Despite the apparent homogeneity of the Amazon forest, this region presents a wide variety of ecosystems, some with a high level of biological specificity. This is the case of white-sand forests or varillales, as they are known regionally in Colombia, which are fragile ecosystems, unique and relatively small in extent, with vegetation established on very old substrates, sandy and poor in nutrients (Anderson 1981; Capurucho et al. 2020). These characteristics, coupled with the sparse distribution of white-sand forests in patches across the Amazon, make it a habitat with high endemicity that significantly contribute to spatial heterogeneity and the Amazonian biodiversity (Adeney et al., 2016). White-sand forests, with their unique and isolated ecosystems, are critical habitats for many bird species, some of which are newly discovered (e.g., Whitney and Alonso 2005, Stiles and Avendaño 2019). Studies from Peru (e.g., Álvarez et al., 2013), Brazil (e.g., Guilherme 2012; Laranjeiras et al., 2014; Borges et al. 2016a, b), Venezuela (e.g., Kvarnbäck and Bosque 2017), and Colombia (e.g., Stiles et al., 1995; Stiles and Avendaño 2019) highlight the ecological significance of these forests, stressing the need for further research particularly across the western Amazon, where important knowledge gaps still remain on white-sand and other poor-soil specialist birds (Socolar et al. 2022). The study of white-sand forests can significantly increase the diversity in the Amazon (Borges et al., 2016b), and contribute to understanding biogeographical patterns this bioregion. This research evaluated the richness and composition of birds in a white-sand forest in the southernmost part of the Colombian Amazon.

The white-sand forest under study is located approximately 24 km northeast of Leticia municipality, Amazonas Department (4° 0’34.62”S, 69°53’35.95”W; Figure 1). The forest has approximately 70 ha, ~110 m asl, average annual precipitation of 3,400 mm (data from Vázquez Cobo Airport weather station), immersed in a matrix of terra-firme forest. It has an open canopy, with thin trees (5 - 8 m), a slightly dense understory, with dominance of bromeliads, mosses and ferns (Figure 2). Between February and March 2015, two seven-day visits (i.e. 14-day sampling) were conducted in which the white-sand forest bird community was characterized using complementary techniques of mist netting, transect censuses, recordings, and ad libitum observations. Ten 12 m x 2.5 m mist nets were used to catch birds. Two 150 m transects separated at least 150 m from the forest edges and between them were established. Bird sounds were recorded with a digital recorder. Same methods were applied to characterize the bird fauna in the surrounding terra-firme forest. Sampling locations in this ecosystem were distributed sparsely around the white-sand forest, with sites established at least 200 m from the forest edge. We categorized the species recorded exclusively in the white-sand forest by the degree of association with this ecosystem, according to the classification offered by Borges et al. (2016a). Birds were identified using the illustrated guides of Hilty and Brown (1986) and Schulenberg et al. (1997), and the taxonomic sequence suggested by Remsen et al. (2024) was followed. The number of recorded species was used as a richness index. The degree of similarity between birds of the white-sand forest and the terra-firme forest was assessed using the Jaccard index. An individual-based rarefaction curve using capture data was constructed to assess the effectiveness of sampling, and the Chao 1 and Jacknife estimators were calculated to estimate the expected species richness. These analyses were conducted using EstimateS (Colwell 2006). A total of 38 bird species were recorded in the white-sand forest and 81 in the terra-firme forest. Best represented families were Psittacidae (parrots), Thamnophilidae (arboreal antbirds), Trochilidae (hummingbirds), Tyrannidae (flycatchers), and Thraupidae (tanagers). Of the 19 species captured in nets (98 individuals), the most commonly recorded were the Great-billed hermit Phaethornis malaris (23 individuals), and the Saffron-crested tyrant-manakin Neopelma chrysocephalum (10). Most of the species caught (63%) were represented by a maximum of three individuals. The individual-based rarefaction curve showed that species richness in the white-sand forest had not yet reached stabilization (Figure 3). The non-parametric estimator Chao 1 revealed that the estimated species richness was 48 species (95% Confidence Interval = 41.1 - 55.8), while the Jacknife estimator suggested a richness of 61 species (95% Confidence Interval = 59.2 - 62.7).

Figure 1
White-sand forest, Colombian Amazon. The dark line marks the boundary of the white-sand forest, which is embedded within the surrounding terra-firme forest. Based on Google Earth.

Figure 2
White-sand forest appearance and bird species exclusive of this environment in the Colombian Amazon. a) typical appearance with a high density of narrow trees and presence of bromeliads in the understory, b) close-up of the soil and understory dominated by ferns, c) Attila citriniventris, d) Neopelma chrysocephalum.

Figure 3
Individual-based rarefaction curve using capture data for birds of a white-sand forest, Colombian Amazon. Dotted line: 95% confidence intervals.

Species composition differed between the two ecosystems. Of the 95 species in total registered in the two ecosystems (16 orders and 34 families), 14 species were exclusively registered in the white-sand forest (Figure 2c,d), 24 were registered both in the white-sand forest and terra-firme forest, and 57 were recorded only in the terra-firme forest (Jaccard index = 0.25; Table 1). The most represented guild in the white-sand forest was insectivorous, with 7 species, while in the terra-firme forest, frugivores dominated with 23 species. Based on Borges et al. (2016a), of the 14 species recorded exclusively in the white-sand forest, Attila citriniventris and Neopelma chrysocephalum were classified as near-restricted species, and the rest as regular users of this ecosystem.

The white-sand forest showed a low number of species, contrasting with the surrounding terra-firme forest that harbored more than twice bird species. White-sand forests in other localities of Amazonia have shown a larger species richness than that recorded in this study (Stiles et al., 1995: 111 species; Borges 2004: 128 species, Guilherme et al. 2018: 171), with some exceptions (Vásquez-Arévalo et al. 2021: 45-57 species), likely reflecting the more intensive sampling efforts in those studies. However, despite the low bird richness, we found that 37% (14 out of 38 species) of these were exclusively recorded in this ecosystem. Nevertheless, the birds of the white-sand forest showed different levels of specificity. N. chrysocephalum, for example, is a species restricted to white-sand forests (i.e., obligated specialist) in countries such as Colombia (Hilty and Brown 1986) and Brazil (Ridgely and Tudor 1994). In turn, Attila citriniventris was registered in this research only in the white-sand forest, but it is a species of terra-firme forests in Ecuador and Brazil (Ridgely and Greenfield 2001; but see Guilherme and Borges 2011 who associated it with white-sand vegetation in southwestern Brazilian Amazon), and common in both terra-firme forest and white-sand forest in the Eastern region of the Colombian Amazon (Hilty and Brown 1986). Other species such as Claravis pretiosa and Ramphotrigon ruficauda were also found exclusively in the white-sand forest, but are considered as facultative species of this ecosystem. Finally, our study did not detect other diagnostic species of white-sand forests (e.g. Xenopipo atronitens). This may be due to a limited sampling effort, especially for rare and elusive species, as well as the location being at the southernmost edge of the distribution for several white-sand specialists in Colombia, distant from the Guiana Shield. The bird species found in white-sand forests are generally linked to the ecosystem’s structure and diversity, habitat amount in the landscape and biogeographical context (Borges et al. 2016; Capurucho et al. 2020), evolutionary history (Matos et al. 2016), and edaphic and climatic conditions (Fine et al. 2012). Therefore, it is important to examine the relationship of white-sand forest birds with the different environmental attributes, as well as to study ecomorphological and evolutionary aspects of these birds (Lima et al 2023).

This white-sand forest is in a relatively good state of conservation, possibly related to its distance to populated areas. Additionally, poor soils and low biomass of the tree component limit its use for agriculture, livestock or selective logging. However, the small size and isolation of this ecosystem and its biological specificity make it susceptible to disturbances. Consequently, a legal protection status as a strategic ecosystem is needed, as well as the recognition of other white-sand forests in the area that can be studied allowing for a better understanding of the relevance of this ecosystem for Amazon biodiversity.

ACKNOWLEDMENTS

We want to express our gratitude to the Universidad Nacional de Colombia, the Pontificia Universidad Javeriana, the Florida International University, and the Universidad de la Amazonia; to Alejandro Suárez, Santiago Villa and Arcesio and Angel Pijachi for their support in the field, and to the grant received from the Iniciativa Socios para la Conservación de la Amazonia Andina.

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