Evidence of the effects of agriculture on natural systems is widespread, but potential evolutionary responses in non-target species are largely uncharacterized. To explore whether exposure to agrochemicals may influence selective pressures and phenotypic expression in non-agricultural plant populations, we characterized the expression of putative anti-herbivore defense phenotypes in three non-agricultural species found upstream and downstream of irrigated rice fields in Guanacaste Province, Costa Rica. We found that plants downstream of chemically intensive agriculture showed shifts toward reduced expression of putative anti-herbivore defenses relative to upstream counterparts. In two of three tested species, leaf extracts from downstream plants were more palatable to a generalist consumer, suggesting a possible reduction of chemical defenses. In one species with multiple modes of putative defenses, we observed parallel reductions of three metrics of putative biotic and physical defenses. These reductions were concurrent with reduced herbivore damage on downstream plants. Together, these results suggest that agriculture has the potential to alter intraspecific phenotypic expression, ecological interactions, and natural selection in non-target plant populations.

Data regarding palatability of leaf extracts from three plant species (Vachellia collinsii, Malvaviscus arboreus, and Guazuma ulmifolia) were collected using generalist ant bioassays conducted in Palo Verde National Park, Costa Rica. Other phenotypic data (diameter at breast height, herbivore damage, extrafloral nectary size and number, spine size, mutualist ant response to disturbance) on Vachellia collinsii were collected in-situ or from scanned images generated from leaf samples collected in situ in an agroecosystem outside of Palo Verde National Park, Costa Rica. Samples testing for presence of one insecticide (acephate) in water samples in the agroecosystem were collected in situ at both study sites outside of Palo Verde National Park; data were generated from spectrophotometric analysis of an acetylcholinesterase inhibition assay conducted at La Selva Biological Satation. In all datasets, data were not processed. Any derived variables used in analysis are provided along with raw measurements collected in the laboratory or field.

All usage notes are described in the file README.txt archived along with the data. Missing values in the file "2013-Vachellia-herbivory.csv" reflect plant samples from which clear leaf scans were not obtained for measuring herbivore damage.