389 2002. The Journal of Arachnology 30:389–402 ON THE NATURE OF AGROBIONT SPIDERS Ferenc Samu: Plant Protection Institute, Hungarian Academy of Sciences, P.O. Box 102, Budapest, H-1525 Hungary. E-mail: samu@julia-nki.hu Csaba Szineta ´r: Berzsenyi College, P.O.Box 170, Szombathely, H-9701 Hungary ABSTRACT. Results from a 10 year survey of spiders in Hungarian arable and natural grassland habitats are cumulated in order to reveal the key characteristics of agrobiont species. We define agrobionts as species that reach high dominance in agroecosystems. The most dominant species, Pardosa agrestis, on average accounted for 40% of the total spider population in Hungarian arable fields. The presence of agrobionts led to a strong skew in arable spider community species distriibution. Regardless of the over- dominance by agrobionts, arable spider communities had a potential for very high species richness. The agrobiont segment of arable spider communities showed very little field-to-field or regional variation, i.e. the same agrobiont species occurred in all fields. Agrobionts were indicators of arable habitats, and were rare in other habitat types, but in many species preferences for specific natural habitat types could be shown. These natural habitat types were often strongly abiotically driven, frequently disturbed habitats. The life cycle of agrobionts showed synchronization with the arable crop-growing season. While many closely related non-agrobiont species had maturity and reproductive periods either earlier or later than the main crop vegetation period, agrobionts invariably reached adulthood and reproduced during that period. Association with frequently disturbed natural habitats and phenological synchronization with the annual arable disturbance regime are such traits that support the theory that agrobiont species are adapted to predictably ephemeral habitats. Keywords: Community structure, arable fields, cyclic colonization, life history strategy Agricultural habitats are artificially created and maintained by periodic disturbances to be more uniform than most natural habitats. Ag- ricultural systems have been present for only a short evolutionary time period. Thus they are likely to lack co-evolved animal commu- nities. Many ‘empty niches’ may offer them- selves for colonization both by herbivorous and predatory animals from natural habitats. It is still debated as to what degree these hab- itats are recolonized repeatedly, or to what ex- tent they are self-perpetuating systems, at least at the metapopulation level (Duffey 1978; Bishop & Riechert 1990; Wissinger 1997). To study the community assembly rules in agri- cultural areas, and to study the ecological characteristics of the individual species should be revealing for the basic ecological phenom- ena, and may provide opportunities to shift the balance in agricultural communities towards beneficial organisms, and thus promote bio- logical control. Considering communities of predatory ar- thropods in agricultural areas, and those of spiders in particular, it has been observed that a few super-abundant species often dominate these systems. The dominating species, since the seminal paper by Luczak (1979) are called ‘agrobiont’ species. The dominance of agro- bionts has been established in various crops and geographical areas (Richman et al. 1990; Nyffeler & Breene 1992; Blick et al. 2000) but many questions about the ecological strat- egies of agrobionts are still open. Duffey (1978) and Luczak (1979) predicted that agro- bionts are habitat generalists, ‘‘eurytopic’’ species, that occur sometimes in quite con- trasting habitats. Recently Wissinger (1997) proposed that agrobionts are species with an ‘‘adaptation to predictably ephemeral habi- tats’’ (APEH). According to the APEH hy- pothesis agrobionts are not generalist species, rather they evolved a specific strategy, called the ‘‘cyclic colonization’’ strategy. Through cyclic colonization, agrobionts can escape the regularly occurring disturbances by dispersing to permanent refugia. The strategy requires specific life history adaptations, with special regard to synchronization with the periodic disturbances through the timing of reproduc-