Austral Ecology (2002) 27, 416–421 Mechanisms affecting the fate of Prosopis flexuosa (Fabaceae, Mimosoideae) seeds during early secondary dispersal in the Monte Desert, Argentina PABLO E. VILLAGRA, 1 LUIS MARONE 2 * AND MARIANO A. CONY 2 1 Departamento de Dendrocronología e Historia Ambiental, IANIGLA, CONICET, Mendoza, Argentina and 2 Instituto Argentino de Investigaciones de las Zonas Áridas (IADIZA), CONICET, Casilla de Correo 507, 5500 Mendoza, Argentina (Email: lmarone@lanet.com.ar) Abstract The fate of seeds during secondary dispersal is largely unknown for most species in most ecosystems. This paper deals with sources of seed output of Prosopis flexuosa D.C. (Fabaceae, Mimosoideae) from the surface soil seed-bank. Prosopis flexuosa is the main tree species in the central Monte Desert, Argentina. In spite of occasional high fruit production, P. flexuosa seeds are not usually found in the soil, suggesting that this species does not form a persistent soil seed-bank. The magnitude of removal by animals and germination of P. flexuosa seeds was experimentally analysed during the first stage of secondary dispersal (early autumn). The proportion of seeds removed by granivores was assessed by offering different types of diaspores: free seeds, seeds inside intact endocarps, pod segments consisting of 2–3 seeds, and seeds from faeces of one herbivorous hystricognath rodent, the mara (Dolichotis patagonum). The proportion of seeds lost through germination was measured for seeds inside intact endocarps, seeds inside artificially broken endocarps, and free seeds. Removal by ants and mammals is the main factor limiting the formation of a persistent soil seed-bank of P. flexuosa: >90% of the offered seeds were removed within 24 h of exposure to granivores in three of four treatments. Seeds from the faeces of maras, on the other hand, were less vulnerable to granivory than were other types of diaspores. These results suggest that herbivory might be an indirect mechanism promoting seed longevity in the soil (and likely germination) by discouraging granivore attack. On the other hand, germination did not seem to have an important postdispersal impact on the persistence of P. flexuosa seeds in the soil. Both direct and indirect interactions between vertebrate herbivores and plants may foster P. flexuosa’s seed germination in some South American arid zones. Key words: germination, granivory, seed predation, indirect interactions, mutualism, Argentina, arid land. INTRODUCTION In most ecosystems, a major proportion of newly pro- duced seeds are lost from the surface soil seed-bank during secondary dispersal or redistribution (Cham- bers & MacMahon 1994). Major mechanisms that account for such losses are germination, consumption by animals, attack from microorganisms and deep burial. Hence, for a plant species to form persistent soil seed-banks, its seeds must not only be capable of some type of dormancy, but also be able to cope with the several postdispersal challenges imposed by mechan- isms other than germination. In the central Monte Desert of Argentina, annual forb seeds appear to form persistent soil seed-banks, whereas perennial grass seeds, shrub seeds and tree seeds form transient soil seed-banks, with most seed output occurring 1–6 months after production (i.e. in autumn and winter; Marone et al. 1998a). The strong decline of grass seeds in the top 2 cm of Monte Desert soil (Marone et al. 1998a) seems to be a consequence of bird and, secondarily, mammal consumption (approximately 50% of grass seeds are lost to grani- vores), deep burial (30% are lost to burial, including all tiny seeds), germination (<5%), and pathogen attack (Marone et al. 2000b). The postdispersal fate of the larger shrub and tree seeds in the Monte Desert remains largely unknown, despite the strong decline seeds suffer as soon as they land on the ground (Marone et al. 1998a). Prosopis flexuosa D.C. (Fabaceae, Mimosoideae) is the most abundant tree species in the central Monte Desert, and its seeds are among the biggest in this ecosystem (24–40 mg). Prosopis flexuosa production is highly variable from year to year, with records of 80 000–800 000 seeds per hectare in different areas of the Monte Desert (Ffolliot and Thames 1983; Dalmasso and Anconetani 1993). Despite such a high potential input, only a few P. flexuosa seeds were found in the soil seed-bank of the central Monte Desert during the winters and springs of 1993 through 1998 (Marone et al. 1998a; L. Marone, unpubl. data). This period had at least two events of high fruit production *Corresponding author. Accepted for publication January 2002.