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Quantitative differences between the insect faunas of sub-antarctic Marion and Prince Edward islands: A result of human intervention?
Biological Conservation 40 (1987) 255-262
Quantitative Differences Between the Insect Faunas of Sub-Antarctic Marion and Prince Edward Islands: A Result of Human Intervention? J. E. C r a f f o r d & C. H. Scholtz Department of Entomology, University of Pretoria, Pretoria 0002, South Africa (Received 19 September 1986; revised version received 6 November 1986; accepted 7 November 1986)
ABSTRA CT The two islands in the sub-Antarctic Prince Edward archipelago are only 22 km apart and are similar in age, geology, physiognomy and climate, yet their insect faunas show both qualitative and quantitative differences. During three visits in 1983 and 1984, the insect fauna of Prince Edward Island was sampled to compare with that of a survey carried out over one year on Marion Island. Two species of weevil ( Curculionidae ), previously known only from Marion, were recorded on Prince Edward for the first time. The biomass of two major groups of invertebrate decomposers (Lepidoptera and Curculionidae larvae) was significantly higher in heavily manured plant communities on Prince Edward than on Marion. Possible reasons for these quantitative differences are discussed. Although differences in species assemblage play a role, it is concluded that the effects of human influence on Marion lsland may be paramount.
INTRODUCTION The two islands o f the Prince Edward archipelago (46°54'S, 37°45'E) are separated by only 22 km of ocean and probably represent the peaks o f two closely related, coalescing shield volcanoes (Kable et al., 1971). They are the same age and have undergone essentially the same geological history (Verwoerd, 1971). Marion Island is 290km 2 in surface area and rises to a height of 1 230 m above sea level. Prince Edward Island is only one-seventh its area and has a m a x i m u m elevation of 672m. Differences in size, 255 Biol. Conserv. 0006-3207/87/$03-50 © Elsevier Applied Science Publishers Ltd, England, 1987. Printed in Great Britain
256
J. E. Crafford, C. H. Scholtz
topography and elevation, and the absence of a permanent ice cap on Prince Edward Island, may result in different weather regimes on the two islands. Prince Edward appears to have a somewhat milder local climate and experiences drier weather than Marion (e.g. Verwoerd, 1971), but comparative meteorological data are not available. Both islands were visited periodically by sealers during the 19th and early 20th century (Marsh, 1948), though Marion Island has borne the brunt of human invasion because of its greater accessibility and larger seal colonies. Marion Island has been permanently occupied since 1948, and has been the site of intensive scientific research since 1965 (see Siegfried et al. (1979) for a bibliography of scientific research at the Prince Edward Islands). House mice (Mus musculus L.), 13 alien vascular plant species and nine alien invertebrate species have inadvertently been introduced to Marion Island and are presently well established in the island's terrestrial ecosystem (Crafford et al., in press; Watkins & Cooper, in press). A population of feral cats (Felis catus L.) has existed on Marion Island for the past four decades. The cats kill up to about 150 000 small burrowing petrels (Procellariidae) per year (based on one year, 1982; Van Rensburg 1985), of which some species have been brought to local extinction (Van Aarde, 1980; Van Rensburg, 1985). Prince Edward Island, on the other hand, is one of the last relatively pristine sub-Antarctic islands: only one alien plant (the grass, Poa annua) and four alien invertebrate species have been recorded on the island (Gremmen, 1981; Crafford et al., in press). Consequently, it was decided to compare the insect faunas of the two islands. It was hypothesised that any differences might be attributable to the indirect effects of human impact. However, lack of facilities and short, intermittent visits precluded intensive study of the Prince Edward Island biota. The native vegetation and terrestrial fauna of the island differs slightly in species assemblage from that of Marion (Gremmen, 1981; Crafford et al., in press), but what appeared to be substantial quantitative differences in insect numbers had not been investigated. During 1983 and 1984 the insect fauna of Prince Edward Island was sampled on an opportunistic basis, and both quantitative and qualitative differences between the two insect faunas were assessed.
METHODS Prince Edward Island was visited for four-day periods in May 1983, April 1984 and September 1984. On each occasion Agrostis magellanica Lam. mire and heavily manured Poa cookii Hook.f./Cotula plumosa Hook.f. herbfield communities were sampled for soil invertebrates, in sites on both the east
Insect faunas of Marion and Prince Edward Islands
257
(Cave Bay) and west (Kent Crater) sides of the island. Within each community a large, pure stand of the dominant vegetation type was selected. As far as it was possible to determine without detailed study, these were identical to those sampled on Marion. Core samples were taken at 1-m intervals along a 50 m transect line of random origin and direction within each stand. A circular auger, 80 m m in diameter, was used to take the cores to a depth of approximately 100 mm; cores included living vegetation, litter and soil. Sampling was carried out at the same site during each visit. The cores were transported to Marion Island and sorted by hand in the laboratory. Only macro-insects--Lepidoptera larvae (Pringleophaga spp.) and weevil larvae (Coleoptera, Curculionidae)--were removed, counted, and dried at 60°C for four days. Results from both sites were pooled as there were no numerical differences. Biomass of the invertebrates was extrapolated and expressed as g m - 2 in each of the two plant communities. The Prince Edward Island results were compared with those from a similar sampling programme carried out over one year (1976) in the same vegetation types on Marion Island (Burger, 1978). During each visit to Prince Edward Island the insect fauna was also sampled qualitatively. All the lowland biotopes and most of the interior highland ones were sampled. Specimens were collected mainly by hand, but sweep nets were used in tall vegetation under dry conditions.
RESULTS Qualitative sampling on Prince Edward yielded all the insects previously recorded (Dreux, 1971) on the island. Two beetles (Antarctonesiotes elongatus Jeannel and Palirhoeus eatoni (Waterhouse), both Curculionidae), previously known only from Marion Island, were recorded on Prince TABLE 1 Biomass (gm -2 means + S D ) o f Pringleophaga and Curculionidae Larvae in Mire and Manured Vegetation on Marion (Burger, 1978) (n = 8 repetitions x 50 samples) and Prince Edward Islands (n = 6 repetitions × 50 samples). (Results from the Cave Bay and Kent Crater sites on Prince Edward Island are pooled.)
Marion
Pringleophaga Curculionidae
Prince Edward
Mire
Manured vegetation
Mire
Manured vegetation
0"58 _ 0"722 2 0.38 ___0.148 1
1'19 + 0-274 1 0.62 + 0.0340
1-12 __+0-434 5 0-12 + 0-001 5
12"86 + 16"230 2 2.19 + 0-4302
258
J. E. Crafford, C. H. Scholtz
Edward for the first time. The former species is endemic to the Prince Edward Islands; the latter also occurs on other islands in the Kerguelen province. The biomass of Pringleophaga and Curculionidae larvae in mire and manured vegetation on Marion (Burger, 1978) and Prince Edward Island (this study) is given in Table 1. The results from the Kent Crater and Cave Bay sites on Prince Edward Island were pooled. The Pringleophaga and Curculionidae populations in manured areas were significantly higher on Prince Edward than on Marion (P < 0-01; Student's t test). DISCUSSION The survey of Prince Edward Island was of necessity limited in scope, but preliminary results indicate a substantial quantitative difference between at least the Lepidoptera and Curculionidae populations of two communities investigated on the two islands. The larvae of these two groups are primarily detritivorous litter-dwellers, and are the major insect decomposers on the Prince Edward Islands. Burger (1978) found little seasonal variation in the biomass of the two groups on Marion Island; this was assumed to also hold true on Prince Edward Island. There are three, possibly interrelated, reasons for the significantly higher biomass of Pringleophaga and Curculionidae larvae in manured plant communities on Prince Edward Island.
Differences in species composition Marion and Prince Edward are now known to share, with one exception, all the species of their indigenous insect faunas. The flightless m o t h Pringleophaga kerguelensis Enderlein occurs sympatrically with P. marioni Viette on Prince Edward Island (Vari, 1971; this study), but has not yet been recorded on Marion Island. P. kerguelensis is a larger and more conspicuous species than P. marioni, and contributes substantially to the higher biomass of Lepidoptera larvae on Prince Edward Island. P. kerguelensis is also sympatric with P. crozetensis on the Crozet islands; its absence on Marion is curious (Vari, 1971).
The effect of predation House mice became established on Marion Island at least 150 years ago (Gleeson, 1981), but Prince Edward Island has remained mouse-free. The mice are predominantly insectivorous; Pringleophaga marioni larvae
Insect faunas of Marion and Prince Edward Islands
259
contribute 55 and 35% to the diet of the mice on Marion Island in winter and summer respectively (Gleeson & Van Rensburg, 1982), and weevils (adults and larvae) contribute between 10 and 15% to the diet throughout the year. The consumption rates of mice on Marion Island are not known. Gleeson's (1981) figures for mice densities (annual mean of 37 adult mice ha-1) and an estimated individual consumption rate of 2g insects d a y - 1 (dry mass; after comparable data by Mullen & Chew, 1973) were used to calculate annual consumption of insects by mice on Marion Island. Mean annual biomass of Pringleophaga larvae on Marion Island is 64 kg ha - 1 (dry mass; Burger, 1978). Annual consumption of Pringleophaga larvae by mice is estimated to be 13.5 kg h a - 1 year- 1, or 21% of the standing crop of larvae, taking into account that Pringleophaga larvae contribute only 50% to the diet of mice. Given the long larval stage of Pringleophaga larvae ( > 3 years) and an inferred low production rate, the cumulative effect of mice predation over time probably results in a more profound impact on the Pringleophaga population than the figures suggest. P. kerguelensis occurs on lies Kerguelen and lies Crozet, respectively 2000 and 925 km distant from Prince Edward, but not on Marion a mere 2 2 k m away. On Prince Edward Island P. kerguelensis occurs at lower densities than P. marioni. It is hypothesised that predation on the larger P. kerguelensis by mice (which may be strongly sizeselective; Banse & Mosher, 1980) has depleted the population to such an extent that the species may be extinct on Marion Island. The delicate balance between natural populations and their susceptibility to disturbance was emphasised by Block (1985), who stated that the predicted low reproductive rates of arthropods in Antarctic populations suggest a limited capacity for recovery after a decrease, and further that their ability to adjust to physical perturbations 'may not be adequate'. The lesser sheathbill (Chionis minor) is the other major vertebrate insectivore on Marion Island and the primary one on Prince Edward Island, although it is primarily a coastal scavenger. On Marion Island, sheathbills and kelp gulls (Larus dominicanus) consume approximately 8% of the standing crop of invertebrates annually. Pringleophaga larvae and weevils constituted 16-8 % and 21-4% respectively of the gizzard contents (dry mass) of sheathbills in Burger's (1978) study. Sheathbills on Prince Edward Island forage more extensively inland and in larger flocks, and appear to be more dependent on invertebrates for food, than those observed by Burger (1982) on Marion Island, with consequently greater effect on the prey.
Different bird-soil-plant-insect relationships The vegetation in those lowland plant communities on Marion Island which are heavily manured by surface-breeding and burrowing birds shows
260
J. E. Crafford, C. H. Scholtz TABLE 2
of Guano Input to Terrestrial Vegetation on Marion Island through Predation by Cats on Six Speciesof Burrowing Petrels (Procellariidae),Calculated for One Year Only (1982). (Birds assumed to spend at least 100 days per year on the island.) Loss
Prey species
Individual guano production (g day- 1, dry mass) (Fugler, 1985)
Number killed by cats during 1982 (Van Rensburg,
Loss of guano production over 100 days (kg dry mass)
1985) Pachyptila salvini Halobaena caerulea Pterodroma macroptera P. mollis P. brevirostris Procellaria aequinoctialis
0"6 a 0"6 0"8 0"6 0"8 a
83 402 20 397 14 205 21 122 7 286
5 004 1 224 1 136 1 267 583
1-8
1 821
328
Total
9 542
" Own estimate based on comparative body size.
increased cover and enhanced vitality and growth (Smith, 1976; 1978). These communities occupy approximately 25 % of the vegetated lowland on Marion Island. Manured plant communities support a disproportionately high biomass of associated detritivorous and grazing invertebrates (Burger, 1978), compared with vegetation not subject to manuring. The impact of cats on the burrowing-bird population of Marion Island has been assessed (Van Aarde, 1980), but not their indirect effect on the vegetation of the island. Table 2 shows the numbers of six bird species caught by cats during 1982 on Marion Island (Van Rensburg, 1985), the guano output by individuals of each species (Fugler, 1985), and the calculated 'loss' of guano to the island during that year; birds were presumed to spend 100 days per year on the island. The vegetation on Prince Edward Island presents a much more luxuriant aspect than that of Marion; in the presence of greater numbers of burrowing birds occurring at greater densities, the effect of manuring is probably more profound on Prince Edward than on Marion. Consequently, natural factors may result in dissimilarity between the insect faunas of Marion and Prince Edward Islands, but the quantitative differences indicated by the results can be in part ascribed to the advent of man and his synanthropes on Marion Island. Insect populations are valuable barometers of environmental disturbance. As a relatively pristine example of the pre-man sub-Antarctic, Prince Edward Island and its terrestrial fauna warrant extraordinary protection.
Insect faunas of Marion and Prince Edward Islands
261
ACKNOWLEDGEM ENTS D r D. T. R o w e - R o w e (Natal Parks Board) gave valuable advice and assistance in compiling the manuscript. Steven C h o w n and Charles Gilbert assisted with field work. Biological research at the Prince Edward Islands is carried out under the auspices o f the South African Committee for Antarctic Research (SASCAR). Financial and logistical support is provided by the South African Department of Environment Affairs.
REFERENCES Banse, K. & Mosher, S. (1980). Adult body mass and annual production/biomass relationships of field populations. Ecol. Monogr., 50, 355-79. Block, W. (1985). Arthropod interactions in an Antarctic terrestrial community. In Antarctic nutrient cycles and food webs, ed. by W. R. Siegfried, P. R. Condy and R. M. Laws, 614-19. Berlin, Springer. Burger, A. E. (1978). Terrestrial invertebrates: A food resource for birds at Marion Island. S. Afr. J. Antarct. Res., 8, 87-99. Burger, A. E. (1982). Foraging behaviour of lesser sheathbills Chionis minor exploiting invertebrates on a sub-Antarctic island. Oecologia, 52, 236-45. Crafford, J. E., Scholtz, C. H. & Chown, S. L. (in press). The insects of sub-Antarctic Marion and Prince Edward Islands, with a bibliography of entomology of the Kerguelen biogeographical province. S. Afr. J. Antarct. Res. Dreux, Ph. (1971). Insecta. In Marion and Prince Edward Islands; report on the South African biological and geological expedition 1965-1966, ed. by E. M. van Zinderen Bakker, J. M. Winterbottom and R. A. Dyer, 33543. Cape Town, A. A. Balkema. Fugler, S. R. (1985). Chemical composition of guano of burrowing petrel chicks (Procellariidae) at Marion Island. In Antarctic nutrient cycles and food webs, ed. by W. R. Siegfried, P. R. Condy and R. M. Laws, 169-72. Berlin, Springer. Gleeson, J. P. (1981). The ecology of the house mouse Mus musculus L. on Marion Island. MSc thesis, University of Pretoria. Gleeson, J. P. & Van Rensburg, P. J. J. (1982). Feeding ecology of the house mouse Mus musculus on Marion Island. S. Afr. J. Antarct. Res., 12, 34-9. Gremmen, N. J. M. (1981). The vegetation of the subantarctic islands Marion and Prince Edward. The Hague, Junk. Kable, E. D., Erlank, A. J. & Cherry, R. D. (1971). Geochemical features of lavas. In Marion and Prince Edward Islands; report on the South African biological and geological expedition 1965-1966, ed. by E. M. van Zinderen Bakker, J. M. Winterbottom and R. A. Dyer, 78-88. Cape Town, A. A. Balkema. Marsh, J. H. (1948). No pathway here. Cape Town, H. B. Timmins. Mullen, R. K. & Chew, R. M. (I 973). Estimating the energy metabolism of free living Perognathusformosus: A comparison of direct and indirect methods. Ecology, 54, 633-7. Siegfried, W. R., Forbes, P. F. & Condy, P. R. (1979). Scientific research at the Prince Edward Islands, 1847 1979, a bibliography. S. Afr. J. Antarct. Res., 9, 35-41.
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Smith, V. R. (1976). The effect of burrowing species of Procellariidae on the nutrient status of inland tussock grasslands on Marion Island. J. S. Aft. Bot., 42, 265-72. Smith, V. R. (1978). Animal-plant-soil nutrient relationships on Marion Island (subAntarctic). Oecologia, 32, 239-53. Van Aarde, R. J. (1980). The diet and feeding behaviour of feral cats Felis catus at Marion Island. S. Afr. J. Wildl. Res., 10, 123-8. Van Rensburg, P. J. J. (1985). The feeding ecology of a decreasing feral house cat, Felis catus, population at Marion Island. In Antarctic nutrient cycles and food webs, ed. by W. R. Siegfried, P. R. Condy and R. M. Laws, 620-4. Berlin, Springer. Vari, L. (1971). Lepidoptera. In Marion and Prince Edward Islands; report on the South African biological and geological expedition 1965-1966, ed. by E. M. van Zinderen Bakker, J. M. Winterbottom and R. A. Dyer, 349-54, Cape Town, A. A. Balkema. Verwoerd, W. J. (1971). Geology. In Marion and Prince Edward lslands; report on the South African biological and geological expedition 1965-1966, ed. by E. M. van Zinderen Bakker, J. M. Winterbottom and R. A. Dyer, 40-62. Cape Town, A. A. Balkema. Watkins, B. P. & Cooper, J. (in press). Introduction, present status and control of alien species at the Prince Edward Islands, sub-Antarctic. S. Afr. J. Antarct. Res.
Quantitative Differences Between the Insect Faunas of Sub-Antarctic Marion and Prince Edward Islands: A Result of Human Intervention? J. E. C r a f f o r d & C. H. Scholtz Department of Entomology, University of Pretoria, Pretoria 0002, South Africa (Received 19 September 1986; revised version received 6 November 1986; accepted 7 November 1986)
ABSTRA CT The two islands in the sub-Antarctic Prince Edward archipelago are only 22 km apart and are similar in age, geology, physiognomy and climate, yet their insect faunas show both qualitative and quantitative differences. During three visits in 1983 and 1984, the insect fauna of Prince Edward Island was sampled to compare with that of a survey carried out over one year on Marion Island. Two species of weevil ( Curculionidae ), previously known only from Marion, were recorded on Prince Edward for the first time. The biomass of two major groups of invertebrate decomposers (Lepidoptera and Curculionidae larvae) was significantly higher in heavily manured plant communities on Prince Edward than on Marion. Possible reasons for these quantitative differences are discussed. Although differences in species assemblage play a role, it is concluded that the effects of human influence on Marion lsland may be paramount.
INTRODUCTION The two islands o f the Prince Edward archipelago (46°54'S, 37°45'E) are separated by only 22 km of ocean and probably represent the peaks o f two closely related, coalescing shield volcanoes (Kable et al., 1971). They are the same age and have undergone essentially the same geological history (Verwoerd, 1971). Marion Island is 290km 2 in surface area and rises to a height of 1 230 m above sea level. Prince Edward Island is only one-seventh its area and has a m a x i m u m elevation of 672m. Differences in size, 255 Biol. Conserv. 0006-3207/87/$03-50 © Elsevier Applied Science Publishers Ltd, England, 1987. Printed in Great Britain
256
J. E. Crafford, C. H. Scholtz
topography and elevation, and the absence of a permanent ice cap on Prince Edward Island, may result in different weather regimes on the two islands. Prince Edward appears to have a somewhat milder local climate and experiences drier weather than Marion (e.g. Verwoerd, 1971), but comparative meteorological data are not available. Both islands were visited periodically by sealers during the 19th and early 20th century (Marsh, 1948), though Marion Island has borne the brunt of human invasion because of its greater accessibility and larger seal colonies. Marion Island has been permanently occupied since 1948, and has been the site of intensive scientific research since 1965 (see Siegfried et al. (1979) for a bibliography of scientific research at the Prince Edward Islands). House mice (Mus musculus L.), 13 alien vascular plant species and nine alien invertebrate species have inadvertently been introduced to Marion Island and are presently well established in the island's terrestrial ecosystem (Crafford et al., in press; Watkins & Cooper, in press). A population of feral cats (Felis catus L.) has existed on Marion Island for the past four decades. The cats kill up to about 150 000 small burrowing petrels (Procellariidae) per year (based on one year, 1982; Van Rensburg 1985), of which some species have been brought to local extinction (Van Aarde, 1980; Van Rensburg, 1985). Prince Edward Island, on the other hand, is one of the last relatively pristine sub-Antarctic islands: only one alien plant (the grass, Poa annua) and four alien invertebrate species have been recorded on the island (Gremmen, 1981; Crafford et al., in press). Consequently, it was decided to compare the insect faunas of the two islands. It was hypothesised that any differences might be attributable to the indirect effects of human impact. However, lack of facilities and short, intermittent visits precluded intensive study of the Prince Edward Island biota. The native vegetation and terrestrial fauna of the island differs slightly in species assemblage from that of Marion (Gremmen, 1981; Crafford et al., in press), but what appeared to be substantial quantitative differences in insect numbers had not been investigated. During 1983 and 1984 the insect fauna of Prince Edward Island was sampled on an opportunistic basis, and both quantitative and qualitative differences between the two insect faunas were assessed.
METHODS Prince Edward Island was visited for four-day periods in May 1983, April 1984 and September 1984. On each occasion Agrostis magellanica Lam. mire and heavily manured Poa cookii Hook.f./Cotula plumosa Hook.f. herbfield communities were sampled for soil invertebrates, in sites on both the east
Insect faunas of Marion and Prince Edward Islands
257
(Cave Bay) and west (Kent Crater) sides of the island. Within each community a large, pure stand of the dominant vegetation type was selected. As far as it was possible to determine without detailed study, these were identical to those sampled on Marion. Core samples were taken at 1-m intervals along a 50 m transect line of random origin and direction within each stand. A circular auger, 80 m m in diameter, was used to take the cores to a depth of approximately 100 mm; cores included living vegetation, litter and soil. Sampling was carried out at the same site during each visit. The cores were transported to Marion Island and sorted by hand in the laboratory. Only macro-insects--Lepidoptera larvae (Pringleophaga spp.) and weevil larvae (Coleoptera, Curculionidae)--were removed, counted, and dried at 60°C for four days. Results from both sites were pooled as there were no numerical differences. Biomass of the invertebrates was extrapolated and expressed as g m - 2 in each of the two plant communities. The Prince Edward Island results were compared with those from a similar sampling programme carried out over one year (1976) in the same vegetation types on Marion Island (Burger, 1978). During each visit to Prince Edward Island the insect fauna was also sampled qualitatively. All the lowland biotopes and most of the interior highland ones were sampled. Specimens were collected mainly by hand, but sweep nets were used in tall vegetation under dry conditions.
RESULTS Qualitative sampling on Prince Edward yielded all the insects previously recorded (Dreux, 1971) on the island. Two beetles (Antarctonesiotes elongatus Jeannel and Palirhoeus eatoni (Waterhouse), both Curculionidae), previously known only from Marion Island, were recorded on Prince TABLE 1 Biomass (gm -2 means + S D ) o f Pringleophaga and Curculionidae Larvae in Mire and Manured Vegetation on Marion (Burger, 1978) (n = 8 repetitions x 50 samples) and Prince Edward Islands (n = 6 repetitions × 50 samples). (Results from the Cave Bay and Kent Crater sites on Prince Edward Island are pooled.)
Marion
Pringleophaga Curculionidae
Prince Edward
Mire
Manured vegetation
Mire
Manured vegetation
0"58 _ 0"722 2 0.38 ___0.148 1
1'19 + 0-274 1 0.62 + 0.0340
1-12 __+0-434 5 0-12 + 0-001 5
12"86 + 16"230 2 2.19 + 0-4302
258
J. E. Crafford, C. H. Scholtz
Edward for the first time. The former species is endemic to the Prince Edward Islands; the latter also occurs on other islands in the Kerguelen province. The biomass of Pringleophaga and Curculionidae larvae in mire and manured vegetation on Marion (Burger, 1978) and Prince Edward Island (this study) is given in Table 1. The results from the Kent Crater and Cave Bay sites on Prince Edward Island were pooled. The Pringleophaga and Curculionidae populations in manured areas were significantly higher on Prince Edward than on Marion (P < 0-01; Student's t test). DISCUSSION The survey of Prince Edward Island was of necessity limited in scope, but preliminary results indicate a substantial quantitative difference between at least the Lepidoptera and Curculionidae populations of two communities investigated on the two islands. The larvae of these two groups are primarily detritivorous litter-dwellers, and are the major insect decomposers on the Prince Edward Islands. Burger (1978) found little seasonal variation in the biomass of the two groups on Marion Island; this was assumed to also hold true on Prince Edward Island. There are three, possibly interrelated, reasons for the significantly higher biomass of Pringleophaga and Curculionidae larvae in manured plant communities on Prince Edward Island.
Differences in species composition Marion and Prince Edward are now known to share, with one exception, all the species of their indigenous insect faunas. The flightless m o t h Pringleophaga kerguelensis Enderlein occurs sympatrically with P. marioni Viette on Prince Edward Island (Vari, 1971; this study), but has not yet been recorded on Marion Island. P. kerguelensis is a larger and more conspicuous species than P. marioni, and contributes substantially to the higher biomass of Lepidoptera larvae on Prince Edward Island. P. kerguelensis is also sympatric with P. crozetensis on the Crozet islands; its absence on Marion is curious (Vari, 1971).
The effect of predation House mice became established on Marion Island at least 150 years ago (Gleeson, 1981), but Prince Edward Island has remained mouse-free. The mice are predominantly insectivorous; Pringleophaga marioni larvae
Insect faunas of Marion and Prince Edward Islands
259
contribute 55 and 35% to the diet of the mice on Marion Island in winter and summer respectively (Gleeson & Van Rensburg, 1982), and weevils (adults and larvae) contribute between 10 and 15% to the diet throughout the year. The consumption rates of mice on Marion Island are not known. Gleeson's (1981) figures for mice densities (annual mean of 37 adult mice ha-1) and an estimated individual consumption rate of 2g insects d a y - 1 (dry mass; after comparable data by Mullen & Chew, 1973) were used to calculate annual consumption of insects by mice on Marion Island. Mean annual biomass of Pringleophaga larvae on Marion Island is 64 kg ha - 1 (dry mass; Burger, 1978). Annual consumption of Pringleophaga larvae by mice is estimated to be 13.5 kg h a - 1 year- 1, or 21% of the standing crop of larvae, taking into account that Pringleophaga larvae contribute only 50% to the diet of mice. Given the long larval stage of Pringleophaga larvae ( > 3 years) and an inferred low production rate, the cumulative effect of mice predation over time probably results in a more profound impact on the Pringleophaga population than the figures suggest. P. kerguelensis occurs on lies Kerguelen and lies Crozet, respectively 2000 and 925 km distant from Prince Edward, but not on Marion a mere 2 2 k m away. On Prince Edward Island P. kerguelensis occurs at lower densities than P. marioni. It is hypothesised that predation on the larger P. kerguelensis by mice (which may be strongly sizeselective; Banse & Mosher, 1980) has depleted the population to such an extent that the species may be extinct on Marion Island. The delicate balance between natural populations and their susceptibility to disturbance was emphasised by Block (1985), who stated that the predicted low reproductive rates of arthropods in Antarctic populations suggest a limited capacity for recovery after a decrease, and further that their ability to adjust to physical perturbations 'may not be adequate'. The lesser sheathbill (Chionis minor) is the other major vertebrate insectivore on Marion Island and the primary one on Prince Edward Island, although it is primarily a coastal scavenger. On Marion Island, sheathbills and kelp gulls (Larus dominicanus) consume approximately 8% of the standing crop of invertebrates annually. Pringleophaga larvae and weevils constituted 16-8 % and 21-4% respectively of the gizzard contents (dry mass) of sheathbills in Burger's (1978) study. Sheathbills on Prince Edward Island forage more extensively inland and in larger flocks, and appear to be more dependent on invertebrates for food, than those observed by Burger (1982) on Marion Island, with consequently greater effect on the prey.
Different bird-soil-plant-insect relationships The vegetation in those lowland plant communities on Marion Island which are heavily manured by surface-breeding and burrowing birds shows
260
J. E. Crafford, C. H. Scholtz TABLE 2
of Guano Input to Terrestrial Vegetation on Marion Island through Predation by Cats on Six Speciesof Burrowing Petrels (Procellariidae),Calculated for One Year Only (1982). (Birds assumed to spend at least 100 days per year on the island.) Loss
Prey species
Individual guano production (g day- 1, dry mass) (Fugler, 1985)
Number killed by cats during 1982 (Van Rensburg,
Loss of guano production over 100 days (kg dry mass)
1985) Pachyptila salvini Halobaena caerulea Pterodroma macroptera P. mollis P. brevirostris Procellaria aequinoctialis
0"6 a 0"6 0"8 0"6 0"8 a
83 402 20 397 14 205 21 122 7 286
5 004 1 224 1 136 1 267 583
1-8
1 821
328
Total
9 542
" Own estimate based on comparative body size.
increased cover and enhanced vitality and growth (Smith, 1976; 1978). These communities occupy approximately 25 % of the vegetated lowland on Marion Island. Manured plant communities support a disproportionately high biomass of associated detritivorous and grazing invertebrates (Burger, 1978), compared with vegetation not subject to manuring. The impact of cats on the burrowing-bird population of Marion Island has been assessed (Van Aarde, 1980), but not their indirect effect on the vegetation of the island. Table 2 shows the numbers of six bird species caught by cats during 1982 on Marion Island (Van Rensburg, 1985), the guano output by individuals of each species (Fugler, 1985), and the calculated 'loss' of guano to the island during that year; birds were presumed to spend 100 days per year on the island. The vegetation on Prince Edward Island presents a much more luxuriant aspect than that of Marion; in the presence of greater numbers of burrowing birds occurring at greater densities, the effect of manuring is probably more profound on Prince Edward than on Marion. Consequently, natural factors may result in dissimilarity between the insect faunas of Marion and Prince Edward Islands, but the quantitative differences indicated by the results can be in part ascribed to the advent of man and his synanthropes on Marion Island. Insect populations are valuable barometers of environmental disturbance. As a relatively pristine example of the pre-man sub-Antarctic, Prince Edward Island and its terrestrial fauna warrant extraordinary protection.
Insect faunas of Marion and Prince Edward Islands
261
ACKNOWLEDGEM ENTS D r D. T. R o w e - R o w e (Natal Parks Board) gave valuable advice and assistance in compiling the manuscript. Steven C h o w n and Charles Gilbert assisted with field work. Biological research at the Prince Edward Islands is carried out under the auspices o f the South African Committee for Antarctic Research (SASCAR). Financial and logistical support is provided by the South African Department of Environment Affairs.
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