Contrasting Herpetofauna Diversity in Monoculture and Polyculture Oil Palm Farms in Costa Rica

Oil palm (OP) supplies 40% of global vegetable oil demand, more than any other crop. Skyrocketing demand precipitates palm proliferation throughout the global tropics, where oil palm plantations (OPPs) cause habitat fragmentation and biodiversity loss. Considering the potential long-term volatility of industrial OP monocultures, farmers increasingly appreciate the value of assessing the potential benefits of combining palm production with other foods and high-value crops. Local farming families, who have guided the Iniciativa Osa y Golfito (INOGO) since its inception, view OP polycultures as a welcome alternative to monocultures due to the benefits of diversifying their income, growing foods for local consumption, and not being indebted to domineering corporations. My study, part of a grander INOGO effort, quantifies and details how smallholder OP polycultures and paired, adjacent monocultures affect herpetofauna –reptile and amphibian– communities. I conducted over 400 hours of visual encounter surveys in eight pairs of family-owned OP smallholdings, half-hectare monocultures bordering half-hectare polycultures (palm intermingled with cacao, bananas, and Laurel timber trees). The 192 farm surveys were split evenly between the rainy and dry seasons, day and night, and the two aforementioned farming practices. I conducted 15 additional surveys in four local primary and secondary forest sites, to serve as a basis of comparison with the two types of OP farms. I surveyed 48 ha total in each of the three habitats: monocultures, polycultures, and forests. In OP, I recorded 2,166 amphibians across 25 species and 2,271 reptiles across 26 species, with polycultures hosting 61% of the total farm abundance. Amphibians were more species-rich in the polycultures while reptiles were more species-rich in the monocultures, entirely due to the presence of Colubrid and Dipsadid snakes. I show that polycultures significantly restore vital forest features like leaf litter, closed canopies, and increased structural heterogeneity. Consequently, species significantly associated with the polyculture experimental plots are generally endemic and highly dependent on these forest-associated microhabitats for foraging, oviposition, and shelter. Contrastingly, species associated with the monocultures, which resemble open-canopy grasslands, are snakes, disturbance-tolerant frogs, and the newly introduced “grass anole”, A. auratus (for which I document the first stable population in Costa Rica). Herpetofauna serve as an effective lens through which we can assess the impacts of anthropogenic land use changes and the benefits of biological enrichment in the case of the polycultures. It is crucial to consider specific macro and microhabitat alterations driven by land use changes to understand subsequent shifts in biodiversity. I assert that diversified OP polycultures, through inclusion of vital microhabitats via increased crop diversity, offer better support for forest-dependent herpetofauna than their monoculture counterparts. I show both smallholder practices, monocultures and polycultures, to harbor greater herpetofauna diversity than large-scale, industrial plantations previously surveyed in the same region. Nonetheless, neither type of smallholding supports nearly the same level of biodiversity as the region’s secondary and primary forests, emphasizing the importance of preserving as much intact forested land as possible. Given the rapid expansion of OPPs in the tropics, my findings are of broad significance.

Keywords: Oil palm
Amphibians
Reptiles
Costa Rica
Community ecology
Polyculture
Monoculture
Microhabitats
Land use change
Forest-dependent species
Disturbance-tolerant species
Smallholder
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Thesis
Advisor: Dirzo Rodolfo
Daily Gretchen