Insectes soc. 46 (1999) 281–290 0020-1812/99/030281-10 $ 1.50+0.20/0 Insectes Sociaux © Birkhäuser Verlag, Basel, 1999 Research article Impact of tree isolation on arboreal and ground ant communities in cleared pasture in the Atlantic rain forest region of Bahia, Brazil J.D. Majer 1 and J.H.C. Delabie 2 1 School of Environmental Biology, Curtin University of Technology, P.O. Box U 1987, Perth, WA 6845, Australia, e-mail: [email protected] 2 Centro de Pesquisas do Cacau (CEPEC-CEPLAC), 45.600.000, Itabuna, Bahia and Departamento de Ciências Agrárias e Ambientais, Universidade Estadual de Santa Cruz, 45.660.000, Ilhéus, Bahia, Brazil, e-mail: [email protected] Received 13 July 1998; revised 9 November 1998; accepted 20 November 1998. Summary. The Atlantic rain forest of south-eastern Brazil tation. It once occupied about one million square kilometers has been substantially cleared, resulting in the creation of a of the eastern part of Brazil, but less than 9% of this eco- fragmented landscape. In addition to the small fragments of system remains (Cãmara, 1991) and much of it that still forest that remain, the pasture is often scattered with isolated exists is represented in small fragments and linear strips of trees. This paper investigates the capacity of these isolated vegetation (Fonseca, 1985). These fragments of forest within trees to support representatives of the original Atlantic rain the matrix of agricultural land are often the only means for forest ant communities and also how these arboreal ants inter- conserving representatives of the original biota (e.g., Saunders act with the disturbance-associated ant fauna of the pasture et al., 1987). beneath them. Twenty trees in the grassland, representing a Unfortunately fragments are prone to edge effects, such range of distances from the forest, and 10 trees within the as changes in microclimate, vegetation structure and diver- forest, were selected for sampling. Arboreal ants were sampled sity, and weed invasion, all of which influence the vertebrate by hand collection and chemical knockdown, while the ants on and invertebrate fauna (Turner, 1996; Didham, 1997). Majer the ground beneath were sampled by pitfall traps and Winkler et al. (1997) sampled the litter ant fauna along transects sacks. Pasture trees supported a moderately high richness of running through Atlantic rain forest into an adjacent arboreal ant species. The richness of ants on pasture trees grassland. Their findings suggested that species richness was appears to be independent of distance from forest, although lowest in the field and highest at the deepest point within the this might become a significant factor on trees that are more forest, and that certain ant species had preferences for the isolated than those studied here. Ant species richness on field or particular distances into the forest. However, it was pasture trees is higher if the trees are large, support a high concluded that although the ant fauna of Atlantic rain forest epiphyte load and are native to the area. Isolated trees within is severely affected by clearing, provided it is protected the agricultural matrix therefore play some role in conserving from degradation by a robust fence, a forest-like ant fauna elements of the original forest ant fauna. Since some of the is able to persist right up to the interior edge of the forest. species on pasture trees have been observed to reach dominant These findings suggest that relatively small remnants of or sub-dominant status in nearby forest and cocoa farms, they forest (1000 ´ 400 m in this case) can contribute to the con- may play some role in limiting pest outbreaks in the pasture servation of ant communities and probably of other inverte- close to the trees. If this is so, there may be a case for retain- brate groups, such as butterflies (see Brown and Brown, ing an adequate density of trees to enable the influence of arbo- 1992). real ants to extend over as much pasture as possible. In addition to the many small fragments of forest that remain, the pasture is often scattered with isolated trees Key words: Rain forest, fragmentation, isolation, ants, trees. that have been planted, have recolonized or are survivors from the forest that formerly occupied the area. This paper investigates the capacity of these isolated trees to support Introduction representatives of the original Atlantic rain forest ant com- munities and also how these arboreal ants interact with As with other ecosystems throughout the World, Atlantic rain the disturbance-associated ant fauna of the pasture beneath forest has not escaped the problems of clearing and fragmen- them. 282 J.D. Majer and J.H.C. Delabie Ants on isolated trees Methods the Myrmecology Laboratory at CEPLAC. The nomenclature follows Bolton (1995). A full reference collection for this material is deposited Selection and characterization of trees at CEPLAC. Weather conditions were generally sunny during much of the sam- Field work was carried out in February 1998 on the grounds of the Cen- pling period; 1.9 mm rain fell on one night during this time and daily ter for Cocoa Research (CEPLAC), Itabuna, Bahia (14°45¢S, 39°13¢W) maximum temperatures averaged 26.7 °C. at the same site where Majer et al. (1997) investigated the influence of forest edges on ant community structure. The grounds are predo- minantly planted to cocoa, although some areas are maintained as Data analysis grassland. A rectangular shaped botanical reserve of secondary rain forest, measuring about 400 m ´ 1000 m, exists within the grounds and The ant species obtained in both Winkler samples (n = 2) and both adjoins a field on the southern side. Despite the fact that cows were pre- pitfall traps (n = 2) situated at the same distance from the trunk of each sent in the field, the grass and shrubs were at least 60 cm high and litter tree were combined (n = 4) to provide a comprehensive assessment of consisted almost solely of decaying grass material. This grassland mea- the ant species found at 1 m and 5 m from the trunk of each tree. The sures 700 m ´ 1000 m and is flanked on both eastern and western sides hand collections and knockdown samples from each tree were also com- by more cleared land. It contained about 100 trees, scattered in a ran- bined in order to provide an assessment of the ant species found on each dom distribution. With few exceptions, trees were at least 20 m from the tree. neighboring tree. A matrix of ant species on and beneath each tree was first prepared Twenty trees, representing a range of distances (10–275 m) from and used to compile lists of the frequency of ants on and beneath trees, the forest, were sampled in the grassland. Ten trees within the forest out of 20 and 10 for pasture and forest respectively. This enabled com- were also sampled. All selected forest trees were from within the area parisons to be made between both species richness and the actual ant studied by Majer et al. (1997) and were at least 20 m from the border in species present on forest versus pasture trees and also on trees versus the order to minimize edge effects. Within the forest it was not possible to ground beneath. In order to provide an indication of the relationships sample from the canopy of a single tree as the canopies of adjacent trees between ant species richness on trees with tree environmental character- were inter-twined. Therefore samples from the forest represent a portion istics, simple parametric correlations were performed on the ant versus of continuous canopy rather than a single tree. Each tree was identified, environmental parameters. Interrelationships between the various tree noted as exotic or local and, for trees in the grassland, the height, crown and ground characteristics were also investigated by this method to see diameter and distance of each tree from the forest was recorded. The if any were interrelated. The relationship between tree and ground ant shrub layer beneath the grassland trees was measured on a four-point species richness was also investigated by simple correlation. Unless scale: 1, equal to grassland; 2, grass plus up to five emergent shrubs; otherwise indicated, all correlations were performed on the pasture 3, grass plus six to twenty emergent shrubs; and 4, understorey mostly trees alone, since the forest trees were likely to present a discontinuity dicotyledonous shrubs. Similarly, the epiphyte load on trees was record- in the data. ed as: 1, none; 2, one to five per tree; 3, six to twenty per tree; and 4, The species presence/absence data for each of the pasture and forest over twenty per tree. No measurements were made on the forest trees, trees were then ordinated (principal coordinates analysis) using the although all had medium to dense epiphyte loads and the general height SYN-TAX ® computer package (Podani, 1995). Sorensen’s similarity of the forest overstorey was measured as 18–20 m. index was used, as it has been shown to be one of the most reliable mea- sures for use with binary sets of data (Huhta, 1979). Tree environmental parameters were plotted on the ordination diagram to identify whether Ant sampling any of these might explain differences in ant community composition. In the grassland, litter ants were sampled by two complementary methods at distances of 1 m and 5 m from the tree trunk, both in the northerly and southerly direction. These samples were considered to Results represent the ground fauna respectively under and outside the influence of the tree and its associated arboreal ant community.