Bird Community Dynamics along Primary Successional Gradients of an Amazonian Whitewater River

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We used spot-map methods to census birds in two plots (40 ha and 80 ha) representing the early and middle stages of primary succession generated by the meandering of the Manu River in southeastern Peru in 1983 and 1985. We distinguished seven distinct successional stages beginning with the first plant communities growing on open beaches (Tessaria: stage 1) and proceeding through mature floodplain forest (stage 7). Each successional stage was more structurally and floristically complex. One plot was dominated by early successional vegetation (stages 1-5) and the other was domi- nated by middle-to-late stages (5-6). We supplemented spot-map data with mist netting to census nonterritorial species, principally nectarivores and frugivores. Because the ear- liest successional stage (Tessaria) covered only small (<3 ha) areas on any given me- ander tongue, we censused six additional stands to characterize their bird communities. Species richness increased along the successional gradient, but not uniformly. Struc- turally simple Tessaria stands (stage 1) contained more diverse communities of breeding birds than the next two successional stages, which appeared to have few resources avail- able to birds. After stage 3, breeding species richness increased by 31-71 species per successional stage as a distinct canopy layer formed (stage 4), fruiting trees became more available (stages 5-6) and the vertical structure of the forest became fully developed (stage 7: 71 more species than stage 6). Early successional bird communities (stages 1- 3) were dominated by a few very abundant insectivores and omnivores, but wanderers from the adjacent forest plus Nearctic and austral migrants seasonally outnumbered breeding residents. Many species that breed in Tessaria stands were restricted to just one or two of the seven stands censused. The middle stages of succession had the highest estimated richness, abundance, and biomasses of nectarivores and frugivores. Nectarivore abundance reflected the huge stands of Heliconia that dominate the understory of mid- successional stages. Bark foragers reached their highest abundance in late successional stages as the dominant canopy tree species of middle successional stages (e.g., Cedrela odorata) began to die off, but remained standing. Middle successional stages lacked many of the insectivores characteristic of more structurally complex mature forest. Among the missing species were understory Mockers, ant-followers, ground foragers, and many arboreal insectivores that participate in canopy flocks. Species of early and middle stages tended to be more abundant on average than those of mature forest. Early-/and late-successional bird communities had few species in common, and many congeners segregated along this successional gradient. Most species characteristic of early successional vegetation, however, were also found in many other kinds of natural and anthropogenic disturbances. The Tessaria beaches of the Manu lacked most species endemic to the same successional stage on the larger rivers in the Amazon system. Mid-successional stages had relatively fewer species restricted to them, but some of these species are rare and endemic to southwestern Amazonia. Many char- acteristic species of floodplains reached their peak abundances in stages 4-6. Mid-suc- cessional stages, therefore, may play a greater role in contributing to the unusually high species richness of western Amazonia than had previously been recognized. These results point to the importance of preserving natural, undammed meandering rivers in conserving the avian diversity of Amazonia. Logging and agricultural devel- opment of mid-successional floodplain forests probably has a negative impact on regional bird communities. Not all species of the early and middle stages of succession have adapted to human-generated secondary succession. Recovery of the full diversity and richness of tropical bird communities following disturbance may take several hundred