**4. Discussion**

With at least 60 species of snakes and 26 species of lizards documented, the squamate reptile diversity at LABS is the highest local-scale diversity reported for the southwestern Amazon basin. These numbers are comparable to those reported for Tiputini Biodiversity Station (Table 1), which is claimed to hold the global alpha diversity record for reptiles [7]. These numbers are based on broadly-comparable sampling regions; as defined here, the LABS total includes species detected in an area of approximately 10 km<sup>2</sup> in the vicinity of the main research facilities, versus 6.5 km<sup>2</sup> for Tiputini. However, the comparisons o ffered in Table 1 do not address gross discrepancies in survey effort and should nonetheless be treated provisionally. LABS has been the subject of considerable long-term research, and we expect that the site has been more intensively surveyed than Tiputini. The "regional" list for LABS (LABS-X, Table A1) includes 68 snake species and 36 lizard species, and is thus considerably richer than faunas typically reported for the southwestern Amazon basin (Table 1 [11,15]).

Nonetheless, it is clear that the snake fauna of central/northwestern Amazonia is richer than for LABS and other southwestern sites. At least 89 species of snakes are known from the Iquitos region [31], which represents a 29% increase in richness relative to the LABS regional list (Table A1: LABS-X). The regional list for Iquitos encompasses a large spatial scale [31], but it is unlikely that the discrepancies in snake richness between these regions can be explained as an artifact of scale. For example, the Iquitos region includes multiple genera of snakes and lizards that have not been reported from the lowlands of southern Peru (e.g., *Arthrosaura*, *Bothrocophias*, *Iguana*, *Loxopholis*, *Thamnodynastes*), and several other genera are considerably more species-rich in the Iquitos region (e.g., *Amerotyphlops*, *Atractus*, *Helicops*, *Micrurus*, *Erythrolamprus*). Given the high richness of the Iquitos region overall, we predict that lowland sites in the Ecuadorian Amazon (Tiputini, Yasuní; Table 1) are more diverse than reported previously [7]. Southern Peru has been subject to greater sampling e ffort, and the discrepancies between local-scale tabulations for southern and northern sites (Table 1) seem likely to increase with additional sampling in the Rio Napo basin and adjacent regions.


**Table 1.** Species richness for lizards and snakes for selected sites in the western Amazon basin. Localities are shown in Figure 2. LABS, LABS-R, and LABS-X are documented and estimated richness totals for Los Amigos Biological Station, as in Table A1.

1 Higher elevation than other sites (500 m, vs 220–280 for others), presumably accounting for higher precipitation. 2 Bass et al. (2010) list does not include several taxa that are almost certainly present: *Ameiva ameiva, Epictia diaplocia*, and *Xenopholis scalaris*; listed totals include these taxa. 3 Added *Epictia diaplocia* to Iquitos list; vouchered specimens known from region [34].

#### *4.1. A Western Amazonian Richness Gradient: Does It Exist?*

For squamate reptiles, and especially for snakes, our results imply that there is a clear gradient in species richness from north to south in the western Amazon basin. Results parallel those found in a wide variety of taxa, including trees, vascular epiphytes, lianas, and frogs (Table 2). For example, regardless of spatial scale, tropical tree communities in Yasuní contain roughly 40–50% more species than sites at Cocha Cashu or LABS [9]. The causes of this western Amazonian richness gradient remain poorly understood. Pitman et al. [9] noted several structural di fferences between northern (Yasuní) and southern (Cocha Cashu) tree communities, including tree density, average height, leaf size, and seed size. Furthermore, a number of macroecological descriptors di ffer between these communities, with species from northern Amazonian sites having smaller geographic and altitudinal ranges. Interestingly, more mobile taxa—birds and primates in particular—show no evidence for increased diversity in the north: Communities from Cocha Cashu and Los Amigos have e ffectively the same diversity as those from Yasuní, Tiputini, and the Iquitos region (Table 2).

**Table 2.** Species richness for di fferent taxonomic groups in lowland rainforest in the northern equatorial Amazon (Loreto, Peru; Ecuador) and the southwestern Amazon (southern Peru, northwestern Bolivia). Diversity inflation is the proportional increase in species richness at northern sites relative to southern sites.


1 Directly estimated from plot data. 2 Estimated from regional compendia of tree diversity. 3 Estimated from surveys at Parque Nacional Madidi, northern Bolivia (border with Peru)

Pitman et al. [9] reviewed several candidate mechanisms for the western Amazonian richness gradient in trees. They considered it unlikely that higher tree richness in the north could be attributed to greater disturbance, increased biogeographic mixing, and intraspecific density-dependence (e.g., Janzen-Connell e ffects). Nonetheless, they noted that modern climate could play an important role in facilitating higher northern richness, because northern sites are simultaneously wetter and less seasonal than southern sites. Relative to the north, the southern Amazon is characterized by a greatly exacerbated dry season, with little precipitation during the months of June, July, and August. One possibility is that the generally wetter year-round climate in the north facilitates the persistence of water-limited taxa in the understory [9]; this model might also explain why amphibian diversity is substantially higher in the north (Table 2). However, it is unclear how water availability per se contributes to higher squamate richness; lizards in particular have diversity patterns that are e ffectively decoupled from annual precipitation, and exceptionally diverse assemblages can be found in regions with both high (e.g., Amazonia) and low (arid Australia) rainfall.

At present, many putative ecological drivers of the richness gradient for squamate reptiles cannot be assessed, because we lack basic information on population structure and dynamics for nearly all species of Amazonian squamates. For starters, we are not aware of any comparative data on the relative density of squamates at northern and southern sites. In addition, "local" communities (Table 1) might not be comparable in some ways. Some fraction of taxa within a particular local assemblage might be transient or otherwise non-coexisting species, maintained by mass e ffects that occur over larger spatial and temporal scales. Because we have so little information about the processes that generate local assemblages, it is all the more di fficult to understand how those processes might vary in space. Furthermore, the lack of standardization in sampling methodology and scale makes it di fficult to compare species abundance distributions among sites. We sugges<sup>t</sup> that future studies should also consider historical explanations for the richness gradient, in addition to contemporary ecological

processes. The history of landscape-level change in the Amazon basin remains controversial [43,44], but it is nonetheless possible that expansion and contraction of savannah habitats in southwestern Amazonia might have resulted in the loss of some species that have not ye<sup>t</sup> recolonized the region. Some widespread Amazonian taxa (e.g., *Iguana iguana*, *Bothrocophias hyoprora*, *Bothrops taeniautus*) apparently fail to occur in suitable habitat in southern Peru, even as they occur at similar latitudes and similarly seasonal climates in Brazil [29,45].

One site from the south-central Brazilian Amazon, "Samuel", purportedly hosts perhaps the highest snake diversity in the entire Amazon basin [11,46], despite a moderately southern latitude (–8.9) and marked seasonality in rainfall. At first glance, the high richness reported for Samuel would appear to reject local climatic explanations for the north-to-south richness gradient for squamates. However, this site cannot be compared to others listed in Table 1, for several reasons. First, the total richness frequently reported for Samuel (92+ species) is in fact a richness estimate for the Brazilian state of Rondonia [47]. The list includes the results of surveys through two major neotropical biomes (lowland Amazonian rainforest and cerrado) and with a survey area that greatly exceeds 100,000 km2. A total of 68 snake species were reported from the principal survey in the vicinity of Samuel hydroelectric project (near Porto Velho, Brazil), with a survey area of roughly 200 km<sup>2</sup> [47]. Sampling intensity for this site was considerably greater than for nearly any other comparable site in the Amazon, and involved 1507 individual snake captures. Hence, we view the Samuel richness totals as comparable to the regional estimates for the Los Amigos snake fauna (68 species; Tables 1 and 2) and markedly lower than the regional snake fauna for the northwestern and equatorial western Amazon.

## *4.2. Taxonomic Issues*

The taxonomy of many neotropical squamates is in flux: cryptic species appear to be present in many groups [48–51], and truly new species remain to be formally described. We present a species list for LABS with the caveat that nomenclatural boundaries for some taxa are likely to change in the near future. Any future use of the list presented here in a managemen<sup>t</sup> context should exercise caution with respect to names assigned to particular taxa. For this reason, we have provided a list of vouchered specimens from LABS that can be cross-referenced by future studies. Potentially problematic groups include the following snake genera: (1) *Chironius*, including the relationship between *C. carinatus* and *C. exoletus* in southern Peru (listed as *C. exoletus*/*carinatus* in Table A1) as well the potential for cryptic diversity in several other taxa [48,52]; (2) all species in the genus *Erythrolamprus* ( = *Liophis*), which appears to consist of five species at LABS, but where—in light of recent taxonomic work [27]—the connections between the taxa we list and *Erythrolamprus*/*Liophis* taxa reported for other Amazonian sites is unclear; (3) *Atractus*, a megadiverse snake genus that contains at least three species at LABS, but which is in need of comprehensive revision for southern Amazonia; species of this genus are frequently misidentified in both museum and field collections [53,54].
