Threats to Mexico's Karst Forest Endemics: Predicting Distribution Shifts of Hylorchilus Wrens

Analysis based on the study by Malpica et al. (2025) using Ecological Niche Models (ENMs) and Land Use/Land Cover Change (LULCC) dynamics.

I. Introduction: The Vulnerable Wrens

Study Focus

Predicting potential distribution shifts due to climate change (GCC).
Assessing contemporary land cover changes (LULCC) for two endemic bird species of the Mexican karst forests: Hylorchilus sumichrasti and Hylorchilus navai.

Habitat and Status

Both species are endemic to Mexico’s karst mountain forests, a specialized habitat rich in biodiversity that covers only 2.07% of Mexico’s land area. They require specialized habitats, primarily limestone outcrops in karst forest.

Species	Conservation Status (IUCN)	National Status	Current Suitable Area
Hylorchilus sumichrasti	Near Threatened	Lacks protection within protected natural areas.	6,553.67 km²
Hylorchilus navai	Vulnerable	Considered Endangered nationally. Limited coverage within protected zones.	2,440.55 km²

Key Determining Variables: Distributions are heavily influenced by temperature and precipitation variables, with slope also important for local climate refuges.

II. Threat 1: Global Climate Change (Projected Distribution Shifts)

Future projections (2050 and 2090) generally indicate a reduction in environmentally suitable areas, especially under the Pessimistic Scenario (SSP5–8.5).

Projected Suitable Area Remaining (as % of Current)

Species	Scenario	2050 Projected	2090 Projected	Key Insight
H. sumichrasti	Optimistic (SSP1–2.6)	64.69% remaining	71.79% remaining	Vulnerable due to low resilience and limited colonization capacity.
H. sumichrasti	Pessimistic (SSP5–8.5)	69.20% remaining	62.80% remaining (Significant loss)
H. navai	Optimistic (SSP1–2.6)	110.50% (Modest increase/Gain)	103.71% (Modest increase/Gain)	Potential colonization constrained by habitat availability and quality.
H. navai	Pessimistic (SSP5–8.5)	69.54% remaining	58.82% remaining (Severe loss)	Highly sensitive to more extreme climate conditions.

III. Threat 2: Land Use and Land Cover Change (LULCC)

Analysis over three decades (1991 to 2021) shows a transformation of suitable habitats, driven by human activities.

General LULCC Trend (1991–2021)

Decline: Natural vegetation (including evergreen forest, dry forest, and secondary succession—critical habitats).
Expansion: Human-induced categories like cropland and urban built-up areas.
Major Threats: Expansion of cropland and urban areas, along with cattle ranching and limestone extraction.

Specific Habitat Loss (1991 vs. 2021)

Species Area	Natural Vegetation (1991)	Natural Vegetation (2021)	Habitat Change Note
H. sumichrasti	42%	31%	Cropland expanded from 31.5% to 41.6%.
H. navai	64.1%	51.1%	Evergreen forest declined significantly (loss of approximately 4,617 km² over 30 years).

IV. Conservation Implications: Urgent Action Needed

Conservation strategies must target evergreen and secondary succession forests, which are essential habitats, and mitigate the dual effects of LULCC and GCC.

Key Recommendations

Protect Refugia: Prioritize the protection of environmentally stable areas that can act as long-term refugia.
- H. sumichrasti: A core stable area is projected in Veracruz and Oaxaca.
- H. navai: Core stable areas identified in western Chiapas and central Veracruz.
Establish Protected Areas: Establishing new protected areas in karst forests is essential.
- Currently, H. sumichrasti lacks formal protection across its entire range.
- Only about 25% (approximately 560 km²) of H. navai's potential distribution is within the Ocote Biosphere Reserve.
Mitigate Fragmentation: Promote habitat restoration and establishing biological corridors to facilitate dispersion and counteract landscape fragmentation.