Revista Brasileira de Ornitologia - Universidade Federal do Acre

Birds of the Brazilian state of Acre: diversity, zoogeography, and conservationEdson Guilherme407of local species is best understood in an east-west, ratherthan a north-south dimension, with the Purus Riverforming the principal division within the state. The Purusdivides Acre into two main portions, an eastern portionon its right margin, and western portion to its left margin(Figure 2).Of the resident forest taxa in Acre, almost 80% arewidely distributed within the state, being found in bothits eastern and western portions. Among other things, thisreflects the mostly low-lying terrain (except in the westernextreme) and relatively homogeneous habitats foundthroughout most of the state. This low species turnoverwould be expected in the absence of major physicalbarriers, such as rivers or mountains, capable of limitingthe dispersal of most taxa. However, some taxa onlyoccur in one of the state’s two sub-regions, although thisdistribution pattern appears not related to the presenceof the two main rivers that cross the state, the Purus andthe Juruá. This may be primarily due to the fact that thesehighly meandering rivers arise in lowland Amazonia, andso, taxa can cross them often more easily than they wouldrivers whose headwaters are in unsuitable habitat (such asdry country or mountains). Genetic studies of primates(Peres et al. 1996), rodents (Patton et al. 1994; Patton andSilva 1998), birds (Aleixo 2004, Fernandes et al. 2012),and amphibians (Gascon et al. 1998) have all confirmedthat the Juruá is an ineffective zoogeographic barrier forthese groups.Other factors may nevertheless influence thedistribution of different taxa in southwestern Amazonia,including interspecific competition, edaphic conditionsand habitat characteristics (Tuomisto et al. 1995, 2003;Daly & Silveira, 2002; Roig & Martini, 2002; Silveiraet al. 2002). A closer examination of the geographicdistribution of birds within one of the sub-regions(Appendix 1) reveals that the pattern observed in Acre isrepeated on the Peruvian side of the border. Species suchas Phlegopsis erythroptera, Thamnomanes saturninus, Attilacitriniventris, Pipra filicauda, and Dixiphia pipra havealso been recorded only in northeast Peru (Schulenberget al. 2007). Similarly, Poecilotriccus albifacies, Cnipodectessuperrufus, and Micrococcyx cinereus, which were onlyfound in eastern Acre (Appendix 1), were also found onlyin adjacent southeastern Peru (Schulenberg et al. 2007).A similar pattern has also been recorded for palms(Arecaceae) in Acre. Some species, such as Wettiniaaugusta, Syagrus smithii, Socratea salazarii, Iriartellastenocarpa, Hyospathe elegans, Dyctyocaryum ptarianum,Bactris riparia, and Astrocaryum faranae, are foundonly in the western sub-region of Acre (Lorenzi et al.2004). Other species, such as Astrocaryum aculeatum,Astrocaryum murumuru, and Bactris elegans are restrictedto the eastern part of the state (Lorenzi et al. 2004). Eventhough practically all these species of palm also occurin areas adjacent to Acre, their east-west distributionwithin the state is similar to that observed in some birdspecies (Appendix 1). Some of these palm species arealso restricted to specific environments, such as plateaus,swamps, campinas, and campinaranas. This distributionpattern in Acre may be a reflection of the sedimentaryhistory of the Marañon–Ucayali–Acre basin, to the west,and the Madre de Dios basin, to the east. In the westernpart of the state, relatively ancient formations (e.g.,Fm Moa, Rio Azul, Divisor and Cruzeiro do Sul) wereexposed by the uplift of the Andes, and contrast withthe more recent Cenozoic formations in the east partsof the state, e.g., Fm Solimões and Fm Madre de Dios(Milani & Thomaz Filho, 2000, Campbell et al., 2006).In western Acre, the topography and variations in thecomposition of the soil have played an important role inthe formation of distinct ecosystems, which may certainlyhelp to explain the higher number of species of palms andbirds (and probably others groups of organisms) restrictedto western Acre (Lorenzi et al. 2004, Guilherme, 2009).For most of the species restricted to one of the subregions,Acre and the adjacent lowlands of Peru representthe southwestern extreme of their distribution in theAmazon basin. Species such as Thamnomanes saturninus,Pipra filicauda (Figure 6), and Dixiphia pipra are widelydistributed in northern Amazonia, ranging south as faras the southwestern extreme of the Amazon lowlands(Ridgely & Tudor, 2009). There are exceptions, however,such as Thamnophilus divisorius, which is endemic towestern Acre and northeastern Peru. The distribution ofthis species is associated with an area of moderate altitude,the submontane forests of the Serra do Divisor range.Many of the taxa restricted to the eastern sub-region ofAcre are associated with bamboo forests (e.g., Picumnussubtilis, Cnipodectes superrufus, and Poecilotriccus albifacies)or dense rainforest (e.g., Lepidothrix coronata exquisita).Eastern Acre encompasses the greatest concentration ofthese habitats than the western portion of the state. Thissuggests that habitat-related environmental gradientsmay play a more important role in the distribution ofthe species in eastern Acre than the mere presence of aphysical barrier such as the Purus River. Other taxarestricted to eastern Acre, such as Crypturellus undulatusundulatus, Antrostomus sericocaudatus, and Herpsilochmusrufimaginatus, are more widely distributed in the Amazonbasin, although this may represent the southwesternextreme of their distribution.Allopatric/parapatric rangesWhereas five pairs of purported sister taxa appear tohave an allopatric distribution in Acre (Table 1), it seemslikely that further ornithological surveys may confirmthat these taxa are actually parapatric in the state, and thattheir apparent allopatric distribution is merely a samplingartifact. A common feature of the taxa considered to beRevista Brasileira de Ornitologia, 20(4), 2012