Some effects of trampling on Molophilus ater (Meigen) (Diptera, Tipulidae)

SOME EFFECTS OF TRAMPLING ON M O L O P H I L U S A TER (MEIGEN) (DIPTERA, TIPULIDAE)

NEIL BAYFIELD

Institute of Terrestrial Ecology, Hill of Brathens, Banchory, Kincardineshire, AB3 4B Y, Great Britain

ABSTRACT

Numbers of Molophilus ater (Meigen) (Diptera, Tipulidae) were lower in trampled peat along a footpath than in adjacent untrampled ground. Peat cores transplanted from the path into undisturbed ground subsequently contained similar numbers of larvae to undisturbed areas, but coresfrom two larger trampled areas where there was no further disturbance contained fewer larvae;possibly fewer eggs were laid there, or survival was poor. Laboratory observations showed that M. ater imagines tended to spend more time on vegetated ground than bare ground, although egg laying was recorded more often on bare ground," large bare areas might, however, be less attractive. Trampling experiments indicated that physical crushing could kill a high proportion of larvae in peat cores, particularly those near the surface. It was concluded that the low numbers of larvae on the path could be explained by a combination of physical crushing and possibly smaller numbers of eggs or poor survival.

INTRODUCTION

Human trampling is usually considered detrimental to plant and animal communities since it not only tends to result in death or injury to individual plants and animals, but can also adversely modify their growing environment (Littel, 1974). Some of the more important changes include reductions in plant height and cover (Chappell et al., 1971), modification of microclimate (Liddle & Moore, 1974) comminution of litter (Burden & Randerson, 1972), soil compaction, and changes in a wide range of other soil physical and chemical characteristics (Mullen et al., 1974, Crawford & Liddle, 1977). Most studies of the effects of these changes on invertebrates indicate reductions in 219 Biol. Conserv. 0006-3207/79/0016-0219/$02"25 Printed in Great Britain

O Applied Science Publishers Ltd, England, 1979

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NElL BAYFIELD

numbers of species and of individuals. Thus Chappell et al. (1971) and Buchanan (I 976) recorded declines in numbers of a wide range of soil invertebrates in trampled chalk grassland and sand dunes, respectively. Van d.er Ploeg & Van Wingerden (1977) found that spider species varied in their sensitivity to trampling, species of closed vegetation being most affected. Duffey (1975) found reductions in numbers of some but not all invertebrate species inhabiting grassland litter, and was able to relate these to changes in degree of compaction and proportion of air space within the litter sample. The death of individuals due to crushing is probably significant in most trampled habitats, but it is often difficult to distinguish this as the cause of a decline from the effects of habitat modification. In a few cases, however, the direct effects of crushing have been clearly demonstrated, as for example in the case of cross-country vehicles crushing soft-bodied clams (Godfrey et al., 1978). This paper describes a study of a peat-inhabiting tipulid fly Molophilus ater (Meigen), the larvae of which were found in smaller numbers in trampled ground along a path than in untrampled ground nearby. Behavioural, trampling and transplanting experiments were used to discover how far the reduction in numbers on the path had resulted from death by crushing, and how far from habitat modification or other factors. Plant nomenclature follows Clapham et al. (1962). MATERIAL SITE AND METHODS

Molophilus ater Molophilus ater is a sub-apterous short-palped cranefly common in peaty and moorland areas of upland Britain (Hadley, 1969, 1971). Eggs are laid at the beginning of June and the larvae go through four instars by about the beginning of November. They overwinter in the peat and emerge as adults towards the end of May and beginning of June. Male and female adults are easily distinguished, and pairs will mate in captivity. The precise food of the larvae remains uncertain. Hadley found that discrete particles of plant tissues were apparently not ingested and he considered that the larvae probably ingested indiscriminately as they burrowed through the peat, presumably utilising organic matter from the material taken in. At Moor House National Nature Reserve Hadley found densities of M. ater varying from about 400 to 2100 final instar larvae/m 2. The species was largely confined to wet peat sites, dominated by Juncus squarrosus, Carex spp. or Eriophorum spp. M. ater was absent on drier ground dominated by the grasses Nardus stricta, Agrostis tenuis and Festuca ovina. The density of larvae recorded in the present study was lower than at Moor House (about 50-170 final instar larvae/m2). The site

The site consisted of parts of a footpath about two years old that led from a new

EFFECTS OF TRAMPLING ON

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ater

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car park on the Ullapool-Achiltibuie road, to the top of Stac Polly, a hill in the Inverpolly National Nature Reserve, Scotland. Two short sections of path about l 0 m long and 300 m apart were selected on the lower ground, where the path crossed blanket peat (30-120 cm deep). Trampling had eliminated about 90 % of the surface vegetation, but the exposed peat surface was largely unbroken. The vegetation of the site was Trichophoreto-Eriophoretum (McVean & Ratcliffe, 1962), the main species being Trichophorum cespitosum, Calluna vulgaris, Eriophorum vaginatum, E. angustifolium, Erica tetralix, Molinia caerulea, Sphagnum spp. and Narthecium ossifragum. No new plant species had appeared on the path but some, notably Calluna vulgaris, had been eliminated. Although aerial plant parts had been severely affected, sub-surface shoots and roots appeared intact and healthy. Cores of apparently bare peat from the path nearly always produced some vegetative regrowth in the laboratory, usually of more than one species.

Use of the site by visitors Overall use of the path at the peak of the tourist season was estimated using a 'photo flux' counter, an electronic counter that counts people by noting a change in light intensity as they pass a pair of photocells (Bayfield & Pickrell, 1971). This counter was in use from 2 to 20 August 1971. The counter recorded an average of 50 people/day, with a range of 3-84 people/day. The pattern was strongly related to weather conditions, with few visitors during days of heavy rain. Casual observations by the resident warden of the Reserve indicated that most use of the path was in midJuly to mid-August, with much less use in June and September, and few visitors during the rest of the year. The lateral spread of visitors across the path was recorded using 'trampleometer' transects at the two sample areas. Each transect consisted of a row of fine wires projecting vertically from the peat. The positions of wires knocked down by walkers' feet were recorded and they were reset by being bent upright again (Bayfield, 1971). Repeated recording during the period 4-16 August 1971 showed that trampling was largely concentrated within the margins of obvious wear, but with occasional use of the ground beyond (Fig. 1). The spread of use was less wide at the second of the transects, indicating a greater intensity of trampling.

Estimating numbers of larvae and imagines Cores of peat 10cm in diameter and 4-5cm deep were used to estimate populations of M. ater, and similar cores were used in the trampling and transplanting experiments. Cores from the path were taken from areas with 25 9/oor less surviving vegetation. Those from adjacent ground were outside the limits of trampling as indicated by the trampleometer transects. Insect numbers were obtained in two different ways; larvae were extracted using the wet funnel method described by O'Connor (1962), and imagines were counted when they appeared during May; the cores were kept in a cool place and covered with polythene bags to catch emerging insects.

NElL BAYFIELD

222

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Numbers of larvae where trampling had ceased Fifteen cores f r o m the path were transplanted into r a n d o m positions in adjacent undisturbed g r o u n d in M a r c h (1973), prior to egg laying. Each core was placed in a hole o f m a t c h i n g size cut by the corer. In September the cores were removed using the corer and the n u m b e r s o f larvae they contained c o m p a r e d with those in a similar n u m b e r o f undisturbed cores f r o m nearby. They were also c o m p a r e d with cores f r o m two larger areas o f bare ground, 20 m x 1 m, created in M a r c h by two 70 kg

EFFECTS OF TRAMPLING ON

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ater

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men wearing climbing boots, trampling backwards and forwards 500 times. A further comparison was made with cores from the main path, where disturbance was continuing.

Egg laying in bare and vegetated ground To examine possible preferences for laying eggs in bare or vegetated ground, single pairs of insects in copula were introduced into enclosed dishes 10cm in diameter; each containing a composite core, half of which was from the path and almost bare of vegetation, and half from adjacent undisturbed and fully vegetated ground. When the pairs of insects separated, the females were observed at half-hour intervals, to record positions and egg-laying.

Trampling experiments--calibration of the trampling machine In the trampling experiments described below a simple machine was used to provide a uniform repeatable treatment. The machine consisted of a circular 'foot' (faced with a 10 cm diameter cleated rubber'vibram' climbing boot sole) which could be dropped vertically from various heights, onto pots of soil or vegetation (Fig. 2a and b). By adjusting the weight of the foot as well as the height from which it fell, varying levels of impact force could be applied. Before using the device to trample peat cores, it was necessary to select an impact force that was similar to that of a human foot. To compare the impact of the machine with that of a walker, a small platform 17 x 33 cm was built, with a sole of the same dimensions as the trampling machine, mounted on its lower surface. The sole rested lightly on the surface of the pot of core to be trampled, and was mounted in a box flush with the ground surface so that a walker could step on the platform without altering his normal walk or stride length (Fig. 2c). To assess damage at various depths, peat cores were sliced either 1, 2-5 or 4 cm from the surface. Between the slices 10 cover slips (18 mm 2, No. 1) were inserted and covered with a thin sheet of polythene film ('clingwrap'), to assist their relocation. Each core was then trampled either by maching or human foot, and the proportion of broken coverslips used as an arbitrary index of damage. Figure 3a compares coverslip breakage by five impacts of a 75 kg man with the same number of impacts by the machine, the impact force of the machine foot being 5, 8, 12 or 17 joules/square metre x 10- 2. Coverslip breakage by the machine declined with depth below the surface, but generally increased with impact energy. Human trampling resulted in relatively more damage near the surface. Damage at 1 cm was higher than with the heaviest machine blow (97 % breakage), but at 2.5 and 4.0cm (51% and 36 % breakage) it was only equivalent to 10 and 13 Jm- 2 x 10- 2 respectively. These differences presumably reflect the complex lateral, horizontal and torque forces present in human walking (Harper et al., 1967). Figure 3b compares the effect of increasing numbers of impacts on breakage at the I cm depth. In this case a uniform impact force of 5 Jm- 2 x 10- 2 was provided by the machine. The breakage by human trampling was greater than that by the

224

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226

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machine at all depths, but the difference between the two methods was greatest where there were few impacts and became progressively less as numbers increased. Thus machine damage was only 63 ~o of that caused by a human foot after a single impact but 96 ~o after 50 impacts. For the trampling experiments that follow 5 Jm- 2 x 10- 2 was used as a standard force. This impact level, having about two-thirds the crushing effect of a human foot, was considered adequate but not excessive. Simulated trampling in spring and summer Cores collected in the field were subjected to 0, 1,5 or 20 impacts of the trampling machine. This was done first in the spring (early March) and the insects were collected as imagines when they emerged at the end of April. The experiment was repeated in September, surviving larvae being extracted with the wet funnel apparatus three days after trampling (to allow time for injured larvae to die). In the second experiment the cores were cut into three slices (separated by 'clingwrap film') before trampling, so that mortality could be determined separately for each slice. Fifteen cores were used for each treatment in the spring experiment, and twenty five in the summer.

RESULTS

Abundance of M. ater in trampled and untrampled peat At the first path sample area only two out of fourteen cores produced any adult M. ater, compared with nine out of fourteen from undisturbed ground. There was also a larger total of flies from undisturbed ground. A similar pattern was seen at the second area, but the overall density of M. ater was slightly lower (Table 1). Analysis of these results with Student's t test and Fisher's exact test showed that the differences at area 1 were highly significant but those at area 2 barely so. It was nevertheless clear that at both areas the numbers of M. ater from trampled ground were reduced, but the species was not eliminated. TABLE 1 COMPARISON OF NUMBERS OF M . a t e r HATCHED FROM CORES FROM THE PATH AND ADJACENT GROUND. FISHER'S EXACT TEST FOR 2 X 2 CONTINGENCY TABLES WAS USED TO CALCULATED PROBABILITY VALUES, AND STUDENT'S t TEST TO COMPARE TOTAL NUMBERS OF IMAGINES

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EFFECTS OF TRAMPLING

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Numbers of larvae where trampling had ceased The numbers of larvae from the main path where disturbance was continuing showed a similar pattern to the data for adults; there were fewer M. ater, and a lower proportion of cores contained larvae than in samples from undisturbed ground (Table 2). The numbers of transplanted cores containing M. ater, and, of larvae extracted were similar to those from undisturbed ground (Fisher's exact test, p = 0.70), indicating that once disturbance ceased, the peat remained a satisfactory habitat for M. ater larvae. The large trampled areas created in the spring, although having nearly as m a n y cores containing larvae as the adjacent ground (p = 0.45), had less than half as m a n y larvae/core as in the adjacent ground or the transplanted cores. Thus in some way the large trampled areas were less suitable habitat for M. ater than the transplanted cores, although both were better than cores from the path where trampling continued. TABLE 2 COMPARISON OF THE NUMBERS OF TRANSPLANTED CORES CONTAINING EXTRACTABLE LARVAE OF M. ater, WITH CORES PROM THE PATH, THE EXPERIMENTAL TRAMPLED AREAS AND ADJACENT UNDISTURBED GROUND. FIFTEEN CORES WERE SAMPLED IN EACH CASE. PROBABILITIES ARE BASED ON FISHER'S EXACT TEST FOR 2 X 2 CONTINGENCY TABLES

Mean numbers of cores containing larvae

Path

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4

8

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0.26

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9 1

Adjacent ground

11 0-70

0"14

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0.33 0-16

0.60 0.16

1-46 0-47

1.53 0.32

Egg laying in bare and vegetated ground Out of 184 observations on 10 different pairs of M. ater, one or both insects were recorded on the bare halves of the cores less often (17 ~o) than on the vegetated halves (60 ~). On other occasions (22 ~ ) flies were on the sides of the polythene bags surrounding the cores, or could not be seen. Females were recorded ovipositing on twelve occasions, eight times in bare peat and four times in the undisturbed halves.

Simulated trampling in spring Trampling had the expected result of reducing the numbers of emerging insects, and the proportion of cores producing them (Fig. 4a,b). Although the numbers of

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M. ater fell with increasing numbers of impacts, cores given the most severe treatment still produced slightly more insects than cores from the path that had been collected at the same time. Similar patterns of damage increasing with the level of disturbance were shown by data for dry weights of surviving live plant material from the trampled cores (Fig. 4c). Simulated trampling in summer

As in the case of spring trampling, the numbers of surviving M. ater fell with increasing levels of trampling (Table 3). The effect was most pronounced, however, in the surface and middle slices, the bottom slices possibly being cushioned from trampling by the slices above. Thus 12 out of 25 bottom slices contained larvae after the heaviest treatment, compared with only 5 of the surface slices, and 7 of the middle slices; Chi-square tests confirmed that trampling had a less significant effect on the bottom than on the surface and middle slices. TABLE 3 EFFECTS OF TRAMPLING ON NUMBERS OF SLICES OF PEAT CONTAINING LARVAE OF M. aler. THE SLICES WERE TAKEN FROM THE SURFACE, MIDDLE AND BOTTOM OF THE CORES, AND DATA ARE NUMBERS OF CORES OUT OF 25. PROBABILITY VALUES ARE BASED ON COCHRAN'S 0 9 5 4 ) LINEAR REGRESSION CHI-SQUARE TEST

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None I 5 20

10 11 8 5

18 15 12 7

16 17 15 12

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2"18 0" 14

Total cores containing larvae 21 19 20 13

Mean number o f larvae~core +_ SE 4.8 4.1 3.3 1.8

+_ 0.8 +_0.7 + 0.6 + 0.4

DISCUSSION

Simulated trampling of peat cores reduced the numbers of extractable larvae by up to three-quarters, and had a similar effect on numbers of emerging adults. The action of the trampling machine was not the same as the human foot but in most respects the human trample could be regarded as more damaging, since not only was it heavier, but it had considerable elements of twisting, and of horizontal as well as vertical components of force. One possibly important feature was that the simulated trampling was provided on a single occasion; in the field, trampling was spread over three months of the summer, or longer. The risk of trampling was much less in the spring than in summer, but mortalities may still have occurred, due to the high suspectibility of the species to trampling at this time of year. The very damaging effect of single impacts in the spring may have been because a large proportion of the population was near the surface, prior to

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NEIL BAYF1ELD

pupation and emergence. The susceptibility of larvae near the surface was emphasised by the summer trampling study. It also provided a possible explanation for the survival of at least some individuals. Hadley (1971) found larvae down to 9 cm, although the largest numbers were in the 0-3 and 3-6 cm strata. Presumably those below the top few cm would be at a much reduced risk of crushing. Survival might also be possible, or improved, where the remains of a tussock of a species such as Trichophorum cespitosum cushioned the impact of the trampling. The extent to which trampling had damaged the larval habitat was difficult to assess. The surface peat certainly became more friable, at least in places, but most of the loose material was removed by surface runoff. Over a period this would reduce the depth of peat along the path, but it had little effect during the course of the study. The moisture content of peat on the path and adjacent ground was not significantly different on the two occasions it was tested (range 76-85 %) and there was no difference in pH (path 4.7 + 0.04, adjacent 4.72 + 0.04). There may have been changes in food availability, but as the precise food of M. ater is unknown, this could not be assessed. The transplanting experiment indicated that trampled peat from the path was adequate for growth of M. ater larvae when further trampling was prevented. This is a conclusion based on numbers of larvae; any differences in size of larvae were not determined. It was assumed that the transplanted cores were identical to the peat on the path, except that the physical crushing by feet was removed. In fact there may have been changes after transplanting, including decompaction, and limited regrowth of vegetation, that could make the cores more suitable for larvae. The behavioural observations showed that female imagines tended to select disturbed ground for egg laying, but overall spent more time in vegetated ground. The preference for vegetated ground might militate against egg laying in large disturbed areas lacking cover. Possible confirmatory evidence is provided by the low numbers of larvae found in cores from the experimental path that received no further disturbance. Large patches of bare ground are known to have a distinct microclimate by comparison with adjacent vegetated ground (Liddle & Moore, 1974). The more productive transplanted cores represented relatively small patches of disturbance and they were completely surrounded by vegetation. Another possibility is that these small patches were actually preferred for egg laying (this is consistent with the egg laying data) but that survival in bare ground was poor. The effect of disturbance on the behaviour of adults was not investigated. Traffic on the path would have been very light in May and it seems unlikely that this would have a significant effect, although work on Oscinellafrit, for example, indicates how very sensitive to disturbance some species are (Jones, 1968). Most of the interpretation of the data presumes that no significant migration of larvae occurred from or into disturbed ground. Even if migration did occur, the conclusion that transplanted cores were satisfactory habitat for the species, is still

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valid. Although the low numbers of larvae in path cores might have been due in part to movement of larvae away from the area of disturbance this cannot explain the low numbers found in the trampled areas where there was no further disturbance, so there seems to be little direct evidence for migration. In conclusion, the reduced numbers of M . ater on the Stac Polly footpath may be partly explained by physical crushing, but fewer eggs may have been laid on the path than in adjacent less open ground, or it may be that survival was poorer in disturbed ground. Peat from the path appeared to be still a satisfactory habitat for the species, although how long it could remain so with so much of the plant cover dead is uncertain.

ACKNOWLEDGEMENTS

I am grateful for the assistance ofD. Gowans, R. Bowles and S. Moyes with parts of this study, to Dr M. Morris, Dr G. R. Miller, J. M. Nelson, J. O'Hara and Dr S. W. Van der Ploeg for advice and criticism of the manuscript, and to P. Rothery for statistical advice. REFERENCES BAYFIELD,N. G. & PICKRELL,B. G. (1971). The construction and use ofa photoflux people counter. Rec. News. Suppl., 5, 9-12. B^YFIELD, N. G. (1971). A simple method for detecting variations in walker pressure laterally across paths. J. appl. Ecol., 8, 533-5. BUCH^N^~4, K. (1976). Some effects of trampling on the flora and invertebrate fauna of sand dunes. University College London, Discussion Papers in Conservation, 13. BURDEN, R. F. & RANDERSON, P. F. (1972). Quantitative studies of the effects of human trampling on vegetation as an aid to the management of semi-natural areas. J. appl. Ecol., 9, 439-58. CH^PPELL, H. G., AINSWORTri,J. F., CAMERON,R. A. D. & PO~DFERN,M. (1971). The effect of trampling on a chalk grassland ecosystem. J. appl. Ecol., 8, 869-82. CLAPHAM,A. R., TUTIN, T. G. & WARBURG,E. F. (1962). Flora of the British Isles. Cambridge University Press. COCHRAN, W. G. (1954). Some methods for strengthening the common X2 tests. Biometrics, 10, 417-51. CRAWFORD, A. K. & LIDDLE, M. J. (1977). The effect of trampling on neutral grassland. Biol. Conserv., 12, 135~2. DtJFWY, E. (1975). The effects of human trampling on the fauna of grassland litter'. Biol. Conserv., 7, 255-74. GODFREY, P. J., LEATHERMAN,S. P. & BUCKLEY, P. A. (1978). Impact of off-road vehicles on coastal ecosystems. Proc. Coastal Zone Conf. 1978, 581-99. H^OLEY, M. (1969). The adult biology of the cranefly Molophilus ater Meigen. J. Anita. Ecol., 38, 765-90. HADLEY, M. (1971). Aspects of the larval ecology and population dynamics of Molophilus ater Meigen (Diptera:Tipulidae) on Pennine moorland. J. Anim. Ecol., 40, 445--66. H^aPER, F. C., W^RLOW, W. J. & CL^RgE, B. L. (1967). The forces applied to the floor by the foot in walking. 1. Walking on a level surface. National Building Studies Res. Pap., 32. JDN~, M. G. (1968). The effect of moving carabids on oviposition by frit fly (Oscinella frit. L.). Entomologists Mon. Mag, 104, 85-7. LIDDLE, M. J. & MOORE, K. G. (1974). The microclimate of sand dune tracks: the relative contribution of vegetation removal and soil compression. J. appl. Ecol., 11, 1057-68.

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LITTEL,A. (1974). An investigation on trampling in a dune valley. Effects on mesofauna and vegetation. Institute for environmental Studies, Free University, Amsterdam, Series B, 5. MULLEN,G. J., JeLLEV,R. M. & Mc_A~.~sE, D. M. (1974). Effects of animal treading on soil properties and pasture production, lr. J. agric. Res., 13, 171-80. McVEAN, D. N. & R^TCLIFFE, D. A. (1962). Plant communities of the Scottish Highlands. London, HMSO. O'CoNNoR, F. B. (1962). The extraction of Enchytraeidae from soil. Progress in Soil Zoology, 279-85, London, Butterworths. V^N DER PLOEG,S. W. F. & VANWINGERDEN,W. K. (1977). The influence of trampling on spiders. Proc. int. Arachn. Congr. Leiden 1975, 6th.