A arara azul, (I>Andorhynchus hyacinthinus) maior representante dos psitacídeos e está ameaçada de extinção. A maior população da espécie é encontrada no Pantanal. A arara azul possui distribuição. relativamente restrita e um nicho ecólogico bastante especializado. Cerca de 95% de seus ninhos, no Pantanal, localizam-se em troncos de manduvi (Sterculia apetala)), e a disponibilidade de sítios de nidificação pode ser o principal fator limitante para as populações da espécie. O objetivo deste trabalho foi avaliar a densidade de indivíduos jovens de manduvi sob três diferentes condições de manejo no Pantanal. Os resultados sugerem que há uma falha no estabelecimento de novos indivíduos em áreas submetidas à presença de gado e queimadas, o que pode comprometer a conservação da arara azul a longo prazo. Palavras-chave: Anodorhynchus hyacinthinus, arara azul, manduvi, Pantanal, sítios de nidificação, Sterculia apetala
Key words: Anodorhynchus hyacinthinus, hyacinthine macaw, manduvi, nesting sites, Pantanal, Sterculia apetala.
Anodorhynchus hyacinthinus, the hyacinthine macaw, is the largest species of the family Psittacidae, and is threatened by extinction (Sick,1995). This species is particularly sensitive because of its low population size, limited distribution and highly specialized niche. The wild population has been estimated at 3,000 individuals (Munn et al.1987). Collar et al. (1992) and Abramson and Thomsen (1995) describe the distribution of the hyacinthine macaw population in three areas, almost exclusively Brazilian: eastern Amazon state, in the border with the state of Pará; central north Brazilian cerrado (part of the states of Piauí, Maranhão, Bahia, Tocantins, Goias, Mato Grosso and Minas Gerais); and the Pantanal wetland, including part of Bolivia and Paraguay. Most of the hyacinthine macaw population occurs in the Pantanal, and occurrence of the species in Bolivia and Paraguay is rare.
In the Pantanal, the hyacinthine macaw feeds only on the fruits of bocaiúva (Acrocomia aculeata) and acuri (Scheelea phalerata) palms, and nests primarily in only one species of tree, the manduvi (Sterculia apetala) (Guedes and Harper 1995, Guedes 1995). Between 95-100% of hyacinthine macaw nests in the south central Pantanal occur in cavities of adult manduvis (Guedes and Harper 1995). The availability of suitable nesting sites likely constitutes the most important limiting factor for this species. Manduvis with hyacinthine macaw nests are lost to senescence, but windstorms and burning are also responsible for the loss of nesting sites (Guedes,1995). The rate of manduvi recruitment is unknown.
The manduvi occurs within patches (capões) and corridors (cordilheiras) of non-inundable semi-deciduous forests in the Pantanal (Ratter et al.1988). Surrounding the capões and cordilheiras are seasonally flooded grasslands, much of which is used as rangeland for cattle. During the flooding season (January-June) these forest habitats experience increased pressure from cattle. Additionally, fire used for pasture management commonly enters capões and cordilheiras during the dry season.
The objective of this study was to conduct a preliminary evaluation of the density of young manduvis (seedlings and trees that have not yet begun to produce fruit) in three different management conditions: exclusion of cattle for the past five years, cattle present throughout the entire year, and cattle present for six months of the year (June through December). Respective to these management conditions we conducted our survey at Estação Ecológica Nhumirim (NHU), Campo Dora Ranch (CDR), and Baú Ranch (BAU). NHU is a 600 ha preserve located within the Nhumirim ranch (18°59'S 56°39'W), a field station of the Centro de Pesquisa Agropecuária do Pantanal (CPAP-EMBRAPA). CDR is a neighboring ranch 45,000 ha, and our survey was conducted in two pastures comprising about 15,000 ha. Both NHU and CDR are seasonally flooded by local rainfall and the Taquari River. BAU is located south of NHU and CDR (19° 19'S 57° 02' W), and it is seasonally flooded by the Miranda River.
The point-centered-quarter method (PCQ) (Bonham 1989) was used to determine the density of young individuals around adult manduvis. In each study area, ten adult manduvis were located and used as centerpoints. The height, basal diameter (taken at a height of 10 cm), and distance to the centerpoint were recorded for the nearest young manduvi in each quadrant. Young manduvis were searched for within a maximum distance of 80 m from the centerpoint. In quadrants where no young manduvis were found, an 80 m value was used to avoid zero distance values when calculating densities. The signed rank test was applied to our data to compare the average densities of the three study areas.
The average manduvi densities were 1.3139 ± 1.2489 (NHU), 0.0009 ± 0.0003 (CDR), and 0.0026 ± 0.0016 (BAU) individuals m2. Young manduvi densities of BAU and CDR did not differ significantly (P = 0.959, Z = -0.052, d.f. =39), but the density of NHU differed significantly from the other two areas (P < 0.001, Z = -3.730 and -3.544, d.f. =39). Young manduvis at NHU tended to be of lesser height and closer to the centerpoint than at both CDR and BAU (Figure 1). The majority (91.3%) of the young individuals at NHU had a basal diameter less than 10 cm, whereas in both CDR and BAU, over 60% of individuals exhibited basal diameters over 10 cm. Within NHU, approximately 78% of young manduvis were less than 3 m in height, whereas at CDR and BAU, 66% and 75% of the individuals, respectively, were taller than 3 m (Figure 2).
The results suggest that outside NHU there is an establishment gap for manduvi recruits (Figure 2). Within NHU the height distribution shows evidence of higher survivorship of seedlings than in CDR and BAU, where individuals from one to five m in height are relatively rare. It is likely that within NHU, after five years of cattle exclusion, seedling establishment has improved due to a lack of trampling, grazing and burning. Improvement of microhabitat conditions could also be promoting higher seedling survivorship. These results contain two sources of bias: first, the PCQ method does not produce an accurate sample of population structure because it is limited to the nearest neighbor; secondly, by limiting our search for young manduvis at 80 m from the centerpoint, the density estimates are higher than actual. However, because the method used in this study was equally applied among the study areas, the results are comparable. A more detailed survey using random plots would be necessary to assess the actual density and population structure of manduvis under different management conditions.
The most common cause for the decline or extinction of American birds is habitat change (Caughley and Gunn 1996). Hyacinthine macaw populations have historically been affected by an illegal pet trade, but habitat loss has also greatly contributed to this species' decline (Thomsen and Brautigam 1991). Cattle have been present in the Pantanal for more than two centuries, and the cumulative long-term effects of grazing and burning could be disrupting the habitat dynamics of the floodplain. Because the hyacinthine macaw depends heavily on old manduvis to reproduce, this species' conservation is in jeopardy. In that nesting sites are located mostly in old manduvis or those already in senescence, disruptions in the population dynamics of this tree could result in a steady decrease of hyacinthine macaw recruitment throughout the Pantanal.
ACKNOWLEDGMENTS
We thank the owners of the Campo Dora, Cáceres, and Baú ranches as well as CPAP-EMBRAPA for their logistic support to the project and research opportunity. The Pousada Arara Azul, California Community Foundation, CECITEC, Toyota do Brazil, and Hyacinthine Macaw Survival Fund have all been important to Project Arara Azul; we also thank Marcelo de Negri Xavier for his help in the field.
The research work of the Projeto Arara Azul/UNIDERP
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