*4.2. Post-Decline Dynamics*

In this study, we found strong evidence that *Bd* is widespread in Costa Rica. Our results also sugges<sup>t</sup> that post-decline *Bd* exhibited enzootic dynamics, characterized by high prevalence of infection across regions and pathogenic loads below thresholds associated with mass mortalities [39,42]. We found *Bd* in all the herpetological provinces and altitudinal belts surveyed (Figures 3 and 4, Tables S5 and S6), for a total infection rate of 23%. We also found that *Bd* was more prevalent in terrestrial amphibians with an aquatic larval stage and direct-developing frogs exhibited the highest pathogenic loads.

We found the lowest infection rate (<5%) in the Pacific Northwest and the highest (33%) in the Caribbean Lowlands (Figure 3). However, these values may be an effect of sampling periods. A study conducted at La Selva Biological Station in the Caribbean Lowlands [95] reported infection rates varying from <5% during the warmest months (May to early November) to 35% in the coolest months (mid-November to January) in three common amphibian species. In addition, the gradual population declines observed at La Selva over several decades [31] and opportunistic observations of small-scale mortality events during cold periods [96] sugges<sup>t</sup> that *Bd* may be causing mortality in amphibians long after its initial invasion. Similar mortality events in response to seasonality could be occurring in amphibian communities in "refuges from decline" in the Pacific Northwest [62,97] and Pacific Southwest [63] of Costa Rica; however, they have not ye<sup>t</sup> been documented. There, seasonal changes in precipitation and temperature caused *Bd* prevalence to vary from >5% in the peak of the dry season (March and April) to 80% in the coldest months (November–December). Therefore, we recommend follow-up studies at these sites to identify whether seasonal disease dynamics are causing mortality events in regions that have been considered unsuitable for *Bd* [98].

*Bd* was found across all altitudinal belts across Costa Rica. Similar results of high *Bd* prevalence across all elevations has been found in Panama [99–101]. These findings sugges<sup>t</sup> that current environmental conditions are suitable for *Bd* at most elevations in Central America [63]. It is also plausible that *Bd*-driven declines during the 1980s and 1990s were not exclusively restricted to highlands [30] but were relatively undetected at lower elevations. Another hypothesis is that species with high susceptibility historically occupied high elevations sites, but severely declined or went extinct after *Bd* was introduced, leaving only species with mid-to-low susceptibility across elevations [102]. On the other hand, the absence of samples from montane and subalpine belts and uncontrolled variables (e.g., changes in species composition, climatic disturbances) could have reduced the statistical power to determine changes in *Bd* prevalence across altitudinal belts. We sugges<sup>t</sup> that future studies increase sampling at high elevations (>2700 m) to better understand the local spatial dynamics of *Bd* across elevations.

Our results showed that infection with *Bd* was common in amphibians across all life-history traits evaluated in the FRHI (Table 2). However, *Bd* was significantly more prevalent in terrestrial amphibians with a larval stage (Figure 4a), especially those that complete metamorphosis in phytotelma (ITF). All the species within the ITF category belonged to the family Dendrobatidae, which have

previously been shown to easily acquire *Bd* infection [62,95]. The high susceptibly of the Dendrobatidae family is likely due to their preferred habitat (e.g., water-filled bromeliads for many species), as it offers suitable conditions for *Bd* infection [103]. In addition, dendrobatid adults forage in the tropical forest floor and stream-associated low vegetation, which are environments that can sustain *Bd* [104]. Regarding infection intensity (Figure 4b), the FRHI showed similar results to studies that have used the aquatic index [43]. We found that direct-developing species with terrestrial reproduction had significantly higher infection load than other species with different life-history traits (Figure 4b, Table 4). This life-history trait is exhibited by leaf-litter frogs and all the species within the *Craugastor punctariolus* clade (robber frogs), which is one of the clades most affected by chytridiomycosis in Central America [29,105]. Robber frogs spend a majority of their life cycle along fast-flowing streams [106], an aquatic environment that seems highly suitable for *Bd* in Central America [107]. In addition, these frogs appear to be highly susceptible to *Bd*-driven mass mortalities outside warm and dry ecosystems [19,30,108]. Our results sugges<sup>t</sup> the FRHI is particularly useful for identifying taxonomic units that are more susceptible to *Bd* [92].
