Granivore impact on soil‐seed reserves in the central Monte desert, Argentina

Abstract

1. Mechanisms of grass‐ and forb‐seed input and output in the central Monte desert of Argentina were studied in 1995 to test the hypothesis that seed consumption by autumn–winter granivores, especially birds, has qualitative as well as quantitative effects on soil‐seed reserves. 2. The abundance of perennial grass seeds in late summer soils (≈ 2400 seeds m–2 or 0·36 g m–2) remained unchanged the following early spring (≈ 2700 seeds m–2 or 0·39 g m–2), despite the incorporation of about 3000 seeds m–2 or 0·71 g m–2 newly produced grass seeds during autumn–winter. Grass seeds appeared to be heavily consumed, especially the medium‐sized ones. 3. The annual forb‐seed bank was about the same size in late summer (≈ 5500 seeds m–2 or 1·34 g m–2) as in early spring (≈ 6500 seeds m–2 or 1·53 g m–2). Since forb‐seed production had been relatively low (≈ 400 seeds m–2 or 0·12 g m–2), these seeds apparently suffered negligible postdispersal losses. 4. This pattern of grass‐ and forb‐seed loss coincides with the pattern of seed consumption by granivorous birds – on average, 93% of seed mass in bird stomachs was from grass seeds, while only 7% was from forb seeds. 5. Further evidence of a major impact of bird foraging on seed reserves is that the mass of particular grass seeds that was lost from soils was positively correlated with the mass of such seeds in bird diets, and that the main target of bird consumption, i.e. medium‐sized grass seeds, suffered the highest postdispersal loss. 6. It is concluded that autumn–winter granivores in the central Monte desert, particularly birds, mainly consume newly produced grass seeds and that they might have major qualitative as well as quantitative impacts on soil‐seed reserves.

Diet switching of seed-eating birds wintering in grazed habitats of the central Monte Desert, Argentina

“…Así, al menos desde mediados del verano en adelante la abundancia de semillas en el suelo aumenta, alcanzando la acumulación máxima durante el invierno (Marone y Horno 1997). Hay una marcada disminución de la abundancia de semillas en especial de las de gramíneas en el suelo a lo largo del invierno (Marone y Horno 1997, Marone et al 1998b, 2000b. En consecuencia, entre fines de invierno y principios de primavera hay un período de escasez relativa de alimento para los granívoros de Ñacuñán.…”

“…La biomasa de semillas en el suelo medida a mediados del invierno es una subestimación conservativa frente a la hipótesis de la disponibilidad durante todo el invierno, porque en ese momento el banco de semillas disponibles que debería ser considerado (el de principios de invierno) ya sufrió pérdidas por consumo (Marone et al 1998b). Adicionalmente, a esa disponibilidad habría que sumarle la biomasa de semillas que ingresa al banco proveniente de las plantas, aunque ésta es extremadamente baja a principios (Marone et al 1998b) y casi nula después de mediados del invierno (Marone et al 1998a(Marone et al y 1998b. La germinación, por el contrario, es una pérdida potencial de semillas del banco que podría redundar en una sobrestimación de la disponibilidad, pero es prácticamente nula durante los meses del invierno (Marone et al 2000a).…”

Impacts of large herbivorous mammals on bird diversity and abundance in an African savanna

“…Granivorous birds can decrease seed survival (Marone et al 1998;Kelt et al 2004), while insectivores can reduce the abundance of herbivorous arthropods (Strong et al 2000;Sanz 2001;Hooks et al 2003;Van Bael et al 2003), and frugivorous birds can be important agents of seed dispersal (Herrera et al 1994;Tabarelli and Peres 2002;Ingle 2003). Consequently, birds can inXuence the survival and reproduction of herbaceous and woody plants both directly, through seed predation, and indirectly, by reducing the abundance of herbivorous insects (Strong et al 2000; Vangermination sites (Nogales et al 1998).…”

Influence of cattle grazing on population density and species richness of granivorous birds (Emberizidae) in the arid plain of the Monte, Argentina

“…Cattle grazing may affect food supply as well as vegetation structure that provides safe sites to eat and to nest (Ryder, 1980). Grass seeds constitute the main food resource of granivorous birds in the Monte Desert during dry seasons (Schluter & Repasky, 1991;Marone et al, 1998;Marone et al, 2000). Seed abundance is lower in the wet season than in the dry season (Marone & Horno, 1997; this study).…”

“…During this time, seed availability limits their abundances (Schluter & Repasky, 1991). Grass seeds (Poaceae) are the main food resource for these birds and constitute nearly 93% of the seed mass in bird stomachs in the autumn and winter in the Monte Desert Marone et al, 1998;Marone et al, 2000).…”