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Universidad de Buenos Aires (2001)

Guild structure and organisation in an avian assemblage of the monte desert

Lopez de Casenave, Javier Nestor

Titre : Guild structure and organisation in an avian assemblage of the monte desert

Estructura gremial y organización de un ensamble de aves del Desierto del Monte

Auteur : Lopez de Casenave, Javier Nestor.

Université de soutenance : Universidad de Buenos Aires

Grade : Doctor en Ciencias Biológicas 2001

Guild structure and organisation of the avian assemblage of Ñacuñán Biosphere Reserve (central Monte desert, Mendoza Province, Argentina) were studied between May 1993 and February 2000. General objectives of the study were : (1) to analyse the guild structure of the avian assemblage, its association with ecological factors, and its variations ; and (2) to assess the role of interspecific competition as an organising process of this assemblage. In the first part of the study, foraging patterns of 18 bird species were quantified, and on this base they were classified into four guilds (ground foraging birds, arboreal herbivores, foliage foraging birds, and aerial foragers). The use of the ground and height strata, along with the utilisation of certain plant species, were the features determining the classification. Then, seasonal differences in the foraging patterns were assessed. The guild structure showed seasonal changes (three guilds were present during the nonbreeding season, and four guilds were present during the breeding season), due both to the seasonal occurrence of migratory species and to changes in the foraging ecology of resident species. In the second part of the study, observed patterns at Ñacuñán were compared to predictions derived from the hypothesis that interspecific competition is responsible of the structure of the avian assemblage. Medium and long.term variations of the avifauna were evaluated, by comparing presence.absence records for bird species during the last 30 years, and by analysing bird density variations and the degree of association in the species. abundance fluctuations during ten years. Results were no compatible with the competition hypothesis, and the alternative hypothesis that the assemblage is organised in accordance with the .crunch model. was postulated. Then, morphological differences among six granivorous species of Ñacuñán were evaluated. There was no evidence of community.wide character displacement for the studied morphological characters. There was no relationship between the characteristics of seeds consumed by birds and bird morphological characters, neither to population nor to individual level. Again, results were better explained under the .crunch model.. Finally, the hypothesis .derived from the model. that granivorous birds are not limited by food in winters with average seed abundance was evaluated. Populations. food requirements were estimated with bioenergetic models, and then compared to food availability. Results indicate that birds are not usually limited by food during winter. At best, birds could be only occasionally limited, as proposed in the model. Overall, results indicate that interspecific competition is not an important organising process in this assemblage. The .crunch model. appears to be a better hypothesis and deserves a deeper evaluation.


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