Aquatic Biodiversity in the Amazon: Habitat Specialization and Geographic Isolation Promote Species Richness

Abstract

The Neotropical freshwater ichthyofauna has among the highest species richness and density of any vertebrate fauna on Earth, with more than 5,600 species compressed into less than 12% of the world's land surface area, and less than 0.002% of the world's total liquid water supply. How have so many species come to co-exist in such a small amount of total habitat space? Here we report results of an aquatic faunal survey of the Fitzcarrald region in southeastern Peru, an area of low-elevation upland (200-500 m above sea level) rainforest in the Western Amazon, that straddles the headwaters of four large Amazonian tributaries; the Juruá (Yurúa), Ucayali, Purús, and Madre de Dios rivers. All measures of fish species diversity in this region are high; there is high alpha diversity with many species coexisting in the same locality, high beta diversity with high turnover between habitats, and high gamma diversity with high turnover between adjacent tributary basins. Current data show little species endemism, and no known examples of sympatric sister species, within the Fitzcarrald region, suggesting a lack of localized or recent adaptive divergences. These results support the hypothesis that the fish species of the Fitzcarrald region are relatively ancient, predating the Late Miocene-Pliocene (c. 4 Ma) uplift that isolated its several headwater basins. The results also suggest that habitat specialization (phylogenetic niche conservatism) and geographic isolation (dispersal limitation) have contributed to the maintenance of high species richness in this region of the Amazon Basin.

Keywords: freshwater fishes; geodispersal; species diversity; species richness; stream capture; tropical rainforest; vicariance.

Figure 1

Topography of the Fitzcarrald Arch in southwestern Amazonia. (A) Elevational map of tropical South America from Shuttle Radar Topography Mission (SRTM) data in a Digital Elevation Model (DEM). Colors represent 25 m elevation intervals; blue-yellow transition at 200 m. (B) Close up of box in panel A. Purple circles indicate expedition locations (with year) between 212–310 m within the Fitzcarrald Arch. White circle indicates a forthcoming expedition. Orange circles indicate locations of comparable aquatic faunal inventories downstream conducted by the authors with colleagues.

Figure 2

Accumulation curves of fish species collected in three expeditions to the interior drainages of the Fitzcarrald Arch, Peru. Each expedition involved about 20 field days during the period of low water (July) in the Alto Yurúa (2008), Alto Ucayali (2009) and Alto Purús (2010) basins. Note the accumulation curves do not approach asymptotic values.

Figure 3

(A) Total species richness recorded in each basin. (B) Total species richness recorded in each habitat type.

Figure 4

Venn diagrams summarizing shared fish species compositions of subregions (river basins) and major habitat types. (A) Among basins, habitats pooled. (B) Among habitats, basins pooled. (C) Among basins by habitat. Data presented as percentages of total Fitzcarrald species pool (208 species) to facilitate comparisons.

Figure 5

Examples of species collected only in the Alto Yuruá. (A) Roeboides affinis (70 mm). (B) Corydoras stenocephalus (54 mm). (C) Callichthys callichthys (59 mm). (D) Pseudostegophilus nemurus (64 mm). (E) Schizodon fasciatus (86 mm).

Figure 6

Examples of species collected only in the Alto Ucayali. (A) Acestrocephalus boehlkei (79 mm). (B) Geryichthys sterbai (26.7 mm). (C) Attonitus ephimeros (48 mm). (D) Chaetostoma lineopunctatum (52 mm). (E) Pimelodus pictus (78 mm).

Figure 7

Examples of species collected only in the Alto Purús. (A) Cichlasoma boliviense (79 mm). (B) Dianema longibarbis (73 mm). (C) Pimelodus sp. 2 (112 mm). (D) Platysilurus mucosus (87 mm). (E) Brachyhypopomus cf. beebei (56 mm).

Figure 8

Examples of species collected in all three basins. (A) Pseudotylosurus angusticeps (73 mm). (B) Clupeacharax anchoveoides (67 mm). (C) Knodus orteguasae (46 mm). (D) Gymnotus carapo (280 mm). (E) Prochilodus nigricans (190 mm).

Figure 9

Geographic distributions of 208 Fitzcarrald fish species (FFS) among freshwater ecoregions (ER) of tropical South America. Black line indicates Amazon Basin watershed. (A) Total number of FFS per ecoregion. (B) Percent of FFS pool in that ecoregion. Data from Table 1. Note high numbers and proportions of FFS in the adjacent Amazonas Lowlands (ER 316; 88%), Ucayali-Urubamba (ER 317; 49%) and Mamoré-Madre de Dios (ER 318; 20%) Ecoregions.

Figure 10

Species-area relationships for Fitzcarrald fish species (FFS) among the freshwater ecoregions of tropical South America. Species richness and area data from [5]. (A) Absolute numbers of FFS per ecoregion, showing high shared species composition with faunas in the adjacent Amazonas Lowlands, Ucayali-Urubamba, and Madre de Dios ecoregions. (B) Proportional representation of FSS per ecoregion, showing relatively high values in some nearby ecoregions, and low values in most other ecoregions.

Figure 11

Examples of the three major aquatic habitats sampled in the Fitzcarrald region. (A) Channel and flooded beaches of the Rio Purús at San Marcos (9°53′S 70°52′W). (B) A stream emptying into the Mishaua river in the Urubamba basin (11°13′S 72°58′W). (C) A floodplain oxbow lake (Cocha Supiri) in the Purús basin (9°58′S 70°55′W).