Status and Distribution of Asthenes anthoides (Furnariidae), a Species Endemic to Fuego-Patagonia, with Notes on Its Systematic Relationships and Conservation

Abstract

The Austral Canastero Asthenes anthoides (King, 1831; Furnariidae), en- demic to Patagonia and Tierra del Fuego (southern South America south of about 37°S), was studied in 1985-1988. Fieldwork showed the Austral Canastero to be (a) relatively widespread and locally common in mesic shrubsteppes, (b) absent from grasslands, and (c) more common in Tierra del Fuego than it was 90-100 years ago. These observations contrast with reports stating (a) that A. anthoides is vanishing and threatened because its formerly extensive grassland habitat has been virtually destroyed by overgrazing, and (b) that its occurrence in shrublands today reflects a relictual distribution in a marginal habitat. To resolve these divergent points of view, the habitat preferences, status, and distribution of A. anthoides are reviewed. Recent field surveys revealed that A. anthoides is not rare and occurs both on the Patagonian mainland and in Tierra del Fuego. A. anthoides is most frequently found in mesic shrubsteppe receiving 350-450 mm of rain/year, with an upper stratum of shrubs (Chiliotrichum and Berberis), and a ground stratum of tussock-grasses (Festuca). A. anthoides is also present in three other habitats: (1) more humid, tall and dense woodland with small trees (Nothofagus antarctica), (2) drier, low and open shrubsteppe with shrubs (Mulinum spinosum) and some grass tus- socks, and (3) drier, low shrubsteppe with uniform stands of Lepidophyllum cupressifor- me without grass cover. In its mesic shrubsteppe habitat A. anthoides reaches densities of up to one singer per ha. A. anthoides was looked for, but not found in spite of repeated visits, in the extensive grasslands of Magallanes receiving 200-300 mm of rain/year. The geographical range of A. anthoides seems to be disjunct. A population in north- western Patagonia (western Neuquen, western Rio Negro, and western Chubut, Argen- tina; and adjacent Chile in Nuble, Malleco, and Aysen) may be geographically isolated from a southern population inhabiting the Patagonian mainland (southern Santa Cruz, Argentina, and Magallanes, Chile) and northern Tierra del Fuego (in both Argentina and Chile). Nineteenth and early twentieth century reports mentioned that A. anthoides was rare in Tierra del Fuego, whereas the species is common there today. A. anthoides has thus increased numerically in the last 90-100 years and is not vanishing on Tierra del Fuego. On the Patagonian mainland this species is relatively localized and patchily dis- tributed but is not rare where found. There is no evidence suggesting that A. anthoides occupied the formerly more widespread grasslands of Patagonia, that its alleged rarity is due to grassland destruction from overgrazing by sheep, or that its modern shrubland habitat is marginal. All steppe habitats, including grasslands where A. anthoides does not occur, and shrubsteppes, including the Chiliotrichum-Festuca shrubsteppe where it is found most commonly, are overgrazed. In Chilean Fuego-Patagonia, grasslands support on average one sheep per 2 ha, and shrubsteppes twice as many sheep, on average one per hectare. Overgrazing has degraded both grasslands and shrubsteppes, and the shrub- steppe habitat where A. anthoides lives is no less impacted by grazing than the grasslands where it does not occur. The apparent distributional gap between northern and southern populations of A. anthoides is located in a low rainfall area where the climate is too arid for the mesic shrubsteppe (degraded or not) favored by this species. Careful exploration of mesic shrubsteppe pockets along the eastern Andean foothills of western Santa Cruz may therefore reveal the presence of A. anthoides there. Even though the southern pop- ulation of A. anthoides does not seem threatened, its shrubsteppe habitat is not only subject to intensive sheep grazing pressure (too many sheep/ha), but also to overgrazing on a large geographical scale. In Chilean Fuego-Patagonia over 80% of grazing land is