Diversity and Organization of the Ground Foraging Ant Faunas of Forest, Grassland and Tree Crops in Papua New Guinea

- - -- Aust. J. Zool., 1975, 23, 71-89 Diversity and Organization of the Ground Foraging Ant Faunas of Forest, Grassland and Tree Crops in Papua New Guinea P. M. Room Department of Agriculture, Stock and Fisheries, Papua New Guinea; present address: Cotton Research Unit, CSIRO, P.M.B. Myallvale Mail Run, Narrabri, N.S.W. 2390. Abstract Thirty samples of ants were taken in each of seven habitats: primary forest, rubber plantation, coffee plantation, oilpalm plantation, kunai grassland, eucalypt savannah and urban grassland. Sixty samples were taken in cocoa plantations. A total of 156 species was taken, and the frequency of occurrence of each in each habitat is given. Eight stenoecious species are suggested as habitat indicators. Habitats fell into a series according to the similarity of their ant faunas: forest, rubber and coffee, cocoa and oilpalm, kunai and savannah, urban. This series represents an artificial, discontinuous succession from a complex stable ecosystem to a simple unstable one. Availability of species suitably preadapted to occupy habitats did not appear to limit species richness. Habitat heterogeneity and stability as affected by human interference did seem to account for inter-habitat variability in species richness. Species diversity was compared between habitats using four indices: Fisher et al.; Margalef; Shannon; Brillouin. Correlation of diversity index with habitat heterogeneity plus stability was good for the first two, moderate for Shannon, and poor for Brillouin. Greatest diversity was found in rubber, the penultimate in the series of habitats according to heterogeneity plus stability ('maturity'). Equitability exceeded the presumed maximum in rubber, and was close to the maximum in all habitats. The mosaic dispersion pattern found among ants elsewhere also appeared to be present in each habitat. The mean maximum number of territories possible to be overlapping is about 4.6; the number in a particular case probably being a function of the specialization of the dominant ant present. Ecological isolating mechanisms among ants are probably similar to those among birds; size and structure of mouthparts in ant communities warrant further study. Introduction The importance of ants in many tropical ecosystems has long been recognized (Wheeler 1910), and there have been a number of cases in which ants have been used to control pests of tropical crops (Groff and Howard 1925; Meer Mohr 1927; Bruneau de Mire 1969; Stapley 1971). Recently, interest has developed in the general principles governing interactions between ants and tropical tree crops, and ways in which those interactions can be manipulated (Room 1972; Leston 1973). The study reported here was concerned with description and analysis of the ground- foraging components of the ant faunas of a variety of habitats ranging from primary forest to urban grassland. Its principal objective was to examine the extent to which ant faunal diversity was a function of habitat heterogeneity as affected by human interference. Study Area and Habitats Study Area Field work was carried out around Popondetta (148'14'E., 8'45's.) in the Northern District of Papua New Guinea. Fig. 1 shows the major localities in the area, and P. M. Room the position of the area in relation to the whole of the island of New Guinea. All samples were taken within or very close to the triangle subtended by Saiho, Lego and Girua with the exception of those taken in the eucalypt savannah grassland in the hills immediately to the south of Oro Bay. The above triangle is located on the gently sloping outwash plain to the north of Mt Lamington. Numerous steep- sided stream valleys dissect this plain, and the black sandy soil is very well drained. 0 30 -kilometres Solomon Afore Fig. 1. Map of the Popondetta area in the Northern District of Papua New Guinea, showing roads and mountains. Much of the primary, evergreen rain forest is still standing in the Northern District. Dotted about on the outwash plain are a number of quite extensive grassland 'islands', while near human settlements and particularly along roads there is a patchwork of secondary forest, food garden clearings, and cocoa, coffee and rubber plantings. Popondetta has an average annual rainfall of 2425 mm. Monthly mean rainfall and daily sunshine are given by Room and Smith (1974). June, July and August of each year are usually relatively dry, but not so dry that lack of water limits plant growth. Rainfall increases inland towards Mt Lamington and the Wharton Range, and decreases towards the coast. No figures are available, but residents believe that Saiho has a little more rain than Popondetta, while Oro Bay has a great deal less. Mean maximum and minimum temperatures at Popondetta are 32 and 22°C I2"C (7-y means). Haantjens (1964) gives more detailed information on geology, soils, climate and vegetation in the study area. Ground-foraging Ant Faunas of Papua New Guinea Habitats Primary forest. Anisoptera kostermansiana Dilmy-Pometia pinnata Forst. F. association (Taylor 1964). A, B and C storey trees were present, with great diversity of species. The herb layer was sparse and contained many tree seedlings. Leaf litter and rotting wood were abundant on the ground. Light intensity was generally less than 1 % of outside light and the microclimate was warm, humid and stable. Rubber plantation. The rubber trees were between 15 and 25 y old and most had a low C storey of young cocoa trees, bananas or rubber saplings beneath them. Light penetration through the rubber canopy was greater than in the forest and consequently there was a denser layer of herbs on the ground. Pathways were kept clear along the rows of rubber trees for tapping, and periodically bush regrowth between rows was cut back. Leaf litter was present on the ground, but not to such a depth as in the forest. Cocoa plantation. The cocoa trees and the most commonly used shade tree, Leucaena leucocephala (Lam.) de Wit, formed a single C storey canopy in most of the plantations, but in a few others Erythrina spp., coconuts or thinned forest trees formed a thin B or A storey above the cocoa. Light penetration to the ground was generally less than in the rubber but more than in the forest. A thick layer of leaf litter was usually present, and in places where the canopy was thin there was a moderately dense herb layer. Room and Smith (1974) give a detailed account of this cocoa ecosystem. CofSee plantation. Most of the robusta coffee formed a dense C storey canopy beneath scattered Albizia falcataria (L.) Fosberg or Leucaena leucocephala shade trees. Light penetration was very low and the herb stratum consisted almost entirely of seedling coffee trees. A thick layer of wet leaf litter was usually present. Oilpalm plantation. Oilpalm is grown without top shade, and, though more light penetrates the canopy than in cocoa plantations, the ground in the two oilpalm blocks sampled was mostly bare because of frequent weeding. Each block was 19 by 17 palms in size and it is probable that microclimatic conditions within the blocks were not as stable as they would be in more extensive plantings. Kunai grassland. Imperata cylindrica (L.) Beauv.-Saccharum spontaneum L. alliance (Taylor 1964) is maintained as a disclimax by annual burning. Mature stands are up to 3 m in height. Temperatures in full sunlight are very high at 1 m, but lower on the ground due to shading by the tall grasses which tend to lodge. Soil between the grass stems is usually bare. Eucalypt savannah grassland. This is a Themeda australis (R. Br.) Stapf.-Imperata cylindrica alliance (Taylor 1964) which is regularly burned. There are some scattered thin stands of Eucalyptus tereticornis Sm. and the grass is generally below 1.5m in height. The rainfall in this habitat is significantly less than in the others. Urban mown grassland. This is dominated by Chrysopogon aciculatus (Retz.) Trin. and repeatedly cut down to 2 cm in height. Playing fields, an airstrip and garden lawns were sampled. Table I summarizes the main components of each of the above habitats, and for the purpose of later discussion indicates the comparative diversity of the ground ant fauna to be expected in each. These latter projections are based on the hypothesis that faunal diversity is a function of habitat : age by area by heterogeneity by microclimatic stability. The first two factors, and a factor 'availability of suitably preadapted P. M. Room species' which could not be estimated, relate to the number of species successfully invading a habitat. The latter two factors relate to the carrying capacity of the habitat, and themselves vary as some inverse function of the 'degree of repeated human interference'. With the exception of the forest, each of the habitats is maintained as a disclimax by man: rubber, cocoa and coffee by infrequent bush regrowth clearance; oilpalm by regular weeding; kunai and savannah grassland by annual burning; urban grassland by frequent mowing. Table 1. Comparison of factors affecting faunal diversity in eight habitats in Papua New Guinea For 'Strata of vegetation' the numeral indicates the stratum; T, tree; H, herb. Other factors are rated on a five-point scale: I, very low, very young or very small; 11, low, etc.; 111, moderate; IV, high, old or large; V, very high, etc. Factor Primary Rubber Cocoa Coffee Oilpalm Kunai Savannah Urban forest grassland grassland grassland Strata of 3T 1H 2T 1H 1T 1H 1-2T 1T 1H 1H 1H 1H vegetation 1H Tree species Many 1-2 2-3 2-3 1 0 0 0 Herb diversity 111 111 I11 I1 I1 111 111 I Leaf litter Much Some Much Much None None None None Microclimate stability IV IV IV IV 111 I1 I1 I Human interference I 111 111 I11 V IV IV IV Age of habitat V I1 I1 I1 I I11 111 I1 Area of habitat V n I1 I1 I I11 I11 I Expected ground ant faunal diversity V IV IV IV I1 I1 I1 I Sampling Methods Thirty samples were taken in each of the habitats except cocoa plantation, in which 60 were taken as part of a more detailed study of the ground and canopy ant faunas.

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