock at the other sites were much less regular. Soil cores were taken for measurement of pH in aqueous suspension, moisture content as weight loss on drying at 100 °C, and organic content as weight loss on ignition at 450 °C. Earthworms were extracted by hand from 25 cm cubes of soil, each cube being separated into 0 - 1 0 c m and 10-25cm layers. Specimens were preserved immediately in 5 ~ formaldehyde solution and identified, counted and weighed after 3 days' equilibration. For convenience the taxonomy and nomenclature of Gerard (1964) was used. At each plot from which earthworms were extracted, vegetation cover was estimated by eye and a rough measure of vegetation height obtained by placing a 30 cm square polystyrene board on top of the sward and recording its height above the ground, taking the mean of midpoints of opposite sides.

Earthworms and trampling 243

The path, at Barker House Farm (SD 483571) was sampled in September/October 1981 and March/April 1982. Between these times it regularly received approximately 100 human traverses per week. Usage from April to September was a fraction of this. In autum 1981 earthworm samples were taken at 3 m intervals along the path, as before, but with each soil cube divided into 0-4, 4 -8 and 8 - 2 5 c m depths. Additional samples were taken along each of two transects parallel to the path, 10 m on either side. Over the winter, several passages of farm machinery compacted the first 16 m length of path, and in spring 1982 extra samples were taken along this section, other areas being sampled as before. Vegetation and soil properties were measured as above, and soil bulk density calculated from dry weights of soil cores.

The relative abilities of Allolobophora caliginosa (Savigny), A. chlorotica (Savigny) (unpigmented form), A. longa Ude and Lumbricus terrestris L., to create channels in compacted soil were compared as follows. Samples of rich garden loam capable of supporting all four species were sieved to pass a 5 mm mesh and compressed into plastic cylinders 12 cm long and 10 cm diameter, almost completely sealed at the bot tom end with rotproof gauze. One kilogram of soil was placed into each cylinder, compressed to a depth of 9 cm, a further 250 g of soil added and the whole compressed to 10 cm. A hundred grams of water was then added, giving a mean moisture content of 34~o and bulk density of 1.06 g cm-3. Approximately equal weights of adult or large immature earthworms of different species were introduced into the soil at the bot tom of each tube, and the remaining section of gauze glued into place. The tubes were covered and left in the dark at 10 °C for 1 week, after which the surface worm casts were collected, dried at 100°C and weighed. Infiltrability was measured by delivering 150 cc water onto the soil surface over 40 s and recording infiltration time.

RESULTS

The most extensive soil sampling was for the path in 1981. For surface samples there was a positive correlation between ignition loss and moisture content (r = 0.84, P < 0-001) and negative correlations between bulk density and moisture ( r=0 .87 , P<0-001) and ignition loss (r = 0.71, P < 0.001). Although along the path denser surface soil had less moisture and organic matter, this was not generally true at the other sites

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Earthworms and trampling 245

(Table 1). In all cases examined (Table 1) deeper soil was denser and more deficient in moisture and organic matter than surface soil.

In Figs 1 and 2 and Tables 2 and 3 data for individual species have been given only where substantial numbers of specimens were collected. Abundant species were Allolobophora caliginosa (Savigny), A. chlorotica (Savigny) (green and unpigmented forms), A. longa Ude, A. rosea (Savigny), Lumbricus rubellus Hoffmeister and L. terrestris L. Small numbers of five other species were taken, including Eophila icterica (Savigny), which has only infrequently been recorded in Britain (Gerard, 1964), at Barker House Farm site A. Total earthworm density was less in the more heavily trampled zones than in those trampled lightly (Figs 1 and 2, Tables 2 and 3), though for the path in 1981 the difference was only very small and not statistically significant (Fig. 2). Biomass changed slightly less than density on moderate trampling but was greatly reduced on heavy compaction (Figs 1 and 2, Tables 2 and 3). Relatively few cocoons were collected from the heavily trampled zones of pasture entrances (Fig. 1) and along the section of path compacted by farm vehicles (Fig. 2), but such results must be treated with caution in view of the inefficiency of hand sorting in extracting cocoons.

TABLE 2 Ear thworm Popula t ions of Pasture Entrances in Relat ion to Trampl ing Intensi ty

(Pooled data f rom Barker House and Hazelrigg Farms)

Site Densities of individual species Total Total density biomass

Ac AchG Al Ar Lr Lt

or in L M 55 62 117 32 21 100 60 66 Density as /o H 4 8 7 1 0 25 6 6

~o worms below 10cm L 6 3 8 2 0 24 5 15 M 6 4 11 17b 0 10 8 17 H 33a 30a 60a 100a - - 20 37c 49 L 27 34 4 44 53 10 M 31 35 10 33 5 H 33 30 0 - - - - - -

27 35 6 40 - - - - all

52A 67 28B 75 - - - -

°/o mature

"J mature at10_25cm0-10cm

L, M, H light, moderate , heavy trampling. Lr = L. rubellus, other abbrevia t ions as Fig. I. Compar i son with lightly t rampled (X 2 test) a P < 0 - 0 5 , b P < 0 . 0 1 , c P < 0 . 0 0 1 . Compar i son with top 10cm: A P < 0.05, B P < 0.01. n = 30.

246

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T. G. Piearce

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BARKER HOUSE SITE B

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A c AchO A I A r L r L I Coc ~-'B ~r'D Fig. 1. Abundance and biomass of earthworms in Hazelrigg and Barker House Farm pasture entrances. Ac, Allolobophora caliginosa; Ach G, A. chlorotica (green form); A1, A. longa; Ar, A. rosea; Lr, Lurnbricus rubellus; Lt, L. terrestris; Coc, cocoons; Y.B, total biomass; ZD, total density. Open, hatched, solid bars = lightly, moderately, heavily trampled zones. Comparison with lightly trampled (t test): a, P < 0-05; b, P < 0-01 ; c, P <

0.001; n = 10.

Earthworms and trampling 247

125-

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AUTUMN 1981

SPRING 1982

A c A c h U A I A r L f Coc ~'- B

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Fig. 2. Abundance and biomass of earthworms along and near a path at Barker House Farm. AchU, A. chlorotica (unpigmented form), other abbreviations as Fig. 1. Open, hatched, solid bars = lightly, moderately, heavily compacted zones. Comparison with lightly trampled (t test): a, P < 0.05; b, P < 0-01 ; c, P < 0.001. n = 30 for path, 20 off path in autumn 1981, 10 for vehicle (i.e. heavily) compacted path, 20 for rest of path, 20 off

path, spring 1982.

248 T. G. Piearce

TABLE 3 Earthworm Populations of Path and Adjacent Area at Barker House Farm in Relation to

Trampling Intensity

Site Densities o f individual species Total Total density biomass

Ac AchU AI Ar Lt

Density as ~ i n A PI 93 21 158 31 44 P2 26 0 51 46 0

~oworms below 4cm A 34 3 28 26 55 P1 25 0 32 33 33 P2 56 - - 57a 64a - -

~oworms below 8cm A 3 0 7 3 3 P1 5 0 7 4 0 P2 33b - - 21a 18 - -

~ m a t u r e A 22 37 14 39 28 P1 22 25 23 28 7 P2 33 - - 29 36 - -

mature at 0 - 4 c m 1 14 34 13 24 14 4 - 8 c m ~ all 41 - - 31 57 29 8 - 2 5 c m 54 - - 59 - - - -

87 102 36 29

4 7 5 13b

23c 37a

P1, P2 path zones without, and with, vehicular compaction, A adjacent area. Species abbreviations as Figs 1 and 2. Comparison with adjacent area (~2 test): a P < 0.05, b P < 0.01,c P < 0.001. n = 30 for path, 20 f o r A i n autumn 1981,20 for Px, l0 for P2 and 20 for A in spring 1982.

Allolobophora longa declined proportionately less in density on increased trampling than other species; L. rubellus declined most (Figs 1 and 2, Tables 2 and 3). Of eight L. rubellus taken from the path and adjacent field, six were taken only in the lightly trampled zone. The data of Tables 2 and 3 suggest that A. longa and L. terrestris burrow relatively deeply, while A. chlorotica and L. rubellus normally do not, and that within individual species there is a general shift downwards in vertical distribution with increased trampling, paralleled by a downward displacement of total earthworm biomass. The proportion of mature specimens varied between species and was universally less in the surface soil than deeper down (Tables 2 and 3). There was no obvious relationship between the percentage of mature earthworms and trampling intensity. Calculation of Morisita's Index of Dispersion (Morisita, 1959) revealed an aggregated distribution of species wherever substantial numbers were

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250 T. G. Piearce

collected, but no consistent relationship between degree of aggregation and trampling intensity.

Table 4 shows the results of the experiment on burrowing in compacted soil by different species. Cast weights were compared using 'Student's' t test and infiltration rates by Fisher's Exact Probability Test. The weight of surface casts per trial produced by A. longa was significantly greater than for A. chlorotica or L. terrestris (P < 0.05) and also greater than for A. caliginosa (0"1 > P > 0"05). Allolobophora chlorotica produced less cast material per individual than any other species (P < 0.05) while A. caliginosa produced significantly less per individual than A. longa (P < 0-01). The number of trials in which infiltration took <3min was significantly greater for A. longa than A. chlorotica (P < 0.001) or L. terrestris (P < 0.05).

DISCUSSION

The reductions in earthworm density in relation to trampling observed here are consistent with those reported previously (Chappell et al., 1971; Aritajat et al., 1977; Briggs, 1978; Hutchinson & King, 1980). Trampling can cause death by crushing of individuals in surface soil: such crushed specimens were found in heavily trampled soils at Hazelrigg Farm and Barker House Farm site A, and along the vehicle-compacted section of path. Tabor (1974) and others have shown that some species at least will rapidly evacuate vibrated soil. Less immediate in its effect on earthworms, but possibly equally important, is the reduction in soil porosity which will impair the movement of water and air through the soil (reviews by Briggs, 1978; Saini, 1980) and impede earthworm locomotion. Trampling alters the composition and amount of vegetation (review by Liddle, 1975) and hence the quality and quantity of food available to the soil fauna. Vegetation height and cover were greatly reduced at all trampled sites, and this presumably resulted in a harsher microclimate at the soil surface and diminished protection from predators. There were no obvious signs of disproportionate enrichment of the heavily trampled areas of pasture entrances in dung and it has been assumed that if such enrichment had occurred its effect on earthworms would have been minor compared with modifications of soil and vegetation resulting from trampling.

The interspecific differences in vertical distribution found in this study are very similar to those obtained elsewhere (see, e.g. Gerard, 1967).

Earthworms and trampling 251

The shallow burrowing L. rubellus showed a disportionately large decrease in abundance with increased trampling, while the deep burrowing A. longa suffered a disproportionately small decrease. There was clear evidence of a downward shift in vertical distribution within individual species. Gerard (1967) and others have shown that juveniles tend to live nearer to the surface than adults, and the present study has demonstrated a consistent increase in the proportion of mature specimens with depth (Tables 2 and 3). The downward shift in distribution on trampling may therefore partly reflect a change in age/size class distribution.

Earthworms can only increase total porosity by excavating soil and depositing it on the surface. The principal surface-casting species found in this study were A. longa and L. terrestris, the former being particularly prolific in producing surface casts (Evans, 1948). A llolobophora rosea has been shown to deposit surface casts when kept in very compact soil in the laboratory (Thomson & Davies, 1974) and this species, with A. caliginosa and A. chlorotica, produces surface casts on compact silt in the field (Piearce, 1975; Piearce & Wells, 1977). In developing the apparatus to measure the effects of earthworms on infiltration, it was found that specimens of all species tested penetrated the surface of heavily compacted soil with great difficulty but that once beneath the surface locomotion was much more effective. In the field, invasion of trampled soil by earthworms may well be largely from below, both because of the ease of penetration and because of the lack of a suitable surface microhabitat for litter-dwelling species.

The laboratory experiment confirmed the effectiveness of A. longa in excavating compacted soil and promoting infiltration. The laboratory population, at a density of 255 large specimens m -2, produced 0.7 kg m- 2 surface casts in 7 days, equivalent to a layer 0.7 mm deep. The increase in dominance of A. longa on trampling can be expected to facilitate soil recovery once trampling ceases, although a loss of soil surface strength due to the fine texture and high water retention of cast soil (Baker, 1981) may to some extent limit resistance to further trampling.

ACKN OWLEDGEMENTS

I thank Mr A. J. Rhodes and Mr T. Whitaker for their co-operation in the taking of samples, Bettina Philipp and David Knight for assistance with

252 T. G. Piearce

the laboratory experiment on amelioration of soil compaction by earthworms, Peter Flint for help with soil analysis, and for preparing the figures, Geoffrey Halliday for assisting with plant identification, and John Whittaker and Neil Bayfield for invaluable criticism of the manuscript.

REFERENCE S

Aritajat, U., Madge, D. S. & Gooderham, P. T. (1977). The effects of compaction of agricultural soils on soil fauna. 1. Field investigations. Pedobiologia, 17, 262-82.

Baker, S. W. (1981). The effect of earthworm activity on the drainage characteristics of winter sports pitches. J. Sports Turf Res. Inst., 57, 9-23.

Briggs, D. J. (1978). Edaphic effects of poaching by cattle. North of England Soils Discussion Group Proceedings, 14, 51-62.

Chappell, H. G., Ainsworth, J. F., Cameron, R. A. D. & Redfern, M. (1971). The effect of trampling on a chalk grassland ecosystem. J. appl. Ecol., 8, 869-82.

Edwards, C. A. & Lofty, J. R. (1977). Biology of earthworms, 2nd edn. London, Chapman & Hall.

Evans, A. C. (1948). Studies on the relationships between earthworms and soil fertility. II. Some effects of earthworms on soil structure. Ann. appl. Biol., 35, 1-13.

Gerard, B. M. (1964). Lumbricidae (Annelida). Synopses of the British Fauna, No. 6. London, The Linnaean Society of London.

Gerard, B. M. (1967). Factors affecting earthworms in pastures. J. Anita. Ecol., 36, 235-52.

Hutchinson, K. J. & King, K. L. (1980). The effects of sheep stocking level on invertebrate abundance, biomass and energy utilization in a temperate, sown grassland. J. appl. Ecol., 17, 369-87.

Liddle, M. J. (1975). A selective review of the ecological effects of human trampling on natural ecosystems. Biol. Conserv., 7, 17-36.

Morisita, M. (1959). Measuring of the dispersion of individuals and analysis of the distributional patterns. Mere. Fac. Sci., Kyushu Unit,., Ser. E (Biol.), 2, 215-35.

Piearce, T. G. 0975). Observations on the flora and fauna of Ingleborough Cavern, Yorkshire. Trans. Br. Cave Res. Ass., 2, 107-15.

Piearce, T. G. & Wells, E. J. (1977). The activity of Lumbricidae in a northern English cave. Proc. int. Speleol. Congr., 7th, Sheffield, England, 353-5.

Saini, G. R. (1980). Pedogenetic and induced compaction in agricultural soils. Technical Bulletin, Agriculture Canada, Research Station, Fredericton, New Brunswick, 1.

Tabor, R. (1974). Earthworms, crows, vibrations and motorways. New S¢ient., 62, 482-3.

Thomson, A. J. & Davies, D. M. (1974). Production of surface casts by the earthworm Eisenia rosea. Can. J. Zool., 52, 659.