The only known introduction of a squirrel species into South America is the case of the Asiatic red-bellied tree squirrel Callosciurus erythraeus that was introduced in the Pampas Region of Argentina in 1970. To inform management programmes, we analysed the current distribution, expansion rate, and abundance of the red-bellied squirrel in Argentina, and identified invasion pathways. Apart from the first and main focus of invasion, three other invasion foci have originated as a consequence of intentional releases between 1995 and 2000. The main invasion focus already extends over >1300 km 2 , where estimated density averaged 15 ind ha −1 and numbers of squirrels may reach 100,000 individuals. The area invaded in the other three invasion foci varied between <1 and 34 km 2 and mean densities were around 3-5 ind ha −1 . Squirrel abundance and residents' attitude towards this introduced species in the main invasion focus make eradication unfeasible, though management actions such as control, containment and mitigation of damages should be undertaken. Eradication of the relatively small invasion foci must be immediately evaluated, with priority given to valuable conservation areas under high invasion risk.

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The relevance of fluvial systems to process nutrients and carbon is widely accepted, but their role as sinks and sources of nutrients and dissolved organic carbon (DOC) is still under discussion especially in non-forested and highly productive streams. In this study, we used a mass balance approach at a reach scale in a Pampean stream to elucidate the major sources of water, nutrients and DOC as well as to determine net in-stream retention efficiencies of nutrients and DOC under different hydrological conditions. We measured conductivity, conservative ions (chloride and calcium), soluble reactive phosphorus (SRP), nitrate (NO 3), nitrite (NO 2), ammonium (NH 4) and DOC at the end-point of a reach of Las Flores stream (site A), at two upstream tributaries (B1 and B2), and at each potential hydrological contributors to stream flow (groundwater, overland and subsurface flows, and rainfall). In addition, we monitored one storm event where we collected samples during the rising and the recession limb of the hydrograph. Stream flow originated from groundwater (≈ 50%), upstream tributaries (B1 and B2) at baseflow, whereas overland flow contributed >20% during high flows. During baseflow, groundwater provided NO 3 to stream water, while B2, which received a point input of a dairy industry, was the main source of SRP and NH 4. Conversely, SRP and NH 4 were provided by B1, overland flow and subsurface flow during high flows. Overland flow also contributed DOC during high flow periods. Mass balance estimates revealed that the reach acts as a source of DOC, SRP and NO 3 (21.4, 37.4 and 53.5 % mean net in-stream release, respectively) and a sink of NH 4 (-36.8 % mean net instream retention). Relevant in-stream processes may be nutrient uptake (as in the case of SRP and NH 4) and biotic production (DOC), as well as decomposition (SRP) and nitrification (NH 4) in this Pampean stream. Our results stress the relevance of nutrient and DOC generation processes within the channel in non-forested and highly productive streams.

RESUMEN.Las invasiones biológicas son una amenaza para las especies nativas. La ardilla de vientre rojo Callosciurus erythraeus fue introducida en Luján (provincia de Buenos Aires, Argentina) en 1970, donde estableció la principal población silvestre del país. Las ardillas podrían afectar a las aves a través de la depredación de nidadas, de la competencia por alimento y sitios para nidificar, o por interferencia. El objetivo fue comparar la composición, densidad y riqueza de especies de aves en sitios arbolados adentro y afuera del área de distribución de ardillas (680 km 2 en 2004) en los alrededores de Luján. Se seleccionaron siete pares de sitios (cada par con estructura y composición de la vegetación similares) más otros 14 sitios (sin parear), la mitad en cada condición (con y sin ardillas). En cada uno de los 28 sitios se establecieron 4-5 puntos de conteo de 10-20 m de radio y durante dos días de primavera-verano 2008-2009 se registraron todas las aves vistas/oídas. La composición de las comunidades de aves resultó diferente entre sitios con y sin ardillas, más notoriamente al comparar entre sitios pareados. No se detectaron diferencias en la densidad de aves pero la riqueza de especies fue mayor en los sitios pareados sin ardillas. Se hicieron ensayos de cafetería para evaluar el consumo de huevos por parte de las ardillas y determinar el tamaño de huevo preferido. Ante la oferta de fruta y huevos de tres tamaños, de los 25 ensayos en que las ardillas consumieron fruta, solo en 2 consumieron algún huevo. Nuestros resultados sugieren que las ardillas podrían afectar negativamente a algunas especies de aves pero no a través de la depredación de nidadas.[Palabras clave: ardillas introducidas, Callosciurus erythraeus, densidad de aves, especies invasoras, riqueza de aves] ABSTRACT. Impact of red-bellied squirrels on the bird community of the Pampas Region, Argentina: Biological invasions are a major threat to native species. The red-bellied tree squirrel Callosciurus erythraeus was introduced in Luján, Argentina, in 1970, where it spread rapidly. Squirrels can affect birds through nest predation, interference and competition for food and nesting sites. We aimed to compare community composition, density and richness of birds in forested areas within and outside the range distribution (680 km 2 in 2004) of the main population of these squirrels in the country. We selected seven paired sites (each pair had similar vegetation structure and composition) and other 14 sites, half in each condition (with and without squirrels). In each of these 28 sites, we did 4-5 point counts during spring-summer 2008-2009, recording all bird individuals that were seen/heard <10-20 m from the observer. Species richness was rarefied to the smallest number of point counts per site. Community composition differed among sites with and without squirrels, particularly for paired sites. We found no differences in bird density though species richness was higher for paired sites without squirrels. To assess if squirrels do consume bird eggs...