Emigración y mortalidad de los cisnes de cuello negro

Synopsis

Emigration and Mortality of Black-necked Swans (Cygnus melancoryphus) and Disappearance of the Macrophyte Egeria densa in a Ramsar Wetland Site of Southern Chile
INTRODUCTION
Ecosystem services and the function of wetlands are increasingly altered by human actions around the world. Wetland ecosystems are scientifically and socially important because they represent highbiodiversityareas and water reservoirs. In southern Chile (ca. 39À418S), there are several coastal and inland wetlands larger than 500 ha that are currently threatened by adjacent urban locations due to the discharges of industrial wastes, conversion of soil to agriculture, and water extraction. These Chilean wetlands are mostly located on the coastal range of the Valdivian Rainy forest, which is biologicallyunique due to its high plant diversity and is also being affected by fragmentation resulting from extensive land use changes. These characteristics of the wetland and Valdivian forest have been the promoters of national and international efforts that highlight the need for conservation of these ecosystems (1). Here we describe recent significant biotic changes observed in the Ramsar wetland siteof Rı´ o Cruces, a nature sanctuary under Chilean law and located north of Valdivia city (ca. 408S) (Fig. 1). The site originated during May 1960 as a land subsidence from the worst earthquake recorded in history (9.58 on the Richter scale) and ensuing tsunami with wave crests reaching 10 to 15 m (2), flooding a present area of more than 5000 ha mostly composed of shallow water zones represented byswamps and marsh vegetation. This tectonically originated Ramsar site is also unique, because it became the main reproductive site of the charismatic and vulnerable Black-necked swans (Cygnus melancoryphus) in the neotropic area of South America (3–5). Apart from that uniqueness, the high diversity of water birds and aquatic plants (6, 7) was also pointed out by the Chilean government, when thissite was designated a Nature Sanctuary (3 June 1981). The sanctuary was then accepted as the first Neotropical Wetland of International Importance under the Ramsar Convention on 27 July 1981. to the neotropical region (8–10), with its showy presence and abundance becoming a charismatic species and indicator of relative health of the wetlands, such as the Rı´ o Cruces Ramsar site (11, 12). Duringthe late 1980s (1987), the annual mean swan population abundance increased from nearly 1400À2200 to 6100À6300 individuals during the mid 1990s (1995À1996), abundances that remained fairly constant until 2003 (Fig. 2a). However, during the late winter to early spring of 2004, a high emigration rate, mortality from unknown reasons, and absence of nest and chicks of Black-necked swans were observedwithin this Ramsar site (13). In addition, a significant reduction in the spatial extent and even death of the aquatic, submerged macrophyte, locally known as Luchecillo (Egeria densa), were also observed. This plant was the primary food source for the swans and other herbivorous birds, such as coots (Fulica armillata and others), both being the most abundant water birds in the site (nearly 90% ofthe total avian population). In November 2004, the Chilean National Environmental Commission (CONAMA) contracted Universidad Austral de Chile to carry out the study ‘‘On the origin of mortality and population decline in aquatic birds from the Carlos Anwandter Natural Sanctuary, Province of Valdivia.’’ The final report of this study was due in April 2005 (13). Since then, several other studies havebeen performed (see www.humedalriocruces.com). Here we report the main findings related to the emigration and death of swans and the disappearance of E. densa.

HISTORICAL BRIEF
C. melancoryphus (Anatidae) is the only representative of the genus Cygnus native
Ambio Vol. 36, No. 7, November 2007

´ Figure 1. Geographic location of the wetland of Rıo Cruces or the sanctuary (darker areas ´...