Impactos da urbanização sobre as assembleias de peixes de igarapés de Manaus, Amazonas, Brasil: processos ecológicos e perspectivas de conservação

Abstract

The city of Manaus has been growing in an accelerated and disorderly way, which causes the fragmentation of the forest and condemns the urban streams to disappearance. Along with these streams, an important part of the local aquatic biodiversity is lost, even before it is properly known. In this context, fish assemblages from streams of 52 urban forest fragments were studied, in addition to laboratory experiments, aiming to evaluate the effects of forest fragmentation and of alteration in structural and limnological characteristics of streams (pollution) on the composition and diversity of fish assemblages. Sixty-eight fish species from seven orders and 14 families were collected. Richness ranged from one to 14 species per 50-m stream stretch. Streams subjected to anthropic impacts showed changes both in structural and limnological parameters, as well as in the composition and structure of fish assemblages. Streams in a good state of conservation showed higher species richness, associated with high values of dissolved oxygen and low values of electrical conductivity and pH, and the opposite situation was recorded in polluted streams. The species richness per stream (1-14 species) was small in relation to the total richness found (68 species), indicating a high regional diversity. The similarity in fish species composition varied with the degree of environmental integrity of the streams. Well-preserved streams presented richer and more diverse assemblages, while heavily degraded streams presented assemblages with few species and more similar to each other. Differences in the original environmental characteristics of the streams, as well as the subsequent isolation of fish populations by chemical barriers (highly polluted stream tretches among forest fragments) may be responsible for the current differences in species composition in nearby streams. Laboratory experiments showed that water quality affects agonistic interactions between three cichlid species (Aequidens pallidus; native; Cichlasoma amazonarum, non-native and allochthonous; and Oreochromis niloticus, non-native and exotic). In general, agonistic interactions decreased between species in polluted water when compared to interactions in clean water, and this reduction was especially intense in the native species A. pallidus. In polluted waters there were weak interspecific interactions between species when analyzed in mixed groups of two species at a time, but there were strong intraspecific interactions in the three species, both in monospecific and mixed groups. These results indicate that water quality modulates the agonistic interactions of the studies cichlid species and may play an important role in the process of replacing native species with non-native ones in the urban streams of Manaus. The set of results obtained in this thesis indicate that the conservation of forest remnants in the urban area of Manaus is essential for the maintenance of the local diversity of stream fishes, and that the loss of any fragment can result in the local extirpation of several fish species, with loss of regional fish diversity. The continuous population growth brings environmental degradation, as well as social and economic disorder for the city of Manaus, because the process of recovery of these areas is much more onerous and takes more time than their conservation. Thus, it is necessary to implement projects for the recovery of urban streams, as well as the implementation of an efficient system for the collection and treatment of the domestic sewage, so avoinding its direct discharge into the streams. Projects to recompose native riparian vegetation would have highly beneficial effects on the environmental recovery of urban streams, on the conservation of local and regional aquatic biodiversity, and on the formation of ecological corridors for fauna and flora, which could feed back the ecological processes of environmental recovery in a regional spatial scale.