Two Doors into the Same Forest: Rethinking Amphibian Distributions Through Microhabitats and Unexpected Encounters

The forest on the upper slopes of Mount Ijen does not ease you in gently. The air is sharp with sulfur drifting from the crater, the ground uneven with volcanic deposits, and the vegetation carries that distinct character of high-elevation resilience — stunted trees, dense moss, and pockets of moisture hidden in unlikely places. At night, however, the forest softens. The wind quiets, humidity rises, and the understory begins to whisper with life.

It was in this shifting atmosphere, between harsh geology and fragile ecology, that I encountered a frog that did not seem to belong — at least not according to what we thought we knew.

Perched quietly among low vegetation was a small rhacophorid that, upon closer inspection, aligned closely with Philautus cf. candrageni — a taxon more commonly associated with Mount Merapi. This was not a species I expected to see here. And yet, there it was.

Months earlier, in a very different landscape — the humid lowland forests of Borneo — I had been working with another amphibian system entirely. There, the focus was on Metaphrynella sundana, a species so specialized that its entire life unfolds within water-filled cavities in trees (1).

Two species. Two islands. Two very different ecological settings.

But as I would come to understand, they are part of the same story — and what's at stake is how we understand amphibian presence, persistence, and ultimately, conservation.

A practitioner's question

For many amphibian practitioners, species distributions are foundational. They guide surveys, shape conservation priorities, and inform ecological understanding. But these distributions are often built on incomplete data — snapshots of presence rather than full pictures of reality (2).

The question that emerged from my work was simple, but deeply consequential:

Are we misunderstanding where species occur because we are overlooking how they live?

In other words, is the apparent absence of a species in a location truly absent, or is it a failure of detection shaped by microhabitats we do not fully account for?

For a non-specialist, this might sound like looking for birds only at ground level and concluding that trees are empty. For amphibians, the issue is even more complex. Many species occupy microhabitats that are highly specific, spatially limited, and often physically inaccessible (3).

The unexpected presence of Philautus cf. candrageni on Ijen raised one side of the problem: distribution may be broader than we think.

The ecology of Metaphrynella sundana revealed the other: presence may be hidden in places we rarely look.

Together, these form a single challenge — understanding amphibians not just in terms of geography, but in terms of microhabitat.

Two parallel approaches in the field

To explore this, my approach was not a formal experimental design, but a practitioner-led integration of field observation across two systems.

Highland surveys on Mount Ijen

On Ijen, I conducted nocturnal visual encounter surveys along elevational gradients, focusing on areas with sufficient humidity and vegetation complexity. Rather than limiting observations to standard transects, I allowed flexibility in movement — following calls, investigating microhabitats, and adapting to terrain (5).

Particular attention was paid to:

• Vegetation structure (shrubs, mossy branches, epiphytes)
• Moisture-retaining features (leaf litter pockets, small depressions, water-holding plants)
• Vertical space, including low canopy access points

When the suspected Philautus cf. candrageni was encountered, detailed photographic documentation was taken, along with notes on perch height, substrate, and surrounding microhabitat conditions.

Microhabitat-focused work in Borneo

In contrast, my work with Metaphrynella sundana in Borneo was explicitly microhabitat-driven.

Instead of broad surveys, the search was structured around identifying and investigating tree holes and natural cavities capable of holding water. This required:

• Locating suitable trees (often large, mature individuals)
• Inspecting cavities at varying heights
• Listening for acoustic signals originating from within cavities (1, 6)

Observations included:

• Water volume and depth within tree holes
• Internal surface texture (important for climbing grip)
• Presence of calling males and egg deposition

Unlike the Ijen surveys, where microhabitats were one of many considerations, in Borneo they were the entire focus.

Bridging the two approaches

What connected these methods was a shift in attention — from landscape-scale presence to fine-scale habitat use.

Rather than asking, "Is the species here?" the guiding question became: "Where, exactly, within this environment could the species persist?"

What we found

We found that both unexpected distributions and highly specialized microhabitats point to the same underlying reality: amphibian presence is often structured at a scale much smaller than our surveys typically capture.

Evidence from Mount Ijen

The individual identified as Philautus cf. candrageni was not encountered in a random or exposed setting. It occupied a microhabitat characterized by:

• High localized humidity
• Dense vegetation providing cover
• Likely access to small, hidden moisture reservoirs

When I encountered this Philautus individual on the slopes of Mount Ijen, it was not its coloration that stood out, but its structure. Closer inspection revealed a distinctly angular loreal region — the transition between the snout and eye was sharp rather than rounded, closely matching the diagnostic profile of Philautus candrageni. Its overall form also suggested relatively elongated hind limbs and a more streamlined body, lacking the compact build of P. aurifasciatus or the heavier proportions typical of P. jacobsoni (4).

These combined traits pointed strongly toward P. candrageni. However, identification cannot ignore geography. The species is primarily known from Mount Merapi, making its presence on Ijen unexpected.

For this reason, I refer to the individual as Philautus cf. candrageni. The "cf." acknowledges a close morphological match while recognizing uncertainty due to its occurrence outside the documented range. Rather than weakening the record, this tension between form and location strengthens its significance — suggesting either an overlooked distribution or the possibility of a cryptic lineage.

In this way, the observation moves beyond identification. It becomes part of a broader question about how much of amphibian diversity remains hidden — not because it is rare, but because we have yet to fully resolve where and how we look.

This aligns with broader findings that microclimatic conditions strongly influence amphibian occurrence, particularly in montane systems (7).

Importantly, no large population was observed — only isolated encounters. This raises the possibility that such populations may be:

• Patchily distributed
• Low-density
• Easily overlooked

Evidence from Borneo

In the case of Metaphrynella sundana, the dependence on tree holes was absolute.

We found that:

• Individuals were only detected within or immediately adjacent to water-filled cavities
• Calling activity was acoustically amplified by the structure of the tree hole (1, 6)
• Morphological traits (notably the yellow hands) were closely tied to climbing within these confined, vertical spaces

The absence of suitable tree holes correlated strongly with absence of the species — even in otherwise intact forest (8).

The connection

Taken together, these findings suggest that:

• Microhabitats act as filters determining where species can persist
• Distribution maps without microhabitat context are incomplete (2, 3)
• Detection probability is strongly influenced by whether surveys target these microhabitats

In simple terms: we are not just mapping species. We are mapping our ability to find them.

Implications for practitioners

For practitioners, this has several immediate implications.

Rethinking absence

Absence data should be treated with caution, especially in structurally complex environments. A species not observed may still be present — occupying microhabitats outside the scope of the survey (3, 7).

For Philautus cf. candrageni, its presence on Ijen may not represent expansion, but revelation.

Expanding survey design

Field methods should incorporate microhabitat targeting as a core component, not an afterthought. This includes:

• Investigating vertical space (canopy, tree trunks, cavities)
• Prioritizing structurally complex areas
• Allowing adaptive survey movement rather than rigid transects (5)

Conservation at the micro-scale

Protecting habitat at a landscape level is necessary, but not sufficient.

For species like Metaphrynella sundana, conservation must ensure:

• Retention of mature trees capable of forming cavities
• Preservation of hydrological conditions that maintain water within these microhabitats (8)

Similarly, for highland species, maintaining microclimatic conditions — humidity, vegetation cover — may be more critical than previously recognized (7).

Embracing uncertainty

The use of "cf." in Philautus cf. candrageni reflects taxonomic and ecological uncertainty. Rather than viewing this as a limitation, it should be embraced as part of the discovery process.

Uncertainty often signals the edge of understanding — and therefore, opportunity.

Where this leads

These observations open several directions for future work.

First, there is a need for targeted microhabitat surveys across known and suspected ranges of species like Philautus cf. candrageni. This could help determine whether the Ijen population is isolated or part of a broader, previously undetected distribution.

Second, integrating acoustic monitoring may improve detection, particularly for species occupying concealed spaces. Many amphibians reveal their presence through sound before sight (5).

Third, microhabitat mapping — documenting the availability and condition of features like tree holes, epiphyte clusters, and moisture pockets — could become a valuable conservation tool (8).

Finally, there is a role for collaborative observation. Field practitioners, local researchers, and citizen scientists can contribute to a growing understanding of where species occur and how they persist — especially when attention is directed toward overlooked spaces.

Two doors into the same forest

Looking back, the encounter on Ijen and the work in Borneo no longer feel like separate experiences.

They are two entry points into the same realization: amphibian ecology does not unfold only across landscapes; it unfolds within them, in small, specific, and often hidden places.

Unexpected distributions and hidden microhabitats are not competing explanations. They are complementary perspectives.

Two doors into the same forest.

And if we want to understand and protect the species within it, we need to walk through both.

Acknowledgments

The author thanks his friend (Arfiza Daffa) and brother (Shalahuddin Averroes Supartha), who accompanied him in the field searching for frogs.

References

1. Inger, R. F., & Stuebing, R. B. A Field Guide to the Frogs of Borneo. Natural History Publications (1997).
2. Guisan, A., & Thuiller, W. Predicting species distribution: offering more than simple habitat models. Ecology Letters 8, 993–1009 (2005).
3. Wells, K. D. The Ecology and Behavior of Amphibians. University of Chicago Press (2007).
4. Herlambang, A.E.N., Gonggoli, A.D., Racho, A., Asti, H.A., Cahyadi, G., Atmaja, V.Y., Fauzi, M.A., Eprilurahman, R. & Hamidy, A. Revisiting the Taxonomy of Javan Philautus (Anura: Rhacophoridae), with the Description of a New Species. Zootaxa 5768 (1), 29–56 (2026).
5. Heyer, W. R. et al. Measuring and Monitoring Biological Diversity: Standard Methods for Amphibians. Smithsonian Institution Press (1994).
6. Grafe, T. U. The reproductive biology of Metaphrynella sundana. Journal of Herpetology 30, 310–318 (1996).
7. Scheffers, B. R., Edwards, D. P., Diesmos, A., Williams, S. E., & Evans, T. A. Microhabitats reduce animal's exposure to climate extremes. Global Change Biology 20, 495–503 (2014).
8. Whitworth, A. et al. Amphibian responses to microhabitat changes in tropical forests. Biotropica 48, 123–131 (2016).