Overgrazing drives ant diversity loss and community homogenization in the Tumbesian dry forest in Ecuador

Gusman Montalvan, P.; Velez-Mora, D. P.; Ramon, P.; Gusman Montalvan, E.; Dominguez, D.; Donoso, D. A.

doi:10.64898/2026.03.18.712513

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bioRxiv · 10.64898/2026.03.18.712513

Overgrazing drives ant diversity loss and community homogenization in the Tumbesian dry forest in Ecuador

Gusman Montalvan, P.·Velez-Mora, D. P.·Ramon, P.·Gusman Montalvan, E.·Dominguez, D.·Donoso, D. A.

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Abstract

Tropical dry forests are among the worlds most threatened ecosystems, having lost over 60% of their original extent due to agricultural expansion and livestock grazing. Despite a growing body of research on the effects of deforestation and habitat fragmentation, the consequences of overgrazing on ant communities remain poorly documented, particularly in the Tumbesian bioregion, where uncontrolled goat grazing continues to be the primary disturbance with detrimental effects to plant communities and all the life they support. We sampled ant communities across a goat grazing gradient in a Tumbesian dry forest in Loja, Ecuador. Nine sampling clusters were distributed across a 20 x 25 km landscape on three levels of grazing degradation (low, moderate, and high). Within each cluster, we established three 60 x 60 m plots separated by at least 250 m from each other. Eighteen pitfall traps were deployed per plot. Ant community responses to goat grazing were assessed using abundance- and incidence-based analyses and quantified vegetation structure with satellite-derived NDVI. We collected 47,473 individuals (abundance) in 1,871 collection events (incidence) representing 22 species in six ant subfamilies. Species richness declined with grazing degradation (low = 20 spp.; moderate = 15; high = 13), and generalist forager species dominated the more degraded landscape. Beta diversity decomposition revealed a shift from turnover-dominated dissimilarity at the low moderate degradation transition to nestedness-dominated dissimilarity at the moderate high transition, indicating increasing habitat simplification with higher land degradation. While changes in community composition showed a non-significant trend along the land degradation gradient, our trait-based analysis evidenced effects on the functional space occupied by ants: under High degradation level intensity, ants became smaller, smoother and darker. These findings underscore the conservation value of pristine and low degradation forests and support grazing exclusion as a precautionary component of restoration strategies.

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Gusman Montalvan, P., Velez-Mora, D. P., Ramon, P., Gusman Montalvan, E., Dominguez, D., Donoso, D. A.. 2026-03-20. Overgrazing drives ant diversity loss and community homogenization in the Tumbesian dry forest in Ecuador. https://doi.org/10.64898/2026.03.18.712513

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