Some of the fundamental predictions of the theory of Island Biogeography establish that species richness will positively correlate with the island's area and inversely correlate with the distance from the nearest land mass. Coiba National Park (CNP) comprises an offshore archipelago of around 35 islands and islets of different areas off the coast of the Panama Pacific. This archipelago was once attached to the Mainland and separated from it around 12,000 ya, creating a barrier for further species colonization. In this study, the herpetological richness, diversity and community assemblages were assessed along four different islands of CNP. Amphibian richness was lower in the islands (maximum five spp.) compared to the mainland (11 spp.); meanwhile, reptile richness was similar between the mainland and Coiba island (21 spp.). Amphibian diversity was in the same order of magnitude among the mainland (H´= 1.18) and the islands of CNP (maximum H´=1.36), except for Canales de Afuera island, which was deprived of amphibians. Interestingly, reptile diversity on the mainland was higher (H´= 2.67) than on the rest of the CNP islands (maximum H´= 2.06). The area of the islands was positively correlated with amphibian species richness but not with reptile richness and no effect of distance was found. The herpetological communities were less similar between the Mainland and the islands of CNP, possibly reflecting not only dispersal limitations but also specific adaptation to the more pristine habitats of the latest. Based on these results, CNP may harbor a unique herpetological fauna with local adaptations to the habitat, influenced by biogeographical elements and past anthropogenic factors that need to be investigated in more detail.

Herpetological Surveys

Surveys were conducted between July 2019 and August 2023 along transects in 18 sites from the Mainland in front of CNP and on four islands of CNP archipelago (Figure 1). The islands surveyed included Canales de Afuera, Rancheria, Coiba, and Jicaron (Table 1). Several field trips occurred each year except for 2020 when only one site was visited (Table 1_SuppInfo).

Visual encounter surveys (VES) (Crump & Norman, 1994; Rodda et al., 2007) were used to identify metamorphic amphibians and terrestrial reptiles. VES was conducted along three to five randomly deployed 2 m wide and 100 m long transects at each site during day and night. Each transect was separated at least 20 m from each other, and two people constantly surveyed the transects. Search effort (in person-h) was calculated by multiplying the total search time at each site by the number of observers (Maritz & Alexander, 2012), excluding time allocated to manipulate captured individuals. Each transect was visited between three and four times during each survey, and the counts of individual specimens were averaged across all visits. The capture rate at each site was calculated by dividing the number of identified individuals by the total search time in each site (Linares & Eterovick, 2013). Assuming that some species cannot be detected, pitfall traps were used (Kenneth Dodd Jr., 2010; Todd et al., 2007). When possible, one or two arrays of pitfall traps were deployed at each site and were separated at least 100 m from transects to provide independent sampling units. Individuals were identified in the field at the species level and compared with dichotomic keys (Köhler, 2008, 2011).

Statistical Analysis

To estimate α diversity, species richness (Magurran, 2021) and diversity were estimated by computing the Shannon-Weaver index (Shannon & Weaver, 1949). In addition, individual-based rarefaction curves with 95% confidence intervals based on the Mao Tau function (Colwell, 2006) were produced using the software Estimates v9.1.0 (Colwell, 2006). Occurrences of amphibians and terrestrial reptiles from 2018 to 2023 downloaded from the GBIF website (www.gbif.org) were included in the analysis. In total, 51 individuals (45 reptiles and 6 amphibians) were included and assigned to the closer study site. To determine β diversity the Jaccard dissimilarity index with the presence–absence of species was calculated using the vegan package (Oksanen et al., 2022). Clusters were produced using the hierarchical clustering algorithm using the hclust function and the unweighted pair group method with arithmetic mean to create dendrograms using the package BiodiversityR (Kindt & Coe, 2005). Presence–absence data of additional species of amphibians and terrestrial reptiles reported by De la Riva (1997a, 1997b), and Pérez-Santos and Martínez (1997) for some islands of CNP and by Martinez and Rodríguez (2003) for Mainland, were also included assuming these species are extant species (Table 2_SuppInfo; Table 3_SuppInfo). Pearson product–moment correlations were conducted to test the effect of island area and distance from the Mainland on species richness. Statistical analyses were conducted using the software R v4.2.2 (R Core Team, 2023). Data presented here are mean ± SD (range) unless otherwise indicated.