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Article

Environmental Drivers and Edge Effects on Anuran Diversity in Fragmented Forests of the Southwestern Brazilian Amazon

by
Yara Araújo Pereira
1,*,
Vinicius Guerra
2,3,
Letícia Lima Correia
1,
Thiago Bernardi Vieira
1 and
Moisés Barbosa de Souza
2
1
Programa de Pós-Graduação em Ecologia (UFPA/PPGECO), Universidade Federal do Pará, Belém 66075-110, PA, Brazil
2
Laboratório de Herpetologia, Programa de Pós-Graduação em Ecologia e Manejo de Recursos Naturais (UFAC/PPG-EMRN), Universidade Federal do Acre, Rio Branco 69920-900, AC, Brazil
3
Departamento de Saúde, Centro Universitário Una, Itumbiara 75532-040, GO, Brazil
*
Author to whom correspondence should be addressed.
Diversity 2025, 17(11), 764; https://doi.org/10.3390/d17110764 (registering DOI)
Submission received: 22 September 2025 / Revised: 17 October 2025 / Accepted: 20 October 2025 / Published: 1 November 2025
(This article belongs to the Special Issue Amphibian and Reptile Adaptation: Biodiversity and Monitoring)
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Abstract

Background: We investigate the influence of environmental variables and edge-interior gradients on the diversity and composition of anuran assemblages in four forest fragments in the southwestern Brazilian Amazon. Methods: A total of 590 individuals from 40 species and eight families were recorded, with Leptodactylidae being the most abundant family. Results: The Humaitá Forest Reserve (RFH) exhibited the highest species richness and diversity, while the Raimundo Irineu Serra Environmental Protection Area (APA) had the lowest. Species composition varied significantly among fragments and along the edge-interior gradient, with edges showing higher species richness. Redundancy analysis (RDA) revealed that temperature, humidity, and litter depth were the most important environmental variables structuring anuran communities. Conclusions: Edge habitats supported disturbance-tolerant species, whereas forest interiors harbored moisture-dependent specialists. These findings underscore the importance of conserving larger, less disturbed fragments and implementing management strategies that account for environmental heterogeneity. This study provides critical insights into the factors shaping anuran distribution in fragmented Amazonian landscapes, offering valuable guidance for biodiversity conservation in the region.

1. Introduction

Litter-dwelling anurans play key ecological roles as both predators of invertebrates [1,2] and prey for various vertebrates [3]. These species exhibit either aquatic or terrestrial reproductive modes [4]. Aquatic species depend on the availability and distribution of water bodies [5,6], while terrestrial species are influenced by microclimatic and environmental factors [7,8,9]. In the central Amazon, their distribution may be determined by topographic and edaphic factors, such as soil texture, pH, slope, tree density, and litter volume [10], as well as primary productivity [11].
The Amazon harbors one of the world’s richest assemblages of anuran amphibians, whose ecological roles are well-documented as vital to biotic community dynamics [1,3]. Comparative herpetofaunal studies reveal that anuran species richness is significantly higher in the southwestern Amazon than in the central biome [12,13,14]. This regional disparity likely stems from the southwestern Amazon’s elevated precipitation rates and greater thermal stability [15], environmental conditions strongly associated with heightened amphibian diversity [15].
Despite the known richness of amphibian communities in the western Amazon, information remains fragmented, and few studies have assessed the influence of spatiotemporal and environmental variations on community structure [10,16,17,18]. Additionally, the region has undergone intense transformations due to deforestation and fragmentation of natural habitats, particularly in the “Arc of Deforestation” zone [15,19]. Anthropogenic impacts in the Amazon may compromise species survival [20] and individual dispersal [21], leading to species richness loss [22,23] and altered anuran community composition [24]. Forest fragmentation modifies habitat structure, creating edges that affect microclimates, resource availability, and ecological interactions, directly impacting species distribution and abundance [25,26].
Environmental and biotic variables play a fundamental role in structuring Neotropical amphibian assemblages, directly influencing species abundance, richness, and composition [8,10,14]. Among biotic factors, both predator–prey dynamics and phylogenetic relationships among species can significantly affect community structure, potentially limiting species persistence and spatial co-occurrence patterns [27,28,29,30]. Abiotic factors—including temperature, humidity, and precipitation—directly affect species abundance and richness [8,9,31], as do habitat features like litter depth, vegetation structure, and surrounding matrix type [10,31].
Edge effects, occurring at transitions between distinct habitats, are widely recognized as crucial drivers of biological community structure for both animal and plant groups [25,26,32]. For amphibians, edge effects can alter species composition, with some taxa benefiting from edge conditions while others remain sensitive and restricted to fragment interiors [33,34]. These responses are often mediated by humidity, temperature, and resource availability, which vary significantly between fragment edges and interiors [25,26].
Given the established influence of environmental variables such as temperature, humidity, and litter depth in structuring anuran assemblages [8,31], and recognizing the well-documented role of edge effects—which occur at transitions between distinct habitats—as critical drivers of species distribution (with some species favored by edge conditions while others remain restricted to fragment interiors [33,34], this study aims to address knowledge gaps regarding amphibian communities in the western Amazon. Particularly, the scarcity of systematic information about how environmental variations shape community structure [10,14] motivates three objectives: (i) provide a detailed inventory of species present in the sampled areas; (ii) estimate and compare taxonomic diversity and species composition both among studied fragments and along the edge-to-interior gradient, identifying species associated with specific fragments or edge distances; and (iii) assess relationships between species occurrence and local environmental variables, determining whether observed patterns reflect fragment-specific characteristics or edge-interior gradients.

2. Materials and Methods

2.1. Study Area

Four forest fragments were sampled between 2017 and 2018 in Acre state, Brazil, located in the southwestern Brazilian Amazon (Figure 1). Two fragments are situated on the periphery of Rio Branco, the state capital and largest urban area: the Raimundo Irineu Serra Environmental Protection Area (APA, 908 ha) and the Zoobotanical Park (PZ, 144 ha). The third fragment, the Humaitá Forest Reserve (RFH, 2000 ha), is located in Porto Acre municipality, 28 km from Rio Branco. The fourth fragment, Catuaba Experimental Farm (FEC, 1260 ha), is situated in Senador Guiomard municipality, 24 km from Rio Branco.
The region has an average altitude of 209 m, annual rainfall of 2156 mm (with a rainy season from November to March, peaking in January and reaching minimum levels in August), and mean annual temperatures ranging between 21.9 °C and 26.2 °C [35]. The vegetation consists of open ombrophilous forest with bamboo patches, embedded in an agricultural-urban matrix of croplands, pastures, and urbanized zones [36].
The fragments exhibit distinct conservation scenarios [37]:
  • APA, located in a highly urbanized area, faces significant anthropogenic pressures from recent housing developments and constant human presence.
  • PZ, though surrounded by residences, represents the best-preserved forest patch within Rio Branco’s urban perimeter.
  • RFH, the largest and most remote fragment, maintains high conservation status.
  • FEC, despite being partially protected, experiences illegal logging, hunting, and fishing pressures.
This gradient of anthropogenic influence across fragments provides an ideal setting. Despite having some level of protection and sustainable use, as they belong to different categories of conservation units, the fragments, except for PZ, have a historical context that has allowed the establishment of settlements in their surroundings, making them areas of dispute due to conflicting interests in land use and occupation [37]. These changes result in serious environmental consequences and consequences for amphibian biodiversity [38,39].

2.2. Sampling Methods

We conducted monthly surveys over three consecutive days in each forest fragment, employing both diurnal and nocturnal sampling efforts. The total sampling period extended for 12 months in APA and PZ (March 2017 to February 2018) and 6 months in RFH and FEC (March to August 2017). Sampling in RFH and FEC was interrupted during the rainy season due to inaccessible roads, preventing data collection at these sites during this period.
We implemented a standardized sampling protocol combining pitfall traps and active visual/auditory surveys. In each fragment, we established a main 450 m transect from which four perpendicular secondary transects (50 m each) were systematically positioned to sample the edge-interior gradient. The secondary transects were spaced at standardized distances: the first at the vegetation edge, followed by additional transects at 50 m, 150 m, and 200 m from the edge. Each pitfall trap station consisted of three 60 L buckets, with the first bucket placed 20 m from the main transect. Following [40], we buried the buckets at ground level, spaced them 10 m apart, and connected them with 1 m high plastic drift fences. To prevent water accumulation and animal drowning, we drilled drainage holes in each bucket and placed a 400 cm2 polystyrene float inside.
Trap monitoring occurred twice daily (07:00–09:00 and 20:00–22:00), coinciding with active search periods. The active search methodology followed [41], involving visual and auditory surveys of all accessible microhabitats including leaf litter (both surface and subsurface), fallen logs, tree cavities, and other potential refugia. Searches covered the 50 m secondary transects with a 1 m width on each side. To avoid recounting individuals during monthly surveys, captured anurans were identified and released at least 200 m from the capture location in similar habitat within the same fragment for examining edge effects and habitat fragmentation impacts on amphibian communities.
For species documentation, we collected voucher specimens that were anesthetized with 5% xylocaine, fixed in formaldehyde for 24 h, and deposited in the UFAC Herpetological Collection. Species identification was conducted using taxonomic keys [42,43,44,45] and by comparison with reference specimens from the UFAC collection. This comprehensive approach ensured rigorous documentation of species presence while accounting for both spatial and temporal variation in detection probabilities across the studied fragments.

2.3. Local Environmental Variables

We quantified key environmental variables at each sampling site to assess their potential influence on anuran distribution patterns. Air temperature and relative humidity were recorded daily during both diurnal and nocturnal active search periods at each pitfall trap station using a digital thermohygrometer. To characterize monthly conditions in each fragment, we calculated the mean of three daily measurements taken at each station. The device was consistently positioned 1 m above ground level and 2 m from the trap array to standardize microclimate measurements.
Litter depth was measured monthly at approximately 1 m distance from each pitfall trap using a Marimon-Hay litter collector gauge [46]. For each fragment, we derived monthly litter depth values by averaging measurements from all four trap stations (one per secondary transect). This standardized approach ensured comparable environmental data across the edge-interior gradient while accounting for fine-scale habitat heterogeneity.

2.4. Data Analysis

To address our first objective, we compiled a species list based on abundance data collected at each sampling point. For the second objective, taxonomic diversity was estimated and compared using observed species richness, Shannon diversity index, and Simpson diversity index. These metrics were calculated using the iNext3D function from the iNext.3D package [47], based on Hill numbers. In this approach, q = 0 corresponds to species richness (considering only total species count regardless of abundance), q = 1 to Shannon diversity (weighting species proportionally to their abundances), and q = 2 to the inverse Simpson diversity (emphasizing dominant species while reducing the influence of rare species).
Species composition was assessed through Permutational Multivariate Analysis of Variance (PERMANOVA) using the adonis2 function from the vegan package [48], based on distance matrices. The species similarity matrix was constructed using the Bray–Curtis index, calculated with the vegdist function from the same package. To visualize community clustering patterns, we performed Principal Coordinates Analysis (PCoA) using the cmdscale function from the stats package [49]. Additionally, to identify species associated with each fragment and different edge distances, we created bar plots displaying species abundances and trophic guilds for each habitat type, including only species with more than two captured individuals and ordered from highest to lowest abundance.
For the third objective, the relationship between species and local environmental variables was evaluated through Redundancy Analysis (RDA) conducted with the rda function from the vegan package. RDA is a constrained ordination technique that examines interactions between species composition, species abundance per sampling point, and local environmental factors (temperature, humidity, and litter depth). The species composition matrix was Hellinger-transformed, and the environmental variables matrix was standardized (mean = 0, variance = 1) using the decostand function from the same package. Site scores were categorized by fragment and edge distance.
All described procedures were applied to compare both fragments and the edge-interior gradient. Analyses were performed in R version 4.2.1 [49] using the aforementioned packages.

3. Results

The study recorded a total of 590 individuals belonging to 37 species across eight families (Table 1). Leptodactylidae was the most abundant family, with Adenomera andreae standing out as the most frequently encountered species. In contrast, Phyllomedusidae showed the lowest representation, with only a single individual of Phyllomedusa camba documented.
The total amphibian diversity across all fragments was estimated at approximately 48 species (Figure 2A, Table 2). Significant differences in species richness were observed when fragments were analyzed separately. RFH showed the highest species richness with 25 observed species (45.04 estimated), while APA exhibited the lowest richness with 16 observed species (16.66 estimated) (Figure 2B, Table 2). This pattern remained consistent when accounting for abundance through Shannon and Simpson diversity indices, with RFH maintaining the highest diversity and APA the lowest (Figure 2B, Table 2).
Edge-distance analysis showed that transects at the forest edge had the highest species richness (25 observed, 27.56 estimated). Species richness decreased to 50 m (21 observed, 23.48 estimated), peaked at 150 m (28 observed, 42.23 estimated), then declined again at 200 m (20 observed, 21.12 estimated) (Figure 2C, Table 2). Overall, vegetation edges showed greater species richness compared to fragment interiors. Diversity indices (Shannon and Simpson) followed an increasing gradient from edge to 150 m, with a sharp decline at 200 m (Figure 2C, Table 2).
The species composition analysis revealed significant patterns both among the sampled fragments (F(3128) = 4.661, p < 0.001) and along the edge-to-interior gradient (F(3128) = 2.433, p < 0.001) (Figure 3). Overall, the fragments showed distinct species compositions (Figure 3A). APA was characterized by high abundance of L. lineatus, T. typhonius, and L. rhodonotus. FEC showed greater abundance of P. fenestratus, L. didymus and E. muiraquitan. PZ was distinguished by high abundance of E. freibergi and A. haneli. RFH exhibited greater abundance of A. femoralis, C. geayi and L. bolivianus (Figure 4A).
Regarding the edge-to-interior gradient, transects at the edge and 50 m from the edge showed similar composition, with high abundance of L. didymus, L. leptodactyloides, L. pentadactylus, P. skydmainos, A. hylaedactyla, A. femoralis, D. minutus, E. muiraquitan, L. lineatus, R. major, L. bolivianus, L. petersii, and S. funereus (Figure 4B). Conversely, transects at 150 and 200 m from the edge formed a distinct group characterized by high abundance of D. acreanus, O. taurinus, Scinax sp., O. quixensis, R. castaneotica, P. fenestratus, Allobates hodli, Ameerega haneli, C. bassleri, D. leucophyllatus, L. knudseni, L. rhodonotus, O. castaneicola, P. camba, E. freibergi, S. garbei, A. trilineatus, H. boliviana, T. typhonius, R. marina, and A. andreae (Figure 4B). Overall, species showed a clearer edge-to-interior gradient (Figure 4B) than among fragments (Figure 4A).
The RDA analyzing the relationship between local environmental variables and species composition explained 85.30% of the variance on the first two axes, with 52.39% on the first axis and 33.91% on the second axis (Figure 5). Temperature (−0.262) and litter quantity (−0.921) showed negative correlations with the first axis, while humidity (0.669) exhibited a positive correlation. S. ruber displayed positive associations with both temperature and litter quantity, being most closely linked to the PZ fragment. In contrast, E. muiraquitan (most abundant in FEC and some APA sites), R. marina, E. freibergi, and H. boliviana (predominantly found in RFH) demonstrated preferences for more humid locations with lower temperatures and reduced litter accumulation (Figure 5A). When examining the edge-to-interior gradient, E. freibergi and H. boliviana showed greater abundance in interior areas, while E. muiraquitan was more abundant in edge transects at 50 and 150 m from the boundary (Figure 5B).

4. Discussion

Our results demonstrate the influence of environmental variables and the edge-interior gradient on the structuring of anuran assemblages in forest fragments of the southwestern Brazilian Amazon. Species richness and composition varied significantly among fragments and along the edge-interior gradient, reflecting the complexity of ecological interactions and the sensitivity of amphibians to microclimatic and structural habitat changes. These findings are consistent with previous studies highlighting the importance of factors such as temperature, humidity, and habitat structure [50] in the distribution of amphibians in fragmented landscapes [8,14,31].

4.1. Species Diversity and Composition Among Fragments

The study recorded 590 individuals from 37 species and 8 families, with a highly uneven abundance distribution across taxa. The Leptodactylidae family dominated the assemblage, with A. andreae emerging as the most abundant species, likely due to its generalized habitat use, fossorial habits, and ability to exploit temporary breeding sites in disturbed environments. In contrast, the Phyllomedusidae family was represented by only one individual (P. camba), suggesting that this arboreal specialist’s strict microhabitat requirements and potential sensitivity to environmental changes make it particularly vulnerable in the study area [51,52]. Four Scinax taxa were recorded in our study, two of which are considered rare (S. funereus and Scinax sp.), and the indeterminate morphospecies likely represents an undescribed lineage or one that requires molecular confirmation. The taxonomy of Scinax in Amazonia is complex and under active revision, with new species and cryptic lineages frequently described through integrative morphological and molecular studies [53]. This pattern featuring a few hyper-abundant species and many rare ones follows the expected logarithmic distribution of ecological communities but may also reflect anthropogenic habitat modifications favoring generalist species over specialists [54,55,56,57]. These findings underscore the importance of considering both abundant and rare species in conservation planning, as they may respond differently to environmental changes and represent distinct components of biodiversity. Future research should investigate the ecological traits driving A. andreae’s dominance and determine whether the apparent rarity of some species reflects true scarcity or sampling limitations, particularly for arboreal and cryptic taxa that may be undersampled by standard methods [58,59,60].
The Humaitá Forest Reserve (RFH) stood out as the fragment with the highest species richness and diversity, followed by the Zoobotanical Park (PZ), Catuaba Experimental Farm (FEC), and Raimundo Irineu Serra Environmental Protection Area (APA). These differences may be attributed to the degree of fragment conservation, anthropogenic pressure, and environmental heterogeneity [60,61,62]. The Humaitá Forest Reserve (RFH) maintained the highest conservation status among studied fragments, with minimal anthropogenic disturbance correlating strongly with its elevated species diversity [36]. This contrasts markedly with the APA, where intensive urbanization has led to pronounced habitat fragmentation and degradation, ultimately reducing amphibian species richness. These findings align with previous studies demonstrating that forest fragmentation and habitat loss are critical factors in amphibian declines in tropical regions [20,23].
Species composition also varied among fragments, with each area hosting a distinct set of dominant species. For instance, the APA was characterized by the abundance of Lithodytes lineatus and Trachycephalus typhonius, species that appear more tolerant of disturbed environments. In contrast, RFH had a higher abundance of species such as Allobates femoralis and Rhinella marina, which may be more sensitive to environmental changes and dependent on well-preserved habitat. These distribution patterns demonstrate that amphibian community composition represents a reliable bioindicator of forest fragment conservation status, consistent with findings from other Amazonian ecosystems [10,14].

4.2. Edge-Interior Gradient and Its Ecological Implications

The edge-interior gradient significantly influenced species’ richness and composition. The forest edges exhibited higher species richness, likely due to greater environmental heterogeneity and resource availability. However, as we moved toward the interior, species richness initially decreased, then increased again at 150 m from the edge, before declining once more at 200 m. This pattern may be explained by microclimatic variations along the gradient, with edges providing more favorable conditions for generalist species, while the interior was more suitable for disturbance-sensitive specialists [25,32].
Species such as E. muiraquitan and R. marina were more abundant near the edge and at 50 m inward, whereas species like E. freibergi and H. boliviana were more associated with the fragment interior. These differences likely reflect variations in microclimatic preferences and resource availability, such as leaf litter depth and relative humidity. The higher abundance of generalists at the edge suggests greater resilience to edge conditions, while interior species may be more vulnerable to habitat fragmentation and loss, as observed in other studies [33,34].
The analysis revealed distinct amphibian communities at different distances from the forest edge. Transects at the edge and 50 m inward shared similar species composition, dominated by taxa such as L. didymus, L. leptodactyloides, P. skydmainos, A. hylaedactyla, A. femoralis, D. minutus, and E. muiraquitan. These species are often associated with disturbed or open habitats, suggesting that edge-affected zones (0–50 m) support communities adapted to microclimatic fluctuations, higher light availability, and altered resource dynamics. Many of these species, particularly those in the genus Leptodactylus, exhibit ecological flexibility, utilizing temporary ponds and moist soils in areas with reduced canopy cover [63,64].
In contrast, transects at 150 and 200 m from the edge formed a distinct group, characterized by species such as D. acreanus, O. taurinus, O. quixensis, P. fenestratus, A. hodli, A. hanhli, E. freibergi, and H. boliviana. These species are typically associated with stable forest interior conditions—higher humidity, lower temperature variability, and denser understory vegetation [65]. Arboreal frogs like O. taurinus and D. leucophyllatus rely on intact canopy structure, while terrestrial species such as O. quixensis and P. fenestratus depend on moist leaf litter and reduced desiccation risk [65]. The presence of moisture-dependent species like E. freibergi and H. boliviana further underscores the importance of undisturbed microhabitats for interior-specialist amphibians [65].
The sharper distinction between edge and interior communities—compared to differences among fragments—suggests that microhabitat conditions along the edge-interior gradient are a stronger driver of amphibian distribution than broader landscape-scale fragmentation effects. These findings are consistent with empirical studies demonstrating that edge effects—particularly increased desiccation risk, pronounced temperature fluctuations, and disrupted predator–prey interactions—function as selective ecological filters, systematically excluding habitat-sensitive interior species from forest edges. Conversely, edge-adapted species may avoid deeper forest zones due to competition, predation pressure, or unsuitable microclimates [55,66,67,68].
Species richness peaked at the immediate forest edge (25 observed species, 27.556 estimated), likely due to the coexistence of both forest specialists and disturbance-tolerant species in this transitional zone. At 50 m inward, richness declined (21 observed, 23.482 estimated), then surprisingly increased at 150 m (28 observed, 42.228 estimated), before dropping again at 200 m (20 observed, 21.118 estimated). This unimodal pattern challenges the assumption of a simple linear decline in diversity from edge to interior, instead suggesting multiple ecological zones within the fragment. The peak at 150 m may represent an ecotone where edge and interior species overlap, creating a biodiversity hotspot.
Diversity indices showed a general increase from the edge to 150 m, followed by a sharp decline at 200 m. While species richness was highest at the edge, true diversity (considering both richness and evenness) peaked in intermediate zones. The abrupt drop at 200 m suggests that the forest core acts as a specialized habitat, filtering out many species and leaving only a few interior-adapted specialists. These findings underscore the importance of preserving large forest areas with minimal edge effects to maintain populations of interior-specialist amphibians. At the same time, edge habitats support a distinct subset of species that may benefit from certain low-intensity disturbances. Conservation strategies should focus on mitigating excessive edge expansion (e.g., through reforestation buffers) while recognizing the ecological value of transitional zones for edge-tolerant taxa [69]. Future research should investigate the mechanisms behind these distribution patterns, including species-specific physiological tolerances and biotic interactions, as well as seasonal and landscape-scale variations in these dynamics.

4.3. Influence of Environmental Variables

Redundancy analysis (RDA) revealed that temperature, humidity, and leaf litter depth were the most important environmental variables structuring anuran assemblages. Species such as S. ruber were associated with warmer sites with abundant leaf litter, while E. muiraquitan and R. marina preferred cooler, more humid environments [55]. These results highlight the critical role of microclimatic conditions and habitat structure in species distribution, supporting previous studies demonstrating amphibian sensitivity to environmental variation [8,31].
Scinax ruber showed a distinct preference for warmer habitats with dense leaf litter, particularly in the Zoobotanical Park (PZ) fragment. This aligns with the ecological traits of many Scinax species, which often thrive in open or disturbed areas where elevated temperatures and accumulated leaf litter provide optimal conditions for foraging, shelter, and reproduction. The thermophilic tendencies of S. ruber may enhance its physiological performance in these microhabitats, while the leaf litter layer likely supports abundant arthropod prey and offers protection from predators. The species’ prevalence in PZ may also reflect its tolerance to edge effects and anthropogenic disturbances, which typically increase temperature and litter accumulation due to canopy openness.
In contrast, species such as E. muiraquitan, R. marina, E. freibergi, and H. boliviana were strongly associated with cooler, more humid sites with sparse leaf litter. These conditions were particularly prominent in the Catuaba Experimental Farm (FEC), Raimundo Irineu Serra Environmental Protection Area (APA), and Humaitá Forest Reserve (RFH) fragments, though their distributions varied along the edge-interior gradient. E. muiraquitan was most abundant in edge zones (50–150 m), suggesting a reliance on edge-specific microhabitats such as ephemeral ponds or disturbed soils critical for breeding and foraging. Its tolerance to edge conditions may also reflect reduced competition from interior-specialist species. Conversely, R. marina, E. freibergi, and H. boliviana were predominantly found in interior zones, where stable humidity and cooler temperatures support their physiological and reproductive needs. For instance, H. boliviana’s burrowing behavior and R. marina’s dependence on moist substrates make them particularly reliant on the buffered microclimates of forest interiors. The reduced leaf litter in these areas may further indicate slower decomposition rates, characteristic of undisturbed, humid forests.
The edge-interior gradient played a key role in segregating species distributions. While E. freibergi and H. boliviana were restricted to interiors, maybe due to their sensitivity to desiccation and habitat specialization, E. muiraquitan thrived in edge zones, likely exploiting transitional resources absent in core habitats [55]. This contrast underscores how microclimatic variability between edges and interiors acts as an ecological filter, with edges favoring generalist or disturbance-adapted species and interiors supporting moisture-dependent specialists. The persistence of moisture-sensitive species like H. boliviana and E. freibergi depends on maintaining intact forest interiors with minimal edge influence, while species such as S. ruber may benefit from heterogeneous landscapes that include open or disturbed habitats [70,71,72]. Conservation strategies should account for these habitat preferences, particularly in fragmented landscapes where edge effects are pervasive and future research should investigate additional factors such as prey availability and interspecific interactions to further clarify the mechanisms driving these distribution patterns.

5. Conclusions

In conclusion, our study enhances the understanding of the factors influencing the distribution and diversity of anurans in forest fragments of the Brazilian Amazon. Our results emphasize the importance of considering both local environmental variables and edge-interior gradients when planning conservation strategies for amphibians in fragmented landscapes. The composition and distribution of anuran assemblages in the southwestern Amazon are influenced not only by microclimatic and structural habitat differences but also by the timing and intensity of ecosystem transformation. Older forest fragments accumulate “extinction debt” and show greater community simplification than recently isolated ones [23]. In addition, recent surveys have detected the chytrid fungus Batrachochytrium dendrobatidis (Bd) in lowland Amazonian populations, suggesting that disease prevalence may interact with habitat degradation to reduce amphibian diversity [73]. Human pressures, including localized harvesting and trade near urbanized zones, further contribute to the decline of conspicuous or range-restricted taxa. Together, these factors reinforce the need for long-term monitoring in both disturbed and continuous-forest control sites to separate the effects of habitat change, disease, and anthropogenic exploitation on Amazonian amphibians. Future research should investigate the long-term effects of fragmentation and climate change on anuran assemblages to preserve these unique ecosystems and their associated biodiversity.
Our findings reveal significant patterns in amphibian community structure across edge-to-interior gradients and among habitat fragments. The high species richness observed at forest edges suggests these transitional zones support both disturbance-tolerant and forest-adapted species, while the distinct assemblages found in interior habitats (150–200 m) highlight the importance of preserving core forest areas for specialist species. The dominance of generalist taxa (e.g., A. andreae) and rarity of specialists (e.g., P. camba) indicate potential environmental filtering linked to habitat fragmentation or degradation. These results demonstrate the need for conservation strategies that maintain both habitat heterogeneity and intact forest interiors to support diverse amphibian communities. Protecting edge habitats benefits disturbance-adapted species, while safeguarding interior zones ensures the survival of moisture-dependent and forest-specialist taxa. Future studies should examine long-term population trends and specific microhabitat requirements of rare species to improve conservation efforts in fragmented landscapes. Ultimately, maintaining connectivity between fragments and minimizing edge effects will be essential for preserving amphibian biodiversity amidst increasing habitat modification.

Author Contributions

Conceptualization, Y.A.P., L.L.C. and M.B.d.S.; methodology, Y.A.P., M.B.d.S. and V.G.; formal analysis, T.B.V.; data curation, Y.A.P. and M.B.d.S.; writing—original draft preparation, Y.A.P.; writing—review and editing, M.B.d.S., V.G., L.L.C. and T.B.V. All authors have read and agreed to the published version of the manuscript.

Funding

This research benefited from Chamada Nº 006/2023 Fundação Amazônia de Amparo a Estudos e Pesquisas (FAPESPA)—Concessão de Cotas Instituicionais de Bolsas de doutorado (DO)—ACT_012/2023, Coordenação de Aperfeiçoamento de Pessoal de Nível Superior—Brasil (CAPES)—Finance Code 001.

Institutional Review Board Statement

The study was conducted in accordance with the Declaration of Helsinki and approved by the Instituto Chico Mendes de Conservação da Biodiversidade, Ministerio do Meio Ambiente (SISBIO N◦37974-1 and 37974-2).

Data Availability Statement

Data are available on request.

Acknowledgments

We thank the BNP Paribas. Foundation’s Climate and Biodiversity initiative (project Bioclimate).

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Spatial distribution of sampled forest fragments from 2017 to 2018. The sampling locations are represented by points, categorized as follows: (A) RFH—Humaitá Forest Reserve; (B) APA—Raimundo Irineu Serra Environmental Protection Area and PZ—Zoobotanical Park; (C) FEC—Catuaba Experimental Farm.
Figure 1. Spatial distribution of sampled forest fragments from 2017 to 2018. The sampling locations are represented by points, categorized as follows: (A) RFH—Humaitá Forest Reserve; (B) APA—Raimundo Irineu Serra Environmental Protection Area and PZ—Zoobotanical Park; (C) FEC—Catuaba Experimental Farm.
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Figure 2. Diversity curves for: (A) overall species richness, (B) individual fragments (Raimundo Irineu Serra Environmental Protection Area—APA, Zoobotanical Park—PZ, Humaitá Forest Reserve—RFH, and Catuaba Experimental Farm—FEC), and (C) edge-to-interior distance gradients (ed1—edge transect; ed2—50 m from edge; ed3—150 m from edge; ed4—200 m from edge). The curves illustrate observed richness (Hill series q = 0), Shannon diversity index (q = 1), and Simpson diversity index (q = 2).
Figure 2. Diversity curves for: (A) overall species richness, (B) individual fragments (Raimundo Irineu Serra Environmental Protection Area—APA, Zoobotanical Park—PZ, Humaitá Forest Reserve—RFH, and Catuaba Experimental Farm—FEC), and (C) edge-to-interior distance gradients (ed1—edge transect; ed2—50 m from edge; ed3—150 m from edge; ed4—200 m from edge). The curves illustrate observed richness (Hill series q = 0), Shannon diversity index (q = 1), and Simpson diversity index (q = 2).
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Figure 3. Principal Coordinates Analysis (PCoA) results showing community composition patterns: (A) across fragments (Raimundo Irineu Serra Environmental Protection Area—APA, Zoobotanical Park—PZ, Humaitá Forest Reserve—RFH, and Catuaba Experimental Farm—FEC) and (B) along the edge-to-interior gradient (ed1—edge transect; ed2—50 m from edge; ed3—150 m from edge; ed4—200 m from edge).
Figure 3. Principal Coordinates Analysis (PCoA) results showing community composition patterns: (A) across fragments (Raimundo Irineu Serra Environmental Protection Area—APA, Zoobotanical Park—PZ, Humaitá Forest Reserve—RFH, and Catuaba Experimental Farm—FEC) and (B) along the edge-to-interior gradient (ed1—edge transect; ed2—50 m from edge; ed3—150 m from edge; ed4—200 m from edge).
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Figure 4. Species abundance patterns across (A) different forest fragments (Raimundo Irineu Serra Environmental Protection Area—APA, Zoobotanical Park—PZ, Humaitá Forest Reserve—RFH, and Catuaba Experimental Farm—FEC) and (B) along the edge-to-interior gradient (ed1—edge transect; ed2—50 m from edge; ed3—150 m from edge; ed4—200 m from edge.
Figure 4. Species abundance patterns across (A) different forest fragments (Raimundo Irineu Serra Environmental Protection Area—APA, Zoobotanical Park—PZ, Humaitá Forest Reserve—RFH, and Catuaba Experimental Farm—FEC) and (B) along the edge-to-interior gradient (ed1—edge transect; ed2—50 m from edge; ed3—150 m from edge; ed4—200 m from edge.
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Figure 5. Redundancy Analysis (RDA) results showing species-environment relationships: (A) across fragments (Raimundo Irineu Serra Environmental Protection Area—APA, Zoobotanical Park—PZ, Humaitá Forest Reserve—RFH, and Catuaba Experimental Farm—FEC) and (B) along the edge-to-interior gradient (ed1—edge transect; ed2—50 m from edge; ed3—150 m from edge; ed4—200 m from edge).
Figure 5. Redundancy Analysis (RDA) results showing species-environment relationships: (A) across fragments (Raimundo Irineu Serra Environmental Protection Area—APA, Zoobotanical Park—PZ, Humaitá Forest Reserve—RFH, and Catuaba Experimental Farm—FEC) and (B) along the edge-to-interior gradient (ed1—edge transect; ed2—50 m from edge; ed3—150 m from edge; ed4—200 m from edge).
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Table 1. Species list of amphibians captured across the four study fragments (Raimundo Irineu Serra Environmental Protection Area—APA, Zoobotanical Park—PZ, Humaitá Forest Reserve—RFH, and Catuaba Experimental Farm—FEC) during the 2017–2018 sampling period. All species are systematically organized by family and alphabetically arranged within each taxonomic group. The IUCN column indicates the global conservation status, while IUCN * refers to the national conservation status.
Table 1. Species list of amphibians captured across the four study fragments (Raimundo Irineu Serra Environmental Protection Area—APA, Zoobotanical Park—PZ, Humaitá Forest Reserve—RFH, and Catuaba Experimental Farm—FEC) during the 2017–2018 sampling period. All species are systematically organized by family and alphabetically arranged within each taxonomic group. The IUCN column indicates the global conservation status, while IUCN * refers to the national conservation status.
Family
Specie		IUCNIUCN *APAFECPZRFHTotal
Aromobatidae 418821
Allobates femoralis (Boulenger, 1884)LCLC 44
Allobates hodli Simões, Lima & Farias, 2010LCLC 1 1
Allobates trilineatus (Boulenger, 1884)LCLC4 8416
Bufonidae 137261864
Rhinella castaneotica (Caldwell, 1991)LCLC12623546
Rhinella major (Müller & Hellmich, 1936)LCLC 3 3
Rhinella marina (Linnaeus, 1758)LCLC11 1315
Craugastoridae 080917
Oreobates quixensis Jiménez de la Espada, 1872LCLC 33
Pristimantis fenestratus (Steindachner, 1864)LCLC 8 412
Pristimantis skydmainos (Flores & Rodríguez, 1997)LCLC 22
Dendrobatidae 20147
Ameerega hahneli (Boulenger, 1884)LCLC 1 1
Ameerega trivittata (Spix, 1824)LCLC2 46
Hylidae 3712508107
Dendropsophus acreanus (Bokermann, 1964)LCLC 1 1
Dendropsophus leucophyllatus (Beireis, 1783)LCLC1 12
Dendropsophus minutus (Peters, 1872)LCLC2 2 4
Osteocephalus castaneicola Moravec, Aparicio, Guerrero-Reinhard, Calderón, Jungfer & Gvozdík, 2009LCLC 1 1
Osteocephalus taurinus Steindachner, 1862LCLC 11
Scinax funereus (Cope, 1874)LCLC 1 1
Scinax garbei (Miranda-Ribeiro, 1926)LCLC3 16120
Scinax ruber (Laurenti, 1768)LCLC21830463
Scinax sp. 11 2
Trachycephalus typhonius (Linnaeus, 1758)LCLC10 1112
Leptodactylidae 50589821227
Adenomera andreae (Müller, 1923)LCLC2933475114
Adenomera hylaedactyla (Cope, 1868)LCLC101512340
Engystomops freibergi (Donoso-Barros, 1969)LCLC 27936
Leptodactylus bolivianus Boulenger, 1898LCLC 11
Leptodactylus didymus Heyer, García-Lopez & Cardoso, 1996LCLC 4217
Leptodactylus knudseni Heyer, 1972LCLC 1 1
Leptodactylus leptodactyloides (Andersson, 1945)LCLC446 14
Leptodactylus pentadactylus (Laurenti, 1768)LCLC 112
Leptodactylus petersii (Steindachner, 1864)LCLC 11
Leptodactylus rhodonotus (Günther, 1869)LCLC2 2
Lithodytes lineatus (Schneider, 1799)LC 522 9
Microhylidae 26742026146
Chiasmocleis bassleri Dunn, 1949LCLC 15 318
Ctenophryne geayi Mocquard, 1904LCLC 22
Elachistocleis muiraquitan Nunes-de-Almeida & Toledo, 2012LCLC1659 1085
Hamptophryne boliviana (Parker, 1927)LCLC10 201141
Phyllomedusidae 00011
Phyllomedusa camba De la Riva, 1999LCLC 11
Abundance 13216020395590
Richeness 1616182537
Table 2. Diversity values (observed and estimated richness) for the overall study area, individual fragments (Raimundo Irineu Serra Environmental Protection Area—APA, Zoobotanical Park—PZ, Humaitá Forest Reserve—RFH, and Catuaba Experimental Farm—FEC), and edge-distance gradients (ed1—edge transect; ed2—50 m from edge; ed3—150 m from edge; ed4—200 m from edge), with Simpson diversity * representing the inverse Simpson diversity (Hill number q = 2 in the iNext3D package).
Table 2. Diversity values (observed and estimated richness) for the overall study area, individual fragments (Raimundo Irineu Serra Environmental Protection Area—APA, Zoobotanical Park—PZ, Humaitá Forest Reserve—RFH, and Catuaba Experimental Farm—FEC), and edge-distance gradients (ed1—edge transect; ed2—50 m from edge; ed3—150 m from edge; ed4—200 m from edge), with Simpson diversity * representing the inverse Simpson diversity (Hill number q = 2 in the iNext3D package).
AssemblageIndexObservedEstimatedSE
TotalSpecies richness40.0047.6313.86
Shannon diversity15.8416.550.79
Simpson diversity *10.6810.860.65
FragmentsAPASpecies richness16.0016.662.95
Shannon diversity10.7211.420.85
Simpson diversity *8.458.961.01
FECSpecies richness17.0041.3515.15
Shannon diversity7.448.290.88
Simpson diversity4.915.030.59
PZSpecies richness19.0022.115.58
Shannon diversity10.1710.780.76
Simpson diversity *7.848.120.65
RFHSpecies richness25.0045.0420.80
Shannon diversity17.8222.432.50
Simpson diversity *13.9916.242.05
Edge Distanceed1Species richness25.0027.566.22
Shannon diversity10.9311.930.95
Simpson diversity *6.716.940.64
ed2Species richness21.0023.487.09
Shannon diversity12.0913.251.07
Simpson diversity *8.689.210.81
ed3Species richness28.0042.2921.24
Shannon diversity16.4719.661.97
Simpson diversity *12.0313.171.33
ed4Species richness20.0021.124.75
Shannon diversity12.2313.120.89
Simpson diversity *8.558.991.00
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Pereira, Y.A.; Guerra, V.; Correia, L.L.; Vieira, T.B.; de Souza, M.B. Environmental Drivers and Edge Effects on Anuran Diversity in Fragmented Forests of the Southwestern Brazilian Amazon. Diversity 2025, 17, 764. https://doi.org/10.3390/d17110764

AMA Style

Pereira YA, Guerra V, Correia LL, Vieira TB, de Souza MB. Environmental Drivers and Edge Effects on Anuran Diversity in Fragmented Forests of the Southwestern Brazilian Amazon. Diversity. 2025; 17(11):764. https://doi.org/10.3390/d17110764

Chicago/Turabian Style

Pereira, Yara Araújo, Vinicius Guerra, Letícia Lima Correia, Thiago Bernardi Vieira, and Moisés Barbosa de Souza. 2025. "Environmental Drivers and Edge Effects on Anuran Diversity in Fragmented Forests of the Southwestern Brazilian Amazon" Diversity 17, no. 11: 764. https://doi.org/10.3390/d17110764

APA Style

Pereira, Y. A., Guerra, V., Correia, L. L., Vieira, T. B., & de Souza, M. B. (2025). Environmental Drivers and Edge Effects on Anuran Diversity in Fragmented Forests of the Southwestern Brazilian Amazon. Diversity, 17(11), 764. https://doi.org/10.3390/d17110764

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