Journal of Biogeography, 28, 1379±1388 Colonization of an island volcano, Long Island, Papua New Guinea, and an emergent island, Motmot, in its caldera lake. VI. The pioneer arthropod community of Motmot John S. Edwards1 and Ian W. B. Thornton21Department of Zoology, University of Washington, Seattle, WA 98195, USA, and 2Department of Zoology, La Trobe University, Bundoora, 3083 Australia ABSTRACT Aim To evaluate the arthropod community of Motmot in relation to primary colonization of young volcanic surfaces. Location Motmot, an island in Lake Wisdom which occupies the caldera of Long Island, Papua New Guinea. Methods Arthropod sampling by means of pitfall, water and tube traps, fallout collectors, and hand collecting. Results At least thirty-®ve species of arthropod were collected in 6 days between 23 June and 3 July 1999. Lycosid spiders and ants dominated in all areas. The predator± scavenger arthropod population is largely or entirely dependent on allochthonous input of aquatic insects from the surrounding lake. Main conclusions Major changes in the arthropod fauna since the pioneer surveys of Ball and his colleagues in the 1970s are the loss of a strand ¯otsam community as the island has eroded to form a predominantly cliffed coastline. Ant and spider diversity has increased. The current colonists include a number of widespread `tramp' species sensu Diamond. Keywords Allochthonous arthropods, ants, arthropod colonists, organic fallout. emergent island, Motmot, in Long Island's freshwater INTRODUCTION caldera lake, Lake Wisdom. In 1999 we took the opportun- The origin of communities is one of the most intriguing ity to extend the pioneer studies of Ball and his colleagues. questions of ecology. How do pioneer plants and animals As a result of volcanic activity in Lake Wisdom between reach and become established on bare ground, and how are 1953 and 1955 an island emerged, which by 1982 had been sustainable communities developed? Opportunities to study reduced by wind and wave erosion to two small, low ash islets. the natural assembly of communities de novo are rare. Long- Resumption of volcanic activity then formed the present term studies have been made of the recovery of communities island, Motmot (see Thornton, 2001), c. 600 sq.m. in area, following devastating volcanic eruptions (Thornton, 1996, with the summit c. 40 m above Lake Wisdom. Motmot is 2000; Edwards, 2002) and of the origin and growth of biotas some 4±5 km from the nearest lake shore (Figs 1 and 2a). in two well known cases of the emergence of marine islands The initial arthropod colonists of Motmot were monitored ± Surtsey (Fridriksson, 1975) and Anak Krakatau (Thornton, by Ball and colleagues (Bassot & Ball, 1972; Ball & 1996). Bassot & Ball (1972) and Ball & Glucksman (1975, Glucksman, 1975) (Table 1). In 1969 lycosid spiders were 1981) made periodic studies on the pioneer biota of an observed as well as odonates. In 1971 an earwig, two species of ant and an anthicid beetle were also present, as well as Correspondence: John S. Edwards, Department of Zoology, University of two species of chironomid Diptera. In the following years Washington, Seattle WA 98105. ®ve more ant species were recorded together with several E-mail: [email protected] Ó 2001 Blackwell Science Ltd 1380 J. S. Edwards and I. W. B. Thornton Figure 1 Sketch map of Motmot as of 1999. more Diptera, an ephydrid shore ¯y, a muscid, a calliphorid island. Arthropods were sampled by passive and active and two more chironomids. Saldid bugs were present in collecting methods and rough estimates of numbers obtained. strand debris and notonectids in the island's short-lived Passive collecting was carried out using pitfall traps made crater pond. Termites were seen ¯ying on Motmot but they from 7 cm diameter plastic cups containing water with must have originated elsewhere. detergent, crawl-in tube traps (Majer, 1978), and water traps Apart from the termites, and freshwater insects derived (20 cm diameter white enamel dishes holding water with from the lake, the pioneer land arthropods ± abundant detergent and ringed with Vaseline). The warm wind rapidly lycosids and (later) earwigs, ants, and anthicid beetles ± were dried water from the dish traps but the residual viscous scavengers and predators. As with other volcanoes (Thornton, 2000; Edwards, 2002), arthropods established populations long before the appearance of vegetation. In the absence of primary productivity these pioneer arthropods must have Figure 2 (a) Pro®le of Motmot from Lake Wisdom, looking east. depended on imported organic material arriving at the site as The caldera wall of Long Island is visible above the low saddle mid ¯otsam and airborne fallout. A quarter of a century later, part right. (b) View toward the south ridge of Motmot from mid-island. of Motmot is sparsely vegetated but much of the volcanic A Zodiac boat is visible on the western shoreline of the sandy wash. surface remains bare. On these mineral surfaces arthropod (c) View to the north-east from the western margin of the sandy populations typical of pioneer communities persist with wash showing stable slopes with sedge vegetation in the middle relatively high biomass, based on allochthonous sources of distance, and aa beds beyond. (d) View of aa ®eld looking east across Lake Wisdom to Long Island. (e) West coast of Motmot immediately organic input. north of the sandy wash showing shelving beach devoid of ¯otsam. (f) View to north-west showing summit crater with protected METHODS vegetation, and northern margin of Lake Wisdom. (g) Tetragnathid spider web on aa beds, with adult to left and immature to right. (h) We visited Motmot on 6 days between 23 June and 3 July Cyperus seed head covered with tetragnathid eggmasses embedded 1999 (12 man-days collection) and spent one night on the in silk. Coastline of Motmot. Ó Blackwell Science Ltd 2001, Journal of Biogeography, 28, 1379±1388 Pioneer arthropods of Motmot 1381 Ó Blackwell Science Ltd 2001, Journal of Biogeography, 28, 1379±1388 1382 J. S. Edwards and I. W. B. Thornton Table 1 Arthropod records from Motmot 1969 1971 1972 1999 based on records of Ball & Glucksman (1975) and the 1999 expedition Arachnida Araneae Lycosidae ?Varacosa tanna (Strand)* x x x `Trochosa' papakula (Strand)* x Tetragnathidae Tetragnatha mandibulata Walckenaer x Tetragnatha nitens Audouin x Salticidae One species indet. x Araneidae Neoscona sp. indet. x Pseudoscorpionida One species, indet. x Insecta Odonata Xiphiagrion cyanomelas Selys x x Orthetrum sabina (Drury) x AK** Pantala ¯avescens (Fabricius) x K, AK Tramea liberata (Lieftinck) x x x x Orthoptera Gryllidae Teleogryllus oceanicus Le Guillou x Acrididae Acridid, immature, sp. indet. x Isoptera Nasutitermes novarum-hebridarum x Mass ¯ight (Holmgren) Dermaptera Labidura sp. indet. x x x AK Labidura riparia (Pallas) x Hemiptera Saldidae sp. indet. x Strand Berytidae sp. indet. x Coccidae sp. indet. x Coleoptera Carabidae sp. indet. x Strand Anthicidae Anthicus sp. x x Staphylinidae, sp. indet. x Strand Tenebrionidae Gonocephalum sp. x Coccinellidae 2 spp. x Trichoptera Triplectides helvolus Morse & Neboiss x Lepidoptera Hesperidae Parnara sp. indet. x Sesiidae sp. indet x Diptera Chironomidae Polypedilum sp. A x x Poypedilum sp. B x Polypedilum nubifer (Sknse) x Chironomus sp. x x x Cryptochironomus griseidorsum Kieffer x? Ablabesmyia hilli Freeman x Ablabesmyia sp. x Cricotopus sp. x x Metriocnema sp. x? Dicrotendipes sp. x? Ó Blackwell Science Ltd 2001, Journal of Biogeography, 28, 1379±1388 Pioneer arthropods of Motmot 1383 Table 1 continued 1969 1971 1972 1999 Cecidomyidae sp. indet. x Ephydridae Hecomede persimils Hendel x Strand Muscidae Musca vetustisima Walker x Calliphoridae Lucilia papuensis Macq. x Flotsam Hymenoptera Formicidae Paratrechina ?bourbonica (Forel) x x Paratrechina sp. x Hyponeura ?biroi (Emery) x Cardiocondyla ?nuda (Mayr) x Cardiocondyla sp. x Trachymesopus darwini (Forel) x Strand Brachymyrmex sp. x Strand Technomyrmex albipes (Fr. Smith) x Strand Technomyrmex sp. x x Alates only `99 Camponotus sp. x x Alates only ``99 Anoplolepis gracilipes (Smith) x K, AK Tetramorium sp. x ?Monomorium sp. x Pheidole sp. x *The lycosid names Varacosa tanna and `Trochosa' papakula may denote the same species. `Trochosa' is used in quotes to denote provisional name, the entire Paci®c lycosid fauna requiring re-evaluation (Cor Vink, pers. comm.). **K signi®es also found on Krakatau. AK signi®es also found on Anak Krakatau. detergent retained specimens. These traps were set out for 2. stabilized slopes to the north of the saddle; 3. open aa lava 10 days on the `sandy wash', a low ¯at saddle towards the ®elds; and 4. sheltered vegetation, as in the summit crater south of the island (Fig. 1). Fallout collectors, modi®ed from `ravine'. those used on Mount St Helens (Edwards, 1986, 1988), using a monolayer of cemented, grey-painted table tennis balls Resident arthropods instead of golf balls, were also deployed on the slopes to the north of the sandy wash. Eight 0.1 m fallout collectors were Sandy wash (Fig. 2b,c) exposed for 10 days on upper east-facing slopes during a A low ¯at saddle between the sharply rising south ridge period when light to moderate winds from the east predom- (Fig. 2a) and the gentler slopes to the north was covered with inated. A Malaise trap was set up and operated for 7 days on sand and grit and devoid of vegetation except at the margins. the upper bench of a stepped beach to the east of the saddle in During the entire period of the expedition moderate to strong order to sample material arriving from Lake Wisdom and winds from the east raked the saddle. Pitfall and tube traps Long Island some 5 km distant. An aerial insect net was set were established in this area. Large numbers of a large lycosid vertically against the prevailing wind at 1.5 m above ground spider `Trochosa' papakula (Strand) were taken in pitfall on the north coast during one night to sample aquatic insects traps, together with some ants of two species Anoplolepis emerging from the lake.