Amazonian forests are extraordinarily diverse, but the estimated species richness is very much debated. Here, we apply an ensemble of parametric estimators and a novel technique that includes conspecific spatial aggregation to an extended database of forest plots with up-to-date taxonomy. We show that the species abundance distribution of Amazonia is best approximated by a logseries with aggregated individuals, where aggregation increases with rarity. By averaging several methods to estimate total richness, we confirm that over 15,000 tree species are expected to occur in Amazonia. We also show that using ten times the number of plots would result in an increase to just ~50% of those 15,000 estimated species. To get a more complete sample of all tree species, rigorous field campaigns may be needed but the number of trees in Amazonia will remain an estimate for years to come.

With its long history of colonization, eastern Amazonia has the highest population density in Brazilian Amazonia and represents the typical pattern of recent human occupation. Between 1991 and1995, we surveyed the mammalian fauna at five sites, representing different degrees of human disturbance. We used line transects (1511 km surveyed) to describe differences in diversity and abundance at each site and to evaluate the effects of environmental factors. Twenty-two of the expected 44 species were recorded during surveys, but no more than 18 were recorded at any one site, and only 3 species were observed at all five sites. Despite a minimum transect length of 202 km, most species were recorded relatively infrequently at all sites, although overall sighting rates at different sites varied by more than 100%. Between-site differences were even more pronounced when we compared specific groups (e.g., arboreal, terrestrial, game, nongame), reflecting the differential effects of factors such as hunting, logging, and forest clearing. In general terms, species diversity, abundance, total biomass, and mean biomass all tended to decrease with increasing human disturbance. Two more specific patterns were also distinguished: decreasing abundance and biomass of game species with increasing hunting pressure and increasing abundance of nongame species with increasing forest disturbance. Intense hunting pressure alone may have deleterious short-term effects on abundance, but not necessarily on diversity, whereas prolonged hunting pressure, combined with forest clearing, results in marked distortions in the mammalian community. Overall, the study emphasizes the relative paucity of the mammalian fauna of eastern Amazonia, in terms of both species diversity and abundance, and its vulnerability to the ongoing process of human colonization in the region. Efectos de la Colonización Humana en la Abundancia y Diversidad de Mamíferos en la Zona Oriental del Amazonas BrasileñoResumen: Con su larga historia de colonización, la zona oriental del Amazonas tiene la población más densa de la región del Amazonas brasileño y representa el patrón típico de ocupación humana reciente. Entre 1991 y 1995 muestreamos la fauna de mamíferos en cinco sitios representativos de diferentes grados de perturbación humana. Utilizamos transectos en línea (1511 km muestreados) para describir las diferencias en diversidad y abundancia en cada sitio y para evaluar los efectos de los factores ambientales. Veintidós de las 44 especies esperadas fueron observadas durante los muestreos, pero no más de 18 fueron observadas en un solo sitio, y solo tres especies fueron observadas en los cinco sitios. No obstante la longitud mínima de transecto de 202 km, la mayoría de las especies fueron relativamente poco frecuentes en todos los sitios, pese a que las tasas de avistamiento en diferentes sitios varió en más de un 100%. Las diferencias entre sitios fueron aún mas pronunciadas cuando se compararon grupos de especies (por ejemplo, arbóreas, terrestres, cinegéticas, no cinegé...

Tropical forests are known for their high diversity. Yet, forest patches do occur in the tropics where a single tree species is dominant. Such “monodominant” forests are known from all of the main tropical regions. For Amazonia, we sampled the occurrence of monodominance in a massive, basin-wide database of forest-inventory plots from the Amazon Tree Diversity Network (ATDN). Utilizing a simple defining metric of at least half of the trees ≥ 10 cm diameter belonging to one species, we found only a few occurrences of monodominance in Amazonia, and the phenomenon was not significantly linked to previously hypothesized life history traits such wood density, seed mass, ectomycorrhizal associations, or Rhizobium nodulation. In our analysis, coppicing (the formation of sprouts at the base of the tree or on roots) was the only trait significantly linked to monodominance. While at specific locales coppicing or ectomycorrhizal associations may confer a considerable advantage to a tree species and lead to its monodominance, very few species have these traits. Mining of the ATDN dataset suggests that monodominance is quite rare in Amazonia, and may be linked primarily to edaphic factors.

With its long history of colonization, eastern Amazonia has the highest population density in Brazilian Amazonia and represents the typical pattern of recent human occupation. Between 1991 and1995, we surveyed the mammalian fauna at five sites, representing different degrees of human disturbance. We used line transects (1511 km surveyed) to describe differences in diversity and abundance at each site and to evaluate the effects of environmental factors. Twenty-two of the expected 44 species were recorded during surveys, but no more than 18 were recorded at any one site, and only 3 species were observed at all five sites. Despite a minimum transect length of 202 km, most species were recorded relatively infrequently at all sites, although overall sighting rates at different sites varied by more than 100%. Between-site differences were even more pronounced when we compared specific groups (e.g., arboreal, terrestrial, game, nongame), reflecting the differential effects of factors such as hunting, logging, and forest clearing. In general terms, species diversity, abundance, total biomass, and mean biomass all tended to decrease with increasing human disturbance. Two more specific patterns were also distinguished: decreasing abundance and biomass of game species with increasing hunting pressure and increasing abundance of nongame species with increasing forest disturbance. Intense hunting pressure alone may have deleterious short-term effects on abundance, but not necessarily on diversity, whereas prolonged hunting pressure, combined with forest clearing, results in marked distortions in the mammalian community. Overall, the study emphasizes the relative paucity of the mammalian fauna of eastern Amazonia, in terms of both species diversity and abundance, and its vulnerability to the ongoing process of human colonization in the region. Efectos de la Colonización Humana en la Abundancia y Diversidad de Mamíferos en la Zona Oriental del Amazonas BrasileñoResumen: Con su larga historia de colonización, la zona oriental del Amazonas tiene la población más densa de la región del Amazonas brasileño y representa el patrón típico de ocupación humana reciente. Entre 1991 y 1995 muestreamos la fauna de mamíferos en cinco sitios representativos de diferentes grados de perturbación humana. Utilizamos transectos en línea (1511 km muestreados) para describir las diferencias en diversidad y abundancia en cada sitio y para evaluar los efectos de los factores ambientales. Veintidós de las 44 especies esperadas fueron observadas durante los muestreos, pero no más de 18 fueron observadas en un solo sitio, y solo tres especies fueron observadas en los cinco sitios. No obstante la longitud mínima de transecto de 202 km, la mayoría de las especies fueron relativamente poco frecuentes en todos los sitios, pese a que las tasas de avistamiento en diferentes sitios varió en más de un 100%. Las diferencias entre sitios fueron aún mas pronunciadas cuando se compararon grupos de especies (por ejemplo, arbóreas, terrestres, cinegéticas, no cinegé...