Biodiversity and Conservation 14: 151–163, 2005. # Springer 2005 Impact of Argentine ants (Linepithema humile) on an arboreal ant community in Don˜ana National Park, Spain S. CARPINTERO1, J. REYES-LO´ PEZ2,* and L. ARIAS DE REYNA3 1Department of Ciencias Ambientales (Zoology), University of Pablo de Olavide (Sevilla), Ctra, de Utrera km 1, E-41013 Seville, Spain; 2Department of Ecology, Faculty of Science, University of Cordoba, E-14071 Cordoba, Spain; 3Department of Animal Biology, Faculty of Science, University of Cordoba, E-14071 Cordoba, Spain; *Author for correspondence (e-mail: [email protected]) Received 19 February 2003; accepted in revised form 1 October 2003 Key words: Arboreal ant community, Competition, Don˜ana National Park, Invasive Argentine ant Abstract. Due to the invasive character of the exotic Argentine ant (Linepithema humile), its use of aphids in trees, and the ecological importance of the Don˜ana National Park (Spain) that is invaded by this species, we designed a study to analyze the extent of the problem with native species of arboreal ants. By searching for ‘de visu’ the species that inhabited 182 cork trees, we found out that the structure of the community of native arboreal ants has been greatly influenced by interspecific competition. The in- troduced species L. humile and the native species Crematogaster scutellaris and Lasius brunneus are dominant, while Camponotus lateralis and Camponotus truncatus are subordinate species associated with C. scutellaris. The distribution of the species in the trees depends on these relationships. Species richness is determined by tree size, thus, when a tree is large enough all native species may appear together. However, in areas colonized by L. humile, this is the only species occupying the tree, regardless of tree size. L. humile is displacing native arboreal ant species as shown by the fact that from 1992 to 2000 the exotic species occupied 23 new cork trees (of the 182 studied) previously inhabited by native species. Introduction Dominant territorial ants typically show mutually exclusive territories, giving rise to a pachy or mosaic distribution, though dominant ants may tolerate some ‘subordinate’ ant species in their territories, but not others. This fact has been extensively studied in arboreal ants (Young 1983; Jackson 1984; Savolainen and Vepsa¨la¨inen 1988, 1989; Adams 1994; Andersen and Patel 1994; Perfecto 1994; Punttila et al. 1994). In consequence, the invasion of aggressive exotic species, such as the Argentine ant (Linepithema humile Mayr, 1868), may greatly disrupt the structure of native ant communities. One of the preferred resources of the exotic Argentine ant is the exudate of homoptera. This exudate provides a large amount of carbohydrates, which are good as fuel or for producing defensive compounds and different types of pheromones (Human and Gordon 1997; Holway 1999). Thus, it is common to find these ants looking for this food in trees and shrubs. A large number of trees (Quercus suber and Pinus pinea) were found to be infested with Argentine ants in Don˜ana National Park 152 (Huelva, Spain). At first sight, this species seemed to remain close to areas inhabited by humans, although we could not exclude the possibility of finding them in natural habitats. Trees in Don˜ana are a scarce and valuable resource for arboreal species (Ferna´ndez 1982); for this reason and because of the invasive character of the Argentine ant, we designed a study to determine the extent of the problem: first to identify the structure of the arboreal native ant community; then to analyze the impact of Argentine ants on this community; and finally, to relate the presence of different species to some characteristics of the trees (such as their size and situation). Materials and methods Study site Don˜ana National Park is located at the southwest coast of the Iberian Peninsula and includes a large area of marshlands at the Guadalquivir and Guadiamar river mouths (378110–368470N, 68100–68470W). The park was created in 1969, mainly to protect aquatic fowl for which it is an important overwintering spot. In the Park there are different ecotopes (Figure 1). One of those, ‘La Vera’, located at the edge of the marshlands, is mainly covered by a eutrophic pastureland with patches of reed beds and scrubland (Juncus spp., Ulex spp., Rubus ulmifolius, Pteridium aquilinum, etc.). In this ecotope, the area known as the ‘Pajareras’ contains several large cork trees (Q. suber) that have been colonized by numerous nesting birds (principally of family Ardeidae). In 1585 a palace was constructed (Don˜ana Palace) in the heart of the current Park, close to ‘La Vera’, when the territory was still private and used for hunting. Today the Palace remains as a residence for researchers and visitors. Other buildings were subsequently added to be used as offices, laboratories and head- quarters of park rangers. A dense population of Argentine ants has been found surrounding this humanized area and this constitutes one of the principal sources of infestation of this species in the Park. Sampling methods In this work, from June to September 1992, we studied and sampled the ant fauna of all cork trees (Q. suber) included in a circular area of 1.5 km radius, with Don˜ana Palace at the center of the circle. There were a total of 182 trees in this area. Sampling was repeated in September 2000. In general, arboreal insect com- munities have been poorly explored, in part due to a scarcity of methods with which to do so (but see Kaspari 2000). In this study, searches for ants were conducted on each cork tree at different times of the day (three visits per tree: morning, midday and evening), each observation during 10 min to detect the ants patrolling trunks and branches from 0.5 to 2 m in height. The following variables were determined for each tree: (1) species of ants present; (2) trunk circumference at 1.5 m height; (3) distance to Palace; (4) distance to nearest road, since the presence of Argentine 153 Figure 1. (Modified from Montes et al. 1998.) (1) Low lands in stable dunes. (2) High humid lands in stable dunes. (3) High dry lands in stable dunes. (4) Semistable dunes. (5) Wandering dunes. (6) Old inland marsh. (7) Marsh of the rivers Rocina and Guadiamar. (8) Meadow between marsh and bushland (‘La Vera’). (9) Beaches. Â Palace of Don˜ana, and inside the circle, the studied area. ants has been associated with the existence of nearby roads in other infested areas (De Kock and Giliomee 1989). Secondly, aggressiveness tests were conducted between species to define the structure of the arboreal ant community. To perform the tests between more abundant species in cork trees (found in N30 trees), worker ants were confronted one to one in a 5.5 cm petri dish for 5 min. Details of the species that initiated encounters and behavioral patterns of both species were registered with a video recorder. The test was repeated 10 times for each pair of species with different individuals. Behavioral patterns detected were as follows: Aggressiveness: (1) Physical attack: when an ant bit another ant’s body, antennae or legs; (2) Chemical attack: the attacker flexed its abdomen and expelled a repellent onto the enemy; (3) Threat: the ant opened its mandible and moved its head towards its opponent. 154 Submission: (1) Retreat: when an ant, following contact, quickly moved away from another ant; (2) Pupal position: when the ant retracted its antennae and legs and remained immobile. Other patterns: Indifference: this occurred when an ant in an encounter did not demonstrate aggressive or submissive behavior. In previous observations, two native species (Camponotus lateralis and C. truncatus) were occasionally seen using trophic trails of another species (C. scutellaris). In order to determine if this behavior was habitual, 106 C. lateralis and 114 C. truncatus workers were observed to record whether they patrolled along a C. scutellaris trail or not. Results Ants in cork trees We found 14 species in the surveyed trees (Table 1). Some of the species nested in trees themselves while others nested in the soil and climbed the trees to forage for food. Argentine ant and arboreal ants (C. scutellaris, C. truncatus, C. lateralis, and Lasius brunneus) were the most abundant species (found in N30 trees). Figure 2 shows a dendrogram with the presence of each species in every tree in 1992, to determine if grouping or segregation occurs between them. The Argentine ant is segregated from the other species, while the native species C. scutellaris, C. lateralis, and C. truncatus form a group independent of L. brunneus. C. lateralis and C. truncatus are not only associated with C. scutellaris because they were in the same trees, but also because they followed C. scutellaris trophic trails (Table 2) (although they were more frequently off the trails) (binomial test for C. lateralis: z ¼ 4.56 p < 0.0001; binomial test for C. truncatus: z ¼ 2.71 p < 0.0001). To analyze relationships between native arboreal species and Argentine ant, aggressiveness tests were conducted (Table 3). Among native species, C. lateralis and C. truncatus are considered submissive since they never attacked and always retreated. C. scutellaris appeared to be the most aggressive species, although it modified its behavior depending on the opponent species. Thus, on the rare occa- sions when it faced C. lateralis or C. truncatus, if an attack occurred, C. scutellaris behaved aggressively and usually did not retreat. In encounters with L. brunneus, C. scutellaris was sometimes attacked and occasionally demonstrated submissive behavior, although in general it was more aggressive than L. brunneus. However, when facing L. humile, it was less aggressive and it was more often attacked. Relationships between species and tree characteristics The following results refer to the relationships found between variables expounded in methodology (presence of species, trunk circumference, distance to closest road 155 Table 1.